Investigation of a canonical all-movable control surface in thickened boundary layers

Submarine aft control surfaces are low aspect ratio appendages that operate, under normal conditions, with a significant portion of their span submerged in the hull boundary layer. These aft control surfaces have a significant gap between the root and hull when placed at incidence. The loads and stall behaviour of these control surfaces are of interest as they directly influence a submarines ability to maneuver, maintain stability and impact the spatial and temporal uniformity of the flow into the propeller. The hull boundary layer and the root-hull gap have a significant influence on the control surface performance.

The cavitation tunnel at the Australian Maritime College (AMC) Cavitation Research Laboratory (CRL) can produce a range of thickened and thinned boundary layers on its ceiling. This capability was used to create a boundary layer with similar momentum deficit to that which occurs at the aft of a submarine. This allows us to obtain force measurements more representative of the conditions on a real submarine for a submarine aft control surface. The loads and stall behaviour of a canonical all-movable control surface were investigated for a range of boundary layer thicknesses and root gap sizes. An increased root gap was observed to increase stall angles and reduce the lift curve slope at low angles of incidence. The thickened boundary layers were observed to have a significant influence on the load measurements during stall, with reduced hysteresis, increased maximum lift and increased stall angle. A momentum based estimate was effective in accounting for the influence of the thickened boundary layer on the measured loads.