Wyniki wyszukiwania - BazTech

1

Hybrid RANS/LES of plane jets impinging on a flat plate at small nozzle-plate distances

Kubacki S., Rokicki J., Dick E., Degroote J., Vierendeels J.

Archives of Mechanics

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2013

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Vol. 65, nr 2

143--166

EN

A k-ω based hybrid RANS/LES (Reynolds-averaged Navier-Stokes/large eddy simulation) model is tested for simulation of plane impinging jets at various nozzle-plate distances (H/B, where H is the distance and B is the slot's width) and various Reynolds numbers (based on the slot's width and the velocity in the symmetry plane). The studied combinations are H/B=2 for Re=10000, H/B=4 for Re=18000 and H/B=9.2 for Re=20000. The focus is on small distance of the nozzle exit to the plate. In LES mode, the hybrid RANS/LES model uses two definitions of the local grid size, one based on the maximum distance between the cell faces in the destruction term of the turbulent kinetic energy equation and one based on the cube root of the cell volume in the eddy-viscosity formula. This allows accounting for flow inhomogeneity on anisotropic grids. In RANS mode, the hybrid model turns into the newest version of the k-ω model by Wilcox.

2

Simulation of the friction factor in a yield-stress slurry flow which exhibits turbulence damping near the pipe wall

Bartosik A.

Journal of Theoretical and Applied Mechanics

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2011

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Vol. 49 nr 2

283-300

EN

The paper deals with the mathematical modelling of fully developed turbulent flow of a Bingham hydromixture in a pipe. The mathematical model is based on time averaged Navier-Stokes equations and uses the apparent viscosity concept. The problem of closure of the turbulence stress tensor was solved by the two-equation turbulence model in which a modified turbulence damping function was taken into account. The final form of the mathematical model constitutes a set of three non-linear partial differential equations. The main aim of the paper is to demonstrate a significant decrease of turbulence near the pipe wall, as the friction factor is below that for a water flow. The paper presents results of numerical simulation of the frictional head loss and friction factor for slurry flows with low, moderate, and high yield stresses. Predicted frictional head losses have been compared with experimental data showing satisfying agreement. It is demonstrated that the frictional head loss and the friction factor substantially depend on the yield stress. The results of numerical simulation are presented as figures and conclusions. Possible causes of turbulence damping near the pipe wall are discussed.

PL

Artykuł dotyczy matematycznego modelowania w pełni rozwiniętego przepływu hydromieszaniny Binghama w przewodzie kołowym. Model matematyczny zbudowano na bazie równań Naviera-Stokesa z zastosowaniem koncepcji lepkości pozornej. Problem domknięcia układu równań, wynikający z dodatkowych składowych tensora naprężeń turbulentnych, rozwiązano poprzez użycie dwu równaniowego modelu turbulencji, w którym zastosowano zmodyfikowaną funkcję tłumiącą turbulencję przy ścianie. Ostateczna postać modelu matematycznego zawiera trzy nieliniowe równania różniczkowe cząstkowe. Głównym celem pracy jest przedstawienie wzmożonego tłumienia turbulencji w obszarze przyściennym, jako że współczynnik strat tarcia hydromieszaniny jest niższy w stosunku do wody. Artykuł przedstawia wyniki numerycznej symulacji współczynnika strat tarcia dla szlamu z niskim, umiarkowanym i wysokim progiem płynięcia. Obliczony jednostkowy spadek ciśnienia porównano z wynikami badań eksperymentalnych, uzyskując satysfakcjonującą dokładność. Wykazano, że jednostkowy spadek ciśnienia i współczynnik strat tarcia zależą istotnie od progu płynięcia. Wyniki numerycznej symulacji przedstawiono w postaci wykresów i wniosków oraz omówiono możliwe przyczyny wzmożonego zjawiska tłumienia turbulencji przy ścianie.

3

A second moment turbulence closure with two scale equations

Stawiarski D., Hanjalić K., Paschedag A. R.

Czasopismo Techniczne. Mechanika

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2004

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R. 101, z. 4-M

255--265

EN

The detailed analysis of the turbulence models is presented with the description of their merits and defects. The new model without split-spectrum was presented.

PL

Najbardziej popularne obecnie modele turbulencji z grupy RANS, mimo że oferują wiele, nie uwzględniają kilku istotnych własności przepływów turbulentnych, na przykład ich spektralnego charakteru. Pierwszą próbą uzupełnienia modeli burzliwości pod tym kątem były tzw. modele wieloskalowe z podzielonym spektrum (split-spectrum multiple-scale models), w których transfer energii w procesie kaskadowym jest modelowany przez dodatkowe równania transportu skalarnej funkcji transferu energii ε[p]. W pracy przedstawiono model wieloskalowy bez podzielonego spektrum, który, w odróżnieniu od klasycznych modeli tej grupy, należy do modeli typu SMC.

