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PHYSMOD 2022 - International Workshop on Physical Modeling of Flow and Dispersion Phenomena
Jurčáková, Klára ; Kellnerová, Radka
The objective of PHYSMOD is to bring together the community active in physical and numerical modeling of flow and dispersion processes occurring in the lowest part of the atmosphere using wind tunnels, water channels, or CFD models. PHYSMOD provides a forum where the most recent advances in fluid modeling, state-of-the-art in experimental work, and newly emerging research areas are discussed in an open-minded and friendly atmosphere. One of the main purposes is to encourage broader collaboration between researchers and transfer knowledge between the laboratories as well as generations. \n\n
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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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Wind-tunnel modelling & development of emergency software
Chaloupecká, Hana ; Kluková, Zuzana ; Jakubcová, Michala ; Kellnerová, Radka ; Jurčáková, Klára
The report describes the procedure of work on the two-year project TACR TJ01000383.\nIn the year 2018, relations between characteristics of short-term and long-term gas leakages for an urban area were searched utilizing the database COST ES1006. Then, releases of short-term and long-term gases for a rural area were studied. A dimensionless database I. was created from the data.\nIn the year 2019, the dispersion of short-term and long-term gas releases in an industrial area was studied. The measured data were analysed and the dimensionless database II. was created. The databases I. and II. were utilized for the validation of the model. We developed software as the main goal of the project.
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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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Core of emergency model for urban canopy & Experiments of gas leakages for rural area
Chaloupecká, Hana ; Jakubcová, Michala ; Kellnerová, Radka ; Jurčáková, Klára
The report consists of two parts.\nThe first part copes with the development of an emergency model core for short-term gas leakages for urban canopy. It is explained a procedure in the development of the model, in which after recording input data describing a gas leakage as an output probability density functions for individual puff characteristics and exposed places are obtained. From the output, one can see for example the most probable puff characteristics, which can occur during an accident as well as extreme cases. This is the biggest difference in comparison with the usually used emergency models, in which only mean puff contours and concentration fields are the output. \nIn the second part, experiments of gas leakages for rural area are described. Fulfilment of similarity criterions in wind-tunnel modelling as well as the experiments themselves and the database development is showed.\n
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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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Impact of Turbulence Generators on Turbulent Characteristics and Structures
Kellnerová, Radka ; Jurčáková, Klára ; Jaňour, Zbyněk
The turbulent generators are routinely used to produce very turbulent flows. Special type of vertically slenderize spires are applied to enhance turbulent diffusion from the surface upwards in order to increase the boundary layer depth. This paper studies the influence of variable upstream distance of generators on formation of both vortical and non-vortical organized structures within a test section. The 2-component 2-dimensional time-resolved particle image velocimetry measurement of the flow was performed in the wind channel with very rough surface. Intermittent flow dynamics was evaluated by means of higher order moments, vorticity, quadrant and wavelet analysis. The downstream evolution of the flow suggests that equilibrium between roughness- and spires-produced turbulence was reached at the distance of 7 spires height.\n
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Neutrally stratified flow modelling over complex terrain at meso-scale: open-cut coal mine study
Nosek, Štěpán ; Jaňour, Zbyněk ; Jurčáková, Klára ; Kellnerová, Radka ; Kukačka, Libor
The objective of this case study was to determine the influence of the coal mine cavity and its vicinity on the main flow field with respect to prevailing wind direction. In order to model the neutrally stratified flow over extremely huge open-cut coal mine Libouš situated at north-west of Czech Republic the laminar flow analogy was applied. The model was scaled down to 1:9000 according to dimensions of wind-tunnel and selected coal mine surroundings. The measurements of longitudinal and lateral velocity components in selected vertical, resp. horizontal planes were performed by LDA. The measured data will be used for validation of CFD simulation and for selections of area of interest at bigger scales where turbulent flow modelling will be performed. The results revealed that not only of cavity shape and deepness but also the surroundings orography has influence on flow pattern, hence on ventilation, within the area of interest.
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Quadrant analysis of turbulent pollution flux above the modelled street intersection
Kukačka, Libor ; Nosek, Štěpán ; Kellnerová, Radka ; Jurčáková, Klára ; Jaňour, Zbyněk
The objective of this experimental study is to determine processes of a vertical turbulent pollution transport above the X-shaped street intersection in an idealised symmetric urban area for several approach flow directions. An experimental set-up for simultaneous measurement of the flow velocity and the tracer gas con-centration in a high temporal resolution is assembled. Vertical turbulent scalar fluxes are computed from the measured data in a horizontal plane above the street intersection. The quadrant analysis was applied to the vertical turbulent pollution fluxes data. Events with dominant contribution to vertical turbulent pollution flux were detected. The mean duration, repetition frequency and the duration percentage were computed for these events. A strong influence of the approach flow direction on the type of dominant events and their characteristics was resolved.
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