Flow Structure Research Articles

Numerical assessment of incoming atmospheric boundary layer effects generated by various approaches on ship airwake turbulence characteristics

The unsteady and extensively separated turbulent airwake flows around a ship’s deck significantly impact helicopter pilot performance during shipboard operations. Hence, incorporating realistic, unsteady incoming mainstream conditions such as the atmospheric boundary layer (ABL), is crucial in numerical simulations of the ship airwake. This study employed delayed detached eddy simulations (DDES) to evaluate the effect of steady and unsteady ABL modeling methods on the turbulent flow characteristics of the ship airwake. A 1:12.5 scaled Simple Frigate Shape 1 (SFS1) ship model was utilized in the numerical simulations, and three distinct ABL profiles (one steady and two unsteady) were simulated at two different wind direction angles (HW and RW30). Synthetic and turbulator modeling methods were utilized to generate the unsteady ABL profiles. Time-averaged and instantaneous flow fields calculated by various ABL profiles were compared with experimental data at multiple points within the ship airwake. The numerical findings indicated that while the time-averaged data remained unaffected by the ABL profile presence, the instantaneous flow quantities experienced significant alterations, especially under the red wind condition. The introduction of an unsteady ABL profile was shown to augment the unsteadiness and fluctuation of the flow field within the ship airwake compared to the steady ABL scenario. Cross-correlation analyses between the velocity fields computed using steady and unsteady ABL profiles unveiled distinct patterns and behaviors of flow structures over the deck. Moreover, evaluation of the velocity and pressure fields of unsteady ABL across time/space domains revealed incremented turbulence and disturbances over the flight deck, intensifying with wind direction angle. The synthetic ABL modeling approach was found to deliver promising outcomes in comparison to the turbulator method, while requiring significantly lower computational resources.

An improved method to process the data of optical Fiber signals for identifying the instantaneous flow structure in a gas-solid fluidized bed

The instantaneous signals recorded with the optical fiber probe (OFP) directly reflect the hydrodynamic behaviors in fluidized beds. A new data-processing method is put forward to calculate the threshold voltage that is used to discern the bubble phase and emulsion phase. It is found that the threshold voltage is not only related to the flow pattern but also to the color of the used particles. Then the data-processing programs including the Fast Fourier filter have been developed for determining the flow characteristics such as the bubble/particle velocity, bubble/agglomerations chord length, etc. By comparing them with other different methods, the suitability of the proposed method for quantifying the hydrodynamic behaviors in bubble/turbulent fluidized beds is verified. Finally, this method is employed to analyze the characteristics of bubbles, agglomerations and particles in a gas-solid fluidized bed. It is found that the hydrodynamic behaviors of bubbling beds and turbulent beds have significant differences.

Toward flow forces acting on a step-pool unit

The flow forces on step-pool units are important to understand the physical processes and flow resistance partitioning in step-pool channels, and build the basis for better prediction of channel evolution and more advanced design of artificial step-pool system. However, the flow forces acting on step-pool units are understudied and poorly understood. To fill this knowledge gap, we applied the approach combining physical experiment and computational fluid dynamics simulation to a step-pool unit made of natural grains at six flow conditions. The topography of the step-pool unit was split into topography components (TCs) covering the entire unit length with the same width. The flow forces from both pressure and shear stress in XYZ directions were examined for the TCs. The results illustrate significant transverse variability of the flow forces from both the shear stress and pressure at all the three directions. The flow forces in both X and Y directions are closely related to the flow structures and morphology in the unit. The ratios between skin and form drag have large variations at low flows while show a relatively limited range of 0.05–0.1 at high flows, suggesting a small proportion occupied by the skin resistance in the total flow resistance in the step-pool channel. The drag and lift coefficient generally increase with discharge and the drag coefficient of the unit is around 0.3 at high flows, which can be used in evaluating the stability of the step-pool units in a sequence.

