14,736 research outputs found
Conditional Markov chain and its application in economic time series analysis
Motivated by the great moderation in major U.S. macroeconomic time series, we formulate the regime switching problem through a conditional Markov chain. We model the long-run volatility change as a recurrent structure change, while short-run changes in the mean growth rate as regime switches. Both structure and regime are unobserved. The structure is assumed to be Markovian. Conditioning on the structure, the regime is also Markovian, whose transition matrix is structure-dependent. This formulation imposes interpretable restrictions on the Hamilton Markov switching model. Empirical studies show that this restricted model well identifies both short-run regime switches and long-run structure changes in the U.S. macroeconomic data.Markov regime switching; Conditional Markov chain
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Developing an Adaptive Strategy for Connected Eco-Driving Under Uncertain Traffic and Signal Conditions
The Eco-Approach and Departure (EAD) application has been proved to be environmentally efficient for a Connected and Automated Vehicles (CAVs) system. In the real-world traffic, traffic conditions and signal timings are usually dynamic and uncertain due to mixed vehicle types, various driving behaviors and limited sensing range, which is challenging in EAD development. This research proposes an adaptive strategy for connected eco-driving towards a signalized intersection under real world conditions. Stochastic graph models are built to link the vehicle and external (e.g., traffic, signal) data and dynamic programing is applied to identify the optimal speed for each vehicle-state efficiently. From energy perspective, adaptive strategy using traffic data could double the effective sensor range in eco-driving. A hybrid reinforcement learning framework is also developed for EAD in mixed traffic condition using both short-term benefit and long-term benefit as the action reward. Micro-simulation is conducted in Unity to validate the method, showing over 20% energy saving.View the NCST Project Webpag
Simple Formula for Marcus-Hush-Chidsey Kinetics
The Marcus-Hush-Chidsey (MHC) model is well known in electro-analytical
chemistry as a successful microscopic theory of outer-sphere electron transfer
at metal electrodes, but it is unfamiliar and rarely used in electrochemical
engineering. One reason may be the difficulty of evaluating the MHC reaction
rate, which is defined as an improper integral of the Marcus rate over the
Fermi distribution of electron energies. Here, we report a simple analytical
approximation of the MHC integral that interpolates between exact asymptotic
limits for large overpotentials, as well as for large or small reorganization
energies, and exhibits less than 5\% relative error for all reasonable
parameter values. This result enables the MHC model to be considered as a
practical alternative to the ubiquitous Butler-Volmer equation for improved
understanding and engineering of electrochemical systems
Over-limiting Current and Control of Dendritic Growth by Surface Conduction in Nanopores
Understanding over-limiting current (faster than diffusion) is a
long-standing challenge in electrochemistry with applications in desalination
and energy storage. Known mechanisms involve either chemical or hydrodynamic
instabilities in unconfined electrolytes. Here, it is shown that over-limiting
current can be sustained by surface conduction in nano pores, without any such
instabilities, and used to control dendritic growth during electrodeposition.
Copper electrode posits are grown in anodized aluminum oxide membranes with
polyelectrolyte coatings to modify the surface charge. At low currents, uniform
electroplating occurs, unaffected by surface modification due to thin electric
double layers, but the morphology changes dramatically above the limiting
current. With negative surface charge, growth is enhanced along the nanopore
surfaces, forming surface dendrites and nanotubes behind a deionization shock.
With positive surface charge, dendrites avoid the surfaces and are either
guided along the nanopore centers or blocked from penetrating the membrane
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