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Pressure-Center-Calculator

Compute the resultant force and pressure center (center of pressure) of any pressure-loaded surface from scattered surface-pressure point clouds (CFD or wind-tunnel / test data).

Applicability

Any 3-D curved surface carrying a distributed pressure load, for example:

• Turbomachinery: rotor/stator blades, compressor/turbine blades, fans, propellers, rotors, wind/tidal/hydro turbine blades, pump impellers, guide vanes;
• Aerospace: wings (upper/lower surfaces), tail fins/stabilizers, control surfaces, fuselage sections, winglets, UAV surfaces, rocket/missile bodies and fins;
• Automotive: rear/front wings, splitters/diffusers, body sections;
• Marine/offshore: hull sections, rudders/fins, hydrofoils, sails, offshore-platform pressure panels;
• Civil / wind engineering: building facades/cladding, roofs (uplift center), canopies/bridges/signboards/solar panels;
• Other: hydrostatic/dynamic pressure faces (gates, dam faces, tank walls), internal-flow components, etc.

Input: 4 columns per line x y z p (x,y,z in [m], p in [Pa]). The point cloud is the surface shape — no extra geometry needed.

Requirements

• Python 3.8+
• numpy, scipy (versions in requirements.txt)

Install: python -m pip install -r requirements.txt

Usage

python pressure_center.py <ps.dat> [ss.dat] [--inside-point X Y Z] [--cstype aero|mech] [--pref P]

• ps.dat: pressure point cloud of one surface (x y z p). For a two-sided body (blade, wing upper/lower, tail surfaces) this is one side.
• ss.dat (optional): the opposing surface. Use it for two-sided bodies; omit for a single surface (single wing skin, wall, sail) or a closed body.
• --inside-point X Y Z: a point inside the body (aero-frame coordinates, m); normals point away from it → robustly fixes the wetted side. Required for single surfaces; not needed for two-sided (two-file) bodies (oriented automatically via the thickness direction).
• --cstype aero|mech: output coordinate system, default mech. The two differ in axes, not only units (see below).
• --pref P: subtract reference pressure P [Pa] to work in gauge; default is absolute pressure.

Output: resultant force F=(Fx,Fy,Fz), |F|, pressure-center point (on the line of action, closest to the body centroid), line-of-action direction, residual couple.

Coordinate systems

Internal computation is in the aero frame (m, N, N·m). --cstype switches the display frame. Note: mech vs aero differ in axes, not only in units (mm vs m):

aero_x =  mech_z / 1000
aero_y = -mech_y / 1000        # y negated
aero_z =  mech_x / 1000        # x and z swapped
i.e. aero = R · mech,  R = [[0,0,1],[0,-1,0],[1,0,0]], then positions /1000 (mm->m)

Force and moment are only rotated by R (units N, N·m unchanged). --cstype affects display only, not the computation.

Verification (optional)

Verify the program against a known external load (force and moment at a reference point). The report is written to verify_report.md in the working directory.

1. Copy external_load.sample.txt to external_load.txt and fill in the reference point, force, and moment (line format, first line sets the coordinate system):

<cs_type>       # coordinate system: aero (positions m) or mech (positions mm)
<x> <y> <z>     # reference point r_ref (mech=mm, aero=m)
<Fx> <Fy> <Fz>  # [N]
<mx> <my> <mz>  # [N·m]

One value per line, 9 values total; # comments allowed. Position units follow cs_type (mech=mm, aero=m); force/moment are always N, N·m. For single-surface / closed-body verification add --inside-point (as above).

2. Run:

python verify.py --ps ps.dat [--ss ss.dat] [--inside-point X Y Z] --ext external_load.txt --tol 1e-3

--tol is the relative tolerance (1e-3 = 0.1%). The report lists four relative errors (force, moment, pressure center, residual couple, shown as percentages) with per-item PASS/FAIL. Normalization scales come from the .dat files themselves (characteristic length = max point-cloud extent; moment scale = |F| × characteristic length) — no preset load. External mx,my,mz must use the right-hand rule M=∮(r-r_ref)×dF.

Output meaning

• Pressure center: the point on the line of action closest to the body centroid (every point on the line of action has zero moment perpendicular to the force).
• Residual couple: a pure torque along the force axis that cannot be removed by choosing the point (inherent to a 3-D distribution). If it is small relative to |F|×structure size, the load is essentially a single force; otherwise include this torque in structural analysis.

Files

File	Purpose
pressure_center.py	main program (load, triangulate, integrate, pressure center, CLI)
verify.py	verify against an external load, outputs verify_report.md
external_load.sample.txt	external-load template (copy to external_load.txt and fill in)
requirements.txt	dependencies
verify_report.md	report generated by verify.py (appears after running)

Input data files (*.dat) must be supplied by the user and are not in the repo (gitignored to avoid leaking real data).

Assumptions / limitations

• Each patch is a graph over its best-fit plane (no folding); suits typical wing/wall surfaces. A fully closed shell (e.g. a full fuselage) must be split into several graph-like patches.
• Two-sided bodies (two files) are oriented automatically via the thickness direction; single/closed surfaces use --inside-point.
• Pressure is treated as absolute by default; for a thin body the constant part nearly cancels between opposing sides, so the force is essentially correct, though the pressure center may be slightly affected. Use --pref for a gauge basis.

License

Unless otherwise stated, the content of this repository and related materials are licensed under CC BY-NC-SA 4.0 (Attribution-NonCommercial-ShareAlike 4.0: non-commercial use, with attribution and ShareAlike). See LICENSE.

Development/testing notes: see README_DEV.