[Nek5000-users] Pressure drop in simple cylinder flow

[nek5000-users at lists.mcs.anl.gov]

Ajay,

You’ve non-dimensionalized your problem incorrectly.  Param(2) = -Re only works if the length scale of your problem is also 1.  In your case it’s 2.  You need to rescale the mesh such that the diameter is 1, not the radius.

-Dillon

From: Nek5000-users [mailto:nek5000-users-bounces at lists.mcs.anl.gov] On Behalf Of nek5000-users at lists.mcs.anl.gov
Sent: Monday, August 14, 2017 1:03 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: [Nek5000-users] Pressure drop in simple cylinder flow

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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