Three Component Doppler Global Velocimeter Measurements of the Flow Above A Delta Wing

A new measurement technique is being developed by NASA to measure off-surface flow fields. This method, Doppler global velocimetry, will allow quantification of complex three-dimensional flow fields at video camera rates. The entire flow field structure within a selected plane is measured simultaneously rather than by scanned, point-by-point measurements using conventional laser velocimetry. To assess the c a p a b i l i t y o f t h i s n ew t e c h n i q u e , t h r e e c omp o n e n t v e l o c i t y measurements of the vortical flow field above a thin 75-degree delta wing were made in the NASA Langley Basic Aerodynamics Research Tunnel. Measurements were made of the flow field at the 70-percent chord loca t i on at ang le s o f a t tack o f 20 .5 and 40 .0 -degrees to investigate unburst and burst vortices. For comparison, previous fringe-type laser velocimeter measurements of the flow field at the same conditions are included. Nomenclature ALF Absorption line filter, Iodine vapor c Speed of light, m/sec î Laser beam propagation direction ô Collected scattered light direction V Velocity of a particle passing through the laser beam, m/sec X Cross tunnel coordinate, m Y Vertical coordinate, m Z Streamwise coordinate, m ∆ν Doppler shift frequency, Hz ν Laser output frequency, Hz θ Angle between the laser propagation direction and the collected scattered light, deg Introduction Of the many fundamental flow structures comprising aircraft fluid dynamics, vortical flows are receiving increased attention. Vortices, if properly utilized, can provide increased lift to enhance maneuverability of modern high-performance aircraft. Devices such as leading edge extensions and extended forebodies have produced vortical flows which have increased the performance of aircraft such as the F-15 and the F/A–18. These devices were developed using the trial-and-error method of aircraft design using force and moment response of wind tunnel models to determine increases in performance. This method provides little, if any, information on dynamic loading on aircraft structures, nor off-body flow field information to correlate flow dynamics with force and moment data. Even if off-body flow field investigations were conducted, classic approaches using probes would change the flow since vortices will avoid obstacles and move around the probes. The only known non i n t r u s i v e mea su r emen t s o f t h e f l ow abou t a h i gh performance configuration were obtained years after the aircraft was operational, reference 1. This investigation, using fringe-type laser velocimetry to measure the off-body flow above a YF-17 at high anglesof-attack, found a complex flow field containing burst vortices and flow separation over 60 percent of the wing. The trajectory of the burst vortex intersected the vertical stabilizer at mid span imparting large dynamic forces on the stabilizers, which may have caused the fatigue damage found in F/A-18’s.