A new Bypass intermittency model incorporating the calming effect

Introduction

The boundary layer on the suction side of an LP turbine blade is subjected to periodic unsteadiness due to the wakes which are shed from an upstream row. Consequently, the boundary layer undergoes a bypass laminar -turbulent transition process. The location of this transition onset is somewhere around the peak suction location; in some instances the onset location is in the accelerating region preceding the peak suction. However, the transition process almost always ends after the peak suction. Current transition zone models do not seem to lead to satisfactory predictions of transition zone parameters in LP turbine suction surface boundary layer where transition begins in different regimes like the accelerating and decelerating regions. The present work is aimed at developing a model which addresses these shortcomings of traditional models, especially in unsteady flows. In the present model, spots are allowed to formed in the transition zone after incorporating the so-called 'calmed region'. This is a region trailing behind the turbulent spot in the transition zone and it is characterised by stable velocity profiles and robustness against separation. In the present model, a spot is allowed to form at a particular location only during the time the flow at that point is laminar; when the location is calmed or turbulent, spots cannot be born. By incorporating these features, a new formulation is arrived at, which also has the merit of being amenable to generalisation to unsteady flows. Moreover, correlations for spot formation rates which are available in the literature in terms of traditional models, can be re-correlated in terms of the new formulation relatively easily.

Papers by Ramesh and Howard Hodson are available in the download section. Press here to down load papers and PhD Theses

Howard Hodson and Ramesh

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