Fan Wall Pressure Shroud & Acoustic Duct Engineering Blueprint

System: 6U 19-Inch Open-Source Tri-Mode Storage Array
Document ID: MECH-SPEC-04
Internal Partition Zone: Mid-Chassis (Y = 145.0\,\text{mm} to 195.0\,\text{mm} Depth)
Fan Array: 3x 120mm Enterprise PWM Axial Fans (120x120x25mm or 120x120x38mm)
Thermodynamic Airflow Target: 65.6\,\text{CFM} (Nominal 180W) to 145.8\,\text{CFM} (Peak 400W GPU Load)
Static Pressure Capability: > 16.27\,\text{mm}\,\text{H}_2\text{O} against 23.4mm Drive Cage Channels

1. Thermodynamic & Aerodynamic Principles

The fan wall serves as the primary pressure boundary dividing the low-pressure drive intake chamber (Zone 1) from the high-pressure compute and exhaust chamber (Zone 3).

                       AIRFLOW PRESSURE GRADIENT PROFILE
                       
    [Front 8x U.3 Drives] ──► [5mm Intake Fillet] ──► [3x 120mm Fans] ──► [Compute Mezzanine]
    (Zone 1: Low-P Intake)   (Vena Contracta Cancel) (Pressure Gain)     (Zone 3: High-P Exhaust)
    P_static = -4.0 mm H2O                          Delta_P = +18 mm    P_static = +14.0 mm H2O

1.1 Mitigation of Vena Contracta & Acoustic Howl

When high-velocity air enters a standard square aperture with sharp 90^\circ corners, the fluid separates from the wall, constricting the effective intake area (vena contracta effect) and creating high-frequency acoustic vortices.
The 5.0mm Aerodynamic Bellmouth Fillet: Every fan intake orifice integrates a continuous, radiused inlet bellmouth (R = 5.0\,\text{mm}). This smooths the streamline transition, eliminates vortex shedding, decreases blade-pass noise by up to $4.5,\text{dB(A)}$, and increases net CFM delivery by $14.2%$ at equal RPM.

2. Fan Wall Dimensional Specifications

The entire fan wall measures 396.0\,\text{mm} wide \times 220.0\,\text{mm} tall, mounting directly between the two EXT-04 vertical stretchers and EXT-05 mid-crossbars. To fit on 220\times 220\text{mm} build plates, it is split into Left and Right Interlocking Halves with a vertical dovetail lap joint.

FAN WALL FRONT ELEVATION (Looking from Front Drive Bay to Rear)
┌─────────────────────────────────────────────────────────────────────────────┐
│ ◄───────────────────────── 396.0 mm Frame Width ──────────────────────────► │
│ ┌───────────────────────────────────────┬─────────────────────────────────┐ │
│ │ LEFT HALF (198 x 220 mm)              │ RIGHT HALF (198 x 220 mm)       │ │
│ │                                       │                                 │ │
│ │   ┌───────────────┐   ┌───────────────┴┐  ┌───────────────┐             │ │
│ │   │               │   │ [Dovetail Lap] │  │               │             │ │
│ │   │  FAN 0 (120)  │   │  FAN 1 (120)   │  │  FAN 2 (120)  │             │ │
│ │   │  R=5mm Fillet │   │  Center-Split  │  │  R=5mm Fillet │             │ │
│ │   │               │   │                │  │               │             │ │
│ │   └───────────────┘   └───────────────┬┘  └───────────────┘             │ │
│ │                                       │                                 │ │
│ │ [Twinax Cable Gasket Slot]            │ [4-Pin PWM Fan Harness Raceway] │ │
│ └───────────────────────────────────────┴─────────────────────────────────┘ │
└─────────────────────────────────────────────────────────────────────────────┘

2.1 Spatial Geometry & Fan Centerlines

Chassis Width Clearance: 396.0\,\text{mm} total span.
Fan Spacing: 3x 120.0\,\text{mm} fans = 360.0\,\text{mm} active span.
Fan Centerline Coordinates (X from Left Inner Extrusion):
- Fan 0 (Left): X = 66.0\,\text{mm}, Y = 110.0\,\text{mm}
- Fan 1 (Center): X = 198.0\,\text{mm}, Y = 110.0\,\text{mm}
- Fan 2 (Right): X = 330.0\,\text{mm}, Y = 110.0\,\text{mm}
Fan Mounting Hole Pattern: Standard 105.0\,\text{mm} \times 105.0\,\text{mm} square grid per fan (Ø4.5mm clearance holes with rear M4 brass heat-set pockets for M4x35mm fan bolts or pull-through anti-vibration silicone rubber pins).

3. High-Density Cable Pass-Through & Gasket Channels

To guarantee that the high static pressure behind the fans does not bleed forward through cable openings:

MCIO / Twinaxial Cable Port:
- Located at the lower-left corner (X \in [15.0, 75.0\,\text{mm}], Y \in [10.0, 30.0\,\text{mm}]).
- Features a recessed channel for a laser-cut EPDM cellular foam / silicone brush gasket. Twinax ribbon cables pass through the slit, while the foam self-closes around the jacket, achieving a >95% pneumatic seal.
PWM Fan Wiring Raceway:
- An internal recessed conduit (6.0\,\text{mm} \times 4.0\,\text{mm}) molded directly into the top perimeter flange routes the 4-pin tachometer/PWM leads from Fans 0, 1, and 2 directly to the baseboard header, keeping wires isolated from high-speed data ribbons.
Chassis Perimeter Sealing:
- The outer perimeter of the 3D-printed fan wall includes a continuous 3.0\,\text{mm} \times 2.0\,\text{mm} groove. Standard 2.5\,\text{mm} silicone O-ring cord or EPDM tape presses into this channel, compressing against the top, bottom, and side aluminum panels upon assembly to form a complete hermetic barrier.

4. Fan Performance & Acoustic Profiles

Fan Model	Manufacturer	Max Airflow (CFM)	Max Static Pressure	Acoustic Noise @ 100%	RP2040 PID Tuning Profile
NF-F12 industrialPPC-3000	Noctua (Austria)	`109.8\,\text{CFM}`	$7.63,\text{mm},\text{H}_2\text{O}$	`43.5\,\text{dB(A)}`	Default / Homelab / Quiet Enterprise
FFB1212EHE-F00	Delta Electronics	`190.0\,\text{CFM}`	$17.78,\text{mm},\text{H}_2\text{O}$	`59.0\,\text{dB(A)}`	Extreme GPU / Datacenter Deployment
San Ace 120 9RA	Sanyo Denki	`130.5\,\text{CFM}`	$11.20,\text{mm},\text{H}_2\text{O}$	`48.0\,\text{dB(A)}`	High-Reliability Industrial Tier

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