Turbulent flow structure around a single submerged angled spur dike under ice cover

Abstract This experimental study examines the velocity fields around the single submerged spur dike in a large-scale flume under three flow conditions: open channel, smooth ice-covered, and rough ice-covered. The effects of dike orientation were investigated for alignment angles of 90°, 120°, and 135°. Instantaneous three-dimensional velocity components were recorded using an acoustic Doppler velocimeter. Results show that the spur dikes generate distinct transverse flow regions in the streamwise, lateral, and vertical directions. Alignment angles greater than 90° reduced streamwise velocity near the dikes, while the frontal surface of the dike tip exhibited increased velocity magnitudes. Downstream, significant variations in Reynolds shear stress were observed, driven by flow separation and the formation of a recirculation wake zone. Quadrant analysis revealed that under ice-covered conditions, turbulent interactions near the dike tip were dominated by ejection and sweep events, whereas sweep events were more prevalent in open channel flows, influencing overall flow dynamics.

Force decomposition of vortex–plate interaction via dynamic mode decomposition

This study investigates the dynamics of two tandem flexible plates flapping in a uniform flow, focusing on how variations in their phase angle affect thrust generation. Considerable emphasis is given to the interactions between the wakes of the two plates and their subsequent effects on the performance of the rear plate. By applying Dynamic Mode Decomposition (DMD) alongside vortex dynamics analysis, the force exerted on the rear plate is quantitatively assessed in relation to the surrounding flow structures. The results show a decline in the rear plate’s thrust when it enters a “locked vortex” state, primarily due to diminished leading-edge suction. The analysis also reveals a strong correlation between the dominant mode f∗=1 and the time-averaged forces, which aligns with the observed vortex–plate interaction patterns. Furthermore, the study demonstrates that the time-averaged horizontal force on the rear plate is largely influenced by low-frequency dominant modes, underscoring the importance of these interactions for optimizing propulsion systems.

The minimal flow unit and origin of two-dimensional elasto-inertial turbulence

The research on elasto-inertial turbulence (EIT), a new type of turbulent flow, has reached the stage of identifying the minimal flow unit (MFU). On this issue, direct numerical simulations of FENE-P fluid flow in two-dimensional channels with variable sizes are conducted in this study. We demonstrate with the increase of channel length that the simulated flow experiences several different flow patterns, and there exists an MFU for EIT to be self-sustained. At Weissenberg number ( $Wi$ ) higher than the one required to excite EIT, when the channel length is relatively small, a steady arrowhead regime (SAR) flow structure and a laminar-like friction coefficient is achieved. However, as the channel length increases, the flow can fully develop into EIT characterized with high flow drag. Close to the size of the MFU, the simulated flow behaves intermittently between the SAR state with low drag and EIT state with high drag. The flow falling back to ‘laminar flow’ is caused by the insufficient channel size below the MFU. Furthermore, we give the relationship between the value of the MFU and the effective $Wi$ , and explain its physical reasons. Moreover, the intermittent flow regime obtained based on the MFU gives us an opportunity to look into the origin and exciting process of EIT. Through capturing the onset process of EIT, we observed that EIT originates from the sheet-like extension structure located near the wall, which is maybe related to the wall mode rather than the centre mode. The fracture and regeneration of this sheet-like structure is the key mechanism for the self-sustaining of EIT.

Friction drag reduction of Taylor–Couette flow over air-filled microgrooves

Reducing drag under high turbulence is a critical but challenging issue that has engendered great concern. This study utilizes hydrophilic tips in superhydrophobic (SHP) grooves to enhance the stability of plastron, which results in a considerable drag reduction ( $DR$ ) up to 62 %, at Reynolds number ( $Re$ ) reaching $2.79 \times 10^{4}$ . The effect of the spacing width $w$ of the microgrooves on both $DR$ and flow structures is investigated. Experimental results demonstrate that $DR$ increases as either microgroove spacing $w$ or $Re$ increases. The velocity fields obtained using particle image velocimetry indicate that the air-filled SHP grooves induce a considerable wall slip. This slip significantly weakens the intensity of Taylor rolls, reduces local momentum transport, and consequently lowers drag. This phenomenon becomes more pronounced with increasing $w$ . Furthermore, to quantify the multiscale relationship between global response and geometrical as well as driving parameters, $DR\sim (w, \phi _s, Re)$ , a theoretical model is established based on angular momentum defect theory and magnitude estimate. It is demonstrated that a decrease in the surface solid fraction can reduce wall shear, and an increase in the groove width can weaken turbulence kinetic energy production, rendering enhanced slip and drag reduction. This research has implications for designing and optimizing turbulent-drag-reducing surfaces in various engineering applications, such as transportation and marine engineering.

