#jetnoise — Public Fediverse posts

🌪️ Turbulence: Chaos or Hidden Order?

For decades, turbulence in jets and mixing layers was treated as random fluctuations.
• Taylor (1935): correlation & diffusion.
• Kolmogorov (1941): energy cascade.
• von Kármán (1948): spectral formulations.

The picture began to change in the 1960s:
• Mollo-Christensen (1967) captured organised jet patterns.
• Kline et al. (1967) revealed streaks near walls.

🔑 The turning point: Brown & Roshko (JFM 1974).
Using spark shadowgraphs, they showed that plane mixing layers are not “pure disorder” but are dominated by large coherent vortices that convect, pair, and drive mixing.

🚀 Why it matters:
• Mixing & entrainment are orchestrated by coherent structures.
• Modern jet-noise research (e.g. Jordan & Colonius 2013) now recognises wave packets (ordered structures) as dominant low-frequency, aft-angle noise sources.

🧮 Equations behind the story:

Kolmogorov cascade: ε ≈ u′³∕ℓ
(ε = dissipation rate, u′ = velocity fluctuation, ℓ = eddy size)

Growth of mixing layer thickness: δ ∝ √(ν·t)
(δ = layer thickness, ν = kinematic viscosity, t = time)

Vortex pairing frequency: f ∝ U∕λ
(f = frequency, U = convection speed, λ = vortex spacing)

🎥 Brown & Roshko’s classic film of vortex pairing:
https://www.youtube.com/watch?v=5sr5s_MvWDA

#FluidDynamics #Turbulence #Aeroacoustics #CoherentStructures #JetNoise