Fluids, Volume 7, Issue 2 (February 2022) – 47 articles

More than 80% of the ocean is not fully mapped or even observed, even though it covers over 70% of our planet’s surface. One of the primary challenges for ocean observation and monitoring is the required power for exploration and monitoring systems, which often operate in remote areas of the ocean. This work addresses the design and development of an ocean wave energy converter that can be installed on observational buoys to provide enough power for sensors, cameras, data acquisition and recording, as well as data transfer units. The initial simulations of the prototype indicate that this system can produce up to 3.7–3.85 watts of power on average, with greater than 12 watts of maximum power in two selected sites in California and Hawaii. The proposed system is simple and low-cost. Further, multiple energy converters can be installed on one buoy to address higher power needs. Full article

Large-scale, ordered magnetic fields in several astrophysical sources are supposed to be originated, and maintained against dissipation, by the combined amplifying action of rotation and small-scale turbulence. For instance, in the solar interior, the so-called $\alpha -\Omega$ mean-field dynamo is known to be responsible for the observed 22-years magnetic cycle. Similar mechanisms could operate in more extreme environments, like proto neutron stars and accretion disks around black holes, for which the physical modelling needs to be translated from the regime of magnetohydrodynamics (MHD) and Newtonian gravity to that of a plasma in a general relativistic curved spacetime (GRMHD). Here we review the theory behind the mean field dynamo in GRMHD, the strategies for the implementation of the relevant equations in numerical conservative schemes, and we show the most important applications to the mentioned astrophysical compact objects obtained by our group in Florence. We also present novel results, such as three-dimensional GRMHD simulations of accretion disks with dynamo and the application of our dynamo model to a super massive neutron star, remnant of a binary neutron star merger as obtained from full numerical relativity simulations. Full article

In this paper, five computational approaches are used to model bulk flow parameters of inclined round negatively buoyant jets. More specifically, an integral model employing Gaussian distributions for velocity and apparent acceleration of gravity, proposed in earlier study, is implemented with two different entrainment formulae. The remaining three computational approaches include an integral model known as EMA, which takes into consideration the fluid detachment occurring in the inner side of the flow near the terminal height, the widely known commercial model Corjet and analytical solutions that were proposed in a previous study. Predictions are provided for the maximum centerline height and its horizontal position, the terminal height of the upper jet boundary, the horizontal distance to the points where the jet centerline and the upper jet boundary return to the source level, the centerline dilution at the maximum height and the centerline dilution at the return point. Detailed comparisons are made in dimensionless form between the estimations provided by the models and a wide range of experimental data for discharge angles between 15° and 90°. Conclusions are drawn regarding the performance of the five computational approaches. Full article

This research paper focuses on a novel coupling of the aerodynamic and structural behaviour of a double-element composite front wing of a Formula One (F1) vehicle, which was simulated and studied for the first time here. To achieve this goal, a modified two-way coupling method was employed in the context of high performance computing (HPC) to simulate a steady-state fluid-structure interaction (FSI) configuration using the ANSYS software package. The front wing plays a key role in generating aerodynamic forces and controlling the fresh airflow to maximise the aerodynamic performance of an F1 car. Therefore, the composite front wing becomes deflected under aerodynamic loading conditions due to its elastic behaviour which can lead to changes in the flow field and the aerodynamic performance of the wing. To reduce the uncertainty of the simulations, a grid sensitivity study and the assessment of different engineering turbulence models were carried out. The practical contribution of our investigations is the quantification of the coupled effect of the aerodynamic and structural performance of the wing and an understanding of the influence of ride heights on the ground effect. It was found that the obtained numerical surface pressure distributions, the aerodynamic forces, and the wake profiles show an accurate agreement with experimental data taken from the literature. Full article

