Re-entry Time Research Articles

Energy consumption and dilution of toxic gases in underground infrastructures: A case study in a railway tunnel under forced ventilation

Toxic fumes are released after blasting in underground excavations. The analysis of safe re-entry times into the work areas is essential for addressing safety and productivity concerns. In this paper, a railway tunnel 2.1 km long under forced ventilation system is selected as a case study to assess the energy consumption and re-entry times after blasting under different ventilation conditions. First, three-dimensional CFD numerical models were conducted in order to predict the migration time to reduce the levels of noxious gases below the limits established in safety regulations. Field measurements were conducted to verify the accuracy of the results obtained in the simulations. Then, additional analytical models were conducted to evaluate the energy consumed by the ventilation system. The results obtained show that the accumulated re-entry time reaches 414 h for the excavation of the tunnel employing an airflow rate of 40 m3 s−1. This time increases by 33 % when the airflow rate decreases to 30 m3 s−1. However, even though the time decreases when the ventilation volume increases, energy consumption increases significantly, reaching 1,306 MWh for a ventilation volume of 40 m3 s−1 compared to 551 MWh using an airflow of 30 m3 s−1.

Reentry trajectory optimization of hypersonic glide vehicle based on improved particle swarm algorithm

The reentry process of hypersonic glide vehicle presents a host of intricate constraint issues. A method for optimizing the reentry trajectory of hypersonic glide vehicle is presented to achieve the shortest possible reentry time. The approach utilizes the improved particle swarm optimization (IPSO) algorithm. Firstly, population initialization is performed using the quasi-oppositional differential evolution (QODE) strategy to promote diversity within the population. Then an adaptive adjustment algorithm is designed for the inertia weight coefficient and learning factor to achieve a balance between global and local search capabilities. Finally, utilizing the IPSO algorithm for optimizing the flight parameters of the reentry trajectory, the reentry time is reduced from 338 seconds to 335 seconds. The simulations results indicate that the IPSO algorithm outperforms both the standard PSO algorithm and genetic algorithm (GA) in terms of optimization performance index.

Study on CO migration characteristics and hazard potential function under ventilation after roof cutting blasting

Multiple toxic gases, such as carbon monoxide (CO), are generated by roof-cutting blasting (RCB) in the automatically formed roadway without coal pillars (AFRCP). To eliminate potential hazards in time, a ventilation model was established based on field data from Dianping Coal Mine. The spatial and temporal evolution characteristics of CO under different conditions after roof-cutting by shaped-charge blasting (SCB) and shaped-charge hydraulic blasting (SCHB), respectively, were investigated by using the Computational Fluid Dynamics (CFD) method. The results demonstrate that the CO gas cloud presents the coupling effect of movement and dilution under ventilation. The charge mainly affects the peak value of CO concentration (PVC). The air velocity affects the PVC and the CO gas cloud's over-limit time (OLT) and migration velocity. Roadway cross-sectional area and corner structure affect the PVC by influencing local air velocity. The CO hazard potential functions of the roadway after RCB were established, indicating a Gaussian-like distribution. The maximum hazard potential of SCHB was found to be 60% lower than that of SCB, and the re-entry time for personnel reduced by 36%. This study can serve as a guide for optimizing the ventilation design of roadways after RCB and contribute to the promotion of AFRCP.

Field measurements on the attenuation characteristics of PM2.5 and toxic gases in a blasting metro tunnel and evaluation of the re-entry time

