Kurt Kaufmann

Numerical Investigation of Three-Dimensional Static and Dynamic Stall on a Finite Wing

Three-dimensional numerical computations using ONERA’s structured elsA code and the unstructured DLR-TAU code are compared with the OA209 finite wing experiments in static stall and dynamic stall conditions at a Mach number of 0.16 and a Reynolds number of 1 × 10^6 . The DLR-TAU computations were run with the Spalart–Allmaras and Menter shear stress transport (SST) turbulence models, and the elsA computations were carried out using the Spalart–Allmaras and the k–ω Kok + SST turbulence models. Although comparable grids were used, the static simulations show large discrepancies in the stall region between the structured and unstructured approaches. Large differences for the three-dimensional dynamic stall case are obtained with the computations using the Spalart–Allmaras turbulence model showing trailing edge separation only in contrast to the leading edge stall in the experiment. The three-dimensional dynamic stall computations with the two- equation turbulence models are in good agreement with the unsteady pressure measurements and flow field visualizations of the experiment, but also show a shift in the stall angle compared to the experiment. The analysis of the flow field around the finite wing using the numerical simulations reveals the evolution of the -shaped vortex, generated by the interaction of the blade tip vortex.

Experimental Investigation of Dynamic Stall on a Pitching Rotor Blade Tip

Measurements on a pitching finite rotor blade tip are performed. Sectional forces obtained from surface pressure measurements show a significant difference in maximum loads and the extent of hystereses between a section near the parabolic blade tip and two sections further inboard. Different characteristics appear also in the flow topology over the suction side of the airfoil investigated by means of Particle Image Velocimetry (PIV) at these locations.

Numerical Investigations of a Back-Flow Flap for Dynamic Stall Control

Numerical investigations with the DLR TAU Code demonstrate that an actuated back-flow flap on the suction side of an OA209 helicopter airfoil is able to improve the dynamic stall behavior. When the flow begins to stall, the flap is extended which interferes with the back-flow. This weakens the dynamic stall vortex and consequently the peak in the pitching moment peak is reduced.

Dynamic Stall Simulations on a Pitching Finite Wing

The aerodynamic effects of dynamic stall on pitching wings and airfoils differ significantly from the static case, as soon as the static stall angle is exceeded. The large drag and pitching moment peaks associated with this phenomenon make it impossible to operate helicopters in flight conditions which trigger dynamic stall over large areas of the rotor blade. To simplify the problem, most of the research on dynamic stall focuses on two-dimensional pitching airfoils. This has lead to an understanding of the process and mechanism of dynamic stall, but has a limited insight into the three-dimensional nature of dynamic stall. There have been a few experimental and numerical studies on full helicopter configurations. These revealed the very complex flow field around the rotor blades, but the combination of downwash, blade-wakevortex interactions and the elasticity of the rotor blades leads to a limited comparability. An approach between these configurations is to use stiff pitching finite wings with defined boundary conditions. In these configurations the blade tip vortex reduces and delays the occurrence of dynamic stall in the surrounding region leading to a strongly three-dimensional flow, see e.g. Le Pape et al. and Lorber.

Spanwise Differences in Static and Dynamic Stall on a Pitching Rotor Blade Tip Model

An experimental investigation of static and dynamic stall on a rotor blade tip model with a parabolic tip geometry and aspect ratio 6.2 at a chord Reynolds number of 900,000 and a Mach number of 0.16 is presented. The resulting flow is analyzed based on unsteady surface pressure measurements and quantitative flow visualizations by high-speed particle image velocimetry. The flow separation is found to be delayed near the parabolic blade tip for static angles of attack as well as for sinusoidal angle of attack motions. The maximum effective angle of attack prior to stall is shifted to approximately two-thirds of the span outboard from the root because of a positive twist of the model with an increasing geometric angle of attack towards the tip. The stall onset is observed near the section with the maximum effective angle of attack, with a subsequent spanwise spreading of the flow separation. Different stages of flow separation for static angles of attack are identified one of them with the occurrence of two stall cells. During dynamic stall, the leading edge vortex formation starts near the maximum effective angle of attack and the pitching moment peak resulting from the passage of the dynamic stall vortex is higher at this section. Further inboard the maximum aerodynamic loads are of comparable magnitude whereas the outboard section shows reduced peaks due to the influence of the wing tip vortex.

Comparison Between Two-Dimensional and Three-Dimensional Dynamic Stall

Numerical computations using the DLR-TAU code investigate the differences and similarities between dynamic stall on the two-dimensional OA209 airfoil and the three-dimensional OA209 finite wing. The mean angle of attack in the two-dimensional computations is reduced to match the effective angle of attack at the spanwise position where in the finite wing computations the dynamic stall vortex starts to evolve. Small variations of the mean angle of attack in the two-dimensional numerical simulations show a change from trailing edge separation only to deep dynamic stall. The analysis of the three-dimensional flow field reveals that after the evolution of the dynamic stall vortex the flow field is split into two parts: 1. High spanwise velocities towards the wing’s root in the region between the plane of the first occurrence of stall and the wing’s root. 2. High spanwise velocities towards the wing’s tip in the region between the plane of the first occurrence of stall and wing tip.