By rotary empirical orthogonal function and coastal-trapped wave mode analyses, we analyzed current velocity data, collected from 2001 to 2016. The data were obtained by an acoustic Doppler current profiler, deployed upward at a location of 41°39.909′ N, 144°20.695′ E, on a 2630-m deep continental slope seabed off the southeastern coast of Hokkaido, Japan. The results indicate that the current intensifies toward the bottom and is directed nearly toward the shore, reaching an average speed of ~2.5 cm s⁻¹ just above the bottom. The thickness of the along-slope northward component of the bottom-intensified current varied within the range of 50–350 m. We found that the current thickness change was caused by oceanic barotropic disturbances, produced by the intensification of the Aleutian Low, largely related to the El Niño–Southern Oscillation and modified through the excitation of bottom-trapped modes of coastal-trapped waves. This finding improves the prediction accuracy of the the bottom-intensified current change, being beneficial for suspended sediment studies, construction and maintenance of marine structures, planning of deep drilling, and so on. Full article

In subcooled boiling flows beyond a certain heat flux, heat transfer is hampered due to a phenomenon known as Departure from Nucleate Boiling (DNB). Conducting DNB experiments at one-to-one nuclear reactor operating conditions is highly challenging and expensive. Another alternative approach is to use Look-up table data. However, its applicability is limited due to its dependence on rod bundle correction factors. In the present investigation, a state-of-the-art Eulerian-Eulerian two-fluid model coupled with an extended heat flux partitioning model is used to predict DNB in tubes and rod bundles with square and hexagonal lattices (relevant to Pressurized Water Reactors). In this approach, bubble departure characteristics are modeled using semi-mechanistic models based on force balance analysis. The predicted DNB values are compared with experimental and Look-up table data and found out to be within 1.8% to 20%. Full article

The dissipation of the kinetic energy (KE) associated with oceanic flows is believed to occur primarily in the oceanic bottom boundary layer (BBL), where bottom drag converts the KE from mean flows to heat loss through irreversible mixing at molecular scales. Due to the practical difficulties associated with direct observations on small-scale turbulence close to the seafloor, most up-to-date estimates on bottom drag rely on a simple bulk formula ($C_d{U}^3$) proposed by G.I. Taylor that relates the integrated BBL dissipation rate to a drag coefficient ($C_d$) as well as a flow magnitude outside of the BBL (U). Using output from several turbulence-resolving direct numerical simulations, it is shown that the true BBL-integrated dissipation rate is approximately 90% of that estimated using the classic bulk formula, applied here to the simplest scenario where a mean flow is present over a flat and hydrodynamically smooth bottom. It is further argued that Taylor’s formula only provides an upper bound estimate and should be applied with caution in the future quantification of BBL dissipation; the performance of the bulk formula depends on the distribution of velocity and shear stress near the bottom, which, in the real ocean, could be disrupted by bottom roughness. Full article

This study presents a hybrid non-linear unsteady vortex lattice method-vortex particle method (NL UVLM-VPM) to investigate the aerodynamics of rotor blades hovering in and out of ground effect. The method is of interest for the fast aerodynamic prediction of helicopter and smaller rotor blades. UVLM models the vorticity along the rotor blades and near field wakes with panels that are then converted into their equivalent vortex particle representations. The standard Vreman subgrid scale model is incorporated in the context of a large eddy simulation for mesh-free VPM to stabilize the wake development via particle strength exchange (PSE). The computation of the pairwise interactions in the VPM are accelerated using the fast-multipole method. Non-linear UVLM is achieved with a low computational cost viscous-inviscid alpha coupling algorithm through a stripwise 2D Reynolds-averaged Navier–Stokes (RANS) or empirical database. The aerodynamics of the scaled S76 rotor blades in and out of ground effect from the hover prediction workshop is investigated with the proposed algorithm. The results are validated with experimental data and various high-fidelity codes. Full article

We present in this paper a model of the transport of human respiratory particles on a Charlotte Area Transit System (CATS) bus to examine the efficacy of interventions to limit exposure to SARS-CoV-2, the virus that causes COVID-19. The methods discussed here utilize a commercial Navier–Stokes flow solver, RavenCFD, using a massively parallel supercomputer to model the flow of air through the bus under varying conditions, such as windows being open or the HVAC flow settings. Lagrangian particles are injected into the RavenCFD predicted flow fields to simulate the respiratory droplets from speaking, coughing, or sneezing. These particles are then traced over time and space until they interact with a surface or are removed via the HVAC system. Finally, a volumetric Viral Mean Exposure Time (VMET) is computed to quantify the risk of exposure to the SARS-CoV-2 under various environmental and occupancy scenarios. Comparing the VMET under varying conditions should help identify viable methods to reduce the risk of viral exposure of CATS bus passengers during the COVID-19 pandemic. Full article