In this study, to reveal the attenuation rules of dust and toxic gases versus time after blasting, nine blasting measurements were conducted in a metro tunnel. Two blasting scenarios were used to investigate the diffusion characteristics of blasting fumes, whilst the remaining seven blasting scenarios were used to analyze the evolution of air parameters [e.g., carbon monoxide (CO), nitrogen dioxide (NO2) and fine particulate matter (PM2.5) concentration, particle size distribution, and air temperature and humidity] versus time after blasting at different positions. The field measurement results showed that the actual throwing length of the blasting fumes was less than half of its empirical value. The attenuation rules of PM2.5, CO, and NO2 conformed to an exponential function with a quartic polynomial of time in the exponent. The decay time of CO and NO2 concentrations was longer than that of PM2.5, and the sizes of particles in the working zone were basically less than 30 μm 5 min after blasting. In addition, the time to peak CO concentration was basically longer than that for PM2.5 and NO2, and the peak concentrations of PM2.5, CO, and NO2 were greatly affected by the supply air volume, explosive mass, forced air duct outlet position, surrounding rock grade, and geological conditions. Moreover, blasting had a weak effect on the air temperature. In contrast, the air temperature increased significantly during slagging operations because of the mechanical heat discharged; the air temperature could be as high as 36.6 ℃, indicating that cooling technologies are needed to achieve an air temperature below 28 ℃. Furthermore, the difficulty of dust control and re-entry time based on dust were significantly greater than that of CO, whilst re-entry time based on CO was significantly larger than that of NO2 by an order of magnitude, and was generally 1,500–2,500 s after ventilation in China. These findings offer theoretical guidance for the cleaner production and safer management of construction tunnels.

Application of Back-Propagation Neural Network in the Post-Blast Re-Entry Time Prediction

Predicting the post-blast re-entry time precisely can improve productivity and reduce accidents significantly. The empirical formulas for the time prediction are practical to implement, but lack accuracy. In this study, a novel method based on the back-propagation neural network (BPNN) was proposed to tackle the drawbacks. A numerical model was constructed and 300 points of sample data were recorded, with consideration to fresh air volume, occupational exposure limit, toxic gas volume per kg of explosives and roadway length. The BPNN model with six neurons in a hidden layer was then developed and prediction performance was discussed in terms of four indicators, namely, the root mean square error (RMSE), the coefficient of determination (R2), the mean absolute error (MAE) and the sum of squares error (SSE). Furthermore, one representative empirical formula was introduced and calibrated for the comparison. The obtained results showed that the BPNN model had a more remarkable performance, with RMSE of 21.45 (R2: 0.99, MAE: 10.78 and SSE: 40934), compared to the empirical formula, with RMSE of 76.89 (R2: 0.90, MAE: 42.06 and SSE: 526147). Hence, the BPNN model is a superior method for predicting the post-blast re-entry time. For better practical application, it was then embedded into the software.

Assessment of in-house algorithms on re-entry time prediction of uncontrolled space objects

Accurate prediction of re-entry time and location of an uncontrolled re-entering space object is crucial but challenging. This paper presents in-house re-entry prediction algorithms: RSMGA, STKOptim, STKLTOptim, and ABPro. Case studies on re-entry prediction of Starlink-26 (COSPAR ID 2019-029F or NORAD ID 44240) and CZ-5B (COSPAR ID 2021-035B or NORAD ID 48275) during the IADC re-entry test campaigns 2021/1 and 2021/2, respectively are presented. Simultaneous application of these re-entry prediction algorithms facilitates decision-making on automation of switching between these algorithms during various phases (short term or long-term) of the re-entry campaign. The effectiveness and applicability of these algorithms are studied based on percentage error. The study focuses on the overall and time-based performance of the four methods. Six more re-entered objects are selected (three Geosynchronous transfer orbit (GTO) objects and 3 Low-Earth orbit (LEO) objects), and their re-entry time prediction has been carried out using the four methods for different time instances before re-entry. Prediction estimates are compared among the methods for all the time instances in terms of percentage error. Also, the study helps in fixing the suitability of the re-entry time prediction algorithm for objects from GTO and LEO.

Procedures for mitigating safety risks associated with post-blast re-entry times

ABSTRACT In this paper, management procedures for post-blast re-entry that offer safety, efficiency, and economic benefits to mining operations are introduced. Such management procedures include use of dedicated and inaccessible exhaust airways for directing blast toxic and explosive gases, supported by use of advanced remote atmospheric monitoring technology and automated monitoring. A decrease in post-blast re-entry time with reliable remote monitoring processes and with effective ventilation management procedures results in safer and more efficient mining process and increased production yield. The presented management programs also aid mines in keeping their re-entry practices in compliance with regulatory bodies and company policies. A practical case study demonstrates the implementation of the program in an operating hard rock mine and provides evidence of actual realized benefits.

