Sharif Digital Repository

Interaction of freestream disturbances with high-speed inviscid flow over a blunt nose is simulated utilizing a shock-fitted spectral collocation method. The unsteady flow computations are made through solving the 2D Euler equations by virtue of such a dissipation-free numerical algorithm for precise unsteady flow simulations. A shock-fitting technique is employed to accurately compute the unsteady shock motions and its interaction with monochromatic freestream disturbances of different conditions. A symmetry condition is proposed to accurately model the both steady and unsteady characters of the symmetry boundary, which allows the use of halved geometries and avoids the extra computational... 

This paper deals with the investigation of the laminar flow past an elliptical cylinder confined in a channel. In this paper, the Lattice Boltzmann (LB) method is used to simulate flow in two dimensions. The present LB method with the used boundary conditions is validated in simulations of the incompressible flow past a circular cylinder. The simulations are carried out in a range of condition, 0≤θ≤90 (angle of incidence), 5≤Re≤100, (Reynolds number) for AR=0.25,0.5 (aspect ratio). The effects of those parameters on the drag and lift coefficients and other flow characteristics of the cylinder are examined in detail. The results demonstrate that the drag and lift coefficients increase with... 

This paper deals with the investigation of the laminar flow past an elliptical cylinder confined in a channel. In this paper, the Lattice Boltzmann (LB) method is used to simulate flow in two dimensions. The present LB method with the used boundary conditions is validated in simulations of the incompressible flow past a circular cylinder. The simulations are carried out in a range of condition, 0 90 (angle of incidence), 5 Re 100, (Reynolds number) for AR=0.25,0.5 (aspect ratio). The effects of those parameters on the drag and lift coefficients and other flow characteristics of the cylinder are examined in detail. The results demonstrate that the drag and lift coefficients increase with the... 

The receptivity of supersonic/hypersonic flows over blunt noses to freestream disturbances is performed by means of spectral collocation methods. The unsteady flow computations are made through solving the full Navier-Stokes equations in 2D. A shock-fitting technique is used to compute unsteady shock motion and its interaction with freestream disturbances accurately in the receptivity study. The computational results for receptivity of a semi-cylinder at Mach 8 is presented and validated by comparison with available theoretical and numerical results. The study shows significant effects of the viscosity on the receptivity process  

A new shock-detecting sensor for properly switching between a second-order and a higher-order filter is developed and assessed. The sensor is designed based on an order analysis. The nonlinear filter with the proposed sensor ensures damping of the high-frequency waves in smooth regions and at the same time removes the Gibbs oscillations around the discontinuities when using high-order compact finite difference schemes. In addition, a suitable scaling is proposed to have dissipation proportional to the shock strength and also to minimize the effects of the second-order filter on the very small scales. Several numerical experiments are carried out and the accuracy of the nonlinear filter with... 

The purpose of this paper is to study and identify suitable outflow boundary conditions for the numerical simulation of viscous supersonic/hypersonic flow over blunt bodies, governed by the compressible Navier-Stokes equations, with an emphasis motivated primarily by the use of spectral methods without any filtering. The subsonic/supersonic composition of the outflow boundary requires a dual boundary treatment for well-posedness. All compatibility relations, modified to undertake the hyperbolic/parabolic behaviour of the governing equations, are used for the supersonic part of the outflow. Regarding the unknown downstream information in the subsonic region, different subsonic outflow... 

A new reduced-order modeling approach is presented. This approach is based on fluid eigenmodes and without using the static correction. The vortex lattice method is used to analyze unsteady flows over two-dimensional airfoils and three-dimensional wings. Eigenanalysis and reduced-order modeling are performed using a conventional method with and without the static correction technique. In addition to the conventional method, eigenanalysis and reduced-order modeling are also performed using the new proposed method, that is, without static correction requirement. Numerical examples are presented to demonstrate the accuracy and computational efficiency of the proposed method. Based on the... 

An analytical procedure for supersonic flutter analysis of two-dimensional panels was developed using unsteady potential flow aerodynamic theory. The local spatial influence was considered as an integral over the plate area. The U-g method was used for flutter prediction. Results indicate a stabilizing effect of the unsteady potential flow aerodynamics theory compared to the quasi-steady first-order piston theory  

An appropriate combination of the thin-layer Navier-Stokes (TLNS) and parabolized Navier-Stokes (PNS) solvers is used to accurately and efficiently compute hypersonic transitional/turbulent flowfields of perfect gas and equilibrium air around blunt-body configurations. The TLNS equations are solved in the nose region to provide the initial data plane needed for the solution of the PNS equations. Then the PNS equations are employed to efficiently compute the flowfield for the afterbody region by using a space marching technique. Both the TLNS and the PNS equations are numerically solved by using the implicit non-iterative finite-difference algorithm of Beam and Warming. A shock fitting... 

Attempts to simulate compressible flows with moderate Mach number to relatively high ones using Lattice Boltzmann Method (LBM) have been made by numerous researchers in the recent decade. The stability of the LBM is a challenging problem in the simulation of compressible flows with different types of embedded discontinuities. The present study proposes an approach for simulation of inviscid flows by a compressible LB model in order to enhance the robustness using a combination of Essentially NonOscillatory (ENO) scheme and Shock-Detecting Sensor (SDS) procedure. A sensor is introduced with adjustable parameters which is active near the discontinuities and affects less on smooth regions. The... 

