Low-speed Flow Research Articles

Early post-settlement events, rather than settlement, drive recruitment and coral recovery at Moorea, French Polynesia.

Understanding population dynamics is a long-standing objective of ecology, but the need for progress in this area has become urgent. For coral reefs, achieving this objective is impeded by a lack of information on settlement versus post-settlement events in determining recruitment and population size. Declines in coral abundance are often inferred to be associated with reduced densities of recruits, which could arise from mechanisms occurring at larval settlement, or throughout post-settlement stages. This study uses annual measurements from 2008 to 2021 of coral cover, the density of coral settlers (S), the density of small corals (SC), and environmental conditions, to evaluate the roles of settlement versus post-settlement events in determining rates of coral recruitment and changes in coral cover at Moorea, French Polynesia. Coral cover, S, SC, and the SC:S ratio (a proxy for post-settlement success), and environmental conditions, were used in generalized additive models (GAMs) to show that: (a) coral cover was more strongly related to SC and SC:S than S, and (b) SC:S was highest when preceded by cool seawater, low concentrations of Chlorophyll a, and low flow speeds, and S showed evidence of declining with elevated temperature. Together, these results suggest that changes in coral cover in Moorea are more strongly influenced by post-settlement events than settlement. The key to understanding coral community resilience may lie in elucidating the factors attenuating the bottleneck between settlers and small corals.

Identifying some regularities of the aerodynamics around wind turbines with a vertical axis of rotation

The design of wind turbines with a vertical axis of rotation is quite simple, which successfully increases the level of efficiency. Existing vane wind turbines have a shortage of currents in the form of negative torque, and installations operating on the Magnus effect have a low lifting force. In this regard, the development and research of installations operating at speeds from 3 m/s, with combined blades with increased work efficiency is an urgent topic. The object of the study is a wind turbine consisting of a system of rotating cylinders and fixed blades operating at low air flow speeds starting from 3 m/s. Numerical studies were carried out using the Ansys Fluent program and the implemented k-ε turbulence model. A special feature of the work is the combined use of two lifting forces: a cylinder and fixed blades, which made it possible to increase the output aerodynamic parameters. Calculations were performed for incoming flow rates of 3 m/s, 9 m/s, 15 m/s and cylinder rotation speeds of 315 rpm, 550 rpm, 720 rpm. It is determined that the period of change of the moment of forces T is 0.5 m/s, which corresponds to 2 revolutions of the wind wheel per minute. It was found that the cylinder rotation frequency in the range from 315 rpm to 720 rpm does not affect the period of change in the moment of forces, but the amplitude of the moment of forces increases with decreasing rotation frequency. The dependences of the rotation speed of the wind wheel on the velocity of the incoming flow, found by the method of sliding grids and 6DOF, are also obtained. It is determined that the installation begins to make revolutions from 3 m/s, with a positive torque of forces. The field of practical application of the numerical results will be useful for further research of wind turbines with combined blades

Design Optimization of Blade Tip in Subsonic and Transonic Turbine Stages—Part I: Stage Design and Preliminary Tip Optimization

