Airflow Dynamics Research Articles

Revolutionizing Spray Drying: An In‐Depth Analysis of Surface Stickiness Trends and the Role of Physicochemical Innovations in Boosting Productivity

Spray drying is a widely utilized method in the pharmaceutical, food, and chemical industries for producing powdered products. However, a recurring challenge in this process is the persistent issue of surface stickiness, which diminishes product quality and operational efficiency. This paper examines the current trend of surface stickiness in spray drying and explores physicochemical approaches to mitigate fouling and enhance overall productivity. The stickiness is often attributed to the formation of lumps on drying chamber surfaces, adversely affecting the final product’s characteristics. The paper reviews existing literature on the causes and consequences of surface stickiness, emphasizing the need for innovative strategies to address this challenge. Promising solutions are emerging in the form of physicochemical approaches, involving the modification of drying chamber materials, applying different operational approaches, optimization of process parameters, and the introduction of drying aids. Researchers aim to alter the surface properties of drying chamber components, minimizing particle and wall deposition to prevent surface stickiness. The present review also delves into the impact of various physicochemical factors on spray drying performance, including temperature, airflow dynamics, and formulation composition. Understanding these factors is crucial for developing effective strategies to reduce fouling and enhance overall productivity in spray drying processes. This review sheds light on the prevalent issue of spray drying surface stickiness and underscores the significance of physicochemical approaches in overcoming this challenge. By addressing stickiness issues through innovative strategies, researchers and industry professionals can improve the efficiency and reliability of spray drying processes, ultimately elevating the quality of powdered products in pharmaceutical, food, and chemical manufacturing.

Impact of Glottis Area on Airflow Dynamics in the Human Trachea Using a Simplified Stenosis Pipe Model

Impact of Glottis Area on Airflow Dynamics in the Human Trachea Using a Simplified Stenosis Pipe Model

МЕТОДИКА ВИЗНАЧЕННЯ ВІДДАЛЕНОГО ДЕСАНТУВАННЯ ПАРАШУТНО-РЕАКТИВНОЇ СИСТЕМИ

The article discusses certain issues and possible ways to partially solve the problem of remote landing, which may be useful in planning airborne operations to improve the safety of traditional parachute landing, as well as in further research and development of remote landing methods for airborne vehicles. Based on the results of the research, a method of remote landing of heavy equipment using a parachute-jet system is proposed, which allows to identify the weakest properties of the system, assess its overall level, characterised by a set of natural and climatic indicators, in particular wind speeds at different altitudes, and develop measures to improve the efficiency of the system's intended use. The results of the study demonstrate the ability of the WFD to move horizontally to the landing site at a considerable distance from the point of discharge by increasing the height of the release. However, it is much more difficult to calculate the specific amount of displacement without current knowledge of the many parameters that influence the wind drift rate. These parameters mainly include the air currents encountered by the WTG, which can help or hinder horizontal movement. Modern technology has made it possible to implement many innovative methods of cargo discharge. In order to reduce the impact of uneven airflows on drop accuracy, HARP (high-altitude release point) systems are now being used to improve the accuracy of high-altitude drops, which take into account airflow dynamics, system ballistics and aircraft speed.The ballistic table (based on the average ballistic characteristics of this parachute system) determines the drop point of the combat vehicle and calculates the free fall trajectory for a high altitude drop, taking into account the typical airflow profiles between the drop point and the landing site. To significantly improve the accuracy of high altitude drops, it is also possible to use an infrared Doppler laser locator to measure airspeed at different altitudes and create real-time 3D wind maps in the area between the aircraft and the landing site surface. Such systems are capable of making measurements with a typical error of less than one metre per second. The application of the research results in practice will increase the operational capabilities of the Airborne Assault Forces of the Armed Forces of Ukraine, namely, to ensure rapid deployment in threatened areas to deter the enemy by remotely landing heavy equipment, which in turn will significantly increase the combat potential of combat units in the operational and tactical depth of the enemy's defence.

