Air Density Research Articles

Influence of seasonal air density fluctuations on wind speed distribution in complex terrains in the context of energy yield

Given the variable nature of wind speed and the importance of accurately determining the energy that can be generated at a given site, understanding the wind speed at different time scales is crucial. In addition to differences within a very short period (i.e., hourly and daily), these changes are also pronounced throughout the seasons. They are affected by the atmospheric conditions and the terrain's complexity. Therefore, this study investigates the seasonal wind speed variability and its impact on the potential energy generation in a representative study case of Koznica, the mountainous region in Kosovo. The wind speed measurements campaign started in May 2019 and ended in April 2020; the measurements were made at a 10 min time scale. Ground measurements show that the wind direction is mainly northwest and southeast. Then, the wind speed and potential energy generation variability analysis were conducted for three different measurement heights. The results show that winter and spring have the highest potential wind energy capacity with an average speed of 6.7 m/s. In comparison, the average wind speed is 6.12 m/s. Potential energy generation for each season (i.e., spring, summer, autumn, and winter is as follows: 64,396.7, 22,040.3, 42,539.3, and 46,417.2 MWh/year, respectively, while the average capacity factor is 25%. Solution-oriented findings from this study might provide valuable insights to policymakers and investors regarding wind power energy exploration in Kosovo and other places with similar geo-climatic conditions.

Enhancing Grain Moisture Prediction with Artificial Neural Networks and Computational Fluid Dynamic

Drying cereals is a vital process to reduce moisture content, enabling efficient storage. This study investigates the convective drying behaviour of cereals through numerical simulations and feedforward neural networks. Key parameters considered include temperature, ambient air speed, relative humidity, porosity, intrinsic permeability, density, thermal capacity, thermal conductivity, and initial relative humidity of stored grains. Training data were generated using numerical methods, solving heat and mass transfer equations based on Whitaker's model within COMSOL Multiphysics® 5.6. Simulation results reveal that moisture in cereals gradually equilibrates with the ambient environment, commencing from the exposed surfaces. The artificial neural network (ANN) demonstrates remarkable predictive accuracy using 100 data points, yielding an overall correlation coefficient of 0.99458 and a mean squared error (MSE) of 3.132 ×10-4. The combination of these two methods offers distinct advantages, with ANN saving computational time and numerical simulations not requiring initial samples. This combined approach proves promising for grain moisture prediction, though results must undergo rigorous validation to ensure reliability and accuracy.

Terahertz radiation from shockwave front irradiated with femtosecond optical pulses

AbstractFemtosecond double optical pulse experiments with nanosecond time delay were conducted in air. Terahertz pulses were generated from laser plasmas produced by the irradiation with the main pulses on shockwave excited by the pre‐pulse illumination. Intensity of the terahertz pulses depends on the time difference Δt and relative position Δy between the pre‐ and main pulses. We found a characteristic dependence of the intensity of the terahertz pulses on Δy at a longer time delay, Δt = 14.7 ns, which has two maxima while only one maximum was observed when delay times were at and shorter than 9.7 ns. The results are discussed taking into account air density distribution of the shockwave. The terahertz emission spectroscopy shown here can be a useful technique for investigating density structure and its time variation of shockwave with high temporal resolution.

Analysis of the Impact of Atmospheric Models on the Orbit Prediction of Space Debris

Atmospheric drag is an important influencing factor in precise orbit determination and the prediction of low-orbit space debris. It has received widespread attention. Currently, calculating atmospheric drag mainly relies on different atmospheric density models. This experiment was designed to explore the impact of different atmospheric density models on the orbit prediction of space debris. In the experiment, satellite laser ranging data published by the ILRS (International Laser Ranging Service) were used as the basis for the precise orbit determination for space debris. The prediction error of space debris orbits at different orbital heights using different atmospheric density models was used as a criterion to evaluate the impact of atmospheric density models on the determination of space-target orbits. Eight atmospheric density models, DTM78, DTM94, DTM2000, J71, RJ71, JB2006, MSIS86, and NRLMSISE00, were compared in the experiment. The experimental results indicated that the DTM2000 atmospheric density model is best for determining and predicting the orbits of LEO (low-Earth-orbit) targets.

