

Investigating the Influence of the Inlet Bypass Recirculation System on the Compressor Performance


Pages :
12951305


Authors :
S. Li,
X. Yang,
W. Li,
M. Tang,
In the present study, an experimental method is proposed to study the influence of different slot widths, positions and structures of the compressor recirculation systems on the compressor performance. Obtained results indicate that the compressor flow range increases with the width of the recirculation slot at the expense of a narrower highefficiency operating region. It is found that when the slot width is set to 3 mm, the surge and choke margins are about 12% higher than that of the 2 mm slot. Moreover, it is found that when the position of the recirculation slot falls in the middle area of the guide wheel and far from the splitter blades, a larger surge margin can be achieved then it near the splitter blade. Adopting the recirculation structure without intake retaining ring structure can effectively broaden the compressor flow range. Compared with a straight slot intake structure, the compressor with an inclined slot intake structure has a greater surge margin in the highspeed region; however, the highefficiency range of the latter scheme is smaller. Furthermore, the efficiency circle moves towards higher mass flow conditions. The present parametric study is expected to provide a guideline to design recirculation devices.





Magnetic Field Effect on Multiplicity of Solutions Induced by Thermosolutal Convection in a Bénard Square Porous Cavity Submitted to Horizontal Concentration Gradient


Pages :
13071316


Authors :
A. Mansour,
M. Ait Ahmed,
A. Amahmid,
M. Hasnaoui,
I. Filahi,
Y. Dahani,
In this work, we present a numerical study of the magnetic field effect on double diffusive natural convection in a square porous cavity saturated with an electrically conducting binary mixture. The cavity is heated from below and cooled from the top, while its vertical walls are adiabatic and maintained at constant but different concentrations. The numerical results are obtained for a Lewis number Le = 10 and the following ranges for the other controlling parameters: 40 ≤ RT ≤ 1000, 0.2 ≤ N ≤ 0.2 and 0.5 ≤ Ha ≤ 0.5, where RT, N and Ha are the thermal Rayleigh number, the buoyancy ratio and the Hartmann parameter, respectively. First, the effect of the Hartmann parameter on the maintenance and disappearance of the multiple steady state solutions obtained in the case of purely thermal convection is examined. Then, the combined effect of N and Ha on the existence of these steady solutions is analyzed. It is found that the critical values of N corresponding to the transitions between the different solutions are modified by the application of a magnetic field. However, the nature of the transitions is unchanged. It is shown that the magnetic field may affect considerably the flow intensity and the heat and mass transfer in the medium.





Drag Reduction Characteristics of Bionic Mucous Membrane Acting on the Turbulent Boundary Layer


Pages :
13171327


Authors :
C. Ma,
K. Zhang,
B. Zhang,
B. Zhao,
Q. Wang,
The mucous membrane on the fish surface has excellent drag reduction performance. The mucous membrane can be regarded as the viscoelastic fluid, and a bionic friction drag reduction model is proposed with the consideration of a Carreau viscoelastic modelbased mucus secretion process. Then, the drag reduction effect of the mucous membrane on the classical wall turbulence boundary layer is investigated by largeeddy simulations. Results show that the bionic mucous membrane is conducive to reducing the turbulence, and can achieve a drag reduction rate of about 14%. This study provides a hydrodynamics understanding of the drag reduction characteristics of the bionic mucous membrane.





Enhancement of the Natural Ventilation within Commercial and Traditional Wind Towers in Arid Environments


Pages :
13291336


Authors :
N. Sakhri,
Y. Menni,
H. Ameur,
Wind tower (catcher) is an old technique used to provide natural ventilation and thermal comfort in arid regions like Iran, and the Middle East. An attempt is made to improve the performance of such techniques by investigating the effects of the windward wall of a traditional wind tower. Four aerodynamic shapes are studied: circular, triangular, U and square shape. Two and threedimensional numerical simulations are carried out to examine the internal and external pressure caused by wind in the region of Bechar (SouthWest of Algeria). The obtained results showed an increase of airflow velocity at the tower outlet connected directly to the ventilated space by 28 and 16% for the circular and triangular models, respectively, and a decrease by 22% for the Umodel. The separation flow zone decreased in both circular and triangular models, in comparison with U and square models. These results can improve the efficiency of natural ventilation of traditional and commercial wind towers.





