Gravure printing is a type of printing method that uses metal cylinders with engraved cells that hold ink. The ink is transferred directly to the paper or other material by pressing it against the cylinder. The flow associated with gravure printing includes a flow in a liquid bridge formed in the contact region of the cylinders and a thin-film coating flow of the ink. The flow is governed by viscous and capillary forces. In many cases, the flow is unstable, which leads to the formation of instability patterns on the printed surfaces. The analysis of these instabilities is a very challenging problem, especially since industrial inks are usually rheologically complex. In this experimental and theoretical study, the flow of inks on a rotating cylinder is analysed, accounting for the shear-rate-dependent liquid viscosity. A theoretical solution for the film flow allows us to predict the width of the liquid bridge between two cylinders. Moreover, it is shown that the measured characteristic size of the printed pattern is of the same order as the predicted liquid bridge width. We observe a nearly linear dependence of pattern size and liquid bridge width.

In the field of experimental fluid dynamics, the direct measurement of vorticity remains a challenge, even though it plays a crucial role in understanding turbulent flows. The present study explores the influence of the rotation of nanoparticles on their luminescence anisotropy as a potential novel measurement method. This relation opens a new field of flow diagnostics, based on the measurement of polarized intensity components. Potentially, the method allows for the direct measurement of the vorticity. For this, the canonical flow in this study is a turbulent round jet at ${{Re}} = {12\,000}$ and 14 400. It is confirmed that the flow regime has an influence on the luminescence anisotropy. Using a model of such deterministic rotations according to another work by the authors (Schmidt & Rösgen, Phys. Rev. Res., vol. 5, no. 3, 2023, 033006), the magnitude of the vorticity components is computed, since the presented set-up is limited to sensing the magnitude of these quantities. The computed components indicate the self-similarity of the vorticity magnitude. A large-eddy simulation is conducted for comparison with the experiments, demonstrating good agreement.

This paper investigates the impact of large-scale turbulence on the aerodynamic characteristics of a pitching wind turbine blade at Reynolds number 135 000, whose cross-section is a NACA0012 airfoil with constant chord length. Large-eddy simulations at reduced frequencies, $k_{red} = 0.05$ and 0.1, were validated against reference data from the literature. An efficient method capable of generating synthetic large-scale turbulence at the inlet was applied by using two streamwise integral length scales $L_{x} = 1c$ and 1.5$c$, which represent energetic turbulence eddies at the height where the wind turbine operates. For $k_{red}= 0.1$, the change in the maximum lift coefficient at the dynamic stall angle near the maximum angle of attack is on average 20 % lower and during the downstroke it is on average 22 % lower, compared with the smooth inflow. A higher reduced frequency ($k_{red} = 0.2$) apparently does not further change the lift, drag and moment coefficients, and the inflow turbulence disordered leading-edge vortices. The turbulent shear stress and the phase-averaged dispersive shear stress in the wake are of the same magnitude, but with negative and positive signs, respectively, suggesting that the large-scale phase-averaged fluctuations transfer momentum in the opposite direction compared with the turbulent fluctuations, reducing the drag on the suction-side flow, and subsequently increasing the averaged lift coefficient. This demonstrates the critical importance of the large-scale non-turbulent unsteadiness in the wake of a pitching wing.

Constriction in the flow passage of the physiological circulatory system is central to the occurrence of several diseased conditions such as thrombosis and is also pivotal towards the understanding of several regulatory processes in the human microvasculature. It is, therefore, imperative to advance a mechanistic insight into the dynamics of lipid vesicles, cell mimicking, fluid-filled compartments, through a physiologically relevant microconfinement, with particular focus on deciphering the role of its mechanophysical properties. Here we bring out the role of membrane bending rigidity and the initial deflation (deviation in shape from sphericity) on the transient shape evolution of a lipid vesicle as it migrates through a microfluidic constriction, a paradigm that is unexplored thus far. Based on our experimental observations as well as theoretical insights, we construct a regime map to elucidate the range of the key dimensionless parameters orchestrating this dynamical transition. Furthermore, our observations on the vesicle's stretching dynamics emerging from selective mapping with viscosity contrast between the encapsulated and the suspending fluid medium offer potential physiologically relevant cues on the impact of cell aging on its deformability across a constricted path. Such mechanistic insights may help in establishing quantitative correlations between the dynamical transition of a lipid vesicle and its membrane mechanics, which may in turn have decisive implications in health and disease while circulating across microvascular fluidic pathways.

