published_or_final_version / abstract / Electrical and Electronic Engineering / Master / Master of Philosophy

Subfilter scalar variance is a key quantity for scalar mixing at the small scales of a turbulent flow and thus plays a crucial role in large eddy simulation (LES) of combustion. While prior studies have mainly focused on the physical aspects of modeling subfilter variance, the current work discusses variance models in conjunction with numerical errors due to their implementation using finite difference methods. Because of the prevalence of grid-based filtering in practical LES, the smallest filtered scales are generally under-resolved. These scales, however, are often important in determining the values of subfilter models. A priori tests on data from direct numerical simulation (DNS) of homogenous isotropic turbulence are performed to evaluate the numerical implications of specific model forms in the context of practical LES evaluated with finite differences. As with other subfilter quantities, such as kinetic energy, subfilter variance can be modeled according to one of two general methodologies. In the first of these, an algebraic equation relating the variance to gradients of the filtered scalar field is coupled with a dynamic procedure for coefficient estimation. Although finite difference methods substantially underpredict the gradient of the filtered scalar field, the dynamic method is shown to mitigate this error through overestimation of the model coefficient. The second group of models utilizes a transport equation for the subfilter variance itself or for the second moment of the scalar. Here, it is shown that the model formulation based on the variance transport equation is consistently biased toward underprediction of the subfilter variance. The numerical issues stem from making discrete approximations to the chain rule manipulations used to derive convective and diffusive terms in the variance transport equation associated with the square of the filtered scalar. This set of approximations can be avoided by solving the equation for the second moment of the scalar, suggesting that model's numerical superiority. / text

在無線區域網路的範疇，IEEE 802.11是一個主要的媒介存取控制協定。然而在隨意式多跳接的網路中，IEEE 802.11面臨更嚴重的hidden terminal和exposed terminal problems，而這些問題主因都源自於過大的訊號干擾範圍與過大的訊號感應範圍。在這篇論文裡，我們提出一個可調式的媒介存取控制協定，針對IEEE 802.11 RTS/CTS handshake機制做簡單地修改，使得IEEE 802.11 devices可以依據週遭的傳收狀態動態調整自身的傳送和接收行為。實驗結果顯示我們的方法使原來的 802.11 減少了互相干擾的情形並且提升了系統的效能。 / The IEEE 802.11 standard is the most popular Medium Access Control (MAC) protocol for wireless local area networks. However, in multihop wireless ad hoc networks, the IEEE 802.11 MAC protocol will suffer from more serious hidden terminal and exposed terminal problems than those in single hop WLANs. More specifically, it is due to the large interference range and the large carrier sensing range. In this thesis, we propose an adaptive IEEE 802.11 MAC (AMAC) that makes two simple modifications of IEEE 802.11 RTS/CTS handshake to dynamically adjust the transmission and reception according to the shared medium status near transmitter and receiver, respectively. Simulation results show that our method can lessen interferences and increase system throughput as compared with IEEE 802.11 MAC.

Une des priorités scientifiques justifiant la construction de télescopes géants (classe 30-50m) est l'étude des premières générations de galaxies situées à de très grands décalages vers le rouge (1

This dissertation describes an innovative approach to the state justification portion of the sequential circuit automatic test pattern generation (ATPG) process. Given the absence of a stored fault an ATPG controller invokes some combinational circuit test generation procedure, such as the D-algorithm, to identify a circuit state (goal state) and input vectors that will sensitize a selected fault. The state justification phase then finds a transfer sequence to the goal from the present state. A forward fault propogation search can be successfully guided through state space from the present state but the forward justification search is less efficient and the failure rate is high. The backward function level search invokes inverse RTL level primitives and exploits easy movement of data vectors in structured VLSI circuits. Examples illustrated are in AHPL. This search is equally applicable to an RTL level subset of VHDL. Combinational logic units are treated as functions and the circuit states are partitioned into control states and data states. The search proceeds backwards over the control state space starting from the goal state node and data states are transformed according to the control flow. Vectorized data paths in VLSI circuits and search guiding heuristics which favor convenient inverse functions keep the number of search nodes low. Partial covers, conceptually similar to singular covers in D-algorithm, model the inverse functions of combinational logic units. The search successfully terminates when a child state node logically matches the present state and the present state values can satisfy all the constraints encountered along the search path.

We present techniques for obtaining precision distance measurements using the baryon acoustic oscillations (BAO) through controlling systematics and reducing statistical uncertainties. Using the resulting distance-redshift relation, we can infer cosmological parameters such as w, the equation of state of dark energy. We introduce a new statistic, ɷ(l)(r(s)), for BAO analysis that affords better control over systematics. It is computed by band-filtering the power spectrum P(k) or the correlation function ξ(r) to extract the BAO signal. This is conducive to several favourable outcomes. We compute ɷ(l)(r(s)) from 44 simulations and compare the results to P(k) and ξ(r). We find that the acoustic scales and theoretical errors we measure are consistent between all three statistics. We demonstrate the first application of reconstruction to a galaxy redshift survey. Reconstruction is designed to partially undo the effects of non-linear structure growth on the BAO, allowing more precise measurements of the acoustic scale. We also present a new method for deriving a smooth covariance matrix based on a Gaussian model. In addition, we develop and perform detailed robustness tests on the ξ(r) model we employ to extract the BAO scale from the data. Using these methods, we obtain spherically-averaged distances to z = 0.35 and z = 0.57 from SDSS DR7 and DR9 with 1.9% and 1.7% precision respectively. Combined with WMAP7 CMB observations, SNLS3 data and BAO measurements from 6dF, we measure w = -1.08 ± 0.08 assuming a wCDM cosmology. This represents a ~8% measurement of w and is consistent with a cosmological constant.The preceding does not capture the expansion history of the universe, H(z), encoded in the line-of-sight distance scale. To disentangle H(z), we exploit the anisotropic BAO signal that arises if we assume the wrong cosmology when calculating the clustering distribution. Since we expect the BAO signal to be isotropic, we can use the magnitude of the anisotropy to separately measure H(z) and D(A)(z). We apply our simple models to SDSS DR7 data and obtain a ~3.6% measurement of D(A)(z=0.35) and a ~8.4% measurement of H(z = 0.35).