Research on Composite Drag Reduction Characteristics of Biomimetic Microstructural Surface


Pages :
13371350


Authors :
M. H. Zhang,
D. B. Zhang,
W. W. Zhuo,
The objective of this work is to investigate the effect of the bionic microstructure surface on flow field structure of the slab. The motivation behind this study is to investigate the effect of the bionic microstructure parameters including the height and intersection angle of microstructure in order to improve the drag reduction characters. The numerical simulation is performed on the bionic microstructure model of the Vshaped and serrated bionic microstructures using the RNG kε model. The drag reduction rates of two bionic microstructures under different dimensionless sizes are ob13.79 tained. The drag reduction efficiency is up to 8.76% when the dimensionless height of microstructure is at and the intersection angle for Vshaped microstructure. In addition, combined with wall temperature control of drag reduction technology, the influence of wall temperature on the drag reduction effect is also analyzed. Compared with the flow field structure of the surface boundary layer of the smooth plate, the wall microstructure divides the surface boundary layer into two parts: the bottom and the tip. The average velocity profile is moved up and the thickness of the linear bottom layer is increased. A large number of "quiet" fluids are gathered at the bottom of the surface boundary layer. In addition, the existence of wall microstructure can weaken the momentum exchange in the boundary layer and restrain the spreading vortex motion of the fluid in the nearwall region. The "secondary vortex pair" on both sides of the tip of the microstructures can effectively limit the lateral pulsation of fluid So as to achieve a good drag reduction effect.





Heat Transfer and Hydraulic Characteristics of Supercritical CO2 in Cooled and Heated Horizontal Semicircular Channels


Pages :
13511362


Authors :
Y. Tu,
Y. Zeng,
Heat transfer characteristics of supercritical CO2 in horizontal semicircular channels are numerically investigated using Computational Fluid Dynamics method validated with experimental data. Comparison study is conducted for semicircular and circular channels with the same hydraulic diameter and boundary condition at the bulk temperature range including pseudocritical point. The results show that the heat transfer coefficients of the semicircular channel are significantly smaller than those of the circular channels due to the blocking effect at the corner area of the channel cross section, and the fluid thermophysical properties near the wall have a significant effect on the convective heat transfer performance in both heating and cooling cases. A modified model was proposed based on Olson correlation of the semicircular channel considering the channel geometry and nearwall fluid viscosity influence. Further study was conducted to discuss the effect of hydraulic diameter and boundary conditions on the heat transfer performance of the semicircular channel and indicate that the modified correlation shows a reasonable prediction of the heat transfer coefficients in the heated semicircular channel.





Experimental and Computational Analysis of BioInspired Winglets for Micro Air Vehicles


Pages :
13631376


Authors :
A. Sathyabhama,
A. M. Rajiv,
I. Sai Sandeep,
S. S. Sandeep Kumar,
C. H. Akash,
Micro Air Vehicles (MAVs) are increasingly being used for civil and military surveillance. As the surveillance requirements are increasing, improving the range of MAVs becomes imperative. The performance of MAVs can be improved if the induced drag due to wingtip vortices can be reduced. In the present study, we try to decrease the induced drag caused by the wingtip vortices, which makes up a major part of the total drag, by introducing a winglet. A unique yet simple design, which has not yet been studied thoroughly, is explored. Inspiration is taken from the feather structures of birds to design the proposed winglet. The performance of a fixedwing MAV at a free stream velocity (U∞) of 20 m/s is studied. Multiple winglet configurations are used to compare the results with the baseline wing. An incompressible, steady threedimensional simulation is carried out using the kω SST turbulence model. The experimental studies carried out for the baseline wing matched well with those obtained from CFD. Since the numerical model is valid, only computational study is done for the modified wing. The stall angle of the baseline wing is around 26°. Numerical results show that when the proposed winglet is used, the stall angle for the wing is increased to around 32°. The use of the winglet did not produce a considerable advantage at the lower Angle of Attack (AOA), but at higher AOA, the lift coefficient (CL) was considerably higher. The overall drag coefficient (CD) was higher at lower AOA when the winglet is used. But at AOAs greater than 5°, the CD reduced. Other effects of the winglet are addressed in terms of improvement in LifttoDrag ratio (L/D) and reduction in vorticity. The effect of the location and number of the feathers was studied to come up with an optimum winglet configuration. The experiments were carried for the wing with optimum winglet configuration and the results agreed fairly with the numerical results.





