Publications by authors named "Goodarz Ahmadi"

30 Publications

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Latest developments in nanofluid flow and heat transfer between parallel surfaces: A critical review.

Adv Colloid Interface Sci 2021 May 21;294:102450. Epub 2021 May 21.

School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China; Department of Mechanical Engineering, Center for Nanotechnology in Renewable Energies, Ferdowsi University of Mashhad, Mashhad, Iran.

The enhancement of heat transfer between parallel surfaces, including parallel plates, parallel disks, and two concentric pipes, is vital because of their wide applications ranging from lubrication systems to water purification processes. Various techniques can be utilized to enhance heat transfer in such systems. Adding nanoparticles to the conventional working fluids is an effective solution that could remarkably enhance the heat transfer rate. No published review article focuses on the recent advances in nanofluid flow between parallel surfaces; therefore, the present paper aims to review the latest experimental and numerical studies on the flow and heat transfer of nanofluids (mixtures of nanoparticles and conventional working fluids) in such configurations. For the performance analysis of thermal systems composed of parallel surfaces and operating with nanofluids, it is necessary to know the physical phenomena and parameters that influence the flow and heat transfer characteristics in these systems. Significant results obtained from this review indicate that, in most cases, the heat transfer rate between parallel surfaces is enhanced with an increase in the Rayleigh number, the Reynolds number, the magnetic number, and Brownian motion. On the other hand, an increase in thermophoresis parameter, as well as flow parameters, including the Eckert number, buoyancy ratio, Hartmann number, and Lewis number, leads to heat transfer rate reduction.
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http://dx.doi.org/10.1016/j.cis.2021.102450DOI Listing
May 2021

Regional deposition of the allergens and micro-aerosols in the healthy human nasal airways.

J Aerosol Sci 2021 Feb 19;152:105700. Epub 2020 Oct 19.

School of Mechanical Engineering, Shiraz University, Shiraz, Iran.

The nasal cavity is the inlet to the human respiratory system and is responsible for the olfactory sensation, filtering pollutant particulate matter, and humidifying the air. Many research studies have been performed to numerically predict allergens, contaminants, and/or drug particle deposition in the human nasal cavity; however, the majority of these investigations studied only one or a small number of nasal passages. In the present study, a series of Computed Tomography (CT) scan images of the nasal cavities from ten healthy subjects were collected and used to reconstruct accurate 3D models. All models were divided into twelve anatomical regions in order to study the transport and deposition features of different regions of the nasal cavity with specific functions. The flow field and micro-particle transport equations were solved, and the total and regional particle deposition fractions were evaluated for the rest and low activity breathing conditions. The results show that there are large variations among different subjects. The standard deviation of the total deposition fraction in the nasal cavities was the highest for
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http://dx.doi.org/10.1016/j.jaerosci.2020.105700DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7569476PMC
February 2021

Investigation of airflow at different activity conditions in a realistic model of human upper respiratory tract.

Comput Methods Biomech Biomed Engin 2021 Feb 17;24(2):173-187. Epub 2020 Sep 17.

Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY, USA.

In the present study, the turbulent flows inside a realistic model of the upper respiratory tract were investigated numerically and experimentally. The airway model included the geometrical details of the oral cavity to the end of the trachea that was based on a series of CT-scan images. The topological data of the respiratory tract were used for generating the computational model as well as the 3D-printed model that was used in the experimental pressure drop measurement. Different airflow rates of 30, 45, and 60 L/min, which correspond to the light, semi-light, and heavy activity breathing conditions, were investigated numerically using turbulence and transition models, as well as experimentally. Simulation results for airflow properties, including velocity vectors, pressure drops, streamlines, eddy viscosity, and turbulent kinetic energy contours in the oral-trachea airway model, were presented. The simulated pressure drop was compared with the experimental data, and reasonable agreement was found. The obtained results showed that the maximum pressure drop occurs in the narrowest part of the larynx region. A comparison between the numerical results and experimental data showed that the transition () SST model predicts higher pressure losses, especially at higher breathing rates. Formations of the secondary flows in the oropharynx and trachea regions were also observed. In addition, the simulation results showed that in the trachea region, the secondary flow structures dissipated faster for the flow rate of 60 L/min compared to the lower breathing rates of 30 and 45 L/min.
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http://dx.doi.org/10.1080/10255842.2020.1819256DOI Listing
February 2021

Comparison between Nucleate Pool Boiling Heat Transfer of Graphene Nanoplatelet- and Carbon Nanotube- Based Aqueous Nanofluids.

ACS Omega 2019 Nov 6;4(21):19183-19192. Epub 2019 Nov 6.

Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, New York 13699, United States.

An increase of nucleate pool boiling with the use of different fluid properties has received much attention. In particular, the presence of nanostructures in fluids to enhance boiling was given special consideration. This study compares the effects of graphene nanoplatelet (GNP), functionalized GNP with polyethylene glycol (PEG), and multiwalled carbon nanotube (CNT) nanofluids on the pool boiling heat transfer coefficient and the critical heat flux (CHF). Our findings showed that at the same concentration, CHF for functionalized GNP with PEG (GNP-PEG)/deionized water (DW) nanofluids was higher in comparison with GNP- and CNT-based nanofluids. The CHF of the GNP/DW nanofluids was also higher than that of CNT/DW nanofluids. The CHF of GNP-PEG was 72% greater than that of DW at the concentration of 0.1 wt %. There is good agreement between measured critical heat fluxes and the Kandlikar correlation. In addition, the current results proved that the GNP-PEG/DW nanofluids are highly stable over 3 months at a concentration of 0.1 wt %.
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http://dx.doi.org/10.1021/acsomega.9b02474DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6868890PMC
November 2019

Laminar flow drag reduction on soft porous media.

