cfd application tutorials 1

Internal flow analysis

tutorial

CFD application tutorials

2

Problem description and analysis purpose

Problem Explanation Analysis Purpose Important Points

25℃ water inlet at 1m/s

Obstacle in the tube of

cylindrical shape

Tubular shape

Flow of 1m/s velocity of Water at 25℃

Existence of obstacle inside the cylindrical

tube

Understand the flow characteristics

inside a mechanical system.

Analyze the displacements and

stresses on the obstacle using

structural analysis

Setting of CFD Analysis options in NFX

Define material and properties

Mesh refinement method for obstacle part

Definition of Boundary Conditions for internal flow

Creation of Analysis Case (Transient CFD)

Methods to check and monitor results

Structural Semi-Coupled Analysis

3

Analysis

SettingsGeometry

Materials/

Properties

Boundary

ConditionsContacts Meshing Analysis Case Solver Results

Change the interface to the Analyst Mode

① Open midas NFX

② Select “Application->Analyst Mode”

CFD Analysis is always performed in

Analyst Mode

4

Check the Units

① Go in the tools>options② Go in the

General>units section and select: “N-m-J-sec”

③ Enter 9.8 m/sec for the acceleration of gravity

④ Click on Apply ②

③

①These are the best units to work in CFD as it is the basic unit of the material DB in NFX

Verify that the value defined is correct

④

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5

Check the Fluid Materials(Incompressible)

① Options>General>Material(CFD)

② Compressibility Solver Type : Incompressible.

③ Compressibility Type : Incompressible

④ Click on apply ② ③

④

Incompressible solver is almost always used, except when the material definition imposes to use compressive solver (natural convection and compressible flow).

Even when using compressible solver, the flow stays incompressible for flows with a Mach number inferior to 0.3

Analysis

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①

6

Geometry and Mesh options setup

① Geometry/Mesh/Connections> Mesh Set>Common > Seed Control>Use Adaptive Seed: True

② Use Geometry Proximity: True

③ Curve Sensitivity: Normal

④ Higher Order Elements: False

⑤ Tetra Mesh ControlAvoid Tetra with all boundary nodes: True

⑥ Apply

② ③

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⑤

①

⑥

When a small edge exists and is close from another small edge the relative distance between the two edges is calculated and the first edge is divided by two.

NFX-CFD is optimized for low order elements

This condition divides automatically the elements which have all their nodes on the boundary surface

Sensibility increased

Analysis

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Boundary


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

Off On

Off On

7

Select the number of processors and the element formulation

① Analysis/Results Tab> Analysis Control Tree

② Number of cores: Enter the number of CPU cores in your computer

Element Formulation:Standard (Stability)

In CFD Analysis, the Standard element

formulation is used to get more stability in the

solution

②

③

Analysis

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①

8

New Project

① Click on New

② Select “3D”

③ Unit System: N-m-J-sec

④ Click OK

①

②

③

N-m-J-sec is the best unit system for CFD analysis.

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9

Import Geometry

① Geometry > Import

② Select “Parasolid” CAD file type

③ Open the folder of the CAD models

④ Import the model “application tutorial 1.x_t”

*If CFD Tutorial Models are not available, please send an Email to [email protected]

①

②

③④

In NFX 2014 R2, the tutorial models can be found in the

installation folder of the software on your computer

“C:\Program Files\midas NFX 2014\Manual

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10

Import the Geometry

① Check the Geometry

※ Inspect the geometry shape by rotating the model with the mouse

② Right-click > Hide the Guiders①

②

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11

Create the fluid domain from the structural part

① Click on “Make Face”

① Select the edges at the inlet of the tube.

② Click on Apply

③ Do the same for the outlet

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②

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The planes created will be used to delimit the fluid volume that will be

generated.