Performance Characteristics and Energy Loss Analyses of a HighSpeed Centrifugal Pump with Straight Blades


Pages :
13771388


Authors :
X. Su,
W. Jin,
Z. Zhu,
Z. Li,
H. Jia,
Along with the rapid growth of cuttingedge petrochemical technology and the pressing demand for efficiency improvement, evaluation of the performance characteristics of highspeed pump is becoming increasingly important. In this paper, numerical simulation is presented on the flow instability of a 16 straightblade highspeed centrifugal pump with flow rate of 3 m3/h and rotating speed of 8500 rpm. Combined with the analysis of flow stability, the entropy production method is introduced to evaluate regions of high mechanical energy loss and its distribution at different flow rates. Results show that approximately 96% of the energy loss of the pump is produced in the volute, gap, and front and back chambers. Large energy loss is observed near the trailing edge of the blade and volute tongue, which are caused by the small region including both the high and low pressure gradients and large momentum exchange by the flow separation, respectively. Moreover, the rotor–stator interaction causes much energy loss at the wall of the volute and front and back chambers. Owing to the circumferential pressure gradient and the 90° leading edge of the straight blade, the fluid tends to form counterrotating recirculation vortices. The large number of blades narrows the passage and limits the formation of large vortices in flow channels, thus the backflow phenomenon seems not to worsen with the rise of flow rates. Hence, the entropy production in most of the flow parts are insensitive to flow rates.





Asymmetric Yshaped Micromixers with Spherical Mixing Chamber for Enhanced Mixing Efficiency and Reduced Flow Impedance


Pages :
13891397


Authors :
E. Cetkin,
A. Miguel,
Microfluidic devices have many attractive qualities such as low cost, small size, and infield use. Micromixers are very important components of these devices because affect their efficiency. In a passive mixer, the structural characteristics of the mixer are crucial and must be analyzed. This paper presents a numerical study of the mixing in passive Yshaped micromixers with a spherical mixing chamber for a volume constrained system. The effect of asymmetric bifurcated ducts, the angle in between the inflow ducts, eccentricity and, obstacles inserted in the mixing sphere, on the mixing efficiency and flow impedance is evaluated. Vortical structures characteristics and the possible occurrence of engulfment are also identified. The results show that flow impedance (pressure drop for unit volumetric flow rate) can be decreased greatly for the same mixing efficiency as the volume of the spherical mixing chamber is 20% of the total volume. Insertion of the obstacles into the sphere mixing chamber decreases the mixing efficiency while they increase the flow impedance. The results also show that spherical mixing chamber enhances mixing efficiency while decreasing flow impedance if the volume reserved for it is greater than a limit value which depends on the diameter and length scale ratios in between the mother and daughter ducts as well as the total volume. Overall, the paper documents the variation of mixing efficiency and flow impedance based on the geometrical parameters of threedimensional asymmetric passive micromixer with sphere mixing chamber.





