

Low Reynolds Number Turbulence Models to Simulate the Bubble Plume Behavior with the EulerEuler Method


Pages :
110


Authors :
S. Besbes,
M. El Hajem,
H. Ben Aissia,
J. Y. Champagne,
This paper presents a comparative assessment of low Reynolds number k models against standard k model in an Eulerian framework. Three different lowRe number k models: LaunderSharma (LS), YangShih (YS) and AbeKondohNagano (AKN) have been used for the description of bubble plume behaviour in stratified water. The contribution of the gas phase movement into the liquid phase turbulence has been achieved by using the Dispersed with Bubble Induced Turbulence approach (DIS+BIT).The results reveal that the oscillation frequency of gasliquid flow are correctly reproduced by standard k and LS models. In fact, we found for standard K and LS a clear dominant peak at a frequency equal to 0.1 Hz. On the other hand, YS and AKN models have predicted chaotic oscillations. The oscillation amplitude of the bubble plume predicted from LS model seems to be in good agreement with the PIV measurements of Besbes et al. (2015). However, for the standard K model the oscillation amplitude is low. The airwater interface shows that the bubble plume mixing with the stratified water is predicted to be stronger compared to standard k model.





A Numerical Study on the Performance of a MagnesiumBased Automotive Cooling Fan with Bead Structure


Pages :
1121


Authors :
K. H. Hur,
B. A. Haider,
C. H. Sohn,
This paper presents the numerical analysis of three types of magnesiumbased, axialflow automotive cooling fans. The numerical modeling is conducted for geometrically modified fan designs with and without bead structure. The effect of geometric modifications of the fan blades on the fan performances (PQ curve), fan efficiency, and energy efficiency is investigated using unsteady ReynoldsAveraged NavierStokes (URANS) equations with the sliding mesh methodology. The baseline fan having nobeads is fabricated using 3D printing technology and tested to measure the flow velocity and volumetric flow rate which shows good agreement to the numerical results. Subsequently, fans with beads are further optimized to achieve a significant increase in fan performances. To investigate the fan vibrations, modal analysis is also carried out using magnesiumalloy AZ31 as the fan material. The modal analysis gives natural frequencies of all types of fans which are beyond the fan rotational frequency and seems satisfactory.





An Investigation of Transition Flow in Porous Media by Event Driven Molecular Dynamics Simulation


Pages :
2336


Authors :
M. Koç,
I. Kandemir,
V. R. Akkaya,
Aim of this study is to investigate the properties of monoatomic gas flow through the porous medium by using EventDriven Molecular Dynamics (EDMD) simulation in the transition regime. The molecules and the solid particles forming the porous structure were modelled as hard spheres hence molecule trajectories, collision partners, interaction times and postcollision velocities were calculated deterministically. The porous medium is formed of spherical particles suspended in the middle of the channel and these particles are distributed into the channel in a regular cubic array. Collisions of gas molecules with porous medium were provided by means of the specular reflection boundary condition. A negative pressure boundary condition was applied to the inlet and outlet of the porous media to ensure gas flow. Porosity, solid sphere diameter and Knudsen number (Kn) were initially input to the simulation for different Cases. Thus, the effects of these parameters on mass flow rate, dynamic viscosity, tortuosity and permeability were calculated by EDMD simulation. The results were compared with the literature and were found to be consistent.





CFD Analysis and Optimization of Effect of Shroud with Multioutlets on Airflow Uniformity in a FrostFree Refrigerator


Pages :
3748


Authors :
X. F. Du,
C. Y. Zong,
Q. D. Fu,
J. R. Zhang,
The shroud is a key component of the frostfree refrigerator and its geometric parameters have great influence on the aerodynamic performance of the whole system. Previous researches mainly focused on the effect of other components, such as the fan, shelves, or plateevaporator. In this paper, the influence of the shroud with multioutlets on the flow distributions of a frostfree refrigerator is studied thoroughly with the help of Computational Fluid Dynamics (CFD) tools. A 1/2 3D CFD model is developed, where the verification of turbulence models and mesh independence tests are performed by comparing the mass flow rate obtained by different model configurations. The standard kepsilon is deemed as the most suitable turbulence model choice and a mesh with Fine level is considered as mesh independence. To obtain the boundaries of the developed CFD model, an airflow velocity test rig is built and constructed. To convert the measured data to CFD model, Structural Response Vector (SRV) method is implemented for velocity profile fitting, and the fitted surface is assigned by User Defined Functions (UDF) macros in simulations. A series of simulations are carried out with the developed model, and the results indicates that no streamline in the middle two cavities of the original freezer compartment and the airflow velocities at the three outlets of the investigated shroud show a certain difference. To optimize the flow distribution, the agent model based on the BP neural network is established, in which four critical parameters of the shroud are adopted as design variables. The results show that the velocity streamlines in the middle two cavities are significantly increased after optimization and the value of the mean square error model constructed in optimization has a reduction of 61.09% compared with the original design.





