Mesh Resolution Research Articles

Optimized Traditional and Leapfrog CDI-FDTD Schemes With Reduced Dispersion and Numerical Properties Analysis

In order to improve the numerical dispersion property of the finite-difference time-domain (FDTD) method further, two optimized low-dispersion schemes are herein proposed, which are based on the complying-divergence implicit (CDI) FDTD method and its one-step leapfrog method. These proposed approaches effectively reduce the numerical dispersion errors by intervening with anisotropic parameters while maintaining unconditional stability. The numerical stability and dispersion properties are analyzed in detail to illustrate their effectiveness. The factors, which impact the numerical dispersion including propagation angles, Courant–Friedrichs–Lewy (CFL) number, nonuniform multiscale, mesh resolution, and frequency are carefully examined. In addition, by using the dual-/triple-band multiple-input multiple-output (MIMO) antennas as numerical examples, the practicality of the optimization schemes is comprehensively validated by comparing the calculation time, memory resource, and relative error with those from the traditional and leapfrog CDI-FDTD methods. It is worth celebrating the superiority of the proposed schemes not only in terms of numerical dispersion, but also in calculation efficiency, solution accuracy, and floating-point operands.

Geometric Deep Learning Using Vascular Surface Meshes for Modality-Independent Unruptured Intracranial Aneurysm Detection.

Early detection of unruptured intracranial aneurysms (UIAs) enables better rupture risk and preventative treatment assessment. UIAs are usually diagnosed on Time-of-Flight Magnetic Resonance Angiographs (TOF-MRA) or contrast-enhanced Computed Tomography Angiographs (CTA). Various automatic voxel-based deep learning UIA detection methods have been developed, but these are limited to a single modality. We propose a modality-independent UIA detection method using a geometric deep learning model with high resolution surface meshes of brain vessels. A mesh convolutional neural network with ResU-Net style architecture was used. UIA detection performance was investigated with different input and pooling mesh resolutions, and including additional edge input features (shape index and curvedness). Both a higher resolution mesh (15,000 edges) and additional curvature edge features improved performance (average sensitivity: 65.6%, false positive count/image (FPC/image): 1.61). UIAs were detected in an independent TOF-MRA test set and a CTA test set with average sensitivity of 52.0% and 48.3% and average FPC/image of 1.04 and 1.05 respectively. We provide modality-independent UIA detection using a deep-learning vascular surface mesh model with comparable performance to state-of-the-art UIA detection methods.

High‐resolution numerical simulations of polymer injection molding: Analysis of mesh size and refinement

AbstractAccurate and high‐resolution numerical simulations are crucial for preventing manufacturing defects during polymer injection molding. However, achieving efficient and precise simulations in this field remains a subject of ongoing research. In this study, we develop a macro‐scale numerical model to describe the behavior of melted polymer during the filling phase. Employing our in‐house finite‐element solver, XNS, we simulate the polymer and air flows as a two‐phase flow using the level‐set method. This formulation involves significant discontinuities across the interface, therefore a high mesh resolution along the interface becomes necessary. Furthermore, to accurately capture the polymer behavior along the wall, we apply a variable Navier slip condition that is dependent on the mesh size. To assess the impact of mesh size on the solution, we compare the results obtained with different levels of mesh refinement. Additionally, we investigate the effects of localized mesh refinement at the wall boundary and conclude that refinement in the interior of the channel is also required. Through our analysis, we highlight the importance of high‐resolution meshing and appropriate wall boundary conditions in achieving accurate simulations of polymer injection molding. This research contributes to the ongoing efforts in improving the understanding of the complex phenomena involved in this manufacturing process.

