Macroturbulence measurements with electromagnetic and ultrasonic sensors: a comparison under high-turbulent flows

 Experimental results of turbulent flows for both, field and laboratory tests are presented. The spatial and temporal resolution is limited by the measuring instruments, which result in “filtering” out the very small scales, thus observing only macroturbulence. The experimental field-results have been obtained during the large-scale surf zone experiments carried out in the Ebro Delta, (Spain), under spilling/plunging breaking waves. The field-measurements include several tests across the surf zone with high vertical resolution. The measured turbulent properties are compared with state-of-art macroturbulence characteristics and length parameterisations. Furthermore, a detailed comparison of two-dimensional electromagnetic and three-dimensional ultrasonic velocity measurements has been carried out for hydraulic jumps in a laboratory flume to validate the field measurements and to study the frequency response of electromagnetic sensors. Received: 22 December 1996/Accepted: 8 September 1998     

Velocity field after wave breaking

The velocity field in breaking water waves is considered in this paper. A numerical simulation describes in detail the transition from a primary overturning and consequent rebounding jets into a bore front, where the vorticity in the coherent large‐scale eddy structures devolves into turbulence. Spatial changes in the frequency spectra of the kinetic energy and the enstrophy are associated with the production, transport and dissipation of the Reynolds stress and the various wave and turbulent mixing length scales. Mean velocity fields and the wave and kinetic energy in a surf zone are evaluated. Fourier and wavelet spectral analysis is applied to study both the surface elevation and energy changes, and the distinction that must be made between spilling and plunging breakers is clarified in this paper. Copyright © 2002 John Wiley & Sons, Ltd.     

Characterization of plunging liquid jets: A combined experimental and numerical investigation

This paper presents a combined experimental and numerical study of the flow characteristics of round vertical liquid jets plunging into a cylindrical liquid bath. The main objective of the experimental work consists in determining the plunging jet flow patterns, entrained air bubble sizes and the influence of the jet velocity and variations of jet falling lengths on the jet penetration depth. The instability of the jet influenced by the jet velocity and falling length is also probed. On the numerical side, two different approaches were used, namely the mixture model approach and interface-tracking approach using the level-set technique with the standard two-equation turbulence model. The numerical results are contrasted with the experimental data. Good agreements were found between experiments and the two modelling approaches on the jet penetration depth and entraining flow characteristics, with interface tracking rendering better predictions. However, visible differences are observed as to the jet instability, free surface deformation and subsequent air bubble entrainment, where interface tracking is seen to be more accurate. The CFD results support the notion that the jet with the higher flow rate thus more susceptible to surface instabilities, entrains more bubbles, reflecting in turn a smaller penetration depth as a result of momentum diffusion due to bubble concentration and generated fluctuations. The liquid average velocity field and air concentration under tank water surface were compared to existing semi-analytical correlations. Noticeable differences were revealed as to the maximum velocity at the jet centreline and associated bubble concentration. The mixture model predicts a higher velocity than the level-set and the theory at the early stage of jet penetration, due to a higher concentration of air that cannot rise to the surface and remain trapped around the jet head. The location of the maximum air content and the peak value of air holdup are also predicted differently.     

Dynamics of large turbulent structures in a steady breaker

The flow near the leading edge of a steady breaker has been studied experimentally using Bubble Image Velocimetry (BIV) with the aim of characterizing the dynamics of the large eddies responsible for air entrainment. It is well reported in the literature, and confirmed by our measurements of the instantaneous velocity field, that this flow shares some important features with the turbulent shear-layer formed between two parallel semi-infinite streams with different velocities. Namely, the formation of a periodic array of coherent vortices, the constant convective velocity of those vortices, the linear relation between their size and their downstream position and the self-similar structure of both mean velocity profiles and Reynolds shear stresses. Nonetheless, important differences exists between the dynamics of the large eddies in a steady breaker and those in a free shear-layer. Particularly, the convective velocity of these large structures is slower in a steady breaker and, consistent with this, their growth rates are larger. A physical interpretation of these differences is provided together with a discussion of their implications. To support our measurements and conclusions, we present a careful analysis of the accuracy of the BIV technique in turbulent flows with large bubbles.     

