[1-Bort-1]

Julia was translated from Python v1.0.0, and Python's v2.3.0 changed its default results. On swept and curved wings, which is every kite, the two had drifted several percent apart. With this branch, Julia matches Python's circulation, angles, per-panel forces and moments, totals and coefficients to six digits on nine reference cases generated by running the Python code.

wing, 8°	CL before	CL now = Python	CD before	CD now = Python
straight	0.6906	0.6910	0.0186	0.0151
swept, tapered	0.8456	0.8256	0.0364	0.0299
arc	0.5947	0.5987	0.0354	0.0325
V3 kite	1.0509	1.0992	0.4910	0.4575

What moved: circulation now follows the effective speed. The chord and chordwise axis are measured square to the bound vortex. Forces turn with the flow at the quarter chord, including a correction for a curved bound vortex, and drag runs along that flow. The force on the chordwise legs is on by default and shared per section boundary. Each wake follows its own panel's inflow, and reference pressure and directions leave out body rates. The defaults also change to 5000 iterations and a relaxation factor of 0.8 of the stability limit, so the curved reference wing that used to diverge now converges.

The same default change ends the lost induced drag of #318: the straight elliptic wing of aspect ratio 4 from Gaunaa, Li & Pirrung 2026 now reaches an Oswald efficiency of 0.9947, against 0.7295 with the quarter-chord force direction off.

Agreeing with Python does not make swept wings right. Against that paper's vortex-lattice results, Oswald efficiency is off by −8.2 % at 15 % sweep, −8.0 % at 10 % sweep with 25 % anhedral and −11.1 % at 25 % sweep, where the solver does not converge. The paper's own model is off by +5.6 %, −0.5 % and +1.1 %. Purely curved wings are off by ±2.4 %. These were measured on the previous head by the task behind #400, whose card plots them, and the chord direction on the swept wings is an assumption. #416 tracks the gap; closing it may mean leaving Python's rule, which is the owner's choice.

Anyone relying on the defaults gets new numbers, including KiteModels wherever its settings do not name these options. Setting the two force options to false and naming a relaxation factor restores those switches, but not the geometry or circulation changes.

Decisions I made:

• Convergence is still measured on the unrelaxed residual at 1e-5, because it does not depend on the relaxation factor. Python's relaxed step at 1e-6 is similar in size, and converged results are the same.
• Warm starting from the previous circulation stays on, because simulation loops rely on it. Past stall this can pick a different solution from Python, which starts from zero.
• Sideslip from angles keeps Julia's unit-length apparent wind. Python's is not unit length (1.00045 at 10° and 10°), so the reference cases pass the inflow as a vector.
• With several wings the curved correction acts only within each wing; Python refuses more than one wing.
• The correction is kept per shape and rebuilt after any shape change. Python's reuse under rigid rotation is not ported.
• Drag beyond a polar table is now held, as in Python, for vector polars only. Matrix polars have no Python counterpart and are unchanged.
• Inside a trailing vortex's core the speed is no longer limited, a deliberate match to Python: closer than 7 micrometres to the line at 10 m/s, the speed is the value on the core boundary, 1.85e5 m/s at 1e-7 m for unit circulation. The nine reference cases still match.

Found on the way: faster convergence exposed that unrefined section coefficients were plain means, so they are now area-weighted, with the same code as #402. A semi-infinite filament now gives zero rather than NaN on its own axis, as #408 also does.

This covers default results only, so the issue stays open. The missing features are queued as parts of it. Left out: bridle drag, which stays in KiteModels as ruled on #113, and the other items declined there (flat area, rotate, cosine circulation, configurable aerodynamic-centre fractions) unless the owner says otherwise.

To act on: stacked on #392, which merges first. It takes a different route from #400, whose rule is not Python's, and from #408's core scaling. It conflicts with #402 in the solver, where the aggregation code is identical. SymbolicAWEModels calls the drag-direction helper with an argument that is now gone, so it needs a one-line change when it raises its bound to the release #392 forces.

Verification

• Reproduced first, unchanged code against Python: V3 CL 1.0509 vs 1.0992, CD 0.4910 vs 0.4575, chord off by up to 0.33 m
• test/verification/test_python_reference.jl: green on nine cases (straight, swept, arc, sideslip, LLT, viscous, body rates, distributed inflow, V3). Red before: run through a script on the unchanged code, since the test itself names functions this branch adds
• test/verification/test_verification.jl: the aspect-ratio-4 test is green, 24/24 in the file; red with the quarter-chord force direction disabled (0.7295, 23/24)
• Green: 20 affected test files, including test/bench.jl, test_verification.jl, test_curved_bound_correction.jl, test_forwarddiff.jl
• Docs build clean in a fresh session · repo has no REUSE lint
• Local full suite: running on this head, no result yet · GitHub CI: not yet reported
• calc_forces! stays at 0 B with the new defaults; one rebuild of the correction took 4.2 ms at 20 panels on a loaded box, growing with panels squared
• Benchmark: not yet run, box saturated (load 213 of 20 cores)
• Tests that follow the change, same system: thesis geometry expectations take the square chord and frame, same tolerance (Python loosened its own to 1e-3). Pinned rectangular-wing values move 0.1 % (force x −117.965 → −117.838) with the circulation map. Tests that compare against section-only formulas now set the leg force off. cl is checked against the control-point angle. The NaN assertion on the filament axis is now zero. One trim call tightens its angle tolerance to 1e-6, since its 1e-5 assertion sat at 1.17e-5.
• Reference: Python a2a91cb, numba 0.68.1, numpy 2.5.4; regenerate with test/data/python_reference/generate_reference.py
• Risk: simulation loops that rebuild the shape every step now pay for the correction every step, and that time is not yet measured.

Closes #318

Opened by 1-Bort-1, an AI agent working for @1-Bart-1.
Refs #410 · task VortexStepMethod.jl-410

[1-Bort-1]

Local full suite: PASS (26 min, Julia 1.13.3, one cell of the matrix)