LES PPT 12 JULY.pptx

8/22/2019 LES PPT 12 JULY.pptx

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LES OF AN AIRSHIP

Date:12-july 13

Manu.C2012AME2625


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HEMISPHERICAL FLOW DOMAIN-SYMMETRY BC

DOMAIN

SPHERE(SYMMETRY)

DIAMETER OF SPHERE : 6 m

DIA.FLOW DOMAIN :120m

395697 nodes

2116087 tetrahedral cells


3/17

PREVIOUS SIMULATIONS-(FROM FINAL THESIS SUBMITTED)

PR.FARFIELD

PR.FARFIELD VEL-IN-OUTFLOW


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CASES Cp Cf Cd

Case 2

(RANS)

0.03 0.006 0.038

Case2

(ILES)

0.051 0.0005 0.0517

Case 2

(LES)

0.032 0.009 0.0421

PRESENT SIMULATION

SYMMETRY-FARFIELD BC

VELOCITY INLET-OUTFLOW BC-SYMMETRY:Works with low reynolds number Solution diverges

with time. Requires fine mesh

MOVE TO BULLET SHAPED FLOW DOMAIN


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BULLET SHAPED FLOW DOMAIN

2095772 nodes,

SPHERE DIA- 6m

CYLINDRICAL PORTION=40*SPHERE DIA

Cooper meshingtgrid

CASES Cp Cf Cd

VEL-IN

(ILES)

0.089 0.0008 0.090

PR FLD(ILES)


6/17

18-july 2013:suitability of upwind

biased FDE for LES(R.Mittal,Moin)

Spectral methods:along with dynamic model gives

good results.utilizes info.from small scales(not

corrupted by numerical errors)

Not suitable for complex geometries.

Use FDE scheme


7/17

Fifth order 1 pint upwind biased scheme thoght to be suitable for complex

geometry LES due to high resolving power & lower numerical disspation

Beaudan and Moin employed it in a series of simulation-

3.9*10^3 Re

Simulation: no SGS model,FIXED SGS,spanwise averaged

DYNAMIC

OBSERVATION:mean wall statistics like drag,cp,tou

wall,separation angle almost same..

SIGNIFICANT FINDING:

1.Region downstream of wake (5


8/17

Seventh order scheme-energy in substantial portion of the

resolvable wave number damped due to numerical disspation

Truncation error analysis:Moin beaudan- High order schemes provide good resolution in about 2/3

range of wave numbers..dissipative in upper half of wave

numbers.

Thought to use CDS with controlled aliasing having lessnumerical dissip..

But CDS sensitive to stretching factors &outflow bc. Resulting

in dispersive errors

So(20-30%finer) need to have much finer mesh than upwindscheme


9/17

Doubt..


10/17

Similar results in near wake in upwind and CDS shows

difference in downstream portion-due to-resolution.

In current simulation of R.Mittal and in Beaudan (earlier)

Span wise resolution and depth wise resol..higher thanstream wise-beaudan-upwind(less fine along streamwise)

x/D>10 similar meshing


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12/17

VELOCITY PROFILE

Both agree well with exper.

STREAMWISE STRESS

x/D 7 upwindBeaudan-

underpredict

x/d-10 both CDS and upwind

underpredict

VERTICAL STRESS PROFILE

Both has peak-to exp.

Profile of upwind best

matching


13/17

Conclusion of this paper

Cds-less dissipation-small scale energetic

No significance of these scales in lower order statistics-mean

velocity and Re stresses remain the same

Reason:most contribution from frequencies centered

But important in cases:flow generated noise,reactive flows

Higher order upwind scheme costly..so finer grid size in CDS is

comparable to this costbut has lesser dissipation


14/17

Numerical and modeling influences on large eddy simulations for

the flow past a circular cylinder

Michael Breuer 1 (1998)(Re-3900)

Simulation C1 without any subgrid scale model shows the shortest

recirculation length and the highest back-pressure and drag coef of all cases

The size of the separation and reattachment regions on the cylinder is similar

in all simulations

turbulent kinetic energy k is reduced when the Smagorinsky model is applied.

Surprisingly, k is further reduced when the Smagorinsky model is replaced by

the dynamic approach.

For all simulations the streamwise Reynolds stress u0u0 is fairly well

predicted


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16/17

From papers..pts to be noted.

Disspipative schemes decrease aliasing error but increases finite differencing

error

On the implicit large eddy simulations of

homogeneous decaying turbulence Ben Thornber,Andrew Mosedale, Dimitris Drikakis *

Using implicit LES (or ILES), excellent results have been gained in simulation of flows as

varied as RayleighTaylor and RichtmyerMeshkov instability [7,8], Free jets [9,10],

channel flow [10], open cavity flow [11,3], geophysical flows [12,13], delta wings [14] and

decaying turbulence [15

20]. Attempts to formalise the development of ILES numerical

schemes is hindered by the inherent complexity of theoretical analysis of non-linear schemes,

however, recent developments show some good agreements between truncation errors due to

the numerical scheme and the required form of the subgrid terms


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Scale separation for implicit large eddy simulation

X.Y. Hu, N.A. Adams (2011)

In this work we have proposed a scale separation approach for ILES. Scaleseparation is accomplished by a simple modification

of the weighting strategy of an existing WENO scheme (WENO-CU6). Basicidea is to counterbalance a stronger bias

towards the central high-order non-dissipative stencil by a higher integerpower of the smoothness-measure contribution to

the weights. This leads to a scale separation of contributions fromresolved scales and non-resolved scales. Model parameters

are the linear-weight bias and the integer power exponent. It was shownthat a straight-forward parameter choice is widely

effective without further tuning. Numerical examples imply that the scale-

separation WENO-CU6 scheme leads to a physically consistent implicit SGS model for incompressible and compressible

turbulence, while the shock-capturing capabilities

of the original WENO-CU6 scheme maintained.

LES PPT 12 JULY.pptx

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