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Old   October 28, 2015, 06:40
Unhappy Moving reference frame ?
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Hello everyone,

I have a fan composed by an intake, a rotor, a stator and an exhaust. I have therefore two volumes : one composed of the rotationnal air around the rotor ("fluid_MRF") and another composed of the other parts ("fluid").

To simulate the rotation of the blade I have placed the fluid_MRF in a moving reference frame. The rotor is consequently stationnary in regard to this reference frame.
The other volume "fluid" is placed in a stationnary frame and the walls in contact with itare also stationnary.
The problem I obtain when I plot the vectors of absolute and relative velocities is that the fluid is purely tangentiel around the rotor and that the velocities at the intake or at the exhaust are not in the right direction (the fluid goes from the outlet to the inlet) and almost null.

Therefore my question is : how do you make the fluid goes properly from the inlet to the outlet (without having a fluid purely tangentiel around the rotor) ?
For information I have a pressure-inlet and a pressure-outlet with a direction of the velocity normal to the boundary. I heard that it might be due to the interface but I don't know more precisely.


Thanks a lot !
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Old   October 31, 2015, 17:29
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HELLO,
first thing to consider is that the mrf follows right hand law,so when you are defining the axis of rotation you should apply this.
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Old   November 2, 2015, 05:54
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Yep I used the right hand law to define the rotation axis. I progressed through some calculations but now I have another problem : with a higher rotationnal velocity (100 rad/s) I have the fluid that comes from the inlet to the outlet. But when I plot the relative velocity I dont get the triangle "absolute velocity equals to the relative velocity plus the rotationnal velocity" (the absolute velocity is more quals to the relative velocity minus the rotationnal velocity). Do you know where can that comes from ?






Also another question : when you don't really know what you have at the outlet of your geometry, is it better to use a pressure-outlet or a outlet-vent ? And what are the criterions to choose either one or another ?
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Old   November 2, 2015, 07:25
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i did a wind turbine recently but i did it srf, because mrf is a little tricky,i had reverse flow with outlet boundary condition,but when i did it with pressure outlet it disappeared.my problem was a little like yours and it became cleared that i have mistaken the rotational axis direction.
about the interface problem,from what i know if you do it wrong,fluent would diverge at the beginning.
another thing that may not be your case but did you use a small enough time step?
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Old   November 2, 2015, 08:25
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I only did steady simulations, with a constant rotationnal speed. At the beginning the rotationnal speed was set to 13rad/s, which corresponded with my geometry to (only) 1.2m/s with a radius at mid-span of 100mm and a span of 30mm. At that time I had the problems I described in my first post (purely tangentiel fluid, abnormal behaviour at the inlet and at the outlet).
But with a rotationnal speed set to 100 rad/s (10m/s) I have a "normal" behaviour of the rotor : the air comes from the inlet to the outlet and I have no reverse flow.

Therefore I don't think that the pb comes from the fact that I work in steady flow, but I'm not sure maybe it takes some time for the flow to stabilize... Do you think I should use unsteady mode ?

Thx for your answer !
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Old   November 2, 2015, 09:13
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this kind of problems are naturally unsteady and if you plot cl in fluent you see even in steady state it is periodic and it shows the flow characteristics,the big difference is the convergence that is better in unsteady,if you post a picture of your mesh which the directions and rotational direction is clear and a picture of your mrf setting may be i could help.
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Old   November 3, 2015, 06:19
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Here they are. Since I am working in 3D, the mesh I present you corresponds to a plane situated at the tip of a rotor blade, with a normal x. The fluid is moving from the Y+ to the Y- and you can see 3 stator blades after the rotor blade, aiming to stabilize the fluid. The red arrow corresponds to the rotation of the rotor blade.
Attached Images
File Type: png mesh_coupe_z-_fleche.png (148.0 KB, 48 views)
File Type: png params_fluide_MRF.PNG (16.3 KB, 40 views)
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Old   November 3, 2015, 13:00
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hello,may be im not seeing correctly but from what im seeing your axis of rotation should be y=1 not y=-1.i hope i have assisted you.
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Old   November 4, 2015, 11:02
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Hi ! Yes the blades are rotating with the direction +y but since the fluide is supposed to turn on the other side (toward the pressure side) I put it in the direction y-. It seems legit for me but maybe I am mistaken !
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Old   November 16, 2015, 08:22
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Hi guys ! Sorry to ask again but if the blades are moving with the direction of the axis of rotation equals to y+, sould I place the rotation of the rotating frame toward y+ or y- ?
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