④

12


① “Others” button > Make CFD Volume Extraction

② Choose the target solid

③ Enter 0.5 for the Width X, the Width Y and the Height

④ Select the Closing Faces

⑤ Uncheck “Delete Target”

⑥ Click on OK

①

②

③

④⑤

⑥

In order to create the CFD volume around the part, the part should be entirely contained in a box with the dimensions defined by the X,Y Width and the Height. Measurement Tool can be used to verify the dimension of the model

If this option is checked, the original structural part will be deleted

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13


① Model > Geometry > Geometry Set-1

② Face> Hide the 2 faces

③ Solid> Hide the pipe & Hide the CFD Volume Extraction (External)

③

①②

[Important] During boundary conditions creation, edges and faces of the fluid solid part only should be selected, so it is better to hide all other faces/parts to avoid wrong assignment of Boundary conditions

Inactivate all the parts except the internal flow part that is studied in this tutorial

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14

Define Fluid Material

① CFD > Material

② Add/ Modify Material > Create (click on the button on the right)> Fluid (CFD)

①

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②

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

15

Define Fluid Material

① Material Database > Select “FRESH_WATER_25’C”

② Click on Apply

③ Click on Close

④ Verify that the material have been added in the Tree Menu

①

②

③By choosing the material in the material database, the density and viscosity will be defined automatically

④

By clicking on the material in the work tree, material properties assigned can be viewed in the property window

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16

Define Properties

① Click on Properties

② Add/Modify properties> Create (Arrow button)> Click on “ 3D...” ①

②

Analysis

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

17

⑤

Define properties

⑤

① CFD 3D Tab

② Material : Select “1: FRESH_WATER_25’C”

③ Click on OK

④ Click on Close

⑤ Check the property in the tree menu

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②

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Material attached to each property can be viewed in the property window

18

Boundary conditions and referential

① Activate the GCS (General Coordinate System)

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19

Define inflow boundary conditions: Inlet

① Click Inlet button

② Object > Type > Select “Face”

③ Select the inflow face

④ “Velocity” > “V” : Enter “1”

⑤ “CFD boundary conditions” > Enter “Inlet”

⑥ Click OK

⑦ Check that the Inlet have been defined correctly in “LBC” > “Inlet”

⑧ Verify the Inlet properties in the properties Window

①

②③

④⑤

⑥

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In NFX-CFD, boundary conditions can be assigned to the mesh surface or to the geometry directly.

The velocity of the fluid is set as 1 m/s at the inlet of the pipe

The name of the CFD boundary set is not important but it is useful to define it if several cases are considered in the analysis. The name will also permit to identify more easily the corresponding boundary condition.

⑧

All the properties of the boundary conditions can be viewed in the tree menu

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20

Define outflow boundary condition: outlet

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②③

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① Click Outlet button

② Object > Type > Select “Face”

③ Select the outflow face

④ “Pressure” > “P” : Enter “0”

⑤ “CFD boundary conditions” > Enter “Outlet”

⑥ Click OK

⑦ Check that the Outlet have been defined correctly in “LBC” > “outlet”

⑧ Window Verify the Inlet properties in the properties

Outflow is at atmospheric pressure so “0 Pa” is defined.

⑥

21

Define wall boundary conditions

① Geometry> right-click> Display mode > Select “line only”

② Click on “Wall”

③ Target Geometry> Type> Select “Face”

④ Select all the faces of the fluid in contact with the geometry

⑤ Wall > Wall Type: Select “No Slip”

⑥ CFD boundary Set: Enter “Wall”

⑦ Click OK ①

②

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④

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⑥

Select icon menu “CFD”->Wall

Click right bottom of mouse to display the menu then you can change display mode to

“Line only”

Select wall as boundary condition , means near the wall velocity is 0 m/s

Select all face of fluid model besides inlet and outlet face

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Contact Condition definition: None

Because this tutorial only focus in single

fluid model analysis, it doesn’t require to

setup contacts

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23

Mesh Generation– Size control definition

① Mesh Menu> Size Control

② Select the edges of the obstacle

③ Interval Length:: 0.003

④ Click on Preview

⑤ Click OK

①

②

③

④

The fluid model includes some small faces around the cylinder so we must define small mesh size on it.NFX provide “Size Ctrl.” to define the small faces to let the user define small mesh size quickly

In this area, the fluid momentum will change drastically, this is why we need to define finer mesh in this area.

