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[617283443]

Hi experts！

My simulation subject is a slender pipe containing air as the working medium. The objective is to calculate the pressure drop generated during the air extraction process within the pipe. The operating pressure is set at 101,325 Pa. The inlet boundary condition is set as a pressure inlet (Gauge Total Pressure = 0), while the outlet is configured as a mass flow rate outlet (Mass Flow Rate = 0.0004 kg/s). The calculation results demonstrate excellent convergence with minimal residuals, mass conservation at both inlets and outlets, and stable monitoring metrics. However, the flow field is unreasonable. The issues are as follows:

1.My simulation results indicate a minimum static pressure of -2.38 ×10⁵ Pa and a minimum absolute pressure of -2.21 ×10⁵ Pa. This is clearly unreasonable, as the theoretical limit for static pressure is -101,325 Pa, and absolute pressure should theoretically be greater than zero. The inconsistency is readily apparent from my contour plots. What could be causing this?

2.In the Fluent ‘Solution limits’ window, the default value for ‘Minimum Absolute Pressure (pascal)’ is 1. Why does my calculation still yield a minimum absolute pressure of -2.21e+05 Pa? Are the solution limits not functioning?

[Essence]

Hello,

Please recheck your boundary conditions. They may have been applied incorrectly. Please check the quality of the mesh too.

[617283443]

Thank you very much for your reply!I employed a two-dimensional axisymmetric model with a hexahedral mesh, and the mesh quality is excellent! The pipe inlet faces the atmosphere, while the outlet connects to a vacuum pump. What issues might arise with the boundary conditions?

[Essence]

Please check the y+ values, growth rate in your entire domain.

Due to unphysical/unrealistic boundary conditions, you will encounter such negative pressures. In a 2D axisymmetric simulation, the total mass flow rate—as it would be in a 3D case—must be entered. It should not be divided by 2π. Fluent automatically incorporates the axisymmetric revolution when calculating areas and volumes.

If the mass flow is incorrectly specified per radian (i.e., divided by 2π), the inlet velocity and total flow will be underestimated, resulting in inaccurate simulation results.

[617283443]

Thank you very much for your reply!You mean that the mass flow rate is too high, causing excessive pressure drop and thus producing this non-physical result?There is no place in Fluent where suppression is applied, such as in the Solution limits?In the Fluent ‘Solution limits’ window, the default value for “Minimum Absolute Pressure (pascal)” is “1”. Why does my calculation still yield a minimum absolute pressure of -2.21e+05 Pa? Are the solution limits not functioning?

[Essence]

Yes, I don't exactly mean that mass flow rate is high. It can be incorrect. Can you share the absolute pressure contour plots?

[617283443]

[Essence]

There are several reasons why solution limits might not be working in your simulation. In Ansys Fluent, if the flow is incompressible, the pressure limiter is ignored because it is designed for compressible flows to prevent negative density values. Additionally, even if the flow is compressible, the pressure limiter is a numerical device that might fail if the numerics are unstable or if the mesh has cells with low orthogonality.

Solution limits in Ansys Fluent may not work if the solver fails to converge due to poor mesh quality, inappropriate boundary conditions, or if the physical model does not accurately represent the real-world scenario. These limits are tied to convergence criteria and numerical stability.