Performance analysis of flat winglet deflector on hybrid solar PV-Wind turbine system: Case study on twisted Savonius turbine

The harnessing of clean energy from solar and wind constitutes the foremost renewable energy source in Indonesia. The amalgamation of these energy modalities holds the promise of heightened energy efficiency coupled with reduced maintenance expenditures. This investigation endeavors to synergize wind turbines with photovoltaic (PV) solar panels in a hybrid configuration, capitalizing on the turbulent effluent from the wind turbine system as a cooling medium for the solar PV panels. Further studies are needed regarding the Solar PV-Wind Turbine hybrid cooling system, as a system needs to be designed to optimize the direction of airflow from the turbine as a cooling medium for the solar PV panels without compromising the turbine's performance. Experimental-scale modeling is implemented in this study, introducing a flat winglet deflector configuration to refine and optimize the airflow dynamics traversing the turbine, directed towards enhancing the performance of the integrated solar PV-Wind Turbine hybrid system. The results showed that the installation of solar PV panels and the addition of a flat winglet deflector configuration could improve the performance of the turbine. The highest Cp and Ct values obtained were 0.18476 and 0.66404 with an increased value of 21.74% and 20.56% respectively. Using the Taguchi method, the most optimal configuration for Cp is obtained for installing a PV solar panel with a height of 10cm with AoA for installing a flat winglet deflector of 5°. In the ANOVA analysis conducted, it is known that AoA has an effect of up to 71.57%, while the panel height has an effect of 24.69% with an error percentage of 3.73%.

Numerical Analysis of Turbulent Air Flow Dynamics in a Rectangular Channel with Perforated Nozzle-Shaped Vertical Baffles

Numerical Analysis of Turbulent Air Flow Dynamics in a Rectangular Channel with Perforated Nozzle-Shaped Vertical Baffles

Nearly zero energy building design and optimization: A residential building transformation in Türkiye

In this study, the energy performance analysis of a representative residential building located in Izmir, Turkey, was carried out, utilizing the DesignBuilder energy simulation program. Heating, ventilation, and air conditioning systems are modeled as detailed in Design Builder in order to consider parameters such as thermal properties of materials, duct layout and airflow dynamics of the system in detail in the analysis. The technical and economic analysis of transforming the building into a nearly zero energy building (NZEB) was performed for eight different retrofit scenarios, including passive and active energy efficiency measures. The most effective scenario, Scenario 8 (S8), reduced the building's annual net primary energy consumption by 96.08% to 7.45 kWh/m²/year. S8's annual CO2 emissions decreased by 100.07% compared with the reference scenario, resulting in −0.042 kg CO2/m²/year. The overall energy performance class of the building was determined using Turkey's national calculation program for the preparation of the energy identity certificate. The energy performance class of the reference building was determined as D, and the class of the building designed according to S8 as A. Investment evaluations were carried out for the retrofit scenarios, revealing that the investment cost for S8, having the lowest net primary energy consumption, amounted to USD 17,565.87. This finding establishes S8 as a more financially viable option in the short term. This study demonstrates the potential of NZEBs in reducing greenhouse gas emissions and achieving sustainable development goals.

Application of a composite, multi-scale COVID-19 mitigation framework: US border use-case

ABSTRACT Airborne pathogen transmission within crowded facilities can be modelled by combining several interrelated mechanisms of spread: movement of people, airflow dynamics, and aerosol dispersion. This paper describes a composite model framework combining analytical models to demonstrate the spread of an airborne pathogen in a crowded, confined space at an immigrant processing centre on the southern US border during the border crisis of March 2021. Recommendations that could reduce current COVID-19 infection rate from 11% to 6.16% at relatively low additional cost to the government are given. These recommendations could also lower the infection rate by approximately five times from 31.14% worst case from long indoor exposures down to 6.35% when immigrant processing times surge to 10 days. This work highlights the challenges of managing COVID-19 in crowded facilities, and provides quantitative decision options with potential both to slow and prevent disease spread, while lessening the economic burden on communities.

