1010 - Sub-Grid Modelling of Turbulent Flows

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    Improving the tools foretter flow simulation

    Part of a series of wider advances in theComputational Fluid Dynamics field, flowsimulation tools have grown increasinglysophisticated and accessible over recentyears, to a point where they are now widelyused in the commercial sector. Specificcomputer programmes are now capableof simulating the interaction betweenliquids and gases at the boundary betweenthe two states, bringing real benefits tocompanies from across the commercialspectrum, including those from theaerospace, automotive and chemicalindustries. However, there is a lower limitto Computational Fluid Dynamics (CFD)

    in terms of the cell size it can simulate,which in turn can have a negative impacton the accuracy of the eventual tool. Assuch all phenomena smaller than thecomputational cells themselves must bemodelled if precision is to be improved,an area that Professor Bengt Anderssonof Chalmers University of Technology inSweden is working to address.

    If for example you have a 10 cubic metrereactor then typically you will divide it into1 million cells. In these circumstances eachcell will be 10 millilitres in size, meaningthat everything which happens on thatscale has to be modelled. We need goodmodels to describe this, the development of

    which is the focus of my research, he says.ere are a number of factors to take intoaccount in this work, including not only thebehaviour of individual chemical agents,but also how they are likely to respond tothe presence of other reactants. If you havetwo competitive reactions one fast andone slow then which reaction will occurearlier? asks Andersson. With the fastreaction the reactant will be consumed quitequickly, so after a short while there will be

    no reactant present, because the diffusionis slow. is means that the slow reactionan then start to dominate. In order to

    understand that kind of process you needquite an accurate model to describe themixing between the reactants.

    While advances in Computational Fluid Dynamics have increased our understanding of

    single phase flow, the development of improved simulation tools for multiphase flow and

    mixing demands further research into basic phenomena, says Professor Bengt Andersson

    Turbulent velocity perpendicular to main flow in a pipe at Re=16000 colored

    with axial vorticity ranging from -150 s-1 to 150 s-1

    Concentration of species Sc=100 in turbulent boundary layer perpendicular to flow

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    Basic phenomenaAn understanding of basic phenomenais crucial to the wider goal of developingmodels suffi ciently accurate to be usedon large scales. Many of the simulationsfor large industrial projects like chemicalreactors currently rely on relatively crudeapproximations of transport betweenthe gas, liquid and solid states, and any

    inaccuracies can have knock-on effectsfurther down the line, particularly in termsof the overall energy effi ciency of theeventual reactor. Commercial companiesare correspondingly keen to understandthe nature of liquid flow in ever-greaterdetail, a context in which Anderssonsresearch into mass transfer in turbulentboundary layers takes on real practicalignificance. If you have a large tank

    reactor and only have laminar flow thenit will take a long time for the elementsto mix together and generate a reaction.In this kind of situation we often want

    to induce turbulence so as to generate achemical reaction. Viscosity usually actsas a stabilising force, but if the inertia orflow is very high then the viscosity mightnot be suffi cient to dampen instabilities.So its likely to become turbulent, heays. is kind of research has long

    informed product design, yet an improvedunderstanding of turbulence and viscositylevels, and how they can be modified,can still bring real improvements to theperformance of everyday applications.Sometimes you want to supress turbulenceand sometimes you want to enhance

    turbulance. Central heating systems workthrough the distribution of hot water. We want the pressure drop associated withthis distribution to be low, and then it canbe decreased further by adding elementswhich increase the viscosity. Laminar flowup to higher velocities will mean a lowerpressure drop, but if you have turbulencethen the pressure drop will be muchhigher, explains Andersson. In chemicalreactors we need good mixing and highvelocities to enhance turbulence. However,if you have a very high velocity then theresidence time will be very short, whichwill have an effect on the way the elementsmix: will those elements have time to reactif you have very high velocity?

    In addressing these kinds of questionsAndersson builds on a core recognitionthat turbulence in mass transport is notrandom, but rather has a clearly definedtructure, and that using only average

    turbulent quantities can lead to inaccuraciesin the model. e development of accurateflow simulation tools thus demands thatthe size, lifetime and velocity distribution

    0 EU Researc

    Break-up of a liquid low viscous high surface tension drop (dodecane) in water

    Break-up of high viscous low surface tension drop (octanol) in water

    Coalescence of two bubbles

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    At a glance

    Full Project Title

    Sub-grid modelling of Turbulent Flows

    Project Funding

    Swedish Science Council

    Contact Details

    Project Coordinator,Professor Bengt Andersson

    Department of Chemical and Biological

    Engineering

    halmers University of Technology

    oteborg

    Sweden

    T: +46 (0) 317 723 026

    E: [email protected]

    W: www.chalmers.se/chem/EN/divisions/

    hemical-reaction

    Project Coordinator

    Professor Bengt Andersson, born in947, did his PhD in Chemical Reac-

    ion Engineering at Chalmers Univer-

    sity 1977. He has divided his researchbetween computational fluid dynam-ics, CFD, and automotive catalysis in

    ompetence Center for Catalysis. He wasboard member of the Science Councilfor Engineering Sciences 1999-2000 and

    hairman for Chemical Engineering inhe Swedish Science Council 2007-2008.

