فهرست مطالب

Applied Fluid Mechanics - Volume:12 Issue:6, 2019
  • Volume:12 Issue:6, 2019
  • تاریخ انتشار: 1398/08/18
  • تعداد عناوین: 30
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  • A. A. Rejniak*, A. Gatto Pages 1729-1744

    Minimising the aerodynamic drag of commercial vehicles is important economically and ecologically. This work demonstrates the effective use of lobed-mixing geometries, traditionally used to enhance flow mixing, as a viable, passive flow control method for reducing base pressure drag of boat-tailed ground vehicles. Experiments were performed on a 1/24th-scale Heavy Goods Vehicle representative model at a Reynolds number of 2.3 × 105 with force and hot-wire anemometry measurements used to quantify drag and wake characteristics. Tests on a baseline (no boat-tail), an unaltered boat-tail, and lobed-mixing configurations with varying pitch and height were compared. Overall, the baseline and unaltered boat-tail exhibited good correlation to previous results. This provided confidence in the methodology adopted. Results using lobed mixers showed up to a 10.2% drag reduction with the added vorticity produced acting to fundamentally shift the nature of the wake. This is manifested principally through the generation of counter-rotating vortical structures which enhance crosswise flow entrainment into the base wake. This action is observed to limit flow entrainment towards the ground leading to a higher wake and a characteristic ‘waist’. Enhanced mixing is also demonstrated. Overall, results suggest the suitability of lobed mixers as an effective means for drag reduction of boat-tailed ground vehicles.

    Keywords: Drag reduction, Lobed mixers, Boat-tailed vehicles
  • Fz. Mirzaei, Fs. Mirzaei, Fs. Mirzaei* Pages 1745-1755

    The dispersion of hazardous gas in the environment presents dangerous risks for people living close to chemical plants or storages. Since heavy gases tend to stay at lower levels and disperse at a slower pace in the atmosphere, they are potentially more dangerous. In this paper, various mathematical models for turbulence (including k-ε, RNG k-ε, EARSM, LES, DES) and their associated parameters have been assessed, compared and validated against the experimental data in various scenarios to find the most suitable one for atmospheric dispersion of dense-gases. This topic has been investigated and validated by a computational fluid dynamics (CFD) simulation of the Kit-Fox experiment. The precision of the CAD models, practicality, computational resource requirements, and some other factors have been considered and addressed in this paper to achieve a comprehensive solution for atmospheric dispersion. The results here suggest that the proper selection of the turbulence model and the turbulent Schmidt number is crucial. Our results indicate that the most promising combination in the case of atmospheric dense-gas dispersion is the RNG k-ε model with the Schmidt number of 0.4, considering the demand for accuracy and computational resource.

    Keywords: Turbulence modeling, Computational Fluid Dynamics, EARSM, RNG k-ε model, Atmosphericdispersion
  • J. Baliti*, M. Hssikou, M. Alaoui Pages 1757-1767

    The heat transfer and flow characteristics of rarefied gas confined within a square cavity are investigated. The cavity upper-wall is subjected to a symmetrical sinusoidal temperature with respect to its midline. Two cases are considered, one of them is that the bottom and two sidewalls are kept adiabatic, while in the other, all the enclosure walls are considered diffusely reflecting. Kinetically, the gas is simulated with the direct simulation Monte Carlo method (DSMC) in the slip and transition regimes. The DSMC results are compared with the Navier-Stokes-Fourier equation, with second order boundary conditions of velocity slip and temperature jump, (NSF) in the slip regime. Main outcomes are presented as velocity streamlines overlaid on temperature contours and macroscopic parameters plots. The Knudsen number (Kn) increase shows other vortices, beside the two classical ones that appear in the hydrodynamic and slip regime. The normal heat flux about the sinusoidally heated wall is strengthened by the flow in DSMC rather than that predicted by NSF method. Good agreement is observed between the NSF theory and DSMC method in the early slip regime, but when Kn=0.1 the NSF approach breaks down to show the secondary counter rotating eddies illustrated by the DSMC method.

    Keywords: DSMC, NSF, Heat transfer, Sinusoidal temperature
  • K. Mathivanan, J. M. Mallikarjuna*, A. Ramesh Pages 1769-1780

    This paper deals with experimental investigations on a four-cylinder, four-stroke, direct injection, turbocharged engine to evaluate effects of fuel injection strategies, exhaust gas recirculation (EGR) and intake boost pressure on the low temperature combustion (LTC) mainly to reduce nitric oxides (NOx) and smoke emissions keeping a concern on indicated mean effective pressure (IMEP). First, single-pulse fuel injection strategy with various fuel injection timings is tried to find the best operating conditions. Then, five fuel injection pulse strategy with a variation in injection timings and fuel quantities of each pulse along with the usage of EGR and intake boost pressure is tried to further improve the performance. Finally, it is found that for a low NOx and smoke emissions with a good brake thermal efficiency, five-pulse injection with the last pulse at closer to top dead centre along with the use of EGR in the LTC mode is a suitable one.

