Air Velocity Profiles Research Articles

Direct and inverse simulations of hydrodynamic and thermal characteristics in a room with random boundary conditions by feedforward neural network modelling

Artificial Intelligence (AI) or Neural Network (NN) approaches are starting to be used in built environment areas. This study demonstrates that feedforward neural network (FNN) modelling is capable of simulating the hydrodynamic and thermal characteristics of a room with a radiant heating and cooling system. The FNN model was trained and tested to simulate air velocity and temperature profiles based on boundary conditions as direct simulation and to restore boundary conditions as inverse simulation, using a scaled and normalised database generated by a Computational Fluid Dynamics (CFD) model. In the CFD model, the non-dimensional form of the governing equations, including the radiative transfer equation (RTE) and vorticity equation, were solved using the discrete ordinate method (DOM) and finite difference method (FDM) respectively. The impacts of training database size, hidden layers, and the number of neurons in each layer on the accuracy of the FNN results in the direct and inverse simulations were evaluated in terms of the average root mean square error (RMSE) over the testing data. In the direct simulation, using a smaller CFD database with more hidden layers and neurons achieved comparable accuracy to an FNN trained with a larger CFD database and a less complex FNN. Furthermore, in the inverse simulation, where the FNN model was trained to use a subdomain of temperature profiles to simulate or restore the boundary conditions, the effect of the size of the subdomain on the accuracy in terms of RMSE was studied. It demonstrated that the FNN approach could conduct inverse simulations that are typically beyond the capability of conventional CFD modelling approaches. The results show that the boundary conditions could be accurately restored using subdomains of the temperature profile, covering approximately 60% of the main domain.

Analysis of seasonal variations and their impact on the microclimate of soilless glass greenhouses: Numerical and experimental investigations

This article presents a comprehensive examination of the interior environments in a soilless Greenhouse located in Tunis and their response to seasonal variations. The research employs a numerical model in conjunction with an experimental setup to achieve this objective. The model considers various elements, including glass, air, crops, and concrete in combination, to assess their impact on the indoor climate. Rigorous comparisons with experimental data collected from a greenhouse prototype, particularly regarding parameters like air velocity and temperature profiles, confirm the validity of the numerical findings. To closely replicate real conditions, the numerical model underwent optimization, incorporating turbulence and radiation models and undergoing a grid independence analysis. This article provides a thorough analysis of how seasonal fluctuations directly influence the microclimate within a soilless glass greenhouse, utilizing a combination of numerical and experimental data. Additionally, a series of numerical simulations were conducted to evaluate the potential impact of crop quantity on static indoor temperature. The comparative analysis revealed that the temperature difference at the roof level between scenarios with tomato and basil cultivation and those without cultivation was narrowed down to 3.5 K. Consequently, this study sheds light on the implications of seasonal fluctuations on soilless greenhouse climates and emphasizes the importance of accurate modeling and control to optimize agricultural practices in the northern regions of Tunisia.

Coherent Structures Analysis of Methanol and Hydrogen Flames Using the Scale-Adaptive Simulation Model

Computational fluid dynamics techniques were applied to reproduce the characteristics of the liquid methanol burner described in a previous paper by Guevara et al. In this work, the unstable Reynolds-averaged Navier–Stokes (U-RANS) approach known as the Scale-Adaptive Simulation (SAS) model was employed, together with the steady nonadiabatic flamelets combustion model, to characterize and compare methanol and hydrogen flames. These flames were compared to determine whether this model can reproduce the coherent dynamic structures previously obtained using the LES model in other investigations. The LES turbulence model still entails a very high computational cost for many research centers. Conversely, the SAS model allows for local activation and amplification, promoting the transitions of momentum equations from the stationary to the transient mode and leading to a dramatic reduction in computational time. It was found that the global temperature contour of the hydrogen flame was higher than that of methanol. The air velocity profile peaks in the methanol flame were higher than those in hydrogen due to the coherent structures formed in the near field of atomization. Both flames presented coherent structures in the form of PVC; however, in the case of hydrogen, a ring-type vortex surrounding the flame was also developed. The axial, tangential, and radial velocity profiles of the coherent structures along the axial axis of the combustion chamber were analyzed at a criterion of Q = 0.003. The investigation revealed that the radial and tangential components had similar behaviors, while the axial velocity components differed. Finally, it was found that, using the SAS model, the coherent dynamic structures of the methanol flame were different from those obtained in previous works using the LES model.

