Slip Reynolds Number in multiphase simulation

ukbid · April 4, 2011, 13:51

Hello everyone,

I am doing a Euler-Euler two phase simulation (Water with dispersed air bubbles) with CFX. In my Solver Out file there is an "Air l Water Slip Reynolds Number" in the Average Scale Information. What does this number meanns and most important how is it calculated by the Solver???

Any replies will be highly appreciated!

Thanks a lot in advance!

ghorrocks · April 4, 2011, 20:17

It is the Reynolds number of the air bubbles, with the length scale being the bubble diameter and velocity being the slip velocity.

ukbid · April 5, 2011, 10:09

Thanks a lot Glenn!

But I am still not quite clear about the porblem. So for instance, i got the following Average Scale Information:

+--------------------------------------------------------------------+
| Average Scale Information |
+--------------------------------------------------------------------+

Domain Name : Default Domain
Global Length = 1.0873E-01
Minimum Extent = 4.0000E-02
Maximum Extent = 7.7000E-01
H2O.Density = 9.9700E+02
H2O.Dynamic Viscosity = 8.8990E-04
H2O.Velocity = 2.6129E-01
H2O.Advection Time = 4.1611E-01
H2O.Reynolds Number = 3.1828E+04
H2O.Mass (Conservative) = 1.1328E+00
H2O.Mass (Normalised) = 1.1332E+00
H2O.Volume = 1.1366E-03
H2O.Volume Fraction = 8.8434E-01
Air.Density = 1.1850E+00
Air.Dynamic Viscosity = 1.8310E-05
Air.Velocity = 4.9441E-01
Air.Advection Time = 2.1991E-01
Air.Reynolds Number = 3.4790E+03
Air.Mass (Conservative) = 1.7597E-04
Air.Mass (Normalised) = 1.7616E-04
Air.Volume = 1.4866E-04
Air.Volume Fraction = 1.1566E-01
Air | H2O.Slip Reynolds Number = 1.9725E+03

The bubble diamter = 6 mm = constant

So Slip velocity = Air.Velocity - H2O.Velocity = 4.9441E-01 - 2.6129E-01= 0.23312 m/s (am I right?)

Slip Reynolds Number = Slip Velocity * Bubble Diamter * H2O Density / H2O Viscosity = 0.23312 m/s * 0.006 m * 997 kg/m^3 / 0.000899 Pas = 1551.

This is still different than the value 1.9725E+03 given by the Solver.

Looking forward to your reply!!

Thanks a lot!!

ghorrocks · April 5, 2011, 19:47

The slip velocity is a vector. You cannot simply subtract two vectors like that, you need to take the direction into account.

vidhya · April 6, 2011, 10:16

hi,
i need one very basic clarification.. generally euler-euler model is used wherein the stationary frame of reference is chosen and the equations solved. if we have two frames, one rotating and the other stationary then how will it be solved..? eg like a stationary tank and rotating turbine in reactors.. pls clarify

ghorrocks · April 6, 2011, 19:49

The frame of reference is a totally different thing to the multiphase formulation. The multiphase model can be used in rotating frames of reference and all the additional terms are taken care of. See the CFX documentation for details.

ukbid · April 8, 2011, 06:45

First thanks a lot again Glenn, for the help and information

And second to vidhya, I think CFX has a tutorial example about a mixing tank, where gas bubbles flow upwards in a tank and something is also moving inside the tank (sorry I can't remember quite clear). Just check the tutorial documentation, maybe you can find something useful.

vidhya · April 10, 2011, 23:51

hi ya,
thanks.. i got it.

April 4, 2011, 13:51	Slip Reynolds Number in multiphase simulation	#1
ukbid New Member Pang Join Date: Jun 2010 Posts: 9 Rep Power: 16	Hello everyone, I am doing a Euler-Euler two phase simulation (Water with dispersed air bubbles) with CFX. In my Solver Out file there is an "Air l Water Slip Reynolds Number" in the Average Scale Information. What does this number meanns and most important how is it calculated by the Solver??? Any replies will be highly appreciated! Thanks a lot in advance! Last edited by ukbid; April 4, 2011 at 18:26.

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April 4, 2011, 20:17		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	It is the Reynolds number of the air bubbles, with the length scale being the bubble diameter and velocity being the slip velocity.

April 5, 2011, 10:09		#3
ukbid New Member Pang Join Date: Jun 2010 Posts: 9 Rep Power: 16	Thanks a lot Glenn! But I am still not quite clear about the porblem. So for instance, i got the following Average Scale Information: +--------------------------------------------------------------------+ \| Average Scale Information \| +--------------------------------------------------------------------+ Domain Name : Default Domain Global Length = 1.0873E-01 Minimum Extent = 4.0000E-02 Maximum Extent = 7.7000E-01 H2O.Density = 9.9700E+02 H2O.Dynamic Viscosity = 8.8990E-04 H2O.Velocity = 2.6129E-01 H2O.Advection Time = 4.1611E-01 H2O.Reynolds Number = 3.1828E+04 H2O.Mass (Conservative) = 1.1328E+00 H2O.Mass (Normalised) = 1.1332E+00 H2O.Volume = 1.1366E-03 H2O.Volume Fraction = 8.8434E-01 Air.Density = 1.1850E+00 Air.Dynamic Viscosity = 1.8310E-05 Air.Velocity = 4.9441E-01 Air.Advection Time = 2.1991E-01 Air.Reynolds Number = 3.4790E+03 Air.Mass (Conservative) = 1.7597E-04 Air.Mass (Normalised) = 1.7616E-04 Air.Volume = 1.4866E-04 Air.Volume Fraction = 1.1566E-01 Air \| H2O.Slip Reynolds Number = 1.9725E+03 The bubble diamter = 6 mm = constant So Slip velocity = Air.Velocity - H2O.Velocity = 4.9441E-01 - 2.6129E-01= 0.23312 m/s (am I right?) Slip Reynolds Number = Slip Velocity * Bubble Diamter * H2O Density / H2O Viscosity = 0.23312 m/s * 0.006 m * 997 kg/m^3 / 0.000899 Pas = 1551. This is still different than the value 1.9725E+03 given by the Solver. Looking forward to your reply!! Thanks a lot!!

April 5, 2011, 19:47		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	The slip velocity is a vector. You cannot simply subtract two vectors like that, you need to take the direction into account.

April 6, 2011, 10:16		#5
vidhya New Member Join Date: Jan 2011 Posts: 24 Rep Power: 15	hi, i need one very basic clarification.. generally euler-euler model is used wherein the stationary frame of reference is chosen and the equations solved. if we have two frames, one rotating and the other stationary then how will it be solved..? eg like a stationary tank and rotating turbine in reactors.. pls clarify

April 6, 2011, 19:49		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	The frame of reference is a totally different thing to the multiphase formulation. The multiphase model can be used in rotating frames of reference and all the additional terms are taken care of. See the CFX documentation for details.

April 8, 2011, 06:45		#7
ukbid New Member Pang Join Date: Jun 2010 Posts: 9 Rep Power: 16	First thanks a lot again Glenn, for the help and information And second to vidhya, I think CFX has a tutorial example about a mixing tank, where gas bubbles flow upwards in a tank and something is also moving inside the tank (sorry I can't remember quite clear). Just check the tutorial documentation, maybe you can find something useful.

April 10, 2011, 23:51		#8
vidhya New Member Join Date: Jan 2011 Posts: 24 Rep Power: 15	hi ya, thanks.. i got it.