A direct Navier-Stockes solver for turbulent flows over round steps

Direct Numerical Simulations (DNS) provide an increasing data source to improve our understanding of turbulent flows. Such DNS are especially required (together with generic experiments) to support flow physical modeling od separated, two- and three-dimensional vortex flows. In perspective to improve the prediction of such flows (either by new turbulence models or Large Eddy Simulations (LES) data fields are required which resolve turbulence and unsteady separated flow features. This is still a challenging task. However, a fully conservative, second order accurate, boundary correction method for Cartesian grids has been developed and implemented in an existing direct Navier-Stokes (NS) solver. For the case of a round backward facing channel step, laminar flow results generated with the proposed method agree well with those calculated on curvilinear, colocated grids. A Direct Numerical Simulation (DNS) of the turbulent flow in a minimal channel domain is performed for a Reynolds number based on friction velocity and channel height of ReT=265 for an efficiency analysis. Additionally the turbulent flows in a channel and a round backward facing channel step are calculated by means of DNS. The former simulation provides inflow conditions for the latter. The upstream Reynolds is ReT=360. Good agreement is obtained comparing statistically averaged variables of the turbulent channel flow with those of Kim et al. [3]. A mean recirculation zone, with mean separation/reattachment points being located 2.0/7.5 step heights downstream the step entrance, is computed in the DNS through the round backward facing channel step. Instantaneous and statistical flow variables are presented. They give an impression of the complex flow dynamics in the free shear layer, the recirculation and reattachment region

Nonlinear Nonlocal Instability Analyses for DLR Test Case within GARTEUR AG27 -Transition on Airfoils and Infinite Swept Wings with Regard to Nonlocal Instability Investigation-

This report is part of the final report of GARTEUR Action Group (AG) 27 'Transition on airfoils and infinite swept wings with regard to nonlocal instability investigation'. The partners in this Action Group were BAe, DASA, DERA, FFA, ONERA and DLR. Four different test cases have been studied by linear and nonlinear nonlocal instability theory. This report summarizes the results for the DLR swept flat plate test case onl

The extended Görtler-Hämmerlin model for linear instability in the three-dimensional incompressible swept attachment line boundary layer

A simple extension of the classic Görtler-Hämmerlin (1955) (GH) model, essential for three-dimensional linear instability analysis, is presented. The extended Görtler-Hämmerlin model classifies all three-dimensional disturbances in this flow by means of symmetric and antisymmetric polynomials of the chordwise coordinate. It results in one-dimensional linear eigenvalue problems, a temporal or spatial solution of which, presented herein, is demonstrated to recover results otherwise only accessible to the temporal or spatial partial-derivative eigenvalue problem (the former also solved here) or to spatial direct numerical simulation (DNS). From a numerical point of view, the significance of the extended GH model is that it delivers the three-dimensional linear instability characteristics of this flow, discovered by solution of the partial-derivative eigenvalue problem by Lin & Malik (1996a), at a negligible fraction of the computing effort required by either of the aforementioned alternative numerical methodologies. More significant, however, is the physical insight which the model offers into the stability of this technologically interesting flow. On the one hand, the dependence of three-dimensional linear disturbances on the chordwise spatial direction is unravelled analytically. On the other hand, numerical results obtained demonstrate that all linear three-dimensional instability modes possess the same (scaled) dependence on the wallnormal coordinate, that of the well-known GH mode. The latter result may explain why the three-dimensional linear modes have not been detected in past experiments; criteria for experimental identification of three-dimensional disturbances are discussed. Asymptotic analysis based on a multiple-scales method confirms the results of the extended GH model and provides an alternative algorithm for the recovery of three-dimensional linear instability characteristics, also based on solution of onedimensional eigenvalue problems. Finally, the polynomial structure of individual three-dimensional extended GH eigenmodes is demonstrated using three-dimensional DNS, performed here under linear conditions

Practicability of investigating artifically forced instabilities in planar laminar boundary layers at Mach 5 in the Ludwieg-tube Goettingen (RWG)

SIGLEAvailable from TIB Hannover: RR 6926(98A18) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Experimental investigation of artificially generated disturbances in laminar hypersonic boundary layers

Experiments to study artificially generated disturbances in flat plate boundary layers were conducted in the Ludwieg-tube wind tunnel of the DLR in Goettingen at a free-stream Mach number of M_1=5. For the investigations, a 500 mm long and 20 mm thick flat plate model was used which spanned the test section 400 mm in width. As disturbance source a spark device flush mounted to the model surface close to its leading edge was utilized. The hot wire measurements in the boundary layer of the flat plate were conducted at different streamwise positions for natural and forced disturbances. The charge conditions used were almost constant at p_c_h_a_r_g_e=5 bar and T_c_h_a_r_g_e=393 K. This resulted in a unit Reynolds number of Re_1=8.45 x 10"6 1/m and a mass flow rate of #rho#_1 u_1=43.5 kg/m"2s. To check the flow quality of the tunnel and the laminar characteristic of the boundary layer optically, Schlieren pictures and high-speed shadowgraphs were taken prior to each measurement campaign. The artificially inserted wave packets were successfully measured at different distances and spanwise locations downstream of the park source. The main wave parameters, e.g. wave numbers and wave inclination angle were deduced from the obtained results. (orig.)SIGLEAvailable from TIB Hannover: RR 6926(98A29) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman