Guido Dietz

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.

Passive Shock Control Concept for Drag Reduction in Transonic Flow

The operating principle of a novel passive shock control concept for drag reduction on swept wings called D-Strips is introduced, and results of a first experimental proof of concept are presented. Wind-tunnel experiments are conducted at transonic airspeeds using an airfoil VC-Opt (9.2% relative thickness) with forced boundary-layer transition. Schlieren pictures confirm the operating principle. Wake measurements demonstrate that, locally, in the wake of D-Strips the total drag increases. However, more globally, at spanwise locations away from these positions the total drag is measurably reduced if a sufficiently strong shockwave is present above the wing-suction-side surface. In contrast to passive control by ventilation, outside of the wake of the D-Strips a wave and viscous drag reduction and observed. Furthermore, the maximum lift tends to be increased by D-Strips suggesting that the buffet-onset limit is delayed. An application of D-Strips to a swept wing at cruise conditions can yield net aircraft drag reduction because the drag rise is limited to the D-Strips and its wake, but the shock-weakening effect and the drag reduction are distributed in the wide-splayed characteristics upstream of the shock front. The effectiveness of D-Strips is less sensitive to the location of the shock wave than control concepts like two-dimensional bumps, which are only effective at the design conditions. Another application of D-Strips that can be considered is the regions with a distinct shock, for example, between engines or in the wing-root region or at the inboard engine. However, there is a need for further investigations in order to optimize D-Strips and to quantify their impact on aircraft performance. Recent experimental results indicate that airscoop-type configurations might perform better as D-Strips than Velcro-type strips. Because D-Strips are easy to apply on existing wings, an application of D-Strips for retrofit is possible.

Helicopter Aerodynamics with Emphasis Placed on Dynamic Stall

Dynamic Stall is a flow phenomenon which occurs on helicopter rotor blades during forward flight mainly on the retreating side of the rotor disc. This phenomenon limits the speed of the helicopter and its manoeuvrability. Strong excursions in drag and pitching moment are typical unfavourable characteristics of the Dynamic Stall process. However compared to the static polar the lift is considerably increased. Looking more into the flow details it is obvious that a strong concentrated vortex, the Dynamic Stall Vortex, is created during the up-stroke motion of the rotor blade starting very close to the blade leading edge. This vortex is growing very fast, is set into motion along the blade upper surface until it lifts off the surface to be shed into the wake. The process of vortex lift off from the surface leads to the excursions in forces and moment mentioned above. The Dynamic Stall phenomenon does also occur on blades of stall regulated wind turbines under yawing conditions as well as during gust loads. Time scales occurring during this process are comparable on both helicopter and wind turbine blades. In the present paper the different aspects of unsteady flows during the Dynamic Stall process are discussed in some detail. Some possibilities are also pointed out to favourably influence dynamic stall by either static or dynamic flow control devices.

Limit Cycle Oscillations of Aeroelastic Systems with Internal Friction in the Transonic Domain

This paper reports on the design, fabrication, and testing of a friction generating device connected to an airfoil to assess the effects of internal friction on limit cycle oscillations. Preliminary shaker testing in the ASU vibrations laboratory successfully validated the design. Full blown testing was then carried out at DLR in the DNW-TWG transonic wind tunnel and limit cycle oscillations with internal friction effects were observed and measured. These first measurements demonstrated a slight benefit of friction but also showed the subcritical nature of the observed limit cycle oscillations which is clarified. A different design is proposed for future tests that should lead to supercritical limit cycle oscillations.