Turbulent Speed Research Articles

Investigations on conical lean turbulent premixed hydrogenated natural gas flames

Hydrogen's ability to enhance carbon neutrality in combustion processes puts forward the use of hydrogenated fuels, both in the form of fuel and an energy carrier as a potential decarbonization solution. However, because of the nature of hydrogen, blending it with hydrocarbons causes crucial structural changes in the flame structure, including higher flame propagation velocities and higher flame temperatures, decreased instantaneous flame thickness, and increased risks of flame flashback and an increasing potential of NOx emissions due to higher flame temperatures. These attributes encourage a thorough examination of hydrogenated blends of hydrocarbon fuels. Using lean premixed fuels is another technique to achieve efficient and cleaner combustion. However, due to the problem of flame instability in lean premixed combustion, forecasting the design points in terms of flame attributes is critical for better combustor designs.In this study, conical (Bunsen type) lean premixed turbulent flames of hydrogenated natural gas-air mixtures are experimentally studied. Through chemiluminescence measurements of the OH* and CH* radicals and laser-induced Mie scattering, lean natural gas-air premixed flames are examined with subsequently increasing hydrogen addition rates up to 20% by volume and keeping the premixture velocity constant. The obtained data is utilized for exploring the dynamics of the turbulent flame front. The main turbulent premixed flame parameters we identified relate to the instantaneous and average topology of the flame such as the turbulent flame brush thickness and flame height. We also inferred global combustion parameters like the turbulent flame propagation speed from the experimental findings.

Understanding the radio luminosity function of star-forming galaxies and its cosmological evolution

ABSTRACT We explore the redshift evolution of the radio luminosity function (RLF) of star-forming galaxies using galform, a semi-analytic model of galaxy formation and a dynamo model of the magnetic field evolving in a galaxy. Assuming energy equipartition between the magnetic field and cosmic rays, we derive the synchrotron luminosity of each sample galaxy. In a model where the turbulent speed is correlated with the star formation rate, the RLF is in fair agreement with observations in the redshift range 0 ≤ z ≤ 2. At larger redshifts, the structure of galaxies, their interstellar matter, and turbulence appear to be rather different from those at z ≲ 2, so that the turbulence and magnetic field models applicable at low redshifts become inadequate. The strong redshift evolution of the RLF at 0 ≤ z ≤ 2 can be attributed to an increased number, at high redshift, of galaxies with large disc volumes and strong magnetic fields. On the other hand, in models where the turbulent speed is a constant or an explicit function of z, the observed redshift evolution of the RLF is poorly captured. The evolution of the interstellar turbulence and outflow parameters appear to be major (but not the only) drivers of the RLF changes. We find that both the small- and large-scale magnetic fields contribute to the RLF but the small-scale field dominates at high redshifts. Polarization observations will therefore be important to distinguish these two components and understand better the evolution of galaxies and their non-thermal constituents.

Analysis of Tesla CyberTruck Speed on the Velocity and Pressure Distribution Using SimFlow Software

This work presents a comprehensive Computational Fluid Dynamics (CFD) analysis of the external flow around the Tesla CyberTruck, focusing on its aerodynamic characteristics under varying conditions. The primary objectives are to study velocity and pressure distributions. The simulation considers two independent parameters: vehicle speed (20, 40, and 80 m/s) and the type of turbulent flow (k-ω SST and k-ε). The simulation provides insights into complex flow patterns through meticulous meshing, boundary condition setup, and solver configuration, highlighting areas of interest such as flow separation, recirculation, and turbulence. Parametric variations are analyzed to determine how turbulent flow type and speed affect critical parameters like pressure and velocity. The results of this CFD analysis offer valuable information about the vehicle's aerodynamic performance, contributing to design optimization, handling and stability enhancements, and improved fuel efficiency. The findings from this study are expected to enhance visualization and understanding of the aerodynamic aspects of the Tesla CyberTruck.

