Analysis of Planar Measurements of Turbulent Flows

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Abstract

Analysis of turbulent velocity data, obtained by Particle Image Velocimetry (PIV), has been carried out. The reason for applying PIV was its ability to provide instantanerous realizations in a plane of a flow, which facilitates a quantitative study of spatial coherent flow structures. The main tool used for the investigations was the Proper Orthogonal Decomposition (POD) Also, Linear Stochastic Estimation (LSE), as well as traditional statistical analysis, has been applied to explore more data. Following a description of the theory behind the analysis techniques, three cases of engineering relevancy were considered. The results of the investigations can be summarized ad follows. In the first case considered, uncorrelated as well as temporally resolved data was obtained in a plane of a scale model of the Annex 20 room, which is a standard measure room used for investigation of ventilation problems. The data was obtained in a region near the inlet, where a wall jet enters the room along the ceiling below which a region with slowly delveloping flow structures is found. A POD analysis was carried out, which permitted a study of the temporal variation of the influence of the most dominant flow structures occurring in this region of the room. Further, the lenght scales were investigated and the dominant structures derived from the POD were reproduced with the LSE. The results showed that flow structures which differ substancially from the mean velocity field are important to the dynamics of the flow in the Annex 20 room. In the second case, the complex flow found in the near field of the turbulent jet-in-crossflow (JICF) was investigated. Stereoscopic PIV measurements yielding all three velocitycomponents in different planes near the jet exit were carried out. The mean velocity fields were presented to give a picture of the evolution of JICF along and downstream of the jet trajectory. Carrying out POD analyses in a number of mutually perpendicular planes revealed that the dominant structures of the near field of the JICF are found just downstream of the jet trajectory, and that these are associated with velocity fluctuations normal to the symmetry plene. Based on these observations, it was conjectured that the maximum production of turbulent kinetic energy results from the initiation of the wake vortices of the JICF which in turn seems to be connected to the dynamics of the characteristic counter-rotating vortex pair. The third and final case considered was the free axisymmetric turbulent jet. Raw measurement data obtained with PIV and Laser Induced Fluorescence (LIF) were processed to obtain combined velocity and concentration fields in a region centered 80 jet diameters downstream. The mean velocity field was deppicted and compared to the results of a self similarity analysis. Detecting the jet boundary from the LIF data, statistical quantities could be derived relative to the turbulent/non-turbulent interface between the jet fluid and the ambient fluid. The results showed a change in velocity gradient across the interface caused by a vorticity maximum, whic is believed to be caused by the frequent occurence of small scale vortices in this region. Further, it was shown that the velocities and the root mean squared values of their fluctuating parts scale with the mean centerline velocity and the jet half-width, when moving downstream. The same behavior was not observed for the vorticity profile, for which the maximum detected just within the jet boundary seems to increase with downstream position relative to a fixed universal value within the jet region.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherTechnical University of Denmark
Number of pages143
ISBN (Print)87-7475-292-8
Publication statusPublished - Jun 2003
SeriesMEK-FM-PHD
Number2003-01

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