A computational study of the interaction noise from a small axial-flow fan.

Small axial-flow fans used for computer cooling and many other appliances feature a rotor driven by a downstream motor held by several cylindrical struts. This study focuses on the aerodynamic mechanism of rotor-strut interaction for an isolated fan. The three-dimensional, unsteady flow field is calculated using FLUENT, and the sound radiation predicted by acoustic analogy is compared with measurement data. Striking differences are found between the pressure oscillations in various parts of the structural surfaces during an interaction event. The suction surface of the blade experiences a sudden increase in pressure when the blade trailing edge sweeps past a strut, while the process of pressure decrease on the pressure side of the blade is rather gradual during the interaction. The contribution of the latter towards the total thrust force on the structure is cancelled out significantly by that on the strut. In terms of the acoustic contributions from the rotor and strut, the upstream rotor dominates and this feature differs from the usual rotor-stator interaction acoustics in which the downstream part is responsible for most of the noise. It is therefore argued that the dominant interaction mechanism is potential flow in nature.

The effect of inlet velocity profile on the bifurcation COPD airway flow.

The effect of inlet velocity profile on the flow features in obstructed airways is investigated in this study. In reality, the inlet velocity distributions on such models, which are extracted from medial branches of natural human lung, should be neither uniform, nor symmetric parabolic, but skewed-parabolic due to having been skewed by the upper carina ridges. Four different three-dimensional three-generation models based on the 23 generations model of Weibel have been considered, respectively. The fully three-dimensional incompressible laminar Navier-Stokes equations and continuity equation have been solved using CFD solver on unstructured tetrahedral meshes. To reduce the complexity of the simulations, only one Reynolds number of 900 was used in this calculation. Four types of inlet boundary conditions, namely uniform, parabolic, positive-skewed parabolic (skewed to the positive x-direction), and negative-skewed parabolic, were imposed on the obstructed airway models, which were considered to be obstructed at either the second generation or the third generation airways, respectively. The results show that the inlet velocity profile has significant influence on the flow patterns, mass distributions, and pressure drops in either the symmetric model, or the three obstructed models. The three generation airways may not be enough to study the bifurcation flow in chronic obstructive pulmonary disease (COPD) airways, and a four-generation or more airway model is necessary to get better predictive results.

Turbulent jet manipulation using two unsteady azimuthally separated radial minijets.

The active manipulation of a turbulent round jet is experimentally investigated based on the injection of two radial unsteady minijets, prior to the issue of the main jet. The parametric study is conducted for the mass flow ratio Cm of the minijets to the main jet, and the ratio fe/f0 of the minijet frequency to the preferred-mode frequency of the main jet. It is found that the decay rate of the jet centreline mean velocity could be greatly increased if the two minijets are separated azimuthally by an angle θ=60°, instead of by θ=180°. This increase is a consequence of the flapping motion of the jet column, and the formation process and generation mechanism of this flapping motion are unveiled by careful analysis of the experimental data.

Modeling the bifurcating flow in a human lung airway.

The inspiratory flow characteristics in a three-generation lung airway have been numerically investigated using a control volume method to solve the fully three-dimensional laminar Navier-Stokes equations. The three-generation airway is extracted from the fifth to seventh branches of the model of Weibel (Morphometry of the Human Lung, Academic Press, New York, Springer, Berlin, 1963) with in-plane and 90 degrees off-plane configurations. Computations are carried out in the Reynolds number range of 200-1600, corresponding to mouth-air breathing rates ranging from 0.27 to 2.16l/s, or an averaged height of a man breathing from quiet to vigorous state. Particular attention is paid to establishing relations between the Reynolds number and the overall flow characteristics, including flow patterns and pressure drop. The ratio of airflow rate through the medial branch to that of the lateral branch for an in-plane airway increases as Re(0.227). However, the total pressure drop coefficient varies as Re(-0.497) for an in-plane airway and as Re(-0.464) for an off-plane airway. These pressure drop results are in good agreement with the experimentally measured behavior of Re(-0.5) and are more accurate than the numerically determined behavior of Re(-0.61) assuming the airways to be approximated by two-dimensional channels.

Modeling the bifurcating flow in an asymmetric human lung airway.

In a former paper, the inspiratory flow characteristics in a three-generation symmetric bifurcation airway have been numerically investigated using a control volume method to solve the fully three-dimensional laminar Navier-Stokes equations. The present paper extends the work to deal with asymmetric airway extracted from the 5th-11th branches of the model of Weibel (Morphometry of the Human Lung. New York Academic Press, Verlag, 1963) in order to more appropriately model human air passage. Computations are carried out in the Reynolds number range 200-1600, corresponding to mouth-air breathing rates ranging from 0.27 to 2.16l/s, representative for an averaged height man breathing from quiet to vigorous state. Particular attentions are paid to establishing relations between the Reynolds number and the overall flow characteristics, including flow patterns and pressure drop. The study shows that the ratios of airflow rate through the medial branches to that of their mother branches are the same, and this is also true for the ratios of airflow rate through the lateral branches. This partially explains why regular human breathing is not affected by airways of different sizes.

Vortex sound due to a flexible boundary backed by a cavity in a low mach number mean flow.

Low frequency sound radiated due to the unsteady motion of an inviscid vortex in the proximity of a flexible membrane backed by an airtight cavity on an otherwise rigid plane is investigated theoretically. Results show that both monopole and dipole are created but the latter is important only when the vortex is traversing over the membrane. The monopole results from the membrane vibration and the dipole from the transverse motion of the vortex. It is also found that these sound fields tend to counteract each other. The increase in the mean flow speed in general results in a stronger acoustic power radiation, but sound attenuation may be possible if the membrane-cavity system is weak compared with the mean flow momentum.