Turbulent Gas-rich Disks at High Redshift: Bars and Bulges in a Radial Shear Flow

Recent observations of high-redshift galaxies (z ≲ 7) reveal that a substantial fraction have turbulent, gas-rich disks with well-ordered rotation and elevated levels of star formation. In some instances, disks show evidence of spiral arms, with bar-like structures. These remarkable observations have encouraged us to explore a new class of dynamically self-consistent models using our agama/Ramses hydrodynamic N-body simulation framework that mimic a plausible progenitor of the Milky Way at high redshift. We explore disk gas fractions of f gas = 0%, 20%, 40%, 60%, 80%, and 100% and track the creation of stars and metals. The high gas surface densities encourage vigorous star formation, which in turn couples with the gas to drive turbulence. We explore three distinct histories: (i) there is no ongoing accretion and the gas is used up by the star formation, (ii) the star-forming gas is replenished by cooling in the hot halo gas, and (iii) in a companion paper, we revisit these models in the presence of a strong perturbing force. At low f disk (≲0.3), where f disk is the baryon mass fraction of the disk relative to dark matter within 2.2 R disk, a bar does not form in a stellar disk; this remains true even when gas dominates the inner disk potential. For a dominant baryon disk (f disk ≳ 0.5) at all gas fractions, the turbulent gas forms a strong radial shear flow that leads to an intermittent star-forming bar within about 500 Myr; turbulent gas speeds up the formation of bars compared to gas-free models. For f gas ≲ 60%, all bars survive, but for higher gas fractions, the bar devolves into a central bulge after 1 Gyr. The star-forming bars are reminiscent of recent discoveries in high-redshift Atacama Large Millimeter/submillimeter Array observations of gaseous disks.

Machine learning models for the prediction of turbulent combustion speed for hydrogen-natural gas spark ignition engines

The work carried out in this paper focused on “Machine learning models for the prediction of turbulent combustion speed for hydrogen-natural gas spark ignition engines”. The aim of this work is to develop and verify the ability of machine learning models to solve the problem of estimating the turbulent flame speed for a spark-ignition internal combustion engine operating with a hydrogen-natural gas mixture, then evaluate the relevance of these models in relation to the usual approaches. The novelty of this work is the possibility of a direct calculation of turbulent combustion speed with a good precision, using only machine learning model. The obtained models are also compared to each other by considering in turn as a comparison criterion: the precision of the result, calculation time, and the ability to assimilate original data (which has not undergone preprocessing). An important particularity of this work is that the input variables of the machine learning models were chosen among the variables directly measurable experimentally, based on the opinion of experts in combustion in internal combustion engines and not on the usual approaches to dimensionality reduction on a dataset. The data used for this work was taken from a MINSEL 380, a 380-cc single-cylinder engine. The results show that all the machine learning models obtained are significantly faster than the usual approach and Random Forest (R2: R-squared = 0.9939 and RMSE: Root Mean Square Error = 0.4274) gives the best results. With a forecasting accuracy of over 90 %, both approaches can make reasonable predictions for most industrial applications such as designing engine monitoring and control systems, firefighting systems, simulation, and prototyping tools.

Feedback and Galaxy Dynamics: A Study of Turbulence and Star Formation in 34 Galaxies Using the PHANGS Survey

The correlation between interstellar turbulent speed and local star formation rate surface density, ΣSFR, is studied using CO observations in the PHANGS survey. The local velocity dispersion of molecular gas, σ, increases with ΣSFR, but the virial parameter, α vir, is about constant, suggesting the molecular gas remains self-gravitating. The correlation arises because σ depends on the molecular surface density, Σmol, and object cloud mass, M mol, with the usual molecular cloud correlations, while ΣSFR increases with both of these quantities because of a nearly constant star formation efficiency for CO. Pressure fluctuations with ΔΣSFR are also examined. Azimuthal variations of molecular pressure, ΔP mol, have a weaker correlation with ΔΣSFR than expected from the power-law correlation between the total quantities, suggesting slightly enhanced star formation rate (SFR) efficiency per molecule in spiral arms. Dynamical equilibrium pressure and SFR correlate well for the whole sample, as PDE∝ΣSFR1.3 , which is steeper than in other studies. The azimuthal fluctuations, ΔP DE(ΔΣSFR), follow the total correlation P DE(ΣSFR) closely, hinting that some of this correlation may be a precursor to star formation, rather than a reaction. Galactic dynamical processes correlate linearly such that ΣSFR∝(ΣgasR)1.0±0.3 for total gas surface density Σgas and galactic dynamical rates, R, equal to κ, A, or Ω, representing epicyclic frequency, shear rate A, and orbit rate Ω. These results suggest important roles for both feedback and galactic dynamics.

