CFD Application Tutorials 2

7/21/2019 CFD Application Tutorials 2

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Basic example of

exterior flow analysis

CFD application tutorials

This tutorial requires knowledge from the previous internal flow analysistutorial


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Problem description and analysis purpose

Problem Explanation Analysis Purpose Important points

Solar Panel Status Investigation Fixed on the ground

Wind at 10m/s velocity is

applied on all the surface of the

panel

Investigate the flow around anobject

Atmospheric pressure boundaryconditions application method

CFD analysis in steady state

10 m/s wind velocity

Fix area


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Analysis

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Materials/

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ConditionsContacts Meshing Analysis Case Solver Results

Change the interface to the Analyst Mode

Open midas NFX

Select Application->Analyst Mode

CFD Analysis isalways performed in

Analyst Mode


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Check the Units

Go in the tools>options Go in the

General>units sectionand select: N-m-J-sec

Enter 9.8 m/sec for theacceleration of gravity

Click on Apply

These are the bestunits to work in CFDas it is the basicunit of the materialDB in NFX

Verify that the valuedefined is correct

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Check the Fluid Materials(Incompressible)

Options>General>Material(CFD)

Compressibility Solver Type :Incompressible.

Compressibility Type :Incompressible

Click on apply

Incompressible solver is almostalways used, except when thematerial definition imposes touse compressive solver (naturalconvection and compressibleflow).

Even when using compressiblesolver, the flow staysincompressible for flows with aMach number inferior to 0.3

Analysis

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Geometry and Mesh options setup

Geometry/Mesh/Connections> Mesh Set>Common > SeedControl>Use Adaptive Seed: True

Use Geometry Proximity: True

Curve Sensitivity: Normal

Higher Order Elements: False

Tetra Mesh ControlAvoid Tetra with all boundarynodes: True

Apply

When a small edge exists and is close fromanother small edge the relative distancebetween the two edges is calculated and thefirst edge is divided by two.

NFX-CFD is optimized forlow order elements

This condition dividesautomatically the elementswhich have all their nodeson the boundary surface

Sensibility increased

Analysis

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When a small edge exists or when an edge is smallerthan the meshing seed, this feature able the mesherto mesh a second time using an automatic lineargrading size control.

Off On

Off On


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Select the number of processors and the element formulation

Analysis/Results Tab> Analysis Control Tree

Number of cores:Enter the number of CPUcores in your computer

Element Formulation:

Standard (Stability)

In CFD Analysis, theStandard element

formulation is used toget more stability in the

solution

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Import Geometry

Geometry > Import

Select Parasolid CAD filetype

Open the folder of the CADmodels

Import the modelapplication tutorial 2.x_t

*If CFD Tutorial Models are notavailable, please send anEmail [email protected]

In NFX 2014 R2, the tutorialmodels can be found in the

installation folder of thesoftware on your computer

C:\Program Files\midas NFX2014\Manual

Analysis

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Hide All Guiders

Show the model on thescreen.

Click right bottom of mouseand select Hide All Guiders

Analysis

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Create the fluid model use the Box

Select Geometry

Select Box

Set box size :Origin PT.[OP] is -2,-2,0Width X[WX] is 4Width Y[WY] is 6

Height[H] is 3.5

Select Preview icon

Click OK

Select the box part andright-click with the mousethen select Display Mode -

> Line Only

This box willrepresent theexternal fluidvolume around themodel

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Create the fluid model use the Boolean cut

Select Boolean -> Solid

Select Cut

Click Select Target Object to select the box model

Click Select Tool Object(s)

to select the Solar Panelmodel

Unselect Delete Tool

Click OK

Unselect the SolarPanelpart to check the cutted

box model.

The solid part inside thebow have to be cut fromthe box part to createthe external fluid part

The inner part can beselected by creating a

drag window with themouse. Part can also beselected directly from

the work tree (pressingthe keyboard [] or []is useful to change theselected part quickly)

The Delete Tool commanddetermine if the Tool partshould be deleted after

performing the geometryoperation. In this tutorial, we

dont need it for thesimulation, but it can be

conserved for postprocessing purpose.

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Define Fluid Material

CFD > Material

Add/ Modify Material >Create (click on the buttonon the right)> Fluid (CFD)

Analysis

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This is the window in which materials used in the presentanalysis are defined. All constants of material that arerequired in CFD analysis (density, viscosity, conductivity,specific heat) are defined here.


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Select AIR 25

Click OK

Click Close

By choosing thematerial in thematerial database,the density andviscosity will bedefined automatically

Define Fluid MaterialAnalysis

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Define Properties

Click on Properties

Add/Modify properties> Create (Arrow button)> Click on 3D...

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During the Mesh creation phase, the properties assigned to themesh will have to be defined as well. This property will bring tothe mesh the assigned material information.Properties gather together material information, porous materialusage and properties, MRF (Multi-reference Frame) applicationArea definition, etc..


