Influence of staff number and internal constellation on surgical site infection in an operating room

Prediction of bacteria-carrying particle (BCP) dispersion and particle distribution released from staff members in an operating room (OR) is very important for creating and sustaining a safe indoor environment. Postoperative wound infections cause significant morbidity and mortality, and contribute to increased hospitalization time. Increasing the number of personnel within the OR disrupts the ventilation airflow pattern and causes enhanced contamination risk in the area of an open wound. Whether the amount of staff within the OR influences the BCP distribution in the surgical zone has rarely been investigated. This study was conducted to explore the influence of the number of personnel in the OR on the airflow field and the BCP distribution. This was performed by applying a numerical calculation to map the airflow field and Lagrangian particle tracking (LPT) for the BCP phase. The results are reported both for active sampling and passive monitoring approaches. Not surprisingly, a growing trend in the BCP concentration (cfu/m³) was observed as the amount of staff in the OR increased. Passive sampling shows unpredictable results due to the sedimentation rate, especially for small particles (5–10 μm). Risk factors for surgical site infections (SSIs) must be well understood to develop more effective prevention programs.     

在轴向气流作用下液体轴对称抛撒二次破碎的实验研究

液体轴对称抛撒的实验研究是以云雾爆轰武器的研制为背景的。为了研究轴向气流作用下液体轴对称抛撒二次破碎所形成的雾化场特性,本文利用两台激波管并对之加以改造,成功地在实验室实现了轴向气流作用下液体的轴对称抛撒。为了研究其雾化场的远场特性,本文利用激光粒子测量仪获得了在不同实验工况和不同位置下的雾化场SMD分布曲线。实验数据表明,由于轴向气流速度的增加,液体破碎的Weber数得到了提高,导致二次破碎初期雾化场的SMD随之减小;随着抛撒驱动压力的提高,二次破碎初期雾化场的SMD也随之减小;在同一工况下,雾化场SMD随着测量位置与喷口距离的增加而变大。     

水平表面上超细气溶胶粒子的沉积

采用二维模型，讨论了气流水平流动时，超细气溶胶粒子受重力、热泳和扩散效应的作用在工业洁净室中水平工作台表面上的沉积情况。分析了沿流动方向的局部沉积速度与粒径和工作台边缘距离之间的关系，指出了工作台表面上的平均沉积速度分布情况。     

Numerical modeling of electrostatic spray painting transfer processes in rotary bell cup for automotive painting

Here we implemented a 3D comprehensive Eulerian-Lagrangian model in order to investigate the electrostatic spray transfer processes in the high-speed rotary bell sprayer. This efficient algorithm contains spray dynamics, airflow, paint droplets tracking and an electrostatic effect to simulate atomization. The algorithm is implemented using the OpenFOAM package. A solver for the particle trajectory was used to illustrate the process of spray transport and also the interaction of the airflow and the particle that is solved by momentum coupling. Creating an initial condition of the particle approach has been proposed that is matched with practical applications. The fluid-dynamics is simulated by solving the unsteady 3D compressible Navier-Stokes equations. Unsteady flow is computed by using a Large eddy simulation (LES) turbulence approach, while the motion of the particles is simulated by tracking the droplet size distribution approach. The model correctly predicts that the bell cup spin forces the paint particles to fall off from the bell surface towards the high-velocity airflow. The present work illustrates a tentative benchmark and contains a systematic analysis of the recirculation zone length, the toroidal vortex, the overspray phenomena and the flowfield characteristics like mean velocity, pressure, turbulent kinetic energy and velocity fluctuation. The results indicate as dominant operating parameter the air-paint flow rate with voltage level deeply affecting the spray shape. A more uniform distribution of the coating is obtained by growing this high-velocity shaping airflow, although the values of the transfer efficiency (TE) are reduced. The distribution of the particle size is very sensitive to changes in the rotational speed. Experimental results obtained in this study put forward a clear link between the shaping air flow rate and the rotation frequency on the aerodynamics and also provide valuable insights to design modern ERBS. The paint spray distribution obtained in the present work is validated against coating experimental results with suitable accuracy.     

