Background Flow Field Research Articles

The Effect of Model Input Uncertainty on the Simulation of Typical Pollutant Transport in the Coastal Waters of China

Route planning to evade potential pollution holds critical importance for aquaculture vessels. This study establishes a fish-feed pollutant drift model based on the Lagrangian particle tracking algorithm and designs four sets of sensitivity experiments in the East China Sea. The research investigates the impact of model input uncertainties on the drift trajectory, centroid position, and sweeping area of the fish-feed pollutants. Numerical results indicate that the uncertainty in the background flow field significantly affects the uncertainty in the centroid position and sweeping area in the numerical simulations. Specifically, when a 35% random error is added to the background flow field, the centroid shift distance reaches its maximum, and the sweeping area also attains its largest value. The uncertainty in the background wind field affects the centroid position of particles but to a much lesser extent compared to the background flow field. When considering only the uncertainty of the background wind field, the sweeping area does not significantly differ from the control experiment as the uncertainty of the background wind field increases. The initial release position has little effect on the drift direction of the fish-feed pollutants but does affect the drift distance; it has minimal impact on the trajectory but significantly affects the final position of the pollutant centroid. By analyzing the model uncertainties, this study reveals the key factors influencing the drift of fish-feed pollutants. This information is crucial for aquaculture vessels in planning routes, considering environmental factors, and reducing potential pollution risks.

Characterizing the internal wave wakes and synthetic aperture radar image features of underwater moving objects

In this study, a refined and improved theoretical model was proposed for calculating the synthetic aperture radar (SAR) images of internal wave wakes. This model was based on an equivalent source model of internal wave wakes and the velocity-bunching principle. Wake field, scattering field, and SAR image characteristics were systematically calculated for multiple scenarios. The model described the transfer and solution processes by inputting the background flow field temperature and salinity profiles to map the radar image intensity distribution. The physical mechanisms of the internal wave wake generation and its capture by SAR were comprehensively explained. The effects of various parameters on the internal wave wake SAR images were analyzed comprehensively. The SAR image features containing internal wave wakes were determined based on the gray-level co-occurrence matrix and image power spectrum, which revealed that the maximum surface current induced by the internal wave modes of the same amplitude was considerably larger than that induced by surface-wave mode. When traveling far from the water surface, the rate of change in the total scattering coefficient because of the Bragg effect was approximately 20 times that of the slope effect. In SAR images, the recognizability of wakes depends primarily on the maximum intensity rather than the average disturbance on the sea surface. Therefore, SAR images exhibit considerable differences during downwind radar observations, with the most prominent contrast differences occurring at the maximum spreading angle of wake scattering and 1500 m behind underwater objects. In the image power spectrum, the frequency range of the first modal internal waves was approximately 10–30 Hz, whereas the higher-order modal internal waves had frequencies lower than 10 Hz. Furthermore, the frequency range of the surface-wave mode was larger than that of internal-wave modes, approximately 50–60 Hz.

The Dynamics of Magnetic Rossby Waves in the Quasigeostrophic Shallow Water Magnetohydrodynamic Theory

The dynamics of magnetic Rossby waves are investigated by applying a quasigeostrophic shallow water magnetohydrodynamic system, which is linearized with respect to both uniform background flow and uniform magnetic field. Due to the influence of the free surface divergence, the phase speed for magnetic Rossby waves can be either a monotonically increasing or a monotonically decreasing function, and the resulting difference between the group velocity and the phase speed can be either positive or negative. This is determined by whether the corresponding Alfvén wave speed is the upper limit or not. Differently, the phase speed is always a monotonically increasing function and the difference between the group velocity and the phase speed is always positive for incompressible magnetic Rossby waves. Multiplying a factor, the wavenumber vector shares the same endpoint with the group velocity vector. The endpoint moves on a cycle that has a center at the k-axis and is tangent to the l-axis in the wavenumber space. The circle is quite similar to the Longuet-Higgins circle for Rossby waves on Earth’s atmosphere and ocean. The fundamental dynamics is the theoretical basis for deeply understanding the meridional energy transport by waves and the interaction between waves and the background states.

Biologically generated turbulent energy flux in shear flow depends on tensor geometry.

