Observations of a sweeping jet actuator for flow separation control of a backward-facing ramp

Flow separation control of a sweeping jet actuator in a backward-facing ramp was studied using two-dimensional and stereoscopic particle-image velocimetry. The actuator was operated at a single supply rate pertaining to a jet velocity of 7.3 m/s and positioned at the streamwise location of 0.8 and 3.7 boundary layer thickness upstream of the separating edge. Three wind tunnel reference velocities corresponding to Reynolds numbers based on the boundary layer thickness of ${\mathrm{Re}}_{\delta }=2400$, 5200, and 9800 (jet-to-reference velocity ratios of 1, 0.5, and 0.3) were investigated, and the control effectiveness was assessed based on the size of the separation bubble. For all three Reynolds numbers, positioning the actuator nearer to the separating edge offered poorer flow separation control for measurement planes near the jet centerline. This was attributed to the upwash effect of the jet which directed high momentum fluid away from the wall leading to lower levels of entrainment, and the formation of a localized low-pressure region that coincided with the sharp change in ramp geometry leading to stronger adverse pressure gradients and lower streamwise velocities that were more likely to undergo flow reversal. At high jet-to-reference velocity ratios, the effect was accentuated, and a secondary separation bubble was observed. When the actuator was positioned further upstream, the primary streamwise vortices produced by the sweeping jet lifted away from the wall, and secondary flow structures induced near the wall contributed to higher levels of near-wall entrainment and improved flow control along the jet centerline. In contrast, for measurement planes far from the jet centerline, positioning the actuator nearer the separating edge was preferable. This was attributed to milder decay of the primary streamwise vortices, leading to stronger downwash effect and higher entrainment levels that were able to affect a wider spanwise area.

Figure 1

Schematic of the experimental setup during 2D PIV measurements.

Figure 2

Images of the experimental setup during stereoscopic PIV measurements.

Figure 3

Schematic of the sweeping jet actuator and its mounting configuration in the wind tunnel. Dimensions are in millimeters.

Figure 4

Baseline ramp flow at $y/D=0$ measurement plane for (a) $U_{\mathrm{ref}}=7.4\mathrm{m}/\mathrm{s}$, (b) $U_{\mathrm{ref}}=15.1\mathrm{m}/\mathrm{s}$, and (c) $U_{\mathrm{ref}}=25.1\mathrm{m}/\mathrm{s}$. (i) Ensemble-averaged velocity contours with superimposed streamlines and (ii) the separation bubble.

Figure 5

Qualitative characterization of the boundary layer using a single continuous HWA measurement. (a) Measured voltage signals for the entire acquisition time and (b) magnified view of the reference and $U_{\mathrm{ref}}=7.4\mathrm{m}/\mathrm{s}$ test cases.

Figure 6

Ramp flow with control at the $y/D=0$ measurement plane for (a) P1-U7.4 ($\mathrm{VR}=1$), (b) P1-U15.1 ($\mathrm{VR}=0.5$), and (c) P1-U25.1 ($\mathrm{VR}=0.3$) test cases. (i) Ensemble-averaged velocity contours with superimposed streamlines and (ii) the separation bubble.

Figure 7

The separation bubble at (a) P1-U7.4, (b) P1-U15.1, and (c) P1-U25.1 test cases. Measurement planes at (i) $y/D=1$, (ii) $y/D=2$, and (iii) $y/D=3$.

Figure 8

(a) Time-averaged nondimensional reattachment lengths ($x$/$H$) and (b) cross-sectional ($xz$ plane) normalized area of the separation bubble.

Figure 9

Ramp flow with control at the $y/D=0$ measurement plane for (a) P2-U7.4 ($\mathrm{VR}=1$), (b) P2-U15.1 ($\mathrm{VR}=0.5$), and (c) P2-U25.1 ($\mathrm{VR}=0.3$) test cases. (i) Ensemble-averaged velocity contours with superimposed streamlines and (ii) the separation bubble.

Figure 10

The separation bubble at (a) P2-U7.4, (b) P2-U15.1, and (c) P2-U25.1 test cases. Measurement planes at (i) $y/D=1$, (ii) $y/D=2$, and (iii) $y/D=3$.

Figure 11

Effect of SWJ actuator streamwise position on $x/H=0.2$ measurement plane for (a) P1-U7.4 and (b) P2-U7.4 test cases. Nondimensional velocity contours for (i) streamwise, (ii) spanwise, and (iii) vertical velocity components.

Figure 12

Ensemble-averaged streamwise vorticity contours in the $x/H=0.2$ measurement plane for (a) P1-U7.4 ($\mathrm{VR}=1$) (b) P2-U7.4 ($\mathrm{VR}=1$), (c) P1-U15.1 ($\mathrm{VR}=0.5$), and (d) P1-U25.1 ($\mathrm{VR}=0.3$) test cases.

Figure 13

Effect of VR on $x/H=0.2$ measurement plane for (a) P1-U15.1 ($\mathrm{VR}=0.5$) and (b) P1-U25.1 ($\mathrm{VR}=0.3$) test cases. Nondimensional velocity contours for (i) streamwise, (ii) spanwise, and (iii) vertical velocity components.

Figure 14

Cross-stream results at $x/H=0.2$ measurement plane for the P1-U7.4 test case. Sweeping jet directed to the (a) right, (b) left, and (c) center positions. Instantaneous nondimensional (i) vertical and (ii) streamwise velocity fluctuations contours, and (iii) instantaneous streamwise vorticity contours.

Figure 15

Cross-stream results at $x/H=0.2$ measurement plane for the P2-U7.4 test case. Arbitrary time instance (a) no. 1, (b) no. 2, and (c) no. 3. Instantaneous nondimensional (i) vertical and (ii) streamwise velocity fluctuations contours, and (iii) instantaneous streamwise vorticity contours.

Figure 16

Cross-stream results at $x/H=0.2$ measurement plane for P1-U15.1 test case. Arbitrary time instance (a) no. 1, (b) no. 2, and (c) no. 3. Instantaneous nondimensional (i) vertical and (ii) streamwise velocity fluctuations contours, and (iii) instantaneous streamwise vorticity contours.

Figure 17

Cross-stream results at $x/H=0.2$ measurement plane for P1-U25.1 test case. Arbitrary time instance (a) no. 1, (b) no. 2, and (c) no. 3. Instantaneous nondimensional (i) vertical and (ii) streamwise velocity fluctuations contours, and (iii) instantaneous streamwise vorticity contours.

Figure 18

Entrainment at the $z/H=0$ measurement plane for (a) $U_{\mathrm{ref}}=7.4\mathrm{m}/\mathrm{s}$, (b) $U_{\mathrm{ref}}=15.1\mathrm{m}/\mathrm{s}$, and (c) $U_{\mathrm{ref}}=25.1\mathrm{m}/\mathrm{s}$ test cases. (i) Baseline without flow control and SWJ at positions (ii) P1 and (iii) P2.

Figure 19

Entrainment extracted along the $y/H=0$ line for (a) $U_{\mathrm{ref}}=7.4\mathrm{m}/\mathrm{s}$, (b) $U_{\mathrm{ref}}=15.1\mathrm{m}/\mathrm{s}$, and (c) $U_{\mathrm{ref}}=25.1\mathrm{m}/\mathrm{s}$ test cases. Measurement planes at (i) $z/H=0$ and (ii) $z/H=1$, 2, and 3.