CROSS-CODE VALIDATION OF UN

CROSS-CODE VALIDATION OF UN-STEADY,
THREE-DIMENSIONAL FLOWS PAST BLUFF
BODIES USING UNSTRUCTURED GRIDS
A. Bonfiglioli,† R. Paciorri¶
†
Università della Basilicata, Scuola di Ingegneria, Potenza, Italy
¶
Università di Roma La Sapienza, Dip. di Ing. Meccanica e
Aerospaziale, Rome, Italy
EFMC9
9th European Fluid Mechanics Conference
9-13 Sept 2012 / Rome, Italy
A. Bonfiglioli, R. Paciorri
Aim of the study and Outline of the talk
• Perform a code-to-code (same grid) validation exercise for a non-trivial 3D,
un-steady (laminar) flow configuration
– We have chosen one of the test-cases belonging to the iCFD db hosted at
CINECA: 3D un-steady flow around a confined square cylinder (contributed
by Camarri et al. )1
• Outline
–
–
–
–
1
Test-case definition.
Features of the two codes.
Cross-comparison.
Physical flow features.
Camarri et al. XVII Congresso di Meccanica Teorica (2005)
EFMC9
9-13 Sept 2012 / Rome, Italy
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A. Bonfiglioli, R. Paciorri
3D channel flow past a square obstacle
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9-13 Sept 2012 / Rome, Italy
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A. Bonfiglioli, R. Paciorri
Mathematical & numerical models
• Model equations:
– Compressible (low Mach), laminar (ReD = 200) Navier-Stokes equations.
• Numerical models confronted:
1. The AERO code2 (provides the reference calculation3 available in the iCFD
db)
2. The in-house developed eulfs code
Both codes are vertex-centred and used tetrahedral meshes; therefore the nof
degrees of freedom is the same in both calculations (approx. 32.5M dofs per
physical time step).
2
C. Farhat, B. Koobus, H. Tran - Computational Methods for Fluid-Structure Interaction (1999)
3
Camarri et al. XVII Congresso di Meccanica Teorica (2005)
EFMC9
9-13 Sept 2012 / Rome, Italy
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A. Bonfiglioli, R. Paciorri
Distinctive features of the two numerical models
The AERO code
The eulfs code
• Mixed
FEM-FVM
space
discretization (O(h2) accurate)
(distance-2 neighbours stencil)
• Fluctuation
Splitting
space
discretization (O (h2) accurate)
(distance-1 neighbours stencil)
• Turkel’s
low
preconditioning
• Merkle’s
low
preconditioning
speed
• Implicit time integration (O(∆t2)
accurate)
EFMC9
9-13 Sept 2012 / Rome, Italy
speed
• Implicit (3 time levels) time
integration (O(∆t2) accurate)
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A. Bonfiglioli, R. Paciorri
Initial/boundary conditions
Inflow/outflow boundary conditions are not trivial due to the need of prescribing
a Poiseuille inflow profile with maximum (centreline) Mach number equal to 0.1.
The AERO code
• Initial conditions correspond to an
impulsive start
EFMC9
9-13 Sept 2012 / Rome, Italy
The eulfs code
• Initial conditions are given by a
snapshot of the 2D calculation
(with w = 0)
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A. Bonfiglioli, R. Paciorri
Time evolution of the aerodynamic coefficients
2
Aerodynamic coefficients
1.5
CD AERO code
CL AERO code
CL eulfs code
CD eulfs code
1
0.5
0
-0.5
0
100
200
300
400
500
t Um/D
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9-13 Sept 2012 / Rome, Italy
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A. Bonfiglioli, R. Paciorri
Time evolution of the aerodynamic coefficients
2
Aerodynamic coefficients
1.5
CD AERO code
CL AERO code
CL eulfs code
CD eulfs code
1
0.5
0
-0.5
300
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9-13 Sept 2012 / Rome, Italy
400
t Um/D
500
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A. Bonfiglioli, R. Paciorri
Onset of three-dimensionality
The eulfs code
The AERO code
eulfs code
AERO code
time evolution of the transverse velocity component
Time evolution of the crossflow (w) velocity component
0.6
0.6
0.5
0.5
max(w)
-min(w)
mean(w) x 10
w/Um
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
100
150
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9-13 Sept 2012 / Rome, Italy
200
t Um/D
250
300
max(w)
-min(w)
avg(w) x 10
350
0
200
250
300
350
t Um/D
400
450
500
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A. Bonfiglioli, R. Paciorri
Three-dimensionality instabilities (un-bounded flow)4
2D flow
(Re=155)
4
Mode A instability
(Re=188)
Mode B instability
(Re=244)
Experimental observations from Luo, Thong, Khoo J. of Fluids and Structures 23 (2007)
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9-13 Sept 2012 / Rome, Italy
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A. Bonfiglioli, R. Paciorri
Mode A or mode B ?
The above figure shows mode A
instability for the un-bounded flow
past a square cylinder at Re = 210,
from Sheard et al. J. Fluid Mech.
(2009)
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9-13 Sept 2012 / Rome, Italy
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A. Bonfiglioli, R. Paciorri
Movies
Here we show movies ..........
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A. Bonfiglioli, R. Paciorri
Conclusions
• Numerics:
– differences btw. the results (e.g. time evolution of the aerodynamic
coefficients) obtained using the two codes have been observed, despite
the use of the same spatial grid.
– 2D grid convergence studies (un-feasible in 3D) have shown that the spatial
resolution (mesh size, preconditioning parameter) has a larger impact on the
results than the temporal one.
• Flow physics:
– are we observing both mode A and mode B instabilities? Or what else?
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9-13 Sept 2012 / Rome, Italy
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A. Bonfiglioli, R. Paciorri
Aknowledgments
Computational resources on the Matrix cluster have been made available by
CASPUR (Consorzio interuniversitario per le Applicazioni di Supercalcolo per
Università e Ricerca) under grants Standard HPC 2010 and Standard HPC 2011.
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9-13 Sept 2012 / Rome, Italy
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