Negative Pressure Observed before outlet

  • Last Post 6 hours ago
bushrar posted this 6 days ago


My simulation consists of material flowing down a barrel to a contraction to  counterbore and then through a capillary. 

Pressure at the capillary outlet is supposed to zero, since flow is directed downstream, which was even observed with the resulting vectors.  However it dropped to a negative pressure at the outlet (z=1.5mm).

I used adaptive sizing, and set element size to 0.0075 mm.  Using this , I obtained fairly accurate shear rates and velocities.  I was wondering what changes could be made to make sure the pressure drops to 0 at the lowest.

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Raef.Kobeissi posted this 6 days ago

Hello, Before answering why is has dropped below 0 may I ask you this:

  • How did you collect this pressure data? Is it from a vertical line ?
  • what are your boundary comditions?
  • Did you check the average pressure at the outlet ? Is it 0?
  • is there any backflow ? Any eddies near the line you took?
  • can you attach a pressure and velocity contour ?

Best Regards

Raef Kobeissi

bushrar posted this 5 days ago

(Part 1)

Mr. Kobeissi, 

I did collect pressure data vertically starting from the top of the barrel to the end of the capillary (Figure 1- shown with a red line).  The points are located at the intersection of the planes of symmetry [(0,0,-1.9) to (0,0,1.6)].

Figure 1: 3-D Pressure contour 

By "average pressure at the outlet", are you referring to various points at the cross section of the outlet (Figure 2)?  

Figure 2: 3-D Pressure contour-outlet cross section



bushrar posted this 5 days ago

(Part 2):  Velocity

There is no back flow or eddies around the capillary area as indicated by vector and contour (Figures 3 and 4 respectively) 

Figure 3

Figure 4


bushrar posted this 5 days ago

(Part 3) Boundary Conditions

Inlet: Volumetric Flow rate- Q=2.0837e-9 m3/s

Planes of Symmetry


outlet: fn=fs=0

Raef.Kobeissi posted this 4 days ago

Hello, At the bottom (Capillary area) and based on the velocity contour there is an area with very high velocity due to the narrowing of the channel. This could be causing. Negative pressure area. In general I would say your average pressure at the outlet is indeed zero but it is gradient due to the narrowing of the channel and high velocities at the edge. Yes when I say average pressure I mean the cross section at the outlet. I believe what you’re seeing is normal. More mesh refinement will probably enhance your result. If you calculate the pressure outlet using the a erage function or average are favet for static pressure you will discover that it will be zero in average.

Raef Kobeissi

bushrar posted this 4 days ago

For mesh refinement, I started off with sweeping around the capillary as well as creating 20 edge divisions at the outlet.  For global parameters, I used adaptive meshing with an element size of 0.0075 mm.  This is total yielded around 2.5 million elements,  compared to the 2 million elements yielded from using only adaptive meshing.  

However, not much of a difference was observed with the addition of sweeping and edge divisions alone.

Are there any other suggestions as to how I can refine the mesh to yield a zero average pressure? 

bushrar posted this 12 hours ago

Update:  I tried inflation layers on conical convergence and capillary (together and separately), which produced a failed mesh

peteroznewman posted this 11 hours ago

Sometimes if you Generate Mesh, you will get a failed mesh, but if you selectively mesh each body from one end to the other, it might succeed. Start at the end that has the capillary and work you way up.

Raef.Kobeissi posted this 6 hours ago

As long as the averaged outlet face is zero pressure I believe you’re doing the right thing. It is expected to have negative pressure in the narrow area.

Raef Kobeissi