We deal with the numerical simulation of a flow of solid-liquid-gas slurries with the virtual mass effect. The governing systmn of equations is strongly nonlinear hyperbolic with nonconservative terms. We propose a numerical scheme which belongs to the class of finite volume methods. In order to increase the order of convergence we apply a higher order reconstruction technique. Several numerical examples demonstrating the efficiency of the schemes are presented. and Obsahuje seznam literatury
The article deals with the numerical solution of transitional flows. The single-point k-kL-ω model of [7] based on the use of a laminar kinetic energy transport equation is considered. The model doesn‘t require to evaluate integral boundary layer parameters (e.g. boundary layer thickness) and is therefore suitable for implementation into codes working with general unstructured meshes. The performance of the model has been tested for the case of flows over a flat plate with zero and non-zero pressure gradients. The results obtained with our implementation of the model are compared to the experimental data of ERCOFTAC. and Obsahuje seznam literatury
This study deals with the numerical solution of a 2D unsteady flow of a compressible viscous fluid in a channel for low inlet airflow velocity. The unsteadiness of the flow is caused by a prescribed periodic motion of a part of the channel wall with large amplitudes, nearly closing the channel during oscillations. The channel is a simplified model of the glottal space in the human vocal tract and the flow can represent a model of airflow coming from the trachea, through the glottal region with periodically vibrating vocal folds to the human vocal tract.
The flow is described by the system of Navier-Stokes equations for laminar flows. The numerical solution is implemented using the finite volume method (FVM) and the predictor-corrector MacCormack scheme with Jameson artificial viscosity using a grid of quadrilateral cells. Due to the motion of the grid, the basic system of conservation laws is considered in the Arbitary Lagrangian-Eulerian (ALE) form.
The authors present the numerical simulations of flow fields in the channel acquired from a program developed exclusively for this purpose. The numerical results for unsteady flows in the channel are presented for inlet Mach number M∞ = 0.012, Reynolds number Re∞ = 5x103 and the wall motion frequency 100 Hz. and Obsahuje seznam literatury
The paper deals with a modeling of a flow in hydraulic parts of a turbo machine operating in a pump and in a turbine mode. The results are Q-H (Q-Y) performance characteristics of the pump operating in both modes. An important piece of knowledge is a shift of the pump‘s and turbine‘s BEP and the change of specific speeds of the turbo machine in a pump and in a turbine mode, which emerges from it. The goal of this paper was to find a shift of pump‘s and turbine‘s BEP and to obtain information about hydraulic efficiency of the pump operating in a pump and turbine mode. Anyway the aim of CFD simulation is to evaluate Q-H (Q-Y) characteristic from the point of view of a stability of Q-H curve. The analysis of flow patterns in the impeller and in the vane diffuser in a pump and turbine mode is a valuable contribution to the topic of a hydraulic interaction between an impeller and a vane diffuser. and Obsahuje seznam literatury
The article presents a numerical model of a coupled electro-magneto-mechanical system - an electromagnet exposed to vibration of a yoke. Operation of a multi-physical (an electro-magneto-mechanical) model is simulated under different working and excitation conditions and a response of the system is analyzed. Simscape, a tool of MATLAB programming environment, is used for numerical analysis of the problem. It is shown, that there exists a combination of operation parameters, which can lead to a substantial attenuation ot the yoke vibration. Furthermore, there exists a critical magnitude of the current, which corresponds to a permanent attraction of the yoke to the electromagnet. An analysis of electormagnet‘s initialization shows an induction of high voltages in electric circuit, which can damage the electromagnet and need to be avoided by a proper choice of parameters. and Obsahuje seznam literatury
The paper makes a sketch of an SDOF system response analysis subjected to a random excitation having a form of the additive Poisson driven independent random impulses. A special generalised Fokker-Planck equation having a form of an integro-differential equation is presented together with boundary and initial conditions. Later the Galerkin-Petrov process as a method of a numerical solution of the respective evolutionary integro-differential equation for the probability density function (PDF) is presented in general. Various analytic and semi-analytic solution methods have been developed for various systems to obtain results requested. However numerical approaches offer a powerful altemative. In particular the Finite Element Method (FEM) seems to be very effective. Shape and weighting functions for purposes of a numerical solution procedure are carred out and corresponding ordinary differential system for PDF values in nodes is deduced. As a demonstration particular SDOF systems are investigated. Resulting PDFs are analysed and mutually compared. and Obsahuje seznam literatury
