Steady Inflow Research Articles

Analysis of the Coal Spontaneous Combustion Risk Space Evolution in the Goaf Under Ventilation and Extraction Coupling

ABSTRACT The risk of coal spontaneous combustion in goaf is one of the disasters in coal mining, ventilation and extraction often aggravate air leakage from the goaf. Due to the unclear rule of expansion law and air leakage, the risk space for coal spontaneous combustion continues to occur. To investigate the influence of ventilation and extraction on the evolution of the risk space of coal spontaneous combustion (CSC) in goaf, further reveal the characteristics of air leakage and extraction in goaf and obtain the expansion law of CSC risk space. A three-dimensional model of goaf under surface drilling and gas extraction was developed. Using FLUENT numerical simulation software, taking a coal mine in Anhui Province, this study analyzes the influence factors, evolution law, and influence characteristics of CSC through field observations and numerical simulation. The research indicates that ventilation keeps the risk space of CSC expanding. The extraction of single drilling showed a local space expansion, while the extraction of double drilling shows the overall space expansion of the goaf. The air leakage flow path in goaf, air flow velocity vector in working face, gas concentration loss, and gas extraction characteristic parameters are determined. The loss of air leakage along the working surface meets four-stage: “strong in – flow - mixed flow – steady inflow – sudden outflow.” It can be determined that the extraction intensity of single drilling should be greater than 34.8 m3/min, and the double drilling should be greater than 18.7 m3/min. The relationship between the coupling influence of ventilation and extraction and the expansion law of CSC risk space is determined, and the maximum risk space width of CSC can be predicted. The research results are of guiding significance for CSC prevention and gas efficient extraction.

Double flows anchored in a Kerr black hole horizon – I. Meridionally self-similar MHD models with loading terms

ABSTRACT Recent observations of supermassive black holes have brought us new information on their magnetospheres. In this study, we attempt a theoretical modelling of the coupling of black holes with their jets and discs, via three innovations. First, we propose a semi-analytical MHD description of a steady relativistic inflow–outflow structure characteristic to the extraction of the hole rotational energy. The mass-loading is ensured in a thin layer, the stagnation surface, by a two-photon pair production originating to a gamma-ray emission from the surrounding disc. The double flow is described near the polar axis by an axisymmetric meridionally self-similar MHD model. Secondly, the inflow and outflow solutions are crossing the MHD critical points and are matched at the stagnation surface. Knowledge of the MHD field on the horizon gives us the angular momentum and energy extracted from the black hole. Finally, we illustrate the model with three specific examples of double-flow solutions by varying the energetic interaction between the MHD field and the rotating black hole. When the isorotation frequency is half of the black hole one, the extracted Poynting flux is comparable to the one obtained using the force-free assumption. In two of the presented solutions, the Penrose process dominates at large colatitudes, while the third is Poynting flux dominated at mid-colatitudes. Mass injection rate estimations, from disc luminosity and inner radius, give an upper limit just above the values obtained for two solutions. This model is pertinent to describe the flows near the polar axis, where pair production is more efficient.

Comparison of a mid-fidelity free vortex wake method to a high-fidelity actuator line model large eddy simulation for wind turbine wake simulations

The need for mid-fidelity wind turbine simulations is increasing due to their balance of accuracy and computational efficiency. A common form of mid-fidelity code is the free wake vortex method (FVWM), but these are known to inaccurately represent the far-wake depending on their implementation. Herein, we compare CACTUS, a FVWM code, to Nalu-Wind, an actuator line model large eddy simulation (ALM-LES) code with particular focus on the far wake. Specifically, we compare one CACTUS simulation, which necessarily has steady and uniform inflow, to three Nalu-Wind simulations using a steady and uniform inflow, a low turbulence inflow, and a high turbulence inflow. The comparison reveals interesting parallels between CACTUS and all Nalu-Wind cases, but ultimately demonstrates that CACTUS is not an appropriate tool for the simulation of the far-wake of a wind turbine.

On exposure uncertainty quantification from accidental airborne point releases

In any health risk related event such as an air hazardous release, a key question that needs to be addressed is, to what extent an individual is exposed to a hazardous pollutant/agent for a specific time interval at a specific location downstream the release. Any systematic and reliable approach on this problem especially at the application/operation level, requires the knowledge not only of the exposure itself, but also its associated uncertainty quantified using probability density functions. A radically new approach is proposed (a) by making full use of the real detailed inlet flow and release rate signals, (b) by performing a limited number of flow and dispersion simulations in comparison to straightforward approaches, dealing only with steady state and reference inflow and release conditions and (c) by projection of the steady state/reference results to real conditions via appropriate novel scaling approaches based on experimental evidence and theory. A validation exercise has been performed with remarkable results using the well-studied University of Hamburg S2 Michelstadt Wind Tunnel Experiment with building structures representing distinct characteristics of typical central European cities. The message is for an attractive approach that needs further validation with the help of carefully designed experiments combined with dispersion model adjustments and improvements.

