فهرست مطالب

Applied Fluid Mechanics - Volume:12 Issue: 3, 2019
  • Volume:12 Issue: 3, 2019
  • تاریخ انتشار: 1398/02/12
  • تعداد عناوین: 30
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  • S. Subramanian *, K. S. Sridhar, C. K. Umesh Pages 647-655
    The concept of periodic renewal of boundary layers is a promising technique for the enhancement of heat transfer in microchannels. Extending the above concept, microchannel heatsink with modified hexagonal fins has been proposed for disrupting the flow periodically. An experimental study has been carried out to investigate the heat transfer and fluid flow characteristics of a microchannel heat sink with modified hexagonal fins and a conventional type of microchannels with plate fins. The heat transfer and fluid flow characteristics of the modified hexagonal fin microchannels have been compared with the plate fin microchannels. The heat transfer enhancement factor and pressure drop penalty factors are evaluated and compared. The modified hexagonal fin microchannel heat sink has been found to outperform the plate fin microchannel heat sinks in spite of the lesser heat transfer area.
    Keywords: Microchannel testing, Modified hexagonal fins, Single phase heat transfer, Copper heat sinks, Laminar flow
  • X. Wen *, J. Liu, Z. Liu, Z. Xia, L. Jin Pages 657-665
    This paper addresses the problem of rapid aerodynamic assessment and experimental verification of integrated hypersonic vehicles. Based on the adaptive Cartesian grid system, the aerodynamic performance of the air-breathing hypersonic vehicle is evaluated. Corresponding shrunken experimental model is investigated in FL-28 transient hypersonic wind tunnel. The grid-independent validation confirmes the proposed meshing method and optimal grid parameters. The convergent method is used for the aircraft model to calculate several flight conditions in cold state, and the calculated results are compared with the wind tunnel test data. The results show that in order to validate the accurate non-viscous dynamic characteristics of both internal and external flows, it is necessary to locally encrypt the grids of inner flow channel while ensuring the overall grid density of the aircraft. Although the computational time increases after grid encryption, the rapid prediction method of aerodynamic performance meets the requirements for engineering design. Compared with experimental results, there are several shockwave features invisible in the numerical results due to the simplification of solution procedure. The aerodynamic force coefficients obtained by the numerical method are verified by the experimental data and the same numerical method can be used in the conceptual design phase of aerodynamic shapes, which can greatly shorten the development cycle.
    Keywords: Aerodynamic characteristics, Internal, external flow integration, Rapid simulation assessment, Wind tunnel test
  • C. Kang *, W. Zhang, Y. G. Ji, Y. Cui Pages 667-677
    To investigate the characteristics of the bubbles trapped in liquid cross flow, air was injected into flowing water circulated in a closed loop. High speed photography was used to record bubble images instantaneously. An image-processing code was specifically developed to identify bubbles in the images and to calculate bubble parameters. Effects of the water velocity and the flow rate of the injected air on bubble patterns were investigated. The results indicate that the inclination of bubble trajectory relative to the nozzle axis is enhanced as the water velocity rises. Meanwhile, bubble size varies inversely with the water velocity. The bubble profile tends to be rounded as the water velocity increases. Fluctuations of the bubble velocity are intensified as the water velocity decreases. As the balance between the external forces exerted on the bubble is reached, an approximately linear relationship between the velocities of the bubble and the water is manifested. For a given equivalent bubble diameter, the bubble terminal velocity is higher than that associated with quiescent water. At small Eötvös number, the consistency of the bubble aspect ratio in the liquid flow and quiescent water is revealed. The range of Eötvös number is extended considerably due to the flowing water. Values of Weber number are accumulated in a range within which high bubble aspect ratio is associated with relatively high water velocity.
    Keywords: Bubble, Liquid cross flow, Air injection, Bubble trajectory, Bubble velocity, Bubble size
  • V. Singh *, S. Kumar, S. K. Mohapatra Pages 679-687
    In the present study, erosion wear of a 90o pipe bend has been investigated using the Computational fluid dynamics code FLUENT. Solid particles were tracked to evaluate the erosion rate along with k-ɛ turbulent model for continuous/fluid phase flow field. Spherical shaped sand particles of size 183 μm and 277 μm of density 2631 kg/m3 are injected from the inlet surface at velocity ranging from 0.5 to 8 ms-1 at two different concentrations. By considering the interaction between solid-liquid, effect of velocity, particle size and concentration were studied. Erosion wear was increased exponential with velocity, particles size and concentrations. Predicted results with CFD have revealed well in agreement with experimental results. The magnitude and location of maximum erosion wear were more severe in bend rather than the straight pipe.
