

Dynamic Stall Prediction of a Pitching Airfoil using an Adjusted TwoEquation URANS Turbulence Model


Pages :
110


Authors :
G. Bangga,
H. Sasongko,
The necessity in the analysis of dynamic stall becomes increasingly important due to its impact on many streamlined structures such as helicopter and wind turbine rotor blades. The present paper provides Computational Fluid Dynamics (CFD) predictions of a pitching NACA 0012 airfoil at reduced frequency of 0.1 and at small Reynolds number value of 1.35e5. The simulations were carried out by adjusting the k − ε URANS turbulence model in order to damp the turbulence production in the near wall region. The damping factor was introduced as a function of wall distance in the buffer zone region. Parametric studies on the involving variables were conducted and the effect on the prediction capability was shown. The results were compared with available experimental data and CFD simulations using some selected twoequation turbulence models. An improvement of the lift coefficient prediction was shown even though the results still roughly mimic the experimental data. The flow development under the dynamic stall onset was investigated with regards to the effect of the leading and trailing edge vortices. Furthermore, the characteristics of the flow at several chords length downstream the airfoil were evaluated.





Maximum Pressure Evaluation during Expulsion of Entrapped Air from Pressurized Pipelines


Pages :
1120


Authors :
D. M. Bucur,
G. Dunca,
M. J. Cervantes,
Pressurized pipeline systems may have a wide operating regime. This paper presents the experimental analysis of the transient flow in a horizontal pipe containing an air pocket, which allows the ventilation of the air after the pressurization of the hydraulic system, through an orifice placed at the downstream end. The measurements are made on a laboratory setup, for different supply pressures and various geometries of water column length, air pocket and expulsion orifice diameter. Dimensional analysis is carried out in order to determine a relation between the parameters influencing the maximum pressure value. A two equations model is obtained and a criterion is established for their use. The equations are validated with experimental data from the present laboratory setup and with other data available in the literature. The results presented as nondimensional quantities variations show a good agreement with the previous experimental and analytical researches.





Experimental and Numerical Investigation of a Longfin Inshore Squid’s Flow Characteristics


Pages :
2130


Authors :
A. Bahadir Olcay,
M. Tabatabaei Malazi,
A. Okbaz,
H. Heperkan,
E. Fırat,
V. Ozbolat,
M. Gokhan Gokcen,
B. Sahin,
In the present study, a threedimensional numerical squid model was generated from a computed tomography images of a longfin inshore squid to investigate fluid flow characteristics around the squid. The threedimensional squid model obtained from a 3Dprinter was utilized in digital particle image velocimetry (DPIV) measurements to acquire velocity contours in the region of interest. Once the threedimensional numerical squid model was validated with DPIV results, drag force and coefficient, required jet velocity to reach desired swimming velocity for the squid and propulsion efficiencies were calculated for different nozzle diameters. Besides, velocity and pressure contour plots showed the variation of velocity over the squid body and flow separation zone near the head of the squid model, respectively. The study revealed that viscous drag was nearly two times larger than the pressure drag for the squid’s Reynolds numbers of 442500, 949900 and 1510400. It was also found that the propulsion efficiency increases by 20% when the nozzle diameter of a squid was enlarged from 1 cm to 2 cm.





Acoustic Detection of Cavitation Inception


Pages :
3140


Authors :
M. A. Hosien,
S. M. Selim,
Cavitation phenomenon can cause deterioration of the hydraulic performance, damage by pitting, material erosion, structure vibration and noise in fluid machinery, turbomachinery, ship propellers and in many other applications. Therefore, it is important to detect inception of cavitation phenomenon. An experimental study has been carried out in order to investigate the noise radiated by various cavitating sources to determine the validity of noise measurements for detecting the onset of cavitation. Measurements have been made measuring the noise radia¬ted by a number of configurations in a water tunnel at various operating condition to determine the onset of cavita¬tion. The measurements have been conducted over a frequency range of 31.5 Hz to 31.5 kHz in onethird octave bands. The on¬set of cavitation was measured visually through a Perspex side of the working section of the water tunnel. Moreover, a theoretical estimate of the pressure radiated from the cavitation nuclei at their critical radii and their frequency was presented. Tests indicated that, generally, at the point of visual incep¬tion there was a marked rise of the sound pressure level in the highfrequency noise, whilst the lowfrequency noise in¬creased as the cavitation developed. This finding was supported by the theoretical estimate of the pulsating frequency of cavitation nuclei. The results illustrated that the visual observations of inception confirm the noise measurements.





Effect of Reverting Channels on Heat Transfer Performance of Microchannels with Different Geometries


Pages :
4153


Authors :
M. Farzaneh,
M. R. Tavakoli,
M. R. Salimpour,
This study investigated the effect of reverting microchannels inside a heat sink on increase of cooling rate as well as the effect of their different configurations on maximum temperature and pressure drop. Based on the convection heat transfer mechanism, selection of the embedded microchannels’ configurations including circular, square and triangular ones, was studied for geometric optimization of the discussed heat sinks. The goal was to minimize the thermal resistance through optimizing the geometries. The volume ratio was defined as the ratio of the volume taken up by microchannels to the solid volume (portion of the heat sink not occupied by microchannels) and was considered as 0.05. According to the results, in addition to obtaining a more uniform temperature distribution, reverting channels remarkably reduced the maximum temperature. Moreover, the heat sink with square shape showed less thermal resistance as compared to the other two geometries.





A New Approach to Reduce Memory Consumption in Lattice Boltzmann Method on GPU


Pages :
5567


Authors :
M. Sheida,
M. TaeibiRahni,
V. Esfahanian,
Several efforts have been performed to improve LBM defects related to its computational performance. In this work, a new algorithm has been introduced to reduce memory consumption. In the past, most LBM developers have not paid enough attention to retain LBM simplicity in their modified version, while it has been one of the main concerns in developing of the present algorithm. Note, there is also a deficiency in our new algorithm. Besides the memory reduction, because of high memory call back from the main memory, some computational efficiency reduction occurs. To overcome this difficulty, an optimization approach has been introduced, which has recovered this efficiency to the original twosteps twolattice LBM. This is accomplished by a tradeoff between memory reduction and computational performance. To keep a suitable computational efficiency, memory reduction has reached to about 33% in D2Q9 and 42% in D3Q19. In addition, this approach has been implemented on graphical processing unit (GPU) as well. In regard to onboard memory limitation in GPU, the advantage of this new algorithm is enhanced even more (39% in D2Q9 and 45% in D3Q19). Note, because of higher memory bandwidth in GPU, computational performance of our new algorithm using GPU is better than CPU.