Simple or simplistic? Sensitivity of an operating room CFD model to refinement and detailing

Indoor air quality is critical in healthcare facilities since it directly relates to infection risk, mainly in operating rooms (OR). Nowadays, air movement in such environments is often evaluated with computational fluid dynamics (CFD) simulations. However, many users typically oversimplify their OR models by including only major equipment (such as operating tables and surgical lights) that do not require refined computational meshes. This work analyzes the sensitivity of CFD simulations to mesh refinement, air circulation layout, geometry detailing, placement of furniture and equipment, as well as the presence of the surgical staff in the OR. An OR of a Brazilian university hospital was modeled and validated with environmental parameters assessed in-loco. Results showed relative differences up to 375% in air velocity depending on mesh resolution. Including small furniture and changing the OR layout resulted in nine-fold differences in air velocity. Finally, modeling the surgical staff in the OR significantly interfered with air circulation patterns, preventing inflow from the polluted towards the clean and sterile zone. The present work demonstrates the alarming fact that even a validated, sound, but overly simple CFD model may be phenomenologically insufficient to assess real contamination risks in an OR — and, by extension, in other healthcare facilities. Therefore, detailed CFD models regarding the number of elements in the simulation mesh, the positioning and the operating condition of the ventilation system, the inclusion and detailing of furniture, and the presence of the surgical staff in the OR should be employed since early design stages.

Accelerating Polyhedral Discrete Element Method with CUDA

This paper presents an efficient CUDA-based implementation of a nonspherical discrete element method where irregular particles are described by using polyhedrons. Two strategies are employed to exploit the parallelism of the numerical method. One is to perform contact detection based on the contact pair level instead of the traditional particle level. The second is to reduce the computational burden of each kernel function by allocating thread blocks reasonably. Contact detection between potential contact pairs is the most complicated, time-consuming, and essential process for the polyhedral discrete element method. The linear bounding volume hierarchies are introduced to fix this issue. The hierarchies of the bounding volume tree are organized in a spatially coherent way. Such a structure can minimize branch divergence and is very suitable for parallel implementation with GPU. Two numerical examples are presented to show the performance of the code. It is found from the scenario of two sphere collision that improving the mesh resolution of polyhedral particles can reduce the computational error while slowing down the computational speed correspondingly. A trade-off must be made between accuracy and efficiency. The other example of self-gravitating aggregation demonstrates the code is convergent, stable, and highly efficient. Particularly, with a mainstream GPU, the proposed method easily performs hundreds of times faster than the serial CPU code that does the same function.

Impact of artificial topological changes on flow and transport through fractured media due to mesh resolution

We performed a set of numerical simulations to characterize the interplay of fracture network topology, upscaling, and mesh refinement on flow and transport properties in fractured porous media. We generated a set of generic three-dimensional discrete fracture networks at various densities, where the radii of the fractures were sampled from a truncated power-law distribution, and whose parameters were loosely based on field site characterizations. We also considered five network densities, which were defined using a dimensionless version of density based on percolation theory. Once the networks were generated, we upscaled them into a single continuum model using the upscaled discrete fracture matrix model presented by Sweeney et al. (2019). We considered steady, isothermal pressure-driven flow through each domain and then simulated conservative, decaying, and adsorbing tracers using a pulse injection into the domain. For each simulation, we calculated the effective permeability and solute breakthrough curves as quantities of interest to compare between network realizations. We found that selecting a mesh resolution such that the global topology of the upscaled mesh matches the fracture network is essential. If the upscaled mesh has a connected pathway of fracture (higher permeability) cells but the fracture network does not, then the estimates for effective permeability and solute breakthrough will be incorrect. False connections cannot be eliminated entirely, but they can be managed by choosing appropriate mesh resolution and refinement for a given network. Adopting octree meshing to obtain sufficient levels of refinement leads to fewer computational cells (up to a 90% reduction in overall cell count) when compared to using a uniform resolution grid and can result in a more accurate continuum representation of the true fracture network.

An Efficient Approach to Include Transport Effects in Thin Coating Layers in Electrochemo-Mechanical Models for All-Solid-State Batteries

A novel approach is presented to efficiently include transport effects in thin active material coating layers of all-solid-state batteries using a dimensionally reduced formulation embedded into a three-dimensionally resolved coupled electrochemo-mechanical continuum model. In the literature, the effect of coating layers is so far captured by additional zero-dimensional resistances to circumvent the need for an extremely fine mesh resolution. However, a zero-dimensional resistance cannot capture transport phenomena along the coating layer, which can become significant, as we will show in this work. Thus, we propose a model which resolves the thin coating layer in a two-dimensional manifold based on model assumptions in the direction of the thickness. This two-dimensional formulation is monolithically coupled with a three-dimensional model representing the other components of a battery cell. The approach is validated by showing conservation properties and convergence and by comparing the results with those computed with a fully resolved model. Results for realistic microstructures of a battery cell, including coating layers as well as design recommendations for a preferred coating layer, are presented. Based on those results, we show that existing modeling approaches feature remarkable errors when transport along the coating layer is significant, whereas the novel approach resolves this.