Experimental study on the structure and propagation of the stretched hydrogen-rich turbulent premixed flames in the thin-reaction-zone regime

An experimental study on the structure and propagation of the stretched hydrogen-rich premixed turbulent flames in the thin-reaction-zone regime was conducted. A set of optical diagnostic techniques including the 2D/3D-particle image velocimetry, hot wire anemometer, intensified charge-coupled device (ICCD), and laser tomography visualization (LTV) methods were adopted to quantitatively measure the velocity field, turbulent intensity, and thickness of the stretched flame of the under various conditions. A new method to improve the measurements of the flame preheat zone thickness was developed by using the low-temperature available LTV method to identify the onset of the preheat zone and using the peak location of CH* to identify the reaction zone. The results showed that the flow field was in the inertial turbulent subrange and the turbulent flames obtained in the present counterflow configuration were categorized into the thin reaction zone regime in Borghi/Peters turbulent flame diagram. The experimental results showed that the flame structure of a turbulent H2-rich premixed flame was synergistically influenced by the hydrodynamic stretch, turbulence, and preferential diffusion. New evidence of the phenomenon accumulated that large-scale vortices wrinkled the surface of the flame while small-scale vortices penetrated into the flame preheat zone. Scaling laws for the turbulent flame propagation speed, apparent turbulent thermal conductivity, and were given with the consideration of the synergetic effect of hydrodynamic stretch, turbulence, and preferential diffusion. The scaling of the turbulent flame preheat zone thickness turned out to be sensitive, while those of the turbulent flame propagation speed and apparent turbulent thermal conductivity were insensitive, to the bulk flow strain rate.

Estimating dust source activities based on dust RGB images and wind speed

With the aim of improving the WRF/Chem model outputs for dust concentration, the soil erodibility as a representative for dust sources have been recalculated based on remote sensing (MSG-SEVIRI) and some relevant atmospheric data. The frequencies of dust episodes on specific regions have been modified by applying some weights with respect to atmospheric turbulence and wind speed. Several new regions of dust sources have been identified by the new dust source, and several formerly known dust sources have been identified as non-source areas in the new dust source map. The new dust source map have been used in the WRF/Chem model to simulated specific dust storms in the region. WRF/Chem results by the new dust sources were better performance in simulation of dust concentration over the region, in comparison to observations.

A consistent MMC-LES approach for turbulent premixed flames

A multiple mapping conditioning (MMC) approach is coupled with large eddy simulations (LES) for the prediction of the flame propagation, the flame structure and finite rate chemistry effects including NO formation in turbulent premixed flames. The mixing term in MMC relies on a reference field that characterizes the chemical composition within the flame and it is typically taken to be the reaction progress variable. In LES, this reference field needs to be artificially thickened to be resolved on the LES grid. A novel MMC mixing time scale model that had been developed with the aid of direct numerical simulation (DNS), has been adapted to account for the thickening of the reference field. Equally, the particle selection mechanism for particle mixing has been modified to ensure localness of mixing in composition space and to preserve the prediction of a realistic (thin) flame thickness by the stochastic particles. A set of freely propagating turbulent premixed flames with varying equivalence ratios is used to test the performance of the MMC model. Results of MMC-LES are compared with reference DNS that include full chemistry. Additional simulations were carried out where MMC is coupled with quasi-DNS where the flow field is fully resolved but the reaction progress variable field is thickened and its source term is closed with a flamelet approach. This allows to isolate the different modelling assumptions invoked in MMC-LES. Deviations from the exact solution can then be associated with the MMC approach and with thickening of the reference field separately and they are small. The MMC-LES predictions of the turbulent premixed flame propagation speed and flame structure are very good and finite rate chemistry effects can be captured well which is demonstrated by good agreement of predicted NO with DNS data.