Reacting Flow Simulations of a Dual ThroatDual Fuel Thruster


Pages :
4959


Authors :
S. Jayakrishnan,
M. Deepu,
Numerical simulation of the supersonic turbulent reacting flow field in a dual throat dual fuel rocket thrust chamber is presented. Future single stage to orbit and high lift space transportation missions aspire a reliable, efficient, and costeffective propulsion systems. The dual throatdual fuel concept is a simple altitude compensating propulsion alternative with reusable possibilities. Turbulent reacting supersonic flow field emanating from independent thrust chambers needs to be resolved for a better understanding of the flow structures and design modifications for the performance improvement. The operation of a dual throat nozzle brings about a unique shock train in reacting supersonic flow. Twodimensional axissymmetric compressible reacting flow field has been solved using HLLC (Harten, Lax, van Leer, with Contact wave) scheme based finite volume Riemann solver with multistep finite rate chemistry model for hydrocarbon/hydrogenoxygen combustion. The computational procedure has been validated with experimental data for species distribution of a coaxial supersonic combustor. Chemical species distribution in the supersonic free shear layer is analyzed in detail to explore the nature of active reaction zones in the flow field.





Design of an Axial Transonic Rotor by Modified Spanwise Loading Distribution


Pages :
6172


Authors :
A. Shahsavari,
M. NiliAhmadabadi,
E. Shirani,
K. Chun Kim,
Improving the aerodynamic performance of the transonic fan in a turbofan engine can be beneficial for both the fuel consumption and maneuverability of an airplane. Deep insight into the supersonic aerodynamics is needed for the simultaneous improvement of all operating parameters of a transonic rotor, including the pressure ratio, efficiency, and surge margin. A design method was developed for an axial transonic rotor by using a combination of radial equilibrium theory, the free vortex method, and a distributed spanwise diffusion factor. The loading distribution obtained by this method was the highest in the hub section and gradually decreased in the tip section. To evaluate the method, a transonic rotor was designed using the geometric parameters and operating conditions of NASA rotor 67. A code was developed to determine a geometry for the new rotor and to modify it. The code was coupled with a RANS flow solver for 3D modification of the new designed rotor. Only standard multi and doublecircular arc airfoils were applied in different radial sections of the new rotor with no blade profile optimization. The results of the RANS equations solution for the new designed rotor showed 1.5% higher efficiency, 3% higher pressure ratio, and more than 1.5 times larger operating range in comparison to NASA rotor 67. The new designed method seems to be an efficient approach that not only improved the efficiency and pressure ratio but also increased the operating range of an axial transonic rotor.





Numerical Study of Fluidic Thrust Vector Control Using Dual Throat Nozzle


Pages :
7387


Authors :
K. X. Wu,
T. H. Kim,
H. D. Kim,
Compared to a variety of mechanical vectoring nozzles, fluidic vectoring nozzles possess more research value nowadays. The dual throat nozzle is gradually developing into an outstanding technology to handle supersonic and hypersonic aircraft deflections. Threedimensional, steady, compressible, and viscous flows in rectangular dual throat nozzles are numerically investigated by resolving Reynoldsaveraged NavierStokes equations and shear stress transport komega turbulence model. Computational fluid dynamics results are verified against the existing experimental data, where a good consistency is gained. The impacts of nozzle pressure ratio, injectiontomainstream momentum flux ratio, and setup angle of the slot injector on the systemic performance are examined. Useful conclusions are summarized for engineering designers. Firstly, pitching angles decline along with an increasing nozzle pressure ratio, while systemic thrust ratio and thrust efficiency increase. Secondly, thrust vector angles enlarge with an increase of the injectiontomainstream momentum flux ratio, whereas both systemic thrust ratio and thrust efficiency decay. Finally, the setup angle of the slot injector impacts the systemic performance remarkably. Although the pitching angle for the setup angle of 120° is highest, comprehensive characteristics in terms of systemic thrust efficiency and systemic thrust ratio for the setup angle of 150° are more excellent.