In several problems involving fluid flows, computational fluid dynamics (CFD) provides detailed quantitative information and allows the designer to successfully optimize the system by minimizing a cost function. Sometimes, however, one cannot improve the system with CFD alone, because a suitable cost function is not readily available; one notable example is diagnosis in medicine. The application considered here belongs to the field of rhinology; a correct air flow is key for the functioning of the human nose, yet the notion of a functionally normal nose is not available and a cost function cannot be written. An alternative and attractive pathway to diagnosis and surgery planning is offered by data-driven methods. In this work, we consider the machine learning study of nasal impairment caused by anatomic malformations, with the aim of understanding whether fluid dynamic features, available after a CFD analysis, are more effective than purely geometric features at the training of a neural network for regression. Our experiments are carried out on an extremely simplified anatomic model and a correspondingly simple CFD approach; nevertheless, they show that flow-based features perform better than geometry-based ones and allow the training of a neural network with fewer inputs, a crucial advantage in fields like medicine.

This paper reviews 70 years of cricket ball swing literature and reassesses the results in light of new measurements. A comparison of ball tracking data with published experimental results shows that current understanding does not explain the behaviour observed in professional cricket matches. Descriptions of cricket ball boundary layer aerodynamics are updated with new results which show that the seam acts more like a series of vortex generators than a trip and that there is a laminar separation bubble on the seam side of the ball. Previous results for reverse swing are consolidated and compared with new data, showing that different magnitudes of swing occur depending on the condition of both sides of the ball. Variation in pressure and temperature should be included when the Reynolds number, a non-dimensional ball speed, is calculated, while humidity can be neglected. Studies on the effect of wind speed and direction are summarised and results considering the effect of free-stream turbulence are compared with new measurements. Throughout the paper, recommendations for future work are suggested. These include quantitative study of the effect of surface defects and roughness, an assessment of whether atmospheric turbulence can affect swing and investigation into the effect of backspin on swing.

This article presents a broad overview of the fluid mechanics of the human cardiovascular system. Beginning in the heart, we travel through the main features of our circulation to highlight important functions and diseases where fluid mechanics plays a central role. Of particular focus is the role of computational modelling in uncovering the dynamic flow phenomenon throughout our body, its association with cardiovascular disease mechanisms and progression and its importance in clinical treatment planning. We also emphasize the multiscale nature of the cardiovascular system, and associated challenges. The main aim of this review is to highlight progress and ongoing challenges in our understanding of cardiovascular haemodynamics, as well as the future outlook for translating the current state-of-the-art to widespread clinical application and improved patient outcomes.

Considered in this investigation is the three-dimensional, gravity-driven flow of a thin viscous fluid layer down an incline, and spreading over topography. Three depth-integrated models are presented and contrasted. These include an integral-boundary-layer model, a weighted-residual model and a hybrid model. A numerical solution procedure suited for solving three-dimensional flows is also proposed. Numerous simulations have been conducted using the models for various steady subcritical, and unsteady supercritical flows over several topographies. Good agreement among the three models was found. In addition, the models were also validated using experimental results, and, again, good agreement between the three models and with experiments was obtained.

In this study, we propose a novel cooling scheme that utilizes copper inverse opals (CIOs) for surface enhancement in a single-phase impingement jet cooling system. We perform computational fluid dynamics simulations to evaluate the cooling performance of the CIO jet coolers. Our modelling results indicate that the proposed CIO-coated cooler can significantly reduce the average temperature and improve the temperature uniformity across the entire chip surface. The average Nusselt number of the CIO-coated cooler can reach up to 2.8 times that of the flat surface jet cooler. However, the porous structure of the CIO-coated cooler increases the total pressure drop. To determine designs with high cooling performance and low energy consumption, we investigate two crucial design factors, namely the inlet velocity and the nozzle-to-CIO distance. Our analysis reveals that increasing the inlet velocity further enhances the heat transfer, but at the expense of high pressure drop. On the other hand, a larger nozzle-to-CIO distance results in a lower pressure drop but also reduces the heat transfer coefficient. The effects of nozzle-to-CIO distance are further understood by studying the flow resistance network. Furthermore, we present a reduced-order model that accurately captures the thermofluidic characteristics of the proposed design.

