Günter Schewe

Reynolds-number effects in flow around more-or-less bluff bodies

This paper reviews the results of experiments carried out over a wide range of Reynolds numbers (104<Re<107) on 2-D sections in the high-pressure wind tunnel in Gottingen. The bodies are a circular cylinder, a sharp-edged, trapezoidal-shaped bridge element, and a thick airfoil at a high attack angle. In all cases, appropriately shifting the value of Re led to dramatic changes in the force coefficients and St. number. Changes in the topological structure of the separated flow cause the observed phenomena: oil-flow pictures were used to make the phenomena visible. These events, which are triggered by the laminar/turbulent transition and its location, have characteristics that appear to be universally valid because they occur in similar form in flow over the three different bodies. The important results are: (i) flow around 2-D bodies can be highly three-dimensional; (ii) there are 2-D sections that behave as a bluff- or streamlined body, depending on Re; and (iii) slender bodies with sharp-edged cross-sections may also suffer pronounced Reynolds-number effects.

Nonlinear effects in transonic flutter with emphasis on manifestations of limit cycle oscillations

This paper presents flutter and forced oscillation experiments in a transonic wind tunnel. For an aeroelastic supercritical 2-D airfoil configuration we studied typical transonic phenomena in as pure a form as possible. Various manifestations of small-amplitude limit cycle oscillations were observed for different flow conditions as well as coexisting limit cycles. We demonstrated how very small control forces were sufficient to excite or suppress flutter oscillations. Limit cycle oscillations occurred under free and forced turbulent boundary layer transition in a perforated wall test-section. Flutter calculations based on experimental aerodynamic forces yield stability limits which show good agreement with directly measured experimental flutter values. The results indicate that flow separation at the trailing edge, and the interactions between the shock and the marginal region of separated flow beneath it, may be responsible for limiting the amplitude of the observed limit cycle oscillations.

Reynolds number effects in the flow around a bluff bridge deck cross section

A commom assumption in wind engineering is that the flow around bluff and sharp edged bodies is independent of Reynolds number, Re. An important practical implication of this assumption is that aerodynamic parameters such as Strouhal number, lift and drag coefficients may be obtained from low speed wind tunnel model tests and applied directly to the prototype structure.

Three-Dimensional Numerical Simulation of Flow Around a 1 : 5 Rectangular Cylinder

This paper deals with the three-dimensional simulation of the unsteady flow around a stationary 1:5 rectangular cylinder at zero-degree angle of attack, low Mach number and relatively high Reynolds number. The computations have been performed using the DLR-Tau code, a non-commercial finite-volume code developed at the German Aerospace Center, and results obtained with a hybrid mesh are validated against the available experimental data, showing good agreement. Detached-Eddy Simulation (DES), that is a hybrid method combining Large-Eddy Simulation (LES) and Raynolds-Averaged Navier-Stokes (RANS) approaches, has been adopted as strategy of turbulence modelling. The comparison with respect to unsteady RANS results, although obtained with advanced turbulence models, show the improvement that can be expected with the DES technique for this type of massively separated bluff-body flows. The paper also clarifies the key role played in LES, and therefore also in DES simulations, by the artificial dissipation characterizing the numerical scheme used to discretize the filtered Navier-Stokes equations. Finally, the discussed results highlight the effects of the spanwise extension of the computational domain. A distance between the periodic boundary planes equal to the width of the cylinder is not enough to allow the natural loss of correlation of pressures and the free development of large-scale turbulent structures. In contrast, a span equal to the double of the cylinder width fulfill much better this requirement.

URANS and DES Simulation of Flow Around a Rectangular Cylinder

This paper deals with numerical simulation of the flow around a 1:5 rectangular cylinder. The Unsteady Reynolds-Aver aged Navier-Stokes (URANS) and Detached-Eddy Simulation (DES) computational techniques are employed. In the process the influence of various modelling parameters, such as turbulence modelling, and flow parameters, such as Reynolds number and incidence angle, is investigated. Simulations with both stationary and harmonically oscillating body are performed. Validation of computed results with experimental data shows that the URANS-based computational approach is capable of predicting the basic unsteady flow phenomena in the considered cases. These results are further confirmed by the DES method, which provides information about the instantaneous flow variables and offers a deeper insight into the flow physics.

Coupled CFD-CSM simulations of aerostabil windtunnel experiments, considering structural shell modelling

Steady CFD-CSM coupled simulations of the Aerostabil windtunnel experiments comprising a beam finite element model and RANS computations have not shown a satisfying agreement with the experimental pressure data.