Active rotor coning for a 25 MW downwind offshore wind turbine

A two-bladed downwind turbine system was upscaled from 13.2 MW to 25 MW by redesigning aerodynamics, structures, and controls. In particular, three 25-MW rotors were developed, and the final version is a fully redesigned model of the original rotor. Despite their radically large sizes, it was found that these 25-MW turbine rotors satisfy this limited set of structural design drivers at the rated condition and that larger blade lengths are possible with conewise load-alignment. In addition, flapwise morphing (varying the cone angle with a wind-speed schedule) was investigated to minimize mean and fluctuating blade root bending loads using steady inflow proxies for the maximum and lifetime damage equivalent load moments. Compared to the fixed coned rotor case, morphing can provide an Annual Energy Production (AEP) increase of 6%, and the maximum blade root flapwise bending moment increases 21% (still under the constraint, i.e., 10% of the ultimate moments) as a trade-off. The resulting series of 25-MW rotors can be a valuable baseline for further development and assessment of ultra-large-scale wind turbines.

Study of Effect of Boundary Conditions on Patient-Specific Aortic Hemodynamics

Cardiovascular computational fluid dynamics (CFD) based on patient-specific modeling is increasingly used to predict changes in hemodynamic parameters before or after surgery/interventional treatment for aortic dissection (AD). This study investigated the effects of flow boundary conditions (BCs) on patient-specific aortic hemodynamics. We compared the changes in hemodynamic parameters in a type A dissection model and normal aortic model under different BCs: inflow from the auxiliary and truncated structures at aortic valve, pressure control and Windkessel model outflow conditions, and steady and unsteady inflow conditions. The auxiliary entrance remarkably enhanced the physiological authenticity of numerical simulations of flow in the ascending aortic cavity. Thus, the auxiliary entrance can well reproduce the injection flow from the aortic valve. In addition, simulations of the aortic model reconstructed with an auxiliary inflow structure and pressure control and the Windkessel model outflow conditions exhibited highly similar flow patterns and wall shear stress distribution in the ascending aorta under steady and unsteady inflow conditions. Therefore, the inflow structure at the valve plays a crucial role in the hemodynamics of the aorta. Under limited time and calculation cost, the steady-state study with an auxiliary inflow valve can reasonably reflect the blood flow state in the ascending aorta and aortic arch. With reasonable BC settings, cardiovascular CFD based on patient-specific AD models can aid physicians in noninvasive and rapid diagnosis.

A review of experiments on stationary bluff body wakes

Experimental studies dealing with the wake of isolated stationary bluff bodies are reviewed. After briefly recalling the pioneering works in this domain, the paper focuses on recent research conducted with the latest experimental methods and techniques. The review encompasses a range of topics, including the effects of bluff body geometry (non-circular cross sections and non-uniformity in spanwise direction), steady and unsteady (periodic and non-periodic) inflow conditions, surface proximity (rigid wall, confinement, and water free surface) and non-Newtonian fluids. Focus is brought to the flow physics of the wakes, especially the complex three-dimensional and oscillatory behaviors induced by the periodic vortex shedding phenomenon. The paper aims to offer a critical and systematic review of new knowledge and findings on the subject area, as well as the most frequently adopted experimental techniques. The review also helps identifying knowledge gaps in the literature that need to be addressed in future investigations.

Assessment of BEMT-Based Engineering Models for Wind Turbine Rotors Under Uniform Steady Inflow

Assessment of BEMT-Based Engineering Models for Wind Turbine Rotors Under Uniform Steady Inflow

Cooling flows around cold clouds in the circumgalactic medium: steady-state models and comparison with TNG50