Numerical investigation of greenhouse climate considering external environmental factors and crop position in Sfax central region of Tunisia

This paper presents a comprehensive study on the influence of external factors and the crop positioning on indoor environments in Sfax, a central region of Tunisia, during summer climate conditions. To achieve this, an experimental setup is implemented, and a numerical model was developed. The model incorporates air, crops, and soil components to analyze their individual and combined effects on the indoor environment. The numerical results were validated through a comparison with test data obtained from a greenhouse prototype, specifically concerning air velocity and temperature profiles. An optimization of the numerical model was performed considering turbulence and radiation models and grid independence analysis to ensure accurate simulations. Moreover, both with and without crops configurations as well as the impact of the crop position were investigated through a series of numerical simulations. Based on the numerical and experimental findings, the study establishes robust correlations to capture the variations resulting from these factors. The study reveals that external factors, such as ambient air temperature and solar radiation, play a crucial role in determining the temperature inside the greenhouse. The comparative study revealed an increase of less than 1% with the integration of crops in the greenhouse. The central position of the crop emerged as optimal, resulting in a slight temperature reduction of around 2 K compared to the left and right positions. Overall, this study provides valuable insights into the impact of external factors on greenhouse climates and highlights the importance of accurate modeling and control for optimal agricultural management in the central region of Sfax, Tunisia.

Turbulent boundary layer profiles in airflow over young wind waves in co- and counter-wind water current

Understanding the air velocity profile over random and three-dimensional wind-waves is crucial for evaluating momentum and energy transfer between air and water. Unfortunately, determining accurate air velocity profiles in field conditions is nearly impossible. It is well accepted, though, that the airflow above water waves has a logarithmic profile usually expressed in terms of the effective roughness parameter. However, this parameter cannot be evaluated directly from the wave measurements at the water surface. For young wind waves, it was recently shown that the airflow over the waves maintains wall similarity. In this case, the airflow vertical velocity distribution can be described by the Nikuradse fully rough logarithmic profile, using the root mean square value of the water surface elevation as the equivalent sand grain roughness height. The existence of mean current in water in either co- or counter-wind direction may significantly modify the wave field, the vertical wind-velocity profile and thus the momentum and energy transfer from air to water. The effect of the water current on the spatially developing boundary layer over young wind waves and the wall similarity is examined. Combined laboratory measurements at several fetches of finely resolved mean air velocity profile above the water surface and of the characteristics of the wind-wave field are performed at multiple wind-forcing and mean water current conditions. The shear stress at the air–water interface estimated using two independent approaches is weakly dependent on current, while the resultant wave field differs significantly.

Novel optimal structure design and testing of air-cooled open-cathode proton exchange membrane fuel cell

Air-cooled open-cathode proton exchange membrane fuel cells are miniature but effective energy conversion devices for both the mobile power station and unmanned equipment applications. However, the structure of the air-cooled open-cathode proton exchange membrane fuel cells still needs to be optimized to improve the system compactness and cell performance. In this study, an annular structure is designed, which offers a more uniform air velocity profile, higher air pressure difference, as well as more uniform temperature distribution with a minimum temperature gradient. Benefiting from the optimized annular structure, the assembly of the annular stack has an internal air duct, which could not only reduces the volume and weight of the stack, but also allocates the air more evenly. Results further indicate that the annular design increases the uniformity of velocity and temperature by around 3 times compared with traditional rectangular stack, it contributes significantly to the fuel cell performance by a 15% increase in power. This study provides an efficient strategy to improve the performance and extend the system reliability for future engineering application.

