R8-e: the coupled 3D FSI's small-dt instability located in the load/kinematics transfer, not a lagged fluid response
- tests/fsi2_embedded3_impulse.rs (new, ignored, cuda): the added-mass timing instrument — the R8-a fluid with a prescribed centreline velocity step (a one-step acceleration delta), baseline and impulse passes from one device snapshot, the load difference per step. Pitch shape on the slab ny 62: generalized added mass 39.11 / 39.09 / 39.07 kg/m at dt x 1 / 0.75 / 0.5, the step after carries 1.7 / 1.3 / 0.9 % above the steady tail — the cut-cell fluid answers in the same step (the lagged-added-mass hypothesis refuted). - fsi2_embedded3.rs: `RTX_E3FSI_LINE_CSV` (per-step centreline + mid/bottom/top y per station) showed the growing 2dt mode is a through-thickness mode — the edge nodes move against the mid-plane nodes (~4:1): the fluid reads the mid-plane, the loads land on the edges, so the added mass acts on that mode with the wrong sign (a negative effective added mass; the scalar Newmark model is unstable for it at omega dt below ~1-2 with gamma 0.7, stable at larger dt or gamma 0.9 — R8-a's observed pattern). `RTX_E3FSI_KIN=surface` (default off, byte-identical when unset): the fluid's centreline = the mean of the bottom and top edge nodes (the nodes the loads land on) — dt x 0.75 / 0.5 / 0.25 stable at gamma 0.7. Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
This commit is contained in:
Omar Sobh
co-authored by
Claude Opus 5.5
Claude Opus 5.5
parent
171da41ed1
commit
74fac32a91
@@ -0,0 +1,220 @@
|
||||
//! R8-e: the added-mass timing instrument for the coupled 3D FSI's small-dt
|
||||
//! instability. The R8-a fluid (the embedded3 cut-cell device path, the
|
||||
//! slab by default) with the flag's centreline PRESCRIBED: rigid to `T0`,
|
||||
//! then from one snapshot two passes of `N` steps each — the baseline (the
|
||||
//! flag at rest) and the impulse (the centreline's velocity steps from 0 to
|
||||
//! `V` × shape at step `N_PRE`: its wall acceleration is a single-step
|
||||
//! delta V/dt; `MODE=pulse` returns the velocity to 0 one step later, a
|
||||
//! +V/dt, −V/dt pair). The difference of the two passes' loads, step by
|
||||
//! step, is the fluid's response to the impulse: an added-mass response
|
||||
//! lives in the impulse's own step; a lagged part shows one step later.
|
||||
//!
|
||||
//! Knobs `RTX_E3IMP_*`: `NY` (62), `NZ` (4), `T0` (0.2 s), `N_PRE` (4),
|
||||
//! `N` (24), `V` (0.05 m/s at the tip), `SHAPE` (`pitch` about the root,
|
||||
//! 1 at A; `heave`; `zig`, `tip`), `MODE` (`step`, `pulse`), `CSV` (per-step series).
|
||||
//! The fluid's own knobs as the R8-a harness (`RTX_E3FSI_DT_SCALE`, …).
|
||||
#![cfg(feature = "cuda")]
|
||||
|
||||
#[path = "fsi2_embedded3/fluid.rs"]
|
||||
mod fluid;
|
||||
#[path = "fsi2_embedded3/state.rs"]
|
||||
mod state;
|
||||
|
||||
use std::io::Write as _;
|
||||
|
||||
use fluid::{CX, CY, Contribution, E3Fluid, HALF, Line, R_CYL};
|
||||
|
||||
pub fn env_f(name: &str, default: f64) -> f64 {
|
||||
std::env::var(name)
|
||||
.ok()
|
||||
.and_then(|v| v.parse().ok())
|
||||
.unwrap_or(default)
|
||||
}
|
||||
|
||||
const X0: f64 = 0.25;
|
||||
const STATIONS: usize = 35;
|
||||
const CHORD: f64 = 0.35;
|
||||
|
||||
fn line(t: f64, c: &[f64], v: &[f64]) -> Line {
|
||||
Line {
|
||||
t,
|
||||
pts: (0..STATIONS)
|
||||
.map(|k| [X0 + 0.01 * k as f64, 0.2 + c[k]])
|
||||
.collect(),
|
||||
vel: (0..STATIONS).map(|k| [0.0, v[k]]).collect(),
|
||||
}
|
||||
}
|
||||
|
||||
/// The flag's loads from the route's contributions (per unit span): fy on
|
||||
/// the flag, fy by part, and the generalized force Σ fy φ(x) with the
|
||||
/// prescribed shape's φ.