Abstract Rotor blade tip has significant influence on turbine stage aerodynamics and heat transfer. Most previous efforts have been based on low-speed cascade settings. However, more recent research on transonic blade tips exhibits distinctive flow features with qualitatively different performance sensitivities. These prompt two key issues of interest on the related flow conditioning. First, the contrast between a low-speed flow and a transonic regime highlights the less studied high-subsonic flow regime, closely relevant to many realistic turbine designs. Second, the relative casing movement and upstream inflow conditions, known to have non-negligible effects, indicate the need to examine a rotor blade tip in a realistic stator–rotor stage environment, which is also lacking. To elaborate the Mach number effect in the flow regimes of practical interest, we aim to examine a high subsonic stage in a direct and consistent comparison with a transonic one. To this end, a high subsonic stage (exit Mach number of 0.7) and a transonic (exit Mach number of 1.1) are designed at the same Reynolds number with a three-dimensional parameterization and meshing system. The tip squealer height is used as a representative parameter to investigate the sensitivity of the stage aerothermal performance. The multi-objective optimization using the Kriging surrogated model is employed to identify the Pareto fronts for the stage efficiency and the heat transfer. The comparison of the optimized results between these two stages shows distinctively different trends in the performance variation with the squealer height. The efficiency of the subsonic stage increases with the squealer height reaching a plateau. In contrast, the efficiency in the transonic stage first increases and then drops to the level comparable to that of a flat tip. Significantly, the present results indicate, for the first time, that the squealer tip in a transonic stage may not be as effective as in a subsonic stage. On the other hand, for heat transfer, sensitivity variations are more complex. The overall heat load and the local nonuniformity lead to qualitatively different sensitivities with the squealer height, as well as completely incomparable Pareto fronts. These observed heat transfer sensitivities raise the question on how to effectively conduct a combined aerodynamic and heat transfer performance design optimization. The authors subsequently resort further aerothermal physics analyses described in a companion paper as Part II of the two-part article. In Part II, the physical interpretation of the contrasting aero-efficiency sensitivities for the two stages, as well as a physical understanding leveraged selection of the objective function for such combined blade tip aerothermal optimization will be presented.

Towards the effect of cracks on the instability of a plate loaded by low-speed axial flow

This paper presents a model of a cantilevered plate with arbitrary cracks in an axial airflow and the crack’s effect on the plate’s aeroelastic instability. It employs the Dirac function to delineate the effect of cracks on the plate’s bending stiffness and utilizes Fourier series coupled with a mirroring technique to calculate the slope function of the cracked plate. The Theodorsen aerodynamic theory gives the fluid pressure, and the Galerkin method applies to the discretization of the fluid–structure interaction equation. The investigation encompasses three scenarios: single, double, and multiple arbitrary cracks. The flutter boundary of the structure is calculated and analyzed under different parameters. The modal participation factor is used to explore the effect of each structure mode on the plate instability. The results show that cracks significantly affect the plate’s stability. The existence of cracks not only affects the critical flow velocity but also affects the characteristics of the instability mode. When the number of cracks is small, some special positions of the cracks will trigger the structure’s higher-order modes and increase the critical speed. When the number of cracks is large, and the distribution of cracks is uniform, the structural stiffness decreases and critical speed.

Ultrasound microflow patterns help in distinguishing malignant from benign thyroid nodules

BackgroundVascular features are not commonly used to evaluate thyroid nodules by conventional ultrasound due to the low sensitivity. Superb Microvascular Imaging (SMI) is a new ultrasonic Doppler technology that specializes in depicting microvessels and low-speed flow. The objective of this study was to explore the value of microflow features on SMI in differentiating malignant from benign thyroid nodules.MethodsOne hundred and seventy-seven adult patients with thyroid nodules in our center from October 2021 to June 2022 with available histopathological results were recruited, including 125 malignant nodules and 123 benign nodules. Preoperative ultrasound was performed using greyscale, Color Doppler Flow Imaging (CDFI), monochrome SMI (mSMI) and color SMI (cSMI). Vascular features such as flow richness, microflow distribution and microflow patterns of malignant thyroid nodules were compared with those of benign nodules.ResultsPenetrating vessel ≥ 1 (82.4% in the malignant group vs. 30.9% in the benign group, P < 0.001), the crab claw-like pattern (64.0% vs. 10.6%, P < 0.001) and the root hair-like pattern (8.0% vs. 2.4%, P = 0.049) were common in malignant thyroid nodules, among which the crab claw-like pattern was an independent risk factor for malignant thyroid nodules. The wheel-like pattern (1.6% in the malignant group vs. 33.3% in the benign group, P < 0.001) and the arborescent pattern (0 vs. 19.5%, P < 0.001) were more likely to appear in benign nodules. The diagnostic specificities of the crab claw-like pattern and the root hair-like pattern for malignant thyroid nodules were 0.894, 0.976, and the positive predictive values were 0.860, 0.769. The diagnostic specificities of the wheel-like pattern and the arborescent pattern for benign thyroid nodules were 0.984, 1.000, and the positive predictive values were 0.953, 1.000.ConclusionsThe crab claw-like pattern and the root hair-like pattern were microflow characteristics of malignant thyroid nodules. The wheel-like pattern and the arborescent pattern could help exclude the diagnosis of thyroid cancer.

