

On the Low Frequency Pressure Fluctuation in a 3/4 Open Jet Automotive Wind Tunnel


Pages :
13591369


Authors :
Q. Jia,
Y. Zhu,
D. Bao,
M. M. Rashidi,
Z. Yang,
In the present research, a possible generation mechanism of lowfrequency buffeting phenomenon based on a 1:15 open jet automotive wind tunnel was investigated. Evolution of vortex structures and pressure field in the plenum chamber have been visualized and analyzed by Largeeddy simulation (LES). It is shown that the low frequency pressure fluctuation is caused by the largescale structures and their interaction. Multiple proper orthogonal decomposition was adopted to analyze the flow field in the plenum chamber. The characteristic frequencies of the vortexrings after pairing is the same as the dominant resonance of buffeting at this wind velocity.





A Study of the TaylorCouette Flow with Finned Surface Rotation


Pages :
13711382


Authors :
M. Carlos Álvarez,
W. Vicente,
F. Solorio,
E. Mancilla,
M. Salinas,
V. R. Zenit,
In this study, the TaylorCouette flow was disturbed by incorporating annular fins over the inner rotating surface. The finned surface had three parameters: height, width, and length between fins. In this work, seven different fin configurations, in which only the fin height varied, were examined and compared using experimental and numerical techniques. We found that annular fins disturbed the flow behavior by reducing the smooth critical Taylor number (Ta=57.18), but more important, we noticed a vortex enlargement induced by the incorporation of a relatively modest perturbation (b´<0.5) and this change remained in a wider range (0.57<T<14.18). On the other hand, it was identified the appearance of smaller secondary vortices just over the fins, it is as a result of an increment in fin height. The relevance of this finding lies on the field of micromixing processes.





Aerodynamic Shape Design of Pantograph Network Monitoring Device on HighSpeed Trains


Pages :
13831394


Authors :
P. Ji,
F. Wu,
B. Qian,
L. Yan,
The pantograph monitoring device on highspeed trains bears not only its own strength but also the aerodynamic load applied by the air flow when the train is running at high speed. A well designed shape of the pantograph monitoring device on highspeed trains reduces the loads and pressure fluctuations acting on it, and therefore, increases its function stability and life cycle. In this paper, we present an aerodynamic shape design method for such device. Firstly, an efficient and reliable numerical simulation approach is established for the evaluation of the aerodynamic loads acting on the device. According to the numerical computations, a basic shape for the monitoring device is formed, with which the minimum functional space of the device is reserved. Then, the corners of the basic shape are smoothed out with three types of continuous transitions. By comparing the numerical results of the three smoothed shapes, we obtain an optimal aerodynamic shape for the pantograph monitoring device. The design method is not limited to the monitoring device studied in this manuscript. The aerodynamic shape of other small functional devices on highspeed trains can also be generated or optimized with the method presented herein.





The Use of Dam Break Model to Simulate Tsunami Runup and Scouring Around a Vertical Cylinder


Pages :
13951406


Authors :
. Kuswandi,
R. Triatmadja,
This research paper deals with the use of a dam break system to generate a surge model to study tsunami runup, rundown and scouring around a vertical cylinder. The dam break system was provided with one or two gates to store water at a predefined depth; the water could then be quickly released by opening the gates to create a tsunami surge that runs up on land. In addition, numerical simulations of dam break surges resulting from various lengths of reservoirs were conducted to obtain more findings for further analysis regarding the characteristics of the dam break surges. A vertical cylinder model was installed on the beach at 6 meter downstream of the main gate to study the scour caused by the tsunami surge. The bed material was fine sand with a 0.19 mm diameter. The results were compared with existing experimental results. The comparison indicated that the dam break surge can be used to simulate tsunami surges by adjusting the reservoir length, the reservoir depth and the water depth downstream of the gate. The ratio of the difference between the upstream and downstream water depth on one side to the length of the reservoir on the other affect the runup height and duration. Thus, this ratio should be considered when simulating tsunami based on dam break systems. Although different in magnitude, the shape of both the surge mareogram and the velocity time history of the tsunami surge generated using dam break system was comparable with the tsunami surge induced by a solitary wave generated using a long flume. For the relatively large cylinder located at the bore location, the separation flow was strongly directed to the wall which produced significantly unsymmetrical scour result.





