View
1
Download
0
Category
Preview:
Citation preview
The University of Sydney Page 1
1st Automotive
Aerodynamics workshop
The University of Sydney
Mr Liang Yu
Dr. Asiful Islam (now JLR)
Dr. Sammy Diasinos (Macquarie)
A/Prof Ben Thornber
The University of Sydney Page 2
Outline
1. Introduction to research group
2. Numerical methods, models and meshes
3. Notchback Results
4. Case 2a
5. Conclusions.
The University of Sydney Page 3
1. Introduction to Research Group
1. Develop governing models for compressible turbulent flows
2. Explore novel numerical methods for unsteady turbulent flows.
3. Unsteady applied aerodynamics computations (aero and auto)
The University of Sydney Page 4
1. Prior relevant research – 2013-2016
– Developed hybrid RANS-ILES methods with flow-adaptive
blending, incorporating wall functions1
– Targeting Automotive Aeroacoustics (compressible solver)
– Smooth transition from Spalart Allmaras to ILES – can use the
equation to keep regions RANS or ILES when desired
– Utilises an additional scalar transport equation to ‘label’ the
boundary layer – simple solution to remove some sensitivity to
gridding
1Islam, A. & Thornber, B, Computers and Fluids, 167, 292-312, 2018.
The University of Sydney Page 5
1. Prior relevant research– 2013-2016
– Undertook hybrid RANS-ILES and Powerflow computations of
the notchback in ~2015.
PIV RANS-ILES
Flamenco
Powerflow
The University of Sydney Page 6
1. Prior relevant research– 2013-2016
Powerflow captured experimental pressure drop at rear of the
backlight (y=102).
Flamenco captured the base flow better
The University of Sydney Page 7
2. Numerical Methods: Fractional step method for
low-Mach number
1) Momentum predictor to get intermediate velocity
2) Pressure gradient added – similar to Rhie-Chow interpolation
3) Pressure corrector
4) Final velocity update
Transient SIMPLE (PimpleFOAM): (1 -> 2 -> 3 -> 4) -> (1 -> 2 -> 3 -> 4)…(usually 5-8
iterations) until converged per time step
Fractional step method: (1 -> 2 -> 3 -> 4) once only per time step
The University of Sydney Page 8
2. Test Case One: linear gas dynamics
Propagation of a Gaussian pulse of amplitude 0.09Pa (solid lines)
Splitting into two waves at late time (dash-dotted lines)
Formal convergence study against analytical solution from characteristics
Pre
ssure
(P
a)
Distance (m)
The University of Sydney Page 9
2. Test Case One: linear gas dynamics
L2 e
rror
norm
dx (m) Computational time (s)
On the same grid, L2 errors are equal
For the same grid, the fractional step method is ~5x faster than rhoPimpleFoam
Solid lines: CFL=4
Dashed lines: CFL=2e-3
Fractional step (circles)
rhoPimpleFoam (asterix)
The University of Sydney Page 10
2. Test Case Two: Isentropic vortex convection
Advection of an isentropic vortex
with a mean flow:
• Aligned with the mesh (0 deg)
• Angled to the mesh (45 deg)
Run for both the fractional step
method (cFSM) and existing
rhoPimpleFoam
The University of Sydney Page 11
2. Test Case Two: isentropic vortex convection
Dashed lines: 0 deg
Dotted lines: 45 Deg
Fractional step (squares)
rhoPimpleFoam (plus)
L2 e
rror
norm
dx (m) Computational time (s)
On the same grid, L2 errors are equal
For the same error, the fractional step method is ~5x faster than rhoPimpleFoam
Benefit does not diminish when the flow direction is not aligned with the grid
The University of Sydney Page 12
2. Mesh for Case 1
Refinement zones
Stationary ground (non-slip)
SAE notchback (non-slip)
𝑼∞
The University of Sydney Page 13
2. Mesh for Case 1
Total Cells Size on car Prism layer
Coarse 6 Million 2.3mm 16 layers, 1st height = 0.0115mm
Growth rate 1.313(leg) and
1.360
Medium 9 Million 2.05mm 16 layers, 1st height =
0.01025mm
Growth rate 1.313(leg) and
1.360
Fine 12 Million 1.875mm 16 layers, 1st height =
0.009375mm
Growth rate 1.313(leg) and
1.360
SAE car body
𝑼∞
Stationary ground
The University of Sydney Page 14
2. Mesh for Case 1
16 layers
1.36 growth
16 layers
1.313 growthlegs
SAE body
Stationary ground
16 layers
1.36 growth
𝑼∞
1) First layer y+ ~ 1 for a low-Re
mesh.
