Computational flow modeling of the equine upper airway

Introduction

Objectives Development of a computational turbulence model for modeling flow through the equine airway Prediction of the minimum larynx size required for normal airflow Determination of flow and pressure characteristics on the soft palate Basis for investigation of new management/ therapeutic approaches for many ailments affecting the equine respiratory system

Modeling: Schematic of the Steps CT Scan 3D Reconstruction Mimics® Geometry Manipulation Magics® Surface Mesh Magics® Change Abduction Volume Mesh TGrid® Solve Flow Equations Fluent®

Geometry Acquisition – CATSCAN Picker PQS CT scanner Slice thickness: 5 mm, Table index: 5 mm

3D Reconstruction Mimics Nasal cavity, Sinuses, Nasopharynx, Pharynx, Larynx and Cranial trachea Export Format: STL, Triangle defined 3D Geometry

Geometry Manipulation Magics Removal of Unwanted parts Smoothing Creation of inflow and outflow surfaces

Surface Mesh Magics Mesh Refinement to improve Mesh Quality Skewness should be less than 0.85 for subsequent interior mesh generation and numerical analysis

Volume Mesh TGrid

Schematic

Solve Flow Equations

Governing Equations Reynolds Averaged Navier-Stokes Equation: Boussinesq Approximation:

Standard k-e Model

Boundary Conditions Atmospheric pressure at inlet Outlet Pressures (30 cm from the larynx) shown in the Figure Reynolds Number ~ 80000 Turbulence intensity Inlet: 1% Outlet: 5% Hydraulic Diameter Inlet: 0.057 Outlet: 0.071 Fig. Tracheal Pressures in exercised horses ( Nielan, Rehder, Ducharme, & Hackett, 1992)

Results

Model Validation Table 1: Comparison of modeled and measured flows Inspiratory Peak Flow Rate (L/s) Expiratory Peak Flow Rate (L/s) Model Computed Values 65.2 31.2 Experimental Values (Nielan et al, 1992) 65.5 84.0

Model Validation (contd..) Figure : Volume flow across the tracheal outlet (L/s). Inspiratory flows are negative, expiratory are positive. Figure : Flow trace of a galloping horse. The corresponding pressure trace for this horse has two-fold higher exhalation pressures than those used for the model.

Flow Profile

Flow Profile- Discussion Flow velocities are higher at the bottom of the nasal passage. Eddies are formed in the sinuses

Flow across the Larynx Average Flow Velocity = 26.9 m/s Pressure = -4300 Pa Reynolds Number = 63000 Velocities are mostly uniform across the larynx. The velocity near the walls are lower as expected.

Velocity Figure 10: Computed average velocities at different cross sections in the nasal cavity

Summary Limitations Enhancement in the model by changing the geometry to improve prediction in the exhalation phase of breathing Future Work Incorporate Temperature and Moisture Transport Analysis for different degree of abduction of the larynx 50% Abduction of the larynx 75% Abduction