Three-dimensional green water velocity on a model structure

Flow kinematics of green water due to plunging breaking waves impinging on a simplified, 3D model structure was investigated in the laboratory. Two breaking wave conditions were tested: one with waves impinging on the vertical wall of the model at still water level, and the other with waves impinging on the horizontal deck surface. The bubble image velocimetry (BIV) technique was used to measure flow velocities. Measurements were taken on both vertical and horizontal planes. Evolution of green water flow kinematics in time and space was revealed and was found to be quite different between the two wave conditions, even though the incoming waves are essentially identical. The time history of maximum velocity is demonstrated and compared. In both cases, the maximum velocity occurs near the green water front and beneath the free surface. The maximum horizontal velocity for the deck impinging case is 1.44C with C being the wave phase speed, which is greater than 1.24C for the wall impingement case. The overall turbulence level is about 0.3 of the corresponding maximum velocity in each wave condition. The results were also compared with 2D experimental results to examine the 3D effect. It was found that the magnitude of the maximum vertical velocity during the runup process is 1.7C in the 3D model study and 2.9C in the 2D model study, whereas the maximum horizontal velocity on the deck is similar, 1.2C in both 3D and 2D model studies.     

Runup and green water velocities due to breaking wave impinging and overtopping

The present study investigates, through measurements in a 2D wave tank, the velocity fields of a plunging breaking wave impinging on a structure. As the wave breaks and overtops the structure, so-called green water is generated. The flow becomes multi-phased and chaotic as a large aerated region is formed in the flow in the vicinity of the structure while water runs up onto the structure. In this study, particle image velocimetry (PIV) and its derivative, bubble image velocimetry (BIV), were employed to measure the velocity field in front and on top of the structure. Mean and turbulence properties were obtained through ensemble averaging repeated tests. The dominant and maximum velocity of the breaking wave and associated green water are discussed for the three distinct phases of the impingement–runup–overtopping sequence. Initially the flow is mainly horizontal right before the breaking wave impinges on the structure. The flow then becomes primarily vertical and rushes upward along the front wall of the structure right after the impingement. Subsequently, the flow becomes mainly horizontal on top of the structure as the remaining momentum in the wave crest carries the green water through. The distribution of the green water velocity along the top of the structure has a nonlinear profile and the maximum velocity occurs near the front of the fast moving water. Using the measured data and applying dimensional analysis, a similarity profile for the green water flow on top of the structure was obtained, and a prediction equation was formulated. The prediction equation may be used to predict the green water velocity caused by extreme waves in a hurricane.     

Fully three-dimensional direct numerical simulation of a plunging breakerSimulation numérique directe tridimensionnelle du déferlement plongeant

The scope of this paper is to show the results obtained for simulating three-dimensional breaking waves by solving the Navier–Stokes equations in air and water. The interface tracking is achieved by a Lax–Wendroff TVD scheme (Total Variation Diminishing), which is able to handle interface reconnections. We first present the equations and the numerical methods used in this work. We then proceed to the study of a three-dimensional plunging breaking wave, using initial conditions corresponding to unstable periodic sinusoidal waves of large amplitudes. We compare the results obtained for two simulations, a longshore depth perturbation has been introduced in the solution of the flow equations in order to see the transition from a two-dimensional velocity field to a fully three-dimensional one after plunging. Breaking processes including overturning, splash-up and breaking induced vortex-like motion beneath the surface are presented and discussed. To cite this article: P. Lubin et al., C. R. Mecanique 331 (2003).     

Numerical analysis of breaking waves using the moving particle semi-implicit method

The numerical method used in this study is the moving particle semi-implicit (MPS) method, which is based on particles and their interactions. The particle number density is implicitly required to be constant to satisfy incompressibility. A semi-implicit algorithm is used for two-dimensional incompressible non-viscous flow analysis. The particles whose particle number densities are below a set point are considered as on the free surface. Grids are not necessary in any calculation steps. It is estimated that most of computation time is used in generation of the list of neighboring particles in a large problem. An algorithm to enhance the computation speed is proposed. The MPS method is applied to numerical simulation of breaking waves on slopes. Two types of breaking waves, plunging and spilling breakers, are observed in the calculation results. The breaker types are classified by using the minimum angular momentum at the wave front. The surf similarity parameter which separates the types agrees well with references. Breaking waves are also calculated with a passively moving float which is modelled by particles. Artificial friction due to the disturbed motion of particles causes errors in the flow velocity distribution which is shown in comparison with the theoretical solution of a cnoidal wave. © 1998 John Wiley & Sons, Ltd.     