    Keywords: Computational Fluid Dynamics (CFD), Erosion wear, Discrete Phase Model (DPM), Pipe bend
  • P. Zhou *, Z. Wu, J. Mou, D. Wu, S. Zheng, Y. Gu Pages 689-699
    The reflux hole has a large effect on the performance of self-priming centrifugal pumps. In order to study the effects of the reflux hole on the performance and transient flow characteristics of the self-priming centrifugal pump, four different areas of the reflux hole inside an external mixed self-priming pump were proposed. The 3D transient flow was numerically simulated under different operating conditions for the investigated pump. The differential pressure, reflux quantity and transient flow characteristics near the reflux hole were analysed, and then the effects of the reflux hole area on the pressure fluctuation characteristics and performance of the pump were further researched. The results show that the differential pressure and reflux quantity is zero around the best-efficiency point. The vorticity magnitude near the exit of the reflux hole is significant, and the unsymmetrical flow structures represent periodic motion over time in the cross-section. The pressure fluctuation intensities of monitoring points P2-P5 upstream of the reflux hole were generally larger than others and decreased with a decrease in reflux hole area. With a decrease of the reflux hole area, the performance of the pump improved to some extent.
    Keywords: Centrifugal pump, Reflux hole, Transient flow characteristics, Pressure fluctuation
  • K. Li, X. Chen, H. S. Dou *, Z. Zhu, L. Zheng, X. Luo Pages 701-713
    Flow instability in a miniature centrifugal pump is numerically simulated with the RANS equations and the SST k-ω turbulence model. The energy gradient method is adopted to analyze the flow instability at design load and two off-design loads, and the results are compared with those analyzed by Q-criterion. The regions with large magnitude of energy gradient function (K) indicate pronounced turbulent intensity and poor flow stability. Internal flow stability is investigated in details for both the near blade surfaces region and the impeller passages. To study the mechanism of energy gradient method, internal flow parameters such as the velocity and total pressure, the transverse gradient of total mechanical energy and the work done by shear stresses are investigated respectively. The results show that the energy dissipation reaches its maximum around the leading edge of suction surface. The value of the energy gradient function K presents a different magnitude for the near blade surfaces region and the impeller passages, and the K in the impeller passage is much larger. Regions with maximum of the work done by shear stresses are concentrated on the suction surface, regions with large transverse gradient of total mechanical energy is concentrated on the hub surfaces or shroud surfaces. It is further found that the K can reflect the influence of the outer boundaries of vortex on the flow near blade surface.
    Keywords: Miniature centrifugal pump, Energy gradient method, Flow instability, Total mechanical energy
  • J. Stelmach *, R. Musoski, C. Kuncewicz, M. G?ogowski Pages 715-728
    Instantaneous radial and axial velocitieques of water in the tank with a self-aspirating disk impeller operating without gas dispersion were measured by the PIV method. A comparison of mean square velocity pulsations confirmed previous observations that the area in which turbulence is non-isotropic is small and extends about 3 mm above and under the impeller and radially 12,5 mm from the impeller blade tip. Based on velocity measurements, the distributions of energy dissipation rates were determined using the dimensional equation  = C·u’3/D and Smagorinsky model. Adoption of the results of the dimensional equation as a reference value allowed us to determine the Smagorinsky constant value. This value appeared to be smaller than the values given in the literature. It has been shown that eddies in a small space near the impeller had sufficient energy to break up gas bubbles flowing out of the impeller. Based on the obtained energy dissipation rate distributions, appropriate turbulence scales were determined.
    Keywords: Energy dissipation rate, Turbulence scales, Self-aspirating disk impeller
  • W. Medaouar , L. Loukarfi , M. Braikia , A. Khelil , H. Naji* Pages 729-742
    A combined experimental and computational study of a turbulent multiple jet from lobed diffusers is performed. The main interest of these multiple lobed jets is to come up with the best configuration that improves the thermal and dynamic homogenization in air diffusion units that can be used for ventilation, heating and air conditioning of residential premises. Herein, the configuration of a central lobed jet surrounded by six equidistant peripheral lobed jets has been investigated. On the experimental level, flow velocities and temperatures were measured by a multifunctional thermo-anemometer. In terms of numerical simulation, the conservation equations of mass, momentum and energy are solved while involving four turbulence models, viz., the k-ϵ model, the k-ω, the shear stress transport (SST) k-ω model and the Reynolds Stress Model (RSM). The findings are compared with thermo-anemometer measurements. It turns out that the SST k- ω model is most appropriate for predicting the average flow characteristics.
