Hood Geometry Comparison

This diagram compares hood configurations over the same cooking source, showing why overhang — the distance the hood extends beyond the cooking surface along the axis the wind is blowing — is the single most important design parameter for outdoor capture performance.

Reference readings

Representative values below are computed directly from the same capture-fraction module (physics/capture.mjs) driving the width-versus-mount comparison above — a medium gas grill (24 × 21-inch cooking surface) at a fixed 30-inch rise in an 8 mph wind, wall hoods 36 inches deep and island hoods 40 inches deep, the depths the instruments draw; live values update as you move the width and wind controls. The table is regenerated by scripts/generate-reference-tables.mjs, never typed by hand.

Under a rear wind — the instrument’s comparison axis — widening the hood from 42 to 72 inches changes nothing: the wall-mounted hood holds about 88% and the island hood about 12% across the entire lineup, because a wind blowing front-to-back is resisted only by the hood’s front-to-back overhang, and width adds none [RB-006 §3.4]. The 76-point gap between the two mounts at the same width is the wall’s doing: RB-006 credits a solid rear wall with a 60–80% reduction in the wind the plume feels [RB-006 §3.9.2], and RB-005 describes the same rear-boundary redirection of escaped plume gas back into the hood [RB-005 §3.4.4]. Under a side wind, width is exactly the lever: at the same 8 mph, modeled capture rises from about 11% at 42 inches to 74% at 72 inches, and the wall is parallel to the flow, so both mounts read the same.

Key Findings

The no-JS fallback diagram for this instrument compares three hood widths over the papers’ reference source — a medium gas grill (24-inch cooking surface) at a 30-inch mounting height, where the plume capture diameter has grown to 41 inches [RB-002 Table 3.3a, Table 3.9]:

No overhang. A hood the same width as the cooking surface (24 inches) sits 17 inches inside the 41-inch plume — escape on both sides is guaranteed by geometry alone. Inadequate overhang is failure mode FM-1, which RB-010 finds present from the day of installation in consumer hoods paired with medium or larger sources [RB-010 Gap S-5].

Minimum geometry (9-inch overhang per side). Hood width equal to the plume capture diameter (W_min = d_capture, 41 inches here) covers the 98% time-averaged flux contour in quiescent conditions — but provides no margin for turbulence, wind, or instantaneous plume fluctuation [RB-005 §3.1, Table 3.1a; RB-002 §3.6]. In the capture model, a 42-inch hood in still air reads about 74% along the side-wind axis, RB-008 Table 3.10’s “65–75% max” [RB-008 §3.10].

Recommended outdoor (17-inch overhang per side). The base width-sizing margin K ≈ 1.38 from RB-002 (turbulent intermittency × puffing oscillations) yields a 57-inch recommended hood width for this source and height, which the model reads at about 96% in still air along the side-wind axis; wind-exposed and open sites scale up toward the wind-inclusive margin K = 1.70 per RB-002 §3.5 [RB-002 Table 3.7; RB-005 Table 3.1b].

Design Priority

RB-005 established the hierarchy: geometry precedes airflow. No amount of exhaust CFM can capture plume gas that is physically beyond the hood’s boundary. Overhang must be sized first — along the axis the prevailing wind will push the plume — and CFM is determined after geometry is set [RB-005 §4.1].

Source Papers

• RB-005: Hood Geometry and Capture — Overhang analysis (§3.1), wall-mount capture (§3.4.4), design hierarchy (§4.1)
• RB-002: Entrainment and Lateral Plume Spread — Plume width data and hood width/overhang margins (base K≈1.38, wind-inclusive K=1.70)