4

One-equation turbulence model solution of developed pipe flow

Tesar V.

Zeszyty Naukowe. Mechanika / Akademia Techniczno-Rolnicza w Bydgoszczy

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2002

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Z. 53(241)

243-254

EN

Flow in constant cross section circular pipe is one of the basic shear flows. Somewhat surprisingly, considering the importance in engineering and the relative simplicity (no advective terms in the equations for the fully developed flow), no exact solution of the flowfield equations has been know until recently for the turbulent flow (as opposed to the well known Poiseuille solution for the laminar case). In 1994, the present author derived a closed form analytic solution using algebraic mixing-length model of turbulence. This remarkable result is unfortunately limited by the well-known weakness of the algebraic model: not taking into account the spatial transport of turbulence. As a result, the analytic solution fails in the paraxial region, where the transported turbulence dominates. The present paper improves upon the previous result by using a more advanced model of turbulence. No analytic solution has been known so far and the derived system of equations is here solved numerically - the earlier algebraic-model analytic solution serving as the wall functions, better than the usual logarithmic-law expressions. The present results provide a very instructive insight into various effects encountered in turbulence and taking of the same into account by progressively mode advanced turbulence modelling.

PL

Zagadnienie przepływu burzliwego jest bardzo złożone. Praktycznie pola przepływu są najczęściej rozwiązywane w sposób przybliżony numerycznie. Określono równania różniczkowe opisujące przepływ. Następnie wprowadzono współrzędne bezwymiarowe. Wyniki pokazano na wykresach. Metodą numeryczną określono bezwymiarowe turbulencję energii przepływu cieczy w rurze.

5

Prediction of turbomachinery flow physics from CFD - review of recent computations of APPACET test cases

Gregory-Smith D. G., Crossland S. C.

TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

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2001

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Vol. 5, No 4

407-431

EN

In order to maintain a competitive edge, the turbomachinery industry has to rely increasingly on design and analysis methods based on numerical simulation of flow. The European funded APPACET was set up to study the application of turbulence modelling and the simulation of unsteady interactions to provide guidelines for the application of CFD to design. This paper shows the results of computations of four of the test cases studied by the project. The importance of grid refinement has been clearly demonstrated, but no evidence was found that one family of turbulence models could be definitely better than the other. Compared to steady results, unsteady time-averaged solutions have not shown any major improvement in accuracy. However unsteady flow phenomena generating losses within each blade row have been captured and partly explained.

6

On the prediction of flow patterns and losses in HP axial turbine stages using 3D RANS solver with two turbulence models

Lampart P., Świrdyczuk J., Gardzielewicz A.

TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

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2001

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Vol. 5, No 2

191-206

EN

An experimentally tested air turbine stage and a real high-pressure (HP) steam turbine stage are calculated using the 3D RANS solver FlowER supplemented with the Baldwin-Lomax and Menter shear stress transport (SST) models. The computations of the model air turbine stage show that the Menter SST model gives better agreement with the experimental data as far as the span-wise distribution of exit velocities and swirl angle. The comparison of performance of the two turbulence models exhibits differences in predicting flow patterns and losses in the considered HP turbine stage. The main differences concern the development of secondary flows and separations. There is a significant span-wise redistribution of losses between these two models. The tendency is that for the same relatively refined grid resolutions, the level of pitch/span averaged losses for the Menter SST turbulence model is slightly above that of Baldwin-Lomax.

7

A Two-dimensional Superheated Steam Impinging Stream Dryer: A Computational Model

Mujumdar A., Hosseinalipour S., Devahastin S.

Bulletin of the Polish Academy of Sciences. Technical Sciences

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2000

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Vol. 48, nr 3

287-299

EN

A computational study was performed to predict the drying performance of a novel two-dimensional impinging stream dryer using superheated steam as the drying medium. The dryer is basically a flash dryer consisting of an opposing jet configuration. Assuming constant rate period drying kinetics, the drying behaviour of superheated steam-particle mixtures flowing in a turbulent two-phase flow is modelled by solving the coupled mass, momentum and energy conservation equations along with a turbulence model for the gas-particle flow. The effects of various operating parameters on the performance of the impinging stream dryer are investigated and discussed. It is evident from the simulation that a considerable amount of moisture can be removed even over a short overall residence time of the particles in the impinging stream system.