    Keywords: Lobed jets, Multiple jets, Experimental study, Numerical simulation, Turbulence modelling, RANS
  • M. Ostad, R. Kamali * Pages 743-749
    In the present study, the numerical analysis of blade tip geometry effect on the performance of a single-stage axial compressor has been the focus of attention. The studied geometries included a rotor with variable tip clearance. For the first model, the tip clearance increases as it moves toward the blade trailing edge. The tip clearance of the second model reduces as it approaches the trailing edge, whereas in the third model, the tip clearance remains constant. The results indicated that the tip clearance of the first sample, as the worst tip clearance case, creates a 10% reduction in the stall margin with respect to the third standard model, and the second sample tip clearance brings about a stall margin reduction of 4% efficiency with respect to the third standard model. Then, the effect of blade tip clearance geometry on outlet flow angle of the rotor was inspected. The results showed that in the first model, the outlet flow angle has the largest deviation than the third standard model and the second model performance places somewhere between the two other tip clearance geometries. Also evident from the result is that taking advantage of the variable blade tip clearance is not an appropriate method for improving compressor performance.
    Keywords: Axial compressor, Variable tip clearance, Stall margin, Flow angle, Compressor performance
  • R. J. Singh, T. B. Gohil * Pages 751-762
    The present analysis emphasized the presence of Lorentz force and its directional effect on the fluid flow and its structure in the channel with two differently shaped orifices. The flow through orifice causes the generation of the bubbles or eddies in the downstream flow. In this study, the numerical code is developed in the open source CFD tool kit OpenFOAM. The magnetohydrodynamics (MHD) principle is adopted to achieve the present objectives. Direct numerical simulation (DNS) has been carried out to predict the flow features at fixed Reynolds number of Re = 1000 and blockage ratio of 1:4 with the varying magnetic field. The magnetic field is varied in term of Hartmann number (Ha) in the direction normal to the flow of fluid. The induced Lorentz force considerably occupies the wake flow area downstream of the throat and hence suppressed down the vortices in the flow. The results obtained has the promising effect of suppressing down the vortex flow past two different orifices produced by the electromagnetic pressure gradient. The present study shows the MHD based flow can be significantly employed for the flow past orifice or any arbitrary obstacle in order to achieve the flow without wake region. The current analysis suggests the method of vortex control by producing Lorentz force using magnetic field without modification of geometry or additional use of devices into the system.
    Keywords: OpenFOAM, Magnetohydrodynamics, DNS, Vortex control, Pressure drop
  • S. Sarmast, M. Ziabasharhagh, A. Salavati, Zadeh *, H. R. Fajri Pages 763-776
    The effects of spark and injection characteristics as well as split injection on the performance and emissions of a spray-guided gasoline direct injection (SG-GDI) engine operating close to stoichiometric conditions are assessed. To accomplish this, a 6-holes injector is simulated and the results are validated against available experimental data for spray penetration length. In addition, an open-cycle multi-dimensional model is developed for a port fuel injection (PFI) engine and the model outcomes are verified against in-cylinder pressure profile and normalized heat release rate. The GDI engine model is yielded under the light of embedment of the above-mentioned models. The model is then employed for investigation of the effects of injector angle, injection pressure, start of first and second injections and two-stage fuel injection with different fuel mass ratios at first and second injections, i.e., split injection, on mixture formation, combustion and engine emissions. The results show the pivotal role of the injector angle on formation of the mixture and output power. On the other hand, it is indicated that while practicing the split injection strategy, the flammability of the relatively stratified lean mixture with fuel to air equivalence ratio of 1.15 around the spark plug, surpasses that of stratified mixture.
