In my case,a model is built up with one inlet and two outlets.Before applying pressure outlet conditions at the two outlets. I used "outflow" to carry out some iterations, while the split division ratio (under outlet/up outlet) is resulted from an experience formula, which is to some degree in agreement to the real data in my case. Then I changed it to pressure outlet condition, continuing calculation. Again some iterations, I find the split division ratio is reduced because always there is reversed flow at the under outlet, and the calculation result does not agree well with experiment data. Could anyone give me some advice?

Is this water? Air? Compressible? How far are the two outlets apart? How far is the inlet?

water,incompressible. the two outlets are 0.4m apart,inlet and up outlet is about 0.04m apart, distance between under outlet and inlet is over 0.4m. inlet velocity is vertical to the two outlets. Thanks!

Assuming viscous case, I suspect that reverse flow is near the wall. There is a finite static pressure at the outlet corner, which may be lower than the specified outlet pressure. Make sure your gravity vector is pointed correctly. I suppose your outlet pressure is well known; i.e, 1 atm!

Yes, I set the operation pressure to be zero, and at the pressure outlet boundary conditions, I let the pressure to be 1 atm at both outlets. And the result static is negative somewhere.

With regard to the negative pressures, I would make sure I'm using the correct atmosheric pressure for the given input mass flow. I'm sure you know you don't need to run the case again. Simply subtract and add! Assuming you have the right solution!