Probability density functions in turbulent channel flow

The probability density functions (pdf's) of the fluctuating velocity components, as well as their first and second derivatives, are calculated using data from the direct numerical simulations (DNS) of fully developed turbulent channel flow. It is observed that, beyond the buffer region, the pdf of each of these quantities is independent of the distance from the channel wall. It is further observed that, beyond the buffer region, the pdf's for all the first derivatives collapse onto a single universal curve and those of the second derivatives also collapse onto another universal curve, irrespective of the distance from the wall. The kinetic-energy dissipation rate exhibits log normal behavior

Improved Carrier Blocking Properties of Interface in Cryogenic Particle Detectors

Sensitivity of cryogenic particle detectors suffers from various loss and leakage mechanisms which influence carrier transport in the bulk and interface layers of the detectors. Suppressing- and wherever possible, eliminating- such loss mechanisms is imperative to lowering the noise floor of detectors to enable detection of characteristically weak energy signatures of exotic particle interactions. This work investigates one such loss mechanism– tunneling driven carrier leakage through the interface stack in particle detectors– and focuses on remodeling the stack composition and associated fabrication processes to mitigate such leakage. Measures to improve carrier blocking properties in the interface are explored with an aim to lower the steady state leakage and thereby improve detector sensitivity. This study aims at identifying and implementing measures to combat carrier tunneling through the interface. As part of such efforts, novel distributions of 40nm interface-thickness budget of SiOv2 and poly-crystalline Silicon have been tested for their capabilities of suppressing tunneling mechanisms. This comes as a modification to the previously proposed 20nm+20nm configuration of SiO2+pc-Si which, while being a significant improvement over the traditionally used amorphous-Si interface, has still been shown to be inadequate in blocking carriers. Interface processing modifications aimed at suppressing trap-mediated tunneling mechanisms by way of annealing the devices following SiO2 deposition have been explored. Rapid thermal processing at 600 degrees C has been found to decrease the leakage by over an order of magnitude at 22K, which while being promising, still leaves room for improvement. CO2 laser annealing has been identified as an option for selectively annealing only the SiO2 film at high temperatures (~1500 degrees C) thereby leaving the substrate purity uncompromised. Laser annealing has been performed at different power levels and scan speeds to identify the highest usable temperature that does not result in pinhole defects in the film. Room temperature and cryogenic characterization has been performed on these devices to evaluate their carrier blocking properties. At 20K, laser annealed devices have been found to exhibit leakage three orders of magnitude lower than as deposited samples and two orders lower than rapid thermal processed devices

Testing of transition-region models: Test cases and data

Mean flow quantities in the laminar turbulent transition region and in the fully turbulent region are predicted with different models incorporated into a 3-D boundary layer code. The predicted quantities are compared with experimental data for a large number of different flows and the suitability of the models for each flow is evaluated

Improved Carrier Blocking Properties of Interface in Cryogenic Particle Detectors

Sensitivity of cryogenic particle detectors suffers from various loss and leakage mechanisms which influence carrier transport in the bulk and interface layers of the detectors. Suppressing- and wherever possible, eliminating- such loss mechanisms is imperative to lowering the noise floor of detectors to enable detection of characteristically weak energy signatures of exotic particle interactions. This work investigates one such loss mechanism– tunneling driven carrier leakage through the interface stack in particle detectors– and focuses on remodeling the stack composition and associated fabrication processes to mitigate such leakage. Measures to improve carrier blocking properties in the interface are explored with an aim to lower the steady state leakage and thereby improve detector sensitivity. This study aims at identifying and implementing measures to combat carrier tunneling through the interface. As part of such efforts, novel distributions of 40nm interface-thickness budget of SiOv2 and poly-crystalline Silicon have been tested for their capabilities of suppressing tunneling mechanisms. This comes as a modification to the previously proposed 20nm+20nm configuration of SiO2+pc-Si which, while being a significant improvement over the traditionally used amorphous-Si interface, has still been shown to be inadequate in blocking carriers. Interface processing modifications aimed at suppressing trap-mediated tunneling mechanisms by way of annealing the devices following SiO2 deposition have been explored. Rapid thermal processing at 600 degrees C has been found to decrease the leakage by over an order of magnitude at 22K, which while being promising, still leaves room for improvement. CO2 laser annealing has been identified as an option for selectively annealing only the SiO2 film at high temperatures (~1500 degrees C) thereby leaving the substrate purity uncompromised. Laser annealing has been performed at different power levels and scan speeds to identify the highest usable temperature that does not result in pinhole defects in the film. Room temperature and cryogenic characterization has been performed on these devices to evaluate their carrier blocking properties. At 20K, laser annealed devices have been found to exhibit leakage three orders of magnitude lower than as deposited samples and two orders lower than rapid thermal processed devices