Numerical Study of Double Diffusive Convection in a Lid Driven Cavity with Linearly Salted Side Walls


Pages :
6979


Authors :
N. Reddy,
K. Murugesan,
Double diffusive convection phenomenon is widely seen in process industries, where the interplay between thermal and solutal (mass) buoyancy forces play a crucial role in governing the outcome. In the current work, double diffusive convection phenomenon in a lid driven cavity model with linearly salted side walls has been studied numerically using Finite element simulations. Top and bottom walls of the cavity are assumed cold and hot respectively while other boundaries are set adiabatic to heat and mass flow. The calculations of energy and momentum transport in the cavity is done using velocityvorticity form of NavierStokes equations consisting of velocity Poisson equations, vorticity transport, energy and concentration equations. Galerkin’s weighted residual method has been implemented to approximate the governing equations. Simulation results are obtained for convective heat transfer for 100<Re<500, 50<N<50 and 0.1<Ri<3.0. The average Nusselt number along the hot wall of the cavity is observed to be higher for higher Richardson number when buoyancy ratio is positive and vice versa. Maximum Nusselt number is recorded at buoyancy ratio 50 and Richardson number 3.0, on the other hand low Nusselt number is witnessed for buoyancy ratio 50.





Numerical Investigation of Temporal Variation of Density Flow and Parameters


Pages :
8194


Authors :
F. Üneş,
N. Ağiralioğlu,
Experimental investigations and observations indicate that water quality modeling is related to the formation of flows in the dam reservoirs. Correct estimation of dam reservoir flow, plunging point and plunging depth are very important for the dam reservoir sedimentation and water quality problem. Therefore, inflow riverwater into a dam is modeled in two dimensions through a reservoir with sloping bottom. The model is developed using nonlinear and unsteady continuity, momentum, energy and kε turbulence model equations. The equations of the model are solved based on the initial and boundary conditions of the dam reservoir flow for a range of bottom slopes. In addition to velocity, temperature and turbulence viscosity variation through the dam reservoir, the effects of density flow parameters such as plunging depths, plunging points, mixing rate are determined from the simulation results. The results of the present model are compared to the previous experimental works and the mathematical models.





Experimental Investigation on Supercavitating Flow over Parabolic Cavitators


Pages :
95102


Authors :
M. Moghimi,
N. M. Nouri,
E. Molavi,
In this paper experimental study was carried out to investigate supercavitation around parabolic cavitators. Various types of cavitators, such as disk, cone, and parabolic, were designed and manufactured. Also, the shape of the cavities formed behind these bodies were considered and compared. Dimensionless parameters such as dimensionless length and the diameter of the cavity as well as the dimensionless required air flow on the cavitators were obtained. The results showed that parabolic cavitators have an optimum design in comparison with the disk and cone cavitators due to their insignificant capability to reduce the drag force, yet the cavity’s length has a moderate size. It was also observed that this type of cavitator is capable of forming a cavity with a dimensionless length up to L/d= 33 and a dimensionless width up to D/d= 3.6. Moreover, parabolic cavitators require the highest amount of air injected in comparison with the cone and disk types; therefore, they operate in lower cavitation numbers. Since no other experimental data has been reported so far, this work reports the experimental characteristic behavior of parabolic cavitators.





Flow Control of Transonic Airfoils using Optimum Suction and Injection Parameters


Pages :
103115


Authors :
Z. Seifollahi Moghadam,
A. Jahangirian,
In this paper, the application of the surface mass transfer optimization in shock waveboundary layer interaction control at offdesign conditions of transonic aircraft wing is presented. The suction or injection parameters include for example its position on the airfoil, its angle, the length of the hole and the rate of the injected or sucked flow. The optimization process is carried out using an efficient Genetic Algorithm (GA) method. The compressible viscous flow equations in Reynolds Averaged form are solved together with a twoequation kepsilon turbulence model to accurately compute the objective function. Four different objective functions are introduced including maximum lift to drag ratio, minimum drag coefficient, maximum lift to drag ratio with no drag increment and minimum drag coefficient with no lift decrement. Effectiveness of each objective function is examined by comparing the optimum results in terms of the flow control parameters and flow characteristics.





Analysis of Interrupted Rectangular Microchannel Heat Sink with High Aspect Ratio


Pages :
117126


Authors :
H. Kamal,
A. Dewan,
A computational modelling of microchannel heat sinks with high aspect ratio has been performed to compare the geometrical features in the plane parallel to the heating surface and to determine the optimum configuration for the best heat transfer characteristics. A periodic thermal development of flow can cause significant heat transfer enhancement. A consensus on a particular geometrical configuration that provides the best heat transfer characteristics has not been reached in the literature, although many novel ideas have been proposed recently. Firstly the validity and applicability of microchannel sink modelling is presented followed by an optimization of parameters of interrupted microchannel heat sink. Consequences of the multichannel effect due to the introduction of transverse microchamber are also presented. It has been shown that the average Nusselt number of the microchannel heat sink increases by the introduction of a transverse microchamber with the additional advantage of a lower pressure drop. There exists an optimum width for the transverse microchamber for which the interrupted microchannel heat sink shows optimum characteristics.