Effects of TwoWay Turbulence Interaction on the Evaporating Fuel Sprays


Pages :
14071415


Authors :
H. Khaleghi,
M. Ahmadi,
H. Farani,
This article discusses the importance of using different turbulence modulation models in simulation of evaporating sprays. An inhouse CFD code has been modified to take into account the effect of considering turbulence modulation by standard or consistent models. These models may predict an augmentation (consistent model) or a reduction (standard model) in the turbulence kinetic energy of continuous phase. Calculations are done in a EulerianLagrangian framework and the effect of injected droplets on turbulent kinetic energy and its rate of dissipation is included in the equations of the continuous phase. Results are shown to be valid by comparing them to Sandia spray A configuration experimental data. Results show that considering the effect of existing droplets in a turbulent combustion chamber can play a major role in having a more accurate CFD simulation. These models can alter the velocity field drastically when droplets are injected into the chamber with a high velocity. As a result, spray characteristics such as evaporation rate is also altered. It can be concluded that modulation models should be used in the simulation of evaporating sprays in order to attain more accurate and realistic results.





Experimental Study of the Effect of Displacement of Vanes Submerged at Channel Width on Distribution of Velocity and Shear Stress in a 180 Degree Bend


Pages :
14171428


Authors :
Ch. Abdi Chooplou,
M. Vaghefi,
Shear stress is a parameter of high significance. Through knowledge of this parameter, assessment of scour or sedimentations at different points of bed is made viable. Therefore, this paper investigated alterations in shear stress along the bend, specifically around a bridge pier, under the influence of applying submerged vanes at the upstream side of the bridge pier. With the aim of modeling submerged vanes, vanes of Plexiglas with a thickness of 20% of the pier diameter, a length of 1.5 times the pier diameter, and submergence ratio of 75% were utilized. The vanes were installed at a distance equal to 5 times the pier diameter from the pier center at a distance of 40 to 60% of the channel width from the inner bank at the upstream side of the bridge pier. AcousticDoppler Velocity velocimeter device was utilized for measuring threedimensional velocity components. The experiments were conducted in a 1meterwide flume with a degree of curvature of 180. The results of the study suggested that upon reaching the bend apex, the maximum flow turbulence rate occurred in a transverse direction in the case of installing submerged vanes at a distance of 40% of the channel width from the inner bank towards the inner wall; while in the case of installing submerged vanes at a distance of 60% of the channel width from the inner bank, it occurred towards the outer wall, and it could be observed that the maximum longitudinal and vertical components of turbulence rate increased by 16 and 5.5% respectively upon increase in the distance of submerged vanes from the inner bank. Furthermore, the values of and turbulence shear stresses at the outer bank in the case of installing the vanes at a distance of 40% of the channel width from the inner bank were smaller than those in the case of installing the submerged vanes at a distance of 60% of the channel width from the inner bank.





Optimization of Synthetic Jet Position for Heat Transfer Enhancement and Temperature Uniformity of a Heated Wall in MicroChannels


Pages :
14291437


Authors :
J. Khaleghinia,
F. Kowsary,
C. Aghanajafi,
In this study, the synthetic jet position was optimized to obtain the maximum rate of heat transfer and the best state of temperature uniformity on a heated surface in microchannels. Based on microchannel length, several cases were simulated to investigate the effects of synthetic jet position on the heat transfer rate and temperature uniformity. After that, the synthetic jet position was optimized using the CFD results and the GMDHMOGA optimization code. The obtained results show that the synthetic jet placement in all longitudinal positions of microchannel increases the heat transfer rate, although the improvement of temperature uniformity of heated surface decreases at some positions as compared to the microchannel without synthetic jet. The optimization results show that for obtaining the maximum value of heat transfer and the best state of temperature uniformity on the heated surface, the dimensionless longitudinal position of synthetic should be between 0.45 and 0.65. The maximum rate of heat transfer and the best state of temperature uniformity have been observed in the vicinity of lower and upper bounds of this range, respectively.





