

Numerical Modelling of Transport Phenomena and Macrosegregation during Ternary Alloy Solidification: Solutal Undercooling Effects


Pages :
319326


Authors :
S. Ganguly,
In this paper, a macroscopic mathematical model is developed for simulation of transport phenomena during ternary alloy solidification processes, taking into account nonequilibrium effects due to solutal undercooling. The model is based on a fixedgrid, enthalpybased, control volume approach. Microscopic features pertaining to nonequilibrium effects on account of solutal undercooling are incorporated through a novel formulation of a modified partition coefficient. The effective partition coefficient is numerically modeled by means of macroscopic parameters related to the solidifying domain. Numerical simulations are performed for ternary steel alloy by employing the present model and the resulting convection and macrosegregation patterns are analyzed. It is observed that the consideration of nonequilibrium solidification in the present mathematical approach is able to capture the thermosolutal convection and leads to prediction of accurate value of macrosegregation. The results from the present model matches well with the experimental observations published in the literature.





An Analysis on the Flow Field Structures and the Aerodynamic Noise of Airfoils with Serrated Trailing Edges Based on Embedded Large Eddy Flow Simulations


Pages :
327339


Authors :
Z. G. Zuo,
Q. Huang,
S. Liu,
This paper presents the numerical analysis on the aerodynamic flows and noise of airfoils with serrated trailing edges at . Flow simulations were performed with an embedded large eddy simulation (ELES) method. Two modified airfoils with serrated trailing edges (same widths, different lengths) were studied and compared with the baseline airfoil baseline NACA0018 airfoil. It is seen that the unsteady lift and drag coefficients of the baseline airfoil A0 have a peak at about 2270Hz, which is close to the tonal noise frequency experimentally observed. Under the flow conditions studied in this research, the longer saw tooth serrations changed the flow fields near the trailing edge, which provides the potential of suppressing the tonal noise. Predictions based on acoustic analogy indicate that the longer saw tooth serrations decreases the overall sound pressure levels. This paper provides a basic understanding of the noise reduction mechanism in the airfoils with serrated trailing edges.





Performance Improvement and TwoPhase Flow Study of a Piezoelectric Micropump with Tesla NozzleDiffuser Microvalves


Pages :
341350


Authors :
S. Derakhshan,
B. Beigzadeh,
M. Rashidi,
H. Pourrahmani,
The Present article aims to design a piezoelectric micropump using a combinational form of microvalves with sufficient diodicity in lowpressure gradients. The goal is to enhance the capability of piezoelectric micropumps with Teslatype valves in order to deliver insulin. Teslatype valves are in the category of passive valves which have sufficient diodicity in case of highpressure gradients. However, low mass flow rates are often required in drug delivery devices. In this paper, the performance of MT135 Teslatype valve in low pressuregradient flows has been investigated and a range of reunion angles, which have not been studied before has been examined by numerical solutions. Inspired by nozzlediffuser valve types, some changes in the bypass path of the microvalve have been exerted to boost the diodicity of the valve in lowpressure conditions that resulted in 9.97% increase of diodicity. At last but not least, the velocity gradients in singlephase flow of water has been attained and performance of micropump toward other kinds of flows has been investigated by a volume of fluid (VOF) model including water as the primary phase and air as the secondary one. To complete the analysis, a VOF model consisting of an arbitrary kind of Casson fluid with the primary phase of water was reached and discussed.





Flow over Flexible Vegetated Bed: Evaluation of Analytical Models


Pages :
351359


Authors :
H. Romdhane,
A. Soualmia,
L. Cassan,
D. Dartus,
The development of vegetation in the river bed and in the banks can affect the hydrodynamic conditions and the flow behavior of a watercourse. This can increase the risk of flooding and sediment transport. Therefore, it is important to develop analytical approaches to predict the resistance caused by vegetation and model its effect on the flow. This is the objective of this work which investigates the ability of different analytical models to predict the vertical velocity profile as well as the resistance induced by flexible submerged vegetation in open channels. Then it is possible to select the appropriate model that will be applied in the real case of rivers. The model validation is determined after a comparison between the data measured in the different experiments carried out and those from literature. For dense vegetation, the role of the Reynolds number is emphasized in particular with a model using the DarcyBrinkman equation in the canopy. With a simple permeability, this model is relevant to estimate friction. However, for larger Reynolds number, models based on the fully turbulent flow assumption provide better results.





