High Shear Regions Research Articles

A Method for Interpreting the Role of Parameterized Turbulence on Global Metrics in the Community Earth System Model

AbstractThe parameterization of subgrid‐scale processes such as boundary layer (PBL) turbulence introduces uncertainty in Earth System Model (ESM) results. This uncertainty can contribute to or exacerbate existing biases in representing key physical processes. This study analyzes the influence of tunable parameters in an experimental version of the Cloud Layers Unified by Binormals (CLUBBX) scheme. CLUBB is the operational PBL parameterization in the Community Atmosphere Model version 6 (CAM6), the atmospheric component of the Community ESM version 2 (CESM2). We perform the Morris one‐at‐a‐time (MOAT) parameter sensitivity analysis using short‐term (3‐day), initialized hindcasts of CAM6‐CLUBBX with 24 unique initial conditions. Several input parameters modulating vertical momentum flux appear most influential for various regionally‐averaged quantities, namely surface stress and shortwave cloud forcing (SWCF). These parameter sensitivities have a spatial dependence, with parameters governing momentum flux most influential in regions of high vertical wind shear (e.g., the mid‐latitude storm tracks). We next evaluate several experimental 20‐year simulations of CAM6‐CLUBBX with targeted parameter perturbations. We find that parameter perturbations produce similar physical mechanisms in both short‐term and long‐term simulations, but these physical responses can be muted due to nonlinear feedbacks manifesting over time scales longer than 3 days, thus causing differences in how output metrics respond in the long‐term simulations. Analysis of turbulent fluxes in CLUBBX indicates that the influential parameters affect vertical fluxes of heat, moisture, and momentum, providing physical pathways for the sensitivities identified in this study.

Experimental Validation of the Modified Holmén Vortex Identification Method

The iterative vortex identification method originally proposed by Holmén was modified to improve its performance and to add the ability to distinguish between counter-rotating vortices. Then, the resulting modified Holmén method (MHM) was evaluated using experimental data from five different experiments, including planar and three-dimensional flowfield data. The Q, λ2, and Γ2 criteria were used for comparison. No currently available vortex identification method is perfect (otherwise, proposing a new scheme would be a pointless exercise), and the MHM is no exception; however, the MHM was found to accurately identify vortical structures when the number of passes is chosen correctly while minimizing the experimental noise. In particular, it was found to be useful in capturing large vortical structures occurring in a noisy or high-shear region, cases where the Q and Γ2 criteria sometimes fail. On the other hand, the method is limited by the spatial resolution of the experimental data with respect to the structure’s size and its inability to resolve rotating structures near the edges of the measurement domain.

A Modeling Framework for the Thermoforming of Carbon Fiber Reinforced Thermoplastic Composites.

A comprehensive modeling framework for the thermoforming of polymer matrix woven laminate composite was developed. Two numerical indicators, the slip path length and traction magnitude, have been identified to be positively correlated to matrix smearing and wrinkling defects. The material model has been calibrated with picture-frame experimental results, and the prediction accuracy for intra-ply shear and thickness distribution was examined with measurements of the physically formed parts. Specifically, thickness prediction for most locations on the formed parts was accurate within an 11.6% error margin. However, at two points with significant intra-ply shear, the prediction errors increased to around 20%. Finally, a parametric study was conducted to determine the relationship between various process parameters and the quality of the formed part. For the trapezoidal part, orienting the laminate at 45 degrees to the mold axis reduces the likelihood of matrix smear and wrinkling defects. Although this laminate orientation yielded a greater spatial variation in part thickness, the thickness deviation is lower than that for the 0-degree orientation case. Two forming analyses were conducted with ramp rates of 25 mm/s and 80 mm/s to match the equipment's operational limits. It was observed that higher forming rates led to a greater likelihood of defects, as evidenced by a 15% and 10% increase in the formed part areas with longer slip paths and higher traction magnitudes, respectively. It was discovered that shallower molds benefit from faster ramp rates, while deeper molds require slower rates to manage extensive shearing, stretching and bending. Faster forming rates lead to smaller thickness increases at high intra-ply shear regions, indicating a shift from intra-ply shear to out-of-plane bending due to the visco-plastic effect of the molten laminate and can negatively impact part quality. Lastly, it was shown that a well-conceived strategy using darts could improve the part quality by reducing the magnitude of the defect indicators.

