IEEE 802.11 has become the standard of medium access control (MAC) in wireless ad hoc networks. However, due to the embedded binary exponential backoff algorithm, the packet delay and jitter incurred by access collisions and frame retransmissions may grow drastically. The lack of time-constraint considerations in IEEE 802.11 makes it very difficult to provide QoS (Quality of Service) guarantees for multimedia services. Therefore, a lot of research works focusing on priority-based MAC protocols for wireless ad hoc networks have been proposed. Yet, no standards come out until now. This thesis presents a priority-based MAC scheme in wireless ad hoc networks, which not only provides differentiated services, but also improves the QoS limitations of the previously proposed schemes. The main idea of the proposed JMAC (jamming-based MAC) mechanism is that traffic flows with different priorities can be differentiated by transmitting jamming noises of different lengths to interfere with one another. The one with the longest length of jamming noise can start data transmission. Besides, in our design, priority can be dynamically adjusted to allow each MH to change its contention priority and the length of jamming noise in accordance with network congestions. To implement the proposed JMAC, three modules are developed in this thesis: Collision Avoidance, Starvation Prevention, and Deadlock Prevention. For the purpose of evaluation, we perform simulations on the well-known network simulator, NS-2. Our scheme is compared with the EDCF (enhanced distributed coordination function) of IEEE 802.11e¡]draft¡^and one of the existing works. The simulation results demonstrate the effectiveness and superiority of our scheme.

Competitive reaction to entry is an important issue in the marketing and economics areas. The monopolistic incumbent of a market often faces competitive entry if the market is profitable. This is a common issue that often occurs in the market transition from monopoly to competition. The entrant not only enters the market, but also wants to signal to tell consumers his quality level. The incumbent's reaction is also a signal to the consumers. Thus, how should the incumbent do? The paper "Response to Competitive Entry: A Rationale for Delayed Defensive Reaction" which is written by Ajay Kalra, Surendra Rajiv and Kannan Srinivasan (1998) and published in Marketing Science provides a completed and clear explanation of this issue. They claim that the incumbent would delay the defensive reaction to the competitive entry under information asymmetry. But it is still incorrect. Hence, we want to provide a revision to support that delayed defensive reaction is really a rational response to competitive entry, and explain this issue correctly and logically. We analyze the strategic interactions of the incumbent and the entrants which were omitted previously. Finally, we would obtain the conclusion that is also supporting this point of view¡G delayed defensive reaction is really a rational response to competitive. And it is really a Nash equilibrium, no firms would deviate.

In order to meet production requirements of small quantity and large variety for versatile market demands, industrial robots with dexterous end-effectors are usually applied to the flexible manufacturing systems. However, owing to constraints of robot¡¦s accuracy, repeatability, and resolution, assembled parts may experience collision during the insertion process. Both positional and angular errors, which cannot be easily predicted because of indeterminate collision situations, may cause failure of the assembly. One of the frequently applied strategies is to use a passive remote center compliance device. Most traditional remote center compliance (RCC) devices aim to solve insertion difficulty for round peg insertion. This dissertation is devoted to analyze the insertion behavior and develop a new remote center compliance device for square pegs, which lack of the axial symmetry property of round pegs. The presented Passive Multiple Remote Center Compliance Device (MRCC) introduces a new azimuthal compliance over traditional passive compliance mechanisms that can effectively compensate the peg¡¦s orientation deviation for polygonal assembly. Besides, a special feature of the adjustable compliance provides capability to overcome the gravity effect. Non-vertical insertions therefore become possible. A spring-supported object in space is also adopted for stability analysis of the compliant device. Actual experimental assembly processes demonstrate promising results on polygonal insertions in both traditional top-down and horizontal directions. The assembly process of a square peg consists of approach, one-point contact, two-point contact, three-point contact, four-point contact, rotation, departure from chamfer crossing, and insertion. Full analysis of the square peg mating process, using a quasi-static approach will be presented. Constraints that can avoid the jamming and wedging phenomenon for successful assembly will also be established. Furthermore, a novel geometric insertion map, which is able to predict regions of failure and success before actual insertion takes place, is developed to improve efficiency of successful assembly for square pegs.

