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This study aims to optimize the influence of the inlet inclination angle on the Indoor Air Quality (IAQ), heat, and temperature distribution in mixed convection within a two-dimensional square cavity filled with an air-CO2 mixture. The air-CO2 mixture enters the cavity through two inlet openings positioned at the top wall, which is set at the ambient temperature (TC). Three values of the Reynolds numbers, ranging from 1000 to 2000, are considered, while the Prandtl number is kept constant (Pr = 0.71). The temperature distribution and streamlines are shown for Rayleigh number (Ra) equal to 104, three inlet inclination angles ϕ (0, π/6 and π/4) and three CO2 concentrations values (1500, 2500, 3500 ppm) applied at both hot vertical walls (maintained at a constant temperature TH). A finite volume method is used under the assumption of two-dimensional laminar flow to solve the Navier-Stokes and energy equations. The results indicate that inlet inclination angle has an impact on the indoor air quality (IAQ), which, in turn, affects the heat transfer distribution and thermal comfort within the cavity. [ABSTRACT FROM AUTHOR]

Data-driven approaches offer novel opportunities for improving the performance of turbulent flow simulations, which are critical to wide-ranging applications from wind farms and aerodynamic designs to weather and climate forecasting. However, current methods for these simulations often require large amounts of data and computational resources. While data-driven methods have been extensively applied to the continuum Navier-Stokes equations, limited work has been done to integrate these methods with the highly scalable lattice Boltzmann method. Here, we present a physics-informed neural network framework for improving lattice Boltzmann-based simulations of near-wall turbulent flow. Using a small amount of data and integrating physical constraints, our model accurately predicts flow behaviour at a wide range of friction Reynolds numbers up to 1.0 × 106. In contradistinction with other models that use direct numerical simulation datasets, this approach reduces data requirements by three orders of magnitude and allows for sparse grid configurations. Our work broadens the scope of lattice Boltzmann applications, enabling efficient large-scale simulations of turbulent flow in diverse contexts. The authors provide a data-driven near-wall modelling framework for the lattice Boltzmann method using IDDES data. Their model can predict flows with friction Reynolds numbers up to 1,000,000 and effectively handle sparse near-wall grids. [ABSTRACT FROM AUTHOR]

The article "Spreading of a viscous drop after impact onto a spherical target" in the Journal of Fluid Mechanics explores the dynamics of liquid droplets colliding with solid spherical particles. It investigates both non-axisymmetric and axisymmetric impacts, analyzing parameters such as drop shape, residual film thickness, and spreading angle. The study introduces a new method for measuring liquid viscosity during drop impacts, with potential applications in industrial processes. The research sheds light on optimizing processes involving drop collisions and offers valuable insights for various industries. [Extracted from the article]

The article "Dynamics of turbulent energy and dissipation in channel flow" delves into the intricate relationship between turbulent kinetic energy (TKE), turbulence dissipation rate (TDR), and turbulence production rate (TPR) in fully developed turbulent channel flow. The research uncovers time lags between these variables, with correlations that vary based on Reynolds number. The findings emphasize the complex nature of energy transfer and dissipation within different regions of the channel, showcasing correlations that involve both time and wall-distance lags. This study offers valuable insights into the self-sustaining process of turbulent flows, with implications for understanding turbulence dynamics across various contexts. [Extracted from the article]

The development of propulsion devices by using the propulsive mechanisms of aquatic animals such as fish is a challenging task. An attempt has been made to replicate the fishtailed motion that caused thrust in the current investigation. At a low Reynolds number of Re = 1173, the propulsive performance of National Advisory Committee for Aeronautics 0012 flapping foils enduring distinct flapping trajectories (fishtailed and elliptical) is evaluated. A dynamic mesh arbitrary Lagrangian-Eulerian framework is used to understand the desired flapping motion of the foils while solving the flow via incompressible Navier-Stokes equations. Along with the flapping trajectory, the effects of the Strouhal number (St), inter-foils distance (Lx), and phase angle (f) between the foils on the produced thrust are examined. The results demonstrate that using tandem configuration flapping foil with elliptical and fishtailed flapping can significantly increase the induced thrust and propulsive efficiency. [ABSTRACT FROM AUTHOR]

It is well known the Oseen iteration for the stationary Navier–Stokes equations is unconditionally stable. However, it is a coupled type scheme where the velocity u$$ \boldsymbol{u} $$ and pressure p$$ p $$ are coupled together at each iteration. By treating pressure p$$ p $$ explicitly would lead to a decoupled iteration, but this treatment is unstable. In this article, we construct a decoupled and unconditionally stable iteration method to solve the stationary Navier–Stokes equations by adopting the pressure projection method to the temporal disturbed Navier–Stokes system whose solution approximates the steady state solution over time (t→+∞$$ t\to +\infty $$). We also rigorously prove its unconditional stability. Numerical simulations demonstrate that our iterative method is more efficient and stable than the extensively used T‐S and Oseen iterations, and could solve the fluid flow with high Reynolds number. [ABSTRACT FROM AUTHOR]

In this study, the effects of surface roughness and Reynolds number (Re) on fluid flow and solute transport are investigated based on a double rough-walled fracture model that precisely represents the natural geometries of rock fractures. The double rough-walled fracture model is composed of two three-dimensional(3D) self-affine fracture surfaces generated using the improved successive random additions (SRA). Simulation of fluid flow and solute transport through the models were conducted by directly solving the Navier-Stokes equation and advection-diffusion equation (ADE), respectively. The results indicate that as the Re increases from 0.1 to 200, the flow regime changes from linear flow to nonlinear flow accompanied with the tortuous streamlines and significant eddies. Those eddies lead to the temporary stagnant zones that delay the solute migration. The increment of Re enhances the transport heterogeneity with the transport mode changing from the diffusion-dominated to the advection-dominated behavior, which is more significant in the fracture with a larger joint roughness coefficient (JRC). All breakthrough curves (BTCs) of rough-walled fractures exhibited typical non-Fickian transport characteristics with "early arrival" and "long tailing" of BTCs. Increasing the JRC and/or Re will enhances the non-Fickian transport characteristics. The ADE model is able to accurately fit the numerical BTCs and residence time distributions (RTDs) at a low Re, but fails to capture the non-Fickian transport characteristics at a large Re. In contrast, the continuous time random walk (CTRW) model provides a better fit to the numerical simulation results over the whole range of Re. Whereas, the fitting error gradually increases with increasing Re. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of a moving flapping wing is investigated numerically using a fluid–solid interaction model. The flow field is assumed to be two-dimensional, viscous, unsteady, and incompressible. Galerkin-least/squares finite element method is used to account for the convective nature of the flow field. The flow solver is coupled with the rigid body equations of motion describing the movement of the flapping wing. An interface-capturing technique is developed and tailored to the present model to capture the flapping wing during its movement. The developed algorithm is validated against published data with a great degree of success. Then, simulations for a flapping wing with a stationary center of gravity and free center of gravity motion, are performed. This is followed by a parametric study through which the effects of the wing’s initial position, maximum flapping angle, Reynolds number, the mass of the wing, the gravitational acceleration, and the critical frequency for each Reynolds number are studied. Finally, the voluntary vertical takeoff of a fruit fly, Drosophila Melanogaster is simulated using the proposed model. [ABSTRACT FROM AUTHOR]