Protective Airflow including ‘First Air’ in GMP Aseptic manufacturing

Good Manufacturing Practices (GMP) related to sterile product aseptic manufacturing environments rely on protective airflow mechanisms to ensure the control over airborne contamination. Alongside the protective airflow, pressure differentials that drive airflow cascade ensure GMP compliant contamination control of airborne contamination that may include Microbial carrying particles (MCPs). Airflow visualisation qualification by smoke studies has been clarified as a GMP requirement in EU & PICS Annex 1. The scope of Annex 1 applies to manufacture of sterile medicinal products together with bioburden control processes where bioburden in intermediates, substances, APIs and non-sterile products can also impact patient safety so a wide scope of protective airflow applications is applied. As a GMP requirement (included in 2008 version of Annex 1): It should be demonstrated that air-flow patterns do not present a contamination risk, e.g. care should be taken to ensure that air flows do not distribute particles from a particle generating person, operation or machine to a zone of higher product risk.' Within Annex 1: 2023, the concept of First Air has emerged as a critical element. First Air is defined as “filtered air that has not been interrupted prior to contacting exposed product and product contact surfaces with the potential to add contamination to the air prior to reaching the critical zone.” The requirement for extensive smoke studies and the integration of First Air protection as a mandated aspect in GMP regulations has brought to the fore an imperative need for a deeper comprehension of airflow dynamics and their implications on sterile product Aseptic manufacturing. This shift beckons us to explore the nuances of protective airflow, particularly in the context of Computational Fluid Dynamics (CFD) analyses, to discern the efficacy of airflow patterns in different scenarios. Stakeholders involved in Annex 1 implementation have been impacted by introduction of the First Air protection requirement, considering design and in qualification via smoke study airflow visualisation. Such requirements must follow QRM principles where process understanding and knowledge of contamination hazards are essential to mitigate risks in compromise of product quality and potential harm to patients. Risk mitigations must take a Quality by Design (QbD) approach as monitoring alone does not provide assurance of product sterility or in meeting defined bioburden limits.

Field-Measurement of Surface Wind and Sediment Transport Patterns in a Coastal Dune Environment, Case Study of Cala Tirant (Menorca, Spain)

Blowouts are integral features of coastal dune fields. Their presence enhances both geomorphological and ecological diversity and enables the movement of sand by wind. Their role as a ‘transport corridor’ may be, however, considered negative from a coastal management perspective in heavily touristic areas, where the existence of blowouts close to the foredune can enhance the loss of sediment from the beach. This paper investigated the relationship between airflow dynamics and patterns of sediment transport from the beach to established dunes through a trough blowout located on the foredune. Seven three-cup anemometers were used to measure wind speed and direction over a 24 h sampling period at a frequency of 1 min under onshore (parallel to the blowout axis) medium and high wind speeds (max of 17.9 ms−1). To measure sediment transport, a total of 12 vertical sand traps were located at three positions along the length of the deflation basin. The results indicated that small amounts of sediments went into the blowout from the beach and that the highest rates of sediment remobilization took place within the deflation basin. These results highlight two processes: (a) flow channelization induced by the blowout topography caused an increase in wind speed and sediment transport toward the depositional lobe, and (b) the presence of embryo dunes and herbaceous vegetation at the beach–blowout boundary effectively reduced the amount of sediment transport from the beach to the landform. The results confirmed the significant role that vegetation plays in controlling sediment movement and conserving the beach–dune system.

