Jean-Francois Dietiker

Numerical investigation of high Reynolds number flows over square and circular cylinders

Numerical solutions of flows at high Reynolds numbers are investigated by detached-eddy simulation (DES). Two cylinders in crossflow are selected as the test cases; flow around a circular cylinder is simulated at Reynolds numbers of 1.4 × 10 5 and 3.6 × 10 6 and simulation for a square cylinder is performed at a Reynolds number of 2.2 × 10 4 . These simple geometries produce complex flow phenomena such as recirculation, vortex shedding, and unsteady turbulent separation, which are very common flows associated with complex geometries. However, most numerical simulations have been performed at low Reynolds numbers, and only a few are reported at high Reynolds numbers ranges. DES with the Spalart‐Allmaras turbulence model is used for turbulent treatment. It functions as a Reynolds averaged approach in the near-wall region and transfers to large eddy simulation (LES) far from the wall. This procedure requires fewer grid points compared to LES. To assess the quality of solutions, the results are evaluated by comparison with experimental data and other numerical results. In addition, laminar solutions and trip functions are also investigated in the circular cylinder cases. Even though fewer grid points are used, most of the results compare well with experimental data and other numerical solutions.

Numerical simulations of pulsatile non-Newtonian flow in an end-to-side anastomosis model

Abstract A potential interaction between the local hemodynamics and the artery wall response has been suggested for vascular graft failure by intimal hyperplasia (IH). Among the various hemodynamic factors, wall shear has been implicated as the primary factor responsible for the development of IH. In order to explore the role of hemodynamics in the formation of IH in end-to-side anastomosis, computational fluid dynamics is employed. To validate the numerical simulations, comparisons with existing experimental data are performed for both steady and pulsatile flows. Generally, good agreement is observed with the velocity profiles whereas some discrepancies are found in wall shear stress (WSS) distributions. Using the same end-to-side anastomosis geometry, numerical simulations are extended using a femoral artery waveform to identify the possible role of unsteady hemodynamics. In the current simulations, Carreau–Yasuda model is used to account for the non-Newtonian nature of the blood. Computations indicated a disturbed flow field at the artery-graft junction leading to locally elevated shear stresses on the vascular wall. Furthermore, the shear stress distribution followed the same behavior with oscillating magnitude over the entire flow cycle. Thus, distal IH observed in end-to-side artery-graft models may be caused by the fluctuations in WSS’s along the wall.

Validity of Low Magnetic Reynolds Number Formulation of Magnetofluiddynamics

*† ‡ Validity of low magnetic Reynolds number approximation has been evaluated by conducting numerical experimentation with both the full magnetofluiddynamic (MFD) formulation and the low magnetic Reynolds number formulation. MFD equations in their classical form and under low magnetic Reynolds number approximation are presented and numerically solved using four-stage modified Runge-Kutta scheme augmented with the Total Variation Diminishing model in post-processing stage. An attempt has been made to compare the results obtained by the two available approaches. The results obtained from low magnetic number approximation compare well with the results obtained by solving the full MFD equations for low ranges of magnetic Reynolds number.

Numerical simulation of hypersonic MHD applications

The possibility of controlling the flow field structure at hypersonic regime by application of an external magnetic field is investigated in this paper. Two-dimensional, compressible, unsteady magnetohydrodynamic flows of ideal plasma are considered. The numerical method used is a modified four-stage Runge-Kutta scheme, augmented by a total variation diminishing model in a post-process stage at each iteration level. A series of numerical experiments for the forebody 1 propulsion system of a hypersonic aircraft have been conducted by varying the intensity of the magnetic field, the Mach number, and the free stream conditions. In addition, the effect of chemistry has been studied in the case of blunt body flows.

Predicting Wall Pressure Fluctuation over a Backward-Facing Step Using Detached Eddy Simulation

Numerical simulations of a turbulent flow over a backward-facing step are performed. The governing equations are solved by the finite volume code Cobalt. Unsteady three-dimensional detached eddy simulations are carried out with Menters shear stress transport turbulence model acting as a subgrid-scale model. Mean flow quantities such as pressure, velocity, and skin-friction coefficients are accurately predicted. Velocity and pressure fluctuations are resolved by the three-dimensional computations. The dominant frequency is in good agreement with existing experimental data, and power spectral analysis of wall pressure fluctuation is consistent with empirical relations found in the literature for several operating conditions.

