Behavioral momentum theory states that resistance to change of operant behavior is governed by the Pavlovian stimulus-reinforcer relation in a given discriminative stimulus situation. That is, higher reinforcer rates in the presence of a discriminative stimulus result in a stronger stimulus-reinforcer relation and, thereby, greater resistance to change. Within the momentum-based quantitative framework of resistance to change, the construct relating persistence to pre-disruption reinforcer rates is termed “behavioral mass.” All research on which momentum theory is based has examined resistance to change following prolonged exposure to stable reinforcer rates in multiple schedules of reinforcement. Thus, at present little is known about the time frame over which behavioral mass accumulates or the manner by which newly experienced stimulus reinforcer relations are incorporated into mass when these rates change. The experiments described in this dissertation aimed to clarify these facets of the construct. Chapters 1 and 2 provide a detailed overview behavioral momentum theory and resistance to change. Topics discussed include quantitative models of resistance to change, clinical implications of resistance-to-change research, some notable limitations of behavioral momentum theory, and extensions of the theory to account for diverse behavioral outcomes. A recently published study is presented in Chapter 3 that aimed to determine how resistance to change and behavioral mass of pigeons’ key pecking adapts in the face of stimulus-reinforcer relations that change across time during baseline. Results suggest that resistance to change is a function of recently experienced stimulus-reinforcer relations and that behavioral mass depends most heavily on these recent experiences. The experiment described in Chapter 4 extended the findings reported in Chapter 3 by examining whether behavioral mass changes during operant extinction. Pre-exposure to extinction in an alternative multiple-schedule component decreased resistance to extinction of target-component key pecking relative to conditions without pre-exposure to extinction. Between-condition differences in extinction were well accounted for quantitatively by either variation in behavioral mass or changes in the magnitude of factors that are assumed to disrupt responding during extinction. Chapter 5 offers an integrative discussion of this research and emphasizes theoretical implications, practical applications, and areas for future research.

Behavioral momentum theory (BMT) provides a theoretical framework for studying the persistence of behavior when challenged. The typical experimental arrangement to study persistence involves reinforcing a behavior according to a multiple schedules design. Unique schedules of reinforcement are programmed to each component. When steady-state responding occurs, the schedules of reinforcement are disrupted by a challenge condition (e.g., extinction, distraction, or prefeeding). The multiple schedules component that maintains the greatest level of responding during disruption is described as being more persistent. Basic research has shown that rate of reinforcement is a reliable predictor of persistence. The multiple schedules component associated with the higher rate of reinforcement persists longer than the multiple schedules component associated with the lower rate of reinforcement during disruption. Applied researchers have recently begun translating BMT to problems of social significance. The success of these initial translations suggests that relations between other dimensions of reinforcement and persistence should be studied. The current two-experiment study investigated the effect of quality of reinforcement on the persistence of task completion. Three participants with a history of engaging in problem behavior to escape from demands participated in Experiment I. After showing the conditions under which participants would and would not allocate away from a work task to engage with a preferred item, a baseline measure of task completion was obtained. Task completion was then reinforced with attention or tangibles within a multiple schedules design. Orange tokens signaled access to tangible reinforcement and yellow tokens signaled access to attention reinforcement. After steady-state responding occurred, preference for attention and tangibles was assessed within a concurrent schedules design. Extinction was then implemented to disrupt task completion within each component of the multiple schedules design. Results showed modest differences in the persistence of task completion with task completion in the multiple schedules component associated with the delivery of the more preferred reinforcer persisting longest. The modest differences in persistence were smaller than what has previously been shown in the literature. Thus, a follow-up experiment was conducted to evaluate the effect of reinforcer potency on the persistence of task completion. Three participants with a history of engaging in problem behavior to escape from demands participated in Experiment II. After identifying relatively more and less preferred stimuli with a multiple stimulus without replacement (MSWO) preference assessment, a unit price analysis was conducted to evaluate the potency of these two items within a concurrent schedules design. Task completion was then reinforced with the more and less potent reinforcers according to a multiple schedules design. After showing steady-state responding, task completion was disrupted by extinction. Results clearly showed greater persistence of task completion under the component associated with the delivery of the more potent reinforcer for two of three participants. Results from both experiments are discussed in terms of their conceptual and applied implications.