Computational analysis of cancer cell adhesion in curved vessels affected by wall shear stress for prediction of metastatic spreading.

Introduction: The dynamics of circulating tumor cells (CTCs) within blood vessels play a pivotal role in predicting metastatic spreading of cancer within the body. However, the limited understanding and method to quantitatively investigate the influence of vascular architecture on CTC dynamics hinders our ability to predict metastatic process effectively. To address this limitation, the present study was conducted to investigate the influence of blood vessel tortuosity on the behaviour of CTCs, focusing specifically on establishing methods and examining the role of shear stress in CTC-vessel wall interactions and its subsequent impact on metastasis. Methods: We computationally simulated CTC behaviour under various shear stress conditions induced by vessel tortuosity. Our computational model, based on the lattice Boltzmann method (LBM) and a coarse-grained spectrin-link membrane model, efficiently simulates blood plasma dynamics and CTC deformability. The model incorporates fluid-structure interactions and receptor-ligand interactions crucial for CTC adhesion using the immersed boundary method (IBM). Results: Our findings reveal that uniform shear stress in straight vessels leads to predictable CTC-vessel interactions, whereas in curved vessels, asymmetrical flow patterns and altered shear stress create distinct adhesion dynamics, potentially influencing CTC extravasation. Quantitative analysis shows a 25% decrease in the wall shear stress in low-shear regions and a 58.5% increase in the high-shear region. We observed high-shear regions in curved vessels to be potential sites for increased CTC adhesion and extravasation, facilitated by elevated endothelial expression of adhesion molecules. This phenomenon correlates with the increased number of adhesion bonds, which rises to approximately 40 in high-shear regions, compared to around 12 for straight vessels and approximately 5-6 in low-shear regions. The findings also indicate an optimal cellular stiffness necessary for successful CTC extravasation in curved vessels. Discussion: By the quantitative assessment of the risk of CTC extravasation as a function of vessel tortuosity, our study offers a novel tool for the prediction of metastasis risk to support the development of personalized therapeutic interventions based on individual vascular characteristics and tumor cell properties.

A continuum model for the elongation and orientation of Von Willebrand factor with applications in arterial flow

The blood protein Von Willebrand factor (VWF) is critical in facilitating arterial thrombosis. At pathologically high shear rates, the protein unfolds and binds to the arterial wall, enabling the rapid deposition of platelets from the blood. We present a novel continuum model for VWF dynamics in flow based on a modified viscoelastic fluid model that incorporates a single constitutive relation to describe the propensity of VWF to unfold as a function of the scalar shear rate. Using experimental data of VWF unfolding in pure shear flow, we fix the parameters for VWF’s unfolding propensity and the maximum VWF length, so that the protein is half unfolded at a shear rate of approximately 5000s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$5000\\,\ ext {s}^{-1}$$\\end{document}. We then use the theoretical model to predict VWF’s behaviour in two complex flows where experimental data are challenging to obtain: pure elongational flow and stenotic arterial flow. In pure elongational flow, our model predicts that VWF is 50% unfolded at approximately 2000s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2000\\,\ ext {s}^{-1}$$\\end{document}, matching the established hypothesis that VWF unfolds at lower shear rates in elongational flow than in shear flow. We demonstrate the sensitivity of this elongational flow prediction to the value of maximum VWF length used in the model, which varies significantly across experimental studies, predicting that VWF can unfold between 2000and3200s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2000\ ext { and }3200\\,\ ext {s}^{-1}$$\\end{document} depending on the selected value. Finally, we examine VWF dynamics in a range of idealised arterial stenoses, predicting the relative extension of VWF in elongational flow structures in the centre of the artery compared to high shear regions near the arterial walls.

