GUST-WING INTERACTION
5.4 Interaction with Pitching Airfoil Gusts
5.4.3 Discussion
Figure 5.9: Mean-subtractedCLdue to gusts from the pitching airfoil impacting the test article atα =0°, scaled by the quasi-steady estimate of the lift peak amplitude.
For theyupstr eam = 0 case, the lift is only scaled byα2. For comparison, the results of the different models are presented. Each row is a different yupstr eam, as noted in the title of each panel.
-10 0 10 20 -1
0 1
Scaled C L
(α, α
2, yupstream): (5, -13, 1 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (5, 13, 1 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (5, -13, 0.5 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (5, 13, 0.5 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (5, -13, 0 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (5, 13, 0 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (5, -13, -0.5 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (5, 13, -0.5 ca )
-10 0 10 20
τa
-1 0 1
Scaled C L
(α, α
2, yupstream): (5, -13, -1 ca )
-10 0 10 20
τa
-1 0 1
Scaled C L
(α, α
2, yupstream): (5, 13, -1 ca )
low-pass avg CL W-TAT QS-TAT E-TL UPM
Figure 5.10: Mean-subtractedCLdue to gusts from the pitching airfoil impacting the test article atα= 5°, scaled by the quasi-steady estimate of the lift peak amplitude.
For theyupstr eam = 0 case, the lift is only scaled byα2. For comparison, the results of the different models are presented. Each row is a different yupstr eam, and the columns are different pitching directions, as noted in the title of each panel.
-10 0 10 20 -1
0 1
Scaled C L
(α, α
2, yupstream): (10, -13, 1 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (10, 13, 1 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (10, -13, 0.5 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (10, 13, 0.5 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (10, -13, 0 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (10, 13, 0 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (10, -13, -0.5 ca )
-10 0 10 20
-1 0 1
Scaled C L
(α, α
2, yupstream): (10, 13, -0.5 ca )
-10 0 10 20
τ
a
-1 0 1
Scaled C L
(α, α
2, yupstream): (10, -13, -1 ca )
-10 0 10 20
τ
a
-1 0 1
Scaled C L
(α, α
2, yupstream): (10, 13, -1 ca )
low-pass avg CL W-TAT QS-TAT E-TL UPM
Figure 5.11: Mean-subtractedCLdue to gusts from the pitching airfoil impacting the test article atα= 10°, scaled by the quasi-steady estimate of the lift peak amplitude.
For theyupstr eam = 0 case, the lift is only scaled byα2. For comparison, the results of the different models are presented. Each row is a different yupstr eam, and the columns are different pitching directions, as noted in the title of each panel.
any specific change in the forces. This appears to be because the perturbed boundary layers were quickly shed, and the released clump of higher vorticity continued to act as a vortex as it convected downstream.
The angle of attack of the downstream airfoil had a significant impact on the forces after the vortex passed, and the timescales associated with their changes. The airfoil at α=0° could not support a significantly asymmetric flow, so any boundary layer disturbances were quickly shed, and so it recovered on a timescale comparable to the Wagner function. The asymmetric flow around theα=5° airfoil responded more strongly to such disturbances, and required more time to shed the slightly separated flow on its suction side. The greater asymmetry, and incipiently stalled flow, of the airfoil at α=10° led to more extreme reactions to perturbations, as well as a longer recovery time. In some cases, the large change in lift was due in part to the separated flow being ‘blown off’ by the incoming gust, temporarily reattaching the flow.
The aforementioned region of additional vorticity was likely due to the evolution of the flow around the upstream airfoil. After its rapid pitching, the airfoil was at an angle of -13°, so it was stalled in a static sense. This meant that the flow began to develop a large separated region. The development of the stalled flow is an unsteady process. On its way toward its final behavior, the airfoil shed much of the separated flow as a large region of strong vorticity. This became the strong disturbance that impacted the downstream airfoil. Were the airfoil pitched to a more moderate angle, it is likely that such a large vortical region would not have been shed. The impact of this additional vorticity was seen as a change in the lift on the downstream airfoil about 5-10tc after the passing of the vortex.
The difference in the upstream airfoil’s pre- and post-pitching wake resulted in permanent changes in the forces on the test article. Both the average value and magnitude of oscillation of those forces changed permanently. This was far from the ideal of a single transient interaction with a vortex. These permanent changes were due to multiple effects. In some cases, the thicker wake perturbed the flow around the downstream airfoil enough that the flow remained attached, resulting in increased lift and/or decreased drag. In others, the wake impacted only one side of the airfoil, causing an asymmetry in the pressure, and so a change in lift.
Each of the numerical and theoretical models with a wake adequately predicted the magnitude and temporal position of the initial lift peak. The models were unable to predict the later viscous effects. The lack of viscosity precludes prediction of the wake of the upstream airfoil. The simulations have no model of flow separation, and
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.