Stress Τw Research Articles

Relative size matters: Spawning substrate roughness size and spacing affect egg dislodgement and retention in the benthos

AbstractThe consequences of hydrodynamic effects on survival of embryonic benthic organisms remain unknown. This is because the interaction of hydrodynamics and substrate roughness is complex, but it can be conceptualized in two dimensions by roughness flow regimes (isolated, wake interference, and skimming flow) or roughness types (k‐ and d‐type). We examined the effect of different roughness flow regimes/roughness types and roughness heights (k) on the dislodgement of walleye (Sander vitreus) eggs (2 mm diameter, ⌀) using a wall jet apparatus. We expected that lower wall shear stress (τw) would be required for dislodgement in more‐exposed/less‐sheltered roughness type/flow regimes (i.e., k‐type/isolated roughness flow) and when k ≤ ⌀. Surprisingly, the opposite was found, indicating that other mechanism(s) are also responsible for egg dislodgement on these rough surfaces. Hydrodynamic sheltering occurred when flow was directed over exposed eggs (k ≤ ⌀) adjacent to roughness elements effectively increasing their width, and thus requiring higher τw for rolling and then ejection. When k > ⌀, eggs dislodgement likely occurred via the lower pressure in the eddies recirculating in the groove width between roughness elements, rather than due to τw. These results indicate the relevance of heterogeneous sorting and sizing of substrate elements to provide suitable spawning habitats in benthic environments.

Hemodynamic characteristics of pulsatile blood flow through bifurcated stenosed carotid artery

PurposeCarotid artery is often associated with plaque deposition because of its shape and associated flow features. The shape of stenosed bifurcation is characterised by bifurcation angle (ß), planarity angle (α) and severity of stenosis (b). In the present work, three-dimensional numerical computations have been performed to analyse the effect of these geometrical parameters of carotid bifurcation on the characteristics of flow.Design/methodology/approachGoverning equations of this study were solved using ANSYS Fluent 20.1 and the blood flow was considered as laminar, pulsatile and non-Newtonian. Instantaneous flow behaviour has been illustrated using vorticity, velocity and helicity contours, whereas the time-averaged wall shear stress (τw¯) and oscillatory shear index (OSI) quantify the time-averaged behaviour.FindingsThe recirculation zone and secondary flow are ascertained to be stronger for higher bifurcation angle as compared to the lower bifurcation angle. Strength of the secondary flow is found to reduce with increase in α from 0° to 10°, whereas it grows as α varies from 10° to 20°. For higher bifurcation angles, τw¯ is lower than 2 Pa and OSI is greater than 0.2 on the outer walls. Similar observations were made for τw¯ and OSI distribution on bottom wall in non-planar cases, which predicted atherogenic locations.Originality/valueThe values for ß were taken as 30°, 45°, 60° and 75°, whereas for α, range of 0°–20° was chosen. The stenosis was considered on the outer wall of internal carotid artery and its severity was considered within the range of 0%–60%.

Influence of solvent viscosity ratio on the creeping flow of viscoelastic fluid over a channel confined circular cylinder

In this study, the role of solvent viscosity ratio (β) on the creeping flow characteristics of Oldroyd-B fluid over a channel-confined circular cylinder has been explored numerically. The flow governing equations have been solved by RheoTool, an open-source toolbox based on OpenFOAM, employing the finite volume method for extensive ranges of Deborah number (De=0.025–1.5) and solvent viscosity ratio (β=0.1–0.9) at a fixed wall blockage (B = 0.5). The present investigation has undergone extensive validation, with available literature under specific limited conditions, before obtaining detailed results for the relevant flow phenomena, such as stream function, pressure and stress contour profiles, pressure coefficient (Cp), wall shear stress (τw), normal stress (τxx), first normal stress difference (N1), and drag coefficient (CD). The flow profiles have exhibited a distinctive behavior characterized by a loss of symmetry in the presence of pronounced viscoelastic effects. The results for low De notably align closely with those for Newtonian fluids, and the drag coefficient (CD) remains relatively constant regardless of β, as the viscoelastic influence is somewhat subdued. These observations indicate that at high De and low β, viscoelasticity causes asymmetry in creeping flow around a circular cylinder. With an increase in De, the maximum velocity in gap between cylinder and channel walls increases; however, the cylinder experiences significantly less drag force. Within this parameter range, the prevailing force governing the flow is the pressure drag force.