Analysis of Unsteady Flow Structures in a Centrifugal Impeller Using Proper Orthogonal Decomposition


Pages :
89101


Authors :
Z. Y. Liao,
J. Yang,
X. H. Liu,
W. L. Hu,
X. R. Deng,
The occurrence and development of the dominant unsteady flow structures in a vanless centrifugal pump impeller are revealed by the proper orthogonal decomposition (POD) method. The pressure and velocity data of four radial surfaces is selected as the variables of decomposition. The results show that this method is beneficial to the analysis of flow field when there is no strong interaction of flow structures. When the flow rate starts to decrease from the design flow rate, unstable flow phenomenon such as flow separation and wake begin to appear and develop in the impeller. The POD analysis reveals the influence of the main unsteady structures on the flow field when there is no mixed or have little interaction among flow structures. It outlines the development of flow separation near the suction side of impeller and the wake near the trailing edge as the flow rate changes. However, the flow field inside the impeller becomes more and more complex as the operation condition is far away from the design condition, which needs to be combined with other methods to better analyze the flow field.





Effect of Pipeline External Geometry on Local Scour and SelfBurial Time Scales in Current


Pages :
103115


Authors :
M. Damroudi,
K. Esmaili,
S. H. Rajaie,
Changes in the external geometry of the pipeline laid on the erodible beds may affect the local scour around pipe. If the scouring process below the pipe is accelerated, the pipe buries on the bed (known as selfburial of the pipe) and can be used as a cheap alternative to mechanical trench digging. In this study, the effect of changes in the external geometry of the pipe in order to accelerate the scouring and selfburial pipe processes by spoiler and piggy back under unidirectional flow on the erodible bed is investigated. The results showed that the time scale of selfburial process is less than that of the scouring process. At the angles of 180, 135 and 225°, the Spoiler and piggy back reduced the time scale of selfburial and scouring process and increased the scour and selfburial depths, as well as the lee erosion length as compared with the simple pipe. At angles of 90 and 270°, the scale time of scour and selfburial processes are increased, the scour and selfburial depths and the lee erosion length are decreased as compared with the simple pipe. It can be concluded that the performance of piggy back is similar to the spoiler, therefore, they are of similar application and could be a suitable alternative for the spoiler.





Investigation on Stability and Galloping Characteristics of Iced Quad Bundle Conductor


Pages :
117129


Authors :
X. Liu,
G. Min,
C. Sun,
M. Cai,
The stability and galloping characteristics of iced quad bundle conductor are studied in this paper. Firstly, the aerodynamic coefficients of iced quad bundle conductor and single conductor under four different working conditions are obtained by wind tunnel test. Secondly, the equivalent aerodynamic coefficients at the central axis of the quad bundle conductor are obtained, and the equivalent aerodynamic coefficients are compared with the aerodynamic coefficients of each subconductor of the quad bundle conductor. Then, based on the Den Hartog instability mechanism and Nigol instability mechanism, the stable and unstable range of the equivalent coefficients of the quad bundle conductor are analyzed. Finally, the galloping characteristics of the quad bundle conductor are studied by combining with the equivalent aerodynamic coefficients at the central axis of quad bundle conductor. The results of the wind tunnel test show that the aerodynamic coefficients increase with the decreasing of the wind speed. The stability analyses show that the higher the wind speed is, the smaller the Den Hartog coefficient is the easier the Den Hartog’ galloping would occur. Furthermore, the higher the wind speed is, the smaller the Nigol coefficient is, the easier the Nigol’ galloping would occur. The analysis of galloping characteristics shows that when the conductor is located at stable state, the displacement in the yaxis direction would be much greater than the displacement in the zaxis direction.





