An adaptive error-controlled hybrid fast solver for regularized vortex methods

In this paper, an adaptive error-controlled hybrid fast solver that combines both O(N) and O(Nlog⁡N)schemes is proposed. For a given accuracy, the adaptive solver is used in the context of regularized vortex methods to optimize the speed of the velocity and vortex stretching calculation. This is accomplished by introducing criteria for cell division in building of the tree, conversion of multipole to local expansion in the downward pass, stopping of the downward pass and choosing between direct and fast summation to compute the vector fields. These criteria are based on key parameters (p,nF,nT,dσ) which take into account the elements distribution, choice of the regularization function, and the computer architecture. The proposed solver automatically adapts to the evolving flow-field by periodically updating the optimal values of these parameters to maximize the speed, while meeting the accuracy constraints, by balancing far and near-field calculations. Performance of the proposed scheme is investigated in terms of the dependence of cost and accuracy on the various controlling parameters. The evolution of the optimal values of these parameters along with the associated computational savings are presented for the case of collision of two vortex rings over a reasonable time span. © 2022 Elsevier Inc

A novel spatio-temporally adaptive parallel three-dimensional DSMC solver for unsteady rarefied micro/nano gas flows

An efficient parallel multi-scale direct simulation Monte Carlo algorithm to simulate three-dimensional rarefied gas flows over complex geometries is presented. The proposed algorithm employs a novel spatio-temporal adaptivity scheme. Based on the gradients of flow macro-properties, the spatio-temporal adaptivity scheme computes the cell size distribution and assigns the appropriate number of time sub-steps for each cell. The temporal adaptivity scheme provides local time step adaptation through different temporal levels employed in different cells. Spatial representation is based on a hierarchical octree Cartesian grid with low memory storage requirement. The hierarchical octree grid endows the method with straightforward and efficient data management suitable for particle ray tracing and dynamic grid refinement and coarsening. Solid objects, represented by triangulated surfaces, are incorporated using a cut-cell algorithm. A new parallelization scheme suitable for simulating strongly unsteady, non-equilibrium flows is proposed. The parallelization scheme, implemented for multi-core Central Processing Units (CPUs), significantly reduces the computational cost of modeling these flows. Performance of the method is assessed by comparing with benchmarked test cases for various rarefied gas flows. © 2019 Elsevier Lt

Proper evaluation of spherical harmonics-based expressions for the velocity and vortex stretching vectors in three-dimensional grid-free vortex methods

We present expressions for approximating the velocity and vortex stretching vectors induced by a far-field collection of point vortices and we propose a set of recurrence relations to properly evaluate these expressions. Expressed as truncated series of spherical harmonics, these approximations are used in the context of O(Nlog⁡N) and O(N)-type fast multipole methods to reduce the computational cost of the advection step in three-dimensional grid-free vortex methods. These methods typically rely on operator splitting to handle diffusion and convection separately. In our implementation, the convection step employs a second order Runge-Kutta time integration scheme, where the particles velocities and vortex stretching vectors are computed using a fast multipole method that employs the proposed expressions. To model diffusion, we introduce an extension of the smoothed redistribution scheme to 3D unbounded flows. We check the accuracy of the expressions by inspecting the convergence of the velocity and the vortex stretching vectors as a function of the expansion order. The performance of the grid-free three-dimensional vortex method is assessed by simulating the collision of two vortex rings, over a long period of time, for different values of Reynolds number covering the range 500−2000. © 2020 Elsevier Inc

Self Sustained Thermally Induced Gas-Damped Oscillations of Bimetal Cantilevers with Application to the Design of a New Pyroelectric Micro Energy Harvester

Low efficiency is the main drawback of many MEMS thermal energy harvesters. Recently, energy harvesting micro-devices that operate using the pyroelectric effect gained attention due to their potential superior performance. Operation of these devices is based on the cyclic motion of a pyroelectric capacitor that operates between a high temperature and a low temperature reservoirs. In this paper, we investigate the dynamics of oscillations of a pyroelectric capacitor self sustained by thermally actuated bimetal micro-cantilevers, a topic which is so far under investigated. In addition to highlighting key thermodynamic aspects of the operation, we explore conditions for self-sustained oscillations and discuss the viability of operation at the mechanical resonance frequency. The analysis is presented for a new design inspired by the device proposed in Refs.\cite{2011,2012}, where in contrast, our proposed design boasts the following features: The pyroelectric capacitor remains parallel to the heat reservoirs, by virtue of its symmetric support by two bimetallic cantilever beams; In addition, the cyclic operation of the device does not require physical contact, thus lowering the risk of mechanical failure; To adjust the damping force imparted by the surrounding gas, the thermal reservoirs are equipped with trenches. To study the dynamic operation of the device, we developed a physically based reduced order, yet accurate, model that accounts for the heat transfer between and within the different components, and for the various forces including the gas damping force. The model is embedded within an optimization algorithm to produce optimal designs over the range 26-38 C of temperature difference between the two reservoirs. The corresponding range of harvested power density is 0.4-0.65 mW/cm2

