Two-dimensional structure of the wake behind a pitching airfoil with higher non-dimensional pitching rate

Two-dimensional flow patterns have been visualized by two kinds of dyes and a VTR camera in a water tunnel atRe=4.0×103 in order to study how flow patterns of vortex changed behind a pitching airfoil with various pitching rates and pitching amplitudes. The tested airfoil was NACA0010. The experiment was performed under the conditions such as the pitching motions with sinusoidal or triangular wave around its mid- or quarter-chord axis. The non-dimensional pitching rates werek=1.97, 2.81 and 4.22. The pitching amplitudes were changed asA=±2, ±6 and ±12, however the mean angle of attack was fixed atμm=0°. As a result, the followings were clarified. The flow pattern of thrust producing vortex street was observed in the wake behind the pitching airfoil with a higher nondimensional pitching rate. Similar flow patterns of the vortex were observed in spite of the differences in the location of pitching motion axis, in the pitching motion wave profile and in the pitching amplitude. Moreover, the pitching motion around the quarter-chord axis with triangular wave could realize the thrust producing vortex street more easily.