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Define properties

CFD 3D Tab

Material : Select 2:AIR_25C

Click on OK

Click on Close

Analysis

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*porous media and MRF Analysis will be available from NFX 2014R2 (end of may 2014)


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Click Inlet

Type select Face

Select Object : select face infront of box model

Input V : 10 m/sec

CFD BC Set : Inlet

Click OK

Inlet condition corresponding to awind velocity of 10 m/s is applied

on the front face.

The name of the CFDboundary set is notimportant but it isuseful to define it ifseveral cases areconsidered in theanalysis.The name will alsopermit to identify moreeasily the correspondingboundary condition.

In NFX-CFD, boundary conditionscan be assigned to the meshsurface or to the geometrydirectly.

Define outflow boundary conditions: InletAnalysis

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Select Outlet

Type change to Face

Select Objetct(s) : select facein back of box model

Input Pressure value is 0

CFD BC Set : Outlet

Click OK

Outflow is at atmospheric pressureso 0 Pa is defined.

When analysis is conducted usinguncompressible fluid model and thereal value of the pressure at theboundary condition is calculated,some differences with the supposed0 value can appear.

Define outflow boundary conditions: OutletAnalysis

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Click Wall

Type change to Face

Select Object : select bottomface of box model and allfaces of SolarPanel model

total faces are 11(11Object(s))

Wall type change to No Slip

CFD BC Set : Wall

Click OK

CFD Analysis is the analysis of liquid or gas flow, thus solidparts is not directly considered in the analysis and wallcondition should be used on the faces which are in contact withsolid parts.

Define outflow boundary conditions: WallAnalysis

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Click Velocity

Type change to Face

Select top face of box model

Component Vx : off

Select Vz and Velocity : 0m/sec

CFD BC Set : top face ofenvironment

Click OK

If the value is unchecked, the Vx velocity will be calculatedautomatically according to the previous step value.

Only the Vz coordinate (according to the Global Coordinate System)will be defined constant equal to 0 m/sec

In addition to inlet, outlet and wall conditions, velocity or pressureconditions should be applied on external model faces in contactwith air. These conditions represent the fact that the air around isalmost infinitely large. This condition can de defined by applying anormal velocity 0 condition to the face at the boundary with theatmosphere.

Define outflow boundary conditions: WallAnalysis

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Click Velocity

Type change to Face

Select Object(s) : select 2 sidefaces on the box model

Vx : on andVelcity input 0m/sec

CFD BC Set : side face ofenvironment

Click OK

In addition to inlet, outlet and wall conditions, velocity or pressureconditions should be applied on external model faces in contact withair. These conditions represent the fact that the air around is almostinfinitely large. This condition can de defined by applying a normalvelocity 0 condition to the face at the boundary with theatmosphere.

Define outflow boundary conditions: VelocityAnalysis

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Contact Condition definition: None

Because this tutorial only focus on single

fluid model analysis so we dont need to

setup contact.

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Contact Condition definition: NoneAnalysis

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Select Size Ctri.

Select Objetct(s) : select allfaces of SolarPanel modeltotal face is 24 Object(s)

Mesh Size : 0.05 m

Click Preview icon to shownode distribution

Click OK

To mesh the model, a certain mesh size is required.Nevertheless, we may require better accuracy on certainparts of the model which are relatively small or complex. Tobe able to do that, the size control allow to select somespecific edges and assign a certain defined mesh size on itcalled also a seed. This seed will be used later on to getrefined mesh on the seeded part.

The preview option helps to see the mesh seed that willbe generated before the application. It is useful to check ifthe mesh size is appropriate or not on the considered area.

In this area, the fluid momentumwill change drastically, this is whywe need to define finer mesh inthis area.

Mesh Generation Size control definitionAnalysis

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Click 3D

Select Object(s) : Select thebox modeltotal is 1 Object(s)

Size Method is 0.2

Property select 1:3D Property

Click OK

Lets create the meshelements required toperform the CFD analysis.

The propertydefinedpreviously isused here

Mesh Generation Size control definitionAnalysis

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In the All sets work tree on the left appear all the meshsets, CFD boundary conditions and contacts that havebeen defined in the analysis model. By pressing the >>button, all these mesh sets, BCs and contacts will beassigned to the current analysis case and activated. Theactive mesh sets appear in the Active Part Sets treemenu and the active boundary conditions and contactsappear in the CFD Analysis Settings Tree Menu. Theseconditions and mesh sets can be activated or inactivatedby simple mouse drag and drop.