This paper is concerned with the nonlinear stability of planar Couette flow for the three‐dimensional NS‐NS equations, which are used to model the motion of two‐phase flow. Our result shows that the planar Couette flow is asymptotically stable for the initial perturbations sufficiently small in some Sobolev space if the background velocity and the Reynolds number are sufficiently small. Moreover, it is proved that the solution converges to the stationary state (ρ∗,us,n∗,ws)$$ \left({\rho}_{\ast },{u}_s,{n}_{\ast },{w}_s\right) $$ at an algebraic time decay rate, and we also give the decay rate of the relative velocity. [ABSTRACT FROM AUTHOR]

Nonmodal development of stationary three-dimensional disturbances in a circular jet is numerically investigated at the Reynolds number Re = 2850. The operating conditions of a laboratory experiment performed earlier in the Institute of Mechanics of Moscow State University are reproduced. A method for calculating optimal disturbances under the conditions of downstream developing main flow is developed. The disturbances associated with different azimuthal numbers are calculated. The shape, character of development, and growth degree of optimal disturbances are determined. [ABSTRACT FROM AUTHOR]

In this article, a numerical study was carried out to study the dynamic adaptive grid method, based on the concept of the equidistribution method. The article explores a method for adapting the computational grid to solving two-dimensional Navier-Stokes differential equations, which describe the physical processes of gas dynamics specifically for the problem of a two-dimensional channel with an expansion coefficient (H/h) = 2. Different flow characteristics were calculated at different Reynolds numbers Re = from 100 to 1000, to get the actual thread behavior. Calculations are performed for laminar flow mode. The results of the longitudinal velocity profiles in different sections of the channel and the length of the primary and secondary vortices are obtained with a change in the Reynolds number after the ledge. For the numerical solution of this problem, a second-order accuracy McCormack scheme was used. To confirm the adequacy and reliability of the numerical results, a careful comparison was made with the experimental data of Armaly V.F. et al., taken from the literature. It is also shown that as a result of using this method of adaptive grids, it is possible to improve the numerical accuracy obtained for a given number of node points. It is shown that the existing method of multiple 2D adaptive meshes makes it easier to concentrate meshes in the required areas. This method should prove useful for many Navier-Stokes flow calculations. [ABSTRACT FROM AUTHOR]

We study the stationary Navier–Stokes equations in the region between two rotating concentric cylinders. We first prove that, for a small Reynolds number, if the fluid flow is axisymmetric and if its velocity is sufficiently small in the $L^\infty$ -norm, then it is necessarily the Taylor–Couette–Poiseuille flow. If, in addition, the associated pressure is bounded or periodic in the $z$ axis, then it coincides with the well-known canonical Taylor–Couette flow. We discuss the relation between uniqueness and stability of such a flow in terms of the Taylor number in the case of narrow gap of two cylinders. The investigation in comparison with two Reynolds numbers based on inner and outer cylinder rotational velocities is also conducted. Next, we give a certain bound of the Reynolds number and the $L^\infty$ -norm of the velocity such that the fluid is, indeed, necessarily axisymmetric. As a result, it is clarified that smallness of Reynolds number of the fluid in the two rotating concentric cylinders governs both axisymmetry and the Taylor–Couette–Poiseuille flow with the exact form of the pressure. [ABSTRACT FROM AUTHOR]

The initial flow past an impulsively started rotating and translating circular cylinder is asymptotically analysed using a Brinkman penalization method on the Navier-Stokes equation. In our previous study (J. Fluid Mech., vol. 929, 2021, A31), the asymptotic solution was obtained within the second approximation with respect to the small parameter, ϵ, that is of the order of 1/λ. Here, λ is the penalization parameter. In addition, the Reynolds number based on the cylinder radius and the translating velocity is assumed to be of the order of ϵ. The previous study asymptotically analysed the initial flow past an impulsively started translating circular cylinder and investigated the influence of the penalization parameter λ on the drag coefficient. It was concluded that the drag coefficient calculated from the integration of the penalization term exhibits a half-value of the results of Bar-Lev & Yang (J. Fluid Mech., vol. 72, 1975, pp. 625-647) as λ→∞. Furthermore, the derivative of vorticity in the normal direction was found to be discontinuous on the cylinder surface, which is caused by the tangential gradient of the pressure on the cylinder surface. The present study hence aims to investigate the variance on the drag coefficient against the result of Bar-Lev & Yang (1975). First, we investigate the problem of an impulsively started rotating circular cylinder. In this situation, the moment coefficient is independent of the pressure on the cylinder surface so that we can elucidate the role of the pressure to the hydrodynamic coefficients. Then, the problem of an impulsively started rotating and translating circular cylinder is investigated. In this situation, the pressure force induced by the unsteady flow far from the cylinder is found to play a key role on the drag force for the agreement with the result of Bar-Lev & Yang (1975), whereas the variance still exists on the lift force. To resolve the variance, an alternative formula to calculate the hydrodynamic force is derived, assuming that there is the pressure jump between the outside and inside of the cylinder surface. The pressure jump is obtained in this analysis asymptotically. Of particular interest is the fact that this pressure jump can cause the variance on the lift force calculated by the integration of the penalization term. [ABSTRACT FROM AUTHOR]

In this study, we investigate a handful of equilibrium states and their hydrodynamic stability for the incompressible flow in a square cavity driven by the tangential simultaneous motion of all of its lids at the same speed. This problem has been investigated in the recent past, albeit only partially and always disregarding the singular boundary conditions at the corners. The implications of the system symmetries have not been rigorously addressed, either. In our study, we introduce a regularized version of the boundary conditions to avoid the corner singularities, in a way that preserves the main features of the original setup. In addition, the Navier–Stokes equations are discretized by means of highly accurate Chebyshev spectral methods that provide exponential convergence of all computed flows and consistently eliminate any potential source of structural instability of the bifurcation scenario. We employ Newton–Krylov solvers, implemented within continuation algorithms, to accurately compute equilibrium solutions. Linear stability analysis of both the primary symmetric base flow and the secondary asymmetric states uncovers new branches of fully asymmetric steady states. The analysis has allowed identification of six previously undetected bifurcations, all of which are associated with the disruption of either the rotational invariance or the reflection symmetry. Some of these bifurcations have been found to be quite clustered in some regions of the parameter space, which points at the underlying action of higher codimension mechanisms. Notably, all the bifurcations reported occur within the range of low to moderate Reynolds numbers, making the regularized four-sided lid-driven cavity flow a reliable benchmark for assessing different numerical schemes in the context of Navier–Stokes equivariant bifurcation theory. [ABSTRACT FROM AUTHOR]

Time-dependent flow fields are typically generated by a computational fluid dynamics method, which is an extremely time-consuming process. However, the latent relationship between the flow fields is governed by the Navier–Stokes equations and can be described by an operator. We therefore train a deep operator network (DeepONet) to learn the temporal evolution between flow snapshots. Once properly trained, given a few consecutive snapshots as input, the network has a great potential to generate the next snapshot accurately and quickly. Using the output as a new input, the network iterates the process, generating a series of successive snapshots with little wall time. Specifically, we consider two-dimensional flow around a circular cylinder at Reynolds number 1000 and prepare a set of high-fidelity data using a high-order spectral/hp element method as ground truth. Although the flow fields are periodic, there are many small-scale features in the wake flow that are difficult to generate accurately. Furthermore, any discrepancy between the prediction and the ground truth for the first snapshots can easily accumulate during the iterative process, which eventually amplifies the overall deviations. Therefore, we propose two alternative techniques to improve the training of DeepONet. The first one enhances the feature extraction of the network by harnessing the "multi-head non-local block." The second one refines the network parameters by leveraging the local smooth optimization technique. Both techniques prove to be highly effective in reducing the cumulative errors, and our results outperform those of the dynamic mode decomposition method. [ABSTRACT FROM AUTHOR]

