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© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

Recent Enhancements

Made in CFD Solver

Capabilities to Accelerate

Convergence and Reduce

Solution Time

• Robustness

• Efficiency

• Accuracy

David Mann,

ANSYS UK

[email protected]

© 2010 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary

ANSYS CFD Solver Advances @ 13.0

• Major CFD solver performance increase @ 13.0:

– FLUENT, CFX

– Evolution of methods from release 12.0

– Revolutionary methods... 1st exposure with 13.0

• Performance improvement categories:

– Robustness: numerics, initialization

– Efficiency: linear, non-linear solution methods

– Accuracy: numerics, mesh sensitivity

• Each of these topics covered in detail


ANSYS CFD

Solver Performance:

Robustness


Robustness

• Changes for 13.0

– Treatment for “poor quality” meshes

– Hybrid initialization

– Particle tracking robustness

– Other numerics robustness improvements


ANSYS CFD

Solver Performance:

Robustness

FLUENT


Poor Quality Mesh Treatment

• Improves the numerical behavior of the solution algorithm on

meshes containing poor quality cells

– Beta capability in release 13.0

• Apply local solution correction targeting the poor quality cells in

the mesh

• Poor quality mesh defined as (highly skewed, highly non-

orthogonal cells, non-convex cells which include left-handed

faces, Vol/Area < 0)

• Three strategies

– 0th-Order

• Cell center value = Average of “good” neighbour cell values

– 1st-Order

• Local 1st order discretization + drop non-orthogonal viscous terms

– 2nd-Order

• Cell gradient = Average of “good” neighbour cell gradients

FLUENT


• TUI only:

– Enable beta features:/define/beta-feature-access yes

– Select solution order/solve/set/bad-mesh-numerics

enable? [= yes, no]

corrected solution order = ? [= 0, 1, 2]

– Brief summary of grid quality via meshing TUI

/mesh/repair-improve print-repair-improve-solver-statistics

• Recommended strategy:

1. Make good meshes

2. Use robust solving algorithms, eg Coupled p-V solver, pseudo-transient method for steady state simulations, etc.

3. Repair mesh using /mesh/repair-improve TUI if 1) doesn’t help

4. Enable bad mesh numerics if 2) doesn’t help


FLUENT



Example: bad concave cells

4 thick concave cells

“in a row”

b4c

4 thin concave cells

“in a row”

b4n

5 thick concave cells

“encircled bad cell”

b5c

5 thin concave cells

“encircled bad cell”

b5n

Domain: 12 3 m, 750 cells ... admittedly coarse.

FLUENT



Example: 2D Scalar Diffusion

Standard numerics 2nd order correction

4 concave cells

5 squeezed

concave cells

10 times faster

URF=0.9

FLUENT

Diverges



Example: Submarine

• N CELLS = 2,634643 e+6

• N BADS = 2282

• Pressure-based coupled algorithm

• Gradient scheme = Green-Gauss

• Pressure scheme = Standard

• Other = Second order upwind

0th order correction

Cl = 0.15236233

1st order correction

Cl = 0.15258488

2nd order correction

Cl = oscilating

• Linear system from standard discretization diverges inside first iteration

FLUENT


Hybrid Initialization

• Initialization procedure based on collection of

recipes and boundary interpolation methods

to obtain a reasonable initial guessed flow

field.

• Solves Laplace's equation to determine the

velocity and pressure fields

• Other flow variables will be automatically

patched based on domain averaged values or

a particular interpolation recipe

• Can be customized

• First Step toward automatic initialization

FLUENT



Example: Multiphase Heat Exchanger

MFINLET(Primary In)

MFR = 1.14; T0 = 322.04 K

POUTLET(Primary Out)

P = 0.0

MFINLET(Auxiliary In)

MFR = 0.5; T0 = 388.7098 KPOUTLET(Auxiliary Out)

P = 0.0

Case Setup :

• PBNS, SIMPLE Scheme

• Viscous – Laminar,

• Heat Exchanger - ON

• LSQ Cell Based, First Order accurate

WALL: Inviscid, Adiabatic

Initialization Fields

FLUENT


Std Init:

• Iterations = 279

• URF

•Mom 0.7, Press 0.3, Den 1.0

•Energy 0.99

Hybrid Init:

• Iterations = 102

• URF

•Mom 0.7, Press 0.3, Den 1.0

•Energy 1.0


Example: Multiphase Heat Exchanger

FLUENT


Other Numerics Features &

Enhancements

• Extending Velocity BC to compressible flows

• Average Pressure Specification (PB solver)

for pressure outlet

• Rothalpy Transport: For rotating frame of

reference solve transport equation of rothalpy

rather than energy so that conservation of

rothalpy is enforced.

