David Greenblatt

The control of flow separation by periodic excitation

Abstract This paper presents a review of the control of flow separation from solid surfaces by periodic excitation. The emphasis is placed on experimentation relating to hydrodynamic excitation, although acoustic methods as well as traditional boundary layer control, such as steady blowing and suction, are discussed in order to provide an appropriate historical context for recent developments. The review examines some aspects of the excited plane mixing-layer and shows how its development lays the foundation for a basic understanding of the problem. Flow attachment to, and separation from, a deflected flap is then shown to be a paradigm for isolating controlling parameters as well as understanding the basic mechanisms involved. Particular attention is paid to separation control on airfoils by considering controlling parameters such as optimum reduced frequencies and excitation levels, performance enhancement, efficiency, reduction of post-stall unsteadiness, compressibility and other important features. Additional topics covered include excitation of separation bubbles, control and exploitation of diffuser flows, three-dimensional effects, the influence of longitudinal curvature and possible applications to unmanned air vehicles. The review closes with some recent developments in the control and understanding of incompressible dynamic stall, specifically illustrating the control of dynamic stall on oscillating airfoils and identifying the crucial time-scale disparity between dynamic stall and periodic excitation.

Use of Piezoelectric Actuators for Airfoil Separation Control

Surface-mounted piezoelectric actuators are used to excite the turbulent boundary layer upstream of separation, where the actuators interact directly with the boundary layer. The actuators are rigid and do not attenuate with increased aerodynamic loading up to the maximum tested speed of 30 m/s

Dynamic Stall Control by Periodic Excitation, Part 1: NACA 0015 Parametric Study

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.

Experimental Investigation of Separation Control Part 1: Baseline and Steady Suction

Low-speed flow separation over a wall-mounted hump, and its control using steady suction, were studied experimentally in order to generate a data set for the development and evaluation of computational methods. The baseline and controlled data sets comprised time-mean and unsteady surface pressure measurements, flowfield measurements using particle image velocimetry, and wall shear stress obtained via oil-film interferometry. In addition to the specific test cases studied, surface pressures for a wide variety of conditions were acquired for different Reynolds numbers and suction rates. Stereoscopic particle image velocimetry and oil-film flow visualization indicated that the baseline time-averaged separated flowfield was two-dimensional. With the application of control, mild three-dimensionality was evident in the spanwise variation of pressure recovery, reattachment location, and spanwise pressure fluctuations.

Steady and Unsteady Plasma Wall Jets for Separation and Circulation Control

An experimental investigation of separation and circulation control was carried out using corona discharge as well as dielectric barrier discharge actuators at typical micro air vehicle (MAV) Reynolds numbers. All actuators were calibrated by direct measurement and their limitations were assessed on the basis of conventional low Reynolds number active flow control data. Aerodynamic data from corona discharge and high frequency dielectric barrier discharge actuators highlighted their applicability at MAV-type Reynolds numbers. Modulating the dielectric barrier discharge actuators at frequencies corresponding to reduced frequencies of O(1), resulted in significant improvements to Cl,max, which increased with decreasing Re. At the low end of the MAV Reynolds number range (Re~20,000) modulation increased Cl,max by more than a factor of 2 and typical low Re hysteresis was eliminated. Of particular interest from an applications perspective was that performance, measured here by Cl,max, was shown to increase with decreasing duty cycle, and hence power input. In fact, duty cycles of around 0.66% were sufficient for effective separation control, corresponding to power inputs on the order of 1.2 milliwatts per centimeter.

Dynamic Stall Control by Periodic Excitation, Part 2: Mechanisms

Dynamic e ow separation and its control over a stationary dee ected surface are used to demonstrate the timescale disparity between the process of dynamic stall, which is dominated by the dynamic stall vortex (DSV), and the excitation-induced large coherent structures that effect its control. Appreciation of this disparity provided a framework for analyzing dynamic stall control on a NACA 0015 airfoil, where leading-edge excitation had effectively eliminated the DSV and signie cantly attenuated trailing-edge separation. Within this framework, a comparisonofstaticandairfoilphase-lockeddynamicpressuredataacquiredin thevicinity ofmaximum incidence (® o 25 deg) revealed that chordwise pressure distributions were independent of the airfoil pitching frequency and that the generation and advection of LCSs were not signie cantly affected by the dynamic airfoil pitching motion.Furthermore,disparitiesbetweenstaticanddynamicdatadiminishedastheexcitationfrequencyincreased relative to the airfoil pitching frequency. Oscillations of the aerodynamic coefe cients induced by the excitations were negligibly small but served to regulateairfoil cycle-to-cycle disparities typical of the baselinepoststall regime.