ThreeDimensional Simulations and Economical Solutions for Cavitation in HollowJet Dispersive Valves


Pages :
13991410


Authors :
T. C. L. Xavier,
J. P. Ortiz,
Cavitation characteristics in HollowJet valves and possible solutions were investigated in the present work. Threedimensional numerical simulations – CFD (Computational Fluid Dynamics) were carried out in an unsteady state, considering homogenous multiphase flow, to identify the phenomenon in these components. Different turbulence and cavitation models were assessed to reach the best compromise of models. The results confirmed the occurrence of cavities with a mixture of vapor and liquid at the valve tip. The cavities are followed by a vortex generation near the same region. These vortices are the result of a highvelocity gradients, especially in the shear region of the discharge pipe wall, and they are followed by the detachment of the cavities from the valve tip. The methods and models were validated by a reproduction attempt of the results from a similar work on literature. Solutions to avoid or reduce cavitation were proposed and analyzed. Refurbishment and protective coatings against cavitation were particularly described, to envisage an economical solution to reuse the valves, avoiding their disposal or replacement.





Influence of Turbulence in Aortalike Tube: Computational and Experimental Study


Pages :
14111420


Authors :
J. Jagos,
J. Kohut,
M. Kotek,
P. Skacel,
J. Bursa,
The presented paper aims at comparison of modelling approaches to a pulsatile fluid flow in aortalike tube; it investigates their influence on the shape of the velocity profiles and waveforms, and consequently on wall shear stress. Comparisons of computational results between rigid and compliant tubes with laminar and low Re turbulent models of fluid are presented. The results were validated with PIV experimental data through the velocity profile in the halflength section of the tube for both cases (rigid and compliant) and the overall agreement was very good, almost perfect for the rigid case. Frequency of the pulse pump in the experimental circuit was 1Hz, the diameter of the tube ~ 20 mm, and maximum deformation of the compliant tube during a period was 12%. The turbulent model improved the agreement with the experimental data by flattening the velocity profiles in both cases, but the effect was much more pronounced for the compliant tube, especially during the deceleration phase. This work confirms the hypothesis stated by Brindise and Vlachos (2018) that a longer deceleration phase triggers transition to turbulence. We put foundations for extension of this hypothesis to compliant tubes where this conclusion was confirmed for physiological Reynolds and Womersley numbers. The main outputs of this study are: (i) the length of deceleration phase should be considered (in addition to the geometry or severity of stenosis) in decision whether fluid simulations should be performed with or without laminar flow assumption; (ii) for fluid simulations of blood vessels considering their compliance, a special care should be devoted to time synchronization between BCs to prevent unphysiological waveforms.





Analysis of Separation Control Authority of DBD Plasma Actuator using HotFilm Sensor Array


Pages :
14211435


Authors :
A. Daliri,
M. J. Maghrebi,
M. R. Soltani,
The boundarylayer control authority of a DBD plasma actuator using surface mounted hotfilm sensors is evaluated. Wind tunnel experiments on a windturbine blade section were established at a Reynolds number of 0.27 × 106. Aerodynamic performance of the windturbine blade section for both plasmaON and plasmaOFF modes are evaluated using measurements made by both surface pressure and wake survey behind the model. Two distinct boundarylayer states are recognized. A state which occurs at the onset and in proximity of the deep stall, which is affected by the lowfrequency instabilities of the separated flow. In this case, the steady actuation of plasma imparts local momentum on the nearby flow, eliminating the instabilities, hence, reattaching the detached flow. The other state happens beyond the static stall angle of attack of the airfoil where the flow over the suction side of the airfoil is fully separated and coexistence of both the leading edge and the trailing edge shearlayer instabilities and natural trailing edge vortex shedding is the underlying mechanism. In this case, although the plasma actuator eliminates the instabilities, to some extent, but the corresponding momentum injection is not efficient to stabilize and reattach the flow.