Particle-laden flows occur in a variety of natural and industrial situations. As particulate suspensions flow through a medium, particles are often retained at constrictions such as pore throats, outlets or orifices. This occurs not only with oversized particles, but also with particles smaller than the constriction. For instance, jams are caused by the formation of particle bridges/arches when several particles attempt to flow through a constriction simultaneously. In many instances the success of an operation depends on our ability to either ensure or stop the flow of particles in the flow stream. Managing the flow of sand into wellbores during hydrocarbon production from poorly consolidated sandstone reservoirs, also referred to as sand control, is one such application in the oil and gas industry. This dissertation presents a multi-pronged effort at modeling the flow of granular suspensions of different concentrations, and through pore openings of different shapes, with two main objectives: (1) predicting the mass and size-distribution of the particles that are produced before jamming, and (2) investigating the underlying factors that influence the onset and stability of particle arches. Since, the dominant interactions and retention mechanisms are concentration dependent, we divided particulate suspensions into three groups based on the volumetric particle concentration ([phi]). High-concentration suspension flows ([phi]>~50%) are dominated by particle-particle interactions. We modeled polydisperse sand packs flowing through screens with rectangular and woven-square openings using 3D discrete element method (DEM). Simulations were validated against experimental data for a wide range of screen opening and sand size distributions. From the experiments and DEM simulations, a new scaling relation is identified, in which the number of different sized particles produced before retention follows a power-law correlation with the particle-to-outlet size ratio. This correlation is explained with a simple probabilistic model of bridging in polydisperse systems and a particle-size dependent jamming probability calculated from experimental data. A new method is presented to estimate the mass and size distribution of the produced solids through screens. The method uses the entire particle size distribution (PSD) of the formation sand, is validated with experimental data and numerical simulations, and provides more quantitative and accurate predictions of screen performance compared to past methods. It is also found that the stability of particle arches is compromised when adjacent outlets are less than three particle diameters away from each other. Low-concentration suspension flows ([phi]<~1%) are dominated by particle-fluid interactions. They were modeled using analytical and stochastic methods to predict sand production through screens with slot and woven-square openings. Analytical expressions were derived for screens with a constant outlet size or with a known outlet size distribution. Monte Carlo simulations showed excellent agreement with the analytical solutions. Based on experiments, we have demonstrated that the models presented here are predictive, provided that an accurate representation of the formation sand PSD and the screen pore size distribution are available. In the intermediate-concentration regime (~1%<[phi]<~50%), the particle trajectories and the flow field are both influenced by each other. The onset of particle bridging due to hydrodynamic forces was studied for monodisperse systems, in a rectangular channel with a single constriction, using coupled computational fluid dynamics (CFD) and DEM simulations. It is shown that the probability of jamming increases with [phi], and there is a critical particle concentration ([phi, superscript asterisk]) for spontaneous bridging. The outlet-to-particle size ratio is the most critical parameter affecting [phi, superscript asterisk]. The effect of inlet-to-particle size ratio, fluid velocity, particle stiffness, particle-to-fluid density ratio, and the effect of convergence in flow geometry were also studied quantitatively. Finally, the application of micro-tomography images in constructing accurate 3D representations and calculating the pore size distribution of complex filter media is demonstrated. A simulation tool is presented that allows one to evaluate the performance of different screens without running expensive and sometimes inconclusive experiments, and enhances our understanding of screen performance. This helps to improve sand screen selection to meet performance criteria under a wide variety of conditions. / text

The interfaces of bubbles and droplets imbue foams and emulsions with extraordinary mechanical and chemical properties. The remarkably large interfacial area of these structures controls their thermodynamics and makes them practical and functional materials. When these interfaces are forced to touch, they can turn a dispersion of one fluid in another into a solid. These solid-like properties are evident in common household products such as shaving foam and mayonnaise, and our ability to control the fluid and solid properties of these materials is essential to their function. / Physics