• Optional Local Residual Scaling:

compatibility with CFX residual reporting

FLUENT


ANSYS CFD

Solver Performance:

Robustness

CFX


Particle Tracking –

Eulerian Numerics Robustness

• Particle Tracking – Robustness enhancement with particle source bounding for energy and

momentum

• Correction factor to account for the fact particles are not tracked

every time step

• Set particle source terms based on

last available heat transfer coefficient

and particle temperature, and current

fluid temperature

• Bound this to avoid new min/max

• Improved robustness with high loading

• Examples applications

– Coal combustion

– Spray injection

– ...

CFX


Particle Tracking –

Eulerian Numerics Robustness

• To improve convergence particle source terms can be

linearized

– CS = Source Coefficient

– F = Fluid value at particle position

– RS,V = Source Value

• Note:

– CFX-12: Only particle momentum and energy sources are linearized.

– CFX-13: Option is added to also linearize particle mass sources

(including the liquid evaporation model).

VSFSS

pRCR

dt

dS,

CFX


0

10

20

30

40

50

0 50 100 150 200

coef

fici

ent

loo

ps

Time Step

12

13

Source Term Linearization:

Particle Tracking Efficiency

Time per Timestep

CFX-12.1 1831

CFX-13 513.2

Factor 3.6!!

Particle Wall

contact

Start

injection

CFX

Iterations required per timestep

• In CFX 12, generally 50

iterations required per

timestep after particle wall

contact (the max)

• In CFX 13, due to particle

source term linearization,

generally only 10 iterations

per timestep required


Case: Gas Washer with Heat and

Mass Transfer

• Compressible flow

• = 0.2

• Heat transfer, Tp = 63

[C], TFl = 168 [C]

• Mass transfer: Liquid

evaporation model

flP mm

CFX


Gas Washer with Heat and Mass

Transfer

Run Temperatures at Domain Outlet

Standard Solver

Default settings (relaxation, iteration

frequency)

Initial temperature rise to 600 [C]

Linearized sources enabled:

linearization of particle mass sources

(mass fraction only)

Temperature drop, no significant

overshoots, good convergence

CFX


ANSYS CFD

Solver Performance:

Efficiency


Efficiency

• Review of important changes for 12.0

– Coupled p-V solver improvements

– Faster linear solver options

– Broad set of parallel solve & I/O improvements


– “Pseudo transient” method for “fast steady state”

– Broad set of parallel improvements

– Coupled solver for p-V-g for free surface flows

– Particle tracking speedup


ANSYS CFD

Solver Performance:

Efficiency

FLUENT


• What is it:

– Definition: PBCS solves momentum and mass equations

simultaneously rather than in segregated (PBNS) fashion

– Status: released since FLUENT 6.3, but significantly evolved !!

• Anticipated benefit

– Significant decrease in time to converged solution due to overall

startup robustness and asymptotic convergence rates

– Speedup achieved for most steady & some transient simulations

– Modest increase in memory required

– Major benefits are often realized... but only if it is used

– If you are not using it regularly, make sure you try it again!

• Instructions for use, key limitations

– Enable COUPLED under P-V coupling in Solution Methods

– Documented since FLUENT 6.3

Faster Linear Convergence in release 12.0:

Coupled p-V Solver

FLUENT


Flow Diagram of Solution Process

Flow

Diagram

of

FLUENT

Solvers

Segregated PBCS

Exit Loop Repeat

Check Convergence

Update Properties

Solve Turbulence Equation(s)

Solve Species

Solve Energy

Solve Mass Continuity;Update Velocity

Solve U-Momentum

Solve V-Momentum

Solve W-Momentum

Solve Mass Momentum Energy &Species

Initialize Begin Loop

Solve Mass & Momentum Pressure

Correction Variables

DBCS

Solve Other Transport Equations as required

Solver?


• Segregated

– Solves momentum equations for u, v, w velocities

– Solves each transport equation in turn, communicating via the flowfield

– Continuity equation recast as pressure correction equation p‟

– Pressure correction terms used to correct pressure field to match current velocity solution (inner iterative loop)

– Long solution times required to dampen out pressure-velocity decoupling errors

What are the solvers doing?


