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Thesis (PhD (Civil Engineering))--Stellenbosch University, 2008. / The velocity, density and turbulence profiles of a horizontal, saline gravity current were measured experimentally. Stable stratfication damped the turbulence and prevented the gravity current from becoming self-similar. The velocity and density prfiles were measured simultaneously and non-intrusively with particle image velocimetry scalar (PIV-S) technology. The application of the PIV-S technology had to be extended in order to measure the continuously stratified gravity current. Measurement of the Reynolds fluxes and Reynolds stresses revealed the anisotropic turbulent transport of mass and momentum within the gravity current body. These measurements also allowed the interaction between turbulence and stratification to be studied. The measured profiles were used to evaluate the accuracy of a gravity current model which did not assume self-similarity. The gravity current model was based on a Reynolds-averaged Navier-Stokes (RANS) multispecies mixture model. The Reynolds flux and Reynolds stress profiles did not show self-similarity with increasing downstream distance. Comparison of the vertical and horizontal Reynolds fluxes showed that gravity strongly damped the vertical flux. At a downstream location, where the bulk Richardson number was supercritical, the shear production profile had a positive inner (near bed) peak and a positive outer peak, while the buoyancy production pro le had a negative outer peak. Further downstream, where the bulk Richardson number was near-critical, the outer shear and buoyancy production peaks disappeared, due to the continuous damping of the turbulence intensities by the stable stratification. However, near bed shearing allowed the inner shear production peak to remain. Sensitivity analyses of different turbulence models for the gravity current model showed that the standard k -e turbulence model, as well as the Renormalization Group theory (RNG) k -e turbulence model, generally underpredicted the mean streamwise velocity profile and overpredicted the excess density pro le. The flux-gradient hypothesis, used to provide closure for the Reynolds uxes, modelled the vertical Reynolds ux reasonably, but not the horizontal flux. This did not compromise the results, since the horizontal gravity current had the characteristics of a boundary-layer ow, where the horizontal flux does not contribute significantly to the flow structure. It was shown that the gravity current model, implementing the standard k -e turbulence model with a constant turbulent Schmidt number of ot = 1;3, produced profiles which were within 10% - 20% of the measured profiles.

Localised QoS routing is a relatively new, alternative and viable approach to solve the problems of traditional QoS routing algorithms which use global state information resulting in the imposition of a large communication overhead and route flapping. They make use of a localised view of the network QoS state in source nodes to select paths and route flows to destination nodes. Proportional Sticky Routing (PSR) and Credit Based Routing (CBR) have been proposed as localised QoS routing schemes and these can offer comparable performances. However, since network state information for a specific path is only updated when the path is used, PSR and CBR operate with decision criteria that are often stale for paths that are used infrequently. The aim of this thesis is to focus on localised QoS routing and contribute to enhancing the scalability of QoS routing algorithms. In this thesis we have developed three new localised QoS routing schemes which are called Score Based QoS Routing (SBR), Bandwidth Based QoS Routing (BBR) and Delay Based Routing (DBR). In some of these schemes, the path setup procedure is distributed and uses the current network state to make decisions thus avoiding problems of staleness. The methods also avoid any complicated calculations. Both SBR and BBR use bandwidth as the QoS metric and mean delay is used as the QoS metric in DBR. Extensive simulations are applied to compare the performance of our proposed algorithms with CBR and the global Dijkstra's algorithm for different update intervals of link state, different network topologies and using different flow arrival distributions under a wide range of traffic loads. It is demonstrated by simulation that the three proposed algorithms offer a superior performance under comparable conditions to the other localised and global algorithms.

