QUANTIFICATION OF ACOUSTIC AND HYDRODYNAMIC FIELDS IN FLOW DUCT SYSTEMS

Internal wall pressure fluctuations downstream of a flow disturbance i n a cylindrical duct consist of a propagating acoustic field of both p lane wave and higher-order acoustic modes superimposed onto hydrodynam ic turbulence. Various methods exist in both the time and the frequenc y domain to separate the hydrodynamic from the acoustic components. Co herence functions and cross-spectra may be used to separate the correl ated parts from the uncorrelated parts of the internal wall pressure f luctuations for frequencies less than the (1,0) mode. Data collected f or airflow through a 40 mm diameter pipe downstream of a flow disturba nce has been examined. Changes to the degree of spatial coherence of t he turbulent field at different axial locations were evident. In addit ion, a comparison of the acoustic spectrum between the separation, flo w recovery and fully developed regions reveals the presence of standin g waves and a change in spectral peak density of the propagating waves . (C) 1997 Elsevier Science Ltd.