Unsteady force production on a flat plate wing by large transverse gusts and plunging maneuvers

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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.