Excitation and Linear Instability of Stationary Modes in FalknerSkanCooke Boundary Layer


Pages :
14371445


Authors :
L. Shen,
C. Lu,
Past studies showed that a micronsized surface roughness may cause the generation of a significant unstable, stationary wave in a crossflow boundary layer, and consequently promote or delay the laminarturbulent transition. The crossflow boundary layer is usually driven by the favorable pressure gradient which is produced by accelerated inviscid velocity. Hence, for a fixed sweep angle, the magnitude of pressure gradient is the key parameter for the excitation and evolution of the stationary crossflow mode. In order to study the effect of pressure gradient on the excitation and subsequent linear development of stationary mode, a classical FalknerSkanCooke boundary layer is introduced so that the magnitude of pressure gradient can be easily parameterized by an acceleration coefficient. Numerical simulation is performed to induce the stationary perturbation by chordwiseisolated, spanwiseperiodic roughness at the lower branch of neutral curve. Then the excited waves develop into Rayleigh modes in the downstream region. The stationary modes with different spanwise wavenumbers in various favorablepressuregradient boundary layers are simulated and analysed to determine the effect of pressure gradient. And the corresponding coupling coefficients are calculated to connect the initial amplitude and the eigenmode of linear stability theory for implementing the existing prediction method of laminarturbulent transition.





A Numerical Investigation of the Characteristics of Porous Vacuum Preloaded Air Bearings


Pages :
14471457


Authors :
K. Ermis,
R. Sener,
Air bearings are widely used in devices that require highprecision. Vacuum preloading is a compact solution, as it does not add extra weight to the air bearing. In addition, the usage of porous material ensures that the pressure is distributed homogeneously to the air film. In this study, a numerical analysis was performed using three vacuum preloaded porous air bearings with different diameters (50, 75, and 90 mm). The effect of applied tensile and compression loads on the air film thickness of the air bearings was determined using experimental methods. It was found that air film thickness increases with increasing tensile load and decreases with increasing compressive load. CFD model parameters were set to be the same as those for experimental conditions. It was concluded that the CFD model is consistent with experimental results. The pressure distribution in the air film and the loadcarrying capacity of the air bearings were obtained using the CFD simulation.





MHD Double Diffusive Convection of Al2O3Water Nanofluid in A Porous Medium Filled an Annular Space Inside Two Vertical Concentric Cylinders with Discrete Heat Flux


Pages :
14591468


Authors :
M. Sammouda,
K. Gueraoui,
The magnetoconvection phenomenon of a double diffusive free convection in an annular porous space inside two concentric cylinders saturated by a (Al2O3, water) nanofluid has been investigated in the current study. The transport equations for vorticity, energy and concentration as well as stream function are solved using the finite difference method. At lower temperatures and concentrations, the outer cylinder is sustained, the discrete heat flux with the unheated adiabatic portions as well as the higher concentrations are imposed in the inside walls of the central cylinder. The base walls are insulated and impermeable. In the vertical direction, external magnetic field (MF) with uniform intensity is applied. The results of the obtained numerical simulation are presented to exhibit the consequences of various numbers such: Rayleigh RaTh, Hartmann Ha, Buoyancy forces ratio N and the solid nanoparticles (NPs) volume fraction on the pattern of the nanofluid flow and the transferred mass and thermal energy in the active wall. It is noted that the transferred mass and thermal flux in the active wall augments as the RaTh and N augments. The thermal energy transferred augments with the growth of , while the transferred mass in the active wall decreases. The greater magnetism declines the rate of the mass and thermal energy transport in the active wall.





