Aeroacoustic source identification in gas transport pipe system

Whistling due to Flow-Induced Pulsations can occur in gas transport and gas export pipe systems with low Mach number flow. These flow-induced pulsations can lead to piping vibration or acoustic fatigue of piping elements. The study presents a simple numerical pulsation source identification method applied to restriction orifices. In case of whistling, small acoustic perturbations incident on some pipe elements such as restriction orifices and T-junctions are amplified by the shear layer flow at certain frequencies, propagate downstream and upstream of the elements, and are reflected by the ends of the piping or by large section changes. The element acts as an acoustic amplifier for the incident acoustic energy at certain frequencies. The source identification method, earlier presented by Martinez-Lera et al [1] for the aeroacoustic source identification of T-junctions and improved by Nakiboglu et al [2] for circumferential cavities in corrugated pipes, combines incompressible numerical simulations with vortex sound theory. It is applied here to restriction orifices, commonly used in industrial pipe systems as measuring devices, to induce a pressure drop, or to reduce low frequency pulsations. Incompressible, laminar CFD simulations are used for the source identification instead of fully compressible LES. These simplifications enable much less CPU intensive computations, but require extra post processing. A particularly sensitive point is the estimation of the pressure drop due to the potential flow. This is estimated with a simulation made on a straight, empty pipe of same characteristics. This method is applied to restriction orifices in this paper, and the specific points for this particular geometry are reviewed. The results are compared to the experimental and numerical results of Testud et al [3] and Lacombe et al [4]. The application of the method to the source identification of whistling restriction orifices predicts whistling at Strouhal numbers similar to earlier experimental and numerical studies, with limited computational effort.