ABSTRACT Cold, non-self-gravitating clumps occur in various astrophysical systems, ranging from the interstellar and circumgalactic medium (CGM), to active galactic nucleus outflows and solar coronal loops. Cold gas has diverse origins such as turbulent mixing or precipitation from hotter phases. We obtain the analytical solution for a steady pressure-driven 1D cooling flow around cold, local overdensities, irrespective of their origin. Our solutions describe the slow and steady radiative cooling-driven gas inflow in the saturated regime of non-linear thermal instability in clouds, sheets, and filaments. Such a cooling flow develops when the gas around small clumps undergoes radiative cooling. These small-scale, cold ‘seeds’ are embedded in a large volume-filling hot CGM maintained by feedback. We use a simple two-fluid treatment to include magnetic fields as an additional polytropic fluid. To test the limits of applicability of these analytical solutions, we compare with the gas structure found in and around small-scale cold clouds in the CGM of massive haloes in the TNG50 cosmological magnetohydrodynamic simulation from the IllustrisTNG suite. Despite qualitative resemblance of the gas structure, we find deviations from steady-state profiles generated by our model. Complex geometries and turbulence all add complexity beyond our analytical solutions. We derive an exact relation between the mass cooling rate ($\dot{\rm M}_{\rm cool}$) and the radiative cooling rate ($\dot{\rm E}_{\rm cool}$) for a steady cooling flow. A comparison with the TNG50 clouds shows that this cooling flow relation only applies in a narrow temperature range around $\rm \sim 10^{4.5}$ K where the isobaric cooling time is the shortest. In general, turbulence and mixing, instead of radiative cooling, may dominate the transition of gas between different temperature phases.

Performance investigation of Wells turbine for wave energy conversion with stall cylinders

The wave energy conversion of oscillation water column (OWC) systems is limited by the narrow range of efficient operation of Wells turbines. Effective control methods are critical to improving performance. In this study, spanwise stall cylinders were mounted near the blade leading edge to delay the stall using dynamic mode decomposition (DMD) analysis. The performance of the Computational Fluid Dynamics simulation with steady inflow conditions is in good agreement with the experimental data. The results indicate that the aerodynamics and turbine performance is significantly influenced by cylinder wakes, which are associated with the location and radius of the stall cylinders. The relatively optimal location of the stall cylinders is determined at a 10% chord of the blade with a radius of 1% of the chord, producing 19.15% and 26.57% increases in the operation range and maximum torque coefficient, respectively. The cylinders act by intensifying the turbulence in the boundary layer and weakening the tip leakage vortex, causing the cylinder wake vortices to interact with vortices near the boundary layer to guide the flow near the suction side. A larger cylinder radius produces stronger wake vortices, but the efficiency decreases due to increased dissipation losses.

Employment Prospects of Humanitarian Migrants in Australia: Does Gender Inequality in the Origin Country Matter?

Despite a steady inflow of humanitarian migrants to Australia, their socio-economic outcomes remain visibly poorer than those of other migrants and the general population. Therefore, understanding the factors contributing to refugee integration is an important endeavor. In this paper, we focus on refugees’ labor-market outcomes and the role of a potentially important macro-level factor: gender inequality in the origin country. To accomplish this, we leverage high-quality longitudinal data from a probability survey (Building a New Life in Australia, n = 9,268) augmented with country-level data from the UNDP. Our results bear implications for gender and migration scholarship, and equity policy and practice.

Aeroelastic Characterization of a Flexible Wing Using Particle Tracking Velocimetry Measurements

The aerodynamic, elastic, and inertial components in Collar’s triangle of forces acting on a flexible wing with span width and chord are determined based on integrated optical measurements of the structural and aerodynamic response to steady and unsteady periodic inflow conditions at a chord-based Reynolds number of . The measurement device is a coaxial volumetric velocimeter mounted on a robotic arm, which is used to perform optical measurements of fiducial markers on the wing surface, and helium-filled soap bubbles, which are used as flow tracers. The optical measurements of the structural markers and the flow tracers are both processed with the Lagrangian particle tracking algorithm Shake-the-Box. Subsequently, physical models are used to determine the inertial and elastic forces of the aeroelastic interaction from the marker tracking results, and to determine the unsteady aerodynamic lift force from the flow velocity fields. The results of this integrated aeroelastic characterization approach are in physical agreement with each other according to the equilibrium of forces in Collar’s triangle and good agreement with external reference measurements.