On the origin of counter-gradient transport in turbulent scalar flux: physics interpretation and adjoint data assimilation

The present study offers a twofold contribution on counter-gradient transport (CGT) of turbulent scalar flux. First, by examining turbulent scalar mixing through synchronized particle image velocimetry and planar laser-induced fluorescence on an inclined jet in cross-flow, we clarify the previously unexplained phenomenon of CGT, revealing key flow structures, their spatial distribution and modelling implications. Statistical analysis identifies two distinct CGT regions: local cross-gradient transport in the windward shear layer and non-local effects near the wall after injection. These behaviours are driven by specific flow structures, namely Kelvin–Helmholtz vortices (local) and wake vortices (non-local), suggesting that scalar flux can be decomposed into a gradient-type term for gradient diffusion and a term for large-eddy stirring. Second, we propose a new approach for reconstruction of turbulent mean flow and scalar fields using continuous adjoint data assimilation (DA). By rectifying model-form errors through anisotropic correction under observational constraints, our DA model minimizes discrepancies between experimental measurements and numerical predictions. As expected, the introduced forcing term effectively identifies regions where traditional models fall short, particularly in the jet centreline and near-wall regions, thereby enhancing the accuracy of the mean scalar field. These enhancements occur not only within the observation region but also in unseen regions, underscoring present DA approach's reliability and practicality for reproducing mean flow behaviours from limited data. These findings lay a solid foundation for adjoint-based model-consistent data-driven methods, offering promising potential for accurately predicting complex flow scenarios like film cooling.

Three-dimensional simulation of film boiling on a horizontal surface with magnetic field

This study conducts a numerical investigation into the three-dimensional film boiling of liquid under the influence of external magnetic fields. The numerical method incorporates a sharp phase-change model based on the volume-of-fluid approach to track the liquid–vapour interface. Additionally, a consistent and conservative scheme is employed to calculate the induced current densities and electromagnetic forces. We investigate the magnetohydrodynamic effects on film boiling, particularly examining the pattern transition of the vapour bubble and the evolution of heat transfer characteristics, exposed to either a vertical or horizontal magnetic field. In single-mode scenarios, film boiling under a vertical magnetic field displays an isotropic flow structure, forming a columnar vapour jet at higher magnetic field intensities. In contrast, horizontal magnetic fields result in anisotropic flow, creating a two-dimensional vapour sheet as the magnetic strength increases. In multi-mode scenarios, the patterns observed in single-mode film boiling persist, with the interaction of vapour bubbles introducing additional complexity to the magnetohydrodynamic flow. More importantly, our comprehensive analysis reveals how and why distinct boiling effects are generated by various orientations of magnetic fields, which induce directional electromagnetic forces to suppress flow vortices within the cross-sectional plane.

Ecosystem Structure and Function in the Sea Area of Zhongjieshan Islands Based on Ecopath Model

Based on the field survey and reference data of the sea area of the Zhongjieshan Islands from 2021 to 2022, the Ecopath model was used to analyze the energy flow structure of the marine ecosystem of the sea area of the Zhongjieshan Islands; the energy structure of the marine ecosystem was divided into 21 functional groups, and its nutrient structure, energy flow, and total system characteristics were analyzed. The results show that the credibility of the model is 0.414, which is at a medium level. The trophic level of each functional group of the ecosystem in the sea area of Zhongjieshan Islands was 1–3.48, the energy flow structure of the system was mainly concentrated in the first five grades, and the trophic level was relatively simple, with the average energy transfer efficiency of the system being 8.11%, the energy flow range being 2.81–13.04%, the energy transfer efficiency of the primary producers of the system being 7.25%, and the energy conversion efficiency of the system debris being 9.12%. The total system throughput was 2125.96 t·km−2; The analysis of the overall characteristics of the ecosystem showed that the system connectance index and the system omnivory index were 0.45 and 0.24, respectively, while the Finn’s cycling index was 8.24, the Finn’s mean path length of the system was 2.72, and the total primary production/total respiration was 1.71. In this study, the marine ecosystem model of the sea area of the Zhongjieshan Islands was studied to understand the trophic structure and ecosystem status of the sea area, which is conducive to the sustainable utilization and scientific management of fishery resources in the sea area.