Paper presents the results of numerical modelling of re-suspension of cohesive sediments settled behind spurs and dikes at the end of backwater of Střekov river reservoir. The monitoring of water, sediment and suspended load quality of Czech Elbe between 1991 and 2001 showed a considerable improvement of environment quality in the main channel of Elbe and even in small weir reservoirs along the waterway, but, in the bights of reservoirs of higher weirs, there are layers of sediments containing old pollutants. These are resuspended during catastrophic floods and then transported downstream like suspended load make the water quality worse. Sediments containing dangerous pollutants (mostly heavy metals) in the Czech reach of Elbe river were assessed in Rudiš, 2000. Its re-suspension may be predicted using at least two-dimensional mathematical model of the water flow. Such model must be “open” for imputation of the criterion of re-suspension Rudiš et al., 1999. Using the model FAST 2D (Wenka, Valenta, 1991) together with the data processing program (Valenta, Wenka, 1996), the authors obtained a tool for modelling of behaviour of cohesive sediments during a definite flood. It is to note that Elbe, from its confluence with Vltava (Moldau) river as far as to the state border, was, in the past, equipped by a system of dikes and spurs originally built up for maintenance of navigable depth. These constructions are submerged now by water elevated by weirs constructed afterwards. Behind these submerged dikes and spurs, water flows very slowly at low flow rates and thus sedimentation occurs there. After some time of sediments being at rest, they attain the cohesive properties and the re-suspension requires shear stress the more increasing with time. As a practical example, the locality at the end of the backwater of the reservoir Střekov was chosen for computation of the time procedure of re-suspension of cohesive sediments from the area behind dikes and spurs. A further reason for computation in this locality was in fact that these sediments may be polluted by effluents from Lovosice chemical factory (Lovochemie) and, being re-suspended, may influence the quality of Elbe water as far as to the German reach of Elbe river. and Článek předkládá výsledky numerického modelování resuspendace kohesivních sedimentů, které jsou usazeny za výhony a hrázkami na začátku vzdutí nádrže Střekov. Sledování kvality vody, sedimentů a plavenin v českém Labi v letech 1991 až 2001 ukázalo významný vzestup kvality prostředí v hlavním korytě toku a také v malých zdržích vodní cesty, avšak v zálivech větších zdrží zůstaly vrstvy kohesivních sedimentů obsahující staré zátěže. Ty se dostanou do vznosu působením vysokých rychlostí vody za katastrofálních průtoků, jsou dále unášeny tokem ve formě plavenin a svým obsahem zhoršují kvalitu vody. Sedimenty v Labi obsahující nebezpečné polutanty (jedná se převážně o těžké kovy) byly stanoveny v práci Rudiš, 2000. Jejich resuspendaci je možno předpovídat s použitím nejméně dvourozměrného numerického modelu proudění, který musí být ''otevřený'', aby bylo možno do něho vložit kriteriální vztah pro resuspendaci (Rudiš a kol., 1999). Použitím modelu FAST 2D (Wenka, Valenta, 1991) spolu s programem pro zpracování dat (Valenta, Wenka, 1996) dostali autoři nástroj modelování kohesivních sedimentů při definované povodni. Je třeba poznamenat, že Labe od soutoku s Vltavou až po státní hranici bylo vybaveno systémem podélných hrázek a výhonů vybudovaných pro zajištění plavební hloubky za nízkých průtoků. Tyto konstrukce jsou nyní ponořeny pod hladinou jezů vybudovaných nebo rekonstruovaných později. Za nimi ovšem voda proudí ve srovnání s hlavním korytem velice pomalu, takže tam dochází k sedimentaci. Sedimenty jsou po dobu trvání nízkých průtoků v klidu, nabudou kohesivních vlastností a jejich resuspendace bude vyžadovat s časem stále vyšší hodnoty smykového napětí. Jako praktický případ předpovědi průběhu resuspendace za povodně ve složitých geometriích kombinace hlavního koryta a oboustranného systému hrázek rozdělených příčnými výhony byla vybrána lokalita na začátku vzdutí nádrže Střekov. Dalším důvodem výběru této lokality byl fakt, že se v ní usazují plaveniny z odpadních vod závodu Lovochemie v Lovosicích. Tento sediment může proto obsahovat nebezpečné polutanty a kdyby byl resuspendován, může ve formě plavenin ohrozit kvalitu vody až na německém území.
The focus of this paper is on the analysis of the influence of near-bed turbulence structures with the inclusion of existing coherent structures on the entrainment of saltating particles in a water stream from the Lagrangian perspective. The interactions between turbulence structures and initiation of particles movement is the key for better understanding of the physics of sediment transport and particles behaviour. These aims are addressed by use of a 3D relevant model of spherical saltating particles, in which a special procedure has been designed to produce coherent structures. In this method, the spectra of turbulent kinetic energy, consisting of four ranges, are used to generate the time series of turbulent velocities in the streamwise, vertical and transversal directions. Numerical results suggest that the initiation of sediment movement is strongly correlated to positive streamwise velocity fluctuations and as such, supports earlier laboratory experimental and field observations, showing that the sweeps and outward interactions play a crucial role in the initiation of saltating particles’ movement.
Naše pracovní skupina vytvořila v letech 2002-2007 globální numerický model magnetosféry, který v sobě obsahuje popis kinetických vlastností iontů v plazmatu, zejména pak popis přenosu energie mezi vlnami a nabitými částicemi v plazmatu. Model je v současné době využíván americkým Národním úřadem pro letectví a kosmonautiku (NASA) k interpretaci pozorování družice MESSENGER, jejímž cílem je výzkum planety Merkur. Pomocí našich simulací jsme se pokusili vysvětlit některé z jevů pozorovaných družicí MESSENGER., Pavel M. Trávníček, Petr Hellinger., and Obsahuje seznam literatury
Numerické metódy majú v skúmaní seizmického pohybu nezastupiteľnú a stále dôležitejšiu úlohu. Metóda konečných diferencií je v súčasnosti dominantnou metódou. Pokrok v jej aplikácii na seizmologické problémy súvisí s úsilím o realistickejšie modely a výpočtovú efektívnosť., Peter Moczo, Jozef Kristek., and Obsahuje seznam literatury