Sci Rep 2017 12 8;7(1):17263. Epub 2017 Dec 8.

Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, New York, United States.

While researches have focused on drag reduction of various coated surfaces such as superhydrophobic structures and polymer brushes, the insights tso understand the fundamental physics of the laminar skin friction coefficient and the related drag reduction due to the formation of finite velocity at porous surfaces is still relatively unknown. Herein, we quantitatively investigated the flow over a porous medium by developing a framework to model flow of a Newtonian fluid in a channel where the lower surface was replaced by various porous media. We showed that the flow drag reduction induced by the presence of the porous media depends on the values of the permeability parameter α = L/(MK) and the height ratio δ = H/L, where L is the half thickness of the free flow region, H is the thickness and K is the permeability of the fiber layer, and M is the ratio of the fluid effective dynamic viscosity μ in porous media to its dynamic viscosity μ. We also examined the velocity and shear stress profiles for flow over the permeable layer for the limiting cases of α → 0 and α → ∞. The model predictions were compared with the experimental data for specific porous media and good agreement was found.
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http://dx.doi.org/10.1038/s41598-017-17141-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5722956PMC
December 2017

A facile, bio-based, novel approach for synthesis of covalently functionalized graphene nanoplatelet nano-coolants toward improved thermo-physical and heat transfer properties.

J Colloid Interface Sci 2018 Jan 17;509:140-152. Epub 2017 Jul 17.

Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia.

In this study, we synthesized covalently functionalized graphene nanoplatelet (GNP) aqueous suspensions that are highly stable and environmentally friendly for use as coolants in heat transfer systems. We evaluated the heat transfer and hydrodynamic properties of these nano-coolants flowing through a horizontal stainless steel tube subjected to a uniform heat flux at its outer surface. The GNPs functionalized with clove buds using the one-pot technique. We characterized the clove-treated GNPs (CGNPs) using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). We then dispersed the CGNPs in distilled water at three particle concentrations (0.025, 0.075 and 0.1wt%) in order to prepare the CGNP-water nanofluids (nano-coolants). We used ultraviolet-visible (UV-vis) spectroscopy to examine the stability and solubility of the CGNPs in the distilled water. There is significant enhancement in thermo-physical properties of CGNPs nanofluids relative those for distilled water. We validated our experimental set-up by comparing the friction factor and Nusselt number for distilled water obtained from experiments with those determined from empirical correlations, indeed, our experimental set-up is reliable and produces results with reasonable accuracy. We conducted heat transfer experiments for the CGNP-water nano-coolants flowing through the horizontal heated tube in fully developed turbulent condition. Our results are indeed promising since there is a significant enhancement in the Nusselt number and convective heat transfer coefficient for the CGNP-water nanofluids, with only a negligible increase in the friction factor and pumping power. More importantly, we found that there is a significant increase in the performance index, which is a positive indicator that our nanofluids have potential to substitute conventional coolants in heat transfer systems because of their overall thermal performance and energy savings benefits.
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http://dx.doi.org/10.1016/j.jcis.2017.07.052DOI Listing
January 2018

Numerical and analytical investigation of irrigant penetration into dentinal microtubules.

Comput Biol Med 2017 10 24;89:1-17. Epub 2017 Jul 24.

Department of Aeronautical and Mechanical Engineering, Clarkson University, NY, USA.

Irrigation is one of the most important steps in root canal therapy. Sodium hypochlorite is inserted into the root canal to eliminate bacteria and dissolve necrotic tissue. Dentinal tubules are micrometer sized channels along the dentin thickness. An irrigant should have the ability to penetrate into these tubules to remove bacteria residing in them. The difference between the concentrations of the inserted irrigant and the dentinal tubule fluid is the main factor of penetration. This study attempts to model dentinal tubules with precise dimensions and to study the time dependent irrigant penetration into them by using Computational Fluid Dynamics (CFD). The effects of needle type and position in the dentinal tubule were also considered. The results showed that concentration distribution would be different when the tubule was modeled as a frustum compared to the cylindrical shape tubule. Dentinal tubule curvature, however, did not have a noticeable effect in irrigant penetration. It was also concluded that when the needle working length is 3 mm, concentration can be considered constant at the tubule's entrance for tubules located at more than 1 mm from the apex. Moreover, by irrigating the root canal with a side-vented needle instead of an open-ended one, the concentration would be less for the tubules located in the apex region. Analytical solutions for different cases were also obtained, and their predictions were found to be in good agreement with the numerical results. Therefore, the presented analytical solutions can be directly used to obtain irrigant concentration in tubules with no need for additional computer simulations.
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http://dx.doi.org/10.1016/j.compbiomed.2017.07.013DOI Listing
October 2017

Numerical investigation of transient transport and deposition of microparticles under unsteady inspiratory flow in human upper airways.

Respir Physiol Neurobiol 2017 10 1;244:56-72. Epub 2017 Jul 1.

Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY, USA.

