Flat Plates: Excuses For The Wrong Answer Or Legitimate Justifications?

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EveningSkyPerson
Posts: 7
Joined: Thu Oct 20, 2022 10:45 pm

Flat Plates: Excuses For The Wrong Answer Or Legitimate Justifications?

Post by EveningSkyPerson »

In the interest of using the simplest shape possible, I thought a Flat Plate might be the easiest for a beginner such as myself, to draw in FreeCAD, the easiest to analyze in FreeCAD CFDof, the easiest to bring in information about, for comparative analysis, from the old, analog, outside aeronautical world, and the easiest for an expert to check to see if it is done correctly. As it turns out, Flat Plates are not that simple. Derek Lisoski wrote an article about the subject
https://thesis.library.caltech.edu/1265 ... a_1993.pdf -- it has eight chapters. Continuing forward anyway, I drew a small (one square foot, .0929 Sq M) plate on the ground with a 100 MPH (44.7 M/sec) wind, using Thomas Schrader's parameters for 'Flow Around A Car'. I'm not sure if those same parameters can be used with different wind velocities. With a Cd of 1.28 I was expecting 32.76 lbs per sq ft (159.95 Kg/Sq M).
After running a simulation I got 29.72 lbs per sq ft (145.09 Kg/Sq M). I sort of assumed that the slippage around the edges would be about the same measurement on a small plate or a big plate, but proportionally greater on a small plate, which would possibly explain why the drag was less than expected. So I drew a larger plate (On the ground, 100 sq ft, 9.29 Sq M 100 MPH wind 44.7 M/sec). Expecting 32.76 lbs per square foot ( 159.95 Kg/ sq M), now I got 34.79 lbs per sq ft (194.25 Kg/Sq M). So then I thought maybe it's greater than expected because it's on the ground and there is no slippage around the bottom edge. So I drew a suspended, one sq ft (.0929 Sq M, 100 MPH wind, 44.7 M/sec) plate. Again expecting 32.76 lbs/sq ft, now I got 26.84 lbs/ sq ft (131.03 Kg/Sq M). I thought possibly it is less than expected because all the edges are exposed to slippage. Then I drew a suspended 100 sq ft (9.29 Sq M) 100 MPH wind (44.7 M/sec) plate. I got 32.13 lbs per sq ft (156.85 Kg/sq M) which is the closest to the expected 32.76 lbs/sq ft (159.95 Kg/sq M). So I was wondering, did I make mistakes in the set up or is this a reasonable explanation of flat plate scale effect?

Code: Select all

OS: CAElinux2020 (XFCE/xubuntu)
Word size of FreeCAD: 64-bit
Version: 0.20.1.29410 (Git) AppImage
Build type: Release
Branch: (HEAD detached at 0.20.1)
Hash: f5d13554ecc7a456fb6e970568ae5c74ba727563
Python 3.10.5, Qt 5.15.4, Coin 4.0.0, Vtk 9.1.0, OCC 7.6.2
Locale: English/United States (en_US)
Installed mods: 
  * CfdOF 1.20.4
  * Help 1.0.3
  * Plot 2022.4.17
One Sq Ft On Ground Flat Plate 100MPH Wind.FCStd
(75.74 KiB) Downloaded 36 times
100sqF On Groundt100MPH.FCStd
(38.17 KiB) Downloaded 40 times
One Sq Ft Suspended Flat Plate 100 MPH.FCStd
(39.24 KiB) Downloaded 36 times
100 Sq Ft Suspended 100 MPH.FCStd
(39.51 KiB) Downloaded 40 times
OneSqFt100SqFt.png
OneSqFt100SqFt.png (540.56 KiB) Viewed 723 times
BobHope
Posts: 22
Joined: Fri Dec 16, 2022 9:10 am

Re: Flat Plates: Excuses For The Wrong Answer Or Legitimate Justifications?

Post by BobHope »

Calcs look correct

q = (1.2 kg/m^3) / 2 * (44.7 m/s)^2
= (1.2 / 2) * (44.7 m/s ^2)
= 0.6 * 1998.09
= 1198.8 N/m^2

For the love of god, switch to SI units my friends, its a lot easier.

Some comments

- plate is still too close the wall. For accuracy, a figure of ten times the object width to the nearest is recommended, however I have found 3-4 times is often good enough.

The box you have drawn is not a plate, it has thickness. This may be altering your result.
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