Destination Marketing: Optimizing Resource Allocation Using Modern Portfolio Theory

Despite the significant contribution that tourism generates for national economic health, the tourism industry is challenged by seasonal and periodic volatility in arrivals. This volatility causes inefficiencies in the allocation of a destination’s resources. Policy makers and operators prefer a steady and constant inflow of tourists. Though prior studies have applied a portfolio optimization approach to inbound tourism flows, this study is the first to consider the destination’s marketing budget and each tourism markets’ heterogeneous marketing expenditure–demand elasticity. Our model provides destination marketing organizations with direct guidance as to how to allocate their marketing budget to facilitate portfolio profiles that are efficient from a risk-reward perspective, and that are attainable given budget constraints and known marketing expenditure–demand elasticity patterns. Using Tourism Research Australia International Visitor Survey data, we find that variability in tourism fluctuations can be reduced and inbound tourism numbers/spending significantly increased by implementing smarter (data-driven) marketing budget allocations.

The effect of Dean, Reynolds, and Womersley number on the flow in a spherical cavity on a curved round pipe. Part 1. Fluid mechanics in the cavity as a canonical flow representing intracranial aneurysms.

Flow in side-wall cerebral aneurysms can be ideally modelled as the combination of flow over a spherical cavity and flow in a curved circular pipe, two canonical flows. Flow in a curved pipe is known to depend on the Dean number De, combining the effects of Reynolds number, Re, and of the curvature along the pipe centreline, κ. Pulsatility in the flow introduces a dependency on the Womersley number Wo. Using stereo PIV measurements, this study investigated the effect of these three key non-dimensional parameters, by modifying pipe curvature (De), flow-rate (Re), and pulsatility frequency (Wo), on the flow patterns in a spherical cavity. A single counter-rotating vortex was observed in the cavity for all values of pipe curvature κ and Re, for both steady and pulsatile inflow conditions. Increasing the pipe curvature impacted both the flow patterns in the pipe and the cavity, by shifting the velocity profile towards the cavity opening and increasing the flow rate into the cavity. The circulation in the cavity was found to collapse well with only the Dean number, for both steady and pulsatile inflows. For pulsatile inflow, the counter-rotating vortex was unstable and the location of its centre over time was impacted by the curvature of the pipe, as well as the Re and the Wo in the freestream. The circulation in the cavity was higher for steady inflow than for the equivalent average Reynolds and Dean number pulsatile inflow, with very limited impact of the Womersley in the range studied.

Aerodynamic modeling of wind turbine loads exposed to turbulent inflow and validation with experimental data

The present paper is intended to assess the ability of the state-of-the-art computational fluid dynamics (CFD) and blade element momentum (BEM) approaches for accurate load predictions on a 2.3 MW wind turbine rotor. Three different cases are considered, a steady uniform inflow condition, a turbulent uniform inflow condition and a turbulent inflow case in combination with shear and yaw. The CFD computations employ a delayed-detached eddy simulation (DDES) approach in combination with a high order (5th) WENO method for flux discretization. The BEM calculations apply several correction factors including recently developed dynamic stall and yaw models. Furthermore, a well established procedure at IAG to set-up BEM calculations consistent to CFD will be presented and verified. The results are compared with the field experimental data of the turbine for these three different flow conditions. The studies show that both CFD and BEM results are in a very good agreement with the experimental data not only on the mean load levels but also with regards to the load fluctuations. The differences between BEM, CFD and experimental data for most radial stations are less than 10%.

Nexus between Foreign Direct Investment Inflow and EPI Score in Brazil

Before 1990, foreign investors could not extensively invest in Brazil because of its stringent and conservative trade policy. However, in 1990 Brazil took a remarkable shift in its trade policy from its previous trends by taking a liberal stance in terms of FDI inflow. The liberal trade policy that Brazil took in 1990 remove all the trade barriers that previously hindered the free flow of FDI in Brazil, the sign of which had been reflected in the steady and gradual incremental FDI inflow in the country after that. This New opening of trade atmosphere had brought with it two very different repercussions. In one hand, it accelerated the economic growth by bringing in investment in the needed sectors; on the other hand this acceleration spurred industrial activities that are accompanied by air and water pollution and deforestation as a byproduct. Since then, environmental health and ecosystem of Brazil have been facing tremendous threat owing to these pollution problems which may indirectly distress the EPI score of Brazil. Keeping these issues in mind, the present study investigates the relationship between FDI inflow and EPI score in Brazil. With the help of descriptive statistical tools as well as Pearson correlation test, this study finds the negative relationship between FDI inflow and EPI score in Brazil.