A small-eddy-dissipation mechanism for turbulence modeling and application to wall-bounded flows

A small-eddy-dissipation (SED) mechanism is proposed in the present study for the development of turbulence models. According to the SED mechanism, a turbulence model introduces artificial dissipation to filter out small eddies so that a lower resolution mesh can be used in the simulation. In addition, the artificial dissipation should be applied outside the energy-containing range so that the large-scale motions are not affected. A small-eddy-dissipation mixing length (SED-ML) model is developed based on the SED mechanism to calculate wall-bounded flows. A local Reynolds number ys+=|det(∇u)|1/3/s is introduced in the SED-ML model to distinguish laminar flows from turbulent flows. Therefore, in addition to fully turbulent flows, the SED-ML model can also be used to calculate weakly turbulent or laminar flows. To demonstrate the performance of the SED-ML model, turbulent channel flows with the Reynolds numbers Reτ up to 4200 are simulated. The numerical results are extensively compared with the large eddy simulation (LES) results using the classical subgrid-scale models. The numerical results show that the SED-ML model predicts the statistical results with a good accuracy, while requiring a lower mesh resolution than the classical LES models. The accuracy of the calculated statistical results can be further improved by the parameter extension. The friction coefficient f for channel flows can be extended directly from the reference solution according to the relation ∂f/∂ϕ/f=−9, where ϕ is a dissipative strength coefficient. The energy and dissipation spectra confirm that the SED-ML model introduces significant dissipation at high wavenumbers. This feature is beneficial in maintaining the high accuracy of the simulation results and reducing the computational cost. The numerical study also shows that a sharp filtering of the turbulent kinetic energy in the wavenumber space is essential for the development of the turbulence model with high performance.

Tropical Cyclones and Equatorial Waves in a Convection‐Permitting Aquaplanet Simulation With Off‐Equatorial SST Maximum

AbstractTropical weather phenomena—including tropical cyclones (TCs) and equatorial waves—are influenced by planetary‐to‐convective‐scale processes; yet, existing data sets and tools can only capture a subset of those processes. This study introduces a convection‐permitting aquaplanet simulation that can be used as a laboratory to study TCs, equatorial waves, and their interactions. The simulation was produced with the Model for Prediction Across Scales‐Atmosphere (MPAS‐A) using a variable resolution mesh with convection‐permitting resolution (i.e., 3‐km cell spacing) between 10°S and 30°N. The underlying sea‐surface temperature is given by a zonally symmetric profile with a peak at 10°N, which allows for the formation of TCs. A comparison between the simulation and satellite, reanalysis, and airborne dropsonde data is presented to determine the realism of the simulated phenomena. The simulation captures a realistic TC intensity distribution, including major hurricanes, but their lifetime maximum intensities may be limited by the stronger vertical wind shear in the simulation compared to the observed tropical Pacific region. The simulation also captures convectively coupled equatorial waves, including Kelvin waves and easterly waves. Despite the idealization of the aquaplanet setup, the simulated three‐dimensional structure of both groups of waves is consistent with their observed structure as deduced from satellite and reanalysis data. Easterly waves, however, have peak rotation and meridional winds at a slightly higher altitude than in the reanalysis. Future studies may use this simulation to understand how convectively coupled equatorial waves influence the multi‐scale processes leading to tropical cyclogenesis.