It has been proposed that biologically generated turbulence plays an important role in material transport and ocean mixing. Both experimental and numerical studies have reported evidence of the nonnegligible mixing by moderate Reynolds number swimmers, such as zooplankton, in quiescent water, especially at aggregation scales. However, the interaction between biologically generated agitation and the background flow, as a key factor in biologically generated turbulence that could reshape our previous knowledge of biologically generated turbulence, has long been ignored. Here, we show that the geometry between the biologically generated agitation and the background hydrodynamic shear can determine both the intensity and direction of biologically generated turbulent energy flux. Measuring the migration of a centimeter-scale swimmer-as represented by the brine shrimp Artemia salina-in a shear flow and verifying through an analog experiment with an artificial jet revealed that different geometries between the biologically generated agitation and the background shear can result in spectral energy transferring toward larger or smaller scales, which consequently intensifies or attenuates the large-scale hydrodynamic shear. Our results suggest that the long ignored geometry between the biologically generated agitation and the background flow field is an important factor that should be taken into consideration in future studies of biologically generated turbulence.

Numerical prediction of the whistling potentiality of a turbulent channel flow with corrugated walls

This study explores the turbulent flow-induced whistling phenomena in a channel with corrugated wall surfaces, which is crucial for mitigating the acoustic fatigue problem in the aerospace field. By solving a compressible linearized Navier–Stokes equation in the frequency domain, the interference between the turbulent flow field along the corrugated wall and the incident acoustic field is studied, including the acoustic wave scattering phenomenon caused by turbulence and the fluid perturbation induced by acoustic waves. In conjunction with this, the acoustic two-ports method is utilized to investigate the transfer-function model and predict the whistling potentiality of the turbulent flow along corrugated walls. Experimental validations through the literature results confirm the numerical accuracy of this aeroacoustic simulation strategy. Subsequently, the investigation extends to different cavity configurations with different cavity profiles and numbers, and the two-port scattering matrix is applied to quantify the acoustic transmission and damping coefficients caused by the background flow field and turbulent eddy viscosity. The acoustic power conversion mechanism between the turbulent flow field and the incident acoustic field is established, allowing for quick prediction and effective analysis of the generation frequency range of the whistling phenomenon. Furthermore, the modulation effect of sound waves on the fluid is studied by analyzing the response of the incident sound wave frequency to the phase interference momentum and shear layer of different configurations of corrugated cavities. The results show that compared with the right-edge configuration, the rounded-edge configuration produces whistling at a lower frequency due to the turbulence effect, and the number of cavities adjusts the intensity, not the frequency, of the sound power generated. In addition, the oscillation of the shear layer caused by sound waves is related to the cavity configuration and the sound wave frequency.

Euphotic Zone Depth Anomaly in Global Mesoscale Eddies by Multi-Mission Fusion Data

As the waters of marine primary production, the euphotic zone is the primary living environment for aquatic organisms. Eddies account for 90% of the ocean’s kinetic energy and they affect marine organisms’ habitats by the excitation of vertical velocities and the horizontal advection of nutrients and ecosystems. Satellite observations indicate that anticyclones mainly deepen the euphotic zone depth, while cyclones do the opposite. The anomalies reach 5 m on average in the region of high eddy amplitude and frequent eddy occurrence. In addition, we found that the anomalies have an extreme value in each of the 5°–23° and 23°–55° and reach a maximum at around 38 degrees with the increase in latitude. In the eddy-center coordinate system, the minus gradient direction of the negative anomaly is consistent with the background flow field and the direction of the eddy movement. Meanwhile, the anomaly increases along the radial direction to about 0.2r and then decreases. Finally, there is a significant linear correlation between the anomaly magnitude and the eddy amplitude. The conclusion of this research and related mechanism explanation contributes to marine biology research and conservation, the estimates of marine primary productivity, and the understanding of the biogeochemical properties of eddy modulation in the upper water column.