    Keywords: LTC, Single injection, Multiple injections, Combustion, EG
  • S. Ghatreh Samani, A. H. Meghdadi Isfahani* Pages 1781-1790

    The standard LBM with the relaxation time is only able to simulate the flow features in continuum and slip regimes. In the present paper, a new relaxation time formulation considering the rarefaction effect on the viscosity for the lattice Boltzmann simulation of shear driven flows is presented in order to cover wide range of the flow regimes. The results show that in spite of the standard Lattice Boltzmann Method, LBM, the presented relaxation time equation is able to predict flow features in wide range of flow regimes including slip, transition and to some extend free molecular flow regimes. The velocity profiles, slip length and shear stress agree very well with DSMC (Direct Simulation Monte Carlo) and linear Boltzmann results.

    Keywords: Lattice Boltzmann method, Micro, nano- Couette, Transitional regime, Knudsen number, Relaxation time, Rarefaction
  • S. Abolfazl Moussavi Torshizi* Pages 1791-1799

    The impeller of a centrifugal compressor is traditionally designed using some formula for only one design point which makes it less efficient in all other situations. This is especially important for compressors not experiencing a constant working condition. To improve the performance at low mass flow rates and retard the surge, an innovative concept is introduced for a centrifugal compressor. In this method pressurized air is injected at the compressor inlet to improve the flow field. With better incidence angle, related losses at off design conditions are minimized and the surge is delayed. This system is designed, modeled and adjusted for providing an optimal flow pattern at the inlet. Its implementation on a compressor has shown an increase of efficiency at low mass flow rates. It has also improved flow pattern in impeller passages and decreased the blade loading near surge condition. It is also shown that the swirl generator system can be fed up from the compressor volute or diffuser, and thus widening the compressor performance map by retarding the surge margin.

    Keywords: Centrifugal compressor, Flow injection, Surge, Performance map, Incidence loss
  • A. A. Barral Jr., R. B. Minussi, M. V. Canhoto Alves* Pages 1801-1812

    This study addresses the characteristics of the interpolation functions and interface reconstruction routines for the VOF – Volume of Fluid method available in the commercial CFD software ANSYSFLUENT. This software was used because it has both implicit and explicit VOF approaches along with diverse interpolation functions. Some of these functions were compared from different viewpoints: the quality of the reconstructed interface; the ability to preserve the initial mass inside the system (numerical diffusion); and the computing time. To undertake the qualitative and quantitative comparisons, a test problem that combines the classical dam break and slosh tank benchmark problems was used. No analytical solution available was found for this problem, in which the most interesting feature is a high interaction between the velocity field and volume fraction, thus making it ideal for addressing the issue of interface smearing. ANSYS-FLUENT permits using 5 interpolation functions for transient simulations: PLIC, CICSAM, HRIC (explicit and implicit) and the UPWIND scheme, and four when performing steady state ones: BGM, modified HRIC, COMPRESSIVE and UPWIND schemes. Both transient and steady state solutions were analyzed in this study, using all the above schemes, except the UPWIND one for steady state simulations. It was found that, for thinner grids, PLIC, CISAM and the explicit HRIC schemes had similar performances concerning the quality of the reconstructed interface and mass conservation. On the other hand, PLIC shows the best results for coarser grids, being the only to conserve mass for all tests. The computation time was similar for all transient simulation (within each grid). Concerning the steady state simulations, which are, in fact, distorted transient simulations, the BGM and the COMPRESSIVE schemes produced similar results, but BGM consumed more computational time.

    Keywords: Volume of Fluid, Numerical Diffusion, Interfacial Smearing
  • F. Chaibina*, G. Bellakhal, J. Chahed Pages 1813-1823

    First and second turbulence models for turbulent bubbly flows are implemented in the CFD code. In the second order turbulence closure, the Reynolds stress tensor of the continuous phase is split into two parts: a turbulent part produced by the gradient of the mean velocity and a pseudo-turbulent part induced by the bubbles displacements; each part is predetermined by a transport equation. The turbulent viscosity issued from this modeling takes into account the excess of the agitation and the supplementary eddies stretching due to the bubbles displacements. First order turbulence closure based on this turbulent viscosity is deduced and a three-equation turbulence model (k, ks, epsilon) is developed. We present the most prominent steps of the modeling and of its implementation in the CFD code then we comment the application of the model in the two homogeneous turbulent flows (uniform and uniformly sheared bubbly flows).