ФИЗИЧЕСКИЕ ПРИНЦИПЫ И МАТЕМАТИЧЕСКОЕ ОПИСАНИЕ ФОРМИРОВАНИЯ ПОЛЕЙ СКОРОСТИ ВОЗДУХА И КОНЦЕНТРАЦИЙ ГАЗОВЫХ ПРИМЕСЕЙ В ГОРНЫХ ВЫРАБОТКАХ УГОЛЬНЫХ ШАХТ

The air composition of the mine atmosphere in coal mines; Safety Rules regulate heat-humidity and speed conditions created by the ventilation system. At the same time, physic-chemical, aero-gas-dynamic and thermos-physical processes occur, which affect the level of aerological safety. A system of equations recommended that allows one to calculate longitudinal air velocity profiles at any point in the mine ventilation network. These recommended describing the transfer of gas impurities in the mine atmosphere by the equation of turbulent-convective diffusion.

3D Computational Fluid Dynamics Analysis of a Convective Drying Chamber

The drying industry has grown considerably due to the tremendous demand for non-perishable food. Convective drying is one of the most popular equipment in the drying industry (food, chemical, pharmaceutical, etc.). One of the drawbacks of this equipment, when used for convective drying, is the non-uniformity in the final product quality. This study presents the development of a numerical model through Computational Fluid Dynamics (CFD). The drying chamber of a heat pump dryer is assessed from the perspective of drying air velocity and temperature profiles. The model was developed by solving different transport phenomena-related equations. The established methodology was set up to evaluate how the drying air velocity and temperature distribution affect the drying chamber. These results will define if there is a need to redesign it. The air velocity and temperature profile results show a need to redesign the chamber. Only trays 2, 3, and 4 are the ones that would achieve the drying of the products. The proposed solution is to implement air distributors or modify the tray positioning to make the drying air and temperature distribution homogeneous.

Air flow inside a vertical pipe induced by a free-falling water jet

The air flow induced by a water jet freely falling inside a vertical pipe with its top and bottom both open to the atmosphere was investigated experimentally and numerically. In the experiments, the radial air velocity distribution and the air pressure variation along the vertical pipe were measured. The air drag of the falling water jet was related to the jet surface disturbance and analyzed by introducing the equivalent friction factor. A predictive model was developed for the air flow inside a 3-m-high pipe based on the momentum equation and its results compared well with the experimental measurements. Numerical simulations were also conducted by approximating the free-falling water jet as a continuous moving solid with diameter and velocity varying in the direction of motion. The effects of pipe size on the air velocity profile and the induced air flow rate were examined. The simulation results showed that the streamwise air velocity profiles inside pipes of different sizes approached the same after a certain traveling distance. The maximum induced air flow rate was found at the pipe diameter of about 20 times of initial water jet diameter.

Method of Designing a Distortion Gauze for Testing a Boundary Layer Ingesting Fan

Abstract As global trends aim to reduce emissions of pollutants, boundary layer ingesting (BLI) propulsions are attracting more and more attention. As such, N+2 generation aircraft with propulsion placed in the aft of the aircraft are gaining in popularity. The boundary layer is formed on the fuselage before entering the engine located in the aft of the aircraft. Due to significant difficulties in performing experimental tests of BLI propulsors with full-size aircraft, distortion gauzes are one of the methods to provide the desired air velocity profile at the inlet. This paper describes a novel method of designing such gauzes, a topic which is not well covered in the existing literature. In the first stage of the presented method, single orifices of different sizes were calculated using CFD tools. The relationship between their size and the gauze resistance coefficient was identified, making it possible to model the distortion gauze using porous media. An iterative approach was used to design a gauze that meets the requirements. This is, to our knowledge, the first distortion gauze design description where a porous media model has been used. Experimental tests demonstrated that the produced distortion gauze yields a velocity profile comparable to the desired one. This indicates the great potential of using the presented approach in further research on boundary layer ingesting propulsions. It offers an opportunity to reduce substantially both the costs of experimental research and the time required to design a distortion-tolerant fan.

