Effect of Oscillation on Boundary-Layer Development with Adverse Pressure Gradients K. Kontis * and M. Amir † University of Manchester, Manchester, M60 1QD England, United Kingdom DOI: 10.2514/1.25450 An experimental study has been conducted to investigate the effects of oscillation, of varying amplitude (0 to 0.03 m) and frequency (0 to 6 Hz), on the boundary-layer development of a flat plate under the influence of adverse pressure gradients. The Reynolds number was 10 6 based on the flat plate chord length. The study employed a subsonic wind tunnel facility at a freestream velocity of 16 m=s. A cylinder placed at various locations over the flat plate within the boundary layer was used to create the effects of adverse pressure gradients. Surface pressures were measured along the centerline of the flat plate to study the flow quantitatively. Oil flow visualizations were also obtained to examine the flow qualitatively. Boundary-layer surveys were conducted to examine the effect of oscillation on the mean velocity, turbulence intensity, and boundary-layer thickness profiles. Nomenclature A = amplitude of oscillation D = cylinder diameter F = frequency of oscillation ‘ = length of the flat plate P = local static pressure P 1 = freestream static pressure Re crit = critical Reynolds number Re N = Reynolds number S t = Strouhal number, (F  A=U 1 ) t = time U rms = root mean square velocity U rms =U 1 = turbulence intensity U 1 = freestream velocity u = mean velocity x = horizontal distance along the flat plate x tran = transition point y = vertical distance above the flat plate  = increase in boundary-layer thickness  = boundary-layer thickness  = kinematic viscosity of air ! = angular velocity I. Introduction S INCE the introduction of the boundary-layer concept by Prandtl, there has been a constant challenge faced by scientists and engineers to minimize its adverse effects and control it to their advantage. A main objective of a control procedure is to prevent or at least delay the separation of the boundary layer from the wall. Methods employing suction, blowing, vortex generators, turbulence promoters, etc., have been investigated and applied extensively with a varying degree of success. However, the use of a moving or oscillating wall for boundary-layer control has received relatively little attention and there are many unanswered questions associated with the effects of oscillation on the boundary-layer development, especially under the presence of adverse pressure gradients. A moving surface attempts to accomplish this in two ways: 1) it retards the growth of the boundary layer by minimizing the relative motion between the surface and the freestream; 2) it adds momentum into the existing boundary layer. Boundary-layer control by a moving surface was first demonstrated by Favre [1]. Favre ran an endless belt forming a portion of the upper surface of an aerofoil and delayed separation until high angles of attack. Since that initial study, boundary-layer control by surface motion has been examined and successfully demonstrated in a number of similar applications [2,3]. In those studies, however, the moving wall effects on the boundary layer itself have not been thoroughly examined. For flows over moving walls, the point or line of vanishing wall shear does not necessarily coincide with separation, and this greatly complicates the problem. This was first observed by Rott [4] while analyzing the unsteady flow in the vicinity of a stagnation point. He observed that, although the wall shear vanished with an accompanying reverse flow, there was no singularity or breakdown of the boundary-layer assumptions. In seeking a generalized model for separation, Sears [5] postulated that the unsteady separation point is characterized by the simultaneous vanishing of the shear and the velocity at a point within the boundary later as seen by an observer moving with the velocity of the separation point. Moore [6], while investigating a steady flow over a moving wall, arrived at the same model for unsteady separation. On the basis of an intuitive relationship between steady flow over a moving wall and unsteady flow over a fixed wall, Moore was able to sketch the expected velocity profiles for both cases. He considered the possibility that a Goldstein-type singularity occurred at the location where the velocity profile simultaneously had zero velocity and a shear at a Fig. 1 Schematic of oscillation mechanism. Received 26 May 2006; revision received 6 December 2006; accepted for publication 11 December 2006. Copyright © 2006 by M. Amir and K. Kontis. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/07 $10.00 in correspondence with the CCC. * Associate Professor (Senior Lecturer) of Aerodynamics and Ground Testing, Special Interest Group and Laboratory Head, School of Mechanical, Aerospace and Civil Engineering, Aero-Physics and Advanced Measurement Technology Laboratory. Member AIAA. † Research Student, School of Mechanical, Aerospace and Civil Engineering, Aero-Physics and Advanced Measurement Technology Laboratory. JOURNAL OF AIRCRAFT Vol. 44, No. 3, May–June 2007 875