Experimental observations of the three-dimensional wake structures and dynamics generated by a rigid, bioinspired pitching panel

The effects of changing Strouhal number on the three-dimensional wake produced by a rigid, bioinspired trapezoidal pitching panel are analyzed through the use of stereoscopic particle image velocimetry over a Strouhal number range of 0.17–0.56. The results show that for all cases, at least some section of the wake comprises an alternating series of interacting vortex rings. The behavior of the flows induced by these vortex rings is consistent with the wake phenomena of spanwise compression and transverse expansion. Increases in Strouhal number correspond to an increased rate of spanwise compression, a greater amount of transverse expansion, and the movement of the location of wake breakdown onset upstream.

Figure 1

Water tunnel test section and stereoscopic PIV setup with coordinate axis definitions. Orientation of panel and NACA-0012 fairing are also shown.

Figure 2

Geometric details of the trapezoidal pitching panel.

Figure 3

Isometric views of $Q$ isosurfaces at a value of 1% $Q_{\max }$ at $t/T=0$ (left column) and $t/T=0.25$ (right column) for all five St. $Q$ isosurfaces are colored by the local value of ${\omega }_z$.

Figure 4

Side views of $Q$ isosurfaces at a level of 1% $Q_{\max }$ at $t/T=0$ (left column) and $t/T=0.25$ (right column) for all five St. $Q$ isosurfaces are colored by the local value of ${\omega }_z$.

Figure 5

Zoom-in of side views of $Q$ isosurfaces at a level of 1% $Q_{\max }$ at $t/T=0$ (left column) and $t/T=0.25$ (right column) for $\text{St}=0.27$ and $\text{St}=0.37$. Isosurfaces of $Q$ are colored by the local value of ${\omega }_z$ in the same manner as Fig. 4.

Figure 6

Top views of $Q$ isosurfaces at a level of 1% $Q_{\max }$ at $t/T=0$ (left column) and $t/T=0.25$ (right column) for all five St. $Q$ isosurfaces are colored by the local value of ${\omega }_z$.

Figure 7

Top views of ${\omega }_z$ in the plane of the midspan at $t/T=0$ (left column) and $t/T=0.25$ (right column) for all five St. The plots have been thresholded in order to only display regions of the wake where $|{\omega }_z|\ge 5{\text{s}}^{-1}$.

Figure 8

Schematic of general wake features and vortex-induced flow fields, and isosurfaces obtained from PIV data. Isosurfaces of ${\omega }_x,{\omega }_z,v$, and $w$ are shown for $\text{St}=0.27$ at $t/T=0$.

Figure 9

Side views of $w$ isosurfaces at $t/T=0$ (left column) and $t/T=0.25$ (right column) for all five St. Gray isosurfaces are presented at $w=25$ mm/s and black isosurfaces are presented at $w=-25$ mm/s.

Figure 10

Top views of $w$ isosurfaces at $t/T=0$ (left column) and $t/T=0.25$ (right column) for all five St. Gray isosurfaces are presented at $w=25$ mm/s and black isosurfaces are presented at $w=-25$ mm/s.

Figure 11

Top views (left column) and side views (right column) of isosurfaces of ${\omega }_x$ and $w$ at $t/T=0$ (left column) for all five St. Red and blue isosurfaces of ${\omega }_x$ are presented at ${\omega }_x=10{\text{s}}^{-1}$ and ${\omega }_x=-10{\text{s}}^{-1}$, respectively. Gray and black isosurfaces of $w$ are presented at $w=40$ mm/s and $w=-40$ mm/s, respectively.

Figure 12

Side views of $v$ isosurfaces at $t/T=0$ (left column) and $t/T=0.25$ (right column) for all five St. Gray isosurfaces are presented at $v=20$ mm/s and black isosurfaces are presented at $v=-20$ mm/s.

Figure 13

Top views of $v$ isosurfaces at $t/T=0$ (left column) and $t/T=0.25$ (right column) for all five St. Gray isosurfaces are presented at $v=20$ mm/s and black isosurfaces are presented at $v=-20$ mm/s.