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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Article
  • Peer Reviewed
UC Berkeley Previously Published Works (2022)
The transport of droplets expelled when coughing is of critical importance for understanding and preventing airborne disease transmission. However, the cough flow structure is complex, and numerous simplifications are often made to the initial flow condition in laboratory and numerical studies. We aim to challenge some of these assumptions, through particle transport experiments from a highly repeatable cough generator with a realistic oral cavity. In the present study the simultaneous transport and deposition of 22–27 μm, 45–53 μm, and 180–212 μm particles was investigated. Hot-wire measurements of the flow field showed the formation of two parallel jets on either side of the tongue, which produced a free shear layer downstream. Quantitative deposition measurements show that particle deposition patterns are strongly dependent on the formed shear layer for the smaller particles but not the largest size. The effect of circulation was characterized using a modified Stokes number of the form given by Davila and Hunt (2001). The scaling analysis and quantitative data both support the conclusion that while the 180–212 μm particles are not strongly affected, the transport and deposition of the smaller particles are influenced by orifice geometry. These results demonstrate that particle transport from a cough, for which most released droplets are below 22 μm, is a strong function of coherent flow structures present in the 3D flow field.
    • Thesis
    • Peer Reviewed
    UC Berkeley Electronic Theses and Dissertations (2022)

    The COVID-19 pandemic heightened the interest in particle-laden turbulent jets generated by breathing, talking, coughing, and sneezing, and how these can contribute to disease transmission. Predicting the transport distance of the expelled droplets remains a critical, and still open, question. In the first segment of my work, I study the effect of ejection geometry and environmental boundary conditions on the transport of water density-matched particles. The study of particle transport from a canonical jet geometry illustrates the influence of secondary flows from the environmental boundary conditions (e.g. thermal gradients and wall effects) on particle dispersal. Subsequent experiments on the particle transport from a repeatable cough generator reveal that vortex structures created by the mouth and tongue structure have a large role in dispersal, due to the ejection of particles from regions of high vorticity.

    In the second segment of my work, I study bubbly flows, in which the dispersed and carrier fluid phases are reversed. This flipped density ratio causes bubbles to accumulate in regions of high vorticity. Bubble transport is studied in the context of flow over a cylinder, for which the induced vibration is an important design consideration in devices from flow meters to nuclear reactors. While past researchers have shown that introducing bubbles to the flow can decrease vibration amplitude while increasing shedding frequency, the nonlinear shift in shedding frequency with bubble size has not been explained. Through experimental and numerical study of size-dependent bubble transport, I provide insight into the mechanismcausing this change in shedding frequency.

    The study of both projects in tandem allows for the exploration of the effect of preferential accumulation/ejection on dispersed multiphase transport. In addition to complementary flow behaviour, the study of interchanged dispersed/carrier phases necessitates the development of highly specialized measurement techniques. I hope that the methods developed in this work see increased use in literature when developing validation datasets, as I look to challenge some of the common assumptions made in existing multiphase transport studies. A common theme through both projects is that typically second-order effects (e.g. spanwise velocity differences) have a significant influence on dispersed multiphase transport. As dispersed multiphase transport draws increased research interest, I hope that this work illustrates that (until proven otherwise) the collective understanding of the dominant mechanisms that exist for the single phase flow do not necessarily apply to multiphase transport.

      Cover page: Selected Topics in Dispersed Multiphase Flow Transport
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2019)
      Unmanned aerial-underwater vehicles (UAUVs) provide the potential for working on missions in complex multi-domain environments. To achieve amphibian mobility, current UAUV designs rely on additional mechanical components such as multiple layers of propeller blades, water ballast, buoys or wings. This paper presents a miniature UAUV which has a simple mechanical design that resembles a traditional quadcopter. The paper discusses the dynamic modelling, state estimation and control strategy for this UAUV, as well as a detailed characterization of the quadcopter blades operating in the air and water regimes. A strategy for the UAUV to breach calm water surface is then proposed and experimentally tested. The results demonstrate that the UAUV can successfully breach the still water surface, but also show tracking error and breaching delay that are not fully characterized by the model. This suggests the need to carry out further analysis on the dynamics of the UAUV both underwater and in the transition regime.
        Cover page: Towards breaching a still water surface with a miniature unmanned aerial underwater vehicle
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2021)
        The SARS-CoV-2 pandemic has heightened the interest in particle-laden turbulent jets generated by breathing, talking, coughing and sneezing, and how these can contribute to disease transmission. We present quantitative measurement methods for such flows, while exploring and offering improvements for common shortcomings. We generate jets consisting of either liquid droplets or solid particles in an isothermal, quiescent and electrically isopotential experimental chamber that was constructed to control the effects of ambient forcing on jet behavior. For liquid droplets, we find promise in surface deposition analysis based on fluorescent tracer use. For particles, we explore the performance of commercially available adhesive sampling strips and develop conductive grounded carbon tape based sampling strips. We explore ways in which the smallest of thermal gradients or electrostatic charge issues can affect particle dispersion, and suggest practical methods to address these issues. The developed methods are applied to study the simultaneous deposition of (Formula presented.) 25, 50 and 200 μm solid particles from a particle laden turbulent jet with a mean velocity of 33.2 m/s. The deposition location as a function of particle size was compared to results from a simple numerical RANS model, and illustrates ways in which imprecise initial or boundary conditions can lead to a notable deviation from experimental results. The differences in deposition pattern seen in experimental and numerical results despite a carefully controlled environment and characterized particle ejection indicate the need for a more stringent numerical model validation, especially when studying fate and transport of mid-range (neither purely aerosol or ballistic) sized particles.
          Cover page: Droplet and particle methods to investigate turbulent particle laden jets
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2024)
          Vortex induced vibration (VIV) experienced during flow past a cylinder can reduce equipment performance and in some cases lead to failure. Previous studies have shown that the injection of bubbles in the flow over a cylinder typically leads to a monotonic increase in shedding frequency with void fraction, however, a satisfactory explanation for this phenomenon has not been proposed. Unexplained scatter in the data exists, including that the increase in shedding frequency is not universal. More research is needed to characterize the influence of bubbles on the wake structure, and subsequent shift in shedding frequency. To this aim, the effect of bubbles on the structure of the wake and VIV was examined over two values of Reynolds number, ReD=100,000 and 160,000. Time-resolved particle image velocimetry (TR-PIV), proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) of the wake structures, vibration of the cylinder, and bubble image velocimetry (BIV) were used to assess the flow topology changes under the influence of gas injection. Using SPOD/POD analysis in the near wake, it was found that the primary Karman shedding frequency decreased with the injection of gas, from a Strouhal number of St = 0.2 to St = 0.17−0.18; the width of the spectral peak was found to increase with void fraction. Notably, the vibration of the cylinder at the primary Karman shedding frequency was suppressed following the injection of gas, even at spanwise-averaged volumetric qualities below 0.01%. This suppression occurred regardless of if gas was concentrated locally near the centerline of the channel, or along the span. BIV data suggests that gas accumulation in the near wake, driven by the high velocity vertical motion of gas, serves to uncouple the cylinder motion from the formation of the vortex street downstream while promoting faster wake recovery.
            Creative Commons 'BY' version 4.0 license
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