Multi-Objective Predictive Taxi Dispatch via Network Flow Optimization

In this paper, we discuss a large-scale fleet management problem in a multi-objective setting. We aim to seek a receding horizon taxi dispatch solution that serves as many ride requests as possible while minimizing the cost of relocating vehicles. To obtain the desired solution, we first convert the multi-objective taxi dispatch problem into a network flow problem, which can be solved using the classical minimum cost maximum flow (MCMF) algorithm. We show that a solution obtained using the MCMF algorithm is integer-valued; thus, it does not require any additional rounding procedure that may introduce undesirable numerical errors. Furthermore, we prove the time-greedy property of the proposed solution, which justifies the use of receding horizon optimization. For computational efficiency, we propose a linear programming method to obtain an optimal solution in near real time. The results of our simulation studies using real-world data for the metropolitan area of Seoul, South Korea indicate that the performance of the proposed predictive method is almost as good as that of the oracle that foresees the future.Comment: 28 pages, 12 figures, Published in IEEE Acces

Scaling of Turbulence and Microphysics in a Convection–Cloud Chamber of Varying Height

The convection–cloud chamber enables measurement of aerosol and cloud microphysics, as well as their interactions, within a turbulent environment under steady-state conditions. Increasing the size of a convection–cloud chamber, while holding the imposed temperature difference constant, leads to increased Rayleigh, Reynolds and Nusselt numbers. Large–eddy simulation coupled with a bin microphysics model allows the influence of increased velocity, time, and spatial scales on cloud microphysical properties to be explored. Simulations of a convection–cloud chamber, with fixed aspect ratio and increasing heights of H = 1, 2, 4, and (for dry conditions only) 8 m are performed. The key findings are: Velocity fluctuations scale as H1/3, consistent with the Deardorff expression for convective velocity, and implying that the turbulence correlation time scales as H2/3. Temperature and other scalar fluctuations scale as H−3/7. Droplet size distributions from chambers of different sizes can be matched by adjusting the total aerosol injection rate as the horizontal cross-sectional area (i.e., as H2 for constant aspect ratio). Injection of aerosols at a point versus distributed throughout the volume makes no difference for polluted conditions, but can lead to cloud droplet size distribution broadening in clean conditions. Cloud droplet growth by collision and coalescence leads to a broader right tail of the distribution compared to condensation growth alone, and this tail increases in magnitude and extent monotonically as the increase of chamber height. These results also have implications for scaling within turbulent, cloudy mixed-layers in the atmosphere, such as fog layers

Ultrafast dynamics of fractional particles in α\alpha-RuCl3_3

In a Kitaev spin liquid, electron spins can break into fractional particles known as Majorana fermions and Z$_2$ fluxes. Recent experiments have indicated the existence of such fractional particles in a two-dimensional Kitaev material candidate, $\alpha$-RuCl$_3$. These exotic particles can be used in topological quantum computations when braided within their lifetimes. However, the lifetimes of these particles, critical for applications in topological quantum computing, have not been reported. Here we study ultrafast dynamics of photoinduced excitations in single crystals of $\alpha$-RuCl$_3$ using pump-probe transient grating spectroscopy. We observe intriguing photoexcited nonequilibrium states in the Kitaev paramagnetic regime between $T_N$~7 K and $T_H$~100 K, where $T_N$ is the N\'eel temperature and $T_H$ is set by the Kitaev interaction. Two distinct lifetimes are detected: a longer lifetime of ~50 ps, independent of temperature; a shorter lifetime of 1-20 ps, with a strong temperature dependence, $T^{-1.40}$. We analyze the transient grating signals using coupled differential equations and propose that the long and short lifetimes are associated with fractional particles in the Kitaev paramagnetic regime, Z$_2$ fluxes and Majorana fermions, respectively

Fast and slow microphysics regimes in a minimalist model of cloudy Rayleigh-Bénard convection

