Quantification of acoustic and hydrodynamic fields in flow duct systems

Internal wall pressure fluctuations downstream of a flow disturbance in a cylindrical duct consist of a propagating acoustic field of both plane wave and higher-order acoustic modes superimposed onto hydrodynamic turbulence. Various methods exist in both the time and the frequency domain to separate the hydrodynamic from the acoustic components. Coherence functions and cross-spectra may be used to separate the correlated parts from the uncorrelated parts of the internal wall pressure fluctuations for frequencies less than the (1,0) mode. Data collected for air flow through a 40 mm diameter pipe downstream of a flow disturbance has been examined. Changes to the degree of spatial coherence of the turbulent field at different axial locations were evident. In addition, a comparison of the acoustic spectrum between the separation, flow recovery and fully developed regions reveals the presence of standing waves and a change in spectral peak density of the propagating waves.