Hamid Rahai

CFD Modeling and Analysis of Pulmonary Airways/Particles Transport and Deposition

Unsteady numerical simulations of air flow, mixed with micron particles, through a human lung conducting zone during inhalation have been performed. The process included importing images from a high resolution CT-Scan into a CFD software, generation of the CFD model and then CFD simulation over a 4 seconds cycle (2 seconds for inhalation and 2 seconds for exhalation). The inlet diameter was 11 mm and the flow rates were 5, 7.5, and 15 liters/ min. Only results for the highest flow rate are presented. The implicit-unsteady Reynolds Average Navier-Stokes equations with the Wilcox K-ω turbulence model were used for the simulation. The micron particles were solid round coal with 1000 Kg/m 3 density. Results indicate high correlation between regions of high vorticity and secondary flow and particle deposits. This was mostly evident in the main bronchus. While most particles should exit the lung during the exhalation process, however, areas of recirculating flow and near the walls continue to have some particle deposits.

The distortion of a passive scalar by two‐dimensional objects

The response of a heated, nearly homogeneous and isotropic turbulent field to a nonuniform rapid distortion upstream of two‐dimensional objects placed in a homogeneous grid generated flow is investigated experimentally. In particular, the effect of the nonuniform distortion on the single‐point statistical properties of the velocity and a passive scalar (temperature), the axial heat flux and higher‐order cross moments are presented. The nearly homogeneous, turbulent flow is produced by a biplane grid of rods and a square mesh grid of electrically heated wire that is placed downstream of the turbulence producing grid. Spatially and temporally resolved, simultaneous measurements of the streamwise turbulent velocity and temperature are obtained upstream of several two‐dimensional objects along the mean stagnation streamline. The effects of the blocking and vortex stretching mechanisms on the root‐mean‐square (RMS) velocity for various ratios of the integral length scale to the characteristic length of the obj...

Tracheal Stenosis: A CFD Approach for Evaluation of Drug Delivery

The existence of obstructions such as tracheal stenosis has major impacts on respiratory functions. Therapeutic effectiveness of inhaled medications is influenced by tracheal stenosis, and particle transport and deposition pattern are modified. The majority of studies have focused on obstructions in branches of the airways, where the flow is diverted to the other branches to meet the needed oxygen intake. In this study we have investigated the effects of trachea with and without stenosis/obstruction on particle depositions and air flow in a human respiratory system. Patient specific CFD simulations were conducted; CT-scans of a patient with tracheal stenosis were used to create 3D models of bronchial tree up to 8 generations. The section of the stenosis was manually modified to create a healthy trachea. Comparisons between CFD simulations before and after intervention demonstrate the impact of the stenosis on flow characteristics and particles distribution. The numerical investigations were performed using the implicit Unsteady Reynolds-Averaged Navier-Stokes equation (U-RANS), using the commercially available software (STAR-CCM+) from CD-Adapco, along with K-ω; shear stress transport model. Two sets of CT-images of inhalation and exhalation were used for assigning Patient-specific boundary conditions at the outlets. Lagrangian Phase model was used to simulate particle transport and depositions of 10, 5 and 2.5 micron diameter particles. Results of the particle depositions for 10 micron particles highlight the difference in depositions and ultimately inhaled medications in patients with and without tracheal stenosis. Particle deposition for normal Tidal volume increased due to stenosis from 47% to 51% for 10 Micron particles and not a significant change for the 2.5 Micron particles (from 4.5% to 4.7%).Comparisons of pressure drop in each generation between patient with tracheal stenosis and the healthy lung showed significant increase in pressure drop after the stenosis, which were experienced in all generations downstream. Experimental validation of the CFD results were made with a model of healthy trachea up to 3rd generation, manufactured using Additive Layer Manufacturing (ALM) from CT-images and pressure results were compared with the corresponding CFD results. Good agreements were found.Copyright

Turbulent Junction Vortex with Upstream Ribbed Surface

Results of the experimental investigations of the effect s upstream flat plate ribbed surface on turbulent junction flow and downstream wake are presente d. The junction flow was developed using a NACA 0012 airfoil mounted normal to a flat plate downstream of its leading edge. The experiments were carried out at four control planes of 50%, 100%, 133%, and 166% of the chord length. Results show that the rible ts displace the location of the horseshoe vortex away from the corner and toward the flat plate edge and reduce its strength. There are significant re ductions in mean streamwise circulation downstream at 133% and 166% planes.

Particulates Depositions in Patient-Specific Simulations of Respiratory System

Ambient particulates depositions have major impacts on respiratory functions. Patient-specific simulations of the respiratory system of a patient were performed to investigate the relationship between the flow characteristics and particulate depositions in the upper respiratory domain. CT scan images were imported to create a 3D model of the bronchial tree and then transferred to a computational fluid dynamics (CFD) software for simulation. Appropriate boundary conditions were assigned to simulate the sinusoidal behavior of the normal breathing cycle with the corresponding pressures at the outlets. Lagrangian phase model was used to simulate the micron solid round particulates transport and depositions. The simulations were performed for 2.5 micron and 10 micron particles. The implicit-unsteady Reynolds Averaged Navier-Stokes equations with K-ω turbulence model were used for these simulations. Results indicate high correlation between regions of high vortices, secondary flow and high wall shear stress and particulate depositions. The total deposition number for 10-micron particles was higher than that for the 2.5-micron particles. The differences in the locations of depositions at various generations of the lung illustrate the importance of the patient-specific simulations.Copyright