Contact angle of ethanol, water, and their mixtures on stainless steel surfaces in dense carbon dioxide

HypothesisContact angle can be a key parameter in chemical engineering. However, the development and the optimization of numerous processes using supercritical CO2, considered as environmentally friendly, require new measurements under dense CO2 atmosphere. Besides, the influence of the roughness or the wetting regime on the contact angle is known at ambient conditions but remains to be discussed for systems under high pressure. ExperimentalContact angle measurements of ethanol, water, and their mixtures, with ethanol mass fractions ranging from 0.25 to 0.75, on two stainless steels in saturated CO2 at pressures ranging from 0.1 MPa to 15.1 MPa, and at313 K and 333 K were carried out in a set-up improving mass transfer between the studied liquid and the continuous fluid phase. Stainless steel surfaces have been characterized by atomic force and scanning electron microscopies allowing the application of the Wenzel equation. FindingsEthanol wetted totally both stainless steels while contact angles of all other liquids were increased by the rise of pressure, with contact angles up to 128° for water at 15.1 MPa. Trapped bubbles were observed at the solid/liquid interface and the bubble formation is discussed. Furthermore, the potential influence of bubble presence on the wetting regime is prospected through the question: could the pressure rise modify the wetting regime?

High-speed and scalable combustion fabrication of ultralight carbon nanotube aerogels below air density

In this study, we introduce a fast, scalable method to fabricate ultralight carbon nanotube (CNT) aerogels with a bulk density (ρ) lower than air, using a combustion process. Typical fabrication methods for ultralight nanocarbon aerogels (ρ ≤ 1.0 mg cm−3) often involve intricate steps, such as high-temperature annealing and thermal reduction. These processes are time-consuming and pose scalability challenges. Here, we propose a high-speed fabrication process for ultralight CNT aerogels by combusting cellulose nanofibers (CNF) as a sacrificial template. The combustion process for CNT/CNF composite aerogels in an open environment takes only a few seconds, and the fabricated CNT aerogels with a lower-than-air density (ρ ≥ 0.48 mg cm−3) have high light absorption and good thermal and mechanical stability. We further illustrate the capability of these ultralight CNT aerogels to achieve levitation upon heating like a hot air balloon. Ultralight CNT aerogels capable of levitation by heating, fabricated by our fast and scalable method, hold the potential to replace traditional motor-driven unmanned aerial vehicles and conventional stratospheric platforms, such as high-altitude balloons, with sunlight-driven materials.

Digital model of the drag force of the Earth’s upper atmosphere for the design of low-orbit spacecraft

A digital model is described for estimating the drag force of the Earth's upper atmosphere acting on a low-orbit spacecraft. Unlike the classical model of aerodynamic force, the digital model is the result of a computer algorithm. This algorithm calculates the field of aerodynamic force values, taking into account changes in the flight altitude, aerodynamic coefficient and the area of the spacecraft midsection over time. The calculations also use a digital model of the dynamic density of the Earth's upper atmosphere. The digital model of the drag force of the Earth's upper atmosphere is useful in the design of low-orbit spacecraft with a low-thrust jet propulsion system (thrust order – millinewtons). On the basis of the calculated values of the aerodynamic force, it is possible to form requirements for the geometry of the spacecraft, attitude modes and arrangement of low-thrust jet engines (for example, electric jet engines). With the accumulation of big data in the form of tabular functions of aerodynamic force from the space of various design and ballistic parameters of low-orbit spacecraft, it is possible to create a machine model (ML-model). It will make forecasts of the values of the design characteristics for the low-orbit spacecraft being developed according to a limited set of initial requirements.

