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LDA measurements and turbulence spectral analysis in an agitated vessel
Kysela, Bohuš ; Konfršt, Jiří ; Chára, Zdeněk
During the last years considerable improvement of the derivation of turbulence power spectrum from Laser Doppler Anemometry (LDA) has been achieved. The irregularly sampled LDA data is proposed to approximate by several methods e.g. Lomb-Scargle method, which estimates amplitude and phase of spectral lines from missing data, methods based on the reconstruction of the auto-correlation function (referred to as correlation slotting technique), methods based on the reconstruction of the time series using interpolation between the uneven sampling and subsequent resampling etc. These different methods were used on the LDA data measured in an agitated vessel and the results of the power spectrum calculations were compared. The measurements were performed in the mixing vessel with flat bottom. The vessel was equipped with four baffles and agitated with a six-blade pitched blade impeller. Three values of the impeller speed (Reynolds number) were tested. Long time series of the axial velocity component were measured in selected points. In each point the time series were analyzed and evaluated in a form of power spectrum.
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Analýza finančních dat metodami ekonofyziky
Šubrt, Jiří ; Kodera, Jan (vedoucí práce) ; Málek, Jiří (oponent)
Vzhledem k naléhavosti finančních expertů na důkladnější analýzu předpovědí finančních krizí se objevují koncepty využívající teorie z jiných než ekonomických disciplín. Významná je oblast ekonofyziky uplatňující teorie z přírodních věd. Smyslem této práce je poukázat na jednu z mnoha metod zkoumání finančních dat opírající se o teorii turbulence tekutin a deterministického chaosu. V práci paralelně analyzujeme vysokofrekvenční záznamy akciového indexu a rychlostí proudění kapaliny. Jsou použity principy statistické matematiky, teorie pravděpodobnosti a procesů škálování. Výsledkem jsou odlišnosti obou zkoumaných systémů, ale smyslem je použití zvolené metodologie i na finanční data.
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Decomposition Methods in Turbulence Research
Uruba, Václav
Nowadays we have the dynamical velocity vector field of turbulent flow at our disposal coming thanks advances of either mathematical simulation (DNS) or of experiment (time-resolved PIV). Unfortunately there is no standard method for analysis of such data describing complicated extended dynamical systems, which is characterized by excessive number of degrees of freedom. An overview of candidate methods convenient to spatiotemporal analysis for such systems is to be presented. Special attention will be paid to energetic methods including Proper Orthogonal Decomposition (POD) in regular and snapshot variants as well as the Bi-Orthogonal Decomposition (BOD) for joint space-time analysis. Then, stability analysis using Principal Oscillation Patterns (POPs) will be introduced. Finally, the Independent Component Analysis (ICA) method will be proposed for detection of coherent structures in turbulent flow-field defined by time-dependent velocity vector field. Principle and some practical aspects of the methods are to be shown. Special attention is to be paid to physical interpretation of outputs of the methods listed above.
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METHODS OF IDENTIFICATION OF COUPLED VORTEX STRUCTURES IN MOTION
Něnička, Václav ; Tesař, Václav ; Šonský, Jiří
Flow visualisation was performed of sections perpendicular to the axis through a jet. It reveaedl surprising complexity in the mixing region (outside the jet core. The images were processed by correlation in sub-images showing vortical motions suggesting the twisting and braiding movements of hair when a plait hairdo is made. For evaluation authors found useful the theory that measures entanglement in fluids by the corresponding algebra of braiding movements.
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Hairpin Vortices in Turbulent Boundary Layer
Uruba, Václav ; Hladík, Ondřej
The experiments on dynamics of turbulent boundary layer on the plane rough wall without pressure gradient are to be presented. Sand roughness of the wall is considered. Measurements are carried out using Time-Resolved PIV technique in plains parallel and perpendicular to the wall. The results on rough wall are compared with the base case of boundary layer on smooth wall. Hairpin vortices and theirs packets have been detected. Topology and typical size of those structures substantially differ in the cases in question.
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AXIAL VARIATION OF A SYNTHETIC JET FREQUENCY SPECTRUM
Tesař, Václav ; Kordík, Jozef
Frequency spectra of synthetic jets vary along the jet axis and there is sudden change of the character and structure of axisymmetric synthetic jets at the relative axial distance from the virtual origin X1*/d ~50. Processing the spectra into the compensated form made obvious a number of surprising facts – one of them the discovery that what wasexpected to be the Komogorov constant obviously varies quite significantly along the jet, its variations exhibiting a marked change in its behaviour at the critical transition.
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PIV investigations of an excited air jet
Tesař, Václav ; Uruba, Václav ; Něnička, Václav
Authors investigated an axisymmetric jet of air issuing from a 40 mm dia. nozzle using 3D particle image velocimetry system. The jet was excited by a rotating acousticpressure field in the nozzle exit plane to ensure generation of two helical instability modes chasing one another and mutually interacting. The excitation signal was also used for ensuring PIV data acquisition at a precise phase position
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