Aerodynamic Forces Acting on a Race Car for Various Ground Clearances and Rake Angles


Pages :
361368


Authors :
I. Džijan,
A. Pašić,
A. Buljac,
H. Kozmar,
Computational model was developed to investigate aerodynamic forces acting on a closedwheel race car. A particular focus was on the effects of ground clearance and rake angle on aerodynamic drag and lift forces. Computations were performed for a steady viscous fluid flow using the realizable kε turbulence model and nonequilibrium wall functions. The computational results indicate a strong influence of ground clearance and rake angle on aerodynamic loading of a race car. The largest drag force coefficient was obtained for the largest ground clearance. The drag force coefficient for the squatting car is larger by 5% compared to the reference case, where the both front and rear ground clearances are 100 mm. For the nosediving car, the drag force coefficient is equal to the reference case. Increasing the ground clearance caused a negligible increase in the lift force coefficient in comparison with the reference case. A decrease in the ground clearance yielded an increase in the lift force coefficient. The largest positive lift force coefficient was obtained for a squatting car, whereas the largest negative lift force coefficient was observed for a nosediving car. While the favorable aerodynamic downforce acting on front wheels is larger for a nosediving car, for rear wheels it is larger for a squatting car.





Numerical and Neural Network Analysis of Natural Convection from a Cold Horizontal Cylinder above an Adiabatic Wall


Pages :
369377


Authors :
Ali Reza Tahavvor,
M. Nazari,
Free convection around cold circular cylinder above an adiabatic plate at steadystate condition has been investigated both numerically and by artificial neural networks. There is a growing demand for a better understanding of free convection from a horizontal cylinder in the areas like air cooling, refrigeration and air conditioning system, etc. Governing equations are solved in some specified cases by finite volume method to generate the database for training the neural network in the range of Rayleigh numbers of 105 to 108 and a range of cylinder distance from adiabatic plate (L/D) of 1/4, 1/2, 1/1, 3/2 and 4/2, thereafter a MultiLayer Perceptron network is used to capture the behavior of flow and temperature fields and then generalized this behavior to predict the flow and temperature fields for other Rayleigh numbers. Different training algorithms are used and it is found that the backpropagation method with LevenbergMarquardt learning rule is the best algorithm regarding the faster training procedure. It is observed that ANN can be used more efficiently to determine cold plume and thermal field in less computational time and with an excellent agreement. From obtained results, average Nusselt number of the cylinder investigated to study the effect of adiabatic wall on the isothermal cylinder. It also observed that in spaces farther than L/D = 3/2, average Nusselt number is almost constant, so the affect is renouncement and it works like a cylinder in an infinite environment.





Numerical Investigation of the Effects of Sand Collision on the Aerodynamic Behaviour of a HighSpeed Train Subjected to Yaw Angles


Pages :
379389


Authors :
J. Wang,
D. Liu,
G. Gao,
Y. Zhang,
J. Zhang,
In this paper, the aerodynamic performance of the head car of a CRH2 train running in sandstorms was investigated. A numerical simulation method based on Realizable kε turbulence model was used to explore the flow features around the highspeed train. The accuracy of mesh resolution and methodology of CFD was validated by wind tunnel tests. A discrete phase model (DPM) was adopted to investigate the effects of sand particle properties (diameter and restitution coefficient) on the aerodynamic performance of the head car. Yaw angle effects with the sandladen flow on the aerodynamic coefficient were also discussed. The results show that the drag force, lift force, lateral force, and overturning moment of the head car increase significantly due to the sand, and the sand particle properties have dominant effects on the aerodynamic performance of the head car. The impact probability of sand particles on the vehicle increases with the sand particle diameter and the yaw angle increasing. Larger restitution coefficients lead to lager forces of the head car, resulting in more contribution to the aerodynamic coefficients. Owing to the sand collision, a larger yaw angle causes more contribution to the aerodynamic performance of the head car, and the influence of sand properties on the drag force, lateral force and overturning moment are enhanced with the increase of the yaw angle. Using appropriate coatings around the highspeed train can not only reduce the energy consumption, but also improve the lateral stability and the critical operational speed of the highspeed train in the sandstorms.





A Numerical Analysis of Laminar Forced Convection and Entropy Generation of a DiamondFe3O4/Water Hybrid Nanofluid in a Rectangular Minichannel


Pages :
391402


Authors :
C. Uysal,
E. Gedik,
A. J. Chamkha,
The convective heat transfer and entropy generation of diamondFe3O4/water hybrid nanofluid through a rectangular minichannel is numerically investigated under laminar flow conditions. Nanoparticle volume fractions for diamondFe3O4/water hybrid nanofluid are in the range 0.050.20% and Reynolds number varies from 100 to 1000. The finite volume method is used in the numerical computation. The results obtained for diamondFe3O4/water hybrid nanofluid are compared with those of diamond/water and Fe3O4/water conventional nanofluids. It is found that 0.2% diamondFe3O4 hybrid nanoparticle addition to pure water provides convective heat transfer coefficient enhancement of 29.96%, at Re=1000. The results show that diamondFe3O4/water hybrid nanofluid has higher convective heat transfer coefficient and Nusselt number when compared with diamond/water and Fe3O4/water conventional nanofluids. For diamondFe3O4/water hybrid nanofluid, until Re=600, the lowest total entropy generation rate values are obtained for 0.20% nanoparticle volume fraction. However, after Re=800, diamondFe3O4/water hybrid nanofluid with 0.20% nanoparticle volume fraction has the highest total entropy generation rate compared to other nanoparticle volume fractions. A similar pattern emerges from the comparison with diamond/water and Fe3O4/water conventional nanofluids. For 0.2% nanoparticle volume fraction, diamondFe3O4/water hybrid nanofluid and diamond/water nanofluid have their minimum entropy generation rate at Re=500 and at Re=900, respectively. Moreover, this minimum entropy generation rate point changes with nanoparticle volume fraction values of nanofluids.