Nanoemulsions with extremely small droplet sizes (<100 nm) have shown several advantages over conventional emulsions. However, almost all nanoemulsions in usage are liquids that restrict their use in many soft materials. The aim of this thesis is to understand the formation and long-term stability of viscoelastic nanogels developed from liquid nanoemulsions. At first, gelation in 40 wt% canola oil-in-water nanoemulsions were investigated as a function of emulsifier type (anionic sodium dodecyl sulfate (SDS) or nonionic Tween 20) and concentration. Three different regimes of colloidal interactions were observed as a function of SDS concentration. 1) At low SDS concentration (0.5 – 2 times CMC) the counterion shell layer increased the effective volume fraction of the dispersed phase (eff) close to the random jamming, resulting in repulsive gelation. 2) At SDS concentration between 5 – 15 times CMC, micelle induced depletion attractions led to extensive droplet aggregation and gelation. 3) At very high SDS concentration, however, oscillatory structural forces (OSF) due to layered-structuring of excess micelles in the interdroplet regions led to loss of gelation. In repulsive gelation, reduction in droplet size coupled with the electrical double layer resulted in a linear increase of Gʹ. On the contrary, attractive nanoemulsions showed rapid increase in gel strength below a critical droplet radius, and was explained by transformation of OSF into depletion attraction. No gelation was seen in Tween 20 nanoemulsions, due to lack of repulsive interactions and weak depletion attraction. Next the influence of the dispersed phase volume fraction () on repulsive nanoemulsion gelation was investigated and the Gʹ values were modeled using empirical scaling law developed by Mason et al. (1995). It was found that an initial liquid regime transformed into glassy phase at a eff = g ~ 0.58, where droplets are entrapped in a cage of neighbouring droplets due to crowding. It was followed by jamming transition at a critical volume fraction (j), where droplet deformation led to large increase in elasticity. The model predicted j = 0.7, which is close to the predictions for repulsive polydispersed emulsions found in the literature. In the final phase long-term stability of the nanogels was evaluated until 90 days, during which the nanogels remained stable to creaming and coalescence. However, repulsive nanogels showed a significant decrease in Gʹ and the gels converted into flowable liquids over time. For attractive nanogels decrease in Gʹ was much less, although given enough time they would also transformed into weak gels. It was hypothesized that surface active compounds generated due to lipid oxidation altered interfacial charge cloud leading to loss of gel strength for repulsive nanogels. For attractive nanogels slippery bonds in the aggregates permitted rotational and translational diffusion of nanodroplets on the surface of each other leading to network compactness and a decrease in gel strength with time. Overall, it was concluded that it is possible to form nanogels from canola oil nanoemulsions using ionic emulsifiers. The gel strength and stability of the nanogels depends on emulsifier concentration, droplet size,  and the chemical stability of the oil used. More investigation is needed in order to improve the long-term stability of the nanogels. The nanogels possess high potential for use in low-fat foods, pharmaceuticals, and cosmetic products.

The purpose of this thesis is to linearize a power amplifier using digital predistortion. A power amplifier is a nonlinear system, meaning that when fed with a pure input signal the output will be distorted. The idea behind digital predistortion is to distort the signal before feeding it to the power amplifier. The combined distortions from the predistorter and the power amplifier will then ideally cancel each other. In this thesis, two different approaches are investigated and implemented on an FPGA. The first approach uses a nonlinear model that tries to cancel out the nonlinearities of the power amplifier. The second approach is model-free and instead makes use of a look-up table that maps the input to a distorted output. Both approaches are made adaptive so that the parameters are continuously updated using adaptive algorithms. First the two approaches are simulated and tested thoroughly with different parameters and with a power amplifier model extracted from the real amplifier. The results are shown satisfactory in the simulations, giving good linearization for both the model and the model-free technique. The two techniques are then implemented on an FPGA and tested on the power amplifier. Even though the results are not as well as in the simulations, the system gets more linear for both the approaches. The results vary widely due to different circumstances such as input frequency and power. Typically, the distortions can be attenuated with around 10 dB. When comparing the two techniques with each other, the model-free method shows slightly better results.

