turbulence modelling prof. paul tucker given by tom hynestph/4a2/turbulence.pdf · one is quantum...
TRANSCRIPT
1
TURBULENCE MODELLING
Prof. Paul Tucker
given by Tom Hynes
2
STRUCTURE
The formidable turbulence modelling
task
Overview RANS modelling
Overview LES
Discuss mixing LES & RANS models
3
KEY ROLE OF TURBULENCE
Drag generation
Heat transfer
Particle dispersion
Scalar mixing
Sound generation
4
TURBULENCE
da Vinci - describes the
“clouds as scattered and torn” Van Gogh Transition
l
y+
5
FORMIDABLE TASK
“I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic” Sir Horace Lamb
FRS (1849-1934) 2nd Wrangler Trinity College
“Turbulence is the last great unsolved problem in classical physics” Richard Feynman (Nobel Prize in Physics - quantum
electrodynamics )
Do not even know the Karman constant (l = y – 0.38 < < 0.45) or if it is a constant!!! – Spalart (2006) 2% decrease in gives 1%
decrease in predicted aircraft drag
6
MODEL BASIS
Phenomenological – but we do not fully understand the phenomena!!!
Spalart & Allmaras (1994) – La Recherche Aerospatiale, No 1, 5-21
Abstract – A transport equation for turbulent viscosity is assembled based
on empiricism and arguments of dimensional analysis ……
7
DICTIONARY DEFINITION
Empiricism
- Philosophy. the doctrine that all
knowledge is derived from sense
experience.
- Undue reliance upon experience, as in
medicine; quackery.
8
WHAT IS RANS MODELLING
NS(u)=0
NS(U+u’)=0 – time average
RANS(U)=0
Identical to NS(U) but μ = μt + μ
u = U + u’
9
SA MODEL BASIS
DtDt
C1St Diffusion Term[C2,]
Shearing for production
tt μS
Dt
μDρ 2Γ
10
CALIBRATION
2D mixing layer
Wake
Calibration suggests 0.6<σ<1; 0.1375< C1<0.1275 & 0.6< C2<0.7
Pick:2/3, 0.1355, 0.622. Acknowledge plane
jet spreading rate 38% too high
( )2010= UΔ.max
( )2060= UΔ.max
11
HEAT TRANSFER
12
URANS
Linear
models
Non-linear
models
OK - has spectral
gap - unusual Liu and Tucker (2007)
IJNME
13
URANS
T [K]
14
The Resolved Solution in Different Approaches
By Strelets group
15
WHAT IS LES?
LES = Resolve all large eddies RANS = Resolve time average of flow
x
y
Resolved/solved forModelled < 2
l
16
DNS, LES & RANS IN A CHANNEL
From K. Hanjalic
17
L. F. Richardson’s (1922) Rhyme & Kolmogorov (1941)
•Big whorls have little whorls,
which feed on their velocity,
and little whorls have lesser whorls,
and so on to viscosity (in the molecular
sense).
•Kolmogorov (1941),
smaller whorls or
eddies isotropic
Energy α k-5/3
Big whorls
Scale separation
18
LES FILTERING
19
KEY LES PROBLEM
Hinze (1975)
•Resolving streaks
•Trent 1000 fan at cruise 107
•LES Cost α Re2.5*
•Hybrid LES-RANS Cost α Re0.5
y+=90
*Piomelli, AIAA-2008-396 DES type problem –
By Forsythe, Wurtzler,
Squires, Cobalt
20
CHAPMAN (1975) – THE DREAM
1014 flops N = 109 -> Road
Runner (2008) 1015 flops
Chow and Moin (2012)
confirmed Chapman’s estimates
GPUs provide cheap computing
21
LES RESOLUTION REQUIREMENTS
Adapted from Leschziner (2009), Piomelli and Balaras (2002)
LES
Hybrid
Fan –
eng
ine s
ca
le
N ≠ f(Re)
22
GRID REQUIREMENTS
23
WING-FLAP [Re = 23 x 106, 3.3 million cells]
ZONAL ILES-RANS vorticity contours
Model CL
% Error
RANS +24
Zonal (I)LES-RANS -5
(I)LES -16
24
PROBLEM AEROSPACE FLOWS
25
CHEVRON ILES-RANS
26
Flow Visualization
U
u’u’
u’v’
27
6 x 106
12 x 106
50 x 106
Vorticity Contours
28
SENSITIVITY TO REAL INFLOW/
GEOMETRY
Geometry and Near Nozzle
Blocking Structure
Vorticity Contours
29
Pylon Geometry
Instantaneous Streamwise Velocity Time Averaged Streamwise Velocity
30
JET PYLON-WING-FLAP INTERACTION AND
MORE
u
|grad(ρ)|
Mesh
Blocking
Topology
Rig tests difficult!!
Big noise impact
31
COMPRESSOR/TURBINE LES
s
DONE USING ‘YOUR’ CODE
32
NEW PHYSICS AND
PALLIATIVES . Vorticity magnitude
isosurfaces
RANS SA
RANS-SA-HJ
LES
Cpt,PA
s
33
NEW PHYSICS - ENDWALLS
34
FAN BLADE
35
CUTBACK TRAILING EDGES
36
RIBBED PASSAGE
37
HIGH PRESSURE COMPRESSOR DRUM & LAB SEALS
38
TURBINE BLADE
Interface
Re ≈ 0.6 million, N > 5 million
10% DS
39
LPT LES/DNS
Iso-surface contours of Vorticity Magnitude
(From DNS)
40
EXPLORE NEW PHYSICS
Turbine blade surface topography
damaged by Spallation
Blade vibration alters reattchment location by 8%
41
DES of F-15 Post-Stall
By
Forsythe,
Wurtzler,
Squires,
Cobalt
42
Work of L. Hedges, NASA Funded
URANS
DES
Vorticity
Magnitude
SRANS, Partly Converged
43
Generic Heavy Truck in Cross-Wind
By Wurtzler, Forsythe, Cobalt
44
RUNWAY IN CROSSWIND
45
LES SIMULATIONS
46
Combustion Noise
URANS + LES + High-
Fidelity Models
47
Open rotor engines
•Today’s aircraft
fly from London to
Berlin, a distance
of 581 miles, on
six tonnes of fuel.
•Planes with
open-rotor
engines could fly
nearly 900 miles,
or from London to
Rome, on the
same amount.
48
BFM: WAKE MODELLING
49 Surface Mesh: Full View
BIFURCATED INTAKE
50
Surface Mesh: Lips detail
BIFURCATED INTAKE
51
Surface Mesh: Vanes and Branches
BIFURCATED INTAKE
52
Surface Mesh: Vanes detail
BIFURCATED INTAKE
53
Y0 PLANE
Total Pressure Contours
54
Y0 PLANE
Vtheta Contours showing asymmetric flow in XZ and XY planes and due to wake
entrainment
55
BIFURCATED INTAKE
Surface Mesh: Full View
56
CONCLUSIONS
Vast number of RANS models, choice can have substantial impact - CFD use a specialist activity
CFD predict correct delta’s
To predict exact levels extreme insights into turbulence model and many other CFD aspects + calibration data
Many practical flows are highly three dimensional in which inviscid pressure driven structures occur and then turbulence stresses become less important
However, if the 3D structures are unsteady in nature, other challenges arise
57
CONCLUSIONS
URANS can help
Zonal RANS-LES & LES with take over
but when?
Depends on HPC/GPU developments
Zonal RANS-LES & LES still need
physical insight by analyst
58
SCALE SEPARATION