Experimental Investigation of MultiJet Air Impingement in Various Conditions and Analysis using Desirability Based Response Surface Methodology


Pages :
131145


Authors :
P. Chandramohan,
S. N. Murugesan,
S. Arivazhagan,
This experimental analysis encapsulates the influence of Reynolds number (Re), diameter of nozzle, height to diameter (H/D) ratio and position of nozzle such as inline and staggered over the responses heat transfer coefficient, temperature and Nusselt number of a hot flat plate exposed to cooling by multijet air impingement. For this analysis, a 15 x 10 cm flat plate is being heated using a heating coil having a heat flux of 7666.67 W/m2 which is maintained as constant through entire experiment. An H/D ratio of 2D, 4D and 6D is considered along with pipe diameters of 4, 6 and 8 mm and Reynolds number are changed between 18000 to 22000. Experimental design was performed with response surface methodology based central composite design. For all output responses, a quadratic model is chosen for analysis and a second order mathematical model is evolved for predicting with a higher R2 value. Desirability analysis is performed for multiobjective optimization and the optimum input parameters obtained are Reynolds no. of 20347, pipe diameter of 8 mm, H/D ratio of 2 and inline nozzle position with the maximum heat transfer coefficient of 189.411 W/m2 K, Nusselt number of 28.8712 and minimum temperature of 56.983°C. Optimum conditionbased confirmation experiments result in enhanced Nusselt number and heat transfer coefficient.





A Simple Passive Device for the Drag Reduction of an Ahmed Body


Pages :
147164


Authors :
N. A. Siddiqui,
M. A. Chaab,
In this paper, a simple passive device is proposed for drag reduction on the 35° Ahmed body. The device is a simple rectangular flap installed at the slant surface of the model to investigate the effect of slant volume, formed between the device and the slant surface, on the flow behaviour. The slant volume can be varied by changing the flap angle. This investigation is performed using the FLUENT software at a Reynolds number of 7.8 ×〖10〗^5 based on the height of the model. The SST komega model is used to solve the Navierstokes equations. It is found that this passive device influences the separation bubbles created inside the slant volume and provides a maximum drag reduction of approximately 14% at the flap angle of 10°. Moreover, the device delays the main separation point, which changes the flow conditions at the back of the model. The drag reduction was found to mainly dependent on the suppression of the separation bubbles formed inside the slant volume, which leads to faster pressure recovery. The cause of this pressure recovery is found to be the reduction in recirculation length and width. Also, the addition of a flap reduces the turbulent kinetic energy, which lessened the wake entrainment in the recirculation region, leading to a drag reduction. Also, it hinders the formation of horseshoe vortex that provides a pressure recovery and influence the wake width. However, the investigation also reveals that this device does not reduce the induced drag due to longitudinal vortex from the side edges.





A New Modeling Method to Reveal Pumping Mechanism of Turbomolecular Pump


Pages :
165173


Authors :
K. Sun,
S. W. Zhang,
F. Han,
F. Zhao,
Z. J. Zhang,
J. Han,
In order to meet the calculating requirements of high speed and miniaturized turbomolecular pump (TMP), a new modeling method is proposed for a rotorstator row. In the same Cartesian coordinate system, the analytical equations of blade row are derived according to the real threedimensional geometric model. A selfdefining procedure is written to simulate the flow of gas molecules in TMP based on test particle Monte Carlo method. The procedure not only can calculate transmission probability of rotor row, stator row and a rotorstator row but also evaluate the pumping performance of different blade parameters. The simulation results and known experimental data have a good agreement to confirm the feasibility of presented modeling method. The flow analysis shows that molecules at outlet of rotor row tend to accumulate in large radius and this phenomenon is obvious for high rotational speed. The differences were found between a singlestage row and a rotorstator row. In rotor row, the molecular density at the rear blades is the highest. This is beneficial for pumping speed of TMP because 57% of molecules at the rear blades are likely to reach outlet. This will provide a direction for the structural optimization design of the blades in the future. In stator row, the molecular density reaching outlet does not significantly increase with the increase of blade velocity ratio, which indicates that the stator row is mainly used to increase the pressure ratio of TMP. The analysis of molecular density in each region reveals the pumping mechanism of TMP.





Numerical Investigation on Film Cooling Mechanism with Different Coolant Delivery Configurations


Pages :
175185


Authors :
Y. T. Jiang,
H. F. Deng,
X. L. You,
H. J. Zhao,
G. Q. Yue,
Kidney vortex has significant impact on film cooling effectiveness, and different kinds of film cooling hole geometry and configuration are developed to weaken or eliminate kidney vortex. This paper is focus on the mechanism of eliminating kidney vortex by optimizing the coolant delivery configuration, seven coolant delivery configurations are designed to conduct a comparative study with different blowing ratios. At high blowing ratio, the strong kidneyshaped vortex is formed outside the film cooling hole causing the low cooling effectiveness for β≤0˚. For β>0˚, the coolant ejection interacts with mainstream hot gas, and the coolant gas in low momentum region of upstream bypasses the large jet momentum coolant to attach film cooling surface at downstream. It increases the distance between the vortexes to weaken mutually reinforcing effect, resulting in high film cooling effectiveness. When the blowing ratio is 1.5, the average adiabatic film cooling effectiveness of β=+15˚ and β=+30˚ is increased by about 130% and 70% compared to case of β=60˚, respectively.





