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Vortex lift and insect flight
Dvořák, Rudolf
Vortex lift is usually associated with swept wings at high angles of attack. Insect wings oscillating at high frequencies develop a leading edge vortex which behaves at very low Reynolds numbers in the same way as on swept wings. Results are of interest for design of MAV (micro-air-vehicles).
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On Principle of the Rolling Turbine
Falta, Jiří ; Maršík, František ; Sedláček, M.
A new principle of a fluid machine called "rolling turbine" is analyzed using two different approaches. The first one utilizes the stability theory for inviscid flow to explain the origin of the vorticity which is crucial for existence of the driving force. The second approach uses the perturbation theory for an approximate solution of the problem describing the machine working in its "pump" mode. The slender-channel theory is applied to simplify the Navier-Stokes equations.
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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.
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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
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Modification of an algebraic bypass-transition model in region near the stagnation point
Straka, P. ; Příhoda, Jaromír
An algebraic model of bypass transition was applied for the numerical simulation of flow over the NACA 0012 airfoil. The system of governing equations is formed by the averaged Navier-Stokes equations closed by the k-omega turbulence model. The intemittent character of the transitional flow is described by the algebraic relation for the intermittency coefficient and by empirical relations for the onset and the length of the transition region. The constitutive relation for the turbulent velocity was modified to improve the prediction near the wall and near the leading edge. The prediction of transitional flow was carried out for the Reynolds number Re = 6x10^5 and free stream turbulence Tu = 0.3%. The agreement of numerical results with experimental data is quite good.
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2D Numerical simulations of wake behind cylinder using spectral element method
Pech, Jan
We investigated 2D numerical simulation of wake in the flow behind circular cylinder. The main object of observance was the frequency of vortex shedding (Strouhal number (St)) depending on the Reynolds number (Re). Using spectral element method we performed a set of simulations to check the onset of vortex shedding, e.g. the critical Reynolds number and relation St-Re. Model of wake behind heated cylinder was computed using commercial software.
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The interaction of continual and synthetic jets
Broučková, Zuzana ; Trávníček, Zdeněk ; Šafařík, Pavel
This work deals with an interaction of continual and cross synthetic jets. Both jets are axisymmetric. All measurements have been performed in air by means of the Pitot tube and hot-wire anemometry. Flow visualization has been also performed. The possibility of flow control is shown and quantified. For the moderate momentum ratio, the mean jet deflection reasonably agrees with the momentum conservation.
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