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The Transitional Regime of Backward-Facing Step Flows
Tihon, Jaroslav ; Pěnkavová, Věra ; Havlica, Jaromír ; Šimčík, Miroslav
The backward-facing step flow is investigated experimentally (electrodiffusion technique) and numerically (CFD software Fluent) at moderate Reynolds numbers achieved in a water channel. The direction sensitive sensors are used to measure the wall shear rate behind the step and detect the near-wall extent of different flow-recirculation regions. 2D numerical simulations provide additional information on the global flow rearrangement caused by the change of operational parameters (channel expansion ratios, inlet flow conditions). At the transitional flow regime, the near-wall flow exhibits an unsteady character with a high sensitivity to external low-frequency perturbations (intrinsic flow instability). The inlet pulsatile forcing is found to affect strongly the overall flow structure behind the step, resulting in the reattachment length reduction and the reverse flow intensification.
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Detailed analysis of POD method applied on turbulent flow
Kellnerová, Radka ; Kukačka, Libor ; Uruba, Václav ; Jurčáková, Klára ; Jaňour, Zbyněk
Proper orthogonal decomposition (POD) on very turbulent flow inside the street canyon is performed. The energy contribution of each mode is yielded up. Also, physical meaning of the POD method is clarified. Particular modes of POD are assigned to the particular flow events like sweep event, vortex behind the roof or vor- tex at the bottom of street. Test of POD sensitivity on the length of data recording is done when the acquisition time increases up to its triple value. Test with decreasing sample frequency is executed. Further, interpola- tion of POD expansion coefficient is attempted in order to test possible increase in sample frequency and get new information about flow from the POD analysis. We tested linear and spline sort of interpolation and the linear one brought slightly better results.
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HWA Measurement of Turbulent Diffusion of a Scalar Quantity
Antoš, Pavel
The paper deals with simultaneous measurement of the velocity and the scalar quantity by means of hot-wire anemometry. Statistical moments of the scalar quantity fluctuations can be obtained employing a dual hot-wire probe. An evaluation procedure of the mean values and the fluctuations is described. Results from the molar concentration measurement in binary-gas mixture are shown.
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Numerical Simulation of Turbulent Flow Affected by Vortex Generators in Straight Channel
Součková, Natálie ; Šimurda, David ; Uruba, Václav
The presented work is the next step after several experimental examinations of the vortex generator (VG) influence on flow separation occurring on a model of the NACA 63A421 airfoil with a deflected simple flap. The other purpose of this simulation is to obtain beneficial information that can be utilized for the preparation of the experimental investigation of the same configuration using Particle Image Velocimetry method (PIV) in the future. The numerical simulation was performed for one single pair and two pairs of low-profile VGs of the same size, whose heights were smaller than the boundary layer thickness. The rectangular vane type VGs in such configuration, which generates counter-rotating vortices, was examined. The behaviour of vortices produced by VG pair or pairs in several positions downstream the VGs is investigated and will be used as a background of the measurement.
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Visualisation of Boundary Layer Separation and Passive Flow Control on Airfoils and Bodies in Wind-Tunnel and In-Flight Experiments
Popelka, Lukáš ; Kuklová, J. ; Šimurda, David ; Součková, Natálie ; Matějka, Milan ; Uruba, Václav
Infrared camera, Particle Image Velocimetry, smoke-wire, tuft filaments and oil-flow visualization techniques were used for wind-tunnel and in-flight investigation of boundary layer separation, both stall and separation bubbles, related to the low-Reynolds numbers transition mechanism. Airfoils of Wortmann FX66 series and FX66 series wing-fuselage interaction, as well as modern airfoils and their wing-fuselage geometry were subject to study. The presence of previously identified structures in the CFD modelling, such as horse-shoe vortices, was confirmed in the flow.
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On Hairpin Vortices in a Transitional Boundary Layer
Hladík, Ondřej ; Jonáš, Pavel ; Uruba, Václav
In the presented paper the results of experiments on transitional boundary layer are presented. The boundary layer was generated on smooth flat wall with zero pressure gradient forming one side of the channel of rectangular cross section. The hairpin vortices, packets of hairpin vortices, turbulent spots and calmed regions were experimentally investigated using time-resolved PIV technique.
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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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Experimental investigation of turbulent flow in a channel with the backward-facing inclined step
Příhoda, Jaromír ; Kotek, M. ; Uruba, Václav ; Kopecký, V. ; Hladík, Ondřej
The work deals with the experimental investigation of turbulent flow in a closed channel with the backward-facing inclined step. Experiments were carried by means of the PIV optical measuring method in the channel of the rectangular cross-section in the inlet part and with inclined steps of the constant height and various inclination angles for a wide range of the Reynolds number. The attention was paid especially to the separation region behind the step and to the relaxation of the shear layer after the reattachment in the outlet part of the channel. The dependence of the length of the separation region on the inclination step angle. Optical measurements were completed by the measurement of static pressure distribution in the inlet and outlet part of the channel to estimate energy losses.
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Modelling of the effect of the foregoing wake on the bypass transitionon the airfoil
Straka, P. ; Příhoda, Jaromír ; Šimurda, David
A modified algebraic model of the bypass transition was used for the simulation of the flow around the symmetrical airfoil NACA 0012 in the tandem configuration. The transition model is based on local variables only to enable its application for complex flow geometry using unstructured computational grids. The attention was especially focused on the flow near the leading edge where the boundary layer is not yet fully developed while used empirical correlations were established for the boundary-layer flow. The numerical simulation was compared with experiments of Lee and Kang (2000) carried out for the Reynolds number Re = (2x6)x10^5, the zero angle of attack and various distance between both airfoils. The agreement of numerical simulation with experimental data is quite satisfactory.
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