The mesh(number, size and distribution of controlvolumes) is probably the
singlemost importantpartof CFD simulationsas this makes the basisfor
discretisationof the flowdomain. The geometry is eitherimporteddirectlyfrom
a commercialcomputer-aided design(CAD)program or createdusingan in-the-
CFD-code CADenvironment. Alternatively,a meshcan be importedfrom
external mesh generatorssuchas ICEM (http://www-berkeley.ansys.com)or
GAMBIT (www.fluent.com). Meshgenerationcan be a verytime-consuming
processof trial and error.A meshstructuresuitedfor a particular geometry and
flow condition might not be appropriate for another, so refinement and
coarsening of the meshis needed (for example flowat different temperatures
influencesthe meshnearthe wall± seeyáin nextsection).
11.3.1 Near-walltreatment
Chenand Patel (1988)stated the importanceof near-wall treatmentfor the
overallsuccessof turbulencemodels.An importantparameterin the success of
the different near-wall treatments available is the distance fromthe wallto the
centrepointof the first cell,whennormalised, calledyá:
yáàypu
àypÅÅÅÅÅ
w
rÖ 11 : 8 Üwhereu is the frictionvelocity (m/s),ypis the distancefromthe wallto the
centrepointof the near-wallcell (m) andis the kinematicviscosity(m^2 /s).
Limitsforyádependon the choiceof near-walltreatment.The wallfunction
approach has beenusedwithsuccessfor modelling flowswhere near-wall
phenomena are less important.Recently,more advancedtwo-layer modelsand
lowRe k±models havebeen implemented into commercialCFD codes,
improving prediction of flow in confined, separatingand attachingflows
encounteredin evenslightlycomplexequipment.
In simulationsof flowswithadversepressuregradients and recirculation
zonesor whereflow nearthe walls,heat transfer, wallshearstressor frictionis
of specialinterest, as in the caseof hygienicdesign,Rodi(1991)suggeststhe
two-layer approach to describe flow in the near-wall layer.In contrast to the wall
function, the flowin the buffer zoneand the logarithmiclayerare resolved by a
number of cells.A transportequation for turbulentkinetic energyis solvedin the
near-wall layerand dissipationof turbulentkineticenergy is expressedby an
algebraic function. A shiftto, for example, the standardk±turbulencemodelis
done at a distance fromthe wallwhere viscouseffects become negligible
compared withinertiaeffects.yá shouldbe around3, and approximately 15
pointsshouldbe placed within the near-wall layer(Anon,1999).As the flow is
modelledall the way to the viscous sublayer, wall shearstressis calculatedfrom
the generaldefinitiongivein equation (11.1)
Improvingthe hygienicdesignof closedequipment 199