A minimalist model of microphysical properties in cloudy Rayleigh-Bénard convection is developed based on mass and number balances for cloud droplets growing by vapor condensation. The model is relevant to a turbulent mixed-layer in which a steady forcing of supersaturation can be defined, e.g., a model of the cloudy boundary layer or a convection-cloud chamber. The model assumes steady injection of aerosol particles that are activated to form cloud droplets, and the removal of cloud droplets through sedimentation. Simplifying assumptions include the consideration of mean properties in steady state, neglect of coalescence growth, and no detailed representation of the droplet size distribution. Closed-form expressions for cloud droplet radius, number concentration, and liquid water content are derived. Limits of fast and slow microphysics, compared to the turbulent mixing time scale, are explored, and resulting expressions for the scaling of microphysical properties in fast and slow regimes are obtained. Scaling of microphysics with layer thickness is also explored, suggesting that liquid water content and cloud droplet number concentration increase, and mean droplet radius decreases with increasing layer thickness. Finally, the analytical model is shown to compare favorably to solutions of the fully-coupled set of governing ordinary differential equations that describe the system, and the predicted power law for liquid water mixing ratio versus droplet activation rate is observed to be consistent with measurements from the Pi convection-cloud chamber

Large-Eddy Simulations of a Convection Cloud Chamber: Sensitivity to Bin Microphysics and Advection

Bin microphysics schemes are useful tools for cloud simulations and are often considered to provide a benchmark for model intercomparison. However, they may experience issues with numerical diffusion, which are not well quantified, and the transport of hydrometeors depends on the choice of advection scheme, which can also change cloud simulation results. Here, an atmospheric large-eddy simulation model is adapted to simulate a statistically steady-state cloud in a convection cloud chamber under well-constrained conditions. Two bin microphysics schemes, a spectral bin method and the method of moments, as well as several advection methods for the transport of the microphysical variables are employed for model intercomparison. Results show that different combinations of microphysics and advection schemes can lead to considerable differences in simulated cloud properties, such as cloud droplet number concentration. We find that simulations using the advection scheme that suffers more from numerical diffusion tends to have a smaller droplet number concentration and liquid water content, while simulation with the microphysics scheme that suffers more from numerical diffusion tends to have a broader size distribution and thus larger mean droplet sizes. Sensitivities of simulations to bin resolution, spatial resolution, and temporal resolution are also tested. We find that refining the microphysical bin resolution leads to a broader cloud droplet size distribution due to the advection of hydrometeors. Our results provide insight for using different advection and microphysics schemes in cloud chamber simulations, which might also help understand the uncertainties of the schemes used in atmospheric cloud simulations

Scaling of an atmospheric model to simulate turbulence and cloud microphysics in the Pi Chamber

The Pi Cloud Chamber offers a unique opportunity to study aerosol-cloud microphysics interactions in a steady-state, turbulent environment. In this work, an atmospheric large-eddy simulation (LES) model with spectral bin microphysics is scaled down to simulate these interactions, allowing comparison with experimental results. A simple scalar flux budget model is developed and used to explore the effect of sidewalls on the bulk mixing temperature, water vapor mixing ratio, and supersaturation. The scaled simulation and the simple scalar flux budget model produce comparable bulk mixing scalar values. The LES dynamics results are compared with particle image velocimetry measurements of turbulent kinetic energy, energy dissipation rates, and large-scale oscillation frequencies from the cloud chamber. These simulated results match quantitatively to experimental results. Finally, with the bin microphysics included the LES is able to simulate steady-state cloud conditions and broadening of the cloud droplet size distributions with decreasing droplet number concentration, as observed in the experiments. The results further suggest that collision-coalescence does not contribute significantly to this broadening. This opens a path for further detailed intercomparison of laboratory and simulation results for model validation and exploration of specific physical processes

A Microchip For Studying the Effects of Dopamine and its Precursor On Neurospheroids

This paper reports a MEMS chip for studying the effects of the neurotransmitter dopamine (DA) and its precursor L-DOPA on adult rat hippocampal progenitor cell (AHPC) neurospheroids for the first time. This chip allows DA or L-DOPA in one chamber to diffuse to AHPC neurospheroids, a promising in vitro brain model, cultured in an adjacent chamber through an integrated diffusion barrier, used to mimic the blood-brain barrier (BBB) by an array of intentionally misaligned micropillars. After cell fixation and immunostaining were conducted, the fluorescence images of AHPC neurospheroids were analyzed. AHPC neurospheroids cultured in these devices remained highly viable following DA or L-DOPA treatment. Cell proliferation and neuronal differentiation have also been shown following DA or L-DOPA treatment, indicating the AHPC neurospheroids as a valuable in vitro brain model for neurogenesis research.This is a manuscript of a proceeding published as S. Mao et al., "A Microchip For Studying the Effects of Dopamine and its Precursor On Neurospheroids," 2022 IEEE 35th International Conference on Micro Electro Mechanical Systems Conference (MEMS), Tokyo, Japan, 2022, pp. 305-308, doi: 10.1109/MEMS51670.2022.9699585.© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works