CFD modelování dvoufázového proudění v rotačně symetrických tělesech

The work summarizes the basic findings which result from numerical modelling of flow at the mixture of air and water, with consideration of laminar and turbulent flow. The attention is focused on the development of the velocity profile of the liquid, depending on adhesion coefficient and the degree of hydrophobicity of the surface. We considered the geometry of a straight circular pipe cal and horizontal position. The solution focuses on the finite element method and the tool utilized to evaluate the results was numerical program ANSYS Fluent.Práce sumarizuje základní poznatky, které vyplývají z numerického modelování proudění směsi vody a vzduchu suvažováním laminárnírychlostního profilu kapaliny v závislosti na adhesním součiniteli, a stupni hydrofobie povrchu. Bylo uvažováno o geometrií přímého kruhového potrubí, které bylo uloženo ve vertikální a horizontální poloze. Řešení je zaměřeno na metodu konečných prvků a nástrojem využitým k zhodnocení výsledků je numerický program ANSYS Fluent

Constitutive Equations for Magnetic Active Liquids

This article is focused on the derivation of constitutive equations for magnetic liquids. The results can be used for both ferromagnetic and magnetorheological fluids after the introduced simplifications. The formulation of constitutive equations is based on two approaches. The intuitive approach is based on experimental experience of non-Newtonian fluids, which exhibit a generally non-linear dependence of mechanical stress on shear rate; this is consistent with experimental experience with magnetic liquids. In these general equations, it is necessary to determine the viscosity of a liquid as a function of magnetic induction; however, these equations only apply to the symmetric stress tensor and can only be used for an incompressible fluid. As a result of this limitation, in the next part of the work, this approach is extended by the asymmetry of the stress tensor, depending on the angular velocity tensor. All constitutive equations are formulated in Cartesian coordinates in 3D space. The second approach to determining constitutive equations is more general: it takes the basis of non-equilibrium thermodynamics and is based on the physical approach, using the definition of density of the entropy production. The production of entropy is expressed by irreversible thermodynamic flows, which are caused by the effect of generalized thermodynamic forces after disturbance of the thermodynamic equilibrium. The dependence between fluxes and forces determines the constitutive equations between stress tensors, depending on the strain rate tensor and the magnetization vector, which depends on the intensity of the magnetic field. Their interdependencies are described in this article on the basis of the Curie principle and on the Onsager conditions of symmetry.</p

Vliv částečně smáčivého povrchu na proudové pole v mezeře mezi dvěma souosými válci

The influence of the partial surface wetting on the flow field between the two coaxial cylinders, the inner of which rotates, has been investigated numerically. Wall boundary condition was modified to account for partial wettability based on the equation proposed by Pochylý for the general curved surface [1-10]. FLUENT software was applied to model the stationary viscous fluid flow in a narrow gap. Different boundary conditions were applied on the rotating wall of the inner cylinder. The results obtained by numerical modelling were compared with theoretical assumptions.Vliv částečně smáčivého povrchu na proudové pole v mezeře mezi dvěma souosými válci, z nichž vnitřní rotuje, byl zkoumán s využitím numerického modelování. Modifikace okrajové podmínky vychází z rovnice, kterou pro částečně smáčivý obecně zakřivený povrch definoval Pochylý [1-10]. V software FLUENT bylo simulováno stacionární proudění nestlačitelné viskózní tekutiny v úzké mezeře, kdy na stěnu vnitřního válce byla aplikována okrajová podmínka zahrnující vliv částečné smáčivosti. Výsledky z numerické simulace byly následně porovnány s teoretickými vztahy

New boundary conditions for fluid interaction with hydrophobic surface

Solution of both laminar and turbulent flow with consideration of hydrophobic surface is based on the original Navier assumption that the shear stress on the hydrophobic surface is directly proportional to the slipping velocity. In the previous work a laminar flow analysis with different boundary conditions was performed. The shear stress value on the tube walls directly depends on the pressure gradient. In the solution of the turbulent flow by the k- model, the occurrence of the fluctuation components of velocity on the hydrophobic surface is considered. The fluctuation components of the velocity affect the size of the adhesive forces. We assume that the boundary condition for depending on the velocity gradients will not need to be changed. When the liquid slips over the surface, non-zero fluctuation velocity components occur in the turbulent flow. These determine the non-zero value of the turbulent kinetic energy K. In addition, the fluctuation velocity components also influence the value of the adhesive forces, so it is necessary to include these in the formulation of new boundary conditions for turbulent flow on the hydrophobic surface

Numerical computation of the damping and stiffness coefficients of the classical and magnetorheological squeeze film damper

Technological solution, frequently used to suppress vibrations in rotating machines, consists in adding damping devices between the rotor and its frame. This is enabled by dampers working on the principle of a squeezing thin classical or magnetorheological fluid film. The Navier-Stokes equations, Reynolds equation, and modified Navier-Stokes equations are used to determine the pressure distribution in the thin fluid film. The damping and stiffness coefficients are computed by the developed procedure presented in this paper. The proposed computational approach is based on the perturbation of the synchronous circular whirling motion. The carriedout computational simulations show that the investigated mathematical models of the squeeze film damper and magnetorheological squeeze film damper allowed computation of the damping and stiffness coefficients. It has been found that the stiffness coefficients computed by the proposed mathematical models may be different

Transition of cavitating flow to supercavitation within Venturi nozzle – hysteresis investigation

Cavitation is usually considered as undesirable phenomena. On the other hand, it can be utilized in many applications. One of the technical applications is using cavitation in water treatment, where hydrodynamic cavitation seems to be effective way how to reduce cyanobacteria within large bulks of water. The main scope of this paper is investigation of the cavitation within Venturi nozzle during the transition from fully developed cavitation to supercavitation regime and vice versa. Dynamics of cavitation was investigated using experimental data of pressure pulsations and analysis of high speed videos, where FFT of the pixel intensity and Proper Orthogonal Decomposition (POD) of the records were done to identify dominant frequencies connected with the presence of cavitation. The methodology of the high speed (HS) records semiautomated analysis using the FFT was described. Obtained results were correlated and above that the possible presence of hysteresis was discussed

Aerační zařízení pro malé hloubky.

Deficit of air in water causes complications with cyanobacteria mainly in the summer months. Cyanobacteria is a bacteria that produces poison called cyanotoxin. When the concentration of cyanobacteria increases, the phenomena „algal bloom“ appears, which is very toxic and may kill all the organisms. This article describes new equipment for aeration of water in dams, ponds and reservoirs with small depth. This equipment is mobile and it is able to work without any human factor because its control is provided by a GPS module. The main part of this equipment consists of a floating pump which pumps water from the surface. Another important part of this equipment is an aerator where water and air are blended. Final aeration process runs in the nozzles which provide movement of all this equipment and aeration of the water. Simulations of the flow are solved by multiphase flow with diffusion in open source program called OpenFOAM. Results will be verified by an experiment.Článek popisuje nové aerační zařízení pro nádrže, přehrady a rybníky s malou hloubkou hladiny. Vynalezené zařízení může být použito pouze pro destratifikaci nebo pro destratifikaci kombinovanou s aerací. Tento článek je obsahuje konstrukční řešení aeračního zařízení, simulaci ve open source programu OpenFOAM a experimentální ověření