Publication

Predicting unsteady flow separation in response to a flow disturbance

Abstract

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=\num9.2e5\Rey=\num{9.2e5}, a sinusoidally pitching NACA0015 airfoil profile with a trailing edge flap in a wind tunnel at \Rey=\num5.5e5\Rey= \num{5.5e5}, and a sharp-edged flat plate undergoing a ramp-up motion in a recirculating water channel at \Rey=\num77500\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.

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Related concepts (36)
Stall (fluid dynamics)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil, and Reynolds number. Stalls in fixed-wing flight are often experienced as a sudden reduction in lift as the pilot increases the wing's angle of attack and exceeds its critical angle of attack (which may be due to slowing down below stall speed in level flight).
Airfoil
An airfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more lift than drag. Wings, sails and propeller blades are examples of airfoils. Foils of similar function designed with water as the working fluid are called hydrofoils. When oriented at a suitable angle, a solid body moving through a fluid deflects the oncoming fluid (for fixed-wing aircraft, a downward force), resulting in a force on the airfoil in the direction opposite to the deflection.
Lift (force)
A fluid flowing around an object exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it is defined to act perpendicular to the flow and therefore can act in any direction. If the surrounding fluid is air, the force is called an aerodynamic force.
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