Abstract. The paper deals with the stochastic generation of synthesized turbulence, which may be used for a generating of an inlet boundary condition for unsteady simulations, e.g. Direct Numerical Simulation (DNS) or Large Eddy Simulation (LES). Assumptions for the generated turbulence are isotropy and homogeneity. The described method produces a stochastic turbulent velocity field using the synthesis of a finite sum of random Fourier modes. The calculation of individual Fourier modes is based on known energy spectrum of turbulent flow, and some turbulent quantities, e.g. turbulent kinetic energy and turbulent dissipation rate. A division of wave number range of the energy spectrum determines directly the number of Fourier modes, and has a direct impact on accuracy and speed of this calculation. Therefore, this work will examine the influence of the number of Fourier modes on a conservation of the first and second statistical moments of turbulent velocity components, which are prespecified. It is important to ensure a sufficient size of a computational domain, and a sufficient number of cells for meaningful comparative results. Dimensionless parameters characterizing the resolution and size of the computational domain according to a turbulent length scale will be introduced for this purpose. Subsequently, the sufficient values of this parameters will be shown for individual numbers of Fourier modes.
Abstract. Correct definition of boundary conditions is crucial for the appropriate simulation of a flow. It is a common practice that simulation of sufficiently long upstream entrance section is performed instead of experimental investigation of the actual conditions at the boundary of the examined area, in the case that the measurement is either impossible or extremely demanding. We focused on the case of a benchmark channel with ventilation outlet, which models a regular automotive ventilation system. At first, measurements of air velocity and turbulence intensity were performed at the boundary of the examined area, i.e. in the rectangular channel 272.5 mm upstream the ventilation outlet. Then, the experimentally acquired results were compared with results obtained by numerical simulation of further upstream entrance section defined according to generally approved theoretical suggestions. The comparison showed that despite the simple geometry and general agreement of average axial velocity, certain difference was found in the shape of the velocity profile. The difference was attributed to the simplifications of the numerical model and the isotropic turbulence assumption of the used turbulence model. The appropriate recommendations were stated for the future work.
Abstract. The paper deals with the convergence problem of the SPH (Smoothed Particle Hydrodynamics) meshfree method for the solution of fluid dynamics tasks. In the introductory part, fundamental aspects of meshfree methods, their definition, computational approaches and classification are discussed. In the following part, the methods of local integral representation, where SPH belongs are analyzed and specifically the method RKPM (Reproducing Kernel Particle Method) is described. In the contribution, also the influence of boundary conditions on the SPH approximation consistence is analyzed, which has a direct impact on the convergence of the method. A classical boundary condition in the form of virtual particles does not ensure a sufficient order of consistence near the boundary of the definition domain of the task. This problem is solved by using ghost particles as a boundary condition, which was implemented into the SPH code as part of this work. Further, several numerical aspects linked with the SPH method are described. In the concluding part, results are presented of the application of the SPH method with ghost particles to the 2D shock tube example. Also results of tests of several parameters and modifications of the SPH code are shown.
Abstract. The paper is concerned with the comparison of two inlet boundary approaches of numerical simulations. The first approach is based on the simulation of a sufficiently long duct upstream the area of interest with known volumetric flow rate. The second approach rests on the experimental measurement of the section closer to the area of interest. The experimental measurement provides velocity profile and in addition turbulent intensity compared to the first approach where only a velocity profile can be computed according to known volumetric flow rate. A simplified model of car ventilation outlet was chosen as a test case. The model consists of circular and rectangular duct, area of interest (closing flap, vertical slats and horizontal slats) and outlet box. We have compared the two mentioned inlet boundary approaches for two distinct values of volumetric flow rate for which the experimental data of the section upstream of the area of interest are available. The velocity and the turbulent kinetic energy profiles downstream of the area of interest were chosen as comparative characteristics. OutlineThe development in computer technology still increases the advantage of numerical simulations compared to experimental measurements. The advantage is meant in the sense of time savings. The experimental measurements are still necessary for tuning of numerical models.Computational fluid dynamics (CFD) is a tool for simulation of fluid flow [1]. The simulations can be divided into two categories. The first involves simulations which solve Navier-Stokes equations directly, e.g. Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). Due to enormous computational demands, these methods are still unsuitable for most engineering applications. More feasible is the second category of Reynolds Averaged Navier-Stokes (RANS) simulations which solve timeaveraged Navier-Stokes equations and has to be closed with a turbulence model [2]. The most popular turbulence model for a wide area of applications is k-Ȧ SST model of Menter [3]. MethodologyAs a computation method for the CFD simulations, RANS simulation closed with the k-Ȧ SST turbulence model was chosen. This turbulence model combines the k-Ȧ turbulence model and k-İ turbulence model (where k is turbulent kinetic energy, İ is turbulent dissipation, and Ȧ is specific dissipation rate) such that the k-Ȧ model is used in the inner region of the boundary layer and switches into the k-İ model in the free shear flow region. A special blending function based on rates of turbulent kinetic energy, cell size, specific dissipation and viscosity is used for the switching. In addition, the k-Ȧ turbulence model is suitable mostly for internal flows, while the k-İ turbulence model fits well to external flows [1]. In our case, internal flow occurs in the pipe, whereas free shear flow is present downstream of the ventilation outlet. ResultsIn this section, the test cases as well as the numerical setup are described. Also the identification sign of each case is introduced for the c...
Abstract. This paper presents numerical simulations of the acoustic wave propagation phenomenon modelled via Linearized Euler equations. A meshless method based on collocation of the strong form of the equation system is adopted. Moreover, the Weighted least squares method is used for local approximation of derivatives as well as stabilization technique in a form of spatial filtering. The accuracy and robustness of the method is examined on several benchmark problems.
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