AI-based shape optimization of galloping micro-power generators: exploring the benefits of curved surfaces

The advent of flow micro-power generation has resparked the interest in researching the galloping instability with the objective of determining the shape of the bluff body that is most prone to galloping. Such shape, which is sought to maximize the efficacy of galloping micro-power generators (GMPGs), must possess a very low cut-in flow speed while achieving large-amplitude steady-state oscillations beyond it. Additionally, since GMPGs can operate in environments with fluctuating flow conditions, the optimal shape must also have a very short rise time to its steady-state amplitude. In this work, we utilize computational fluid dynamics in conjunction with machine learning to optimize the shape of the bluff body of GMPGs for both steady-state and transient performance. We investigate a continuum shape description which encapsulates most of the cases studied earlier in the literature. The continuum has a straight frontal and dorsal faces with varying lengths, and side faces described by surfaces of different curvatures. The optimization study reveals that a curved-trapezoidal bluff body with the highest side surface curvature and frontal-to-dorsal ratio is the perfect shape for steady flow conditions. On the other hand, a square profile with the highest side surface curvature is the ideal choice for highly-fluctuating flow conditions because of its shortest rise time. The theoretical findings are replicated experimentally using wind tunnel tests.

Improved VIV energy harvesting with a virtual damper–spring system

In recent years, energy harvesting systems driven by vortex induced vibrations (VIV) emerged as potential alternative to conventional rotor-based installations, in particular for low flow speeds in water. Best energy outputs are obtained when the flow-dependent VIV frequency locks in the natural frequency of the energy harvesting structure. However, placed in tidal currents or rivers with variable flow rates, the VIV frequency changes, leading to frequency mismatch with efficiency losses. In this paper, we present a new mechatronic experimental set-up to study the harvesting efficiency of a VIV driven cylinder under various flow rates. In particular, we explore options to improve harvesting efficiency by adding computer controlled virtual damping and stiffness to the mechanical system. The setup comprises an elastically mounted, single-degree-of-freedom cylinder within a low-speed water tunnel. Virtual damping and stiffness are applied by controlling the force exerted through a mechanically coupled DC motor. The experiments reveal that optimal values for damping and stiffness are consistent with previous results from the literature. The maximum energy conversion efficiency of around 18% is achieved when oscillation and vortex shedding frequencies are synchronized. The explored parameter space highlights in particular that mismatches between oscillation and vortex shedding frequency strongly decrease the achievable efficiency. By contrast, the peak efficiency is less sensitive to an increase of damping due to energy harvesting. All these observations suggest that VIV driven energy harvesting systems need a real-time structural stiffness control to maintain an economically acceptable efficiency under various flow conditions. In the near future, we plan to automate the structural stiffness control under various flow rates by testing Machine Learning algorithms with our mechatronic set-up.

A normal-mode approach for high-speed rarefied plane Couette flow

Based on gas kinetic theory, a linear stability analysis method for low-speed rarefied flows was developed by Zou et al. [“A new linear stability analysis approach for microchannel flow based on the Boltzmann Bhatnagar–Gross–Krook equation,” Phys. Fluids 34, 124114 (2022) and “A novel linear stability analysis method for plane Couette flow considering rarefaction effects,” J. Fluid Mech. 963, A33 (2023)]. In the present study, we extended the method to high-speed rarefied flows using the Bhatnagar–Gross–Krook model. The Chebyshev spectral method is employed to discretize physical space, and the Gauss–Hermite and fourth-order Newton–Cotes quadrature methods are used to discretize velocity space. The fourth-order Newton–Cotes quadrature method was found to have sufficient accuracy for the stability analysis, laying the foundation for future research on hypersonic flows. The stability analysis of compressible rarefied Couette flow showed that acoustic modes are reflected between the wall and the relative sonic line, and the variation in their phase speed and growth rate with the wavenumber is not affected by the Mach number (Ma) and the Knudsen number (Kn). Increasing Kn has a stabilizing effect on both the acoustic and viscous modes, but as Ma increases, the attenuation rate of each mode's growth rate gradually decreases. In subsonic and sonic flows, the least stable viscous mode dominates in the case of small numbers. As Kn increases, the viscous mode gradually dominates over all wavenumber ranges considered in subsonic flow. In sonic flow, mode 1 is dominant in the region beyond the range of small wavenumbers. In supersonic flow, mode 2 is the least stable in the large wavenumber ranges, while mode 1 is the least stable in other wavenumber ranges. At a fixed wavenumber, as Kn increases, the decay rate of the growth rate of mode 2 is the highest. Additionally, under different Knudsen numbers, the growth rates of mode 1, mode 2, and the least stable viscous mode monotonically increase with an increase in Ma, with mode 2 showing the most significant increase.