Enhancement of Mixing Performance of NonNewtonian Fluids using Curving and Grooving of Microchannels


Pages :
127141


Authors :
S. Baheri Islami,
M. Khezerloo,
In this study, a numerical investigation was performed on the mixing of nonNewtonian powerlaw fluids in curved micromixers with powerlaw indices between 0.49 and 1 and Reynolds numbers between 0.1300. The properties of water and CMC solution were used for simulation of Newtonian and nonNewtonian fluid flows, respectively. The effects of grooves embedded on the bottom wall of micromixers and geometrical parameters such as depth and angle of grooves on mixing performance were examined. The mixing of nonNewtonian fluids using this kind of micromixers has not been studied before. Eventually, using of inclined grooves with 30° inclination angle was studied. Open source CFD code of OpenFOAM was utilized to simulate the mixing process. The results showed that the grooves caused chaotic advection and improved the mixing performance but had no significant effect on dimensionless pressure drop. Also, the grooves with 30◦ angle showed better mixing index for all values of powerlaw indices.





Flow Characteristics of a Pipe Diffuser for Centrifugal Compressors


Pages :
143155


Authors :
Z. Sun,
X. Zheng,
Z. Linghu,
The pipe diffuser, an efficient kind of radial bladed diffuser, is widely used in centrifugal compressors for gas turbine engines. This paper investigates flow characteristics of a pipe diffuser for centrifugal compressors by solving threedimensional Reynoldsaveraged NavierStokes equations. The results show that the pipe diffuser is adaptable to high Mach number incoming flows, and its unique leading edge could uniform the flow distortion. Numerical analysis indicates that the choke in pipe diffuser occurs suddenly, which leads to the dramatically steep performance curves near choke condition. Besides, it is found that the first half flow passage is particularly important to the pipe diffuser performance as it influences the choking behavior, the static pressure distribution, and the matching, so more attention should be paid to this region when designing or optimizing a pipe diffuser. Two counterrotating vortices generated in the diffuser inlet region are captured by numerical simulation, and they can exist in the downstream of the diffuser passage. More detailed analysis show that these two vortices dominate the flow structure in the whole diffuser passage by shifting flow to certain positions and forming highmomentum flow cells and wake flow cells. The leading edge formed by the intersection of adjacent diffuser passages significantly affects this pair of vortices. In addition, these two vortices also affect the flow separation in pipe diffuser flow passages, they suppress separation near the front wall and back wall while facilitate separation at center locations. Therefore, it is recommended to design the leading edge of the pipe diffuser carefully to control the vortices and obtain a better flow field.





Numerical Investigation of Optimization of Injection Angle Effects on Fluidic Thrust Vectoring


Pages :
157167


Authors :
F. Forghany,
M. TaeibiRahni,
A. AsadollahiGhohieh,
A computational investigation was conducted to optimize the fluidic injection angle effects on thrust vectoring. Numerical simulation of fluidic injection for shock vector control, with a convergentdivergent nozzle concept was performed, using URANS approach with SpalartAllmaras turbulence model. The fluidic injection angles from 60º to 120º were investigated at different aerodynamic and geometric conditions. The current investigation demonstrated that secondary injection angle is an essential parameter in fluidic thrust vectoring. Computational results indicated that, optimizing secondary injection angle would have positive impact on thrust vectoring performance. Furthermore, in most cases, decreasing expansion ratio of the nozzle with increasing NPR has negative impact on pitch thrust vector angle and thrust vectoring efficiency. That is, the highest pitch thrust vector angle is obtained by decreasing nozzle expansion ratio with increasing SPR in smaller fluidic injection angles. In addition, the current investigation attempted to initiate a database of optimized injection angles with different essential parameter effects on thrust vectoring, in order to guide the design and development of an efficient propulsion system.





Heat Transfer Enhancement in a Stagnant Dielectric Liquid by the Up and Down Motion of Conductive Particles Induced by Coulomb Forces


Pages :
169182


Authors :
G. Eslami,
E. Esmaeilzadeh,
P. GarcíaSánchez,
A. Behzadmehr,
S. Baheri ,
When charged particles are exposed to an electric field the wellknown Coulomb force acts on them. In this investigation, this force is utilized to induce vertical motion of spherical steel particles submerged in a dielectric liquid. The interelectrode space of a two parallel electrode system is filled with the liquid and dispersed steel particles, which become charged after contact with the electrodes. Experiments were carried out to measure the effect of this particle motion on the heat transfer between an electrode surface and an adjacent stagnant dielectric liquid. In order to interpret the experimental data, the dynamics of particles was analytically studied for low particle volume concentrations. Experimental results demonstrate significant heat transfer enhancement on low viscosity dielectric liquids. A detailed discussion is presented on the possible mechanisms responsible for such an enhancement.





Cross Flow past Circular Cylinder with Waviness in Confining Walls near the Cylinder


Pages :
183197


Authors :
R. Deepakkumar,
S. Jayavel,
S. Tiwari,
Two dimensional flow past circular cylinder confined by walls with local waviness near the cylinder has been studied. The aim of the present study is to identify the ability of the waviness to control vortex shedding, for which two different waviness configurations such as inphase configurations (IPC) and outphase configurations (OPC) are considered. Further, the effect of location of the local waviness with respect to the cylinder has also been studied. Air is the working fluid and the flow is assumed to be laminar and incompressible at Re=200. The finite volume based CFD solver Ansys Fluent (Version 15.0) is used for the computations. Flow characteristics such as drag, lift and Strouhal number are computed. Interesting shedding characteristics and drag reduction are observed due to the presence of local waviness. However, the significant factor is the location of waviness in the confining walls that leads to complete suppression of shedding. Among various locations and configurations of waviness studied, waviness in downstream with OPC3 suppresses the vortex shedding completely with reduced drag.





Scattering of Fexural Gravity Waves by a TwoDimensional Thin Plate


Pages :
199208


Authors :
S. Banerjee,
P. Maiti,
D. Mondal,
An approximate analysis based on standard perturbation technique together with an application of Green’s integral theorem is used in this paper to study the problem of scattering of water waves by a two dimensional thin plate submerged in deep ocean with ice cover. The reﬂection and transmission coefﬁcients upto ﬁrst order are obtained in terms of the shape function describing the plate and are studied graphically for different shapes of the plate.