A Robust Grid‐Based Meshing Algorithm for Embedding Self‐Intersecting Surfaces

AbstractThe creation of a volumetric mesh representing the interior of an input polygonal mesh is a common requirement in graphics and computational mechanics applications. Most mesh creation techniques assume that the input surface is not self‐intersecting. However, due to numerical and/or user error, input surfaces are commonly self‐intersecting to some degree. The removal of self‐intersection is a burdensome task that complicates workflow and generally slows down the process of creating simulation‐ready digital assets. We present a method for the creation of a volumetric embedding hexahedron mesh from a self‐intersecting input triangle mesh. Our method is designed for efficiency by minimizing use of computationally expensive exact/adaptive precision arithmetic. Although our approach allows for nearly no limit on the degree of self‐intersection in the input surface, our focus is on efficiency in the most common case: many minimal self‐intersections. The embedding hexahedron mesh is created from a uniform background grid and consists of hexahedron elements that are geometrical copies of grid cells. Multiple copies of a single grid cell are used to resolve regions of self‐intersection/overlap. Lastly, we develop a novel topology‐aware embedding mesh coarsening technique to allow for user‐specified mesh resolution as well as a topology‐aware tetrahedralization of the hexahedron mesh.

Effect of triangular mesh resolution on the geometrical trueness of segmented CBCT maxillofacial data into STL format

ObjectivesTo determine the optimal level of mesh reduction that would maintain acceptable levels of geometrical trueness while also minimizing the impact on other parameters such as file size and processing time. MethodsIntraoral and extraoral maxillofacial defects were created on 8 cadaver heads and scanned by using a CBCT scanner (NewTom 3D Imaging, Verona). DICOM data were segmented to produce head (n=8) and skull models (n=8) saved as standard tessellation language (STL) files. A further processing of head models was preformed to produce face (n=8) and ear models (n=8). A mesh reduction process was performed for each STL model (reference, R0) by generating 50% (R1), 75% (R2), and 90% (R3) reductions. The 3 datasets were compared to the R0 file using 3D evaluation software (GOM Inspect) using a global best-fit algorithm, to calculate the root mean square (RMS) deviations. Statistical analyses were performed at a level of significance of α=0.05. ResultsThere was no 3D deviation after the 50% triangular mesh reduction in the 4 datasets. Minor 3D deviations were observed after 75% reduction, in all groups. After 90% reduction, higher 3D deviations were observed, and especially in head and skull. Statistically significant increase in 3D deviations was observed with higher degrees of mesh reduction (p < 0.001). ConclusionThe resolution of CBCT-based maxillofacial defect models can be reduced up to 50%, with neglectable concern to inaccuracy. Clinical SignificanceAccurate maxillofacial models can be obtained from CBCT DICOM files after segmentation and export as STL files, even when the mesh resolution is reduced up to 50%. This information can be valuable for practitioners and researchers working with 3D models of maxillofacial defects.

Finite Element Simulation and Experimental Assessment of Laser Cutting Unidirectional CFRP at Cutting Angles of 45° and 90°.

Laser cutting of carbon fibre-reinforced plastics (CFRP) is a promising alternative to traditional manufacturing methods due to its non-contact nature and high automation potential. To establish the process for an industrial application, it is necessary to predict the temperature fields arising as a result of the laser energy input. Elevated temperatures during the cutting process can lead to damage in the composite's matrix material, resulting in local changes in the structural properties and reduced material strength. To address this, a three-dimensional finite element model is developed to predict the temporal and spatial temperature evolution during laser cutting. Experimental values are compared with simulated temperatures, and the cutting kerf geometry is examined. Experiments are conducted at 45° and 90° cutting angles relative to the main fibre orientation using a 1.1 mm thick epoxy-based laminate. The simulation accurately captures the overall temperature field expansion caused by multiple laser beam passes over the workpiece. The influence of fibre orientation is evident, with deviations in specific temperature data indicating differences between the estimated and real material properties. The model tends to overestimate the ablation rate in the kerf geometry, attributed to mesh resolution limitations. Within the parameters investigated, hardly any expansion of a heat affected zone (HAZ) is visible, which is confirmed by the simulation results.