A computational study of cell membrane damage and intracellular delivery in a cross-slot microchannel.

We propose a three-dimensional computational framework to simulate the flow-induced cell membrane damage and the resulting enhanced intracellular mass transport in a cross-slot microchannel. We model the cell as a liquid droplet enclosed by a viscoelastic membrane and solve the cell deformation using a well-tested immersed-boundary lattice-Boltzmann method. The cell membrane damage, which is directly related to the membrane permeability, is considered using continuum damage mechanics. The transport of the diffusive solute into the cell is solved by a lattice-Boltzmann model. After validating the computational framework against several benchmark cases, we consider a cell flowing through a cross-slot microchannel, focusing on the effects of the flow strength, channel fluid viscosity and cell membrane viscosity on the membrane damage and enhanced intracellular transport. Interestingly, we find that under a comparable pressure drop across the device, for cells with low membrane viscosity, the inertial flow regime, which can be achieved by driving a low-viscosity liquid at a high speed, often leads to much larger membrane damage, compared with the high-viscosity low-speed viscous flow regime. However, the enhancement can be significantly reduced or even reversed by an increase of the cell membrane viscosity, which limits cell deformation, particularly in the inertial flow regime. Our computational framework and simulation results may guide the design and optimisation of microfluidic devices, which use cross-slot geometry to disrupt cell membranes to enhance intracellular delivery of solutes.

Термическое воздействие на приземный слой тропосферы как способ управления рисками природных чрезвычайных ситуаций

Purpose. Natural emergencies that occur in mountainous regions annually damage ecosystems, the country's economy and lead to the death of people. On leeward mountain slopes, where precipitation is insufficient, there is an increased fire hazard of territories, complicating water supply of the population and ecosystems. On windward slopes excessive precipitation leads to the risk of avalanches, floods, and mudflows. The search for ways to redistribute the volume of precipitation between the windward and leeward slopes to control the risks of natural emergencies is relevant. The intensity of precipitation is significantly affected by the dew point height (DPH), which determines the position of the lower cloud boundary (the lower the dew point height, the greater the probability of precipitation). The article discusses the possibility of local thermal impact on the ground layer of the troposphere by heating a section of the mountain windward slope in order to increase the dew point height for the subsequent natural transfer of the precipitation zone to its leeward slope. The purpose of the study is to determine the values intervals of the difference in average temperatures of the tropospheric ground layer at its opposite boundaries and values intervals of average wind speed over the relief, where the thermal interaction of the tropospheric ground layer with a local area of the earth's surface can be used to effectively control the dew point height over the entire windward slope of the mountain. Methods. Three-dimensional field modeling is applied using the Fire Dinamic Simulator software product, which provides a numerical solution of the complete nonlinear Navier-Stokes thermohydrodynamic equations for low-speed temperature-dependent flows. Findings. It has been established that the thermal effect on the ground layer of the troposphere above the foot of the mountain windward slope under forced convection conditions can be used to control the distribution of the dew point height over this entire slope. Research application field. The conducted numerical study is the initial stage of a new scientific direction related to natural emergencies risks control caused by excess or lack of mountain slopes moisture (floods, droughts, forest and steppe fires). The obtained results indicate the fundamental possibility of such control by means of local temperature impact from the earth's surface to the ground layer of the troposphere. The practical significance of the results, including financial costs, required for the specified impact on the ground layer of the troposphere, can be revealed in subsequent publications after conducting full-scale experiments. Conclusions. The possibility of reducing the natural emergencies risks caused by excess or lack of moisture in mountain slopes territories by controlling the dew point height value over windward mountain slopes is shown. A solution to this problem is proposed through thermal impact on the ground layer of the troposphere from the earth's surface.