In the present study, unsteady airflow patterns and particle deposition in healthy human upper airways were simulated. A realistic 3-D computational model of the upper airways including the vestibule to the end of the trachea was developed using a series of CT scan images of a healthy human. Unsteady simulations of the inhaled and exhaled airflow fields in the upper airway passages were performed by solving the Navier-Stokes and continuity equations for low breathing rates corresponding to low and moderate activities. The Lagrangian trajectory analysis approach was utilized to investigate the transient particle transport and deposition under cyclic breathing condition. Particles were released uniformly at the nostrils' entrance during the inhalation phase, and the total and regional depositions for various micro-particle sizes were evaluated. The transient particle deposition fractions for various regions of the human upper airways were compared with those obtained from the equivalent steady flow condition. The presented results revealed that the equivalent constant airflow simulation can approximately predict the total particle deposition during cyclic breathing in human upper airways. While the trends of steady and unsteady model predictions for local deposition were similar, there were noticeable differences in the predicted amount of deposition. In addition, it was shown that a steady simulation cannot properly predict some critical parameters, such as the penetration fraction. Finally, the presented results showed that using a detached nasal cavity (commonly used in earlier studies) for evaluation of total deposition fraction of particles in the nasal cavity was reasonably accurate for the steady flow simulations. However, in transient simulation for predicting the deposition fraction in a specific region, such as the nasal cavity, using the full airway system geometry becomes necessary.
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http://dx.doi.org/10.1016/j.resp.2017.06.005DOI Listing
October 2017

In silico investigation of cornea deformation during irrigation/aspiration in phacoemulsification in cataract surgery.

Med Eng Phys 2017 05 10;43:77-85. Epub 2017 Mar 10.

Aeronautical and Mechanical Engineering Department, Clarkson University, Potsdam, NY, USA.

To analyze the stress, strain and displacement of the human cornea under the action of negative intraocular pressure, which occurs during phacoemulsification in cataract surgery, a multidisciplinary approach including biomedical engineering, solid mechanics, numerical analysis, and fluid dynamics was used. Fluid-structure interaction method was implemented using 3-dimensional nonlinear finite element analysis of cornea tissue in conjunction with CFD analysis of anterior chamber fluid flow to study the deformation of the cornea under negative gage pressure during irrigation and aspiration (I/A). The computational model of the eye includes both cornea and sclera. To increase the accuracy of the computational model, both cornea hyperelasticity and thickness variation were included in the analysis. The simulation was performed for both coaxial and bimanual I/A systems with different flow rates. The cornea deformations for various flow rates were evaluated, and the possibility of an unstable anterior chamber was assessed. The results show that the critical pressure in the anterior chamber, which may lead to the surge condition due to buckling of the cornea, is sub-ambient (below zero gauge pressure). Anterior chamber instability occurs at higher volume flow rates for coaxial I/A system compared with that for bimanual system, but the deformation of the cornea is more intense for the bimanual system.
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http://dx.doi.org/10.1016/j.medengphy.2017.02.010DOI Listing
May 2017

Mass production of highly-porous graphene for high-performance supercapacitors.

Sci Rep 2016 09 8;6:32686. Epub 2016 Sep 8.

Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia.

This study reports on a facile and economical method for the scalable synthesis of few-layered graphene sheets by the microwave-assisted functionalization. Herein, single-layered and few-layered graphene sheets were produced by dispersion and exfoliation of functionalized graphite in ethylene glycol. Thermal treatment was used to prepare pure graphene without functional groups, and the pure graphene was labeled as thermally-treated graphene (T-GR). The morphological and statistical studies about the distribution of the number of layers showed that more than 90% of the flakes of T-GR had less than two layers and about 84% of T-GR were single-layered. The microwave-assisted exfoliation approach presents us with a possibility for a mass production of graphene at low cost and great potentials in energy storage applications of graphene-based materials. Owing to unique surface chemistry, the T-GR demonstrates an excellent energy storage performance, and the electrochemical capacitance is much higher than that of the other carbon-based nanostructures. The nanoscopic porous morphology of the T-GR-based electrodes made a significant contribution in increasing the BET surface as well as the specific capacitance of graphene. T-GR, with a capacitance of 354.1 Fg(-1) at 5 mVs(-1) and 264 Fg(-1) at 100 mVs(-1), exhibits excellent performance as a supercapacitor.
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http://dx.doi.org/10.1038/srep32686DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5015014PMC
September 2016

Relationship between saccadic eye movements and formation of the Krukenberg's spindle-a CFD study.

Math Med Biol 2017 09;34(3):293-312

Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY 13699-5725, USA.

In this research, a series of numerical simulations for evaluating the effects of saccadic eye movement on the aqueous humour (AH) flow field and movement of pigment particles in the anterior chamber (AC) was performed. To predict the flow field of AH in the AC, the unsteady forms of continuity, momentum balance and conservation of energy equations were solved using the dynamic mesh technique for simulating the saccadic motions. Different orientations of the human eye including horizontal, vertical and angles of 10° and 20° were considered. The Lagrangian particle trajectory analysis approach was used to find the trajectories of pigment particles in the eye. Particular attention was given to the relation between the saccadic eye movement and potential formation of Krukenberg's spindle in the eye. The simulation results revealed that the natural convection flow was an effective mechanism for transferring pigment particles from the iris to near the cornea. In addition, the saccadic eye movement was the dominant mechanism for deposition of pigment particles on the cornea, which could lead to the formation of Krukenberg's spindle. The effect of amplitude of saccade motion angle in addition to the orientation of the eye on the formation of Krukenberg's spindle was investigated.
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http://dx.doi.org/10.1093/imammb/dqw007DOI Listing
September 2017

Computer simulations of pressure and velocity fields in a human upper airway during sneezing.