Patient Specific Modelling of Blood Flow in Coronary Artery


Pages :
14691482


Authors :
B. Thomas,
K. S. Sumam,
N. Sajikumar,
The performance of the heart is considerably affected by the blocks formed because of the deposition of plaque inside the coronary artery. The blocks (stenosis) either in coronary artery or elsewhere force the heart to work harder for pumping the oxygenated blood to the heart muscles and blood vessels. This study analyses the flow through the stenosed coronary arteries via numerical modelling by using ANSYS FLUENT software. Three real cases with different asymmetric stenosis levels (i.e., block level 33%, 66% & 85%) are analysed by considering blood as a nonNewtonian fluid, and blood flow as pulsatile in nature. As the flow regime falls in transition to turbulent region, the transition Shear Stress Transport (SST) kω turbulence model is used to take care of the changeover stage from laminar to turbulent flow and vice versa. The results show large variation both in Wall Shear Stress (WSS) and pressure drop near the stenosis. Pressure drop becomes more significant at severe degrees of stenosis (66% and 85%) compared to the mild case (33%). The study throws light on the critical distribution of shear stress and pressure drop along the artery wall, which are considered as indicators of the commencement of heart disease and further growth of stenosis. An indicator, viz., Fractional Flow Reserve (FFR), which relates the percentage of stenosis to the pressure variations, can be used as an index to diagnose the severity of stenosis. All the three cases with different stenotic levels were analysed under hyperaemic conditions and found that even 45% stenosis case can go near to critical at hyperaemic flow conditions. The effect of severity due to vessel constriction can be estimated by comparing the simulated pressure drop and WSS before and after the stenosis, with the ones for a healthy artery. The present study developed a methodology to calculate FFR value for unknown percentage of stenosis based on the simulated results obtained from 33%, 66% and 85% stenosis. Thus, criticality of a patient with certain percentage stenosis can also be evaluated. This simulation technique can be recommended as a noninvasive diagnostic tool for the early detection of atherosclerosis.





ThreeDimensional Thermocline Dynamics in Thermal Storage Tanks


Pages :
14831495


Authors :
S. S. Rantu,
K. V. Manu,
In this work, a series of threedimensional unsteady numerical simulations are performed to study the stability and interface dynamics of a thermoclinebased labscale single tank Thermal Energy Storage system (TES). The stability of thermocline is analysed by introducing relatively cold fluid for a short period at the inlet of the TES. Numerical simulations are performed for three inlet flow disturbances (weak, medium and strong) and three stratification levels (sharp, moderate and large). The fluid injected at the inlet rollsup and interacts with the thermocline which causes spatiotemporal disruption of the stable stratification inside the TES. It was found that the threedimensional simulations bear some resemblance to the twodimensional case but also show crucial differences. The propagation of the injected cold fluid and the subsequent interaction with the thermocline are analysed. A wide gamut of flow structures is identified inside the TES depending on the degree of stratification and level of disturbance. Finally, the oscillatory nature of interface and associated mixing mechanism are addressed. The simulation indicates that the oscillations at the interface are through the successive generation of countersign vorticity which retards/suppresses the propagation of the vortex ring. In the case of large interface, internal waves are generated by the periodic array of vortices which generates a standing wave pattern near the BruntVa ̈isa ̈la ̈ frequency.





Study of the Aerodynamic Characteristics on the Computed Flowfield During Thrust Reversers Operation


Pages :
14971509


Authors :
S. Bi,
J. Mao,
X. Han,
K. Cai,
F. Wang,
With the increasing of the bypass ratio of modern aeroengines, the problem of wingengine interference is more prominent, especially for the layout design of engine nacelle. The reverse thrust cascade is widely used in turbofan engines with high bypass ratio. In order to meet the requirements of the integrated analysis of airframe, wing, engine nacelle and hanger, the aerodynamic characteristics of aircraft/engine integration configuration with reverse cascade is numerically studied via CFD (Computational Fluid Dynamics) method. The streamline distribution, isosurface of total temperature, vorticity distribution and total reverse thrust efficiency on different engine nacelle layouts are compared and analyzed in details. The results show that the lift coefficient of the wing decreases by 21.2% and 45.02% respectively as the engine moves forwards horizontally by 11%L and 21.2%L. The lift coefficient of the wing decreases by 2.4% and 4.82% respectively as the engine moves subsidence by 3.5%L and 3.5%L. The influential region of the reverser airflow in the radial and circumferential direction without airframe interference is significantly larger than that in the case with aircraft/engine integration. The reverser flow is susceptible to be interfered by the adjacent fuselage and wing sections, and the development of reverse thrust flow is significantly limited. Compared to the baseline nacelle location, the reverse thrust performs badly as engine nacelle moves backward and lateral horizontally. The total reverse thrust efficiency decreases gradually as the engine nacelle moves forward horizontally.