Effects of cartilaginous rings on airflow and particle transport through simplified and realistic models of human upper respiratory tracts

In the present study, computational fluid dynamics （CFD） is used to investigate inspiratory and expiratory airflow characteristics in the human upper respiratory tract for the purpose of identifying the probable locations of particle deposition and the wall injury. Computed tomography （CT） scan data was used to reconstruct a three dimensional respiratory tract from trachea to first generation bronchi. To compare, a simplified model of respiratory tract based on Weibel was also used in the study. The steady state results are obtained for an airflow rate of 45 L/min, corresponding to the heavy breathing condition. The velocity distribution, wall shear stress, static pressure and particle deposition are compared for inspiratory flows in simplified and realistic models and expiratory flows in realistic model only. The results show that the location of cartilaginous rings is susceptible to wall injury and local particle deposition.     

Modeling micro-particle deposition in human upper respiratory tract under steady inhalation

A representative human upper respiratory tract (URT) with idealized oral region and asymmetric tracheo-bronchial (TB) airway has been modeled, and laminar-to-turbulent airflow for typical inhalation modes as well as micro-particle transport and deposition has been simulated using CFX10.0 software from Ansys Inc. on a personal computer. The asymmetric TB airway could not be replaced by an extended straight tube as outlet of the oral region while investigating the tracheal airflow field and particle depositio...     

Stereoscopic PIV measurements of flow in the nasal cavity with high flow therapy

Knowledge of the airflow characteristics within the nasal cavity with nasal high flow (NHF) therapy and during unassisted breathing is essential to understand the treatment’s efficacy. The distribution and velocity of the airflow in the nasal cavity with and without NHF cannula flow has been investigated using stereoscopic particle image velocimetry at steady peak expiration and inspiration. In vivo breathing flows were measured and dimensionally scaled to reproduce physiological conditions in vitro. A scaled model of the complete nasal cavity was constructed in transparent silicone and airflow simulated with an aqueous glycerine solution. NHF modifies nasal cavity flow patterns significantly, altering the proportion of inspiration and expiration through each passageway and producing jets with in vivo velocities up to 17.0 ms⁻¹ for 30 l/min cannula flow. Velocity magnitudes differed appreciably between the left and right sides of the nasal cavity. The importance of using a three-component measurement technique when investigating nasal flows has been highlighted.     

鼻腔加温功能特征及其与气流场关系的研究

为研究鼻腔加温功能特征及其与气流场之间的关系, 选用1例健康国人的鼻腔进行CT扫描. 据CT数据对鼻腔气道进行表面三维重建, 运用计算流体动力学方法分析通气量为12L/min时吸气相0.15s, 0.45s, 0.75s的鼻腔气流场与温度场. 结果显示吸气相0.15s, 0.45s, 0.75s鼻腔气流场主要表现为双侧气流量分布不对称, 其中气流主要流经左侧; 双侧均为总鼻道中、下部气流量较多, 嗅裂、中鼻道和下鼻道气流量少. 吸气相0.15s, 0.45s, 0.75s温度场均表现为温度自鼻腔前端至鼻咽部逐渐增高, 其中温度主要上升区域为鼻内孔-下鼻甲前端-中鼻甲前端对应气道, 且在吸气速度和流量增大后, 这一主要加温区域无明显向后延长征象; 双侧鼻腔及单侧鼻腔不同部位气道气流分布差别较大, 但双侧温度场基本对称, 单侧鼻腔不同气道部位温度差值亦较小, 幅度均小于1℃.     

Comparison of magnetic resonance concentration measurements in water to temperature measurements in compressible air flows