A numerical assessment of the dispersion of dissolved pollutants in the Arabian Gulf associated with the Barakah nuclear power plant

The main goal of this study is to investigate long-term dispersion of quasi-conservative pollutants in the Arabian Gulf, such as anthropogenic radionuclides, associated with regular and potential accidental emissions of those from the recently commissioned Barakah nuclear power plant (BNPP) in the western United Arab Emirates (UAE). In particular, discharge of tritium 3H, which is expected to be present in the cooling water effluent during normal operation of the plant, raises concerns about eventual build-up of tritium and tritiated water plume in the poorly flushed western UAE region. A good understanding of the transport processes in the sea is also essential for contingency planning in case of accidental releases of harmful radionuclides such as Cesium 137Cs, and it provides basis for more comprehensive models of the marine radioactivity. To achieve this goal, a three-dimensional unstructured-grid hydrodynamic and tracer dispersion model of the Arabian Gulf, focusing on the western UAE area, was established using Semi-implicit Cross-scale Hydroscience Integrated System (SCHISM). The model was validated against water level, current and temperature data collected in the UAE waters during 2017–2018. The horizontal resolution of the numerical mesh of this model varied from approximately 4 km in the Sea of Oman to 0.1 km near Barakah to properly account for the effect of numerous small islands, narrow straits between them, and other fine-scale coastal features on the hydrodynamic. Two types of scenarios were considered: (1) direct continuous release of a tracer dissolved in the BNPP's heated effluent, and (2) offshore fallouts from the atmosphere onto the water surface. The simulations were conducted over the period of 2 years. Direct continuous tracer discharge was found to cause a notable accumulation of it in the Southern Shallows region, especially Barakah – Sir Bani Yas – Dalma – Yasat Islands zone, where the volume-averaged concentration converged to approximately 7.4% of the excess concentration of the tracer in the BNPP effluent, with the Gulf-averaged saturation concentration projected at approximately 0.37%. The quasi-half-saturation states for these two regions were reached in approximately 0.4 and 1.8 years, respectively. A pollutant plume resultant from an atmospheric fallout generally tends to move eastward along the UAE coast, with the separation occurring in the western approaches of the Strait of Hormuz, between Dubai and Ras Al Khaimah, where a denser near-bed portion continues sinking down and then exits into the Sea of Oman, while a near-surface portion is entrained into the general counterclockwise circulation in the Gulf, and it is carried along the Iranian coast to the upper northwestern Gulf. After the period of 2 years, 60–70% of the total initial tracer load is estimated to be washed out from the Gulf. Despite relatively low probability of the occurrence of such wind conditions, which could result in radionuclide fallouts north of Dalma Island, a notably faster spread of contaminated waters, with the estimated 5–6 weeks for a peak concentration to reach Abu Dhabi City in the studied case, would make emergency response more challenging.

Stability of delaminated composite beams subjected to retarded periodic follower force

This work deals with the stability problem of elastic composite cantilever beams subjected to a delayed, periodically changing follower force. The equation of motion of the periodic system with time delay is deduced based on some previous works. Composite beams with and without delamination are considered, and the finite element method is applied to carry out the spatial discretization of the structures. Besides, for the delaminated case further two cases are involved. The first case is when the delamination is in the midplane of the beam, while the second case involves an asymmetrically placed delamination, respectively. The Floquet theory is applied to derive the transition matrix of the periodic system. An important aspect is that the time delay and the principal period of the dynamic force are equal to each other. The discretization over the time domain is performed by using the Chebyshev polynomials of the first kind. Basically, there are five parameters governing the dynamic problem including among others the time delay and the static and dynamic forces. The stability behavior is shown for the intact and delaminated beams on the parameter planes for large number of cases by using the unit circle criteria. The presence and absence of structural damping is also analyzed in each case. The results indicate that some planes are sensitive to the mesh resolution, others are not. Moreover, on some planes significant differences may take place between the intact and delaminated beams from the standpoint of stable zones.

A Turbulent Inflow Generation Method for the LES of High Re Flow by Scaling Low Re Flow Data

The rescaling–recycling method (RRM) is usually used to generate turbulent inflow for the LES of compressible wall-bounded flows, which can lead to relatively high computational cost for high Re flows since the mesh resolution increases exponentially with Re number. A turbulent inflow generation method based on the scaling of low Re flow, referred as TIG-LowRe, is proposed, aiming at reducing the computational cost when applying the RRM. To validate the proposed method, the TIG-LowRe method was applied to generate turbulent inflow for the LES of a non-isothermal round jet flow at Re = 86,000. Two cases were carried out with the inflow generated based on two round pipe flows at Re = 10,000 and 24,000. The results show that the mean and fluctuating temperatures of the two cases agree well with the experimental data. In the case of low Re flow at Re = 10,000, the jet flow decays too fast along the axial direction, the mean and fluctuating axial velocities are over-predicted and the radial fluctuating velocity is under-predicted. By increasing the Re of the low Re flow to 24,000, the decay rate of the jet flow decreases and the accuracies of the mean and fluctuating axial velocities are obviously improved, while the radial fluctuating velocity shifts further away from the experimental data. The main reason for the difference between the two cases is that more fine turbulent structure of the inflow in case-Re10000 is lost than in case-Re24000 during the turbulence generation process.

