Motion vs Rotating Region

Analysis Setup

The CAD Model

Simplification

Inlet and Outlet

Adding the Rotating Region

Boundary Conditions

Materials

Rotating Region

Meshing

Capturing the Blade Profiles

The Rotating Region to Surrounding Fluid Interface

Solve

Time Step Calculation

Turbulence Models

Intermediate Time Steps

Turbulence Models

Looping Back To Materials and Boundary Conditions

Monitor Points

During/Post Analysis

Convergence Assessment

Pump API – Automated pump curve creation

Results

Topics (Agenda)

Motion Disadvantages

Running with motion disables the boundary layer mesh, which is not ideal

Turbulence model limitation

Motion requires more mesh to capture the wall profile – because of mesh masking (overlapping mesh)

Longer runtimes

Lower accuracy with motion

Again due to mesh – poorly capturing blade profile

Rotating Region Disadvantage

Impeller must be symmetrical – no single blade pumps

Motion vs Rotating Region

Rotating Region needs to be drawn in CAD

Captures the rotation of the moving solid and the fluid in close proximity to it

Rotating Region should envelop the impeller and a small amount of fluid

We set the motion properties to the Rotating Region and can capture these three scenarios

Rotational speed

Driving torque

Flow-driven (free spinning)

CFD outputs the torque values within the Rotating Region Results and as a .csv file

Motion solver is a requirement

Rotating Region

Impeller and Volute

We can usually remove any shaft attached to the impeller

The CAD Model - Simplification

The CAD Model -

Production Simulation

Add Inlet – 5x diameter in length

Add Outlet – 10x diameter in length

Utilising caps in this manner means that CFD will

create the internal fluid for us when we launch from

CAD

The CAD Model – Inlet and Outlet

Create a Rotating Region (RR)

In a real world model, the RR

should sit exactly halfway

between the impeller and the

volute wall, at the smallest gap

The exception here is if the volute

is very large compared to the

impeller, then we simply make the

RR a little larger than the impeller

Including approximately 2

elements outside the

impeller

In our simplified model, the

impeller is touching the walls and

so the RR will also

Generally the case when

we have very small gaps

and do not need to capture

the leakage paths

From WIKI:

This is best practice although

The CAD Model – Rotating Region

For an open condition, apply a P = 0 at the inlet and

outlet

For a different point on the pump curve, apply a non-zero

pressure or flow rate but assign it transiently – it must

ramp up over the first 50 iterations along with the Rotating

Region

• Can generate reverse flow at the beginning of the

simulation

• Will occur until impeller builds enough kinetic energy

to overcome

• More stable if we ramp the condition over time

• Solves faster with a flow rate at the outlet rather than

a pressure

Actual ramp up time is generated later after the time step

calculation in the solve window

Boundary Conditions

P = 0psi

P

t

P = 100psi

Name the Rotating Region with the correct

RPM and blade number

Take a guess at the time in the table for now

RPM vaue is critical to allow CFD to

calculate the correct time step size

Materials

Unless we need to consider heat transfer,

suppress the impeller and all solids

Add the Leading and Trailing edges to their

own groups (for quick selection later)

Uniform mesh and refine each

Meshing - Impeller

It is also good practice to add mesh

refinement to the tongue of the volute where

flow is most likely to separate

An additional refinement region around this

area of the volute is also useful

Meshing - Volute

As the final stage (always the last stage) – add

a uniform mesh to the inlet and outlet surfaces

surfaces of the Rotating Region

Can then be coarsened if necessary

In some situations, it is well worth considering a

manual mesh – uniform by default

Meshing – Rotation Region

To suit the SST k-omega turbulence model

(applied later), we should increase the number

of boundary layer elements

Meshing – Mesh Enhancement

In the past we have run a pump for two stages

1. Ramp up to full speed over 50 iterations with a blade to

blade timestep and a further 10-20 revolutions

2. Run for a further 3 revolutions with a small timestep,

typically 3 deg/step

From CFD 2015 onwards, simply start the run with

3deg/timestep and run to completion

Use the Time Step Size Calculator

This uses the RPM within the Rotating Region

material

The time step here is therefore 0.0001389s

It is useful to turn on Stream Function within Result

Quantities at this stage, so that we can plot Nodal Aspect

Ratio (refer to Meshing Troubleshooting Hangout)

Ideally this should be below 100 within the Rotating

Region

Utilise Advection Scheme 5

As a cautionary note we might occasionally need to

use a smaller timestep than one that rotates only

3deg Keep an eye on the results, if they look like the Rotating

Region is acting like it is a flow obstruction this is likely the

case

A good rule of thumb is to move the RR’s circumference by

no more than 4 elements at a time

Solver Settings

For rotating machinery SST k-omega is

typically the most reliable turbulence model

Proven to produce the most

accurate solutions

To best capture flow separation

5 -10 layers of mesh enhancement

(ref Slide 17)