In this study, we investigate the thermocapillary rotation of microgears at fluid interfaces and extend the concept of geometric asymmetry to the translational propulsion of micrometer-sized particles. We introduce a transient numerical model that couples the Navier–Stokes equations with heat transfer, displaying particle motion through a moving mesh interface. The model incorporates absorbed light illumination as a heat source and predicts both rotational and translational speeds of particles. Our simulations explore the influence of microgear design geometry and determine the scale at which thermocapillary Marangoni motion could serve as a viable propulsion method. A clear correlation between Reynolds number and rotation efficiency can be recognized. To transfer the asymmetry-based propulsion principle from rotational to directed translational motion, various particle geometries are considered. We demonstrate that, under illumination from above, geometrically asymmetric "Christmas tree"-shaped particles move forward. The exploration of breaking geometric symmetry for translational propulsion is mostly ignored in the existing literature, thus warranting further discussion. Therefore, we analyze expected translational speeds in comparison to corresponding microgears to provide insight into this promising propulsion method. Our simulations indicate that translational propulsion speeds of several particle lengths per second can be expected on the micrometer scale. [ABSTRACT FROM AUTHOR]

This paper explores the potential of Physics-Informed Neural Networks (PINNs) to serve as Reduced Order Models (ROMs) for simulating the flow field within stirred tank reactors (STRs). We solve the two-dimensional stationary Navier-Stokes equations within a geometrically intricate domain and explore methodologies that allow us to integrate additional physical insights into the model. These approaches include imposing the Dirichlet boundary conditions (BCs) strongly and employing domain decomposition (DD), with both overlapping and non-overlapping subdomains. We adapt the Extended Physics-Informed Neural Network (XPINN) approach to solve different sets of equations in distinct subdomains based on the diverse flow characteristics present in each region. Our exploration results in a hierarchy of models spanning various levels of complexity, where the best models exhibit $ \ell_1 $ prediction errors of less than 1% for both pressure and velocity. To illustrate the reproducibility of our approach, we track the errors over repeated independent training runs of the best identified model and show its reliability. Subsequently, by incorporating the stirring rate as a parametric input, we develop a fast-to-evaluate model of the flow capable of interpolating across a wide range of Reynolds numbers. Although we exclusively restrict ourselves to STRs in this work, we conclude that the steps taken to obtain the presented model hierarchy can be transferred to other applications. [ABSTRACT FROM AUTHOR]

The growth and characteristics of linear, oblique instabilities on a highly swept fin on a straight cone in Mach 6 flow are examined. Large streamwise pressure gradients cause doubly inflected cross-flow profiles and reversed flow near the wall, which necessitates using the harmonic linearized Navier-Stokes equations. The cross-flow instability is responsible for the most-amplified disturbances, however, not all disturbances show typical cross-flow characteristics. Distinct differences in perturbation structure are shown between small (~3-5 mm) and large (~10 mm) wavelength disturbances at the unit Reynolds number Re = 11 × 106 m-1. As a result, amplification measurements based solely on wall quantities bias a most-amplified disturbance assessment towards larger wavelengths and lower frequencies than would otherwise be determined by an off-wall total-energy approach. A spatial-amplification energy-budget analysis demonstrates (i) that wall-normal Reynolds-flux terms dictate the local growth rate, despite other terms having a locally larger magnitude and (ii) that the Reynolds-stress terms are responsible for large-wavelength disturbances propagating closer to the wall compared with small-wavelength disturbances. Additionally, the effect of free-stream unit Reynolds number and small yaw angles on the perturbation amplification and energy budget is considered. At a higher Reynolds number (Re° = 22 × 106 m-1), the most-amplified wavelength shrinks. Perturbations do not behave self-similarly in the thinner boundary layer, and the shift in most-amplified wavelength is due to decreased dissipation relative to the lower-Reynolds-number case. Small yaw angles produce a streamwise shift in the boundary layer and disturbance amplification. The yaw results quantify a potential uncertainty source in experiments and flight. [ABSTRACT FROM AUTHOR]

In thermal protection systems (TPS), Darcy's law or Darcy-Forchheimer's law is employed to model the pyrolysis gas flow within the anisotropic porous ablator depending on the flow regime considered. A key challenge with using these laws is first, the knowledge of the validity domain of each flow regime in terms of a critical Reynolds number (R e c ). Secondly, the lack of data on macroscopic properties, namely, the permeability and Forchheimer tensors is particularly challenging for the relevance of the models. The objective of this work is to contribute to overcoming these challenges by performing experimental and X-ray tomographic image-based characterization of Calcarb, a commercial carbon preform used for manufacturing TPS. For this purpose, fluid flow within Calcarb was studied experimentally in the Through-Thickness (TT) and the In-Plane (IP) directions for Reynolds numbers of 0.05 to 10.46 -representative of the TPS application. Tomography image-based micro-scale simulations, involving the direct resolution of the Navier–Stokes equations under both flow regimes, were also performed. Experimental results exhibit the anisotropic nature of Calcarb, namely through R e c values, corresponding to the Darcy flow regime limit, slightly different in the two directions (R e c of 0.31 and 0.43) with measured permeability values of 1. 248 × 1 0 − 10 m 2 and 1. 615 × 1 0 − 10 m 2 for TT and IP directions respectively. In the Forchheimer regime, experimental Forchheimer coefficients β were 2. 0010 × 1 0 5 m−1 (TT) and 1. 4948 × 1 0 5 m−1 (IP). During the simulation process, a numerical strategy was defined to obtain the permeability tensor yielding values within 8% of the experimental ones. The comparison of experimental results vs simulation results in the Forchheimer regime was performed through the analysis of the pressure gradients as functions of R e in the x , y , and z directions. The numerical estimations were compared successfully with experimental measurements, with a discrepancy of 5.2%, for R e values up to 2.4. [Display omitted] • Experimental investigation and tomography analysis of the parameters of a carbon preform. • It is applied in Darcy-Forchheimer flows in thermal protection systems. • The permeability is calculated via direct resolution of the Navier–Stokes equations. • The Darcy flow regime limit in Calcarb is observed at a Reynolds number of 0.43. [ABSTRACT FROM AUTHOR]