|
||||
fn flag_loads(
|
||||
contrib: &[Contribution],
|
||||
pts: &[[f64; 2]],
|
||||
width: f64,
|
||||
phi: impl Fn(f64) -> f64,
|
||||
) -> (f64, f64, [f64; 4]) {
|
||||
let (mut fy, mut q, mut parts) = (0.0, 0.0, [0.0; 4]);
|
||||
for &(pos, c, part, v) in contrib {
|
||||
if c != 1 {
|
||||
continue;
|
||||
}
|
||||
let (x, y) = (pos[0], pos[1]);
|
||||
let mut best = f64::INFINITY;
|
||||
for m in 0..pts.len() - 1 {
|
||||
let (a, b) = (pts[m], pts[m + 1]);
|
||||
let (ex, ey) = (b[0] - a[0], b[1] - a[1]);
|
||||
let u = (((x - a[0]) * ex + (y - a[1]) * ey) / (ex * ex + ey * ey)).clamp(0.0, 1.0);
|
||||
best = best.min(((x - a[0] - u * ex).powi(2) + (y - a[1] - u * ey).powi(2)).sqrt());
|
||||
}
|
||||
let d_cyl = ((x - CX).powi(2) + (y - CY).powi(2)).sqrt() - R_CYL;
|
||||
if d_cyl < best - HALF {
|
||||
continue;
|
||||
}
|
||||
let v = v / width;
|
||||
fy += v;
|
||||
q += v * phi(x);
|
||||
parts[part] += v * phi(x);
|
||||
}
|
||||
(fy, q, parts)
|
||||
}
|
||||
|
||||
#[test]
|
||||
#[ignore = "R8-e: the added-mass timing instrument (GPU, a minute)"]
|
||||
fn impulse_response() {
|
||||
let ny = env_f("RTX_E3IMP_NY", 62.0) as usize;
|
||||
let nz = env_f("RTX_E3IMP_NZ", 4.0) as usize;
|
||||
let t0 = env_f("RTX_E3IMP_T0", 0.2);
|
||||
let n_pre = env_f("RTX_E3IMP_N_PRE", 4.0) as usize;
|
||||
let n = env_f("RTX_E3IMP_N", 24.0) as usize;
|
||||
let vel = env_f("RTX_E3IMP_V", 0.05);
|
||||
let shape = std::env::var("RTX_E3IMP_SHAPE").unwrap_or_else(|_| "pitch".into());
|
||||
let mode = std::env::var("RTX_E3IMP_MODE").unwrap_or_else(|_| "step".into());
|
||||
// The shape per station (`zig`: alternate stations ±, the root clamped;
|
||||
// `tip`: the last 5 stations), linear between stations.
|
||||
let phis: Vec<f64> = (0..STATIONS)
|
||||
.map(|k| {
|
||||
let x = X0 + 0.01 * k as f64;
|
||||
match shape.as_str() {
|
||||
"heave" => 1.0,
|
||||
"zig" => {
|
||||
if k == 0 {
|
||||
0.0
|
||||
} else if k % 2 == 0 {
|
||||
1.0
|
||||
} else {
|
||||
-1.0
|
||||
}
|
||||
}
|
||||
"tip" => {
|
||||
if k + 5 >= STATIONS {
|
||||
1.0
|
||||
} else {
|
||||
0.0
|
||||
}
|
||||
}
|
||||
_ => ((x - X0) / CHORD).clamp(0.0, 1.0),
|
||||
}
|
||||
})
|
||||
.collect();
|
||||
let phi = |x: f64| -> f64 {
|
||||
let s = ((x - X0) / 0.01).clamp(0.0, (STATIONS - 1) as f64);
|
||||
let m = (s.floor() as usize).min(STATIONS - 2);
|
||||
let u = s - m as f64;
|
||||
(1.0 - u) * phis[m] + u * phis[m + 1]
|
||||
};
|
||||
let zero = vec![0.0; STATIONS];
|
||||
let rest = line(0.0, &zero, &zero);
|
||||
let mut fl = E3Fluid::build(ny, nz, 3.0, rest.clone(), None);
|
||||
let dt = fl.dt;
|
||||
let rigid = (t0 / dt).round() as usize;
|
||||
for s in 0..rigid {
|
||||
let r = fl.step();
|
||||
assert!(r.final_residual.is_finite(), "rigid death at {s}");
|
||||
}
|
||||
let t_start = fl.time();
|
||||
println!(
|
||||
" R8-e impulse: ny {ny} nz {nz} dt {dt:.4e}, rigid {rigid} steps to t {t_start:.4}; shape {shape}, mode {mode}, V {vel} m/s (acceleration V/dt {:.2} m/s²) at step {n_pre} of {n}",
|
||||
vel / dt
|
||||
);
|
||||
let snap = fl.snapshot();
|
||||
// Per pass: per step (t, tip velocity, fy, Q, Q parts, lift total).
