(a) Numerical simulation of the axial mean velocity field within the pen cap contraction for a mean velocity U ₀ = 20 m s⁻¹. The black dashed line indicates the position of the shear layer. (b) Amplitude of the acoustic velocity component v _⊥, perpendicular to the shear layer, normalized by the mean acoustic velocity in the contraction u ₀. (c) Schematic representation of the vorticity ω, the mean tangential velocity U _Γ, and the normal acoustic velocity v _⊥. (d) Acoustic pressure field at the first resonance.