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We present a proof-of-concept for the adaptive mesh refinement method applied to atmospheric boundary-layer simulations. Such a method may form an attractive alternative to static grids for studies on atmospheric flows that have a high degree of scale separation in space and/or time. Examples include the diurnal cycle and a convective boundary layer capped by a strong inversion. For such cases, large-eddy simulations using regular grids often have to rely on a subgrid-scale closure for the most challenging regions in the spatial and/or temporal domain. Here we analyze a flow configuration that describes the growth and subsequent decay of a convective boundary layer using direct numerical simulation (DNS). We validate the obtained results and benchmark the performance of the adaptive solver against two runs using fixed regular grids. It appears that the adaptive-mesh algorithm is able to coarsen and refine the grid dynamically whilst maintaining an accurate solution. In particular, during the initial growth of the convective boundary layer a high resolution is required compared to the subsequent stage of decaying turbulence. More specifically, the number of grid cells varies by two orders of magnitude over the course of the simulation. For this specific DNS case, the adaptive solver was not yet more efficient than the more traditional solver that is dedicated to these types of flows. However, the overall analysis shows that the method has a clear potential for numerical investigations of the most challenging atmospheric cases.

The interior oceans of several icy moons are considered as affected by rotation. Observations suggest a larger heat transport around the poles than at the equator. Rotating Rayleigh‐Bénard convection (RRBC) in planar configuration can show an enhanced heat transport compared to the non‐rotating case within this “rotation‐affected” regime. We investigate the potential for such a (polar) heat transport enhancement in these subglacial oceans by direct numerical simulations of RRBC in spherical geometry for Ra = 106 and 0.7 ≤ Pr ≤ 4.38. We find an enhancement up to 28% in the “polar tangent cylinder,” which is globally compensated by a reduced heat transport at low latitudes. As a result, the polar heat transport can exceed the equatorial by up to 50%. The enhancement is mostly insensitive to different radial gravity profiles, but decreases for thinner shells. In general, polar heat transport and its enhancement in spherical RRBC follow the same principles as in planar RRBC. Plain Language Summary The icy moons of Jupiter and Saturn like for example, Europa, Titan, or Enceladus are believed to have a water ocean beneath their ice crust. Several of them show phenomena in their polar regions like active geysers or a thinner crust than at the equator, all of which might be related to a larger heat transport around the poles from the underlying ocean. We simulate the flow dynamics and currents in these subglacial ocean by high‐fidelity simulations, though still at less extreme parameters than in reality, to study the heat transport and provide a possible explanation of such a “polar heat transport enhancement.” We find that the heat transport around the poles can be up to 50% larger than around the equator, and that the believed properties of the icy moons and their oceans would allow polar heat transport enhancement. Therefore, our results may help to improve the understanding of ocean currents and latitudinal variations in the oceanic heat transport and crustal thickness on icy moons. Key Points The polar heat transport in spherical rotating Rayleigh‐Bénard convection experiences an enhancement by rotation The influence of rotation differs at low latitudes: the heat flux is reduced and compensates the polar enhancement on the global average In combination, this strengthens the latitudinal variation between polar and equatorial heat flux for Prandtl numbers larger than unity

Cloud microphysical processes occur at the smallest end of scales among cloud-related processes and thus must be parameterized not only in large-scale global circulation models (GCMs) but also in various higher-resolution limited-area models such as cloud-resolving models (CRMs) and large-eddy simulation (LES) models. Instead of giving a comprehensive review of existing microphysical parameterizations that have been developed over the years, this study concentrates purposely on several topics that we believe are understudied but hold great potential for further advancing bulk microphysics parameterizations: multi-moment bulk microphysics parameterizations and the role of the spectral shape of hydrometeor size distributions; discrete vs “continuous” representation of hydrometeor types; turbulence-microphysics interactions including turbulent entrainment-mixing processes and stochastic condensation; theoretical foundations for the mathematical expressions used to describe hydrometeor size distributions and hydrometeor morphology; and approaches for developing bulk microphysics parameterizations. Also presented are the spectral bin scheme and particle-based scheme (especially, super-droplet method) for representing explicit microphysics. Their advantages and disadvantages are elucidated for constructing cloud models with detailed microphysics that are essential to developing processes understanding and bulk microphysics parameterizations. Particle-resolved direct numerical simulation (DNS) models are described as an emerging technique to investigate turbulence-microphysics interactions at the most fundamental level by tracking individual particles and resolving the smallest turbulent eddies in turbulent clouds. Outstanding challenges and future research directions are explored as well.