Impact of the Speed of Airflow in a Cleanroom on the Degree of Air Pollution

The high-energy requirements of cleanrooms are the main motivation for optimizing their operational conditions. The ventilation system consumes the most energy in order to ensure the precise air conditioning of the room (filtration, temperature, and humidity adjustment). The main function of the ventilation system is to keep particle concentration to a minimum. This work deals with the optimization of an experimental operating room via the optimization of air supply through the distribution element (laminar airflow ceiling) in the range of 0.15–0.25 m·s−1. The laminar airflow between the distribution element and the patient is influenced by the operating light and different airflow velocities. These factors affect changes in particle concentration. Ansys Fluent software was used to investigate the nature of the flow, velocity profiles, and particle trajectories. The results of our numerical simulation demonstrate that a suitable flow rate setting increases the efficiency of particle reduction in the operating table area by up to 54%, which can, in turn, reduce operating costs. The simulated air velocity profile was subsequently verified using the particle image velocimetry (PIV) method. The typical size of particles monitored for in cleanrooms is 0.5 μm according to ISO EN 7. Therefore, the results of this study should be helpful in correctly designing distribution elements for clean rooms.

HEAT-TRANSFER BEHAVIORS OF A SLIGHTLY HORIZONTAL VENTILATED ROOF UNDER VARIABLE CLIMATIC CONDITIONS

This paper illustrates a numerical study to evaluate thermal and fluid dynamic behaviors of natural convection inside a slight inclined ventilated roof for residential building in Mediterranean area. The system is studied considering a roof with a length equal to 6.0 m and three different inclinations with respect to the horizontal plane (0°, 2°, 5°). The ventilated channel, under the roof, has a distance between the flat plates equal to 10 cm. A two-dimensional natural convection with surface radiation model is considered assuming a turbulent air flow. Different conditions are applied on the top wall and the bottom wall of the roof to simulate summer and winter conditions. Different values of solar heat fluxes are applied on the top wall of the ventilated cavity together with heat transfer with the external ambient. Along the bottom wall there is a heat transfer toward an internal ambient by means a thermal conductance. The problem is solved by means of the commercial code Ansys-Fluent. Results are given in terms of temperature and pressure distributions, air velocity and temperature profiles. The increase in inclination of the pitched roof determines a better performance of the component. In fact, during the summer, the efficiency of the system improves because the high heat flux produces an optimal air-convective condition into the ventilated channel. In the winter, the effect of the ventilated layer is more significant to fulfill optimal thermal and hygrometric conditions.

Experimental tests of the air flowing out of a swirl diffuser

The geometry of swirl diffusers affects the shape of the air stream injected into the rooms. In this work, the air flow from the swirl diffuser was tested using the Particle Image Velocimetry (PIV) method. The presented laboratory model was a simplification of the real object and was made at a geometric scale of 1:10. The swirl diffuser was tested with two vanes settings. The measured speed fields are shown. Normalized air velocity profiles obtained at different values of volume flow were compared. This will allow better understanding of the air flow from the swirl diffusers and better shape the air flow in the ventilated rooms.

Challenges in simulating and modeling the airborne virus transmission: A state-of-the-art review.

Recently, the COVID-19 virus pandemic has led to many studies on the airborne transmission of expiratory droplets. While limited experiments and on-site measurements offer qualitative indication of potential virus spread rates and the level of transmission risk, the quantitative understanding and mechanistic insights also indispensably come from careful theoretical modeling and numerical simulation efforts around which a surge of research papers has emerged. However, due to the highly interdisciplinary nature of the topic, numerical simulations of the airborne spread of expiratory droplets face serious challenges. It is essential to examine the assumptions and simplifications made in the existing modeling and simulations, which will be reviewed carefully here to better advance the fidelity of numerical results when compared to the reality. So far, existing review papers have focused on discussing the simulation results without questioning or comparing the model assumptions. This review paper focuses instead on the details of the model simplifications used in the numerical methods and how to properly incorporate important processes associated with respiratory droplet transmission. Specifically, the critical issues reviewed here include modeling of the respiratory droplet evaporation, droplet size distribution, and time-dependent velocity profile of air exhaled from coughing and sneezing. According to the literature review, another problem in numerical simulations is that the virus decay rate and suspended viable viral dose are often not incorporated; therefore here, empirical relationships for the bioactivity of coronavirus are presented. It is hoped that this paper can assist researchers to significantly improve their model fidelity when simulating respiratory droplet transmission.

