We observed two-dimensional vorticity fields on a NACA0012 airfoil in a periodic flow modeled as a gust flow with a sinusoidal velocity profile in the streamwise direction. Experiments were conducted by using particle image velocimetry in a water tunnel under conditions of Re=$3.0\times 10^3$, reduced frequency $k=2.27$ and $15^\circ$ angle of attack. We found that phase-turbulence intensity fields illustrate the separation and reattachment points more quantitatively than do phase-averaged vorticity fields.

This intensity is defined as the ratio of the root mean square of the flow fluctuation $u'(x,y,\phi)$ to the phase-averaged velocity $\overline{u}(x,y,\phi)$, as follows: \begin{equation} I_{\phi}(x,y,\phi)= \frac{|u'(x,y,\phi)|}{\sqrt{2}|\overline{u}(x,y,\phi)|} \tag{6} \end{equation} The separation and reattachment points have high values for phase-turbulence intensity as defined by Eq. (6) because the denominator $|\overline{u}(x,y,\phi)|$ is also nearly 0 m/s. Hence, the phase-turbulence intensity fields can illustrate separation and reattachment points and the vortex core more clearly than do the phase average of vorticity fields.

1. Observation of Two-dimensional Vorticity Field of Airfoil Surface in a Periodic Flow under Low Reynolds Number
   Hirohisa YAMANISHI, Yohsuke TANAKA and Shigeru MURATA
   10th International Symposium on Advanced Science and Technology in Experimental Mechanics, Paper Number:60, Matsue, Shimane, Japan, November 1-4, 2015.

© Measurement System Laboratory, Kyoto Institute of Technology.