GUST-WING INTERACTION
5.6 Summary
an intermediate amount of time to recover from strong perturbations. The recovery time of the vortex shedding was approximately 15 tc for the α = 5° airfoil. This appeared to be unrelated to the return to normalcy of the low-pass-filtered forces.
As with the pitching gust generator, the models were unable to properly predict the wake of the heaving plate, the spatial extent of the vortex, or the behavior of the boundary layers on the airfoil. The over-prediction of the lift in theS =0.25 case is likely a result of two factors: the large size of the generated vortex and the difference between its predicted and true circulation. The larger vortex, and its associated structures, would result in a more drawn-out vortex interaction, as compared to the point-vortex used in the models. The over-prediction of the circulation simply resulted in larger estimated forces.
Since all of the numerical models lack viscosity, they were unable to account for the effects of the oncoming wake. Thus, they were inappropriate for modeling the cases where the plate passed the midline. This lack of viscosity also prevented predictions of the drag, which changed due to the velocity deficit in the wake of the plate, or due to the separated region on the airfoil being swept away by the gust. The different numerical models with wakes performed similarly, again confirming that the initial part of the vortex-wing interaction is an inviscid effect. The unsteady panel method and extended Tchieu-Leonard models were slightly more accurate than the model using the Wagner function in thin airfoil theory. The simplicity of the W-TAT model again recommends it, however, due to the relative expense of the other methods.
The heaving plate gust generator is imperfect. Although its wake only interacted with the test article for a finite time, and only when the plate reached the midline of tunnel, the resulting variations in the forces were of the same magnitude as those from the vortex interactions. This suggests that it is inappropriate to use for vortex generation when it would pass in front of the test article. Unfortunately, this constrains the polarity of the generated vortices to gusts with negative circulation above the airfoil, and positive circulation below it.
(a) Best use of the pitching vortex generator.
(b) Best use of the heaving vortex generator.
Figure 5.19: Suggestions for the best use of the investigated vortex generators: the pitching generator should be transversely far from the test article, and the heaving plate should not pass in front of the test article.
positive circulation. This is similar to the numerical results found by Golubev et al [26], though the lift oscillations were smoother in these experimental results.
The evolution of the vortex itself closely resembled the behaviors described by Rockwell [59]. When it was further than a chord length from the body, the vortex was relatively undisturbed. When it was closer, it was subjected to significant shear, and sometimes split into multiple vortical regions. The closest vortices directly interacted with the boundary layers of the airfoil. The asymmetry of the vortex’s distortion, which was dependent on which side of the test article it passed, is similar to the results from Peng and Gregory [53].
The pitching airfoil created stronger and more compact vortices, but it also had a persistent wake. Additionally, its wake was significantly different before and after pitching. In some cases, this significantly altered the flow around the test article, permanently changing the average forces as well as disrupting the vortex shedding from the test article. As the flow around the upstream airfoil developed after pitching, a second burst of vorticity was released, which again modified the flow around the test article. To reduce these unwanted effects, the pitching vortex generator should
thus be limited to operation with a significant separation in yfrom the test article, as shown in Figure 5.19a.
The heaving plate also released a wake, which significantly interfered with the test article if the plate moved past they-position of the test article. In contrast to the use of the pitching airfoil, the heaving plate’s withdrawal to the edge of the tunnel meant that the flow around the test article returned to its previous state. Efforts to avoid the effects of its wake limit its usefulness to the creation of vortical gusts with negative circulation above the airfoil, or positive circulation below it. This is illustrated in Figure 5.19b.
With both systems, the wakes had significant effects when interacting with the test article. The perturbations in the wakes sometimes caused flow to reattach on the test article. In other instances, the perturbations modified the flow on one side of the airfoil, causing an asymmetry and resulting change in lift. The velocity deficit in the wake of the generators likely had an effect as well. It may have manifested as a decrease in drag, but the complexity of the interaction made attribution difficult.
The evolution of these wakes was dependent on the streamwise distance between the generators and test article, but this distance was not varied in the current experiments.
Thus, each generation method has its benefits and drawbacks. The persistent wake of the airfoil permanently changed the flow around the test article, whereas the heaving plate’s wake significantly disrupted the flow temporarily. The pitching airfoil could create vortices with either sense of rotation, while the heaving plate’s wake limits it to one sense per side of the test article.
With both generators, the time needed for the low-pass-filtered forces on the test article to approach their final state was primarily dependent on the angle of attack of the test article. At low angles, the flow remained attached and the forces responded quickly, on timescales of 5-10tc, which was consistent with the Wagner function. In contrast, atα= 10°, when the flow around the test article was significantly perturbed, the flow re-attached and its approach to its final state required approximately 20tc. This is consistent with timescales reported in control of separated flows [12], and of flow development around impulsively started wings at high angles of attack [47].
In the case of separated flow control, this transient was associated with repeated oscillations of the bound circulation as vorticity was shed into the wake. Similarly, the timescales of the impulsively started wings are strongly related to the formation of a leading edge vortex, and repeated shedding of vorticity as the flow settled into a final separated state. Surprisingly, this long timescale has also been associated with
the reattachment of separated flow through the use of periodic perturbations [14].
The heaving plate was able to reveal one effect that was unobservable with the pitching generator: the reassertion of vortex shedding from the airfoil at α = 5°.
This occurred on a timescale of roughly 15tc.
The numerical methods with wake models provided reasonable estimates for the initial changes in lift, in the cases where the wakes were not overly intrusive. They did not properly capture the behavior after the initial vortex interaction, as the models did not include viscous effects like boundary layer evolution and separation.
Although the UPM allowed the vortical gust to move freely, this was only significant when the vortex was close to the airfoil, which is when viscous effects would render any of these inviscid methods inaccurate. The quasi-steady method lacked models of the wake, and so it failed to capture the slower increase in the lift that this effect causes. The UPM and E-TL were slightly more accurate than the W-TAT method, but significantly more expensive to compute. Overall, the W-TAT model provided estimates that were appropriate for modeling the initial changes in lift due to vortex-gust interaction, with low computational costs.
C h a p t e r 6