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Temporal, Spatial, and Spatio-temporal correlation of the velocity fluctuations
Jurčáková, Klára
Taylor's hypothesis about frozen turbulence [1] says that if the turbulence intensity is small compared to the mean velocity then the advection of a turbulent flow field past a fixed point can be taken entirely due to the mean flow. This means that in the “frozen turbulent field” spatial and temporal dimensions can be interchanged. Although the main assumption of Taylor's hypothesis is not met in the atmospheric boundary layer (the intensity of turbulence is usually between 10 and 20%) the hypothesis is commonly used to calculate integral length scales of turbulent fields from point measurements. Particle image velocimetry with high temporal resolution (TR-PIV) allows us to analyze both temporal and spatial cross-correlations in the turbulent flow fields and to evaluate obtained results.
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Assessing Quality of Boundary Layer Flows With the Diagnostic Plot
Jurčáková, Klára ; Procházka, Pavel P. ; Kellnerová, Radka
We experimentally investigated turbulent boundary layers over 4 rough surfaces and 1 smooth surface by particle image velocimetry for a range a free stream velocities. The obtained mean velocity vertical profiles were universally scaled by roughness length z₀ and friction velocity u*, but both parameters have to be obtained from the fit of the logarithmic or composite velocity pro le to the experimental data. Diagnostic plot, which relates turbulence intensity with the mean velocity showed good collapse of different free stream velocities for each surface. It became fully universal in its modified version when the roughness function was taken into an account. We found that turbulence intensities over our surfaces are higher than any published one and that they dependent on roughness length z₀*.
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Large Scale Motions in Turbulent Flows
Kellnerová, Radka ; Jurčáková, Klára ; Procházka, Pavel P.
Quadrant analysis based on the Reynolds decomposition was performed on the data from turbulent boundary layer flows above variously rough surfaces. An estimation of a convective velocity for events significant in terms of momentum flux, and the statistics of a length and amplitude of the individual events, together with their spectral analysis, was performed. Based on the analyses, we were able to detect a few individual sweep and ejection events with a length longer than 6δ, which could be considered as very large scale motions (VLSM). We also identified number of events with a longitudinal dimension equal to approximately 3δ labelled as large scale motions (LSM). The analyses were executed for various Reynolds numbers. The number of detected structures increases with the momentum of the flow.
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Spatial and temporal correlations in turbulent boundary layers
Jurčáková, Klára ; Kellnerová, Radka
The experimental data from time-resolved particle image velocimetry measurement of the boundary-layer flows over various aerodynamically rough surfaces are presented. Temporal, spatial, and time-space correlation are analyzed and used for calculation of integral length and time scales. Temporal and spatial integral scales are connected via convection velocity. Taylor's hypothesis on frozen turbulence says that the convection velocity is equal to the local mean velocity. The experimental data shows that the convection velocity is larger than local mean velocity in the lower third of the boundary layer and greater in the upper two thirds. The cross-over line is higher over surfaces with higher roughness.
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