Ventilation system in tunnels is of a great importance in providing a safe condition for the people passing through the tunnel in both normal and emergency situations. This is usually performed by injecting fresh air into the tunnel and removing hazardous gases. In this research, the longitudinal ventilation of a 5-kilometer tunnel constructed in the Tehran-North freeway project, Iran, through straight jetfans is investigated. The effects of full congested traffic on the airflow rate imposed by the jetfans in this one-directional tunnel, and therefore, the function of ventilation system in removing the exhaust gases are studied. A 3D realistic computational domain for the tunnel with about 1100 stationary vehicles is developed and CFD simulations are performed. The results showed the traffic arrangement and the location of large vehicles in the tunnel has a significant impact on the performance of the ventilation system. The CFD data also suggested that the designed system is likely to fail in removing heat generated by the stationary vehicles in the tunnel and providing a safe condition for the passengers. © 2019 Taylor & Francis Group, London.
Dastan, A.,
Matsumoto, E.A.,
Frith, W.J.,
Cleaver, D.J. Publication Date: 2018
Molecular Physics (00268976)116(21-22)pp. 2823-2835
Hierarchical self-assembly underpins much of the diversity of form and function seen in soft systems, yet the pathways by which they achieve their final form are not always straightforward–intermediate steps, kinetic effects and finite sizes of aggregates all influence the self-assembly pathways of these systems. In this paper, we use molecular dynamics simulations of binary mixtures of spheres and ellipsoidal discs to investigate the self-assembly of anisotropic aggregates with internal structures. Through this, the full aggregation pathways of spontaneously chiral, multi-bilayer and multi-layer assemblies have been tracked and characterised via a semi-qualitative analysis. This includes the unambiguous identification of first-, second- and third-generation hierarchical assemblies within a single simulation. Given the significant challenge of tracking full aggregation pathways in experimental systems, our findings strongly support the notion that molecular simulation has much to contribute to improving our understanding of hierarchical self-assembling systems. © 2018, © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
Publication Date: 2017
Journal of Physical Chemistry B (15205207)121(42)pp. 9920-9928
In this paper, molecular dynamics simulations of simple disc-shaped particles are used to investigate the free self-assembly of defect-free fibers. Depending on the choice of particle shape and interaction strength, the formed fibers are reproducibly either straight or, for reasons of packing efficiency, spontaneously chiral. As they grow radially, increasing stresses cause chiral fibers to untwist either continuously or via morphological rearrangement. It is also found that, due to the kinetics of fiber initiation, the isotropic solution has to be significantly supercooled before aggregation takes place. As a result, the thermal hysteresis of one formed fiber extends to 13.9% of the formation temperature. In the presence of a three-thread seed cluster of 15 particles, however, monotonic fiber growth is observed 9.3% above the normal formation temperature. Thus, as in many experimental systems, it is the kinetic pathway, rather than the thermodynamic stability of the final assembly, that dominates the observed behavior. © 2017 American Chemical Society.
Rahiminejad, M.,
Haghighi, A.,
Dastan, A.,
Abouali, O.,
Farid, M.,
Ahmadi, G. Publication Date: 2016
Computers in Biology and Medicine (00104825)71pp. 115-127
In this paper, the airflow field including the velocity, pressure and turbulence intensity distributions during sneezing of a female subject was simulated using a computational fluid dynamics model of realistic upper airways including both oral and nasal cavities. The effects of variation of reaction of the subject during sneezing were also investigated. That is, the impacts of holding the nose or closing the mouth during sneezing on the pressure and velocity distributions were studied. Few works have studied the sneeze and therefore different aspects of this phenomenon have remained unknown. To cover more possibilities about the inlet condition of trachea in different sneeze scenarios, it was assumed that the suppressed sneeze happens with either the same inlet pressure or the same flow rate as the normal sneeze. The simulation results showed that during a normal sneeze, the pressure in the trachea reaches about 7000 Pa, which is much higher than the pressure level of about 200 Pa during the high activity exhalation. In addition, the results showed that, suppressing the sneeze by holding the nose or mouth leads to a noticeable increase in pressure difference in the tract. This increase was about 5 to 24 times of that during a normal sneeze. This significant rise in the pressure can justify some reported damage due to suppressing a sneeze. © 2016 Elsevier Ltd.