Morphology and turbulent burning velocity of n-decane/air expanding flames at constant turbulent Reynolds numbers

In this work we experimentally investigated the propagation and the turbulent burning velocity of n-decane/air turbulent expanding flames at 423 K and 0.5-3.0 atm, with constant turbulent Reynold number, ReT,flow = (urmsLI)/ν, ranging from 174 to 4712, using a medium-scale, fan-stirred combustion chamber, where urms and LI are respectively the turbulence intensity and the integral length scale, and ν is the kinematic viscosity of a fresh mixture. It was found that the n-decane/air turbulent expanding flame is self-similar, and the normalized turbulent flame propagation speed scales with the turbulent flame Reynold number, ReT,flame = (urms/SL)(〈r〉/δL), to the one-half power for mixtures characterized by non-unity Lewis numbers, where the mean flame radius (〈r〉) is the length scale, and the thermal diffusivity (α = SLδL) is the transport property. The turbulent burning velocity (ST,c=0.5) at the mean progress variable (〈c〉) of 0.5 decreases with the pressure at a constant ReT,flow, following the similar pressure dependence of the laminar burning velocity (SL). The ST,c=0.5/SL of the fuel-rich n-decane/air flame (ϕ = 1.4) is twice larger than that of the fuel-lean mixture (ϕ = 0.8), which would be attributed to the promotion of local burning rates by coupling between the differential diffusion and the turbulent flame stretch on the local flame fronts. In addition, five useful modified correlations for the turbulent burning velocity with the experimental Lewis number (LeBM) consideration were obtained based on the present experimental data of n-decane/air turbulent expanding flames. Particularly, the successful scaling of the modified correlation based on Damköhler's second hypothesis, ST,c=0.5/SL = 1 + 0.115(ReT,flowLeBM−2)0.5, suggesting that the ST,c=0.5/SL is inversely proportional to the LeBM. The importance of using experimental determined values of the Lewis number in the scaling of turbulent burning velocity with the differential diffusion consideration should be emphasized.

Turbulent burning velocity of hydrogen/n-heptane/air propagating spherical flames: Effects of hydrogen content

This study investigated the flame propagation and turbulent burning velocity of hydrogen/n-heptane/air mixtures subjected to differential diffusion and flame stretch induced by turbulence at different hydrogen molar fractions (XH2 = 0–100%), equivalence ratios (ϕ = 0.5–1.0) and turbulence intensities (u’ = 0–3.62 m/s). The experiments were carried out on a medium-scale, fan-stirred cylindrical combustion chamber generating near-isotropic turbulence. Results show that as hydrogen content increases, the turbulent flame propagation speed tends to be affected by stronger differential diffusion and weakened turbulence, characterized by decreasing Lewis number (Le) and Karlovitz number (Ka) within a similar range of turbulence intensity. Among them, the effect of differential diffusion on the normalized turbulent propagation speed (d<r>/dtσSL) appears to be well correlated with ReT,flameLe-2, where ReT,flame represents the turbulent flame Reynolds number. However, inconsistencies in the fitting parameters for this scale are observed at different XH2, highlighting the impact of substantial variations in flame stretch induced by turbulence. Therefore, the normalized turbulent flame propagation velocity under varying hydrogen contents is proposed to be accurately predicted by d<r>/dtσSL-1 = 0.07Ka0.32(ReT,flameLe−2)0.5. The synergistic effects of Le and Ka can also account for the peak phenomenon in the normalized turbulent burning velocity (ST/SL) observed at 85% hydrogen content. Furthermore, the quantitative characterization of turbulence and differential diffusion under extensive experimental data (Ka = 0.03–90) is studied. As evidenced by the Damköhler theoretical hypothesis, the two-stage trend between ST/SL and (u’/SL)Le-1 is identified with a transition near unity Ka. The modified correlations for turbulent burning velocity in terms of (u’/SL)Le-1 and Ka0.2 are obtained with consideration of the differential diffusion and flame stretch rate induced by turbulence, which show good predictive performance for data at different turbulence regime diagrams regardless of fuel type and equipment size.

Multi-objective optimization design of the wind-to-heat system blades based on the Particle Swarm Optimization algorithm

