Help modeling flow rate of car rotor

tlemonds · March 27, 2013, 21:08

Hey guys,

I am attempting to model the flow rate of a spinning brake rotor on CFX. In a full brake assembly, you have brake pads/caliper/heat shield on one side of the rotor and the wheel on the other. I've attached a picture of the control volume and rotor and I am working with. I have the circular face on the larger cylinder as the outlet and I am hoping to measure mass flow rate from this. Basically, this should be a good gauge of how much air my rotor is pumping. I am designing several different rotors for my capstone project and I was going to categorize them based on their mass flow rate capactiy. The simulation runs and I am getting results but I do not believe they are correct. When I model the velocity as vectors, all the vectors are radial as you can see in the attached picture. I was expecting the vectors to extend from the center of the rotor radially in all 360 degrees. I've tried messing around with switching inlets with openings and a few other things but I can't get any significanlty different results. Any suggestions to get the results I am looking for?

jthiakz · March 28, 2013, 04:12

# Is gravity is included, (-z direction)
# rotation of rotor domain is not clear, all the components are rotating or only the walls are rotating. RPM ?
#why would you expect velocity vector would come straight in radial direction,rather than vector (sprial )comes out with an inclination because of the rotating wall. Velocity at rotating wall will have the same velo of the wall. Between the disc gap is small so flow is well guided by the rotating wall
# I also notice that you have guides in between plate ,is that the reason you are expecting the flow to come out straight out.

tlemonds · March 28, 2013, 12:23

1.) No, gravity is not included. I'm pretty new to CFX but I haven't stumbled upon any feature and/or tutorial mentioning gravity yet. Quick tip on how to apply that?

2.) Just the rotor is rotating at 48 rad/s about the global x-axis which is normal to its center. I have it set as a rotating immersed solid domain inside a stationary fluid domain.

3.) The wall of the enclosure should not be rotating. However, I do see what you're saying about the gap on one side of the rotor. If the air is trapped in a tiny gap between a stationary wall and a rotating wall I can see that it would rotate with it, that makes sense. There is only a tiny gap one between the enclosure and the rotor on its outside face though, but I will change that to optimize my simulation.

4.) Yes, there are vanes inside the rotor and that is why I am expecting it to pump air through it. Basically, since the rotor is spinning, I was expecting it to suck air in through the inlet through the smaller diameter cylinder enclosure and pump it out radially from its center. I know this is how it works in real life but I'm probably messing up the simulation.

ghorrocks · April 1, 2013, 19:49

You do not need gravity for this simulation so do not add it.

To get this simulation to work you need to carefully match the rotating and non-rotating bits. If the entire simulation is rotating (or has little to stop it rotating) then the fluid will just rotate with the domain and not much will happen. You need to make sure your inlet flow does not rotate. You may have to add some of the surrounding stuff to achieve this.

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March 28, 2013, 04:12		#2
jthiakz Member Thiagu Join Date: Oct 2012 Location: India Posts: 60 Rep Power: 14	# Is gravity is included, (-z direction) # rotation of rotor domain is not clear, all the components are rotating or only the walls are rotating. RPM ? #why would you expect velocity vector would come straight in radial direction,rather than vector (sprial )comes out with an inclination because of the rotating wall. Velocity at rotating wall will have the same velo of the wall. Between the disc gap is small so flow is well guided by the rotating wall # I also notice that you have guides in between plate ,is that the reason you are expecting the flow to come out straight out.

March 28, 2013, 12:23		#3
tlemonds New Member T. Lemonds Join Date: Mar 2013 Posts: 2 Rep Power: 0	1.) No, gravity is not included. I'm pretty new to CFX but I haven't stumbled upon any feature and/or tutorial mentioning gravity yet. Quick tip on how to apply that? 2.) Just the rotor is rotating at 48 rad/s about the global x-axis which is normal to its center. I have it set as a rotating immersed solid domain inside a stationary fluid domain. 3.) The wall of the enclosure should not be rotating. However, I do see what you're saying about the gap on one side of the rotor. If the air is trapped in a tiny gap between a stationary wall and a rotating wall I can see that it would rotate with it, that makes sense. There is only a tiny gap one between the enclosure and the rotor on its outside face though, but I will change that to optimize my simulation. 4.) Yes, there are vanes inside the rotor and that is why I am expecting it to pump air through it. Basically, since the rotor is spinning, I was expecting it to suck air in through the inlet through the smaller diameter cylinder enclosure and pump it out radially from its center. I know this is how it works in real life but I'm probably messing up the simulation.

April 1, 2013, 19:49		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You do not need gravity for this simulation so do not add it. To get this simulation to work you need to carefully match the rotating and non-rotating bits. If the entire simulation is rotating (or has little to stop it rotating) then the fluid will just rotate with the domain and not much will happen. You need to make sure your inlet flow does not rotate. You may have to add some of the surrounding stuff to achieve this.