Localized Flow Control Using Counterflow Jets in High Speed Flows

Abstract

Reducing wave drag associated with supersonic flight vehicles and generating vehicle steering forces using a novel flow control technique are investigated both experimentally and computationally. Experimental work involved exploring oblique shock wave attenuation for a pair of oblique shock waves generated due to a supersonic flow past a double wedge system. Computational efforts were focused on flow control over two flows of interest; 1) drag reduction on a 2D wedge surface by counter flow jets issuing from a slit located on the wedge near the leading edge, and 2) steering force generation on a missile fin by jets issuing from on surface slits located at optimum places. Three different slit configurations, at freestream Mach numbers, 0.8, 1.5, at different angles-of-attack (AoA) were studied. The aim of the work was to achieve fin lift obtained at an AoA without moving the fin but using the flow control jet. Results indicate that counterflow jet can reduce drag over a 2-D wedge up to 15%. Results also indicate that while the technique generates significant control forces for Mach=0.8 case, it fails for the Mach=1.5 flow due to flow separation over the fin surface induced by the counter flow jets. I. Introduction Shock waves observed over aerospace vehicles at supersonic speeds result in substantial aerodynamic drag over these vehicles and affects design factors like endurance, payload capacity, fuel consumption, and performance. Over the years, numerous techniques have been proposed to reduce the drag associated with shock waves. Some of these techniques concentrate on modifying or controlling the flow field upstream and adjacent to the vehicle or the area of interest on the surface of the vehicle itself such that the shock wave associated with the vehicle is attenuated, diffused, or dispersed. Counter flow drag reduction (CDR) technique is one such method. In this technique, a jet emanates from the stagnation point on the surface of the flying object in a direction opposite to the flow field. Further, for devices such as tactical missiles an efficient steering system is highly desirable to increase maneuverability. Presently, missiles utilize hinged control surfaces with actuators to bring about the required control surface deflection. These actuators are located in the missile body annulus and impose severe space restrictions on the guidance and propulsion systems of the missile. Further, the actuators with the hinged control surfaces increase the overall weight-induced drag and thereby penalize the endurance and payload capabilities of the missile. The envisioned Airframe Propulsion Steering System (APS) utilizes Aero Control Fins (ACF) to reduce or enhance the aerodynamic characteristics by flow modification and generate aerodynamic control forces without the use of conventional actuators and hinged control surfaces. The present study concentrates on experimental and computational efforts devoted on developing a novel flow control technique aimed at locally modifying the flow over the control surfaces of missiles through jet impingements (counter-flow jets) from the span wise slits located at optimal places on the control surfaces. The experiments explore the possibility of attenuating oblique shock waves by counter flow blowing from locations on the intersecting plane of the oblique shock waves generated due to a supersonic flow past a double wedge system. The computational efforts explore generating the necessary steering forces required to maneuver the missile by asymmetric flow modification on the control surfaces, either to supplement the control forces generated by the missile fin deflections or to independently maneuver the missile with zero control surface deflection using jets momentum. Likewise, the study also explores the possibility of suppressing flow separation and delaying transition and aerodynamic lift enhancement by increasing circulation through jet impingements in the direction of the flow. In the following sections first the experimental setup, schlieren pictures, and the results obtained in the flow control of the shock waves generated by a double-wedge flow in a Mach 3 flow is presented. Computational study of a supersonic flow past a wedge with an included angle of 11.3 0 using commercial CFD solver, FLUENT, is presented next. The drag over the wedge was investigated for supersonic flows for Mach=1.5, 2.0, 2.5, and 3.0. The discussion emphasizes the reduction in drag forces achieved due to counter flow jet impingements from a point on