Experimental Investigation of Laminar Convection Heat Transfer of Al2O3Ethylene GlycolWater Nanofluid as a Coolant in a Car Radiator


Pages :
209219


Authors :
G. A. Sheikhzadeh,
M. M. Fakhari,
H. Khorasanizadeh,
In this experimental study, heat transfer of a coolant nanofluid, obtained by adding alumina nanoparticles to Ethylene Glycolwater (60:40 by mass), in a car radiator has been investigated. For this purpose, an experimental setup has been designed and constructed. Firstly, to investigate the accuracy of the results, the experiments have been done for the base fluid. Then the experiments have been performed for the nanofluid with different nanoparticles volume fractions of 0.003, 0.006, 0.009 and 0.012. To ensure laminar flow regime three coolant flow rates of 9, 11 and 13 lit/min have been tested. The thermophysical properties have been calculated using the recently presented temperature dependent models in the literature. According to the results, both the convective heat transfer coefficient and Nusselt number increase (about 9%) with increasing the coolant flow rate. Also, convective heat transfer coefficient increases with increasing the nanoparticles volume fraction. Although Nusselt number decreases when nanofluid is utilized, it enhances as the nanoparticles volume fraction increases. Based on the experimental results obtained, A new empirical correlation has been developed for average Nusselt number of Al2O3EGwater nanofluid in developing region of flat tubes of car radiator for laminar flow and its maximum error is 3%.





Prediction of VelocityDipPosition at the Central Section of Open Channels using Entropy Theory


Pages :
221229


Authors :
S. Kundu,
An analytical model to predict the velocitydipposition at the central section of open channels is presented in this study. Unlike the previous studies where empirical or semiempirical models were suggested, in this study the model is derived by using entropy theory. Using the principle of maximum entropy, the model for dipposition is derived by maximizing the Shannon entropy function after assuming dimensionless dipposition at the central section as a random variable. No estimation of empirical parameter is required for calculating dipposition from the proposed model. The model is able to predict the location of maximum velocity at the central section of an open channel with any aspect ratio. The developed model of velocitydipposition is tested with experimental data from twentytwo researchers reported in literature for a wide range of aspect ratio. The model is also compared with other existing empirical models. The present model shows good agreement with the observed data and provides least prediction error compared to other models.





Numerical Comparison of the Parameters Influencing the Turbulent Flow using a Tshaped Spur Dike in a 90° Bend


Pages :
231241


Authors :
M. Vaghefi,
Y. Safarpoor,
M. Akbari,
Spur dikes are used for river training purposes. To meet the navigability of rivers, the mean annual flow is considered; hence, in terms of river flooding, spur dikes are necessarily submerged. Considering the importance of submerged spur dikes, this paper studied the effects of a Tshaped spur dike’s submergence ratios on turbulent flow parameters in a 90° bend using the SSIIM as a commercial CFD model. The SSIIM numerical model solves the NavierStokes equations with the k ε model on a threedimensional, almost general, nonorthogonal grid. Submergence ratios of 0 (nonsubmerged), 5, 15, 25 and 50% were evaluated for parameters affecting the turbulent flow such as kinetic energy, pressure, eddy viscosity and the Froude number. It was observed that by increasing the spur dike submergence ratio from 0% (nonsubmerged) to 50%, in addition to changes in the values of pressure and kinetic energy, the Froude number changed in the bend and increased 2.1 times at the inner bank of the bend exit, and the eddy viscosity near the bed, which is the decisive factor of the turbulent flow, reduced by 42%. At the bed near the spur dike wing, the amount and range of kinetic energy reduced by increasing the submergence ratio. Near the bed, for all submergence ratios, the maximum pressure occurred at the upstream end of the spur dike.





A Computational Study of HighSpeed Microdroplet Impact onto a Smooth Solid Surface


Pages :
243256


Authors :
J. Q. Feng,
Numerical solutions of highspeed microdroplet impact onto a smooth solid surface are computed, using the interFoam VoF solver of the OpenFOAM® CFD package. Toward the solid surface, the liquid microdroplet is moving with an impinging gas flow, simulating the situation of ink droplets being deposited onto substrate with a collimated mist jet in the Optomec Aerosol Jet® printing process. For simplicity and computational efficiency, axisymmetric incompressible flow is assumed here for the freesurface fluid dynamic problem. The computed values of maximum spread factor ξ, for the range of parameters relevant to Aerosol Jet® printing, are found in good agreement with some of the correlation formulas proposed by previous authors in the literature. A formula of improved accuracy is then obtained for evaluating ξ of Aerosol Jet® deposited droplets, by combining selected formulas from different authors with appropriate modifications. The computational results also illustrate droplet impact dynamics with lamella shape evolution throughout the spreading, recedingrelaxation, and wetting equilibrium phases, consistent with that observed and described by many authors. This suggests a scaleinvariant nature of the basic droplet impact behavior such that experiments with larger droplets at the same nondimensional parameter values may be applicable for studying microdroplet impact dynamics. Significant free surface oscillations can be observed with low viscosity droplets. The border line between free surface oscillations and aperiodic creeping to the capillary equilibrium shape appears at Oh ∼ 0.25. Droplet bouncing after receding is prompted with large contact angles at solid surface (as consistent with findings reported in the literature), but can be suppressed by increasing the droplet viscosity.





Numerical Study on the Effect of Single Shallow Circumferential Groove Casing Treatment on the Flow Field and the Stability of a Transonic Compressor


Pages :
257265


Authors :
M. Agha Seyed Mirzabozorg,
M. Bazazzadeh,
M. Hamzezade,
The present research investigates the effect of the location and the width of single shallow circumferential groove casing treatment on the flow field and the stability improvement of NASA Rotor 37 utilizing the help of computational fluid dynamics. At first, steady state simulation of Rotor37 was presented for smooth casing (without groove). Then, forty five various grooved casing were simulated and compared with the smooth casing. The results indicated that narrow grooves had slight effect on the adiabatic efficiency but as the width of the groove was increased, a decline in efficiency was observed. The investigation on the stall margin revealed that narrow grooves next to the leading edge could improve the stall margin by a reduction in the size of vortex breakdown zone. Mediumwidth grooves displayed an effective role in delaying the separation produced by shock wave and boundary layer interaction on the blade suction side near the casing. This type of grooves could improve the stall margin more than narrow grooves when located on the top of separation zone near the blade suction side. Wide grooves had negative effect on the stall margin and caused a significant drop in the efficiency and the total pressure ratio of the compressor.