    Keywords: 1D-3D CFD modeling, Homogeneous-stratified charge, Injection characteristics, SG-GDI engine, Split injection
  • N. K. Singh * Pages 777-788
    Large-eddy simulation of a laminar separation bubble on a flat plate has been performed and compared with the data in the literature. Suitability of different subgrid-scale models has been examined for simulation of transition. Comparison of various parameters and three-dimensional visualization of instantaneous flow fields indicate that standard Smagorinsky model, being too dissipative, is not suitable for this kind of problem and fails to properly resolve transition. With the application of low Reynolds number correction and a reduced model constant, a good agreement with the dynamic model is obtained at a lower computational cost. Of the three SGS models investigated, dynamic model gives the most physically accurate description of transition. The simulations illustrate that the appearance of Λ-vortices, vortex stretching and break down of longitudinal streaks characterize the transition process. Low values of reverse flow make it clear that a convective instability is involved. It is concluded that the initial amplification of disturbances is due to Tollmien - Schlichting mechanism while the roll-up of the shear layer takes place due to Kelvin-Helmholtz instability. It is observed that the universal log-law profile is not reached by the velocity profiles even far downstream.
    Keywords: Separation bubble, Large-eddy simulation, Smagorinsky model, Dynamic model
  • A. Menouer *, N. Sad Chemloul, K. Chaib, A. Kadari Pages 789-801
    Laminar flow of non-Newtonian fluid (shear-thinning) through a 1:3 planar gradual expansion is numerically investigated, for various Power-Law index (0.6, 0.8 and 1.0) and expansion angles (15, 30, 45, 60 and 90°) at different generalized Reynolds number (1 ≤ Reg ≤ 400). The study of these parameters effect on the flow pattern allowed the determination of the two critical generalized Reynolds numbers (Regcr1 and Regcr2), which correspond to the transition from the symmetric to the asymmetric flow and the appearance of the third recirculation zone respectively. The results showed that decreasing the Power-Law index or the expansion angle stabilizes the flow by increasing significantly the two critical generalized Reynolds numbers. In order to predict the two critical generalized Reynolds numbers, two correlations have been proposed.
    Keywords: Gradual expansion, Bifurcation, Shear-thinning fluid, Critical Reynolds Number, Recirculation
  • S. Mohamed Illyas *, B. R. Ramesh Bapu, V. Venkata Subba Rao Pages 803-817
    The influence of swirling flow on distribution of wall heat transfer on a flat plate with helicoid swirl inserts is experimentally studied. The focus of the study is on the swirling effect imposed by helicoid surfaces. Six helicoid swirl inserts of single vane, double vanes and triple vanes with swirl number (Sw) of 0.75 and 1.1 are used in this study. The heat transfer measurements are made for the Reynolds number range of 12700 - 32700 and for the nozzle exit to impinging plate distance (H/D) of 1, 2, 3 and 4 using thermo chromic liquid crystal technique. The swirling impinging jet is also compared with circular impinging jet on the heat transfer performance. The obtained experimental results provide the information on the behavior of single, double and triple helicoid swirl inserts on the heat transfer performance. The experimental values are analyzed with multi objective optimization technique of principle component analysis by computing multi response performance index (MRPI). Their performance is presented in terms of heat transfer rate through evaluation of Nusselt number on the impinging surface and heat transfer uniformity and decay of Nusselt number. The principle component analysis reveals that the double helicoid with higher H/D ratio improves performance of the swirling jet with relatively higher computed MRPI. It is found from the analysis of variance (ANOVA) that the H/D ratio contributes significant effect on the output followed by number of helicoid vanes and swirl number.
    Keywords: Heat transfer, Swirling impinging jet, Thermochromic liquid crystal, Optimization
  • M. Amiri , M. Kahrom *, A. R. Teymourtash Pages 819-829
    In this study, a three-bladed pivoted vertical axis Savonius wind turbine is subjected to numerical and experimental studies. The experiments are carried out in a subsonic open-jet type wind tunnel, where the instantaneous position of the opening/closing blades are also determined via high speed imaging. The effects of adding end plates and the rotor aspect ratio on the turbine torque and power coefficients are investigated experimentally. Results show that adding end plates greatly enhances the rotor aerodynamic performance, in terms of both the maximum power coefficient and also the working range of the turbine. Similar effects are also observed for the effects of increasing the aspect ratio. Comparing numerical results with the experimental data demonstrated that the numerical results are in a convincing agreement with the experimental data of a wind rotor with an aspect ratio of 2.0 equipped with end plates. Although there are several two-dimensional numerical simulations for the drag-based vertical axis wind turbines in the literature, the results of the current study suggests that two-dimensional numerical results are not comparable with the experimental data of the rotors with small aspect ratios, especially without end plates.
