Inlet - problem with volumetric flow rate
I would like to ask about settings of inlet condition. In my problem, I have box filled with fluid. At the bottom of the tank is tube that fills the tank, sketch attached. (It's a simulation of weir.)
Inflow is located as pointed in figure. I know volumetric flow rate and diameter of inlet pipe as input data. Buffer was fitted (so particles infront couldn't go backward around the buffer) to inlet pipe and inlet velocity was chosen to satisfy prescribed VFR.
From simulation was obvious, that volume flow is significantly lower than has been set. (Due to knowledge of the real system.) After that observation, three methods have been used to determine actual volumetric flow rate. First of all, I measured velocity in the inlet pipe (as shown in figure), then I added box and measured volume of fluid flowing to the box over the weir and finally I used outlet channel to measure velocity of water flowing away from system to compute volume flow. So I could be pretty sure the volumetric flow rate goes wrong.
I tried to play with inlet velocity and diameter of inlet buffer. (Each time I also changed model, specifically diameter of the pipe, so buffer was fitted perfectly. Square profile was also tried, results are same.) If I used smaller area for buffer with higher inlet
velocity I obtained described volumetric flow rate. But with this solution came other problems like rapids and "air capsules" and I don't think that's ideal way how to solve it.
I tested different inlet options in DualSPH v5 (I tried both refilling options, 1:simple below zsurf, 2:advanced for reverse flows) but the results were even worse then results obtained by v4.4. I noticed same problem also in case of vertically oriented pipe with normal oriented against gravity vector.
With proper velocity and smaller diameter of inlet, I'm able to achieve right volumetric flow rate. But if the diameter of inlet pipe is higher (with adjustment for inlet velocity, buffer perfectly fitted), the VFR drops below set value.
I would like to ask you about some ideas of settings for this type of tasks. Is it something what can be solved with proper inlet settings or is this caused by any incompatibility or something?
I will be grateful for your answers.