Stall Delay and Leading-Edge Suction for a Pitching Airfoil with Trailing-Edge Flap

The flow around a pitching NACA0015 airfoil with an oscillating trailing-edge flap is experimentally investigated to characterize the influence of the flap kinematics on the development of dynamic stall. We specifically focus on the timing of the stall development and critical values of the leading-edge suction parameter as potential stall triggers. In general, static or dynamic trailing-edge flap motions strongly affect the entire lift response but preserve the characteristic evolution of the leading-edge suction parameter. The influence of the flap on the critical values of the leading-edge suction parameter and their timing is more subtle. Variations in the magnitude of the leading-edge suction are related to the instantaneous geometric effective angle of attack. A thin-airfoil theory-based model that linearly superimposes the effect of the flap on the response for a fixed flap in its neutral position correctly predicts the maximum values of the leading-edge suction parameter. The timing at which these maxima are reached corresponds to the stall delay and is dominated by the prestall temporal evolution of the effective angle of attack. Our results suggest that dynamic stall development is governed by characteristic stall delays rather than critical values of the maximum leading-edge suction parameter.

Predicting unsteady flow separation in response to a flow disturbance

Julien Dominique Claude Deparday, Guosheng He, Sabrina Henne, Karen Ann J Mulleners

We present here a summary of the activities and results of dynamically stalling airfoils from the UNFoLD lab at EPFL that have been presented at the meetings of the NATO AVT-282 discussion group on Unsteady aerodynamic response of rigid wings in gust encounters during the past three years. The results are based on experimental data for a sinusoidally pitching OA209 airfoil in a wind tunnel at

\Rey=\num{9.2e5}

, a sinusoidally pitching NACA0015 airfoil profile with a trailing edge flap in a wind tunnel at

\Rey= \num{5.5e5}

, and a sharp-edged flat plate undergoing a ramp-up motion in a recirculating water channel at

\Rey= \num{77 500}

. The first two data sets are used to study the onset of dynamic stall and the third one is used to analyse post-stall load fluctuations. Based on the experimental data, we derived a new model of the leading edge suction parameter that accurately predicts the value and the timing of the maximum leading edge suction and dynamic stall onset on a pitching airfoil. The model is based on thin airfoil theory and links the evolution of the leading edge suction and the shear layer during stall development. By including an oscillating trailing edge flap, the lift polars can be significantly altered but the dynamic stall time delay is only marginally affected by the kinematics of the flap. The maximum lift coefficient is strongly affected by both the main wing and the trailing edge flap kinematics. For a flat plate without flap, the maximum lift coefficient and the subsequent post stall peak values increase with increasing pitch rate up to a critical pitch rate beyond which the entire lift response become independent of the pitch rate. The convective time delay to reach the primary lift peak decreases with increasing pitch rate up to the critical pitch rate and the time delay between subsequent peaks slightly decreases until the limit cycle oscillation period is reached.

American Institute of Aeronautics and Astronautics, Inc2020

Predicting unsteady flow separation in response to a flow disturbance

Julien Dominique Claude Deparday, Guosheng He, Sabrina Henne, Karen Ann J Mulleners

\Rey=\num{9.2e5}

, a sinusoidally pitching NACA0015 airfoil profile with a trailing edge flap in a wind tunnel at

\Rey= \num{5.5e5}

, and a sharp-edged flat plate undergoing a ramp-up motion in a recirculating water channel at

\Rey= \num{77 500}

American Institute of Aeronautics and Astronautics, Inc2020