Learning Forum

[natalya.vorobtsova]

Hello,

I am trying a simple model (straight pipe) to setup internal flow problem in Discovery Live. I set 300m/s velocity at the inlet and 0MPa pressure outlet. The results for velocity that I get are shown on the screenshot. Max value of the velocity is higher that 300m/s. Why is that? It should be lower than 300m/s everywhere considering there will be some pressure drop along the pipe.

Natalya Vorobtsova

[Brian Bueno]

Natalya Vorobtsova

300 m/s is a very high velocity.  It may be approaching the limitations of an analysis in Discovery Live.

I suspect that increasing the fidelity will reduce the inaccuracy, but most likely the reported maximum will always be slightly higher.

[natalya.vorobtsova]

Brian, 

Thank you for your response. HUnfortunately, this does not answer my question. I tried the same example with much lower velocity 10m/s. The max velocity I get is ~13m/s.

 

Best regards,

Natalya

[Brian Bueno]

Natalya Vorobtsova 

Can you send me that model to test here?  13 m/s for an inlet velocity of 10 m/s is well beyond what I would expect for any kind of inaccuracy.

[Brian Bueno]

Natalya Vorobtsova

It seems this is the intended behavior because of the use of the default no slip conditions at the wall location. With the particles sticking at the wall due to no slip, the net cross section area area through which fluid flows is reduced. This requires an increase in velocity to maintain the mass flow rate as pert he continuity equation A1V1 = A2V2.

The Discovery team has cross checked this in AIM and the same observations were recorded there as well with no slip wall defined.

[natalya.vorobtsova]

Brian, 

What is the conclusion then? How should I account fro that when I setup the velocity inlet? What should be the adjustment factor for the inlet velocity? Haw can it be calculated? 

Natalya

[Gaurav Sharma]

Natalya Vorobtsova 

It is important to understand here that the no-slip condition is actually a more realistic condition that represents the fluid behavior more accurately. Experimentally, it is known that for all real fluids, the fluid particles with immediate contact with a solid surface, move with the velocity of surface itself. When the wall is stationary, the particles near the boundaries will have zero velocity. According to Newton's law of viscosity, the particles to a depth near the wall, will experience resistance and will observe a decrease in velocity. Further, as stated by Brian, this requires the velocity of particles away from wall to increase to keep the mass flow rate constant.

This is not a limitation on Discovery Live's part and I believe this is the correct representation of a practical application as this establishes the required consistency in mass flow rate. Had the particle velocity not increased, the net mass flow rate would have experienced an unintended drop owing to the reduction of speed of fluid particles near the wall.

I tried a sample case and could see that though the max velocity increases from 10 m/s to ~14. 5 m/s before stabilizing, the inlet and outlet mass flow rate still remains almost constant. It can also be seen from the picture below that the particles close to the wall have velocity < 10m/s and this is compensated by the particles near the center, in order to ensure the inlet mass is equal to the outlet mass, which is must have as per the continuity condition.

 

A free-slip condition on the other hand (though not exposed in DL), would give a more uniform distribution of velocity and seems to be what you are looking for. I must add, such a distribution is approximate and no-slip condition is always a better representation, closer to practical scenario. The free-slip condition is not exposed in Discovery Live and in case if you are willing to use it for some reason, you may have to opt for Discovery AIM. Below is a snapshot of the flow developed using free-slip condition in Discovery AIM (just for your reference), and it can be seen that the velocity if fairly consistent across the section.

  

Thanks & Regards,

Gaurav