Studying the airflows and the resultant aerodynamic pressure/force in the pharyngeal airway is critical for understanding the pathophysiology of snoring and sleep apnea. In this work, an experiment-driven computational study was conducted to examine the aerodynamics in human pharyngeal airway. An anatomically accurate pharynx model associated with different uvula kinematics was reconstructed from human magnetic resonance image (MRI) and high-speed photography. An immersed-boundary-method (IBM)-based direct numerical simulation (DNS) flow solver was adopted to simulate the corresponding unsteady flows in all their complexity. Analyses were performed on vortex dynamics and pressure fluctuations in the pharyngeal airway and force oscillations on the pharyngeal wall under the influence of varying airway obstructions, uvula flapping mode, and uvula flapping frequencies. It was found the vortex formation, aerodynamic pressure, and pharyngeal wall force were significantly affected by the width of the pharyngeal airway. By contrast, the influences from the uvula flapping mode were insignificant when other parameters were similar. Fast Fourier transformation (FFT) and continuous wavelet transform (CWT) analysis of the pressure time history revealed the existence of higher order harmonics of base frequency with significant pressure amplitudes and energy intensities. It was also found the airway pressure and pharyngeal wall force oscillate more dramatically at higher uvula flapping frequencies, which tends to promote the collapse of pharyngeal wall and initiates sleep apnea.

It has long been known that under the right circumstances, inertial particles (such as sand, dust, pollen, or water droplets) settling through the atmospheric boundary layer can experience a net enhancement in their average settling velocity due to their inertia. Since this enhancement arises due to their interactions with the surrounding turbulence it must be modelled at coarse scales. Models for the enhanced settling velocity (or deposition) of the dispersed phase that find practical use in mesoscale weather models are often ad hoc or are built on phenomenological closure assumptions, meaning that the general deposition rate of particles is a key uncertainty in these models. Instead of taking a phenomenological approach, exact phase-space methods can be used to model the physical mechanisms responsible for the enhanced settling, and these individual mechanisms can be estimated or modelled to build a more general parameterization of the enhanced settling of inertial particles. In this work, we use direct numerical simulations (DNS) and phase-space methods as tools to evaluate the efficacy of phenomenological modeling approaches for the enhanced settling velocity of inertial particles for particles with varying friction Stokes numbers and settling velocity parameters. We use the DNS data to estimate profiles of a drift–diffusion based parameterization of the fluid velocity sampled by the particles, which is key for determining the settling velocity behaviour of particles with low to moderate Stokes number. We find that by increasing the settling velocity parameter at moderate friction Stokes number, the magnitude of preferential sweeping is modified, and this behaviour is explained by the drift component of the aforementioned parameterization. These profiles indicate that that when eddy-diffusivity-like closures are used to represent turbulent transport, empirical corrections used in phenomenological models may be potentially compensating for their incompleteness. Finally, we discuss opportunities for reinterpreting phenomenological approaches for use in coarse-scale weather models in terms of the exact phase-space approach.

Purpose Fluid flows in pipes whose cross-sectional area are increasing in the stream-wise direction are prone to separation of the recirculation region. This paper aims to investigate such fluid flow in expansion pipe systems using direct numerical simulations. The flow in circular diverging pipes with different diverging half angles, namely, 45, 26, 14, 7.2 and 4.7 degrees, are considered. The flow is fed by a fully developed laminar parabolic velocity profile at its inlet and is connected to a long straight circular pipe at its downstream to characterise recirculation zone and skin friction coefficient in the laminar regime. The flow is considered linearly stable for Reynolds numbers sufficiently below natural transition. A perturbation is added to the inlet fully developed laminar velocity profile to test the flow response to finite amplitude disturbances and to characterise sub-critical transition. Design/methodology/approach Direct numerical simulations of the Navier–Stokes equations have been solved using a spectral element method. Findings It is found that the onset of disordered motion and the dynamics of the localised turbulence patch are controlled by the Reynolds number, the perturbation amplitude and the half angle of the pipe. Originality/value The authors clarify different stages of flow behaviour under the finite amplitude perturbations and shed more light to flow physics such as existence of Kelvin–Helmholtz instabilities as well as mechanism of turbulent puff shedding in diverging pipe flows.

This study examines the aerodynamic behavior of the symmetric National Advisory Committee for Aeronautics (NACA) 0018 airfoil at a chord-based Reynolds number of 30,000, utilizing Direct Numerical Simulation (DNS) as the primary tool, supplemented by selected Large Eddy Simulation (LES) cases. DNS covers a full sweep of angles of attack from 0° to 10°, while LES was conducted at α = 4° and 8° to verify consistency between the two approaches. Despite the limited LES dataset-restricted by computational cost, the results showed nearly identical aerodynamic loads and surface pressure distributions, thereby reinforcing confidence in the DNS database. To further assess robustness, two additional DNS cases were performed for the NACA 0012 airfoil at α = 3° and 6° under identical grid resolution and flow conditions. These simulations demonstrated very good agreement with published experiments, extending validation beyond a single geometry. In both DNS and LES, clean inflow conditions were imposed, ensuring that the domain inlet did not introduce resolved turbulence. The results captured laminar boundary-layer separation on the suction side without reattachment, producing bluff-body-like wake dynamics dominated by periodic vortex shedding, while classical Kelvin–Helmholtz instabilities were not significant. Computed aerodynamic coefficients, including time-averaged lift, showed good agreement with measurements. The novelty of this work lies in providing the first DNS dataset for a thick symmetric airfoil (NACA 0018) at Re = 30,000, further validated through NACA 0012 comparisons. The findings establish a benchmark for future investigations of unsteady aerodynamics, particularly dynamic stall in pitching motions, and are directly relevant for the design of low-Reynolds-number devices such as micro air vehicles, unmanned aerial vehicles, and vertical-axis wind turbines.

