The multiple side‐branch system as a model for a corrugated pipe Free

Corrugated pipes are used as flexible risers in offshore natural gas production and for vacuum cleaners. Such pipes can display whistling. As theoretical models are available to predict the aero‐acoustical behaviour of closed side branches [1], we consider a row of equally spaced closed side branches along a pipe, as a model for a corrugated pipe. We consider side branches with a diameter D and depth L equal to the main pipe diameter, placed at a distance of three diameters from each other. For systems of 11 or 12 side branches the lowest resonance modes are reasonably well predicted by assuming n(λ/2) standing wave modes with an effective speed of sound c_eff[2]. Whistling is observed for the n=2 and n=3 modes with a pressure fluctuation amplitude p'/(ρ₀c₀U₀)=O(2×10^‐2) similar to that observed for sharp edges corrugations in a corrugated pipe (with ρ₀ the fluid density, c₀ the speed of sound and U₀ the main flow velocity). The Strouhal number of these oscillations is Sr_D=(f⋅D)/U₀≃0,70. This is higher than typical values observed for corrugated tubes [3]. References: [1] J.C. Bruggeman et al., J. Sound and Vibration 1991 150(4) 371‐393. [2] J.W. Elliot, in "Lecture Notes on the Mathematics of Acoustics" M.C.M. Wrigth, Imperial College Press, London (2005). [3] S. Belfroid et al., ASME paper PVP2007‐26503.