In this paper TR (Time-Resolved)-PIV measurements on an SD7003 aerofoil at Re = 66000 were presented to quantitatively estimate the dominant frequencies and magnitudes of the KH (Kelvin-Helmholtz) instabilities associated with an LSB (Laminar Separation Bubble). It was shown that this technique is well suited for reliably determining frequency spectra. Contrary to other measurement techniques like hot-wire anemometry or LDA which were applied for measurements on LSBs in the past, TR-PIV makes it possible to obtain temporal information for a two-dimensional field. This is of particular interest when the fluid mechanic process is not periodic. In this case flow phenomena are not repeatable, and the information must be captured in a single sequence. This applies to the presented experiment m which no artificial disturbance is introduced to achieve a periodic flow. The results clearly show that the KH instabilities lead to a vortex formation in the shear layer above the separation bubble. By means of the Fourier transformation a periodicity of the vortex formation was found. Compared to experiments with introduced disturbances (forced TS waves) more than a single vortex shedding frequency was found here. However, dominant structures which lead to large magnitudes in the frequency spectrum were observed. In addition the flapping of the separation bubble was measured. It seems that the flapping in this experiment is associated with a low-frequency variation of the most amplified disturbance. This frequency was also observed in the oncoming flow without the aerofoil in the tunnel. The investigations with a light sheet aligned nearly parallel to the aerofoil surface gave information about the vortex dimension and interaction in the spanwise direction. The size of the vortices in this direction is in the range 0.05,..., 0.3.c. A regular formation of the vortices in spanwise direction is not observed, not unlike previous investigations from Burgmann et al. (2006) at a lower Reynolds number and from Hain & Kahler (2005) at Ke = 60000.


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    Title :

    Dynamics of laminar separation bubbles at low-Reynolds-number aerofoils


    Additional title:

    Dynamik laminarer Ablösungsblasen an Flügelprofilen bei kleinen Reynolds-Zahlen


    Contributors:
    Hain, R. (author) / Kähler, C.J. (author) / Radespiel, R. (author)

    Published in:

    Publication date :

    2009


    Size :

    25 Seiten, 27 Bilder, 2 Tabellen, 27 Quellen




    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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