A laboratory study of irregular shoaling waves

The present research aims to investigate the dynamics of a single laboratory irregular wave, characterized by a narrow-banded spectrum and developing on a sloping sand bottom, in intermediate waters up to the surf zone. Experiments focused on the wave shoaling region, in order to examine how the wave is affected by breaking induced turbulence offshore the surf zone. A 3D acoustic Doppler velocimeter was used to measure the three wave velocity components, which were all processed to evaluate the time-averaged vertical distributions of orbital velocities, wave and turbulent Reynolds shear stresses and turbulent intensities. The vertical distributions of the phase-averaged velocity components, turbulent kinetic energy and transport of turbulence were also analysed. The adopted phase-averaging technique was applied to each investigated measurement point. Therefore, the crucial element of the study is that all the analysed values derive directly from real measurements and are not approximated by any kind of interpolation. The study confirmed some dynamic behaviour in the shoaling zone already known in the literature, such as the typical cell-type flow pattern of the mean flow and the necessity to evaluate the turbulent kinetic energy with all the three velocity components, when available, which would otherwise be underestimated. Referring to the time-averaged wave and Reynolds shear stresses, a contribution was added to the open debate on their order of magnitude. The measured wave Reynolds shear stresses were also compared with the results of the model by Zou et al. (J Geophys Res 111:C09032, 2006), confirming the behaviour typical of dissipative breaking waves. The analysis of turbulence transport in the shoaling zone revealed that it is seaward directed close to the surface and landward directed close to the bottom. The results presented in the paper can be extended only to other analogous flow conditions.     

Fiber-optic probe measurements of void fraction and bubble size distributions beneath breaking waves

Experiments were performed to determine the accuracy of single-tip fiber-optic probes for making simultaneous measurements of the void fraction and bubble size distributions under breaking waves. Tests in a vertical bubble column showed that the normalized RMS error in the void fraction measurements was ∼10%. The relationship between bubble rise time and bubble velocity was investigated in a unidirectional flow cell. Similar to previous studies the rise time and bubble velocity were found to be related by a power law equation. The probes can provide mean bubble velocities accurate to ±10% when a minimum of ∼15 individual bubble velocities are averaged. The fiber-optic probes were deployed beneath a plunging breaking wave in a laboratory wave channel. The slope and shape of the bubble cord length size distribution measured with the probes was found to agree closely with the size distribution measured from digital video recordings. The probes were then positioned in the splash-up zone of a plunging breaker and the resulting cord length distribution had a shape and slope that was in agreement with previous measurements. These results demonstrate that single-tip fiber optic probes can provide accurate simultaneous measurements of the void fraction and bubble sizes inside the dense bubble clouds entrained by breaking waves.     

Green water void fraction due to breaking wave impinging and overtopping

The present study uses laboratory measurements to investigate the void fraction of an overtopping flow on a structure. The overtopping flow, also called green water, was generated by the impingement of a plunging breaking wave on the structure following the Froude similarity of an extreme hurricane wave and a simplified offshore structure. The flow is multi-phased and turbulent with significant aeration. A fiber optic reflectometer (FOR) and bubble image velocimetry (BIV) were employed to measure the void fraction and velocity in the flow, respectively, and to determine the water level on the deck. Mean properties of void fraction and velocity were obtained by ensemble-averaging and time-averaging the repeated instantaneous measurements. The temporal and spatial distributions of void fraction reveal that the flow is very highly aerated near the front of green water and has relatively low aeration near the deck surface. The mean void fraction and velocity distributions were also depth-averaged for simplicity and potential use in engineering applications. Using the measured data, similarity profiles for depth-averaged void fraction, depth-averaged velocity, and water level were found. The study suggests that using only the velocity data is insufficient if the flow momentum or the flow rate is to be determined. The accuracy of the void fraction measurements was validated by comparing the directly measured water volume of the overtopping flow with the calculated water volume based on the measured velocity and void fraction.     