Wave, flow, and suspended sediment dynamics under strong winds on a tidal beach

Tidal beaches are shaped by intense wave-current interactions, leading to dynamic sediment erosion and accretion, which can be further complicated by strong winds. In this study, field observations were conducted on a tidal beach at the junction of Yangtze Estuary and Hangzhou Bay (China), to investigate the dynamics of wave, flow, and suspended sediment under strong winds. Observations were carried out synchronously at two sites on the middle and lower intertidal beach covering 4 semidiurnal tides (52 h) including a ∼20 h-long strong wind period. During the strong wind period, wave motions and flood currents were significantly enhanced, leading to a high combined wave-current bed shear stress (τcw). The wave-induced bed shear stress (τw) greatly exceeded current-induced bed shear stress (τc). Breaking waves generated turbulence at the surface and amplified the near-bed turbulent Reynolds stress (−u′w′‾ and −v′w′‾) at the shallow, middle intertidal beach. The intense hydrodynamics and turbulence kept sediments in suspension and hindered any settling, resulting in net erosion of the intertidal beach. These findings provide valuable insights into the complex interplay among waves, currents, and sediment dynamics under short-duration strong winds on tidal beaches. A key emphasis is placed on the significant role of breaking waves, underscoring the critical importance of their contribution to near-bed turbulence.

Numerical analysis of novel wavy wall based control of turbulent boundary layer separation

In this work, a comprehensive insight into the new passive flow control method, i.e. a two-dimensional streamwise waviness (recently proposed by Dróżdż et al. [1]) is provided using Large Eddy Simulations. This approach was demonstrated to be effective in postponing turbulent boundary layer separation at high Re, which is out of reach for other commonly known passive flow control strategies. Although the effectiveness of the wavy wall in delaying flow separation has been confirmed, the physical mechanisms standing behind the skin friction enhancement are not fully understood which opens a space for future work. Of particular interest is an insight into the fluid flow in the near-wall region just above the waviness and this paper is aimed at doing so. To separate the effect of the global adverse pressure gradient, the results were confronted with the ones obtained under zero pressure gradient conditions for the identical wavy wall geometry and the same Reτ=2500. Also additional simulations of the turbulent boundary layer building up on the flat wall were conducted to have an additional reference for the flow fields obtained above the wavy wall under zero and adverse pressure gradients respectively. A detailed inspection of the streamwise velocity and its fluctuation profiles indicated that the contribution of the global adverse pressure gradient to the flow statistics begins when the Rotta-Clauser pressure gradient parameter β≥3. The paper confirms an enhancement of the wall shear stress τw downstream the corrugation, which is the necessary condition to postpone the separation. Interestingly, the effect of increased τw was not observed for the case with the same waviness and without pressure gradient.

Observation and Parameterization of Bottom Shear Stress and Sediment Resuspension in a Large Shallow Lake

AbstractParameterizations for bottom shear stress are required to predict sediment resuspension from field observations and within numerical models that do not resolve flow within the viscous sublayer. This study assessed three observation‐based bottom shear stress (τb) parameterizations, including (a) the sum of surface wave stress and mean current (quadratic) stress (); (b) the log‐law (τb = τL); and (c) the turbulent kinetic energy (τb = τTKE); using 2 years of observations from a large shallow lake. For this system, the parameterization τb = τw + τc was sufficient to qualitatively predict resuspension, since bottom currents and surface wave orbitals were the two major processes found to resuspend bottom sediments. However, the τL and τTKE parameterizations also captured the development of a nepheloid layer within the hypolimnion associated with high‐frequency internal waves. Reynolds‐averaged Navier‐Stokes (RANS) equation models parameterize τb as the summation of modeled current‐induced bottom stress (τc,m) and modeled surface wave‐induced bottom stress (τw,m). The performance of different parameterizations for τw,m and τc,m in RANS models was assessed against the observations. The optimal parameterizations yielded root‐mean‐square errors of 0.031 and 0.025 Pa, respectively, when τw,m, and τc,m were set using a constant canonical drag coefficient. A RANS‐based τL parameterization was developed; however, the grid‐averaged modeled dissipation did not always match local observations, leading to O(10) errors in prediction of bottom stress. Turbulence‐based parameterizations should be further developed for application to flows with mean shear‐free boundary turbulence.

