Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence
Jones, L.E., Sandberg, R.D. and Sandham, N.D. (2008) Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence. Journal of Fluid Mechanics, 602, 175-207. (doi:10.1017/S0022112008000864).
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Direct numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Re-c = 5 x 10(4) and incidence 5 degrees are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles.
|Keywords:||direct numerical simulation, airfoil, aerofoil, laminar separation bubble, transitional separation bubble|
|Subjects:||T Technology > TL Motor vehicles. Aeronautics. Astronautics|
|Divisions:||University Structure - Pre August 2011 > School of Engineering Sciences > Aerodynamics & Flight Mechanics
|Date Deposited:||05 Aug 2008|
|Last Modified:||27 Mar 2014 18:38|
UK Turbulence Consortium
Funded by: EPSRC (EP/D044073/1)
Led by: G Coleman
8 March 2006 to 7 May 2009
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