Experiments have been made on a thin two-dimensional aerofoil with sharp leading edge and with a jet blowing through a mid-chord slot on the upper surface in order to control the boundary layer and circulation around it. The measurements were made at low Mach number. The effect of changing the angle of blowing was studied in anticipation that this would help to control the leading edge separation bubble. It transpired that variation of the blowing angle caused little change in lift coefficient but produced an appreciable change in moment coefficient. A low pressure separation bubble was formed downstream of the slot with the jet discharging at an angle and subsequently re-attaching to the surface. This reduced the nose-up pitching moment about the quarter-chord point. The experimental results are in fair agreement with the “jet-flap” and jet entrainment theory.

The flow in a two-dimensional plane wall jet with different initial gaps between the nozzle exit and the leading edge of the wall was probed at various stations along the jet. The jet slot thickness and the velocity were kept constant. It was found that the region close to the leading edge of the wall behaved like a transforming region where the type of flow changed from a free jet to a wall jet. The length of this region, which depended directly on the gap size, was so short for small gaps that the gap effects were found to be negligible. In addition, it was found that the inner layer velocity distribution of a wall jet did not follow the classic one-seventh power law.

In a recent paper, Gibbings has made an interesting comparison of the length of two-dimensional contractions based on three criteria, namely:

1. (1) the rapidity with which the boundary slope approaches its zero value;

2. (2) the rapidity with which the centre-line velocity approaches its asymptotic value; and

3. (3) the degree of uniformity of the velocity expressed as a percentage of its mean value.

The note by W. T. F. Lau upon the effective “length” of two-dimensional contracting ducts has raised further points of interest.

Lau's paper presents three different criteria which can control a contraction length. Of these three, the second and third, that respectively are based upon the approach of the centre-line velocity to its asymptotic value and upon the approach of the flow to uniformity, were compared in ref. 1. The first criterion that is based upon the approach of the boundary slope to zero is not in general identical with the second.