Resuspension of trace explosive particle residues by planar impinging jet: Effects of exposure duration and wall shear stress

Aerodynamic particle resuspension is important in many practical applications, such as surface cleaning, understanding particle fate in the environment, and surface sampling. In practical application, the probability of particle resuspension from the surface depends on several parameters, such as particle size, morphology, chemical composition, surface properties, environmental and flow conditions. The wall shear stress (τw) links fluid motion and forces acting on the particle and can be used to characterize particle resuspension. The literature reports that the resuspension rates decay as 1/t at long exposure times; however, the effect of very short exposures to aerodynamic forces on the resuspension of irregular microparticles has not been characterized. This work examines the effect of τw and exposure duration utilizing planar jet pulses. We evaluated resuspension silica spheres and irregular Trimethylenetrinitramine (RDX) microparticles from a smooth glass surface with a 10–100 ms pulse duration and extended exposure (60 s) for the same wall shear stresses. Reducing the jet duration shows that the removal rate is inversely proportional to exposure time; the trend is more visible at lower τw. At higher shear stresses, up to 80% of particles are removed in the first 10 ms of exposure to aerodynamic force.

Parameters of Flow through Paravalvular Leak Channels from Computational Fluid Dynamics Simulations—Data from Real-Life Cases and Comparison with a Simplified Model

Background: Shear forces affecting erythrocytes in PVL channels can be calculated with computational fluid dynamics (CFD). The presence of PVLs is always associated with some degree of hemolysis in a simplified model of the left ventricle (LV); however, data from real-life examples is lacking. Methods: Blood flow through PVL channels was assessed in two variants. Firstly, a PVL channel, extracted from cardiac computed tomography (CCT), was placed in a simplified model of the LV. Secondly, a real-life model of the LV was created based on CCT data from a subject with a PVL. The following variables were assessed: wall shear stress (τw) shear stress in fluid (τ), volume of PVL channel with wall shear stress above 300 Pa (V300), duration of exposure of erythrocytes to shear stress above 300 Pa (Vt300) and compared with lactate dehydrogenase (LDH) activity levels. Results: τw and τ were higher in the simplified model. V300 and Vt300 were almost identical in both models. Conclusions: Parameters that describe blood flow through PVL channels can be reliably assessed in a simplified model. LDH levels in subjects with PVLs may be related to V300 and Vt300. Length and location of PVL channels may contribute to a risk of hemolysis in mitral PVLs.

Early biofilm and streamer formation is mediated by wall shear stress and surface wettability: A multifactorial microfluidic study.

Biofilms are intricate communities of microorganisms encapsulated within a self‐produced matrix of extra‐polymeric substances (EPS), creating complex three‐dimensional structures allowing for liquid and nutrient transport through them. These aggregations offer constituent microorganisms enhanced protection from environmental stimuli—like fluid flow—and are also associated with higher resistance to antimicrobial compounds, providing a persistent cause of concern in numerous sectors like the marine (biofouling and aquaculture), medical (infections and antimicrobial resistance), dentistry (plaque on teeth), food safety, as well as causing energy loss and corrosion. Recent studies have demonstrated that biofilms interact with microplastics, often influencing their pathway to higher trophic levels. Previous research has shown that initial bacterial attachment is affected by surface properties. Using a microfluidic flow cell, we have investigated the relationship between both wall shear stress (τw) and surface properties (surface wettability) upon biofilm formation of two species (Cobetia marina and Pseudomonas aeruginosa). We investigated biofilm development on low‐density polyethylene (LDPE) membranes, Permanox® slides, and glass slides, using nucleic acid staining and end‐point confocal laser scanning microscopy. The results show that flow conditions affect biomass, maximum thickness, and surface area of biofilms, with higher τw (5.6 Pa) resulting in thinner biofilms than lower τw (0.2 Pa). In addition, we observed differences in biofilm development across the surfaces tested, with LDPE typically demonstrating more overall biofilm in comparison to Permanox® and glass. Moreover, we demonstrate the formation of biofilm streamers under laminar flow conditions within straight micro‐channels.