Experimental Study on the Discharge Coefficient of BiSwirl Coaxial Injectors


Pages :
14391447


Authors :
K. Ahn,
B. J. Lee,
An experimental study was carried out to investigate the effects of recess length and mixture ratio on the discharge coefficient of biswirl coaxial injectors with inner closedtype and outer opentype swirl injectors. Ten biswirl coaxial injectors were classified into two groups with different recess lengths. By independently varying the mass flow rates through the inner and outer injectors, the discharge coefficients of the injectors were obtained. Singleinjection coldflow tests indicated that the discharge coefficients of both the inner and outer swirl injectors were only marginally affected by the recess length and mass flow rate. Biinjection coldflow tests showed that the discharge coefficients of the inner swirl injectors were also almost constant, regardless of the recess length and mixture ratio. On the other hand, those of the outer swirl injectors in the tipmixing and internalmixing biswirl coaxial injectors with long recess lengths had significantly decreased with the increase in mixture ratio. A novel empirical equation for the discharge coefficient of the outer swirl injector in the internalmixing biswirl coaxial injector is suggested through a linear regression analysis of the present test data. It was found that the present empirical equation could accurately predict the experimental data.





Study on the Accuracy of RANS Modelling of the Turbulent Flow Developed in a Kaplan Turbine Operated at BEP. Part 1  Velocity Field


Pages :
14491461


Authors :
R. G. Iovănel,
D. M. Bucur,
M. J. Cervantes,
This paper investigates the accuracy of Reynoldsaveraged NavierStokes (RANS) turbulence modelling applied to complex industrial applications. In the context of the increasing instability of the energy market, hydropower plants are frequently working at offdesign parameters. Such operation conditions have a strong impact on the efficiency and life span of hydraulic turbines. Therefore, research is currently focused on improving the design and increasing the operating range of the turbines. Numerical simulations represent an accessible and cost efficient alternative to model testing. The presented test case is the Porjus U9 Kaplan turbine model operated at best efficiency point (BEP). Both steady and unsteady numerical simulations are carried out using different turbulence models: kepsilon, RNG kepsilon and komega Shear Stress Transport (SST). The curvature correction method applied to the SST turbulence model is also evaluated showing nearly no sensitivity to the different values of the production correction coefficient Cscale. The simulations are validated against measurements performed in the turbine runner and draft tube. The numerical results are in good agreement with the experimental timedependent velocity profiles. The advantages and limitations of RANS modelling are discussed. The most accurate results were provided by the simulations using the kepsilon and the SSTCC turbulence models but very small differences were obtained between the different tested models. The precision of the numerical simulations decreased towards the outlet of the computational domain. In a companion paper, the pressure profiles obtained numerically are investigated and compared to experimental data.





Study on the Accuracy of RANS Modelling of the Turbulent Flow Developed in a Kaplan Turbine Operated at BEP. Part 2  Pressure Fluctuations


Pages :
14631473


Authors :
R. G. Iovănel,
G. Dunca,
M. J. Cervantes,
The aim of the paper is to investigate the limitations of unsteady Reynoldsaveraged NavierStokes (RANS) simulations of the flow in an axial turbine. The study is focused on modelling the pressure pulsations monitored on the runner blades. The scanned blade geometry renders the meshing process more difficult. As the pressure monitor points are defined on the blade surface the simulation relies on the wall functions to capture the flow and the pressure oscillations. In addition to the classical turbulence models, a curvature correction model is evaluated aiming to better capture the rotating flow near curved, concave wall boundaries. Given the limitations of Reynoldsaveraged NavierStokes models to predict pressure fluctuations, the Scale Adaptive SimulationShear Stress Transport (SASSST) turbulence model is employed as well. The considered test case is the Porjus U9, a Kaplan turbine model, for which pressure measurements are available in the rotating and stationary frames of reference. The simulations are validated against timedependent experimental data. Despite the frequencies of the pressure fluctuations recorded on the runner blades being accurately captured, the amplitudes are considerably underestimated. All turbulence models estimate the correct mean wall pressure recovery coefficient in the upper part of the draft tube.