Effect of Inclination of Twin Jets Impinging a Heated Wall


Pages :
403411


Authors :
F. Bentarzi,
A. Mataoui,
M. Rebay,
This study examines the interaction of twin oblique turbulent slotjets of different directions (divergent, convergent or parallel) impinging a heated wall. A comparison of the results is done between the cases of perpendicular jets and three cases of twinned jets (parallel, convergent and divergent).The twin slot jets are located on a confining adiabatic wall at a distance of 8 slot jet width. Convective heat is investigated numerically examining the effect of Reynolds number (Re) and jet inclination angle (). This problem is relevant to a wide range of practical applications including nuclear engineering devices, manufacturing, material processing, electronic cooling, drying paper or textile, tempering of glass, etc. The numerical investigation is performed using two dimensional large eddy simulations (LES) approach with Smagorinsky subgrid scale (SGS) models. The results show the presence of a complex flow resulting from the interaction of the two jets. When the impingement angle is reduced from 0° (perpendicular impingement) to 60°, the position of the stagnation points are modified and therefore the peaks of the Nusselt number locations on the impingement surface and their magnitude, vary. For largest Reynolds number Nusselt number is enhanced for all types of inclination. The averaged Nusselt number shows that the perpendicular impingement gives better heat transfer than that of the oblique jets. The poor heat transfer is obtained for the parallel oblique jets. For the same angle, divergent jets give smallest heat transfer than the convergent jets.





A Study of the Effect of Nanoparticle Concentration on the Characteristics of Nanofluid Sprays


Pages :
413420


Authors :
B. Kang,
M. Marengo,
S. Begg,
Nanofluids are metallic or nonmetallic, nanometersized particles dispersed in liquid. They can be used in various fields to increase heat transfer rates, as the thermal conductivity of nanofluids can be increased significantly. Nanofluids may be used as a good alternative coolant in spray cooling applications. This study conducted experiments to compare spray characteristics, such as droplet diameters and velocities, between water and alumina nanofluid sprays. The mass ratio of alumina nanoparticles was varied from 0.2 to 0.5 weight percentages (wt.%) and the spray injection pressure was varied between 0.2 and 0.3 MPa. The local distributions of droplet sizes and velocities along the spray axial and radial directions were measured by a laser doppler instrument. Generally, the spray characteristics of nanofluid sprays is significantly different from that of water sprays. The average droplet diameters of the fluids tested increased in an approximately linear manner with the increase in the mass ratio of nanoparticles up to 0.4 wt.%, whereas the average droplet velocities decreased. In the case of the nanofluid spray of 0.5 wt.%, the increase in droplet diameters and the decrease in droplet velocities were much more marked, departing from the linear relationship. This unusual behavior could also be observed in the local distributions of droplet diameters and velocities along the axial and radial directions. Further research studies are required to reveal how the addition of nanoparticles affects the atomization mechanism of nanofluids. The difference in the spray characteristics of nanofluid sprays from that of water sprays should be taken into consideration when the cooling effectiveness of nanofluids and water in spray cooling is compared.





Effect of Inlet Air PreCooling of Water Injection on Compressor Performance at High Flight Mach


Pages :
421431


Authors :
A. Q. Lin,
Q. Zheng,
L. Yang,
H. Zhang,
In high altitude and Mach number, the inflow air with the high temperature will influence on the aeroengine performance while the mass injection precompressor cooling (MIPCC) technology is one of the problemsolving ways to reduce high temperature. To explore the convection coupling process between droplet and inflow air, the compressible Reynolds average NS equations in the compressor coupled with the precooling section is solved by the finite volume method to analyze its performance changes at different water injection rates and droplet sizes. Results show that, in the flight of 3.5 Mach number, the larger water injection rate easily form the shock wave due to the disturbance of droplets in the precooling section. Furthermore, the temperature on the pressure surface near the trailing edge of the rotor blade aggravates along the radial migration, leading to uneven temperature distribution in the radial direction. Within the water injection rates of 08% and the particle sizes of 1020 µm, the inflow mass flow of air improves by 15.331.4%; the temperature ratio of compressor drops by 3.616.14%, which results in the decrease of specific compression work of the compressor and the changing trend from “increasing” to “decreasing” for the compressor efficiency.





