Learning Forum

[DavideDiPasquale]

I would like to know when using a Multi Reference frame in steady mode to simulate a rotating propeller if I have to consider the rotation of the blade or the rotation of the fluid (MRF) when input my RPM and axis of rotation. I mean if my blade rotate anticlockwise , should I make the fluid around the blade rotating clockwise?

[Rob]

In the mrf system we move the fluid (or rather make the solver think the fluid is moving) and move the walls at zero velocity relative to the cell zone. So, if the blade should move clockwise at 20rpm we'd set the fluid to be 20rpm clockwise and the impellor walls to be rotating at zero rpm relative to the fluid zone. The reference zone model is very simple, but confuses everyone at first: once it clicks you'll not understand why you initially struggled with it.

[DavideDiPasquale]

yes thanks

[DavideDiPasquale]

Hi Rob but I have the following warning:
Info: Interface zones, 12 and 13, penetrate each other.
This could adversely affect your solution.
Where zone 12 is my source contact region (interface) and zone 13 is my target contact region (interface).
Look at the picture:



This is my set up:


I have an inlet set up as velocity-inlet and an outlet set up as Pressure outlet
Should I set up my side wall as stationary?
Any advice?
Thanks and Regards

[Rob]

Side walls shouldn't need a setting as they're adjacent to the stationary zone.
Check for contact regions in the Meshing tool, given the domain size and likely proximity of the blade faces and surrounding volume it may be a tolerance issue rather than an interface.

[DavideDiPasquale]

Hi Rob but whatever sense of rotation I input in for the rotating part I obtain negative thrust.
I am using ANSYS 2021R1 version.
I mean z is the propeller rotation axis, and if I use +3000RPM or -3000RPM my thrust is always negative.
Do you have any idea why?
Thanks again and Regards

[Rob]

We use the right hand rule for rotation direction, but most people get the direction wrong...... My excuse is being left handed....
Thrust is also linked to the local flow: how fast is it passing the blades? Is it forced?

[DavideDiPasquale]

the propeller is made of three blades rotating with a rotational speed of 3000RPM and subject to a freestream velocity of 10 m/s perpendicular to the propeller plane

[Rob]

So, if the wind is 10m/s should the blades be spinning at that rate?

[DavideDiPasquale]

HI Rob, no matter what is the rotational speed (I have tried RPM =1500), the thrust is always negative for both sense of rotation.
I do really do not understand the reason for that!

[Rob]

Does the level of negative change? Ie if you plot what you have already seen when should the thrust be zero?

[DavideDiPasquale]

the negative level slightly change. I do not understand your 2nd question

[Rob]

We know the blades have a thrust force. We know the flow velocity. If you plot thrust against flow velocity at what velocity will the thrust be zero?

[DavideDiPasquale]

The thrust is always negative, it is a steady simulation

[Rob]

Can you post some results?

[DavideDiPasquale]

Which kind of results you would like to look at?
Thanks and Regards Davide

[Rob]

Thrust v velocity graph
Pressure fields, velocity contours.