Real-time rotor effective wind speed estimation using Gaussian process regression and Kalman filtering

The use of state estimation technique offers a means of inferring the rotor effective wind speed based upon solely standard measurements of wind turbines. For the ease of design and computational concerns, typical wind speed estimators rely on a pre-computed mapping that describes the relationship from tip-speed ratio and pitch angle to the power coefficient. Typically, this mapping is built using numerical simulations under steady inflow conditions. Thus, the mapping built by traditional methods does not well capture the influence of other turbine dynamics and atmospheric variations, thus, inevitably resulting in poor performance of the wind speed estimator. Therefore, the paper presents a framework of rotor effective wind speed estimator design that obviates the need for a pre-computed power coefficient mapping. Specifically, the proposed method reconstructs the mapping using Gaussian process regression with a small set of real-time turbine measurement data. Subsequently, the wind speed estimator is built based upon the regression-based model and an extended Kalman filter, enabling optimal estimation from standard turbine measurements. The proposed method was evaluated in normal operation and down-regulation, against benchmark models obtained from an aero-elastic code. The estimation errors of the power coefficient and wind speed were significantly reduced by the regression-based approach.

GenRadar: Self-Supervised Probabilistic Camera Synthesis Based on Radar Frequencies

Autonomous systems require a continuous and dependable environment perception for navigation and decision-making, which is best achieved by combining different sensor types. Radar continues to function robustly in compromised circumstances in which cameras become impaired, guaranteeing a steady inflow of information. Yet, camera images provide a more intuitive and readily applicable impression of the world. This work combines the complementary strengths of both sensor types in a unique self-learning fusion approach for a probabilistic scene reconstruction in adverse surrounding conditions. After reducing the memory requirements of both high-dimensional measurements through a decoupled stochastic self-supervised compression technique, the proposed algorithm exploits similarities and establishes correspondences between both domains at different feature levels during training. Then, at inference time, relying exclusively on radio frequencies, the model successively predicts camera constituents in an autoregressive and self-contained process. These discrete tokens are finally transformed back into an instructive view of the respective surrounding, allowing to visually perceive potential dangers for important tasks downstream.

High-fidelity simulations and field measurements for characterizing wind fields in a utility-scale wind farm

Characterizing wind farm flow fields at high temporal and spatial resolutions is critical prerequisite for the optimal design and operation of utility-scale wind farms and for reducing the levelized cost of energy. However, due to the large disparity of underlying scales, measurements or simulations alone cannot provide high resolution wind fields, which are informed by and account for the effect of both large scale (i.e. hour, day, month and year) and small scale (i.e. second and minute) site-specific variations in the atmosphere. We explore the feasibility of integrating field measurements and high-fidelity large-eddy simulation (LES) to characterize the wind field in a utility-scale wind farm while accounting for flow phenomena across multiple temporal scales. Specifically, we employ field measurements to characterize the monthly wind speed and wind direction distributions and investigate the wind characteristics in turbine wakes. It was found that the probability density function (PDF) of the wind speed in turbine wakes can be reasonably represented using the Weibull distribution but with shape factors smaller than those not in the wake. LES of the wind farm under statistically steady inflow is subsequently carried out for one wind direction. The LES predictions are compared with the measured data conditionally averaged based on the wind speed, wind direction and the root-mean-square of wind speed fluctuations over time intervals of 30 min. Good agreement is obtained for both mean wind speed and turbulence intensity. The present work shows the possibility of integrating field measurements and high-fidelity simulations for improved characterization of the site-specific wind fields in utility-scale wind farms.

Characterization of anisotropic turbulence behavior in pulsatile blood flow

Turbulent-like hemodynamics with prominent cycle-to-cycle flow variations have received increased attention as a potential stimulus for cardiovascular diseases. These turbulent conditions are typically evaluated in a statistical sense from single scalars extracted from ensemble-averaged tensors (such as the Reynolds stress tensor), limiting the amount of information that can be used for physical interpretations and quality assessments of numerical models. In this study, barycentric anisotropy invariant mapping was used to demonstrate an efficient and comprehensive approach to characterize turbulence-related tensor fields in patient-specific cardiovascular flows, obtained from scale-resolving large eddy simulations. These techniques were also used to analyze some common modeling compromises as well as MRI turbulence measurements through an idealized constriction. The proposed method found explicit sites of elevated turbulence anisotropy, including a broad but time-varying spectrum of characteristics over the flow deceleration phase, which was different for both the steady inflow and Reynolds-averaged Navier–Stokes modeling assumptions. Qualitatively, the MRI results showed overall expected post-stenotic turbulence characteristics, however, also with apparent regions of unrealizable or conceivably physically unrealistic conditions, including the highest turbulence intensity ranges. These findings suggest that more detailed studies of MRI-measured turbulence fields are needed, which hopefully can be assisted by more comprehensive evaluation tools such as the once described herein.