Gordon Taylor

An Investigation of Vortex Flows over Low Sweep Delta Wings

This paper presents the results of a recent investigation into the vortex structure over a nonslender delta wing with leading edge sweep, Λ = 50°. A flow visualisation study in water tunnel experiments has shown profound sensitivity of the vortex structure to Reynolds number. As Reynolds number was reduced, the trajectory of the vortex core moved inboard toward the wing centre-line, and the onset of breakdown was noticeably delayed. The results provide the first experimental evidence of the dual vortex structure that has been observed in previous computational studies. The formation of these dual vortices was also sensitive to Reynolds number, and its formation was not observed at the very low end of values considered. Digital Particle Image Velocimetry (DPIV) measurements of the cross-flow have yielded axial vorticity data at a number of streamwise stations and incidences. At low incidences only a very weak vortex structure was observed with a strong shear layer forming very close to the wing surface. As incidence was increased the shear layer lifted from the surface but reattachment was still observed. Increasing incidence also resulted in a movement of the reattachment line towards the model centre-line, until ultimately reattachment failed.

Buffeting Flows over a Low-Sweep Delta Wing

An experimental study was conducted with the aim of understanding the unsteady vortex flows and buffeting response of a nonslender delta wing with 50-deg leading-edge sweep angle. Particle image velocimetry and laser Doppler velocimetry measurements, surface flow visualization, force balance measurements, and wing-tip acceleration measurements were used. It was found that there is a profound effect of Reynolds number on the structure of vortical flows. The breakdown of the leading-edge vortices is delayed significantly, and the vortices form more inboard at low Reynolds numbers. The secondary vortex effectively splits the primary vortex into two separate concentrations of vorticity, resulting in a dual vortex structure at small incidences. This dual vortex structure diminishes, and a single primary vortex is observed at higher incidences. At higher Reynolds numbers (on the order of 3 × 10 4) the flow approaches an asymptotic state, with further increases in the Reynolds number resulting in only small variations in the location of vortex core and breakdown. Weak vortex breakdown observed at low incidences is replaced by a conical breakdown with increasing incidences. However, the maximum buffeting occurs prior to the stall, after the vortex breakdown has reached the apex of the wing. The largest velocity fluctuations near the wing surface are observed along the reattachment line. Hence, the shear-layer reattachment, rather than the vortex breakdown phenomenon, is the most important source of increasing buffet in the prestall region as incidence is increased. The velocity fluctuations in the reattachment region have similar dominant frequencies as slender wings in spite of the differences in the physical nature of the flow. With further increase in incidence, the shear-layer reattachment becomes impossible, resulting in very low velocity fluctuations near the wing surface and a precipitous fall in the rms wing-tip acceleration.

Lift enhancement over flexible nonslender delta wings

Unsteady aerodynamics of flexible nonslender delta wings is investigated in an experimental study using various techniques. Dramatic fluid/structure interactions emerge with increasing wing flexibility and result in substantial lift enhancement in the poststall region. This recently discovered phenomenon appears to be a feature of nonslender wings. Self-excited antisymmetric vibrations of the wing promote reattachment of the shear layer, which results in the lift enhancement. These self-excited vibrations are not observed for a half-model. The Strouhal number of the dominant frequency of the structural vibration is on the order of unity for all nonslender wings, which also corresponds to the frequency of the shear-layer instabilities for the rigid wing. Velocity measurements demonstrate the striking difference between the flows over the flexible and rigid wings in the poststall region. The effect of flexibility is to promote the reattachment of the shear layer near or downstream of the apex, depending on the incidence. There are substantial effects on the vortical flow with increasing wing flexibility, which might lead to the axial flow developing within the reattached region. The time-averaged vorticity flux increases due to the oscillating leading edge, which leads to increased circulation.

Passive Flow Control over Flexible Non-Slender Delta Wings

∗† ‡ Force measurements over a range of non-slender delta wings have demonstrated the ability of a flexible wing to enhance lift and delay stall compared with a rigid wing of similar geometry. The work has extended the results of a recent study to include delta wings with a range of sweep angles. It has been shown that the greatest lift enhancement was observed over the wing with smallest leading edge sweep. Additionally for the only slender wing considered no lift enhancement was observed indicating that it is a feature of the fundamentally different flow that occurs over low-sweep wings that is responsible for the phenomenon. The variation of RMS lift force and rolling moment coefficients with incidence for all the wings concerned suggest that the wings are vibrating in an antisymmetric structural mode in the lift enhancement region. This supports previous evidence for the 50° wing that suggests the same. Further, experiments of a half-wing model suggest that this wing does not undergo lift enhancement, and as such it may be that the antisymmetric vibration is a necessary condition for the lift enhancement to exist. Wing-tip deflections for some of the wings were also measured, which showed that the slender wing undergoes much higher time-averaged deflections than the non-slender wings, although it also experiences a much lower level of buffet. PIV and LDV measurements have demonstrated the striking difference between the surface flows over the flexible and rigid wings in the post-stall region at the same incidence. Implementing flexibility on a low sweep wing results in continued reattachment of the shear layer to much higher incidences than would otherwise be observed over a rigid wing.