Surf Zone Currents

A large fraction of the water-wave energy incident on beaches is dissipated as the waves break and travel towards the shore through the surf zone. However, the momentum associated with the incident waves is not destroyed and drives other motions within the surf zone. An analysis is given of the unsteady and irregular currents that may occur in the surf zone. The forcing due to a discrete group of waves, and the vorticity of the surf-zone currents are the major topics. In particular, the almost two-dimensional nature of the flow implies that significant eddies are likely to be generated. There is evidence that eddies arise from long-shore currents, and the combination of eddies of opposite sign gives rip currents. It is noted that generation of vorticity by non-uniformities in bores may be a useful way of considering the forcing of surf zone motion. Expressions for the rate of increase in the circulation about material circuits and the vorticity generated at bores are derived. Received 30 January 1997 and accepted 16 May 1997     

Particle velocity field measurements in a near-wall flow using evanescent wave illumination

Evanescent waves from the total internal reflection of a 488 nm argon-ion laser beam at a glass-water interface were used to measure velocity fields in creeping rotating Couette flow within 380 nm of the stationary solid surface. Images of fluorescent 300 and 500 nm diameter polystyrene and silica particles suspended in water recorded at 30 Hz were processed using cross-correlation particle image velocimetry to determine the two in-plane velocity components with an in-plane spatial resolution of 40Ꮀ µm over a 200 µm (h)쏦 µm (v) field of view. The results are in reasonable agreement with the exact solution for the corresponding single-phase Stokesian flow. These data are, to our knowledge, the first velocity field measurements with this small out-of-plane spatial resolution (in all cases less than 380 nm), and the first such measurements in this interfacial or near-wall region. This paper describes the novel experimental diagnostic technique used to obtain these results.     

Numerical study of breaking waves by a two‐phase flow model

A two‐phase flow model, which solves the flow in the air and water simultaneously, is presented for modelling breaking waves in deep and shallow water, including wave pre‐breaking, overturning and post‐breaking processes. The model is based on the Reynolds‐averaged Navier–Stokes equations with the k ?ε turbulence model. The governing equations are solved by the finite volume method in a Cartesian staggered grid and the partial cell treatment is implemented to deal with complex geometries. The SIMPLE algorithm is utilised for the pressure‐velocity coupling and the air‐water interface is modelled by the interface capturing method via a high resolution volume of fluid scheme. The numerical model is validated by simulating overturning waves on a sloping beach and over a reef, and deep‐water breaking waves in a periodic domain, in which good agreement between numerical results and available experimental measurements for the water surface profiles during wave overturning is obtained. The overturning jet, air entrainment and splash‐up during wave breaking have been captured by the two‐phase flow model, which demonstrates the capability of the model to simulate free surface flow and wave breaking problems.Copyright © 2011 John Wiley & Sons, Ltd.     

URANS simulations for a high-speed transom stern ship with breaking waves

An exploratory study of high-speed surface ship flows is performed to identify modelling and numerical issues, to test the predictive capability of an unsteady RANS method for such flows, to explain flow features observed experimentally, and to document results obtained in conjunction with the 2005 ONR Wave Breaking Workshop. Simulations are performed for a high-speed transom stern ship (R/V Athena I) at three speeds Froude number (Fr) = 0.25, 0.43 and 0.62 with the URANS code CFDSHIP-IOWA, which utilizes a single-phase level set method for free surface modelling. The two largest Fr are considered to be high-speed cases and exhibit strong breaking plunging bow waves. Structured overset grids are used for local refinement of the unsteady transom flow at medium speed and for small scale breaking bow and transom waves at high-speeds. All simulations are performed in a time accurate manner and an examination of time histories of resistance and free surface contours is used to assess the degree to which the solutions reach a steady state. The medium speed simulation shows a classical steady Kelvin wave pattern without breaking and a wetted naturally unsteady transom flow with shedding of vortices from the transom corner. At higher speeds, the solutions reach an essentially steady state and display intense bow wave breaking with repeated reconnection of the plunging breaker with the free surface, resulting in multiple free surface scars. The high-speed simulations also show a dry transom and an inboard breaking wave, followed by outboard breaking waves downstream. In comparison to an earlier dataset, resistance is well predicted over the three speeds. The free surface predictions are compared with recent measurements at the two lowest speeds and show good agreement for both non-breaking and breaking waves.     