Analysis

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⑤

You can select to preview the mesh size (node distribution on the edges of the cylinder)

24

Mesh Generation– Size control definition

① Click on 3D button

② Select the fluid geometry

③ Size Method > Size: Enter 0.0122

④ Property: Select “1:3D Property”

⑤ Click “>>”

⑥ No select “Higher-Order Element

⑦ Click “ok”

⑧ Click “ok”

①

②③

④

⑤

You can select “3D” to create fluid mesh model.

The property defined previously is assigned to the mesh set

⑥

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⑦

⑧

“G” represents the number of geometric parts, “N” represents the number of nodes and “E” represents the number of elements.

25

Mesh Generation– Verify the Quality

①① Check> Check Mesh Quality

② Dialogue box> Skew Angle: Off> Warpage : Off

③ Click on Apply

④ Check the highest value in the Output window

③

Element aspect ratio checking threshold value can be chosen ；default aspect ratio is 8 ; if < 8 , the analysis convergence will be better so it is always better to check element quality before going further

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Aspect ratio represent the ratio of the longest mesh

edge over the smallest mesh edge in the model. When

this ratio is too large, it can cause convergence problems

during the analysis.

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④

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26

Create the Analysis Case

①

④

① CFD Menu Tab> Transient CFD

② “Add/Modify Analysis Case”>Analysis Case Settings > Title: Enter “Analysis Case 1”

③ Click on “>>” Button

④ Click on Analysis Control

①

②

③ ④

The Analysis Case regroups all the conditions of the analysis defined previously.The “Transient” CFD Analysis is used when results in function of time are required.“Steady” State Analysis is used when only the last result at the steady state is important. Another difference is that it is required to define the time increment for the transient analysis, whereas for steady state analysis, the increment input can be automatically changed by the solver.

Analysis

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In the “All sets” work tree on the left appear all the mesh sets, CFD boundary conditions and contacts that have been defined in the analysis model. By pressing the “>>” button, all these mesh sets, BCs and contacts will be assigned to the current analysis case and activated. The active mesh sets appear in the “Active Part Sets” tree menu and the active boundary conditions and contacts appear in the “CFD Analysis Settings” Tree Menu. These conditions and mesh sets can be activated or inactivated by simple mouse drag and drop.

Drag by mouse

27

Define the Analysis Case– Analysis Control

①① Time : “0.0002”

② Time increment: Enter “100”

③ Intermediate Output Request > Enter: 5

④ Click Field Definition

In the Analysis Control Window are defined all the general parameters of the analysis.

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

①②

③

④

It defines the time duration of one analysis step. If the time increment is too high, the convergence rate will decrease and if the time increment is too short, the analysis will take too much time.

Recommended value for the time increment = 0.01 × representative model length ÷ velocity

In the present analysis, the representative model length can be chosen as 0.02m, the

length of the obstacle

The number of steps with a time duration equal to the previously defined time increment.

Total Analysis Duration = “time increment” ×“Number of steps”

After defining a sufficient number of time steps and ran the analysis (see the following page), the analysis can be stopped and the results can be checked. If the Convergence criteria is not met during the analysis, the analysis can be started again from the last increment using the “restart” function.

Intermediate Output request > “Start Step” represents the first step for which the results will be registered, while “Interval Step” represents the interval on which results will be output.

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Define Analysis Case– Analysis Control: Field Definition

①① Field Definition> Turbulence> Eddy Kinetic Energy: Enter “0.00375”

② Field Definition> Turbulence> Eddy Length Scale: Enter “0.0084”

③ Click OK

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

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

To calculate accurate values of the turbulence, the eddy kinetic energy and eddy length scale need to be defined according to the equation below:

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

<turbulence intensity level>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 length × 0.07External flow length scale = 10×viscosity÷(density×[eddy kinetic energy]1/2)

①②

③

※the representative length of the pipe can

be chosen as its diameter

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29

Define Analysis Case– Analysis Control: Turbulence definition

①① Click the “Module Data” Tab

② Turbulence model: Select “2equation k-ε”

③ Click OK④ Click OK⑤ Check that the analysis case 1

has been added in the “Analysis & Results Work tree)

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NFX-CFD is optimized for the “2-Equation k-ε” turbulence Model .