Assessment of cavitation erosion risk by Eulerian–Lagrangian multiscale modeling

Cavitation erosion in fluid machineries is a complicated physicochemical and instantaneous process, leading to material loss and reduced efficiency. As the cavitation and the consequent erosion are difficult to be measured and modeled, the present work aims to incorporate an improved energy balance approach into a new Eulerian–Lagrangian multiscale model for addressing the risk of cavitation erosion. The volume of fluid (VOF) and discrete bubble model (DBM) are coupled by a transforming algorithm to develop the multiscale cavitation model. With using the large eddy simulation (LES) approach, the multiscale cavitation features including the morphological evolution of large-cavities and characteristics of microbubbles are well reproduced. To assess the cavitation erosion, a modified aggressive indicator using a modified moving time-averaged pressure field as the driving pressure is proposed to consider the evolution of local pressure and vapor volume fraction. Furthermore, the erosion powers produced from large cavities and microbubbles are completely taken into account. The results show that the VOF-DBM multiscale model can better simulate the cavitation features around a twisted hydrofoil including the secondary shedding and the horse-shoe vapor structure, compared with the VOF method using the same mesh resolution. From the results of discrete bubbles, it is found that the primary shedding leads to the rapid growth of the number of discrete bubbles, which decreases during the transportation of the bubble cloud generated from the primary shedding. By taking the collapsing power of discrete bubbles into account, the prediction of cavitation erosion distribution is quite consistent with the experimental one, whereas the VOF method fails to predict the erosion region near the trailing edge.

On tidal array layout sensitivity to regional and device model representation

In-stream tidal devices are ready to be deployed, yet the largest operational commercial array is limited to 6MW. Upcoming government support should see the size of such arrays increase by orders of magnitude, and thus the optimal placement of turbines within tidal arrays is an emerging challenge for successful commercial integration. Hydrodynamic models are required to predict the power produced by an array and the impact on the surrounding environment. The influence of common model inputs to layout optimisation are investigated herein. This is achieved using a shallow water equation based tidal array modelling framework, Thetis, coupled with a low cost analytical wake model (FLORIS) that allows for rapid assessment of the impact of small changes in hydrodynamic results on array micro-siting. The sensitivity of array optimisation at an intermediate development point (43 turbines) is interrogated through both artificial flow field manipulation and the variation of model input pertinent to the optimisation. A small margin exists in which an optimised array layout will perform efficiently for a deviation in flow prediction accuracy. However, incorrect flow predictions by a range sensitive to model inputs had a reduction of almost 8% on array efficiency relative to a control case. The sensitivity of flow field variance by model input changes, on extractable energy and array layout are substantial. Comparing arrays sited on flow fields using different bathymetry resolution leads to a discrepancy on average of over 2% to average array power. Arrays sited for different mesh resolution and friction representation also see average changes up to and exceeding 0.75%. For array developers and the future of this nascent industry, re-calibration of the model at every stage of data collection coupled with early acquisition of bathymetry data is critical not just for accurate power quantification, but also array efficiency. At a regional scale, quantification of the potential tidal resource must consider the consequences of uncertainty in model input data and site constraints that cannot yet be accounted for.

