000186633 001__ 186633
000186633 005__ 20180317094426.0
000186633 02470 $$2ISI$$a000329178300021
000186633 0247_ $$2doi$$a10.1063/1.4846916
000186633 037__ $$aARTICLE 000186633 245__$$aOpen-loop control of noise amplification in separated boundary layers
000186633 269__ $$a2013 000186633 260__$$c2013
000186633 336__ $$aJournal Articles 000186633 520__$$aLinear optimal gains are computed for the subcritical two-dimensional separated boundary-layer flow past a bump. Very large optimal gain values are found, making it possible for small-amplitude noise to be strongly amplified and to destabilize the flow. The optimal forcing is located close the summit of the bump, while the optimal response is the largest in the shear layer. The largest amplification occurs at frequencies corresponding to eigenvalues which first become unstable at higher Reynolds number. Non-linear direct numerical simulations show that a low level of noise is indeed sufficient to trigger random flow unsteadiness, characterized here by large-scale vortex shedding. Next, a variational technique is used to compute efficiently the sensitivity of optimal gains to steady control (through source of momentum in the flow, or blowing/suction at the wall). The bump summit is identified as the most sensitive region for control with wall actuation. Based on these results, a simple open-loop control strategy is designed, with steady wall suction at the bump summit. Calculations on controlled base flows confirm that optimal gains can be drastically reduced at all frequencies. Forced direct numerical simulations show that such a control allows the flow to withstand a higher level of noise without becoming non-linearly unstable, thereby postponing bypass transition. Finally, in the supercritical regime, the same open-loop control is able to fully restabilize the flow, as evidenced by direct numerical simulations and supported by a linear stability analysis.
000186633 6531_ $$aflow control : boundary layers 000186633 6531_$$ainstabilities
000186633 6531_ $$anonlinear dynamical systems : bifurcation 000186633 700__$$0245025$$aBoujo, Edouard$$g197960
000186633 700__ $$aEhrenstein, Uwe 000186633 700__$$0244721$$aGallaire, François$$g189938
000186633 773__ $$j25$$k12$$tPhysics of Fluids 000186633 8564_$$uhttp://scitation.aip.org/content/aip/journal/pof2/25/12/10.1063/1.4846916$$zURL 000186633 909CO$$ooai:infoscience.tind.io:186633$$particle$$pSTI
000186633 909C0 $$0252358$$pLFMI$$xU12052 000186633 917Z8$$x197960
000186633 917Z8 $$x197960 000186633 917Z8$$x197960
000186633 937__ $$aEPFL-ARTICLE-186633 000186633 973__$$aEPFL$$rREVIEWED$$sPUBLISHED
000186633 980__ aARTICLE