Analytical Solution to the Boundary Layer Slip Flow and Heat Transfer over a Flat Plate using the Switching Differential Transform Method


Pages :
433444


Authors :
H. H. Mehne,
M. Esmaeili,
In this paper, the twodimensional steady boundary layer flow and heat transfer over a flat plate with slip velocity and temperature jump conditions at the walls were analyzed. Using similarity transforms, the governing equations were reduced to a system of ordinary differential equations. Semianalytical solutions to the resulting boundary value problem were obtained using the differential transform method (DTM). In order to cover the asymptotic boundary conditions, a method of switching curves was proposed. In this switching approach, the traditional solution to the DTM, which is valid for finite horizons, was followed by another path that was also an analytical solution to the problem. The main preference of the resulting closed form solution with respect to numerical solution is the possibility of parametric studies. The method was verified using some available numerical data, and the results showed that our proposed method had reasonable efficiency and accuracy.





Monitoring the Performance of Centrifugal Pump under SinglePhase and Cavitation Condition: A CFD Analysis of the Number of Impeller Blades


Pages :
445459


Authors :
A. Ramadhan AlObaidi,
In this current study, the transient numerical calculations using CFD are carried out under different number of impeller blades for the flow field within a centrifugal pump under singlephase and cavitation condition. Both qualitative and quantitative analyses have been carried out on all of these results in order to better understand the flow structure within a centrifugal pump under both singlephase and cavitation. Also, the investigation using different number of impeller blades relating to the static pressure, velocity magnitude and vapour volume fraction variations have been analysed. Fluctuations pressure in both time and frequency domains at the impeller and volute of the pump also investigated. As a result, the pressure and velocity were gradually increased from inlet to outlet of the pump. Pressure at the impeller outlet was higher than the pressure at other parts due to high interaction between impeller and volute tongue region. The distribution of volume fraction first occurs at the inlet eye of impeller. Furthermore, the cavitation increases as the number of impeller blades and flow rate increase. The length of the cavity was increased when low pressure at the inlet impeller (eye) decreased at Z=5 blades cavitation was affected highly at the suction of impeller compared to other number of blades particularly at high flow rate.





Visualization of Shock Wave Phenomenon around a Sharp Cone Model at Hypersonic Mach Number in a Shock Tunnel using High Speed Schlieren Facility


Pages :
461468


Authors :
M. Saiprakash,
C. Senthil Kumar,
G. K. Sunil,
S. P. Rampratap,
V. Shanmugam,
G. Balu,
The flow field around a Sharp cone model configuration has been investigated by means of Schlieren facility in hypersonic shock tunnel. The time dependent evolution of flow around a cone of angle 11.38° with base radius of 150mm has been visualized for a flow Mach number M = 6.5. Experiments have been carried out with Helium as driver gas and air as test gas to visualize the hypersonic flow field. The flow establishment, steady state, and termination process of the hypersonic flow have been visualized for two different angles of attack, namely 0°&5°. Experimentally measured shock angle compares well with the theoretical and the computational study. The measured shock layer thickness compares well with the numerical simulation for both angles of attack.





Linear Instability of Throughflow in a Rectangular Box Saturated by Nanofluid


Pages :
469475


Authors :
S. Saini,
Y. D. Sharma,
An analysis is made for the effect of throughflow on the onset of convection in a rectangular box under the assumption that total flux (sum of diffusive, thermophoretic, and convective) is zero on the boundaries. A linear stability analysis and Galerkin weighted residual method are used to obtain the Rayleigh number and stability curves for the onset of convection. Three dominating combination of parameters are extracted from the nondimensional analysis. All rescaled parameters promote the convection. Aspect ratios, throughflow, and nanoparticles play an important role in the formulation of cell distribution and development of convection. Oscillatory convection is possible for permissible range of nanofluid parameters. It is also found that the size of a cellular mode is altered by throughflow and nanoparticles.





Determination of Flow Patterns in Vertical Upward TwoPhase Flow Channel via Void Fraction Profile


Pages :
474483


Authors :
M. A. Zubir,
R. Ramli,
M. Z. Zainon,
Twophase flow happens widely in the industrial plants and certain equipment. This paper attempts to study the characteristics of twophase flow in a vertical piping system. This was achieved by comparing the void fraction, in the working fluid, by employing Constant Electric Current Method (CECM) with the actual observation using highspeed camera. The experiment requires a complete set of twophase flow system information and was conducted based on various flow conditions. In order to carry out this experiment, the twophase flow loop was constructed using a specific experimental apparatus and components. The flow channels were constructed using three pipes with three different inner diameters of 21.0 mm, 47.0 mm and 95.0 mm. The flow direction was vertical upward cocurrent flow with liquid superficial velocity range of 0.025 m/s to 3.0 m/s and gas superficial velocity range of 0.025 m/s to 3.0 m/s, depending on the size of the pipe. The flow pattern investigation focuses on experimental work, which was based on systematic observation and measurements using a highspeed camera and some measuring apparatus. The void fraction measurement using the CECM sensor was integrated into twophase flow system with constant electric current running in the pipe and data acquisition system controlled virtually via LabVIEW software. Both result of the flow pattern and void fraction graph were then compared to determine the type of flow pattern from the void fraction graph. Information from the previous studies and experiments were collected and the assumption of any theoretical simplifications were used as a reference. According to the result, the flow pattern in pipe can be easily determined using CECM.