    Keywords: Bubbly flows, Turbulence, Pseudo turbulence, Second-order turbulence model, K-epsilon model
  • S. Motahari, I. Chitsaz* Pages 1825-1836

    Iran is located at the high altitude region and has a diverse four season climate. The temperature difference of two locations at the same time reaches to 50° C. Therefore, the modern direct injection turbocharged engines are highly affected at this condition. This paper deals with the effects of temperature and pressure variations on the engine performance and fuel consumption of turbocharged gasoline direct injection engine. Ford ecoboost is selected for this study and the base experiments are performed at the sea level. At the next step, a comprehensive one-dimensional model of the engine is constructed in GT power and validated with experimental data. Validated model is implemented to investigate the effects of ambient air variations on the engine performance and fuel consumption. The simulations revealed that low end torque is not highly affected by the temperature increase due to the turbocharging compensation while engine torque is significantly dropped at high engine speeds in the elevated temperatures. At constant air temperature, brake specific fuel consumption is decreased for higher intake pressure up to 3000 rpm and does not change up to 3500 rpm.

    Keywords: Temperature variations, Altitude effects, Turbocharged, GT power
  • A. Q. Lin*, X. Y. Chang, L. H. Cao, H. Zhang, d L. X. Sun Pages 1837-1845

    The condensation of wet steam has important effects on the behavior of the flow field. To evaluate the aerodynamic performance of exhaust passage influenced by wet steam phase change condensation, a numerical investigation was conducted. Taking a 600 MW steam turbine as an example with consideration of the wet steam from the last stage blade and the steam exhaust of the BFPT (boiler feed water pump turbine), the governing equations of wet steam two-phase flow were adopted by the Eulerian-Eulerian approach. Results show that the wetness in the stator domain increases gradually while the wetness in the rotor domain varies little on the pressure surface and is in small increment on the suction surface. The velocity uniformity can be improved at condenser throat outlet as the mass flow or wetness increases. Moreover, the trend to improve the aerodynamic performance of exhaust passage benefits from the improvement of wetness at the last stage blade inlet. Conversely, with the increment of wetness at the BFPT inlet, the static pressure recovery coefficient reduces by 5.8% and the total pressure loss coefficient increases by 2.4%, resulting in a reduction of aerodynamic performance of exhaust passage.

    Keywords: Exhaust passage, Wet steam, Aerodynamic performance, Last stage blade
  • D. Buczkowski*, G. Nowak Pages 1847-1854

    The purpose of the investigation was to improve the hydraulic damper valve to meet the automotive customer requirements. The original design was not good enough to achieve the damping forces in the range demanded. The main goal was to lower the minimum achievable forces at the compression stroke by reducing flow restrictions of piston valve. CFD analysis was used to verify proposed variants without the expense of prototyping and experimental testing. Design restrictions of present components were measured on the flow test bench and compared with previously done CFD analysis to ensure a proper correlation with a numerical model. The model was used to predict pressure drop over developed designs.

    Keywords: Shock absorber, CFD analysis, Damping forces, Oil flow, Damper valve, Flow restriction
  • C. W. Ong, F. Kabinejadian, F. Xiong, Y. R. Wong, M. Toma, Y. N. Nguyen, K. J.Chua, F. S. Cui, P. Ho, H. Leo* Pages 1855-1872

    Thoracic aortic aneurysm (TAA) is a severe cardiovascular disease with a high mortality rate, if left untreated. Clinical observations show that aneurysm growth can be linked to undesirable hemodynamic conditions of the aortic aneurysm. In order to gain more insight on TAA formation, we developed a computational framework in vitro to investigate and compare the flow patterns between pre-aneurismal and post-aneurismal aorta using a deformable wall model. This numerical framework was validated by an in vitro experiment accounting for the patient-specific geometrical features and the physiological conditions. The complex flow behaviors in the preaneurismal and post-aneurismal aorta were evaluated experimentally by particle image velocimetry (PIV). Our experimental results demonstrated flow behaviors similar to those observed in the fluid-structure interaction (FSI) numerical study. We observed a small vortex induced by the non-planarity of pre-aneurismal aorta near the aortic arch in pre-aneurysmal aorta may explain the aneurysm formation at the aortic arch. We found that high endothelial cell action potential (ECAP) correlates with the recirculation regions, which might indicate possible thrombus development. The promising image-based fluid-structure interaction model, accompanied with an in vitro experimental study, has the potential to be used for performing virtual implantation of newly developed stent graft for treatment of TAA.

