EXPERIMENTAL STUDIES OF A DISTORTED TURBULENT SPOT IN A 3-DIMENSIONALFLOW

We report here on the results of a series of experiments carried out o n a turbulent spot in a distorted duct to study the effects of a diver gence with straight streamlines preceded by a short stretch of transve rse streamline curvature, both in the absence of any pressure gradient . It is found that the distortion produces substantial asymmetry in th e spot: the angles at which the spot cuts across the local streamlines are altered dramatically (in contradiction of a hypothesis commonly m ade in transition zone modelling), and the Tollmien-Schlichting waves that accompany the wing tips of the spot are much stronger on the outs ide of the bend than on the inside. However there is no strong effect on the internal structure of the spot and the eddies therein, or on su ch propagation characteristics as overall spread rate and the celeriti es of the leading and trailing edges. Both lateral streamline curvatur e and non-homogeneity of the laminar boundary layer into which the spo t propagates are shown to be strong factors responsible for the observ ed asymmetry. It is concluded that these factors produce chiefly a geo metric distortion of the coherent structure in the spot, but do not ot herwise affect its dynamics in any significant way.