Numerical Analysis on Pipeline Leakage Characteristics for Incompressible Flow


Pages :
485494


Authors :
Z. Zeng,
R. Luo,
Systematic Computational Fluid Dynamics (CFD) simulations on incompressible water pipe flow with leakage were conducted in the present study. The aim is to provide the understanding of how different parameters, including the leakage pipe diameter, inlet mass flow rate, and main pipe length, affect the flow phenomena at the vicinity of the leakage location. The present CFD data show that the leakage pipe diameter has dominant effect on the leak mass quantity, pressure change at the vicinity of leak location, total pressure drop and pressure gradient along the main pipe. The effects of both inlet mass flow rate and the main pipe length on leak mass quantity are comparably important. Due to existence of the leakage pipe, larger velocity but lower pressure at upstream, and lower velocity but larger pressure at downstream occur at the vicinity of leakage, which causes adverse pressure at this region. The pressure change resulted from the adverse pressure increases approximately linear with the leak size ratio (ratio of leakage pipe diameter to main pipe diameter) when it is smaller than approximately 40%, at which the maximum pressure change at the leak location occurs. When the leak size ratio is smaller than approximately 5%, the pressure change at the leak location is seen to be approximately zero, implying negligible pressure difference at the two boundary points of leakage pipe. There is sudden change in the pressure gradient along the flow direction at the leak location, which results from a local pressure increase there. When farther away from the leakage, the magnitude of the maximum pressure gradient along the flow direction is reduced due to attenuation of leakage effect. The present study proves that CFD analysis could be an effective and lesscostly way to investigate pipe flow with leakage, so as to provide scientific understanding of the physics on pipe flows with leakage.





Fluidized Bed Granulation Parameters Effect on Urea Granule Physical Properties


Pages :
495503


Authors :
M. F. Dimin,
M. K. Loh,
M. R. Jamli,
M. K. Sued,
R. F. Munawar,
The aim of this study is to determine the effect of different airflow patterns and process conditions on the physical properties of granules produced using fluidized bed granulation technique. It was observed that spiral airflow was the most important factor to produce granules with required size and density under similar process conditions if compared with normal airflow. From ANOVA, binder spray pressure and bag shake duration showed the strongest influence on hardness of granules on both types of airflow patterns. Optimization studies for spiral and normal airflow proved that granules with desired density and hardness can be produced at middle level of wind velocity and binder spray pressure together with high level bag shake duration. For spiral airflow, the optimum value for wind velocity, binder spray pressure and bag shake duration was 28 m/s, 0.31 MPa and 60 s respectively; for normal airflow, it was 25 m/s, 0.20 MPa and 15 s respectively.





NonGaussian Wind Pressure Characteristics of HAWT Tower System with and Without Rotor


Pages :
505514


Authors :
N. I. Haroon Rashid,
S. Nadaraja Pillai,
S. Selvi Rajan,
C. Senthil Kumar,
Generally, the Gaussian assumption has been considered in analyzing the data pertaining to the wind effects on the structures or bluff bodies due to the abundance of the statistical information. In this study, Horizontal Axis Wind Turbine (HAWT) tower system with dimension of 1:330 scale is studied in order to understand their peak pressure behavior for wind resistant design. Generally, tower systems are constructed of various geometrical structures such as lattice towers, tubular steel towers, concrete towers, but in this present study tubular cylindrical tower is only considered. Simultaneous pressure measurements on the surface of the tower were performed in the lowspeed boundary layer wind tunnel with test section dimension of 18 m × 2.5 m × 2.15 m having Reynolds number ranging from 102 to 104. The peak pressures acting on the tower systems are calculated for a number of tenminute samples on various locations of the wind turbine. Peak value calculations based on Gaussian and Non – Gaussian processes are discussed mathematically and applied to the data collected from the wind tunnel tests. A mathematical model of Davenport and Kareem – Zhou is used in calculating the peak factor for Gaussian and non – Gaussian processes, respectively. The results indicate that higher moments dominate as most of the distribution is skewed and with kurtosis value. Henceforth, a study on extreme value analysis is deemed necessary in designing wind resistant structures or bluff bodies. Considering Gaussian nature alone may underrepresent the peak value of the HAWT tower.