    Keywords: : Thoracic aortic aneurysm, Particle image velocimetry, Thrombus, Fluid structure interaction (FSI), Hemodynamics
  • H. Zhou, M. Xiang, W. Zhang*, X. Xu, K. Zhao, S. Zhao Pages 1873-1883

    In more recent years, supercavitation has attracted intensive attention due to its potentials in drag reduction for underwater vehicles. Ventilation is acknowledged as an efficient way to enhance cavitation when vehicles work under low speed. That means natural and ventilated cavitation may coexist in the flow and the interaction between the natural cavitation and ventilated cavitation has to be considered. In this paper, ventilated cavitating flow with natural cavitation around a base-ventilated hydrofoil is solved by a multi-phase cavitation solver based on OpenFOAM. The Partially-Averaged Navier-Stokes method is utilized for resolving turbulence. Lengths of the natural cavities are investigated under non-ventilation and ventilation conditions. Cavity shape evolution and interface deformation have also been studied under different angle of attack. Results show that ventilation cavitation at the base of the hydrofoil tends to depress the natural cavitation on the hydrofoil surface. As the increase of the attack angle, the shedding cavity of natural cavitation have a great impact on the interface shape of the ventilation cavity. Furthermore, the research also finds that the re-entry jet is the reason for natural cavitation shedding process and the interface deformation of the ventilated cavity arises from the vortex structures induced by the shedding natural cavitation.

    Keywords: Natural cavitation, Ventilated cavitation, Interaction, PANS model, Numerical simulation, OpenFOAM
  • M. Aelaei*, S. Karimian, F. Ommi Pages 1885-1903

    This paper explores the effect of design variables on the objective functions of clipped delta wing with a modified double-wedge airfoil section based on parametric analysis and CFD-based optimization using response surface method. This type of wing is used in air-launch-to-orbit vehicles. The thickness, wing-span, tip chord, leading edge radius, front diagonal edge and rear diagonal edge lengths are defined as design variables and aerodynamic efficiency, drag and lift coefficients as objective functions. The analysis was performed at Mach 0.85 and 1.2 and for several angle of attack (AOA). The optimization process is performed by numerical stimulation of the flow around the wing at different Mach numbers and AOAs for the deformed geometry at each step including 368 cases. Minimizing the drag force and maximizing both lift coefficient and aerodynamic efficiency have been selected as optimization goal. The evolutionary optimization technique of NSGA-II (Nondominated Sorted Genetic Algorithm-II) in combination with the RSM has been used, which leads to distinct but very close candidates for each flight conditions. Defining the critical design point, it can be deduced the aerodynamic efficiency will be increased by 50% compared with base wing model. Finally, it is shown that the best point for optimizing the air-launched vehicle equipped with delta wing in the ascent trajectory, is the maximum angle of attack that occurs at Mach 1.2.

    Keywords: Delta wing, Air launch-to-orbit vehicle, aerodynamic efficiency, NSGA-II Optimization, ResponseSurface Methodology
  • S. P. Shah*, S. A. Channiwala, D. B. Kulshreshtha, G. C. Chaudhari Pages 1905-1917

    The work arose initially from an interest in design of radial turbine for small scale gas turbine applications typically suitable for distributed power generation system which demands compact installations. The paper describes an investigation in to the design and performance of radial inflow turbines having a capacity of 25kW at 1,50,000 rpm. First a non-dimensional design philosophy is deduced to design a turbine rotor. The design approach is largely one dimensional along with empirical correlations for estimating losses used to obtain the main geometric parameters of turbine. From the proposed design approach, turbine total-to-static efficiency is calculated as 84.91% which is reasonably good. After that a modified vortex design procedure is developed to derive the non-dimensional volute geometry as a function of azimuth angle for actual flow condition. Once a specific turbine is designed, the flow is analyzed in detail using a three-dimensional Computational Fluid Dynamics (CFD) code in order to assess how accurately the performance is predicted by simple meanline analysis. Finally, a fully instrumented experimental setup is developed. The experimental investigations have been carried out to study the temperature and pressure distribution across turbine and total-to-static efficiency is calculated. The limitations of surging and choking in compressor as well as in the bearings to take up load at such high speed has allowed the tests to be conducted upto 70000 rpm only, with turbine inlet temperatures ranging from 900 K to 1000 K and a pressure ratio upto 1.79, which developed power in the range of 1.69 kW to 10.22 kW. The uncertainty bands are in order of ±13.76% to ±3.12%. It is observed that the CFD results are in good agreement with test results at off design condition. CFD models over predicted total to static efficiency by order of 7-8% at lower speed. These deviations are reduced as turbine runs close to design point.