• Density Based Coupled Solver (DBCS)

– Solves continuity, momentum, energy

equations in a coupled fashion by inverting

a matrix to find the values for each

– 5 equations and 5 unknowns yields solution

for u, v, w, ρ, T

– Ideal gas law yields pressure field

– Works well when there is a strong

interdependence of momentum, energy

and density

What are the solvers doing? (cont.)


• Pressure Based Coupled Solver (PBCS)

– Solves momentum and continuity in a coupled fashion

– Momentum and continuity equations rewritten in terms of velocity correction and pressure correction variables u‟, v‟, w‟, p‟ (no additional correction step)

– 4 equations and 4 unknowns

– Additional transport equations solved in a segregated manner

What are the solvers doing? (cont.)


Pseudo Transient Method

• Form of local implicit under-relaxation for

solving steady-state problems using

discretization of transient terms in transport

equations

• Improve the convergence on highly

anisotropic meshes and for rotating reference

frame (turbo-machinery flow)

• Available in:

– Pressure-based coupled solver

– Density-based implicit solver

• The pseudo-time step

– Automatic

– User Specified

FLUENT


Pseudo-transient relaxation

study Cases

Courant number coupled:

# Iterations

Pseudo-transient coupled:

# Iterations

Backward facing step

(turbulent: SST) 750 75Film cooling benchmark

(turbulent: SA) 2300 1350Flat plate, SST transition model

1200 100Rotor/Stator with the mixing plane

model 500 250Centrifugal pump

220 50Axial compressor stage

400 110

Pseudo Transient Method: speedup

CPU time savings almost directly proportional to #iteration savings

FLUENT


Pseudo-Transient Method:

High-Lift workshop example

• Pressure based coupled solver

• SST Transition model

• Full 2nd order with node based gradients

• AOA =13 degrees

• ICEM CFD mesh (6M cells)

• Max cell aspect ratio 30.62x106

FLUENT

Extreme skew

Extreme aspect

30 million-to-1


FLUENT R12 Scalability

Broke Barriers…

• External flow around a truck body

• 111 million mixed cells

• Turbulence

• pressure-based segregated solver

• Intel Quadcore Xeon Harpertown, Infiniband

• Much improved scalability compared to FLUENT 6.3

• FLUENT-12 scaled linearly to 1024 cores!

TRUCK_111M Infiniband

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

0 256 512 768 1024

Number of cores

RA

TIN

G

IDEAL

6.3.35

12.0.5


R13 Parallel/HPC Enhancements

• File-IO

– Case-IO speedup

– Parallel-IO speedup

– Support for Lustre, EMC/MPFS, AIX/GPFS

– Faster asynchronous data compression

• Mesh

– Parallel mesh and data append

– Replace zone in parallel

– Sliding mesh performance improvements

– Fast shell conduction zone creation/deletion

– Hexcore mesh performance improvement

Pdat write R12 vs 13

BMW -68%

FL5L2 4M -63%

Circuit -97%

Truck 14M -64%

Sliding

Interface

50 Mesh

Preview (secs)

R12.1 R13.0

8x 9 10

16x 82 25

24x 129 25

32x 170 24

3D case, 4 sliding interfaces, 2.1M

Case Read Times V12.0.19 V13.0.1 % diff

Truck_14m (96x) 153 92 -40%

F1_30m (128x) 474 269 -43%

Truck_111m (128x) 1011 726 -28%

F1_150m (128x) 1549 930 -40%

FLUENT


More “Realistic” Benchmark

• Complex open wheeled racecar

• Hybrid prism-hexcore mesh

• 50m/s for:

– Inlet flow velocity

– MRFs on wheels (ω = 175 rad/s)

– Rolling road

• Turbulence Intensity = 0.5%, TVR=10

• Realisable KE turbulence

• Isothermal, incompressible

• PBCS with pseudo-transient under-relaxation

• PRESTO! For pressure equation, 2nd order upwind for all other

equations.

• Benchmark for full convergence at 800 iterations

• Include writing Cd and Cl to text files


FLUENT 13 Speed

F1 Benchmark – 140M Cells

• F1 Size & Complexity Benchmark

• 140M Hexcore

• Generic Racer

• Run to convergence – 800

iterations

• Include monitor file write for Cd

and Cl and MRFs on wheels

Standard File I/O Times


• Conclusions:

– Solve a 140M cell case on ~250 cores in ~2.5 hours?