A Numerical Study of the Frontal System between the Inflow and Outflow Waters in the Persian Gulf


Pages :
14751486


Authors :
A. Rahnemania,
A. A. Bidokhti,
M. Ezam,
K. Lari,
S. Ghader,
In this study the dynamical characteristics of the salinity front between the Persian Gulf inflow and outflow were studied using the HYCOM numerical model. This model was integrated for 5 years from the beginning of 2011 to the end of 2015and the results of 2015 were discussed. The results of the model clearly showed seasonal variations in the salinity front in which the intrusion of the salinity front extends much farther into the Persian Gulf in summer. The salinity front appears to be prone to baroclinic instability with maximum intensity in spring and summer months (with a strong density stratification), forming cyclonic eddies (saline center) and anticyclones (sweeter center), that peaks in August. Results showed that some anticascade processes occur in mesoscale eddy activity, in agreement with the quasitwodimensional turbulence behavior. Spectral analysis of salinity time series in the front showed eddies with time scales ranging from a few hours to about 3 months. The result also showed that there was a reasonable relation between mixed layer depth and the formation of mesoscale eddies, so that mesoscale eddies disappeared when the thickness of mixed layer was increased in winter.





Empirical and Numerical Analysis of Aerodynamic Drag on a Typical SUV Car Model at Different Locations of Vortex Generator


Pages :
14871496


Authors :
P. N. Selvaraju,
K. M. Parammasivam,
The aerodynamic characteristics are concerned with the fuel consumption rate and the stability of a high speed vehicle. The current research aims at studying the aerodynamic behavior of a typical SUV vehicle model mounted with the vortex generator (VG) at various linear positions with reference to its rear roof edge. The flow field around the vehicle model was observed at different wind speed conditions. It had been determined that at the instance of lower wind speed, the VG had minimal effects of aerodynamic drag on the vehicle body. However, at the instance of higher wind speed conditions the magnitude of the drag force decreased significantly. Vehicles move at higher speeds in the highways, location of the VG varied towards the upstream of the vehicle due to early flow separation. Therefore test were conducted at different wind speeds and locations of VG. The numerical simulation conduced in this study provides flow characteristics around the vehicle model for different wind speeds. The realizable k−ε model was used to simulate and validate the empirical results in an effective manner. By using experimental data, the drag was reduced by 9.04 % at the optimized VG location. The results revealed that the induced aerodynamic drag would determine the best car shape. This paper provides a better understanding of VG positioning for enhanced flow separation control.





Simulation of Pigging with a Brake Unit in Hilly Gas Pipeline


Pages :
14971509


Authors :
H. He,
Z. Liang,
Pigging is a routine operation in the oil and gas industry. In this paper, the governing equation of pig speed was combined with the gas flow equations. The transient equations of gas flow are solved by the method of characteristics (MOC). An experiment was carried out to test the proposed pigging model. The measured speed of the pig coincides with the calculated speed well. The process of a pig carrying a brake unit to pass over a hilly gas pipeline is simulated. The results indicate that the brake unit would lead to a sharp increase of the pressure on the tail of the pig, because the pig is dragged by the brake unit and thus prevented to accelerate together with the gas column in a downhill gas pipeline. This way, the pig speed in a downhill gas pipeline is much lower by using a brake unit, but the speed of pig still can hardly be controlled in the desired range. Furthermore, response surface methodology (RSM) is used to study the maximum speed of pig with/without a brake unit in downhill gas pipeline. Based on the results of the RSM simulations, two equations are present to predict the maximum speed of a pig in a downhill gas pipeline.





Instability and Transition in a Laminar Separation Bubble


Pages :
15111525


Authors :
N. K. Singh,
In this numerical study, a laminar separation bubble is simulated by imposition of suction to create an adverse pressure gradient. The DNS elucidates the entire transition process over the separation bubble leading to turbulence. Several important conclusions are drawn from the simulations regarding the origins of transition and evolution of turbulence. Break down to turbulence, preceded by threedimensional motions and nonlinear interactions, occurs in the second half of the mean bubble length. Two topological structures of the bubble causing vortex shedding are suggested; one for the normal shedding and the other for the low frequency flapping. The normal shedding frequency can be attributed to the regular shedding of smaller vortices while shedding of large vortices formed due to coalescence of smaller vortices results in the lowfrequency flapping. Due to the shedding of bigger vortices, the instantaneous reattachment point varies greatly resulting in large variation in the instantaneous bubble length. Break down of longitudinal streaks, appearing via Λvortices and vortex stretching mechanism, characterizes the transition process. Low values of reverse flow suggest that a convective instability is involved. The instability analysis indicates that the initial amplification of disturbances is due to TS mechanism while the rollup of the shear layer takes place due to KelvinHelmholtz instability.