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

The mean flowfield and time-dependent characteristics of a Mach 5 cylinder-induced transitional shock-wave/boundary-layer interaction have been studied experimentally. The objectives of the study were to: (i) provide a detailed description of the mean flow structure of the interaction, and (ii) characterize the unsteadiness of the interaction based on fluctuating pressure measurements. / text

<p>Haematology instruments using optical and fluorescence techniques have improved the platelet count in domestic animals. There are still some difficulties present, especially when counting cat thrombocytes due to their ability to aggregate and the occurrence of large platelets.</p><p>The objective of this study was to evaluate and compare the platelet count, mean platelet volume and platelet crit in dogs, horses and cats on Advia 2120 and Sysmex XT-2000iV.</p><p>Fresh blood samples from 64 dogs, 40 horses and 39 cats with various medical conditions were analysed on both instruments. Manual blood smears of all feline samples were scrutiniously analysed to evaluate the aggregation warning flag from Advia.</p><p>There was good agreement between the instruments for the optical platelet count in dogs and cats. Slightly higher values were reported from Advia. Samples from horses presented poor correlations for all studied parameters. Platelet clumps appeared in 70% of the 37 scrutinized feline blood smears, while 46% of the samples generated aggregation warning flags from the Advia instrument.</p><p>Advia and Sysmex showed good agreement for platelet counts in blood from dogs and cats. Mean platelet volume and platelet crit need further evaluation before conclusions can be made concerning their clinical relevance. The sensitivity of the platelet aggregation warning flag from the Advia instument needs further elevation.</p>

Relays can be used in wireless communication systems to provide cell coverage extension, reduce coverage holes and increase throughput. Full duplex (FD) relays, which transmit and receive in the same time slot, can have a higher transmission rate compared with half duplex (HD) relays. However, FD relays suffer from self interference (SI) problems, which are caused by the transmitted relay signal being received by the relay receiver. This can reduce the performance of FD relays. In the literature, the SI channel is commonly nulled and removed as it simplifies the problem considerably. In practice, complete nulling is impossible due to channel estimation errors. Therefore, in this thesis, we consider the leakage of the SI from the FD relay. Our goal is to reduce the SI and increase the signal to noise ratio (SNR) of the relay system. Hence, we propose different precoder and weight vector designs. These designs may increase the end to end (e2e) signal to interference and noise ratio (SINR) at the destination. Here, a precoder is multiplied to a signal before transmission and a weight vector is multiplied to the received signal after reception. Initially, we consider an academic example where it uses a two path FD multiple input and multiple output (MIMO) system. The analysis of the SINR with the implementation of precoders and weight vectors shows that the SI component has the same underlying signal as the source signal when a relay processing delay is not being considered. Hence, to simulate the SI problem more realistically, we alter our relay design and focus on a one path FD MIMO relay system with a relay processing delay. For the implementation of precoders and weight vectors, choosing the optimal scheme is numerically challenging. Thus, we design the precoders and weight vectors using ad-hoc and near-optimal schemes. The ad-hoc schemes for the precoders are singular value decomposition (SVD), minimising the signal to leakage plus noise ratio (SLNR) using the Rayleigh Ritz (RR) method and zero forcing (ZF). The ad-hoc schemes for the weight vectors are SVD, minimum mean squared error (MMSE) and ZF. The near-optimal scheme uses an iterative RR method to compute the source precoder and destination weight vector and the relay precoder and weight vector are computed using the ad-hoc methods which provide the best performance. The average power and the instantaneous power normalisations are the two methods to constrain the relay precoder power. The average power normalisation method uses a novel closed form covariance matrix with an optimisation approach to constrain the relay precoder. This closed form covariance matrix is mathematically derived using matrix vectorization techniques. For the instantaneous power normalisation method, the constraint process does not require an optimisation approach. However, using this method the e2e SINR is difficult to calculate, therefore we use symbol error rate (SER) as a measure of performance. The results from the different precoder and weight vector designs suggest that reducing the SI using the relay weight vector instead of the relay precoder results in a higher e2e SINR. Consequently, to increase the e2e SINR, performing complicated processing at the relay receiver is more effective than at the relay transmitter.