Hydrodynamics of vertical upward and downward flow boiling in a millimetric tube

Upward and downward vapor-liquid vertical flows inside a vertical 6 mm sapphire tube was investigated using HFE-7000 as working fluid. The goal was to investigate the effect of flow direction and wall heating on two-phase parameters such as flow pattern, wall shear stress (τw), interfacial shear stress (τi) and interfacial wave structures. The mass flux, heat flux and vapor quality ranges were 50≤G≤400kg/m2.s,0≤q≤3W/cm2 and 0≤x≤0.7 respectively. Bubbly, slug, churn, annular flow patterns were observed in upward and downward flows. A falling film regime occurred in downward flow at low mass fluxes. Void fractions were higher in downward flow than in upward flow due to gravity effect, and were in good agreement with drift flux models of the literature. The wall shear stress increased with the wall heat flux due to the bubble nucleation at the wall. Following a similar approach to Kim and Mudawar (2013b), a correlation for the wall shear stress taking into account the forced convection and the bubble nucleation was derived and provided a good estimation of the experimental data within ±20%. From image processing of the high-speed visualizations, velocities (Uw) and frequencies (Fw) of the disturbance waves in annular flow were measured. The interfacial shear stress was found to directly depend of the product Uw×Fwand a prediction of the interfacial friction factor was proposed in flow boiling for both upward and downward flows.

Build of Machine Learning Proxy Model for Prediction of Wax Deposition Rate in Two Phase Flow Water-Oil

Wax deposit is one of the major fl ow assurance experienced in the process of oil production and transportation from sub- surface to surface. Large amounts of data are required to perform modeling using existing thermodynamic models such as carbon number data from HGTC. In this paper, a machine learning algorithm using unifi ed model approach from Huang (2008). Two types of input are implemented in order to simulate infl uence of feature selection used in training and testing machine learning which are input A consists of water volume fraction (fw), shear stress (τw), effective viscosity (μe), wax concentration gradient (dC/dT), and temperature gradient (dT/dR) and input B consists of water volume fraction (fw), shear stress (τw), effective viscosity (μe), wax concentration gradient (dC/dT), temperature gradient (dT/dR), shear stripping variable (SV) dan diffusion variable (DV). The random forest with Ntree = 500 known to be the best machine learning method compared to others. Based on accuracy parameter it achieves error parameter R-squared (R2) for training, testing and total data for input A and B are 0.999, 0.992, 0.9975 and 0.999, 0.993, 0.9977, respectively.

Large-eddy simulation for solid particle transport and deposition in a helically rib-roughened pipe using an Euler-Lagrange approach

Multiphase large-eddy simulations of a particle-laden turbulent flow in a helically rib-roughened pipe were performed for Re=10000,16000, and 20000 by using an Euler–Lagrange approach. Cyclic boundary conditions were imposed to the continuous as well as the dispersed phase, to get a fully developed particle-laden turbulent flow and a constant deposition rate Ṅd of the solid particles. The validation of the computation of the turbulent continuous and the disperse phase by the cyclic Euler–Lagrange approach, as well as the deposition of the particles, is based on particle-laden backward-facing step and cyclic duct flow simulations. An adhesion model, dependent on a critical particle velocity up,crit, and a removal model, dependent on a critical wall-shear stress τw,crit, were added to the ribbed pipe flow simulations to take the physical effect of particle rebound and particle removal into account. Simulation results of deposition rates Ṅd and deposition velocities ud+ for a variety of particle diameters Dp and model parameters are investigated. The simulations showed that significantly less particle adhere to the wall when taking into account the adhesion and removal model. A self-cleaning effect of the almost entire pipe wall at specific particle diameters Dp or particle relaxation times τp+ due to wall-shear stress removal could be found as well as a limit value from which the effect of this kind of removal is reduced.

