Wind Tunnel Effects on Wingtip Vortices

Abstract

The influence of wind tunnel boundaries on a wing tip vortex is studied experimentally and by computation. Computations show that vortex is similar to that which would exist without tunnel walls. The experimental data are corrected for meandering. Then computations are in close agreement to data. A computation with the wall moved closer to the tip shows the primary effect to be displacement of the vortex core by the image effect. Wind tunnel experiments have studied the dynamics of the initial rollup from a stationary wing, and the subsequent development of axial and tangential velocities and turbulence with downstream distance. Devenport et al. (1996) reported that the flow outside the vortex core was dominated by the remainder of the wing wake, which wound into an ever increasing spiral, and that the turbulence stress levels varied along the wake spiral in response to the varying rate of strain imposed by the vortex. Chow et al. (1997) investigated the wing-tip vortex of a NACA 0012 airfoil model. They indicated a high level of axial velocity, in excess of 1.7U ∞ at all measurement locations. They also reported that the turbulence intensity in the vortex can be as high as 24%, but it decayed quickly with streamwise distance because of the stabilizing effect of the nearly solid-body rotation within the vortex-core. Many other detailed properties of the tip vortex have been measured in wind tunnel experiments. These studies have uncovered a good deal of useful information. However, the majority of them used point-wise flow measurement techniques. A common shortcoming of point-wise measurements is the inability to provide spatial structure of the unsteady vortices. Full field measurements are needed to effectively reveal the transient behavior of the wing-tip vortex structures. Temporally synchronized and spatially resolved flow field measurements are highly desirable in order to elucidate underlying physics. In a companion paper we describe the instrumentation for our current experiments. It consists of high resolution, stereoscopic PIV. Here we report preliminary data and CFD comparisons. It is well known that wind tunnel walls interfere with the flow around a test model. That is a major source of uncertainty in simulating free flight conditions. Various correction methods have been proposed for flows with concentrated vortices. For instance, Wang and Coton (2000) suggested combining a low-order panel method and a prescribed wake vortex into a coupled model to assess the basic affect of wind tunnel walls on wind turbine flow and performance. They reported that the numerical results from the coupled model compared well with the wind tunnel test data, although some discrepancies were noted. However, very little can be found in the literature to evaluate wind tunnel confinement effects on wing-tip vortices. Tunnel effects include test model blockage, inlet flow non-uniformities and inlet turbulence levels. Questions about how similar measured characteristics are to those occurring in free flight remain. Confinement effects provide one focus of the present experimental and numerical study.