UNSTEADY VORTEX FLOWS AND BUFFETING OF A LOW SWEEP DELTA WING

An experimental study was conducted to understand the unsteady vortex flows and buffeting response of a nonslender delta wing with 50° leading edge sweep angle. Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) measurements, surface flow visualization, force balance measurements, and wing-tip acceleration measurements were used. Weak vortex breakdown observed at low incidences is replaced by a conical breakdown with large unsteadiness with increasing incidence. However, the maximum buffeting occurs prior to the stall, after the vortex breakdown has reached the apex of the wing. The largest velocity fluctuations near the wing surface are observed along the reattachment line. Hence, the shear layer reattachment, rather than the vortex breakdown phenomenon, is the most important source of increasing buffet in the pre-stall region as incidence is increased. Velocity fluctuations near the wing surface in the reattachment region exhibit ‘dual-peak’ frequency spectra. With further increase in incidence, the shear layer reattachment fails resulting in very low velocity fluctuations near the wing surface and a precipitous fall in the rms wing tip acceleration.

Investigation of support interference in high-angle-of-attack testing

An investigation was undertaken to understand support interference in high-angle-of-attack testing with particular emphasis on premature vortex breakdown induced by struts. In static experiments efforts concentrated on the support geometry and its location with respect to leading-edge vortices generated by delta wings. Extensive e ow visualization studies show that vortex breakdown induced by a dummy support might move over the wing depending on theangleofattack andthelocation of thesupport.As thelateral distancebetweenthevortex axisand support is varied, static hysteresis of vortex breakdown location was observed, which will have very important implications on forcemeasurements in wind-tunnel testing. The results also suggest that support interference is more important for slender wings. To separate the effects of time-dependent vortex strength and support interference, the model was kept stationary, and a dummy support was oscillated in the spanwise direction in the oscillatory experiments. It was observed that vortex breakdown location oscillates with large amplitudes at low frequencies, but does not show any response at high frequencies, indicating that the frequency response is similar to that of a low-pass e lter. Variation of phase-averaged breakdown location showed hysteresis loops and time lags, which are largerfor a thin e at platethan a circularcylinder. Theresultssuggestthatsupport interferenceproblems aremore complex in transient experiments.

Physical mechanisms of lift enhancement for flexible delta wings

Passive lift enhancement for flexible nonslender delta wings has been demonstrated as a potential method for the control of vortex-dominated wing flows. Physical mechanisms of lift enhancement and the effect of important variables are discussed. Lift enhancement for flexible wings with low/moderate sweep is a very complex phenomenon, involving self-induced antisymmetric vibrations of leading edges, spanwise camber effect due to the large time-averaged deflection, shear layer instabilities, reattachment, increased mean vorticity flux and circulation, re-formation of the leading edge vortices, possible enhancement of streamwise pressure gradient, and the effects of frequency and edge velocity.

Lift Enhancement over a Flexible Delta Wing

*† Passive lift enhancement over a flexible, low-sweep delta wing has been demonstrated as a potential method for control of vortex-dominated wing flows. The lift enhancement was achieved in the post-stall region of a wing of 50° leading edge sweep. An increase in timeaveraged lift coefficient of up to 45%, and a delay in stall of up to 9 degrees were observed. Large time-averaged structural deflections and vibrations accompany this behavior, along with a switching of dominant mode of vibration from the fundamental to the 2 nd antisymmetric mode. Comparisons of these results with examples of active flow control over delta wings in literature indicate that the mechanism for this behavior may be related to the excitation of sub-scale vortices in the shear layer originating from the leading edges. It is proposed that vibration of the wing energizes the vortices shed into the shear layer, which promotes earlier reattachment on the suction surface, delaying stall and allowing lift coefficient to increase.

Support interference for a maneuvering delta wing

The results of a recent investigation of support interference in dynamic wind-tunnel testing are presented. Particular emphasis has been focused on studying the interaction of vortices generated by delta wing models at high angle of attack with a support structure. A novel approach was used whereby the effects of time-varying vortex strength could be separated from the effects of the vortex-support interaction. With this approach, it was shown that the principal effect of support interference was to cause an upstream shift in the mean breakdown location, regardless of the form of motion or forcing frequency. The magnitude of the fluctuations in the breakdown location was shown to be dictated by support interference at low forcing frequencies, but by time-varying vortex strength effects at high frequencies. A number of important observations were also made of the vortex behavior at high forcing frequencies. In particular, a curious jumping behavior of the vortex breakdown location was observed. The presence of the support reduced the magnitude of the breakdown jump, but jumping was, nevertheless, observed in all of the cases considered at high enough forcing frequencies.