Effect of Temporal Modulation on the Local Kinematic Process of TwoDimensional Chaotic Flow: A Numerical Analysis


Pages :
187199


Authors :
M. Telha,
M. Bachiri,
Y. Lasbet,
T. T. Naas,
In this work, a numerical analysis based on CFD method is carried out to examine an unsteady laminar flow of Newtonian fluids in a twodimensional simulation of a mixer, which is composed of two rods inside a moving cylindrical tank. Three stirring protocols are considered: Nonmodulated “NM”, Continuously modulated “CM” and noncontinuously modulated “ALT” by using the dynamic mesh technique and user defined functions “UDF’s” for the velocity profiles. The chaotic advection is obtained by temporal modulation of the rotational velocity of the cylinder and the rods to enhance the mixing of the fluid for very low Reynolds number. For this purpose, we applied the Poincaré map as a reliable mathematic tool to check mixing quality by tracking particles inside the fluid domain. Additionally, we investigated the evolution of local flow proprieties such as rotation rate, deformation rate and elongation rate at different time periods in order to see the effect of temporal modulation on the fluid kinematics. Among the considered protocols, the results of the mentioned simulation showed that it is possible to obtain a chaotic advection only for noncontinuously modulated protocol which enhance mixing fluid efficiency.





Impact of a Tapered Design in the Performance of Diffusion Flame Reactors


Pages :
201214


Authors :
A. C. Klemz,
É. Fontana,
A. A. U. de Souza,
S. M. A. G. U. de Souza,
Several research projects developed processes for precise nanoparticle positioning with high production rates. Among the gas phase manipulation strategies, the study of the inertial properties received special attention for the fabrication of nanostructured systems. In particular, aerodynamic focusing technique has allowed particles concentration onto a single streamline, improving collimated particle beam formation and opening new perspectives for nanoparticles production in the gas phase. In this paper, the influence of the reactor design was investigated, particularly in respect to the gas phase characteristics, aiming to improve the nanoparticles focusing. It was observed that a concentrated beam can be obtained in the new tapered reactor design without significantly affect the production rate and temperature profile. In addition, the coupling of aerodynamic lenses to the tapered reactor was also investigated, showing that the flow can be better focused at the cost of an increase in the average temperature and pressure drop.





Numerical Study of SingleHole and MultiHoles Orifice Flow Parameters


Pages :
215226


Authors :
M. Đurđević,
M. Bukurov,
S. Tašin,
S. Bikić,
Importance of accurate fluid flow measurement in industry is crucial especially today with rising energy prices. There is no ideal measuring instrument due to numerous errors occurring during process of physical quantities measurement but also due to specific requirements certain instruments have like fluid type, installation requirements, measuring range etc. Each measuring instrument has its pros and cons represented in accuracy, repeatability, resolution, etc. Conventional singlehole orifice (SHO) flow meter is a very popular differentialpressurebased measuring instrument, but it has certain disadvantages that can be overcame by multiholes orifice (MHO) flow meter. Having this in mind, the aim of this paper is to help gain more information about MHO flow meters. Both SHO and MHO gas (air) flow meters with same total orifice area and the pipe area ratio β were numerically studied and compared using computational fluid dynamics (CFD). Simulation results of 16 different orifices with four different β (0.5, 0.55, 0.6 and 0.7) were analysed through pressure drop and singular pressure loss coefficient. Standard kε turbulence model was used as a turbulence model. Beside singular pressure loss coefficient, pressure recovery as well as axial velocity for both the SHO and MHO were reported. Results showed lower (better) singular pressure loss coefficient and pressure drop as well as quicker pressure recovery in favour of the MHO flow meters. Also, centreline axial velocity results were lower for MHO compared to corresponding SHO. CFD simulation results were verified by experimental results where air was used as a working fluid. The influence of geometrical and flow parameters on singular pressure loss coefficient was also reported and results showed that MHO hole distribution did not have significant influence on singular pressure loss coefficient.





