Large Eddy Simulation of Transitional Separated Flow over a Flat Plate and a Compressor Blade

Abstract

The ability of Large-Eddy Simulation (LES) to predict transitional separation bubbles is investigated, with particular emphasis being placed on the response to free-stream-turbulence. The principal objective is to quantify the penalties, relative to Direct Numerical Simulations (DNS), that arise from the coarser resolution and the use of subgrid-scale models. Two flow configurations are considered: a flatplate boundary layer, subjected to different free-stream-turbulence levels, ranging from 0 to 2% (at the point of separation), and the flow over a compressor blade at 0 and 3.25% turbulence levels. For both cases, results are compared with DNS data. A number of challenges associated with the use of LES in transitional flows are addressed, including the representation of the decay of free-stream turbulence and the mesh resolution needed for a correct description of the growth of instability waves in the early stage of transition.

A Bypass Transition Model Based on the Intermittency Function

“…The current model is able to predict locations of transition and reattachment with decent accuracy compared to the LES data of Lardeau et al (2012). The inlet is set half of the "chord" length L upstream of the leading edge.…”

“…In recent turbomachine designs, the suction side boundary layer over a blade is more subject to separation, particularly in the compressor stage (Lardeau et al, 2012). Understanding transition in separated region can therefore impact the design for improvement of the efficiency of the compressor performance.…”

“…U ′ and |U ′ |, U ′′ , and |U ′ | ′ are plotted from left to right. The velocity profile was extracted within the separated region from LES data by Lardeau et al (2012). |U ′ | ′ (i.e.…”

A bypass transition model based on the intermittency function

“…For simplicity, X/L is replaced with X in the following plots. The contour of the upper wall is exactly as Lardeau et al (2012). The bottom surface from the inlet to the leading edge is specified as a symmetric boundary, and after the leading edge, it is a non-slip wall boundary to the outlet.…”

Transition Models for Turbomachinery Boundary Layer Flows: A Review

“…As with natural transition, mean-flow turbulence accelerates and controls the growth of the Kelvin-Helmholtz vortices and makes the process less sensitive to other disturbances. The mechanisms of separation-induced transition were studied by experiments, by LES and by DNS by many researchers [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Generally, the free shear layer, once sufficiently turbulent, reattaches forming a separation bubble.…”