Reversed Magnus effect
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 This topic has 6 replies, 2 voices, and was last updated 3 years, 3 months ago by Martins.Brics.

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July 22, 2020 at 1:44 pm #5076Martins.BricsParticipant
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:
P.S. The same is also observed with cubic particles where rotation direction is better visible.
Best regards
Martins BricsJuly 31, 2020 at 9:49 am #5082robin.trunkKeymasterDear 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 HLBM2D the rotation is defined in a mathematice sense, meaning that a positive velocity leads to counterclockwise 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
RobinJuly 31, 2020 at 4:29 pm #5083Martins.BricsParticipantDear Robin,
Thanks for replay. Indeed I applied positive Torque when got clockwise rotation.
the piece of modified code:
T torque=1.0e7;
_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. videoBest regards
Martins BricsJuly 31, 2020 at 4:52 pm #5084robin.trunkKeymasterHi 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
RobinDecember 2, 2020 at 6:54 pm #5385Martins.BricsParticipantHi 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.
December 2, 2020 at 9:24 pm #5388robin.trunkKeymasterHi Martins,
thank you for your answer. I added your issue to our tracking system.
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 ydirection (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 counterclockwise 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
RobinDecember 3, 2020 at 10:09 am #5391Martins.BricsParticipantHi Robin,
you are absolutely right. This means that falling down ball which rotates counterclockwise 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 counterclockwise 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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