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Abstract Shear layers and corresponding Kelvin Helmholtz‐type Coherent Structures (KHCS) can be generated by rivers discharging into laterally‐unconfined quiescent open water bodies (e.g., lakes). When the river discharge has a greater density than the receiving water due to temperature and/or suspended sediment, both the shear layers and KHCS will be influenced by the negative buoyancy of the plume and thus become highly three‐dimensional (3D). The present study uses a turbulence‐resolving Computational Fluid Dynamics model based on Large Eddy Simulation to simulate the nearshore flow fields of a hyperpycnal river plume entering an unconfined quiescent ambient. Shear layers are observed at both sides of the plume and their growth is suppressed by negative buoyancy arising from the greater density of the river plume. The plume‐ambient interface is tilted by the negative buoyancy and is transformed into a curved face. As a result, the shear layer is also tilted and shear‐induced vorticity progressively changes its direction from vertical near the water surface to transversal near the bottom. Tilted along with the shear layers, KHCS present unique 3D subsurface structures and create strongly mixed and curved “coherent structure regions” in transects. Quadrant analysis shows that the “Ejection” and “Sweep” events associated with KHCS dominate the local mass and momentum exchange between the plume and ambient water. At the plume‐ambient interface, the KHCS generate near‐periodic velocity fluctuations whose non‐dimensionalized frequency (Strouhal number) decreases with increasing local Richardson number.

Lagrangian techniques, such as the finite-time Lyapunov exponent (FTLE) and hyperbolic Lagrangian coherent structures, have become popular tools for analyzing unsteady fluid flows. These techniques identify regions where particles transported by a flow will converge to and diverge from over a finite-time interval, even in a divergence-free flow. Lagrangian analyses, however, are time consuming and computationally expensive, hence unsuitable for quickly assessing short-term material transport. A recently developed method called Objective Eulerian Coherent Structures (OECSs) (Serra and Haller in Chaos Interdiscip J Nonlinear Sci 26(5):053110, 2016) rigorously connected Eulerian quantities to short-term Lagrangian transport. This Eulerian method is faster and less expensive to compute than its Lagrangian counterparts, and needs only a single snapshot of a velocity field. Along the same line, here we define the instantaneous Lyapunov Exponent, the instantaneous counterpart of the FTLE, and connect the Taylor series expansion of the right Cauchy-Green deformation tensor to the infinitesimal integration time limit of the FTLE. We illustrate our results on geophysical fluid flows from numerical models as well as analytical flows, and demonstrate the efficacy of attracting and repelling instantaneous Lyapunov exponent structures in predicting short-term material transport.

The flow-inclined cylinder interaction is an application area in the industry (i.e., offshore wind turbines and pile-supported near-shore structures). Findings of recent studies have revealed the significance of eco-friendly coastal structures that needs the utilization of inclined cylinder. The primary purpose of this study was to better understand the influence of inclination on flow, turbulence, and bed shear stress character. To achieve this objective, a three-dimensional numerical code (the Reynolds-averaged Navier-Stokes model) was used. The numerical model was calibrated based on eleven velocity profiles obtained by point measurements data of the wake region of the inclined cylinder. The mean flow, turbulence, and secondary flow characteristics around the bodies were extensively investigated, particularly at points where experimental measurements are inapplicable with intrusive turbulence measurement devices. The findings of the study revealed that as the inclination of the cylinder increased, the coherent structures that largely control the flow dynamics in the wake zone became stable rather than cyclical. Specifically, it was determined that although vorticity couples underpinned the flow field behind the vertical cylinder, large-scale streamwise vortices replaced the visible coherent structures when the cylinders were inclined (LSCSVs). When the cylinder inclined 42 degrees, the reduction in amplification factor (τ0 / τ∞) over the bed was roughly fifty percent in terms of quantity. This finding shows that inclination is a streamlined form for a cylinder and may reduce the collapse risk due to scour.

