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This research delves into the practical implications of the effect of exhaust gases on the wake region of an Audi car, which includes vortices and separated flowing from its surface. The findings could potentially inform future vehicle design and performance strategies. The car model's rear configuration is akin to the standard DrivAer Estateback car, which the Aerodynamics Institute of the Technical University of Munich has modeled. The SST k-ω turbulence model is selected for simulation using computational fluid dynamics, considering the problem's nature and the flow stability analysis. To validate the solution method, the numerical results obtained from flow simulation at a Reynolds number of 4.87×106 around the standard model are compared with the experimental results obtained from the wind tunnel. After validating the solution method, it was determined in the initial investigation that at the given Reynolds number, the drag coefficient of the Audi car with an exhaust system is 1.15 % lower than the drag coefficient of the car without an exhaust system. Furthermore, the effects of increasing the vehicle speed, the angle of the exhaust pipe outlet, and finally, the presence of an exhaust on one side of it on the value of the drag coefficient are examined.

In this paper, three-dimensional code has been developed to simulate the partial cavitating flow around projectiles with various heads (blunt, hemispherical, and conical) using the BEM. For this purpose, after generating the geometry using quadrilateral elements with four control points, using the integral expression of Green's theory, source and dipole have been distributed on elements, and using an iterative algorithm, simulation is done and results are compared with the available experimental data and other numerical results. Despite the low computational cost of this method, the results have a high accuracy and convergence rate. One of the main contributions of this work is to present a correlation between the properties of cavity around projectiles with different heads (in the limit of 0.075≤σ≤0.5). Also, Analysis of the results shows that the method has a suitable ability to predict the properties of cavitation flow at non-zero angles of attack (up to 8° angle of attack) in the shortest time. Of course, by increasing the angle of attack and getting away from the potential assumption, the results are associated with some errors (15% in geometrical characteristics and 12% in aerodynamic coefficients). Due to high convergence rate and acceptable accuracy, this method can be used for initial design and optimization of subsurface projectiles with cavitation.

Plasma actuator is an active flow control tool, which has been evaluated by the aerodynamic researchers since last decade due to its simple structure, light weight, low energy consumption, and high time response. In this paper effects of plasma on aerodynamic behavior of a rocket at different flight conditions is numerically investigated. Results of plasma effects or variation of attack angle, Mach number, and flight altitude on the drag and lift coefficients are evaluated. Applying plasma increases the vertical velocity under the rocket canards which leads to higher pressures and therefore higher pressure difference and forces are applied on the rocket canards which improves their functionality. Drag and lift coefficients are both increased due to the plasma, but the aerodynamic efficiency (lift to drag ratio) is increased by increasing potential difference. Results shows that plasma effect is reduced with increasing of angle of attack and increased with the flight altitude and aerodynamic efficiency is changed between %3 and %60 by applying plasma.

In recent years, the delta wing configuration has been widely used in subsonic drones in addition to supersonic aircrafts such as fighters. On the other hand, due to the increasing development of acoustic detection systems, it is necessary to investigate the noise of drones. In this article, the behavior of surface pressure fluctuations as a source of noise generation on a delta wing with a 70-degree sweep angle has been investigated experimentally. The obtained results show that at angles of attack above 5 degrees, there was a significant increase in the surface power spectral density values, which could be caused by the formation of leading-edge vertices at the end of the wing. Also, by moving in the direction of the flow, while transferring energy from high frequencies to lower frequencies, the peak of the surface pressure spectrum occurs at lower frequencies. Finally, the obtained results show that at an angle of attack of 30 degrees, there was a significant increase in the surface power spectral density values in the position close to the trailing edge of the wing, which indicates the occurrence of the vortex bursting phenomenon at that position.

