Unsteady lift produced by a flat-plate wing translating past finite obstacles

The unsteady lift of a high-angle-of-attack, flat-plate wing encountering finite-length obstacles is studied using towing-tank force measurements and flow visualization. The wing translates from rest and at 1 chord traveled interacts with a rectangular channel, ceiling, or ground. The angle of attack, obstacle length, and height to the obstacle are varied. As the channel gap height decreases, circulatory-lift peaks attributed to leading-edge vortices (LEVs) become larger, and for the second peak onward occur earlier, from wing blockage enhancing the flow speed. Larger and earlier LEVs are visualized, supporting this, as are secondary vortices off the channel. The lift reduces while exiting a channel, being lowest afterward if exiting during a lift peak. For ceilings, the first circulatorylift peak increases for smaller LE-to-ceiling gaps, but for 0.5 chord gaps or less, later maxima are below the noobstacle case yet still earlier. For grounds, with lower wing height the first circulatory-lift peak is larger but the second peak’s behavior varies with angle of attack, and lift decreases near the ground end. Grounds affect peak timing the least, indicating less influence on the LEV. The lift rises slightly ahead of channels and ceilings, and often lowers before channels and grounds end, providing warnings.