The rim breakup of an impacting drop is experimentally investigated by comparing the impacts on superheated and superhydrophobic surfaces. The objective of the present study is to experimentally examine whether the Bo = 1 criteria holds for the rim breakups of drops impacting on the surfaces. A transparent sapphire plate was heated to achieve the Leidenfrost impact, which enables us to observe with a high-speed camera from below. The characteristics of the rim breakup were evaluated quantitatively using a particle tracking velocimetry method for both the rim and the drops generated. As a result, we clarified that Bo of the rim increases in the spreading phase and marks the highest value of 0.5 on a superheated surface, which is smaller than that on a pillar, where Bo ≈ 1. On a superhydrophobic surface, the highest Bo was 1.2, which is smaller than that on a wettable solid surface, 2.5, but close to the value on a pillar. We also revealed that diameters of generated drops collapse on a master curve when plotted as a function of pinch-off time for both the impacts on superheated and superhydrophobic surfaces. Full article

Fully developed turbulent flow in a pipe was studied by considering experimental and computational methods. The aim of this work was to build on the legacy of the University of Manchester, which is widely regarded as the birthplace of turbulence due to the pioneering work of the prominent academic Professor Osborne Reynolds (1842–1912), by capturing the evolution of fluid turbulence analysis tools over the last 100 years. A classical experimental apparatus was used to measure the mean velocity field and wall shear stress through four historical techniques: static pressure drop; mean square signals measured from a hot-wire; Preston tube; and Clauser plot. Computational Fluid Dynamics (CFD) was used to simulate the pipe flow, utilizing the Reynolds-averaged Navier–Stokes (RANS) method with different two-equation turbulence models. The performance of each approach was assessed to compare the experimental and computational methods. This comparison revealed that the numerical results produced a close agreement with the experiments. The finding shows that, in some cases, CFD simulations could be used as alternative or complementary methods to experimental techniques for analyzing fully developed turbulent pipe flow. Full article

In practice, often devices are ordered rod structures consisting of a large number of rods. Heat exchangers, fuel assemblies of nuclear reactors, and their cores in the case of using caseless assemblies are examples of such devices. Simulation of heat and mass transfer processes in such devices in porous-body approximation can significantly reduce the required resources compared to computational fluid dynamics (CFD) approaches. The paper describes an integral turbulence model developed for defining anisotropic model parameters of a porous body. The parameters of the integral turbulence model were determined by numerical simulations for assemblies of smooth rods, assemblies with spacer grids, and wire-wrapped fuel assemblies. The results of modeling the flow of a liquid metal coolant in an experimental fuel assembly with local blocking of its flow section in anisotropic porous-body approximation using an integral turbulence model are described. The possibility of using the model of an anisotropic porous body with the integral model of turbulence to describe thermal-hydraulic processes during fluid flow in rod structures is confirmed. Full article

A multiscale modeling concept for numerical simulation of multiphysics turbulent flow utilizing map-based advection is described. The approach is outlined with emphasis on its theoretical foundations and physical interpretations in order to establish the context for subsequent presentation of the associated numerical algorithms and the results of validation studies. The model formulation is a synthesis of existing methods, modified and extended in order to obtain a qualitatively new capability. The salient feature of the approach is that time advancement of the flow is fully resolved both spatially and temporally, albeit with modeled advancement processes restricted to one spatial dimension. This one-dimensional advancement is the basis of a bottom-up modeling approach in which three-dimensional space is discretized into under-resolved mesh cells, each of which contains an instantiation of the modeled one-dimensional advancement. Filtering is performed only to provide inputs to a pressure correction that enforces continuity and to obtain mesh-scale-filtered outputs if desired. The one-dimensional advancement, the pressure correction, and coupling of one-dimensional instantiations using a Lagrangian implementation of mesh-resolved volume fluxes is sufficient to advance the three-dimensional flow without time advancing coarse-grained equations, a feature that motivates the designation of the approach as autonomous microscale evolution (AME). In this sense, the one-dimensional treatment is not a closure because there are no unclosed terms to evaluate. However, the approach is additionally suitable for use as a subgrid-scale closure of existing large-eddy-simulation methods. The potential capabilities and limitations of both of these implementations of the approach are assessed conceptually and with reference to demonstrated capabilities of related methods. Full article