Conventional Wrap-Around Fins (WAF) have unique aerodynamic characteristics, such as changing the roll moment direction with increasing Mach number and the nonlinearity of the aerodynamic coefficients. The placement of Wrap-Around Fins against each other eliminates their aberrant and non-linear aerodynamic characteristics and provides the condition with a less complicated guidance system. In this research, the effects of curvature of Opposite Wrap-Around Fins (OWAF) on the aerodynamic performance of the projectile has been investigated. This investigation has been done numerically, and the results were compared with the wind-tunnel accessible test data, which have good accuracy. The simulation has been carried out at a Mach number of 1.5 and an angle of attack of  0 and 5 degrees, assuming a turbulent compressible flow; the physics of the flow around the fins has been investigated. The results showed that increasing fins curvature increases lift and drag coefficients by 7% and pitch moment up to 20%. Also, the fin curvature eliminate flow separation in junction of fin leading edge and body.

In the transportation industry, the vehicles aerodynamics is important because it affects on fuel consumption and stability of cars. In this numerical research by using Ansys Fluent software, the aerodynamic performance of the Iranian car Tara was investigated by creating an air duct with different dimensions and geometries in the modification of front aerodynamics and the modification of rear aerodynamics. The purpose of these changes was to establish air flow between areas of the body that have a high pressure difference, which in both aerodynamic developments reduced drag force and lift force. Also, the effect of creating side curvature on the body was also studied. The results showed that design and direction of the air flow from the sides to the back of the car is very important in aerodynamics forces, however, drag force was reduced in all cases. Finally, the best methods were combined simultaneously on the car. In the most optimal model, through the combination of the the front aerodynamic modifications and rear aerodynamic modifications the drag coefficient and lift coefficient reduced 16.3% and 4.1% respectively as the drag coefficients reduced from 0.332 to 0.278.

In the present research, the experimental and numerical investigation of the starting process of the second throat diffuser with a parabolic nozzle containing expansion ratio of 35 has been conducted at a relatively low total pressure. This investigation uses an experimental setup known as a small-scale high-altitude test facility with compressed air as the working fluid. Using numerical simulation, the physical phenomena occurring in each stage of vacuum generation have been identified and analyzed. The results show that the process of vacuum generation in the high-altitude simulator of the second throat type with the presence of Thrust Optimum Parabolic (TOP) nozzles included four stages. In the first stage, the vacuum was gradually created by using the nozzle in the free shock separation (FSS). In the second stage, which started as soon as the transition from the FSS pattern to restricted shock separation (RSS), the vacuum generation was slow and accompanied by oscillation. Vacuum generation was gradual in the third stage, which corresponds to the beginning of the shock separation with recirculation(SSR) and continues until the end of regular reflection (RR). And finally, the last stage also coincides with the structure of expanded, under-expanded conditions, the impact of the jet exiting the nozzle with the diffuser wall, and the establishment of start-up conditions, creating a vacuum at a slower rate than in other stages.

Underwater vehicles in industrial applications, have various control surfaces for stability, maneuverability, gudance and control. These control surfaces affect the body hydrodynamic characteristics, including the resistance forces and the form of the generated wave due to the motion of the vehicle near the free surface. In this paper the effect of a vertical control surface on the hydrodynamic characteristics of an underwater vehicle near free surface is studied using Boundary Element Method.Results, including characteristics of free surface wave, pressure coefficient and wave resistance are calculated for Froude numbers in the range of 0.1-0.5 and non dimensional submergence depths of 1.25, 2.25, 3.5 & 4.5 for the bare model and model with control surface. Comparing BEM and experimental results shows the discrepancy of less than 3% for maximum free surface wave height and 17% for wave resistance. Predicted results indicates that control surface causes an increase of about 13% in maximum free surface wave height and 16% in wave resistance. This increment is due to the interaction of the main body with control surface and also direct relation of wave resistance with free surface wave height.