In the under-resolved simulation of the high-speed liquid injection into stagnant air, the attention in subgrid-scale models is focused on events of intense gradients of the velocity in turbulent flow, i.e., on effects of intermittency. Three typical flows are considered—the in-nozzle flow, primary atomization zone, and secondary atomization of spray droplets. In the simulation of the first two flows, the filtered Navier–Stokes equations are forced by stochastic processes with properties which incorporate the statistical physics of fluid acceleration at the high Reynolds number—in this way we update the under-resolved acceleration. In the case of in-nozzle flow, the proposed stochastic subgrid acceleration model is combined with wall-damping function, and the ability in prediction of the velocity statistics is demonstrated. In the simulation of primary atomization, the approach with stochastic subgrid acceleration is combined with the volume of fluid method. This leads to intensification of the interface dynamics, resulting in additional corrugation, with more intense shearing and stretching of liquid structures are observed. Thereby, the experimental profiles of the time-averaged liquid volume fraction distribution for four different axial locations are rather well predicted. The secondary atomization of droplets is simulated by a new stochastic model for the breakup rate along the droplet path. To this end, the breakup rate is expressed as a function of turbulent viscous dissipation which evolves along the droplet path according to the proposed stochastic process. Preliminary assessment performed against the recent experiments shows the correct predictability of this model and its low sensibility to the grid density. [ABSTRACT FROM AUTHOR]

Curved free shear layers emerge in many engineering problems involving complex flow geometries, such as the flow over a backward-facing step, flows with wall injection in a boundary layer, the flow inside side-dump combustors, or wakes generated by vertical axis wind turbines, among others. Previous studies involving centrifugal instabilities have mainly focused on wall-flows where Taylor instabilities between two rotating concentric cylinders or Görtler vortices in boundary layers are generated. Curved free shear layer flows, however, have not received sufficient attention, especially in the nonlinear regime. The present work investigates the development of centrifugal instabilities in a curved free shear layer flow in the nonlinear compressible regime. The compressible Navier–Stokes equations are reduced to the nonlinear boundary region equations (BREs) in a high Reynolds number asymptotic framework, wherein the streamwise wavelength of the disturbances is assumed to be much larger than the spanwise and wall-normal counterparts. We study the effect of the freestream Mach number M ∞ , the shear layer thickness δ , the amplitude of the incoming disturbance A, and the relative velocity difference across the shear layer Δ V on the development of these centrifugal instabilities. Our parametric study shows that, among other things, the kinetic energy of the curved shear layer flow increases with increasing Δ V and A decreases with increasing d e l t a . It was also found that increasing the disturbance amplitude of the incoming disturbance leads to significant growth in the mushroom-like structure's amplitude and renders the secondary instability structures more prominent, indicating increased mixing for all Mach numbers under consideration. [ABSTRACT FROM AUTHOR]

First-order sensitivities and adjoint analysis are used widely to control the linear stability of unstable flows. Second-order sensitivities have recently helped to increase accuracy. In this paper, a method is presented to calculate arbitrary high-order sensitivities based on Taylor expansions of the incompressible base flow and its eigenproblem around a scalar parameter. For the incompressible Navier–Stokes equations, general expressions for the sensitivities are derived, into which parameter-specific information can be inserted. The computational costs are low since, for all orders, a linear equation system has to be solved, of which the left-hand-side matrix stays constant and thus its preconditioning can be exploited. Two flow scenarios are examined. First, the cylinder flow equations are expanded around the inverse of the Reynolds number, enabling the prediction of the two-dimensional cylinder base flow and its leading eigenvalue as a function of the Reynolds number. This approach computes accurately the base flow and eigenvalue even in the unstable regime, providing, when executed subsequently, a mean to calculate unstable base flows. This case gives a clear introduction into the method and allows us to discuss its constraints regarding convergence behaviour. Second, a small control cylinder is introduced into the domain of the cylinder flow for stabilization. Higher-order sensitivity maps are calculated by modelling the small cylinder with a steady forcing. These maps help to identify stabilizing areas of the flow field for Reynolds numbers within the laminar vortex shedding regime, with the required number of orders increasing as the Reynolds number rises. The results obtained through the proposed method align well with numerically calculated eigenvalues that incorporate the cylinder directly into the grid. [ABSTRACT FROM AUTHOR]

We performed numerical simulations of a homogeneous swarm of bubbles rising at large Reynolds number, $Re=760$ , with volume fractions ranging from 1 % to 10 %. We consider a simplified model in which the interfaces are not resolved, but which allows us to simulate flows with a large number of bubbles and to emphasize the interactions between bubble wakes. The liquid phase is described by solving, on an Eulerian grid, the Navier–Stokes equations, including sources of momentum which model the effect of the bubbles. The dynamics of each bubble is determined within the Lagrangian framework by solving an equation of motion involving the hydrodynamic forces exerted by the fluid accounting for the correction of the fictitious self-interaction of a bubble with its own wake. The comparison with experiments shows that this coarse-grained simulations approach can reliably describe the dynamics of the resolved flow scales. We use conditional averaging to characterize the mean bubble wakes and obtain in particular the typical shear imposed by the rising bubbles. On the basis of the spectral decomposition of the energy budget, we observe that the flow is dominated by production at large scales and by dissipation at small scales and we rule out the presence of an intermediate range in which the production and dissipation are locally in balance. We propose that the $k^{-3}$ subrange of the energy spectra results from the mean shear rate imposed by the bubbles, which controls the rate of return to isotropy. [ABSTRACT FROM AUTHOR]

In the Aerodynamic drag analysis, the predominant factor is pressure drag which reduces the vehicle speed and fuel efficiency. By delaying the flow separation at trailing edge in roof of the car body and creating turbulence in the vacuum region available at rear (of car body), the pressure difference between front and rear can be minimized, thus reducing the pressure drag. This study deals with the computational analysis of a simplified car model using Ansys FLUENT software at a Reynolds number of 3.281e5 based on the body length. To solve the Navier-stokes equations k - e turbulence model is used. A base car model and a car model with vortex generator (VG) placed at three different locations on either side of rear slant are developed using NX modelling software. By using ICEM meshing software, developed car models are meshed. For all model's coefficient of drag is analyzed using Ansys-FLUENT at different velocities such as 20m/s, 25m/s, 30m/s and 35m/s. From the computational analysis, a considerable reduction in the coefficient of drag is achieved in all three models. In particular, the model which had vortex generator placed at middle of rear slant of either side provides improved drag reduction comparing with other two models. While calculating the drag force using coefficient of drag, it is observed that for all velocities around 11% reduction in drag force are achieved in car model with vortex generator on comparing with the base car model. [ABSTRACT FROM AUTHOR]

Hydrodynamic forces on small diameter subsea pipelines and cables placed near seabed are important for their on-bottom stability design. In offshore environments, these pipelines are usually subjected to extreme wave conditions. The present study investigates hydrodynamic forces acting on a pipeline near a flat seabed subjected to a wave-induced boundary layer flow. The Keulegan-Carpenter numbers of the wave-induced boundary layer flow are 20, 140, and 200, defined based on the pipeline diameter (D), the maximum velocity of the undisturbed near-bed orbital velocity (Uw), and the period of the incoming oscillatory flow (Tw). Reynolds number is 1 × 104 based on Uw and D. A seabed roughness ratio ks/D (ks is the Nikuradse equivalent sand roughness) of up to 0.1 and different gap ratios of G/D = 0.05-0.5 between the pipeline and the seabed are considered. Numerical simulations have been carried out based on two-dimensional (2D) unsteady Reynolds-averaged Navier-Stokes equations combined with the k-ω shear stress transport turbulence model. A preliminary one-dimensional (1D) simulation is carried out to obtain a fully developed wave-induced boundary layer velocity profile, which is used as inlet flow for the 2D simulations. The numerical model is validated against the experimental data reported by Sumer et al. [1991, "Effect of a Plane Boundary on Oscillatory Flow Around a Circular Cylinder," J. Fluid Mech., 225, pp. 271-300] at KC = 10. Influences of KC, ks/D, and G/D on the hydrodynamic forces and the surrounding flows are discussed in detail. [ABSTRACT FROM AUTHOR]