Magnetic resonance imaging (MRI) measurements in liquid flows provide highly detailed 3D mean velocity and concentration data in complex turbulent mixing flow applications. The scalar transport analogy is applied to infer the mean temperature distribution in high speed gas flows directly from the MRI concentration measurements in liquid. Compressibility effects on turbulent mixing are known to be weak for simple flows at high subsonic Mach number, and it was not known if this would hold in more complex flows characteristic of practical applications. Furthermore, the MRI measurements are often done at lower Reynolds number than the compressible application, although both are generally done in fully turbulent flows. The hypothesis is that the conclusions from MRI measurements performed in water are transferable to high subsonic Mach number applications. The present experiment is designed to compare stagnation temperature measurements in high speed airflow (M = 0.7) to concentration measurements in an identical water flow apparatus. The flow configuration was a low aspect ratio wall jet with a thick splitter plate producing a 3D complex downstream flow mixing the wall-jet fluid with the mainstream flow. The three-dimensional velocity field is documented using magnetic resonance velocimetry in the water experiment, and the mixing is quantified by measuring the mean concentration distribution of wall-jet fluid marked with dissolved copper sulfate. The airflow experiments are operated with a temperature difference between the main stream and the wall jet. Profiles of the stagnation temperature are measured with a shielded thermocouple probe. The results show excellent agreement between normalized temperature and concentration profiles after correction of the temperature measurements for the effects of energy separation. The agreement is within 1 % near the edges of the mixing layer, which suggests that the mixing characteristics of the large scale turbulence structures are the same in the two flows.     

Effect of source location on particle dispersion in displacement ventilation rooms

A proposed computer model for predicting aerosol particle dispersion in indoor spaces was validated with experimental data found in the literature,and is then used to study the effect of the area and point source locations on particle dispersion in displacement ventilation (DV) rooms.The results show that aerosol source location has a strong impact on the spatial distribution and removal rate of indoor particles.Particle removal performance depends strongly on ventilation efficiency and particle deposition rate on indoor surfaces.Important consideration for both relative ventilation efficiency and deposition rate consists of the position of the aerosol source relative to the main airflow pattern and the occupied zone.     

Effect of source location on particle dispersion in displacement ventilation rooms

A proposed computer model for predicting aerosol particle dispersion in indoor spaces was validated with experimental data found in the literature, and is then used to study the effect of the area and point source locations on particle dispersion in displacement ventilation （DV） rooms. The results show that aerosol source location has a strong impact on the spatial distribution and removal rate of indoor particles. Particle removal performance depends strongly on ventilation efficiency and particle deposition rate on indoor surfaces. Important consideration for both relative ventilation efficiency and deposition rate consists of the position of the aerosol source relative to the main airflow pattern and the occupied zone.     

Two-phase PIV method using two excitation and two emission spectra

To be able to characterize the airflow in the presence of liquid droplets in a confined geometry, a new two-phase particle image velocimetry (PIV) method is developed. It is based on a two-colour YAG laser and two different fluorescent dyes dissolved in the gas-phase tracers and droplets. This approach permits to separate the images (and thus the information) of the two phases optically and simultaneously. When experiments need to be carried out in a confined geometry (such as in a wind tunnel) with uniform droplet distribution and high turbulence, which are the case in the present investigation, one should be able to deal with continuous droplet deposition on the lateral walls through which the cameras acquire images. It requires the adaptation of the experimental conditions and the development of a dynamic background subtraction algorithm. The typical results reveal the influence of the presence of liquid droplets on the airflow by comparing single-phase flow field to the air-phase motion in two-phase flow configuration. Furthermore, by analysing the continuous-phase and the discrete-phase properties, some aspects of the interaction between the two phases are shown.     

Two-component Brownian coagulation: Monte Carlo simulation and process characterization

The compositional distribution within aggregates of a given size is essential to the functionality of composite aggregates that are usually enlarged by rapid Brownian coagulation. There is no analytical solution for the process of such two-component systems. Monte Carlo method is an effective numerical approach for two-component coagulation. In this paper, the differentially weighted Monte Carlo method is used to investigate two-component Brownian coagulation, respectively, in the continuum regime, the free-molecular regime and the transition regime. It is found that (1) for Brownian coagulation in the continuum regime and in the free-molecular regime, the mono-variate compositional distribution, i.e., the number density distribution function of one component amount (in the form of volume of the component in aggregates) satisfies self-preserving form the same as particle size distribution in mono-component Brownian coagulation; (2) however, for Brownian coagulation in the transition regime the mono-variate compositional distribution cannot reach self-similarity; and (3) the bivariate compositional distribution, i.e., the combined number density distribution function of two component amounts in the three regimes satisfies a semi self-preserving form. Moreover, other new features inherent to aggregative mixing are also demonstrated; e.g., the degree of mixing between components, which is largely controlled by the initial compositional mass fraction, improves as aggregate size increases.     

