Direct Numerical Simulations of Transitional Separation-Bubble Development in Swept-Blade Flow Conditions

Author:

Brinkerhoff Joshua R.1,Yaras Metin I.2

Affiliation:

1. Ph.D. Candidate

2. Professor e-mail:  Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa, ON K1S 5B6, Canada

Abstract

This paper describes numerical simulations of the instability mechanisms in a separation bubble subjected to a three-dimensional freestream pressure distribution. Two direct numerical simulations are performed of a separation bubble with laminar separation and turbulent reattachment under low freestream turbulence at flow Reynolds numbers and streamwise pressure distributions that approximate the conditions encountered on the suction side of typical low-pressure gas-turbine blades with blade sweep angles of 0 deg and 45 deg. The three-dimensional (3D) pressure field in the swept configuration produces a crossflow-velocity component in the laminar boundary layer upstream of the separation point that is unstable to a crossflow instability mode. The simulation results show that crossflow instability does not play a role in the development of the boundary layer upstream of separation. An increase in the amplification rate and the most amplified disturbance frequency is observed in the separated-flow region of the swept configuration and is attributed to boundary-layer conditions at the point of separation that are modified by the spanwise pressure gradient. This results in a slight upstream movement of the location where the shear layer breaks down to small-scale turbulence and modifies the turbulent mixing of the separated shear layer to yield a downstream shift in the time-averaged reattachment location. The results demonstrate that although crossflow instability does not appear to have a noticeable effect on the development of the transitional separation bubble, the 3D pressure field does indirectly alter the separation-bubble development by modifying the flow conditions at separation.

Publisher

ASME International

Subject

Mechanical Engineering

Reference49 articles.

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3. Instability and Transition in a Separation Bubble Under a Three-Dimensional Freestream Pressure Distribution;ASME J. Turbomach.,2012

4. Large-Eddy Simulation of Boundary-Layer Separation and Transition at a Change of Surface Curvature;J. Fluid Mech.,2001

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