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Research project: Aerofoil separation bubbles

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The stall behaviour of aerofoils at low and moderate Reynolds numbers is often determined by the behaviour of transitional separation bubbles, in which a laminar boundary layer separates, undergoes transition to turbulence and subsequently reattaches as a turbulent boundary layer. Such separation bubbles act as a site of laminar-turbulent transition and the ultimate breakdown of the reattachment process then dictates the maximum lift possible with the aerofoil.

Project Overview

Iso-surfaces of streamwise (top) and spanwise (bottom) vorticity, showing the amplification of vorticity fluctuations in the braid region between successive vortices — Iso-surfaces

The red regions are saddle points where vorticity is amplified before being convected upstream as part of the shedding cycle — Schematic of the transition process

In this work the governing equations are solved by direct numerical simulation using 170 million grid points. The simulation databases were used to identify a new mechanism of transition to turbulence which depends on a global instability of the unsteady vortex shedding that occurs behind the separation bubble. This mechanism explains the observed self-sustaining nature of transition, even though the flow is locally only convectively unstable. The simulations additionally contain information on the sound radiation from the aerofoil, with strong sources located near the reattachment point and at the trailing edge.

Linear stability rates for three different aerofoil configurations, showing that all cases are absolutely stable (imaginary part of the frequency w1<0 for all x up to the end of the separation bubble — Linear stability rates

Transition to turbulence in a 3D separation bubble — Transition to turbulence

Aerofoil spanwise vorticity showing vortex shedding behind a 2D separation bubble — Aerofoil spanwise vorticity

Aerofoil flow visualisation including the sound radiation — Aerofoil flow visualisation

All the figures are for a NACA-0012 aerofoil at an angle of incidence of 5°, a Reynolds number of 50,000 and a Mach number of 0.4

Related research groups

Aerodynamics and Flight Mechanic (AFM)

Staff

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