Kenneth S. Wittmer

Perpendicular blade vortex interaction

The interaction between a streamwise vortex and a spanwise blade was studied in incompressible flow for blade-vortex separations between ±1/8 chord. Three-component velocity and turbulence measurements were made from 4 chord lengths upstream to 15 chord lengths downstream of the blade using miniature four-sensor hot-wire probes. The interaction of the vortex with the blade causes an almost immediate loss in vortex core circulation. Downstream of the blade the core becomes embedded in the blade wake and begins to grow rapidly. Core radius increases and peak tangential velocity decreases but circulation remains roughly constant. True turbulence levels within the core are much larger downstream than upstream of the blade. Outside of the core the interaction produces a substantial region of turbulent flow associated with the blade and vortex generator wakes. Overall, perpendicular blade vortex interaction substantially alters the flow and produces a much larger and more intense region of turbulent flow than that presented by the undisturbed vortex. These results have significant implications for the prediction of both impulsive and broadband helicopter noise.

Wake of a Compressor Cascade with Tip Gap, Part 3: Two Point Statistics

Velocity spectra and space-time correlations have been measured downstream of a low-speed linear compressor cascade with tip gap. The objective of this work is an improved understanding of the coherent turbulence structures present in fan-tip wakes and of the turbulent source terms responsible for broadband stator noise

Effects of perpendicular blade-vortex interaction. Part 2 : Parameter study

Experiments have been performed to document the turbulent flow produced downstream of an airfoil encountering an intense streamwise vortex. In the second part of two-part paper, measurements revealing the effects of blade-vortex separation, blade angle of attack, and vortex strength are presented. The purpose is to provide the data needed to improve blade-vortex and blade-wake interaction noise prediction schemes to account for these effects. A total of 48 flows were studied by measuring three-component velocity and turbulence stress profiles and spectra through the vortex core downstream of the interaction. Most measurements were made 15 chordlengths downstream of the blade trailing edge. These measurements show, consistent with an inviscid model, that the effects of the interaction are only weakly dependent on blade angle of attack and vortex strength and on whether the vortex passes to the suction or pressure side of the blade. The interaction has little influence on the properties of the vortex core for separations greater than about 0.3 chords

Turbulence structure of the flow downstream of a compressor cascade with tip leakage

Detailed measurements of the mean flow and turbulence stress field downstream of a linear compressor with tip gap have been made. The cascade, consisting of 8 GE Rotor B section blades produces a highly periodic flow with 11.5 degrees of flow turning. Measurements were made for an approach free stream velocity of 87 ft/s. The tip-leakage vortex dominates the endwall flow region. Away from the endwall, the blade wakes exhibit characteristics similar to those of a two-dimensional wake. The mean flow fields of the blade wakes and vortex decay at similar rates as the flow travels downstream. The turbulence stress field of the wake decays at a rate consistent with the rate of decay of its mean flow field, but the decay rate of the turbulence generated by the vortex is much slower. The vortex generates intense turbulent fluctuations in an arch-shaped region surrounding the core that includes the region where flow is being lifted away from the endwall. This turbulence is apparently generated by the gradients of streamwise velocity associated with the vortex, rather than its circulating motion.