A particulate suspension under shear is a classic example of a system driven out of equilibrium. While it is possible to predict the equilibrium phase behaviour of a quiescent suspension, linking microscopic details to bulk properties under flow remains an open challenge. Our current understanding of sheared suspensions is restricted to two disparate regimes, the colloidal regime, for particle sizes d < 1 μm and the granular regime, for d > 50 μm. The physics of the industrially-relevant intermediate size regime, 1 μm ≲ d ≲ 50 μm, is unclear and has not been explored previously. In this thesis, we use conventional rheometry on a range of model spheres to develop the foundations of a predictive understanding of suspension flow across the entire size spectrum. In the first part of the thesis, we show that in repulsive particulate systems the rheology is characterised by two viscosity "branches" diverging at different volume fractions φRCP and φm, which represent states of flow with lubricated (frictionless) and frictional interactions between particles. In the intermediate size regime, there is a transition between these two branches above a critical onset stress σ* which manifests as shear thickening. This σ* is related to a barrier (invariably due to the charge or steric stabilisation) keeping particle surfaces apart. Our data are quantitatively fit by the Wyart and Cates theory for frictional thickening [1] if we assume that probability distribution of forces in the system is similar to in dry granular media. The onset stress for shear thickening is found to decrease with the inverse square of the particle size σ* / d ̄ 2 for diverse systems. We show that it is the competition between the scaling of σ*(d) and the size dependence of the entropic stress scale (~ d ̄ 3) that controls the crossover from colloidal to granular rheology with increasing size. Granular systems are "always shear thickened" under typical experimental conditions, while colloidal systems are always in a frictionless state. In the second part of the thesis, we explore the validity of the frictional framework for shear thickening. Although it quantitatively predicts our steady-state rheology, the frictional framework contradicts traditional fluid-mechanical thinking and has yet to be rigorously tested experimentally. In fact, there is a large body of literature that attributes thickening to purely hydrodynamic effects. Using dimensional analysis and simple physical arguments we examine possible physical origins for thickening and show that previously-proposed mechanisms can be subdivided into three types: two-particle hydrodynamic thickening, many-particle hydrodynamic thickening ("hydroclusters") and frictional-contact driven thickening. Many of these mechanisms can are inconsistent with the experimental two-branch phenomenology and can be disregarded. We further narrow down possible causes of thickening using the technique of flow reversal, which disentangles the relative contributions of contact and hydrodynamic forces to the viscosity. Consistent with recent simulations [2] and theory [1], we find that in each case thickening is dominated by the formation of frictional contacts and that hydrodynamic thickening, if present, is subdominant.

Purpose - Research on punk culture often falls prey to three main dilemmas. First, an ageist bias exists in most popular music research, resulting in the continued equating of music and youth. Second, punk culture research often uses a Marxist economic lens that implies fieldwork reveals already known conceptions of class and culture. Third, research on punk culture lacks ethnographic and narrative examinations. This ethnographic project explores my reentry into punk culture as an adult, exploring it from a new researcher perspective. It provides an insider's view of emerging cultural themes at the site that disrupts these traditional research approaches. Methodology/approach - This ethnography examines punk culture at an inner city nonprofit arts establishment. Through grounded theory and using a fictional literary account, this research probes how rituals and cultural narratives pervade and maintain the scene. Findings - Concepts such as carnival, jamming as an organizing process - and as an aesthetic moment - emerged through the research process. This ethnography found narratives constituted personal and communal identity. Research limitations/implications - As a personal ethnography, this research contains experiences in one local arts center, and therefore is not necessarily generalizable to other sites or experiences. Originality/value of paper - Using ethnography, I explored punk as one of my primary identities in tandem with younger members of the scene. It critiques Marxist and youth approaches that have stunted music scene research for decades.

The purpose of this study is to shed light on the issues surrounding the marketing of white snus, which employs various means to downplay and beautify the product, thus normalizing it as part of everyday life. Using our analytical framework, we have developed spoof advertisements for white snus that convey this problematic nature. We then examine the strength of the message conveyed by using these parody advertisements as stimuli in two interview sessions to measure the experiences of active snus users. The aim of the study within a societal context is to investigate how parody advertisements can challenge prevailing societal structures by applying them in the context of the issues surrounding white snus.  This is done through the research questions: How can parody advertisements for white snus be designed to convey the issues in the marketing of white snus? How do users of white snus perceive the messages in the parody advertisements?. The analytical framework of the study encompasses semiotics, framing theory, storytelling in advertising and a model of caricature,  as well as previous research on culture jamming and parody advertisements. Using the analytical framework, parody advertisements were created based on two commercials produced by popular manufacturers of white snus, LOOP and VELO, which were then used as stimulus material during the interview sessions. The design process was initiated with a preliminary study in which suitable brands were identified and their advertisements analyzed semiotically, followed by the creation of the parody advertisements. Through focus group interviews with users of white snus, it was observed that the respondents generally understood and reacted to the message conveyed in the parody advertisement for LOOP. However, the discussion regarding the message and the feeling of captivity was limited due to the minimal elements and fictional representation in the advertisement. The parody advertisement for VELO prompted a more in-depth discussion, where respondents could relate to the depicted situation and gain a broader understanding of the consequences of their nicotine addiction. The responses were divided; some found the parody advertisement for VELO impactful, generating negative feelings towards the portrayed lifestyle, while others found it comforting, emphasizing positive sentiments towards the lifestyle.