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SUMMARY:Effects off free-stream turbulence intensity and length scale on d
 isturbance growth on an airfoil and its relation to optimal perturbations 
 - Ardeshir Hanifi (KTH - Royal Institute of Technology)
DTSTART:20220328T103000Z
DTEND:20220328T110000Z
UID:TALK171116@talks.cam.ac.uk
DESCRIPTION:The common feature of boundary-layer flows subjected to free-s
 tream turbulence (FST) is dominance of streamwise elongated structures \, 
 often called streaks or Klebanoff modes\, whose secondary instability is p
 recursor of laminar-turbulent transition. These structures are observed bo
 th experimentally and in the numerical simulations. Theoretically\, appear
 ance of these structures are explained by non-normality of the operator de
 scribing evaluation of small disturbances. Here\, the streaks are the opti
 mal respond of the boundary layer to initial perturbations or forcing. How
 ever\, the role of optimal perturbations in generation of streaks in exper
 iments and numerical simulations has been questioned arguing such optimal 
 initial perturbations have not been observed in the experiments. In the pr
 esent study\, by means of a series of numerical simulations of flow over a
 n airfoil subjected to FST\, we quantify the role of optimal disturbance i
 n generation of streaks observed in the simulations. This is done through 
 projection of the flow fields at the leading edge onto optimal disturbance
 s. Our results demonstrate that optimal disturbance growth is the main cau
 se of growth of disturbances on the wing boundary layer. We consider four 
 different FST conditions which are characterised by their intensity and in
 tegral length scales. In all cases the perturbed flow develops into elonga
 ted disturbances of high and low streamwise velocity inside the boundary l
 ayer\, where their spacing has been found to be strongly dependent on the 
 scales of the incoming free-stream vorticity. The disturbance growth is fo
 und to depend not only on the turbulence level\, but also on the FST lengt
 h scale. Particularly\, higher disturbance growth is observed for our case
 s with larger length scales. This behaviour is attributed to the preferred
  wavenumbers that can exhibit maximum transient growth.
LOCATION:Seminar Room 1\, Newton Institute
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