38475503-CFD

SC07 Education ProgramDesktop to Grid Parallel Session - CFD November 10, 2007

Jim GiulianiClient and Technology Support ManagerOhio Supercomputer [email protected]

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Introduction to Fluid Dynamics

Computational Fluid Dynamics Motivation & Processes

State of the Art in CFD and its Role in Education

Agenda

CFD Tools•Java Applets•MATLAB Application•FLUENT

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Introduction• A brief introduction will be given to the field of Fluid

Mechanics and the types of physical systems that can be modeled

• Computational Fluid Dynamics (CFD): – Why CFD– How it is applied– What is the current state of the art

• CFD’s role in Education– Design Tool for the engineer...doctor…???– Educational tool for understanding

physical phenomena“The story behind the simulation”

• Applications will be explored that allow computers to help convey fundamental concepts in fluid dynamics that allow for increased knowledge discovery

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What is Fluid Dynamics

• Roughly defined: Fluid Mechanics is the study of a system in which a fluid is the working medium

• Other fields– Solid mechanics– Thermodynamics

• Statics is the study of systems at rest

• Dynamics is the study of systems in motion

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Classification of Fluid MechanicsContinuum Fluid Mechanics

• Inviscid

• Viscous– Laminar– Turbulent

• Compressible/Incompressible

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Classification of Fluid MechanicsProblems are also classified in terms of the number

of dimensions of the problem

• 1-Dimensional

• 2-Dimensional

• 3-Dimensional

Most problems are 3-D, but analysis based on fewer dimensions is often meaningful

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Differential vs. Integral ApproachDifferential Equations

– Equations formulated in terms of infinitesimal control volumes

– Solution of differential equations determines the detailed, point by point, behavior of the flow

Integral Equations– System under study does not require a detailed

knowledge of the flow– Equations formulated in terms of finite systems and

control volumes– Often easier to treat analytically

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Navier-Stokes Equations

yw

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xu

yv

yyu

xv

xypg

DtDv

y 232

zu

xw

zxv

yu

yzw

yv

xu

xxpg

DtDu

x 232

Assumptions/simplifications allow us to reduce complexity•For an inviscid flow, where we neglect viscosity, we simplify the equations to the Euler equations•For an incompressible, constant viscosity flow, the viscous terms simplify significantly (more applicable to gasses than fluids)

yv

xu

zw

zyw

zv

yzu

xw

xzpg

DtDw

z 232

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Why Computational Fluid Dynamics (CFD)

• Once we have classified our problem, we can derive equations that describe the fluid system

• Due to complex geometries, or lack of realistic analytical equations, we use numerical methods to approximate the fluid equations

• CFD uses computers to develop solutions to fluid mechanic problems

– Ability to solve more complex geometries

– Ability investigate regions of the flow where sensors cannot be placed

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The CFD Process• Once the governing equations for a problem have been

derived, the task of solving them numerically can be started

• A fundamental aspect of CFD is to discretize the spatial domain into small cells to form a volume mesh or grid

• A suitable algorithm to solve the equations is then applied

• Implicit tech.solve the eqns.directly

• Time marchingexplicit tech.

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The CFD Process: Gridding Limitations

• Only 1.5” gap between belly and ground• To properly represent flow through any gap, should have at least 5 cells between wall surfaces• Turbulence modeling requires first layer of cells on road and body to be ~0.5”

– This leaves only ~0.5” for remaining 3 layers; results in very high aspect ratio cells and misrepresented ground/vehicle boundary layer interaction

Boundary layer cells

Boundary layer cells

0.5”

0.5”1.5”

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State of the Art in CFD• Large companies understand the impact of numerical

simulations on R&D and are leading the charge– SC07 Masterworks series

• Advances in mesh generation allow complex CAD geometries to reliably be meshed from solid models