Comput Biol Med 2016 Apr 2;71:115-27. Epub 2016 Feb 2.

Aeronautical & Mechanical Engineering Department, Clarkson University, Potsdam, NY, USA.

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 7000Pa, which is much higher than the pressure level of about 200Pa 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.
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http://dx.doi.org/10.1016/j.compbiomed.2016.01.022DOI Listing
April 2016

Thermostatic and rheological responses of DPD fluid to extreme shear under modified Lees-Edwards boundary condition.

Eur Phys J E Soft Matter 2015 Dec 28;38(12):134. Epub 2015 Dec 28.

School of Aerospace, Mechanical and Mechatronic Eng., The University of Sydney, 2006, NSW, Australia.

Thermodynamic, hydrodynamic and rheological interactions between velocity-dependent thermostats of Lowe-Andersen (LA) and Nosé-Hoover-Lowe-Andersen (NHLA), and modified Lees-Edwards (M-LEC) boundary condition were studied in the context of Dissipative Particle Dynamics method. Comparisons were made with original Lees-Edwards method to characterise the improvements that M-LEC offers in conserving the induced shear momentum. Different imposed shear velocities, heat bath collision/exchange frequencies and thermostating probabilities were considered. The presented analyses addressed an unusual discontinuity in momentum transfer that appeared in form of nonphysical jumps in velocity and temperature profiles. The usefulness of M-LEC was then quantified by evaluating the enhancements in obtained effective shear velocity, effective shear rate, Péclet number, and dynamic viscosity. System exchange frequency (Γ) with Maxwellian heat bath was found to play an important role, in that its larger values facilitated achieving higher shear rates with proper temperature control at the cost of deviation from an ideal momentum transfer. Similar dynamic viscosities were obtained under both shearing modes between LA and NHLA thermostats up to Γ = 10, whilst about twice the range of viscosity (1 < η < 20) was calculated for M-LEC at larger probabilities (ΓΔt > %). The main benefits of this modification were to facilitate momentum flow from shear boundaries to the system bulk. In addition, it was found that there exist upper thresholds for imposing shear on the system beyond which temperature cannot be controlled properly and nonphysical jumps reappear.
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http://dx.doi.org/10.1140/epje/i2015-15134-0DOI Listing
December 2015

Microwave-Assisted Synthesis of Highly-Crumpled, Few-Layered Graphene and Nitrogen-Doped Graphene for Use as High-Performance Electrodes in Capacitive Deionization.

Sci Rep 2015 Dec 8;5:17503. Epub 2015 Dec 8.

Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia.

Capacitive deionization (CDI) is a promising procedure for removing various charged ionic species from brackish water. The performance of graphene-based material in capacitive deionization is lower than the expectation of the industry, so highly-crumpled, few-layered graphene (HCG) and highly-crumpled nitrogen-doped graphene (HCNDG) with high surface area have been introduced as promising candidates for CDI electrodes. Thus, HCG and HCNDG were prepared by exfoliation of graphite in the presence of liquid-phase, microwave-assisted methods. An industrially-scalable, cost-effective, and simple approach was employed to synthesize HCG and HCNDG, resulting in few-layered graphene and nitrogen-doped graphene with large specific surface area. Then, HCG and HCNDG were utilized for manufacturing a new class of carbon nanostructure-based electrodes for use in large-scale CDI equipment. The electrosorption results indicated that both the HCG and HCNDG have fairly large specific surface areas, indicating their huge potential for capacitive deionization applications.
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http://dx.doi.org/10.1038/srep17503DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4672334PMC
December 2015

Evaluation of Residence Time on Nitrogen Oxides Removal in Non-Thermal Plasma Reactor.

PLoS One 2015 23;10(10):e0140897. Epub 2015 Oct 23.

Biofuel Engine Research Facility, Queensland University of Technology, Brisbane, Queensland, Australia.

Non-thermal plasma (NTP) has been introduced over the last few years as a promising after- treatment system for nitrogen oxides and particulate matter removal from diesel exhaust. NTP technology has not been commercialised as yet, due to its high rate of energy consumption. Therefore, it is important to seek out new methods to improve NTP performance. Residence time is a crucial parameter in engine exhaust emissions treatment. In this paper, different electrode shapes are analysed and the corresponding residence time and NOx removal efficiency are studied. An axisymmetric laminar model is used for obtaining residence time distribution numerically using FLUENT software. If the mean residence time in a NTP plasma reactor increases, there will be a corresponding increase in the reaction time and consequently the pollutant removal efficiency increases. Three different screw thread electrodes and a rod electrode are examined. The results show the advantage of screw thread electrodes in comparison with the rod electrode. Furthermore, between the screw thread electrodes, the electrode with the thread width of 1 mm has the highest NOx removal due to higher residence time and a greater number of micro-discharges. The results show that the residence time of the screw thread electrode with a thread width of 1 mm is 21% more than for the rod electrode.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0140897PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4619676PMC
June 2016

Numerical simulation of airflow and micro-particle deposition in human nasal airway pre- and post-virtual sphenoidotomy surgery.

Comput Biol Med 2015 Jun 24;61:8-18. Epub 2015 Mar 24.

Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY, USA.

