Experimental particle paths and drift velocity in steep waves at finite water depth

The Lagrangian paths, horizontal Lagrangian drift velocity, U L , and the Lagrangian excess period, T L − T 0 , where T L is the Lagrangian period and T 0 the Eulerian linear period, are obtained by particle tracking velocimetry (PTV) in non-breaking periodic laboratory waves at a finite water depth of h = 0 . 2 m, wave height of H = 0 . 49 h and wavenumber of k = 0 . 785 / h . Both U L and T L − T 0 are functions of the average vertical position of the paths, ̄ Y , where − 1 < ̄ Y / h < 0. The functional relationships U L ( ̄ Y ) and T L − T 0 = f ( ̄ Y ) are very similar. Comparisons to calculations by the inviscid strongly nonlinear Fenton method and the second-order theory show that the streaming velocities in the boundary layers below the wave surface and above the fluid bottom contribute to a strongly enhanced forward drift velocity and excess period. The experimental drift velocity shear becomes more than twice that obtained by the Fenton method, which again is approximately twice that of the second-order theory close to the surface. There is no mass flux of the periodic experimental waves and no pressure gradient. The results from a total number of 80 000 experimental particle paths in the different phases and vertical positions of the waves show a strong collapse. The particle paths are closed at the two vertical positions where U L = 0

Visualization and measurements of flows in micro silicon Y-channels

Velocities and accelerations are measured and visualized in silicon microchannels using particle tracking velocimetry (PTV). Both pulsatile and stationary flows are generated in channels with different geometry. Distinct differences between flow regimes and geometries are shown. Flow separation occurred at Re = 84 for the channel with an expanded bifurcation shown by streamlines from long exposed images. Moving least squares are used to find the ensemble-averaged positions of the measured velocities from tracking. This is needed to find the local and convective accelerations

Velocity fields in breaking-limited waves of finite depth

The kinematics below the strongest possible periodic water waves on intermediate depth, for wave periods Tg/h=8.75 and 11.7 (g acceleration of gravity, h water depth), is measured by PTV. The largest possible uniform waves far away from the wave maker have a height of H/h≃0.49 and a fluid velocity up to 0.5gh for these periods. Moderately breaking waves measured close to the wave maker have a turbulent surface region riding on top of a smooth flow with horizontal fluid velocity of 0.62gh at maximum, and wave height up to H/h=0.63. Strongly breaking waves have a thicker turbulent surface region, smaller maximum height (H/h=0.56) and a horizontal fluid velocity of 0.72gh at maximum. Measurement of the flow below 72 breaking wave crests illustrate the range and variation of the elevation and kinematics. Experiments are compared to fully nonlinear and second-order theories, where the former is valid for regular nonbreaking waves, and the latter gives conservative predictions for the very strong waves. Secondary streaming in the bottom boundary layer below the waves is measured

Performing particle image velocimetry using artificial neural networks: a proof-of-concept

Traditional programs based on feature engineering are under performing on a steadily increasing number of tasks compared with Artificial Neural Networks (ANNs), in particular for image analysis. Image analysis is widely used in Fluid Mechanics when performing Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV), and therefore it is natural to test the ability of ANNs to perform such tasks. We report for the first time the use of Convolutional Neural Networks (CNNs) and Fully Connected Neural Networks (FCNNs) for performing end-to-end PIV. Realistic synthetic images are used for training the networks and several synthetic test cases are used to assess the quality of each network predictions and compare them with state-of-the-art PIV software. In addition, we present tests on real-world data that prove that ANNs can be used not only with synthetic images but also with more noisy, imperfect images obtained in a real experimental setup. While the ANNs we present have slightly higher Root Mean Square (RMS) error than state-of-the-art cross-correlation methods, they perform better near edges and allow for higher spatial resolution than such methods. In addition, it is likely that one could with further work develop ANNs which perform better that the proof-of-concept we offer. The final version of this research has been published in Measurement Science and Technology. © 2017 IOP Publishin

A PIV investigation of stratified gas–liquid flow in a horizontal pipe

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/m3 and viscosity 18.4 µPa·s) and water (density 996 kg/m3 and viscosity 1.0 mPa·s) as test fluids. For the purpose of validation of the experimental set-up 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

Bichromatic synthetic schlieren applied to surface wave measurements

acceptedVersio

X-ray measurements of plunging breaking solitary waves

The aim of this study is to examine and measure the characteristics of air cavities generated by breaking solitary waves by utilizing a novel tomographic X-ray system. Small scale experiments of solitary waves that propagate on a (1:10) beach are conducted. Waves with amplitude normalized with the water depth, on a flat bottom are investigated by two perpendicular X-ray systems. Images are captured at locations from the surf zone to the swash zone and at maximum runup. A large air tube is observed right after the plunger impacts the dry beach. Void velocity and the shape of the large air tube, are measured and reported. The large air tube evolves from a symmetrical shape with two large air pockets located close to the walls of the wave tank, to an asymmetrical shape. Contrast enhanced X-ray images reveal that the swash tongue surface is unstable and that secondary mixing of air and water occurs. X-ray images from the maximum runup reveal that the air is still entrapped by the thin swash tongue at times close to maximum runup