Combined Heat and Mass Transfer during Condensation of Vapours Mixture and NonCondensable Gas in a Vertical Tube


Pages :
515526


Authors :
A. Charef,
M. Feddaoui,
A. Nait Alla,
M. Najim,
The problem of laminar film condensation from binary vapours mixture with the presence of noncondensable gas (air) flowing in a vertical tube is numerically investigated. The set of the nonlinear parabolic equations expressing the mass conservation, momentum, energy, and species diffusion in both phases with the boundary conditions are resolved by using a finite difference numerical scheme. A comparative study between the results obtained for three cases (waterethanolair, watermethanolair, and ethanolmethanolair) under the same conditions is made. The impact of varying the wall temperature, the inlet vapour mass fractions, and the inlet liquid mass flow rate on the conjugate the heat and mass transfer during the condensation of the studied mixtures are examined. It is found that the condensation of watermethanolair corresponds to a higher latent heat flux QL2 and accumulated condensation rate Mr2 when compared with waterethanolair and ethanolmethanolair. Moreover, the nature of the fluid plays an important role in the heat and mass exchanges.





Experimental Study of an Acoustically Excited Plane Jet at Low Reynolds Numbers


Pages :
527537


Authors :
S. Marzouk,
N. Hnaien,
The dynamics of a vertical twodimensional air jet under acoustic excitations at a low Reynolds number are investigated experimentally. The perturbation is introduced by means of a loudspeaker located in a settling chamber before the nozzle exit. The experiments are operated at Strouhal number St ranging from 0 to 1 and for different pulsation amplitudes. A laser plan is used to visualize the flow and the hotwire anemometry for more specific measures of the mean and fluctuation velocity. The discussion is focused on the influence of two parameters governing the flow: the Strouhal number St and pulsing amplitude. The main results show that the flow consisting of the vortex propagating downstream a nozzle exit is strongly affected by the excitation. Indeed, the introduction of an external perturbation introduces a more rapid degeneration of the potential core with the appearance of vortices near the nozzle as the pulsation amplitude increases. These vortices are amplified and become larger than the nozzle width which induces the enhancing of the entrainment and mixing effects of the shear layers. Another very important phenomenon is observed: the excitation has led to the formation of a switching from the asymmetric mode (sinuous mode) to the symmetric mode (varicose mode).





A NavierStokes Solver for Compressible Turbulent Flows on Quadtree and Octree Based Cartesian Grids


Pages :
539549


Authors :
E. Kara,
A. İ. Kutlar,
M. H. Aksel,
Cartesian grids represent a special extent in unstructured grid literature. They employ chiefly created algorithms to produce automatic meshing while simulating flows around complex geometries without considering shape of the bodies. In this article, firstly, it is intended to produce regionally developed Cartesian meshes for two dimensional and three dimensional, disordered geometries to provide solutions hierarchically. Secondly, accurate results for turbulent flows are developed by finite volume solver (GeULERNaTURe) with both geometric and solution adaptations. As a result, a “handsoff” flow solver based on Cartesian grids as the preprocessor is performed using objectoriented programming. SpalartAllmaras turbulence model added Reynolds Averaged Navier Stokes equations are solved for the flows around airfoils and wings. The solutions are validated and verified by one two dimensional and one three dimensional turbulent flow common test cases in literature. Both case studies disclose the efficaciousness of the developed codes and qualify in convergence and accuracy.





On the ReynoldsAveraged NavierStokes Modelling of the Flow around a Simplified Train in Crosswinds


Pages :
551563


Authors :
T. Li,
J. Zhang,
M. M. Rashidi,
M. Yu,
Currently, there are different computational fluid dynamic (CFD) techniques used to obtain the flow around trains. One of these techniques is the Reynoldsaveraged NavierStokes (RANS), which is commonly and widely used by industry to obtain the mean flow field around trains in different operating conditions. In order to assess the performance of RANS turbulence modelling for train aerodynamics, five different common RANS modelling have been used in this paper to obtain the flow and the surface pressure around a simplified train model subjected to crosswind; the standard k model, the realisable k model, the ReNormalisation Group (RNG) k model, the standard k model and Shear Stress Transport (SST) k model. The train model was stationary and subjected to crosswind with a 90o yaw angle. The effects of mesh size and spatial discretization scheme on the aerodynamic characteristics of the train were also investigated. The results obtained from the different RANS models were compared to those from published experimental data. In general, all the RANS models provided the pressure distribution trend. However, all k models overestimate the surface pressure on the train body except the bottom face. The standard k model underestimates the surface pressure on the train body except the streamwise face. It was shown that the simulation using SST k model together with a second order discretization scheme provides the closest results to the experimental surface pressure. It could be concluded from the present study that the SST kmodel with a second order discretization scheme and y+ around 1.0 is the most appropriate RANS model for simulating the flow around trains subjected to crosswinds.