Soaring over open waters: horizontal winds provide lift to soaring migrants in weak thermal conditions

BackgroundFor soaring birds, the ability to benefit from variable airflow dynamics is crucial, especially while crossing natural barriers such as vast water bodies during migration. Soaring birds also take advantage of warm rising air, so-called thermals, that allow birds to ascend passively to higher altitudes with reduced energy costs. Although it is well known that soaring migrants generally benefit from supportive winds and thermals, the potential of uplifts and other weather factors enabling soaring behavior remains unsolved.MethodsIn this study, we GPS-tracked 19 Red Kites, Milvus milvus, from the Central European population that crossed the Adriatic Sea on their autumn migration. Migratory tracks were annotated with weather data (wind support, side wind, temperature difference between air and surface—proxy for thermal uplift, cloud cover, and precipitation) to assess their effect on Red Kites' decisions and soaring performance along their migration across the Adriatic Sea and land.ResultsWind support affected the timing of crossing over the Adriatic Sea. We found that temperature differences and horizontal winds positively affected soaring sea movement by providing lift support in otherwise weak thermals. Furthermore, we found that the soaring patterns of tracked raptors were affected by the strength and direction of prevailing winds.ConclusionThanks to modern GPS–GSM telemetry devices and available data from online databases, we explored the effect of different weather variables on the occurrence of soaring behavior and soaring patterns of migratory raptors. We revealed how wind affected the soaring pattern and that tracked birds could soar in weak thermals by utilizing horizontal winds, thus reducing energy costs of active flapping flight over vast water bodies.

Experimental characterization of exhaled flow dynamics of human breathing and vocalization

During the onging pandemic of COVID-19, there are numerous asymptomatic patients who are infectious. The exhaled droplets from their daily respiratory activities like breathing or speaking can be the sources of airborne disease transmission of COVID-19. The understanding of the airflow dynamics of these respiratory activities may be helpful to develop effective measures to prevent and control the spread of the disease. In this study, the exhaled flows from human breathing and vocalization of specific syllables are characterized using particle image velocimetry (PIV) and smoke visualization. The exhaled flow generated by ten phonemes including vowels, fricatives, affricates, plosives and nasals as well as mouth breathing are studied. The dynamic developments of the airflow processes are described by a series of parameters, including peak velocity, peak velocity time, duration time, propagation velocity and distance. Results show that vocalization of affricates and plosives as well as mouth breathing tend to have higher peak velocities and propagation distances. The evolutions of exhaled flows generated from these respiratory processes are found to have different jet structures, which are related to the stroke ratio (L/D). The flow field of a small L/D only has a pair of dominant vortices. Whereas that of a large L/D presents both a pair of dominant vortex ring and a trailing jet. Certain phoneme (e.g., /t/) is found to display a two-stage jet similar to a cough with the starting jet and an interrupted jet. The characterization of human exhaled flow of this study may be helpful to provide basis of CFD simulations and a better understanding of the spread of airborne diseases from human breathing.

2330. Ventilation Strategies Based on an Aerodynamic Analysis During a Large-Scale SARS-CoV-2 Outbreak in an Acute-Care Hospital

Abstract Background The severity of coronavirus disease-2019 (COVID-19) has decreased owing to antiviral agents and herd immunity; however, transmission among hospitalised patients could be fatal. Therefore, strategies to prevent the spread of COVID-19 in medical institutions are crucial. This study aimed to investigate ventilation strategies to prevent nosocomial transmission of COVID-19. Methods We retrospectively conducted an epidemiological investigation of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak between 12 February and 5 March 2021 in a referral teaching hospital in Korea. The pressure difference and air change per hour (ACH) of the rooms were measured in the largest outbreak ward. Airflow dynamics were measured based on the opening or closing of the outside window and door by using an oil droplet generator, indoor air quality sensor, and particle image velocimetry in the index patient’s room, corridor, and opposite rooms. Results During the outbreak, 283 COVID-19 cases were identified. The largest outbreak occurred at the 8th floor followed by the 9th and 7th floors of the main building. According to an epidemiological investigation, the SARS-CoV-2 spread occurred sequentially from the index room to the nearest room, especially the opposite. The aerodynamic study demonstrated that droplet-like particles in the index room diffused through the corridor and the opposite room through the opening door (Figure 1). The mean ACH of the rooms on the 8th floor was 1.44; the air supply volume was 15.9% larger than the exhaust volume, forming a positive pressure. Compared with the open-door condition, droplet-like particles did not diffuse into an adjacent room facing each other in the closed-door condition. The concentration of droplet-like particles in the ward was lowered during natural ventilation, and their spread to an adjacent room facing the corridor was also reduced (Figure 2). Location of the index patient and detection sensor of the droplet-like particles Relative ratio in measurement of droplet-like particles, PM2.5, concentration according to natural ventilation conditions (A) Window closed/indoor-open (usual condition: ventilation with the heating, ventilation, and air conditioning [HVAC] system without natural ventilation), (B) Window open/indoors closed (condition for improvement: close the door and perform natural ventilation for each room), (C) Window and indoor opening (condition for improvement: opening all doors and windows in all the rooms and maximising natural ventilation). Conclusion The spread of droplet-like particles between rooms could be attributed to the pressure difference between the rooms and corridor. To prevent the spread of transmission between rooms, increasing the ACH in the room by maximising natural and mechanical ventilation is essential. Disclosures All Authors: No reported disclosures