    Keywords: Pivoted savonius, Aspect ratio, End plate, Multiple reference frame, Vertical axis
  • F. Jiang*, Y. Xu, J. Song, H. Lu Pages 831-843
    The flow-focusing method is a technology for microfluidic droplet control, and the temperature can effect on the droplet formation. In this study, the droplet formation in the flow-focusing method during the squeezing of dispersed phase by the continuous phase is simulated using CLSVOF, with the consideration of the effects of temperature on droplet size, shape and frequency. The simulation results are consistent with experimental data. The simulated results demonstrate that the droplet size increases with the increase of inlet phase temperature, while the shape regularity and forming frequency decrease, the maximum increase of droplet size is 16%, the biggest drop of droplets number is 29%, and the biggest drop of the roughness parameter is 5%. When the inlet temperatures of the continuous phase are not equal, dripping and jetting are observed in the flow regime of droplet dispersed phase. The mechanism of the temperature influence on droplet formation and the detailed process of droplet formation under different flow regimes are discussed. At the same time, the radial size of droplet breakup point under different flow regimes is compared. The simulation results provide insights in better selection of the control parameters for droplet formation technology.
    Keywords: Numerical simulation, Droplet formation, Flow focusing, Temperature, CLSVOF
  • A. Q. Lin, J. Zhou, X. J. Tian, Q. Zheng, H. Zhang * Pages 845-855
    Flow characteristics inside the compressor are of great importance for aeroengine performance under the high adverse pressure gradient. To meet the need for quick performance estimation, the numerical simulation is a meaningful investigation method to predict the similarity flow characteristics of aeroengine compressor at high Mach flight. Thus, this paper aims to satisfy the accuracy of compressor flow field at high altitude based on the similarity criterion. The accuracy for solution results by the application of the similarity criterion and the derivation of compressor boundary conditions is verified with the experimental data. Then, the parametric definitions of air intake are put forward to get the inlet boundary conditions of compressor. The comparative simulation results are conducted between similarity and prototype flow fields at design boundary conditions. The results show that among the most important dimensionless criterion is Mach number at high-speed flow, so the same equivalent mass flow and equivalent speed are recommended. In addition, the flow characteristics of the compressor at high altitude and high Mach number have a good similarity. Consequently, it can extend to further study compressor performance of aeroengine at different flight altitudes and Mach numbers.
    Keywords: Aeroengine compressor, High Mach, Similarity criterion, Parametric definition
  • D. Surekha R. S.* , A. Khandelwal, R. Rajasekar Pages 857-863
    This paper presents the investigation on the phenomenon of a deep dynamic stall at the Reynolds number of the order of 105 over an oscillating NACA 0012 model. Wind tunnel experiments are conducted to investigate the aerodynamic characteristics of the upstroke and downstroke phase associated with the sinusoidal pitching motion of the airfoil using the technique of surface pressure measurements and Particle Image Velocimetry. The validation of the lift curve slope of upstroke and downstroke with the Prandtl’s thin airfoil theory reveals the fact of massive flow separation during the deep dynamic stall regime. Numerical simulations are performed using Reynolds averaged Navier Stokes turbulence models such as RNG K-є and SST models. The data obtained from these models have been compared with the experimental data to investigate the aerodynamic features of the deep dynamic stall regime. The comparison shows that the URANS with K-ε model is in good agreement with the experimental data within the reasonable regime.
    Keywords: Sinusoidal oscillation, Unsteady flow, Pitching airfoil, Numerical simulation
  • L. Chilambarasan, R. Prakash, J. P. Shanu, P. Murugasen * Pages 865-870
    Nanotechnology research has proved sustainable results for a wide range of applications from engineering to medical science. Nanotechnology corresponds to the engineering of materials in nanosize (10-9m) whose material properties differ from of bulk properties. Nanofluid is one category of applications reported for its use as thermal management in cooling of electronic devices and fuel cell applications. In most literature, electrical conductivity studies were used as a basis to define the stability of nano-suspensions. In the present paper, the electrical conductivity studies of two glycol based nanofluids dispersed with ZnO nanoparticles of 50nm average diameter in the temperature range of 30-550C are reported. ZnO nanoparticles are added to the aqueous glycol base fluid prepared with (30 EG: 70 Water) and (30 PG: 70 Water) composition at a low volume concentration of 0.01 to 0.05%. Correlations are developed using experimental results for each volume concentration to predict electrical conductivity (EC) of nanofluids with temperature. From obtained results, the electrical conductivity of aqueous propylene glycol shows a decrement in EC after adding ZnO nanoparticles (except at 0.04% volume concentration) and vice versa for aqueous ethylene glycol. For aqueous propylene and ethylene glycol nanofluids, electrical conductivity enhancement up to 20% and 12% is obtained at a volume concentration of 0.04% and 0.01% at 550C temperature respectively. The electrical conductivity of both nanofluids increases with increase in temperature at all volume concentrations.
