CFD Lecture (7/8): Best Practices in CFD

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Best Practices
Practices to be followed to get reasonable results

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©ZeusNumerixPvtLtd:ConfidentialDocument
Overview
23-Sep-20 Best Practices in CFD: Zeus Numerix
 Zeus Numerix: Introduction
 Assumption
 What is a Best Practice
 Surface Grid
 Volume Grid
 Sample Problems
 Pressure Drop
 Skin Friction
 Heat driven flows
 Initial and Boundary Conditions
 General Gyan
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Assumption
 Participants know what CFD is about
 Know the basics of Compressible and Incompressible flow
 Know what are Initial Conditions and Boundary Conditions
 Know about Meshes
 Surface Mesh
 Volume Mesh
 Clustering
 Smoothening
 Have basic idea of Fluid Mechanics
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Best Practices
 Best Practice is a misnomer
 It should be good practices
 Before touching computer know
 Problem statement
 Aim of the problem
 Accuracy required
 Computing power available
 No blind trust on the software
 Validation
 http://www.grc.nasa.gov/WWW/wind/valid/tutorial/glossary.html (a must read)
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Grid generation
Rules to be followed in grid generation
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Grid Generation Essentials
 Knowledge of Physics is essential
 Grid for pressure drop estimation is very different from heat flux estimation
 Accuracy required should be understood
 Preliminary design calculations or design improvement
 Very accurate may not be cost effective
 Will the grid be moving, if yes where
 Parameters that may be changed in design
 Good grid is half CFD done
 Bad grid is full CFD repeated
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Grid contd…
 What size of mesh is good mesh size
 For any new problem or new software grid convergence has to be repeated
 Recipe for failing
 This software is well established, it will work
 Physics may be different but this software works
 Automatic mesh will be good
 Grid read by solver so it must be OK
 Make a large mesh and coarsen it, till you get 2 meshes with reasonably identical
results
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Grid contd…
 Always smoothen a grid before doing any other operation
 A grid parallel and perpendicular to the flow is the best grid
 Clustering must never precede smoothening
 More clustering makes convergence slower
 Less clustering does not resolve flow features
 5-10 grid points in boundary layer based on Reynolds number
 Refinement of grid at sharp corners, bends and locations where shocks are expected
 Check – Skewness, smoothness and aspect ratio
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Grid contd…
 Coordination with solver
 Velocity regime
 Euler, Laminar or Turbulent flow
 Which turbulent model
 Proper labeling and boundary conditions
 What BCs are required
 Proper labels to various components
 Units must be specified when grid is made
 If not made in SI units, COMMUNICATE
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Simple problems
Problems requiring less accuracy and attention
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Problem Types
 Problem desiring less accuracy
 Pressure drop across regions
 Supersonic flow – lift estimation
 Preliminary design calculations
 Design of low cost mechanical equipment
 High rise buildings
 Ventilation of huge spaces
 These problems require only estimation of gross number
 Spatial resolution of physical properties may not be accurate, however gross
properties are reasonably accurate
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Case Study: High Rise
 Aim 1: Estimation of wind loads on a high rise building
 Aim 2: Estimation of discomfort due to air flow on balconies
 Data required
 Building drawings – usually given in 2D format
 Wind data – Collection of data of metrology dept
 Data on nearby buildings
 Comfort data
 Step 1: Selection of number of simulation
 Select severest wind conditions for three seasons
 Also select any other worst case scenario as seen in the metrology table
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CFD of Buildings
 Input is received in format as shown below
 Convert format to 3D using Revit® or other tools
 Remove small features like grill, ventilators, balcony designs etc
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Buildingisusually
representedinAutoCAD®
format

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Meshes
 Meshes and cleaned building CFD model
 Mesh very coarse even near wall
 Good mesh = billions of cells 
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Simulation Table
 Following Simulations selected from the Metrology data
 Data measured near Colaba by Metrology dept
 Site near the sea hence relative humidity is high
 Ground Boundary layer approximated in simulation
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North-WestWestNorth-WestEastWind Direction
43.264.825.227Wind Speed (Kmph)
70877570Relative Humidity
37.434.842.236.2Temperature (C)
NovemberJulyAprilJanuary