Wireless broadband acousto-mechanical sensing system for continuous physiological monitoring.

The human body generates various forms of subtle, broadband acousto-mechanical signals that contain information on cardiorespiratory and gastrointestinal health with potential application for continuous physiological monitoring. Existing device options, ranging from digital stethoscopes to inertial measurement units, offer useful capabilities but have disadvantages such as restricted measurement locations that prevent continuous, longitudinal tracking and that constrain their use to controlled environments. Here we present a wireless, broadband acousto-mechanical sensing network that circumvents these limitations and provides information on processes including slow movements within the body, digestive activity, respiratory sounds and cardiac cycles, all with clinical grade accuracy and independent of artifacts from ambient sounds. This system can also perform spatiotemporal mapping of the dynamics of gastrointestinal processes and airflow into and out of the lungs. To demonstrate the capabilities of this system we used it to monitor constrained respiratory airflow and intestinal motility in neonates in the neonatal intensive care unit (n = 15), and to assess regional lung function in patients undergoing thoracic surgery (n = 55). This broadband acousto-mechanical sensing system holds the potential to help mitigate cardiorespiratory instability and manage disease progression in patients through continuous monitoring of physiological signals, in both the clinical and nonclinical setting.

Strategic Placement of Portable Air Cleaners for Enhanced Aerosol Control in Dental Treatment Rooms: A Computational Fluid Dynamics (CFD) Analysis

Background. Adequate ventilation is imperative for controlling respiratory transmission, particularly in the context of the COVID-19 pandemic. The commercial portable air cleaners have emerged as practical solutions to reduce contaminated aerosols in dental treatment rooms. This study employs computational fluid dynamics (CFD) to assess their impact on airflow dynamics. Methods. Dental treatment room models were constructed using SolidWorks software, encompassing two distinct air conditioner grille orientations (straightening and 45-degree downward directions) and five different positions for the portable air cleaner (two located at the rear left/right of the dental unit and three at the foot end of the dental unit—center, left, and right corners). The study examined alterations in airflow direction and residual aerosol concentrations using ANSYS Fluent software. Results. The incorporation of portable air cleaners in dental treatment rooms significantly reduced aerosol levels across all model configurations. Notably, the placement of the portable air cleaner emerged as a critical factor influencing airflow patterns. In models with straightening and 45-degree downward air conditioner grille orientations, optimal positioning was near the operating field and at the foot end of the dental chair, respectively. Conclusion. This investigation highlights the pivotal role of strategic portable air cleaner placement in dental treatment rooms for effective aerosol removal. Placing the air cleaner near the operating field or at the foot end of the dental chair not only improved airflow patterns but also enhanced aerosol removal efficiency, ultimately promoting superior air quality within dental treatment environments.