In the present study, the effects of endoscopic sphenoidotomy surgery on the flow patterns and deposition of micro-particles in the human nasal airway and sphenoid sinus were investigated. A realistic model of a human nasal passage including nasal cavity and paranasal sinuses was constructed using a series of CT scan images of a healthy subject. Then, a virtual sphenoidotomy by endoscopic sinus surgery was performed in the left nasal passage and sphenoid sinus. Transient airflow patterns pre- and post-surgery during a full breathing cycle (inhalation and exhalation) were simulated numerically under cyclic flow condition. The Lagrangian approach was used for evaluating the transport and deposition of inhaled micro-particles. An unsteady particle tracking was performed for the inhalation phase of the breathing cycle for the case that particles were continuously entering into the nasal airway. The total deposition pattern and sphenoid deposition fraction of micro-particles were evaluated and compared for pre- and post-surgery cases. The presented results show that sphenoidotomy increased the airflow into the sphenoid sinus, which led to increased deposition of micro-particles in this region. Particles up to 25 μm were able to penetrate into the sphenoid in the post-operation case, and the highest deposition in the sphenoid for the resting breathing rate occurred for 10 μm particles at about 1.5%.
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http://dx.doi.org/10.1016/j.compbiomed.2015.03.015DOI Listing
June 2015

Numerical simulation of wave propagation in a realistic model of the human external ear.

Comput Methods Biomech Biomed Engin 2015 16;18(16):1797-810. Epub 2014 Dec 16.

a School of Mechanical Engineering, Shiraz University , Shiraz , Iran.

In this study, a numerical investigation is performed to evaluate the effects of high-pressure sinusoidal and blast wave's propagation around and inside of a human external ear. A series of computed tomography images are used to reconstruct a realistic three-dimensional (3D) model of a human ear canal and the auricle. The airflow field is then computed by solving the governing differential equations in the time domain using a computational fluid dynamics software. An unsteady algorithm is used to obtain the high-pressure wave propagation throughout the ear canal which is validated against the available analytical and numerical data in literature. The effects of frequency, wave shape, and the auricle on pressure distribution are then evaluated and discussed. The results clearly indicate that the frequency plays a key role on pressure distribution within the ear canal. At 4 kHz frequency, the pressure magnitude is much more amplified within the ear canal than the frequencies of 2 and 6 kHz, for the incident wave angle of 90° investigated in this study, attributable to the '4-kHz notch' in patients with noise-induced hearing loss. According to the results, the pressure distribution patterns at the ear canal are very similar for both sinusoidal pressure waveform with the frequency of 2 kHz and blast wave. The ratio of the peak pressure value at the eardrum to that at the canal entrance increases from about 8% to 30% as the peak pressure value of the blast wave increases from 5 to 100 kPa for the incident wave angle of 90° investigated in this study. Furthermore, incorporation of the auricle to the ear canal model is associated with centerline pressure magnitudes of about 50% and 7% more than those of the ear canal model without the auricle throughout the ear canal for sinusoidal and blast waves, respectively, without any significant effect on pressure distribution pattern along the ear canal for the incident wave angle of 90° investigated in this study.
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http://dx.doi.org/10.1080/10255842.2014.974578DOI Listing
November 2015

Numerical investigation of regional particle deposition in the upper airway of a standing male mannequin in calm air surroundings.

Comput Biol Med 2014 Sep 19;52:73-81. Epub 2014 Jun 19.

Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY, USA.

A 3-D realistic computational model of the airway system integrated into a standing male mannequin was developed. The computational domain includes the regions around the mannequin and the inside of the airway passages. The simulation was performed for low activity breathing rates with calm air around the mannequin. The flowfield of the inhaled air was first obtained from solving the Navier-Stokes and continuity equations. Then the particles were released in the domain around the mannequin and their trajectories were evaluated by using the Lagrangian approach for solving the particle equation of motion. The regional aerosols deposition was evaluated for different parts of the human airway system and the results were compared with those obtained from the separate modeling of the airway system without the interaction of the airflow with the mannequin external face. The results showed when the upper airway is integrated into the mannequin, the regional deposition of inhaled particles mainly changes in the airway system.
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http://dx.doi.org/10.1016/j.compbiomed.2014.06.007DOI Listing
September 2014

Numerical investigation of the flow field in realistic nasal septal perforation geometry.

Allergy Rhinol (Providence) 2014 Jul 1;5(2):70-7. Epub 2014 Jul 1.

Department of Otolaryngology, Head and Neck Surgery, Shiraz University of Medical Sciences, Shiraz, Iran.

The computational fluid dynamics (CFD) are used to evaluate the physiological function of the nose. We evaluated the aerodynamics of the nasal cavity in a patient with septal perforation (SP), pre- and postvirtual repair. Three-dimensional nasal models were reconstructed, and then a wide range of the pressure drops and flow rates were analyzed. The airflow velocity is higher in the central region and is lower around the boundary of the SP. The air velocity in the SP increases as the pressure drop increases. Furthermore, at the anterior part of the SP, the shear stress is higher in the upper part. In addition, the repair of SP does not affect the total nasal airflow rate and the velocity contour patterns. The potential usage of the CFD technique as a predictive technique to explore the details and a preoperative assessment tool to help in clinical decision making in nasal surgery is emphasized.
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http://dx.doi.org/10.2500/ar.2014.5.0090DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4124581PMC
July 2014

Physiology of aqueous humor dynamic in the anterior chamber due to rapid eye movement.

Physiol Behav 2014 Aug 24;135:112-8. Epub 2014 May 24.