Numerical Investigation on the Response Characteristics of Hypersonic Boundary Layer under Different Types of Finite Amplitude Pulse Disturbance Waves


Pages :
227241


Authors :
X. Tang,
J. Yu,
H. Zhang,
H. Li,
M. Shi,
The hypersonic transient flow pass a blunt cone under three types of pulse disturbances is calculated using DNS. The response characteristic of hypersonic boundary layer among different types of pulse disturbance is compared. The distribution and evolution characteristics of disturbance modes are investigated by mode analysis. Results indicate that the receptivity characteristics induced by freestream pulse wave have both similarities and differences with that induced by freestream continuous wave. The interactions of different types of pulse waves with boundary layer and bow shock present different characteristics. The boundary layer thermodynamic characteristics under pulse fast acoustic wave are sensitive to mainstream disturbance wave, and that under pulse slow acoustic wave are sensitive to residual reflection wave. The type of pulse disturbance wave has a great influence on the production and mode distribution of boundary layer disturbance wave. In general, the disturbance amplitude in the pulse fast acoustic wave situation is the largest, the case of entropy wave is the second, and the case of slow acoustic wave is the smallest. For regional influence, the type of pulse disturbance has a huge impact on the disturbance modes in both the head and the nonhead. For the three cases of pulse wave, the main mode group attenuation phenomenon which narrows the disturbance frequency band exists in the boundary layer. This group attenuation is the fastest for freestream slow acoustic wave, followed by entropy wave, and then fast acoustic wave. Under the action of pulse slow acoustic waves, the disturbance wave evolution of each order mode in the boundary layer along the streamwise is relatively stable, followed by entropy wave, and the case of fast acoustic wave is the most active.





Hydrodynamic Simulation of TwoPhase Flow in an Industrial Electrowinning Cell with New Scheme


Pages :
243257


Authors :
S. A. A. Pourahmadi ,
S. Talebi,
Electrowinning is the process of depositing copper of the electrolyte solution inside the cell to the cathode. In the present study, the hydrodynamic simulation of the electrowinning cell of Miduk Copper Complex is studied using computational fluid dynamics. The software used is Ansys CFX. The NavierStokes and continuity equations are considered in the form of two phases of fluid and gas, turbulent, incompressible and steady state, and the equation for copper concentration in the electrolyte is solved with consideration of its specific boundary condition. The flow turbulence is modeled using Kω relationships. Due to large variations in the properties near cathode and anode, and also the large size of the electrowinning cell, to create a good grid, and increase the speed and accuracy of the results, global and local simulations are used together. First, in global simulation, the entire geometry of the cell is modeled by creating an appropriate grid, then, in the local simulation, the volume between two cathodes of the cell only is considered and modeled with a much smaller mesh. Data on boundary conditions of the common border plates in the local simulation are obtained from global simulation data, which increases the accuracy of modeling. The results of this simulation are the velocity vectors, the concentrations of acid and copper, turbulence Intensity, the amount of pressure, and the volume ratio of the oxygen phase in the entire electrowinning cell domain. Finally, for model validation, the model is compared with experiments conducted on actual cells in the industry. Results show high accuracy with less than 2.5% deviation of this modeling technique. Then, the mass transfer coefficient values for the different electrode intervals are obtained by this modeling and the results are validated using the results of the experimental relations, indicating a deviation of only 0.5%. In the next step, the electrolyte mixture containing different mass fractions of oxygen is sprayed into the electrowinning cell from the inlet of the simulated cell. The results demonstrate that spraying 16 liters per second of oxygen at the inlet can increase the overall mass transfer coefficient of the electrode plates up to 7%. The effect of changing inlet temperature and flow rate of the electrolyte on the mass transfer coefficient is also investigated by the obtained model.





