Simultaneous Particle Image Velocimetry (PIV) measurements of stratified turbulent air/water flow in a horizontal pipe have been performed using water droplets as tracers in the gas-phase. The use of water droplets as tracers ensures that the water surface tension remains unaffected and thus allows small scale interfacial structures, such as capillary waves to occur naturally. Experiments have been conducted in a 31 m long, 100 mm diameter PVC pipe using air (density 1.20 kg/m 3 and viscosity 18.4 µPa•s) and water (density 996 kg/m 3 and viscosity 1.0 mPa•s) as test fluids. For the purpose of validation of the experimental setup and the suggested seeding technique, single-phase measurements of both air and water were compared to each other and to DNS results provided by "Wu X. and Moin P., 2008, A direct numerical simulation study on the mean velocity characteristics in turbulent pipe flow, J. Fluid Mechanics, Vol. 608.", showing very good agreement. The two-phase measurements are presented in terms of mean-and rms-profiles. These measurements offer a qualitative demonstration of the behavior of the interfacial turbulence and its correlation with the various interfacial flow patterns. The observations made in this paper are in agreement with the conclusions drawn from the DNS study of "Lakehal D., Fulgosi M., Banerjee S. and De Angelis, Direct numerical simulation of turbulence in a sheared air water flow with a deformable interface, 2003, J. Fluid Mechanics, Vol. 482.". The present results may eventually provide a better explanation to many important phenomena related to the physical characteristics of stratified two-phase flow such as scalar mixing between phases, and to challenges related to its modeling.

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We perform an experimental analysis of two-phase stratified wavy pipe flow, with the aim to detect and quantify the effect of small scale wave breaking. Particle image velocimetry (PIV) is employed to analyze the velocity fields below individual waves, and a threshold for the vorticity on the leeward side of the crest is used to assess active wave breaking. Keeping the liquid flow rate constant, we analyze five experimental cases with increasing gas flow rates. The cases span the flow map from when first interfacial waves are observed, to the "amplitude saturation" regime, where the rms interface elevation is independent of the gas flow rate. While some wave breaking events are observed also in the wave-growth regime, wave breaking is found to be much more frequent when the gas flow rate is increased into the amplitude saturation regime, and 35-40 % of the waves passing the measurement section are assessed to be in a state of active breaking in this regime. A conditional averaging of the flow field is performed, and the turbulent dissipation rate below breaking and nonbreaking waves is estimated. The effect of microscale breaking is observed down to a depth of 10 mm below the water surface. Below the crest of microscale breaking waves the turbulent dissipation rate is increased by a factor 2.5 to 4 compared with non-breaking waves. This fraction increases with U sg , implying that the breaking events become more energetic as the gas flow rate is increased.

Abstract. We introduce an improved formulation of the double-multiple streamtube (DMST) model for the prediction of the flow quantities of vertical axis wind turbines (VAWT). The improvement of the new formulation lies in that it renders the DMST valid for any induction factor, i.e., for any combination of rotor solidity and tip speed ratio. This is done by replacing the Rankine–Froude momentum theory of the DMST, which is invalid for moderate and high induction factors, with a new momentum theory recently proposed, which provides sensible results for any induction factor. The predictions of the two DMST formulations are compared with VAWT power measurements obtained at Princeton's High Reynolds number Test Facility, over a range of tip speed ratios, rotor solidities, and Reynolds numbers, including those experienced by full-scale turbines. The results show that the new DMST formulation demonstrates a better overall performance, compared to the conventional one, when the rotor loading is moderate or high.