Department of Aeronautical and Mechanical Engineering, Clarkson University, Potsdam, NY, USA.

The nature of aqueous humor (AH) mixing in the anterior chamber (AC) of the human eye due to rapid eye movement (REM) has not been fully understood and has been somewhat a controversial issue. This study uses a computational modeling approach to shed light on this issue. For this purpose a numerical method was developed and used to solve the mathematical equations governing the flow and mixing of aqueous humor motion in the eye subjected to such movements. Based on the experimental measurements available in the literature for the average and maximum amplitudes of the eye movements, a harmonic model for the REM was developed. The corresponding instantaneous and time-averaged velocity fields were evaluated. The simulation results showed that, contrary to earlier reports, the REM led to complex flow structures and a 3-D mixing of AH in the anterior chamber. In addition, the mixing velocity increased in direct proportion to the REM amplitudes. Thus, the AC flow generated by REM could carry nutrients to the posterior surface of the cornea during the sleep. Furthermore, the shear stress acting on the corneal endothelial cells due to REM was computed and compared with that of buoyancy driven flow in the AC due to temperature gradient. It was found that the shear stress generated by REM is much higher than that introduced by the natural convection. A video file for providing a better understanding of the AH mixing process in the AC was also prepared. This video is available on the web.
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http://dx.doi.org/10.1016/j.physbeh.2014.05.017DOI Listing
August 2014

An experimental study on thermal conductivity and viscosity of nanofluids containing carbon nanotubes.

Nanoscale Res Lett 2014 28;9(1):151. Epub 2014 Mar 28.

Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia.

Recently, there has been considerable interest in the use of nanofluids for enhancing thermal performance. It has been shown that carbon nanotubes (CNTs) are capable of enhancing the thermal performance of conventional working liquids. Although much work has been devoted on the impact of CNT concentrations on the thermo-physical properties of nanofluids, the effects of preparation methods on the stability, thermal conductivity and viscosity of CNT suspensions are not well understood. This study is focused on providing experimental data on the effects of ultrasonication, temperature and surfactant on the thermo-physical properties of multi-walled carbon nanotube (MWCNT) nanofluids. Three types of surfactants were used in the experiments, namely, gum arabic (GA), sodium dodecylbenzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS). The thermal conductivity and viscosity of the nanofluid suspensions were measured at various temperatures. The results showed that the use of GA in the nanofluid leads to superior thermal conductivity compared to the use of SDBS and SDS. With distilled water as the base liquid, the samples were prepared with 0.5 wt.% MWCNTs and 0.25% GA and sonicated at various times. The results showed that the sonication time influences the thermal conductivity, viscosity and dispersion of nanofluids. The thermal conductivity of nanofluids was typically enhanced with an increase in temperature and sonication time. In the present study, the maximum thermal conductivity enhancement was found to be 22.31% (the ratio of 1.22) at temperature of 45°C and sonication time of 40 min. The viscosity of nanofluids exhibited non-Newtonian shear-thinning behaviour. It was found that the viscosity of MWCNT nanofluids increases to a maximum value at a sonication time of 7 min and subsequently decreases with a further increase in sonication time. The presented data clearly indicated that the viscosity and thermal conductivity of nanofluids are influenced by the sonication time. Image analysis was carried out using TEM in order to observe the dispersion characteristics of all samples. The findings revealed that the CNT agglomerates breakup with increasing sonication time. At high sonication times, all agglomerates disappear and the CNTs are fragmented and their mean length decreases.
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http://dx.doi.org/10.1186/1556-276X-9-151DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4006636PMC
May 2014

Blood flow vectoring control in aortic arch using full and partial clamps.

Comput Biol Med 2013 Sep 3;43(9):1134-41. Epub 2013 Jun 3.

VFE Research Institute, University of Tehran, Tehran, Iran.

Background: Early diagnosis and treatment of aneurysm plays an important role in reducing the mortality risk of rupture. The aneurysm is a complex phenomenon and caused by different reasons, such as arteriosclerosis and heredity. In addition, pressure and Wall Shear Stress are two known factors influencing the establishment of an aneurysm. The aim of this study is to investigate the effect of using a full or partial clamp to control the blood flow streamlines and hence the location of stress concentration in a clean configuration of aorta. The main question is how to control the stresses distribution in order to reduce the possibility of aneurysm growth with less negative effects on the other sides.

Methods And Results: A simple form of aortic arch with three branches is considered to simulate the effect of changing blood flow streamlines directions. A parameter study has been performed on the main characteristics of clamp, i.e. size, location, and the percentage of coverage. The Shear Stress Transport model is employed to simulate steady-state Newtonian blood flow when the Reynolds number is about 6500. Simulations are conducted using the commercial CFD solver ANSYS Fluent. The obtained results show that the location of clamp is more effective than the size. It is also found that increasing the depth of clamp has a negative impact on mean velocity field and hence on stress concentration.

Conclusion: The present results demonstrate that the Blood Flow Vectoring Control (BFVC) can change the main form of flow streamlines and consequently the distributions of pressure and Wall Shear Stress. A partial clamp leads to better results.
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http://dx.doi.org/10.1016/j.compbiomed.2013.05.016DOI Listing
September 2013

Saccade movements effect on the intravitreal drug delivery in vitreous substitutes: a numerical study.

Biomech Model Mechanobiol 2013 Apr 12;12(2):281-90. Epub 2012 May 12.