A streakline method for computing two-dimensional vortical flows

A forward time-stepping method is developed for computing free-streamline flows past two-dimensional bodies. The field vorticity is represented by vortex sheets. The kinematic boundary condition of no flow across the field vortex sheets is satisfied temporally by shifting the sheets normal to themselves as dictated by the local field velocities. The Helmholtz vorticity equation is reformulated as a dynamic boundary condition to be applied along the field vortex sheets. It is demonstrated that for a specified separation point, the solution evolves toward the Kirchhoff steady free-streamline flow. It is further demonstrated that in shedding vortex sheets from both corners of a polygonal contour with two corners, the method picks the forward corner as the separation point. For a circular polygonal contour with eleven corner points, five of which are allowed to shed vortex sheets, the method picks the corner point at 54 degrees from the forward stagnation point as the separation point. It is observed that this is close to 55.04 degrees, which is the forwardmost separation point allowed by the classical free-streamline theory for the circle.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30997/1/0000672.pd

Resilient dynamic state estimation for multi-machine power system with partial missing measurements

Accurate tracking the dynamics of power system plays a significant role in its reliability, resilience and security. To achieve the reliable and precise estimation results, many advanced estimation methods have been developed. However, most of them are aiming at filtering the measurement noise, while the adverse affect of partial measurement missing is rarely taken into account. To deal with this issue, a discrete distribution in the interval [0,1] is introduced to depict mechanism of partial measurement data loss that caused by the sensor failure. Then, a resilient fault tolerant extended Kalman filter (FTEKF) is designed in the recursive filter framework. Eventually, extensive simulations are carried on the different scale test systems. Numerical experimental results illustrate that the resilience and robustness of the proposed fault tolerant EKF method against partial measurement data loss

Resilient dynamic state estimation for power system using Cauchy-kernel-based maximum correntropy cubature Kalman filter

Accurate estimation of dynamic states is the key to monitoring power system operating conditions and controlling transient stability. The inevitable non-Gaussian noise and randomly occurring denial-of-service (DoS) attacks may, however, deteriorate the performance of standard filters seriously. To deal with these issues, a novel resilient cubature Kalman filter based on the Cauchy kernel maximum correntropy (CKMC) optimal criterion approach (termed CKMC-CKF) is developed, in which the Cauchy kernel function is used to describe the distance between vectors. Specifically, the errors of state and measurement in the cost function are unified by a statistical linearization technique, and the optimal estimated state is acquired by the fixed-point iteration method. Because of the salient thick-tailed feature and the insensitivity to the kernel bandwidth (KB) of Cauchy kernel function, the proposed CKMC-CKF can effectively mitigate the adverse effect of non-Gaussian noise and DoS attacks with better numerical stability. Finally, the efficacy of the proposed method is demonstrated on the standard IEEE 39-bus system under various abnormal conditions. Compared with standard cubature Kalman filter (CKF) and maximum correntropy criterion CKF (MCC-CKF), the proposed algorithm reveals better estimation accuracy and stronger resilience

Evaluation of a Hybrid Boltzmann-Continuum Method for High Speed Nonequilibrium Flows

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83556/1/AIAA-2010-1569-683.pd