    Keywords: Turbine design, Volute design, Small gas turbine, Radial turbine, Alternate design
  • B. Zina , A. Filali*, S. Laouedj, N. Benamara Pages 1919-1928

    In the present study, numerical investigation of two-dimensional incompressible air flow through a solar air heater (SAH) with a triangular artificial roughness having a curved top corner is performed using ANSYS Fluent 15.0 based finite volume method. The geometrical parameters of the triangular ribs having a curved top corner such as the roughness height ratio (e/D = 0.021, 0.03 and 0.042) and the roughness pitch ratio (p/e =7.14, 10.70, 14.28 and 17.86) have been investigated for a varied Reynolds number between 3800 to 18000. Flow and energy governing equations were solved with the accosiation of two transport equation for the turbulence kinetic energy k and the dissipation rate ɛ. The RNG k-ε turbulent model have been selected to be the more appropriate turbulence model for the present study. Results indicates that the values of Nusselt number and friction factor strongly depend on the roughness relative height e/D, relative pitch p/e and the value of Re number. The best solar air heater performance could be obtained for e/D=0.042 and p/e=7.14.

    Keywords: Solar air heater, Roughened artificial, Turbulent flow, THPP
  • A. B. Osman*, M. Ovinis, M. H. Fakhruldin, I. Faye Pages 1929-1943

    Opaque fluid flow estimation is a challenging problem due to the complex nature of this flow type. Deepwater Horizon oil spill is one of the real examples of opaque fluid flow. Due to the complicated spill flow and the lack of dedicated flow measurement technique its flow rate was estimated with high uncertainty. In this paper, a simulation of jet flow is conducted experimentally and numerically. This is in order to analyze the difference between them. First, a turbulent buoyant jet was experimentally simulated considering various ranges of nozzle flow rates including laminar and turbulent flow. A video camera was used to capture the jet flow. Then, Fast Fourier Transform (FFT) based method was developed to estimate velocity field from video sequence. The outcomes of experimental results were compared to the outcomes of numerical simulation. As a result, the FFT-based method was estimated the nozzle flow rates with a relative error of 18.2% when it was compared to the measured experimental values. Despite this poor accuracy, a good agreement between experimental and numerical simulation outcomes was found in term of overall velocity field, centerline velocity, axial velocity as well as the distribution of radial velocity.

    Keywords: Deepwater horizon, Optical technique, Cross-correlation, K-epsilon model
  • G. Xu, H. Li, J. Zhang, X. Liang Pages 1945-1955

    A time-dependent simulation method, DES (detached eddy simulation), combined with Realizable k-ε turbulence model, has been adopted to study the underbody flow and near wake structures of a high-speed train with two bogie cavity configurations laid on the stationary ground. The numerical data, including timeaveraged aerodynamic drag forces and pressure coefficients, were compared with experimental results from previous wind tunnel tests. A detail comparison of the instantaneous flow structures, mean velocity vector contours, velocity and pressure profiles under the train bottom in the symmetry plane and velocity contours overlaid with streamlines in the wake has been conducted in the two configurations. Also the aerodynamic drag coefficients for the two cases are discussed herein. The two cases show that the bogie cavity configurations contribute to the differences of velocity and pressure distributions in each bogie region, as well as the complex vortex structures around the bogie regions. Compared to the inclined bogie cavity configuration, the train with straight plates experiences a lower drag force by 2.8% for a three-car model in the stationary ground. Thus, an effective simplification criterion for the train model will contribute to an accurate prediction of forces of trains in simulations.

    Keywords: High-speed train, Drag force, Bogie cavity configuration, Pressure distribution, DES
  • A. Tabrizian, M. Masdari*, M. Tahani Pages 1957-1966

    This paper describes the experimental study of surface pressure over a supercritical airfoil which was oscillated in pure pitching, pure plunging and combined pitch-plunge motions at the Reynolds number of 8.76*105 . While the surface pressure distribution is of significant importance in stability and performance of an airfoil, not sufficient information is available on the pressure distribution in dynamic stall. The experiments were conducted in a closed-loop wind tunnel utilizing pressure transducers array. The motions were designed to maintain constant reduced frequency, Strouhal number and phase difference. Three different regions were assumed to represent the pressure distribution over the airfoil. The results showed that LEV formed on the upper surface manifested different behavior. In the attached flow region the LEV grew and shrunk over the upper surface but in the light stall region the LEV spilled on the airfoil while a small partial LEV remained at the leading edge. In the deep stall region the LEV spilled entirely and the flow was fully separated. The formation of Laminar Separation Bubbles and suction peaks were also reported in low angles of attack. Besides, the pitching moment Damping Factor was studied to determine the level of airfoil stall flutter stability. For lower amplitudes of pitching motion the airfoil seemed to be stable except where deep stall occurred. However for high amplitudes the airfoil had a tendency to enter the stall flutter. Nevertheless, forcing the airfoil to undergo a combined motion improved the stability condition in all cases.

