48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition 4-7 January 2010, Orlando, Florida AIAA 2010-325 Wind tunnel effects on wingtip vortices Xin Huang 1, Hirofumi Igarashi 2, Paul Durbin 3 and Hui Hu 4 Iowa State University, Ames, IA, 50011 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. N I. Introduction umerous experimental, theoretical and computational investigations have been conducted to improve the basic understanding of wing-tip vortex structure: Chow et al. (1997) and Devenport et al. (1996), for instance. A good physical understanding is essential in order to guide a variety of applications. For these purposes, a detailed knowledge of the fluid dynamics of wing-tip vortices in the near field is desired. A particular concern is how the fluid dynamics are affected by wind tunnel confinement. 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. 1 PhD Student, Aerospace Engineering 2 PhD Student, Aerospace Engineering. 3 Professor, Aerospace Engineering. Senior Member AIAA 4 Associate Professor, Aerospace Engineering, Member AIAA. 1 Copyright 2010 by the, Inc. All rights reserved.
Our work is a combined experimental/numerical collaboration. A wing tip vortex is positioned at various distances from a wind tunnel wall. The wall is an insert that can be positioned above the wing tip. Computations of the flow field are carried out before the measurements to help configure the apparatus, and afterwards to compare CFD to the experimental results. The wind tunnel test section dimensions are 39 x 30 x 30cm in the streamwise and cross-stream directions. The chord and span of the wing are 10.2 and 15.2cm, respectively. Hence, the wing tip is at the middle of the test section, when there is no wall insert. The geometry is shown in figure 1. The wing is mounted on a side wall. Measurements are made with stereoscopic PIV. In the geometry of figure 1 the distance to the wall opposing the wing is to be varied in order to investigate effects of confinement. The right portion of figure 1 shows the mesh at the tip of the wing. It is well resolved to capture the original vortex. Fig. 1: Geometry and mesh around wing tip. A laser sheet is fixed at 13in upstream from the test section outlet. Downstream sweeps are effected by repositioning the wing. Data on the vortex development are taken at 1, 2, 3 and 4 chord lengths after the trailing edge. II. Results For the initial measurements, the laser sheet is 3 chord lengths after the trailing edge. The experimental flow field develops similarly to the computed field. The computations are validated against experimental measurements downstream of the wing, with the wing well away from the wall of the wind tunnel. Figure 2 compares CFD results without the wind tunnel walls and with them. They are very close, suggesting that walls are having little effect on the experiment in this configuration. 2
Fig. 2: Flow with and without wind tunnel walls. Figure 3 compares streamwise vorticity contours. The computation captures the correct level. This is a good test that the grid resolution is adequate. The computations invoke the SST eddy viscosity closure. That model does not have an explicit correction for the effect of swirl on the turbulent energy; nevertheless the vortex appears to have the right strength. We will see later that the turbulent kinetic energy is not predicted well. Experiment Fig. 3: Streamwise vorticity. The laser sheet is 3 chord lengths downstream of the trailing edge. The wing tip is at the lower right. 3
Experiment Fig. 4: Streamwise velocity. The laser sheet is 3 chord lengths downstream of the trailing edge. The wing tip is at the lower right. Figure 4 compares predicted and measured contours of streamwise velocity. Figure 5 provides a comparison of the spanwise velocity. In both cases, again, the CFD is predicting about the correct level and pattern of the flow: the computed streamwise velocity at the core of the vortex is about 8% above the experiment. Experiment Fig. 5: Spanwise velocity. The laser sheet is 3 chord lengths downstream of the trailing edge. The wing tip is at the lower right. Despite the overall correct flow field, figure 6 shows that the turbulent kinetic energy is not well predicted. The experiment is showing a high intensity at the center of the vortex. These data have been corrected for meander. Were centrifugal stabilization substantial, the center of the vortex would have low intensity, with higher levels around the edge. The computation shows about 50 times higher magnitude, over a larger area. The cause of this discrepancy is under investigation. It is not consistent with the generally close agreement between the mean velocity fields. As a first step, the influence of prescribed inlet turbulent intensity was examined. These are shown in figure 7. In previous figures the inlet intensity was 1%. Decreasing it to 0.5% or increasing it to 1.5% has little effect on the flow. 4
Experiment Fig. 6: Turbulent kinetic energy. The laser sheet is 3 chord lengths downstream of the trailing edge. The wing tip is at the lower right. Figure 7. Streamwise vorticity and turbulent kinetic energy with the inlet turbulent intensities of 0.5%, 1% and 1.5% from top to bottom 5
Finally, figure 8 shows how the vortical field varies as the opposing wall is moved closer to the wing tip. Experiments are in progress to test the CFD predictions. Starting with panel (a) the left most wall is moved in from 15 to 10 to 5 to 2.5cm from the wing tip. Since the chord is 10.2 cm, the closest approach is 0.25 chords. The primary effect is that the position of the vortex core moves upward --- increasing y/c. It stays at about the same z/c location. The upward movement is consistent with the effect of the image vortex. Fig. 7: Flow fields with the outer wall wind tunnel wall at 15, 10, 5 and 2.5cm, cases a-d, from the wing tip. Acknowledgment This work is sponsored by the AFOSR, grant FA9550-08-1-0294. References 1 Chow, J. S., Zilliac, G. C., and Bradshaw, P. B., Mean and turbulence measurements in the near field of a wingtip vortex, AIAA Journal, Vol. 35, 1997, pp. 1561-1567 2 Devenport, W. J., Rife, M. C., Lapis, S. I. and Follin, G. J, The structure and development of a wingtip vortex, J. Fluid Mechanics, Vol. 312, 1996, pp. 67-106. 3 Wang, T. and Coton, F. C. Numerical simulation of wind tunnel wall effects on wind turbine flows, Wind Energy, Vol. 3, 2000, pp. 135-148. 6