Effect of Liquid Viscosity and Solid Inventory on Hydrodynamics in a Liquid  solid Circulating Fluidized Bed


Pages :
267274


Authors :
N. Gnanasundaram,
A. Venugopal,
G. Ullas,
Y. Katragadda,
A comprehensive hydrodynamic study of a Liquid  Solid Circulating Fluidized Bed (LSCFB) is conducted with changes in viscosity of the fluidizing medium and the inventory height of solids initially fed into the system. An LSCFB of height 2.95m and riser outer diameter 0.1m was chosen for experimentation. The three liquid media systems with varying viscosities that were chosen were water, glycerol 10% (v/v) and glycerol 20% (v/v). Effect of inventory on the hydrodynamics was also studied, by taking initial heights of inventory to be 15cm, 25cm and 35cm. The hydrodynamic studies concentrated on pressure gradients along the axial pressure tapings, axial solid holdup, average solid holdup, solid circulation rate and slip velocity. Uniformity in axial solid holdup and average solid holdup was validated for changes in viscosity and inventory. Solid flux was seen to follow an inverse relationship to holdup. The changes in slip velocity with varying viscosity and inventory were studied, and found to decrease with both variables. The distribution parameter, Co of the drift flux model was found to be in the range of 0.9830.994, suggesting nonuniformity in radial solid distribution, with higher solid concentration by the walls compared to the core of the column.





Identification of Drag Force of the Underwater Vehicles


Pages :
275281


Authors :
Y. Jahangardy,
R. Madoliat,
N. M. Nouri,
An inverse analysis is conducted for the estimation of drag coefficient and wake’s width in incompressible turbulent flows over the moving underwater bodies. The inverse analysis uses the laws of momentum and mass conservation for a control volume to estimate the drag coefficient and the wake’s width from the measured velocity in the wake. The drag coefficient and wake’s width are determined as unknown parameters by the Levenberg–Marquardt algorithm. The proposed inverse method is applicable for an environment without boundaries (e.g., the sea). Several experiments are conducted to evaluate the developed inverse algorithm. The wake velocity behind a cylinder located in the flow field is measured by a calibrated pitot tube and is used as an input to the algorithm. The cylinder is selected as the test body, because its hydrodynamic information is available in the literature. The effects of the tunnel’s wall and the turbulence intensity are considered in the results of the algorithm. The estimated drag coefficient is validated by the values presented in the literature. The estimated wakevelocity profiles are fitted favorably with the measured velocities at the corresponding locations. It is shown that the proposed inverse method can be used to estimate the drag coefficient and wake’s width of the underwater vehicles with very good accuracy.





Effect of Geometry Modifications on the Vectoring Performance of a Controlled Jet


Pages :
283291


Authors :
M. N. Tomac,
Jet vectoring performances of ten different designs with various depths and geometrical outlines were quantified through constant temperature anemometry measurements for a Reynolds number range from 10,000 to 30,000 by using passive and active flow control methods at cold flow. The reference design was based on NASA’s double throat nozzle concept and a selfinjection double throat nozzle design that uses similar flow control concept as the reference design, were also tested for performance comparison. Furthermore, jet vectoring performance of a single throat design, utilizing Coanda effect for jet vectoring, was also quantified. Results indicated jet vectoring angles starting from 2° up to 47° for a control jet flow rate range from 1% up to 10% with respect to the primary jet flow rate in the investigated Re range. Maximum jet vectoring angle was achieved with a single throat design which incorporates small step geometry before the Coanda surface for more effective flow attachment and these results were compared with the vectoring performance of the double throat nozzle designs.





Heat Transfer Analysis of Flat Plate Subjected to MultiJet Air Impingement using Principal Component Analysis and Computational Technique


Pages :
293306


Authors :
P. Chandramohan,
S. N. Murugesan,
S. Arivazhagan,
The aim of this work is to investigate experimentally the variation in temperature, heat transfer coefficient and Nusselt number of a hot plate subjected to multijet air impingement cooling to use the multiobjective optimization technique to arrive at optimum conditions. A flat plate of 15 cm x 10 cm is heated through a heating foil with a constant heat flux of 7667 W/m2. Air jets with and without swirling action are considered, fixing the distance of target surface from nozzle exit at 2D, 4D and 6D. Reynolds numbers 18000, 20000and 22000 and pipe diameters 8mm, 10mm and 12 mm have been considered for investigation. Experiments are designed and analyzed using Taguchi’s technique, coupled with principal component analysis for multivariate optimization by calculating multiresponse performance index (MRPI). Based on the observations made, it is concluded that lower H/D ratio and higher Reynolds number result in higher heat transfer coefficient, in accordance with the first principles. Heat transfer coefficient obtained for jets with swirl is compared with that of jet without swirling for the same Reynolds number and H/D ratio. Furthermore, it is concluded that introducing swirl results in increase of heat transfer coefficients for all the test conditions for 10mm and 12mm diameter jets. However for 8mm jet, introduction of swirl reduced the heat transfer rate for all the test conditions. From Analysis of Variance (ANOVA), it is found that significant contributions on outputs are due to the effect of H/D ratio and Reynolds number. Confirmation experiments with optimum condition result in improved heat transfer coefficient and Nusselt number. Numerical simulation has also been performed with the optimum condition. It is observed that the simulation results are in consistence with the experimental results.





