Comparison of turbulence models in simulating swirling pipe flows

Swirling flow is a common phenomenon in engineering applications. A numerical study of the swirling flow inside a straight pipe was carried out in the present work with the aid of the commercial CFD code fluent. Two-dimensional simulations were performed, and two turbulence models were used, namely, the RNG k–ε model and the Reynolds stress model. Results at various swirl numbers were obtained and compared with available experimental data to determine if the numerical method is valid when modeling swirling flows. It has been shown that the RNG k–ε model is in better agreement with experimental velocity profiles for low swirl, while the Reynolds stress model becomes more appropriate as the swirl is increased. However, both turbulence models predict an unrealistic decay of the turbulence quantities for the flows considered here, indicating the inadequacy of such models in simulating developing pipe flows with swirl.     

河渠非定常流的精确边界能控性

借助于一阶拟线性双曲型方程组混合初边值问题的半整体C^1解理论对单个河道及弦状网络河道中的非定常流动分别讨论了在闸门边界条件下的精确边界能控性问题,并对在泄洪边界条件下的精确边界能控性进行了相应的讨论。     

Exact boundary controllability of unsteady flows in a network of open canals

By means of the general results on the exact boundary controllability for quasilinear hyperbolic systems, the author establishes the exact boundary controllability of unsteady flows in both a single open canal and a network of open canals with star configuration respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simulation of the flow past a circular cylinder using an unsteady panel method

In the present work, an in-house UnSteady Double Wake Model (USDWM) is developed for simulating general flow problems behind bodies. The model is presented and used to simulate flows past a circular cylinder at subcritical, supercritical, and transcritical flows. The flow model is a two-dimensional panel method which uses the unsteady double wake technique to model flow separation and its dynamics. In the present work the separation location is obtained from experimental data and fixed in time. The highly unsteady flow field behind the cylinder is analyzed in detail. The results are compared with experiments and Unsteady Reynolds-Averaged Navier Stokes (URANS) simulations and show good agreement in terms of the vortex shedding characteristics, drag, and pressure coefficients for the different flow regimes.     

Exact boundary observability of unsteady flows in a tree‐like network of open canals

In this paper we establish the exact boundary observability of unsteady flows in a tree‐like network of open canals with general topology. Copyright © 2008 John Wiley & Sons, Ltd.     

A model of unsteady flow separation from a cylinder without boundary layer equations

Presented at the seminar of the Department of Computational Mathematics, May 26, 1983.     

Exact controllability for quasilinear hyperbolic systems and its application to unsteady flows in a network of open canals

In this paper we establish the exact boundary controllability for quasilinear hyperbolic systems with interface conditions. As an application, we get the exact boundary controllability of unsteady flows in a string‐like network of open canals. Copyright © 2004 John Wiley & Sons, Ltd.     

Exact boundary controllability of unsteady supercritical flows in a tree‐like network of open canals

In this paper we establish the exact boundary controllability of unsteady supercritical flows in a tree‐like network of open canals with general topology. Copyright © 2008 John Wiley & Sons, Ltd.     

Approximate computation of unsteady moving-boundary flows in a porous medium

The approximate method of integral relations is used to compute two problems of unsteady moving-boundary flow in a porous medium in the cases of linear parallel and plane radial motion. The problems are associated with the design and operation of underground gas storage facilities in horizontal and slightly sloping aquifers. It is assumed that the process is isothermal and the gas is ideal. Numerical results obtained in various approximations are described and analyzed. The results are presented as plots.     

Numerical simulation of cavitation around a two-dimensional hydrofoil using VOF method and LES turbulence model

In this paper simulation of cavitating flow over the Clark-Y hydrofoil is reported using the large eddy simulation (LES) turbulence model and volume of fluid (VOF) technique. We applied an incompressible LES modelling approach based on an implicit method for the subgrid terms. To apply the cavitation model, the flow has been considered as a single fluid, two-phase mixture. A transport equation model for the local volume fraction of vapour is solved and a finite rate mass transfer model is used for the vapourization and condensation processes. A compressive volume of fluid (VOF) method is applied to track the interface of liquid and vapour phases. This simulation is performed using a finite volume, two phase solver available in the framework of the OpenFOAM (Open Field Operation and Manipulation) software package. Simulation is performed for the cloud and super-cavitation regimes, i.e., σ = 0.8, 0.4, 0.28. We compared the results of two different mass transfer models, namely Kunz and Sauer models. The results of our simulation are compared for cavitation dynamics, starting point of cavitation, cavity’s diameter and force coefficients with the experimental data, where available. For both of steady state and transient conditions, suitable accuracy has been observed for cavitation dynamics and force coefficients.     