Validation of a RANS Turbulence Model for a S833 Wind Turbine Airfoil With a Trailing Edge Flap Using Oil Visualization and Pressure Taps

Abstract The aerodynamics of a small wind turbine blade was captured using a γ–Reθ k–ω shear stress transport transitional turbulence model tuned with production limiter coefficients at a Reynolds number of 1.70×105. The computational fluid dynamics simulations were validated against wind tunnel experiments that included airfoil pressure tap measurements and surface oil flow visualization (SOFV) to capture the flow field. The uniqueness of this blade included a trailing edge flap that was 20% of the chord controlled using a servomotor. The test matrix included angles of attack (AOA) between 1 deg and 7 deg with flap angles of 10 deg in the upward and downward position. Two locations were always observed on the airfoil: a leading edge region of high shear and a midsection of flow separation. Within the flow separation section, two distinct regions existed: a complete detachment of flow from the airfoil surface creating a stagnation region which was followed by a reverse flow region. A third location of flow reattachment near the trailing edge was observed for all cases excluding a downward angled trailing edge flap. The utilization of the flap resulted in changes to the size of the separation zone and the movement of the separation zone along the chord. The numerical skin friction coefficient, oil residue profiles from the SOFV, and pressure tap measurements all showed onset of separation locations on the chord within 10%. The computational fluid dynamics model also predicted the coefficient of pressure across the chord of the airfoil within 10% in comparison to the experimental measurements.

Spatiotemporally Mapped Endothelial Dysfunction at Bifurcations in a Coronary Artery‐on‐a‐Chip

AbstractAtherosclerotic plaques are commonly observed at low shear regions, in particular, the left main coronary artery (LMCA) bifurcation. Low shear regions at bifurcations promote endothelial dysfunction, a key factor initiating atherogenesis, however, mechanisms underlying this process are poorly understood. Dynamic in vitro models are critical to investigate endothelial dysfunction, but current static and vessels‐on‐chip systems typically lack physiologically complex geometries and local shear changes. Here, a bifurcating coronary artery‐on‐a‐chip is developed, mimicking the human LMCA, displaying reduced shear near the bifurcation, verified using computational fluid dynamics simulations. Over 7 days of dynamic culture, human coronary artery endothelial cells aligned with the flow and expressed more Endothelial nitric oxide synthase (eNOS) and intercellular cell adheison molecule‐1 (ICAM‐1) at high shear regions (12.7 dyn cm−2) adjacent to, but not at the bifurcation (0–3 dyn cm−2). After tumor necrosis factor‐alpha (TNFα) stimulation to induce endothelial dysfunction, spatially mapping cellular changes and shear gradients revealed cell alignment is disrupted over a larger area surrounding the bifurcation at a higher shear, and ICAM‐1 expression is increased closer to the bifurcation at a lower shear. This coronary artery‐on‐a‐chip establishes a system to spatially map endothelial cell behavior in response to differential shear and vessel geometry, enabling future studies into plaque initiation events, treatment targets, and drug screening.

Direct numerical simulation of a wall source dispersion in a turbulent channel flow

A comprehensive direct numerical simulation (DNS) is conducted to analyze the turbulent mixing process of a surface source dispersion emitting from the bottom wall in a turbulent channel flow. A comparative analysis of ten test cases has been conducted to examine the turbulent mixing process and its relationship with the source strength. The study involves an examination of the dispersion process in both physical and spectral spaces, which includes an evaluation of statistical moments of the concentration field, pre-multiplied spatial spectra of the velocity and concentration fields, as well as an analysis of the coherent turbulent structures in the flow. Upon normalization by friction concentration, the DNS analysis reveals that the first- and second-order statistical moments of the concentration field remain unaffected by variations in the source strength. Conversely, the relative intensity of concentration fluctuations and the thickness of the concentration boundary layer exhibit significant susceptibility to variations in the source strength. It has been observed that the dispersion process within the channel is significantly influenced by the small-scale eddies in cases where the source strength is not particularly intense. When confronted with high source strength scenarios, analysis of the simulation results establishes that both small- and large-scale eddies are crucial players in the dispersion phenomena, as evinced by a comparison of the pre-multiplied spectra of concentration and velocity fields. Furthermore, hairpin vortices have been recognized as the primary source of turbulence in the dispersal of substances from regions of high shear to the center of the channel.