Predictions are presented of mean values of statistical variables of large-scale turbulent flow of the widely used basic k- ϵ model, and of a new model, which is based on general statistical descriptions of turbulence. The predictions are verified against published results of direct numerical simulations (DNSs) of Navier–Stokes equations. The verification concerns turbulent channel flow at shear Reynolds numbers of 950, 2000, and 10 4 . The basic k- ϵ model is largely based on empirical formulations accompanied by calibration constants. This contrasts with the new model, where descriptions of leading statistical quantities are based on the general principles of statistical turbulence at a large Reynolds number and stochastic theory. Predicted values of major output variables such as turbulent viscosity, diffusivity of passive admixture, temperature, and fluid velocities compare well with DNS for the new model. Significant differences are seen for the basic k- ϵ model. [ABSTRACT FROM AUTHOR]

The classical vorticity‐streamfunction formulation (VSF) can avoid the difficulty in the calculation of pressure gradient term of the Navier Stokes equation via eliminating pressure gradient term from the theoretical basis. Within this context we propose a general VSF, together with redefined vorticity and streamfunction, so as to realize numerically stable and reliable simulations of binary fluids with an arbitrary density contrast. By incorporating the interface‐tracking phase‐field model based on the conservative Allen‐Cahn equation [Phys. Rev. E 94, 023311 (2016)], the binary flow simulation framework is established. Numerical tests are conducted using the Lattice Boltzmann method (LBM), which is usually regarded as an easy‐to‐use tool for solving the Navier–Stokes equation but generally suffers from the drawback of not being capable of enforcing incompressibility. The LBM herein functions as a numerical tool for solving the vorticity transport equation, the streamfunction equation, and the conservative Allen‐Cahn equation. Three two‐dimensional benchmark cases, i.e., the Capillary wave, the Rayleigh–Taylor instability, and the droplet splashing on a thin liquid film, are discussed in detail to verify the present methodology. Results show good agreements with both analytical predictions and literature data, as well as good numerical stability in terms of high density ratio and high Reynolds number. Overall, the general VSF inherits the intrinsic superiority of the classical VSF in enforcing incompressibility, and offers a useful and reliable alternative for binary flow modeling. [ABSTRACT FROM AUTHOR]

A comprehensive 2D numerical model was conscientiously developed to investigate the vortex-induced vibration phenomena in a cylindrical structure with rounded corners. The Navier-Stokes equation was adeptly solved under the specific condition of a Reynolds number (Re) of 150. The investigation reveals intricate details of the phenomena. The study aimed to systematically analyze the interaction between drag and lift force coefficients, cylinder vibration amplitude, and the patterns of vortex shedding modes under various conditions. This study systematically altered the radius of the cylinder's rounded corners to evaluate their effects on both structural and hydrodynamic responses. This variation was crucial in comprehending how slight alterations in the cylinder's geometry impact significant changes in the flow dynamics and correlated vibration behavior. The model's numerical results revealed the significant impact of the curved edge ratio on both the hydrodynamic forces acting on the cylinder and its vibration response. The variation in edge curvature resulted in changes in drag and lift coefficients, leading to a significant impact on the amplitude of vibration. This elucidates the crucial role of geometric design in controlling and optimizing the structural behavior of cylindrical structures under fluid flow conditions. [ABSTRACT FROM AUTHOR]

The numerical solution of the two–dimensional, time–dependent, incompressible Navier–Stokes equations (NSE) in curvilinear coordinates using the nodal integral method (NIM) is presented in this paper. The developed numerical scheme is applied to solve fluid flow problems in domains discretized using quadrilateral cells. Three test problems are numerically solved to assess the accuracy and efficiency of the method. The scheme is second-order accurate in space for all considered deformation levels and Re numbers. The results of the current scheme are compared with the other numerical results in the literature, and good agreement is obtained for all considered cases. The NIM demonstrates superior accuracy per cell compared to other second–order finite–volume schemes considered in this work. New benchmark results obtained from the solution on fine meshes are presented. • The time-dependent, 2D, incompressible Navier-Stokes equations are numerically solved in curvilinear coordinates using nodal integral method. • The accuracy and efficiency of the numerical scheme are assessed through three numerical test problems. • The scheme is found to be second-order accurate in space for all considered deformation levels and Reynolds numbers. • The developed scheme shows superior per-cell accuracy compared to other considered second-order finite-volume schemes. • The developed scheme is capable of simulating incompressible flows accurately using coarse meshes. [ABSTRACT FROM AUTHOR]

The investigation of the thermodynamic efficacy of an infrared suppression system for ocean liners, battleships, and warships, is the focus of the current study. The governing equations are worked out using ANSYS Fluent 15.0. (Navier–Stokes equation, energy equation, and equation of turbulence). The outcomes are then utilized to create a more thorough and precise design methodology. Relevant factors can vary, including nozzle inlet temperature, inclination angle, nozzle overlapping, and Reynolds number ( 6.12 × 10 5 to 3.11 × 10 6). Various quantities of heat transfer and entropy production are taken into consideration, as well as the influence of different conical shape IRS systems (RLCF and ELCF) and various funnel wall conditions. The IRS system's suggested design is verified by the entropy generation research, which also assists in evaluating which model is optimal. To clarify the thermo-fluid behavior, the Bejan number plots, the entropy generation contours, and both the thermal and frictional irreversibility are presented. [ABSTRACT FROM AUTHOR]

Due to the correlation of design parameters of the counter-flow jet in addition to the complexity of the flow field, understanding the mechanism of the counter-flow jet for drag reduction and flow control remains challenging. Furthermore, to satisfy the demands of the space transportation system, investigating the counter-flow jet's suitability for a range of flight conditions is critical. To solve these problems, a study was performed by varying the pressure ratio (PR) and exit Mach number of the counter-flow jet at hypersonic low-Reynolds number regime. For numerical simulations, laminar, axisymmetric Navier–Stokes equations were solved by the total variation diminishing scheme with second-order accuracy in space and the explicit strong stability preserving the Runge–Kutta method. With given numerical conditions, the flow field was categorized as the long penetration mode (LPM) based on the penetration length and the fluctuation of the flow field at the high-Reynolds number regime. By reducing the free-stream flow Reynolds number while keeping other parameters unchanged, the flow field transitioned from the LPM to a stable LPM, short penetration mode, or long penetration with periodically oscillation mode. The critical Reynolds number for the transition of the flow field is highly dependent on the exit Mach number of the counter-flow jet and PR. The extended jet layer was the primary reason for the fluctuation of the drag coefficient. Furthermore, with the counter-flow jet at certain flight conditions, drag can be reduced by up to 78% regardless of the stability of the flow field. [ABSTRACT FROM AUTHOR]

