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SUMMARY:Feedforward and Feedback Control of Boundary Layer Streaks Induced
  by Freestream Turbulence - Dr. Kevin Gouder\, Imperial College\, London
DTSTART:20170224T130000Z
DTEND:20170224T140000Z
UID:TALK70384@talks.cam.ac.uk
CONTACT:Akshath Sharma
DESCRIPTION:Sensing and cancellation of streaks early within their growth 
 extent is key to the delay of\nbypass transition and eventual turbulence. 
 Hanson et al.\, [1]\, [2]\, utilised feedforward and/or\nfeedback control 
 to weaken artificially induced streaks in a Blasius boundary layer\; Lunde
 ll [3]\ndemonstrated bypass transition delay\, utilising wall-shear sensin
 g\, but manually-tuned feedforward\ncontrol. In contrast\, the current wor
 k aims to delay bypass transition utilising feedforward and\nfeedback cont
 rollers driving plasma actuators. The experimental setup\, Figure 1\, comp
 rises a\nturbulence-generating grid ahead of a flat plate with a sharp lea
 ding edge and a small trailing\nedge flap. Naturally-occurring high and lo
 w-speed streaks exhibiting linear transient growth were\nobserved within t
 he first 300 mm\, eventually followed by turbulent spot formations first d
 etected\nat a streamwise location x ≈ 350 mm from the leading edge. Two 
 wall-shear-stress sensors (SU and\nSD) and two plasma actuators (capable o
 f producing positive and negative wall-normal forcing to\noppose high and 
 low-speed streaks respectively) were placed in the linear growth region.\n
 SU detects streaks in their early stages of growth. A Feedforward (FF) con
 trol system makes\nuse of the output from SU and combined with single-poin
 t Linear Stochastic Estimation (LSE)\, an\nestimate of the streaks’ stre
 amwise shear disturbance τ at the x location of SD is obtained. The\nesti
 mate is used to modulate the plasma actuator voltage to produce a counter-
 disturbance which\,\nat SD \, is equal in magnitude and opposite in sign t
 o the estimate. A Feedback (FB) loop uses any\noutput from SD in a PI cont
 roller to correct for remaining\, uncancelled disturbances resulting from\
 ,\nfor example\, inaccuracies in the LSE model of the dynamics of streak g
 rowth. Notable changes in\nthe mean and rms wall normal velocity profiles\
 , turbulent spot intermittency and energy spectra\,\nfor FF\, and combined
  FF + FB control\, demonstrate the viability of the control scheme to weak
 en\nboundary layer streaks and delay bypass transition.\n
LOCATION:JDB Seminar Room\, CUED
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