2) With growth rate of 1.313 and
1.36, the total layer thickness is
more than y+ = 250.
3) Final layer thickness is 0.3-0.5 of
the adjacent cell size for smooth
transition.
Final layer
Adjacent cell
The University of Sydney Page 15
2. Mesh for Case 1
legs
SAE body
Stationary ground
𝑼∞
1) snappyHexMesh allows mesh
distortion to add prism layer
2) Mesh quality needs to be carefully
controlled for max prism layer
coverage
3) Influence of mesh qualities on flow
solution:
Face pyramids -> discretisation
conservation
Non-orthonality -> diffusive term
accuracy
Skewnesss -> convective term
interpolation accuracy
Cell determinate -> gradient term
accuracy
Tet decomposition -> Affects
lagrangian computation
The University of Sydney Page 16
2. Mesh for Case 1
SAE body
𝑼∞
1) The distance at which distortion is
allows is also controlled so that the
distortion does not affect all interior
cells.
2) The distance is important for prism
layer coverage around corners
SAE body𝑼∞
Distortion
distance
The University of Sydney Page 17
2. Mesh for Case 1: y+
Needs much smaller surface mesh here for thicker prism layer coverage.
The wall boundary condition of the turbulent viscosity is the continuous
nutUSpaldingWallFunction in OpenFOAM.
The University of Sydney Page 18
2. Mesh for Case 2a
Contact
patch
Low-Re, 16 layers
on car body
High-Re, 4 layers
on wheels
High-Re, 4 layers on contact patch
and moving ground
Rear wheel
well
The University of Sydney Page 19
2. Mesh for Case 2a
Number of prism layers reduces at some ‘sharp’ locations.
Currently under testing: snappyHexMesh addLayer process modified to force
generate one layer on all surfaces
The University of Sydney Page 20
2. Computation time summary
Case 1
Number of Cells 12 Million
Inflow Velocity 40 m/s
Number of processors 960
Time Step 1.0e-05 s
Simulation Time 2.0s
Clock Time 14.7 h
Case 2a
Number of Cells 80 Million
Inflow Velocity 16 m/s
Number of
processors
960
Time Step 1.0e-04 s
Simulation Time 2.0s
Clock Time 10 h
The University of Sydney Page 21
2. Other computational details
Turbulence model:
compressible SST-DDES in OpenFOAM-v1612+
Discretisation:
Temporal terms: Second-order backward differencing
Gradient terms: Gauss linear with a limiter (Minmod)
Turbulence quantities: Second-order upwind
Diffusive term discretisation: Central differencing with full non-orthogonality
correction
Momentum convective terms: Hybrid convection scheme of Travin et al. for
hybrid RANS/LES calculations
Central differencing in
LES-like regionSecond-order upwind elsewhere
The University of Sydney Page 22
3. Workshop Case 1
Iso-surface of Q = 100000 1/s
Second-order upwind
Second-order upwind
Second-order central differencing
The University of Sydney Page 23
3. Workshop Case 1
Cd Cl Cm
Coarse mesh 0.197 -0.076 -0.102
Medium mesh 0.194 -0.075 -0.115
Fine mesh 0.195 -0.079 -0.114
Experiment 0.207 0.054 -0.073
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
Cp [
-]
X [m]
Cp-centreline over the car
Cp-experiment
Cp-coarse mesh
Cp-medium
Cp-fine
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 10 20 30 40 50
Cp [
-]
Sensor number
Cp-backlight
Cp-experiment
Cp-coarse mesh
Cp-medium
Cp-fine
The University of Sydney Page 24
3. Workshop Case 1
Time-averaged Cp on the SAE model, left Flamenco (in-house, very high order
compressible solver), right OpenFOAM Fractional step.
The University of Sydney Page 25
4. Workshop Case 2a
Whole car Car no wheels
Cd 0.260 0.185
Cl -0.070 -0.073
Clf -0.145 -0.151
Clr 0.075 0.078
The University of Sydney Page 26
4. Workshop Case 2a
Iso-surface of Ux = -0.001 m/s
The University of Sydney Page 27
4. Aeroacoustics of side mirror
Iso-surface of Q =
100000 1/s
Wall shear stress
magnitude
The University of Sydney Page 28
5. Conclusions and Acknowledgements
Gave a quick introduction to our recent developments of a
compressible fractional step method
– No reduction in accuracy compared to current best
options
– Considerable speed saving in practical configurations
– Weakly compressible – may be used for direct
aeroacoustics computations
Preliminary results gained for Case 1 and Case 2a. Grid
convergence and further analysis for Case 2a still
underway.
The University of Sydney Page 29
Thank-you for listening
Recommended