基于交变电场的月表除尘方法研究进展

实施月球表面探测时, 月表尘埃物质容易在探测设备表面沉积并造成严重不利影响. 近年来交变电场 除尘方法被认为是在月表进行尘埃防护的有效手段, 但要使该方法实用化, 仍需要弄清除尘的机理, 确定相关 影响的因素, 以便进行优化设计. 本文从实现交变电场的电帘出发, 分别介绍电帘表面的交变电场分布、颗粒 在电帘表面的受力状态、颗粒群运动过程中的摩擦电荷分布、颗粒运动的模拟方法、颗粒运动过程、除尘效 率和能耗的理论和实验研究, 并对自清洁太阳能电池板系统的设计与制备进行了总结. 对颗粒的起跳过程、静 电的生成和除尘效率影响因素等普遍关注的问题进行了细致的讨论和总结, 指出了颗粒的带电量对起跳过程 的关键作用. 最后, 对该除尘防尘存在的问题进行了归纳, 并展望了该领域的发展趋势.      

Measurement of electrical charges carried by airborne bacteria laboratory-generated using a single-pass bubbling aerosolizer

Widely used bioaerosol generators like Collison nebulizer probably produce electrostatically charged particles, but the electrical charges carried by laboratory-generated airborne microorganisms using bubbling aerosolizers are poorly understood. In this study, we measured the fraction of neutral particles and number of elementary charges per particle as a function of the aerodynamic diameter of airborne bacteria (Escherichia coli and Enterococcus hirae). Bioaerosols were produced by a liquid sparging aerosolizer-type bubbling generator, with particle sizes ranging from roughly 0.6 to 2 μm. The experimental setup included an electrostatic precipitator and real-time devices including an electrometer, aerodynamic particle sizer, and electrical low-pressure impactor. Experimental results obtained for various operating conditions showed that aerosols produced with a higher bubbling airflow contained a larger proportion of neutral particles (from around 30% to 50%) and that bacteria carried a greater average absolute number of elementary charges (from around –10 to –60 elementary units) than those under lower airflow. Under the investigated conditions, a neutralization step is unnecessary because it may have a negative effect on the viability of sensitive microorganisms. Our results suggest that the neutral fraction can be used downstream of an electrostatic precipitator, and that this setup may have advantages over bipolar neutralizers.     

Experimental Investigation of Three-Dimensional Vortex Structures Downstream of Vortex Generators Over a Backward-Facing Step

An experimental investigation of vortex generators has been carried out in turbulent backward-facing step (BFS) flow. The Reynolds number, based on a freestream velocity U₀ = 10 m/s and a step height h = 30 mm, was Re_h = 2.0 × 10⁴. Low-profile wedge-type vortex generators (VGs) were implemented on the horizontal surface upstream of the step. High-resolution planar particle image velocimetry (2D-2C PIV) was used to measure the separated shear layer, recirculation region and reattachment area downstream of the BFS in a single field of view. Besides, time-resolved tomographic particle image velocimetry (TR-Tomo-PIV) was also employed to measure the flow flied of the turbulent shear layer downstream of the BFS within a three-dimensional volume of 50 × 50 × 10 mm³ at a sampling frequency of 1 kHz. The flow control result shows that time-averaged reattachment length downstream of the BFS is reduced by 29.1 % due to the application of the VGs. Meanwhile, the Reynolds shear stress downstream of the VGs is considerably increased. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) have been applied to the 3D velocity vector fields to analyze the complex vortex structures in the spatial and temporal approaches, respectively. A coherent bandwidth of Strouhal number 0.3 < St_h < 0.6 is found in the VG-induced vortices, and moreover, Λ-shaped three-dimensional vortex structures at St_h = 0.37 are revealed in the energy and dynamic approaches complementarily.     