Computations of Turbulent Flow over a Backstep

*† Numerical simulations of a turbulent flow over a backward facing step are performed. The governing equations are solved by the finite volume code Cobalt. Fast two-dimensional Reynolds-Averaged and more computationally intensive three-dimensional Detached-Eddy Simulations are carried out, with Menter’s Shear Stress Transport turbulence model. Mean flow quantities such as pressure, velocity and skin friction coefficients are accurately predicted by both techniques. Pressure fluctuations can only be captured by the threedimensional computation. The dominant frequency is in good agreement with existing experimental data, and power spectral analysis is consistent with empirical relations found in the literature. I. Introduction ALL pressure fluctuations beneath a turbulent boundary layer are associated with noise generation and can lead to structural vibrations. It is desirable to understand the nature of pressure fluctuations to limit their impact in several engineering applications such as commercial airplanes and turbines. Turbulent boundary layer measurement has been the subject of numerous investigations, and precious information can be obtained from experiments [1-7], which can be used to validate available semi-empirical models. W With the advent of Computational Fluid Dynamics, numerical tools have been developed to study and understand a wide range of turbulent flow fields. There are mainly three approaches for the computation of turbulent flows. The Direct Numerical Simulation (DNS) approach is an “exact method” in the sense that the original governing equations are solved without any modifications, or filtering process. The second approach for turbulent flow computation is the Large Eddy Simulation (LES) [8]. Large scales are numerically computed, whereas the small scales are modeled by simple eddy viscosity models, known as Sub Grid Scale models (SGS). Algebraic models are sufficient, because the imperfections of these simple models should not greatly affect the solution. The two methods described above are very costly in terms of computational time and storage requirement. A more affordable method consists in averaging the Navier-Stokes equations in time, resulting in the Reynolds Averaged Navier-Stokes (RANS) equations. A turbulence model is required to close the system. Several different turbulence models exist, ranging from simple algebraic models to more sophisticated multi-equation models [9-13]. A hybrid method has been recently developed to take advantage of existing techniques. The Detached Eddy Simulation (DES) combines the RANS approach in regions of thin boundary layer where no separation occur, because this does not constitute a real challenge for RANS and switches to LES in region of massive separation [14]. This method allows a reduction of the prohibitive cost of LES method and therefore, the solution of a turbulent flow field can be obtained within a reasonable computational time.

Computational aspects of high-speed flows with applied magnetic field

High-speed flows over the surface of hypersonic airfoil subjected to several types of applied magnetic field distributions are numerically simulated. The governing equations are composed of the Euler equation modified to include the effect of magnetic field. In the current applications, the low magnetic Reynolds number approximation is utilized and the Hall effect and ion slip have been neglected. A fourth-order modified Runge-Kutta scheme augmented with the Davis-Yee symmetric Total Variation Diminishing model in post-processing stage is used to solve the magnetogasdynamics equations. The flow simulations are compared to the existing solutions. A good agreement between the present analysis and the available normal shock data is demonstrated. It has been found that the location and distribution of the imposed magnetic field have dominant effects on the flow parameters and the shock standoff distance.

Numerical Investigation of Magnetogasdynamic High Speed Flows

High speed flows over leading edge of hypersonic airfoil subject to an applied magnetic field is numerically simulated. The governing equations are composed of the Euler equation modified to include the effect of magnetic field. In the current applications, the low magnetic Reynolds number approximation is utilized. A four-stage modified Runge-Kutta scheme augmented with the Davis-Yee symmetric Total Variation Diminishing model in post-processing stage is used to solve the magnetogasdynamic equations. The flow simulations are compared to the existing solutions.

Numerical Simulation of Magnetohydrodynamic Flows

The development of a versatile computational tool for the solution of turbulent magnetohydrodynamic flows is presented. The flow solver can simulate the full magnetohydrodynamic equations or simplified equations based on the low magnetic Reynolds number approximation. Both laminar and turbulent flows are investigated. The Baldwin‐Lomax turbulence model evaluates the eddy viscosity to represent turbulence. Modifications accounting for the presence of a magnetic field are presented to extend its range of application to magnetohydrodynamic flows. The modification of the turbulence model is performed based on the turbulent Hartmann flow. The numerical solutions are compared with existing analytical solutions and experimental data, for low- and high-speed magnetohydrodynamic flows.

Assessment of Computational Boundary Conditions for Hyperbolic Systems

Specification of bqundary conditions is essential when solving partial differential equations. For Hyperbolic equations, a locally one dimensional approach can be used tit the boundaries to reduce the non physical reflectio& that may occur when the boundary conditions are not well Ispecified. The application of this non- reflecting boundary condition to Eulers equations is presented here for a non uniform grid system. Appending an additional layer to the domain of interest where the equations are modified to absorb, and elimi&e the reflections is another approach. Both methods show substantial improvement over the simple extrapolation of variables at the boundaries