Near-surface turbulent dissipation at a laboratory-scale confluence: implications on gas transfer

AbstractRiver confluences contribute to the outflux of saturated dissolved gases in the water resulting from high dam discharges. This process is related to gas transfer across the water–air interface, which is primarily controlled by turbulent dissipation near the water surface. However, the near-surface turbulence dissipation is rarely reported in confluence hydrodynamics studies. This study conducted experiments with different discharge ratios to investigate near-surface turbulent motions at a laboratory-scale confluence. The higher dissipation rate $$\varepsilon H/U_{m}^{3}$$ ε H / U m 3 of near-surface turbulence was mainly located inside the interfacial shear layer between the two incoming streams (~ 10–4) and the bank separation zone (10–4–10–3) where high shear was found in the mean flow. By contrast, the dissipation rates were much lower inside the incoming flows and outside the two regions of high shear (~ 10–5). The magnitudes of the dissipation rate inside the shear layer were comparable in experiments where the mixing interface was in the Kelvin–Helmholtz mode or in the wake mode. The dissipation rate was found to increase away from the free surface outside the shear layer, while it was more uniformly distributed over the depth inside the layer possibly due to the presence of strongly-coherent, vertically-orientated vortices. In the far field, the mean shear within the shear layer was largely weakened. Nonetheless, the effects of flow separation persisted and laterally expanded to occupy the entire cross section. The dissipation rate $$\varepsilon H/U_{m}^{3}$$ ε H / U m 3 of the confluent flow was more than 10–4 even at a distance of 10 times the channel width in the post-confluence channel.

Vertical shear instability in two-moment radiation-hydrodynamical simulations of irradiated protoplanetary disks

Context. The vertical shear instability (VSI) is a hydrodynamical instability predicted to produce turbulence in magnetically inactive regions of protoplanetary disks. The regions in which this instability can occur and the physical phenomena leading to its saturation are a current matter of research. Aims. We explore the secondary instabilities triggered by the nonlinear evolution of the VSI and their role in its saturation. We also expand on previous investigations on stability regions by considering temperature stratifications enforced by stellar irradiation and radiative cooling, and including the effects of dust-gas collisions and molecular line emission. Methods. We modeled the gas-dust mixture in a circumstellar disk around a T Tauri star by means of high-resolution axisymmetric radiation-hydrodynamical simulations including stellar irradiation with frequency-dependent opacities, considering different degrees of depletion of small dust grains. Results. The flow pattern produced by the interplay of the axisymmetric VSI modes and the baroclinic torque forms bands of nearly uniform specific angular momentum. In the high-shear regions in between these bands, the Kelvin–Helmholtz instability (KHI) is triggered. A third instability mechanism, consisting of an amplification of eddies by baroclinic torques, forms meridional vortices with Mach numbers up to ∼0.4. Our stability analysis suggests that protoplanetary disks can be VSI-unstable in surface layers up to tens of au for reasonably high gas emissivities. Conclusions. The significant transfer of kinetic energy to small-scale eddies produced by the KHI and possibly even the baroclinic acceleration of eddies limit the maximum energy of the VSI modes, likely leading to the saturation of the VSI. Depending on the gas molecular composition, the VSI can operate at surface layers even in regions where the midplane is stable. This picture is consistent with current observations of disks showing thin midplane millimeter-sized dust layers while appearing vertically extended in optical and near-infrared wavelengths.

Modelling the effect of roughness density on turbulent forced convection

By examining a systematic set of direct numerical simulations, we develop a model which captures the effect of roughness density on global and local heat transfer in forced convection. The surfaces considered are zero-skewed three-dimensional sinusoidal rough walls with solidities, $\varLambda$ (defined as the frontal area divided by the total plan area), ranging from low $\varLambda = 0.09$ , medium $\varLambda = 0.18$ to high $\varLambda = 0.36$ . For each solidity, we vary the roughness height characterised by the roughness Reynolds number, $k^+$ , from transitionally rough to fully rough conditions. The findings indicate that, as the fully rough regime is approached, there is a pronounced breakdown in the analogy between heat and momentum transfer, whereby the velocity roughness function $\Delta U^+$ continues to increase and the temperature roughness function $\Delta \varTheta ^+$ attains a peak with increasing $k^+$ . This breakdown occurs at higher sand-grain roughness Reynolds numbers ( $k_s^+$ ) with increasing solidity. Locally, we find that the heat transfer can be meaningfully partitioned into two categories: exposed, high-shear regions experiencing higher heat transfer obeying a local Reynolds analogy and sheltered, reversed-flow regions experiencing lower and spatially uniform heat transfer. The relative contribution of these distinct mechanisms to the global heat transfer depends on the fraction of the total surface area covered by these regions, which ultimately depends on $\varLambda$ . These insights enable us to develop a model for the rough-wall heat-transfer coefficient, ${C_{h,k}(k^+, \varLambda, Pr)}$ , where $Pr$ is the molecular Prandtl number, that assumes different heat-transfer laws in exposed and sheltered regions. We show that the exposed–sheltered surface-area fractions can be modelled through simple ray tracing that is solely dependent on the surface topography and a prescribed sheltering angle. Model predictions compare well when applied to heat-transfer data of traverse ribs from the literature.

