Recent Volume
Vol10 , No 3
Electrohydrodynamic Instability in a Heat Generating Porous Layer Saturated by a Dielectric Nanofluid
Pages : 763-776
Authors : D. Yadav, 
In this paper, a theoretical investigation has been carried out to study the combined effect of AC electric field and temperature depended internal heat source on the onset of convection in a porous medium layer saturated by a dielectric nanofluid. The model used for nanofluid incorporates the combined effect of Brownian diffusion, thermophoresis and electrophoresis, while for porous medium Darcy model is employed. The flux of volume fraction of a nanoparticle with the effect of thermophoresis is taken to be zero on the boundaries and the eigenvalue problem is solved using the Galerkin method. Principle of exchange of stabilities is found to be valid and subcritical instability is ruled out. The results show that increase in the internal heat source parameter , AC electric Rayleigh-Darcy number , the Lewis number , the modified diffusivity ratio and the concentration Rayleigh-Darcy number are to hasten the onset of convection. The size of convection cells decreases with increasing the internal heat source parameter and the AC electric Rayleigh-Darcy number .

CFD Transient Simulation of a Breathing Cycle in an Oral-Nasal Extrathoracic Model
Pages : 777-784
Authors : C. Paz,  E. Suarez,  M. Concheiro,  J. Porteiro, 
Knowledge of respiratory flow behaviour is important in many respiratory medical fields. The usefulness of numerical models in providing a better understanding of flow phenomena has made the Computational Fluid Dynamics (CFD) an indispensable research tool due to the difficulty of measuring in vivo data. In this research, the extrathoracic airways and the upper tracheobronchial region, trachea and main bronchus bifurcation were modelled. Oral and nasal breathing routes have been considered under steady and cyclic unsteady conditions. A realistic far boundary condition was imposed as the flow inlet. Different ventilation levels and frequencies were simulated. The model presented has been validated successfully by two parts: nasal and oral models. The airflow distributions through oral and nasal routes were determined, analysed and compared under different breathing conditions. The flow behaviour and respiratory effort during inhalation and exhalation phases change from rest to high activity; the flow can increase 40% with the same respiratory effort, opening the mouth during the inspiration. Significant differences in turbulent intensity contours in steady and unsteady cases have been observed. This study demonstrated the relevance of considering different breathing patterns and more realistic unsteady conditions.

Numerical Studies on the Effect of Diffuser Rotational Speeds on Low Pressure Ratio Centrifugal Compressor Performance
Pages : 785-799
Authors : S. Seralathan,  D. G. Roy Chowdhury, 
Numerical investigations are conducted to analyze the effect of rotational speeds of rotating vaneless diffuser on performance of a low-pressure ratio centrifugal compressor stage at off-design and design flow coefficients. Four different rotational speeds are selected for the rotating vaneless diffuser. Free type rotating vaneless diffusers are rotated at speed ratios SR0.25, SR0.50 and SR0.75. Total temperature of the fluid at stage exit increases with increase in rotational speeds of the free rotating vaneless diffuser. This affects the efficiency of free rotating vaneless diffusers. Its flow angles are smaller resulting in a shorter flow path length thereby reducing the frictional losses substantially. Gain in stagnation pressure is observed for all free rotating vaneless diffuser configurations. The rotational speeds determine the extent of net gain in energy level of the fluid and drop in total pressure losses. Based on static and stagnation pressure distributions at stage exit, the flow in FreeRVDSR0.75 undergoes a comparatively better diffusion process. In general, efficacy of diffusion process in a compressor stage with free rotating vaneless diffuser is better at speed ratio above 0.50. Subsequently, the speed ratio is increased to SR1.0 thereby the rotating vaneless diffuser behave like a forced type. Various free rotating vaneless diffuser configurations are compared with the forced type of rotating vaneless diffuser. Based on this study, it is understood that an optimum rotational speed of the rotating vaneless diffuser plays an important role in facilitating the effective diffusion process within the diffuser passage.