VLA monitoring of LS V +44 17 reveals scatter in the X-ray–radio correlation of Be/X-ray binaries

ABSTRACT LS V +44 17 is a persistent Be/X-ray binary (BeXRB) that displayed a bright, double-peaked period of X-ray activity in late 2022/early 2023. We present a radio monitoring campaign of this outburst using the Very Large Array. Radio emission was detected, but only during the second, X-ray brightest, peak, where the radio emission followed the rise and decay of the X-ray outburst. LS V +44 17 is therefore the third neutron star BeXRB with a radio counterpart. Similar to the other two systems (Swift J0243.6+6124 and 1A 0535+262), its X-ray and radio luminosity are correlated: we measure a power-law slope $\beta = 1.25^{+0.64}_{-0.30}$ and a radio luminosity of LR = (1.6 ± 0.2) × 1026 erg s−1 at a 0.5–10 keV X-ray luminosity of 2 × 1036 erg s−1 (i.e. $\sim 1~{{\ \rm per\ cent}}$LEdd). This correlation index is slightly steeper than measured for the other two sources, while its radio luminosity is higher. We discuss the origin of the radio emission, specifically in the context of jet launching. The enhanced radio brightness compared to the other two BeXRBs is the first evidence of scatter in the giant BeXRB outburst X-ray–radio correlation, similar to the scatter observed in subclasses of low-mass X-ray binaries. While a universal explanation for such scatter is not known, we explore several options: we conclude that the three sources do not follow proposed scalings between jet power and neutron star spin or magnetic field, and instead briefly explore the effects that ambient stellar wind density may have on BeXRB jet luminosity.

Study of the structure of turbulent underexpanded supersonic jets by laser transillumination

The paper analyzes the spatial distribution of the average air density in a supersonic jet based on the results of laser transillumination. The algorithm to retrieve the average air density from transverse (to the jet axis) deviations of the transilluminating wavefront was tested in experiments on the Vertical jet setup of ITAM SB RAS. The results of retrieval are compared with the known literature data of contact measurements and with results of numerical simulation. The good sensitivity of local wavefront tilts to oscillations in the air density at frequencies of discrete acoustic tones has been demonstrated. This opens up the possibility of experimental study of the spatial structure of air density inhomogeneities in the jet channel.

Study and analysis of lift to drag ratio performance of supercritical wing based on computational fluid dynamics method

As the main airfoil of large fixed-wing aircraft at present, the supercritical wing has better performance than the traditional airfoil, especially its aerodynamic performance. Therefore, it is necessary to calculate and experiment with the supercritical wing to optimize its structure. In this paper, computational fluid dynamics (CFD) technology is used to evaluate and compare the performance of a supercritical wing (half wingspan) and a flat wing (half wingspan) with similar size and the same boundary conditions. First, shrink modelling of the two airfoils is performed using the similarity principle. Then it was imported into the simulation environment to calculate the lift-drag ratio under different boundary conditions and draw a corresponding dot plot for comparison. The results show that the aerodynamic indexes of the supercritical wing are better than those of the flat wing of the same size at the cruise stage. Under the standard cruise condition (the angle of attack of the flow is 2°, the air density is 0.5259 kg/m3, and the incoming flow velocity is 800 km/h, for example), the lift-drag ratio of the supercritical wing is increased from 15.0855 to 28.1962 by 1.869 times.

Comparisons of Rainfall Microphysical Characteristics between the Southeastern Tibetan Plateau and Low-Altitude Areas

