Abstract
PurposeThis study aims to present flow control over the backward-facing step with specially designed right-angle-shaped plasma actuator and analyzed the influence of various scales of flow structures on the Reynolds stress through snapshot proper orthogonal decomposition (POD).Methods2D particle image velocimetry measurements were conducted on region (x/h = 0–2.25) and reattachment zone in the x–y plane over the backward-facing step at a Reynolds number of Reh = 27,766 (based on step height h {= 40 text{ mm }} and free stream velocity U_{infty } {= 11 text{ m/s}}). The separated shear layer was excited by specially designed right-angle-shaped plasma actuator under the normalized excitation frequency Sth ≈ 0.345 along the 45° direction. The spatial distribution of each Reynolds stress component was reconstructed using an increasing number of POD modes.ResultsThe POD analysis indicated that the flow dynamic downstream of the step was dominated by large-scale flow structures, which contributed to streamwise Reynolds stress and Reynolds shear stress. The intense Reynolds stress localized to a narrow strip within the shear layer was mainly affected by small-scale flow structures, which were responsible for the recovery of the Reynolds stress peak. With plasma excitation, a significant increase was obtained in the vertical Reynolds stress peak.ConclusionsUnder the dimensionless frequencies Sth ≈ 0.345 and St_{theta } approx 0.0183, which are based on the step height and momentum thickness, the effectiveness of the flow control forced by the plasma actuator along the 45° direction was ordinary. Only the vertical Reynolds stress was significantly affected.
Highlights
The separated and reattached flows have been the research hot topic in the aerodynamics field, which are an important flow phenomenon, and would be met frequently in diverse practical fluid engineering applications
This study mainly aims to analyze the influence of different scales of flow structures on Reynolds stress by snapshot Proper orthogonal decomposition (POD) method for flow over backward-facing step (BFS) with and without rightangle-shaped plasma actuator
Flow characteristics induced by right‐angle‐shaped plasma actuator The natural instability frequencies at different streamwise positions over BFS were measured in advance by hot-wire anemometry and power spectra density (PSD) through fast Fourier transform method (FFT)
Summary
The separated and reattached flows have been the research hot topic in the aerodynamics field, which are an important flow phenomenon, and would be met frequently in diverse practical fluid engineering applications. It is generally accepted that a better cognition of coherent structures is the key to research turbulence (Kostas et al 2005). An in-depth understanding of coherent structures would provide the possibility of elaborating the physical dissipated mechanism of turbulent energy and its control (Shah and Tachie 2009). It is believed that the Reynolds stress plays a crucial role in the turbulence flow, which can affect turbulent energy transfer over the wall bounded turbulence flow. The skin friction and the occurrence of flow separation have direct relationship to the Reynolds stress. There is a relative little of present understanding of the inter-relationship between Reynolds stress and various scales flow structures in the turbulent flow. If inter-relationship understanding can be improved, it would be positive meaning for flow separation control and drag reduction
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