The aerodynamic forces and moments on an axisymmetric body of revolution are controlled in a low-speed wind tunnel by induced local flow attachment. Control is effected by an array of aft-facing synthetic jets emanating from narrow, azimuthally segmented slots embedded within an axisymmetric backward facing step. The actuation results in a localized, segmented vectoring of the separated base flow along a rear Coanda surface and induced asymmetric aerodynamic forces and moments. The observed effects are investigated in both quasi-steady and transient states, with emphasis on parametric dependence. It is shown that the magnitude of the effected forces can be substantially increased by slight variations of the Coanda surface geometry. Force and velocity measurements are used to elucidate the mechanisms by which the synthetic jets produce asymmetric aerodynamic forces and moments, demonstrating a novel method to steer axisymmetric bodies during flight.

The objective of the research is to develop a cost-effective, dynamic grid controller called the controllable network transformer (CNT) that can be implemented by augmenting existing load tap changing (LTC) transformers with an AC-AC converter. The concept is based on using a fractionally rated direct AC-AC converter to control the power through an existing passive LTC. By using a modulation strategy based on virtual quadrature sources (VQS), it is possible to control both the magnitude and the phase angle of the output voltage of the CNT without having any inter-phase connections. The CNT architecture has many advantages over existing power flow controllers, like absence of low frequency storage, fractional converter rating, retro-fitting existing assets and independent per-phase operation making it potentially attractive for utility applications. The independent control of the magnitude and the phase angle of the output voltage allow independent real and reactive power flow control through the CNT-controlled line. In a meshed network with asymmetric network stresses this functionality can be used to redirect power from critically loaded assets to other relatively under-utilized parallel paths. The power flow controllability of CNT can thus be used to lower the overall cost of generation of power. The solid state switches in the CNT with fast response capability enable incorporation of various additional critical functionalities like grid fault ride through, bypassing internal faults and dynamic damping. This bouquet of features makes the CNT useful under both steady state and transient conditions without compromising the grid reliability.

Rapidly increasing transistor densities have led to the emergence of richly-integrated substrates in the form of chip multiprocessors and systems-on-a-chip. These devices integrate a variety of discrete resources, such as processing cores and cache memories, on a single die with the degree of integration growing in accordance with Moore's law. In this dissertation, we address challenges of scalability and quality-of-service (QOS) in network architectures of highly-integrated chips. The proposed techniques address the principal sources of inefficiency in networks-on-chip (NOCs) in the form of performance, area, and energy overheads. We also present a comprehensive network architecture capable of interconnecting over a thousand discrete resources with high efficiency and strong guarantees. We first show that mesh networks, commonly employed in existing chips, fall significantly short of achieving their performance potential due to transient congestion effects that diminish network performance. Adaptive routing has the potential to improve performance through better load distribution. However, we find that existing approaches are myopic in that they only consider local congestion indicators and fail to take global network state into account. Our approach, called Regional Congestion Awareness (RCA), improves network visibility in adaptive routers via a light-weight mechanism for propagating and integrating congestion information. By leveraging both local and non-local congestion indicators, RCA improves network load balance and boosts throughput. Under a set of parallel workloads running on a 49-node substrate, RCA reduces on-chip network latency by 16%, on average, compared to a locally-adaptive router. Next, we target NOC latency and energy efficiency through a novel point-to-multipoint topology. Ring and mesh networks, favored in existing on-chip interconnects, often require packets to go through a number of intermediate routers between source and destination nodes, resulting in significant latency and energy overheads. Topologies that improve connectivity, such as fat tree and flattened butterfly, eliminate much of the router overhead, but require non-minimal channel lengths or large channel count, reducing energy-efficiency and/or performance as a result. We propose a new topology, called Multidrop Express Channels (MECS), that augments minimally-routed express channels with multi-drop capability. The resulting richly-connected NOC enjoys a low hop count with favorable delay and energy characteristics, while improving wire utilization over prior proposals. Applications such as virtualized servers-on-a-chip and real-time systems require chip-level quality-of-service (QOS) support to provide fairness, service differentiation, and guarantees. Existing network QOS approaches suffer from considerable performance and area overheads that limit their usefulness in a resource-limited on-die network. In this dissertation, we propose a new QOS scheme called Preemptive Virtual Clock (PVC). PVC uses a preemptive approach to provide hard guarantees and strong performance isolation while dramatically reducing queuing requirements that burden prior proposals. Finally, we introduce a comprehensive network architecture that overcomes the bottlenecks of earlier designs with respect to area, energy, and QOS in future highly-integrated chips. The proposed NOC uses a topology-centric QOS approach that restricts the extent of hardware QOS support to a fraction of the network without compromising guarantees. In doing so, network area and energy efficiency are significantly improved. Further improvements are derived through a novel flow-control mechanism, along with switch- and link-level optimizations. In concert, these techniques yield a network capable of interconnecting over a thousand terminals on a die while consuming 47% less area and 26% less power than a state-of-the-art QOS-enabled NOC. The mechanisms proposed in this dissertation are synergistic and enable efficient, high-performance interconnects for future chips integrating hundreds or thousands of on-die resources. They address deficiencies in routing, topologies, and flow control of existing architectures with respect to area, energy, and performance scalability. They also serve as a building block for cost-effective advanced services, such as QOS guarantees at the die level. / text

