Flow Control of Vortical Structures and Vortex Breakdown Over Slender Delta Wings

An understanding of the vortical structures and vortex breakdown is essential for the development of highly maneuverable and high angle of attack flight. This is primarily due to the physical limits these phenomena impose on aircraft and missiles at extreme flight conditions. In today’s competitive world, demands for more maneuverable and stealthy air vehicles have encouraged the development of new control concepts for separated flows. The goal of this paper is to describe experimental flow control techniques used to manipulate the vortical structures and vortex breakdown over slender delta wings at high angles of attack. The paper begins with a review of the experimental vortical flow control techniques implemented and tested over the past 50 years. This is by no means a comprehensive review, but is representative of the various flow control techniques examined. Beyond the brief historical review, this paper will examine more closely, two promising and different pneumatic flow control methods for the control of vortex breakdown over slender delta wings: open-loop, along-the-core blowing and periodic blowing and suction along the leading edges. These studies were performed at Onera and at the US Air Force Academy and consist of both experimental and computational analysis of subsonic flow fields around 70° delta wings over a broad range of angles of attack (20° < α < 40°) and root-chord Reynolds numbers (2x10 < Rec < 2.6x10). INTRODUCTION The delta wing flow field is dominated by vortical structures, the most prominent called leading-edge vortices. As angle of attack increases, these leading-edge vortices experience a sudden disorganization, known as vortex breakdown which can be described by a rapid deceleration of both the axial and swirl components of the mean velocity and, at the same time, a dramatic expansion of the vortex core. Henri Werlé first photographed the vortex breakdown phenomenon in 1954, during water tunnel tests of a slender delta wing model at Onera. This work was quickly confirmed by Peckham and Atkinson, Elle and Lambourne and Bryer and spawned a large number of experimental, computational and theoretical studies which continue today. These investigations led to the development of several theories governing vortex breakdown, although none have been universally accepted. Despite this lack of a unified theoretical interpretation, several forms of vortex breakdown have been identified and the global characteristics of the phenomena are understood. During the breakdown process, the mean axial velocity component rapidly decreases until it reaches a stagnation point and/or becomes negative on the vortex axis. This stagnation point, called the breakdown location, is unsteady and typically oscillates about some mean position along the axis of the vortex core (see Fig. 1). As angle of attack is increased, the vortex breakdown location moves upstream over the delta wing (from the trailing edge toward the apex). Werlé was one of the first researchers to implement an active technique of controlling the vortical flow around a delta wing using either suction aft of the trailing edge or by injecting a mass flow over the delta wing. His original, qualitative study confirmed the capability of external methods to manipulate both the vortical structure and the vortex breakdown location. Additional flow control research initiated during the 1960's, at Onera and elsewhere has led to a vast and varying Paper presented at the RTO AVT Symposium on “Advanced Flow Management: Part A – Vortex Flows and High Angle of Attack for Military Vehicles”, held in Loen, Norway, 7-11 May 2001, and published in RTO-MP-069(I). Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 00 MAR 2003 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Flow Control of Vortical Structures and Vortex Breakdown Over Slender Delta Wings 5a. CONTRACT NUMBER