Experimental and Numerical Investigation on Laminar Pipe Flow of Magneto-Rheological Fluids under Applied External Magnetic Field
Pages : 801-811
Authors : E. Gedik, 
An experimental and numerical study of Magnetorheological (MR) fluids flow in circular pipes under the influence of uniform magnetic field is considered. In the experiments, an electromagnetic device was manufactured to generate the magnetic field. The experiments were performed using magnetic fields B= 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.12 and 0.15 T. Numerical study was performed to show the accuracy of the results obtained from experimental study. In numerical study, Computational Fluid Dynamics (CFD) analysis was used. The ANSYS Fluent 14.0 code based on the finite volume method was used for the CFD analysis. In the experiments, the applied magnetic field decreased the flow rate of the fluids by increasing viscosity. In case of 10 mm pipe diameter, the flow velocity of the A, B and C fluids were obtained as 0.593, 0.749 and 0.938 m/s respectively in situation B=0 T. When magnetic field was applied as B=0.15 T, decreases have occurred in the velocity of A, B and C fluids as 95.27%, 90.24% and 85.6% respectively. Similarly, in case of 15 mm pipe diameter, 96.87%, 95.06% and 90.76% decreases have occurred in the flow velocity of A, B and C fluids having 0.301, 0.363 and 0.445 m/s flow velocity respectively. The results were compared for the magnetic field values B=0, 0.05, 0.10 and 0.15 T. It was found that the differences between experimental and numerical study were found as 6.10% and 1.71% for the B=0 T and B≠0 T situations respectively when the pipe has 10 mm pipe diameter. In case of 15 mm pipe diameter, the differences were found as 2.31% and 0.89%. As a result, it was found that the results obtained from experimental and numerical study were qualitatively and quantitatively in good agreement.

Numerical Simulation for Freeze Drying of Skimmed Milk with Moving Sublimation Front using Tri-Diagonal Matrix Algorithm
Pages : 813-818
Authors : R. Naik,  G. Arunsandeep,  V. P. Chandramohan, 
Freeze drying is a highly advanced dehydration technique used for preserving pharmaceuticals, human organs transplanted to others and highly heat sensitive food products. During the freeze drying, there are two layers formed namely dried region and frozen region. In this present work, a numerical model is developed to estimate the temperature distribution of both regions. The sample object considered is skimmed milk. The transient heat conduction equations are solved for both regions of dried and frozen region. The interface layer between the two region is considered as moving sublimation front as same as the realistic case. Radiative boundary condition at the top and convective boundary condition at the bottom are considered. The model has been solved by finite difference method and the scheme used is backward difference in time and central difference in space (implicit scheme), which generates set of finite difference equations forming a Tri-Diagonal Matrix. A computer program is developed in MATLAB to solve the tri-diagonal matrix. The temperature distribution along the length of the product with varying chamber pressures and the sublimation front temperature with time are estimated. The transient effect of sublimation front movement was estimated with different applied chamber pressure. It was noticed that at lower pressure the sublimation rate is very fast.

Entropy Generation and Optimization of Laminar Forced Convection Air Cooling in a Horizontal Channel Containing Heated Sources
Pages : 819-831
Authors : M. Bensouici,  F. Z. Bensouici, 
A forced convection air-cooling of two identical heat sources mounted in a horizontal channel is numerically studied. Four effects of Reynolds number, separation distance, height and width of the components on the flow structure and heat transfer inside the channel have been examined. The entropy generation minimization method (EGM) is employed to optimize the heat transfer and fluid flow in the channel. The flow field is governed by the Navier–Stokes equation and the thermal field by the energy equation. The finite volume method and the SIMPLER algorithm are used to solve the continuity, momentum, energy and entropy generation equations. Results show that the mean Nusselt number increases with increase of the following parameters: Reynolds number, separation distance, height and width of the components. However, these parameters increase the total entropy generation, and thus provokes the degradation of the fan energy. The optimal values of separation distance, height and width heat source are: [(Sopt= 1 with W=0.25, C=0.25, Re=50, η=1.134), (Copt=0.3 with W=0.25, S=0.25, Re=100, η=0.895) and (Wopt= 0.1 S=0.25, C=0.25, Re=200, η=1.004)], respectively, where η is the optimization factor (=Num/S_T^*) and is defined as the ratio of Nusselt number to the total entropy generation. Finally, the optimal and the best configuration for maximum heat transfer and minimum entropy generation is observed at Re=50, S=1, C=0.25 and W=0.25.