Complex flow separation in thrust optimized parabolic nozzles in the over-expanded condition is one of the challenging issues of many numerical investigations. The correct estimation of a thrust optimized parabolic nozzle performance extremely depends upon the accurate estimation of the onset of flow separation. Literature review indicates that conventional Reynolds-averaged Navier–Stokes turbulence models have a significant error in predicting the onset of flow separation in these types of nozzles due to the overestimating of turbulent kinetic energy production. Recently proposed generalized k-omega has made it possible to rectify numerical simulations based on governing physics and using limited experimental results. In the present study, the flow physics in the LEA_TOC nozzle has been investigated with the numerical simulation approach. At the first, the significant error of conventional Reynolds-averaged Navier–Stokes turbulence models is shown to simulate flow separation in this type of problem. Then, the generalized k-omega parameters are modified based on the limited experimental result of the LEA_TOC nozzle, and the ability of this model has been evaluated to estimate the flow physics under different pressure ratios. Numerical investigations show that generalized k-omega has a high capability for accurately estimating the onset of flow separation at a wide range of nozzle pressure ratios. Applying the corrected generalized k-omega has resulted in an improvement of about 30% in the estimation of the onset of separation in the over-expanded LEA_TOC nozzle compared to the k-ω-SST model.

In hydrodynamic applications, accurately predicting fluid flows with cavitation is very important. In this regard, prediction of the cavity dimensions and the pressure distribution and the flow dynamics, inside and around the cavity, specifically at the closing point has frequently been under consideration. In this study, cavitating flow around cylindrical projectiles with flat or hemispherical heads is considered numerically. To this end, four turbulence models of k-ε-Realizeable, k-ω Standard, k-ω SST, and GEKO, in combination with the Zwart cavitation model are considered using the Fluent software. Flows with a vast range of cavitation numbers (0.1-1.8) are considered in comparison with the experimental and numerical results of other researchers. Our results show that the last turbulence model proposed by Menter, namely the GEnelarilized-KOmega )GEKO( model, in which two extra equations are solved, predicts the results much better, particularly for higher cavitation numbers.

In this paper the research has been performed on the cross-axis flow Lucid spherical turbine. Three-dimensional steady numerical simulation is used to determine power output and performance of this kind of turbines for low velocities in a channel. The k-ω SST turbulence model is used to perform the turbulent steady flows around the turbine. Further, Bachant and Wosnik's experimental data reported for spherical Lucid turbine, is employed to confirm the simulations. The influence of three effective parameters of blades, including the chord length, the number of blades and the type of airfoil section on the turbine performance are investigated over a range of tip-speed-ratios. It was found that increasing the blade chord length in lower speed tip ratios cause to increase the power coefficient up to %15 compared to the original version. Also, for the turbine with three blades, the best results were obtained by which, the power coefficient increased by 12.5%. Finally, it is obtained that using asymmetric airfoil section for blades has a positive effect on the turbine performance.

Improving the aerodynamic performance of vehicles due to the impact on fuel consumption, stability and controllability of vehicles has always been on the attention of engineers. In this study, in order to obtain a brief view of the effectiveness of methods to improve the aerodynamic performance of vehicles, a review of research conducted to reduce aerodynamic forces using the passive flow control method, which can be effective in consideration, implementation and selection of the proper method. The results show the high importance of car body design in reducing drag and lift such that implementing the channel and curvature on the sides of the car to transfer air flow from high pressure areas in front of the body to low pressure areas behind the body has the highest value of reduction in drag force. Also, using a spoiler at the end of the body to delay separation and to direct airflow upwards reduces the lifting force. The effect of these methods on reducing aerodynamic forces is more than other mechanisms, but the exact extent of the effectiveness of each method depends on the type of vehicle and the way of its implementation on the body.

