Direct numerical simulations of transition in a compressor cascade: the influence of free-stream turbulence

By Tamer A. Zaki, Jan G. Wissink, Wolfgang Rodi, P. Durbin. Journal of Fluid Mechanics, 2010

132 citations recorded by the source corpus.

The flow through a compressor passage without and with incoming free-stream grid turbulence is simulated. At moderate Reynolds number, laminar-to-turbulence transition can take place on both sides of the aerofoil, but proceeds in distinctly different manners. The direct numerical simulations (DNS) of this flow reveal the mechanics of breakdown to turbulence on both surfaces of the blade. The pressure surface boundary layer undergoes laminar separation in the absence of free-stream disturbances. When exposed to free-stream forcing, the boundary layer remains attached due to transition to turbulence upstream of the laminar separation point. Three types of breakdowns are observed; they combine characteristics of natural and bypass transition. In particular, instability waves, which trace back to discrete modes of the base flow, can be observed, but their development is not independent of the Klebanoff distortions that are caused by free-stream turbulent forcing. At a higher turbulence intensity, the transition mechanism shifts to a purely bypass scenario. Unlike the pressure side, the suction surface boundary layer separates independent of the free-stream condition, be it laminar or a moderate free-stream turbulence of intensity Tu ~ 3%. Upstream of the separation, the amplification of the Klebanoff distortions is suppressed in the favourable pressure gradient (FPG) region. This suppression is in agreement with simulations of constant pressure gradient boundary layers. FPG is normally stabilizing with respect to bypass transition to turbulence, but is, thereby, unfavourable with respect to separation. Downstream of the FPG section, a strong adverse pressure gradient (APG) on the suction surface of the blade causes the laminar boundary layer to separate. The separation surface is modulated in the instantaneous fields of the Klebanoff distortion inside the shear layer, which consists of forward and backward jet-like perturbations. Separation is followed by breakdown to turbulence and reattachment. As the free-stream turbulence intensity is increased, Tu ~ 6.5%, transitional turbulent patches are initiated, and interact with the downstream separated flow, causing local attachment. The calming effect, or delayed re-establishment of the boundary layer separation, is observed in the wake of the turbulent events.

Subfields: Aerospace Engineering, Computational Mechanics, Mechanical Engineering

View Paper: https://www.semanticscholar.org/paper/74b9c1794c59e079cef88f9fc958e0ed4a7a39e7

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Citations in the corpus

The lift-up effect : The linear mechanism behind transition and turbulence in shear flows (2014, background): “Note that disturbances can also be advected directly inside the shear layer at the leading edge of, e.g. a compressor or turbine blade (Zaki et al., 2010), as studied numerically in Schrader et al. (2010, 2012).”
Stability of zero-pressure-gradient boundary layer distorted by unsteady Klebanoff streaks (2011, background): “It was also noted by Zaki et al. (2010) in simulations of transition in compressor passages, where the overall pressure gradient is adverse and the interaction of streak and instability waves leads to breakdown to turbulence.”