karalit cfd validation: ahmed body. · fig 2. streamlines for ﬂow at rear of ahmed body ‐...

INTRODUCTION. The innova*ve Immersed Mesh technology in KARALIT CFD has been used to perform valida*ons against the Ahmed Body for two different flow scenarios. Although the valida*on presented here is based on a 2D representa*on, it is evident that some of the key flow‐field features for the two differing 25‐degree and 35‐degree cases have been captured. CASE SETUP. Using the power of the Immersed Boundary (IB) method in KARALIT, the Ahmed Body model geometry (STL format) is directly immersed into a background Cartesian grid. Preprocessing takes just seconds, as the user does not have to deal with *me‐consuming mesh genera*on. Fig 1. Immersed Mesh representa3on of the Ahmed Body. FINDINGS. The computa*ons accurately predict the two very different flow regimes at the rear of the body. The following key observa*ons were made. KARALIT CFD VALIDATION: Ahmed Body. Ahmed Validation_june 2012 pag. 01 • 2D mesh comprising 77k cells • 15‐cell extrusion layer on surface • Ground‐confined External Flow app • Spalart‐Allmaras turbulence model • Wall func*on, y+ ~ 100 • 2 nd ‐order symmetric TVD discre*za*on • 40m/s inflow velocity • Re = 2.8x106 based on body height • Implicit coupled solver • 7‐core parallel computa*ons • Full case setup from scratch in about 5‐10 minutes • The flow separates over the slant for the 35‐degree case, whereas in the 25‐degree case it mostly remains adached. • As a result of the flow separa*on over the slant, the Wake in the 35‐degree case is larger than for the 25‐ degree scenario. • The U‐velocity profiles over the slant and in the wake of the body compare favorably to experiments and previous calcula*ons (eg., Crae et. al. 2001).

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Page 1: KARALIT CFD VALIDATION: Ahmed Body. · Fig 2. Streamlines for ﬂow at rear of Ahmed Body ‐ 25‐degree case(top)and35‐ degree

INTRODUCTION.

Theinnova*ve ImmersedMeshtechnologyinKARALITCFDhasbeenusedtoperformvalida*onsagainsttheAhmedBodyfor twodifferentflowscenarios.Althoughthe valida*on presented here is based on a 2Drepresenta*on,it isevidentthatsomeofthekeyflow‐fieldfeatures for the twodiffering 25‐degreeand35‐ degreecaseshavebeencaptured.

CASESETUP.

Usingthe powerof the ImmersedBoundary (IB) method in KARALIT,the AhmedBodymodelgeometry(STL format) isdirectly immersedinto a backgroundCartesiangrid.Preprocessingtakesjustseconds, as theuserdoesnothave todealwith*me‐consumingmeshgenera*on.

Fig1.ImmersedMeshrepresenta3onoftheAhmedBody.

FINDINGS.

The computa*ons accurately predict thetwoverydifferentflowregimesattherearo f t he body. The fo l l ow ing keyobserva*onsweremade.

KARALIT CFD VALIDATION:Ahmed Body.

Ahmed Validation_june 2012

pag. 01

• 2Dmeshcomprising77kcells• 15‐cellextrusionlayeronsurface• Ground‐confinedExternalFlowapp• Spalart‐Allmarasturbulencemodel• Wallfunc*on,y+~100• 2nd‐ordersymmetricTVDdiscre*za*on• 40m/sinflowvelocity• Re=2.8x106basedonbodyheight• Implicitcoupledsolver• 7‐coreparallelcomputa*ons• Fullcasesetupfromscratchinabout5‐10minutes

• Theflowseparatesover the slant forthe35‐degreecase,whereas in the 25‐degreecase it mostly remainsadached.

• Asa resultofthe flowsepara*onovertheslant,theWakeinthe35‐degreecase is largerthanforthe25‐degreescenario.

• The U‐velocityprofilesovertheslantandinthewakeofthebodycompare favorablytoexperimentsandpreviouscalcula*ons(eg.,Craeet.al.2001).

Page 2: KARALIT CFD VALIDATION: Ahmed Body. · Fig 2. Streamlines for ﬂow at rear of Ahmed Body ‐ 25‐degree case(top)and35‐ degree

Fig2. Streamlines forflowatrear of Ahmed Body ‐25‐degreecase(top)and35‐degreecase(boEom).

CONCLUSION.

The calcula*ons undertaken with the KARALIT CFD code using the Immersed Mesh technique predict the correctdistribu*onsandtrendsofthestreamlinesandvelocityfieldattherearofthebody.

Thesecalcula*onsare extendedtothefull 3Dcomputa*ons.Representa*onoftheside vor*cesandtheirimpactsontheflowboundarylayerovertheslantareexpectedtohaveasignificantimpactontheresults.

Fig 3. Comparisons of Uvelocityatslantand inthewake of the Ahmed Bodyforthe25‐degreecase.

Fig 4. Comparisons of Uvelocityatslantandinthewake of the Ahmed Bodyfor35‐degreecase.

Further inves*ga*on of the Ahmed Body flow field using different turbulence models available in the code is alsoan*cipated.

Thesecalcula*onsdemonstratetheaccuracyof theKARALITCFDcodepredic*ons,withminimalsetupeffort forquitechallengingflow‐fieldscenarios.

Exploit the immersed value

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Ahmed Validation_june 2012

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