Start with 5

Save Intervals

Mainly used to create an animation

Memory intensive. Save no more than 20-

30 steps for a full analysis

Often better to utilise Monitor Points to

capture what happens in real-time

To create an animation, once the simulation

is complete, run a further revolution

capturing intervals

Be sure to set the value small

enough so that the pump both

appears to move and also appears

to rotate in the correct direction

Solver Settings Continued

Now we know the time step we are using,

we can re-visit the Rotating Region and

Boundary Condition setups in order to have

it ramp up to their full value over 50

iterations. A value from experience we have

found to be quick and stable

The time step used is 0.0001389, so we

multiply by 50 and enter this in the table

We also need to enter an additional line to

ensure the Rotating Region and Boundary

Condition will continue as we continue the

solution

Materials and Boundary Conditions

A useful method of monitoring the outlet

pressure/flow rate during the analysis is via

a Monitor Point.

To add one, simply right click during any of

the setup stages.

These can be viewed during the run within

the convergence plot

Change the plot from Global to the

desired Monitor Point (multiples can

be assigned)

Choose the variable you wish to

monitor

This is a memory free method and can also

be plotted after the run is complete. Unless

an animation is needed, this is a wise

approach, rather than saving out Save

Intervals.

Monitor Points

The convergence plot from a Rotating Region

analysis will look something like this

Global results

It is clear that CFD will not automatically stop at a

converged solution

Utilise the Monitor Point(s) to track the

convergence of a variable at a point

Check the Rotating Region Results

The Torque can be plotted

Convergence Assessment

-700

-600

-500

-400

-300

-200

-100

0

1

19

37

55

73

91

10

9

12

7

14

5

16

3

18

1

19

9

21

7

23

5

25

3

27

1

28

9

30

7

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5

34

3

36

1

37

9

39

7

41

5

43

3

45

1

46

9

48

7

Hydraulic Torque (N-m)

Ability to create a full pump curve

Start by setting up as normal but without

assigning Boundary Conditions

Add a single inlet surface and a single outlet

surface into a group each (Inlet and Outlet)

Fill out the table and press ‘Run’

Run Simulation CFD as administrator

to ensure that the script is able to run

CFD will create a scenario for each point on

the curve

Solve all the scenarios that are produced

Pump Curve API

These settings give 10 points on a curve running 3deg/step from the beginning and for 20 impeller revolutions

Use ‘Process Data’ to produce an Excel file

with the results from each scenario

Pump Performance Curve will exist within

the Excel file

Results - Pump API

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0

100

200

300

400

500

600

700

800

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Effi

cie

ncy

Pre

ssu

re H

ead

(ft

. H2

O)

Flow Rate (GPM)

Pump Performance Curve

Name Flow Rate (GPM) Pressure Head (ft. H2O) Efficiency

Full Flow 3903.712685 0 0%

OP2 - 3555.5556 GPM 3555.322582 156.215975 39%

OP3 - 3111.1111 GPM 3110.911222 443.4494256 95%

OP4 - 2666.6667 GPM 2666.499862 523.6416756 96%

OP5 - 2222.2222 GPM 2222.072651 597.0291264 96%

OP6 - 1777.7778 GPM 1777.661291 725.2029053 103%

OP7 - 1333.3333 GPM 1333.248346 731.5794772 98%

OP8 - 888.8889 GPM 888.8322306 643.665754 74%

-1200

-1000

-800

-600

-400

-200

0

200

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Torq

ue

(N

-m)

Time (sec)

Torque Curves

Full Flow

OP2 - 3555.5556 GPM

OP3 - 3111.1111 GPM

OP4 - 2666.6667 GPM

OP5 - 2222.2222 GPM

OP6 - 1777.7778 GPM

OP7 - 1333.3333 GPM

OP8 - 888.8889 GPM

OP9 - 444.4444 GPM

Results - Numerical

-700

-600

-500

-400

-300

-200

-100

0

1

19

37

55

73

91

10

9

12

7

14

5

16

3

18

1

19

9

21

7

23

5

25

3

27

1

28

9

30

7

32

5

34

3

36

1

37

9

39

7

41

5

43

3

45

1

46

9

48

7

Hydraulic Torque (N-m)

Extracted from the Rotating Region Results

The Torque can be plotted

The Bulk Calculator will show average flow

rates or pressure at over a cut-plane

Results – Localised Investigation

Cut Planes can show high levels of

detail – vectors are especially useful

ISO Surfaces are useful for finding

regions of interest

Highest flow

Cavitation

Nodal Aspect Ratio

Results – Animating From Startup

Next week: Autodesk Simulation Moldflow Hangout – Matching Simulation to Actual Molding

· Gain more confidence in conveying information from your simulation to the press

· Learn best-practices and steps for matching up molding processes to Moldflow simulations