Airflow Simulation in a Turbofan Engine: A Study of Flow Behavior

The efficient functioning of modern turbofan engines relies heavily on a deep understanding of airflow dynamics within critical components. This research paper presents a comprehensive investigation into airflow simulation within a turbofan engine, employing advanced computational techniques. The study focuses on flow behavior and makes use of SolidWorks for 3D modeling and ANSYS for simulation. The investigation centers on analyzing key flow parameters such as velocity, pressure, and temperature. The methodology involves creating an accurate 3D model of the turbofan engine excluding the compressor, combustion chamber, and turbine using SolidWorks to capture fine geometry and details. Subsequently, ANSYS is utilized to simulate the airflow within the turbofan engine, simulating realistic conditions and enabling the detailed analysis of flow behavior. The results of this study advance knowledge of turbofan engine technology and lay the groundwork for additional study and advancement in the area of aviation propulsion systems. In response to the evolving requirements of the aviation sector, the knowledge acquired from this research will serve as a priceless asset in the development of engines that are characterized by enhanced dependability, a reduced ecological footprint, and heightened fuel efficiency.

Investigating unsteady airflow characteristics in the human upper airway based on the clinical inspiration data

To enhance understanding of the airflow characteristics in the human respiratory system during realistic inspiration, we investigated the airflow field in a human upper airway model using large eddy simulation and the dynamic grid method, taking into account clinically measured inspiratory characteristics. The results reveal the following novel findings: (1) The laryngeal jet and recirculation zone exhibit significant unsteadiness, with their dynamic characteristics primarily influenced by the transient inspiration flow rate and glottis motion. This pattern holds true for other airflow characteristics as well. (2) Glottis expansion reduces the energy consumed during inhalation for both steady and unsteady inspiratory flow rates, with the degree of expansion being directly related to the reduction in energy. We can accurately predict power loss by considering the glottis area and inspiratory flow rate. (3) Analysis of spectral entropy clearly demonstrates that the flow transitions from the laminar to turbulence earlier when using clinical inspiration data. Turbulence intensity in the trachea increases when either glottis motion or the transient inspiratory is ignored. In conclusion, the airflow dynamics are significantly more unsteady compared to cases where we ignore either glottis motion or the transient inspiratory flow rate. A precise understanding of realistic respiratory airflow cannot be achieved by assuming either a rigid glottis or a steady inspiration pattern. Therefore, it is crucial to use accurate inspiratory data when studying the properties of airflow structures in the human respiratory system. Moreover, incorporating more physiological data is also essential to obtain realistic respiratory airflow characteristics.

Droplet dispersion characteristics during human walking in a queue

The dispersion of respiratory droplets is strongly influenced by the complex airflow induced by human activities, such as walking in a queue. Understanding the relationship between local airflow disturbances during queue walking and droplet dispersion is crucial. This study investigates the effects of following distance (1.0, 1.5, 2.0, and 2.5 m), walking speed (0.8, 1.0, 1.2, and 1.4 m/s), and droplet diameter (1, 10, 50, 80, and 120 μm) on droplet dispersion. The findings reveal that the interaction between wake vortex and forward airflow provides a foundation for cross-infection among individuals. An increased following distance leads to an initial rise and subsequent decrease in the concentration in the breathing zone of the susceptible individual. The social distances of 1.0 and 1.5 m are insufficient to mitigate the risk of cross-infection, warranting a recommended following distance of at least two meters. The effect of walking speed on droplet dispersion varies depending on the scenario. In cases involving standing and walking cycles, the infection risk of the susceptible individual gradually increases with higher walking speeds. Conversely, when individuals walk continuously in a queue, the infection risk of the susceptible individual decreases with increased walking speed. Moreover, intermediate-sized droplets play a critical role in the transmission of respiratory infectious diseases and demand heightened attention. This study expounds the intricate airflow dynamics during queue walking and emphasizes the significance of following distance, walking speed, and droplet diameter in minimizing the risk of cross-infection.