Flow field characteristics of Rayleigh streaming in a two-dimensional rectangular channel under the background physical field

The purpose of this paper is to explore the flow field characteristics of Rayleigh streaming in a two-dimensional rectangular channel under the background physical field. Based on the Nyborg perturbation theory, the nonlinear acoustic streaming control equation of the coupled background physical field is derived. Furthermore, the finite element method is used to establish the numerical model of Rayleigh streaming in a two-dimensional rectangular channel under a background physical field. Finally, the Reynolds stress method is used to study the flow field characteristics of Rayleigh streaming in the uniform directional shear flow field and temperature gradient field. The reliability of the numerical method is verified by comparing it with the analytical solution of the classical Rayleigh streaming. The results show that the background flow field can restrain the Rayleigh streaming. When the average velocity of the background flow is greater than the particle velocity amplitude of the first-order sound field, the Rayleigh streaming is easily submerged by the background flow field. The streaming intensity decreases exponentially with the increase of the average velocity of the background flow. Moreover, Rayleigh streaming is very sensitive to the temperature gradient field. With the increase in a temperature gradient, the Rayleigh streaming vortex near the high-temperature region expands rapidly, and its streaming intensity increases exponentially. The conclusion of this paper provides a certain explanation for revealing the mechanism of flow heat transfer regulated by strong sound waves under the background physical field.

Tracking and analysis of interfaces and flow structures in multiphase flows

A methodology is introduced to study the dynamics of fluid interfaces in multiphase flows, emphasizing their break-up and coalescence. The algorithm tracks surfaces, here obtained by isocontouring an interface-describing scalar field (e.g., VOF) from a time series of volumetric snapshots. Physical and geometric information of the surfaces is used to find correspondences in a higher-dimensional space. Events are derived from found correspondences to describe the interactions among isosurfaces of closed fluid structures extracted at consecutive tracking time steps. The correspondences and events are filtered based on physical realizability, accounting for geometric constraints between consecutive time instances, as well as temporal constraints on the relations between surfaces in previous tracking steps. The resulting events are used to map the time evolution of all surfaces and their interactions into a graph, which is then queried to retrieve information on the dynamics of the fluid interfaces. The methodology is applied to a DNS dataset of droplet break-up in forced homogeneous isotropic turbulence (HIT). Emphasis is placed on the statistics of split and merge events, the lifetime of surfaces, and their geometric evolution in relation to the background flow fields.

Acoustic black hole analogy to analyze nonlinear acoustic wave dynamics in accelerating flow fields

We present a physical model and a numerical method based on a space- and time-dependent Galilean-type coordinate transformation to simulate acoustic waves in the presence of an accelerating background flow field with sonic transition. Kinematically, the coordinate transformation is designed so as to maintain the well-posedness of the transformed wave equation, which is solved in a fixed computational domain using standard finite differences. Considering an acoustic black hole analogy, we analyze the nonlinear dynamics of acoustic waves in a stationary but non-uniformly accelerating flow field under the assumption of spherical symmetry. The choice of the acoustic black hole analogy is motivated by the fact that the steady-state spherical sonic horizon allows us to parameterize the wave-flow configuration in terms of a Helmholtz number He=c2/(λagh), which is expressed as a function of the speed of sound c, the emitted wavelength λa, and the flow acceleration at the sonic horizon, that is, the acoustic surface gravity gh. The results of the numerical simulations show that He describes geometrically similar sets of wave characteristics for different combinations of gh and λa. However, we also observe nonlinear variations of the wave amplitude along the wave characteristics, which are attributed to nonlinear Doppler modulations. It appears that these amplitude modulations depend on the acceleration of the flow field and can, therefore, differ for geometrically similar characteristics.

Incoherent signatures of internal tides in the Tokara Strait modulated by the Kuroshio

