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Propeller disc theory in Star CCM+

Hey guys,

I am currently doing my master thesis in rotor aerodynamics for a small scaled helicopter. The first step before I include the main rotors in my mesh is to make a simplified verison of the model.

My idea is to create a infinite thin disc over my helicopter body to simulate the flow of the rotor blade. The model will be simulated in a box which acts as a windtunnel (with a certain freestream velocity).

Is there a easy way to model a disc in Star CCM+? I was wondering about this due to the fact that adding the disc will create a discontinuity in velocity and pressure around the rotor.

Great forum btw

//Christian

I'm not too sure about an infinite thin disk. Maybe it works with a fan interface, but I haven't tried that.
Another option would be to create a disk with a finite thickness. Create a region from that and put a momentum source. That should work for sure (we did something similar for wind park simulations to simplify the turbines).

I've used the finite thickness method described by abdul099 to model marine propellers. You can use a field function to set the strength of the momentum source based on some known propeller curve (ie advance ratio vs thrust and torque output).

I am new to CCM+ and have some problems implementing an actuator disc to simulate a propellor/wind turbine. Need some help here please as i have spent very long hours but not getting anywhere.

I have seen many of the given tutorials and I thought the following tutorials are relevant to what I wanna achieve.

Quote:

1. Motion \ Marine Resistance Prediction: KCS Hull with a Rudder
and
2. Motion \ Moving Reference Frames: Marine Propellor in Open Water
and
3. Incompressible Flow \ Porous Resistance: Isotropic Media

My current situation

Quote:

I have managed to simulate a a solid disc inside a wind tunnel test section.
I used Mesh Models described in 1. KCS_Geometry Tutorial, SURFACE REMESHER | TRIMMER | PRISM LAYER MESHER.
Reference Values | Base size was set as the my Disc's diameter size.

Other than that, Mesh Default mesh settings was DEFAULT as my whole mesh got worse after I started playing around with the settings.

All the sides of the wind tunnels were set as WALLS as they should.
At this stage, the disc was set to WALLS on the inlet, outlet and sides.
Solver was IMPLICIT UNSTEADY.

Inlet Region was 10 m/s with turbulent intensity 2% and turbulent length scale 0.01.
Attachment 35945

These are my results (pardon poor pics, I can't find a straighforward way in CCM+ to output pictures nicely).

Velocity Scene.
Attachment 35946
Results don't look well especially at the wall of the wind tunnel and the disc, I would expect lower velocity due to the no-slip condition.

I still am abit lost on how I should tweak my mesh settings. Any ideas please.

Vorticity Scene.
Attachment 35947

EDIT: My base size was previously set to the size of my disc. I guess that was too big, which let to large mesh grids. I now set the base to 1/2 of my diameter. Mesh looks better now, but my solution doesn't converge. I guess it must be something to do with the Courant number. So, I set my time step from 0.02 to 0.01 and I have convergence now. Any wisdow on how to how low my base size or timestep, or is it a trial and error thing? I wondering how low should I go to get a fine solution at the walls.

My situation ahead

Quote:

Now I want to implement a a pressure drop across the disc.

Based on what I saw from the tutorial 3. Incompressible Flow \ Porous Resistance: Isotropic Media, I believe I need to use define a Porous region for my disc
Attachment 35948

In my mind, I have a self-determined thrust coefficient (C_T) of my disc and then I can apply T = C_T * 0.5* 1/2 * rho * Vinf^2 * A.
Since I am assuming uniform thrust across my disc, my thrust per unit area would be t = T/A = C_T * 0.5* 1/2 * rho * Vinf^2.
But now, I am a bit lost on how I should implement this field function. spent fustrating hours and hours and getting nowhere.

Also, when I tried to regenerate the mesh under this case, an error occurs.
Attachment 35950

I believe my error occured because the way the region has been defined. To define 2 regions (1 static region - flow outside ) and (1 rotating region - disc), I had to apply "ASSIGN PARTS TO REGION" to the PARTS on (1) Disc and (2) Wind Tunnel separately.

However, this resulted in the disc REGION being inside the Wind Tunnel REGION, and which I suspect, caused this error.

how should I go about this defining my disc in this manner? I couldn't find any specific tutorial that teaches this.

your guidance and help in part or whole is really really appreciated and would really make this whole work more enjoyable.

thanks!

In my mind, to create an Actuator Disc simulation in a wind tunnel, the aim is to create 2 regions:
a) Rotating Region (representing the Disc)
b) Fluid Region (representing the wind coming onto the Disc)

Quote:

I have decided to adopt a Rigid Body Frame just like in the tutorial Motion \ Rigid Body Motion: Rotating Fan. This means, the disc will rotate.

So I created a DISC in a TEST SECTION in 3D-CAD.
Attachment 35955

a) Since they are essentially 2 PARTS (disc and wind tunnel box) intersecting each other, I need to SUBTRACT the volume of the DISC from the TEST SECTION.
b) Since I need to specify interaction between the rotating disc and the rest of the fluid, I need to use the INTERSECT operation and obtain the intersecting surfaces of the disc (i.e. the upstream surface, downstream surface and the sides).

Quote:

With PARTS: SUBTRACT and INTERSECT created, I proceeded to assign regions (i.e. 2 regions)to these parts. I also created INTERFACE relationship on those disc surfaces.

So I thought I was right up to this point. Then I wanted to proceed to generate the VOLUME MESH but got this error:

Code:

Trimmer cannot currently accommodate multiple regions in one continuum.

I really do not understand why.

Ok so I made a bit of progress after much fiddling around over and over.

Quote:

I managed to get to past the problem with defining separate regions for Rotating Fluid and Static Fluid.

I also implemented a rotating disc based on a Moving Reference Frame (MRF) approach and specified some rpm value for my disc.

Quote:

Attachment 35987
Here you can see the velocity of the fluid at the disc upstream surface. My disc is rotating clockwise, so I thought the velocity field should be flipped the other way around.

Quote:

Attachment 35986
Across the disc, the expected pressure drop is observed.

Moving ahead, I have now specified a porous region for the disc.

Under porosity option, there were a few options of inputing porosity: Constant, Field Function etc...

I tried an extreme case of porosity 0 as a constant value. So I expected the flow to just pass through my disc as if it was non-existent. But this did not happen and there was absolutely no change in my thrust coefficient.

How should I specify a specifc thrust for my disc?

Thank you!