A NEW EMPIRICAL CORRELATION FOR TRANSITION IN BOTH SHORT AND LONG SEPARATION BUBBLES

With the aim of improving the predicting capability of current correlation-based transition models, a new correlation for transition in separated flows is presented, which is based only on local quantities. The peak value of the vorticity-based Reynolds number at the detachment position has been chosen as a local indicator of the boundary layer state. As largely discussed in the literature [1], this facilitates the implementation of transition models into modern numerical schemes, characterized by parallel execution and unstructured grids. The current experimental database consists of a total amount of almost 90 flow conditions concerning a flat plate boundary layer developing under variable Reynolds number, free-stream turbulence intensity and adverse pressure gradient. The typical operating conditions of low-pressure turbine profiles have been reproduced. Due to the wide range of Reynolds numbers tested, the bursting process of the laminar separation bubble has been observed. Using the present database, a proportionality coefficient between the momentum-thickness and the vorticity-based Reynolds numbers at the separation position has been computed, which is found to be the same for both short and long bubbles. This provides the link between local and integral quantities of the boundary layer. Then, the vorticity-based Reynolds number is adopted as main ingredient of a correlation for the prediction of the transition onset in separated boundary layers. The influence of other factors such as the free-stream turbulence and the streamwise pressure gradient has been also considered. The present work is thought to improve the code ability in predicting the profile losses and the blade loading distribution in case of separated flows, for both short and long bubble states.