Numerical simulations of flows in an elastic cylinder with two chambers

We present a mathematical model of valveless pumping in a tube with two elastic chambers, which are motivated by the Liebau's two-tank model. The tube consists of partially soft and partially (almost) rigid. We employ a two-dimensional model using the immersed boundary method. In this new model, we have showed the important features of valveless pumping as the previous experiments and mathematical models have been discovered. For instance, we have observed that the direction and the amount of a net flow are sensitively dependent on the pumping frequency. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Note on unsteady viscous flow on the outside of an expanding or contracting cylinder

In this paper, the viscous flow on the outside of an expanding or contracting cylinder is studied. The governing Navier-Stokes equations are transformed into a similarity equation, which is solved by a shooting method. The solution is an exact solution to the unsteady Navier-Stokes equations. Results show both trivial and non-trivial solutions. For trivial solutions, there is no axial flow induced during the cylinder expansion or contraction. However, for the non-trivial solutions which only exist for cylinder expansion, an axial flow is generated and its strength increases with the increase in expansion speed.     

A simplified model for unsteady pressure driven flows in circular microchannels of variable cross-section

In this paper, we present a fast and accurate model for unsteady pressure-driven flows in circular microchannels of variable cross-section. The model is developed for channels of small diameter to length ratio, but allows for large variations in the channel’s diameter along the axis. A key feature of the model is that it puts no restriction on the time dependence of the forcing, in terms of shape and frequency. The only condition on the forcing is such that the advective component of the inertia term is small. This is a major departure from many previous expositions which assume harmonic forcing. The model is based on an extended and unsteady lubrication approximation in the aspect ratio of the channel. The resulting equations for each order are solved analytically using a finite Hankel transform, except for the implicit pressure profile, which is solved numerically with a recursive time scheme. Compared to classical CFD simulations, the reduced order semi-analytic method is two orders of magnitude faster, owing in part to the fact that the number of modes required for the convergence of these expressions is not too large. The numerical simulations reveal that the model is accurate for a large class of channels and a fairly wide range of Reynolds numbers. This, combined with the fact that it imposes no conditions on the shape and frequency of the unsteady forcing, renders the model a valuable tool for rapidly simulating large fluidic circuits (as in lab-on-a-chip, μTAS and the human body), thereby allowing significant reduction in the design parameters space.     

Stochastic flows of diffeomorphisms on an open set in rn

In this paper, we will give sufficient conditions for the solution to a stochastic differential equation (SDE) on an open set D in R" to define a stochastic flow of diffeomorphisms of D onto itself. Since a necessary and sufficient condition for the solution to determine a stochastic flow of diffeomorphisms is that the original SDE and its adjoint SDE are both strictly conservative, we will concentrate our attention on finding sufficient conditions for the SDE to be strictly conservative. It will be etablished that the strict conservativeness follows if the vector fields governing the SDE decay suitably near the boundary dD in the direction transversal to 3D and some additional assumptions are satisfied.     

Numerical study of unsteady rarefied diatomic gas flows in a plane microchannel

The numerical solution of a kinetic equation for a diatomic gas (nitrogen) is used to study two-dimensional unsteady gas flows in a plane microchannel caused by discontinuous in the initial distributions of macroscopic gas parameters. The plane discontinuity fronts are perpendicular to the walls of the channel. The arising flows are model ones for gas flows in a shock tube and a microchannel. The reflection of an incident shock wave from a flat end face is studied. It is found that the gas piles up at the cold wall, which slows down the shock wave detachment. The numerical results are in qualitative agreement with experimental data.     

POD and CVT Galerkin reduced-order modeling of the flow around a cylinder

Reduced-order models are applied to the laminar vortex-shedding flow around a circular cylinder. The models rely on approximations of the solutions in the space spanned by a set of POD or CVT reduced basis vectors, which are computed from snapshots of a numerical solution. To enable the computation of drag and lift forces, the modeling of the velocity and pressure is realized via a one-way coupling. A comparison of the results of the CVT and POD reduced-order models is presented. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Boundary layer analysis of oscillating cylinder flows in a viscoelastic liquid

A boundary layer analysis is applied to the oscillating cylinder system in a viscoelastic liquid. The effect of the inherent elasticity of the liquid is to increase the thickness of the inner vortex system of the steady streaming secondary flows, which is consistent with the experimental observation reported earlier.

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Résumé L'analyse d'un système composé d'un cylindre oscillant dans un liquide visco-élastique est approchée tel un problème de couche limite. L'effet de l'élasticité inhérente du liquide est d'augmenter l'épaisseur du vortex interne des écoulements secondaires en régime permanent, effet en accord avec l'observation expérimentale décrite dans une publication antérieure.

     

Propagation of unsteady waves in a hollow magnetoelastic cylinder in contact with liquid media

A numerical-analytic solution is constructed for the problem of magnetoelasticity for a hollow cylinder immersed in a liquid and loaded from inside by an impulse-type axisymmetric mechanical pressure. Nonconducting and compressible internal and external media have different densities and elastic moduli, with their motion described by wave equations. The hollow cylinder is assumed to be an ideal conductor, and its motion is described by a linearized system of equations of magnetoelasticity; on internal and external boundaries, the conditions of conjugation hold. The problem is solved by the method of integral Laplace transforms in the time domain, and the inverse transforms are found by numerical inversion. The solutions obtained for the bounded problem are compared with solutions for a simplified unbounded problem.Translated from Teoreticheskaya i Prikladnaya Mekhanika, No. 18, pp. 83–87, 1987.