Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
Browse
3 results
Search Results
Item Experimental Investigation of Force Transients during Gust Encounters(2021) Biler, Hulya; Jones, Anya; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The small size and low speed of Micro Air Vehicles make them vulnerable to wind gusts such that sustaining controlled flight becomes a challenge in the unsteady environments. A better understanding of the gust flow is crucial to develop models capable of predicting unsteady forcing. Therefore, this study aims to improve understanding of fundamental flow physics behind the unsteady force production during gust encounters. The bulk of the results presented here were obtained experimentally for a flat plate passing through a transverse gust created in a water towing tank and found to have a sine-squared velocity profile. The effects of 4 different parameters, namely angle of attack, gust ratio, effective angle of attack, and aspect ratio, were explored. A wide investigation range was used for each parameter. The forces increased significantly from their steady-state values during the encounter. 2D flowfields showed the formation and shedding of vortices from the leading and trailing edges of the wing. The flow was found to stay nominally two-dimensional until the forces peak. Only thereafter, spanwise variations were observed in the 3D flowfields. The accuracy and limitations of Kussner's model were evaluated by comparing the sine-squared and top-hat transverse gusts (the latter experiments performed by collaborators). The gradients in the transverse velocity profile were found to significantly affect the force response such that sharper gradients cause higher nonlinearities. Kussner's model was found to provide accurate predictions for the sine-squared gust even when the flow is highly nonlinear, whereas it failed to do so for the top-hat gust. A momentum flux-based normalization was proposed and found to scale the responses of different velocity profiles as long as the response can be predicted by Kussner's model. The effect of gust type on the unsteady forcing was examined by comparing the sine-squared transverse and vortex gusts (the latter experiments performed by collaborators). The results showed that both gust encounters result in large transients in the lift. The increase in the lift force and the leading-edge vortex strength for the transverse gust was found to be steeper than the vortex gust. A flowfield-based force prediction method was proposed and found to be effective for low-to-moderate effective angles of attack.Item MICRO AIR VEHICLE SCALE GUST-WING INTERACTION IN A WIND TUNNEL(2018) Smith, Zachary Francis; Jones, Anya R; Hrynuk, John T; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Studying isolated gust effects on simple airfoil models in a controlled environment is a necessity to further the development of MAV gust response and control laws. This work describes the creation of a vertical gust generator in a low speed, low turbulence wind tunnel through the use of an actuated fan placed below the tunnel and ducted through its floor. Gusts of up to 40% of the freestream velocity were created. Characterization of the gust generator is shown, and its interaction with a stationary wing at several angles of attack is evaluated. The actuated gust profile is also compared to that of a pitched wing in a gust-less environment with many visible similarities.Item Unsteady force production on a flat plate wing by large transverse gusts and plunging maneuvers(2017) Perrotta, Gino; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The transient forces produced by large-amplitude transverse gust encounters and plunge maneuvers were studied experimentally in a water-filled towing tank. Forces were measured as a flat plate wing with an aspect ratio of four was towed through a fluid gust and as the same wing performed plunge maneuvers which matched the shape of the gust velocity profile. The transient velocity in each case conformed to the sine-squared profile, and the peak transient velocities were of the same order of magnitude as the steady towing velocities. In most cases, the wing pitch angle was high enough to cause constant flow separation. Even at low wing pitch angles, the increase in flow incidence angle by the transverse gust or plunge velocity was enough to cause flow separation. Transient force magnitudes were shown to increase with increasing stream-normal velocity for both the gust encounters and plunge maneuvers. Transient forces varied with increasing wing pitch angle during gust encounters but not during plunge maneuvers. Force histories in each case were compared to predictions made by existing small-perturbation force models, and adaptations were made to those models based on physical interpretation of the observed characteristics. Measured forces in both the gust encounters and the plunge maneuvers were found to correspond more closely to predictions made based on attached flow than on separated flow, which supports the suggestion that the presence of a leading edge vortex significantly augments the transient lift. Additionally, a large trailing edge vortex forms at the end of the gust encounter which temporarily reduces the force production below the steady-state values. This was not observed in the plunge maneuver force histories, which were much closer to quasi-steady than were the gust encounter force histories. This analysis contributes to the understanding of unsteady force production in large-amplitude events, and in particular in conditions with separated flow, the behaviors of which are not adequately captured by existing small-perturbation models.