Two active flow control methods are investigated numerically to understand the mechanism by which they control aerodynamics in the presence of severe flow separation on an airfoil. In particular, synthetic jets are applied to separated flows generated by additional surface feature (the actuators) near the trailing edge to obtain Coanda-like effects, and an impulse jet is used to control a stalled flow over an airfoil. A moving-grid scheme is developed, verified and validated to support simulations of external flow over moving bodies. Turbulent flow is modeled using detached eddy simulation (DES) turbulence models in the CFD code CDP (34) developed by Lopez (54). Synthetic jet actuation enhances turbulent mixing in flow separation regions, reduces the size of the separation, deflects stream lines closer to the surface and changes pressure distributions on the surface, all of which lead to bi-directional changes in the aerodynamic lift and moment. The external flow responds to actuation within about one convective time, which is significantly faster than for conventional control surfaces. Simulation of pitching airfoils shows that high-frequency synthetic jet affects the flow independently of the baseline frequencies associated with vortex shedding and airfoil dynamics. These unique features of synthetic jets are studied on a dynamically maneuvering airfoil with a closed-loop control system, which represents the response of the airfoil in wind-tunnel experiments and examines the controller for a rapidly maneuvering free-flight airfoil. An impulse jet, which is applied upstream of a nominal flow separation point, generates vortices that convect downstream, interact with the separating shear layer, dismantle the layer and allow following vortices to propagate along the surface in the separation region. These following vortices delay the separation point reattaching the boundary layer, which returns slowly to its initial stall condition, as observed in wind-tunnel experiments. A simple model of the impulse jet actuator used herein is found to be sufficient to represent the global effects of the jet on the stalled flow because it correctly represents the momentum injected into the flow. / text

As computer systems support more aspects of modern life, from finance to health care, security is becoming increasingly important. However, building secure systems remains a challenge. Software continues to have security vulnerabilities due to reasons ranging from programmer errors to inadequate programming tools. Because of these vulnerabilities we need mechanisms that protect sensitive data even when the software is untrusted. This dissertation shows that secure and practical frameworks can be built for protecting users' data from untrusted applications in both desktop and cloud computing environment. Laminar is a new framework that secures desktop applications by enforcing policies written as information flow rules. Information flow control, a form of mandatory access control, enables programmers to write powerful, end-to-end security guarantees while reducing the amount of trusted code. Current programming abstractions and implementations of this model either compromise end-to-end security guarantees or require substantial modifications to applications, thus deterring adoption. Laminar addresses these shortcomings by exporting a single set of abstractions to control information flows through operating system resources and heap-allocated objects. Programmers express security policies by labeling data and represent access restrictions on code using a new abstraction called a security region. The Laminar programming model eases incremental deployment, limits dynamic security checks, and supports multithreaded programs that can access heterogeneously labeled data. In large scale, distributed computations safeguarding information requires solutions beyond mandatory access control. An important challenge is to ensure that the computation, including its output, does not leak sensitive information about the inputs. For untrusted code, access control cannot guarantee that the output does not leak information. This dissertation proposes Airavat, a MapReduce-based system which augments mandatory access control with differential privacy to guarantee security and privacy for distributed computations. Data providers control the security policy for their sensitive data, including a mathematical bound on potential privacy violations. Users without security expertise can perform computations on the data; Airavat prevents information leakage beyond the data provider's policy. Our prototype implementation of Airavat demonstrates that several data mining tasks can be performed in a privacy preserving fashion with modest performance overheads. / text