Experimental investigation of separation control. Part 2: Zero mass-flux oscillatory blowing

The control of a separated flow over a wall-mounted hump, by means of two-dimensional zero mass-flux perturbations, was studied experimentally to generate a data set for the development and evaluation of computational methods. The companion paper (Part 1) considered details of the baseline (uncontrolled) case and a steady-suction control case. The data set for a specific zero mass-flux control case comprised static surface pressures together with phase-averaged unsteady surface pressures and particle image velocimetry flowfield measurements. Additional surface pressures were acquired for a variety of control frequencies, control amplitudes and Reynolds numbers. Due consideration was given to characterizing the flow in the vicinity of the control slot, with and without external flow, and to perturbation two-dimensionality. Triple-decomposition of the fluctuating velocity and pressure fields was employed for presenting and analyzing the experimental data. This facilitated an assessment of the mechanism of separation control and the quantification of the coherent and turbulent surface pressures, Reynolds stresses, and energy fluxes. Spanwise surface pressures and phase-averaged stereoscopic particle image velocimetry data revealed an effectively two-dimensional flowfield despite highly three-dimensional instantaneous flow structures.

A Separation Control CFD Validation Test Case. Part 1; Baseline and Steady Suction

Low speed flow separation over a wall-mounted hump, and its control using steady suction, were studied experimentally in order to generate a data set for a workshop aimed at validating CFD turbulence models. The baseline and controlled data sets comprised static and dynamic surface pressure measurements, flow field measurements using Particle Image Velocimetry (PIV) and wall shear stress obtained via oil-film interferometry. In addition to the specific test cases studied, surface pressures for a wide variety of conditions were reported for different Reynolds numbers and suction rates. Stereoscopic PIV and oil-film flow visualization indicated that the baseline separated flow field was mainly twodimensional. With the application of control, some three-dimensionality was evident in the spanwise variation of pressure recovery, reattachment location and spanwise pressure fluctuations. Part 2 of this paper, under preparation for the AIAA Meeting in Reno 2005, considers separation control by means of zero-efflux oscillatory blowing.

Influence of Finite Span and Sweep on Active Flow Control Efficacy

Active flow control efficacy was investigated by means of leading-edge and flap-shoulder zero mass-flux blowing slots on a semispan wing model that was tested in unswept (standard) and swept configurations. On the standard configuration, stall commenced inboard, but with sweep the wing stalled initially near the tip. On both configurations, leading-edge perturbations increased CL,max and post stall lift, both with and without deflected flaps. Without sweep, the effect of control was approximately uniform across the wing span but remained effective to high angles of attack near the tip; when sweep was introduced a significant effect was noted inboard, but this effect degraded along the span and produced virtually no meaningful lift enhancement near the tip, irrespective of the tip configuration. In the former case, control strengthened the wingtip vortex; in the latter case, a simple semi-empirical model, based on the trajectory or “streamline” of the evolving perturbation, served to explain the observations. Control on finite-span flaps did not differ significantly from their two-dimensional counterpart, while control over a tip flap produced significant variations to all three moments in the presence of large deflection and these variations were linear with input slot momentum. Control from the flap produced expected lift enhancement and CL,max improvements in the absence of sweep, but these improvements degraded with the introduction of sweep.

Effect of leading-edge curvature on airfoil separation control

Separation control on NACA 0012 and NACA 0015 airfoils was compared under incompressible conditions, using leading-edgeperiodicexcitation, in orderto assess the effect of leading-edgecurvature. Both lift and moment coefe cients were considered to compare and analyse control effectiveness. In contrast to the relatively mild NACA 0015 trailing-edge stall, NACA 0012 stall was dominated by a leading-edge bubble-bursting mechanism that gave rise to alternating intervals of partial attachment and separation, but with no regular frequency. Low-amplitude excitation downstream of the bubble enhanced poststall lift and signie cantly attenuated the associated unsteadiness. In general, larger momentum coefe cients were required for NACA 0012 separation control due to the large centrifugal acceleration of the e ow around the leading edge. Because of the different stalling characteristics, relatively high- and low-excitation frequencies were effectivefor the NACA 0012 and NACA 0015 airfoils, respectively. However, the combination of high-excitation amplitudes with relatively low frequencieswas effective on the NACA 0012, and this was believed to be associated with the large harmonic content of the evolving perturbations.