[zizie]

Hi everyone,

I'm trying to simulate the propeller blade in fluent using MRF. Basically, I'm trying to get a similar result with the experimental data, to get the thrust and torque. However, the torque that I'm getting is way to much difference than the experimental value. I'm not sure what could have gone wrong.

I'll start with explaining the method that I'm using. After some search and reading in the forum, I found that to study a propeller blade, it need to be segregated into three different sections, the front zone for translation velocity, rotating zone that contains the propeller blade, and the downstream outlet section. I'm using k-eps model.
Figure below shows the setup that I'm currently using for my analysis, Kindly please advice me if there is anything wrong.

[zizie]

More images here.

[NachoVidal]

What about the boundary conditions of the propeller wall?
Also, shouldn't the rotational speed of your outside wall be 0 in your rotating zone?

I cannot really help you that much, I have a similar problem with the propeller not giving me the expected thrust.

[zizie]

Originally Posted by NachoVidal

What about the boundary conditions of the propeller wall?
Also, shouldn't the rotational speed of your outside wall be 0 in your rotating zone?

I cannot really help you that much, I have a similar problem with the propeller not giving me the expected thrust.

I'm quite confused on that, rotational speed of the outside wall should be 0 because the wall is not rotating?


Thank you.

[duri]

Since your zone is rotating, propeller blade should be stationary relative to rotating zone. Either make rotation as zero in relative frame or give omega to actual value in absolute frame. Is there an outer wall to propeller like a ducted propeller? In such case give absolution rotation as zero or relative rotation as -omega.

[zizie]

Thank you for your advice.

There is no ducted / outer wall for my analysis.
I'm a bit confused on your statement.
So currently in boundary condition, for rotation section wall, I should make it zero?
I've attached some photos to show what I did now. Is it correct?
Basically I tried to imitate the results for a propeller blade running in wind tunnel, with the condition of certain range of inlet velocity, and the blade is rotating at 3008 rpm.

[duri]

This looks to be correct. In relative to the rotating zone wall is stationary. Hence it has to be zero. Check the axis of rotation, at zone it is -1 and wall is +1. Axis specified at wall should not have any impact on the solution.

[zizie]

So I should also put it as -1 in wall as well right?
Thanks for pointing out my mistake.
Another question is,
I know that if the solution is not converge yet, the result might not be correct,
but in my analysis, even if the solution does converged, the results (thrust and moment) shows the value almost similar with experimental result (with error approximately 0-5%).
Can I just use this result? Or should I work until the analysis is converge?

Thank you so much.

[duri]

Do you mean residual convergence? It should show some level of convergence if flow is steady. Check mass flow and force convergence.

[zizie]

Yes, the residual convergence.
The closest that I get is as in the figure attached here.
I'm reluctant to use the results, even the error compared to experimental is acceptable.
Kindly please advice.

[NachoVidal]

I'm curious about the inlet ant outlet boundary conditions. Did you set them as velocity inlet and pressure outlet?

My problem is quite similar to yours, but I'm simulating a ducted propeller as a fan in a duct, without farfield.

However, the thrust is about the half it should be. Any advice?

[zizie]

Hi NachoVidal,

I've change my flow domain since the last time I've posted here. I've attached the figure here. This method give me a better result, for both thrust and torque.

I'm using velocity inlet, with certain velocity and turbulence intensity of 0.1% and viscosity ratio of 10. The paper that I'm referring to specifically mention that its 0.1%.

For outlet,I set it as outflow. This is because, based on my read around the forum, it is much safer to use outflow.
I've tried using pressure outlet previously. I set it at 0 Pa, but along the analysis I'm having some issue with backflow.

Hope that this might help you.
Thanks!

[NachoVidal]

Thanks zizie! surely I can try some of your settings

[duri]

Originally Posted by zizie

Yes, the residual convergence.
The closest that I get is as in the figure attached here.
I'm reluctant to use the results, even the error compared to experimental is acceptable.
Kindly please advice.

The convergence looks good. I would not expect more from fluent.

[zizie]

Thank you so much for your advice.

[zizie]

Originally Posted by duri

The convergence looks good. I would not expect more from fluent.

Greetings Sir,

I've some more inquires regarding the rotation axis.
Based on my reading, Fluent is using right handed coordinate system , so the rotation will be counter clockwise on specified rotation axis.
Thus, based on this, I set my coordinate system to be (0,0,1) with 3008 RPM, as my blade should have normal rotation (counter-clockwise when viewed from the front).
I did my analysis, and by observing the streamline animation (on CFD-post), it shows that it rotates clockwise.
Then I did another testing, with (0,0,-1), with RPM 3008, the results of streamline animation shows its counter-clockwise.

Which is actually correct? The result is much different when its clockwise.

Kindly please advice. Thank you in advance.

[vion]

hi, i am new to Fluent, i am doing the marine propeller performance using Fluent. May i know ur other setting like reference value, reference zone and which initialisation you have used?

[zizie]

Hi, reference value depends on the configuration of your propeller. But in my case, I did not change the default value, because I'm concern in the force value, not the coefficient.
and for solution initialization, I'm computing from inlet, and initialize it.
I'm also a beginner in FLUENT. So I might do things wrong as well.
After some trying and error, you'll find the best way of doing it.

[vion]

Thanks for ur explanation. i am also a newbie in fluent and i am now doing the ship propeller analysis for thrust and torque which i think the simulation is similar with you. Can u guide me to setup the simulation? should i need the rotating domain in my simulation or just the channel and the propeller only?
which one is preferable?

[zizie]

I'll further explain what I did on my analysis:

1) Geometry:

I did my propeller drawing in CATIA, and insert in workbench in .stp files (name it blade). Then, I created the first enclosure, enclosing the propeller blade (I name it rotating). Next, square enclosure to imitate the flow in wind tunnel (name it stationary). I did 2 different boolean, first by subtracting stationary with rotating, and preserving the tool bodies. Then, boolean subtracting of rotating and blade.

Meshing:
I only modified some of the default setting. (You need to do the meshing depends on your propeller). Mine is smaller around the blade area. I include my boundary in this stage (inlet, outlet, wall, interface between rotating and stationary section). The number of meshing also depends on the accuracy that you required and also time.

Setup:
I use standard k-omega.
With two zones, stationary and rotating.
For rotating section, I use the frame motion and insert my rotational speed.
Also for boundary condition, on the rotating- I use rotating default with 0 rpm. As stated by duri on the discussion above.

There's a few tutorial available on youtube that helps me to do the analysis. You might need to check it out, to clearly understand.

Try this one Ansys CFX Tutorial Airboat propeller