Promising method for improving the accuracy of estimates in space debris tasks - Optimal filtering of measurements

Space debris poses an increasing threat to functioning spacecraft, as well as people and property on Earth. Every year, dozens of large spacecraft re-entry into atmosphere and reach Earth's surface, while there is always a risk of damage to people or property. To date, several collisions have occurred in near-Earth space, which can be avoided in future if appropriate characteristics of warning systems of such events are provided.The main method of parrying threats associated with space debris is warn about dangerous situations – time and place of large objects re-entry, a possible collision between spacecraft and space debris or another spacecraft. To implement this method and solve corresponding problems, it is necessary to have updated data on parameters of space objects orbits by measurements. Improving accuracy of estimating spacecraft orbits parameters and their subsequent prediction is necessary to ensure safety of space activities in conditions of a large number of space debris.The article presents main mathematical relations of optimal filtering of measurements method, it is shown that use of this method can significantly improve results accuracy of time of re-entry and space object as collision probability estimates, in comparison with the traditionally used least squares method. The results of using optimal filtering of measurements method are shown when predicting re-entry time and place of Tiangong-1 orbital station using available open data. A posteriori evaluation of accuracy prediction results showed that use of optimal filtering of measurements allows increasing accuracy of estimates up to several times, while not increasing computational complexity.One of ways to mitigate new space debris is active removal. Currently, work on space debris active removal has moved from research to being implemented in daily practice of space activities. In coming years, a number of projects will be implemented to remove spent upper stages, rocket bodies and spacecraft from orbits. The article present results of comparing areas for space debris active removal obtained by a method that takes into account optimal filtering of measurements method with a specific list of objects obtained by a group of international experts. As can be seen from the comparison - out of 50 objects, 48 fall into calculated areas, which indicate a good correspondence of results obtained earlier with estimates of international specialists group. In this regard, it can be assumed that in order to prevent collisions that can lead to a large formation of new objects in near future, both the ranges of orbits in altitudes and inclinations, and specific objects have been determined.The use of optimal filtering of measurements method in monitoring and warning systems for hazardous events related to space debris will increase efficiency of their functioning with existing measuring instruments.

Concentration, Propagation and Dilution of Toxic Gases in Underground Excavations under Different Ventilation Modes.

The drill-and-blast method is widely used for the excavation of hard rock tunnels. Toxic gases such as carbon monoxide and nitrogen oxides are released immediately after blasting by the detonation of explosives. To provide a safe working environment, the concentration of noxious gases must be reduced below the threshold limit value according to health and safety regulations. In this paper, one-dimensional mathematical models and three-dimensional CFD numerical simulations were conducted to analyze the concentration, propagation and dilution of the blasting fumes under different operating conditions. Forced, exhaust and mixed ventilation modes were compared to determine the safe re-entry times after blasting in a 200 m-long tunnel excavated using the top-heading-and-benching method. Based on the numerical simulations, carbon monoxide was the most critical gas, as it required a longer ventilation time to reduce its concentration below the threshold limit value. The safe re-entry time reached 480 s under the typical forced ventilation mode, but was reduced to 155 s when a mixed ventilation system was used after blasting, reducing the operating costs. The reduction of the re-entry time represents a significant improvement in the excavation cycle. In addition, the results obtained show that 1D models can be used to preliminary analyze the migration of toxic gases. However, to reliably determine the safe re-entry times, 3D numerical models should be developed. Finally, to verify the accuracy of the CFD results, field measurements were carried out in a railway tunnel using gas sensors. In general, good agreements were obtained between the 3D numerical simulations and the measured values.

Assessing Collision Probability in Low-Thrust Deorbit

End-of-life support of satellites is necessary to improve post-mission-disposal compliance rates for maintaining space environment. Deorbit mission with low thrust, e.g. a laser, induces a low-level deceleration on the target object that gradually lowers the target altitude. Since such a low-thrust trajectory is time-consuming, the risk of collision greatly influences the mission success rate. In this context, this paper assesses the collision risk during deorbit trajectories with low thrust. Furthermore, parametric studies for the relationship between the re-entry time and the risk of collision are performed.

Consensus Report by the Italian Academy of Osseointegration on the Use of Graft Materials in Postextraction Sites.