    Keywords: Combined pitch-plunge motion, Dynamic stall, Experimental unsteady aerodynamics, Supercriticalairfoil
  • A. V. F. Modolo, B. V. Loureiro, E. J. Soares, R. L. Thompson* Pages 1967-1978

    Dam break problems occur in a variety of applications. In the present paper we are especially concerned with the mining industry, where a dam break can be a catastrophic event with significant harm to the environment. In this case, the materials involved have a yield stress property, i.e., they flow only when a threshold is overcome by the stress that acts on the material. The plastic number, which measures the importance of the yield stress in the overall characteristic stress, is the main dimensionless number analyzed, and carbopol solutions are the kind of material employed. Since slip is common in motions of yield stress materials, the influence of this phenomenon is investigated by a comparison between the flow over a smooth and a rough surface. An image processing that captures the evolution of the shape of the interface as well as particle image velocimetry measurements were employed as tools to understand the role played by the plastic number in the problem. A number of cases presented a triangular depression that originated from a difference between the flow below the initial yield surface position and a rigid body motion above the initial surface position.

    Keywords: Yield stress material, Free surface flow, Gravity flow
  • A. Shahzad*, M. N. Mumtaz Qadri, S. Ahmad Pages 1979-1988

    Wing geometry, kinematics and flexibility are the fundamental components which contribute towards the aerodynamics performance of micro aerial vehicles. This research focuses on determining the role of isotropic flexibility in the aerodynamic performance of high aspect ratio (AR = 6.0) wings with different shapes in hovering flight. Three shapes are chosen, defined by the radius of the first moment of wing area 𝑟̅1, which are 0.43, 0.53 and 0.63, where low (resp. high) value of 𝑟̅1 corresponds to less (resp. more) spanwise area distribution towards the wingtip. The leading edges of flexible wings are modelled as rigid and the wings, therefore, predominantly deform in the chordwise direction. Flexible wings are categorized as flexible FX2 and more flexible MFX2 for brevity. The governing equations of fluid flow are solved using a sharp interface immersed boundary method, coupled with an in-house finite element structure solver for simulations of flexible wings. The results indicate that the rigid wings produce one lift peak per stroke during the mid-stroke and its magnitude increases with an increase in 𝑟̅1 due to strong leading-edge vortex. For flexible wings, the numbers of lift peaks per stroke and their timings during a flapping cycle depend on the deformation that affects the pitch angle and pitch rotation rate of the wings. The lift coefficient for a given shape decreases as flexibility increases because the pitch angle decreases during the mid-stroke. This decrease in lift coefficient with flexibility is pronounced for 𝑟̅1= 0.63 wing (up to 66 % less lift as compared to rigid equivalent) due to pitch down rotation at the commencement of the stroke, resulting in vortical structures on the bottom surface of the wing. For more flexible wings at high AR considered in this study, a wing with low 𝑟̅1 (= 0.43) may be suitable for the wing design of micro-aerial vehicle, as in general, it has better aerodynamic performance (24.5 % more power economy and similar lift coefficient) than high 𝑟̅1 (= 0.63) wing.

    Keywords: Flapping wings, Wing shape, Aspect ratio, Fluid-structure interaction, Micro aerial vehicle
  • T. B. V. Chetan*, S. Chakrabarti Pages 1989-2002

    In the present article, numerical analysis has been performed on a dump diffuser model, to study the effect of sidewall expansion angle (SWA), on its performance aspects. SWA has been varied from 90° to 1° and performance has been evaluated in terms of major influencing aspects. It is observed that, at SWA of magnitudes greater than 11°, there is no significant change in the performance. But at SWA below 11°, significant changes, which enhance the performance are observed. It is noticed that at SWA in range, from 3.57° to 8°, higher static pressure recovery (almost from 25 to 33% of inlet dynamic pressure) happens in the dump and annular regions. SWA of magnitudes less than 11° have resulted in smaller, low dense and higher intense recirculation zones. At the SWA of 3.57°, static pressure recovered is maximum and total pressure lost is minimum. But that SWA causes too much delay in pressure stabilization on the liner wall. However, SWA of magnitudes less than 3.57° have resulted in comparatively poor performance. Eventually, sidewall angle in the range from 5° to 7° is found to be optimum as it yields higher static pressure recovery and low total pressure loss. This range also results in early stabilization of pressure both on the liner and casing walls.

    Keywords: Sidewall Angle, Corner recirculation zone, Converging, diverging area, Static pressure recovery, Total pressure loss
  • M. Falsafioon*, Z. Aidoun, K. Ameur Pages 2003-2015

    Recent work on ejector performance enhancement indicates that more information on ejector internal flow structure is needed to have a clearer picture of factors and conditions affecting operation and performance of these devices. This paper relies on experimental studies and CFD simulations to identify flow structures occurring under typical ejector refrigeration conditions and primary nozzle geometry and position. Effects on parameter distributions and the resulting operation of the device are given particular attention. The CFD model used for this purpose was validated by using in-house data, generated from an experimental prototype and over a wide range of conditions. The experiments for the selected condition were predicted very satisfactorily by numerical model. The study then focused on the role of the primary nozzle geometry and the distance of the nozzle from the beginning of the mixing chamber (NXP), in locally shaping the flow structure and the related consequences on ejector operation. Simulations on NXP for given operating conditions have shown that an optimum value was always found, and slightly varied the operating conditions within the range considered. Primary nozzle shape changes in terms of outlet diameters for given upstream conditions directly affected the expansion level of the flow. The simulations showed that an optimum range of nozzle exit diameters could be found, for which ejector performance was highest. Moreover, under these conditions it was observed that pressure fluctuations inside the ejector were reduced.

