The Separating Shear Layer Development with the Artificial Disturbances

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The separation and reattachment phenomena of turbulent shear layers have been an important subject in the engineering application. A backward-facing step flow has been investigated as a model of separated flows with the reattachment process. The perturbed shear layers can be obtained, when an artificial disturbance is provided upstream of the step edge in the backward facing step flow. For example, a fence installation upstream of the backward-facing step corresponds to the multi-step concept to enhance the turbulent mixing as in a combustor design. The aim of the present project is to understand the reattachment process by using a fence installed upstream of the separation point or by providing the step wall vibrations with a particular frequency and amplitude. Therefore, a local turbulence structure is changed drastically at the step and there might be the possibilities to control the separation and reattachment process.The following fruitful results and conclusions were obtained. The reattachment length has a strong correlation with a fence height or the vibration frequency and the amplitude. The reattachment process can be effectively controlled by changing the disturbance strength. The strong perturbation occures even in the case of a smaller disturbance, in particular, in the turbulence quantities, while the flow with a stronger disturbance involves overwhelming perturbation. The structures in the inner shear layer are strongly affected by the inherent mechanism of the turbulent mixing downstream of the step, although those of the outer shear layer are not altered even in the downstream location.