School of Mechanical Engineering, Shiraz University, Shiraz, Iran.

In this study, the distributions of intravitreal injected drugs in post-vitrectomy human eyes, which are subjected to periodic saccade movements, are investigated. The computational model for the vitreous cavity of human eye is a sphere with one side truncated by the eye lens. A dynamic mesh technique was used to model the eye motion and the unsteady 3-D forms of continuity; Navier-Stokes and concentration transport of drug equations were solved numerically. The numerical model was validated earlier for the vitreous liquid flow field. The predicted drug concentration for idealized geometry was compared with the available analytic solution and excellent agreement was observed. The validated computer model was then used to simulate a real vitreous cavity filled with Balanced Salt Solution or aqueous humor as a vitreous substitute in order to obtain distribution of drugs in the post-vitrectomy eyes or liquefied vitreous. Additionally, effects of locations of drug injection, drug diffusion coefficients and saccade amplitude on the drug distribution and its uniformity were investigated. Although the earlier findings in the literature reported a day or a week as a needed time for drug uniform distribution in the vitreous substitutes, the present work depicts that saccade movements augment the transport of the drug in a way that the uniformity of the drug distribution can be achieved in a matter of minutes. Furthermore, in a vitreous cavity subjected to the saccade movements, the diffusion coefficient of drugs does not significantly affect their distribution after a few minutes. Even the injection location does not matter as uniform distribution is achieved after some time.
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http://dx.doi.org/10.1007/s10237-012-0398-3DOI Listing
April 2013

Micro and nanoparticle deposition in human nasal passage pre and post virtual maxillary sinus endoscopic surgery.

Respir Physiol Neurobiol 2012 May 24;181(3):335-45. Epub 2012 Mar 24.

School of Mechanical Engineering, Shiraz University, Shiraz, Iran.

Realistic 3-D models of the human nasal passages were developed pre and post virtual uncinectomy and Middle Meatal Antrostomy. A 3-D computational domain was constructed by a series of coronal CT scan images from a healthy subject. Then a virtual uncinectomy intervention and maxillary antrostomy were performed on the left nasal passage by removing the uncinate process and exposing the maxillary sinus antrum. For several breathing rates corresponding to low or moderate activities, the airflows in the nasal passages were simulated numerically pre and post virtual routine maxillary sinus endoscopic surgery. The airflow distribution in the nasal airway, maxillary and frontal sinuses were analyzed and compared between pre and post surgery cases. A Lagrangian trajectory analysis approach was used for evaluating the path and deposition of microparticles in the nasal passages and maxillary sinuses. A diffusion model was used for nanoparticle transport and deposition analysis. The deposition rate of the inhaled micro and nanoparticles in the sinuses were evaluated and compared for pre and post operation conditions. The results showed that after maxillary sinus endoscopic surgery, the inhaled nano and microparticles can easily enter this sinus due to penetration of the airflow into the sinus cavity. This was in contrast to the preoperative condition in which almost no particles entered the sinuses. These results could be of importance for a better understanding of the effect of sinus endoscopic surgery on patient exposure to particulate pollution and inhalation drug delivery. The significantly higher airflow rate and particle deposition in the sinus could be a reason for the discomfort reported by some patient after maxillary sinus endoscopic surgery.
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http://dx.doi.org/10.1016/j.resp.2012.03.002DOI Listing
May 2012

Two-phase flow in a rough fracture: experiment and modeling.

Phys Rev E Stat Nonlin Soft Matter Phys 2011 Jul 28;84(1 Pt 2):016316. Epub 2011 Jul 28.

US DOE, National Energy Technology Laboratory, Morgantown, West Virginia 26507-0880, USA.

To develop and test theory-based procedures for modeling two-phase flow through fractures, it is important to be able to compare computational results for a fracture with experiments performed on the exact same fracture. Unfortunately for real fractures, any attempt to image the fracture and to produce a numerical model of the fracture accessible to computer modeling unavoidably results in a coarsening of the resolution, with the very small-scale features of the imaged fracture averaged to produce the numerical representation used in modeling. Contrary to the hope that these high-resolution features would be unimportant, several modeling efforts have shown that such changes in resolution do affect the flow. Therefore, the numerical representation is different from the real fracture because of this unavoidable coarsening of the resolution. To remove the problems caused by the use of different fractures in the experiment and in the model, the fracture used in our experiments was stereographically constructed from the same numerical representation used in the modeling so that the only difference between the experimental "fracture" and the modeling "fracture" is a manufacturing error of approximately 3% or less in the aperture sizes of the manufactured experimental model. Using several models not unlike others in the literature, we modeled injection of air into the water-saturated fracture. The modeling results are compared to experimental results for injection of air into the water-saturated stereolithographically constructed fracture. A comparison between modeling and experimental results for the essentially identical fractures shows a much better detailed agreement than obtained in other studies, which compared experimental flows on the real fracture with modeling results for a lower resolution representation of the real fracture. This suggests that many of the differences between experiment and modeling in previous work resulted from the differences between the experimental and modeling fractures. For our low capillary-number cases, the best agreement with experiment is for a modification of invasion percolation with trapping (IPwt) that included approximations to viscosity ratio effects and to the interfacial tension effects in reducing very short-range curvature.
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http://dx.doi.org/10.1103/PhysRevE.84.016316DOI Listing
July 2011

Simulation of flow field during irrigation/aspiration in phacoemulsification using computational fluid dynamics.