Investigation of Corrugated Wing in Unsteady Motion
Pages : 833-845
Authors : A. Shahzad,  H. R. Hamdani,  Ahmad Aizaz, 
Delayed stall is the most dominant lift enhancing factor in insect flapping motion. Micro air vehicle operates at Reynolds number 104-105; slightly higher than the insects’ Reynolds number (Re). In the present research, thefocus is to investigate “stall-absent” phenomenon at Re representative of the micro air vehicles, the effect of spanwiseflow on the leading edge vortex and also to study the effect of geometry variations on the aerodynamic performance of the wing in unsteady motion. Corrugated dragonfly airfoil with rectangular wing planform is used, however, with wing kinematics restricted to azimuth rotation only. Three-dimensionalfinite-volume method is used, through commercial software Fluent, to numerically solve time-dependent incompressible Navier-Stokes equations. Computed results at Re 34000 and 100,000 reveal the same phenomenon of delayed stall, as observed in the case of insects. Furthermore, the performance of flat plate, profiled and corrugated wing in a sweeping motion at a high angle of attack is also compared.

Analysis of Turbulent Natural Convection by an Elliptic Relaxation Model in Tall Vertical Cavities with Linear Temperatures on Sidewalls
Pages : 847-859
Authors : L. El Moutaouakil,  Z. Zrikem,  A. Abdelbaki, 
Turbulent natural convection of air is studied, by the elliptic-relaxation model v^2-f, in a tall vertical cavity whose hot and cold walls are maintained at linear temperatures of slopes γ_1 and γ_2, respectively. The average temperatures of the active walls are located at mid-height of the cavity. Four situations are analyzed, corresponding to γ_1=γ_2=γ (case I), γ_1=-γ_2=γ (case II), γ_1=0 and γ_2=γ (case III), γ_1=γ and γ_2=0 (case IV). These boundary conditions may be more representative or used to control heat transfer for certain systems. The effects of the slope (-1≤γ≤1), the aspect ratio of the cavity (10≤A≤80) and the average Rayleigh number (5×〖10〗^4≤〖Ra〗_m≤〖10〗^6 ) on the streamlines, isotherms, contours of the turbulent kinetic energy, heatlines, local and average Nusselt numbers are investigated. It is shown that the local and average heat transfers of cases III and IV can be deducted from those of cases I and II. The obtained dynamic and thermal fields as well as local and average heat transfers of the studied cases are quite different of those of the classical case corresponding to γ=0. A simplified procedure for calculating the average Nusselt number is also developed for each case.

Numerical and Experimental Investigation of Wiper System Performance at High Speeds
Pages : 861-870
Authors : S. Cadirci,  E. S. Ak,  B. Selenbas,  H. Gunes, 
In this study, aerodynamic forces acting on the windshield wiper system at critical wiper angles are simulated using different wiper blade geometries, i.e., wiper and spoiler modifications, to solve the wiping problem occurring at high speeds due to lifting forces. Undesired aerodynamic lift forces reach a peak at critical blade angles, thus turbulent air flow around the wiper blades at critical angles on a car model is investigated numerically in detail to solve this problem. Previous experimental studies have shown that the front windshield wiper blades can be lifted up by aerodynamic forces between wiper blade angles of 30-40°, if no geometric modifications are done to prevent this. The possible modifications which can have a positive effect on wiper’s performance include wiper’s profile (also spoiler’s curvature), wiper’s height and connection type of the rubber part to the metal part. Aerodynamic lift and drag forces acting on the wiper blade and wiper arm are calculated for both driver’s and passenger’s sides. It is revealed that for both wiper blades on the driver’s and passenger’s sides, an increased wiper height with a blunt connection type can supply most satisfactory results in terms of decreased lift forces, in other words negative lift forces. Utilizing the output of the numerical analysis, the new wiper-blade-spoiler profile is selected and then manufactured to test its wiping performance in a thermal wind tunnel by soiling tests. Numerical studies are validated by experimental tests, since the new wiper profile has been proven as a more efficient prototype in terms of wiping performance compared to the original one.