Flow structure of radiatively driven convection in inertial and rotating frames under steady and periodic radiative forcing

Flow structure of radiatively driven convection in inertial and rotating frames under steady and periodic radiative forcing

Efficient ROUND schemes on non-uniform grids applied to discontinuous Galerkin schemes with Godunov-type finite volume sub-cell limiting

Developing accurate, efficient and robust shock-capturing schemes on non-uniform grids remains challenging particularly when facing strong non-uniformity. Thus, we extend the unified normalised-variable diagram (UND) initially proposed by Deng (2023) [30] on uniform grids to non-uniform grids, and propose essentially non-oscillatory (ENO) and high-resolution regions in non-uniform grid UND. Based on the proposed UND, we formulate a high-resolution shock-capturing scheme termed ROUND (Reconstruction Operators on Unified-Normalise-variable Diagram) on non-uniform meshes. Unlike classic WENO (Weighted Essentially Non-Oscillatory) schemes applied to non-uniform grids, the ROUND scheme avoids the expensive calculation of smoothness indicators. The ROUND scheme is first applied to solve the scalar convection equation. The results reveal that the ROUND scheme significantly improves the numerical resolution and preserves the structure of the passively convected scalar compared to the TVD (Total Variation Diminishing) limiters. For capturing discontinuous solutions, the proposed ROUND scheme on non-uniform meshes surpasses the performance of the 5th-order WENO-JS scheme. The ROUND scheme is then integrated into discontinuous Galerkin (DG) with the FV subcell limiting method to enhance the numerical resolution at the subcell level while adhering to the discrete conservation law. The compactness and simplicity of the ROUND scheme on non-uniform grids are compatible with the DG method, known for its features such as compactness and flexibility of hp-refinement. The resulting DG method, utilising finite volume ROUND subcell limiting, is denoted as the DG/FV-ROUND scheme. To assess the accuracy and robustness of the DG/FV-ROUND scheme, we simulate high-speed compressible flows characterized by strong shock waves and small-scale flow structures. Comparative studies show the improved numerical resolution achieved by DG/FV-ROUND. Thus, this research demonstrates the efficacy and robustness of the ROUND scheme on non-uniform grids and affirms that incorporating high-resolution ROUND as subcell shock-capturing schemes can enhance the resolution of DG/FV methods.

Impact of freestream turbulence integral length scale on wind farm flows and power generation

The impact of freestream turbulence integral length scale on wind farm flow and power production is investigated by conducting Large Eddy Simulations on wind farms with two spacings, Sx=8R and Sx=12R (turbine radius R). The integral length scale of inflow turbulence Lu is varied, Lu∈[3.2R,12.0R], while maintaining identical turbulence intensity and velocity. Shorter integral length scales lead to a faster near wake breakdown and improved wake recovery in the wake of the first turbine, causing substantial increases in the second turbine power output; 42% and 18% for the two spacings. Over the first four turbines, total power output increases by 8.6% and 6.0% respectively. Spectra, cross-correlations and entrainment scales are also examined and show that the first turbine breaks down inflow scales and wake-generated turbulence dominates the inflow to the second turbine. Further into the turbine row, dominant flow structures and entrainment scales are associated with both wake turbulence and larger wind farm-generated structures matching the turbine spacing. These results show that the freestream turbulence integral length scale has a significant impact on wind farm flows and power generation, mainly by impacting the development of wakes in the farm entrance.

Impact of spanwise rotation on flow separation and recovery behind a bulge in channel flows

Direct numerical simulations of spanwise-rotating turbulent channel flow with a parabolic bump on the bottom wall are employed to investigate the effects of rotation on flow separation. Four rotation rates, $Ro_b := 2\varOmega H/U_b = \pm 0.42$ , $\pm$ 1.0, are compared with the non-rotating scenario. The mild adverse pressure gradient induced by the lee side of the bump allows for a variable pressure-induced separation. The separation region is reduced (increased) when the bump is on the anti-cyclonic (cyclonic) side of the channel, compared with the non-rotating separation. The total drag is reduced in all rotating cases. Through several mechanisms, rotation alters the onset of separation, reattachment and wake recovery. The mean momentum deficit is found to be the key. A physical interpretation of the ratio between the system rotation and mean shear vorticity, $S:=\varOmega /\varOmega _s$ , provides the mechanisms regarding stability thresholds $S=-0.5$ and $-$ 1. The rotation effects are explained accordingly, with reference to the dynamics of several flow structures. For anti-cyclonic separation, particularly, the interaction between the Taylor–Görtler vortices and hairpin vortices of wall-bounded turbulence is proven to be responsible for the breakdown of the separating shear layer. A generalized argument is made regarding the essential role of near-wall deceleration and resultant ejection of enhanced hairpin vortices in destabilizing an anti-cyclonic flow. This mechanism is anticipated to have broad impacts on other applications in analogy to rotating shear flows, such as thermal convection and boundary layers over concave walls.