Cross-streamline migration and near-wall depletion of elastic fibers in micro-channel flows.

The complex dynamics of elastic fibers in viscous fluids are central to many biological and industrial systems. Fluid-structure interactions underlying these dynamics govern the shape and transport of flexible fibers, and understanding these interactions can help tune flow properties in applications such as microfluidic separation, printing and clogging. In this work, we use slender-body theory to study micromechanical dynamics that arise from the coupling between the elastic backbone of a fiber and the local straining flow that contributes to filament flipping and cross-streamline migration. The resulting transverse drift is unbiased in either direction in simple shear flow. However, a non-uniform shear rate results in bias towards regions of high shear, which we connect to the shape transitions during flips. We discover a depletion layer that forms near the boundaries of pressure-driven channel flow due to the competition between such a cross-streamline drift and steric exclusion from the walls. Finally, we develop scaling laws for the curvature of filaments during flip events, demonstrating the origin of the drift bias in non-uniform flows, and confirm this behavior from our simulations. Put together, these results shed light on the role of a local and dominant coupling between elasticity and viscous resistance in dictating long-term dynamics and transport of elastic fibers in confined flows.

Momentum and mass transport and flow structure characteristics for the unsteady flow in compound channel

In recent decades, the study of steady flow in the compound channel has received much attention, while the flow characteristics of flood in the compound channel are rarely reported. In this paper, the characteristics of the flood propagation under different initial main channel depths are studied using the laboratory experiments and numerical simulation. Results show that the overflowing water from the main channel soon forms the reverse flow after hitting the floodplain sidewall. This reverse flow influences the subsequent overflowing process and significantly enhances the bed shear stress on the floodplain. However, this effect gradually decreases with the flood evolution distance and the increase of the initial downstream water depth. Unsteady transverse transport during flooding inhibits the formation of coherent eddies at the interface between the main channel and floodplain, while the high shear region will extend to the internal of the main channel or floodplain. Complex multi-vortex structures are found on the floodplain due to transverse interaction behavior. The main transverse mass transport takes place in the first half of the period after the flood arrival. It is found that the weir flow equations used in the previous studies are unable to accurately describe the transverse mass transfer between the main channel and floodplain. Secondary currents have a little contribution to the unsteady transverse momentum transport, while the transverse currents and transverse Reynolds stresses have a significant effect. The time when the peak transverse current arrives is later than that when the peak transverse Reynolds stresses does in the current study.

Development and validation of a mathematical model for evaluating shear-induced damage of von Willebrand factor