Steady MHD Flow of Nano-Fluids over a Rotating Porous Disk in the Presence of Heat Generation/Absorption: a Numerical Study using PSO
Pages : 871-879
Authors : Z. Uddin,  R. Asthana,  M. K. Awasthi,  S. Gupta, 
In this paper, a numerical integration technique, based on particle swarm optimization is proposed to investigate the effects of internal heat generation/absorption, on MHD boundary-layer flow of different types of nano-fluids over a rotating disk with uniform suction. Thermo-physical properties are modeled based on a wide range of experimental data. In the model, effect of nature of base fluid, nature of nano-particle material, size of nano particle, concentration of nanoparticle in the base fluid, nano-thermal layer formed around the nano particle etc. are taken into consideration. The two dimensional non-linear partial differential equations governing the flow are reduced to a system of coupled non-linear ordinary differential equations by using similarity transformations. These non-linear equations have been solved by using shooting based integration technique along with particle swarm optimization. In this study, four different types of water bases nanofluids are compared with respect to heat transfer enhancement, and the effects of nano-particle concentration, nano-particle size and heat generation/absorption parameters are studied in detail. The effects of different parameters on the dimensionless velocity profile and temperature distribution are discussed graphically. It is found that out of the four nano fluids considered, the heat transfer rate for CuO water based nano fluid is highest. It is also concluded that small sized nano-particles, high suction and high heat absorption increase the heat transfer rate.

Vorticity-Based Study of Apparent Mass of Body of Revolution
Pages : 881-887
Authors : X. Fu,  A. Y. Mao,  Z. Z. Jin, 
The apparent mass effect is enhanced significantly when the motion of a body changes quickly, such as a flapping wing or an impulsively started plate. Previous method for calculating the apparent mass of a given body needs to adopt the assumption of ideal flow and know the potential of velocity field arising in the fluid due to the motion of the body. However, the assumption of ideal flow is contrary to real fluid field and it is hard to obtain the potential of velocity field in most cases. In this paper, a new method based on the vorticity moment theorem for calculating the apparent mass of the body of revolution in the axial direction due to axial acceleration is developed. This method has no assumption of ideal flow and establishes the relationship between the apparent mass and the vorticity loops adjacent to the surface of the body. Using this method, the value of the apparent mass can be easily figured out and the physical mechanism of the apparent mass can be revealed from the view of the vorticity loop. The comparisons between different bodies have shown the influences of the fineness ratio (the ratio of the length to the maximum diameter) and the trailing edge type on the apparent mass.

A Finite Volume Study on the Effects of Electro-Hydro-Dynamic Fluid Acceleration on Airflow Around a Cylinder at Low Reynolds Numbers
Pages : 889-897
Authors : G. Tathiri,  H. Parishani,  E. Esmaeilzadeh, 
Applying a high-voltage electric field in a flow (electro-hydrodynamics or EHD) is a highly effective approach to locally accelerate a fluid to a desired speed. In this paper, a finite volume implementation is used to study the fluid flow around a fixed cylinder at a low Reynolds number. Two pairs of wire-plate electrodes are used to generate the electric field for fluid acceleration. Two Reynolds numbers are considered: Re=40, 100. We show that by increasing the Reynolds number, the relative effect of EHD is decreased. Further, we study the change in drag force due to EHD actuation. Finally, we showed that under certain voltage, electrode placement and Reynolds number the EHD fluid acceleration does not increase the total drag on the cylinder and yet leads to an increase in the total streamwise momentum transfer by augmenting the velocity at the top of the boundary layer.

On the Response of a Freely Vibrating Thick Elliptic Cylinder of Low Mass Ratio
Pages : 899-913
Authors : K. Sourav,  S. Sen, 
Two-dimensional space-time finite-element simulations are carried out to study the free vibrations of a rigid elliptic cylinder of aspect ratio 1.11 and low non-dimensional mass of unity. Undamped transverse-only as well as two-degrees-of-freedom oscillations are considered. The effect of damping is investigated on transverse-only motion. For all three cases, results for cylinder response are presented for 50≤Re≤180. In the absence of damping, transverse oscillations are mostly periodic except for a very narrow region near the end of lock-in. In contrast, a damping of 0.044 removes quasi-periodicity as well as secondary hysteresis from flow and body motion. For undamped motion, inclusion of in-line oscillations excites high amplitude oscillations, widens the range of synchronization and delays phase shift between lift and cross-stream response. In each case, synchronization between cylinder oscillations and vortex-shedding is 1:1. In addition, drag-lift phase plots are symmetric about mean lift (= 0) line. Thus, symmetrical shedding of two equally strong alternate vortices per oscillation cycle forms 2S, CNW(2S) or C(2S) modes. For each case, the lower branch initiates at Re = 65 where the oscillation or shedding frequency is found to be locally maximum.

