Doppler Global Velocimeter Measurements of the Vortical Flow Above A Thin 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. Data obtained using this technique will be used to correlate with other data sets for verification, and following verification, provide a quantified, highly detailed definition of the flow field. This will help to improve the understanding of fluid physics, supplement and broaden the database required to validate and refine computational fluid dynamics (CFD) models , and improve aircraft design methodology. To assess the capability of the technique, velocity measurements of the vortical flow field above a thin 75-degree delta wing were made in the NASA Langley Basic Aerodynamics Research Tunnel. Preliminary comparisons of the results were made with similar measurements obtained using a three component laser velocimeter indicate that this technique is capable of d e s c r i b i n g t h e e n t i r e t h r e e c om p o n e n t v e l o c i t y f l o w f i e l d simultaneously within a measurement plane in real time. 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 Dn Doppler shift frequency, Hz n Laser output frequency, Hz q Angle between the laser propagation direction and the collected scattered light, deg Introduction Designers of modern aircraft need improved design methods which e x p l o i t c om p l e x t h r e e d i m e n s i o n a l f l o w f i e l d s t o e n h a n c e maneuverability and increase lift at certain critical stages in the flight regime. Fighter aircraft, for example, operating at high angles of attack generate vortices to aid in increasing performance. However, these vortices may also interact with the airframe or other external systems resulting in buffeting and the potential for fatigue-related structural problems. With increased knowledge of the flow field, designers will be able to exploit the increase in performance resulting from vortical flow while avoiding the problems. Similarly, high speed civil transports flying at low speed may require higher lift than available from classic wing designs. Designers have solved this problem by using leading-edge vortex flow control devices to obtain high lift and low drag on slender, highly swept wings. The development of design methods which take advantage of these complex flow fields has been hampered in the past b e c a u s e o f t h e l a c k o f q u a n t i t a t i v e n o n i n t r u s i v e f l o w f i e l d measurements.