The influence of airfoil thickness and Reynolds number on separation

• DOI: 10.1007/bf00944968
• Publication Date: 2005-01-07T06:28:58Z
• Authors: H. K. Cheng; F. T. Smith
• Keywords: 0103 physical sciences; 01 natural sciences

A theoretical description is sought for the mechanisms whereby the attached motion past a finite flat plate is gradually transformed into the quite different grossly separated motion past a bluff body, as the body's thickness increases from zero to a finite value at high Reynolds numbers. The theory is based largely on the free stream line inviscid approach allied to the viscous triple-deck requirement at separation, for laminar symmetric flow of an incompressible fluid. Some surprising and potentially significant phenomena are found to arise as the body thickness increases; and the geometries of the trailing and leading edges have a crucial effect on the occurrence and position of separation. The main properties found are: the match with the Kirchhoff theoryat one extreme; at the other extreme, the match with the triple-deck account for wedged trailing edges; in-between, a “cut-off” stage during which the wake dimensions and the drag abruptly increase; and, also in-between, a nonlinear stage in whichleading edge separation can occur, depending delicately on the precise shape of the leading edge. These properties arise from the strong viscous effect at separation. Again, whether the closure of the recirculatory flow region is blunt or not, the theory is consistent with the results of earlier work for the above extremes.Nonuniqueness/bifurcation of the separated flow solutions is also found to arise for certain profiles.