Learning Forum

[Guojun]

Hello,

I was simulating counter-current flow in a tube using VOF method. The gas and liquid velocities are -1 and 1.2 m/s, respectively. And there is an acceleration (9.8m/s) at z direction.

[Konstantin]

1) Your plot is counter-intuitive. For gas, you are inducing gas out of the domain through the inlet, yet pressure at the inlet is higher than at the outlet. My expectation is for the flow to move from the region of high pressure to low pressure, and not the other way around. Are your inlets (or phases) inverted by chance?n2) Is your simulation axisymmetric? You use r-z coordinate axis, but in Fluent the axis is always x-axis in axisymmetric problems. Just checking if this is simply your convention of naming the axis.n

[Guojun]

Thank you for your answer. I double-checked the case, and the gas flows from right to the left. In addition, the axis of this model is the x-axis. n

[Guojun]

As you mentioned, it is really strange that the gas flows from the region of low pressure to high pressure. But it is the simulation results.n

[Konstantin]

can you post a picture of the vector field and its zoomed views near the inlets? n

[Guojun]

Sure, the following two pictures are the velocity vector in the whole domain and near the inlets, respectively. Thank you very much.nn

[Rob]

Can you plot the phases? Also plot the area around the inlet in more detail (zoom in) with node values off so we can see the actual data and avoid any interpolation effects. n

[Guojun]

Sure, the first three pictures are the liquid phase, pressure and velocity magnitude in the domain. The last three pictures are the liquid phase, pressure, and velocity magnitude near the liquid inlet, respectively. If you need more information, please let me know. Thanks.nnnnn

[Rob]

Can you replot the last 3 images with node values off? How is gravity in the z-direction? What operating density are you using?n

[Guojun]

1 The last 3 images( liquid phase, pressure and velocity magnitude).nnn

[Guojun]

Sorry, Rob. My expression makes you confuse. Therefore, I'm going to describe it again. And then I will answer your questions.nIt's a 2D axisymmetric model using VOF method. The two phases are air and water, and their velocities are -1.052 and 1.19m/s, respectively(counter-current flow). The gravity is in the X-direction. nAnd in the solver, the gravity is defined as follows.nThe operation conditions are shown as follows.nThank you for your help and patience. Looking forward to hearing from you.n

[Rob]

OK. Move the reference pressure to be somewhere in the middle of the air region and fix the operating density to be exactly that of the air. It may not do anything but it's one thing that can cause issues. n

[Guojun]

Thanks for the comment. I did it as you suggested. The reference pressure lies in the center of the domain. The last two pictures are the pressure with different scales. The results show that the pressure of the air region is almost the same. However, the pressure near the liquid inlet is still low. Is it an error caused by Fluent? Many thanks.nnn

[Rob]

I don't think it's an error as such as a physics problem. Water enters the domain, so has a velocity and the boundary pressure is altered to give the flow: that's correct on the boundary. Water then accelerates due to gravity, again all good. The question (and where I need to be careful as Ansys staff because of the Forum rules) is how the solver deals with the cell-inlet interaction as the water next to the inlet is moving faster than the inlet velocity is set. n

[Anthony Bowers]

Hi,

 

In your pressure outlet what is the back flow volume fraction of your gas phase?

[Anthony Bowers]

Also, how did you initialize your problem? Did you patch the entire domain with the gas phase or vice versa?