|
| |
|
|
Study of the Dissipation in Spiraling Vortical Structures
Štefan, David ; Drábková, Sylva (referee) ; Koutník,, Jiří (referee) ; Skoták, Aleš (referee) ; Rudolf, Pavel (advisor)
This work deals with study of swirling flows where the spiral vortical structure appears. The main relation is to flow seen in the draft tube cone of hydraulic turbines operated out of the design point (i.e. best efficiency point). In this cases large coherent vortex structure (vortex rope) appears and consequently high pressure pulsations are propagated to the whole machine system leading to possible restriction of turbine operation. This flow features are consequence of flow instability called vortex breakdown in case of Francis turbine operated at part load (flow rate lower than optimal one). The present study is carried out using simplified device of swirl generator in order to access similar flow conditions as can be found in real hydraulic turbines. Both the dynamic and dissipation effect of spiral vortex breakdown are investigated. The first part of thesis deals with spiral form of vortex breakdown. The experimentally measured velocity profiles (LDA) and wall static pressures are correlated with numerical simulations carried out using open-source CFD package OpenFOAM 2.2.2. The high speed camera recording of cavitating vortex core is used to obtain image ensemble for further post-processing. The dissipation effect of spiral vortex structure is in detail discussed based on computed flow fields. The second part of thesis is dedicated to the application of POD decomposition to the study of spatio-temporal features of spiral vortex dynamics. Firstly the POD is applied to the both the experimentally obtained image ensemble of cavitating vortex and numerically computed static pressure fields. Secondly the comprehensive analysis of spiral vortex mitigation effect by the axial water jet is analyzed. The collaborative study employing the swirl generator apparatus designed by the researchers from Politehnica University of Timisoara in Romania is performed and changes in spatio-temporal vortex dynamic are studied. In this study the numerical data (in a form of three-dimensional pressure and velocity fields) are obtained using commercial CFD software ANSYS Fluent R14.
|
|
|
Stereoscopic PIV measurement of boundary layer affected by DBD actuator
Procházka, Pavel P. ; Uruba, Václav
The effect of ionic wind generated by plasma actuator on developed boundary layer inside a narrow channel was investigated recently. Since the main investigated plane was parallel to the channel axis, the description of flow field was not evaluated credibly. This paper is dealing with cross-section planes downstream the actuator measured via 3D time-resolved PIV. The actuator position is in spanwise or in streamwise orientation so that ionic wind is blown in the same direction as the main flow or in opposite direction or perpendicularly. The interaction between boundary layer and ionic wind is evaluated for three different velocities of main flow and several parameters of plasma actuation (steady and unsteady regime, frequency etc.). Statistical properties of the flow are shown as well as dynamical behaviour of arising longitudinal vortices are discussed via phase-locked measurement and decomposition method.
|
|
|
Mathematical Modelling of Flow over Periodic Structures
Bauer, Petr
We examine the influence of roofs' shapes on the boundary layer of a simplified urban canopy by computing non-stationary Navier-Stokes flow over a periodic pattern. The solution is obtained by means of finite element method (FEM). The resulting linear system is solved by the multigrid method. We present computational studies of the problem.
|
|
|
Proper Orthogonal Decomposition of Turbulent Flow above Urban Area
Kellnerová, Radka ; Kukačka, Libor ; Uruba, Václav ; Jurčáková, Klára ; Jaňour, Zbyněk
This contribution describes the character of turbulent flow generated above series of street canyons in wind-channel. Main concern is to clarify how intermittent dynamics of the flow determines the ventilation process inside the canyon. Since geometry of building around street canyon significantly affects the flow regime, two shapes of building are studied and comparison of these two regimes is done. During experiment, velocity data are measured using PIV with high-repetition rate. Over than three thousand snapshots of instantaneous velocity are then analysed by Proper Orthogonal Decomposition (POD). This method reveals the measure of coherence of the flow and decomposes the individual snapshots into dynamics modes. For example, the first mode in shear layer (localised at the roof level) contains almost 55% of global turbulent kinetic energy, what is an in
|
|
|
A New Method for Vector Fields Dynamic Analysis
Uruba, Václav
A method for analysis of spatio-temporal data is proposed. This is based on combination of Proper Orthogonal Decomposition and Principal Oscil-lation Patterns methods. The method provides dynamical characteristics of spatial modes evaluated either on both energetic or stability basis. Example on TR-PIV results on unsteady separation is shown.
|
|
| |
guest ::
