Abstract
An algebraic intermittency model for boundary layer flow transition from laminar to turbulent state, is extended using an experimental data base on boundary layer flows with various transition types and results by large eddy simulation of transition in a separated boundary layer. The originating algebraic transition model functions well for bypass transition in an attached boundary layer under a moderately high or elevated free-stream turbulence level, and for transition by Kelvin–Helmholtz instability in a separated boundary layer under a low free-stream turbulence level. It also functions well for transition in a separated layer, caused by a very strong adverse pressure gradient under a moderately high or elevated free-stream turbulence level. It is not accurate for transition in a separated layer under a moderately strong adverse pressure gradient, in the presence of a moderately high or elevated free-stream turbulence level. The extension repairs this deficiency. Therefore, a sensor function for detection of the front part of a separated boundary layer activates two terms that express the effect of free-stream turbulence on the breakdown of a separated layer, without changing the functioning of the model in other flow regions. The sensor and the breakdown terms use only local variables.
Highlights
Laminar boundary layer separation, followed by transition to turbulence in the separated layer, is a common phenomenon in low Reynolds number flows over aerofoils and turbomachinery blades.A separated boundary layer becomes unstable and breaks down with the generation of fine-scale turbulence [1]
The transition may still be categorised as being of bypass type, but with the meaning that the spontaneous secondary instability phase by spanwise patterns of full-span Kelvin–Helmholtz rolls, which occurs under a low free-stream turbulence level, is bypassed, but not the primary instability of the separated boundary layer leading to the creation of the rolls themselves
The conclusion from the above description of the phenomena is that a transition model connected to the Reynolds-averaged Navier–Stokes equations (RANS), meant for transition in a separated boundary layer subjected to an adverse pressure gradient and to moderately high or elevated free-stream turbulence, has to express the growth of perturbations by the combined effects of Kelvin–Helmholtz rolls and Klebanoff streaks and has to express when the distortion of the separated boundary layer becomes sufficiently strong for causing transition
Summary
Laminar boundary layer separation, followed by transition to turbulence in the separated layer, is a common phenomenon in low Reynolds number flows over aerofoils and turbomachinery blades. In a free stream with a low turbulence level, the inherent instability of the separated layer leads to transition after separation. A small increase in flow incidence may cause an abrupt increase in the bubble length and a significant loss increase in an aerodynamic flow with low turbulence level. This process is typically referred as bubble bursting [4]. Proper accounting for transition in attached and separated boundary layers is important for turbomachinery flows
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