We numerically investigate Taylor–Couette flows within a system consisting of an elliptical outer cylinder and a rotating inner circular cylinder, with particular emphasis on the behavior of Taylor cells. The three-dimensional unsteady Navier–Stokes equations are solved under the assumption of axial periodicity. Also, a scalar transport equation is solved for the heat transfer. Our methodology employs a Fourier-spectral meshless discretization technique, which interpolates variables at scattered points using polyharmonic splines and appended polynomials. A pressure-projection algorithm achieves the time advancement of the flow equations. We present findings for an elliptical enclosure with an aspect ratio of two, examining a range of Reynolds numbers (Re) from subcritical to 300. Our analysis includes streamlines, axial velocity contours, pressure, vorticity, and temperature profiles. The results indicate that the flow remains steady up to R e ≈ 300 before transitioning to an unsteady state at R e ≈ 350. [ABSTRACT FROM AUTHOR]

A prototype cleanroom for hazardous testing and handling of satellites prior to launcher encapsulation, satisfying the ISO8 standard has been designed and analyzed in terms of performances. Unsteady Reynolds Averaged Navier-Stokes (URANS) models have been used to study the related flow field and particulate matter (PM) dispersion. The outcomes of the URANS models have been validated through comparison with equivalent large-eddy simulations. Special attention has been paid to the location and shape of the air intakes and their orientation in space, in order to balance the PM convection and diffusion inside the cleanroom. Forming a cyclone-type flow pattern inside the cleanroom is a key to maintaining a high ventilation efficiency. [ABSTRACT FROM AUTHOR]

A data-driven implementation of a quasi-linear approximation is presented, extending a minimal quasi-linear approximation (MQLA) (Hwang & Ekchardt, J. Fluid Mech., vol. 894, 2020, p. A23) to incorporate non-zero streamwise Fourier modes. A data-based approach is proposed, matching the two-dimensional wavenumber spectra for a fixed spanwise wavenumber between a direct numerical simulation (DNS) (Lee & Moser, J. Fluid Mech., vol. 774, 2015, pp. 395-415) and that generated by the eddy viscosity enhanced linearised Navier-Stokes equations at Reτ ≈ 5200, where Reτ is the friction Reynolds number. Leveraging the self-similar nature of the energy-containing part in the DNS velocity spectra, a universal self-similar streamwise wavenumber weight is determined for the linearised fluctuation equations at Reτ ≃ 5200. The data-driven quasi-linear approximation (DQLA) provides noteworthy enhancements in the wall-normal and spanwise turbulence intensity profiles. It exhibits a qualitatively similar structure in the spanwise wavenumber velocity spectra compared with the MQLA. Additionally, the DQLA offers extra statistical outputs in the streamwise wavenumber coordinates, enabling a comprehensive global analysis of this modelling approach. By comparing the DQLA results with DNS results, the limitations of the presented framework are discussed, mainly pertaining to the lack of the streak instability (or transient growth) mechanism and energy cascade from the linearised model. The DQLA is subsequently employed over a range of Reynolds numbers up to Reτ = 105. Overall, the turbulence statistics and spectra produced by the DQLA scale consistently with the available DNS and experimental data, with the Townsend-Perry constants displaying a mild Reynolds dependence (Hwang, Hutchins & Marusic, J. Fluid Mech., vol. 933, 2022, p. A8). The scaling behaviour of the turbulence intensity profiles deviates away from the classic ln(Reτ) scaling, following the inverse centreline velocity scaling for the higher Reynolds numbers. [ABSTRACT FROM AUTHOR]

In this study, a modified Cahn–Hilliard equation with a very simple format was proposed, which can be used to simulate immiscible multi-component/multiphase flow with a super large density ratio. In addition, based on this modified equation and the Navier–Stokes equations, an improved multiphase lattice Boltzmann flux solver (IMLBFS) has been proposed, and its computational ability has been tested by multiple numerical examples, including Laplace law, two bubbles merging, contact angle, bubble rising, and droplet splashing on a thin film. The results show that the proposed IMLBFS can simulate immiscible two-phase flow with a very large density ratio up to 1:5000 or 1:10 000 under various operating conditions, including the Reynolds number reaching 10 000. In addition, IMLBFS also has excellent features such as clear physical properties, freely adjustable source term strength, and effective suppression of mass loss. [ABSTRACT FROM AUTHOR]

This study introduces an analytical solution for the laminar entrance flow in circular pipes, aiming to confirm the occurrence of velocity overshoot. Velocity overshoot is characterised by the maximum axial velocity appearing near the pipe wall instead of the central axis. Similar to the previous studies, the analytical solution is derived from the parabolised Navier-Stokes equation; however, the specific approach used in linearising the momentum equation has not been attempted before. The accuracy of this analytical solution has been verified through a comprehensive comparison with various published experimental data. The existence of velocity overshoot at a short distance from the inlet, which is evident in numerous numerical calculations based on the full Navier-Stokes equations and corroborated by recent magnetic resonance (MR) velocimetry experiments, is identified analytically for the first time. The parabolised Navier-Stokes equation has inherent self-similarity with respect to the Reynolds number, implying that Re is incorporated into the dimensionless variables rather than serving as an independent flow parameter. According to both MR velocimetry measurements and the present analytical solution, the self-similarity is not valid immediately following the pipe inlet, and this becomes more evident as Re decreases; hence, the analytical solution derived from the parabolised Navier-Stokes equation cannot accurately predict the evolution of the velocity profile within this region near the pipe inlet. [ABSTRACT FROM AUTHOR]

With the rapid development of artificial intelligence technology, the physics-informed neural network (PINN) has gradually emerged as an effective and potential method for solving N-S equations. The treatment of constraints is vital to the PINN prediction accuracy. Compared to soft constraints, hard constraints are advantageous for the avoidance of difficulties in guaranteeing definite conditions and determining penalty coefficients. However, the principles on the formulation of hard constraints of PINN currently remain to be formed, which hinders the application of PINN in engineering fields. In this study, hard-constraint-based PINN models are constructed for Couette flow, plate shear flow and stenotic/aneurysmal flow with curved geometries. Particular efforts have been devoted to assessing the impact of the model parameters of hard constraints, i.e., degree and scaling factor, on the prediction accuracy of PINN at different Reynolds numbers. The results show that the degree is the most important factor that influences the prediction accuracy, followed by the scaling factor. As for the N-S equations, the degree of hard constraints should be at least two, while the scaling factor is recommended to be maintained around 1.0. The outcomes of the present work are of reference value for the development of PINN methods in fluid mechanics. [ABSTRACT FROM AUTHOR]

We examine the effects of the history force on the preferential concentration of small particles in homogeneous isotropic turbulence under a wide range of the particle-to-fluid mass density ratio γ (from 0 to 10000) by direct simulations of the Navier–Stokes equations. As to the history integral kernel, we introduce two expressions: i.e., the classical Basset kernel valid only in the limit of creeping flow conditions and for short times, and the Mei–Adrian kernel (Mei and Adrian, 1992) applicable for finite Reynolds numbers and large times. It is revealed that the presence of the history force weakens the level of preferential concentration to some extent, specifically under the conditions of the mass density ratio of around 1. 5 – 10 for heavy particles (e.g., solid particles in liquid) and smaller than 0.7 for light particles (e.g., bubbles in liquid) and that the overprediction of particle accumulation by neglecting the history force is much more remarkable for light particles. A comparison between the Basset kernel and the Mei–Adrian kernel has shown that the Basset kernel overestimates the history effect but only slightly. [ABSTRACT FROM AUTHOR]

