National Repository of Grey Literature 8 records found  Search took 0.00 seconds. 
Wind-tunnel Modelling of Turbulent Flow Inside the Street Canyon
Kellnerová, Radka ; Jaňour, Zbyněk (advisor) ; Brechler, Josef (referee) ; Jonáš, Pavel (referee)
Turbulent flow inside a street canyon was investigated in an open circuit wind tunnel and in a blow-down wind channel. Two geometries were used for comparison purposes: buildings with pitched roofs and with flat roofs. Both generate the flow of a different category, so the induced ventilation regimes are fundamentally different. Quadrant, Fourier and Wavelet analysis, Proper Orthogonal Decomposition (POD) and vortex detection methods are used to identify coherent structures in the flow and establish their impact on the ventilation of pollution. Two types of the organised motions are detected: the compact areas of sweep and ejection with the scale comparable to the size of building and the small vortices generated in the shear layer behind the building roof. POD identifies the most dominant modes with high coherency in the flow and evaluates the relative contributions of each mode to the overall kinetic energy of turbulence. Rigorous analysis of the correctness of the physical interpretation for such a decomposition is carried out. Wavelet analysis is applied to the time-series of the POD expansion coefficients in order to reveal control mechanism of the dynamics of the modes. Vorticity, calculated from the original velocity data, is decomposed by POD as well. Finally, the correlation between the vorticity...
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.
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
Wind-tunnel Modelling of Turbulent Flow Inside the Street Canyon
Kellnerová, Radka ; Jaňour, Zbyněk (advisor) ; Brechler, Josef (referee) ; Jonáš, Pavel (referee)
Turbulent flow inside a street canyon was investigated in an open circuit wind tunnel and in a blow-down wind channel. Two geometries were used for comparison purposes: buildings with pitched roofs and with flat roofs. Both generate the flow of a different category, so the induced ventilation regimes are fundamentally different. Quadrant, Fourier and Wavelet analysis, Proper Orthogonal Decomposition (POD) and vortex detection methods are used to identify coherent structures in the flow and establish their impact on the ventilation of pollution. Two types of the organised motions are detected: the compact areas of sweep and ejection with the scale comparable to the size of building and the small vortices generated in the shear layer behind the building roof. POD identifies the most dominant modes with high coherency in the flow and evaluates the relative contributions of each mode to the overall kinetic energy of turbulence. Rigorous analysis of the correctness of the physical interpretation for such a decomposition is carried out. Wavelet analysis is applied to the time-series of the POD expansion coefficients in order to reveal control mechanism of the dynamics of the modes. Vorticity, calculated from the original velocity data, is decomposed by POD as well. Finally, the correlation between the vorticity...
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.
Analýza nestacionárního proudění uvnitř ulice a dvoru
Kellnerová, Radka ; Jaňour, Zbyněk
Spatial distribution of momentum flux and moments of third order of velocity components obtained from one-point measurement are investigated. The area of interest covers an inner part of street canyon and adjacent intersection. Time-averaged momentum flux were divided into particular quadrants using Quadrant analysis method. Comparison between flux events and skewness provides some insight into the spatial distribution of strong intermittent tendency and explain better the spatially local dominance of sweep and ejection.
Turbulent Boundary Layer generated Over Two Urban Roughnesses
Kellnerová, Radka ; Jaňour, Zbyněk
As a basis for PIV measurement, turbulent characteristics of a flow were investigated by LDA over two different types of urban roughness in wind-tunnel. The roughness consists from long series of street canyons with unit aspect ratio. Different shapes of roof - flat and pitched - provide a different internal boundary layer above canyons. The latter one, generated by pitched roofs, is significantly more turbulent and thus slower then above flat roofs. Momentum flux and corresponding transport exhibits the largest deviation specially at the roof level. Ventilation in the streets is therefore distinct from each other. Quadrant analysis was applied for vertical profiles and cross-sectional areas in canyons in order to detect a domination of sweep or ejection events and to find a linkage to the third moment of velocity for both cases. The overall goal of the project is to find any of coherent structure produced by the roof edge as a low momentum region or an ejection formed by hairpin.
Dynamika proudění uvnitř městské křižovatky: charakteristiky šikmosti a špičatosti
Kellnerová, Radka ; Jaňour, Zbyněk
The experiment uses wind-tunnel modeling method to investigate an intermittent ventilation of pollutants within an urban intersection and adjacent canyon. The skewness and kurtosis of velocity components were obtained in order to identify an area with strong intermittent tendency. Pollutants have released from line source and mean concentration was obtained using slow flame ionization detector. Vertical vortices as well as corner vortices are responsible for accumulation of emissions emitted by vehicle traffic. However, flow instabilities corresponding to enhancement of skewness can reduce or enhance the instantaneous level of exhalations. Vertical and horizontal momentum flux was analyzed by quadrant analysis. The method has detected the localities with highest momentum transfer from free stream down to the canyon. The connection between sweep and skewness in this area was found.

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