A novel wind turbine direct driving heat pump (WTDHP) system is proposed, in which the heat pump is employed to replace the wind turbine's generator. The impacts of the blades with different tip speed ratios on WTDHP system are analyzed in this study. The inverse design method, verified by importing the design shape into FAST for simulation and comparing the simulated tip speed ratio and axial induction factor, was employed to create blades with different tip speed ratios. The heating performances of different blade shape designs at various turbulence and wind speeds are understood. The conclusion is drawn that the blades with a low tip speed ratio (λ=6–8) are clearly superior for heating in the turbulence categories IEC A at wind speeds between 6 and 14 m/s. However, the dynamic responses of the system are weakened by low turbulence and wind speeds. Taking into account the cost and manufacturing process, the blades with a high tip speed ratio can be chosen. Furthermore, the multi-objective optimization of blades was carried out using the enhanced Particle Swarm Optimization (PSO) algorithm, considering heating capacity, flapping load and blade mass. Based on the Wilson method, initial blade of PSO optimization algorithm is generated. Obtaining the heating capacity, flapping load and blade mass in real time from the simulation, the objective functions is calculated to update the status of particle swarm. After optimization using the improved multi-objective algorithm under turbulent wind with an average wind speed of 7 m/s, the blade mass is reduced by 2.96%, the flapping moment is reduced by 3.17%, and the heating capacity is increased by 1.22%. The results show that the optimization effect is more obvious at higher wind speeds.

Astroclimatic parameters characterization at lenghu site with ERA5 products

ABSTRACT Atmospheric turbulence and wind speed distributions are essential in determining the quality of astronomical sites and implementing adaptive optics systems. In this study, the optical turbulence with a general turbulence model and the wind speed characteristics at Lenghu are investigated by employing the 21-yr European Centre for Medium-Range Weather Forecasts’ fifth set of reanalysis (ERA5) data. The vertical distributions and seasonal behaviours of the wind speed and atmospheric turbulence are analysed. The wind speed values at 200 hPa pressure level (V200) are low in spring and summer except for June, and high in autumn and winter. The highest and lowest values of V200 at Lenghu are 39.93 m s−1 in September and 27.48 m s−1 in April, respectively. Additionally, atmospheric conditions are relatively stable above 20 km throughout the year. The locations of the peaks in the $C_n^2$ profiles correspond to the tropopause and the jet wind stream regions at the Lenghu site. Furthermore, the median seeing value from ERA5 is 0.72 arcsec, generally consistent with the statistically measured value of 0.75 arcsec. Moreover, the astronomical parameters (r0, θAO, and τAO) were initially provided at the Lenghu site. The median values of r0, ε, θAO, and τAO are 15.17 cm, 0.72 arcsec, 1.19 arcsec, and 2.83 ms, respectively, thus providing the potential reference for astronomical applications.

Effect of Turbulence and Diffusional–Thermal Instability on Hydrogen‐Rich Syngas Turbulent Premixed Flame

As a kind of fuel with low‐carbon emissions, hydrogen‐rich syngas has a large resource potential and is a promising alternative energy resource. A series of experiments in a constant volume combustion bomb are carried out to investigate the effects of turbulence and diffusional–thermal (DT) instability on a hydrogen‐rich syngas turbulent premixed flame. The flow field in the constant volume combustion bomb is calibrated, and the turbulent flow field's homogeneity and isotropy were investigated. The effects of different speeds of fan (1366–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0–3.0 bar), and equivalence ratios (0.6–1.0) on the hydrogen‐rich syngas turbulent premixed flame are studied. According to the analyses, the flow field in the experimental device exhibits good homogeneous and isotropic characteristics. With the increase of turbulence intensity, equivalence ratio, hydrogen fraction, or pressure, the turbulent flame propagation speed increases gradually. As the hydrogen fraction increases or the equivalence ratio decreases, the effective Lewis number decreases, and the effect of DT instability on the flame increases. The research in this article is crucial for the clean and efficient utilization of hydrogen‐rich syngas and the design of related burners.

Evaluating the aeration efficiency of hydraulic flume on photo-oxidative disinfection of wastewater

Photosensitized oxidation has been considered to offer better disinfection mechanism under natural sunlight and the efficiency of its inactivation pathway depends on the concentration of dissolving oxygen, the solar irradiance, and the reactive nature of the liquid. This study aimed to evaluate the hydraulic mixer's aeration performance during photo-oxidation treatment. A hydraulic jump was created to aerate the sewage prior to its passage into a reactor, and the amount of dissolved oxygen generated was measured. The effect of mixing intensity on sunlight inactivation of faecal coliforms in a reactor was investigated for 60 min using velocity gradients of 520 s−1, 640 s−1, 720 s−1 and 960 s−1 produced under flow rates of 3 L min−1, 5 L min−1 and 7 L min−1 and 10 L min−1 respectively. The photo-oxidation kinetics of faecal coliform contained in settled and raw sewage samples were also studied under two dark control tests (static and dynamic tests) and a light test. The system achieved a 2.0–2.34 log reduction of faecal coliforms and dissolved oxygen was within the range of 1.9 mg L−1 - 3.0 mg L−1. In the dynamic control test, the inactivation rate constant of bacteria indicated a reduction of 1.11 log (92%) for settled sewage and 1.0 log (90%) for raw sewage. The coliform inactivation rate constant, due to the combined effect of turbulent mixing speed and dark inactivation rate constant, was 2.3 h−1, and the solar irradiance inactivation rate constant was 2.4 h−1. These results have demonstrated that induced aeration is an economically friendly approach to enhance wastewater treatment in developing nations with long sunshine hours.