• Node and element counts in the 10 to 100 million range

(Cd = Drag coefficient)Stock Fusion Cd 0.34

Lowering vehicle -0.08

Closing off cooling -0.025

Removing mirrors -0.015

Adding full bellypan -0.01

XV1 Cd = 0.21

XV1 – 0.21

BASE – 0.34

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State of the Art in CFD

Solution driven by velocity inlets on domain walls

Flow exits domain through pressure outlet

• Many commercial CFD applications now support distributed memory parallel processing

• Production runs common in the 50 to 500 processor range

• Leading edge runs in the 1000 to 5000 processor range

• Example: Virtual wind tunnel

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Roll of CFD in Education• Fluid mechanics courses are in many undergraduate

curriculum

• Computational techniques, specifically visualization, offer many educational benefits in helping convey fundamental fluid mechanic concepts

• Computer simulations can allow some lab experiments to be replaced with on-line tutorials

• CFD courses are mostly found at the graduate level

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• We will consider several fundamental problems in fluid mechanics and examine how different computational tools can be used to solved the problem and highlight characteristics

• Applying the different tools to the same problem will help compare an contrast the capabilities of the different utilities

• Flow over a cylinder is a fundamental fluid mechanics problem of practical importance. The flow field over the cylinder is symmetric at low values of Reynolds number. As the Reynolds number increases, flow begins to separate behind the cylinder causing vortex shedding which is an unsteady phenomenon (and a COOL visualization!)

Case Studies

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CFD Tools OverviewThree tools will be discussed in the workshop

• Java applications and Java applets– Programs written in the Java programming language and

often integrated with on-line web pages

• MATLAB– Commercial software package for general mathematical

modeling

• FLUENT– FlowLab (educational product) integrates commercial mesh

generator (GAMBIT) and solver (FLUENT) with simplified GUIs for students

While only scratching the surface, these applications cover the range of simulation ability, from simple Java

app, to cutting edge parallel CFD solver

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Java AppletsJava is a very useful tool for education

Benefits– Applets allow for easy integration into web based lecture

material and on line exercises.– Interactive and often include graphics– Excellent portability and distribution– Many good applets already exist (some funded by NSF)

Drawbacks– High degree of programming effort– Java is not fast enough for many computational tasks

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engAPPLETS– NSF funded project at Virginia Tech

http://www.engapplets.vt.edu/

– Fluid Dynamics– Statics– Dynamics

Applications that automatecalculations with data visualization allow students to interact withscience, not just observetrends

Java Applets - Example

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Ideal Flow Machine– Ideal Flow Machine is designed for students learning the

foundations of fluid mechanics– The term ‘ideal flow' describes the way in which a fluid

(liquid or gas) moves when the effects of compressibility and viscosity are negligible

– Removing these terms from the Navier-Stokes equations reduces the problem to one that can be solved analytically or numerically

• Go to the engAPPLET web page

• Click on Ideal Flow Machine, which is in the left hand column

• Click on “Launch Ideal Flow Machine”

Java Applets – Ideal Flow Machine

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Java Applets – Ideal Flow Machine

We will use the following options from the pull down menu to examine flow around a cylinder:

•Free stream

•Circle

•Draw Streamline

•Vortex

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Java Applets – Ideal Flow Machine• Choose the Free Stream option from the pull down

menu

• Type in 10.0 in the Strength input box

• Click anywhere on the mesh. An arrow will appear on the left hand side showing the free stream

• Choose the Circle option from the pull down menu

• Click on one of the cross hairs and hold down the mouse button

• Pull the mouse to the neighboring cross hare and release

• You should see a green circle appear on the mesh

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Java Applets – Ideal Flow Machine• Choose the “Draw Streamline” option from the pull down

menu

• Click anywhereon the mesh and you will see a streamline from that point

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• Choose Vortex from the pull down menu and click anywhere on the mesh to add in a vortex (flow circulating around a point)

• Click on Draw Streamline to visualize the new flow

• Click on New Flow and redo the exercise but with10.0 strength and 10.0 angl

Java Applets – Ideal Flow Machine

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• NASA Glenn Research Center – “The Beginner’s Guide to Aeronautics”

http://www.grc.nasa.gov/WWW/K-12/airplane/• Java applications for:

– Aerodynamics• FoilSim Interactive Simulator (very good)• CurveBall Interactive Simulator• Atmosphere Modeler

– Propulsion– Hypersonics– Model Rockets

Java Applets - Example

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FoilSim Choose between ideal flow and flow with separation

Sliders allow students to dynamically change the orientation and design of the airfoil and see real time changes in lift, airflow and stall performance

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Curveball

Students to change spin of ball and see quantitative (lbs of force, change in streamlines) as well as qualitative results (ball / strike)

•Extends the ideal flow simulator by adding viscosity

•Provides real world connection (i.e. viscosity makes curve balls possible)

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MATLAB

Benefits

• Graphical User Interfaces can be designed to lead students through exercises

• Good quality and easy to implement graphics

• Software can be used for other academic purposes

Drawbacks

• Not as fast as native code

• Software has a financial cost to students

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Navier2d – Example MATLAB CFD Program

• Navier2d is a set of MATLAB functions designed to simulate the motion of incompressible fluids via numerical solution of the 2D, unsteady Navier-Stokes equations

• Available from the MATLAB Central file exchange– Go to http://www.mathworks.com– Click on User Community– Type Navier2d in the search box and click Search

• Solver reads in mesh, which allows flexibility to solve different problems

• Mesh generator is available in a separate MATLAB program

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Navier2d

• Graphical User Interface designed into the application to lead the student through the different steps of the solution

– Allows for easy step-by-step example problems– Separate mesh generator allow students to go

beyond the “caned” examples

• Allows students to gain experiences with the terminology and work flow of computational fluid dynamics, while still learning about the flow problem at hand

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Navier2d – How to Run• Download the zip file and unpack on your local

machine

• Start up MATLAB and in the Current Directory window, move to thedirectory where Navier2d resides

• Right click onNavier2d.m and click Run

• You should see the applications main menu

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Navier2d - MeshRather than build a mesh from scratch, we will use a pre-

defined mesh of a cylinder in a free stream

• In the “Mesh Options” window, click Load

• In the file browser that pops up, select cylinder.mat and Open

• Click View to see the mesh

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Navier2d – Boundary Conditions

• To set boundary conditions, click the Velocity/Pressure button in the Boundary Conditions window

• To set boundary conditions, click the Select button

• With your mouse, drag a box around the nodes you want to select and then press the right mouse button when done

• With the nodes highlighted in red, click the Set button to apply the boundary condition to those nodes.

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Navier2d – Boundary Conditions• For this example, we will choose the following initial

and boundary conditions– Inlet (left hand side)

• Velocity B/C• U=5m/s, V=0m/s

– Top and bottom• Velocity B/C• U=5m/s, V=0m/s

– Outlet (right side)• Outflow

(extrapolated)– Surface of cylinder

• Velocity B/C• U=0m/s, V=0m/s

• Click File->Close when done

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Navier2d – Initial and Solver Settings

• Under Integration Settings, click on Initial and set the U velocity to the initial value of 5m/s

• Under Integration Settings, click on Set to set solver options

• Accept defaults, but change Maximum number of steps to 2000 and then click OK

• Click Run to begin the solution

• Watch the vortex structure form as the simulation progresses

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Navier2d - Results

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FLUENT• One of the leading CFD

applications for researchand product development

• A wide range of physical systems and processes can be simulated using numerous techniques and equations

– Compressible/Incompressible– Combustion– Porous media– Flow with heat transfer– Adaptive mesh refinement– Parallel solver

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FLUENT Capabilities

Simulation of a 1 gallon milk container being formed by blow molding. Simulation includes the arison extrusion, pinch-off, mold closing and inflation steps

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FlowLab – Academic Interface for Fluent• Integrates the following three steps:

– Preprocessor• Meshing the object or domain to be analyzed• Applying boundary conditions• Set application/solver control parameters• Usually graphic intensive