Vortex asymmetry, dynamics, and breakdown in the wake of an axisymmetric cone have been investigated using direct numerical simulation for a wide range of angles of attack. The immersed boundary method is employed with pseudo-body-conformal grids to ensure the accuracy and resolution requirements near the body while being able to account for topology changes near the cone tip. The separated shear layer originated from the surface of the cone swirls into a strong primary vortex. Beneath the primary vortex on the leeward surface of the cone, a well-coherent counter-rotating secondary vorticity is generated. Beyond a particular threshold of swirl, the attached vortex structure breaks and the flow undergoes a chaotic transformation. Depending on the angle of attack, the flow shows different levels of instabilities and the topology of the vortices changes in the wake. In addition to swirl, spiral vortices that revolve around the primary vortex core often merge with the core and play a role in developing the double-helix mode of instability at the onset of the vortex breakdown. At the angle of attack of 60∘, the time-averaged side force becomes asymmetric at the stage where the drag overcomes the lift. At the angle of attack of 75∘, the primary vortex governs the flow asymmetry and the side force. Flow asymmetry is independent of the vortex breakdown. Finally, the contribution of primary vortices to the total forces is quantified using a force partitioning method.

Wind turbines (WTs) face a high risk of failure due to environmental factors like erosion, particularly in high-precipitation areas and offshore scenarios. In this paper we introduce a novel computational tool for the fast prediction of rain erosion damage on WT blades that is useful in operation and maintenance decision making tasks. The approach is as follows: Pseudo-Direct Numerical Simulation (P-DNS) simulations of the droplet-laden flow around the blade section profile are employed to build a high-fidelity data set of impact statistics for potential operating conditions. Using this database as training data, a machine learning-based surrogate model provides the feature of the impact pattern over the 2-D section for given wind and rain conditions. With this information, a fatigue-based model estimates the remaining lifetime and erosion damage for both homogeneous and coating-substrate blade materials. This prediction is done by quantifying the accumulated droplet impact energy and evaluating operative conditions over time periods for which the weather at the installation site is known. In this work, we describe the modules that compose the prediction method, namely the database creation, the training of the surrogate model and their coupling to build the prediction tool. Then, the method is applied to predict the remaining lifetime and erosion damage to the blade sections of a reference WT. To evaluate the reliability of the tool, several site locations (offshore, coastal, and inland), the coating material and the coating thickness of the blade are investigated. In few minutes we are able to estimate erosion after many years of operation. The results are in good agreement with field observations, showing the promise of the new rain erosion prediction approach.

This study investigates turbulent particle-laden channel flows using direct numerical simulations employing the Eulerian-Lagrangian method. A two-way coupling approach is adopted to explore the mutual interaction between particles and fluid flow. The considered cases include flow with particle Stokes number varying from St = 2 up to St = 100 while maintaining a constant Reynolds number of Re τ = 180 across all cases. A novel vortex identification method, Liutex (Rortex), is employed to assess its efficacy in capturing near-wall turbulent coherent structures and their interactions with particles. The Liutex method provides valuable information on vortex strength and vectors at each location, enabling a detailed examination of the complex interaction between fluid and particulate phases. As widely acknowledged, the interplay between clockwise and counterclockwise vortices in the near-wall region gives rise to low-speed streaks along the wall. These low-speed streaks serve as preferential zones for particle concentration, depending upon the particle Stokes number. It is shown that the Liutex method can capture these vortices and identify the location of low-speed streaks. Additionally, it is observed that the particle Stokes number (size) significantly affects both the strength of these vortices and the streaky structure exhibited by particles. Furthermore, a quantitative analysis of particle behavior in the near-wall region and the formation of elongated particle lines was carried out. This involved examining the average fluid streamwise velocity fluctuations at particle locations, average particle concentration, and the normal velocity of particles for each set of particle Stokes numbers. The investigation reveals the intricate interplay between particles and near-wall structures and the significant influence of particles Stokes number. This study contributes to a deeper understanding of turbulent particle-laden channel flow dynamics.