Laminar Flow over a Square Cylinder Undergoing Combined Rotational and Transverse Oscillations


Pages :
259273


Authors :
B. Anirudh Narayanan,
G. Lakshmanan,
A. Mohammad,
V. Ratna Kishore,
This work numerically investigates the effects of combined rotational and transverse oscillations of a square cylinder on the flow field and force coefficients. The primary nondimensional parameters that were varied are frequency ratio fR (0.5, 0.8), Re (50200), phase difference (ϕ) between the motions and rotational amplitude (θ0) with the influence of the last three parameters being discussed in detail. The amplitude of transverse oscillations is fixed at 0.2D, where D is the cylinder width. The study has been validated using the mean drag coefficient for stationary and transversely oscillating square cylinders from literature. Output data was obtained in the form of force coefficient (Cd), vorticity and pressure contours. The governing equations for the 2dimensional model were solved from an inertial frame of reference (overset meshing) using finite volume method. The interplay between the convective field and prescribed motion has been used to explain many of the results obtained. The relative dominance of cylinder motion over the flow stream was determined using Discrete Fast Fourier Transform. The influence of Re on Cd disappears when the motions are completely out of phase (ϕ = π). In general, the Cd for low Re flows exhibited low sensitivity to change in other parameters. Direct correlation has been observed between frontal area, vortex patterns and drag coefficient





Numerical Investigation of Bile Secretion and Pressure Rise in Obstructed Human Common Bile Duct


Pages :
275286


Authors :
M. Baghaei,
M. Kavian,
S. Ghodsi,
S. E. Razavi,
A fully obstructed Common Bile Duct (CBD) could lead to severe implications such as jaundice, cholangitis, and pancreatitis. A 2D CFD model with the employment of Fluidstructure Interaction (FSI) formulations is established to investigate the interactions of bile with its surroundings. Ascertaining bile secretion against a total CBD obstruction is a major interest of this study. Therefore, a function is assigned to bile secretion pattern such that the resulting intraluminal pressure complies with clinical data. To cover the variation in the parameters representing the mechanical properties of the biliary system as well as bile secretion, specific piecewise ranges are given for each of them and consequently, numerous cases are simulated. Models which after simulation lead to pressure rises in the interval of 700 Pa to 1300 Pa are picked. This interval could roughly include the actual pressure rise of a vast majority of patients. It is determined that among numerous cases, higher distention does not necessarily correlate with higher pressure increase. Furthermore, the effect of alteration in each parameter in pressure rise is determined. This model is the first numerical step towards understanding the pathogenesis of complications resulting from a fully obstructed CBD and deformation of the CBD in general.





Numerical Study on the Effects of AntiSnow Deflector on the WindSnow Flow Underneath a HighSpeed Train


Pages :
287299


Authors :
L. Cai,
Z. Lou,
T. Li,
J. Zhang,
In order to study the effect of the antisnow deflector on the windsnow flow underneath a highspeed train, Detached Eddy Simulation (DES) approach and discrete phase model (DPM) are used to simulate the windsnow flow around the train. The distribution of snow particles underneath the train body is analyzed. Meanwhile, the influence of deflectors on the movement of snow particles around the train is investigated. The results show that lots of vortices shed from the bogie, and the entrainment vortices near the ground actuates the movement of the snow particles on the snowcovered track, which forms a windsnow flow. The snow smoke around the train develops gradually from the bottom of the first bogie to the end of the tail car. The deflector installed in the front of the bogie will guide the vortices off the bogie region to the ground, which results in flying up more downstream snow particles and correspondingly the number of snow particles accumulated in the bottom of the rear car and around the rear skirt plate is increased. The installation position for the deflector has a certain effect on the snow accretion in the bogie region. When the deflector is installed in the front of the 2nd and 4th bogies, the snow particles captured in the bogie region are reduced by 42.3% and 15.6%, respectively.





Stability of Oblique Water Entry of Cylindrical Projectiles


Pages :
301314


Authors :
M. A. Akbari,
J. Mohammadi,
J. Fereidooni,
Water entry is an interesting subject but many of its physical aspects have remained unknown so far. Using computational fluid dynamics (CFD), this study investigates the dynamic stability of cylindrical projectiles in the oblique water entry at shallow angles in the presence of three phases of air, water and water vapor. The threedimensional and transient numerical model has been verified using the former experimental results in the literature. In this study, the effects of projectile lengthtodiameter ratio (L/D) and water entry angle on the projectile stability within cavity were investigated. Accordingly, the water entry of six projectiles was simulated with aspect ratios of 2 to 6 at three water entry angles of 6, 9 and 12 degrees with respect to the free surface with an initial velocity of 280 m/s. At each of the aforementioned angles, the critical L/D, where the projectile avoids tumbling inside the cavity at a larger value, was determined. This study showed that in the oblique water entry of a cylindrical projectile at the angles of 6, 9 and 12 degrees, the projectile tumbled within the cavity with a L/D of less than 5, 4 and 3.5, respectively. The simulation results showed that increasing the L/D as well as the water entry angle relative to the free surface resulted in the improvement of the cylindrical projectile motion stability, which is in agreement with the experimental results. By analyzing the details of each simulation, it was found that the projectile stability within the cavity is correlated with the magnitude of the angular momentum which is generated in the projectile by the impact of the cavitator on the free surface and it was shown that the projectile with a specific L/D can withstand destabilizing angular momentum to a certain extent. Considering the fact that the atmospheric ballistics of gyroscopically stabilized projectiles lead to a limit for increasing L/D, this study showed that, for aluminum cylindrical projectiles in which air stability is achieved via the gyroscopic effect, the minimum water entry angle is 6° to attain the gyroscopic stability of the projectile in the air and stable motion inside the cavity. This fact is very important from a practical point of view.