Thermal characterization of a high-temperature industrial bread-baking oven: A comprehensive experimental and numerical study

This study presents a complete experimental campaign and the first three-dimensional numerical model developed for a high-temperature bread-baking oven. A typical baking tunnel oven prototype is designed and constructed for the study. The prototype has appropriate temperature sensors to investigate temperature profiles at several points inside the baking chamber. A specific device is developed to measure the heat flux received by bread during baking, showing values ranging between 6.6 and 29.3 kW/m2. Computational fluid dynamic software models the non-premixed combustion of diesel fuel occurring at the burner and the airflow characteristics inside the baking chamber. Numerical results show the heterogeneity of temperatures (450–1000 °C) inside the tunnel oven and agree with the experimentally measured values (maximal variation of 9 %) also shows complex airflow dynamics and the existence of multiple velocity zones. The model is applied to the specific case of Lebanese bread baking. An energy efficiency analysis of the oven reveals an efficiency range of around 16 %. This foundational work sets the stage for future research and optimization of oven design, energy recovery systems, and bread-baking processes. Also, the insights gained from this study will be integral in developing future numerical models for flatbread baking coupled with large deformation.

Three-dimensional critical points and flow patterns in pulmonary alveoli with rhythmic wall motion

The dynamics of airflow in the pulmonary acini are of broad interest in understanding respiratory diseases and the fate of inhaled particles. This study investigates the three-dimensional (3d) alveolar flows with rhythmic cavity wall motion, using a finite element method based computational fluid dynamics. This study reports the new research findings on the critical points and associated flow patterns. The locations of critical points are found based on the Brouwer degree theory and Broyden’s method. The phase portrait is used to evaluate the flow patterns around the critical points and the stability (repelling/attracting property) of the critical points on the symmetry plane of the alveolus. Based on the Poincare–Bendixson theorem, the closed orbits on the symmetry plane are found which have the capability to alter the spiral direction of the spiral streamlines. In the 3d space, the alveolar flow is symmetric about the geometric symmetry plane of the alveolus. Different types of 3d critical points, including saddle, spiral, and spiral saddle, are revealed. There are only one saddle point and at least one spiral point or spiral saddle in the alveolar flow. Spiral points and spiral saddles are located on the vortex core line and their number is dependent on the Reynolds number and varies with time. The study of critical points and their evolution helps us to understand the mechanism of irreversible transport of particle tracers from a new perspective.

An implicit time integration approach for simulation of corona discharges

Corona discharge is a type of electric discharge that occurs when a small conductor carries a voltage strong enough to ionize the surrounding air. The resultant ions are drifted by the electric field and transfer their momentum to neutral particles via elastic collisions, creating a flow effect known as ionic wind that has promising application in flow control. Over the last two decades, a large amount of numerical works have been realized to study this effect. However, simulation of corona discharges with explicit time integration usually requires an excessive amount of time to finish, ranging from days to months in two dimensions, depending on mesh size and physical end time. The reason lies in the profound disparity between physical time scales in a discharge: for example, electron density evolves on the order of picoseconds while the time scale of airflow dynamics is on the order of milliseconds. The objective of this work is therefore to reduce the CPU time of corona discharges. We propose an implicit time strategy in the scope of the Local Field Approximation plasma model. We investigate some algorithms, namely the Lie operator splitting, the Douglas-Rachford method and a nonlinear variant of the Gauss-Seidel algorithm, to solve the system of discrete equations, which are adapted to take into consideration a constraint on the electron density. This constraint is introduced to compensate for sources of electrons from physical processes that are not accounted for in the plasma model; it is expressed by a minimum density, or floor density, imposed on the electrons. Therefore, simplified plasma models can be employed for discharge simulations, for instance a four-specie, four-reaction model is used in this article. The investigation is done on a one-dimensional wire-to-wire corona discharge. After having identified the most suitable algorithm, we carry on to two-dimensional simulations and compare the numerical results with experiment data for different levels of floor density.