PurposeTo develop a mathematical model for predicting shear-induced von Willebrand factor (vWF) function modification which can be used to guide ventricular assist devices (VADs) design, and evaluate the damage of high molecular weight multimers (HMWM)-vWF in VAD patients for reducing clinical complications. MethodsMathematical models were constructed based on three morphological variations (globular vWF, unfolded vWF and degraded vWF) of vWF under shear stress conditions, in which parameters were obtained from previous studies or fitted by experimental data. Different clinical support modes (pediatric vs. adult mode), different VAD operating states (pulsation vs. constant mode) and different clinical VADs (HeartMate II, HeartWare and CentriMag) were utilized to analyze shear-induced damage of HMWM-vWF based on our vWF model. The accuracy and feasibility of the models were evaluated using various experimental and clinical cases, and the biomechanical mechanisms of HMWM-vWF degradation induced by VADs were further explained. ResultsThe mathematical model developed in this study predicted VAD-induced HMWM-vWF degradation with high accuracy (correlation with experimental data r2 > 0.99). The numerical results showed that VAD in the pediatric mode resulted in more HMWM-vWF degradation per unit time and per unit flow rate than in the adult mode. However, the total degradation of HMWM-vWF is less in the pediatric mode than in the adult mode because the pediatric mode has fewer times of blood circulation than the adult mode in the same amount of time. The ratio of HMWM-vWF degradation was lower in the pulsation mode than in the constant mode. This is due to the increased flushing of VADs in the pulsation mode, which avoids prolonged stagnation of blood in high shear regions. This study also found that the design feature, rotor size and volume of the VADs, and the superimposed regions of high shear stress and long residence time inside VADs affect the degradation of HMWM-vWF. The axial flow VADs (HeartMate II) showed higher degradation of HMWM-vWF compared to centrifugal VADs (HeartWare and CentriMag). Compared to fully magnetically suspended VADs (CentriMag), hydrodynamic suspended VADs (HeartWare) produced extremely high degradation of HWMW-vWF in its narrow hydrodynamic clearance. Finally, the study used a mathematical model of HMWM-vWF degradation to interpret the clinical statistics from a biomechanical perspective and found that minimizing the rotating speed of VADs within reasonable limits helps to reduce HWMW-vWF degradation. All predicted conclusions are supported by the experimental and clinical data. ConclusionThis study provides a validated mathematical model to assess the shear-induced degradation of HMWM-vWF, which can help to evaluate the damage of HMWM-vWF in patients implanted with VADs for reducing clinical complications, and to guide the optimization of VADs for improving hemocompatibility.

Micromixing Performance in a Taylor–Couette Reactor with Ribbed Rotors

The Taylor–Couette reactor (TCR) is becoming an increasingly significant topic in chemical industry. This study investigates the micromixing performance of a ribbed TCR with axial flow in the Villermaux–Dushman reaction system. The local micromixing mechanism of the ribbed TCR was analyzed, and the volume-averaged energy dissipation rate was calculated using CFD. The effects of operating parameters and rib structural parameters on micromixing performance were investigated. The results show that the introduction of ribs eliminates the high shear region between the vortex pairs, resulting in the strong micromixing region being situated on the inner and outer cylinder wall surfaces and the ribbed surface region. Smaller rib spacing, larger rib width, and rib height can strengthen micromixing and result in a smaller segregation index. Micromixing times of ribbed TCRs were calculated using the incorporation model, tm, in the range of 2.0 × 10−5 to 8.0 × 10−3. The results show that ribbed TCRs require a lower energy consumption to achieve a lower tm than other rotating reactors. A correlation equation between tm and five parameters was developed, with a correlation coefficient of 0.951. The accuracy of the volume-averaged energy dissipation rate obtained via CFD was verified through experimental analysis. The correlation between the micromixing time and the volume-averaged energy dissipation rate was established in a form that satisfies Kolmogorov’s turbulence theory for tm. To convert the volume-averaged energy dissipation rate into a local energy dissipation rate, a factor ϕ was introduced and solved using the engulfing diffusion model. This study provides insights into the design and optimization of ribbed TCRs.

A Numerical Study to Compare the Antifouling Potential of two Bio-inspired Antifouling Topographies Derived from the Pilot whale skin and the Zebra mussels