The Influence of Rotating Cage Geometry and Baffles on the Wall Shear Stress Equation in Sweet/Sour Corrosion

Flow-accelerated corrosion has been studied by means of computational fluid dynamics (CFD) techniques and rotating cage (RC) autoclave tests according to ASTM G170. This study evaluated through CFD and laboratory tests the influence of RC geometry and baffles on the wall shear stress (τw) proposed by the aforementioned standard. The experimental were performed at 120 °C and 2 angular velocities (500 and 800 rpm) during 25 h in a solution with 600 mg/L of Cl- ions and 10-3 mol/L of sodium thiosulfate. Corrosion rates were obtained by ASTM G31 and the surfaces were evaluated by OM and XRD. Corrosion rate values and XRD analyses showed divergence between samples in different positions in the RC and between distinct regions in the same sample. The numerical simulations and the experimental tests showed a great influence of the baffles on τw. The conservatism in the values obtained with this technique is caused by excessive corrosion close to the samples edges and the non-uniformity of τw on the internal samples surfaces. Thus, the main conclusion of the study is related to the importance of better knowledge on experimental methodology and mathematical approximations for updating the criterion for the correct τw calculation on sample surfaces.

Boundary Layer Flow and Heat Transfer Analysis by Using the Correlation Coefficient and Regression Model Over a Stretching Bullet-Shaped Object

The boundary layer theory is important when fluid flows over a solid surface that is moving or stationary. In presence of the boundary layer, the effective shape of the body may change leading to changes in pressure distribution, as a result, the overall lift and drag forces change. Therefore, the Boundary layer theory helps in designing aerofoil’s, to compute the lift and drag forces for the aerospace and automobile designers, to control the heat transfer rate from the device, etc. So, the present problem will help design the various types of bullet-shaped objects in the field of automobile engineering. Therefore, the current problem has focused on the two-dimensional axisymmetric BL flow over a stretching bullet-shaped object for the effect of magnetic field strength (M), linear stretching parameter (M), and surface thickness parameter (s). Therefore, the main goal of this work is to determine the relation by applying the correlation coefficient among the physical parameters and velocity field, temperature field, shear stress (τw), Nusselt number (Nux). Hence, the novelty of the current paper is to develop the relationship among the dependent and independent parameters by the correlation coefficient and also developed the numerical method to solve these highly nonlinear equations. The numerical results are discussed for the three different values of the stretching ratio parameter and two values of the surface thickness parameter. The velocity and temperature distribution equations are compressed into a system of ODEs with similarity transformations. These ODEs are then solved using a spectral quasi-linearization method (SQLM) by applying Taylor series expansions that can be used to linearize the non-linear terms in the equations. These resulting linearized systems of equations are determined by the spectral collocation method. The convergence of the numerical solutions was performed by using the residual error of the PDEs. The error analysis is established for the validity of the present model. This error norm is applied to establish the validity and convergence of the numerical solution. The outcome of the mentioned dimensionless parameters over the fluid velocity field, temperature field, skin friction coefficient (Cf), and Nusselt number (Nux) are displayed graphically. It is observed that the parameters M and M are positively correlated with fluid velocity distribution within the BL but the surface thickness parameter(s) are negatively correlated. The rate of temperature increases for the parameter M and Pr but decreases for M and s. Therefore, the boundary layer thickness reduces for increasing the values of M and M but increases for increasing the values of s. The velocity of the fluid is about 80% higher in the case of a thinner surface (s = 0.2) than the thicker surface (s = 2.0) and the heat transfer rate is also igher in the case of a thinner surface comparatively thicker surface. The innovation of this present problem lies in the unification of more physical parameters into the governing equations and an attempt to give a thorough analysis of how the flow properties are affected by these parameters.