    Keywords: Ejector performance, Ejector configuration, CFD simulation, Performance improvement
  • R. Askari, M. R. Soltani , K. Mostoufi , A. Khajeh Fard, M. Abedi Pages 2017-2030

    An extensive experimental investigation to study the effects of angle of attack (AOA) on the performance of a body-integrated supersonic inlet has been carried out. The present inlet, known as Diverterless Supersonic Inlet (DSI), is utilized with a three-dimensional bump to provide both supersonic flow compression and boundary layer diversion. Experiments were conducted at the presence of a typical fore-body including an elliptical nose to further contemplate the effects of fore-body geometry on the approaching flow. All tests were conducted at a constant free stream Mach number, 𝑀∞ = 1.65 zero degrees angle of sideslip (AOS), and at various angles of attack (AOA) ranging from -2 to 6 degrees. The results showed that the present DSI had acceptable performance characteristics for all ranges of AOA tested. It should be noted that the present DSI does not have any moving, adjustable or auxiliary mechanisms as such systems or mechanism are used to improve the performance of an inlet.

    Keywords: Supersonic flow, Diverterless inlet, Bump, Angle of attack, Boundary layer control
  • G. Erfort*, T. W. von Backström, G. Venter Pages 2031-2038

    Turbulence models in computational fluid dynamics (CFD) aim to capture a complex phenomenon through simplified mathematical models. The models themselves range in terms of application, complexity and methodology. This work looked at a transitional model for Reynolds averaged Navier Stokes equations. In particular the focus was on the correlation based intermittency and momentum thickness Reynolds number (γ - R̃eθt) model. The original model has high order correlations, that were determined and calibrated from flat plate tests of various pressure gradients. In this work the correlations were simplified to reduce the number of calibration coefficients and help in understanding the effect of each parameter. Flat plate test data, from the European Research Community on Flow, Turbulence and Combustion (ERCOFTAC) T3A series, were used to verify the lower order approximations through OpenFOAM simulations. The open source CFD package OpenFOAM was used for its easy access to the base code. The reduced order model was then applied to a National Advisory Committee for Aeronautics (NACA) 0012 foil at a transitional Reynolds number of 360 000 as a means of validation. The reduced order, the original γ - R̃eθt and the fully turbulent k - omega shear stress transport (k − ω SST) turbulence models are compared over a range of angles of attack to highlight the difference between models. The proposed model reduced the runtime of simulation by approximately 6%. The reduction in model coefficients meant a step by step adjustment could be implemented to increase model accuracy. In addition the adjusted model increased the accuracy of drag prediction on a NACA0012 airfoil, while maintaining a similar lift prediction as the original.

    Keywords: Transitional turbulence model, Optimization, NACA foil, OpenFOAM, Flat plate
  • R. Taherinezhad*, G. Zarepour Pages 2039-2055

    In this paper, a novel pyrogenic pulser was designed both analytically and numerically and evaluated with empirical tests. The motivation of this study was the need for active control of the aero acoustic pressure oscillations by injecting the secondary flow into the solid rocket motor. First, in brief, pyrotechnic and pyrogenic pulsers have been introduced, and then analytical governing equations have been presented in three transient, sinusoidal and Hercules methods. In order to understand the internal pressure of the pulsar and its plume length, the injection flow field has been evaluated using the ANSYS-Fluent software with both 𝑘 − 𝜔 SST and 𝑘 − 𝜀 Realizable models both at ambient and motor pressure. After that, the design and manufacturing of the pulser hardware and the test process have been described. Finally, analytical, numerical and experimental results have been discussed. The results show that there is a good correlation between the transient analysis in theory and the numerical solution by the k-ω SST model and the empirical test data. In addition, pyrogenic pulsers design depends on various parameters of motor and pulser charge performance prediction. The quality of pulser charge bonding to its insulator and erosion of its throat path due to injection have an important role to obtain a desirable pulser mass flow rate and plume length.

