National Repository of Grey Literature 36 records found  beginprevious21 - 30next  jump to record: Search took 0.01 seconds. 
Proper Orthogonal Decomposition Analysis of an Air Jet with Exicited Helical Instabilities
Tesař, Václav ; Kellnerová, Radka ; Uruba, Václav ; Něnička, Václav
Authors performed analysis by the Proper Orthogonal Decomposition procedure of two air jets, one of them naturally evolving and the other with excited two helical intsanilities in the shear leyer surriunding the core. Several modes of cogherent structures were identified, differing significantly in the case with the excitation
Wavelet and POD Analysis of Turbulent Flow Within Street Canyon
Kellnerová, Radka ; Kukačka, Libor ; Uruba, Václav ; Antoš, Pavel ; Odin, J. ; Jaňour, Zbyněk
Wavelet analysis of flow inside the street canyon is applied on highrepetition PIV and high-frequency hot wire data. Experiment is performed in windchannel with floor covered by series of street canyons.The passage of sweep and ejection events is detected as being long-term signature in lower frequencies. The passage of vortex is recognizable as circular patterns in higher frequencies on Wavelet spectrum.When using velocity vectors as input data, proper orthogonal decomposition (POD) reveals the most dominant modes in terms of turbulent kinetic energy.
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.
Fyzikální modelování změn difúze znečišťujících látek v oblasti křižovatky v závislosti na směru nabíhajícího proudění
Kukačka, Libor ; Jurčáková, Klára ; Kellnerová, Radka ; Jaňour, Zbyněk
The objective of this study is physical modelling of emissions from vehicle exhausts in street canyons within an urban area and the influence of the approach flow direction on contaminant spreading. The requirements of the similarity of the atmospheric boundary layer and a boundary layer in the wind tunnel are satisfied. For several flow directions mean velocity and concentration fields are measured within street canyon intersection inside symmetrical urban built-up area. Scalar fluxes are computed from measured data to quantify pollutant spreading. Results of the experiment show a significant sensitivity of the mean velocity field on the approach flow direction. The approach flow extensively influence contaminant spreading within the built-up area and therefore intensity of street canyons ventilation vary with the approach flow direction. The approximate flow direction with the most favourable dispersion conditions in the studied area is found out.
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.
Generating a very rough turbulent boundary layer in wind tunnel Wotan, Meteorological Institute, Hamburg
Kellnerová, Radka ; Jaňour, Zbyněk
Contribution poses flow characteristics of symmetric very rough turbulent layer in wind-tunnel Wotan in Hamburg. Measurement was conducted using 2-D component LDA with high temporal resolution. Requirements for generating a top quality boundary layer with consistent parameters that follow up-to-date science of physical modeling are described and explained. Main attention was focused on linkage between integral length scale and turbulence intensity Ix and Iz according to guideline VDI.
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.
Vlastnosti trysek – experimentální ověření na zažízení stavěném pro výzkum šroubovicových nestabilit
Tesař, Václav ; Kellnerová, Radka ; Kukačka, Libor ; Kordík, Jozef
Nozzle characterisations in current use may be described as zero-order (neglected loss) and first-order (constant loss coefficient). The first author (V. Tesar) introduced in 1985 a second-order theory, fully predicting all the variations of nozzle properties with Reynolds number Re. The basic idea is an assumption of the loss being entirely specified by displacement effect of wall boundary layer in the exit. The other assumption is the layer thickness being governed by laminar boundary layer growth law. All evidence for this theory so far was only indirect. This paper presents recent direct experimental verification by measurements of boundary layer profiles on a large-scale nozzle model across a wide range of Re. The predictions of the 1985 model are fully supported.
Turbulentní charakteristika uvnitř městské křižovatky
Kellnerová, Radka ; Jaňour, Zbyněk
The wind-tunnel experiment checked the central symmetry for the inlet conditions and the flow inside an one case of symmetric model. For approaching flow, central asymmetry of y- and z-component of velocity between the right and left side of tube was detected. Nonetheless, the model of urban intersection has a dominant role in flow up to the 1.5 height of building and just mild asymmetry was found here. For elimination of these discrepancies the influence of wind direction and model arrangement suppressing the lateral flow was tested. The main reason for asymmetry was appeared to be a position of the mounted probe of LDA measuring facility. The turbulent characteristics obtained inside the model were analysed using Quadrant analysis technique. This helped to understand the structure of momentum flux in lower surface layer called the roughness sublayer.
Symetrie turbulentních charakteristik uvnitř městské křižovatky
Kellnerová, Radka ; Jaňour, Zbyněk
The wind-tunnel modelling of dispersion is very sensitive to central symmetry. The asymmetry of concentration field inside the symmetrical model is often caused by two causes: inappropriate inlet conditions or non-symmetric set-up of model. The project has carried out the flow measurements by Laser Doppler Anemometer's probe merged into the tunnel. The aim was to investigate the flow processes inside the tunnel body-tube itself – i.e. without the model and inside the model of a symmetrical urban intersection. The influence of wind direction and model arrangement suppressing the lateral flow was tested. The largest deviations from symmetry were caused by the circumfluence of the measuring devices.

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