Insect wings are generally constructed from veins and solid membranes. However, in the case of the smallest flying insects, the wing membrane is often replaced by hair-like bristles. In contrast to large insects, it is possible for both bristled and membranous wings to be simultaneously present in small insect species. There is therefore a continuing debate about the advantages and disadvantages of bristled wings for flight. In this study, we experimentally tested bristled robotic wing models on their ability to generate vertical forces and scored aerodynamic efficiency at Reynolds numbers that are typical for flight in miniature insects. The tested wings ranged from a solid membrane to a few bristles. A generic lift-based wing kinematic pattern moved the wings around their root. The results show that the lift coefficients, power coefficients and Froude efficiency decreased with increasing bristle spacing. Skin friction significantly attenuates lift production, which may even result in negative coefficients at elevated bristle spacing and low Reynolds numbers. The experimental data confirm previous findings from numerical simulations. These had suggested that for small insects, flying with bristled instead of membranous wings involved less change in energetic costs than for large insects. In sum, our findings highlight the aerodynamic changes associated with bristled wing designs and are thus significant for assessing the biological fitness and dispersal of flying insects. Full article

The natural convection of incompressible flow confined within an enclosed right-angled triangular and isosceles cavity was investigated numerically using the multirelaxation time lattice Boltzmann method (MRT-LBM). According to the left and inclined walls thermal boundary conditions, two cases were considered in this study. In the first case, the inclined side of the enclosure was adiabatic, and the horizontal wall was heated, while the left one was kept at a cold temperature. However, the states of the left and inclined walls were interchanged in the second case. As the flow is only transported under the convection force, this study was carried out for the Rayleigh number ranging from $Ra={10}^3$ to ${10}^6$. The effects of the Rayleigh number on velocity and temperature profiles, streamlines, isotherms, and average Nusselt number were investigated. The position of cold and adiabatic walls had a great effect on the results. The results obtained are in good agreement with those of the literature and show the robustness of the MRT-LBM approach. In both cases, the heat-transfer rate increases with the increase in the Rayleigh number. Full article

Published theories and observations have shown that dissipation of gravity waves implies frequency downshifting of wave energy. Hence, for wind-waves, the wind energy input to the highest frequencies is of special interest. Here it is shown that this input is vital, because the direct wind energy input obtained by the air-pressure’s work on most gravity waves is slightly less than what the waves need to grow. Further, the wind’s input of the angular momentum that waves need to grow is found to be absent at most gravity wave frequencies. The capillary waves that appear at the surface of the sea when the wind is blowing solve these problems. To demonstrate this, an extension of linear wave theory is established to study possibilities and limitations for transfer of energy and angular momentum from the wind to waves through these frequencies. The theory describes regular, gravity–capillary waves with constant amplitude under laminar conditions. It includes surface tensions, viscosity, gravity and a wind-generated shear current, and shows that these waves—contrary to most gravity waves—receive more energy from the wind than they dissipate and angular momentum they cannot keep. Hence, the problem of the missing input of energy and angular momentum from wind to gravity waves is solved by transfers through the capillary waves. This implies that capillary waves are vital to obtain growing gravity waves. Full article

Slipstream wake structures generated by the passing of high-speed rail vehicles represent a hazard to passengers and workers in close proximity. In this article, the possibility of reducing peak slipstream velocities through the implementation of angled fins or swirling flow injection is assessed on the basis of improved delayed detached eddy simulations (IDDES). The key to improving slipstream velocities involves redirecting and/or reducing the internal energy, a pair of meandering counter-rotating vortex cores that are associated with large wake slipstream deviations. It is demonstrated that the danger imposed by slipstream wake structures, as measured by the induced velocity measures recorded at a series of test points adjacent to the passing train, could be significantly reduced, with decreases from 10% up to 23%, recorded across a range of sampling locations. The means by which these reductions are generated and the corresponding changes in the flow are also explored through analysis of the modified wakes. As such, these devices show promise at improving the operational safety of high-speed rail vehicles. Full article

