Particle-scale modelling of fluid velocity distribution near the particles surface in sand filtration.

Sand filtration is widely used in drinking water treatment processes, yet the hydraulic fundamentals at particle-scale are not well defined, especially the fluid velocity profile near the sand particles surface. In this study, a numerical model is developed by combining the Lattice Boltzmann (LBM) and the Discrete Element Method (DEM), used to describe the fluid flow over the sand particles surface and the micro-structure details of the sand packed bed respectively. The model is validated by comparing the simulation results with the experimental measurements using two systems, showing that the model can describe the fluid velocity distribution around the particles surface. Critical flow velocity is introduced as the balance between hydrodynamic and adhesive torques acting on sand particle surface. Furthermore, a new concept - effective filter surface (EFS), is defined as the area where the velocity near sand particles surface is less than the critical flow velocity, aiming for indirectly evaluating the performance of sand filtration. It is quantitatively demonstrated that increasing the sand particle size or feed flow velocity results in the decrease of both critical flow velocity and EFS under the given tested conditions. The LBM-DEM model provides a useful tool for understanding the fundamentals of liquid flow distribution and also estimating sand filtration performance under different operation conditions.

[Influence of uncinate process on aerodynamic characteristics of nasal cavity and maxillary sinus].

OBJECTIVE: This study aimed to investigate the influence of uncinate process on air flow velocity, trace, distribution, air pressure, as well as the air flow exchange of nasal cavity and paranasal sinuses. METHODS: Fluent software was used to simulate two nasal cavity and paranasal sinus structures following CT scanning, one had normal nasal cavity, the another had the nasal cavity with uncinate process removed. Air flow velocity, pressure, distribution and trace lines were calculated and compared by Navier-Stokes equation and numerically visualized between two models. RESULTS: Air flow of two models in the common and middle meatus accounted for more than 50% and 30% of total nasal cavity flow. Flow velocity of two models were maximal in the common meatus, followed by the middle meatus. The maximal velocity existed on the left nasal district between limen nasi and head of inferior turbinate. The flow traces of two models were similar. In the normal model, the air flow velocity of the district around uncinate process was almost the same in inhale and exhale. In the model with the uncinate process removed, the air flow velocity of the district around uncinate process was faster, the air flow velocity in expiratory phase was quicker. Compared with the normal nasal cavity, there was more exchange of maxillary sinus in the model with cut uncinate process. CONCLUSIONS: In the view of flow dynamics, the uncinate process effects the air flow velocity of the district around uncinate process and the exchange of maxillary sinus, the contribution of nasal flow is connected with the morphosis of the uncinate process.

[Effect of endoscopic sinus surgery on airflow of the nasal cavity and paranasal sinuses: a computational fluid dynamics study.].

OBJECTIVE: To study the airflow velocity, trace, distribution, pressure, as well as the airflow exchange between the nasal cavity and paranasal sinuses in a computer simulation of nasal cavity pre and post virtual endoscopic sinus surgery (ESS). METHODS: Computational fluid dynamics (CFD) technique was applied to construct an anatomically and proportionally accurate three-dimensional nasal model based on a healthy adult woman's nasal CT scans. A virtual ESS intervention was performed numerically on the normal nasal model using Fluent 6.1.22 software. Navier-Stokes and continuity equations were used to calculate and compare the airflow characteristics between pre and post ESS models. RESULTS: (1) After ESS flux in the common meatus decreased significantly. Flux in the middle meatus and the connected area of opened ethmoid sinus increased by 10% during stable inhalation and by 9% during exhalation. (2) Airflow velocity in the nasal sinus complex increased significantly after ESS. (3) After ESS airflow trace was significantly changed in the middle meatus. Wide-ranging vortices formed at the maxillary sinus, the connected area of ethmoid sinus and the sphenoid sinus. (4) Total nasal cavity resistance was decreased after ESS. (5) After ESS airflow exchange increased in the nasal sinuses, most markedly in the maxillary sinus. CONCLUSIONS: After ESS airflow velocity, flux and trace were altered. Airflow exchange increased in each nasal sinus, especially in the maxillary sinus.

Computational fluid dynamics simulation of airflow in the normal nasal cavity and paranasal sinuses.

BACKGROUND: This study aimed to investigate airflow velocity, trace, distribution, and air pressure, as well as the airflow exchange between the nasal cavity and paranasal sinus in a normal subject using computational fluid dynamics. METHODS: Fluent software is used to simulate nasal cavity and paranasal sinus structure after CT scanning of a normal adult subject. Airflow velocity, pressure, distribution, and trace lines were calculated by Navier-Stokes equation and numerically visualized. RESULTS: Airflow in the common and middle meatus accounted for >50 and 30% of total nasal cavity flow. Flow velocity was maximal in the common meatus, followed by the middle meatus. Flow velocity and flux in each paranasal sinus was extremely low. The flow trace in the inferior and lower part of the common meatus was predominately straight in form. Flow was parabolic in the middle and superior meatus and the middle and upper parts of the common meatus. Air pressure was high at the front end of the inferior and middle turbinate and the uncinate process. There was little pressure difference/flow exchange between inner and outer aspects of the paranasal sinus. CONCLUSION: The major airflow forms are straight (lower common and inferior meatus) and parabolic (middle and upper common meatus and middle superior). Flow force is strongest at the front end of the inferior and middle turbinate and uncinate process. There is very little exchange between the paranasal sinus and the nasal cavity during stable airflow.

Numerical flow simulation in the post-endoscopic sinus surgery nasal cavity.

In this study we utilized computational fluid dynamic (CFD) techniques to construct a numerical simulation of nasal cavity airflow pre and post virtual functional endoscopic surgery (FESS). A healthy subject was selected, and CFD techniques were then applied to construct an anatomically and proportionally accurate three-dimensional nasal model based on nasal CT scans. A virtual FESS intervention was performed numerically on the normal nasal model using Fluent software. Navier-Stokes and continuity equations were used to calculate and compare airflow, velocity, distribution and pressure in both the pre and post FESS models. In the post-FESS model, there was an increase in airflow distribution in the maxillary, ethmoid and sphenoid sinuses, and a 13% increase through the area connecting the middle meatus and the surgically opened ethmoid. There was a gradual decrease in nasal resistance in the posterior ethmoid sinus region following FESS. These findings highlight the potential of this technique as a powerful preoperative assessment tool to aid clinical decision-making.