Impact of boundary conditions in a microclimate model on mitigation strategies affecting temperature, relative humidity, and wind speed in a Mediterranean city

This paper investigates several mitigation strategies to detect which has the biggest effect on thermal comfort parameters, namely temperature, relative humidity, and wind speed in a neighborhood in Beirut, the capital city of Lebanon. The strategies include changing the ground surface albedo, replacing old buildings with new ones, and adding vegetation. The simulations, conducted on ENVI-met V4 software, were run on the 20th of January and August representing the two major seasons in the country, winter and summer, respectively. The simulations were carried out on a 200 x 200 x 46-m domain at a resolution of 2 m, which permits the study of small-scale interfaces between buildings, surfaces, and vegetation. The paper then represents various types of lateral boundary conditions (LBC) and illustrates the effects of expanding the area of buildings surrounding the field of interest. The present study will allow urban planners to determine the policies required to ameliorate the urban living environment in cities like Beirut. The simulation exhibits that the greatest impact on thermal comfort parameters is vegetation. It was eminent that the occupancy of vegetation is the most effective mitigation strategy given its capability of enhancing the thermal comfort conditions at the pedestrian's pathway sidewalks. The maximum decrease in air temperature was 2.4 °C at 17:00 on August 20th, while the maximum decrease in the mean radiant temperature was 10.5 °C at 12 at noon on the 20th of August. © 2021 Elsevier Lt

The spread of Indo-Pacific origin fish species in the Mediterranean Sea is influenced by sea currents, habitat factors, and increasingly by shipping

The construction of the Suez Canal initiated a profound marine biogeographic rearrangement by connecting two marine regions that had been separated for about 16 million years. To date, more than 100 fish species native to the Indo-West Pacific Ocean have crossed the Suez Canal and become established in the Mediterranean Sea. The purpose of this study is to identify the major factors that influence how fish species of Indo-Pacific origin (FSIPO) spread within the Mediterranean. We analyzed Mediterranean records of 136 FSIPOs, divided the Mediterranean Sea into polygons, and examined how sea currents, shipping, environmental variables, and species characteristics affect the probability that an FSIPO record is found in a polygon. Our analysis showed that any particular FSIPO is more likely to colonize regions that receive currents from the entrance of the Suez Canal, as well as regions that are close to – or receive currents or cargo ships from – other regions already occupied by the same species. Higher salinity is linked to increased colonization rates, and colonization rates overall show a significant increase over time. In addition, colonization due to spatial proximity to occupied regions and due to shipping increased significantly over time. Comparisons between simulated and observed rates of spread showed that there is additional variation among fish species that our analysis did not capture

A modified k-ω turbulence model for improved predictions of neutral atmospheric boundary layer flows

A modified k-ω turbulence model is developed for neutral atmospheric boundary layer flows by introducing additional source terms to the equations of the standard model to remove its inconsistency with the inlet conditions. Rough wall functions based on aerodynamic roughness are adopted to overcome the limitations of the standard wall functions that are based on sand grain roughness. Moreover, the diagnostic and prognostic approaches are considered when simulating the flow. The diagnostic approach is approximate, relying on satisfying mass conservation by solving an optimization problem, whereas the prognostic approach resolves the Navier-Stokes equations with a turbulence model. The performances of the developed modified k-ω model, an existing modified k-ε model, and the standard SST k-ω model are compared. To calculate the concentration field, the Eulerian approach which solves a concentration transport equation, and the Lagrangian approach which adopts the particle method with core radius spreading for computing diffusion are used. Results generated with the diagnostic approach demonstrate its unsuitability to calculate wind flows over complex geometries. On the other hand, the prognostic approach, with the three mentioned turbulence models, was successful in reproducing the flow data of CEDVAL A1 and B1 experiments in domains composed of one building and an array of buildings. In addition, a good agreement with the concentration measurements is obtained when using the newly modified k-ω and the SST k-ω models to account for turbulence, with the Eulerian method outperforming the Lagrangian dispersion approach. © 2022 Elsevier Lt

Eddy Detection Using Reanalysis Datasets

Oceanic eddies are ubiquitous in oceans and play a major role in several parameters that include ocean energy transfer, nutrients distribution and air-sea interaction. Typically, eddy detection algorithms are based on single physical parameter, geometrics or other handcrafted features. To achieve better performances, we aim to develop a new approach to fuse multi-variable features for eddy detection. We will investigate lumping satellite datasets of Sea surface height, Sea surface temperature, Salinity in addition to full model solution velocity field through the inclusion of information (correlation) between the datasets