We develop a novel approach to detect cloud‐subcloud coupling during the cloud life cycle and analyze a large eddy simulation of marine shallow cumulus based on the Barbados oceanographic and meteorological experiment campaign. Our results demonstrate how the activity of sub‐cloud coherent updrafts (SCUs) affect the evolution of shallow cloud properties during their life cycles, from triggering to development, and through to dissipation. Most clouds (∼80%)$(\\sim 80\\%)$are related to SCUs during their lifetime but not every SCU (∼20%${\\sim} 20\\%$for short‐lived ones) leads to cloud formation. The fastest growing SCUs in a relatively moist region are most likely to initiate clouds. The evolution of cloud base mass‐flux depends on cloud lifetime. Compared with short‐lived clouds, longer lived clouds have longer periods of development, even normalized by the full lifetime, and tend to increase their cloud base mass‐flux to a stronger maximum. This is consistent with the evolution of mass flux near the top of SCU, indicating that the development of clouds is closely related to the sub‐cloud activity. When the SCUs decay and detach from the lifting condensation level, the corresponding cloud base starts to rise, signifying the start of cloud dissipation, during which the cloud top lowers to approach the rising cloud base. Previous studies have described similar conceptual pieces of this relationship but here we provide a continuous framework to cover all the stages of cloud‐subcloud coupling. Our findings provide quantitative evidence to supplement the conceptual model of shallow cloud life cycle and is critical to improve the steady‐state assumption in parameterization. Plain Language Summary The coupling between shallow trade wind cumulus clouds and sub‐cloud processes is not well understood and remains one of the roadblocks to improving our climate modeling capability. As model resolution increases, the steady‐state assumption adopted in conventional convection schemes no longer holds and the knowledge about the evolution of clouds is necessary to improve the parameterization. Observations have provided evidence of sub‐cloud coherent updrafts being the roots of shallow clouds, but it is currently not practical to continuously detect how the shallow clouds evolve alongside the sub‐cloud coherent structures. Our study takes a step forward to demonstrate how the sub‐cloud coherent structures interact with the shallow clouds. It is shown that the evolution of sub‐cloud coherent structures plays an important role from cloud initiation, development and through to dissipation. The comprehensive physical picture of the whole cloud life cycle uncovered in this study provides useful insights to improve the representation of cloud evolution in convection parameterizations with an explicit treatment of the cloud life cycle in the future. Key Points The evolution of maritime shallow cloud properties during the life cycle is connected to the activity of sub‐cloud coherent structures Clouds are likely to be triggered by the humid faster growing sub‐cloud coherent structures and start to dissipate without their ongoing support The evolution of cloud base mass‐flux depends on cloud lifetime and is asymmetric around the middle of life for short and mid‐lived clouds

Large eddy simulations of a three-dimensional (3D) compressible parallel jet flow at Mach number of 0.9 and Reynolds number 2000 are carried out. Four subgrid-scale (SGS) models, namely, the standard Smagorinsky model (SM), the selective mixed scale model (SMSM), the coherent-structure Smagorinsky model (CSM) and the coherent-structure kinetic-energy model (CKM) are employed, respectively, and compared. The purpose of the study is to compare the SGS models and to find their suitability of predicting the flow transition in the potential core of the jet, and so as to provide a reference for selecting SGS models in simulating compressible jet flows, which is a kind of proto-type flow in fluid dynamics and aeroacoustics. A finite difference code with fourth-order spatial and very low storage third-order explicit Runge-Kutta temporal schemes is introduced and employed for calculation. The code, which was previously designed for simulating shock/boundary-layer interactions and had been widely validated in simulating a variety of compressible flows, is rewritten and changed into parallelized using the OpenMP protocol so that it can be run on memory-shared multi-core workstations. The computational domain size and the index of LES resolution quality are checked to validate the simulations. Detailed comparisons of the four SGS models are carried out. The results of averaged flow-field including the velocity profiles and the developments of shear-layer, the instantaneous vortical flows and the viscous dissipation, the predicted turbulence statistics and the balances of momentum equation are studied and compared. The results show that although the normalized developed velocity profiles are well predicted by the four SGS models, the length of the potential core and the development of the shear-layer reveal that the SM has excessive SGS viscosity and is therefore too dissipative to correctly predict the flow transition and shear-layer expansion. The model smears small vortical scales and lowers down the effective Reynolds number of the flow because of the over-predicted SGS viscosity and dissipation. The turbulence statistics and the balances of momentum equation have also confirmed the excessive dissipation of the SM. The CKM is also found to over-predict the SGS viscosity. Compared with these two models, the SMSM and the CSM have performed well in predicting both the averaged and the instantaneous flow-fields of the compressible jet. And they are localized models which are computationally efficient and easy for coding. Therefore, the SMSM and the CSM are recommended for the LES of the compressible Jet.