Behavioral momentum theory provides a conceptual framework for the study of the recurrence of previously extinguished operant behavior. Commonly referred to as treatment relapse, this is the failure to maintain treatment gains (e.g., reductions in challenging behavior) when there is a change in conditions under which these gains were achieved. One treatment relapse paradigm previously examined in basic and applied research is reinstatement. Reinstatement of challenging behavior has been shown to occur when functional reinforcers are delivered on a fixed-time schedule following extinction of challenging behavior. Although examinations appropriate behavior have applied value, analyses of reinstatement have been conducted almost exclusively with challenging behavior. During the current study, a reinstatement methodology was applied to communicative responses with five children diagnosed with developmental disabilities who exhibited comorbid communication deficits, as well as challenging behavior maintained by positive reinforcement. In the first phase of the reinstatement evaluation, each child received functional communication training (FCT) within a positive reinforcement context within a two-component multiple schedule design with each schedule paired with a distinct communicative response. After achieving steady-state responding in the first phase, in which all participants were independently emitting both communicative responses, all appropriate communication was placed on extinction in the second phase. Extinction continued until rates of appropriate communication were at or near zero. In the third phase, positive reinforcement was delivered and the recurrence of appropriate communication was evaluated. For two of five participants, communicative responding recurred following the fixed-time delivery of the functional reinforcer, indicating a successful demonstration of reinstatement. For three of five participants, communicative responding recurred prior to the delivery of fixed-time reinforcement, indicating that an alternative recurrence phenomenon likely occurred. These results suggest that reinstatement methodologies can be applied to cases in which FCT treatment failures have occurred to efficiently restore clinical gains for some participants. Implications for clinical practice and future directions of this line of research are discussed.

Interventions based on a token economy effectively reduce problematic behavior. Yet, treatment gains deteriorate once an intervention is discontinued. It is important to better understand the persistence of behavior maintained by token reinforcement in simple experimental procedures. A Pavlovian association with primary reinforcement is said to endow neutral stimuli (e.g., coins, poker chips, lights, signs, stickers, etc.) with their own function to strengthen behavior as conditioned reinforcers. Behavioral momentum theory suggests that resistance to change under conditions of disruption is the appropriate measure of response strength. However, some animal studies have suggested that conditioned reinforcement may not affect resistance to change of a response. Here, a novel token reinforcement procedure was developed to investigate the resistance to change of responding maintained by token reinforcement. Pigeons responded on a key to produce tokens displayed on a touchscreen monitor in two signaled token-production components. Tokens accumulated over the two production components prior to a common exchange component where pecks to the tokens on the touchscreen produced food reinforcement. Resistance to change of responding maintained by different rates of token reinforcement was assessed by disrupting baseline token-production responding with presession feeding. Token reinforcement rates had inconsistent effects on baseline token-production response rates. However, small effects of token reinforcement rate on resistance to change were found. Results provide weak support for a response-strengthening account of conditioned reinforcement and insightful directions for future studies of token reinforcement in related procedures.

<p><p>This paper is the result of a master thesis project on helicopter rotor downwash effects using computational fluid dynamics (CFD). The work was performed at the department of Aerodynamics and Flight Mechanics at Saab AB, Linköping in 2008. It completes the author’s studies for a M.Sc degree in Applied Physics and Electrical Engineering at the Department of Electrical Engineering at the Linköping institute of technology (LiTH), Linköping, Sweden.</p><p> </p><p>The aim of the project was to study the rotor downwash effects and its influence on the helicopter fuselage. To fulfil this purpose, several CFD calculations were carried out and the aerodynamic forces and moments resulting from the calculations were implemented in an existing simulation model, developed in-house at Saab. The original (existing) model was compared to the updated model by studying step responses in MATLAB, Simulink. For some step commands, the comparisions indicated that the updated model was more damped in yaw compared to the original model for the hovering helicopter. When the helicopter was trimmed for a steady turn, the states in the updated model diverged much faster than the states in the original model for any given step command.</p><p> </p><p> </p><p>In order to investigate the differences between the original helicopter model and the updated model from a controlling perspective, a linear quadratic (LQ) state feedback controller was synthesized to stabilize the vehicle in a steady turn. The LQ method was chosen as it is a modern design technique with good robustness and sensitivity properties and since it is easily implemented in MATLAB.  Before synthesising, a simplification of the helicopter model was made by reducing states and splitting them into lateral and longitudinal ones. Step responses from simulations with the original and the updated model were studied, showing an almost identical behavior.</p><p> </p><p>It can be concluded that the aerodynamic coefficients obtained from the CFD calculations can be used for determining the aerodynamic characteristics of the helicopter. Some further validation is needed though, for example by comparing the results with flight test data. In order to build an aerodynamic data base that covers the whole flight envelop, additional CFD calculations are required.</p><p> </p></p>