Biofouling occurs naturally in aquatic environments when microorganisms attach to surfaces and form colonies. The destruction of equipment has occurred in a number of well-known economic sectors, including the power and water treatment industries. Large ships and boats face performance issues due to biofouling, such as increased fuel costs and drag. Inhibiting surfaces, toxic coatings, foul release coatings, and physical removal are just a few of the techniques that have been created and used to prevent biofouling. Some of these strategies are known to be expensive, not eco-friendly, and time-consuming. Inspired by the surface topography of zebra mussel shells and pilot whale’s skin, it stays biofoul free. This study’s objective is to assess the two bio-inspired topographies’ ability to prevent fouling. This study looks at how fouling organisms would respond to the hydrodynamic properties of the aforementioned topographies as well as whether geometric size and features have an effect on them. This study aims to clarify or advance our current understanding of biomimicry and antifouling technology because testing of biomimicry-based antifouling solutions is lacking. This research focuses on explaining the nature of microorganisms in water that settle on surfaces and how they react to disturbances caused by fluid flow. They don’t tend to build up on the surface due to the relationship between flow and the different sizes of the surface geometry. Test the hypothesis that the non-bounded high shear stress ripple features on the Zebra mussel’s shell have lower antifouling efficiency compared to the Pilot whale’s skin, which contributes to antifouling efficiency because of its high shear nanoridges, micropores, and gel-coated skin properties. Therefore, if the terrain size is smaller than the invasive organism, the effective range of antifouling is expected to be in the range of 100 µm - 300 µm. The performance of the two bio-inspired topographies was assessed using CFD ANSYS (Fluent) in an entirely numerical study. Two CAD-created representations of the bio-inspired topographies were positioned inside a fluid domain (5 mm, 5 mm, and 50 mm) under specific boundary conditions, including no slip walls assigned to the fluid domain walls, an inlet velocity of 0.05 ms-1, and the flowing fluid being water. Detailed surface meshing and mesh quality are necessary to prove a reliable solution. The velocity profile, vorticity, and wall shear stress of the surface were examined in order to determine the antifouling effectiveness. Utilizing contour and streamline plots, the flow over the topography models was examined. Based on the hypothesis, the results showed that not only does geometry affect microbial settlement but also size. Topographic patterns with larger sizes were associated with higher settlements of algae and barnacles in low velocity areas. Microbial settling may also be aided by the emergence of low shear stress and vorticity in the lower part of the topography. However, the high shear region that forms close to kink sites or sharp edges and the hydrodynamic disturbance near the surface both have an impact on reducing bacterial attachment. To add to this finding, it is still important for science to consider and conduct research on the novel strategy of using surfaces that mimic biological systems to reduce biofouling and other alternative methods to develop sustainable technology.

Tidal turbine power performance assessments following IEC TS 62600-200 using measured and modelled power outputs

Demonstrating power performance in real conditions is vital for verifying tidal turbine design. The International Electrotechnical Commission’s Technical Specification IEC TS 62600-200 provides guidance for tidal developers to evaluate machine performance. This study evaluates the performance of the operational 1 MW DEEP-Gen IV commercial-scale tidal turbine deployed during the ReDAPT project at the European Marine Energy Centre’s tidal test site in Orkney, Scotland. IEC TS 62600-200 states that the power performance should be measured relative to two independently located current profilers deployed in one of two orientations; ‘in-line’ (preferred) or ‘adjacent’ (least preferred), relative to the turbine. Two measurement campaigns are used to assess the impact of instrument placement on the measured power curve and annual energy production (AEP). The ambient current flow and vertical profile form differed at each current profiler location (due to topography and bathymetry), leading to AEP variability. The results reinforce the IEC TS preferred orientation for instrument placement, with inline measurements showing smaller variation in AEP estimates compared with adjacent measurements. An additional analysis assesses the sensitivity of AEP estimates on the vertical alignment of the flow profiles with rotor positioning. For this we consider three turbine scenarios: the DEEP-Gen IV and two modelled turbine rotors occupying different regions in the water column. Results show that for regions of high vertical shear, AEP estimates can be misrepresented by up to 5.5% under an imposed vertical misalignment of 2 m. This work has quantified, at this site, the variation in AEP estimates from different flow measurement campaigns informed by IEC TS 62600-200 and has shown the need to ensure good vertical alignment between measurements and turbine rotor positioning.

Slow wind belt in the quiet solar corona

The slow solar wind belt in the quiet corona, observed with the Metis coronagraph on board Solar Orbiter on May 15, 2020, during the activity minimum of the cycle 24, in a field of view extending from 3.8 R⊙ to 7.0 R⊙, is formed by a slow and dense wind stream running along the coronal current sheet, accelerating in the radial direction and reaching at 6.8 R⊙ a speed within 150 and 190 km s−1, depending on the assumptions on the velocity distribution of the neutral hydrogen atoms in the coronal plasma. The slow stream is separated by thin regions of high velocity shear from faster streams, almost symmetric relative to the current sheet, with peak velocity within 175 and 230 km s−1 at the same coronal level. The density–velocity structure of the slow wind zone is discussed in terms of the expansion factor of the open magnetic field lines that is known to be related to the speed of the quasi-steady solar wind, and in relation to the presence of a web of quasi-separatrix layers, S-web, the potential sites of reconnection that release coronal plasma into the wind. The parameters characterizing the coronal magnetic field lines are derived from 3D MHD model calculations. The S-web is found to coincide with the latitudinal region where the slow wind is observed in the outer corona and is surrounded by thin layers of open field lines expanding in a non-monotonic way.