    Keywords: Electrical conductivity, ZnO-nanofluids, Aqueous glycol, Volume concentration, Temperature, Ion condensation effect
  • D. W. Jia, X. W. Deng, Y. Wang, J. L. Lei * Pages 871-881
    Intake port structure directly affects the flow characteristics and combustion process of diesel engine, and then affects the comprehensive performance of diesel engine. To the intake ports of a four-valve direct injection diesel engine, the flow characteristics are analyzed on the four combined intake ports : (1) helical (left) and tangential (right), (2) tangential (left) and helical (right); (3) helical (left) and helical (right); and (4) tangential (left) and tangential (right).And the influence of air flow in four combined intake ports to in-cylinder gas flow is also analyzed. Results show that the helical and tangential combination intake ports flow velocity increases with the valve lift increases, and small turbulence arises at the valve guide lug, and the intake flow velocity of the minimum cross-section of the junction of the guiding section and the helical section is the maximum. The air flow in- cylinder moves from top cylinder head bottom to the cylinder bottom, the air flow is enlarged gradually by the small-scale irregular swirl, which eventually converges to a single swirl. The turbulence kinetic energy is very big when the air is just entering the cylinder, the flow space expands rapidly, and the dissipation of turbulent kinetic energy is very significant with the gas moves to the bottom of the cylinder.
    Keywords: Four-valve diesel engine, Combined intake ports, Air flow in intake ports, Air flow in-cylinder
  • N. Alam *, K. M. Pandey, K. K. Sharma Pages 883-890
    The present computational analysis reports the results of combustion phenomenon in 1200 mm long and 60 mm internal circular diameter (D) of three dimensional obstructed combustion chamber (combustor) of the pulse detonation engine (PDE). The simulation is carried out for stoichiometric mixture of two fuels Kerosene-air and Butane-air mixture at atmospheric pressure and temperature of 1 atm and 300 K respectively along with preheated air. The chemical species of Kerosene and Butane (C12H26 and C4H10) fuel are solved by species transport equation and irreversible one-step chemical kinetics model. The propagation speed of flame, detonation wave pressure and deflagration-to-detonation transition (DDT) run-up length are analyzed by three dimensional reactive Navier–Stokes algorithm along with realizable k-ɛ turbulence equation model. The obstacles are placed inside the combustor tube at spacing (s) of 60 mm (1D) and obstacles having blockage ratio (BR) 0.5 for creating perturbation in propagating combustion flame. This resulted in increase of the surface area of propagating flame and reduces deflagration-to-detonation transition (DDT) run-up length.
    Keywords: Obstacles, Turbulent flame, Detonation, PDE
  • N. K. Singh * Pages 891-905
    In this paper, the performance of sub boundary layer vortex generators and conventional vortex generators in controlling the separation bubble has been compared and the resultant highly three-dimensional flow has been studied. Two pairs of vortex generators mounted symmetrically along the spanwise direction are placed upstream of separation point to produce counter-rotating vortices. Effect of these three-dimensional vortex generators on the separation bubble and the flow downstream has been examined. The simulations show that the length of the separation bubble is reduced by sixty two per cent due to the deployment of vortex generators of height 0.33 δ while the original separation bubble is completely eliminated by the vortex generators of height 0.66 δ. However the presence of larger height vortex generators by itself causes a small mean separation bubble downstream. The flow downstream of vortex generators is highly three-dimensional and zones of recirculation can be observed between regions of attached flow. Presence of adverse pressure gradient results in greater interaction between counter-rotating vortices, leading to their early breakup and higher vortex decay rate compared to the zero pressure gradient case. Further, it is seen from the simulations that the counter-rotating array of vortices does not move away from the wall even far downstream.