Trends in the development of border regions in the face of global challenges

Subject. This article discusses the importance of border regions in the economic development of the country in the context of international sanctions. Objectives. The article aims to identify trends and areas of development of border regions in the context of global challenges. Methods. For the study, we used the general scientific research methods. Results. Based on the analysis of the economic indicators of the regions of the Russian-Kazakh border-zone, the article offers the author-developed recommendations for the development of the structure of export-import flows in the regions under study. Conclusions. The border regions of the Russian Federation are experiencing strong sanctions pressure. This has a negative impact on the weakest regions in economic terms, while more developed and economically stable regions adapt more effectively to negative trends.

The Evolution of Wisteria Vein Mosaic Virus: A Case Study Approach to Track the Emergence of New Potyvirus Threats.

Wisteria vein mosaic virus (WVMV, Potyvirus wisteriae), a virus belonging to the genus Potyvirus, is responsible for Wisteria vein mosaic disease (WMD), a severe disease that affects Wisteria, a genus of garden plants acclaimed worldwide. Although probably originating in the Far East, WVMV infection was first reported in the US, and subsequently in numerous countries. Following the first molecular detection of an Italian isolate, WVMV Bari, its full-length genome was achieved using NGS barcoding technology. A PhyML phylogenetic analysis, supported by clustering algorithm validation, identified a clear separation between two phylogroups. One major clade comprised WVMV strains isolated from Wisteria spp. A second clade grouped three highly divergent strains, at the borderline species threshold, all found in non-wisteria hosts. Relying on a Relative Time Dated Tips (RTDT) molecular clock, the first emergence of WVMV clades has been traced back to around the 17th century. A network inference analysis confirmed the sharp separation between the two host-related phylogroups, also highlighting the presence of potential intermediate variants. Inter-population genetic parameters revealed a very high genetic differentiation in both populations, which was made reliable by statistically significant permutation tests. The migrant number (Nm) and fixation index (FST) evidenced a restricted gene flow and strong population structures. According to the dN/dS ratio and negative neutrality tests, it was derived that purifying selection at the expense of non-silent variants is underway within WVMV populations. Targeting WVMV evolutionary traits, the present effort raised interesting questions about the underestimated potential of this culpably neglected species to spread in economically relevant crops. The main intention of our study is, therefore, to propose an evolution-based analysis approach that serves as a case study to investigate how other potyviruses or newly emerging viruses may spread.

Structural and hydrodynamic controls on fluid travel time distributions across fracture networks

Fracture networks are preferential flow paths playing a critical role in a wide range of environmental and industrial problems. Their complex multiscale structure leads to broad distributions of fluid travel times, affecting many biogeochemical processes. Yet, the relationship between the fracture network structures, their hydrodynamic properties, and the resulting anomalous transport dynamics remains unclear. We use a large database of fracture network models to investigate the factors controlling fluid velocity and travel-time distributions across a wide range of networks, from synthetic to field-calibrated models, with aperture variability at both fracture and network scales. Analysis reveals that transport statistics have generic properties across investigated networks, including notably heavy-tailed travel time distributions. Networks of increasing complexity and heterogeneity lead to broader velocity distributions and more channeled velocity fields, where flow concentrates in a narrow channel web in the three-dimensional (3D) fracture structure. While heterogeneity in point-velocity statistics increases travel-time variability, channeling tends to reduce it. This counterintuitive phenomenon challenges current theories, which assume that long travel time power law exponents are determined solely by point-velocity statistics. By analyzing velocity and travel time statistics for different flow structures, we develop a coupled Continuous Time Random Walk framework capturing the unexpected control of the velocity field's spatial structure on anomalous transport in fracture networks. This leads to a unique class of random walk models capturing the respective roles of velocity heterogeneity and spatial structure on transport in networks. These findings open a prospective for characterizing, modeling, and predicting transport dynamics in complex networks, with potential applications to geological, biological, and engineered networks.