Enhancing Wind Farm Performance through Axial Induction and Tilt Control: Insights from Wind Tunnel Experiments

Static axial induction control and tilt control are two strategies that have the potential to increase power production in wind farms, mitigating wake effects and increasing the available power for downstream turbines. In this study, wind tunnel experiments are performed to evaluate the efficiency of these two techniques. First, the axial induction of upstream turbines in wind farms comprising two, three, and five turbines is modified through the tip-speed ratio. This strategy is found to be ineffective in increasing power extraction. Next, the power extraction and flow through a two-turbine wind farm are evaluated, considering different tilt angles for the upstream turbine, under two levels of incoming flow turbulence intensities and turbine spacing distances. It is shown that forward tilting increases the overall power extraction by deflecting the wake downwards and promoting the entrainment of high-speed fluid in the upper shear layer, regardless of the turbine spacing distance and turbulence intensity level. Also, the wake is seen to recover faster due to the increased shear between the wake and the outer flow. Tilting a turbine backward deflects the wake upwards and pulls low-speed flow from under the turbine into the wake space, increasing the available power for downstream turbines, but it is not enough to increase global power extraction. Moreover, since the wake deflection under backward tilting is not limited by ground blockage, it leads to larger secondary steering compared with forward tilting. Finally, it is demonstrated that the secondary steering of the downstream turbine’s wake influences the flow encountered by a turbine positioned farther downstream.

Effect of overlapping and space between stages of a three-bladed double-stage Savonius hydrokinetic turbine for low flow speed perennial river application

ABSTRACT Savonius-like hydrokinetic turbine (SAHT) in low-speed flow streams of perennial rivers is not much explored. In this paper, a triple-bladed double-stage SAHT is designed, and its performance is investigated for low flow speeds within 0.45–0.65 m/s under the effect of blade overlapping (within 10–20% in each stage), turbine diameter (within 208–234 mm), and without and with space between the stages (5.0 mm) at different operating speed ratios. Different combinations of design and operating input parameters are studied to obtain a reliable assessment of SAHT performance. After analyzing the results, the best conditions of input parameters and output performance parameters are reported in their nondimensional forms for future use of the obtained results. The torque produced by the turbine increased with the applied brake load, leading to its maximum value at the highest brake load 0.83 times the maximum load. The highest coefficient of performance is obtained as 0.086 at a space 0.25 times the maximum space (i.e. 20 mm), overlapping 15%, brake load 0.83 times the maximum load, tip speed ratio 0.276. And for this, the corresponding power 0.85 times the maximum power (i.e. 0.201 W) at free-stream flow speed of 0.85 times the maximum flow speed (i.e. 0.65 m/s) is obtained. The present turbine with space and overlapping exhibits better performance than some available SAHT designs of single/double stage, two-blade/multi-blade with/without overlap configurations. The novelty of the work is that an improved SAHT has been developed for low-flow speed perennial river applications in the band of low tip speed ratios by combining design conditions like double-stage, space between stages, and overlapping.

Explicit and implicit large eddy simulations of variable-density low-speed flows

The application of the Non-oscillatory Forward-in-Time (NFT) class of solvers to facilitate conventional explicit Large Eddy Simulations (LES) and implicit LES (ILES) of low-speed turbulent flow encompassing substantial local density variations is explored. Schemes solving two different sets of Low Mach Number (LMN) equations are proposed with the aim of capturing variable-density effects arising from either a non-isothermal distribution in single-species flow or compositional variation occurring in a binary-species mixing under isothermal conditions. Both schemes employ the semi-implicit NFT integrators based on the Multidimensional Positive Definite Advection Transport Algorithm and a non-symmetric Krylov subspace solver for the variable-coefficient Poisson problems arising from different treatments applied to the non-zero velocity divergence term in the two sets of the LMN equations. The developments are successfully validated using three diverse test cases, differentially heated cavity flow, non-isothermal free-jets, and a helium plume. The reported simulations show that both the dynamic Smagorinsky model in LES and ILES subgrid-scale treatments yield similar accuracy in capturing turbulent effects for the considered flows.