Profiles of Two Side-By-Side Jets with Different Momentum Ratios in a Cross Flow

−k turbulence model. The experimental measurements were performed using a double hot wire probe in X formation. The distance between the two side-by-side jets was two-jet diameter and the jets’ nozzles were flush with the surface within the turbulent boundary layer. The investigations were carried out at a cross flow free stream mean velocity of 4.65 m/sec. The ratios of jet to cross flow momentums for the first jet, rj1, was 3.3 and for the second jet, rj2, were 6.6 and 13.2. The Reynolds numbers based on jet diameter were nearly 6200 for the first jet and 12,400 and 24,800 for the second jet. Investigations were carried out at two different planes of x/d = 1 and 5. In addition, the trajectories and other turbulence statistics were obtained along axial midplanes of the two jets and the axial mid-plane between the two jets up to x/d=15. Here x is the streamwise direction along the cross flow and d is the inside diameter of the jet nozzle. Results indicate reduced rate of penetration and axial vorticity for the variable momentum jets when compared with similar side-by-side jets with constant momentums.

EXPERIMENTAL INVESTIGATIONS OF TWO SIDE -BY -SIDE JETS IN A CROSS FLOW

Single poi nt measurements of three components of turbulent velocity and their cross moments were made downstream of a single and two side -by -side jets in a cross flow. For both cases, the ratio of jet to cross flow momentum was 3.3. The Reynolds number based on jet diameter was nearly 6200. The distance between the two side -by -side jets was two -jet diameter. The jets’ nozzles were flush with the wind tunnel surface of the working area within the turbulent boundary layer. Measurements were carried out at six different planes of x/d = 0.5, 1, 2, 5, 10.7, and 15.7. Here x is the streamwise direction along the cross flow and d is the inside diameter of the jet nozzle. Results showed that for the double jets there were increased regions of high turbulence intensity, secon dary flow and turbulent shear stress, near the flat plate surface in the near field region, resulting in enhanced mixing in these areas. The trajectories of the double jets showed reduced penetrations in the near field when compared with the corresponding results for the single jet. However, in the far field, the penetration rates of the single and the double jets were nearly identical. NOMENCLATURE

Distortion of a turbulent scalar upstream of axisymmetric objects

TECHNICAL NOTES are short manuscripts describing neve developments or important results of a preliminary nature. These Notes cannot exceed 6 manuscript pages and 3 figures; a page of text may be substituted for a figure and vice versa. After informal review by the editors, they may be published within a few months of the date of receipt. Style requirements are the same as for regular contributions (see inside back cover).

PARTICULATE DEPOSITION IN A PATIENT WITH TRACHEAL STENOSIS

The presence of obstructions such as tracheal stenosis has important effects on respiratory functions. Tracheal stenosis impacts the therapeutic efficacy of inhaled medications as a result of alterations in particle transport and deposition pattern. This study explores the effects of the presence and absence of stenosis/obstruction in the trachea on air flow characteristics and particle depositions. Computational fluid dynamics (CFD) simulations were performed on three-dimensional (3D) patient-specific models created from computed tomography (CT) images. The analyzed model was generated from a subject with tracheal stenosis and includes the airway tree up to eight generations. CT scans of expiratory and inspiratory phases were used for patient-specific boundary conditions. Preand post-intervention CFD simulations’ comparison reveals the effect of the stenosis on the characteristics of air flow, transport, and depositions of particles with diameters of 1, 2.5, 4, 6, 8, and 10 lm. Results indicate that the existence of the stenosis inflicts a major pressure force on the flow of inhaled air, leading to an increased deposition of particles both above and below the stenosis. Comparisons of the decrease in pressure in each generation between preand post-tracheal stenosis intervention demonstrated a significant reduction in pressure following the stenosis, which was maintained in all downstream generations. Good agreements were found using experimental validation of CFD findings with a model of the control subject up to the third generation, constructed via additive layer manufacturing from CT images. [DOI: 10.1115/1.4038260]

Computational fluid dynamics evaluation of excessive dynamic airway collapse

Background: Excessive dynamic airway collapse, which is often caused by the collapse of the posterior membrane wall during exhalation, is often misdiagnosed with other diseases; stents can provide support for the collapsing airways. The standard pulmonary function tests do not necessarily show change in functional breathing condition for evaluation of these type of diseases. Methods: Flow characteristics through a patients airways with excessive dynamic airway collapse have been numerically investigated. A stent was placed to support the collapsing airway and to improve breathing conditions. Computed tomography images of the patients pre‐ and post‐stenting were used for generating 3‐Dimensional models of the airways, and were imported into a computational fluid dynamics software for simulation of realistic air flow behavior. Unsteady simulations of the inspiratory phase and expiratory phase were performed with patient‐specific boundary conditions for pre‐ and post‐intervention cases to investigate the effect of stent placement on flow characteristic and possible improvements. Findings: Results of post‐stent condition show reduced pressure, velocity magnitude and wall shear stress during expiration. The variation in wall shear stress, velocity magnitude and pressure drop is negligible during inspiration. Interpretation: Although Spirometry tests do not show significant improvements, computational fluid dynamics results show significant improvements in pre‐ and post‐treatment results, suggesting improvement in breathing condition. HighlightsInspiratory‐expiratory CT‐based models were used for patient‐specific simulations.Spirometry results do not necessarily show symptom improvement post‐intervention.Fluid simulations show significant improvements in airflow and functional breathing.Post‐stent results show reduced pressure and velocity magnitude during expiration.