|
||||
let mut rows: Vec<Vec<[f64; 9]>> = Vec::new();
|
||||
for pass in 0..2 {
|
||||
fl.restore(&snap);
|
||||
let mut c = zero.clone();
|
||||
let mut prev = rest.clone();
|
||||
prev.t = t_start;
|
||||
let mut out = Vec::new();
|
||||
for s in 0..n {
|
||||
let t_new = t_start + (s + 1) as f64 * dt;
|
||||
// The step's centreline velocity (constant over the step).
|
||||
let v_tip = if pass == 0 || s < n_pre {
|
||||
0.0
|
||||
} else if mode == "pulse" {
|
||||
if s == n_pre { vel } else { 0.0 }
|
||||
} else {
|
||||
vel
|
||||
};
|
||||
let v: Vec<f64> = phis.iter().map(|p| p * v_tip).collect();
|
||||
for k in 0..STATIONS {
|
||||
c[k] += dt * v[k];
|
||||
}
|
||||
let next = line(t_new, &c, &v);
|
||||
fl.set_lines(prev.clone(), next.clone());
|
||||
let r = fl.step();
|
||||
assert!(
|
||||
r.final_residual.is_finite(),
|
||||
"death at pass {pass} step {s}"
|
||||
);
|
||||
let (tot, contrib) = fl.loads();
|
||||
let (fy, q, parts) = flag_loads(&contrib, &next.pts, fl.load_width, &phi);
|
||||
out.push([
|
||||
t_new, v_tip, fy, q, parts[0], parts[1], parts[2], parts[3], tot[1],
|
||||
]);
|
||||
prev = next;
|
||||
}
|
||||
rows.push(out);
|
||||
}
|
||||
let mut csv = std::env::var("RTX_E3IMP_CSV")
|
||||
.ok()
|
||||
.map(|p| std::fs::File::create(p).expect("csv"));
|
||||
if let Some(f) = csv.as_mut() {
|
||||
writeln!(
|
||||
f,
|
||||
"k,t,v_tip,fy0,q0,fy,q,dfy,dq,dq_p,dq_shear,dq_xdiff,dq_xconv,dlift"
|
||||
)
|
||||
.unwrap();
|
||||
}
|
||||
let q_imp = rows[1][n_pre][3] - rows[0][n_pre][3];
|
||||
for s in 0..n {
|
||||
let (a, b) = (&rows[0][s], &rows[1][s]);
|
||||
let k = s as i64 - n_pre as i64;
|
||||
let d = |i: usize| b[i] - a[i];
|
||||
if let Some(f) = csv.as_mut() {
|
||||
writeln!(
|
||||
f,
|
||||
"{k},{:.6},{:.4e},{:.6},{:.6},{:.6},{:.6},{:.6e},{:.6e},{:.6e},{:.6e},{:.6e},{:.6e},{:.6e}",
|
||||
b[0], b[1], a[2], a[3], b[2], b[3], d(2), d(3), d(4), d(5), d(6), d(7), d(8)
|
||||
)
|
||||
.unwrap();
|
||||
}
|
||||
if (-1..=6).contains(&k) {
|
||||
println!(
|
||||
" k {k:+}: v_tip {:.3e} ΔQ {:+.5e} ({:+.4} of the impulse step) parts p {:+.4e} shear {:+.4e} xdiff {:+.4e} xconv {:+.4e} Δfy {:+.5e}",
|
||||
b[1],
|
||||
d(3),
|
||||
d(3) / q_imp,
|
||||
d(4),
|
||||
d(5),
|
||||
d(6),
|
||||
d(7),
|
||||
d(2)
|
||||
);
|
||||
}
|
||||
}
|
||||
println!(
|
||||
" IMPULSE ΔQ(k=0) {q_imp:+.5e} N/m → generalized added mass −ΔQ·dt/V {:.4} kg/m",
|
||||
-q_imp * dt / vel
|
||||
);
|
||||
}
|
||||
Reference in New Issue
Block a user