In this study, the impact of geometric parameters of rectangular baffles with varying location angles and heights is investigated on the heat transfer and fluid flow characteristics of cross-triangular grooved channels. Computational methods are employed to explore these effects, utilizing the Ansys-Fluent program to solve the Navier-Stokes and energy equations, incorporating the k-e turbulence model for numerical simulations. The inlet temperature of the air, serving as the working fluid, is set at 293 K, while the wall surface temperature of the lower triangular grooved channel remains fixed at 373 K. Rectangular baffles are tested with angles of 30°, 60°, and 90°, and heights of 0.25H, 0.5H, and 0.75H, respectively. The numerical results show good agreement with a 3.53% deviation compared to existing empirical data in the literature. The obtained findings are presented in terms of mean Nusselt (Num) number, fluid temperature, and Performance Evaluation Criterion (PEC) number variations taking into consideration of pressure drop for each rectangular baffle angle and height. Additionally, contour distributions of temperature and velocity are evaluated for different Reynolds numbers (Re) and arrangements of rectangular baffles. It has been determined that the Nu number value increases by 197.56% at a 90° angle and 0.75H height, compared to the 0.25H baffle height at Re=6000. Furthermore, at Re=1000, the PEC number is 84.50% higher with a baffle height of 0.25H and a baffle angle of 30° compared to the condition with a 90° angle. [ABSTRACT FROM AUTHOR]

Unsteady laminar confined and unconfined fluid flow and mixed (forced and free) convection heat transfer around equilateral triangular cylinders are investigated numerically. The computation model is a two-dimensional domain with blockage ratios of BR=0.5, 0.25, 0.2, 0.1, 0.05, and 0.0333, with the Reynolds numbers ranging from 100 to 200. The working fluid is water (Pr = 7). The effects of aiding and opposing thermal buoyancy are incorporated into the Navier-Stokes equations using the Boussinesq approximation. The Richardson number, which is a relative measure of free convection, is varied in the range -2 = Ri = 2. The governing equations are solved by using the Finite Volume Method with a second-order upwind scheme used for differencing of the convection terms, and the SIMPLE algorithm is used for the velocity-pressure coupling. A discussion of the effect of the blockage ratio on the mean drag, mean rms lift coefficients, the Strouhal number, and the mean Nusselt number is also presented. The iso-vorticity contours and dimensionless temperature field are generated to interpret and understand the underlying physical mechanisms. The results reveal that, in addition to the Richardson and Reynolds numbers, the blockage rate is effective in the vortex distribution in the channel. It has been determined that the vortices formed behind the cylinder spread to the channel with a decreasing blockage rate. Especially at high Reynolds numbers, both the drag coefficient and the mean Nusselt number are significantly affected by the blockage ratio. For Ri=0, the drag coefficients for BR=0.25 in comparison to the BR=0.05 case are about 9% and 29% larger for Re= 100 and 200, respectively. For BR<0.1, two-column vortex formation at the back of the cylinder gave way to single vortexes in the aiding thermal buoyancy condition (Ri=2) compared to Ri=0 and -2. Also, useful correlations for flow characteristics and heat transfer are derived using the computed data. [ABSTRACT FROM AUTHOR]

Finding ways to identify and quantify the losses from various sources in turbomachinery is significant for understanding the physical loss mechanisms and improving aerodynamic performance. However, traditional loss-assessment methods fail to reveal the local losses and decouple the flow field. In this paper, a new power-loss methodology is proposed. This methodology defines local and accumulated power losses, and a new method of averaging the total outlet pressure is presented. This establishes a direct relationship between the well-known total pressure loss and the accumulated power loss. The method was verified based on experimental results, the Reynolds-averaged Navier–Stokes equations, and large-eddy simulations of a compressor cascade at various Reynolds numbers. By applying this method, the boundary-layer loss, separation loss, and trailing-edge mixing loss of the compressor cascade were successfully distinguished and quantitatively accounted for. The method has been shown to be a valuable tool for understanding and quantifying the losses experienced in different flow regimes. In conclusion, the power-loss methodology demonstrates the potential for accurate quantitative analysis of local and global loss generation, the investigation of physical mechanisms, and the development of physical models for diverse complex flows beyond just the compressor cascade flow. [ABSTRACT FROM AUTHOR]

An adjoint approach is developed to undertake a receptivity study of the rotating disk boundary layer. The adjoint linearized Navier–Stokes equations are first derived in cylindrical coordinates. A receptivity formula is then formulated that specifies the response of stationary and traveling linear perturbations to an external force, including sources of momenta and mass and unsteady wall motion. Using the parallel flow approximation, in which the radial dependence of the undisturbed flow is ignored, receptivity characteristics are computed for a broad range of temporal frequencies, radial wavenumbers, azimuthal mode numbers, and Reynolds numbers. The type-I crossflow instability attains a maximum amplitude for external forces fixed near the wall-normal location of the critical layer (i.e., α ¯ r F + β G = ω), and the type-II Coriolis instability achieves larger amplitudes when external forces are located in the vicinity of a vanishing effective shear stress (i.e., α ¯ r F ′ + β G ′ = 0). Sources of radial momenta fixed about these wall-normal locations establish larger-sized disturbances than equivalent-sized sources of azimuthal momenta, wall-normal momenta, and mass. At the disk surface, motion along the wall-normal direction induces a stronger receptivity response than wall motions acting along the radial and azimuthal directions. In general, the crossflow instability achieves larger-sized amplitudes than the Coriolis instability, with the peak response realized for Reynolds numbers near the critical conditions for linear instability. [ABSTRACT FROM AUTHOR]

A numerical simulation on a six-row helicalsegmented finned tube in a staggered layout is performed with the Reynolds Averaged Navier-Stokes (RANS) approach. The turbulence effect is represented with the k-e RNG model, and a fixed temperature in each tube row considers the inside fluid temperature. The tube bank is represented by a small finned tube bank and a single interacted module. The small finned tube bank considers symmetry conditions on the sides of the computational domain. The single interacted module considers periodic boundary conditions for velocity, pressure, and temperature. Both simulations are compared to develop the basis of complex simulations such as repetitive finned tube modules. Average values on parallel planes to the streamwise direction for velocities, pressures, and temperatures are compared in both simulations. The comparative analyses show deviations lower than 11% for the velocity field, 12% for the pressure field, and 10% single for the temperature field. Therefore, results show an appropriate flow representation with periodic boundary conditions. [ABSTRACT FROM AUTHOR]

