Drag reduction and thrust generation by tangential surface motion in flow past a cylinder

Xuerui Mao*, Emily Pearson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

2 Citations (Scopus)

Abstract

Sensitivity of drag to tangential surface motion is calculated in flow past a circular cylinder in both two- and three-dimensional conditions at Reynolds number Re≤ 1000. The magnitude of the sensitivity maximises in the region slightly upstream of the separation points where the contour lines of spanwise vorticity are normal to the cylinder surface. A control to reduce drag can be obtained by (negatively) scaling the sensitivity. The high correlation of sensitivities of controlled and uncontrolled flow indicates that the scaled sensitivity is a good approximation of the nonlinear optimal control. It is validated through direct numerical simulations that the linear range of the steady control is much higher than the unsteady control, which synchronises the vortex shedding and induces lock-in effects. The steady control injects angular momentum into the separating boundary layer, stabilises the flow and increases the base pressure significantly. At Re= 100 , when the maximum tangential motion reaches 50% of the free-stream velocity, the vortex shedding, boundary-layer separation and recirculation bubbles are eliminated and 32% of the drag is reduced. When the maximum tangential motion reaches 2.5 times of the free-stream velocity, thrust is generated and the power savings ratio, defined as the ratio of the reduced drag power to the control input power, reaches 19.6. The mechanism of drag reduction is attributed to the change of the radial gradient of spanwise vorticity (∂r (Formula presented.)) and the subsequent accelerated pressure recovery from the uncontrolled separation points to the rear stagnation point.

Original languageEnglish
Pages (from-to)307-323
Number of pages17
JournalTheoretical and Computational Fluid Dynamics
Volume32
Issue number3
DOIs
Publication statusPublished - 1 Jun 2018
Externally publishedYes

Keywords

  • Bluff body flow
  • Drag reduction
  • Sensitivity

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Mao, X., & Pearson, E. (2018). Drag reduction and thrust generation by tangential surface motion in flow past a cylinder. Theoretical and Computational Fluid Dynamics, 32(3), 307-323. https://doi.org/10.1007/s00162-017-0452-y