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• #5076
Martins.Brics
Participant

Hi,

I wanted to simulate settling rotating particle, so in dkt2d example I removed one particle and applied external torque. However, I observe always reverse Magnus effect: i.e.: that particle moves in opposite direction in liquid as it should. To my knowledge due to Bernoulli’s principle (Magnus effect) the direction of motion in the liquid and when particle roles on the bottom surface should be the same and not opposite as I observe. For details see videos:

See videos here

P.S. The same is also observed with cubic particles where rotation direction is better visible.

Best regards
Martins Brics

#5082
robin.trunk
Keymaster

Dear Martins Brics,

did you apply this rotation by settling a positive value for the angular velocity or acceleration for the particle?
It seems that for HLBM-2D the rotation is defined in a mathematice sense, meaning that a positive velocity leads to counter-clockwise rotation.
You can verify this by setting gravitation to zero and having a look at the velocity field.
We will have a look at that for the next release to make this more intutitive / consistent.

Best regards
Robin

#5083
Martins.Brics
Participant

Dear Robin,

Thanks for replay. Indeed I applied positive Torque when got clockwise rotation.

the piece of modified code:
T torque=1.0e-7;
_vecOfExt_Torque.push_back(torque);
_vecOfHydroDyn_Torque.push_back(sumF[2+4*iInd]);
indicator[iInd]->setAlpha2( (sumF[2+4*iInd]+torque) / indicator[iInd]->getMofi() );

To determine how the angle is defined I repeated simulation with cube, where I can see rotation direction. with cube for a positive external torque I got that angle theta also increased so I assumed there is the same definition of angle – positive for clockwise rotation.
Later I will look at velocity fields, but next week I am on vacation.However, I am bit confused today as I got that cuboid2d as circle deflects in correct direction now. video

Best regards
Martins Brics

#5084
robin.trunk
Keymaster

Hi Martins Brics,

if you refer to deflection after the particle reaches the bottom, it is probably not caused by “rolling” on the floor. The bottom wall hast no roughness, so there is no friction between particle and wall in this sense. It is most likely just pushed away from the wall by the fluid, when reaching the corner.

Best regards & a nice vacation
Robin

#5385
Martins.Brics
Participant

Hi Robin,

sorry for long not response. After vacation I had to deal with another project.

But I finally understood why the effect suddenly disappeared. I changed the ration matrix from

rotationMatrix[0] = cos;
rotationMatrix[1] = -sin;
rotationMatrix[2] = sin;
rotationMatrix[3] = cos;

to

rotationMatrix[0] = cos;
rotationMatrix[2] = -sin;
rotationMatrix[1] = sin;
rotationMatrix[3] = cos;

and forgot that I did the changes. Changed indicis [1]->[2] and [2]->[1]. This is equivalent to rotation by negative angle. In version 1.4 I have to change to do the same trik in order not to observe visually reversed Magnus effect.

videos can be seen here

#5388
robin.trunk
Keymaster

Hi Martins,

I have a question on your videos, so correct me if I’m wrong:

I assume the flow is induced by the cube settling under gravity. This should induce a flow in positive y-direction (I can’t verify as the magnitude of velocity is depicted). The Magnus effect states that the particle moves to the side where rotation and flow align. This means a counter-clockwise rotation should lead to the particle drafting to the right. This means your results with the original rotation matrix in the first video are already correct?

Best regards
Robin

#5391
Martins.Brics
Participant

Hi Robin,

you are absolutely right. This means that falling down ball which rotates counter-clockwise will lean right as seen in the video. This means that everything is fine with your code, if original rotation matrix is used. Just by manipulating the rotation matrix I introduced an error and got lost with what is correct outcome of the Magnus effect. Somehow I thought that the falling ball in front of itself increases the density of media it falls in. Thus, by rotating counter-clockwise the ball at the bottom moves more media particles to rihgt than to left at the top. Therefore, do to 3rd Newtons’s law, the ball should move left. But that is obviously wrong. Bernoulli effect at the sides of the ball is lager. I just ignored it and thus predict wrong behavior. Somehow I was convinced that I am correct and was able to convince my colleges that I am right.

Sorry for wasted time!

Best regards
Martins Brics

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