Computational fluid dynamics simulations of respiratory airflow in human nasal cavity and its characteristic dimension study

To study the airflow distribution in human nasal cavity during respiration and the characteristic parameters of nasal structure, three-dimensional, anatomically accurate representations of 30 adult nasal cavity models were recons- tructed based on processed tomography images collected from normal people. The airflow fields in nasal cavities were simulated by fluid dynamics with finite element software ANSYS. The results showed that the difference of human nasal cavity structure led to different airflow distribution in the nasal cavities and variation of the main airstream passing through the common nasal meatus. The nasal resistance in the regions of nasal valve and nasal vestibule accounted for more than half of the overall resistance. The characteristic model of nasal cavity was extracted on the basis of characteristic points and dimensions deduced from the original models. It showed that either the geometric structure or the airflow field of the two kinds of models was similar. The characteristic dimensions were the characteristic parameters of nasal cavity that could properly represent the original model in model studies on nasal cavity.     

Effect of a portable ultra-clean exponential airflow unit on the particle distribution in an operating room

The effects of a mobile laminar airflow unit on the concentration, deposition and distribution of bacteria-carrying particles in an operating room are investigated. The exploration is carried out using numerical calculation schemes (computational fluid dynamics approach). The model validation was performed through result comparisons with published measurement data from literature. Two types of mobile screen units were evaluated as an extension of turbulent-mixing operating-room ventilation. Airborne particle concentration/sedimentation was recorded with and without a screen unit on the operating table and two instrument tables. Both active and passive air sampling were examined and the results are compared. It was found that the additional mobile ultra-clean laminar airflow unit reduces the counts of airborne bacteria and surface contamination to a level acceptable for infection-prone surgeries.     

A direct‐forcing pressure‐based lattice Boltzmann method for solving fluid–particle interaction problems

A direct‐forcing immersed boundary‐lattice Boltzmann method (IB–LBM) is developed to simulate fluid–particle interaction problems. This method uses the pressure‐based LBM to solve the incompressible flow field and the immersed boundary method to handle the fluid–particle interactions. The pressure‐based LBM uses the pressure distribution functions instead of the density distribution functions as the independent dynamic variables. The main idea is to explicitly eliminate the compressible effect due to the density fluctuation. In the IB method, a direct‐forcing method is introduced to capture the particle motion. It directly computes an IB force density at each lattice grid from the differences between the pressure distribution functions obtained by the LBM and the equilibrium pressure distribution functions computed from the particle velocity. By applying this direct‐forcing method, the IB–LBM becomes a purely LBM version. Also, by applying the Gauss theorem, the formulas for computing the force and the torque acting on the particle from the flows are derived from the volume integrals over the particle volume instead of from the surface integrals over the particle surface. The order of accuracy of the IB–LBM is demonstrated on the errors of velocity field, wall stress, and gradients of velocity and pressure. As a demonstration of the efficiency and capabilities of the new method, sedimentation of a large number of spherical particles in an enclosure is simulated. Copyright © 2010 John Wiley & Sons, Ltd.     

Techniques of PIV in stratified two-phase headers

The stratification of two fluid phases, namely gas and liquid, within flow distribution devices, such as headers, that have side or bottom oriented fluid pipe connections, or discharges, has shown relevance to loss-of-coolant accidents in nuclear power plants. Under critical conditions the gas phase could entrain into the predominantly liquid discharge flow causing the fluid quality to be dramatically affected. This condition is referred to as the onset of gas entrainment (OGE) phenomenon and it occurs at a specific critical liquid height which changes with the Froude number. The liquid velocity field at the OGE is of importance, for example, to theorists who may find a semi-empirical approach to model this phenomenon. Stereoscopic particle image velocimetry (PIV) technique is an excellent candidate for non-intrusively investigating the velocity field. The liquid-phase velocity field was investigated for three discharge Froude numbers at the OGE. It was found that the stereoscopic PIV could be used to extract the velocity field experimentally, yet a high degree of error was found in the region closest to the discharge. The relative error was determined through conservation of mass by comparing the flow rate obtained with the PIV data to that obtained using a flow meter. In summary it was found that the number of image planes used, the resolution of the image planes, and consequently the number of vectors used to calculate the flow rate, all contributed a great deal to the relative error.