Airport capacity is constrained, in part, by spacing requirements associated with the wake vortex hazard. NASA's Wake Vortex Avoidance Project has a goal to establish the feasibility of reducing this spacing while maintaining safety. Passive acoustic phased array sensors, if shown to have operational potential, may aid in this effort by detecting and tracking the vortices. During August/September 2003, NASA and the USDOT sponsored a wake acoustics test at the Denver International Airport. The central instrument of the test was a large microphone phased array. This paper describes the test in general terms and gives an overview of the array hardware. It outlines one of the analysis techniques that is being applied to the data and gives sample results. The technique is able to clearly resolve the wake vortices of landing aircraft and measure their separation, height, and sinking rate. These observations permit an indirect estimate of the vortex circulation. The array also provides visualization of the vortex evolution, including the Crow instability.

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Blade-vortex interaction is the source mechanism for a prominent impulsive noise created by rotorcraft. An experimental study of two-dimensional blade-vortex interaction was performed to determine controlling parameters governing the strength of the blade-vortex interaction process. The interaction process is shown to involve a combination of effects on the vortex, including changes in trajectory and distortion of the vortex core shape while the blade undergoes significant transient loading. Calculations of the acoustic field produced by the interaction using measured unsteady blade surface pressure data are presented. The effects of the interaction process on the blade and the vortex are shown to be increased by both reduction of blade-to-vortex spacing and increase in blade loading. Nomenclature a/c = nondimensional vortex major axis length c = blade model chord length, 20.32 cm c g = vortex generator chord length, 15.24 cm C/ = blade unsteady lift coefficient c 0 = acoustic velocity in still air, m/s / = vortex generator oscillation frequency, Hz k = reduced frequency, ir/Cg/UÎ = force per unit area on the fluid, Pa M -Mach number n = unit vector normal to blade panel p = pressure, Pa p ' = acoustic pressure, Pa r = length of radiation vector, m r = unit vector along radiation direction S = blade surface, m 2 t = time, s U,U m = test section total and mean velocity, respectively, m/s u = streamwise perturbation velocity, m/s v = velocity in y/c direction, m/s x/c,y/c = nondimensional length in streamwise direction and normal to streamwise direction, respectively a. = blade angle of attack, deg F = vortex circulation, mVs co = vorticity vector, rad/s Subscripts L = loading term n = normal to the panel surface r = component in the direction of the observer ret = retarded time T = thickness term

The use of an Active Twist Rotor system to provide both vibration reduction and performance enhancement has been explored in recent analytical and experimental studies. Effects of activetwist control on rotor noise, however, had not been determined.During a recent wind tunnel test of an active-twist rotor system, a set of acoustic measurements were obtained to assess the effects of active-twist control on noise produced by the rotor, especially blade-vortex interaction (BVI) noise. It was found that for rotor operating conditions where BVI noise is dominant, active-twist control provided a reduction in BVI noise level. This BVI noise reduction was almost, but not quite, as large as that obtained in a similar test using HHC. However, vibration levels were usually adversely affected at operating conditions favoring minimum BVI noise. Conversely, operating conditions favoring minimum vibration levels affected BVI noise levels, but not always adversely.

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