This study presents a comprehensive investigation of wellbore storage effects and non‐Darcy flow in pumping well systems using a multiphysics numerical model. The model incorporates the Reynolds‐averaged Navier‐Stokes equations coupled with a moving free surface to simulate the flow field and drawdown in the wellbore. Hydraulic behavior of the system, including well and aquifer drawdown, pumped water ratio, aquifer flow nonlinearity, and wellbore flow field, is analyzed and compared with simplified 1‐D models. This study highlights the presence of a vortex in the wellbore flow field induced by large Reynolds number and the uneven stress distribution at the screen. This vortex influences the wellbore drawdown and introduces 2‐D flow in the surrounding aquifer, which further contributes to the longer travel paths for groundwater particles and increased hydraulic pressure consumption. The results show that non‐Darcy flow, coupled with wellbore storage effects, leads to higher drawdown in the pumping well compared to Darcy flow cases. The aquifer drawdown exhibits a similar temporal pattern but with decreasing deviation at larger distances due to the cone of depression. The pumped water ratio indicates the challenge of supplying outflow at the intake from aquifer storage, with nonlinear flow resulting in a smaller ratio compared to linear cases. A nonlinearity ratio is defined to illustrate the expansion of nonlinear flow regions over time, their convergence to a predictable quasi‐steady‐state shape, which is influenced by the Forchheimer parameter, and the gradual transition to Darcy flow away from the wellbore. Plain Language Summary: This study focuses on understanding the behavior of water flow in pumping wells considering two important factors: wellbore storage and non‐Darcy flow. Wellbore storage refers to the amount of water stored inside the well, while non‐Darcy flow describes the flow behavior in the surrounding aquifer. Using a computer model, the researchers found that as pumping continues, the areas of nonlinear flow expand and eventually settle into a stable pattern. This transition from nonlinear flow to a more predictable behavior is influenced by the properties of the aquifer. The study also revealed that the presence of wellbore storage and the flow patterns near the well have a significant impact on the overall hydraulic performance. These findings have practical implications for improving the design and operation of pumping wells. Key Points: This study investigates the combined effects of wellbore storage and non‐Darcy flow in pumping well systems using a multiphysics modelThe findings reveal the expansion of nonlinear flow regions, transition to a quasi‐steady‐state, and the gradual shift toward Darcy flowThe study highlights the formation of vortices and 2D flow near the wellbore, which affect flow paths and hydraulic pressure consumption [ABSTRACT FROM AUTHOR]

Vortex-induced vibrations on wind turbine blades are a complex phenomenon not predictable by standard engineering models. For this reason, higher-fidelity computational fluid dynamics (CFD) methods are needed. However, the term CFD covers a broad range of fidelities, and this study investigates which choices have to be made when wanting to capture the vortex-induced vibration (VIV) phenomenon to a satisfying degree. The method studied is the so-called forced-motion (FM) approach, where the structural motion is imposed on the CFD blade surface through mode shape assumptions rather than fully coupled two-way fluid-structure interaction. In the study, two independent CFD solvers, EllipSys3D and Ansys CFX, are used and five different turbulence models of varying fidelities are tested. Varying flow scenarios are studied with low to high inclination angles, which determine the component of the flow in the spanwise direction. In all scenarios, the cross-sectional component of the flow is close to perpendicular to the chord of the blade. It is found that the low-inclinationangle and high-inclination-angle scenarios, despite having a difference equivalent to up to only a 30°C azimuth, have quite different requirements of both grid resolution and turbulence models. For high inclination angles, where the flow has a large spanwise component from the tip towards the root, satisfying results are found from quite affordable grid sizes, and even with unsteady Reynolds-averaged Navier-Stokes (URANS) k-! turbulence, the result is quite consistent with models resolving more of the turbulent scales. For low inclination, which has a high degree of natural vortex shedding, the picture is the opposite. Here, even for scale-resolving turbulence models, a much finer grid resolution is needed. This allows us to capture the many incoherent vortices, which have a large impact on the coherent vortices, which in turn inject power into the blade or extract power. It is found that a good consistency is seen using different variations of the higher-fidelity hybrid RANS-large eddy simulation (LES) turbulence models, like improved delayed detached eddy simulation (IDDES), stressblended eddy simulation (SBES) and k-ω scale-adaptive simulation (SAS) models, which agree well for various flow conditions and imposed amplitudes. This study shows that extensive care and consideration are needed when modeling 3D VIVs using CFD, as the flow phenomena, and thereby solver requirements, rapidly change for different scenarios. [ABSTRACT FROM AUTHOR]

The full Navier–Stokes equations are used to study the linear stability of plane Poiseuille flow in a channel with the lower wall corrugated along the flow, due to which the flow has two velocity components. A generalized eigenvalue problem is solved numerically. Three types of perturbations are considered: plane periodic (zero Floquet parameter), plane doubly periodic (finite values of the Floquet parameter), and spatial perturbations. Neutral curves are analyzed in a wide range of the corrugation parameter and Reynolds number. It is found that the critical Reynolds number above which time-growing perturbations appear depends in a complex way on the dimensionless amplitude and period of corrugation. It is shown that in the case of flow in a channel with corrugated wall, three-dimensional perturbations are usually more dangerous. The exception is the small amplitude of corrugation at which plane perturbations are more dangerous. [ABSTRACT FROM AUTHOR]

We study Lyapunov exponents of tracers in compressible homogeneous isotropic turbulence at different turbulent Mach numbers Mt and Taylor-scale Reynolds numbers R e λ . We demonstrate that statistics of finite-time Lyapunov exponents have the same form as that in incompressible flow due to density-velocity coupling. The modulus of the smallest Lyapunov exponent λ3 provides the principal Lyapunov exponent of the time-reversed flow, which is usually wrong in a compressible flow. This exponent, along with the principal Lyapunov exponent λ1, determines all the exponents due to vanishing of the sum of all Lyapunov exponents. Numerical results by high-order schemes for solving the Navier–Stokes equations and tracking particles verify these theoretical predictions. We found that (1) the largest normalized Lyapunov exponent λ 1 τ η , where τ η is the Kolmogorov timescale, is a decreasing function of Mt. Its dependence on R e λ is weak when the driving force is solenoidal, while it is an increasing function of R e λ when the solenoidal and compressible forces are comparable. Similar facts hold for | λ 3 | , in contrast to well-studied short-correlated model; (2) the ratio of the first two Lyapunov exponents λ 1 / λ 2 decreases with R e λ and is virtually independent of Mt for M t ≤ 1 in the case of solenoidal force but decreases as Mt increases when solenoidal and compressible forces are comparable; (3) for purely solenoidal force, λ 1 : λ 2 : λ 3 ≈ 4 : 1 : − 5 for R e λ > 80 , which is consistent with incompressible turbulence studies; and (4) the ratio of dilation-to-vorticity is a more suitable parameter to characterize Lyapunov exponents than Mt. [ABSTRACT FROM AUTHOR]

Owls' silent flight is commonly attributed to their special wing morphology combined with wingbeat kinematics. One of these special morphological features is known as the leading-edge serrations: rigid miniature hook-like patterns found at the primaries of the wings' leading-edge. It has been hypothesized that leading-edge serrations function as a passive flow control mechanism, impacting the aerodynamic performance. To elucidate the flow physics associated with owls' leading-edge serrations, we investigate the flow-field characteristic around a barn owl wing with serrated leading-edge geometry positioned at 20° angle of attack for a Reynolds number of 40 000. We use direct numerical simulations, where the incompressible Navier–Stokes equations are solved on a Cartesian grid with sufficient resolution to resolve all the relevant flow scales, while the wing is represented using an immersed boundary method. We have simulated two wing planforms: with serrations and without. Our findings suggest that the serrations improve suction surface flow by promoting sustained flow reattachment via streamwise vorticity generation at the shear layer, prompting weaker reverse flow, thus augmenting stall resistance. Aerodynamic performance is negatively impacted due to the shear layer passing through the serration array, which results in altered surface pressure distribution over the upper surface. In addition, we found that serrations increase turbulence level in the downstream flow. Turbulent momentum transfer near the trailing edge increased due to the presence of serrations upstream the flow, which also influences the mechanisms associated with separation vortex formation and its subsequent development over the upper surface of the wing. [ABSTRACT FROM AUTHOR]