Sharif Digital Repository

In this work, the spectral difference lattice Boltzmann method (SDLBM) is extended and applied for accurately computing two-dimensional viscous compressible flows on structured meshes. Here, the compressible form of the discrete Boltzmann-BGK equation with the Watari model is considered and the numerical solution of the resulting LB equation is obtained by using the spectral difference method. The main benefit of the use of the LB method in simulating compressible flows is that a same formulation can be applied to compute the inviscid and viscous portions of the flowfield. Note that the LB formulation for simulating the viscous flows is the same as that used for the inviscid ones, however,... 

In this study, a high-order accurate numerical method is applied and examined for the simulation of the inviscid/viscous cavitating flows by solving the preconditioned multiphase Euler/Navier-Stokes equations on triangle elements. The formulation used here is based on the homogeneous equilibrium model considering the continuity and momentum equations together with the transport equation for the vapor phase with applying appropriate mass transfer terms for calculating the evaporation/condensation of the liquid/vapor phase. The spatial derivative terms in the resulting system of equations are discretized by the nodal discontinuous Galerkin method (NDGM) and an implicit dual-time stepping... 

In this work, a high-order nodal discontinuous Galerkin method (NDGM) is developed and assessed for the simulation of 2D incompressible flows on triangle elements. The governing equations are the 2D incompressible Navier–Stokes equations with the artificial compressibility method. The discretization of the spatial derivatives in the resulting system of equations is made by the NDGM and the time integration is performed by applying the implicit dual-time stepping method. Three numerical fluxes, namely, the local Lax–Friedrich, Roe and AUSM+-up are formulated and applied to assess and compare their accuracy and performance in the simulation of incompressible flows using the NDGM. Several... 

In this study, a numerical model of unsteady potential flow around submerged marine propellers has been developed. The boundary element approach in combination with time stepping method to model free wake dynamics has been implemented. An important feature of this method in the simulation of pressure-dominant problems is a proper balance between time and accuracy in the numerical process. Another advantage of time stepping method is that there is no need to define wake geometry before modeling. Due to inherent instability of boundary integral equations, a smoothing function to damp the effect of singularities is imposed to the solution. The main innovative idea of this work is that the... 

The unsteady preconditioned characteristic boundary conditions (UPCBCs) based on the artificial compressibility (AC) method are formulated and applied at artificial boundaries for the direct numerical simulation (DNS) of incompressible flows. The compatibility equations including the unsteady terms are mathematically derived in the generalized curvilinear coordinates and then incorporated as boundary conditions (BCs) in a high-order accurate incompressible flowsolver. The spatial derivative terms of the systemof equations are discretized using the fourth-order compact finite difference (FD) scheme, consistent with the high-order accuracy required for the DNS. The time integration is carried... 

A high-order compact finite-difference scheme is applied and assessed for the numerical simulation of structural dynamics. The two-dimensional elastic stress-strain equations are considered in the generalized curvilinear coordinates and the spatial derivatives in the resulting equations are discretized by a fourth-order compact finite-difference scheme. For the time integration, an implicit second-order dual time-stepping method is utilized in which a fourth-order Runge-Kutta scheme is used to integrate in the pseudo-time level. The accuracy and robustness of the solution procedure proposed are investigated through simulating different two-dimensional benchmark test cases in structural... 

Heave and pitch motion of an oscillating airfoil in uniform flow will cause generation of forwarding thrust. Applying a combination of these two motions on flexible foil, one can increase thrust and therefore the efficiency. This is the way that most fishes and other flying animals uses to consume less energy. In this paper, hydrodynamic forces and efficiency of an oscillating airfoil is investigated. A code is developed based on potential flow formulation in combination with Time Stepping Method (TSM) with nonlinear free shear layer dynamic approach to predict the wake behind the lifting bodies. A linear Morino type Kutta condition has been implemented on panels adjacent to trailing edge.... 

Flow movement in unsaturated soil can be expressed by a partial differential equation, named Richards equation. The objective of this study is the finding of an appropriate implicit numerical solution for head based Richards equation. Some of the well known finite difference schemes (fully implicit, Crank Nicolson and Runge-Kutta) have been utilized in this study. In addition, the effects of different approximations of moisture capacity function, convergence criteria and time stepping methods were evaluated. Two different infiltration problems were solved to investigate the performance of different schemes. These problems include of vertical water flow in a wet and very dry soils. The... 

The objective of this study is to develop and apply an arbitrary Lagrangian-Eulerian unstructured finite-volume lattice-Boltzmann method (ALE-FVLBM) for solving two-dimensional compressible inviscid flows around moving bodies. The two-dimensional compressible form of the LB equation is considered and the resulting LB equation is formulated in the ALE framework on an unstructured body-fitted mesh to correctly model the body shape and properly incorporate the mesh movement due to the body motion. The spatial discretization of the resulting system of equations is performed by a second-order cell-centered finite-volume method on arbitrary quadrilateral meshes and an implicit dual-time stepping...