The problem of a geophysical western boundary current negotiating a gap in its supporting boundary is considered. For traditional straight, parallel gaps, such systems are known to exhibit two dominant states, gap penetrating and leaping, with the transitional dynamics between states displaying hysteresis. However, for more complex geometries, such as angled or offset gap configurations, the question of multiple states and hysteresis is unresolved. In such cases, the inertia of the western boundary current is oriented into the gap, hence the assumption that increased inertia promotes gap penetrating loop current states. Here we address the problem numerically in an idealized setting. It is found that despite the inertia of the current being directed into the gap, for large western boundary current transport values, leaping states will be present. That is, we show here that the presence of multiple states with hysteresis for gap-leaping western boundary current systems is robust to both angled and offset gap geometries. Full article

The issues of leakage with respect to the clearance between the pump plunger outer surface and the pump barrel inner surface and other operational conditions have been revisited in this paper. Both Poiseuille flow rate due to the pressure difference and Couette flow rate due to the plunger motion have been considered. The purpose of this study is to explore the possibility of representing the entire downhole pump system with a simple viscoelastic model. We have explored both Kelvin and Maxwell viscoelastic models along with the dynamic behaviors of a mass point attached to the viscoelastic model. By using the time-dependent polished rod force measured with a dynamometer as the input to the viscoelastic models, we have obtained the displacement responses, which match closely with the actual measurements in experiments and operations. Further study and experiments have been planned and partially implemented in the McCoy School of Engineering at Midwestern State University, a member of the Texas Tech University System. Full article

When pressurized with a fluid, the sweeping jet actuator (SWJA) emits a self-induced and self-sustained temporally continuous, but spatially oscillating bi-stable jet at the outlet. The SWJA adds up local momentum using the Coanda extension without any moving parts and, therefore, can be a promising tool for suppressing aerodynamic flow separation. However, the SWJA needs to be integrated into curved aerodynamic surfaces with an angle. The present study focuses on investigating the effects of various exit nozzle geometries on the flow field. The geometric parameters considered were the exit nozzle angle, diffuser arm length, and curvature. The working fluid was air, and the mass flow rate was 0.015 lb/s. A set of time-dependent flow fields was computed using a two-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) simulation. The time history of pressure was recorded inside the upper and lower feedback channels. The jet oscillation frequency was obtained by employing the fast Fourier transform (FFT) for all datasets. The results were compared against the baseline case and data available in the literature. The results showed that external geometric variations at the nozzle exit had a negligible impact on the oscillation frequency. However, there were notable effects on the pressure and velocity distribution in the flow field, indicating that the actuator had sensitivity towards the geometric variation of the exit nozzle—the wider the exit nozzle, the lower the downstream velocity. Notably, we observed that the mean velocity at the exit nozzle downstream for the curvature case was 40.3% higher than the reference SWJA. Full article

The paper considers the numerical modeling of the processes of homogeneous and heterogeneous condensation and evaporation in multiphase flows using the method of moments. Nonstationary processes of gas dynamics and phase transitions in the two-dimensional plane and axisymmetric regions are described by a general system of equations. The system of equations is expanded by adding two equations. One describes the evolution of the total mass fraction of the condensing substance; the other describes the evolution of the mass fraction of solid particles. An instant wetting model is used to model heterogeneous nucleation. The Gyarmathy model is used for the approximation of the average droplet growth rate. Heterogeneous condensation is modeled based on the distribution function of foreign impurities. An approach to calculating evaporation in the heterogeneous case is proposed. A comparison of the proposed models with a numerical experiment is given. Numerical simulation of homogeneous-heterogeneous condensation in a gas-dynamic ejector is carried out. Full article

