The cloud disc that slides along an aircraft’s fuselage in acceleration across Mach I might be explained by the physics of an ultrasound field generated by the Doppler Effect.
Sound, mechanical and aerodynamic, generated by an aircraft accelerating towards Mach I, is confined within a sound cone, the apex of which is at a point at a decreasing distance in front of the nose. At Mach I, the aircraft nose is the cone’s apex. Aircraft-generated sound energy in the sound cone travels at the speed of sound – laterally, perpendicular to the line of flight, and anteriorly where it builds up in intensity in front of the leading edges.
As air speed increases, the sound energy in the anterior sound cone displays rising sound frequency with the wavelength decreasing proportionately (Doppler Effect). Reducing the aircraft to point size, the aerodynamic and aircraft mechanical sounds are confined within a perfect cone. At Mach I, the angle of the cone margins to the line of flight would be 45 degrees (the sound radiating laterally from the line of flight the same distance as the plane moves forwards), creating a 90-degree angle between the cone margins.
Cloud disc formation requires moisture as ice-crystal mist, water mist or rain drops (ultrasound “atomizes” water). Doppler-induced ultrasound propels (sweeps) suspended particles (including droplets) forwards away from the ultrasound source (Liebermann L.N. 1949. The second viscosity of liquids. Phys. Rev. 75, 1415-1422), which is the aircraft; this creates a disc-shaped cloud, its rounded margins defined by the shape of the cross-section of the sound cone. At Mach I, the cloud disc touches the aircraft nose. While accelerating beyond Mach I, , the cloud disc slides along the fuselage (Figure 1), leaving aircraft sound behind.