30

Perform Calculation– Define Monitoring nodes to assess the convergence

①① Click on Result Monitoring

② Select a node on the inlet face

③ Pressure Checkbox: On

④ Click Apply

①

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③

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The velocity is fixed at 1 m/s at the inlet so let’s investigate and monitor the pressure instead

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

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

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①① Select a node on the outlet face

② Pressure checkbox: OffTotal Velocity checkbox: On

③ Click Apply

At the outlet, the pressure is fixed at 0, so the Total velocity can be monitored instead.

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①① Select a point on the wall boundary condition

② Total Velocity checkbox: OffPressure checkbox: On

③ Click Apply ①②

③The flow is changing suddenly its direction at this position, so it is useful to monitor the Pressure at this point.

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33

Perform calculation – Save the file

①① Main Menu> Save As…

② File Name: “CFD application tutorial1.nfx”

③ Click Save ①

②

③

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34

Perform calculation – Perform Analysis Case

①① Analysis & Results> Analysis Case>

“Analysis Case1”: Mouse right-click> Solve

If several Analysis are present, keep [Ctrl] pressed while selecting will allow to select several subcases at the same time.

①

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35

Perform Calculation– Calculation process and convergence assessment

①① Select CFD Graph

② Select “CFD Norm Graph”

③ Velocity and nom graph 0.001 (Norm =VEL(3.715E-004)

④ If the value reach 0.001 will be a “CONVERGED” solution

①

②

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

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

2. When the monitored value in the area of interest stays stable and doesn’t undergo very large variation (can be checked using monitoring or by stopping the analysis and verifying the results).

④

The Norm graph is displaying the last value of the norm which is calculated numerically at each step. The default value of the norm defining the convergence is 0.001, so if the curve goes below, it means that the analysis is converging and that the results can be displayed.

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36

Observation Monitoring nodes –velocity curve result

①

②

① Check Monitoring nodes to access stable value

② Velocity keep at about 1.18 m/sec

①

This graph helps to monitor the value at a specific position of the model during the analysis to check if the value in the area of interest stays stable and doesn’t undergo very large variation (2e condition of convergence)

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37

Check the model results during the analysis

① Right click on the analysis case

② Select “Open result file”③ Select “CFD Result Files

(*.rst)” as the file type to be opened

④ Select the result file of your current analysis

⑤ Click on OK

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When the analysis cannot be stopped, there is a method to check the results without stopping the analysis by opening the result file on the current analysis

38

※ Restart previous .rst file to restart

①① Analysis Case1 : Transient CFD

② Mouse right-click ,select “Edit”

③ Select “Analysis Control”

④ Check “Restart”

⑤ Select the path of the restart file of your analysis (same folder as the nfx file)

⑥ Select “ok”

①

②

④⑤

. When the analysis is restarted, it will continue to run for the number of

steps defined

Restart is generally used when some supplementary steps are required to obtain the convergence of the analysis. Analysis which is restarted starts back from the last step where it was stopped.

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③

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39

Observe the flow path result of the fluid model

①① Select “Result”

② Select “Fluid->Flow Path”

③ Select “Transient CFD(Required) CFD”

④ Select random node(you can select several nodes to view the flow path)

⑤ Select ”Apply”

① ②

③④

⑤

By clicking on the nodes, the flow line will be calculated

starting from these flow lines

If the first time step which didn’t

converge is selected here,

the flow line will not be visible

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40

Observe Flow Quantity on section plane

① Select “Fluid->Flow Quantity

② Select “Transient CFD(Required) CFD”(the last step)

③ “Select 1 part(s)”

④ Select “<<“

⑤ Input z text box “0.3” m

⑥ Click “Add”

⑦ Click “Close”

⑧ Show “Plane1” in Plane List

⑨ Click “Plot”

⑩ Click “Close”

The Flux unit ism3/s.

