Evaluation of Turbulent Spot Production Rate in Boundary Layers Under Variable Pressure Gradients for Gas Turbine Applications

Abstract

Abstract The paper presents an experimental data base on transitional boundary layers developing on a flat plate installed within a variable area opening endwall channel. Measurements have been carried out by means of time-resolved PIV. The overall test matrix spans 3 Reynolds numbers, 4 free-stream turbulence intensity levels and 4 different flow adverse pressure gradients. For each condition, 16000 instantaneous flow fields have been acquired in order to obtain high statistical accuracy. The flow parameters have been varied in order to provide a gradual shift of the mode of transition from a bypass process occurring with mild adverse pressure gradients at high free-stream turbulence, to separated flow transition, occurring with low Reynolds number, low free-stream turbulence intensity and elevated adverse pressure gradient. In order to quantify the influence of the flow parameter variation on the boundary layer transition process, the transition onset and end positions, and the turbulent spot production rate have been evaluated with a wavelet based intermittency detection technique. This post-processing technique is in fact able to identify the vortical structures developing within the boundary layer, the intermittency function is then automatically evaluated for each tested condition counting the number of such structures and defining the cumulative probability function. The by-pass transition mode has the longest transition length that decreases with increasing the Reynolds number. The transition length of the separated flow case is smaller than the by-pass one, and the variation of the flow parameters has a similar impact. Similarly, the dimensionless turbulent spot production rate reduces when the Reynolds number is increasing. The variation of the inlet turbulence intensity has a small influence on this parameter except for the condition at the highest turbulence intensity, that always shows the lowest turbulent spot production rate because a by-pass type transition occurs. This large amount of data has been used to develop new correlations used to predict the spot production rate and the transition length in attached and separated flows.