Free Surface Thin Film Flow of a Sisko’s Fluid over a Surface Topography


Pages :
307317


Authors :
R. A. Shah,
P. Gaskel,
S. Veremieiev,
The flow of a thin film down an inclined surface over topography is considered for the case of liquids with Sisko’s model viscosity. For the first time lubrication theory is used to reduce the governing equations to a nonlinear evolution equation for a current of a Sisko’s model nonNewtonian fluid on an inclined plane under the action of gravity and the viscous stresses. This model is solved numerically using an efficient Full Approximation Storage (FAS) multigrid algorithm. Free surface results are plotted and carefully examined near the topography for different values of powerlaw index np, viscosity parameter m, the aspect ratio A and for different inclination angle of the plane with the horizontal. Number of complications and additional physical effects are discussed that enrich real situations. It is observed that the flows into narrow trenches develop a capillary ridge just in front of the upstream edge of a trench followed by a small trough. For relatively small width trenches, the free surface is almost everywhere flat as the dimensional width of the trench is much smaller than the capillary length scale. In this region, surface tension dominates the solution and acts so as to stretch a membrane across the trench leading to smaller height deviations. The ridge originates from the topographic forcing which works to force fluid upstream immediately prior to the trench before helping to accelerate it over. The upstream forcing slows down the fluid locally and increases the layer thickness.





TwoDimensional Transient Modeling of Energy and Mass Transfer in Porous Building Components using COMSOL Multiphysics


Pages :
319328


Authors :
M. Maliki,
N. Laredj,
K. Bendani,
H. Missoum,
This paper reports on a transient heat, air and moisture transfer (HAM) model. The governing partialdifferential equations are simultaneously solved for temperature and capillary pressure through multilayered porous media, including the nonlinear transfer and storage properties of materials. Using partial differential equations functions, some thermophysical properties of porous media are converted into coefficients depending on temperature and capillary pressure. Major features of the model are multidimensional and transient coupling of heat, air and moisture transport. The coupled equations are solved using the COMSOL Multiphysics timedependent solver. This solver enables HAM (Heat, Air, Moisture) modeling in porous media. Besides, the good agreements obtained with a 2D benchmark suggest that the model can be used to assess the hygrothermal performance of building envelope components. This paper concludes that the total heat flux in the insulated wall represents only the quarter of that crossing the uninsulated concrete roof. On the other hand, the concrete having the lowest water vapour permeability of all used materials allows maintaining the vapour pressure levels close to the initial value (103 Pa). This induces a situation of interstitial condensation within the concrete of the roof. Being able to evaluate the hygrothermal behaviour, the proposed model may turn out to be a valuable tool to solve other building problems.





Similarity Solution for a Cylindrical Shock Wave in a Selfgravitating, Rotating Axisymmetric Dusty Gas with Heat Conduction and Radiation Heat Flux


Pages :
329341


Authors :
R. Bajargaan,
A. Patel,
Similarity solutions are obtained for unsteady adiabatic propagation of a cylindrical shock wave in a self gravitating, rotating, axisymmetric dusty gas with heat conduction and radiation heat flux in which variable energy input is continuously supplied by the piston. The dusty gas is taken to be a mixture of nonideal gas and small solid particles. Azimuthal fluid velocity and axial fluid velocity in the ambient medium are taken to be variable. The equilibrium flow conditions are assumed to be maintained. The initial density is assumed to be constant. The heat conduction is expressed in terms of Fourier’s law and the radiation is taken to be of the diffusion type for an optically thick grey gas model. The thermal conductivity and the absorption coefficient are assumed to vary with temperature and density. The effects of the variation of the gravitational parameter and the heat transfer parameters on the shock strength and the flow variables such as radial velocity, azimuthal velocity, axial velocity, density, pressure, total heat flux, mass behind the shock front, azimuthal vorticity vector, axial vorticity vector, isothermal speed of sound and adiabatic compressibility are studied. It is found that the presence of gravitation effect in the medium modify the radiation and conduction effect on the flow variables.





Experimental and Numerical Investigation of Drag Force at High Speed Diver Motion in Different Depths from Free Surface


Pages :
343352


Authors :
M. R. Sadeghizadeh,
B. Saranjam,
R. Kamali,
In this study, the drag force exerted on diver body is investigated by computational fluid dynamic and then the results validity is compared with towing tank experiments. The final target of this research is to evaluate thruster power for diver to reach the high speed motion underwater. In the last decade, the low speed diver motion has been studied by researchers for sport purposes and improvement of diving time, while in this research authors have tried to increase diver speed up to 10 ms1 by use of electrical thruster mechanism. The numerical simulations are done by computational fluid dynamics considering three dimensional two phase turbulent flow. The geometry of the swimmers' bodies was generated by 3D scanning and image modeling of real divers in experimental diving test. Different turbulent models might be applied in specific case but the favorable one (k– method) is selected for estimating the forces on the divers. The experiments are done by five divers and one mannequin with 5 depths and 9 speeds. The numerical simulation results agree well with those of experimental tests in range up to 8 ms1 but because of lab limitations more results between 8 ms1 and 10 ms1 were not accessible.





Asymmetric Water Entry of Twin Wedges with Different Deadrises, Heel Angles, and Wedge Separations using Finite Element Based Finite Volume Method and VOF


Pages :
353368


Authors :
R. Shademani,
P. Ghadimi,
Asymmetric water entry of twin wedges is investigated for deadrise angles of 30 and 50 degrees, and heel angles of 5, 10, 15, and 20 degrees as well as wedge separation ratios of 1 and 2. Finite Element based Finite Volume method (FEMFVM) is used in conjunction with Volume of Fluid (VOF) scheme for the targeted analyses. Free surface evolution and impact forces versus time are determined and comparisons of the maximum force of the wedges against each other are presented for all the considered cases. It is demonstrated that the impact force on the second wedge is always greater than the first one by a minimum of 6% and maximum of 146% which is a very significant increase in the impact force and may cause high accelerations and damage to the structure. It is also observed that the mentioned effects increase with decreasing deadrise angle and increasing heel angle.





Analytical and Numerical Studies of a Steam Ejector on the Effect of Nozzle Exit Position and Suction Chamber Angle to Fluid Flow and System Performance


Pages :
369378


Authors :
A. S. Ramesh,
S. Joseph Sekhar,
Nozzle exit position [NXP] plays a vital role in the performance of the ejector, but its values are specified in a range for the required operating condition. In this study instead of the range of values, a specific value, named as entrainment diameter is developed and its effect on the performance of the ejector is studied for several combinations of suction chamber angle using numerical method. The effect of the condenser and boiler pressures on the performance of the ejector are also studied to ensure the offdesign operating conditions. The entrainment diameter of an ejector is derived analytically by solving one dimensional compressible fluid flow equations using MATLAB. To study the effect of entrainment diameter on the performance of the ejector, CFD technique is employed. Analytical and numerical results are validated with experimental data available in the previous studies. For 7 kW refrigeration capacity, it is inferred that the suction chamber angle of 18° and the corresponding entrainment diameter 90.8 mm with the NXP of 23.62 mm yield the maximum entrainment ratio. The study predicts that the performance of the ejector is highly influenced by the pressure increment at the exit of the nozzle, while the suction chamber angle is between 12° to 21°.