    Keywords: Laminar separation bubble, Rectangular vortex generators, Direct numerical simulation, Immersed boundary method
  • W. Jing *, X. Cheng Pages 907-919
    Under far-field long-period earthquake, liquid storage tanks are easy to be failure because of large amplitude liquid sloshing. In this paper, nonlinear contact is used to simulate behavior of sliding isolation bearing, nonlinear dynamic equation is used to solve fluid-structure interaction, bilinear material model is used to simulate limiting-device, and 3-D calculation model of sliding isolation concrete rectangular liquid storage tank (CRLST) with limiting-devices is established. Firstly, artificial far-field long-period earthquake waves are synthesized based on the existing seismic records. Secondly, dynamic responses of sliding isolation CRLST under the action of short-period and far-field long-period earthquakes are studied. Thirdly, effects of bi-directional earthquake and structure size on dynamic responses are investigated. Lastly, displacement control measures are discussed. Results show that far-field long-period earthquakes mainly affect horizontal displacement of structure and liquid sloshing wave height, and sliding isolation has obvious control effect on liquid sloshing wave height. Besides, horizontal displacement of structure and liquid sloshing wave height are increased with increase of seismic dimension and structure size. The reasonable designs of sliding isolation bearing and limiting-device can solve the problem that the maximum horizontal displacement of sliding isolation CRLST may exceed the limit under far-field long-period earthquake.
    Keywords: Sliding isolation, Concrete rectangular liquid storage tank (CRLST), Far-field long-period ground motion, Fluid-structure interaction, Liquid sloshing
  • D. Joachimiak *, P. Krzy??ak Pages 921-930
    The paper discusses the results of investigations performed for the segments of straight-through labyrinth seals of constant length. Increasing the number of teeth of a segment resulted in a reduction of the pitch length to obtain the slot seals. The phenomena occurring during gas flow in labyrinth and slot seals differ significantly. They are described with different calculation models. The analysis presented in this paper is related to the change of the tightness and the nature of the flow from a straight-through labyrinth seal to a slot seal. The paper includes the results of experimental research and CFD calculations. Models applied for the Neumann and Scharer labyrinth seals as well as the model of the Salzman and Fravi slot seals were discussed. For the Neumann and Scharer models, correction coefficients for the tested geometry were proposed. Based on the assumptions for the said models and the obtained results, the phenomena responsible for the minimization of the leakage were discussed. The leakage rate in segments of different gap heights depending on the number of teeth and the pressure ratio upstream and downstream of the segment has been analyzed. Based on the experimental data, an optimum number of teeth in the segment for minimum leakage was determined. CFD calculations allowed determining the minimum leakage geometry. The experimental data contained in this paper confirm that the determined optimum pitch range is independent of the pressure drop.
    Keywords: Slot seal, Flow machines, Leakage, CFD, Optimization
  • A. Sedaghatkish, S. Mortazavi * Pages 931-946
    Film boiling has various industrial applications especially in heat exchangers. Studying this phenomenon on complex geometries and investigating heat transfer coefficient is desired by many industries. The numerical method used here is a finite difference/front tracking method which is developed independently for film boiling in complex geometries. The film boiling over one, two or more cylinders is simulated using this method. The effect of dimensionless parameters namely Grashof and Jacob numbers are investigated for one cylinder. The effects of spacing, angle, and diameter are investigated for two cylinders. For the case with many cylinders, the effects of different geometrical configurations (regular and staggered) and number of rows are investigated by calculating the average Nusselt number on each cylinder. It is observed that the cylinder spacing does not have any significant effect on the Nusselt number for the upper cylinder. However the angle and cylinder diameter significantly affect the Nusselt number for the upper cylinder. In regular configuration, the Nusselt numbers for the upper cylinders are relatively uniform and higher than lower cylinders. In the staggered configuration, however, the Nusselt numbers of the upper cylinders are different, non-uniform, and higher than those of the regular arrangement.
    Keywords: Film boiling, Front tracking method, Complex geometries, Heat transfer
  • E. Javanmard *, Sh. Mansoorzadeh Pages 947-959
    The accuracy of experimental procedure used to calculate the drag coefficient of an Autonomous underwater vehicle (AUV) in a towing tank is investigated using computational fluid dynamics. Effects of struts, used to connect the AUV model to towing carriage, on the hydrodynamics coefficient of the AUV at various relative submergence depths, at AUV speeds of 1.5 and 2.5 m/s are numerically simulated. Various numerical modeling are performed to investigate the effects of free surface with and without presence of struts on the drag coefficient of the AUV. Volume of fluid (VOF) model is used to solve the two phase flow RANS equations. The drag coefficients obtained from two phase flow simulations are compared with those obtained from single phase flow at corresponding velocities. The results obtained from experiments conducted in the towing tank of the Subsea Science and Technology centre, on a full-scale model of the AUV developed in this Centre, agreed well with those obtained by numerical simulations.
