Session W10: General Fluid Dynamics (10:00am - 10:45am CST)

Why does water shoot higher if we partially block the garden hose outlet?

It is a common experience that water shoots higher when we block a garden hose outlet by our thumb. But what causes this? How high does the water go? Does water from our neighbor's garden hose reach the same height? Is there an optimum outlet blockage that results in the highest height that water can reach? Here, we show that a competition between viscous friction along the hose and the viscous dissipation at the thumb-generated constriction results in a variable water shooting height. Through systematic analysis we demonstrate that depending on the municipal water main pressure, and length and diameter of the hose, the maximum water height may increase, decrease, or gain an optimum as the blockage ratio of the outlet varies.   

Image-based flow decomposition using empirical wavelet transform

We propose an image-based flow decomposition developed from the 2D tensor empirical wavelet transform (EWT). The idea is to decompose the instantaneous flow data, or its visualisation, adaptively according to the averaged Fourier supports for the identification of spatially localised structures. The resulting EWT modes stand for the decomposed flows, and each accounts for part of the spectrum, illustrating fluid physics with different scales superimposed in the original flow. With the proposed method, decomposition of an instantaneous 3D flow becomes feasible without resorting to its time series. In contrast to proper orthogonal decomposition or dynamic modal decomposition that extract spatial modes according to energy or frequency, EWT provides a new strategy as to decompose an instantaneous flow from its spatial scales.   

Fluid-Guided Chemical Vapor Deposition Growth for Large-Scale Monolayer Two-Dimensional Materials

Atmospheric pressure chemical vapor deposition (APCVD) has been used extensively for synthesizing two-dimensional (2D) materials because of its low cost and promise for high-quality monolayer crystal synthesis. However, the understanding of the reaction mechanism and the key parameters affecting the APCVD processes is still in its embryonic stage. Hence, the scalability of the APCVD method in achieving large-scale continuous film remains very poor. Here, we use MoSe2 as a model system and present a fluid guided growth strategy for understanding and controlling the growth of 2D materials. Through the integration of experiment and computational fluid dynamics (CFD) analysis in the full-reactor scale, we identified three key parameters, precursor mixing, fluid velocity, and shear stress, which play a critical role in the APCVD process. By modifying the geometry of the growth setup to enhance precursor mixing and decrease nearby velocity shear rate and adjusting flow direction, we have successfully obtained inch-scale monolayer MoSe2. This unprecedented success of achieving scalable 2D materials through fluidic design lays the foundation for designing new CVD systems to achieve the scalable synthesis of nanomaterials.   

Simulation-based study of airborne transmission of COVID-19 in two practical settings

COVID-19 has shown a high potential of transmission via virus-carrying aerosols as supported by growing evidence. However, detailed investigations that draw direct links between aerosol transport and virus infection are still lacking. There is a dire need for quantitative assessment of the risks of such airborne infection, which can significantly reduce the uncertainties and inconsistencies in our current preventive measures. To fill in the gap, we conducted large-eddy simulations (LES) of indoor airflow and the associated aerosol transport in two practical settings, where likely cases of airborne infection caused by asymptomatic individuals were reported and the detailed information of infection process through contact tracing/surface viral samplings are available. Our simulation resolves turbulence eddy transport, models subgrid-scale effects, and incorporates thermal effect to enable a more accurate assessment of spatiotemporal variation in aerosol exposure and surface contamination due to aerosol deposition. Our results show LES predicts the hot spots of aerosol exposure and surface contamination that coincide with the reported locations of infected individuals and viral positivity, providing strong support and in-depth insights to airborne transmission pathway of COVID-19.   

Surface morphology and flow dynamics in biomimetic wind-driven fog harvesting

The physical and behavioral adaptations evolved through eons by plants and animals can provide practical solutions to enhance the efficacy of the current technologies. Using fog as a source of fresh water has been observed in a few species of the Namib desert beetles. One of these species, O.unguicularis, climbs to the top of the sand dunes on foggy days and uses its own body to collide with the inertial drops carried by the wind. The influence of the surface wettability on transportation of the already deposited drops has been extensively studied. However, the interplay between surface morphology and fluid dynamics that can enhance impaction of the fog drops is not fully understood. Careful experiments complimented by numerical flow simulations show that surface morphology governs deposition efficiency and inherent effect of lubrication forces can be manipulated by surface texture and mechanics. Altering drops motion and inducing collisions with minimal geometrical modifications can offer design guidelines for enhanced fog collectors and can be a possible driver of the physical adaptation of O.unguicularis.   

Risk Assessment of Airborne Disease Transmission during Wind Instrument Plays

The potential airborne transmission of COVID-19 has raised significant concerns regarding the safety of wind instrument play. Such problem involves multiple flow process such as aerosol generation from different breathing techniques and aerosol transport into, within, and outside the instrument. By collaborating with 15 musicians from Minnesota Orchestra, we provide the first systematic examination of aerosol generation and transport from 10 types of wind instruments under different dynamic levels and articulation patterns. We conduct the in situ measurements of flow field and aerosol generation during instrument play at the orchestra. We find the aerosol concentration of different instruments exhibits two orders of magnitude variation. The dependence of aerosol production upon dynamic level and articulation pattern varies for different instruments. We obtain the flow field and spatial variation of aerosol concentration associated with different instrument plays. Our results suggest aerosol generation from wind instrument is influenced by a combination of breathing techniques, instrument inlet design, and the tube structure. Based on our findings, we provide suggestions on orchestra seating arrangement to mitigate the risk of airborne disease transmission.   

Numerical investigation of the role of air ventilation rate in reducing healthcare worker exposure to infectious aerosols in a hospital isolation room

Aerosol dispersion from a coughing patient in a hospital isolation room is investigated via Reynolds-averaged Navier-Stokes simulation (RANSS). Dispersion in the room is caused by the air circulation driven by the room's ventilation. Healthcare worker (HCW) exposure to the aerosol contaminant will be assessed through residence time analysis of the imperfect mixing induced by inflow-outflow channeling (i.e. flow channeling between the supply and exhaust air vents) and dead zone regions in the room. It is observed that dead zones can trap aerosols over periods of time comparable to or greater than the theoretical residence time of the room (approximately 10 mins. for typical isolation rooms). Furthermore, increasing the ACH leads to more efficient mixing by weakening dead zones resulting in lower amounts of trapped contaminant. HCW exposure to aerosol contaminant will be quantified as a function of ACH. Finally, the RANSS results will be used to calibrate a less computationally intensive zonal compartment model able to predict average aerosol concentration throughout the room as well as HCW exposure to the infectious aerosols.