An experimental study of the breakdown of leading-edge vortices on diamond, cropped, delta, and double delta wings during dynamic pitching

The effect of delta wing shape on leadingedge vortex breakdown was investigated in the 2 ft x 3 ft water tunnel at Wichita State University. It is well known that a vehicle's performance at high angles of attack is greatly influenced by the development of leading-edge vortices on a delta-shaped wing. In this experiment, the aft Vs of a 76° swept delta wing was modified to obtain diamond, cropped, standard delta, and double delta shapes. The vortex breakdown location during dynamic pitch-up and pitch-down motion was observed using dye flow visualization. Amongst the four shapes tested, the cropped delta wing had the longest unbursted leading-edge vortex during dynamic pitching while the double delta wing had the earliest vortex breakdown. NOMENCLATURE AR Wing Aspect Ratio, b /S b Wing Span cr Wing Root Chord q Dynamic Pressure, V&pUa, Re Reynolds Number, Uwcr Iv S Wing Area Uo,, Freestream Velocity a Angle of Attack a' Pitch Rate K Non-Dimensional Pitch Rate, a'cr /2UC A Sweep-back Angle v Freestream Flow Kinematic Viscosity p Freestream Flow Density INTRODUCTION Recent interest in high angle of attack aerodynamics has refocussed attention on delta shaped wings. Vortices are formed at non-zero angles of attack as flow separates along the leading edges of a delta shaped wing. Very low pressure is associated with these leading-edge vortices, and they can account for up to 30% of the total lift at moderate angles of attack. For example, lift continues to increase until about 40° angle of attack on a 76° swept delta wing. In comparison, symmetric two dimensional airfoils typically stall out around 10-15° angle of attack. Unfortunately, there are limits to the benefits produced by these delta wing vortices. As the angle of attack is increased, there is a sudden breakdown in vortex structure. This phenomena, also known as vortex "bursting" results in a sudden stagnation in core axial flow and an expansion in radial size. Once this occurs, lift is no longer enhanced aft of the burst point. Thus, the development and subsequent breakdown of leading-edge vortices is crucial to the performance of delta wing aircraft. There have been a number of attempts to control delta wing vortices including the use of blowing', suction', flaps", and canards'. The *Assistant Professor, Senior Member AIAA. Student, present address: 26A Lengkong Satu, Singapore 417500, Singapore. *Student, Student Member AIAA, present address: Department of Aerospace Engineering, University of Southern California, Los Angeles, CA 90089-1191. Associate Professor, Member AIAA. Copyright ® 1997 by Roy Y. Myose, Boon-Kiat Lee, Shigeo Hayashibara, and L. Scott Miller. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. reader is referred to Lee and Ho for a more complete review on delta wing vortices. As the angle of attack is increased on delta wings, the unburst part of the leading-edge vortices become shorter. Under dynamic conditions, there is a hysteresis or phase lag in the vortex burst location. For example, the vortex burst location is further aft compared to the static case (at a given a) under pitch-up motion and further forward under pitch-down motion. This phase lag is larger as the pitch rate is increased.' Thus, fast pitch-up and slow pitch-down is desired in order to delay vortex breakdown. It is well known that the sweep-back angle on a delta wing affects the development and breakdown of the leading-edge vortices. For example, full stall angle of attack (under static conditions) occurs at 27° on a 55° swept delta wing while it is 38°on a 65° swept delta wing and 54° on a 75° swept delta wing. Thus, high sweep-back angles provide enhanced lift until high angles of attack. This principle is employed on modern day fighter aircraft using strakes in front of the main wing to form a double delta shape. In this case, the strakes provide enhanced lift in addition to the main whig. A number of unique delta whig shapes were also investigated by Gatlin and McGrath. However, all of these studies on the effect of delta wing shape were conducted under static conditions. Under dynamic conditions, investigations on only the basic shapes such as the delta'' and double delta wings have been conducted. Modern day military aircraft often employ novel wing shapes hi order to incorporate stealth technology. Furthermore, enhanced performance at high angles of attack and under unsteady conditions may be required of these military aircraft. Thus, a series of experiments were conducted on the effect of different delta wing shapes on vortex breakdown under dynamic pitching conditions. EXPERIMENTAL METHOD The experiment was conducted in the 2 ft x 3 ft water tunnel located at Wichita State University, National Institute for Aviation a) Diamond b) Crop c) Delta d) Double delta Fig. 1. Test model shapes. Research (NIAR). The facility is a closed-loop water tunnel containing a total of 3500 gallons of water. The flow velocity is adjustable up to 1.0 ft/s using an impeller pump driven by a 5 hp variable speed motor. The facility has excellent optical access providing two side views, a bottom view, and an end view. Fig. 1 shows a sketch of the four different whig shapes which were tested. All four shapes have a sweep-back angle of 76° at the wing apex and a root chord length of 9". The sweep-back angle of the aft Vs of each wing is different, corresponding to diamond, cropped, standard delta, and double delta shapes. Each whig is made out of 0.063" thick aluminum alloy, and both starboard and port sides are symmetrically beveled at a 45° angle. The wings are painted white to enhance visual contrast, but black reference grid lines perpendicular to the whig centerline are marked in 5% chord intervals to determine the vortex breakdown locations. Table 1 lists the specifications for each whig shape. A dynamic test mount consisting of a rotating turntable was used to obtain the dynamic pitch motion (see fig. 2). The position and rotational speed of this belt-driven turntable are controlled by a variable speed DC motor. The turntable is capable of 360° of rotation at rates up to 30 deg/s. The pitching motion produced by this dynamic test mount is a constant angular rate of change, i.e., a "ramp-type" pitching motion. Additional details about the dynamic test mount are presented by Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. Table 1: Test model specifications.