Cavitation Status Recognition Method of Centrifugal Pump Based on MultiPointand MultiResolution Analysis


Pages :
315329


Authors :
L. Dong,
J. C. Zhu,
K. Wu,
C. Dai,
H. L. Liu,
L. X. Zhang,
J. N. Guo,
H. B. Lin,
Cavitation monitoring is particularly important for pump efficiency and stability. It is easy to misjudge cavitation by using a given threshold of a single eigenvalue. In this work, based on the vibration signal, a method for multiresolution cavitation status recognition of centrifugal pump is proposed to improve the accuracy and universality of cavitation status recognition., wavelet packet decomposition (WPD) is used to extract the statistical eigenvalues of multiscale timevarying moment of cavitation signal after reducing the clutter, such as root mean square value, energy entropy value and so on. The characteristic matrix is constructed. Principal component analysis method (PCA) is employed to reduce the dimension of the characteristic matrix and remove the redundancy, which constructs the radial basis function (RBF) neural network as the input. The results show that the overall recognition rate of noncavitation, inception cavitation and serious cavitation by using the vibration signal of one measuring point is more than 97.7%. The recognition rate of inception cavitation is more than 80%. Based on the vibration signal information fusion method of two measuring points, the recognition rate of centrifugal pump inception cavitation status reaches more than 99%, and the recognition rate of vibration signal information fusion method of three measuring points reaches 100% for all three cavitation statuses. Due to the influence of factors such as change of external excitation and abrupt change of working conditions, sensor data acquisition is often subjected to unpredictable disturbance. To study the ability of singlepoint cavitation status recognition method to resist unknown disturbances, by constantly adjusting the value of the interference coefficient of the interference term. It is found that the recognition rate of cavitation status using single measuring point decreases almost linearly with the increase of the interference coefficient. When five measuring points are used for information fusion cavitation status recognition, the cavitation status recognition rate still reaches over 90% even if the interference factor of one measuring point reaches 50%.





Innovations in NonLinear Oscillations of a Pendent Drop From a Capillary Tip During Formation and Detachment  An LBM Simulation


Pages :
331344


Authors :
S. Ghorbanifar,
M. TaeibiRahni,
M. Zareh,
Individual drops are suitable tools to study the liquidfluid interfacial properties. In this work, forcedisplacement equation and nonlinear oscillations of a pendent drop are numerically investigated. The presented novel forcedisplacement function allows following the dynamics of a pendent drop and realizing its elastic behavior. The growth and detachment of drop, which is pending due to gravity from a capillary tip, is considered (assuming high density and high viscosity ratios and immiscible twophase flows). Twodimensional multirelaxation time lattice Boltzmann method (MRTLBM) was used to simulate growth, detachment, and oscillations of the drop using a conservative model for highdensity ratio. The forcedisplacement function of a pendent drop (FDFPD), which is nonlinear, was introduced. Using FDFPD, the nonlinear elastic specifications of the pendent drop were determined. It was realized that the drop shows three different elastic behaviors simultaneously (hardening, linear, and softening). The drop superharmonic and subharmonic frequencies were calculated, using the natural frequency of the linear portion of FDFPD. Besides, the drop would grow as long as its displacement is between the extrema of FDFPD. In addition, a dynamic criterion for the onset of detachment was established. Also, increasing the Bond number from 0.11 to 1.96, while keeping Reynolds number equal to 0.023, accelerates the drop detachment and increases the linear portion of FDFPD. It was shown that increasing Capillary number from 1.8E5 to 7.3E4, while keeping Reynolds number equal to 0.023, accelerates the drop detachment and increases the nonlinear portions of FDFPD.