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Results
 Load on the whole building is integrated by adding individual forces on surface cells
 Major focus on vortices formed in the buildings
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Results
 Vortices near NW apartments uncomfortable
 Construct small structures to suppress them
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Accuracy Medium
Problems requiring care in making mesh and
simulation
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Problem Types
 Problem desiring higher accuracy
 Flow around automobiles
 Flow around aircraft and missiles
 Flow in process industry – cyclone separator, piping, ducts
 These problems require only estimation of properties at specific locations with
reasonable accuracy
 Grid generation requires attention
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Case Study: Flow Past Automobile
 Aim 1: Estimation of aerodynamic forces on an automobile
 Aim 2: Estimation of discomfort due to dust ingress
 Data required
 IGES file of the automobile
 Wind Conditions for the problem
 Step 1: Cleaning of CAD data
 Remove the components that are small in size
 Close the gaps of handle, doors
 Assumptions
 Tyre rotation effect is not modeled
 Dust as particles is not modeled
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CFD of Automobile
 Input is received in format as shown below
 Remove features that are small compared to the vehicle size like grill, door handle ,
headlight protrusions etc
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CADDrawingofanSUV
Withallfeatures
15000components

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Grid Generation
 CAD cleaned to remove surfaces not exposed to aerodynamics or insignificant
surfaces
 Surface divided into patches for generation of hex grid
 Clustering done to 0.1 mm to capture boundary layer
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Grid
 View showing clustered grid at the surface
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Grid density is
high near vehicle
surface

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Results
 Streamlines important; seeds on correct locations
 Dust ingress known by streamlines curving inside
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Path lines seen to
point inward

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Iso-surfaces
 Zero axial velocity on green surfaces
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Iso-surfaces shown
by Green

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Accuracy High
Problems to be attempted by experienced
personnel
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Problem Types
 Problem desiring highest accuracy
 Aerothermal considerations in high speed flow
 Heat driven flows
 Reactive flows
 Turbomachinery (rotating flows)
 These problems require estimation of properties accurately to serve the aim of
simulation
 Grid generation requires high degree of care as the simulations are very sensitive
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Heat Flux
 Aim 1: Estimation of heat flux of a missile
 Aim 2: Estimation of aerodynamic coefficients
 Data required
 IGES file of the missile
 Atmospheric Conditions for the problem
 Isothermal temperature of the wall
 Step 1: Cleaning of CAD data
 Remove the components that are small in size
 Close the gaps of missile and fins etc
 Assumptions
 Air remains calorifically perfect
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Grid Generation
 Surface grid chosen after extensive analysis
 Requirement of y+ is within bounds of 1-5 at all places
 Since the flow is very high speed, first cell distance has to be 1 micron from the
surface for the given mach number
 VALIDATION is a must before attempting these problems
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Results
 Variation of y+ on the surface
 Variation of heat flux on the surface
 * Heating more in wing than nose
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Mach Number
variation
High
Low
High heat flux regions
Heat Flux
Spalart-Allmaras Turbulence
Model, HLLC Scheme
y+ range 1-5 at all places

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Initial & Boundary Condition
Importance of putting conditions for correct results
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Initial Conditions
 It is a wrong assumption that whatever conditions you give flow will finally converge
to a correct solution
 Sometimes wrong initial condition may have a physical meaning
 Constant Initial Conditions throughout the domain may not work always
 Most obvious initial conditions may not be the fit for certain solvers
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Example
 Consider a case of supersonic CD nozzle with the three ICs
 Flow at zero velocity inside domain
 Flow supersonic everywhere
 Flow supersonic in converging section and gradually decreased to subsonic till the exit
 The representation physically
 Flow shock will enter inside and form a normal shock and subsequent flow will be subsonic
throughout
 Flow has been made supersonic everywhere by use for external machines
 Flow has been made supersonic and now exiting in atmosphere
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Boundary Conditions
 A pressure and velocity boundary is not the same as mass flow boundary for
incompressible flow
 Signal traveling backward may change mass flow rate
 Farfield and outlet are not the same for compressible flow
 Farfield means waves are not affected by flow
 Isothermal wall is significantly different than adiabatic, default expression is
adiabatic wall
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General
 Better to use standard format like CGNS for interoperability
 Research says lot of time wasted in converting file formats
 Aim not written at the beginning is a sure recipe for disaster
 Solution should be doable in reasonable cost
 Try to arrange a computer that can handle the problem instead of taking shortcuts
and solving on existing resource
 CDAC gives good access to computing power for academics
 Make a checklist of activities if same type of problem may come all the time
 Usually engineers make a mistake in routine simulations due to over confidence
 More engineers doesn’t mean faster results; reverse possible
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www.zeusnumerix.com
+91 72760 31511
Abhishek Jain
abhishek@zeusnumerix.com
Thank You !

CFD Lecture (7/8): Best Practices in CFD

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CFD Lecture (7/8): Best Practices in CFD