The control of flow separation by periodic excitation

Modern developments in shear-stress measurement ☆

A parametric study was undertaken to investigate the effect of periodic excitation (with zero net mass e ux ) on a NACA0015airfoilundergoingpitchoscillationsatrotorcraftreducedfrequenciesunderincompressibleconditions. Theprimaryobjectiveofthestudywastomaximizeairfoilperformancewhilelimitingmomentexcursionstotypical prestalled conditions. The incidence angle excursions were limited to § 5 deg, and a wide range of reduced excitation frequencies and amplitudes were considered for 0 :33 10 6 < ‐ Re <‐ 0:93 10 6 with various e ap dee ections and excitation locations. Signie cant increases in maximum lift and reductions in drag were attained while containing themomentexcursions. Oscillatoryexcitation wasfoundto befarsuperiortosteadyblowing,which wasevendetrimental under certain conditions, and e ap-shoulder excitation was found to be superior to leading-edge excitation.

Dynamic stall control by periodic excitation, Part 1: NACA 0015 parametric study

Dynamic stall is an incredibly rich fluid dynamics problem that manifests itself on an airfoil during rapid, transient motion in which the angle of incidence surpasses the static stall limit. It is an important element of many manmade and natural flyers, including helicopters and supermaneuverable aircraft, and low–Reynolds number flapping-wing birds and insects. The fluid dynamic attributes that accompany dynamic stall include an eruption of vorticity that organizes into a well-defined dynamic stall vortex and massive excursions in aerodynamic loads that can couple with the airfoil structural dynamics. The dynamic stall process is highly sensitive to surface roughness that can influence turbulent transition and to local compressibility effects that occur at free-stream Mach numbers that are otherwise incompressible. Under some conditions, dynamic stall can result in negative aerodynamic damping that leads to limit-cycle growth of structural vibrations and rapid mechanical failure. The mechanisms leading to negative damping have been a principal interest of recent experiments and analysis. Computational fluid dynamic simulations and low-order models have not been good predictors so far. Large-eddy simulation could be a viable approach although it remains computationally intensive. The topic is technologically important owing to the desire to develop next-generation rotorcraft that employ adaptive rotor dynamic stall control.

Dynamic Stall in Pitching Airfoils: Aerodynamic Damping and Compressibility Effects

Compressibility effects on dynamic stall

Introduction An experiment documenting the compressible flow over a dynamically deforming airfoil is presented. This airfoil, which has a leading edge radius that can be dynamically changed, was tested at various defor- mation rates for fixed airfoils angle of attack. Selected leading-edge shapes were also tested during airfoil os- cillation. These tests show that for a range of Mach numbers observed on the retreating blades of heli- copter rotors the dynamic stall vortex can be avoided by the judicious variation of leading-edge curvature

Unsteady Stall Control Using Dynamically Deforming Airfoils

The compressible dynamic stall flowfield over a NACA 0012 airfoil transiently pitching from 0 to 60 deg at a constant rate under compressible flow conditions has been studied using real-time interferometry. A quantitative description of the overall flowfield, including the finer details of dynamic stall vortex formation, growth, and the concomitant changes in the airfoil pressure distribution, has been provided by analyzing the interferograms. For Mach numbers above 0.4, small multiple shocks appear near the leading edge and are present through the initial stages of dynamic stall. Dynamic stall was found to occur coincidentally with the bursting of the separation bubble over the airfoil. Compressibility was found to confine the dynamic stall vortical structure closer to the airfoil surface. The measurements show that the peak suction pressure coefficient drops with increasing freestream Mach number, and also it lags the steady flow values at any given angle of attack. As the dynamic stall vortex is shed, an anti-clockwise vortex is induced near the trailing edge, which actively interacts with the post-stall flow.

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Interferometric investigations of compressible dynamic stall over a transiently pitching airfoil

A multielement airfoil designed for helicopter application has been tested for compressible dynamic stall behavior and has been proven to be a robust dynamic stall-free concept. This slotted airfoil has operated into poststall areas without the dynamic stall vortex that is normally present whenever airfoils are tested beyond their static stall boundary. Point diffraction interferogram images of the dynamic flow over the airfoil are presented, showing details of the flow development during the oscillation cycle, and instantaneous pressure distributions on the airfoil and slat during dynamic airfoil motion are included

Effect of Compressibility on Suppression of Dynamic Stall Using a Slotted Airfoil

Earlier experiments have documented the onset of compressible dynamic stall either from the bursting of a leading-edge laminar separation bubble or from a leading-edge shock, depending on the Reynolds number and Mach number. For certain combinations of conditions, the supersonic flow and the bubble dynamics compete with each other. The consequent complex interactions lead to a newly discovered mechanism of dynamic stall onset. Details of these various mechanisms are discussed

Competing Mechanisms of Compressible Dynamic Stall

Research conducted at NASA Ames Research Center has shown that the color-change response of a shear-sensitive liquid crystal coating (SSLCC) to aerodynamic shear depends on both the magnitude of the local shear vector and its direction relative to the observer's in-plane line of sight. In conventional applications, the surface of the SSLCC exposed to aerodynamic shear is illuminated with white light from the normal direction and observed from an oblique above-plane view angle of order 30 deg. In this top-light/top-view mode, shear vectors with components directed away from the observer cause the SSLCC to exhibit color-change responses. At any surface point, the maximum color change (measured from the no-shear red or orange color) always occurs when the local vector is aligned with, and directed away from, the observer. The magnitude of the color change at this vector-observer-aligned orientation scales directly with shear stress magnitude. Conversely, any surface point exposed to a shear vector with a component directed toward the observer exhibits a non-color-change response, always characterized by a rusty-red or brown color, independent of both shear magnitude and direction. These unique, highly directional color-change responses of SSLCCs to aerodynamic shear allow for the full-surface visualization and measurement of continuous shear stress vector distributions. The objective of the present research was to investigate application of the SSLCC method through a transparent test surface. In this new back-light/back-view mode, the exposed surface of the SSLCC would be subjected to aerodynamic shear stress while the contact surface between the SSLCC and the solid, transparent wall would be illuminated and viewed in the same geometrical arrangement as applied in conventional applications. It was unknown at the outset whether or not color-change responses would be observable from the contact surface of the SSLCC, and, if seen, how these color-change responses might relate to those observed in standard practice.

/pdf/shear-sensitive-liquid-crystal-coating-method-applied-3k44r1gzm3.pdf

M. C. Wilder

Papers

Unsteady Stall Control Using Dynamically Deforming Airfoils

Interferometric investigations of compressible dynamic stall over a transiently pitching airfoil

Effect of Compressibility on Suppression of Dynamic Stall Using a Slotted Airfoil

Competing Mechanisms of Compressible Dynamic Stall

Shear-Sensitive Liquid Crystal Coating Method Applied Through Transparent Test Surfaces