Impact of Spanwise Rotation on Flow Separation and Recovery Behind a Bulge in Channel Flows
arxiv(2024)
摘要
Direct numerical simulations of spanwise-rotating turbulent channel flow with
a parabolic bump on the bottom wall are employed to investigate the effects of
rotation on flow separation. Four rotation rates of Ro_b := 2Ω H/U_b =
± 0.42, ± 1.0 are compared with the non-rotating scenario. The mild
adverse pressure gradient induced by the lee side of the bump allows for a
variable pressure-induced separation. The separation region is reduced
(increased) when the bump is on the anti-cyclonic (cyclonic) side of the
channel, compared with the non-rotating separation. The total drag is reduced
in all rotating cases. Through several mechanisms, rotation alters the onset of
separation, reattachment, and wake recovery. The mean momentum deficit is found
to be the key. A physical interpretation of the ratio between the system
rotation and mean shear vorticity, S:=Ω/Ω_s, provides the
mechanisms regarding stability thresholds of S=-0.5 and -1. The rotation
effects are explained accordingly with reference to the dynamics of several
flow structures. For anti-cyclonic separation, particularly, the interaction
between the Taylor-Görtler vortices and hairpin vortices of wall-bounded
turbulence is proven to be responsible for the breakdown of the separating
shear layer. A generalized argument is made regarding the essential role of
near-wall deceleration and resultant ejection of enhanced hairpin vortices in
destabilizing an anti-cyclonic flow. This mechanism is anticipated to have
broad impacts on other applications in analogy to rotating shear flows, such as
thermal convection and boundary layers over concave walls.
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