Effect of Gurney Flap Configuration on the Performance of a Centrifugal Fan


Pages :
565571


Authors :
M. Suresh,
N. Sitaram,
The present investigation is conducted to study the effect of Gurney flap configuration on the performance of a centrifugal fan at different Reynolds numbers. Gurney flaps of different configurations, such as angle, quarter round and half round corresponding to a nominal height of 1.2 mm (1.94% of the impeller blade spacing at tip) height are attached to the pressure surface of the centrifugal fan impeller blade tip. Performance tests are carried out on the centrifugal fan with a vaneless diffuser at five Reynolds numbers viz. 0.30, 0.41, 0.55, 0.69 and 0.80×105 based on the impeller tip speed, impeller blade exit height and kinematic viscosity, both with and without Gurney flaps. From the performance curves it is found that fan performance improves significantly with Gurney flaps at low Reynolds numbers and improves marginally at high Reynolds numbers. Gurney flap of angle configuration of height as small as 1.94% of the impeller blade spacing at tip increases head coefficient by 5.2% and increases the volume flow rate across the fan by 5.4% at the lowest Reynolds number of 0.30×105. Even though there is increase in both head and flow coefficients with other Gurney flap configurations (quarter round and half round), they are always less than that for the angle Gurney flap. The effect of Reynolds number on the performance curves is found to be negligible with Gurney flaps, whereas the effect of Reynolds number on the performance curves of the impeller without Gurney flap is found to be considerable. Additional experiments conducted with Gurney flaps of two configurations, viz. angle and quarter round, with a larger height of 2.5 mm (4.05% of the impeller blade spacing at tip) attached on the pressure surface of the centrifugal fan impeller blade tip have shown that the performance of the fan with quarter round GF is better than the performance of the fan with angle GF.





Performance Evaluation of Insoluble Surfactants on the Behavior of Two Electric Layers


Pages :
573586


Authors :
S. A. Alkharashi,
A. Assaf,
K. AlHamad,
A. Alrashidi,
The purpose of this study is to establish the effects of insoluble surfactants on the stability of two layers flow down an inclined wall in the limit of Stokes and longwavelength approximations. The dynamics of the liquidliquid interface is described for arbitrary amplitudes by evolution equations derived from the basic hydrodynamic equations, in which the fluids are subjected to a uniform electric field. The principle aim of this work is to investigate the interfacial stability as well as the growth rate in the presence of insoluble surfactants. The parameters governing the flow system, such as Marangoni, Weber, capillary numbers and the inclined substrate strongly affect the waveforms and their amplitudes and hence the stability of the fluid. Approximate solutions of this system of linear evolution equations are performed. epending on the selected parameters, the phenomenon of the dual role is found with respect to the electric Weber number as well as the viscosity ratio. The interfacial waves will be more stable due to the growth of the Marangoni number while, while the opposite effect is found for the increase in capillary number. In the longwave perturbations, the stability process is found to confirm the stabilizing effect of the Marangoni number and the destabilizing influence of both capillary and Reynolds numbers, whereas the dual role is observed for the dielectric ratio.





Inertia Effects in Rheodynamic Lubrication of an Externally Pressurized Converging Thrust Bearing using Bingham Fluids


Pages :
587594


Authors :
G. Alexander Raymand,
I. Jayakaran Amalraj,
Thrust bearing are innately developed to withstand axial load. When the bearing is subjected to high speed operations, heavy load, high stiffness etc., suggesting a change in the design of the bearing plays a vital role in its performance. Friction is developed between the circular plates while the bearing operates. To reduce this friction, the bearing is lubricated with lubricants such as mineral oil, greases etc., Generally, lubricants are classiﬁed into two types that is Newtonian and nonNewtonian. However, nonNewtonian ﬂuids characterized by an yield value such as Bingham, Casson and Herschel Bulkley, are attracting the tribologists, at present. And also, the study of ﬂuid inertia on thrust bearing is required to optimize the performance of the bearings. In this investigation, we have ventured to analyze the performance of the bearing by considering the combined effects of ﬂuid inertia forces and nonNewtonian characteristic with Bingham ﬂuid as lubricant in an externally pressurized converging circular thrust bearing. Such studies will be useful in the design of the bearing for the optimum performance using the appropriate lubricant in various machineries operating in an extreme condition in the industries. Averaging the inertia terms over ﬁlm thickness and deﬁning a modiﬁed pressure gradient, the rheodynamic lubrication equation containing inertia terms has been analyzed. Using the appropriate boundary conditions and considering externally pressurized ﬂow in narrow clearance between two converging discs is symmetric w.r.t r and z axis, the velocity distributions, the modiﬁed pressure gradient and thereby the ﬁlm pressure and the load capacity of the bearing have been obtained numerically for different values of Bingham number, Reynolds number and angle of convergence. In addition to that, the effects of the inertia forces, nonNewtonian characteristics and angle of convergence on the bearing performances have been discussed.





