Control of wall-separated flow by internal acoustic excitation

Abstract

This paper explores the control of wall-separated flow on a NACA 63 3 -018 airfoil and a circular cylinder by using the internal acoustic excitation technique. Experimental study of the characteristics of the flow under internally emanating acoustic waves is performed in an open-type, suction wind tunnel. Tests are carried out at the Reynolds number ranging from 6.3 x 10 3 to 5.0 x 10 5 based on the relevant characteristic lengths, the airfoil chord, and the cylinder diameter. The control effectiveness is verified by the measurements of parameters such as the excitation frequency, the excitation level, and the forcing location. Data indicate that the excitation frequency and the forcing location are the key parameters for controlling the separated flow, and the forcing level is the least-effective parameter for the study. As long as the emanating acoustics is "locked in" to the separated shear-layer instability frequency and forcing is applied at the separation point, the separated flow is controlled most effectively. Moreover, the b'ft is increased and the drag reduced dramatically. Nomenclaturecircular cylinder f e = excitation frequency f s = vortex shedding frequency f t = shear layer instability frequency P = static pressure P t -total pressure p tao = total pressure in freestream P^ = static pressure in freestream Re c = Reynolds number based on chord, Re c = U^ C/v Re D = Reynolds number based on diameter, Re D -U^D/v St = Strouhal number, (/,<:/#«,) or (f.D/U^) U^ = freestream velocity X = coordinate along the freestream direction Y = coordinate normal to the freestream direction p = fluid density v = kinematic viscosity a = sound emission angle of the circular cylinder 0 = polar coordinate in the azimuthal direction

Active separation control: an overview of Reynolds and Mach numbers effects

“…Historically, these studies set the scene for current advances, but it is now appreciated that hydrodynamic, and not acoustic, disturbances are far more practical, effective and facility independent. This was demonstrated using a vibrating ribbon [34], wire [4] or two-dimensional blowing slot [19] (see [11] for a summary of relevant investigations).…”

Active Control of a Circular Cylinder Flow at Transitional Reynolds Numbers

“…Later studies focused on hydrodynamic excitation, for example from a slot (e.g. Hsiao et al [11]; Williams et al [31]; Schewe [25]; Pal and Sinha [18]; Heine et al [10]; Amitay et al [2]; Liu and Brodie [14]; Béra et al [4]), producing profound effects on transition and separation, with consequences for lift, drag and vortex shedding. For example, Hsiao et al [11] showed that a suction peak could be generated on one side resulting in C l =0.6 with the excitation location at 100°when F þ f e D=U 1 % 1; similar observations were made by Amitay et al [2] for F + =1.5 who also showed that the separation point could be moved by approximately 60°, resulting in a 25% reduction in form-drag.…”

Experimental Study of Flow Separation over NACA633018 Wing with Synthetic Jet Control at Low Reynolds Numbers

“…Flow control with the piezoelectric-driven SJAs has a low velocity which can be applied to resist flow separation on the surface of wing at relevant frequencies, but there is still no literature referring to where the piezoelectric disks are placed on the surface of a specific wing for effective flow control. Hsiao et al [3] showed that the investigation of a NACA 63 3 018 airfoil where the separation bubble was significant to exist at a low Reynolds number, and acoustic excitation can effectively affect the airfoil performance in a beneficial way. When the freestream flows over the surface of wing or airfoil at low Reynolds numbers, the separation bubble may be formed on the surface of wing or airfoil.…”

“…The synthetic jet actuators (SJAs) have been applied to control flow separation in boundary layer, virtual shaping, enhancing mixing layer, etc. In recent years, the investigations of aerodynamic performance in the low Reynolds numbers of wing and airfoil has attracted much attention, especially, flow control with some kinds of techniques including vibrating ribbons or flaps [1], steady or unsteady blowing [2] and external and acoustic excitation [3] to enhance the total lift and reduce drag of a wing or airfoil. However, recent years, a lot of experts have studied fundamental characteristics of SJA at cavity of resonance.…”