This thesis studies different aspects of firm decisions by using a dynamic model. I estimate a dynamic model of the firm based on the trade-off theory of capital structure that endogenizes investment, leverage, and payout decisions. For the estimation of the model I use Efficient Method of Moments (EMM), which allows me to recover the structural parameters that best replicate the characteristics of the data. I start analyzing the question of whether target leverage varies over time. While this is a central issue in finance, there is no consensus in the literature on this point. I propose an explanation that reconciles some of the seemingly contradictory empirical evidence. The dynamic model generates a target leverage that changes over time and consistently reproduces the results of Lemmon, Roberts, and Zender (2008). These findings suggest that the time-varying target leverage assumption of the big bulk of the previous literature is not incompatible with the evidence presented by Lemmon, Roberts, and Zender (2008). Then I study how corporate income tax payments vary with the corporate income tax rate. The dynamic model produces a bell-shaped relationship between tax revenue and the tax rate that is consistent with the notion of the Laffer curve. The dynamic model generates the maximum tax revenue for a tax rate between 36% and 41%. Finally, I investigate the impact of financial constraints on investment decisions by firms. Model results show that investment-cash flow sensitivity is higher for less financially constrained firms. This result is consistent with Kaplan and Zingales (1997). The dynamic model also rationalizes why large and mature firms have a positive and significant investment-cash flow sensitivity.

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本研究的目的，在於檢視購併案產生後，企業的報酬是否會受到平均數復歸（Mean reversion）的影響，而逐漸回復到產業的平均報酬。並且分析主併公司與被併公司佔雙方總資產比例的高低差異，所受到的不同影響程度。 本次研究的樣本採用1987～2006年中的美國電子電機產業共77件購併案，並依照主併公司佔雙方總資產的比例不同，分成為兩個群集。將兩群集做比較分析。實證結果顯示，兩個群集均有顯著的平均數復歸現象。而主併佔雙方總資產比例較高的群集，在購併第一年即有顯著的優於產業平均報酬，而主併與被併資產相近的群集，則是於第四年開始才有顯著的績效改善。根據實證結果，我們做出以下的結論： 1.企業的報酬具有平均數復歸的特性，表現優良的公司會被他人學習，而使整體產業平均報酬逐漸提升，漸漸蠶食原先與產業平均之間的差距；相反的，表現不佳的公司亦透過學習，逐漸改善自己的績效，也使的整體產業平均報酬上升，最後雙方都將趨向產業的平均水準。 2.當研究者要分析企業遭受偶發性事件（如購併）的影響時，應當考慮平均數復歸的所帶來的效果。如果忽略了平均數復歸所帶來的效果，將會對事件產生的影響給予錯誤的解讀。 3.面對購併績效的研究時，必須移除平均數復歸的負面效果，如此才得以正確估計購併後的績效表現。且此時主併公司資產佔雙方資產比例較高者，於購併第一年就有明顯的績效改善的效果，而雙方資產相近者，則在購併成立後第四年開始才有較佳的績效表現。 / The purpose of this research is to look over the return of enterprise can receive influence from mean reversion on post-merger performance. And it analyzes the degree of influence on acquirer companies with different proportion in total assets of both acquirer and target companies. The sample of this research adopts 77 cases of electronic and electrical equipment industry of U.S.A. in 1987- 200, take according to the proportion of total assets of acquirer company in both total assets of acquirer and target company, divide into two clusters. Make comparative analysis of these two clusters. According to the empirical results, we make the following conclusions: 1.The return of enterprises has characteristic of mean reversion. Both well performed companies and worse performed companies will move toward the whole industry's average return. 2.As the researcher wants to analyze the influence of enterprises suffered from the sporadic incident (such as M&A), they should consider the phenomenon of mean reversion. Take off the negative result of the mean reversion in order to estimate correctly for the exactly performance after M&A. 3.The acquirer companies which total assets have a higher proportion in both acquirer and target companies, will have superior to industry average return apparently in first year, and the acquirer companies which total assets close to the target companies, will have apparent improvement in performances until the fourth year.