– Per 1000 nodes you can run 4 x 140M cell cases in 2.5

hours including file I/O

• Not included which may slow overall time:

• File I/O export of 3rd party PP files (Ensight, FVW, etc)

• Further text files may be required – Cz, Integrals for

other quantities, could slow down by few more %

• FLUENT14 will be optimised further for monitor writing

in parallel to improve performance further

F1 Benchmark – 140M Cells


Coupled VOF method (beta)

Wigely Hull case (Coupled VOF solver with Pseudo-transient method )

• Provides implicit coupling of pressure, momentum and volume fraction equation.

• Aims to provide faster steady state solution compared to segregated way of solution

of VOF and flow

• Can be beneficial for unsteady problems when large time step is required due to

practical reasons

Unsteady Flow over an obstacle

(Coupled VOF method )

FLUENT


ANSYS CFD

Solver Performance:

Efficiency

CFX


CFX – Particle Database Access

Time to create and

delete n particles

Old

Time [s]

New

Time [s]

Speed

up

100 0.006 0.001 6

1.000 0.059 0.005 12

10.000 0.562 0.044 13

100.000 5.715 0.261 22

1.000.000 58.145 2.410 24

• Particle database access times in CFX are expensive due to heavy

Memory Management System (MMS) usage

– Particularly noticeable for transient simulations, where particles are only

tracked for a short time and spend most time being checked in and out

• Reduction of MMS related work in particle database by accessing

particle groups

– Significant speed-up possible wrt. CFX 12

CFX


ANSYS CFD

Solver Performance:

Accuracy


Accuracy


– Bounded 2nd order transient

– Bounded 2nd order CDS (for LES)

– Compressive numerics for free surface (VOF)

– Nested rotating frames of reference

– Transient blade row specialized numerics

– Multiphase flow specialized numerics


ANSYS CFD

Solver Performance:

Accuracy

FLUENT


Bounded Second Order Transient

• Bounded second order time formulation provides the second order

accuracy with better stability compared to Adams-Bashforth second order

time formulation.

• This formulation allows to use larger time step size compared to first

order and second order implicit (Adams Bashforth) scheme.

• It is available with all models (single phase/multiphase) using pressure

based solver.

• This scheme is bounded by the lower and upper bounds for any variable

based on availability of bounds.

FLUENT


Bounded Second Order Transient

Scheme : Free surface wave

Surface waves T – time period

First order: dt =T/500, Amplitude decays

Bounded second order: dt =T/500

(Numerical vs Analytical)

Bounded second order: dt = T/80, T/40, T20

Large timestep results comparible to first order

results.

FLUENT


VOF : Compressive Scheme:

• Compressive scheme is a second order reconstruction scheme based on a slope limiter.

• This scheme is available for both Implicit and Explicit formulation using following models

• VOF model

• Eulerian multiphase with “immiscible fluid model”

• Mixture multiphase with expert option

• This scheme provides much sharper and accurate interface compared to high resolution interface

capturing schemes like CICSAM or Modified HRIC

Pure advection of shapes

FLUENT


VOF : Zonal Discretization

• Zone based VOF discretization based on Compressive scheme

• This options provides diffusive or sharp interface modeling in different zones

based on the value of zone dependent slope limiter.

(Zone 1) (Zone 2) (Zone 3)

FLUENT


VOF : BGM Scheme:

• The Bounded Gradient Maximization (BGM) scheme is introduced to obtain sharp

interfaces with the VOF model, comparable to that obtained by the Geometric

Reconstruction scheme.

• Currently (R13) this scheme is available only with the steady state solver and cannot

be used for transient problems.

• In the BGM scheme, discretization occurs in such a way so as to maximize the local

value of the gradient, by maximizing the degree to which the face value is weighted

towards the extrapolated downwind value.

HRIC BGM

FLUENT


Nested Reference Frame &

Mesh Motion

• Simplified specification for complex motion of one zone relative to others

• Independent specification of moving reference frame and moving mesh for a

single zone

• Superimposed complex relative motion of Reference Frames and Moving

Meshes with different rotation axes

• Transient profiles and UDF for complex motion

FLUENT


ANSYS CFD

Solver Performance:

Accuracy

CFX


Conditional GGI Interfaces

• Domain Interfaces

– Conditional GGI connection β

– „Open‟ or „close‟ GGI based on

CEL expression

Windows breaking as heat from a fire

spreads through a double skin façade

CFX


Turbulence – Numerics Accuracy

• Turbulence

– Bounded Central Difference (BCD) scheme

• Prevent wiggles in simulations with scale-resolving

models (DES/SAS/LES)

Unphysical wiggles

due to CD and

skewed meshBCD: no wiggles!