New VLSI and VHSIC devices require increased performance from electronic packages. The major challenge that must be met in materials/process development for high complexity and high speed integrated circuits is the processing of even larger amounts of signals with low propagation delay. Hence, materials with low dielectric constant and low dissipation factor are being sought. In this investigation the dielectric properties of the most commonly used composite materials for printed circuit boards, Teflon-glass and Epoxy-glass, were measured in the frequency and temperature intervals of 100 HZ-1 GHZ and 25-260°C, respectively. From the measured results, it is concluded that Teflon-glass is more suitable for the board level packaging of high performance circuits due to its lower dielectric constant and low dissipation factor.

This thesis addresses the problem of constructing a flawless subarray from a defective VLSI/WSI array consists of identical cells such as memory cells or processors. In contrast to the redundancy approach in which some cells are dedicated as spares, all the cells in the degradation approach are treated in a uniform way. Each cell can be either fault-free or defective and a subarray which contains no faulty cell is derived under constraints of switching and routing mechanisms. Although extensive literatures exist concerning spare allocation and reconfiguration in arrays with redundancy, little research has been published on optimal reconfiguration in a degradable array. A systematic method based on graph theoretic models is developed to deal with the problem. The complexities of reconfiguration are analyzed for schemes using different switching mechanisms. Efficient heuristic algorithms are presented to determine a target subarray from the defective host array.

The aim of the thesis is to develop and validate subgrid-scale models that are relevant for large eddy simulations of complex flows including scalar mixing. A stochastic Smagorinsky model with adjustable variance and time scale is developed by adding a stochastic component to the Smagorinsky constant. The stochastic model is shown to provide for backscatter of both kinetic energy and scalar variance without causing numerical instabilities. In addition, new models for the subgrid-scale stress and passive scalar flux are derived from modelled subgrid scale transport equations. These models properly account for the anisotropy of the subgrid scales and have potentials wall bounded flows. The proposed models are validated in wall bounded flows with and without rotation and show potential or significantly improve predictions for such cases. / <p>QC 20100826</p>

All executives strive for better results in their organisations. They are always dependent on others to achieve these results and this dependency is particularly evident in large organisations. This thesis is concerned with the ways in which these better results might be achieved and the role senior executives might play in this process. The traditional view is that senior executives design and control the way their organisations function and better results therefore depend upon getting the design and the controls ‘right’. My personal experience, supported by many authors, is that this view is often far from reality. In this thesis I therefore draw on an alternative view of how organisations function, namely, the theory of complex responsive processes, in order to explore how senior executives can be more effective given their very limited ability to design and control their organisations. From a complex responsive processes perspective (Stacey, Griffin and Shaw, 2000; Stacey, 2003a), an organisation is understood, by analogy with the complexity sciences, to be processes of self-organising interaction between agents. The abstract analogy from the complexity sciences is interpreted in the case of human interaction according to the thinking of the American pragmatist G. H. Mead (1934). Mead explains the simultaneous emergence of mind and society in terms of the social act in which one person gestures to another and in doing so calls forth a response from that other in ongoing conversational processes in which patterns of communication (meaning) emerge across the organisational population. Work in organisations is accomplished in these conversational processes. In their conscious, self-conscious and responsive interaction, human agents depend on each other; according to the process sociologist N. Elias (1978), this means that all human relating is simultaneously constraining and enabling. Elias defines power as these enabling constraints between people, so that power is an aspect of all human relating. According to Elias, values, norms and ideology are the basis of power. Human choice and intention influence the shifting of power balances in which conflict, as a normal aspect of human interaction, plays an important role. Power, ideology and identity are then seen as central aspects of organisations. 4 People only interact locally with a small proportion of the total population they are part of, and do so on the basis of their own local organising principles (communication, power and choice) rather than simply obeying centrally set rules. This can be understood as self-organisation. The global patterns of communicative interaction and power relations across the organisation emerge in these local interactions rather than following a specific plan, programme or blueprint. The global patterns are unpredictable and are not under the control of any member of the organisation. Global – that is, company-wide – results are thus not directly determined by global design or control, but emerge in this local interaction. This approach means re-thinking what is involved in leadership and the roles of senior executives. From this perspective, senior executives are paradoxically in control and not in control at the same time (Streatfield, 2001). In this thesis I draw on my own personal experience over the past three years as a senior executive in a large services and transport company to identify the role a senior executive can actively play in potentially achieving better results despite not being fully in control. I emphasise the active contribution of senior executives in many local interactions in which global company-wide results emerge. Through the manner in which they participate in, and inspire, the development of local conversational interaction, senior executives can actively encourage front-line staff to take local responsibility for contributing to global, company-wide improvement of results. During these local interactions a chain reaction of local responsibilities can emerge that can contribute to the improvement of global company-wide performance. It is the responsibility of senior executives to communicate clearly in the organisation about demands on performance and results by customers and stakeholders in the market, and to encourage the taking of local responsibility for them. From a complexity view, the impact of leaders on the organisation is not less but different, with potentially better results.