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Fix Papaevangelo_2010 friction factor: log10 not natural log in numerator #97

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Merged
CalebBell merged 2 commits into from
Jul 26, 2026
Merged

Fix Papaevangelo_2010 friction factor: log10 not natural log in numerator #97

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355b6ee
Fix Papaevangelo_2010: use log10, not natural log, in the numerator
binggao1230 Jul 21, 2026
1d3f3c1
Drop tol=0 from the Colebrook reference so the test doesn't need mpmath
binggao1230 Jul 26, 2026
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6 changes: 3 additions & 3 deletions fluids/friction.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1655,7 +1655,7 @@ def Papaevangelo_2010(Re: float, eD: float) -> float:
(2010) [2]_ as shown in [1]_.

.. math::
f_D = \frac{0.2479 - 0.0000947(7-\ln Re)^4}{\left[\log_{10}\left
f_D = \frac{0.2479 - 0.0000947(7-\log_{10} Re)^4}{\left[\log_{10}\left
(\frac{\epsilon}{3.615D} + \frac{7.366}{Re^{0.9142}}\right)\right]^2}

Parameters
Expand All @@ -1677,7 +1677,7 @@ def Papaevangelo_2010(Re: float, eD: float) -> float:
Examples
--------
>>> Papaevangelo_2010(1E5, 1E-4)
0.015685600818488177
0.018525128421514474

References
----------
Expand All @@ -1691,7 +1691,7 @@ def Papaevangelo_2010(Re: float, eD: float) -> float:
Corfu, Greece: University of Ioannina Greece and Stevens Institute of
Technology New Jersey (2010)
"""
x1 = (7.0-log(Re))
x1 = (7.0-log10(Re))
term = (log10(eD*(1.0/3.615) + 7.366*Re**-0.9142))
return (0.2479-0.0000947*x1*x1*x1*x1)/(term*term)

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21 changes: 20 additions & 1 deletion tests/test_friction.py
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Expand Up @@ -137,7 +137,7 @@ def test_friction_basic():
assert_close(Rao_Kumar_2007(1E5, 1E-4), 0.01197759334600925)
assert_close(Buzzelli_2008(1E5, 1E-4), 0.018513948401365277)
assert_close(Avci_Karagoz_2009(1E5, 1E-4), 0.01857058061066499)
assert_close(Papaevangelo_2010(1E5, 1E-4), 0.015685600818488177)
assert_close(Papaevangelo_2010(1E5, 1E-4), 0.018525128421514474)
assert_close(Brkic_2011_1(1E5, 1E-4), 0.01812455874141297)
assert_close(Brkic_2011_2(1E5, 1E-4), 0.018619745410688716)
assert_close(Fang_2011(1E5, 1E-4), 0.018481390682985432)
Expand All @@ -152,6 +152,25 @@ def test_friction_basic():
assert_close(fd, 0.01628845962146481)
assert_close(ft_Crane(1e-5), 604.8402578042682)

def test_Papaevangelo_2010_range():
# Papaevangelo_2010 is an explicit Colebrook approximation. Across its registered
# range (Re 1e4-1e7, eD 1e-5-1e-3) it must stay positive and track Colebrook within
# its ~1% literature accuracy. A natural-log-for-log10 slip in the numerator
# previously gave errors up to ~260% and negative friction factors above Re~3e6.
worst = 0.0
for Re in logspace(4, 7, 16):
for eD in logspace(-5, -3, 9):
fd = Papaevangelo_2010(Re, eD)
assert fd > 0.0
ref = Colebrook(Re, eD)
worst = max(worst, abs(fd - ref)/ref)
assert worst < 0.01

# the fix must also reach the public dispatcher and the once-negative Re=1e7 corner
assert Papaevangelo_2010(1e7, 1e-4) > 0.0
assert_close(friction_factor(1e7, 1e-4, Method="Papaevangelo_2010"),
Papaevangelo_2010(1e7, 1e-4), rtol=0)

def test_friction():
assert_close(sum(_roughness.values()), 0.01504508)

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