Three-dimensional behavior of low-frequency unsteadiness in the incompressible turbulent separation bubble (TSB) produced by a wall-mounted hump is investigated using time-resolved planar and stereoscopic particle image velocimetry measurements at several planes across the separated region. The aspect ratio (wind tunnel width/separation length, L z , WT / L b = 4.4 ) provides nominally two-dimensional flow for more than half of the spanwise extent of the test section. Analysis in the streamwise/wall-normal plane along the center of the test section shows low-frequency ( S t < 0.1 ) large-scale motion of the separated region. The flowfield contains features of both geometry-induced and pressure-gradient-induced separation, but unsteady dynamics produce dominant frequencies closer to geometry-induced TSBs ( S t 0.1 ) compared to purely pressure-gradient-induced TSBs ( S t 0.01 ). Measurements along the spanwise direction and parallel to the initial shear layer development show strong evidence that the low-frequency motion is inherently three-dimensional, providing an additional dimension to the understanding of the flapping/breathing typically observed in planar streamwise/wall-normal measurements. Spectral proper orthogonal decomposition and low-order modeling identify spanwise undulations with wavelengths of the order of L b and frequencies of S t < 0.1 . The three-dimensional behavior causes a peak/valley formation along the span and a more localized expansion/contraction, leading to only small variations in the integral volume of the TSB.


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    Title :

    Three-Dimensional Nature of Low-Frequency Unsteadiness in a Turbulent Separation Bubble


    Contributors:

    Published in:

    AIAA Journal ; 62 , 11 ; 4349-4363


    Publication date :

    2024-11-01




    Type of media :

    Article (Journal)


    Type of material :

    Electronic Resource


    Language :

    English




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