    Professor Bengt Andersson

    f small-scale phenomena be takeninto account, and while the analysis ofurbulent eddies turbulence indicators

    range from micrometers to meters inlarge chemical reactors is enormouslyomplex, Andersson is confident that it will

    bring tangible benefits. Understandinghe structure of turbulence will help us

    understand the basic phenomena that

    ccur in chemical reactors, he stresses.With the project also studying the break-up and coalescence of bubbles and drops,and utilising sophisticated experimentalechniques, they are well-placed to develop

    models reflecting the complexities of flow ynamics. Indeed, the use of high-speed,

    microscope-equipped cameras, planar laserinduced fluorescence (PLIF), and particleimage velocimetry (PIV), allows forunprecedented levels of analytical depth. You need to introduce energy to formurbulent eddies; turbulent eddies attractnergy from velocity gradients. In the case

    f pipes, velocity gradients are locatedlose to walls, at the furthest point from

    its centre, says Andersson. Turbulence ismostly generated close to boundary layersr between phases mainly solid anduid phases and when we understandurbulence we will also be better able to

    benefit from it. is is of great interesto industry,in general; better combustion

    will allow he automotive and energy

    industries to reduce fuel consumption, while the chemical industry will benefit

    rom higher yields and reduced use of rawmaterials You lose a lot of engine powerif you have a downstream pressure drop.Our understanding of basic phenomenaan help reduce turbulence and pressurerop, although in general we are aimingor the opposite.

    Commercial links is approach has developed in directresponse to commercial needs, and whileit still takes time to process large volumes

    f data, advances in computational powermean companies today can use flowimulation tools to gain data within a

    reasonable timeframe.

    Car manufacturers in particular recognisethat simulation tools are a cheaper way oftesting prototypes than conventional means,which has helped the project attract interestfrom across the commercial spectrum,

    and not just the automotive sector. Wehave Scania, Volvo, Saab and VolvoCarsautomotive companies in our competenceentre. ey dont want ready solutions,

    they want the basic understanding. Weprovide manufacturers with simulationtools which they can then use to developproducts themselves. We have also beeninvolved with a large project with AlfaLaval where they developed completelynew chemical reactors, which they now selln the commercial marketplace. ey hadome clever ideas about how to design the

    reactor so that it would generate turbulence

    at lower velocities, says Andersson. Whilethe commercial relevance of his work isundeniable, Anderssons research agenda isin no way limited by commercial interests,and includes a number of fundamentaluestions. We focus more on generic issues

    in the flow dynamics field than specificproblems, and work across a diverse rangef areas, he continues. One large project,

    for which Im about to recruit one assistant

    professor and some PhD students, involveslooking at instantaneous measurements ofturbulence. I will introduce bubbles and

    rops into the turbulent eddys, or aroundthe turbulent eddys, and follow theway they move around it. How are theyattracted into the eddys? How are theybroken up, what role does the velocitygradient and shear rate play? e questionf mixing is also extremely important,

    how do you get the different compoundsto be entrained into the turbulent eddys? ese are the areas Im trying to moveinto now.

    Turbulence is of great interest to industry;

    better combustion will allow theautomotive and energy industriesto reduce fuel consumption, whilethe chemical industry will benefit from higher

    yields and reduced use of raw materials

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    experimental techniques, they are well-

    placed to develop models reflecting the

    complexities of flow dynamics. Indeed, the

    use of high-speed, microscope-equipped

    cameras, planar laser induced fluorescence

    (PLIF), and particle image velocimetry

    (PIV), allows for unprecedented levels of

    analytical depth. You need to introduce

    energy to form turbulent eddies; turbulent

    eddies attract energy from velocity

    radients. In the case of pipes, velocity

    radients are located close to walls, at the

    furthest point from its centre, says

    Andersson. Turbulence is mostly generated

    close to boundary layers or between phases

    mainly solid and fluid phases and when

    we understand turbulence we will also be

    better able to benefit from it. This is of great

    interest to industry,in general; better

    combustion will allow the automotive and

    energy industries to reduce fuel

    consumption, while the chemical industry

    will benefit from higher yields and reduced

    use of raw materia ls You lose a lot of engine

    power if you have a downstream pressure

    drop. Our understanding of basic phenomenacan help reduce turbulence and pressure

    drop, although in general we are aiming for

    the opposite.

    Commercial links

    This approach has developed in direct

    response to commercial needs, and while

    it still takes time to process large volumes

    www.projects.eu.com

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