CUED Publications database

Global linear and nonlinear stability of viscous confined plane wakes with co-flow

TAMMISOLA, O and LUNDELL, F and SCHLATTER, P and WEHRFRITZ, A and SÖDERBERG, LD (2011) Global linear and nonlinear stability of viscous confined plane wakes with co-flow. Journal of Fluid Mechanics. pp. 1-38. ISSN 0022-1120

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The global stability of confined uniform density wakes is studied numerically, using two-dimensional linear global modes and nonlinear direct numerical simulations. The wake inflow velocity is varied between different amounts of co-flow (base bleed). In accordance with previous studies, we find that the frequencies of both the most unstable linear and the saturated nonlinear global mode increase with confinement. For wake Reynolds number Re = 100 we find the confinement to be stabilising, decreasing the growth rate of the linear and the saturation amplitude of the nonlinear modes. The dampening effect is connected to the streamwise development of the base flow, and decreases for more parallel flows at higher Re. The linear analysis reveals that the critical wake velocities are almost identical for unconfined and confined wakes at Re ≈ 400. Further, the results are compared with literature data for an inviscid parallel wake. The confined wake is found to be more stable than its inviscid counterpart, whereas the unconfined wake is more unstable than the inviscid wake. The main reason for both is the base flow development. A detailed comparison of the linear and nonlinear results reveals that the most unstable linear global mode gives in all cases an excellent prediction of the initial nonlinear behaviour and therefore the stability boundary. However, the nonlinear saturated state is different, mainly for higher Re. For Re = 100, the saturated frequency differs less than 5% from the linear frequency, and trends regarding confinement observed in the linear analysis are confirmed.

Item Type: Article
Divisions: Div A > Energy
Depositing User: Cron Job
Date Deposited: 17 Jul 2017 20:37
Last Modified: 27 Jul 2017 05:53