Modeling of Fluid-Solid Two-Phase Geophysical Flows

Fluid-solid two-phase flows are frequently encountered in geophysical flow problems such as sediment transport and submarine landslides. It is still a challenge to the current experiment techniques to provide information such as detailed flow and pressure fields of each phase, which however is easily obtainable through numerical simulations using fluid-solid two-phase flow models. This chapter focuses on the Eulerian-Eulerian approach to two-phase geophysical flows. Brief derivations of the governing equations and some closure models are provided, and the numerical implementation in the finite-volume framework of OpenFOAM® is described. Two applications in sediment transport and submarine landslides are also included at the end of the chapter

Optimal Equilibria for Multi-dimensional Time-inconsistent Stopping Problems

We study an optimal stopping problem under non-exponential discounting, where the state process is a multi-dimensional continuous strong Markov process. The discount function is taken to be log sub-additive, capturing decreasing impatience in behavioral economics. On strength of probabilistic potential theory, we establish the existence of an optimal equilibrium among a sufficiently large collection of equilibria, consisting of finely closed equilibria satisfying a boundary condition. This generalizes the existence of optimal equilibria for one-dimensional stopping problems in prior literature

NMR studies of the relationship between the changes of membrane lipids and the cisplatin-resistance of A549/DDP cells

Changes of membrane lipids in cisplatin-sensitive A549 and cisplatin-resistant A549/DDP cells during the apoptotic process induced by a clinical dose of cisplatin (30 μM) were detected by (1)H and (31)P-NMR spectroscopy and by membrane fluidity measurement. The apoptotic phenotypes of the two cell lines were monitored with flow cytometry. The assays of apoptosis showed that significant apoptotic characteristics of the A549 cells were induced when the cells were cultured for 24 hours after treatment with cisplatin, while no apoptotic characteristic could be detected for the resistant A549/DDP cells even after 48 hours. The results of (1)H-NMR spectroscopy demonstrated that the CH(2)/CH(3 )and Glu/Ct ratios of the membrane of A549 cells increased significantly, but those in A549/DDP cell membranes decreased. In addition, the Chol/CH(3 )and Eth/Ct ratios decreased for the former but increased for the latter cells under the same conditions. (31)P-NMR spectroscopy indicated levels of phosphomonoesters (PME) and ATP decreased in A549 but increased in A549/DDP cells after being treated with cisplatin. These results were supported with the data obtained from (1)H-NMR measurements. The results clearly indicated that components and properties of membrane phospholipids of the two cell lines were significantly different during the apoptotic process when they were treated with a clinical dose of cisplatin. Plasma membrane fluidity changes during cisplatin treatment as detected with the fluorescence probe TMA-DPH also indicate marked difference between the two cell lines. We provided evidence that there are significant differences in plasma membrane changes during treatment of cisplatin sensitive A549 and resistant A549/DDP cells

Automatic Curriculum Learning With Over-repetition Penalty for Dialogue Policy Learning

Dialogue policy learning based on reinforcement learning is difficult to be applied to real users to train dialogue agents from scratch because of the high cost. User simulators, which choose random user goals for the dialogue agent to train on, have been considered as an affordable substitute for real users. However, this random sampling method ignores the law of human learning, making the learned dialogue policy inefficient and unstable. We propose a novel framework, Automatic Curriculum Learning-based Deep Q-Network (ACL-DQN), which replaces the traditional random sampling method with a teacher policy model to realize the dialogue policy for automatic curriculum learning. The teacher model arranges a meaningful ordered curriculum and automatically adjusts it by monitoring the learning progress of the dialogue agent and the over-repetition penalty without any requirement of prior knowledge. The learning progress of the dialogue agent reflects the relationship between the dialogue agent's ability and the sampled goals' difficulty for sample efficiency. The over-repetition penalty guarantees the sampled diversity. Experiments show that the ACL-DQN significantly improves the effectiveness and stability of dialogue tasks with a statistically significant margin. Furthermore, the framework can be further improved by equipping with different curriculum schedules, which demonstrates that the framework has strong generalizability

Convergence of Policy Improvement for Entropy-Regularized Stochastic Control Problems

For a general entropy-regularized stochastic control problem on an infinite horizon, we prove that a policy improvement algorithm (PIA) converges to an optimal relaxed control. Contrary to the standard stochastic control literature, classical H\"{o}lder estimates of value functions do not ensure the convergence of the PIA, due to the added entropy-regularizing term. To circumvent this, we carry out a delicate estimation by moving back and forth between appropriate H\"{o}lder and Sobolev spaces. This requires new Sobolev estimates designed specifically for the purpose of policy improvement and a nontrivial technique to contain the entropy growth. Ultimately, we obtain a uniform H\"{o}lder bound for the sequence of value functions generated by the PIA, thereby achieving the desired convergence result. Characterization of the optimal value function as the unique solution to an exploratory Hamilton-Jacobi-Bellman equation comes as a by-product

Wave-current interaction in water of finite depth

Thesis (Ph. D. in Environmental Fluid Mechanics)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2004.Includes bibliographical references (p. 263-269).In this thesis, the nonlinear interaction of waves and current in water of finite depth is studied. Wind is not included. In the first part, a 2D theory for the wave effect on a turbulent current over rough or smooth bottom is presented. The logarithmic profile of the basic current is modified by the waves due to an effective mean shear stress on the free surface. Surface distortion of the eddy viscosity is shown to be important for the change of the mean velocity profile by waves. Both wave-following and wave-opposing current are studied. Comparisons are made with some existing laboratory experiments. In the second part, an instability theory is presented for the initiation of Langmuir cells due to waves interacting with a turbulent current maintained by tides or by an external pressure gradient. With an infinitesimal span-wise disturbance, the free surface experiences a new mean stress, which generates new vorticity to be diffused downward, and induces further growth. Various contributions to the unstable growth of Langmuir circulation are analyzed by examining the mechanical energy budget. Evidences will be shown that the surface stress contributes significantly to instability. Both wave-following current and wave-opposing current are studied. For the wave-following current, two types of Langmuir cells can grow in time; while for the wave-opposing current, only one can grow. Effects of current strength, wave conditions, and water depth on the growth of Langmuir circulation will be studied by numerical examples. Remarks on existing laboratory experiments are madeby Zhenhua Huang.Ph.D.in Environmental Fluid Mechanic