Biphasic parameter determination of sheep articular cartilage from stress relaxation data using an optimized 3D FE-based method

Abstract Mechanical characteristics of hard and soft tissues are central features for the quantitative description of tissue properties. In this study, we present a biphasic 3D-FE-based method to determine the biomechanical properties of sheep stifle joint articular cartilage (load-bearing area of the medial and lateral femur condyle, n = 28) from stress relaxation indentation tests (s = 0.1 mm, t = 180 s). The FE-model computation was optimized by exploiting the axial symmetry and mesh resolution. Parameters were determined using the Levenberg-Marquardt-algorithm. Results showed significant differences between the biomechanical parameters of the lateral (n = 11) and medial (n = 17) femur condyle of the sheep stifle joint articular cartilage. R² of the fit results varied between 0.91 and 0.99. Overall values for the Young’s modulus were 1.316 ± 0.778 MPa, for the Poisson’s ratio 0.106 ± 0.088 and for the permeability 0.008 ± 0.004 mm4/Ns. Future work will focus on studying the influence of experimental settings on the results obtained with the biphasic 3D-FE-model.

A parameter-free LES model for anisotropic mesh adaptivity

Balancing accuracy and computational cost is a challenge in the modelling of turbulent flows. A widely used method for turbulence modelling is large–eddy simulation (LES). LES allows one to describe large scale flow features at a reasonable computational cost compared to the more accurate direct numerical simulation (DNS), making it a popular choice for engineering applications. One strategy to balance accuracy and cost with LES is through the use of mesh adaptivity, which allows the degrees of freedom in a problem to be reduced by changing spatial discretisation. However, mesh adaptivity can affect accuracy when using the standard Smagorinsky LES model with an implicit filter, considering that the parameter Cs is highly dependent on the filter width, which depends on mesh resolution. This work is aimed to develop an LES model that does not require any user–defined parameters and is suitable for mesh adaptivity with implicit filter. In this study we introduce a parameter–free LES model incorporating an anisotropic eddy–viscosity formulation combined with anisotropic mesh adaptivity. In our model, the parameter Cs in the eddy–viscosity formulation of the Smagorinsky model, is replaced by a function that evaluates the relative location of turbulence fluctuations in each element with respect to the turbulence spectrum inertial range. The anisotropic formulation of the eddy–viscosity allows for the application of an appropriate filter width in different directions, improving accuracy. Additionally, the mesh adaptivity algorithm assesses the local turbulence fluctuations via local Reynolds number and vortex identification criteria. This assessment leads to the refinement of regions with higher turbulence fluctuations down to the smallest scale limit in the inertial range in the corresponding direction, and also leads to the coarsening of regions without turbulence fluctuation up to largest scale limit in the inertial range. This method is tested using a flow past a sphere test case. The results are compared both qualitatively and quantitatively to results obtained with the standard Smagorinsky model, and demonstrate the better performance of our method with lower computational cost. This allows us to simulate large Reynolds number cases and compare our quantitative results to experimental results found in the literature, emphasising that our method produces good results at reasonable computational cost.

Sensitivity of fish habitat suitability to multi-resolution hydraulic modeling and field-based description of meso-scale river habitats

In-stream habitat models at the meso-scale are increasingly used to quantify the effects of hydro-morphological pressures in rivers. The spatial distributions of water depth and velocity represent key attributes of physical habitat. Choosing between field surveys, hydraulic modeling or their integration is made depending on available tools, technical skills, budget and time. However, the sensitivity to such choices of estimated habitat conditions suitable for biological organisms, such as fish, is poorly known.In this study, three commonly used approaches in hydraulic-habitat modeling were compared and tested on a mountain stream, the Mareta River (NE Italy). Two approaches were based on 2D hydraulic modeling, calculated on computational meshes with varying resolution and quality: (1) high-resolution meshes derived from topographical data obtained from Airborne Bathymetric LiDAR; (2) a mesh extrapolated from topographical cross-sectional profiles. The third approach (3) was based on in-stream surveys. From these, suitable channel-area for two fish species, the marble trout (juvenile and adult), and the European bullhead (adult), were estimated.Results showed that decreasing mesh resolution and quality affects the simulated water depth and velocity distributions, both in terms of their average and their standard deviation. The largest differences were found for the in-stream survey-based results. Morphologically complex unit types, such as steps, rapids and pools were more sensitive than simpler mesohabitats, such as glides and riffles. The most sensitive hydro-morphological unit types to the chosen approach were backwaters, glides being the least sensitive, also in terms of their suitability as mesohabitats. Despite that, a key finding is that errors are minimized when deriving habitat - streamflow rating curves at the reach scale, for which all approaches were largely able to reproduce the main characteristics of the curve, i.e. maxima, minima and inflection points.