Experimental examination of vorticity stripping from a wing-tip vortex in free-stream turbulence

Time-resolved stereoscopic particle image velocimetry measurements were conducted of a wing-tip vortex decaying in free-stream turbulence. The objective of the research was to experimentally investigate the mechanism causing the increased rate of decay of the vortex in the presence of turbulence. It was observed that the circulation of the vortex core experienced periods of rapid loss and recovery when immersed in free-stream turbulence. These events were not observed when the vortex was in a laminar free stream. A connection was made between these events and distortion of the vortex, coinciding with stripping of core fluid from the vortex core. Specifically, vortex stripping events were connected to asymmetry in the vortex core, and this asymmetry was associated with instances of rapid circulation loss. The increased rate of decay of the vortex in turbulence coincided with the formation of secondary vortical structures which wrapped azimuthally around the primary vortex.

Figure 1

Experiment configuration.

Figure 2

Comparison of (a) ensemble-averaged core circulation $\langle {\mathrm{\Gamma }}_c\rangle /\langle {\mathrm{\Gamma }}_0\rangle$ and (b) inner circulation ${\mathrm{\Gamma }}_1/\langle {\mathrm{\Gamma }}_0\rangle$ for the no-grid, small-grid, and large-grid cases.

Figure 3

Isosurfaces showing evolution of $U_{\theta }r/\langle {\mathrm{\Gamma }}_0\rangle =0.035$ with $U_{t}t/C$ for a single member of the ensemble of the (a) no-grid, (b) small-grid, and (c) large-grid cases.

Figure 4

Contour plots at constant angle of $U_{\theta }r/\langle {\mathrm{\Gamma }}_0\rangle$ showing its evolution with $U_{t}t/C$ for a single member of the ensemble taken during measurement of the (a) no-grid, (b) small-grid, and (c) large-grid cases.

Figure 5

Isosurface plots of normalized swirling strength $\lambda /{\langle {\mathrm{\Omega }}_x^{★}\rangle }^2=0.008$ showing its evolution with $U_{t}t/C$ for a single member of the ensemble taken from measurements of (a) no-grid, (b) small-grid, and (c) large-grid cases.

Figure 6

Contour plots at constant angle showing the change in $\lambda /{\langle {\mathrm{\Omega }}_x^{★}\rangle }^2$ with $U_{t}t/C$ for a single member of the ensemble taken from measurements of (a) no-grid, (b) small-grid, and (c) large-grid cases.

Figure 7

Contour plots of swirling strength $\lambda /{\langle {\mathrm{\Omega }}_x^{★}\rangle }^2$ covering time interval $\mathrm{\Delta }t=0.021C/U_t$ for (a)–(c) no-grid, (d)–(f) small-grid, and (g)–(i) large-grid cases with the initial at $U_{t}t/C=1.02$.

Figure 8

Scatter plots of normalized $d{\mathrm{\Gamma }}_1/dt$ plotted against $C_1$ for (a) no-grid, (b) small-grid, and (c) large-grid cases. Dashed red lines indicate the mean value of $C_1$ for the no-grid case and zero level of circulation decay. Dashed blue lines in (b) and (c) indicate the mean value of $C_1$ for small-grid and large-grid cases.

Figure 9

Isosurface plots of pseudo azimuthal vorticity ${\mathrm{\Omega }}_{\theta \theta }/\langle {\mathrm{\Omega }}_x^{★}\rangle =0.09$ (red), ${\mathrm{\Omega }}_{\theta \theta }/\langle {\mathrm{\Omega }}_x^{★}\rangle =0.05$ (green), and ${\mathrm{\Omega }}_{\theta \theta }/\langle {\mathrm{\Omega }}_x^{★}\rangle =-0.05$ (blue) with $U_{t}t/C$ for (a) no-grid, (b) small-grid, and (c) large-grid cases.