Numerical Simulation of Pitching Sloshing under Microgravity


Pages :
15271537


Authors :
W. J. Yang,
T. T. Zhang,
C. Li,
S. M. Li,
X. Xu,
In this paper, the fluid characteristics of pitching sloshing under microgravity condition are investigated. A numerical method by solving the NavierStokes equations to study threedimensional (3D) nonlinear liquid sloshing is developed with OpenFOAM, a Computational Fluid Dynamics (CFD) tool. The computational method is validated against existing experimental data in rectangular tank under ordinary gravitational field. However under low gravity conditions, the sloshing liquid shows seemingly chaotic behavior and a considerable volume of liquid attaches on the sidewall due to the effect of surface tension, which is verified in simulation experiment. Besides, the nonlinear liquid behaviors in hemispherically bottom tank are firstly studied in this paper. It is found that the wave evolution becomes divergent with the decrease of gravitational acceleration. The natural frequency reaches a constant magnitude quickly with the increase of liquid height and then increases again until the filling level exceeds 70%. Meanwhile, the liquid dynamics of forced pitching sloshing under resonant and offresonant condition are demonstrated respectively. The numerical techniques for 3D simulation are hopeful to provide valuable guidance for efficient liquid management in space.





Influence of Heat and Mass Transfer on Free Convection of Micropolar Fluid between Vertical Concentric Cylinders


Pages :
15391545


Authors :
A. K. Singh,
A. K. Singh,
Natural convective flow of a micropolar ﬂuid is examined analytically in order to see the effect of heat and mass transfer between two concentric vertical cylinders of infinite length. The governing equations of model in nondimensional form corresponding to the temperature, velocity and microrotational velocity, using the Boussinesq approximation and Eringen equation with suitable boundary conditions are expressed in terms of cylindrical coordinate system and then their exact solutions are obtained. The influence of the nondimensional physical parameters such as the material and vortex viscosity parameters on the velocity, microrotational velocity is evaluated by showing on the graphs while the values of skin friction in nondimensional form at the outer and inner surfaces of inner and outer cylinders have been presented in the tabular form.





An Experimental Investigation of a Passively Flapping Foil in Energy Harvesting Mode


Pages :
15471561


Authors :
M. N. Mumtaz Qadri,
A. Shahzad,
F. Zhao,
H. Tang,
Energy extraction through flapping foils is a new concept in the domain of renewable energy, especially when the system is fully driven by incoming freestream flow, a phenomenon known as flowinduced vibration. To investigate this concept, a water tunnel testrig was designed and fabricated, where a flat plate foil made from plexiglass performs twodegrees of freedom pitch and plunge motion under the influence of incoming water flow. For this study a powertakeoff system was not introduced, hence energy harvesting performance was evaluated through realtime force and motion measurements with the help of sensors. The energy harvester performed selfsustained flapping motions when the freestream velocity reached a threshold value, known as the cutoff velocity, which for this testrig is 0.40 m/s (without sensors) and 0.50 m/s (with sensors). To support these selfsustained flapping motions, inertial mass blocks were placed to provide the necessary inertia especially when the flat plate foil performed the pitching or stroke reversal action. Different inertial mass units (mib = 0.45, 0.90 & 1.35 kg/block) were tested to analyze their effect on the flat plate foil kinematics and its energy harvesting performance. Other parameters such as pitching amplitude (θo = 30°, 43° & 60°) and freestream velocity (U∞ = 0.57 m/s, 0.65 m/s and 0.78 m/s) were varied at fixed pivot location (xp = 0.65 chords (c)) to augment the varying inertial mass unit study. In the first section at fixed mib of 0.45 kg/block and xp = 0.65c from leading edge, energy harvesting performance (C̅p & η) was observed to increase with increase in pitching amplitude, while it degraded as the freestream velocity increased. Best energy harvesting performance of η = 52.5% and C̅p = 1.124 was achieved with mib = 0.45 kg/block, θo = 60° and U∞ = 0.57 m/s. Varying mib also had a considerable effect on the energy harvesting performance of the testrig, where the mib = 0.90 kg/block case showed a 36.5% and 21.13% decline in performance compared to the mib = 0.45 and 1.35 kg/block cases, respectively at θo = 60° and U∞ = 0.57 m/s. This shows that the energy harvester is sensitive to changes in inertial loads, affecting the forcemotion synchronization which eventually affects its performance.