Abstract The low air pressure and density over the Tibetan Plateau may have an impact on the microphysical features of rainfall. Using a two-dimensional video disdrometer (2DVD), a Micro Rain Radar (MRR), and a microwave radiometer (MWR), the features of the raindrop size distribution (DSD) on the southeastern Tibetan Plateau (SETP) are explored and compared with those in low-altitude regions. The falling speed of raindrops on the SETP is higher than that in low-altitude areas. Under different rainfall-rate categories, the number concentration and the maximum diameter of raindrops on the SETP are smaller than those in low-altitude regions. The convective rainfall on the SETP is more maritime-like because the South Asian summer monsoon brings water vapor from the ocean here. For stratiform and convective rainfall, the SETP has more small-sized raindrops than low-altitude locations. The mass-weighted mean diameters (Dm) on the SETP are the smallest among six sites. The generalized intercept parameter (Nw) of stratiform rainfall is balanced at a low rainfall rate, while that of convective rainfall is balanced at a high rainfall rate. Furthermore, for a given μ (the shape parameter of gamma distribution) value, the λ (the slope parameter of gamma distribution) value on the SETP is the highest of the six sites. Significance Statement For the occurrence and progression of rainfall, microphysical processes (for instance, collision, fragmentation, coalescence, and evaporation) that take place when rainfall particles descend are crucial. A key factor in the microphysical features of rainfall that varies with rainfall rates and types is the raindrop size distribution (DSD). The southeastern Tibetan Plateau (SETP)’s unique terrain ensures that there is enough moisture for rain to fall there, but little is known about the microphysical aspects of this type of precipitation. There has not been enough research done on how the high altitude affects the microphysical features of rainfall. The microphysical features of rainfall in this area must thus be studied.

Study on Temperature Field in Common-rail Diesel Engine Combustion Chamber at Different Altitudes

The plateau covers 26% of China’s land area, the higher altitude, the lower atmospheric pressure, the smaller air density, the lower average temperature. Therefore diesel engine power decreases significantly, fuel consumption increases, starting performance deteriorates, and soot emissions increase. In this paper, virtual simulation and simulation test methods are used to study the temperature field change in the combustion chamber of CA6DL2-35E3R common-rail diesel engine at different altitudes and atmospheric pressures. The results show that the altitude has a great influence on the temperature field in the combustion chamber. The maximum combustion temperature gradually increases with increase of altitude, post-combustion is serious and the exhaust temperature increases.

Experimental study on the spark ignition characteristics at elevated temperature and pressure under flow conditions

This study investigates the fundamental spark-ignition characteristics under engine-relevant environments, focusing on spark discharge energy and the shapes of spark channels. The pressure, temperature, and mean flow velocity conditions were independently controlled (up to 20 bar, 567 K, and 18 m/s, respectively) to study the individual effects on spark channels. In terms of analyzing spark discharge energy, the breakdown and arc/glow phases were individually examined. Results showed that the breakdown voltage and energy are predominantly affected by ambient density, which is consistent with Paschen's law. During the arc/glow phases, the total discharge energy increased with an increase in mean flow velocity, whereas relatively minor effects on the total discharge energy were observed with pressure and temperature variances. In terms of the shapes of spark channels, it was found that the shapes vary significantly under different thermodynamic and flow conditions. Specifically, ambient pressure and spark channel stretching by flow had considerable effects on the shapes of spark channels, such as the thickness of spark channels and the extent of corrugation. This study also proposes a mechanism for forming short circuits, which explicates the spark behaviors under flow conditions, based on the prior analyses of spark characteristics. In the process of examining the mechanism, the formulae for the required voltage for short circuits and the resistance of spark channels are additionally suggested.

Offshore Wind Energy Assessment with a Clustering Approach to Mixture Model Parameter Estimation

In wind resource assessment research, mixture models are gaining importance due to the complex characteristics of wind data. The precision of parameter estimations for these models is paramount, as it directly affects the reliability of wind energy forecasts. Traditionally, the expectation–maximization (EM) algorithm has served as a primary tool for such estimations. However, challenges are often encountered with this method when handling complex probability distributions. Given these limitations, the objective of this study is to propose a new clustering algorithm, designed to transform mixture distribution models into simpler probability clusters. To validate its efficacy, a numerical experiment was conducted, and its outcomes were compared with those derived from the established EM algorithm. The results demonstrated a significant alignment between the new method and the traditional EM approach, indicating that comparable accuracy can be achieved without the need for solving complex nonlinear equations. Moreover, the new algorithm was utilized to examine the joint probabilistic structure of wind speed and air density in China’s coastal regions. Notably, the clustering algorithm demonstrated its robustness, with the root mean square error value being notably minimal and the coefficient of determination exceeding 0.9. The proposed approach is suggested as a compelling alternative for parameter estimation in mixture models, particularly when dealing with complex probability models.

