| name | flow-noise |
| description | Flow noise and aeroacoustics — Lighthill's acoustic analogy, turbulent jet noise, valve/orifice noise (IEC 60534-8), duct noise, aerodynamic noise sources, octave band spectra, noise prediction, attenuation, OSHA limits. |
| metadata | {"priority":7,"promptSignals":{"phrases":["flow noise","aeroacoustic noise","valve noise","IEC 60534 noise","turbulent jet noise","duct noise prediction"],"minScore":3}} |
Flow Noise and Aeroacoustics — Complete Skill
Lighthill's Acoustic Analogy
Lighthill equation (exact rearrangement of Navier-Stokes):
∂²ρ'/∂t² - c₀² ∇²ρ' = ∂²T_ij/∂x_i∂x_j
T_ij = ρu_iu_j + δ_ij(p - c₀²ρ') - τ_ij [Lighthill stress tensor; ρu_iu_j = Reynolds stress; τ_ij = viscous stress]
Sound power scaling laws:
- Subsonic jet (monopole absent): W_acoustic ∝ ρ U⁸ D² / c₀⁵ [Lighthill U⁸ law; strong velocity dependence]
- Dipole source (flow over solid body): W ∝ U⁶ D² ρ / c₀³
- Quadrupole (free turbulence): W ∝ U⁸ (dominates at high U)
Acoustic efficiency (subsonic jet):
η = W_acoustic / W_jet = K × M⁵ [M = U/c; K ≈ 10⁻⁴ for clean jets; M = Mach number]
Implies reducing jet velocity 10× reduces acoustic power by 10⁸ (80 dB)
Aerodynamic Noise Sources
Turbulent Boundary Layer and Trailing Edge Noise
Trailing edge noise (dominant mechanism for fans/aerofoils):
SPL ∝ U⁵ × δ* / R² [δ* = boundary layer displacement thickness; R = observer distance]
Frequency: f_peak ≈ U / (5δ*) [peak in spectrum near this frequency]
BPF noise (blade passing frequency):
f_BPF = N_blades × RPM / 60 [Hz]
SPL at BPF depends on blade loading and inlet turbulence intensity; dominant tonal component
Jet Noise
Octave-band sound power level (Lighthill, corrected):
Lw_octave = 10 log₁₀(W_octave / W_ref) [dB re 10⁻¹² W]
Total jet noise (A-weighted approximation, subsonic):
Lw_A ≈ 10 log₁₀(ρ U⁸ D² / c⁵) + constant [constant depends on jet geometry]
Coanda nozzle / high-aspect ratio jet: lower noise per unit flow (higher mixing efficiency)
Chevron nozzles: reduce jet noise by 2–5 dB through mixing enhancement at trailing edge
Control Valve Noise (IEC 60534-8)
Hydrodynamic Noise (Liquid Service)
Cavitation noise (dominant in liquid valves):
Cavitation index: σ = (P₁ - P_v) / (P₁ - P₂) [P_v = vapor pressure]
Incipient cavitation: σ_i ≈ F_L² [F_L = liquid pressure recovery factor from IEC]
Cavitation noise: Lp_cavitation ≈ Lp_turbulent + 10 log₁₀(1/(σ - σ_i)) when σ > σ_i
Turbulent noise (no cavitation):
IEC 60534-8-4 prediction:
Lp = Lw - 10 log₁₀(4πr²) + α_atm × r [r = distance from valve; α_atm = air absorption coefficient]
Lw = Lp_valve_exit + correction terms from IEC tables (pipe wall insertion loss, etc.)
