Hi,

Thank you Thomas, I found this in a cfd forum. I should read OpenFOAM documents in more detail.

"OpenFOAM uses the rho-normalized pressure p*=p/rho

[p*] = {kg/(m.s**2)} /(kg/m**3) = m**2/s**2 = [0 2 -2 0 0 0 0]

in your BC you just have to divide your real pressure with your density"

I run the simulation again with inlet pressure set to 290 Pa / 1000 kg/m3 and obtained the following result. Although I still not able to get pressure residual down below 0.1. The simulation results are very close to that calculated from formulas.

I will try to fillet the strainer openings' edges and run the simulation again with different settings to see if the residual can drop.

**One thing I want to ask, why the model display below in ParaView has some stranger walls inside the pipe?** It should only be a simple cylinder with 25 holes at the middle of the pipe.

**Simulation Result**
Inlet Flow Velocity: 0.17 m/s

Peak Flow Velocity at Strainer Opening:

**0.83 m/s**
Outlet Opening Velocity: 0.53 m/s

Volume Flow Rate:

**0.00019 m3 / s**
**Calculated from formulas**
Outlet velocity when draining a tank or container: v = Cv * (2 g H )^0.5

v = outlet velocity (m/s)

Cv = velocity coefficient (water 0.97)

g = acceleration of gravity (9.81 m/s2)

H = height (m)

v = 0.97 * (2 * 9.81 * 0.03)^0.5 =

**0.74 m/s**
which is very close to the velocity simulation result at the strainer opening.

Liquid volume flow rate when draining a tank or container: V = N * Cd * A * (2 g H)^0.5

V = volume flow (m3/s)

N = number of apertures

A = area of aperture - flow outlet (m2)

Cd = discharge coefficient = Cc * Cv

Cc = contraction coefficient (sharp edge aperture 0.62, well rounded aperture 0.97)

V = 25 * 0.62 * 0.97 * 0.00001256 * (2 * 9.81 * 0.03)^0.5 =

**0.00014 m3 / s**
which is very close to the volume flow rate obtained in ParaView from the simulation result.

*Note: Correction, the strainner hole diameter is 4 mm each, not 2.5 mm as mentioned in the first post.*