– Solver• Read in the mesh and supporting input files and solve• Longest step, usually CPU, memory and disk intensive

– Post Processor• Read in solution and visually display data• X – Y plots, histograms, contour plots, animations

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FlowLab• Provides a virtual fluids lab for teaching and

virtually reinforcing concepts in fluid flow and heat transfer

• Acquaints students with the following:– Terminology of CFD– Basics of geometry and mesh generation– Setting up a problem– Concepts of accuracy and convergence– Post processing

• Several fluid mechanics text books include FlowLab modules

http://flowlab.fluent.com/collaborations/index.htm

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FLUENT – High Performance Computing Example

• Now that we have seen examples of CFD on the desktop, let’s examine a larger application

• Motivation– Examine more complex flow systems– Experience the full modeling process

• Pre-processing• Solving• Post-processing

– Introduce the concept of parallel processing• Solve models quicker• Solve bigger models

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Mixing Problem in 90o Elbow*

A cold fluid at 20oC flows into the pipe through a large inlet, and mixes with a warmer fluid at 40oC that enters through a smaller inlet located at the elbow

• Heat transfer, 3-D, Turbulent flow

• Will run in parallel on 4 processors

* Fluent 6.3 Tutorial Guide

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FLUENT – High Performance Computing Example

• Workshop accounts will be provided for access to OhioSupercomputer CentersIBM 1350 Opteron cluster

• Due to network load, slow graphic performance is very possible

1. Open up a CYGWIN terminal

2. SSH to the Opteron cluster:

ssh –X wrk###@opt-login1.osc.edu

where you will replace ### with the number given to you

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First Steps – Starting a Remote Application• To run parallel FLUENT, we are going to request 1 machine

and all 4 processors within that machine

• A graphics window will return after we start FLUENT– It is possible to do perform tasks through menu interface– For speed, we will type in most commands

• To request 4 processors

qsub –I –V –l walltime=1:00:00 –l nodes=1:ppn=4

• To start FLUENTmodule load fluentrm -f pnodescat $PBS_NODEFILE | sort > pnodessetenv ncpus `cat pnodes | wc -l`fluent 3d -t$ncpus -pinfiniband.ofed -cnf=pnodes

In this exercise, text in a bubble as seen here are commands you will type into the FLUENT command window.

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FLUENT Command Window

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Read in Model and Partitionparallel partition auto use-case-file-method

yes

rc elbow3.cas.gz

print-active-partitions

Solve initialize initialize-flow

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Display Parallel Decomposition(NOTE: These are mouse/keyboard commands)

• Click on Display

• Click on Contours

• Change “Contours of” selectionto “Cell Info…”

• Chose “Active Cell Partition”from the box below

• Change “Levels” to 4, whichis the number of processorswe are using for this run

• Choose “symmetry” from the “Surfaces” selection

• Click Display

• Click Close

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Parallel Decomposition

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Set Initial Conditions(NOTE: These are mouse/keyboard commands)

Type command:file write-case elbow4

Click on Solve

Click on Initialize

Click on Initialize

Choose “velicity-inlet-5” from “Compute From”

Change “Y velocity” to 0

Click INIT

Click OK

Click CLOSE

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Solve the Modelsolve iterate 200

(when finished)

file write-data elbow5

parallel timer usage

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Display Results(NOTE: These are mouse/keyboard commands)Click on DisplayClick on ContoursChange “Contours of” selection

to “Custom Field Functions…”Chose “dynamic-head”

from the box belowChange “Levels” to 80Choose “symmetry” from

the “Surfaces” selectionClick DisplayClick Close

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Simulation Results

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FLUENT Observations• Many problems have static solutions

• Students not familiar with Fluid Mechanics seem to like the interactive process and obtaining the solution, as long as step-by-step instructions are given

• Students with Fluid Mechanic behavior will benefit from being able to interact with solution results and see system characteristics that have been covered in lecture

• High school students can handle geometry design and meshing, along with setting initial and boundary conditions

• Junior undergraduate students are able to develop and solve complete models