A Novel Approach for Measurement of Peak Expiratory Velocity


Pages :
379387


Authors :
C. C. Wu,
F. M. Yu,
It is well known that behind the orifice in a pipe, flow velocity increases and pressure decreases simultaneously. The generated sound appears that is caused by the pressure fluctuations that occur as the flow passes through the orifice. Then the flow velocity is averaged over a pipe crosssection and is considered as a constant, it can be seen that the amplitude of sound increases with increases in the expiratory velocity. An experimental study of the quantitative analysis of sound pressure level correlated with expiratory velocity in a pipe was conducted using an apparatus that includes an air pump in conjunction with a pipe, a microphone, and an orifice plate, among other instruments. The regression and analysis of the results shows that the pressure fluctuation of sound spectra can be correlated to the expiratory velocity of a pipe. The experiment is conducted under conditions where the air passing through the orifice has an averaged expiratory velocity ranging from 0.88 m/sec to 1.35 m/sec, an inlet temperature of 298.15 K, and where the outlet pressure is that of the atmosphere. In this experiment, the Mach number is very low, and the compressibility effects can be ignored. The obstacle orifice plate was placed in the center of the pipe, and a microphone was mounted flush downstream to acquire the sound pressure data on the pipe wall. The measured results show that the approach for measuring the expiratory velocity using a microphone can be justified, and there exists a good correlation between the Power Spectral Density (PSD) of sound pressure fluctuation and the peak expiratory velocity.





Mathematical Modeling of the Flow of DieselCNG Fuel Mixture in a Pipe under the Influence of a Magnetic Field


Pages :
389396


Authors :
H. A. Abdul Wahhab,
A. R. A. Aziz,
H. H. AlKayiem,
M. S. Nasif,
Horizontal bubbly flow is encountered in various gas and oil facilities and industrial systems. Bubbly flow is characterized by the ability to provide large interfacial areas for heat and mass transfer. Nonetheless, horizontal bubbly flow orientation has received less attention when compared to vertical bubbly flow. This paper presents development of mathematical model and discusses the results obtained from the simulation of hydromagnetic flow of DieselCNG fuel mixture in a horizontal pipe, as predicted by the developed model. The fundamental equations of unsteady, twophase liquidgas under an imposed magnetic field were derived and presented. Derivation procedure of the velocity distribution of the liquid and gas phases and the inverse Stokes’ number of a bubbly flow are presented. These governing nonlinear partialdifferential equations have been solved numerically using a Fourier–Bessel series. Results obtained from the model solution show that the axial velocities of liquid and gas, in laminar flow, have decreased and the slip ratio has increased with the increase of the magnetic field intensity. While, the magnetic field parameter, Ha increased the probability of decreasing the bubbles radii and increasing the bubbles number (n.Rb).





Force Generation Mechanisms by an Insect Wing in an Idealized Hovering Motion


Pages :
397411


Authors :
H. R. Hamdani,
A. Aizaz,
M. A. Naqvi,
An Unsteady force generation mechanisms (delayed stall, wake capture and rotational lift) during idealized hovering of insect flight at Reynolds number (Re) of 136 have been identified in this research. Dependence of flow physics on Re forms the basis of present study to observe the dependence of unsteady force generation mechanisms on Re. A systematic study has been carried out by increasing Re from 136 to 4000 to investigate persistence of delayed stall, wake capture and rotational lift phenomenon. Using the solution of 3D NavierStokes equations, the aerodynamic force and the detailed flow structure around the wing are obtained which can provide useful insights into mechanism of unsteady force generation during idealized hovering at Re=4000. After grid and Mach number sensitivity analysis, the results are compared with previous studies at Re=136 for the code validation. The aerodynamic force and flow structure of a wing performing hovering motion at Re=4000 is calculated by solving NavierStokes equations. Re=4000 is selected on the premise that the length scale (mean aerodynamic chord) becomes closer to a Micro Air Vehicle (MAV); furthermore 30 times increase in Re (from 136 to 4000) is considered sufficient to assess changes in flow physics while remaining in laminar flow regime. Calculations are conducted for idealized hovering motion during which stroke 1 is initiated in still air, followed by flipping motion for reversing the direction and then stroke 2 (similar to stroke 1 but in opposite direction). Results obtained from this research are helpful for future work where they can be compared with those obtained from actual wing kinematics to assess the impact of kinematics on unsteady mechanisms.





ThreeDimensional Axisymmetric Solidification of a Viscous Incompressible Flow in the Stagnation Point Region


Pages :
413420


Authors :
A. Shokrgozar Abbasi,
The history of the study of fluid solidification in stagnation flow is very limited. Among these studies, only one twodimensional Cartesian coordinate case has considered fluid viscosity and pressure variation along the boundary layer. In the present paper, the solidification process of an incompressible viscous fluid in a threedimensional axisymmetric coordinate system is considered. The solidification is modeled by solving the momentum equations governing a problem in which a plate is moving toward an impinging fluid with a variable velocity and acceleration. The unsteady momentum equations are transformed to ordinary differential equations by using properly introduced similarity variable. Furthermore, pressure variations along the boundary layer thickness are taken into account. The energy equation is solved by numerical method as well as similarity solution. Interestingly, similarity solution of the energy equation is used for validation of the numerical solution. In this research, distributions of the fluid temperature, transient distributions of the velocity components and, most importantly, the solidification rate are presented for different values of nondimensional governing parameters including Prandtl number and Stefan number. A comparison is made between the solidification processes of axisymmetric threedimensional and twodimensional cases to justify the achieved results in a better way. The obtained results reveal that there is a difference between the final solid thickness, when the process has reached to its steady condition, of threedimensional axisymmetric and twodimensional cases. Also the results show that increase the Prandtl number up to 10 times or increase the heat diffusivity ratio up to 2 times lead to decrease the ultimate frozen thickness almost by half. While, the Stefan number has no effect on the value of thickness and its effect is captured only on the freezing time. Prediction the ultimate thickness of solid before obtaining solution and introducing a new method for validation of numerical results are achievements in this research.