Conceptual design and experimental study of a flexible oscillating hydrokinetic turbine

Harnessing the energy of deep ocean currents to power deep-sea observation equipment has emerged as a significant focus in recent years. Given the extremely low flow speeds in the deep sea, hydrokinetic turbines need to demonstrate reliable self-starting ability. A novel flexible oscillating hydrokinetic turbine is proposed in the present study to address this challenge. The advantage of the flexible oscillating hydrokinetic turbine lies in the ability to adjust the posture of the flexible blades to create optimal attack angles at different positions. Particle Image Velocimetry measurements are employed to reveal the internal flow field. The results indicate that the incoming flow through the turbine cavity induces two substantial deflections in the inflow and outflow regions, contributing to the generation of significant positive torque. The flexible blades can achieve suitable attack angles at various positions as per the expectations. Especially in the downstream region, the blades adopt a diversion state, thereby avoiding the generation of excessive negative torque. The parametric study was conducted through a water flume and a dynamic testing platform. The results indicate that when the number of blades is six and the installation angle of the blade frame is 20°, the power coefficient of the flexible oscillating hydrokinetic turbine reaches its maximum, exceeding 0.18. A full-process interdisciplinary simulation platform test showed that a flexible oscillating hydrokinetic turbine with a radius of 1 m can self-start in conditions as low as 0.1 m/s. At a flow speed of 0.5 m/s, the turbine can generate over 900 Wh of power per day, enough to power sensors or support an Autonomous Underwater Vehicle’s monthly cruise.

Investigation of the influence of the dimensionless centrifugal work number on spanwise rotating channel low-speed compressible flow

Investigation of the influence of the dimensionless centrifugal work number on spanwise rotating channel low-speed compressible flow

Numerical Simulation of Supersonic Turbulent Separated Flows Based on k–ω Turbulence Models with Different Compressibility Corrections

The accurate prediction of supersonic turbulent separated flows involved in aerospace vehicles is a great challenge for current numerical simulations. Based on the k–ω equations, several different compressibility corrections are incorporated in turbulence models to improve their prediction capabilities. Two benchmark test cases, namely the ramped cavity and the compression corner, are adopted for the numerical validation. Detailed comparisons between simulations and experiments are conducted to evaluate the effect of compressibility corrections on turbulence models. The computed results indicate that compressibility corrections have a significant impact on turbulence model performance. The compressibility correction, considering the effects of both dilatation dissipation and pressure dilatation, is suitable for the prediction of compressible free shear layers, but it may have a negative impact on the prediction of low-speed flows in the near-wall region due to the severe underprediction of the wall skin friction coefficient. In comparison, the compressibility correction only considering the effects of dilatation dissipation is conservative, with decreased predictability of free shear layers in supersonic flows, although it improves the predictions of the original models without corrections.

Design and performance test of series underwater Savonius rotors with horizontal axis

&lt;span lang="EN-US"&gt;The energy of river water flow or irrigation canals can be utilized by using a horizontal axis Savonius turbine, which converts low-speed river flow into electrical power with good design. The design of this turbine needs to pay attention to several parameters, namely, the deflector angle, the number of blades, the diameter and thickness of the blades, and the diameter and thickness of the end plates. However, the problem usually encountered is imperfect turbine construction due to the large drag force that occurs so that the power generated is low. Based on this background, it is proposed to manufacture and test the performance of a series Savonius underwater rotors with a horizontal axis. The research results found that the generator voltage without load was 25.6 V when the turbine only rotated at 47.2 rpm, whereas when under load, the average power produced was 8.5 watts with an average turbine speed of 31 rpm. The highest efficiency value on the rotor is 86.73%, with a torque value of 3.36 at a turbine speed of 29.9 rpm. This indicates that the tool can generate large torque at low speeds.&lt;/span&gt;