Turbulent valve noise (simplified):
Lw ≈ 10 + 10 log₁₀(W_mechanical) where W_mechanical = ΔP × Q × (1 - F_L²) / (1 + F_L²)
Aerodynamic Noise (Gas/Steam Service) — IEC 60534-8-3
Sound power level at valve exit:
Lw = Lw_internal + transmission loss through pipe wall
Internal sound power (subsonic, x < F_γ x_T):
W_internal = η × (ΔP² × Q) / (C_v × P₁) [η = acoustic efficiency factor from IEC; typically 10⁻⁴–10⁻²]
Mach number effect on noise:
x = ΔP/P₁ (pressure drop ratio); at x ≥ F_γ x_T: choked flow; higher Mach → more noise
Post-choke aerodynamic noise: SPL increases by ~8 dB per doubling of inlet pressure
Pipe wall insertion loss (IEC 60534-8-3):
TL = 10 log₁₀(m_pipe × f / (ρ_air × c_air)) [TL = transmission loss; m_pipe = pipe wall mass per unit area]
Typical: DN100 schedule 40 steel → TL ≈ 35–45 dB at 1 kHz
Predicted external SPL:
Lp = Lw_internal - TL - 10 log₁₀(2π × r × L_pipe) [at distance r from pipe centerline; L_pipe = pipe length]
Duct Noise Propagation
Duct insertion loss (lined duct — rectangular):
IL = 1.05 × α_lining × P_perimeter / A_cross [dB/m; α = sound absorption coefficient of liner material]
Fan noise (AMCA 301):
Lw_total = Kw + 10 log₁₀(Q) + 20 log₁₀(ΔP) [Kw = specific sound power level from AMCA test data]
AMCA 300: standard test for duct fan noise
Duct breakout noise:
TL_breakout ≈ -10 log₁₀(S_duct / S_room) + TL_wall [S = surface area]
Breakout dominates at low frequencies where duct radiation is efficient
Silencer (attenuator) insertion loss:
Reactive (expansion chamber): IL = 10 log₁₀(1 + (m × sin(kL))²) [m = expansion ratio; kL = acoustic length]
Dissipative: IL = α_liner × L_attenuator [typical: 1–5 dB/100 mm at target frequency]
Occupational Noise Limits
OSHA 29 CFR 1910.95 (USA):
| Duration [hr/day] | Permissible SPL [dBA] |
|---|
| 8 | 90 |
| 4 | 95 |
| 2 | 100 |
| 1 | 105 |
| 0.5 | 110 |
| ≤0.25 | 115 |
Hearing conservation action level: 85 dBA TWA (8-hour time-weighted average)
Noise dose: D = Σ (C_i / T_i) ≤ 1.0 [C_i = actual exposure time; T_i = permissible time at that level]
NIOSH REL (more conservative): 85 dBA TWA (3 dB exchange rate vs. OSHA 5 dB)
Noise Measurement
Sound pressure level (SPL):
Lp = 20 log₁₀(p_rms / p_ref) [dB; p_ref = 20 μPa]
Sound power level:
Lw = Lp + 10 log₁₀(S/S_ref) [S = measurement surface area; S_ref = 1 m²]
A-weighting: approximates human ear frequency response; adds -26 dB at 63 Hz, -3 dB at 1 kHz, +1.2 dB at 4 kHz
Octave bands (center frequencies): 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz
Distance attenuation (free field, point source):
ΔLp = 20 log₁₀(r₂/r₁) [dB; doubling distance = -6 dB]
Line source (pipe): ΔLp = 10 log₁₀(r₂/r₁) [doubling distance = -3 dB]
Noise Reduction Strategies
Source reduction (most effective):
- Reduce velocity: W ∝ U⁸ → halving velocity drops W by 24 dB
- Reduce pressure drop across valve (staged pressure reduction)
- Use low-noise trim (labyrinth paths in control valves → subsonic expansion)
Path reduction:
- Pipe wall thickness increase: +6 dB per doubling of wall mass (≈ 10% noise reduction per kg/m² increase)
- Acoustic wrapping/lagging: 10–20 dB at higher frequencies
- Silencers/attenuators in duct systems
Receiver protection:
- Hearing protection devices (HPD): 25–35 dB attenuation (earmuffs)
- Acoustic enclosures: 20–40 dB depending on construction
Standards
| Standard | Scope |
|---|
| IEC 60534-8-3 | Control valve aerodynamic noise prediction |
| IEC 60534-8-4 | Control valve hydrodynamic noise prediction |
| OSHA 29 CFR 1910.95 | Occupational noise exposure |
| ISO 9614 | Sound intensity measurement |
| AMCA 300/301 | Fan noise testing and rating |
| ISO 3746 | Sound power level measurement |
Output
Provide: noise source type (aerodynamic/hydrodynamic/mechanical), dominant mechanism (turbulence/cavitation/tonal/BPF), sound power level Lw [dB re 10⁻¹² W], predicted external SPL Lp [dBA] at 1 m, octave band spectrum (63 Hz–8 kHz center frequencies), IEC 60534-8 calculated Lp at 1 m (for valves), pipe wall transmission loss TL [dB], occupational exposure level vs. OSHA limit, noise reduction measure recommended (reduce velocity/staged pressure drop/acoustic treatment), reduction achieved [dB], and applicable standard (IEC 60534-8-3/-8-4, OSHA 1910.95).
