Experimental characterization of non-linear interfacial wave interaction in stratified gas-liquid pipe flow

Abstract

We present a statistical and spectral study of interfacial waves in stratified turbulent gas-liquid flow in a horizontal pipe. The main objective of this paper is to report on experimental findings with respect to the influence of the gas flow rate on statistical and spectral properties of interfacial waves. Emphasis is put on the assessment of non-linear interfacial mode interactions by means of higher-order spectra. Interfacial elevation time series were acquired using conductance probes with sampling frequency Fs = 500 Hz. The probes were placed at the pipe centerline and 270D downstream of the pipe inlet. Local wave parameters such as wavelengths, heights, and phase speeds were extracted by means of a zero-crossing technique. The evolution of their statistical properties is assessed for a range of flow conditions in which the liquid flow rate was kept constant at Usl = 0.1 m/s, whilst the gas flow rate was gradually increased from 1.3 to 4 m/s. The mode wave speed is compared to linear and weakly non-linear theory. Interfacial mode interactions are investigated through an assessment of first and second-order spectra, i.e., power spectral density and bispectrum. The results indicate that weakly non-linear (triadic) interactions in the form of overtone and sub-harmonic interactions play an important role and may partially be responsible for the observed transition to the wave saturation regime reported in our previous studies. Finally, a flow map is presented in terms of a Ursell-vs-Froude number plot, in which observed wave regimes are classified according to underlying linear/non-linear mechanisms.