Marine aquaculture has proliferated over the past decade, expanding into new, untapped open-water cultivation areas, such as lakes, rivers and deeper offshore environments, in response to increasing demand for seafood by consumers. However, to ensure sustainable development, it is necessary to minimize the impact of other ocean activities and the environment through science-based spatial planning. The choice of the primary site (physical carrying capacity) depends mainly on the aquaculture system, which varies around the world. However, the site is considered one of the factors (production, ecological and social) keys to any aquaculture operation, especially in the African continent. This choice affects both the success and sustainability of the products cultivated and the resolution of conflicts between different activities as well as the rational use of space. This study aims to identify suitable areas (primary site selection) for aquaculture in the Moroccan Atlantic continental shelf focused on the sub-area located between Cap Ghir 31.25° and Tarfaya 27.47°, based on the assessment of the dominant wave energy by implementing the hydrodynamical SWAN (Simulating Waves Nearshore) model dedicated for this kind of study. We derived the inputs for the SWAN model from WW3 (WAVEWATCH III model), which the AVISO data-products have extensively validated. The results show that, even if the Atlantic area is known for the agitation of its seas, there is the possibility of having adequate areas for aquaculture with an overall capacity that could extinguish the 389 ha in the study area if aquatic cultivation manages to exploit the offshore areas. At the level of the sub-zone belonging to the sous-Massa region (zone 1), the results show a strong coherence between the values of the surfaces estimated by the study and the actual values resulting from the development plan, with a value of 69 Ha for the first and 75 for the second, i.e., equal to 6 Ha, due to the geomorphology of the coast and natural coastal shelters, which play favorably on the environment for aquaculture development. These areas may attract the greed of investors, although they are in the process of being the subject of an aquaculture development plan. Full article

Mechanical circulatory support (MCS) devices are designed to perform the functional needs of organs and to meet clinical hemocompability criteria. Critical complications have been reported with their long-term use such as thrombosis, anemia and gastrointestinal bleeding. Damage to red blood cells (RBCs), which occurs with nonphysiological blood flow conditions such as contact with foreign surfaces, high shear stress, and turbulence, is a major problem for the design and development of these systems. Even in the absence of hemolysis, cardiovascular devices (CAD) still cause cell injury and shortened RBC lifespans. This review summarizes various effects that occur to erythrocytes exposed to supraphysiological but sublethal stresses. Full article

Turbulence control in the form of a streamwise travelling wave of transverse wall motion was studied numerically by employing direct numerical simulations (DNS). Both total and phase averaging were utilised to examine the statistical behaviour of the turbulence affected by the wall forcing, with a focus on the skin friction. Comparison with results from pure temporal and spatial wall forcing are conducted, and a compilation of data is used to explore analogies with drag-reduced channel flow. Full article

The analytical review has shown that the scientific inquiry for effective technologies for high-viscosity oil field development is a critical task of the present-day oil industry. The paper presents a technique for determining the expediency and effectiveness of deploying the near-wellbore cyclic steam stimulation technology for oil recovery enhancement. The method involves the calculation of process parameters of the technology cycle and the comparative analysis of cumulative oil production before the treatment (base case) and after its deployment. Separately, the work focuses on studying the impact of dynamic oil viscosity over the entire temperature range on the technology effectiveness and expediency. The laboratory studies showed dynamic viscosity correlation dependencies for six different oils of the Nozhovskaya group of oil fields (Russian Federation) characterized as viscous and highly viscous. As a case study of the proposed method application, a numerical simulation of the technology deployment was carried out for six oil samples. The calculations determined inexpediency of cyclic steam stimulation for one of the samples since oil well downtime for workover operation prevailed over the time of near-wellbore cooling. Full article

