Sum FX vs. TOTAL FX vs. pressx in Wall Calculator vs. Report Generator vs. Summary File - help me understand

milos28HR3 — Mon, 18 Jul 2022 01:07:11 GMT

I'm running a 2D sim. It is a simple closed curve in Cartesian 2D coordinates. I'm comparing the results from the Wall Calculator, Fluid Forces on Walls section of the Report Generator, and Sum of Fluid Forces on Walls section of the Summary File.

The Wall Calculator provides sum FX and TOTAL FX (and FY, FZ) that match in BOUNDARY ID and Summary sections of the calculator. The forces on the wall (PressX, Y, Z) also match between the Report Generator and the Summary File. However, FX and PressX (Y, Z) forces do not match.

I cannot find any explanation in help files on what these are, exactly. Why are the values of FX and pressX different? Why is PressX named after the pressure but has a unit of force? Does anyone has a better idea what these are, precisely?

I'm pasting the relevant sections of the outputs here for the reference. Emphasis is mine.

Thanks.

1) Wall Calculator (for the selected curve)

BOUNDARY ID 1
----------------
Area 0.267634 mm^2
Avg. Pressure -2.74055 Pa
sum FX 0.672718 micro-Newton
sum FY -3.41277 micro-Newton
sum FZ 0 micro-Newton
Center of Force about X-Axis (Y-Z) 0.0139965 0
Center of Force about Y-Axis (X-Z) -0.00249274 0
Center of Force about Z-Axis (X-Y) 0 0
sum MX 0 microN-mm
sum MY 0 microN-mm
sum MZ -0.000908579 microN-mm

Summary
-------------------
Total area 0.267634 mm^2
TOTAL FX 0.672718 micro-Newton
TOTAL FY -3.41277 micro-Newton
TOTAL FZ 0 micro-Newton
Center of Force about X-Axis (Y-Z) 0.0139965 0
Center of Force about Y-Axis (X-Z) -0.00249274 0
Center of Force about Z-Axis (X-Y) 0 0
TOTAL MX 0 microN-mm
TOTAL MY 0 microN-mm
TOTAL MZ -0.000908579 microN-mm
Torque -0.000908579 microN-mm

2) Fluid Forces on Walls section of the Report

pressx 0.68435 microNewtons
pressy -2.8762 microNewtons
pressz 0 microNewtons
shearx -0.011635 microNewtons
sheary -0.53653 microNewtons
shearz 0 microNewtons

3) Sum of Fluid Forces on Walls section from the Summary File

ShearX, PressX = -0.011635 0.68435 microNewtons
ShearY, PressY = -0.53653 -2.8762 microNewtons
ShearZ, PressZ = 0 0 microNewtons

Re: Sum FX vs. TOTAL FX vs. pressx in Wall Calculator vs. Report Generator vs. Summary File - help me understand

karol.suchon — Mon, 18 Jul 2022 07:43:52 GMT

Hello,

We have two different forces generated by the fluid, which generates by the pressure gradient which is called CFD

Press[Direction] and the Shear stress which is generated by the fluid viscosity called Shear[Direction]. Adding these two forces will give us a real vector of the force which works on the object.

So the difference between the Overall Force and the pressure is connected with the shear stress which just has the opposite direction.

ShearX + PressX = -0.011635 + 0.68435 = 0.672718

If you need more information about that I could search for some information in the documentation or the overall article about the phenomenon.

Best regards,

Karol

Re: Sum FX vs. TOTAL FX vs. pressx in Wall Calculator vs. Report Generator vs. Summary File - help me understand

milos28HR3 — Mon, 18 Jul 2022 16:34:14 GMT

Oh, I see, "pressure" and "shear" are what is generating the forces. The units make sense too now.

Thanks Karol!

topic Sum FX vs. TOTAL FX vs. pressx in Wall Calculator vs. Report Generator vs. Summary File - help me understand in CFD Forum

Sum FX vs. TOTAL FX vs. pressx in Wall Calculator vs. Report Generator vs. Summary File - help me understand

Re: Sum FX vs. TOTAL FX vs. pressx in Wall Calculator vs. Report Generator vs. Summary File - help me understand

Re: Sum FX vs. TOTAL FX vs. pressx in Wall Calculator vs. Report Generator vs. Summary File - help me understand