NACA 0021: Re=2.7e5 , a=60deg (EU project DESIDER)

CFX


Turbomachinery – Radial

Equilibrium Outlet BC

• Turbomachinery

– Radial equilibrium outlet

boundary condition

• User specifies reference

pressure at a radial location

• Radial equilibrium of the

pressure gradient and the

centrifugal force

Can reduce need for

extended outlets or stage

simulation

More efficient simulations

cr

u

dr

dp2

CFX


Unequal Pitch Problem

• The blade passages in rotor and stator rows have different pitch

• Instantaneous periodicity can not be enforced

• The flow is periodic but with a phase shift.

– along the pitchwise periodics

– along the rotor-stator interface

S2

S1

R2

R1

ROTOR STATOR

VRPR PS

S2

S1

R2

R1

ROTOR STATOR

VRPR

PS

2 passages 3 passages

Periodic boundary conditions between R1/R2 and S1/S2 cannot be applied

Periodic boundary conditions between R1/R2 and S1/S2 can be applied if ensemble pitch ratio is unity

BUT More memory required

CFX


13.0: Transient Blade Row

Two years of

R&D comes to

fruition at 13.0:

• Time

transform

• Fourier

transform

• Gust analysis

• R-S analysis

• Flutter

analysis

13.0CFX


Transient Blade Row

Flow Problems

Turbine

Gust pitch

Blade Passage pitchGust speed

Gust Analysis

Multi-Stage

Forced Response Blade Flutter

Period

dis

pla

cem

ent

102

NbjjNb

IBPA

IBPAD

am

pin

g C

oef.

Single-Stage

TBR Simulations

CFX


Gust Analysis

T106 Turbine Cascade

CPU

Effort

Time

Transform1

Fourier

Transform3

True

Transient16

• Verify Time-transformation & Fourier-transformation,

to true full-domain transient

• Full domain: 8 blade passages vs. 21 wakes

• Submitted for publications (GT2010-22762)

• Works extremely well, massive time savings!

MP1Ref 21-8

FT

TT

MP2

Ref 21-8

FT

TT

CFX


Forcing function on IGV: Integration of pressure

distribution at 90% span from experiment and CFD

Off-Design PointDesign Point

• Validating Time-Transformation to true full domain

transient solution

• Full domain 180o: 10 IGV / 9 R

• TT : 1 IGV/1R

• Published work GT2010-22762

Large Scale Single-Stage

Purdue Compressor

CFX


Monitor Point of static pressure at 90% span:

MP

Off-Design PointDesign Point

Large Scale Single-Stage

Purdue Compressor

CPU Effort Trans/ Steady

TRS-TT 1.0 38

TRS-PT 1.0 38

Full Domain 10.5 399

CFX


• Blade forced response under bending vibration

• IBPA= 90 deg., full domain 4 passages

• Amplitude = 5% chord

• Forced response mesh morphing

• Frequency = 100 Hz

• Inlet disturbance wake profile

•Total pressure Profile

• Gaussian,10% amplitude

• Frequency = 200 Hz

Flutter Multi-Disturbance

Full Domain

FT Solution Domain

MP1 MP2 MP3

MP1 MP2 MP3

CFX


Summary: CFD Performance @ 13.0

• The improvements covered lead to significant

gains in speed and accuracy of our solvers

which in many cases can be stacked up, for

example in FLUENT, the following measures in

combination can achieve up to 2 orders of

magnitude speed up– Hybrid or FMG Initialisation (~2-5 times speed up)

– Pressure Based Coupled Solver (~3-5 times speed up)

– Pseudo Transient Running (~2-10 times speed up)

• Technology such as the new Transient Blade

Row capability in CFX can reduce the required

domain size (and thereby CPU effort) by a similar

factor


Summary: CFD Performance @ 13.0

• Major CFD solver performance improvements

– Release 13.0 sees major performance gains

– Both evolutionary & revolutionary changes

– Generic & physics-specific improvements

• Performance improved in these areas

– Robustness

– Efficiency

– Accuracy

• Learn more, take advantage, gain performance!

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