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8. Through the contribution of these authors the development and lifetime of the fully developed wing-tip vortex is relatively well understood. Many of these experimental methods have had difficulties resolving the vortex core. These difficulties stem from the high velocity gradients, and flow fluctuations that typically accompany the vortex core. In particular it has shown to be difficult to resolve the out-of-plane or axial velocity component of the vortex core. Therefore, it is beneficial to instead examine the in-plane velocity components from which much can be learned about the vortex flow field. Additionally, exact solutions to the Navier-Stokes equations exist for vortex flows. The most basic of these solutions involves superimposing a stagnation point flow and a potential vortex, resulting in axial, circumferential, and radial potential solutions. Also, steady and unsteady viscous solutions for a vortex in stretched flow have been developed. An unsteady solution for a viscous vortex in stretched flow was discovered by Rott 9 , who independently arrived at Burgers’ famous solution 10 . Rott’s solution is circumferentially symmetric, and has time and radial dependence. This effort adapted Rott’s solution into a steady solution for circumferential velocity that is dependent on radial and axial coordinates. This experiment sought to detail the evolution of a wing-tip vortex shed into a favorable pressure gradient. Cross sectional velocities were measured at sequential locations in a mild contraction in the Cal Poly 3 x 4 ft lowspeed wind tunnel to examine the circumferential velocity and vorticity evolutions of the vortex in the streamwise direction. Afterwards, these experimental results were compared with previous experiments as well as an approximate analytical solution to a vortex in stretched flow. Test description The experiment was conducted in the California Polytechnic State University 3 ft by 4 ft low-speed open-circuit indraft wind tunnel. The tunnel inlet has an 11:1 contraction ratio, it has a honeycomb of soda straws followed by three screens at the entrance. The variation in mean velocity across an empty test section was less than 1% and the mean free-stream turbulence intensity was less than 0.5% at a centerline speed of 20 m/s. The wind tunnel was powered by a belt driven, 150 hp, 440 volt three-phase motor. A one-dimensional contraction was designed to fit inside the wind tunnel test section. The contraction created a favorable pressure gradient that would cause stretching in a wing-tip vortex passing through the section. It had increasing curvature for the first 30% of the length, and then |
