Comment on 'Dynamic airfoil flow separation and reattachment'

I an article by Ericsson, 1 extensive use is made of normal force coefficient data from wind-tunnel tests done at the University of Glasgow. We would like to comment on Ericsson's methodology. In addition, we are somewhat concerned that the author has not considered, except in passing, any of the arguments put forward by ourselves in published work. A paper by ourselves, which is referred to in Ericsson's work, clearly shows some serious anomalies with his own methodology and conclusions. In his article, Ericsson makes the assumption that incipient reattachment is indicated by the local minimum normal force. Fig. 7a of Ref. 2, however, clearly indicates that the flow may be as much as 50% attached at the minimum normal force condition. This would obviously have serious implications on the remainder of Ericsson's analysis. We would like to stress that the analysis carried out by Ericsson is of normal force coefficient data only, and has not been done with any reference to the behavior of individual pressure transducer traces. A great deal of work by ourselves," however, has concentrated upon a detailed analysis of surface pressure data from a number of aerofoil models performing ramp-down motions (the data that Ericsson analyzed was derived from essentially the same tests). We have, therefore, been able to resolve the attachment information much more lucidly. Most importantly, we have come to the conclusion that attachment alone is not responsible for all of the phenomena observed on the CN ~ a data. 2 We have proposed that the dominant effect during the ramp-down motion is the convection of the wake fluid off the aerofoil surface, and our pressure data analysis and other experimental data give this suggestion very firm backing. In particular, the local minimum normal force (that Ericsson has taken as incipient reattachment), in fact, coincides with the passage of a pressure wave over the trailing edge, and this wave is likely to be the result of wake fluid convection effects.' Attachment meanwhile proceeds slowly up to x/c ~ 0.15, and then proceeds at a constant, but pitch-rate-dependent, rate along the rest of the chord. All of these data were available to Ericsson before submission of his article. Ericcson has given great significance to a comment by Ranke in an International Council of the Aeronautical Sciences paper that "flow reattaches uniformly" during the downstroke of an aerofoil oscillating in pitch. We stress that Ranke's paper contains next to no technical detail on the rate of reattachment. Ranke's comment is therefore wide open to interpretation, and it would be inappropriate to use this single comment in preference to a large body of highly detailed, pub-