Numerical Optimization of an Array of Triangular Microchannels using Constructal Theory


Pages :
595601


Authors :
M. Mardani,
M. R. Salimpour,
In the present study, an arrangement of triangular microchannels with different contact angles is analyzed and optimized following the guidelines provided by the constructal theory to reach to the maximum heat removal rate. This investigation is performed analytically and numerically. Based on the obtained results, it is emerged that this optimization is independent of the number and the type of the arrangement of the microchannels. It is also observed that increasing the pressure drop through the triangular microchannels decreases the optimal hydraulic diameter. Numerical results recommend that the microchannel with contact angle of 60° possesses the highest heat transfer rate at a given pressure drop, and decreasing the contact angle of the triangular crosssection leads to lower heat transfer rates. Comparing the analytical and numerical results of the optimal hydraulic diameter of the microchannel heat sinks, a reasonable agreement is observed; however, due to some assumptions which are considered at the analytical method, the analytical predictions of the configurations having the highest heat transfer rate are inaccurate. Therefore, the numerical optimization should be used to choose the configuration with the highest cooling capacity.





A Magnetohydrodynamic Time Dependent Model of Immiscible Newtonian and Micropolar Fluids through a Porous Channel: a Numerical Approach


Pages :
603615


Authors :
M. Devakar,
Ankush Raje,
The objective of the present article is to study the magnetohydrodynamic(MHD) unsteady flow and heat transfer of two immiscible micropolar and Newtonian fluids through horizontal channel occupied with porous medium. Initially, fluids in both regions as well as both plates are at rest. At an instant of time, the flow in both regions is generated by a constant pressure gradient. The governing nonlinear and coupled partial differential equations of Eringen’s micropolar fluid and Newtonian fluid are solved subject to suitable initial, boundary and interface conditions. The numerical results for velocity, microrotation and temperature are obtained using CrankNicolson finite difference approach. The results obtained for velocities, microrotation and temperatures are presented through figures. The analysis regarding volume flow rate, skinfriction coefficient and Nusselt number is also done and is presented through tables. It is explored that, velocity, microrotation and temperature are increasing with time and accomplishing steady state at higher time level. Velocity is decreasing with micropolarity parameter and Hartmann number, and increasing with Darcy number. Temperature enhances with increasing Brinkmann number, and declines with Prandtl number and ratio of thermal conductivities.





Study of Conjugate Heat Transfer from Heated Plate by Turbulent Offset Jet in Presence of Freestream Motion using LowReynolds Number Modeling


Pages :
617630


Authors :
Sushil Rathore,
The present study deals with conjugate heat transfer from a heated ﬂat plate by a turbulent offset jet in presence of freestream motion. The turbulent convection in ﬂuid and conduction in solid is solved in a coupled manner by simultaneously satisfying the equality of temperature and heat ﬂux at the solidﬂuid interface. The computations have been carried out using lowReynolds number (LRN) k −ω SST model in the ﬂuid region. The capability of LRN modeling have enabled to solve the entire boundary layer including the thin viscous sublayer due to which Moffatt vortices (secondary recirculation regions) have been captured near the corner of the wall where the turbulence Reynolds number is low. The bottom surface of solid plate is maintained at a constant temperature higher than the jet inlet temperature whereas the jet inlet temperature is same as that of the ambient. The present investigation reports the effects of offset ratio of jet (OR), Reynolds number of ﬂow (Re), solid to ﬂuid thermal conductivity ratio (K), solid slab thickness (S) and freestream velocity (U∞) on conjugate heat transfer arises due to solid and ﬂuid interaction. The offset ratio is varied in the range OR = 3 − 11, Reynolds number in the range Re = 10000 − 25000, solid to ﬂuid thermal conductivity ratio in the range K = 1 − 2000, solid slab thickness in the range S = 1−20 and freestream velocity in the range U∞ = 0.1 − 0.25. The effects of various parameters on the nearwall velocity proﬁle, solidﬂuid interface temperature, local Nusselt number variation along the plate, heat ﬂux variation along the plate, etc. have been discussed in detail.





Computational Simulation of ShockBubble Interaction, using a FrontTracking/Ghost Fluid Method


Pages :
631645


Authors :
A. Razmi,
M. TaeibiRahni,
H. R. Massah,
H. Terashima,
H. Moezzi,
A front tracking/ghost fluid method was used to simulate fluid interfaces in a shock–bubble interaction problem. The method captures fluid interfaces, using explicit fronttracking and defines interface conditions, using the ghostfluid method. In order to demonstrate the accuracy and the capability tracking of the approach used, an airhelium and anairR22 shockbubble interaction cases were simulated. The computational results were compared with reliable experimental and computational studies, showing close agreements.