Synthetic jet actuators for flow control applications have been an active topic of experimental research since the 90’s. Numerical simulations have become an important complement of that experimental work, providing detailed information of the dynamics of the controlled flow. This study is part of the AVOCET (Adaptive VOrticity Control Enabled flighT) project and is intended to provide computational support for the design and evaluation of closed-loop flow control with synthetic jet actuators for small scale Unmanned Aerial Vehicles (UAVs). The main objective is to analyze active flow control of a NACA4415 airfoil with tangential synthetic jets via computational modeling. A hybrid Reynolds-Averaged Navier-Stokes/Large Eddy Simulation (RANS/LES) turbulent model (called Delayed Detached-Eddy Simulation-DDES) was implemented in CDP, a kinetic energy conserving Computational Fluid Dynamics (CFD) code. CDP is a parallel unstructured grid incompressible flow solver, developed at the Center for Integrated Turbulence Simulations (CITS) at Stanford University. Two models of synthetic jet actuators have been developed and validated. The first is a detailed model in which the flow in and out of the actuator cavity is modeled. A second less costly model (RSSJ) was also developed in which the Reynolds stress produced by the actuator is modeled, based on information from the detailed model. Several static validation test cases at different angle of attack with modified NACA 4415 and Dragon Eye airfoils were performed. Numerical results show the effects of the actuators on the vortical structure of the flow, as well as on the aerodynamic properties. The main effect of the actuation on the time averaged vorticity field is a bending of the separation shear layer from the actuator toward the airfoil surface, resulting in changes in the aerodynamic properties. Full actuation of the suction side actuator reduces the pitching moment and increases the lift force, while the pressure side actuator increases the pitching moment and reduces the lift force. These observations are in agreement with experimental results. The effectiveness of the actuator is measured by the change in the aerodynamic properties of the airfoil in particular the lift ([Delta]C[subscript t]) and moment ([Delta]C[subscript m]) coefficients. Computational results for the actuator effectiveness show very good agreement with the experimental values (over the range of −2° to 10°). While the actuation modifies the global pressure distribution, the most pronounced effects are near the trailing edge in which a spike in the pressure coefficient (C[subscript p]) is observed. The local reduction of C[subscript p], for both the suction side and pressure side actuators, at x/c = 0.96 (the position of the actuators) is about 0.9 with respect to the unactuated case. This local reduction of the pressure is associated with the trapped vorticity and flow acceleration close to the trailing edge. The RSSJ model is designed to capture the synthetic jet time averaged behavior so that the high actuation frequencies are eliminated. This allows the time step to be increased by a factor of 5. This ad hoc model is also tested in dynamic simulations, in which its capacity to capture the detail model average performance was demonstrated. Finally, the RSSJ model was extended to a different airfoil profile (Dragon Eye) with good results. / text

No description available.

Recent interest in Micro Aerial Vehicles (MAVs) and Unmanned Aerial Vehicles (UAVs) have revived research on the performance of airfoils at relatively low Reynolds numbers. A common problem with low Reynolds number flow is that separation is almost inevitable without the application of some means of flow control, but understanding the nature of the separated flow is critical to designing an optimal flow control system. The current research presents results from a joint effort coupling numerical simulation and wind tunnel testing to investigate this flow regime. The primary airfoil for these studies is a modified 4415 with an adaptive actuator mounted internally such that the camber of the airfoil may be changed in a static or oscillatory fashion. A series of simulations are performed in static mode for Reynolds numbers of 25,000 to 100,000 and over a range of angles of attack to predict the characteristics of the flow separation and the coefficients of lift, drag, and moment. Preliminary simulations were performed for dynamic mode and it demonstrates a definitive ability to control separation across the range of Re and AoA. The earlier experimental work showed that separation reduction is gradual until a critical oscillation frequency is reached, after which increases in frequency have little additional impact on the flow. Present numerical simulation results were compared with the previous experiments results which were performed on the airfoil in like flow conditions and these comparisons allow the accuracy of both systems to be determined.

The flow field around a low pressure turbine (LPT) blade cascade model with and without flow control is examined using ejector nozzle (EN) and vortex generator jet (VGJ) geometries for separation control. The cascade model consists of 6 Pak-B Pratt andamp; Whitney low pressure turbine blades with Re = 30,000-50,000 at a free-stream turbulence intensity of 0.6%. The EN geometry consists of combined suction and blowing slots near the point of separation. The VGJs consist of a row of holes placed at an angle to the free-stream, and are tested at two locations of 69% and 10.5% of the suction surface length (SSL). Results are compared between flow control on and flow control off states, as well as between the EN, VGJs, and a baseline cascade with no flow control geometry for steady and pulsatile blowing. The EN geometry is shown to control separation with both steady and pulsatile blowing. The VGJs at 69% SSL are shown to be much more aggressive than the EN geometry, achieving the same level of separation control with lower energy input. Pulsed VGJs (PVGJ) have been shown to be just as effective as steady VGJs, and results show that a 10% duty cycle is almost as effective as a 50% duty cycle. The VGJs at 10.5% SSL are shown to be inefficient at controlling separation. No combination of duty cycle and pulsing frequency tested can eliminate the separation region, with only higher steady blowing rates achieving separation control. Thus, the VGJs at 69% SSL are shown to be the most effective in controlling separation.

A low-cost power router (PR), capable of dynamic, independent control of active- and reactive-power flows on meshed grids is presented. The operating principle, detailed schematics, and various possible implementations of the proposed power router are discussed. Various operating modes are identified and a control algorithm has been proposed and verified through simulations. Small-signal and frequency-domain models of the power router from basic time-domain equations are developed. A three-tier protection system based on the fail-normal switch to avoid single point-of-failure is presented. The operation of proposed protection system in isolating the converter and the grid in the event of faults is verified through simulation. An analytical method to evaluate the stability of a system with multiple power routers is proposed. Necessary conditions for the PR-controller design to ensure stable operation of a system with multiple power routers is proposed. These necessary conditions are verified through simulation studies. Potential applications of proposed power router in distribution system and the associated challenges in implementation are presented. The functionality and advantages of the proposed power router are experimentally demonstrated at 13 kV, 1 MVA. The proposed power router can result in a low cost power routing solution that can reduce electric grid congestion and efficient implementation of RPS mandates.