The Coefficient of contraction of a jet of fluid flowing through an orifice has been widely studied in the field of hydraulics, using a jet of water discharging through an orifice into the atmosphere and measuring the area of the jet at the vena contracta. the coefficient of contraction cr is then defined as:

Two-Dimensional Stress concentration factors may be obtained more quickly and simply than the corresponding three-dimensional factors, either by experiment or mathematical analysis. It would be convenient to obtain information, for varying geometry in the two-dimensional case of a particular type of stress raiser, e.g. a shoulder, groove or hole, and use this either to predict the three-dimensional stress concentration factors or to extend the range of existing three-dimensional results. Clearly a comparison is only possible if the three-dimensional stress raiser embodies a plane of symmetry (which gives the geometry of the similar two-dimensional stress raiser), and if the loading conditions can be reproduced in both the two- and three-dimensional cases. The latter requirement restricts the correlation to the stress concentration factors obtained in tension and in bending. The three-dimensional torsional loading system has no plane of symmetry which can be simulated in two dimensions.

Recent Researches have led to some possible explanations for thin aerofoil stalling behaviour. Apart from the Owen Klanfer criterion these are the reverse flow breakdown hypothesis of McGregor and Wallis's turbulent separation theory.

This note describes simple theoretical boundary layer calculations which indicate the feasibility of Wallis's hypothesis. In addition the results of some experiments on a thin two-dimensional aerofoil with various leading edge configurations with Reynolds number, based on model chord, of 1.8 million and 1 million support either of these hypotheses, depending on the leading edge configuration. It is concluded that thin aerofoil stall can occur broadly, through either of the suggested mechanisms, depending on conditions in the nose region.