Define CFD analysis case

Click Steady

Title name input CFDapplication tutorial2

Click >>

Click Analysis Control

The Analysis Caseregroups all theconditions of theanalysis definedpreviously.The Transient CFDAnalysis is used whenresults in function oftime are required.Steady StateAnalysis is used when

only the last result atthe steady state isimportant. Anotherdifference is that it isrequired to definethe time incrementfor the transientanalysis, whereas forsteady state analysis,the increment inputcan be automaticallychanged by thesolver

Analysis

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Drag and Drop


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Time Increment : 1 sec

Number of Steps : 1000

Intermediate OutputRequest : Interval : 10 Step

Click Field Definition...

In the Analysis Control Window are definedall the general parameters of the analysis.

Ex) Module used, Time information, Symmetryconditions, Initial conditions, turbulence, etc.

In the previous tutorial, the time increment wasimportant as it represented the real duration of a timestep in transient analysis. In Steady State analysis, it isa bit different, as the time increment doesnt represent

the real time duration of a time step but simply aparameter used by the solver to compute the finalsteady state value. If the time increment is too large,the solver will automatically decrease it. This value isset here in case the user wants to set manually a timestep smaller than the time increment used by thesolver.

It defines the number of times thesolution will be calculated using the

defined time increment.

Calculation time= time increment number oftime step

After entering a large enough number, the calculationcan be launched and depending on the convergencestatus (see next page) the calculation can be stoppedto check the results. If there is no convergence afterthe number of steps defined, number of steps can beincreased and calculation repeated.

Results will be outputevery 10 steps bydefining this intermediateoutput request.

Define CFD analysis caseAnalysis

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Input Eddy Kinetic Energy :0.00135 m2/sec2

Eddy Length Scale : 0.0034

Click OK

In CFD Analysis, the result of the previous step isused to calculate the next step. This is why the initial

value is very important. This initial value can bedefined in this field definition window.

To calculate accurate values of the turbulence, the eddykinetic energy and eddy length scale need to bedefined according to the equation below:

Eddy Kinetic Energy = 1.5*(Velocity*TurbulenceIntensity Level)^2

Planes,Cars, Submarine : 0.003 (Under 0.01)Atmosphere : 0.3Internal flow, Heat exchanger, Rotative machinery : 0.05~0.15Pipe,exhast chimney, low reynolds (Simple model) : 0.01~0.05

Pipe eddy length scale= representative model length0.07External flow length scale =10viscosity(density[eddy kinetic energy]1/2)

Define Analysis Case Analysis Control: Field Definition

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Click Module Data

Turbulence Model select 2-Equation k-e

Click OK

Click OK

Under Analysis Case willshow CFD applicationtutorial2 : Steady State CFD

NFX-CFD is optimizedfor the 2-Equation k-turbulence Model .

Analysis

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Define Analysis Case : Turburlence Model Definition


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Click Result Monitoring

Select Object(s) : Select thenode on box mesh modeltotal is 1 Object(s) (Inlet face)

Pressure : On

Click OK

This monitoring options gives the possibility to check the value atsome specific node during the analysis. The purpose of thismonitoring is to verify that the 2 following conditions are verified:

1. Check the value at some specific node when theconvergence norm is greater than 0.0012. Verify that there is no abrupt change in the area of interest

The velocity is fixedat 10 m/s at the inletso lets investigateand monitor thepressure instead

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Perform Calculation Define Monitoring nodes to assess the convergence


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Select Object(s) : Select thenode on box mesh modeltotal is 1 Object(s) (Outletface)

Total Velocity : OnPressure : Off

Click OK

At the outlet, thepressure is fixed at 0, sothe Total velocity can bemonitored instead.

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Perform Calculation Define Monitoring nodes to assess the convergence


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Click Save As...

File name input CFDapplication tutorial2.nfx

Click Save

Perform calculation Save the fileAnalysis

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Select CFD applicationtutorial2 then click rightbottom of mouse to selectsolve

If several Analysis are present,keep [Ctrl] pressed whileselecting will allow to selectseveral subcases at the sametime.

Perform calculation Perform Analysis CaseAnalysis

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Calculated process to review and determine the convergence

We can observe the Pressureand velocity norm curveswhich tend towards a valuesmaller than 0.001(Convergence). The solverwill stop at the step number1000.

We can observe the TOTALVELOCITY curve at the outletposition which tendstowards a stable constantvalue (Value is about 6.8m/sec)

We can observe that the 3curved reached a stablestatus within 400 steps(CONVERGENCE reached)

The norm to evaluate that the analysis is converging and the results arecorrect is:

1. When the norm graph is decreasing under the value 0.001 and staysbelow this value (can be checked through the norm graph)

2. When the monitored value in the area of interest stays stable anddoesnt undergo very large variation (can be checked using monitoringor by stopping the analysis and verifying the results)..

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Observe PRESSURE result oflast step

Observe Total Velocity resultof last step

CFD result : preview Pressure and Velocity contour plot

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Documents

CFD Application Tutorials 2