Internal tides are ubiquitously generated from disturbances between barotropic tides and bottom topography in stratified oceans. Incoherence is routinely considered as non-stationary phase discrepancy between internal and astronomical tides. Internal tides, especially incoherent constituents, contribute substantial energy to dissipation and turbulent mixing. Using ∼8-month near-full-depth moored current observations, we investigated internal tides in the Tokara Strait, where the Kuroshio exits the East China Sea. Semidiurnal internal tides, especially the M2 internal tides, dominate the diurnal internal tides by approximately one-order of magnitude. We decomposed the bandpass-filtered tidal currents into coherent and incoherent constituents, both of which presented a spring–neap cycle; however, the former was largely coupled with local barotropic tides, while the latter had an incoherent phase. The coherent semidiurnal internal tides were intensified and had larger variability near the bottom; nevertheless, the intensity and variability of incoherent internal tides decreased with depth. These results suggest that the incoherent internal tides are exogenous and are possibly affected by the background flow field during propagation. The mean incoherence (above 54 % for both semidiurnal and diurnal internal tides) was expectedly higher than the global mean (∼44 %), implying a robust effect of nonlinear Kuroshio-tide interactions. Moreover, the relationship between the variability of the incoherence and the north–south shift of the Kuroshio further revealed the impact of the background flow intensity variation on internal tides. A ray tracing model demonstrated different refraction indices during the propagation of internal tides, leading to converged (diverged) ray paths in the mooring when the Kuroshio was at the northern (southern) axis position, thus explaining the differential incoherence caused by the variability of the Kuroshio.

Development of a dynamic wake model accounting for wake advection delays and mesoscale wind transients

With tip height often above 200 meters, currently installed wind turbines are increasingly interacting with the atmospheric boundary layer. Therefore, the research intends to develop a new dynamic wake model that considers weather transient inputs. The Flow Redirection and Induction in Steady State (FLORIS) framework has been proven to be a powerful wake modelling tool even though lacking dynamical effects. An extension of the FLORIS framework is presented in order to include wake advection delays between wind turbines, time-varying and spatially heterogeneous wind conditions. The so-called Observation Points (OPs) are generated at the rotor centre location. Following a Lagrangian approach, the OPs are convected downstream, along the wake of the wind turbine, defining the wake centreline. It is during this process that the dynamics of wind turbine wake and background flow field unsteadiness are included. Finally, using the same approach currently implemented for yaw-misalignment wake deflection, the wake is shifted to match the unsteady wake centreline. The developed model is validated against Large Eddy Simulation for unsteady inflow with variable wind turbine control and wake interactions. The new model is applied to a simulation of the Horns Rev wind farm with a sinusoidally time-varying wind inflow direction to show an hysteresis in the wind farm power extraction curve.

Simulating acoustic waves in acoustic black hole analogues

We present a physical model and a numerical method to simulate nonlinear acoustic waves emitted from moving boundaries in a moving background flow field with transition from subsonic to supersonic flow speeds. The physical model is based on a convective form of the Westervelt equation and accounts for the motion of the background medium and the progressive distortion of a finite amplitude acoustic wave. A suitable coordinate transformation in physical space allows us to accommodate the motion of the wave emitting boundary, while solving the governing equation in a fixed computational domain using standard finite difference techniques. We present the case of an oscillating spherical wave emitting boundary with an induced spherical flow as a prototypical example of an acoustic black hole analogue, where the acoustic waves cannot escape from the horizon of sonic flow speed during the contraction phase. It is demonstrated that the accelerating motion of the wave emitting boundary gives rise to frequency sidebands and amplitude modulations of the acoustic wave. The influence of the background flow speed on this nonlinear Doppler effect is investigated with the present methodology, thus contributing to an improved understanding of the acoustic wave behavior in the proximity of a sonic horizon.

Operational Modeling of North Aegean Oil Spills Forced by Real-Time Met-Ocean Forecasts

Over the latest decades, oil marine pollution has posed a vital threat for global ocean health, since spillages of any scale are related to environmental, social and financial impacts. The worldwide increase in oil and gas demand, and the parallel rise in oil and gas production, exploiting particularly coastal and offshore marine deposits, have significantly increased the risk of accidental oil release to the sea. In the present study, an operational oil spill model was applied to test the oil dispersive properties and to reveal the relative magnitude of weathering processes, after an accidental oil spill release along the main tanker transportation route in the North Aegean Sea. Numerical simulations were implemented using the OpenOil transport and fate numerical model, a subclass of the OpenDrift open-source trajectory framework. This model integrates algorithms with several physical processes, such as oil entrainment, vertical mixing, oil resurfacing and oil emulsification. The oil dispersion model was coupled to real-time met-ocean forecasts received from NOAA-GFS and CMEMS. Present simulation results have focused on the impact of turbulent kinetic energy, induced by the background flow field, on the horizontal spreading of particles, as well as on the evolution of oil mass balance and oil mass properties.

