HIGH AMPLITUDE VORTEX-INDUCED PULSATIONS IN A GAS-TRANSPORT SYSTEM

High Reynolds number, low Mach number gas flows in pipe systems with c losed side branches exhibit spectacular low frequency self-sustained p ulsations driven by periodic vortex shedding at specific values of the Strouhal number. A detailed study is presented of the behaviour of th e flow in a system with two opposite closed side branches of equal len gth in a cross configuration. For junctions with both sharp and rounde d edges the acoustic flow velocity amplitude is comparable to the main flow velocity. A two-dimensional potential flow model based on the vo rtex blob method, used to simulate the flow in the junction, describes accurately the flow visualization and laser Doppler data obtained in pipes with square cross-sections and with sharp edged junctions. The n umerical simulation is used to calculate the acoustical power generate d by the vortical flow at a given amplitude of the acoustic velocity f ield and Strouhal number. In reality, for a pulsation with constant am plitude, this power is balanced by the viscothermal losses and acousti c radiation, which is the basis for the indirect measurement of the so urce power. It is shown that, for the acoustic amplitude observed, rad iation losses due to the generation of non-resonating harmonics by wav esteepening has to be taken into account in the energy balance. This f inding is confirmed by the appearance of shock waves in the geometry w ith rounded edges. (C) 1995 Academic Press Limited