Convective Heat Transfer in Uniformly Accelerated and Decelerated Turbulent Pipe Flows

This study presents a detailed investigation of the temporal evolution of the Nusselt number (Nu) in uniformly accelerated and decelerated turbulent pipe flows under a constant heat flux using direct numerical simulations. The influence of different acceleration and deceleration rates on heat transfer is systematically studied, addressing a gap in the previous research. The simulations confirm several key experimental findings, including the presence of three distinct phases in the Nusselt number temporal response—delay, recovery, and quasi-steady phases—as well as the characteristics of thermal structures in unsteady pipe flow. In accelerated flows, the delay in the turbulence response to changes in velocity results in reduced heat transfer, with average Nu values up to 48% lower than those for steady-flow conditions at the same mean Reynolds number. Conversely, decelerated flows exhibit enhanced heat transfer, with average Nu exceeding steady values by up to 42% due to the onset of secondary instabilities that amplify turbulence. To characterize the Nu response across the full range of acceleration and deceleration rates, a new model based on a hyperbolic tangent function is proposed, which provides a more accurate description of the heat transfer response than previous models. The results suggest the potential to design unsteady periodic cycles, combining slow acceleration and rapid deceleration, to enhance heat transfer compared to steady flows.

Turbulent convection in rotating slender cells

Turbulent convection in the interiors of the Sun and the Earth occurs at high Rayleigh numbers $Ra$ , low Prandtl numbers $Pr$ , and different levels of rotation rates. To understand the combined effects better, we study rotating turbulent convection for $Pr = 0.021$ (for which some laboratory data corresponding to liquid metals are available), and varying Rossby numbers $Ro$ , using direct numerical simulations in a slender cylinder of aspect ratio 0.1; this confinement allows us to attain high enough Rayleigh numbers. We are motivated by the earlier finding in the absence of rotation that heat transport at high enough $Ra$ is similar between confined and extended domains. We make comparisons with higher aspect ratio data where possible. We study the effects of rotation on the global transport of heat and momentum as well as flow structures (a) for increasing rotation at a few fixed values of $Ra$ , and (b) for increasing $Ra$ (up to $10^{10}$ ) at the fixed, low Ekman number $1.45 \times 10^{-6}$ . We compare the results with those from unity $Pr$ simulations for the same range of $Ra$ and $Ro$ , and with the non-rotating case over the same range of $Ra$ and low $Pr$ . We find that the effects of rotation diminish with increasing $Ra$ . These results and comparison studies suggest that for high enough $Ra$ , rotation alters convective flows in a similar manner for small and large aspect ratios, so useful insights on the effects of high thermal forcing on convection can be obtained by considering slender domains.

Assessing the Effect of Coriolis Acceleration on the Coherent Structures in the Wake of a Wind Turbine using DMD

Abstract The present work aims to study the effect of veer, namely, the height-dependent lateral deflection of wind velocity due to Coriolis acceleration, on the coherent structures in the wake of the NREL 5-MW reference wind turbine using the sparsity-promoting dynamic mode decomposition (SP-DMD) method for the detection of dynamically-relevant flow structures. Large eddy simulation of the flow impacting the wind turbine is carried out by solving the filtered Navier-Stokes equations for incompressible flows. The effects of both tower and nacelle are included in the simulation using the immersed boundary method. Simulations are performed at Re ≈ 108, generating the inlet velocity profile by a precursor simulation of the atmospheric boundary layer subjected to Coriolis acceleration. The analysis of the DMD results allows to study the effects of the Coriolis acceleration on the most relevant dynamic modes in the turbine wake and to understand the basic mechanisms by which the wind veer significantly affects the wake recovery rate. Moreover, as a result of the SP-DMD methodology, the most relevant modes are extracted from the wake and a limited subset of relevant flow features that optimally approximates the original data sequence is identified. This small number of modes represent the kernel that can be employed for developing an accurate reduced order model of the wake.