Revisiting hydraulics of flowing artesian wells: A perspective from basinal groundwater hydraulics

Causes and water sources of flowing artesian wells attracted the interest of many hydrogeologists throughout history, however, a quantitative model that satisfactorily considers the roles of topography, groundwater recharge/discharge and aquitards on hydraulics of flowing wells is still lacking. In this study, a three-layer river-valley basin with a recharge boundary is used to obtain the basinal flow field, spatial distribution and transient discharge rates of flowing wells. Even if there is an aquitard separating the unconfined and the confined aquifers, groundwater discharge to the river still plays a critical role in occurrence of flowing wells. An aquitard with lower permeability would enlarge the zone with flowing wells, implying that flowing wells are more likely to occur in basins with continuous aquitards. By comparing flowpaths and inter-aquifer leakages before and after numerically installing a flowing well in a three-layer basin, we find decreased upward leakage in the discharge limb plays a much larger role than increased downward leakage in the recharge limb in the stable discharge rate of a flowing well. This recognition is different from previous studies where the stable discharge rate is supported only by downward leakage from the overlying aquifer. By considering well hydraulics in a basinal background flow field, this study improves understanding of the mechanisms of flowing artesian wells and the interaction of surface water and deep groundwater, indicating that flowing wells are valuable for sampling deep groundwater representative of deep and old groundwater in the discharge area of a basin.

Statistical Analysis of Mesoscale Eddies Entering the Continental Shelf of the Northern South China Sea

An Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO) mesoscale eddy trajectory atlas product is used to analyze the path type and temporal variability of the eddies that entered the continental shelf area of the northern South China Sea (SCS) from 1993 to 2016. A total of 184 mesoscale eddies entered the continental shelf area of the northern SCS during a 24-year period. We classify the mesoscale eddies into four types according to the motion trajectories: along-the-isobath type, intrusion-of-continental-shelf type, local wandering type, and shelf-internal-generation type. The occurrence numbers of these four types were 87, 38, 23, and 36, respectively. The mean amplitude and radius of the along-the-isobath type are the largest, about 18 cm and 153 km, respectively; furthermore, their average lifetime is also the longest, about 93 days. The mean amplitude, radius, and lifetime are the smallest for the shelf-internal-generation type, about 16 cm, 146 km, and 74 days, respectively. The direction and velocity of the background flow field affects the intrusion path of the mesoscale eddies onto the continental shelf of the northern SCS. The seasonal distribution of the mesoscale eddies quantity is also related to the direction and velocity of the corresponding background flow field.

Electromagnetic Scattering of Near-Field Turbulent Wake Generated by Accelerated Propeller

The electromagnetic scattering study of the turbulent wake of a moving ship has important application value in target recognition and tracking. However, to date, there has been insufficient research into the electromagnetic characteristics of near-field propeller turbulence. This study presents a new procedure for evaluating the electromagnetic scattering coefficient and imaging characteristics of turbulent wakes in the near field. By controlling the different values of the net momenta, a turbulent wake was generated using the large-eddy simulation method. The results show that the net momentum transferred to the background flow field determines the development of the turbulent wake, which explains the formation mechanism of the turbulence. Combined with the turbulent energy attenuation spectrum, the electromagnetic scattering characteristics of the turbulent wake were calculated using the two-scale facet mode. Using this method, the impact of different parameters on the scattering coefficient and the electromagnetic image of the turbulence wake were investigated, to explain the modulation mechanism and electromagnetic imaging characteristics of the near-field turbulent wake. Moreover, an application for estimating a ship’s heading is proposed based on the electromagnetic imaging characteristics of the turbulent wake.

Learning efficient navigation in vortical flow fields

Efficient point-to-point navigation in the presence of a background flow field is important for robotic applications such as ocean surveying. In such applications, robots may only have knowledge of their immediate surroundings or be faced with time-varying currents, which limits the use of optimal control techniques. Here, we apply a recently introduced Reinforcement Learning algorithm to discover time-efficient navigation policies to steer a fixed-speed swimmer through unsteady two-dimensional flow fields. The algorithm entails inputting environmental cues into a deep neural network that determines the swimmer’s actions, and deploying Remember and Forget Experience Replay. We find that the resulting swimmers successfully exploit the background flow to reach the target, but that this success depends on the sensed environmental cue. Surprisingly, a velocity sensing approach significantly outperformed a bio-mimetic vorticity sensing approach, and achieved a near 100% success rate in reaching the target locations while approaching the time-efficiency of optimal navigation trajectories.