Feasibility Study for the Development of Small Scale Low Speed Wind Tunnel for Supermileage Prototype Car

This paper presents the design and construction of a small-scale low-speed open circuit wind tunnel suitable for experimental testing and analysis of aerodynamic phenomena. The wind tunnel is specifically designed for low-speed applications, focusing on flows within velocities of 35 meters per second with turbulence conditions below 10 percent. The design process of a wind tunnel involves considerations such as dimensions, design parameters, and instrumentation to ensure accurate flow measurements, with key components including a settling chamber, contraction section, test section, diffuser, and drive system working together to remove disturbances, create uniform flow, minimize blockage effects, and generate required airflow for accurate aerodynamic testing. This paper highlights the design considerations, key components, construction techniques, and the significance of the wind tunnel for aerodynamic testing, validation of simulations, and education. The designed wind tunnel is capable of producing controlled and repeatable low-speed flow conditions, making it suitable for a wide range of applications, including aerodynamic testing of scaled- down models, validation of Computational Fluid Dynamics (CFD) simulations, and educational purposes. The small-scale design allows for cost-effective construction and operation while maintaining reliable results.

Kinetic modeling of fluid-induced interactions in compressible, rarefied gas flows for aerodynamically interacting particles

In the study of gas-particulate multiphase systems, the flow of high-speed gas through a distribution of solid particulates is of utmost importance. While these aerodynamically interacting systems have been extensively studied for low-speed gas flows in the gas continuum regime, less attention has been given to high-speed systems where non-continuum effects are significant due to the high flow gradients. To address this, the flow of rarefied gas through an aerodynamically interacting monodisperse spherical particle system is studied using the Direct Simulation Monte Carlo (DSMC) gas-kinetic approach. Since the method provides the best resolution of shocks at supersonic Mach numbers it is used to classify the weak separated shocks and strong collective shocks in these systems based on particle spacing in a two-particulate system at different orientation angles. The study used the two-particle system to help analyze more complex particle distributions of volume fractions, 1%, 5%, and 15%, exposed to gas flows in the slip and transitional gas regime for a free-stream Mach number range of 0.2<Ma∞<2.0. We observe that the weak separated shocks in the 1% distribution allow a higher degree of gas penetration and shock-particle interactions or “hypersonic-surfing”, exposing a major fraction of the particulates to higher force magnitudes. In contrast, the strong collective shock in the 5% and 15% distributions only generates high particulate forces on the flow-facing particles. Finally, a simple stochastic model is proposed for use in large-scale Eulerian–Lagrangian simulations that captures the non-monotonic behavior of average drag and force variability generated by the complicated gas particulate interactions in the compressible gas regime.

Dynamics and scaling of particle streaks in high-Reynolds-number turbulent boundary layers

Inertial particles in wall-bounded turbulence are known to form streaks, but experimental evidence and predictive understanding of this phenomenon is lacking, especially in regimes relevant to atmospheric flows. We carry out wind tunnel measurements to investigate this process, characterizing the transport of microscopic particles suspended in turbulent boundary layers. The friction Reynolds number$Re_\tau = {O}(10^4)$allows for significant scale separation and the emergence of large-scale motions, while the range of viscous Stokes number$St^+=18$–870 is relevant to the transport of dust and fine sand in the atmospheric surface layer. We perform simultaneous imaging of both carrier and dispersed phases along wall-parallel planes in the logarithmic layer, demonstrating that streamwise particle streaks largely overlap with large-scale low-speed flow regions. The fluid–particle slip velocity indicates that with increasing inertia, the particle streaks outlive the low-speed fluid streaks. Moreover, two-point statistics show that the width of the particle streaks increases linearly with Stokes number, bounded by the size of the coherent flow structures. Finally, the particle-sampled flow topology suggests that particle streaks reside between the legs of hairpin packets. From these observations, we infer a conceptual view of the formation of particle streaks in the frame of the attached eddy model. A scaling for the particle streaks’ width is derived as a function of$Re_\tau$and$St^+$, which reproduces the measured trends and predicts widths${O}(0.1)$m in the atmospheric surface layer, comparable to aeolian streamers observed in the field.