Underwater explosions are inherently complex and unique physical phenomena markedly distinct from those occurring above the surface. This distinctiveness is primarily attributed to the relatively incompressible nature of water, which fundamentally alters the propagation and impact of shock waves. The study of underwater explosions is paramount in applications such as underwater demolitions for construction and salvage operations. These applications require a comprehensive understanding in order to mitigate the disturbances’ impact on marine structures and ecosystems. Studying underwater explosions and their mitigation encompasses various disciplines, including fluid mechanics, materials science and structural engineering. The work reviewed in this study contributes significantly to enhancing safety measures in marine structures by providing critical insights into the behaviour of structures under extreme conditions. This includes understanding the behaviour of gas bubbles formed by explosions, the transmission of shock waves through different media and the resultant forces exerted on structures submerged in water. Consequently, this review is meant to aid in designing robust and resilient marine systems capable of withstanding severe loading conditions caused by underwater explosions by providing key engineering considerations. The continuous evolution of this research area is essential for advancing maritime technology, ensuring the safety of undersea operations and protecting marine environments from the adverse effects of extreme subaqueous loadings.

The shoaling of large-amplitude internal waves in the coastal ocean yields a consistent mechanism for both horizontal and vertical transport of material, momentum and energy. This review surveys recent numerical, field and laboratory work on the details of this transport. A particular focus is made on the two issues of how boundary-layer processes are modified during shoaling, and the development of spanwise structure, especially in the trapped cores that form during shoaling. Numerical challenges, including gaps in existing parametrizations are identified using pseudospectral simulations on the laboratory scale. A number of challenges for future work, for simulations, parametrizations and laboratory and field measurements are laid out.

Microstructured surfaces with pillar arrays are widely used to control the wetting morphology and spreading dynamics of droplets. In both simulations and experiments, it is shown that fabricating the surface with various microstructures is a very effective method for achieving the desired symmetry of the moving contact line. However, the method for characterizing miscellaneous pillar-arrayed microstructured surfaces is still insufficient. This paper presents the configurational entropy to characterize the microstructured surfaces with pillar arrays. By calculating the configurational entropy of pillar-arrayed microstructured surfaces, the relationship between the configurational entropy and the wetting morphology of droplets is obtained. For pillar-arrayed microstructured surfaces with the configurational entropy S > 0, the droplet wetting morphology may be much more complex than those with S = 0. The relationship is found to be consistent with the previous results. Furthermore, the wetting dynamics has been analysed. This study may be useful to understand the mechanism of droplet wetting on pillar-arrayed microstructured surfaces and provide insights for the design and manufacture of microstructured surfaces.

This paper describes the scientific features of an innovative technique to mass-produce monolayers of hexagonal close-packed structures (HCP) of particles (280–1100 nm). Our technique differs from a continuous roll-to-roll Langmuir–Blodgett (LB) process. It consists of a thin liquid film flowing down an inclined plane channel, the ramp, and entraining deposited particles floating on its surface to form a compact monolayer. Vertical sidewalls limit the entire flow. The main benefits of this technique in comparison with a standard LB process are a gentler push on the floating particles during the assembly and the prospect of better flexibility and scalability in the design of industrial applications. Our disruptive approach presents new control parameters and surprising but challenging hydraulic phenomena due to the flowing liquid. This paper investigates the hydrodynamics of this new LB-type design theoretically and experimentally. We propose an original theoretical prediction of the thickness of the liquid film flowing down the ramp without or with particles on its surface, including within the hydraulic jump region separating the liquid film whose surface is free of particles and the liquid film whose surface is particle-loaded. The experimental determinations of the film thickness obtained by a confocal chromatic technique and moiré topography agree well with our model. In addition, Bragg diffraction topography and false colour topography allow the HCP structure of the compact monolayer of particles to be quantified.

