Kenneth S. Wittmer
Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
William J. Devenport, Chair
William H. Mason
Wayne L. Neu
Saad A. Ragab
Roger L. Simpson
October 23, 1996
Experiments were performed to document the turbulent flowfield produced downstream of an airfoil encountering an intense streamwise vortex. This type of perpendicular airfoil--vortex interaction commonly occurs in helicopter rotor flows. The experiments presented here thus provide useful information for the prediction of helicopter noise, particularly BWI noise.
Three-component velocity and turbulence measurements were made in unprecedented detail using a computerized miniature four-sensor hot-wire probe system; revealing much about the structure and behavior of this flow over a range of conditions. The interaction between the vortex and the airfoil wake leaves the vortex surrounded by a large region of intense turbulence unlike the turbulence surrounding an isolated vortex. Even for close separations, the vortex core passes the airfoil virtually unchanged. However, vorticity of opposite sign is shed by the airfoil in response to the angle of attack distribution induced by the vortex resulting in an unstable circulation distribution according to Rayleigh's criterion. Simple theoretical models adequately describe the shed vorticity distribution of the airfoil and the unstable circulation distribution it imparts on the vortex.
As the flow develops, the vortex continuously distorts the airfoil wake. The strain rates imparted by the vortex on the spanwise vorticity contained in the airfoil wake result in an anisotropic, turbulence producing stress field. For several chord lengths downstream, the vortex core remains laminar and little change is seen in the unstable circulation distribution. While the vortex core is laminar, turbulent fluctuations measured in the core are the result of inactive wandering motions and the characteristic length and velocity scales of the flat portion of the vortex wake appear to be appropriate scales for the fluctuations. Eventually, the vortex core becomes turbulent as indicated by an increase in high frequency velocity fluctuation levels of more than an order of magnitude. Subsequently, the circulation distribution reorganizes to a stable distribution. A loss in core circulation occurs due to a decrease in the peak tangential velocity which is proportionately larger than the increase in the vortex core radius. The peak tangential velocity decreases to the point where it is exceeded by the axial velocity deficit---another unstable situation. These effects increase with decreased separation between the vortex and the airfoil, but appear to be largely independent of airfoil angle of attack an only weakly dependent upon vortex strength.
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