Noise Reduction on Low Reynolds Number Rotors by Boundary Layer Transition

Gowree, Erwin Ricky; Jaroslawski, Thomas; Mellot, Bertrand; Gojon, Romain

doi:10.2139/ssrn.4331944

Cited by 1 publication

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“…To the best of our knowledge, prior research has not extensively investigated the influence of roughness-generated streaks on the stability and transition mechanisms of LSBs, including the interplay with freestream turbulence effects and roughness. Overall, the application of passive roughness elements and the induction of streaks within the laminar boundary layer offer promising potential for flow control [5,6]. However, further research is necessary to investigate their effects on separated boundary-layer flows, including LSBs.…”

Section: Introductionmentioning

confidence: 99%

“…Particularly, it is of significant practical interest to gain an understanding of the transition mechanisms in the presence of freestream turbulence and surface roughness. For example, in the context of MAVs, recent research indicates that forcing the boundary layer and LSB forming over suction sides of MAV rotor aerofoils with roughness or freestream turbulence can yield both acoustic [5] and aerodynamic [6] benefits.…”

Section: Introductionmentioning

confidence: 99%

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Influence of roughness-generated streaks on laminar separation bubbles

Jaroslawski,

Vermeersch,

Forte

et al. 2024

Preprint

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This experimental investigation studies the impact of streaks on two-dimensional laminar separation bubbles forming over an aerofoil. Streaks are introduced into the boundary layer using cylindrical roughness elements, and the resulting mean and unsteady flow fields are measured using hot-wire anemometry. The observed streaks generated by roughness exhibit analogous behavior to those generated by freestream turbulence, significantly altering the mean-flow characteristics of the bubble, including reductions in its length, height, and the introduction of spanwise velocity gradients. These mean flow modifications have a damping effect on convective disturbance growth. The experiments suggest the coexistence of modal instability due to the laminar separation bubble and non-modal instability due to streaks. We perform linear stability analysis at the fore position of the laminar separation bubble and find that by incorporating a spanwise wavenumber into the calculations, the experimental findings are reasonably modeled, indicating an oblique modal instability. To investigate the combined effect of roughness and the presence of freestream turbulence, we increase the turbulence level from the baseline in the presence of a roughness forcing configuration. We find that increasing the turbulence intensity leads to an enhancement of non-modal instability, accompanied by distinctive chordwise disturbance growth compared to lower freestream turbulence intensity levels. Moreover, increasing the turbulence intensity level in the presence of roughness not only bypasses the modal instability observed without the addition of turbulence but also suppresses laminar separation.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%