Drag Reduction in the Flow of Aqueous Solutions of a Mixture of Cocamidopropyl Betaine and Cocamide DEA

The study presents results of rheological tests and measurements of pressure drops occurring during the flow of aqueous solutions of a mixture of drag reducing surfactants: cocamidopropyl betaine (CAPB, zwitterionic surfactant) and cocamide DEA (nonionic surfactant) through straight pipes. Tests were carried out at different CAPB/DEA weight ratios and different total concentrations of surfactants in the solution. Rheological measurements demonstrate the formation of a shear-induced structure (SIS) in the temperature range below 10 °C, which provides evidence for the presence of wormlike micelles in CAPB/DEA solutions. Drag reduction was observed during the flow of CAPB/DEA solutions in the temperature range from 3 to 45 °C, however, above 25 °C the degree of drag reduction was markedly decreased. The lower temperature limit at which drag reduction occurs depends on the CAPB and DEA weight ratio in the solution. In the range of higher temperatures, during the flow of CAPB/DEA solutions (similarly to flexible-chain polymer solutions) the onset of drag reduction is noted above a certain critical value of the Rec,0 number, whose value depends on the temperature of the solution, diameter of the pipe and the weight ratio of surfactants. At the same time, the critical value of wall shear stress τw,c0 corresponding to the critical value of Rec,0 is approximately independent of pipe diameter. The critical value of the Rec,0 number has been linked to the clouding of CAPB/DEA solutions.

The effect of fluid flow on CO2 corrosion of high-strength API carbon steels

This paper presents and discusses the outcomes of a recent study on the corrosion resistance of API carbon steels (T95 and Q125). The scope includes experimental investigation as well as theoretical modeling. To predict CO2 corrosion under dynamic conditions, an existing model is improved and validated using experimental measurements.Corrosion tests were conducted under static and dynamic conditions. API casings were cut and machined to prepare test specimens used for measuring Average Corrosion Rate (ACR) and Load-Carrying Capacity (LCC). The dynamic test condition was simulated using the flow between two concentric cylinders (Couette type flow). The test solution (2% wt. NaCl) was saturated with a gas mixture containing 50% CO2 and 50% methane. Tests were carried out at 41.37 MPa with varying temperature and the speed of the inner cylinder, and subsequently, changing the wall shear stress (τw). The ACR was determined by applying the Weight Loss (WL) method. A digital microscope was utilized to inspect the morphology of the corrosion scale (product). The change in LCC of the specimens was compared with the theoretically predicted change in LCC to identify the presence of localized corrosion and/or intergranular attack. In addition to LCC, the ultimate tensile strength (UTS) of corroded specimens is determined and compared with that of uncorroded specimens.Results show that the CO2 corrosion of API steels is sensitive to the fluid velocity regardless of temperature. The corrosion of Q125 is more affected by fluid flow than that of T95. Below critical shear stress, CO2 corrosion was strongly affected by fluid flow. Under a static condition, Q125 demonstrated higher corrosion resistance than T95 but under a dynamic condition, T95 exhibited higher corrosion than Q125. At high temperature (71 °C), the occurrence of localized corrosion was observed. Corrosion model predictions show reasonable agreement with measurements.

Suction Controlled Topological Transition in Laminar Juncture Flows

The effect of suction on the topological transition in juncture flow is studied experimentally and numerically. A number of combinations of the positions, where suction is applied, and the flow rates are investigated and their effect on the topological transition of laminar juncture flow is assessed. It is observed that for any particular case, a combination of suction hole positions and volume flow rates can transform the most upstream surface singular point of the horseshoe vortex (HSV) system from a saddle of separation to a saddle of attachment. The topological transition at low suction rates is achieved, when the suction holes are placed close to the separation region. A better HSV control is achieved, when suction is applied close to the juncture at high suction rates. A previously proposed separation/attachment prediction method based on the mass conservation is also confirmed by analyzing the simulated results of suction-controlled and originally-generated wall saddles. The results suggest that the topological transition (separation/attachment) at each wall saddle can be accurately estimated according to the sign of a discriminant coefficient CS/A comprised of the local shear stress τw on the wall and stream-tube width n near the saddle.

On the reversibility of deposit formation in low temperature milk microfiltration with ceramic membranes depending on mode of adjustment of transmembrane pressure and wall shear stress

During the microfiltration of skim milk, flux and protein permeation decrease due to the formation of a deposit layer. For a better understanding of the deposit formation during the microfiltration of skim milk at 10 °C, flux and protein permeation were studied with a ceramic multichannel membrane (nominal pore size (nps) = 0.1 µm) at wall shear stress (τw) values from 45 to 341 Pa. When the limiting transmembrane pressure was applied, the flux showed a nearly linear increase with increasing wall shear stress and peaked at a critical τw. A further increase of τw resulted in a reduction of the flux until it levelled off. Thus, we could define a limiting τw additionally to the critical τw, at which no further changes in flux were observable. The limiting flux indicates that no structural changes in the deposit layer occur beyond the limiting τw. This conclusion is reassured by the development of the protein permeation. Beyond the limiting τw, no changes in the protein permeation could be observed. The critical and limiting τw are useful values for the design of fractionation processes by means of ceramic microfiltration.