Assessment of Five Wind-Farm Parameterizations in the Weather Research and Forecasting Model: A Case Study of Wind Farms in the North Sea

Abstract To simulate the large-scale impacts of wind farms, wind turbines are parameterized within mesoscale models in which grid sizes are typically much larger than turbine scales. Five wind-farm parameterizations were implemented in the Weather Research and Forecasting (WRF) Model v4.3.3 to simulate multiple operational wind farms in the North Sea, which were verified against a satellite image, airborne measurements, and the FINO-1 meteorological mast data on 14 October 2017. The parameterization by Volker et al. underestimated the turbulence and wind speed deficit compared to measurements and to the parameterization of Fitch et al., which is the default in WRF. The Abkar and Porté-Agel parameterization gave close predictions of wind speed to that of Fitch et al. with a lower magnitude of predicted turbulence, although the parameterization was sensitive to a tunable constant. The parameterization by Pan and Archer resulted in turbine-induced thrust and turbulence that were slightly less than that of Fitch et al., but resulted in a substantial drop in power generation due to the magnification of wind speed differences in the power calculation. The parameterization by Redfern et al. was not substantially different from Fitch et al. in the absence of conditions such as strong wind veer. The simulations indicated the need for a turbine-induced turbulence source within a wind-farm parameterization for improved prediction of near-surface wind speed, near-surface temperature, and turbulence. The induced turbulence was responsible for enhancing turbulent momentum flux near the surface, causing a local speed-up of near-surface wind speed inside a wind farm. Our findings highlighted that wakes from large offshore wind farms could extend 100 km downwind, reducing downwind power production as in the case of the 400-MW Bard Offshore 1 wind farm whose power output was reduced by the wakes of the 402-MW Veja Mate wind farm for this case study. Significance Statement Because wind farms are smaller than the common grid spacing of numerical weather prediction models, the impacts of wind farms on the weather have to be indirectly incorporated through parameterizations. Several approaches to parameterization are available and the most appropriate scheme is not always clear. The absence of a turbulence source in a parameterization leads to substantial inaccuracies in predicting near-surface wind speed and turbulence over a wind farm. The impact of large clusters of offshore wind turbines in the wind field can exceed 100 km downwind, resulting in a substantial loss of power for downwind turbines. The prediction of this power loss can be sensitive to the chosen parameterization, contributing to uncertainty in wind-farm economic planning.

High-sensitivity Observations of the H2O Megamasers of NGC 1068: Precise Astrometry and Detailed Kinematics

We present High Sensitivity Array observations of the H2O megamasers of NGC 1068. We obtain absolute astrometry with 0.3 mas precision that confirms the association of the disk masers with the nuclear radio continuum source S1. The new observations reveal two new blueshifted groups of disk masers. We also detect the 22 GHz continuum on short interferometric baselines. The position–velocity diagram of the disk masers shows a curve consistent with a nonaxisymmetric distribution of maser spots. This curve is probably the result of spiral arms with a constant pitch angle ∼5°. The disk kinematics are consistent with Keplerian rotation and low turbulent speeds. The inferred central mass is 17 × 106 M ☉. On the basis of disk stability arguments, the mass of the molecular disk is ≈110 × 103 M ☉. The disk masers further resolve into filamentary structures suggesting an ordered magnetic field threading the maser disk. The magnetic field strengths must be ≳2 mG to withstand turbulent motions in the partially ionized molecular gas. We note apparent asymmetries in the molecular disk that might be explained by anisotropic heating by a misaligned inner accretion disk. The new observations also detect the fainter jet masers north of the disk masers. The distribution and kinematics of the jet masers are consistent with an expanding ring of molecular gas.