水气两相流中稀疏气泡流动速度场的数字图像测量初探

由空压机提供的气体通过—排微小直径的喷嘴进入静止水体，形成水气两相流流场。在单相PIV和PTV技术的基础上，研究稀疏气液两相流情况下气泡的速度场分布。PIV算法采用快速傅立叶互相关分析法，而PTV算法需要获得每幅图像中每个气泡的形心，根据连续图像中的粒子对，计算速度。用PIV和PTV两种算法处理求出气泡的速度并对两种方法进行比较，其最终研究成果可应用于流体及多相流的流量测技术，提高我们进行低密度气液两相流相关研究的测量水平。同时为水气两相流的数值分析和理论研究提供流场测试的数据。     

The prediction of stratified two-phase flow with a two-equation model of turbulence

A two-equation model is applied to a stratified two-phase flow system to predict turbulent transport mechanisms in both phases.In the present analysis, the effects of interfacial waves on the flow field are formulated in terms of boundary conditions for the gas-liquid interface. For the gas phase, the wavy interface has such flow separation effects as a rough surface in a single-phase flow. While for the liquid phase, the waves generate turbulant energy which is transported progressively toward a lower wall region. The analytical results are in good agreement with available data regarding pressure drop, holdup and velocity profiles.     

Improvement of Acoustic Doppler Velocimetry in bubbly flow measurements as applied to river characterization for kinetic turbines

Acoustic Doppler Velocimetry (ADV) can measure flow velocities in three directions in experimental facilities and field applications. Based on the Doppler shift effect, ADV can accurately resolve the quasi-instantaneous flow field at frequencies of up to approximately 200 Hz. However, this technique is sensitive to operating conditions that can lead to contaminated signals containing large amplitude spikes, a disadvantage of ADV. Aliasing of the Doppler signal creates these spikes. Such a situation occurs when large particles intersect the sampling volume or acoustic waves. For example during the characterization of river velocities, sediments floating near the riverbed cause aliasing from particles, and more importantly, surface entrained air bubbles contaminate the ADV signal. Spikes due to air bubbles not only increase the standard deviation of the velocity, but also corrupt the autocorrelation and power spectra. As some of these spikes appear like velocity fluctuations, developing accurate despiking procedures is an important requirement during post-processing of ADV velocity measurements in bubbly flow applications. A new hybrid method is introduced which has advantages over conventional despiking methods such as the acceleration thresholding method and the phase-space thresholding method when using ADV in bubbly flow. ADV river velocity measurements near kinetic turbines demonstrate the proposed method. This method is applicable to other bubbly flow applications to characterize the liquid phase using ADV.     

Evaluation of wall slip effects in the lubricating grease/air two-phase flow along pipelines

Evaluation of wall slip phenomena during the horizontal pipeline flow of air/lubricating grease mixtures was investigated. With this aim, pressure drop measurements have been carried out along pipelines with different diameter and roughness. A modified Jastrzebski's equation for the slip velocity, based on the introduction of the relative roughness, has been used to correct wall slip effects for a lithium lubricating grease/air system. This expression has been introduced in the classical Rabinowitsch–Mooney treatment and applied to the superficial liquid velocity instead of the single-phase average velocity following a single-phase treatment analogy. Thus, the non-slip flow curve data for the two-phase mixture were obtained from roughened pipes and compared with those obtained from pipes with smooth internal surfaces. The effect of air on the extension of wall slip has been established as a function of air flow rate. Thus, the consideration of the reduction of the wetted pipe surface as air is injected allows an adequate explanation of this phenomenon, confirmed by the reduction of the effective slip contribution on the observed apparent shear rate. A power-law relationship between the slip velocity and the wall shear stress has been deduced, although this tends asymptotically to linearity as air flow rate is increased.     

Measurements of gravity and gravity-capillary waves in horizontal gas-liquid pipe flow using PIV in both phases

An experimental study was performed in stratified wavy flow of air and water through a horizontal pipe. The velocity fields in both phases were measured simultaneously using PIV and the interfacial shape was resolved using a profile capturing technique. The objective of the study was to investigate the interfacial characteristics and the velocities of the liquid and gas phases in two wave patterns: ‘3D small amplitude’ and ‘2D large amplitude’ waves. The wave patterns were shown to consist of gravity and gravity-capillary waves, respectively, with substantial differences in the wave characteristics and liquid velocities. Contrary to this, the effect of the waves on the gas velocities was rather similar in both wave regimes, with both wave regimes causing an increase in the velocity fluctuations close to the interface. The current measurements also produced a valuable dataset that can be used to further improve the numerical modeling of the stratified flow pattern.