Non-Uniform Heat Source/Sink and Thermal Radiation Effects on the Stretched Flow of Cylinder in a Thermally Stratified Medium
Pages : 915-924
Authors : T. Hayat,  Sadia Asad,  A. Alsaedi, 
The paper addresses the influence of non-uniform heat source/sink in flow of couple stress fluid by a stretching cylinder in a thermally stratified medium. Thermal radiation effect in heat transfer analysis is also accounted. Conservation laws of mass, linear momentum and energy leads to nonlinear situation. Use of adequate transformations coverts the partial differential equations into the ordinary differential equations. Series solutions of the resulting equations are obtained for the velocity and temperature. Convergence of the solutions is explicitly checked. Impacts of various sundry variable son the velocity, temperature, wall shear stress and Nusselt number are examined through graphical illustrations and numerical values. The effect of β and Re on velocity field is qualitatively similar. For larger values of curvature parameter γ velocity enhances. Influences of S and R on temperature on the temperature distribution are opposite. Heat transfer at the surface decays when A and B increase.

Determining the Thin Film Thickness of Two Phase Flow using Optics and Image Processing
Pages : 925-932
Authors : G. C. Keerthi Vasan,  M. Venkatesan, 
Gas- Liquid flows are by far the most important type of multiphase flow. This can be attributed to the wide range of industrial applications that the gas-liquid flow is discerned in. Popular examples of Gas-liquid flows are oil-gas mixtures, evaporators, boilers, condensers, refrigeration and cryogenics. The measurement of the liquid film thickness in two phase flows is prominent in various heat and mass transfer applications such as in boilers. To determine the thin film thickness is the aim of this study. A glass tube of diameter 4.7 mm is used for conducting the experiment and a laser pointer is used to obtain an image pattern on the screen. Using the principles of Optics, a method has been proposed to determine the thin film thickness and also to characterize the different types of flow. The thin film thickness obtained in the proposed method is validated using Image Processing.

Numerical Analysis of the Turbulent Flow Structure Induced by the Cavitation Shedding using LES
Pages : 933-946
Authors : H. Kanfoudi,  G. Bellakhall,  M. Ennouri,  A. Bel Hadj Taher,  R. Zgoll, 
To analyze the interaction between the turbulent flow structure with the cavitation shedding dynamics, a three-dimensional unsteady cavitating turbulent flow around the three-dimension NACA009 hydrofoil is investigated in this study. The cavitating flow in has been modeled with a homogeneous mixture of liquid and vapor using LES. The interaction between the cavitation and the fluid vortex is analyzed and discussed. The results demonstrate that the vortex stretching is mainly in the center of the cloud cavity and changes quasi-periodically as the cloud cavity evolves. As a result, the mechanism of the inception of cavitation, re-entrant jet and cavitation cloud shedding are accurately captured and predicted by LES in accordance with the experiment data.

Numerical Studies to Predict the Impact of Air Nozzle Position and Inclination on the Performance of Downdraft Gratifier
Pages : 947-955
Authors : P. C. Murugan,  S. Joseph Sekhar, 
Biomass gasification is one of the promising technologies to produce energy from the renewable energy sources, and the downdraft biomass gasifier is a widely used biomass energy conversion device. Among the various components of a gasifier, the position and the inclination of air nozzle have a vital role in the generation of producer gas. Therefore, a proper design is needed to fix the position and angle of the air nozzle. Keeping the above aspects, the present work focuses on the numerical simulation to predict the appropriate position and inclination of the air nozzle in a 50kWth imbert type downdraft gasifier by the species transport approach. The nozzle inclination varies from 0°, 20°, 30°, 45° and 60°, and the nozzle position is considered from 50mm, 100mm, 150mm and 200mm respectively. Experiments were also conducted to validate the numerical study. Both the studies show that the nozzle inclination at 45° and its position at 100mm above the reduction zone gives a reasonable composition of producer gas.