    Keywords: Free surface, AUV, Computational fluid dynamics, VOF, Towing tank, Strut
  • A. Moradmand, M. Saghafian *, B. Moghimi Mofrad Pages 961-969
    In the present study, flow through two-dimensional microchannel under an axial electric field, transverse electric and magnetic fields and with axial pressure gradient has been investigated numerically. Continuity and momentum equations were solved steadily with respect to the non-slip condition by using discrete finite volume method and a numerical code. The results show that in the presence of the axial electric field, applying transverse magnetic field reduces flow velocity. However, when the transverse electric field and axial electric field exist together, applying the transverse magnetic field increases the flow rate to a certain extent and then reduces the flow rate. Hartmann number like this amount of magnetic field is known as critical Hartmann number. Therefore, with the presence of transverse and axial electric fields and transverse magnetic field, the highest possible flow rate is for critical Hartmann number. It was also found that by increasing the pressure gradient within the microchannel, the critical Hartmann number decreases. Moreover, by increasing the transverse electric field, the sensitivity of critical Hartmann number to the pressure gradient decreases and its value tends to a specific number (about 1.5).
    Keywords: Microchannel, Electro-osmotic flow, Electro magneto hydro dynamic, Transverse electrical field, Critical hartmann number
  • T. M. Thomas, N. Vinod * Pages 971-980
    This paper presents the molecular dynamics simulations of unconfined forced convective flow through the nanostructures at steady state condition. A better understanding of forced convective flow through the nanostructures is important because of its wide range of applications in nano-scale devices. Present work focuses on the heat transfer process of argon flow over a carbon nanotube and carbon nanotube arrays with constant surface temperature using molecular dynamics simulations. We consider two elementary configurations for the case of carbon nanotube arrays based on the unit cell structure. The simulation domain consists of fixed carbon nanotubes surrounded with the flowing argon atoms. An extensive study of momentum and thermal transport between carbon nanotube and surrounded argon atoms are analyzed from its microscopic state. The heat transfer coefficient is found in the order of 108 W/m2K. The method proposed in this paper can be an elementary step for the geometry calculation of nano-structured heat sink in the high heat flux electronic chips.
    Keywords: CNT arrays, Heat transfer, Convection
  • S. Kapkin, M. S. Demir *, M. O. Kaya, E. Uzal Pages 981-985
    An analytical solution is given to investigate the vibrations of a membrane under the effect of an incoming fluid flow perpendicular to it. The membrane is located at the stagnation point of the flow and is of finite width but infinite length. A rigid wall extends through the finite width of the membrane to infinity. The flow is considered to be a small perturbation on the two dimensional potential stagnation flow solution due to the vibrations of the membrane, and the membrane is modeled by the linear vibration equation. The resulting coupled problem is solved by a Galerkin procedure and the eigenvalue equation relating the membrane frequency to the other parameters is derived.
    Keywords: Flutter velocity, Membrane, Stability, Stagnation point flow, Vibration
  • S. Debnath, A. K. Saha *, P. G. Siddheshwar, A. K. Roy Pages 987-1000
    The present paper is a study on dispersion of reactive solute in an oscillatory flow of a two-fluid, three-layer Casson-Newtonian continuum using Aris-Barton’s approach. A two-fluid model of blood flow has been considered, the fluid in the central region is taken to be a Casson fluid (a core of red blood cell suspension) and a peripheral layer of plasma modelled as Newtonian fluid. The governing equations for the velocity distribution have been solved using a perturbation technique, and the effective dispersion coefficient has been evaluated numerically (FDM) by solving the moment equations. Using the Hermite polynomial representation of central moments the axial distribution of mean concentration is determined. The main objective is to look into the impact of yield stress, peripheral layer thickness, irreversible and reversible reaction rate on the dispersion process. The study has significant applications on the transport of species in a blood flow system.
    Keywords: Axial-dispersion coefficient, Peripheral layer, Casson fluid, Reaction rate, Two-fluid, Three-layer