A mathematical model for assessing shear induced bleeding risk

PurposeThe objective of this study is to develop a bleeding risk model for assessing device-induced bleeding risk in patients supported with blood contact medical devices (BCMDs). MethodsThe mathematical model for evaluating bleeding risk considers the effects of shear stress on von Willebrand factor (vWF) unfolding, high molecular weight multimers-vWF (HMWM-vWF) degradation, platelet activation and receptor shedding and platelet-vWF binding ability. Functions of the effect of shear stress on the above factors are fitted/employed and solved by the Eulerian transport equation. An axial flow-through Couette device and two clinical VADs which are HeartWare Ventricular Assist Device (HVAD) and HeartMate II (HM II) blood pump were employed to perform the simulation to evaluate platelet receptor shedding (GPIbα and GPIIb/IIIa), loss of HWMW-vWF, platelet-vWF binding ability and bleeding risk for validating the accuracy of our model. ResultsThe platelet-vWF binding ability after being subjected to high shear region in the axial flow-through Couette device predicted by our bleeding model was highly consistent with reported experimental data. As indicated by our CFD simulation results in the axial flow-through Couette device, it can find that an increase in shear stress led to a decrease in the adhesion ability of platelets on vWF, while the binding ability of vWF with platelets first increase and then decrease as shear stress elevates gradually beyond a threshold. The factor of exposure time can enhance the effect of shear stress. Additionally, the shear-induced bleeding risk predicted by our model increases with increasing shear stress and exposure time in an axial flow-through Couette device. As indicated by our numerical model, the bleeding risk in HVAD was higher than HMII, which is highly consistent with the meta-analysis based on clinical statistics. Our simulation investigations in these two clinical VADs also found that HVAD caused a higher rate of platelet receptor shedding and lower damage to HWMW-vWF than HeartMate II. The high shear stress generated in the narrow and turbulent regions of both VADs was the underlying cause of device-induced bleeding. ConclusionIn this study, the shear-induced bleeding risk predicted by our bleeding model in axial flow-through Couette device and two clinical VADs is consistent or highly correlated with experimental and clinical findings, which proves the accuracy of our bleeding model. Our bleeding model can be used to aid the development of new BCMDs with improved functional characteristics and biocompatibility, and help to reduce risk of device-induced adverse events in patients.

Flow behavior analysis in boundary layer transition based on the Liutex–shear decomposition

The Liutex based vortex identification method is superior to previous methods in that it overcomes the issues of threshold problem, shear contamination, etc., with a clear physical meaning that the direction of the Liutex vector represents the local axis of rotation, while the magnitude is equal to twice the angular velocity of the rigid-rotation part of the flow. The current study focuses on the interaction between the Liutex represented rotation and the residual shear part during the development of Λ vortex and hairpin vortex in boundary layer transition. The temporal–spatial evolution and development of typical vortical structures are analyzed based on the Liutex–shear decomposition with particular attention paid to the position and strength changes of Liutex and shear. Vortex core lines are extracted to investigate the mutual interactions between rotation and shear inside vortices. It is demonstrated that for the Λ vortex, spanwise rotating motions are formed at the head region and can persist for a long time under the influence of surrounding shear, while the tail region of the Λ vortex stretches down near the wall but then becomes weaker due to dissipation. High-shear regions tend to be located on top or below the vortices. When the shear layer formed between the legs gets stronger, it will be rolled up to form new vortices. On the one hand, the vortex legs together with the newly formed spanwise vortex consist of a new hairpin vortex, which, in turn, leads to the generation of the second- and third-level hairpin vortices. On the other hand, it also generates many smaller streamwise vortices in the near wall region. The results show that the interaction between Liutex and shear is very important in the development of vortical structures during transition.