A MLPG Meshless Method for Numerical Simulation of Unsteady Incompressible Flows


Pages :
421432


Authors :
I. Saeedpanah,
This article presents a numerical algorithm using the Meshless Local PetrovGalerkin (MLPG) method for numerical simulation of unsteady incompressible flows, governed by the Navier–Stokes equations via the stream function–vorticity (ψ–ω) formulation. The driven flow in a square cavity is used as the model problem. The present method is a truly meshless method based only on a number of randomly scattered nodes. The multiquadrics RBFs are employed for constructing trial functions in the local weighted meshless local Petrov–Galerkin method. The present numerical algorithm is based on a local weighted residual method with the Heaviside step function as the test function over a local subdomain. The efficiency, accuracy and robustness of the numerical algorithm are demonstrated by the standard driven cavity. It is observed that the obtained results agreed very well with the results of Ghia. Therefore the ability and accuracy of the present numerical algorithm was presented by solving the standard driven cavity flow problem with reasonable accuracy when compared to solutions obtained by Ghia. In other words the benchmark computations indicate that the MLPG Meshless method is very effective in the simulation of fluid flow problems.





Force Generation Mechanisms by an Insect Wing in Hovering Motion with Different Flipping Schedules


Pages :
433445


Authors :
H. R. Hamdani,
A. Aizaz,
A. Naqvi,
The aerodynamic force and the flow structure of a wing performing hovering motion at small Reynolds number (Re=4000) is calculated by computationally solving the 3D NavierStokes equations. The computations are performed for the hovering motion which consists of stroke 1, followed by the flipping motion for reversing the direction and then the stroke 2 (similar to stroke 1 but in the opposite direction). The intent of the study is to research the effects of different scheduling of the flip motion between the two strokes. At Re=4000, the delayed stall mechanism is noted during the azimuth rotation of a wing with a high value of CL due to stabilized Leading Edge Vortex. The lift contribution during the flip (pitch rotation for reversing the direction) for the complete stroke is not substantial. During a stroke, the wing encountered the wake from the previous stroke in which, the wake does not contribute positively.





Numerical Study of Unsteady Cavitating Flows around a Hydrofoil


Pages :
447458


Authors :
A. Bel Hadj Taher,
H. Kanfoudi,
M. Ennouri,
R. Zgolli,
In this paper, we report the results of a numerical investigation on unsteady cavitating flows around a circular leading edge (CLE) hydrofoil. The objective of this study is to properly predict the appearance of cavitation pocket, its development and its detachment causing adverse effects on industrial systems such as microscopic plastic deformations at the solid walls. For this reason it is very important to study the influence of turbulence models on simulation results. We present a closing of the hydrodynamic equation system by a transport equation of an active scalar (volume fraction of the vapor phase) with a source terms. The Computational Fluid Dynamics (CFD) code used is ANSYS CFX. Before comparing the capability of the different turbulent models to predict unsteady behavior of cavitating flow along the hydrofoil, the study of the influence of the mesh resolution was performed in cavitating condition. This investigation was performed, on CLE hydrofoil, by monitoring the influence of for progressively finer meshes on the values of the drag CD and lift CL coefficients. Moreover, a study of the influence of the normal dimensionless distance to the wall (y+) was carried out on the hydrofoil surface. For the unsteady flow, a comparison of different turbulence models with the experiment leads to study the interaction of these models with the vapor pocket (detachment and collapse of vapor pocket). Two turbulence models were tested in this study: modified kε model and large eddy simulation (LES). In the present work, the predictions of velocity and pressure evolutions in the vicinity of the hydrofoil are compared to experimental data.





Numerical Study on the Magnetohydrodynamics of a Liquid Metal Oscillatory Flow under Inductionless Approximation


Pages :
459477


Authors :
J. A. RizzoSierra,
A harmonicallydriven, incompressible, electrically conducting, and viscous liquid metal magnetohydrodynamic flow through a thin walled duct of rectangular cross section interacting with a uniform magnetic field traverse to its motion direction is numerically investigated. Chebyshev spectral collocation method is used to solve the NavierStokes equation under the inductionless approximation for the magnetic field in the gradient formulation for the electric field. Flow is considered fully developed in the direction perpendicular to the applied magnetic field and laminar in regime. Validation of numerical calculations respect to analytical calculations is established. Flow structure and key magnetohydrodynamic features regarding eventual alternating power generation application in a rectangular channel liquid metal magnetohydrodynamic generator setup are numerically inquired. Influence of pertinent parameters such as Hartmann number, oscillatory interaction parameter and wall conductance ratio on magnetohydrodynamic flow characteristics is illustrated. Particularly, it is found that in the side layer and its vicinity the emerging flow structures/patterns depend mainly on the Hartmann number and oscillatory interaction parameter ratio, while the situation for the Hartmann layer and its vicinity is less eventful. A similar feature has been discussed in the literature for the steady liquid metal flow case and served as rationale for developing the composite coresidelayer approximation to study the magnetohydrodynamics of liquid metal flows usable in direct power generation. In this study that approximation is not considered and the analysis is performed on liquid metal oscillatory (i., e., unsteady) flows usable in alternating power generation. Conversely, in terms of prospective practical applicability the formulation developed and tested with these calculations admits the implementation of a load resistance and walls conductivity optimization. That means that besides representing a numerical study on the magnetohydrodynamics of the oscillatory flow under consideration, absent in the literature for the parametric ranges reported, the formulation presently implemented can also be applicable to study the performance of an alternating liquid metal magnetohydrodynamic generator in the rectangular channel configuration.





