analysis setup -...
TRANSCRIPT
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
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-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)
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
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-600
-500
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-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