Passive Flow Control on A Backward-Facing Step Flow

摘要

A ubiquitous yet commonly undesirable feature in many engineering applications, are the effects of flow separation. A characteristic separated flow region is observed on bluff bodies which have a steep pressure gradient. The flow over a backward-facing step has strong flow separation and recirculation around the step change in height, to some length downstream of the step. The geometry of the backward facing step is seen throughout various practical application in engineering mechanical components. The frequent occurrence of the backward facing step and the inherent loss it presents to the flow of fluids, provides on-going interest to at least mitigate the effects of separated flow. There have been considerable efforts to effect flow control on a backward facing step, in particular, active flow control and feedback mechanisms with optimization. However, there is a considerable lack of investigative research of passive flow control methodologies. The present work is directed towards the use of a passive flow control technique to investigate the aerodynamic performance on backward facing step. The flow over the backward facing step was studied using computational fluid dynamics. The CFD analysis was conducted in a steady state flow regime for the base reference case and all subsequent cases using the passive flow control modifications to the backward facing step. The boundary conditions were established to ensure they reflected the capabilities possible in the UNSW aerodynamics tunnel for future experimental wind tunnel validation studies. The freestream velocity was set a single velocity of 20 m/s. The geometry of the backward facing step was trialled with three different expansion ratios (H/h) of 2, 2.5, 3 : 1 to provide a more complete study. The width of the model was constructed so-as-to create essentially quasi-2D flow across the model, however 3D effects were strongly observed in certain passive flow control cases. The Reynolds number was based on the inlet flow conditions and the inlet geometry and varied according to the expansion ratio generating, Re in the range of 3.8e5 to 5.9e5. The large Reynolds numbers is a realistic expectation to that used in industrial flow applications. The present work has shown that novel passive flow control techniques can be used effectively and are a suitable means for improving the aerodynamic performance of typical quasi-two-dimensional flow on the backward-facing step. These advances in subsonic internal and external fluid flows should prove useful in gaining control and streamlining these types of separated flows.