The simulation of fusion plasmas in realistic magnetic configurations and tokamak geometries still requires the development of advanced numerical algorithms owing to the complexity of the problem. In this context, we propose a Hybrid Discontinuous Galerkin (HDG) method to solve 2D transport fluid equations in realistic magnetic and tokamak wall geometries. This high-order solver can handle magnetic equilibrium free structured and unstructured meshes allowing a much more accurate discretization of the plasma facing components than current solvers based on magnetic field aligned methods associated with finite-differences (volumes) discretization. In addition, the method allows for handling realistic magnetic equilibrium, eventually non steady, a critical point in the modeling of full discharges including ramp up and ramp down phases. In this paper, we introduce the HDG algorithm with a special focus on recent developments related to the treatment of the cross-field diffusive terms, and to an adaptive mesh refinement technique improving the numerical efficiency and robustness of the scheme. The updated solver is verified with a manufactured solution method, and numerical tests are provided to illustrate the new capabilities of the code. Full article

In this review, we summarize existing trends of flow control used to improve the aerodynamic efficiency of wings. We first discuss active methods to control turbulence, starting with flat-plate geometries and building towards the more complicated flow around wings. Then, we discuss active approaches to control separation, a crucial aspect towards achieving a high aerodynamic efficiency. Furthermore, we highlight methods relying on turbulence simulation, and discuss various levels of modeling. Finally, we thoroughly revise data-driven methods and their application to flow control, and focus on deep reinforcement learning (DRL). We conclude that this methodology has the potential to discover novel control strategies in complex turbulent flows of aerodynamic relevance. Full article

Convective velocity perturbations (CVPs) are known to play an important role in the response of flames to acoustic perturbations and in thermoacoustic combustion instabilities. In order to elucidate the flow-physical origin of CVPs, the present study models the response of laminar premixed slit flames to low amplitude perturbations of the upstream flow velocity with a reduced order flow decomposition approach: A linearized G-equation represents the shape and heat release rate of the perturbed flame, while the velocity perturbation field is decomposed into irrotational and solenoidal contributions. The former are determined with a conformal mapping from geometry and boundary conditions, whereas the latter are governed by flame front curvature and flow expansion across the flame, which generates baroclinic vorticity. High-resolution CFD analysis provides values of model parameters and confirms the plausibility of model results. This flow decomposition approach makes it possible to explicitly evaluate and analyze the respective contributions of irrotational and solenoidal flows to the flame response, and conversely the effect of flame perturbations on the flow. The use of the popular ad hoc hypothesis of convected velocity perturbation is avoided. It is found that convected velocity perturbations do not result from immediate acoustic-to-hydrodynamic mode conversion, but are generated by flame-flow feedback. In this sense, models for flame dynamics that rely on ad-hoc models for CVPs do not respect causality. Furthermore, analysis of the flame impulse response reveals that for the configuration investigated, flame-flow feedback is also responsible for “excess gain” of the flame response, that is, the magnitude of the flame frequency response above unity. Full article

An astrophysical dynamo converts the kinetic energy of fluids into magnetic energy. Dynamo is a non-local process. Here, we consider whether a dynamo can operate at the magnetopauses of magnetic rapidly rotating planets. We analyze the main necessary condition for the work of this type of dynamo—the rotation transfer from the planet to the magnetopause. We show the role of the current disc around a rapidly rotating magnetic planet in the redistribution of angular momentum depending on the direction of the external magnetic field, using the example of the Jupiter’s magnetodisc. Full article

This article deals with the study of the initial water saturation effect of a porous medium on the oil recovery factor using a water-based nanosuspension. The initial water saturation of the porous medium in the computations varied within the range from 0 to 90%. The nanoparticle SiO₂ concentration varied from 0 to 1 wt%. The particle sizes were equal to 5, 18, 22, and 50 nm. Experimentally measured wetting angles and the interfacial tension coefficient depending on the concentration and size of nanoparticles were used in computations. A mathematical model was developed, describing the transfer and diffusion of nanoparticles within the aqueous phase during immiscible displacement of oil by nanosuspension from a porous medium. Using the developed model, a systematic computational study of the effect of the initial water saturation of the core micromodel on the oil recovery factor using nanosuspension was carried out. It was revealed that with an increase in the initial water saturation, the oil recovery factor monotonically decreased in the case of displacement both by water and nanosuspension. It was shown that with an increase in the concentration of nanoparticles and a decrease in their size, the oil recovery factor increased. At that, the relative increase in the recovery factor had a maximum at an initial water saturation equal to 60%. Full article