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Page 1: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Flow StructureIn CFVNs

Ernst von Lavante

University of Duisburg-Essen

Page 2: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Introduction – why again?Transition laminar-turbulentLow and high unchokingShock at the exit?Steady or unsteady?2-D, 2-D axisymmetric, 3-D or what? Is there a hope to predict the flow?Conclusions – if any

Overview

Page 3: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Introduction

The beginning: flow in CFVNs simulated with ACHIEVEPresented at Flomeko ’98Always unsteady!

Next effort: “premature unchoking” – Nakao, Takamoto, Ishibashi

After that: transition laminar-turbulentsimulation & theory (Abu

Ghanam, Mayle, Schlichting, ...)

Current effort: visualize transition

Page 4: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Introduction

Then came Bodo M.: Latest paper with Kramer and LiPresented at 8th ISFFM ’12Definition of “low” and “high” unchokingDiscussion of flow structure

Þ Decision to carry out “good” flow simulation

After that: transition laminar-turbulentsimulation & theory (Abu Ghanam, Mayle, Schlichting, ...)

Current effort: visualize transition

Page 5: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Main goal: numerical simulation of flow fields in flow metering configurations

In all cases, scale sufficiently large to give Kn = λ/L < 0.01with λ 10-8 – 10-9 m => continuum

Notice: Kn M/Re a) flows with M/Re > 1 called rarefiedb) incompressible gas (M0) can not be rarefiedc) small Re flow could mean rarefied fluidd) large Re flows are always continuum

Basics

Page 6: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Physics of the flow:compressible (Ma ≥ 0.3) => mixed hyperbolic-parabolic, coupledincompressible => mixed elliptic-hyperbolic-parabolic,

decoupledlaminar (Re ≤ 2300 !) turbulent => turbulence model (k-ε, k-ω, RNG, realizable, SST,

RSM, LES, DES, DNS)steadyunsteady – periodic (deterministic) or stochastic

Basics

simulation method must have low numerical dissipation, sinceμTot = μPhys + μNum => 1/ReTot = 1/RePhys + 1/ReNum

Page 7: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Considerations in numerical simulation methods (CFD):2-D or 3-D configuration

grid generation: structured multiblock (mutigrid?)unstructured tetrahydral, hexahydral, polyhydralhybridmoving (deforming) grids (adapting to flow)overlapping grids (chimera), immersed body gridsquality of grids: smoothing, continuity, resolution

in time and spaceComputation: time and space accuracy, damping Boundary conditionsMultiprozessing (parallel processing)

Basics

Page 8: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Choice of correct tools:hardware (minimum requirements)competence of staffSoftware: system

preprocessing (grid generation)simulation system (CFX, Fluent, adapco Star

CCM+, my own programs ACHIEVE, trace,Flower, ….)

postprocessing (included, Tecplot, …)

The correct choice will „make you or break you“ !

Basics

Page 9: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

CFVN 1 - ISO

Shape: ISO 9300, toroidal versiondifferent Reynolds numbers and pressure ratios2-D axisymmetric, 3 blocks, structured, laminar

Page 10: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

CFVN 1 - ISO

Resulting Flow, Movies, Re=1.5 106

Page 11: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

CFVN 1 - ISO

Resulting Flow, Movies, Re=0.1 106

Page 12: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

CFVN 1 - ISO

Resulting Flow, Movies, Re=1.5 106

Page 13: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

CFVN 1 - ISO

Resulting Flow, Movies, Re=0.1 106

Page 14: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

CFVN 1 - ISO

- Unsteady effects (Elster, eon)- Premature unchocking- National Calibration Standard at Pigsar (Pigsar, Elster, PTB, eon)- Real gas effects in CFVN (eon)- Influencing of flow fields in CFVN (steps, suction)- Micro nozzles (PTB)- Reynolds number effects in CFVN (transition laminar-turbulent)- Geometric factors (PTB) - Theoretical determination discharge coeff. CD (PTB)- Shock location, influence of condenzation (NRLM)

All simulations with ACHIEVE – accuracy !!

Page 15: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

CFVN 1 - ISO

Page 16: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

CFVN 1 - ISO

Experimental verification by Ishibashi (NRLM)

Page 17: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

CFVN 2 – micro nozzle

Aim of present study: comparison of high resolution CFD simulations with experimental results (PTB)

Two basic shapes: punched and drilled

D ≥

4m

m

0,2mm

d

l=d

α = 34°

a

D ≥

4m

m

0,2mm

d

l=d

5·d α = 34°

b

Utilized in forward (L to R) and backward (R to L) orientation

Page 18: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Present cases:

CFVN 2 – micro nozzle

In our case: Kn = 1.28 κ0.5 Ma/Re

throat diamet

erD in [µm]

Reynolds-

numberRed

B.L. thicknessδ in [µm] ->

ratio ofδ/d

Knudsen number Kn

15 197 5,348 0,3565 0,0153

25 328 6,904 0,2762 0,0092

35 459 8,169 0,2334 0,0066

50 656 9,764 0,1953 0,0046

80 1049 12,351 0,1544 0,0029

Page 19: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Simulation Parameter

Simulation carried out using ACHIEVE solver developed by authorGrid generated by elliptic PDE developed in houseConfiguration: D = 15, 25, 35, 50 and 80 μ , P0 = 0.101325 MPa, T0 = 300 K

Pressure ratios pout/P0 = 0.3 and 0.4

Page 20: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Experimental Work at PTB

Results for D = 25 µm:1. Forward nozzle: choking at p/P0 = 0.35 (ideal nozzle 0.528…)2. Backward nozzle: no apparent choking

Task for numerical simulation: explain phenomenon !!

Page 21: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Numerical Simulations - Results

Forward orientation, pout/p0 = 0,3

Page 22: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Numerical Simulations - Results

Backward orientation, pout/p0 = 0,3

Page 23: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Numerical Simulations

Boundary layer in cylindrical part

large vertical velocity

x/d

x/d

x/d

pout/p0 = 0,4

x/d

x/d

x/d

Page 24: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Numerical Simulations - Results Drilled nozzle, pout/p0 = 0,3

Page 25: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Numerical Simulations - Results Drilled nozzle, pout/p0 = 0,3

Page 26: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Summary

p2/p0 0,4 0,3

Nozzle Cd, exp Cd, num Deviation Cd, exp Cd, num DeviationForward 25 0,662 0,705 6,45 % 0,664 0,711 7,07 %Backward 25 0,670 0,707 5,47 % 0,676 0,743 9,97 %Forward Drilled 25 0,660 0,692 4,90 % 0,662 0,722 9,12 %Backward Drilled 25 0,663 0,697 4,96 % 0,667 0,724 8,57

Discharge Coefficient vs. 1/Re^0,5

0,650,670,690,710,730,750,770,790,810,830,85

0,030 0,040 0,050 0,060 0,070 0,080

1/Re^0,5

Dis

char

ge C

oeff

icie

nt FW 0,4 BW 0,4FW 0,3 BW 0,3

Page 27: Flow Structure In CFVNs

Workshop on CFVNs – Poitiers 2013

Conclusions

Reliable numerical simulation of komplex flows in flow metering configurations possible using low numerical dissipation schemes

Commercial codes should be used with care –

it is not all gold that shinesOpenFoam looks promising in many cases

Present simulations were able to provide an explanation of many flow behaviour questions

Much higher resolution simulations in future – there is never enough computer power (CPU and RAM)


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