The genesis of both coherent structures and reactive flow control strategies is explored. Futuristic control systems that utilize mi-crosensors and microactuators together with artificial intelligence to target specific coherent structures in a transitional or turbulent flow are considered. Of possible interest to the readers of this journal is the concept of smart wings, to be briefly discussed early in the article.

The aortic valve is located at left ventricular outlet and is exposed to the highest pressure in the cardiovascular system. Problems associated with the valve leaflet movement can cause complications for the heart. Specifically, aortic stenosis (AS) arises when aortic leaflets do not efficiently open. In the present study, Lagrangian Coherent Structures (LCSs) were utilized by processing a variety of Computational Fluid Dynamics (CFD) models velocity vector data further to identify the characteristics of AS jets. Particularly, effective orifice areas (EOA) for different cases were accurately identified from unstable manifolds of finite time Lyapunov exponent (FTLE) fields. Calcified leaflets were modeled by setting the leaflet's Young modulus to 10 MPa and 20 MPa for moderately and severely calcified leaflets respectively while a healthy leaflet's Young modulus was assigned to be 2 MPa. Increase in calcification degree of the leaflet caused destruction of the vortex structures near the fibrosa layer of the leaflet indicating a malfunctioning for the movement mechanism of the leaflet. Furthermore, when we analyzed stable manifolds, we identified a blockage region at the flow upstream due to the stagnant blood here. Compared to a healthy case, for the calcified valve, this blockage region was enlarged, implying an increase in AS jet velocity and wall shear stress on leaflets. As a conclusion, results from the present study indicate that aortic leaflet malfunctioning could be accurately evaluated when LCS technique was employed by post processing velocity vector data from CFD. Such precise analysis is not possible using the Eulerian CFD approach or a Doppler echocardiography since these methods are based on only analyzing instantaneous flow quantities and they overlook fluid flow characteristics of highly unsteady flows.

Numerical simulation of nonstationary dissipative structures in 3D double-diffusive convection has been performed by using the previously derived system of complex Ginzburg-Landau type amplitude equations, valid in a neighborhood of Hopf bifurcation points. Simulation has shown that the state of spatiotemporal chaos develops in the system. It has the form of nonstationary structures that depend on the parameters of the system. The shape of structures does not depend on the initial conditions, and a limited number of spectral components participate in their formation.

Coherent structures are essential for the momentum exchange and turbulence production in wall-bounded turbulent flows. Diversified coherent structures have been observed in turbulent boundary layers, and hairpin-based vortices dominate most of the relevant literature. However, there is no consensus yet on the origin and forming mechanism of hairpin vortices. Herein, five cornerstones pertaining to the framework of hairpin-based coherent structures are reviewed, and three different hairpin generation mechanisms are clarified. Next, the time-resolved tomographic particle image velocimetry (Tomo-PIV) is used in an early turbulent boundary layer (Reθ= 420) to investigate the origin of hairpin vortices. The timelines reveal a triangular bulge in the low-speed streak (LSS), and the initial roll-up occurs at two sides of it. Meanwhile, the material surfaces manifest as a three-dimensional (3D) wave structure in the LSS, which may support the model of a soliton-like coherent structure (SCS). Subsequently, the method of Lagrangian-averaged vorticity deviation is used to detect early vortices. We find that the 3D wave structure is flanked by two vortices, thus confirming the roll-up of timelines and demonstrating the advantage of the Lagrangian criteria in capturing structures in complex flows. These results suggest that various coherent structures may evolve from the metamorphosis of 3D wave structures and their later interaction. Finally, the limitations of traditional experimental and post-processing tools are discussed.

Despite a long and rich history of scientific investigation, fluid turbulence remains one of the most challenging problems in science and engineering. One of the key outstanding questions concerns the role of coherent structures that describe frequently observed patterns embedded in turbulence. It has been suggested, but not proved, that coherent structures correspond to unstable, recurrent solutions of the governing equation of fluid dynamics. Here, we present experimental and numerical evidence that three-dimensional turbulent flow tracks, episodically but repeatedly, the spatial and temporal structure of multiple such solutions. Our results provide compelling evidence that coherent structures, grounded in the governing equations, can be harnessed to predict how turbulent flows evolve.