Hydraulic pipe system with branch split

Windows 10, CCX 2.16

Hello,
CalculiX has interesting hydraulic network capabilities, including branches (redirecting flow to two other pipes). There are Calculix examples for gas flow (Branchsplit2) and they work. However, there are difficulties for liquid flows (water). The following input shows a small test example with only one branch split, one entry and two exits.

** Branch Split with IDELCHIK1
*Node,NSET=NALL
1, 0.09, 0., 0.
2, 0.10, 0., 0.
3, 0.15, 0., 0.
4, 0.20, 0., 0.
5, 0.25, 0., 0.
6, 0.30, 0., 0.
7, 0.31, 0., 0.
8, 0.20, 0.05, 0.
9, 0.20, 0.10, 0.
10, 0.20, 0.11, 0.
*Element, type=D, Elset=EALL
1, 0, 1, 2
2, 2, 3, 4
3, 4, 5, 6
4, 6, 7, 0
5, 4, 8, 9
6, 9,10, 0
*Elset, elset=Branch
2,3,5
*Elset, elset=INOUT
1,4,6
*MATERIAL,NAME=WATER
*DENSITY
1000.
*FLUID CONSTANTS
**Cp Eta Temp
4217.,1750.E-6, 273.
*FLUID SECTION,ELSET=InOut,TYPE=PIPE INOUT,MATERIAL=WATER, LIQUID
*FLUID SECTION,MATERIAL=WATER, LIQUID, ELSET=BRANCH,TYPE=BRANCH SPLIT IDELCHIK1
2, 3, 5, 0.008, 0.008, 0.002, 0, 90
0.101, 0.0505, 0, 0, 1, 1, 0
*INITIAL CONDITIONS,TYPE=TOTAL PRESSURE
2, 3E5
4, 2E5
*STEP,INC=1
*HEAT TRANSFER,STEADY STATE
1.,1.
*DLOAD
EALL,GRAV,9.81,0.,0.,-1.
*BOUNDARY,MASS FLOW
1, 1,1, 1.0
*BOUNDARY
6, 2,2, 1E5
9, 2,2, 1E5
*NODE PRINT,NSET=NALL
U,
*NODE PRINT,NSET=NALL
MF,PN
*END STEP

The launcher window says: Error in liquidpipe: loss coefficients for branches do not apply for reversed flow. Due to initial and boundary conditions on pressure DOF 2 there is a pressure gradient downstream that should avoid reversed flow.

If anyone could help, it would be great.
Best regards,
Zweistein

Clearly, the solver claims that there is a reversed flow in your model which is not allowed for branch split type of fluid section.

That’s the system you want to simulate, right ? To make sure that initial and boundary conditions are correct, I would analyze this using standard CFD approach.

Did you check it with gas properties instead of water ?

Hello,
thank you for your quick response. Yes, your plot with the element numbers is correct. Pressure boundary conditions are necessary only for the outlets at nodes 6 and 9 (atmospheric pressure = 1 bar = 1E5 Pa). Initial conditions are helpful for accelerating convergence, they were specified at nodes 2 and 4 in a meaningful way. Trying other combinations at these nodes, all of them result in “reversed flow”. The aim of this test analysis is to find out the pressure drop at the branch split and to gain confidence for larger problems. Theoretical solutions for the 90 degree branch split are available (I.E. Idelchik Handbook of Hydraulic Resistance, Diagram 7.21). It was hoped that alternatives like CFD can be avoided by using 1D-analysis with CalculiX as a first approximation.
I tried the same problem with gas (air properties) → divergence - too many cutbacks.
Thanks and greetings,
Zweistein