Aerodynamic Forces Acting on AhmedType Vehicles under Fluctuating Headwind Conditions


Pages :
15631574


Authors :
M. Sumida,
K. Hayakawa,
Aerodynamic forces of Ahmedtype road vehicles subjected to atmospheric fluctuation were studied in an advanced wind tunnel with programmable settings enabling the generation of pulsating wind speeds. The experiments were performed with a timeaveraged airflow speed of approximately 13 m/s, with the fluctuating speed ranging from 2.58 to 2.90 m/s, and periods ranging from 1.5 to 5.0 s. The results of the timedependent drag and lift forces acting on the vehicle were compared with those under steady wind conditions. Further, the influence of the rear slant angle of the Ahmed model on the forces was addressed. The fluctuation in wind speed showed a greater effect on the aerodynamic forces than predicted. The amplitude of the drag force under the pulsating wind became larger in a vehicle having a shape that experienced a large drag force under steady wind conditions. It is concluded that even under fluctuating wind conditions, there exists a critical angle of 30 at which the vehicle experiences either high or low fluid forces.





Effect of Knudsen Number, Lid Velocity and Velocity Ratio on Flow Features of Single and Double Lid Driven Cavities


Pages :
15751583


Authors :
S. Mukherjee,
V. Shahabi,
R. Gowtham,
K. S. Rajan,
R. K. Velamati,
Effects of Knudsen number, lid velocity and velocity ratio are investigated on the flow features of single lid driven cavity with an aspect ratio of one and double lid driven cavity of aspect ratio two. Knudsen numbers studied are 0.01(early slip regime), 0.1 (slip regime) and 1 (transitional regime). Lid velocities investigated are 100 m/s, 200 m/s and 500 m/s. The velocity ratios explored are 1 and 1. Knudsen number was found to have a huge impact on the flow rigidity. Lid velocity tends to shift the central vortex to the top left of the cavity for a cavity with aspect ratio of one and shifts the upper vortex to the top left of the cavity for a cavity with aspect ratio of two. Lid velocity does not affect the slip to a great extent on the lid. Changing the velocity ratio from 1 to 1 leads to the reversal of the relative vorticity in the top and bottom half of the cavity.





Computational Fluid Dynamics Simulation on Particulate Distribution in Gyro Casting for the Manufacture of Al/SiC Particulate Metal Matrix Composite


Pages :
15851597


Authors :
P. A. Abdul Samad,
P. R. Shalij,
A. Ramesh,
A. K. Mubarak,
The enhanced specific strength of SiC Particulate Metal Matrix Composites (PMMC) has been the major contributing factor which helps to find applications in the aerospace and automotive industries. Uniform distribution of the particulates in PMMC controls the attainment of better mechanical properties. The most accepted method for producing such a composite is stir casting in which the homogeneity of particulate reinforcement is a significant challenge. This research work proposes a new method for mixing the particulate reinforcement with the liquid and semisolid aluminium matrix to ensure a uniform mix of the particulates using a gyro shaker. Gyro shaker is a dual rotation mixer commonly used for mixing high viscous fluids. It rotates about two mutually perpendicular axes which help in thoroughly mixing of the ingredients. Developed Computational Fluid Dynamics (CFD) simulation model of the mixing device in finding the mixing performance while mixing SiC particulates with glycerol. The results of the simulation were also validated by experimentation. Analogue fluid simulation of gyro casting was carried out using water and glycerol/water mixture which are having a closer value of viscosity as that of liquid aluminium and semisolid aluminium. The mixing time obtained in the water system at gyration speeds of 29.63 rpm, 58.18 rpm, 72.73 rpm and 87.27 rpm was 61.84 sec, 43.44 sec, 26.85 sec and 27.24 sec respectively. The mixing time obtained in glycerol/water system at gyration speeds of 58.18 rpm, 87.27 rpm, 116.36 rpm and 145.45 rpm was 26.34 sec, 15.97 sec, 9.8 sec and 6.26 sec respectively. The distribution of the SiC particulates obtained from simulation was compared with stir casting simulations. The homogeneous distribution of particulates was observed in the gyro casting simulation.





