[Magistrane]

Hello Foamers,

I need you. For 2 years now I am modeling a tidal turbine using an AMI and a wall condition on my blades :

{type movingWallVelocity;
value uniform (0 0 0);}

But I noticed yesterday with this wall condition, I have no adherence on my blade : the velocity is not null on my rotor.

So I switched to a noSlip condition.
{
type noSlip;
}
It gives me the velocity repartition in the 2 "noSlip" screen that are joined. Even if the solid normal velocity seems to be 0, the noSlip condition iss not respected in the 0 directions.


At least, I used a fixed value condition :
{
type fixedValue;
value uniform (0 0 0);
}

Here I have at least my 0 velocity on my blade patches.

I am trying to get a reasonable Cp for my turbine using pimpleFoam. So I think that i have to resolve the boundary layer. I am know wondering if I Have to switch all my cases to fixed value 0 because it's the only case where I actually get a boundary layer.

Could you please explain me the difference between this conditions : fixedValue is not defined in the documentation as a wall condition but for now on, it is the closest condition of it.

Thanks,
Have a great day !

Magistrane

NoSlip2.png No Slip2

No_Slip1.png No slip1

Moving_Wall.png Movingwallvelocity

FixedValue.png fixed value(0 0 0)

[Carlo_P]

The most correct should be movingWallVelocity.
It is correct that the velocity is not zero, becasue the velocity at the wall is not zero!
Is the same velocity as the wall that is rotating.


In thi case, the velocity should be around omega*r, where omega is the radiant/sec.


If you see the pics, the velocity is growing with the radius, so it is ok.

[Magistrane]

I don't get it.

Of course the velocity of the rotor isn't zero : it is rotating... But, shouldn't every solid have a 0 FLOW velocity on it's surface ? If it's not, you will never see the boundary layer !

Why should be the flow velocity equal to omega*r with an adherence condition ? :/

[Carlo_P]

Ahderence condition:
In fluid dynamics, the no-slip condition for viscous fluids assumes that at a solid boundary, the fluid will have zero velocity relative to the boundary.
The fluid velocity at all fluid–solid boundaries is equal to that of the solid boundary.[1]



(wiki: https://en.wikipedia.org/wiki/No-slip_condition)




The velocity at the wall should be the same as the wall one.
So, if the wall is moving, the velocity of the air should be the same as the moving wall.



If a car is moving at 100 km/h, the velocity of the air should be 100 km/h next to the wall.
It is a zero velocity relative to the car velocity.


You will have always a bounday layer, because the velocity at the wall is not exactly the same as the velocity at the wall+0.00001 mm.
You will always have a gradient in the direction perpendicolar to the wall.

[Magistrane]

Of course !...

I forgot the referential !
Thanks Carlo for your precious explanations !