superficial velocity
 

hi,

I couldn't find what the exact difference between superficial velocity and physical velocity is. Fluent user guide 6.1 always defines it using porous medium, however my model do not consist of any porous medium. Moreover physical velocity condition gives a much better solution when compared to superficial. can anyone make a short description of their difference?

Thanks for any help

Taskin

Re: superficial velocity
 

The superficial velocity in a monophasic flow is equal to its mean velocity, while in multiphase flows it's defined as the ratio of the velocity and the volume fraction of the considered phase in a multiphase system.

Best regards, ap

Re: superficial velocity
 

ap is right. However, some people prefer to call it as the "Actual velocity" of a phase.

For Instance:

Actual velocity of gas phase = (Superficial velocity of gas phase) / (volume fraction of gas phase)

When you feed velocities and a guess for the volume fraction for a phase in fluent, you use the above equation to determine the 'Actual velocity' and enter this in the "Velocity inlet" panel. For the dispersed phase volume fraction you will have to enter the volume fraction guess that you used to calculate 'Actual velocity'.

Re: superficial velocity
 

hi, you mean in superficial velocity, it doesn't make a varying velocity distrubution along the cross section. It just takes a mean value, but physical velocity models the real situtation, I mean varying velocity distribution. did I understand correctly? Thanks for your help

Regards, Taskin

Re: superficial velocity
 

Well, no. If you wish to specify a distribution profile for velocity across a boundary, you can either you ASCII profile definitions or UDF's to do it. Simply inserting values into the "Velocity inlet" panel just means a uniform constant distribution across the boundary, not a profile.

Re: superficial velocity
 

hi, I mean, varying velocity distribution due to viscous effects. I give a constant velocity but, due to viscous effects, it should have a bell shaped profile after the flow is fully-developed. so is that what superficial velocity takes as a mean? Thanks for you help, Regards, Taskin

Re: superficial velocity
 

Superficial velocity is simply a number (defined as):

Volumetric flow rate

-------------------------

Cross section area

Re: superficial velocity
 

hmm I got it I think.. Thanks for your help Regards Taskin

superficial velocity
 

Superficial velocity is nothing but the velocity of a fluid in a pipe,conduit,column etc in the absence of packing or obstruction. like in packed columns the actual velocity of the fluid through it is actually the volumetric flow rate divided by the cross sectional area. so the velocity achieved by the same fluid in the same column in absence of the packing is called superficial velocity.

In single phase flows in absence of porous medium, the superficial velocity is Q/A, where Q is the volumetric flow rate and A refers to the cross-sectional area.

In multiphase flows, here assuming a gas-liquid two-phase flow, we have:

physical velocities...vg and vl
superficial velocities...Jg=alpha*vg and Jl=(1-alpha)*vl where alpha represents the void fraction and (1-alpha) is the liquid holdup, HL. Thus: alpha+HL=1.

Now, considering a gas-liquid two-phase flow in porous medium, we have:
First, defining the porosity as "por"......por=(V_total-V_solids)/V_total
So,

superficial velocities........vsg=por*alpha*vg and vsl=por*(1-alpha)*vl.

With kind regards,
Chris