This paper focuses on simulating turbulent flow over propagating waves by solving the full Navier–Stokes equations in a moving frame. A careful comparison of flow statistics with previous experimental and numerical results demonstrates, to some extent, the rationality of simplifying wind waves as turbulent flow over moving wave boundaries. The phase-averaging method is then applied to investigate the momentum and energy transfers between turbulent wind and waves propagating at slow, intermediate and fast speeds. The results suggest that the dominant mechanism for producing Reynolds shear stress (RSS) and turbulent kinetic energy (TKE) is related to the wave age. Slow waves produce RSS and TKE similar to a two-dimensional shear turbulence. However, a fast wave enhances the streamwise Reynolds normal stress, the windward side's negative RSS and the gradient of both streamwise and vertical velocities, leading to additional RSS and TKE productions that can be ignored under the slow wave regimes. A strengthening wave–turbulence exchange is also found for fast waves. The intermediate wave can be regarded as a transitional condition determining this change.

This manuscript mainly explores the characteristics of turbulence quantities in the wake of tall building clusters of different array size ($N$) and building spacing ($W_S$) arranged in an aligned and regular grid in the flow direction. Velocity fields are measured in a wind tunnel using three-dimensional laser Doppler anemometry. Results show a delayed recovery of $u_{rms}$ and $v_{rms}$ (defined as the root-mean-square of the streamwise and lateral velocities, respectively) in the wake flow compared with the mean flow. Based on the turbulent fluctuations, the extents of the near-, transition- and far-wake regions in Mishra et al. (Boundary-Layer Meteorol., vol. 189, 2023, pp. 1–25) are revisited. In the near wake, we observe a significant reduction in $u_{rms}$ and $v_{rms}$ in the wake of a $4\times 4$ cluster compared with that of a single building. In the transition region, the turbulence intensity magnitudes within the cluster reduce to below their free-stream counterpart; this reduction is associated with the slowly varying nature of the normalised wake deficit in the streamwise direction. The recovery of the root mean square in the far-wake region is observed for $x \geq 2.5W_A$ (where $W_A$ is the width of the cluster), with the mutual interaction of the wakes formed behind the individual buildings reducing with an increase in $W_S$, resulting in a faster recovery of the turbulent fluctuations. Finally, wavelet analysis suggests the existence of multi-scale vortex-shedding frequencies downwind of tall building clusters.

The Reynolds-averaged Navier–Stokes (RANS) models depend on empirical constants to close the Reynolds stress terms. The empirical constants were obtained using experiments conducted at low Reynolds numbers several decades ago. In this paper, we revisit the turbulent viscosity parameter $C_\mu$, based on the stress–intensity ratio $c^2 = {|\overline {uw}|}/{k}$. Here, $\overline {|uw|}$ and $k$ are the absolute values of the Reynolds stress and turbulent kinetic energy, respectively. Through a priori comparisons, we find that the currently accepted value of $C_\mu = 0.09$ does not agree with the latest direct numerical simulation (DNS) and experimental datasets of wall-bounded turbulent planar flows. Therefore, a new value is suggested by averaging $c^2$ in the equilibrium region, where the production ($\mathcal {P}$) of $k$ is within 10 % of the dissipation rate ($\epsilon$), and consequently, $c^4 \approx C_\mu$. We evaluate flows up to friction Reynolds number $Re_\tau \approx 10\,000$ and find that with increasing $Re_\tau$, $C_\mu$ approaches a value of 0.06, which is almost 50 % lower than the prevalent value of 0.09. Finally, we perform an a priori test with the new (proposed) value of $C_\mu = 0.06$ to show that the estimated turbulent viscosity $\nu _T$ for wall-bounded flows is in much closer agreement with the exact (DNS) values than when $\nu _T$ is estimated using $C_\mu = 0.09$.

