[Nek5000-users] Pressure drop in simple cylinder flow
nek5000-users at lists.mcs.anl.gov
nek5000-users at lists.mcs.anl.gov
Mon Aug 14 13:03:24 CDT 2017
Hello Neks,
As a first excercise learning to mesh using the tools provided in Nek5000,
I ran a simple laminar cylinder flow simulation (geometry obtained by
extending the geometry of cyl3dr.rea from /examples/expansion) and clearly
see a fully-developed flow at the outlet. The maximum velocity is, as
expected, exactly twice the uniform inlet velocity, and I see a parabolic
velocity distribution along the radial direction. However, the pressure
drop is not in line with the theory.
For example, from Cengel, deltaP=8nuLV/(R*R). nu is the kinematic (equal to
dynamic in this case) viscosity, L is the length of the cylinder = 20, V is
the average velocity = 1, and R is the radius of the cylinder = 1.
Also, Re=10=V(2R)/nu, giving nu=(1/5) leading to
deltaP=(8)(1/5)(20)(1)/(1*1)=32. But, the pressure drop (falls linearly, in
line with theory) in my simulation is only 16. I increased the cylinder
length by two times, and again the pressure drop is half of what is
expected. Can anyone tell me if there is something I am not accounting for?
I am new to CFD and this is my first ever problem. I am pasting my input
parameters below.
118 Parameters from file:
1 1.00000 p001 DENSITY
2 -10.0000 p002 VISCOS
8 1.00000 p008 CONDUCT
11 10000.0 p011 NSTEPS
12 -0.100000E-02 p012 DT
15 500.000 p015 IOSTEP
21 0.100000E-05 p021 DIVERGENCE
22 0.100000E-06 p022 HELMHOLTZ
24 0.100000E-01 p024 TOLREL
25 0.100000E-01 p025 TOLABS
26 1.00000 p026 COURANT/NTAU
27 2.00000 p027 TORDER
65 1.00000 p065 #iofiles (eg, 0 or 64); <0 --> sep. dirs
66 6.00000 p066 output : <0=ascii, else binary
67 6.00000 p067 restart: <0=ascii, else binary
93 20.0000 p093 Number of previous pressure solns saved
94 3.00000 p094 start projecting velocity after p94 step
95 5.00000 p095 start projecting pressure after p95 step
99 0.00000 p099 dealiasing: <0--> off/3--> old/4--> new
103 0.100000E-01 p103 weight of stabilizing filter (.01)
Thanks,
Ajay
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