Eftekharian, E.,
Dastan, A.,
Abouali, O.,
Meigolinedjad, J.,
Ahmadi, G. Publication Date: 2014
Tunnelling and Underground Space Technology (08867798)44pp. 56-67
Recent increase in construction of road tunnels in cities, has increased the need for effective ventilation for removing toxic gases emitted by vehicles from the tunnels especially during traffic jams. Severe traffic jam is an inevitable part of the urban life. The objective of this study is to investigate the ventilation effectiveness of the Banana® jet fan and the traditional straight jet fan and also to compare their performance in exhausting the vehicle emissions in severe traffic condition from the tunnel. The effects of adverse wind blowing into the tunnel outlet portal and the impact of mountain blocks, located at the tunnel inlet and outlet, on the airflow field are also investigated. The standard k- ε turbulence model was used in the computational fluid dynamic (CFD) analysis to simulate the ventilation airflow in a 919. m tunnel. For severe congested traffic condition, the simulation results show that the tunnel airflow rate induced by Banana® and traditional jet fans is roughly the same. This leads to an almost similar average of Carbon Monoxide (CO) concentration at the tunnel exit. The performance of the Banana® jet fans, however, is more desirable regarding the local concentration of CO near the human breathing zone of the tunnel. Moreover, it is shown that the effect of adverse wind in decreasing the tunnel airflow rate predicted by the present simulations is much stronger compared with that suggested by the current engineering design approach. © 2014 Elsevier Ltd.
Publication Date: 2014
Journal of Aerosol Science (18791964)69pp. 132-149
In this study, CFD simulations of fibrous particle deposition in different realistic human nasal cavities were performed. The airflow field in the cavity was evaluated by solving the Navier-Stokes and continuity equations using commercial software, while a Lagrangian trajectory analysis approach for solving the coupled translational and rotational equations of motion of ellipsoids was developed and used to investigate fiber transport and deposition in the nasal passages. Different breathing rates in the laminar flow regime in the nose and a range of fiber lengths and diameters were used in these simulations. It was shown that the aerodynamic diameter based on the Stokes equivalent diameter is an appropriate parameter for correlating the fiber deposition rate. Presenting the deposition fraction results versus the Stokes-based and pressure-based impaction parameters collapsed the results of different cases for various nose models roughly to a single curve. The simulated regional fiber deposition results were also presented for different nasal cavities. A simple approach developed earlier for modeling non-spherical particles using the shape factor in the drag force was also studied, and the resulting deposition fraction was compared with the present coupled translational-rotational trajectory analysis approach. © 2013 Elsevier Ltd.
Publication Date: 2013
Computers and Fluids (00457930)71pp. 28-40
A general computer code which solves the motion equations of non-spherical ellipsoidal particles in the fluid flow was developed and fiber motion and web formation at the channel entrance of a microchannel heat sink were investigated numerically. A circular inlet duct, inlet plenum and 15 parallel channels with hydraulic diameter of 225 μm were considered as the computational domain. Water flow field in the microchannel was solved with Eulerian approach. Fiber motion equations consisting of translational and rotational motions were solved by Lagrangian approach assuming a one-way interaction. The numerical approach was validated for different aspects of the model and close agreement was obtained in comparison with experimental and other numerical data. The objective of the present work is to simulate the formation of fiber web at the entrance of the channels, as well as studying the effects of deposited fibers on the flow field and deposition of next fibers. The results show that, the deposited fibers act as a filter that can lead to deposition of more fibers at the channel entrance. The growth of fiber web in time reported in previous experimental works was also observed in this numerical investigation. © 2012 Elsevier Ltd.