    Keywords: : Pyrogenic pulsers, Secondary injection, Solid rocket motors
  • A. R. Al Obaidi, H. Towsyfyan* Pages 2057-2067

    Vibration signatures have been studied for monitoring the condition of centrifugal pumps by many researchers, however, there is limited published information on the application of vibration analysis to incipient pump cavitation. The paper will review the state of the art in the field and develops an effective signal processing approach based on envelope spectral analysis to close this gap. A purpose-built test rig was employed for recording vibration signals from a centrifugal pump at a wide range of operating conditions. The collected data was then processed using time domain and frequency domain analysis methods. The study has shown that the vibration energy concentrated mainly in the frequency range between 8-15 kHz. At the flow rates less than 300(l/min), i.e. in design flow rate range, the vibration amplitudes remain constant and does not show a notable change by the flow rate increase. However, a notable increase in the vibration level is evident when the flow rate exceeds 300 (l/min). Analysis the result of filtered vibration signatures has revealed that vibration signal parameters: peak value, root mean squared (RMS), crest factor along with vibration spectrum allow development of cavitation (for the flow rates higher than 300 (l/min)) to be diagnosed reliably. However, conventional signal processing methods may not produce a clear separation of the incipient cavitation from the healthy baseline. Therefore, envelope spectrum analysis has been carried out on recorded vibration signatures to detect the onset of cavitation from the baseline and satisfactory results have been perceived.

    Keywords: Centrifugal pump, Inception of cavitation, Vibration signal, Envelope spectrum analysis
  • M. U. Yangaz*, G. A. Çiftçioğlu, M. A. N. Kadırgan Pages 2069-2081

    Energy sources must be used efficiently to provide the sufficient amount of energy for the still-growing population in the world, already threatened by the effects of global warming. The significant increase in the use of natural resources causes serious problems due to its unsustainable situation. Therefore, exhaust gases/emissions must be reduced to prevent more damage on the environment. This study aims to provide solutions for a sustainable ecosystem by lowering emissions such as CO, unburnt HC, NOx, and enhancing the combustion efficiency in a certain type/scale industrial burner. In that way, some geometric modifications (on furnace design and the connected burner) have been applied on the conventional type burners to benefit the effects of preheating of combustion air. Modified geometries have been analyzed numerically and compared with the conventional design’s results. Moreover, the comparison between a linear and non-linear turbulence model has been given in terms of simulation results. Major findings indicate that Burner-1 has significantly lower emissions compared to the others. Preheating effect coupled with the flue gas recirculation (FGR) seems to work well in terms of performance and emissions. Also, a significant difference between linear and nonlinear turbulence model appeared on the emission characteristics for the same simulations.

    Keywords: Computational fluid dynamics, Combustion, Preheating, Emissions, Flue gas recirculation
  • S. Louaifi Hamaili, A. Mataoui*, M. Aksouh Pages 2083-2093

    This paper investigates the influence of a weak-jet on the development of a turbulent axisymmetric strong jet. A parametric study is carried out to evaluate simultaneously the effect to the jets-spacing (S/D=3 and S/D=7.5) and their velocity ratios in range 0≤λ≤1. The mixing phenomenon is studied numerically by the finite volume method using the 3D-RANS second-order model, which gives a good agreement with the available data. Three distinct regions of this type of jets interaction are evidenced. This study confirms that the jets spacing affects strongly the converging region and has a minor effect on the combining region. It is found that the weak jet attracts the strong jet and the combining region extends from 30D to 40D where the self-similarity of a single jet is obtained. The jet width decreases when velocity ratio and jets spacing augment. In the combining region, in comparison with the free jet, it is found that the addition of a weak-jet increases the decay rate of the centerline velocity.

    Keywords: Two unequal Jets, Turbulence, Strong jet, CFD, Self similarity
  • J. A. Rizzo Sierra*, C. Isaza, J. P. Zavala de Paz, E. K. Anaya Rivera Pages 2095-2109

    The performance characterization of a liquid metal magnetohydrodynamic alternate generator is numerically investigated via its electric isotropic efficiency. The model consists of a harmonically driven liquid metal oscillatory flow confined to a thin-walled closed rectangular duct interacting with a uniform magnetic field transverse to its motion and attached to a load resistance. Spectral collocation method is used to solve the properly boundary-conditioned Navier-Stokes equation under inductionless approximation for the magnetic field with implementation of gradient formulation for the electric field. Flow is considered fully developed in the direction perpendicular to the applied uniform magnetic field (i.e., motion direction), incompressible, viscous, and laminar in regime. Currently, there are neither purely analytical or experimental results on this problem, but ours were cross-referenced with those from a one-dimensional analytical model as close as possible to it, finding reasonable correspondence. Dimensional estimates on the power production of prospective mesoscale devices having in mind household application are provided for different liquid metals as well.

    Keywords: : Magnetohydrodynamics, Oscillatory liquid metal flow, Laminar fully developed regime, Inductionless approximation, Navier-Stokes equation, Spectral collocation method, Alternative powergeneration