Dynamical friction of a massive black hole in a turbulent gaseous medium

The orbital decay of massive black holes in galaxies in the aftermath of mergers is at the heart of whether massive black holes successfully pair and merge, leading to emission of low-frequency gravitational waves. The role of dynamical friction sourced from the gas distribution has been uncertain because many analytical and numerical studies have either focussed on a homogeneous medium or have not reached resolutions below the scales relevant to the problem, namely the Bondi-Hoyle-Lyttleton radius. We performed numerical simulations of a massive black hole moving in a turbulent medium in order to study dynamical friction from turbulent gas. We find that the black hole slows down to the sound speed, rather than the turbulent speed, and that the orbital decay is well captured if the Bondi-Hoyle-Lyttleton radius is resolved with at least five resolution elements. We find that the larger the turbulent eddies, the larger the scatter in dynamical friction magnitude, because of the stochastic nature of the problem, and also because of the larger over- and under-densities encountered by the black hole along its trajectory. Compared to the classic solution in a homogeneous medium, the magnitude of the force depends more weakly on the Mach number, and dynamical friction is overall more efficient for high Mach numbers, but less efficient towards and at the transonic regime.

Wind turbulence modeling for real-time simulation

This paper proposes a method to design a real-time wind turbulence simulator. The objective of such a simulator is to make dynamical models more accurate in order to develop more robust controls, especially in the case of mechanical systems operating outdoors such as tracking antennas. The models used to generate a random wind speed are based on the wind spectral characteristics rather than time domain ones. Indeed, turbulence is a stochastic and non-stationary process corresponding to the short-term component of wind and therefore is difficult to model in time domain. Wind spectral characteristics are described by the power spectral density whose approximation is used in the real-time simulator to reproduce wind behavior. The von Kármán model is the most commonly used model to approximate the power spectral density of wind turbulence. However, this model was originally designed for aircraft, and therefore for high altitudes and moving systems. In our case, tracking antennas are used under different conditions: low altitude and slow moving systems. This makes the von Kármán model less precise in middle and high range frequency. Consequently, there is a need in more accurately modeling wind turbulence under these specific conditions. As von Karman’s model expression uses fractional calculus, other models with the same range of parameter number are proposed to better approximate the wind power spectral density by using Cole-Cole and Davidson-Cole fractional models. The idea is to use shaping filters issued from these more precise fractional models to generate realistic random wind speed turbulence from a random white noise input. These filters are approximated by rational functions and their parameters are tuned in the same way as those of the von Kármán model with the same inputs (mean speed, standard deviation and length scale). If a rational model is sought, a very high number of parameters (more than six times) is needed to have the same precision as our proposed fractional Cole-Cole model which has only four parameters. Finally, an accurate turbulence wind speed generator is proposed.

Prediction of the mean turbulence intensity with a thermodynamic model for CNG and gasoline fuels

Combustion is the main parameter that affects efficiency and exhaust gas emissions in internal combustion engines. In this study, the burning speed of gasoline and CNG were investigated quantitatively by calculating the mean value of turbulent consumption speed instead of qualitatively as is usually done. A three-zone quasi-dimensional thermodynamic model based on the measured cylinder pressure was created to calculate the mean value of turbulent consumption speed and turbulence intensity. The mass flow rate of air was kept constant in all experiments as much as possible, and the spark advance was kept constant at each relative air/fuel ratio. Thus, the effect of fuel and combustion chamber design on the consumption speed and turbulence intensity was directly determined. MR shape reached the highest consumption speed and turbulence intensity in all conditions. In the flat geometry, without any bowl, speed continuously decreased differently from the other designs. Natural gas burned clearly faster in the ultra-lean mixture. The increase in turbulence intensity has different effects on CNG and gasoline. The highest value of the mean turbulence intensity was calculated as approximately 3.4 m/s in the MR design. In the ultra-lean mixture, although the mass flow rate of air was constant, the mean value of the turbulence intensity changed in the same combustion chamber. Therefore, it has been determined that the combustion process affects the turbulence intensity. Using the quasi-dimensional thermodynamic model, mean values of the turbulent burning speeds and turbulence intensity might be calculated without having any optical observation and CFD analysis.