Václav Uruba

On the receptivity of the by-pass transition to the length scale of the outer stream turbulence

Abstract The effect of length scale on flat-plate by-pass boundary layer transition under free stream turbulence conditions has been investigated. Plane grids generated a constant value of 3 percent intensity turbulent fluctuations and five values of the dissipation length parameter in the range of 2.2 mm to 33.3 mm at 5 m/s flow velocity in the plane of the leading edge of the flat plate. The investigated boundary layer corresponds to the ERCOFTAC Test Case T3A+. Distributions of integral parameters as well as the statistical characteristics of the turbulent fluctuations were measured. They document a significant effect of turbulence length scale on the onset and the end of by-pass transition. It was found that the onset of the final stage of transition comes on later in `fine-grained turbulence than in the case of large free stream turbulence length scale. At the same time, the extent of the transition region shortens with decreasing length scale. Nevertheless, the transition process terminates sooner in a flow with a large turbulence length scale than in a flow with a small one. An attempt has been made to describe these observations quantitatively.

Dynamics of turbulent spots in transitional boundary layer

The turbulent spots appearance, topology and dynamics on a flat-plate boundary layer in zero pressure gradient during its transition to turbulence is investigated experimentally using time-resolved PIV technique. The intermittently appearing turbulent spots are subjected to detailed study. The hairpin packets organized in regular streamwise rows having fractal structure in the same time are identified in spots. Elongated vortical structures have been recognized in the spots rear-part. The calmed region is characterized by presence of low-velocity streaks originating between the elongated structures.

Comparison of wavelet analysis with velocity derivatives for detection of shear layer and vortices inside a turbulent boundary layer

Detection of highly energetic structure in the turbulent flow can be easily done by wavelet analysis, especially with Morlet function as a mother wavelet for detection of repetitious low-frequency structures and Mexican hat for detection of vortices. Since wavelet analysis can be simply applied on one-point measurement, the better understanding of its meaning is welcome. In this project, turbulent boundary layer was investigated with focus on intermittent dynamics. PIV snapshot were used for comparison with Wavelet analysis results, since PIV was performed with high sample frequency (500 Hz). Instantaneous snapshots of velocity and vorticity were studied also by POD and spectra of particular expansion coefficients were computed. Vorticity from planar velocity vectors was used as a suitable methods for recognition of strongly deformed streamlines. Nevertheless, to identify a trajectory of vortex and estimate the true advective velocity of vortex core in the flow, swirling strength or the second invariant could have been used as a proper tool.

Dynamical structure of the turbulent boundary layer on rough surface

The experiments on structure of turbulent boundary layer on the plane rough wall without pressure gradient are presented. Sand roughness of the wall is considered. Measurements are carried out using Time-Resolved PIV technique in planes parallel and perpendicular to the wall. The results on rough wall are compared with the base case of boundary layer on smooth wall. Hairpin vortices have been detected. Topology and typical size of those structures substantially differ in the cases in question. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)