Several efforts have been performed to improve LBM defects related to its computational performance. In this work, a new algorithm has been introduced to reduce memory consumption. In the past, most LBM developers have not paid enough attention to retain LBM simplicity in their modified version, while it has been one of the main concerns in developing of the present algorithm. Note, there is also a deficiency in our new algorithm. Besides the memory reduction, because of high memory call back from the main memory, some computational efficiency reduction occurs. To overcome this difficulty, an optimization approach has been introduced, which has recovered this efficiency to the original... 

An appropriate combination of the thin-layer Navier-Stokes (TLNS) and parabolized Navier-Stokes (PNS) solvers is used to accurately and efficiently compute hypersonic flowfields of equilibrium air around blunt-body configurations. The TLNS equations are solved in the nose region to provide the initial data plane needed for the solution of the PNS equations. Then the PNS equations are employed to efficiently compute the flowfield for the afterbody region by using a space marching procedure. Both the TLNS and PNS equations are numerically solved by using the efficient implicit non-iterative finite-difference algorithm of Beam and Warming. A shock fitting technique is used in both the TLNS and... 

High-order accurate solutions of parabolized Navier-Stokes (PNS) schemes are used as basic flow models for stability analysis of hypersonic axisymmetric flows over blunt and sharp cones at Mach 8. Both the PNS and the globally iterated PNS (IPNS) schemes are utilized. The IPNS scheme can provide the basic flow field and stability results comparable with those of the thin-layer Navier-Stokes (TLNS) scheme. As a result, using the fourth-order compact IPNS scheme, a high-order accurate basic flow model suitable for stability analysis and transition prediction can be efficiently provided. The numerical solution of the PNS equations is based on an implicit algorithm with a shock fitting procedure... 

An appropriate combination of the thin-layer Navier-Stokes (TLNS) and parabolized Navier-Stokes (PNS) solvers is used to accurately and efficiently compute hypersonic transitional/turbulent flowfields of perfect gas and equilibrium air around blunt-body configurations. The TLNS equations are solved in the nose region to provide the initial data plane needed for the solution of the PNS equations. Then the PNS equations are employed to efficiently compute the flowfield for the afterbody region by using a space marching technique. Both the TLNS and the PNS equations are numerically solved by using the implicit non-iterative finite-difference algorithm of Beam and Warming. A shock fitting... 

The trade-off between the fuel consumption and drag coefficient makes the investigations of drag reduction of utmost importance. In this paper, the rear-end shape optimization of Ahmed body is performed. Before changing the geometry, to identify the suitable simulation method and validate it, the standard Ahmed body is simulated using k − ω shear stress transport (SST) and k-epsilon turbulence models. The slant angle, rear box angle, and rear box length as variables were optimized simultaneously. Optimizations conducted by genetic algorithm (GA) and particle swarm optimization (PSO) methods indicate a 26.3% decrease in the drag coefficient. To ensure the validity of the results, a... 

The trade-off between the fuel consumption and drag coefficient makes the investigations of drag reduction of utmost importance. In this paper, the rear-end shape optimization of Ahmed body is performed. Before changing the geometry, to identify the suitable simulation method and validate it, the standard Ahmed body is simulated using k − ω shear stress transport (SST) and k-epsilon turbulence models. The slant angle, rear box angle, and rear box length as variables were optimized simultaneously. Optimizations conducted by genetic algorithm (GA) and particle swarm optimization (PSO) methods indicate a 26.3% decrease in the drag coefficient. To ensure the validity of the results, a... 

A general three-dimensional aeroelastic solver is developed based on coupled finite element and boundary element methods and applied to investigate the flutter boundaries in supersonic flows. The boundary element method is applied to three-dimensional unsteady supersonic potential flow as the aerodynamic model and coupled with the finite element method for structural modelling, in order to construct the system of aeroelastic equations. The aeroelastic equations are solved for the flutter prediction using the frequency domain approach. Flutter boundaries for two types of wing planforms at supersonic speeds are determined and compared with the existing experimental results and previous... 

The trade-off between the fuel consumption and drag coefficient makes the investigations of drag reduction of utmost importance. In this paper, the rear-end shape optimization of Ahmed body is performed. Before changing the geometry, to identify the suitable simulation method and validate it, the standard Ahmed body is simulated using k − ω shear stress transport (SST) and k-epsilon turbulence models. The slant angle, rear box angle, and rear box length as variables were optimized simultaneously. Optimizations conducted by genetic algorithm (GA) and particle swarm optimization (PSO) methods indicate a 26.3% decrease in the drag coefficient. To ensure the validity of the results, a... 

With the stricter limitations on both fuel consumption and air pollution, the advantages of a hybrid electric vehicle are becoming more evident than ever. In the present study, an energy management system for a hybrid electric vehicle is developed. Because the plant under consideration is nonlinear, multi-domain, time-varying, has multiple uncertainties and, in addition, the designed control strategy must be able to obey the driver's commands and achieve the par-internship for a new generation of vehicle regulations, the fuzzy logic approach is chosen. A feed-forward hybrid vehicle simulation model is used to demonstrate the validity and the convenience of the current approach and its...