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At the beginning, all the 5 geometrical parts present in the analysis are selected, whereas we

need to select only the fluid volume part which is actually studied. For that, we can double click on the button to reset the selection and select again

the correct fluid volume part.

①

②

③

④

⑤

⑥ ⑦

The position of the cutting plan can be defined on the screen by a drag of

the mouse

⑧

⑨Appropriate step

should be defined

41

Preview flow force data on wall

①① Select “Fluid->Flow Force on wall and Conform “Wall”

② Click “Apply”

③ Click “Cancel”

④ Click “x” to close

①

② ③

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1) P-Force caused by the pressure, SP-Force caused by Hydrostatic pressure, V-Force caused by viscosity, T-Total force caused by these 3 force together

2) F-Force, M-Moment

3) x, y, z : Coordinate Axis

P F x

1) 2) 3)

The Element called “Wall” here is the name of the CFD boundary Set defined previously in the analysis. If the force on a specific surface have to be retrieved, the wall condition defined on this surface have to be defined separately from the other model wall surfaces by defining it as a new CFD Boundary Set with a different name. The new boundary set will then be listed separately in the “Fluid force on wall” Window and it will become possible to investigate the force value on this surface.

④

42

Use previous CFD pressure result to import structure pressure (FSI analysis)

①① Select “Result->Extract”

② Select Result “PRESSURE”

③ Click “Unselect All”

④ Select last step “Transient CFD(Required):CFD:INCR=0021”

⑤ Select “Node”

⑥ Use “Face” to filter Nodes on cylinder face of hole

⑦ Show nodes on screen

⑧ Click “Table” to show pressure values on table

⑨ Click “Close”

①

②

④

⑤

⑥

⑦

⑧ ⑨

The pressure on the obstacle is selected to be extracted and applying on the structural model later on.

As it is difficult to selected all the nodes one by one, the selection can be done easily by switching the selection mode to “surface” and selecting the geometrical faces around the obstacle (fluid geometric part should be activated).

The output results are consisting of the node number, the coordinates X,Y,Z of the nodes, along with the value of pressure at this node.

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43

Open previous fsi.nfx file to preview boundary condition of structure

①① Click “Open” file

② Select file name “CFD application tutorial1-FSI”

③ Show the boundary condition (fix top and bottom face) ②

The model file “CFD application tutorial1-FSI.nfx”

has been created and defined already as a the structural

simulation of the pipe model. Material, property, Mesh, BCs and analysis case have been

already defined to simplify the explanation process and show

directly how to import the pressure load from CFD results

on the structural model.

※ NFX structural analysis knowledge is required

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③

44

① Select “Static/Heat Analysis”

② Click “From Results”

③ Select “Interpolation

④ Select Result Type “Normal Pressure(Scalar)

⑤ Type select “3D Element Face

⑥ Use “Face” to filter Nodes

⑦ Select arc face of cylinder (total is 4 faces)

⑧ Use pressure values on table in page 42, only select column of X and Y and Z and Transient CFD(Required):CFD:INCR=0021

⑨ Enter “Pressure” in the Load set name

⑩ Click on “OK”

①②

③④

⑤

⑥

⑦⑧

⑨

⑩

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⑧

Use previous CFD pressure result to import structure pressure (FSI analysis)

Copy-paste

Data of the previous CFD Analysis (p 42)

45

Add linear static analysis and include CFD pressure

①① Click “Linear Static-1 :Linear Static”

② Click right bottom of mouse and select “Edit”

③ Click “>>”

④ Click “OK” ①

②

③

④

All the boundary conditions created after the generation of the analysis case have to be activated in the analysis case

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46

Use CFD result get stress and deformation contour plot

①① Select “Linear Static-1: Linear static“and right-click to select “Solve”

② Select “TOTAL TRANSLATION” to observe deformation results

③ Select “SOLID STRS VON MISES” to observe stress results

①

The analysis of the structural results is not detailed in this

tutorial

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② ③

cfd application tutorials 1

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