A comprehensive overview on solar grapes drying: Modeling, energy, environmental and economic analysis

This paper reports different solar dryers primarily designed for grapes drying. Many common solar dryer types, some typical variants, and limitations of traditional grape drying methods are discussed. Functional and economic studies have shown that drying grapes with solar energy is feasible at a lower cost. Due to significant initial investment and low dryer power, solar grape drying commercialization has not, as expected, gained traction. Also, farmers' acceptance of tested solar dryers is not encouraging. Various solar dryers used in grapes drying having a payback period of 1-5 years with an extensive service life of 15–30 years. The indirect solar dryer gives better quality raisins in contrast to the direct type solar dryer because direct exposure to sunlight harms the texture and color of raisins. Mixed-mode type dryers and hybrid type dryers require 15–25% more initial investment than an indirect solar dryer but drying time reduces 30–40% when using mixed-mode and hybrid technology. For the prediction of the drying behavior of grapes, mathematical models such as the Two-term & Midilli model were found best. Simulation software conducts a numerical analysis that predicts air velocity and temperature profiles to dry grapes. It estimates the exact shape and size of the solar dryer and saves money which is expermintally a prolonged exercise. For evaluating the performance of solar dryers, three criteria, i.e., energy analysis, economic analysis, and environmental impacts, are the most important, showing the actual benefits of solar dryers. These solar dryers also promote the reduction of carbon emission and thus helps in earning carbon credit.

Airflow Fluctuation from Linear Diffusers in an Office Building: The Thermal Comfort Analysis

In buildings, the HVAC systems are responsible for a major part of the energy consumption. Incorrect design or selection of the system and improper installation, operation, and maintenance of the systems’ elements may result in increased energy consumption. It is worth remembering that the main aim of the appropriate system is to maintain the high quality of the indoor environment. Appropriate selection of the HVAC solution ensures both thermal and quality parameters of the air, independently of the internal and external heat loads. The microclimate of a room is affected not only by air temperature, humidity, and purity, but also by air velocity in the occupied zone. The proper air velocity distribution prevents discomfort, particularly at workstations. Based on the measurements in the office building, an analysis of velocity profiles of air supplying two different types of linear diffusers was carried out. The analysis was made based on the results of measurements performed with thermoanemometers in the actual facility. During the study, temperature of the supply air was lower that the air in the room. Analysis was focused on the airflow fluctuation and its impact on the users’ comfort. This is an obvious topic but extremely rarely mentioned in publications related to air diffusers. The results show the importance of air fluctuation and its influence on the users’ comfort. During the measurements, the instantaneous air velocity for one of the analyzed types of the diffuser was up to 0.34 m/s, while the average value from the period of 240 s for the same measuring point was relatively low: it was 0.19 m/s. Only including the airflow variability over time allowed for choosing the type of diffuser, which ensures the comfort of users. The measurements carried out for two linear diffusers showed differences in the operation of these diffusers. The velocity in the occupied zone was much higher for one type (0.36 m/s, 3.00 m from diffusers) than for another one (0.22 m/s, 5.00 m from diffusers). The improper selection of the diffuser’s type and its location may increase the risk of the draft in the occupied zone.

