Yaw Angle Effect on Flow Structure over the Nonslender Diamond Wing

D ELTAwings have evolved over the years and are primarily used on many fighter aircraft. As these aircraft become more and more maneuverable, delta-wing vortex dynamics and the understanding of the physics of time-dependent unsteady flows have become substantially important [1]. Several variables affect the deltawing vortex dynamics. As indicated by Yaniktepe [2], some of these variables are angle of attack, leading-edge geometry, wing thickness, sweep angle, Reynolds number, and freestream conditions. Yaniktepe and Rockwell [3] investigated aerodynamics of the delta wing with a sweep angle of 38:7 for the value of Reynolds number based on the chord length C, which was maintained at Re 10. They reported that the nonslender delta wings exhibited more distinctive features than the slender delta wings, especially at a higher angle of attack as a result of the earlier onset of vortex breakdown, which are based on the time-averaged velocity and vorticity distributions in the crossflow plane. Canpolat et al. [4] observed the variation of flow structures on the delta-wing surface with a sweep angle of 40 as a function of the angle of attack and yaw angle , using the dye visualization technique. When the delta wing is under the effect of a yaw angle, the symmetrical flow structure deteriorates, and a vortex breakdown occurs earlier on the windward side of the delta wing, as compared with the leeward side. The main vortices in crossflow planes occur in the inner side close to the central axis of the delta wing. Many small-sized vortices are also evident next to themain rotating vortices. Yayla et al. [5] investigated the flow structure close to the surface of the nonslender diamond wing, both qualitatively and quantitatively, using the dye visualization and the stereoscopic particle image velocimetry (PIV) techniques. It was stated that, when the yaw angle is increased, the locations of vortex breakdowns approach thewing apex, but the other one moves toward the trailing edge. Goruney and Rockwell [6] investigated the near-surface flow structure and topology on a delta wing of low sweep angle having sinusoidal leading edges of varying amplitude and wavelength. Gursul et al. [7] reviewed unsteady aerodynamics of nonslender delta wings, covering topics of shear layer instabilities, structure of nonslender vortices, breakdown, maneuvering wings, and fluid/structure interactions. Yaniktepe and Rockwell [8] characterized the instantaneous and the time-averaged flow structure on the nonslender diamond and lambda planforms by using the PIV technique. Ozgoren et al. [9] investigated the structure of vortex breakdown and the effect in the surface of the wing of the separated flow region in the case of the high angle of attack over the slender delta wing. They declared that the high angle of attack rather affects the onset of vortex breakdown, spiral vortex structure, and separated flow region. Breitsamter [10] presented selected results from extensive experimental investigations on turbulent flowfields and unsteady surface pressures caused by leading-edge vortices, in particular, for vortex breakdown flow. Another important parameter for the delta wing is the yaw angle. The influence of sideslip on the flow about a sharp-edged biconvex delta wing of a unit aspect ratio was investigated by Verhaagen and Naarding [11] using flow visualization techniques as well as pressure and force balance measurements. It was observed that the yaw angle affects the structure of the leading-edge vortex, vortex breakdown, and formation of nonsteady flow structure substantially, which is generated after vortex breakdown. Sohn et al. [12] presented the development and interaction of vortices over a yawed delta wing with leading-edge extension (LEX) through offsurface flow visualization using microwater droplets and a laser beam sheet. By sideslip, the coiling, the merging, and the diffusion of the wing and LEX vortices increase on the windward side, whereas they are delayed significantly on the leeward side. Also, the migration behavior of vortices on the windward and leeward sides of the wing change considerably. A review of experimental data for delta wings under both steady and unsteady conditions was presented from a vortex dynamics point of view by Lee and Ho [13]. Conclusions were derived that vortices on the suction surface provide an important contribution to the lift of a delta wing, especially for the wings with large sweep-back angle. Delery [14] stated that, in three-dimensional flows, boundary-layer separation leads to the formation of vortices formed by the roll up of the viscous flow sheet, previously confined in a thin layer attached to the wall, which suddenly springs into the outer nondissipative flow. Comprehensive reviews of experimental and numerical works on vortex breakdown were reported by Leibovich [15,16], Escudier [17], and Visbal [18]. Sahin et al. [19] concluded that substantial retardation, or delay, in the onset of vortex breakdown, and thereby the development of largescale concentration of vorticity due to the helical mode of vortex breakdown, are attainable when the leading edge of the delta wing is perturbed at a natural frequency of vortex breakdown. They also found that upstream movement of the onset of vortex breakdown is attainable when the period of excitation frequency is sufficiently large. Akilli et al. [20] used the technique of PIV to characterize the alterations and structure of the leading-edge vortex formed from a