After tooth extraction, a modeling and remodeling phase of bone and soft tissues occurs. It has been fully demonstrated that bone resorption as high as 50% can take place regarding ridge width and a variable amount concerning ridge height, making it difficult to perform implant surgery. Active members of the Italian Academy of Osseointegration (IAO) participated in this Consensus Conference, and three systematic reviews were conducted before the meeting to provide guidelines on alveolar ridge preservation procedures. The systematic reviews covered the following topics: (1) What material best preserves the dimensions of the ridge horizontally and vertically?; (2) what material favors the formation of the highest quantity of new bone?; (3) which technique would best seal the socket?; and (4) what effect does alveolar ridge preservation have on soft tissues? The main conclusions reached by the assembly were that alveolar ridge preservation is advisable after dental extraction, particularly in esthetic areas, in proximity of anatomical structures (ie, maxillary sinus, inferior alveolar nerve, and mental foramen), whenever the treatment plan requires delayed placement, and whenever patients ask to postpone implant insertion for various reasons. Socket debridement is advised before the use of a "regenerative material," and xenograft is considered the gold standard material to maintain ridge dimensions. Another indication is antibiotic therapy, which is recommended in the case of alveolar ridge preservation (amoxicillin 2 g 1 hour before the intervention and 1 g every 12 hours for 6 days). A membrane or autologous soft tissue should be used to seal the socket and protect the regenerative material, and the indicated reentry time (implant insertion) is 4 to 6 months. This Consensus Conference agreed that the adoption of alveolar ridge preservation can effectively prevent physiologic bone loss, especially in esthetic areas. It is recommended to cover the xenograft material with a membrane or autologous soft tissue, and antibiotic therapy is advisable.

Optimal Measurements Filtering is a Promising Method for Estimation Accuracy Improving of Re-Entry Time and Collision Probability of Space Objects

Optimal Measurements Filtering is a Promising Method for Estimation Accuracy Improving of Re-Entry Time and Collision Probability of Space Objects

Knowledge and Practice of Farmers on Pesticide Pre-Harvest Interval and Pre-Entry Period and Personal Protective Equipment Use in Vegetable Growing Area of South Gondar, Ethiopia

This study aimed to assess the personal protective equipment use, understanding of farmers on pre-harvest interval, and pre-entry period for pesticides on different vegetables after pesticide application in south Gondar, Ethiopia. Sixty-three farmers were randomly selected and interviewed using a questionnaire. The majority of the farmers were not aware of the negative effects of pesticides on their health and the environment if not well handled. The farmers did not have awareness about the pesticide pre-harvest interval period and they did not read the instruction. Similarly, none of the respondents, followed the instruction of the manufacturer (reentry time specified on pesticide label) to reenter the sprayed field. Furthermore, farmers in the study area do have not enough awareness or they are careless regarding PPE use. Unavailability, inconvenience PPE wearing (e.g. mask affect breath properly), misperception about pesticide long term health effect and cost of personal protective equipment were raised as major causes for different farmers as factors behind their pesticide handling without personal protective equipment. It is recommended that If farmers can not to read the period of reentry on the pesticide container label, it is advised to wait at least 72 hours after the last application. Keywords: pesticide Pre-harvest interval period, re-entry time, personal protective equipment DOI: 10.7176/JBAH/11-23-02 Publication date: December 31 st 2021

창정 5B호 발사체의 재진입 시점 예측 분석

This paper represents a reentry time prediction analysis of CZ-5B rocket-body in China, subject to analysis of the Inter-Agency Space Debris Coordination Committee Reentry (IADC) reentry test campaign conducted in May 2021. Predicting the reentry of space objects is difficult to accurately predict due to the lack of accurate physical information about target, and uncertainty in atmospheric density. Therefore, IADC conducts annual re-entry campaigns to verify analysis techniques by each agency, and the Korea Aerospace Research Institute has also participated in them since 2015. Ballistic coefficient estimation method proposed to predict target reentry time and the result confirmed the difference of 73 seconds, which confirms the accuracy of the proposed method.

Clinical, Radiographic, and Histomorphometric Evaluation of a Vertical Ridge Augmentation Procedure Using a Titanium-Reinforced Microporous Expanded Polytetrafluoroethylene Membrane: A Prospective Case Series with 1-Year Follow-Up.

