flow structure in cfvns
DESCRIPTION
Flow Structure In CFVNs. Ernst von Lavante University of Duisburg-Essen. Overview. Introduction – why again? Transition laminar-turbulent Low and high unchoking Shock at the exit? Steady or unsteady? 2-D, 2-D axisymmetric , 3-D or what? Is there a hope to predict the flow? - PowerPoint PPT PresentationTRANSCRIPT
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Workshop on CFVNs – Poitiers 2013
Flow StructureIn CFVNs
Ernst von Lavante
University of Duisburg-Essen
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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
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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
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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
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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
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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
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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
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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
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Workshop on CFVNs – Poitiers 2013
CFVN 1 - ISO
Shape: ISO 9300, toroidal versiondifferent Reynolds numbers and pressure ratios2-D axisymmetric, 3 blocks, structured, laminar
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Workshop on CFVNs – Poitiers 2013
CFVN 1 - ISO
Resulting Flow, Movies, Re=1.5 106
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Workshop on CFVNs – Poitiers 2013
CFVN 1 - ISO
Resulting Flow, Movies, Re=0.1 106
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Workshop on CFVNs – Poitiers 2013
CFVN 1 - ISO
Resulting Flow, Movies, Re=1.5 106
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Workshop on CFVNs – Poitiers 2013
CFVN 1 - ISO
Resulting Flow, Movies, Re=0.1 106
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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 !!
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Workshop on CFVNs – Poitiers 2013
CFVN 1 - ISO
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Workshop on CFVNs – Poitiers 2013
CFVN 1 - ISO
Experimental verification by Ishibashi (NRLM)
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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
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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
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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
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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 !!
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Workshop on CFVNs – Poitiers 2013
Numerical Simulations - Results
Forward orientation, pout/p0 = 0,3
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Workshop on CFVNs – Poitiers 2013
Numerical Simulations - Results
Backward orientation, pout/p0 = 0,3
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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
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Numerical Simulations - Results Drilled nozzle, pout/p0 = 0,3
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Workshop on CFVNs – Poitiers 2013
Numerical Simulations - Results Drilled nozzle, pout/p0 = 0,3
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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
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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)