Breaking waves aerate seawater surfaces and form whitecaps in the open ocean. The aerated surface area, or whitecap coverage, has been used to macroscopically parametrize air–sea momentum and gas exchange. However, the microscopic mechanisms of the generation, evolution and attenuation of surface bubbles in whitecaps remain poorly understood. In this study, we examined the size distributions and size-dependent lifetimes of surface bubbles generated by water sheet entry and air injection on a porous plate during the clustering, coalescing and bursting processes, depending on surfactant concentrations and bubble mobility. Mechanisms of coalescence through film thinning of adjacent bubble walls owing to the inter-bubble attraction and Marangoni forces experimentally described the surfactant-dependent bubble growth, finally achieving bubble bursting, which were statistically characterized in a population balance analysis. Lagrangian bubble lifetimes were described by the Weibull distribution, providing that surfactant in seawater extended the probabilistic survival periods of surface bubbles two times longer than those of clean bubbles.

Hollow fibre membrane bioreactors provide a fast and efficient method for engineering functional tissue for use in medical treatments. Flow is utilised to overcome mass transport limitations by perfusing a nutrient-rich culture medium through the fibre lumen, which can then transport along the fibre lumen or across the porous membrane wall. Cells seeded at the outer membrane wall consume the nutrient and subsequently produce waste metabolites, which are transported away through an external extra-capillary space (ECS) along with excess nutrient. We present and investigate a two-dimensional axisymmetric model for fluid flow and solute transport through a single-fibre bioreactor configuration, with cells seeded to the external fibre wall. Fluid flow is modelled by steady lubrication and Darcy equations, which are coupled to the solute transport problem modelled by a system of advection–diffusion equations, supplemented with a reaction term to model the cell layer. Our model analysis reveals how spatially varying wall permeability distributions can be utilised to provide uniform nutrient delivery to a spatially uniform, homogeneous cell population. We also reveal how maximising the transmural pressure drop across the membrane wall is the dominant mechanism for waste removal rather than traditional experimental methods of flushing the ECS.

We experimentally investigate the flow through a hollow cube, with an indoor ground-level passive scalar source, immersed in a rough-wall turbulent boundary layer inside a water tunnel. The focus is on characterizing scalar transport within the cube, through simultaneous scalar and flow measurements using planar laser-induced fluorescence and particle image velocimetry. To understand the role of window positioning, three cube configurations, labelled as ‘centre’, ‘up-down’ and ‘down-up’, distinguished by window positions at the upstream and downstream ends, are studied. Varying window position alters the flow characteristics within the cube, resulting in differences in scalar concentration and distribution. The steady-state concentration is highest for ‘centre’, followed by ‘up-down’ and ‘down-up’ configurations. Regarding the scalar distribution, ‘centre’ showed accumulation near the top and bottom walls, while ‘up-down’ and ‘down-up’ exhibited scalar buildup in the lower and upper half of the cube, respectively. The flow patterns and scalar transport mechanisms remained consistent across different Reynolds numbers ($Re=U_{Ref}H/\nu = 20\ 000$, 35 000, 50 000) for each configuration; $U_{Ref}=$ incoming flow velocity at cube height ($H$), and $\nu =\,$ kinematic viscosity of water. The analysis is extended by revising the classical box model, accounting for practical complexities such as non-perfect mixing. Our results can help better understand and model indoor–outdoor pollutant exchange in complex urban environments.

The stirring and mixing of heat and momentum in the ocean surface boundary layer (OSBL) are dominated by 1 to 10 km fluid flows – too small to be resolved in global and regional ocean models. Instead, these processes are parametrized. Two main parametrizations include vertical mixing by surface-forced metre-scale turbulence and overturning by kilometre-scale submesoscale frontal flows and instabilities. In present models, these distinct parametrizations are implemented in tandem, yet ignore meaningful interactions between these two scales that may influence net turbulent fluxes. Using a large-eddy simulation of frontal spin down resolving processes at both scales, this work diagnoses submesoscale and surface-forced turbulence impacts that are the foundation of OSBL parametrizations, following a traditional understanding of these flows. It is shown that frontal circulations act to suppress the vertical buoyancy flux by surface forced turbulence, and that this suppression is not represented by traditional boundary layer turbulence theory. A main result of this work is that current OSBL parametrizations excessively mix buoyancy and overestimate turbulence dissipation rates in the presence of lateral flows. These interactions have a direct influence on the upper ocean potential vorticity and energy budgets with implications for global upper ocean budgets and circulation.