Publication Date: 2012
American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM (08888116)1(PARTS A AND B)pp. 839-846
In this paper, the motion and deposition of micro fibers in different regions of a realistic human nasal airway were studied using a computational modeling approach. The airflow field in the nasal cavity was simulated by solving the Navier-Stokes and continuity equations. The coupled translational and rotation motion of the fibers were analyzed by a Lagrangian approach assuming one-way coupling. The fibers were assumed to be ellipsoids and a computer code was developed for solving the coupled translational and rotational equations of motion of the ellipsoidal fiber. A large number of fibers were injected at the nostril and the deposition pattern and deposition fraction (DF) of the fibers in different regions of the nasal cavity were evaluated for different breathing rates, various fiber diameters and different fiber aspect ratios. The simulation results for ellipsoidal fibers obtained by solving the coupled translational and rotational equations were compared with those obtained by solving only the translational equations of equivalent spherical particles with a shape factor, which were used in some earlier works. Copyright © 2012 by ASME.
Publication Date: 2011
Heat Transfer Engineering (15210537)32(7-8)pp. 554-565
In this paper, pressure drop, heat transfer characteristics, and particle deposition in a microchannel with a fiber web at the inlet are investigated numerically. The fiber web was made up of fibers several hundred micrometers in length caught at the entrance of the channels. Governing equations for the flow field are solved by an Eulerian approach, while the equations of particle motion in the flow are solved by a Lagrangian approach. Assuming the symmetry in the domain, one channel and the corresponding inlet and outlet plenums are selected as the computational domain. Several fiber webs with various fiber numbers, orientations, and dimensions are modeled. The increase in the pressure drop and the decrease of heat transfer due to the fiber web are computed and discussed. A correlation is developed for pressure drop as a function of the fiber web blockage ratio, microchannel geometry, and flow characteristics. The deposition of the microparticles with various diameters on the fiber webs is investigated, as well. Deposition of the particles on the fiber web is because of two different mechanisms, inertial impaction and interception. The numerical results indicate that the fiber webs have no considerable effect on the heat transfer characteristic of the channel under constant pumping power. Copyright © Taylor and Francis Group, LLC.
Publication Date: 2010
(PARTS A AND B)pp. 1547-1554
In this paper the motion of micro fibers in a microchannel is studied numerically. The liquid flow regime is considered to be laminar and it is assumed that the fluid and the fibers have a one-way interaction meaning the effects of fibers on the flow are neglected. An inlet plenum of the microchannel with 15 channels considered as the physical domain. The fluid flow in the model is solved numerically by an Eulerian approach using the conventional SIMPLE algorithm. To study the motion of the micro fibers in the flow, the fibers are considered to be ellipsoids of revolution. A code is developed which uses the fluid flow results and solves the equations of ellipsoid motion in a Lagrangian reference frame. The equations of ellipsoid motion consist of three equations for the translational motion and three equations for the rotational motion. The equations are integrated numerically to find the trajectory and the orientation of the micro fibers in the microchannel. Copyright © 2010 by ASME.
In this paper pressure drop and particle deposition in a microchannel with a hydraulic diameter of 225 micrometer is investigated numerically. Several hundred micron length fibers caught at the entrance of the channels making a "fiber web" also is modeled in this research. Governing equations for the flow field are solved with an Eulerian approach while the equations of particle motion in the flow are solved by a Lagrangian approach. Assuming the symmetry in the domain, one channel and the corresponding plenum are studied in the computational domain. For studying the effects of fibers in the flow, two fiber webs with four and six solid fibers are studied. The increase of pressure drop in the microchannel because of the entrance fiber web is computed and discussed. Also deposition and collection of the particles with various diameters at the fiber webs are also presented. Copyright © 2009 by ASME.