In the present study, the effect of pre-evacuation on starting process of a second throat exhaust diffuser has been investigated experimentally by examining a thrust optimized parabolic nozzle. An experimental setup called high-altitude test facility has been used with compressed air as operating fluid. According to the importance of area ratio parameter (Ad/Ast) of a second throat diffuser, the effect of this parameter variation has been examined on the start-up performance of the nozzle and diffuser. In all of the diffuser geometries, to evaluate the instantaneous performances, the pressure in the nozzle chamber has charged instantly in two modes; with and without pre-evacuation. then, the vacuum chamber pressure and static pressure distribution along the diffuser are measured by a data acquisition system. The results show that pre-evacuation in the test chamber reduces the start-up time of the diffuser by 50 to 60%. In addition, pre-evacuating the test chamber eliminates the destructive transition phenomenon from the flow separation pattern during start-up of the nozzle and diffuser. Also, It is observed that with the narrowing of the diffuser’s second throat duct, the minimum starting pressure of the diffuser increases and eventually flow chocks at the second throat in a certain area ratio.

Investigation of the hydrodynamic parameters of the moving objects under the free surface in order to study the effects of velocity, curvature, thickness and angle of attack of hydrofoils has been initiated since many years. The computational fluid dynamics software, FLUENT, provides the solution of the governing equations with VOF method for two phases flows. The RSM turbulence model is used in this article which is more accurate generally. In the present study, the effects of free surface and finite depth on the hydrofoils characteristics are investigated simultaneously and compared with those derived from the BEM method and validated with the existing and experimental values. The results show that for the hydrofoils in the more depth, the lift and drag coefficients and lift to drag fraction increases. Furthermore, in shallow water flows, the hydrofoils with higher camber and lower angle of attack have better performance and the downstream waves damp earlier.

In this paper, the effect of different kind of constraints on two and three dimensional adjoint-based shape optimization of aerodynamic design in inviscid and turbulent flows has been investigated. To assess the accuracy and efficiency of the adjoint-based approach, a complete set of optimization problem is investigated: from none to fully aerodynamic and geometrical constraints. With the adjoint method, the complete gradient information needed in the design optimization can be obtained by solving the governing flow equations and the corresponding adjoint equations only once for each cost function, regardless of the number of design parameters. Drag forces was chosen to be the objective function. Hicks–Henne shape functions and free form deformation parameters are adopted for the surface geometry perturbations, for two and three dimensional geometries, respectivelyThe drag minimization results show that the adjoint equation converges well and with specifying the suitable constraints, the designed shape approaches to the most optimized level without the loss of performance.

In this paper, the preconditioned nonlinear multigrid algorithm is used with aim of improving convergence rates,reducing CPUtime for solving compressible flow equations. MultiGrid (MG) methods are a group of algorithms that are used for acceleratingconvergence of differential equations using a series of discretizations. This method is based upon the principle that explains whenthe global (low-frequency) error of a fine mesh is represented on a coarse mesh, it retains the same characteristics as of the local(high-frequency) error, thus the same method for removing high frequency errors is also applicable in such situation. When thenonlinear multigrid algorithm drives the external iterations, the choice of smoother on each grid level plays an important role in theperformance. In this research, a preconditioning method has been used to reduce instabilities of the linear system that arise in eachiteration. Some important results of this research are: choosing the best preconditioning scheme to be coupled with MG method,assessment of MG method performance in equations of turbulence, laminar,inviscid flows,analysis of different cyclingschemes.

Nowadays, with regard to the new methods of simulation, the designof airfoils have greatly changed and this allows the evaluation of the effect of various factors on the efficacy of airfoils. Thus, choosing the type of wing would be so important; in another term, designing a proper wing is what an airplane designer is mainly concerned with. In this project, the effect of airfoil wave surface on the changes of coefficients of lift and drag are studied. Graphs of lift and drag coefficients, Cantor Distribution Pressure and Cantor Distribution speed are alsoinspected in different attack angles. The achieved results indicate that wavy airfoils atspecific attack angle and also near the angle of extinction have a better performance than airfoils without wavy surface. The results of airfoil performance in different attack angles shown throughgraphs indicate the proper performance of wavy wing.