Settling versus mixing in stratified shear flows

We study the coupled settling, deformation and mixing dynamics of a dense blob of fluid falling in an axially (vertically) linearly stratified Taylor–Couette cell (operated in a laminar stable regime). This configuration allows the independent analysis of stretching dynamics, driven by radial (horizontal) velocity variations, and settling dynamics, driven by buoyancy forces associated with vertical density variations. As the blob settles, it is stretched in the horizontal plane and forms an elongated lamella. Through the competing effects of transverse compression of the lamella due to this shear-induced stretching and broadening due to diffusion, the lamella irreversibly mixes with ambient fluid, thus progressively adjusting its own density towards that of the ambient fluid. Eventually, the lamella settling stops at a final equilibrium position that depends on the ambient vertical density gradient and the rate at which it has been deformed by the horizontal shear. We show how this final position is determined by stretching-enhanced diffusion, i.e. mixing. We demonstrate that a theoretical mixing model compares favourably with experiments with various Froude numbers (quantifying the relative strength of the horizontal shear and the vertical stratification) and construct a new criterion for the energetic ‘efficiency’ of this mixing process that explicitly captures its inherently diffusive character.

An Airborne Visible Light Lidar’s Methodology for Clear Air Turbulence Detection Based on Weak Optical Signal

A clear air turbulence (CAT) detection method using a 532 nm visible light airborne laser radar (LiDAR) system is proposed to address the urgent challenge in the aviation safety field. This method is based on the indirect detection technique of atmospheric molecular density for CAT and utilizes the strong aerosol scattering absorption characteristics of the iodine molecular 1109 absorption line to eliminate the interference of aerosol scattering and extinction on the weak molecular backscattering signal caused by CAT. This enables CAT detection under conditions where traditional ultraviolet LiDAR systems fail to function properly due to aerosol presence. The influence of axial wind speed and atmospheric temperature variations on the molecular backscattering spectrum in the aircraft flight path is studied, and a formula for vertical wind speed inversion in the CAT field is derived. The 532 nm airborne LiDAR CAT detection theoretical model and system architecture are presented. Through simulation analysis, the CAT detection range of the visible light LiDAR system is evaluated under different aircraft cruising altitudes and turbulence intensities. The results indicate that, with the proposed LiDAR system, the aerosol scattering influence can be effectively suppressed, and CAT can be detected up to 7 km for light-to-moderate turbulence and 10 km for moderate turbulence ahead of the aircraft when traditional ultraviolet LiDAR systems fail as the backscattering coefficient ratio between aerosol and molecule reaches the 10−1 condition. Based on this finding, a suggestion is made to construct a dual-wavelength (ultraviolet-visible) LiDAR system for CAT detection, aiming to solve the full coverage problem of CAT detection under various aerosol conditions. This study has a reference value for promoting the early resolution of CAT detection in the aviation field.

Огляд методів та засобів відведення об’єктів космічного сміття з низьких навколоземних орбіт

The importance of the space debris problem in the today’s world is generally recognized. The number of space debris objects in near-Earth space is rapidly growing. The goal of this paper is to overview existing methods, systems, and means for space debris removal from low-Earth orbits with the aim to contribute to the solution of a topical problem of outer space utilization: the problem of space debris in near-Earth space. Space debris removal systems are under active development in the leading space countries. The overview showed that in scientific publications a great attention is paid to passive and active methods and means for space debris removal from near-Earth space. Relatively recently, a start was made on studying the feasibility of space debris removal systems using a combined method, which simultaneously uses means developed on the basis of passive and active methods. This paper considers a combined contactless space debris removal system with a service spacecraft equipped with electrojet engines and an aerodynamic compensator in the form of two plates. The combined system implements a directional deorbit of space debris objects by acting thereon with an ion beam. The proposed combined space system may be used to remove space debris from low-Earth orbits to the dense atmosphere followed by its burn-up. The combined line in the development of space debris removal systems is yet to be studied; however, its implementation would offer some advantages over active and passive methods used alone. Because of this, the development of the proposed combined space system with an aerodynamic compensator for contactless space debris removal is a promising line, which poses problems for further studie.

