Bug: friction_model="nikuradse" adds laminar + turbulent lambda instead of selecting by Reynolds number

[Haplo98]

Bug description

The default friction model "nikuradse" unconditionally adds lambda_laminar + lambda_nikuradse instead of selecting the appropriate value based on Reynolds number. This produces incorrect friction factors for all flow regimes, with the largest relative error in the laminar/transition zone.

Location

pandapipes/pf/derivative_calculation.py, function calc_lambda(...), the else branch at lines 194-197 (pandapipes 0.13.0):

else:
    # lambda_tot = np.where(re > 2300, lambda_laminar + lambda_nikuradse, lambda_laminar)
    lambda_tot = lambda_laminar + lambda_nikuradse
    return lambda_tot, re

This branch is taken whenever friction_model is neither "colebrook" nor "swamee-jain" — i.e. it is the path used for "nikuradse" (the default).

Expected behaviour

Standard textbook treatment: pick laminar or turbulent, do not sum them.

lambda_tot = np.where(re > 2300, lambda_nikuradse, lambda_laminar)

• Laminar (Re < 2300): lambda_laminar = 64/Re (Hagen-Poiseuille)
• Turbulent (Re >= 2300): lambda_nikuradse (Nikuradse fully-rough approximation)

The commented-out line directly above the bug also has an issue: it would still add both values in the turbulent branch (lambda_laminar + lambda_nikuradse) rather than using only lambda_nikuradse.

Actual behaviour / impact

Re	lambda_laminar	lambda_nikuradse	Current (sum)	Correct	Error
500	0.1280	0.0196	0.1476	0.1280	+15%
2000	0.0320	0.0196	0.0516	0.0320	+61%
80979	0.0008	0.0196	0.0204	0.0196	+4%

The error is most severe near the transition zone (Re ~ 2000-3000) where both terms are similar in magnitude.

Minimal reproducer

import pandapipes as pp

net = pp.create_empty_network(fluid="water")
j0 = pp.create_junction(net, pn_bar=6, tfluid_k=363.15)
j1 = pp.create_junction(net, pn_bar=6, tfluid_k=363.15)
pp.create_pipe_from_parameters(net, j0, j1, length_km=0.5, diameter_m=0.1, k_mm=0.1)
pp.create_ext_grid(net, j0, p_bar=6.0, t_k=363.15)
pp.create_sink(net, j1, mdot_kg_per_s=2.0)

pp.pipeflow(net, mode="hydraulics", friction_model="nikuradse")
dp_nik = net.res_pipe.p_from_bar.iloc[0] - net.res_pipe.p_to_bar.iloc[0]

pp.pipeflow(net, mode="hydraulics", friction_model="colebrook")
dp_cb = net.res_pipe.p_from_bar.iloc[0] - net.res_pipe.p_to_bar.iloc[0]

print(f"dp nikuradse: {dp_nik:.4f} bar")
print(f"dp colebrook: {dp_cb:.4f} bar")
print(f"ratio:        {dp_nik/dp_cb:.3f}")
# Observed ratio ~0.9 at high Re (should be ~1.0); discrepancy comes from the
# spurious +lambda_laminar term that is still added in the fully turbulent regime.

Workaround

Pass friction_model="colebrook" (or "swamee-jain") explicitly — both code paths return early before the buggy else branch.

pp.pipeflow(net, friction_model="colebrook")

Suggested fix

Replace line 196 with:

lambda_tot = np.where(re > 2300, lambda_nikuradse, lambda_laminar)
return lambda_tot, re

I am happy to open a PR if the maintainers agree with the fix.

Environment

• pandapipes 0.13.0 (installed at site-packages/pandapipes/)
• Python 3.14.3
• Windows 10