In this study k-e-v2 turbulence model has been considered and modified for pipe flows in accelerating condition. In order to determine the characteristics of the fluid under the non-periodic accelerating conditions and using water as the working fluid, k-e-v2 model has been under taken. This model imposes the wall effects into the flow field properly. However, most of turbulence models cannot predict this phenomenon appropriately for unsteady flows. Although, k-e-v2 model is the best among the same turbulence models, however, still it needs some modifications. First, the weight coefficient of various parameters effects available in this model have been considered thoroughly. Then the most effective parameter has been obtained. It has been found that the parameter and its differential equation must be modified to correlate the experimental data desirably. Considering the point that v2 has the same dimension of k-equation, its effect is imposed into this equation. Further, this modification has been examined for internal pipe flows with different periods of accelerations. It was found that the results of some characteristics of flow such as turbulence kinetic energy and wall shear stress for different accelerations of flow are improved desirably compared to the experimental data.

The growing and diverse applications of low Reynolds number operating vehicles impose the need for their accurate study. Optimization is an important part of computational science that can improve the performance and increase the efficiency of the initial geometry. most of the research studies on aerodynamic optimization were focused on high Reynolds number airfoils. But for aerodynamic devices that have small dimensions, like MAVs, usually the flow speed is low and thus the unsteady effects caused by boundary layer separation cannot be neglected. In this article, oscillating airfoil with pitching motion in turbulent and low Reynolds flow has been optimized with the continuous adjoint method. Drag to lift ratio was chosen to be the objective function and free form deformation parameters is adopted for the surface geometry perturbations. Since aerodynamic optimization generally consists of two parts, first solving the flow equation and then computing the gradient of the objective function, in this article in order to evaluate the accuracy of the optimization process both has been validated. The results show that the adjoint equation converges well and with specifying the suitable constraints, the designed shape approaches to the most optimized level without the loss of performance.

In this paper, the full two-dimensional Navier–Stokes equations in the external turbulent flow were numerically modeled. Thus, in order to calculate the fluxes, the Ausm method was utilized and for modeling turbulent flows, the single-equation Spalart-Allmaras model was used. The employed Spalart-Allmaras model is a modification of this turbulence model, which prevents the formation of negative dynamic viscosity, a point that has gone generally unnoticed. The suggested modification was conducted on the distributing and the source terms in the single equation of turbulence model. For validation and assessment of accuracy and stability of the turbulence model and the calculation method of fluxes, the totally turbulent flow with Reynolds number of 9106 and Mach number of 0.799 and angle of attack 2.26 and also flow with Reynolds numbers of 6106 and and Mach number of 0.15 and angles of attack zero,10 around NACA 0012 airfoil are investigate. The results of this study show that computational framework designed have a good agreement with experimental data .furthermore the number of iterations required for convergence is reduced.

In this article, cavitation flow around axisymmetric projectiles with ringed and non-ringed cavitator has been investigated using control volume and boundary element methods. In the numerical method, the homogeneous equilibrium approach as well as the zwart model, for modeling the mass transfer and forming the system of equation, have been used. In the boundary element approach with dipole distribution on the body and cavity surfaces and source distribution on the cavity surface, the right conditions were set for using the Green's theorem in solving the potential flow. Moreover, some source components were imposed on the cavitator surface in order to add the hole effects. The validation procedure for both methods has been done by analytical and experimental data. In general, the results of this research are presented in two parts. In the first part, hydrodynamic properties of ringed cavitator such as cavity dimensions, intended forces, flow behavior and etc are analysed deploying the numerical methods based on Navier Stokes equations. In the second part, the boundary element method has been used for the analysis of the cavitation flow around practical geometries with ringed cavitator. The most important finding of this study is reduction of the cavity dimensions and also an increase in the force on the projectile during the use of annular cavitator. In addition, as a result of this study, two equations for maximum length and maximum diameter of the formed cavity on the cylindrical body in relation to the cavitation number and hole diameter have been provided.