Inlet Parameters Effects of Invelox on the Aerodynamic Performance using Numerical Simulation


Pages :
15111520


Authors :
A. Farokhzade,
M. J. Maghrebi,
Invelox is a structure that increases the wind velocity by collecting air from the higher level of ground. Previous studies showed that Invelox has the potential to capture air and increases the speed of wind, which means more power output. Although it was claimed that Invelox can capture air from all directions, its performance greatly depends on the angle of wind. In this study, the performance of Invelox is analyzed and improved for different wind directions by changing the number of blades. The results showed, however, the aerodynamic performance of all proposed Inveloxes has decreased due to increasing the wind angle, the fiveblade Invelox outperforms the original model in output power by about 7%20% in various wind directions. Furthermore, in order to reduce air escaping from the opposite side of Invelox, the length of the upper funnel has been increased. Results showed about 12% improving performance when the length of the upper funnel is equal to its diameter. The three dimensional models were analyzed by AnsysFluent, under steadystate conditions.





Lattice Boltzmann Computations of the Fluid Flow in the Wake of a Wind Turbine Modelled by a Porous Disk


Pages :
15211533


Authors :
D. Hamane,
O. Guerri,
S. Larbi,
The study of wind turbine wakes was the subject of numerous papers. However, most computational studies were based on Reynolds Averaged Navier Stokes (RANS) equations or on large eddy simulations (LES) technique. In this work, a different technique based on Lattice Boltzmann method (LBM) is applied. This approach allows the numerical investigations of the flow field in the wake of a wind turbine modelled by a solid porous disk. The LBM is a mesoscopic simulation method for fluid flow computations. The applied model is based on the Regularized BhatnagarGroosKrook (RBGK) model and an LBMLES method is used to solve the turbulent flow at a Reynolds number Re = 40000. Threedimensional computations are performed using the open source Palabos code. The effects of the lattice velocity schemes D3Q19 and D3Q27, the value of Smagorinsky’s constant and the type of boundary conditions on the solid porous disk are investigated. The potential of 3D LBM computations to describe the far wake of an horizontal axis wind turbine is also shown.





Flow and Heat Transfer Performance of Channels with 45 Degree Ribs in Staggered Array


Pages :
15351546


Authors :
Z. Wang,
Y. Yin,
L. Yang,
L. Yan,
Y. Luan,
The cooling efficiency of blade is growing demand with increasing turbine inlet temperature in gas turbine development. Ribs used in cooling channels is a common cooling structure, therefore, many configurations were studied by previous literatures, including angle, spacing, shape etc. However, there are less research about the dislocation ribs structure. In this paper, the 45deg parallel ribs, crossed ribs and dislocation ribs were investigated by numerical simulation, in order to reveal the heat transfer performance and flow mechanism. Refer to the experiment, SST kω model was applied in steady simulation, at Re from 20000 to 50000. Due to the angled ribs can induce the secondary flow and generate small helical vortices at front corner, heat transfer performance was elevated. The large rotating vortices influenced by the ribs arrangement occupy the center channel, thence the dislocation caused different flow and heat transfer results. The results shown that parallel rib has higher heat transfer enhancement than crossed ribs, but pressure loss possess considerable level. At Re=21587, the averaged turbulent kinetic energy of Case2.2 with dislocation ribs is 22.4% lower than parallel ribs. The all 45deg crossed ribs present higher level of overall thermal performance, and Case2.2 is optimal for the range of Re investigated.