J Cataract Refract Surg 2011 Aug;37(8):1530-8

School of Mechanical Engineering, Shiraz University, Shiraz, Iran.

Purpose: To provide the details of the flow field in the anterior chamber during irrigation/aspiration (I/A) in phacoemulsification using computational fluid-dynamics methods.

Settings: School of Mechanical Engineering, Shiraz University, Dr. Khodadoust Eye Hospital, Shiraz, Iran, and Clarkson University, Potsdam, New York, USA.

Design: Theoretical study.

Methods: A 3-dimensional model for the irrigating cannula, anterior chamber, capsular bag, and aspiration cannula was developed. The corresponding mathematic equations were solved numerically, and the details of the flow field were evaluated. The simulation was performed for coaxial and bimanual I/A systems with various flow rates. The pressure flow rate curve was evaluated, and the possibility of an unstable anterior chamber in different cases was assessed.

Results: The effects of flow turbulence on the corneal endothelium were lower for the coaxial handpiece. That is, inner placement of irrigation cannula tip lowered the effect of turbulence on the cornea.

Conclusions: Assessment of the fluid dynamics of I/A using computational fluid dynamics provided details that cannot be obtained with the available experimental and analytic methods.
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http://dx.doi.org/10.1016/j.jcrs.2011.02.026DOI Listing
August 2011

Numerical simulations investigating the regional and overall deposition efficiency of the human nasal cavity.

Inhal Toxicol 2008 Sep;20(12):1093-100

Department of Mechanical and Aeronautical Engineering Clarkson University, Potsdam, New York 13699-5700, USA.

Numerical simulations have been carried out on a model of the right passageway of an anonymous, adult male's nasal cavity, constructed from magnetic resonance imagery (MRI) scans. Steady, laminar, inspiratory flow was assumed to simulate inhalation. Analysis shows smoothly varying streamlines with a peak in velocity magnitude occurring in the nasal valves and a peak in vorticity magnitude immediately posterior. Dilute, uniform concentrations of inertial (1 microm < or = d(ae) < or = 10 microm) particles were released at the nostril and tracked via a Lagrangian tracking algorithm. Deposition efficiency is shown to increase with particle size and flow rate. Preferential deposition is seen in the anterior third of the nasal cavity for large Stokes number particles. An empirical expression for particle deposition is proposed that incorporates particle size, flow rate, and nose anatomy.
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http://dx.doi.org/10.1080/08958370802130379DOI Listing
September 2008

A new stereolithography experimental porous flow device.

Rev Sci Instrum 2008 Apr;79(4):044501

United States Department of Energy National Energy Technology Laboratory, Morgantown, West Virginia 26507-0880, USA.

A new method for constructing laboratory-scale porous media with increased pore-level variabilities for two-phase flow experiments is presented here. These devices have been created with stereolithography directly on glass, thus improving the stability of the model created with this precision rapid construction technique. The method of construction and improved parameters are discussed in detail, followed by a brief comparison of two-phase drainage results for air invasion into the water-saturated porous medium. Flow through the model porous medium is shown to substantiate theoretical fractal predictions.
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http://dx.doi.org/10.1063/1.2903740DOI Listing
April 2008

Uniformity of the fluid flow velocities within hollow fiber membranes of blood oxygenation devices.

Artif Organs 2006 Jan;30(1):10-5

Mechanical Engineering Department, University of Pittsburgh, Pittsburgh, PA 15261, USA.

A finite volume-based computational model was developed to investigate the uniformity of the fluid flow across the hollow fiber membranes in blood oxygenation devices. A two-dimensional annular cross section of a blood oxygenation device including about 3,300 hollow fiber membranes was used in the computation model. The equations governing the steady incompressible laminar flow in the blood oxygenation device were solved numerically and the results were compared with those obtained from the equivalent porous medium approximation. For the porous medium approximation, the Ergun equation was used for evaluating the permeability. The simulation results showed that the fluid molecules spend about six times longer in the fiber bundle region than that in its equivalent porous medium approximation model. The computational model also provides a more detailed fluid flow pattern in the membrane compartment of the blood oxygenator.
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http://dx.doi.org/10.1111/j.1525-1594.2006.00150.xDOI Listing
January 2006

Crossover from capillary fingering to viscous fingering for immiscible unstable flow:Experiment and modeling.

Phys Rev E Stat Nonlin Soft Matter Phys 2004 8;70(1 Pt 2):016303. Epub 2004 Jul 8.

US D.O.E., National Energy Technology Laboratory, P. O. Box 880, Morgantown, West Virginia 26507-0880, USA.

Invasion percolation with trapping (IPT) and diffusion-limited aggregation (DLA) are simple fractal models, which are known to describe two-phase flow in porous media at well defined, but unphysical limits of the fluid properties and flow conditions. A decade ago, Fernandez, Rangel, and Rivero predicted a crossover from IPT (capillary fingering) to DLA (viscous fingering) for the injection of a zero-viscosity fluid as the injection velocity was increased from zero. [Phys. Rev. Lett. 67, 2958 (1991)]]. We have performed experiments in which air is injected into a glass micromodel to displace water. These experiments clearly demonstrate this crossover as the injection velocity of the air is increased. Furthermore, simulations, using our standard pore-level model, also support the predicted IPT-to-DLA crossover, as well as the predicted power-law behavior of the characteristic crossover length.
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http://dx.doi.org/10.1103/PhysRevE.70.016303DOI Listing
October 2004