GUST-WING INTERACTION
5.5 Interaction with heaving plate gusts
5.5.1 Experimental Results
so are incapable of modeling the creation of the additional shed vorticity, as well as the changes in the boundary layer of the downstream airfoil. The models should thus be compared only on their prediction of that initial lift peak.
The quasi-steady model, as well at the extended Tchieu-Leonard model, estimated faster changes in the forces than were observed. This is likely due to the fact that the QS-TAT model lacked the moderating effect of a wake model. Similarly, the E-TL model had not released a vortex since the beginning of the simulation, so the moderating effect was reduced due to the distance of the wake vortex. The unsteady panel method and the thin airfoil theory model with the Wagner function provided reasonable approximations of the initial peak in the lift. After that, viscosity, and the change in the flow due to the wake of the gust generator, made the estimates less accurate. For predictions of the initial change in forces due to the vortical gust, the W-TAT model appears to give fairly accurate estimates, with much lower computational costs than the panel method simulations.
Overall, the pitching airfoil was not a perfect gust generator. Though it was able to generate compact vortical gusts, its persistent presence upstream of the test article made it difficult to separate the effects of its wake from those of the vortex. Beyond the initial lift peak, it was difficult to attribute further effects solely to the passing vortex. This suggests that it is inappropriate to use for examining vortex-wing interactions when the it is near the midline, as its unwanted effects would strongly affect the test article.
Figure 5.12: Gust from heaving plate interacting with the downstream airfoil: ypeak
= 0.5ca, negative initial motion, α = 10°. The temporal position of the vorticity snapshots at the top are denoted by circles in the middle two plots. The second plot shows the low-pass filteredCL across the different repetitions, the average of those, and the averageCL envelope. The third plot shows that average, as well as the estimates from numerical models. The bottom plot shows the y-position of the upstream airfoil, as well as that position shifted in time to compensate for the travel time of the gust.
deflected downward. The wake of the heaving plate followed the y-position of the plate, and so was only visible in the PIV field of view for a limited amount of time.
In experiments where the heaving plate passed the midline of the tunnel, the plate’s wake passed the airfoil twice. These two factors, the vortical gust and the plate’s wake, are associated with certain force responses. After the gust passed, the flow and forces returned to their initial state.
As with the gust from the pitching airfoil, the response of the forces to the gust from the heaving plate was very repeatable. This can be seen as the close overlap of the red lines in 5.7 when the vortex passed over the airfoil.
The lift forces in each experiment are plotted together in Figure 5.13. Figures C.14
-10 -5 0 5 10 0
0.5 1
C L
positive initial motion to y
peak = 1c a
-10 -5 0 5 10
0 0.5 1
C L
positive initial motion to y
peak = 0.5c a
-10 -5 0 5 10
0 0.5 1
C L
negative initial motion to y
peak = 0c a
-10 -5 0 5 10
0 0.5 1
C L
positive initial motion to y
peak = 0c a
-10 -5 0 5 10
0 0.5 1
C L
negative initial motion to y
peak = -0.5c a
-10 -5 0 5 10
0 0.5 1
C L
positive initial motion to y
peak = -0.5c a
-10 -5 0 5 10
0 0.5 1
C L
negative initial motion to y
peak = -1c a
-10 -5 0 5 10
0 0.5 1
C L
positive initial motion to y
peak = -1c a
α=0o, S=0.1 α=5o, S=0.1 α=5o, S=0.25 α=10o, S=0.1
-10 -5 0 5 10
0 0.5 1
C L
negative initial motion to y
peak = 1c a
-10 -5 0 5 10
0 0.5 1
C L
negative initial motion to y
peak = 0.5c a
Figure 5.13: Lift coefficient due to gusts from the heaving plate interacting with the airfoil. Each panel contains force traces from a single release position and initial direction, but different airfoil angles of attack. The left column moved initially in the−ydirection, and the right in+y. Each row is a different ypeak.
- C.16 show the moment and drag histories as well. These show the evolution of the low-pass-filtered forces over time, with the gust passing the leading edge of the airfoil at t = 0. The left columns show the interaction with gusts generated by the plate initially moving in the−y direction, which created a vortex with negative circulation when it changed direction, and vice-versa for the right column. The average envelopes of the forces are shown in Figures C.17 - C.19.
In response to an oncoming vortex with negative circulation, the lift coefficient
dipped, rose, and returned to the steady-state value. The opposite occurred with vortices of positive circulation.
The other factor in the variation of the forces was the wake of the plate. In the cases where the heaving plate passed the midline (ypeak<0 for negative initial motion, or ypeak>0 for positive initial motion), the lift deviated from its steady-state behavior earlier than the estimated vortex arrival time. These deviations were coincident with the arrival of the wake of the heaving plate.
The arrival of the wake is also visible in the force envelopes for α = 5°, which shrank in response to the perturbations. In the case of the negative initial motion at speed S = 0.1 to ypeak = −ca, both interactions with the wake are seen, as well as recovery between those impacts. In contrast, positive initial motion toypeak = −ca
resulted in no noticeable changes to the force envelopes. When the flow around the airfoil was sufficiently perturbed, the envelopes returned to their original state over approximately 15tc.
The airfoil’s angle of attack also had an effect on the change in the forces. Early in the interaction, the changes inCL were very similar acrossα, but they diverged after the first peak. At higherαthe gusts led to larger and more lasting deviations in the forces. This recovery time was approximately 5-10 tc for the lower angles, and 10-20 tc for α = 10°. A comparison of lift traces in Figure 5.14 illustrates the variation of the recovery timescale with the angle of attack. In the cases where the plate passed the midline, the final passage of the wake set the beginning of this recovery time.