Large Eddy Simulation of Compressible Parallel Jet Flow and Comparison of Four SubgridScale Models


Pages :
15991614


Authors :
Q. Liu,
Y. H. Dong,
H. Lai,
Large eddy simulations of a threedimensional (3D) compressible parallel jet flow at Mach number of 0.9 and Reynolds number 2000 are carried out. Four subgridscale (SGS) models, namely, the standard Smagorinsky model (SM), the selective mixed scale model (SMSM), the coherentstructure Smagorinsky model (CSM) and the coherentstructure kineticenergy model (CKM) are employed, respectively, and compared. The purpose of the study is to compare the SGS models and to find their suitability of predicting the flow transition in the potential core of the jet, and so as to provide a reference for selecting SGS models in simulating compressible jet flows, which is a kind of prototype flow in fluid dynamics and aeroacoustics. A finite difference code with fourthorder spatial and very low storage thirdorder explicit RungeKutta temporal schemes is introduced and employed for calculation. The code, which was previously designed for simulating shock/boundarylayer interactions and had been widely validated in simulating a variety of compressible flows, is rewritten and changed into parallelized using the OpenMP protocol so that it can be run on memoryshared multicore workstations. The computational domain size and the index of LES resolution quality are checked to validate the simulations. Detailed comparisons of the four SGS models are carried out. The results of averaged flowfield including the velocity profiles and the developments of shearlayer, the instantaneous vortical flows and the viscous dissipation, the predicted turbulence statistics and the balances of momentum equation are studied and compared. The results show that although the normalized developed velocity profiles are well predicted by the four SGS models, the length of the potential core and the development of the shearlayer reveal that the SM has excessive SGS viscosity and is therefore too dissipative to correctly predict the flow transition and shearlayer expansion. The model smears small vortical scales and lowers down the effective Reynolds number of the flow because of the overpredicted SGS viscosity and dissipation. The turbulence statistics and the balances of momentum equation have also confirmed the excessive dissipation of the SM. The CKM is also found to overpredict the SGS viscosity. Compared with these two models, the SMSM and the CSM have performed well in predicting both the averaged and the instantaneous flowfields of the compressible jet. And they are localized models which are computationally efficient and easy for coding. Therefore, the SMSM and the CSM are recommended for the LES of the compressible Jet.





Investigation of Parameters Affecting Axial Load in an End Suction Centrifugal Pump by Numerical Analysis


Pages :
16151627


Authors :
H. Pehlivan,
Z. Parlak,
The total force produced in the axial direction on a pump is called axial load and is caused by the pressure difference between the front and rear of the impeller and the hydrostatic force in the suction direction. In a centrifugal pump, 3D computeraided analysis programs are used to design and reduce R&D and manufacturing costs. In this study, parameters affecting axial load of the centrifugal pump with a single suction and closed impeller were investigated by using the Computational Fluid Dynamics (CFD) method. In this context, the flow rate and the some physical properties such as the back gap of the impeller, wear ring and balancing holes, of the centrifugal pump were investigated to determine how much affected the axial load. The results showed that the wear ring and the balancing holes give rise to effective results on the axial load, while the back gap of the impeller does not affect the large extent. With the design changes made with these parameterizations, the axial force was reduced by up to 60%, whereas the efficiency was decreased by 5%. The loss of efficiency due to this decrease in axial force is negligible. However, higher efficiency values were also found at a different point from the working point where the axial load is lowest.





