Boiko A.V. Dovgal A.V. Sorokin A.M. Zverkov I.D.
On some applications of particle image velocimetry in subsonic shear flows
Reporter: Boiko A.V.
Results of measurements of the linear stability of an attached boundary layer, local laminar separated flows, and mean characteristics of turbulent boundary layer using PIV are presented. Peculiarities of application of PIV to the problems under consideration are discussed. The results can be used as a guide in designing aerodynamic experiments to measure instant and averaged velocities in subsonic near-wall shear flows.
The experimental results show that the high-speed PIV used in the study is a reliable tool for characterizing small periodic perturbations of laminar flow, which often have to deal with the experimental simulation of the effects of hydrodynamic instability. The minimum amplitudes of the controlled oscillation of about 0.1% of the free-stream velocity resolved in the zone of the boundary layer separation behind a backward-facing step are significantly lower than their non-linear threshold. Thus, the method of the study, although not as accurate in disturbance amplitude as CTA measurements, can be an effective means of determining the characteristics of the linear instability of shear flows. In addition, it is possible to obtain detailed information about the normal to the wall component of the perturbations that is difficult or impossible with CTA equipment.
The experiments performed in the turbulent boundary layer demonstrated the reliability of PIV to measure mean flow characteristics near a flow-exposed surface, which makes it possible to use the method to determine the local skin friction coefficient. In experimental determining of flow characteristics in a variety of ways, including measurements of the linear part of velocity profile in viscous sublayer similar results were obtained, the experimental error not exceeding its values known for other experimental methods. Improving the accuracy of sublayer measurements in non-canonical flows, in which the behavior of the velocity profiles in the buffer and logarithmic regions may not always be approximated by known laws, is possible with the use high-resolution optics and smaller tracers.
In the last part of the study, disturbance development in boundary layers of classical and wavy wings were compared with the help of PIV measurements. Isosurfaces of disturbance r.m.s. amplitudes demonstrate that the laminar-turbulent transition is triggered in the separation bubbles along the grooves.
The authors are grateful to the Alexander von Humboldt Foundation and the German Aerospace Center for the opportunity of the research. Data analysis and processing is also supported by the Russian Foundation of Basic Research (Project numbers 13-01-00270 and 13-08-00129).
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