URANS-Based Prediction of Vortex Induced Vibrations of Circular Cylinders
Pages : 957-970
Authors : Er. Dobrucali,  O. K. Kinaci, 
Vortex induced vibrations (VIV) are highly nonlinear due to three different frequencies involved in the process; fluid force frequency, vortex shedding frequency and oscillation frequency. It is computationally complex to solve such a chaotic fluid flow but recent progress in numerical algorithms, turbulence models and computer capabilities have made it easier to approach the problem with a nonlinear approach. These developments have paved the way to approach the problem with the simple equation of motion of Newton’s law and when coupled with URANS, which is a commonly used method in solving problems related to fluid flow, the highly nonlinear problem of vortex induced vibrations become solvable. The existing literature computationally can only handle flows for Re>10,000-12,000 but the numerical methodology adopted in this study furthers this limitation. The numerical algorithm is first tried for a stationary cylinder and the boundary layer separation is investigated for higher Re. The generated results are found to be satisfactory to proceed solving for VIV at high Re. The solution strategy is tested in a wide range of Reynolds number with different springs and damping coefficients. Satisfactory agreement is found with the experiments for a cylinder in VIV. The shortcomings of the computational work and why these limitations arise are tried to be explained using the experimental results and the existing mathematical models.

CFD Analysis for Non-Newtonian Pseudo Plastic Liquid Flow Through Small Diameter U-BEND
Pages : 971-987
Authors : Suman Debnath,  T. Bandyopadhyay,  A. K. Saha, 
Experimental investigation has been carried out in order to study the pressure drop for non-Newtonian liquid flow through four different U-bends. Here, we have considered the internal diameter of the bend as 0.0127 m, radius of curvature (m) as 0.06 ≤ Rcb ≤ 0.20, and the non-Newtonian liquid flow rate (Ql×10-4 m3/s) as 1.18 to 4.5. The effects of different variables such as liquid flow rate, radius of curvature, pseudo plasticity of the liquid on the frictional pressure drop have been investigated. Numerical modeling is carried out using Fluent 6.3 software to find its applicability. The computational fluid dynamic (CFD) simulations are carried out using laminar non-Newtonian pseudo plastic power law model. Laminar non-Newtonian pseudo plastic power law model is used here as the SCMC (Sodium Carboxy Methyl Cellulose) solution flow through the bend behaves as non-Newtonian pseudo plastic fluid in laminar condition. The simulated results predict the flow structure, pressure drop, static pressure, shear stress, shear strain, Dean Vortices, friction factor and loss coefficients. The effect of pseudo plasticity, angle, radius of curvature, and Reynolds number on pressure drop, loss coefficient, friction factor and flow behavior inside the bend have also been explained elaborately and compared the results with water. The CFD simulation results are used to compare with the experimental data and observe a very good agreement with the experimental values.

Numerical Investigation of the Performance of Kenics Static Mixers for the Agitation of Shear Thinning Fluids
Pages : 989-999
Authors : A. Mahammedi,  H. Ameur,  A. Ariss, 
The laminar flow of non-Newtonian fluids through a Kenics static mixer is investigated by using the CFD (Computational Fluid Dynamics) tool. The working fluids have a shear thinning behavior modeled by the Ostwald De Waele law. We focus on the effect of Reynolds number, fluid properties, twist angle and blade pitch on the flow characteristics and energy cost. The pressure drop information obtained from the simulations was compared to several experimental correlations and data available in the literature. The numerical results were found in good agreement with the experimental values. From the obtained results, the twist technique is confirmed to be very useful to enhance the mixing with less power consumption and at low Reynolds numbers. A faster axial mixing has been achieved with increased blade length and decreased twist angle. However, the good mixing near the tube walls was obtained with increased twist angle. The power consumption expressed in power drop was found to be increase with increased CMC concentrations, Reynolds number, twist angle and decreased blade length.