Relativistic Runaway Electron Avalanche Development Near the Electric Field Threshold in Inhomogeneous Air

AbstractRelativistic Runaway Electrons Avalanches (RREAs) development depends on the applied electric field and the environment's air density. This dependency controls the RREA exponential growth length scale. The RREA development affects the bremsstrahlung excess occurring due to the passage of charged particles through the thundercloud's electric fields, the gamma‐ray glow. We used Monte Carlo simulations to develop an empirical model showing the RREA behavior in a realistic atmospheric density profile. The new formulation shows how the density variation modulates the electron population under electric field strengths near the RREA electric field threshold. The model limits the initial RREA altitude range as a function of the electric field strength. The new model is valid between ∼0.6 and ∼18 km, covering the relevant heights to investigate the generation of ground‐detected gamma‐ray glows.

Detectability of carbon with ChemCam LIBS: Distinguishing sample from Mars atmospheric carbon, and application to Gale crater

Onboard NASA's Curiosity rover, the ChemCam LIBS instrument has provided a wealth of information on the chemistry of rocks within Gale crater. Here, we use ChemCam in order to search for carbonates among the >3500 individual targets analyzed by this instrument. Because the carbon-lines are a combination of signal from the CO2-rich atmosphere and possible carbon from the targets, we developed a laboratory-based univariate calibration obtained under Mars-like atmosphere. We measured different type of carbon-bearing samples (sediments, coals, carbonates) and their mixture with a basaltic powder. Based on this work, the preferred approach to qualitatively assess carbon under a CO2-rich atmosphere is to use a ratio to an oxygen line (777 nm) and the estimated limit of detection for carbon in a single LIBS point are found to be of 4.5 wt% and 6.9 wt% for reduced and organic carbon, respectively. Considering carbonate, this LOD correspond to about 50 wt% carbonate in the analyzed volume.Analysis of data obtained on Mars by ChemCam up to sol 3350 reveals the presence of a correlation between the intensity of carbon and oxygen lines, as observed in the laboratory, confirming that most carbon signal is related to ionization of the atmosphere. Some variability in the carbon signal appears related to the physical state of the atmosphere (density, temperature).Based on a combined analysis of carbon lines and major element compositions (Ca, Fe, Mg), there was no detection of carbonate in the ChemCam dataset up to sol 3355. Therefore, we conclude that carbonate was not present as a major constituent (>50%) in the ChemCam LIBS targets, and that soils are not enriched in carbon beyond the limit of detection. The dominant salts present are sulfate, chlorides, and the lack of carbonates in Gale, while observed in Jezero, may at least partly be related to a difference in protolith.

Near-field plume-to-plume interaction of multi-hole GDI injector under elevated ambient density conditions

Spray collapse can occur in the multi-hole injector under elevated ambient density conditions due to the plume-to-plume interaction. Numerous studies have investigated spray collapse in high ambient density conditions, but limited literature is available regarding the near-field plume-to-plume interaction triggering the spray collapse due to the optical density of the spray in the near field. In this study, an X-ray phase-contrast imaging (XPCI) technique that can visualize the spray structure and dynamics in the near field is applied to resolve the optical difficulties. With this method, the plume-to-plume interaction of a multi-hole gasoline direct injection injector is analyzed by measuring the transverse spray droplet size and axial velocity distributions under various ambient densities and injection pressures. The results showed that in the high ambient density condition with the low injection pressure, the spray droplet size started to increase, and the axial velocity remained stagnant from around 3 mm axial location as a result of plume-to-plume interaction and following spray collapse. The region of large droplets appeared asymmetric based on the hole arrangement. As the injection pressure increased in the high ambient density condition, the plume-to-plume interaction was suppressed in the near field and the spray collapse appeared farther downstream. Through the results, the spray collapse phenomenon and associated mechanism in the near field were discussed considering the droplet collision/coalescence and pressure structure in the spray.