Region-Based Convolutional Neural Network for Wind Turbine Wake Characterization in Complex Terrain

We present a proof of concept of wind turbine wake identification and characterization using a region-based convolutional neural network (CNN) applied to lidar arc scan images taken at a wind farm in complex terrain. We show that the CNN successfully identifies and characterizes wakes in scans with varying resolutions and geometries, and can capture wake characteristics in spatially heterogeneous fields resulting from data quality control procedures and complex background flow fields. The geometry, spatial extent and locations of wakes and wake fragments exhibit close accord with results from visual inspection. The model exhibits a 95% success rate in identifying wakes when they are present in scans and characterizing their shape. To test model robustness to varying image quality, we reduced the scan density to half the original resolution through down-sampling range gates. This causes a reduction in skill, yet 92% of wakes are still successfully identified. When grouping scans by meteorological conditions and utilizing the CNN for wake characterization under full and half resolution, wake characteristics are consistent with a priori expectations for wake behavior in different inflow and stability conditions.

On the determination of grid size/smoothing distance in un−/semi-resolved CFD-DEM simulation of particulate flows

Unresolved and semi-resolved CFD-DEM coupling are frequently used for modeling particulate flows due to their high efficiency and satisfying accuracy. The grid size (in unresolved CFD-DEM) and smoothing distance (in semi-resolved CFD-DEM) are usually around three times of the particle diameter so as to properly reproduce the background flow field. An over-estimation of the grid size or smoothing distance can lead to low-fidelity flow fields, while under-estimations of these parameters can produce an inaccurate drag force. Therefore in practices, the size ratio is usually empirical, and how does it vary with cases remains unclear. In this paper, we provide an approach to identify the most suitable grid size or smoothing distance in coupled CFD-DEM. Firstly, resolved simulations based on immersed boundary method (IBM) are conducted to compute particles' drag forces. The obtained forces are then compared with those calculated by force models with certain smoothing lengths to identify an optimal smoothing length or effective grid size. During this process, integral scale of each case is also computed. Finally, linearity between integral length and the optimal smoothing distance or grid size is found. This correlation can offer guidance to CFD-DEM modeling in future and provide insights for novel drag force models.

Estimation of initial conditions for surface suspended sediment simulations with the adjoint method: A case study in Hangzhou Bay

In this paper, an improved numerical scheme is developed to estimate the initial conditions (ICs) based on a three-dimensional suspended sediment transport model (3D SSTM) with adjoint data assimilation, and the method is then applied to Hangzhou Bay as an example. Specifically, the ICs are estimated by assimilating artificial observations in twin experiments and suspended sediment concentrations (SSCs) retrieved from the Geostationary Ocean Color Imager (GOCI) in practical experiments. In the twin experiments, the sensitivity of the estimated ICs to several influential factors is discussed. The results demonstrate that the conjugate descent algorithm of Fletcher is proven to be better than the steepest descent, finite memory BFGS, and five other conjugate gradient algorithms in estimating the ICs; the estimated ICs are sensitive to initial guess values, and appropriate initial values are necessary for improving the efficiency of convergence and obtaining good results. Additionally, the errors of observations can significantly influence the estimated results. In contrast, the estimated results are not very sensitive to cloud coverage, errors in the background flow field, and length of the assimilation time window. In practical experiments, according to the conclusions of twin experiments, an improved 3D SSTM with the adjoint method is developed for Hangzhou Bay, and the surface GOCI-retrieved SSCs during typical neap and spring tidal cycles are assimilated to estimate the practical ICs. The experimental results imply that with the present estimation method, more accurate ICs can be obtained, which indicates that the adjoint method is effective in the estimation of the ICs in SSTMs. Furthermore, this study verifies that accurate ICs are critical for the numerical modeling of SSCs on the tidal cycle scale. This study is not only useful for further improving the accuracy of ICs in SSTMs but also suggestive for the initialization schemes of other matter transport models.