Performance and thermal optimization of different length-width ratio for air-cooled open-cathode fuel cell

Open-cathode polymer electrolyte fuel cells with compact design have attracted increasing attention in the field of portable energy equipment. This study explores the impacts of different length-width ratios (RL-W) of the cell with the same active area (50 cm2) on cell performance based on various measurements of polarization curves, air velocity profile, and temperature distribution. Results suggest that RL-W has a significant impact on cell performance. Remarkably, a performance improvement of 7% is achieved with the increase of RL-W (4.14–22.40) due to the enhancement of heat and mass transfer ability. The maximum temperature difference at the cathode outlet surface is less than 2 °C when RL-W is 22.40. It is also indicated that the optimum cathode outlet surface temperature is about 48～52 °C. Under the condition of ensuring the structural strength and performance of the cell simultaneously, the appropriate range of RL-W is confirmed as 10–20. The results provide basic data and technical support for the system design and application of open-cathode fuel cells.

Aerodynamic analysis of unmanned aerial vehicle with hawk shape for monitoring oil leakage

The oil pipeline network requires periodic monitoring to detect pipeline damages, which may cause oil leakage with severe environmental contamination. These damages can be generated by interference from third parties such as construction works, sabotage, vandalism, excavations,and illegal oil theft. Todetect the oil pipeline damages,it canbeusedaerodynamic aerial vehicles (UAVs) with infrared cameras and image processing systems. This paperpresents the aerodynamic analysis of a UAV with a hawk shape (wingspan of 2.20 m and length of 1.49 m) for potential application in the detection of oil pipeline failures. A 1:6.5 scale prototype of the UAV is fabricated using a 3D printer. The aerodynamic coefficients of UAV are determined using computational fluiddynamic (CFD) simulations and experimental testing with a subsonic wind tunnel. In addition, the lift and drag coefficients of UAVsare obtained as a function of Reynolds number and angle of attack. Also, the air velocity profile around UAV is estimated with the CFD model. The proposed UAV could decrease the inspection costs of pipeline networks in comparison with the use of helicopters or light aircraft.

Theoretical study of partially return air flows in vertical mine shafts

This paper is concerned with a theoretical study of the conditions for occurrence of partially return air flows in vertical mine shafts. Such problem arises when the temperature of the air rising in the vertical mine shaft is much higher than the temperature of the shaft walls. This situation can occur during the construction of mine shafts using artificial freezing of surrounding rocks. The uncontrolled return air flows in the shaft under construction may cause an accumulation of harmful impurities released during drilling and blasting operations and a deterioration of microclimate parameters in the atmosphere of mine shafts. We investigate here a two-dimensional, fully turbulent, unsteady air flow in a vertical domain. In accordance with the Boussinesq approximation, the air density is specified as a function of temperature. The results of simulations show that a significant change in the transverse air velocity profile and the occurrence of return air flows are observed at Rayleigh numbers (dimensionless temperature difference) of order of 109. We propose two criteria for quantitative analysis of the flow structure: the critical Rayleigh number which denotes the occurrence of return flows in the cross section of the shaft and the transforming Rayleigh number which serves as indicator of significant changes in the velocity profile. Numerical dependences of the critical and transforming Rayleigh numbers on the Reynolds number of the airflow in the shaft are determined.

Development of a numerical simulation model for droplets and droplet nuclei transportation and experimental verification

The new coronavirus (COVID-19) is spread by droplets and droplet nuclei generated by human coughs, sneezes, and conversations. In order to prevent infection, it is important to accurately predict the scattering behavior of droplets and droplet nuclei by numerical analysis. So far, a lot of numerical simulation using velocity profiles of exhaled air by a spirometer and flow velocity measurements at the mouth by PIV (Particle Image Velocimetry) have been reported. However, there is few cases in which simulation results have been compared and verified with actual measured flow velocity fields. In this study, we have developed a numerical simulation model that reproduces the actual measurements by visualizing coughs emitted by humans using a CW (continuous wave) laser and a high-speed camera, performing PIV analysis, and comparing the arrival distance of the measured cough airflow. In addition, as an application of the numerical model, droplet and droplet nucleus transport have been evaluated considering the effect of partitions.