Preparation and characterization of perfluorocarbon microbubbles using Shirasu Porous Glass (SPG) membranes

Microbubbles are increasingly used in several fields, such as medical imaging for enhanced contrast ultrasound imaging. Theses microbubbles usually consist of a gas core stabilized by surfactant molecules. In this study, a technique using Shirasu Porous Glass (SPG) membranes was used to produce perfluorocarbon microbubbles. The microbubbles obtained were characterized by their size, size distribution, and stability. The effect of several parameters on the microbubble's size was investigated related to the process (transmembrane pressure, ΔP, bubble point pressure, PBP, shear stress, τw), membrane pore size, Dp, and formulation (gas, surfactants in the aqueous phase). The transmembrane pressure nor the shear stress (τw) had influence on the microbubble's size or size distribution for ΔP/PBP <1.5. The decrease of the membrane pore size from 1.1, 0.5, to 0.2 μm led to lower microbubble size 13.3, 6.36, and 4.42 μm, respectively, which was associated with higher size distribution 16%, 24% and 31%, respectively due to the higher Laplace pressure exerted on smaller microbubbles leading to their destabilization. With the 1.1 μm pore size membrane, perfluorocarbon microbubbles were obtained with a diameter of 13.3 μm and coefficient of variation (CV) of 16% when stabilized by sodium dodecyl sulfate (SDS), 15.6 μm with CV% of 23% when stabilized by Tween20, and 16.5 μm with CV% of 26% when stabilized by Polyoxyethylene (40) stearate (PEG40S). These low CV were indication of monodispersity. Perfluorocarbon microbubbles had a smaller size than air microbubbles due to the lower surface tension that decreased the retention force, keeping the microbubbles at the pore opening. The stability study showed that the perfluorocarbon gas greatly increased the lifetime of the microbubbles with a slight increase in size of 1.3 after 90 s compared to 2.2 for air microbubbles. Overall, the membrane technique proved to be an effective, controlled and reproducible method to produce perfluorocarbon microbubbles at a high rate ∼0.6 − 1.5 × 10+10 microbubbles/min. The key factor that determines the microbubbles formation is the adsorption kinetics of the surfactant at the new gas–liquid interface at the pore opening.

Bed shear stress estimation on an open intertidal flat using in situ measurements

Accurate estimations for the bed shear stress are essential to predict the erosion and deposition processes in estuaries and coasts. This study used high-frequency in situ measurements of water depths and near-bed velocities to estimate bed shear stress on an open intertidal flat in the Yangtze Delta, China. To determine the current-induced bed shear stress (τc) the in situ near-bed velocities were first decomposed from the turbulent velocity into separate wave orbital velocities using two approaches: a moving average (MA) and energy spectrum analysis (ESA). τc was then calculated and evaluated using the log-profile (LP), turbulent kinetic energy (TKE), modified TKE (TKEw), Reynolds stress (RS), and inertial dissipation (ID) methods. Wave-induced bed shear stress (τw) was estimated using classic linear wave theory. The total bed shear stress (τcw) was determined based on the Grant–Madsen wave–current interaction model (WCI). The results demonstrate that when the ratio of significant wave height to water depth (Hs/h) is greater than 0.25, τcw is significantly overestimated because the vertical velocity fluctuations are contaminated by the surface waves generated by high winds. In addition, wind enhances the total bed shear stress as a result of the increases in both τw and τc generated by the greater wave height and reinforcing of vertical turbulence, respectively. From a comparison of these various methods, the TKEw method associated with ESA decomposition was found to be the best approach because: (1) this method generates the highest mean index of agreement; (2) it uses vertical velocities that are less affected by Doppler noise; and (3) it is less sensitive to the near-bed stratification structure and uncertainty in bed location and roughness.