Vertical ridge augmentation for long-term implant stability is difficult in severely resorbed areas. We examined the clinical, radiological, and histological outcomes of guided-bone regeneration using novel titanium-reinforced microporous expanded polytetrafluoroethylene (MP-ePTFE) membranes. Eighteen patients who underwent implant placement using a staged approach were enrolled (period: 2018–2019). Vertical ridge augmentation was performed in areas with vertical bone defects ≥4 mm. Twenty-six implant fixtures were placed in 14 patients. At implant placement six fixtures had relatively low stability. On cone-beam computed tomography, the average vertical changes were 4.2 ± 1.9 (buccal), 5.9 ± 2.7 (central), and 4.4 ± 2.8 mm (lingual) at six months after vertical ridge augmentation. Histomorphometric analyses revealed that the average proportions of new bone, residual bone substitute material, and soft tissue were 34.91 ± 11.61%, 7.16 ± 2.74%, and 57.93 ± 11.09%, respectively. Stable marginal bone levels were observed at 1-year post-loading. The residual bone graft material area was significantly lower in the exposed group (p = 0.003). There was no significant difference in the vertical height change in the buccal side between immediately after the augmentation procedure and the implant placement reentry time (p = 0.371). However, all implants functioned well regardless of the exposure during the observation period. Thus, vertical ridge augmentation around implants using titanium-reinforced MP-ePTFE membranes can be successful.

Recurrent neural network model to predict re-entry trajectories of uncontrolled space objects

This paper proposes a novel sequence-to-sequence recurrent neural network model to predict re-entry trajectories of uncontrolled space objects. The epoch and altitude information of the objects from two-line element data were interpolated to equally spaced data and were used as inputs to the model. More than 200 datasets from five different objects with already known re-entry times were used as the training set for the model. Test datasets being not used for training, were chosen to validate the model’s performance. To assess the performance, the model predictions were compared with the ground-truth data of the test objects and with predictions from the Inter-Agency Space Debris Coordination re-entry campaign, in which space agencies from various countries had participated. The results indicate that the proposed approach is highly suitable for re-entry prediction compared to classical dynamics-based approaches, despite the small quantity of training data.

우주상황인식을 위한 인공우주물체 추락 예측 소프트웨어 개발

The high-level Space Situational Awareness (SSA) objective is to provide to the users dependable, accurate and timely information in order to support risk management on orbit and during re-entry and support safe and secure operation of space assets and related services. Therefore the risk assessment for the re-entry of space objects should be managed nationally. In this research, the Software for Re-Entry Prediction of space objects (SREP) was developed for national SSA system. In particular, the rate of change of the drag coefficient is estimated through a newly proposed Drag Scale Factor Estimation (DSFE), and is used for high-precision orbit propagator (HPOP) up to an altitude of 100 km to predict the re-entry time and position of the space object. The effectiveness of this re-entry prediction is shown through the re-entry time window and ground track of space objects falling in real events, Grace-1, Grace-2, Tiangong-1, and Chang Zheng-5B Rocket body. As a result, through analysis 12 hours before the final re-entry time, it is shown that the re-entry time window and crash time can be accurately predicted with an error of less than 20 minutes.

Chemical changes in thirdhand smoke associated with remediation using an ozone generator