CFD Study on the Efficacy of Flow Diverter Stent Placement for Cerebral Aneurysms


Pages :
15471558


Authors :
Y. Ito,
I. Cho,
Y. Sakai,
K. Iwano,
The effect of flow diverter (FD) stent placement as a cerebral aneurysm treatment on hemodynamics was systematically investigated via numerical simulation. The aneurysm diameter was set to 8 mm and 12 mm, with an aspect ratio (AR) of 1.2 and 1.9, respectively. The curvature of the parent artery was also varied for the following three types: straight, inside, and outside. The blockage ratio of the FD stent was set to 31 %. The results reveal that, regardless of artery shape, the FD stent drastically modifies the flow in the aneurysm, including changing the flow direction in the systolic phase. In most cases, the flow rate into the aneurysm is significantly reduced by the stent; however, in the case of a straight artery, the flow rate is increased for the aneurysm model with AR = 1.9. The oscillatory shear index (OSI) generally increases owing to FD stent placement while the wall shear stress is substantially decreased. In particular, a high OSI area is widely distributed in the large aneurysm sac (AR = 1.9) for straight and internal artery cases. Although idealized aneurysm models are employed in the present study and further parametric studies are required, particularly with respect to stent configuration, these facts may explain the unexpected outcome in some (but not all) large aneurysm cases.





An Experimental Study on the Simultaneous Effect of Nanofluid and Tube Insert on the Thermal Performance of an Automobile Cooling System


Pages :
15591566


Authors :
A. Karimdoost Yasuri,
E. Soleimani,
In this research, an experimental study is conducted to investigate the effect of simultaneous use of tube insert and nanofluid on the thermal performance of the automotive cooling system (radiator). Al2O3water based nanofluid is used and the flow regime is laminar (Reynolds 10001800). For this study, an experimental automotive cooling system is designed and built. Thermal performance of Al2O3water nanofluid at three concentrations of 0.1, 0.2 and 0.4% weight fraction of Al2O3water nanofluid compared to water is monitored by variations of flow rate. In this research, a tube insert with a steel wire rod and a sine profile inside the radiator tube is used as a turbulentmaker. The results show that the sole use of nanofluid, without a tube insert inside the radiator, increases the heat transfer coefficient by about 11%, and the use of a tube insert together with distilled water increases heat transfer coefficient by about 35%. Finally, the use of tube insert together with Al2O3water based nanofluid at their best mass concentration increases heat transfer coefficient by about 56%.





Growth of Turbulent Boundary Layer with Varying Inlet Length Scales


Pages :
15671581


Authors :
Y. T. Lee,
H. C. Lim,
This study aims to generate a fully developed turbulent boundary layer in the channel domain using LES (Large Eddy Simulation), suitable inflow conditions along with statistically reliable turbulent characteristics are required. This study clarifies the effect of the integral length scale from the existing data on the generation of turbulent boundary layers. In order to justify the work, an artificially created boundary layer is imposed on the inlet section, which gradually evolves into a fully developed turbulent boundary layer flow inside the numerical domain. In this study, the synthetic inflow method, which is based on an exponential correlation function, is used by imposing the spatial and temporal correlation between two different points on the inlet section. In addition, we conducted parametric length scale studies on the inlet section and compare our results with existing data. The results indicated that the cases of larger length scale in the spanwise direction were not only effective in achieving the target shape of a fully developed turbulent boundary layer, but also developed it faster than the smallerlength scales.





Numerical Research on Effect of Base Bleed Type on Operation Process of Base Bleed Projectile


Pages :
15831591


Authors :
M. Wang,
C. Zhuo,
The edge base bleed type is presented in this paper. Based on the advantages of computational fluid dynamics and the firing range prediction of base bleed projectile, the flight ballistic of base bleed projectile with two kinds of base bleed type is solved by using the computational aerodynamics coupled with particle trajectory, obtained the change laws of operation parameter, flow filed of base bleed projectile with time. The results show that: The performance of the edge base bleed type is better than that of the traditional center base bleed type, and the fire range of the base bleed projectile with edge type is about 2 km higher than that of the center type. The base bleed parameter of the two kinds of base bleed type at the state of drag reduction increase first and then decrease with the increase of time. For the center base bleed type, the drag coefficient increases with time. For the edge base bleed type, the drag coefficient is nearly stable at the reduction state, and is always smaller than that of the center base bleed type. The base flow structure of the edge base bleed type does not change with both the time and base bleed parameter. The base bleed gas always forms the main recirculation zone at the bottom of projectile and plays the role of drag reduction.





