Development of a blockage correction for highly cambered 2D circular arcs

Abstract

Model-scale testing of yacht sails and wings often suffers from blockage due to the physical constraints of experimental facilities. With blockage, a greater increase in flow speed occurs in the vicinity of the geometry compared to an unblocked flow as a direct consequence of the restricted cross-sectional area. This leads to artificially higher forces, making comparison and validation between tests conducted in different facilities difficult, while also flawing performance prediction if the forces are not suitably corrected. Blockage correction for streamlined bodies and bluff bodies such as flat plates normal to the flow, are well-established. However, it is not the case for lift-generating bluff bodies, or lifting body, experiencing high trailing-edge separation, such as highly cambered plates and downwind yacht sails. This study focusses on the development of a blockage correction for highly cambered plates, specifically circular arcs, comparable to horizontal sections of downwind yacht sails. Measurements are undertaken at positive incidences below deep-stall for Reynolds numbers ranging from 53,530 to 218,000 in a towing tank and a water tunnel to devise a blockage correction. The critical impact of the free surface deformation on wake blockage is evidenced. This allows to set a maximum limit to the amount of blockage a cambered plate can experience before blockage correction is no longer accurate, hence the importance of closed measurement sections to prevent free surface deformation. Furthermore, the experiments revealed that flow behaviours such as the laminar-to-turbulent transition are preserved even with high blockage. The angle of attack at which transition occurs is also preserved in pressurized wind tunnel tests. However, the effect on the forces cannot be fully corrected, and thus further work would be needed to extend the applicability of the proposed blockage correction to such facilities. These findings provide experimental insights into the effect of blockage on highly cambered plates, and it is anticipated they will support future force experiments conducted on high-camber plates and downwind sails in water tunnels.