Ozonation is a common remediation approach to eliminate odors from mold, tobacco and fire damage in buildings. Little information exists to: 1) assess its effectiveness; 2) provide guidance on operation conditions; and 3) identify potential risks associated with the presence of indoor ozone and ozonation byproducts. The goal of this study is to evaluate chemical changes in thirdhand smoke (THS) aerosols induced by high levels of ozone, in comparison with THS aerosols aged under similar conditions in the absence of ozone. Samples representing different stages of smoke aging in the absence of ozone, including freshly emitted secondhand smoke (SHS) and THS, were collected inside an 18-m3 room-sized chamber over a period of 42 h after six cigarettes were consumed. The experiments involved collection and analysis of gas phase species including volatile organic compounds (VOCs), volatile carbonyls, semivolatile organic compounds (SVOCs), and particulate matter. VOC analysis was carried out by gas chromatography/mass spectrometry with a thermal desorption inlet (TD-GC/MS), and volatile carbonyls were analyzed by on-line derivatization with dinitrophenylhydrazine (DNPH), followed by liquid chromatography with UV/VIS detection. SVOCs were extracted from XAD-coated denuders and Teflon-coated fiberglass filters in the absence of ozone. In those extracts, tobacco-specific nitrosamines (TSNAs) and other SVOCs were analyzed by gas chromatography with positive chemical ionization-triple quadrupole mass spectrometric detection (GC/PCI-QQQ-MS), and polycyclic aromatic hydrocarbons (PAHs) were quantified by gas chromatography with ion trap mass spectrometric detection (GC/IT-MS) in selected ion monitoring mode. Particulate matter concentration was determined gravimetrically. In a second experiment, a 300 mg h−1 commercial ozone generator was operated during 1 h, one day after smoke was generated, to evaluate the remediation of THS by ozonation. VOCs and volatile carbonyls were analyzed before and after ozonation. Extracts from fabrics that were exposed in the chamber before and after ozonation as surrogates for indoor furnishings were analyzed by GC/IT-MS, and aerosol size distribution was studied with a scanning mobility particle sizer. Ozone concentration was measured with a photometric detector. An estimated 175 mg ozone reacted with THS after 1 h of treatment, corresponding to 58% of the total O3 released during that period. Fabric-bound nicotine was depleted after ozonation, and the surface concentration of PAHs adsorbed to fabric specimens decreased by an order of magnitude due to reaction with ozone, reaching pre-smoking levels. These results suggest that ozonation has the potential to remove harmful THS chemicals from indoor surfaces. However, gas phase concentrations of volatile carbonyls, including formaldehyde, acetaldehyde and acetone were higher immediately after ozonation. Ultrafine particles (UFP, in most cases with size <60 nm) were a major ozonation byproduct. UFP number concentrations peaked shortly after ozonation ended, and remained at higher-than background levels for several hours. Based on these results, minimum re-entry times after ozone treatment were predicted for different indoor scenarios. Clearly defining re-entry times can serve as a practical measure to prevent acute exposures to ozone and harmful ozonation byproducts after treatment. This study evaluated potential benefits and risks associated with THS remediation using ozone, providing insights into this technology.

Migration characteristics of CO under forced ventilation after excavation roadway blasting: A case study in a plateau mine

Multiple toxic gases, such as carbon monoxide (CO), are generated after excavation roadway blasting of plateau mines. To eliminate environmental pollution in a timely manner and avoid potential occupational hazards to miners, a three-dimensional unsteady roadway model was established based on field data of the Pulang copper mine in Yunnan province, China. Computational Fluid Dynamics (CFD) method was used to explore the space-time evolution characteristics of CO under different working conditions, and the grey correlation analysis was applied to study the correlation values of the influencing factors on the CO dispersion coefficient in the roadway. The results demonstrate that in the early stage of the roadway with forced ventilation, the mass concentration of CO in the vortex area near the working area was obviously higher than elsewhere, and in the later stage of ventilation, the mass concentration of CO at the bottom of the roadway was higher than that at the top. The most significant changes of the CO appeared within 500 s before ventilation. The mass-weighted average concentration of CO in the cross-section of the roadway presented a Gaussian-like distribution with respect to the ventilation time. The first stage represented a period of rapid growth, and the second stage exhibited a period of variable speed decline. The ventilation time when the mass concentration of CO in the roadway reached the upper limit of occupational exposure was generally between 700 s and 900 s, and the safe re-entry time was greater than 15 min. The grey correlation analysis revealed that the major influencing factors of CO migration in the roadway of the plateau mine, were, in a descending sequence, as follows: the distance between ventiduct mouth and working face, ventilation volume, and altitude, and the corresponding correlation coefficients were 0.9931, 0.7792, and 0.4366, respectively. The altitude was inversely proportional to the CO dispersion coefficient, and the distance between ventiduct mouth and working face was directly proportional to the ventilation coefficient corresponding to the CO dispersion coefficient. This research can provide guidance on optimising the ventilation design of roadways in plateau mines, thus ensuring the occupational health of cleaners.

Reentry time evaluation for Tiangong-1 spacecraft using different atmosphere models and prediction methods

Reentry time evaluation for Tiangong-1 spacecraft using different atmosphere models and prediction methods