One of the most important issues of applied Hydrodynamics is Analysis of Moving hydrofoil near the free surface. In this paper attention is being paid to the analysis of hydrofoil near the free surface. For this simulation, an iterative method based on Green’s theorem is employed, and the problem is divided to hydrofoil and free surface and the effects on each other is calculated, and then perturbation potential on hydrofoil and free surface are acquired. Next, the values of these potentials are modified with an iterative algorithm until the results converge to real values. Then by having these potentials, Pressure distribution on hydrofoil surface and curve wave on free surface are obtained. Having validated this method, various factors on the hydrofoil performance such as thickness, camber, angle of attack, the Froud number, distance from the free surface, and distance from depth are surveyed. It can be observed that the results of boundary element method with good approximation predict the flow performance. However, the existence of an ideal fluid is assumed.

In this paper simulation of steady super cavitation phenomenon Çhas been considered by using partial non-linear model of Boundary Element Method(BEM).The grid mesh used is fixed and the strength of dipole and source are constant on each element. With the assumption of a partial non-linear model the cavity condition is applied on the body with the assumption that cavity height is low. Thus there is not any calculation on the cavity surface, but it is restricted to only the panels on the body surface. Cavitation number is known at first and the cavity length is determined in every iteration. When the lengths obtained in two successive iterations are very close to each other it assumed to be the answer. Based on this method two Kutta conditions including Morino condition and Iterative Pressure Kutta Condition(IPKC) are studied to satisfy the wake surface condition. The application is a wing with NACA16006 section. Iterative pressure Kutta condition compared to Morino condition needs higher computational costs, but on the other hand leads to more accurate results. It has been shown that the simulation of the flow with super cavitation over wing leads to a pressure difference at the trailing edge of each strip if we use Morino’s Kutta condition. While if Iterative Pressure Kutta Condition is usedthe results are satisfactory. Comparing the results show that this method leads to very accurate predictions for the behavior of flows with cavitation, while significantly lower computational cost is required if we use the simple cavity closure condition.

In recent years, achievement more accurate numerical method in different flow regimes for the discontinuities with less oscillation and numerical errors is the interest of many researchers. What distinguishes this paper is, comparison of the performance of artificial dissipation and upwind methods, in solving the Euler equations for internal compressible flows in a wide range of inlet Mach numbers. In this study we examined the ability of three AUSM upwind method, Scalar and Cusp artificial dissipation methodes from very low Mach up to ultrasound and non-viscous flow in a convergent-divergent nozzle. The ability of AUSM method and Scalar methods in two-dimensional inviscid sonic flow between turbine stator blades at both supersonic and subsonic outlet is also investigated. In this study an excellent performance for AUSM method with more convergence speed and low numerical error in all flow regimes at a converging-diverging nozzle is observed. In the second example, in both flow AUSM method presents the results based on experimental, with low numerical errors and satisfy the mass conservation better than Scalar. It should be mentioned that the AUSM method is highly recommended for higher Mach numbers.

The flight dynamics of nine configurations of supersonic continuous deflectable nose guided missiles have been investigated. The studied configurations consist of a spherical nose tip, a tangent ogive, a set of stabilizing tail fins and a cylindrical body that its mid-section is flexible to form an arc of a circle. So the cylindrical body consists of a fix part in vicinity of nose, middle flexible part and main body with stabilizers. The effects of fix length and flexible length parameters on the flight dynamics of surface to surface, antiaircraft and antimissile missiles have been studied. A code has been developed to solve full Navier-Stokes equations using finite volume and modified Baldwin-Lomax turbulence model. Further, a 3 degree of freedom code has been developed to compare planar flight dynamics of missiles. This code consists of a guidance subroutine based on pure persuit law. The results show that even increase of fixed and flexible lengths enhance the maneuverability of the missile, but in some scenarios this can lead to increased flight time and more errors in the target engagement. Deflected nose relocates mass center away from the axis and a thrust vector torque is created. Study of surface to surface scenario shows that this torque improves accuracy of targeting and the ability of target dislocation. In air defense missiles, increase of Fix and Flex variables, will extend the limits of allowable firing angle. However, a heavy nose increases the role of thrust torque and subsequently decreases the role of nose geometry.