Interventions based on a token economy effectively reduce problematic behavior. Yet, treatment gains deteriorate once an intervention is discontinued. It is important to better understand the persistence of behavior maintained by token reinforcement in simple experimental procedures. A Pavlovian association with primary reinforcement is said to endow neutral stimuli (e.g., coins, poker chips, lights, signs, stickers, etc.) with their own function to strengthen behavior as conditioned reinforcers. Behavioral momentum theory suggests that resistance to change under conditions of disruption is the appropriate measure of response strength. However, some animal studies have suggested that conditioned reinforcement may not affect resistance to change of a response. Here, a novel token reinforcement procedure was developed to investigate the resistance to change of responding maintained by token reinforcement. Pigeons responded on a key to produce tokens displayed on a touchscreen monitor in two signaled token-production components. Tokens accumulated over the two production components prior to a common exchange component where pecks to the tokens on the touchscreen produced food reinforcement. Resistance to change of responding maintained by different rates of token reinforcement was assessed by disrupting baseline token-production responding with presession feeding. Token reinforcement rates had inconsistent effects on baseline token-production response rates. However, small effects of token reinforcement rate on resistance to change were found. Results provide weak support for a response-strengthening account of conditioned reinforcement and insightful directions for future studies of token reinforcement in related procedures.

This paper is the result of a master thesis project on helicopter rotor downwash effects using computational fluid dynamics (CFD). The work was performed at the department of Aerodynamics and Flight Mechanics at Saab AB, Linköping in 2008. It completes the author’s studies for a M.Sc degree in Applied Physics and Electrical Engineering at the Department of Electrical Engineering at the Linköping institute of technology (LiTH), Linköping, Sweden.   The aim of the project was to study the rotor downwash effects and its influence on the helicopter fuselage. To fulfil this purpose, several CFD calculations were carried out and the aerodynamic forces and moments resulting from the calculations were implemented in an existing simulation model, developed in-house at Saab. The original (existing) model was compared to the updated model by studying step responses in MATLAB, Simulink. For some step commands, the comparisions indicated that the updated model was more damped in yaw compared to the original model for the hovering helicopter. When the helicopter was trimmed for a steady turn, the states in the updated model diverged much faster than the states in the original model for any given step command.     In order to investigate the differences between the original helicopter model and the updated model from a controlling perspective, a linear quadratic (LQ) state feedback controller was synthesized to stabilize the vehicle in a steady turn. The LQ method was chosen as it is a modern design technique with good robustness and sensitivity properties and since it is easily implemented in MATLAB.  Before synthesising, a simplification of the helicopter model was made by reducing states and splitting them into lateral and longitudinal ones. Step responses from simulations with the original and the updated model were studied, showing an almost identical behavior.   It can be concluded that the aerodynamic coefficients obtained from the CFD calculations can be used for determining the aerodynamic characteristics of the helicopter. Some further validation is needed though, for example by comparing the results with flight test data. In order to build an aerodynamic data base that covers the whole flight envelop, additional CFD calculations are required.

In this thesis, the turbine blade design eligible for ocean current conditions is proposed using blade element momentum theory. in the beginning, the performance of water turbines is evaluated by CFD (computational fluid dynamics) package code, so as to design the suitable turbine under various conditions. The blade design encompasses parameters of the hydrofoil selection and blade shape which affect the turbine performance. Shortly following the investigation of the aforementioned parameters, the turbine¡¦s performance with radius of two meter is also studied. The current conditions include the yaw and the pitch angle of the turbine relative to the current flow direction, as well as the periodic flow conditions on the performance of the water turbine. Lastly, the electricity generation is estimated by the present device. The results show that hydrofoils with less changes in the angle of attack with respect to the lift-drag ratio help enhance the turbine¡¦s performance. The feedback mechanism is added to the blade design procedure to make sure that the turbine design caters to the best angle of attack. A turbine with two-meter radius can garner 34% of the sea current energy at most, living up to the project goal of exceeding the efficiency of 30%. The simulated test indicates that the adequate enlargement of the blade not only sustains the maximal efficiency, but it also lowers the stress imposed on the blade. Given the ocean current conditions, it is also shown that the turbine¡¦s efficiency is proportional to the cubic cosine incident angle of inflow velocity alongside with the enlargement of the turbine radius. When it comes to the current electricity generation, from the in-situ measurement data, the current maximal velocity near the sea region is around 1.3 m/s. If incorporated with the self excited induction generator with the efficiency of 55%, a one-meter-radius turbine is estimated to be able to generate 530W at most, while a two-meter-radius turbine is estimated to generate 2.5KW. However, the use of the permanent magnet generator can produce 45% more electricity than a self excited induction generator.

Several variables have been shown to influence resistance to change including rate, magnitude, and delay to reinforcement (Nevin, 1974). One variable that has not been studied with humans concerns the evaluation of resistance to change with unconditioned and conditioned reinforcers. In Experiment 1 (Resistance to Extinction Assessment), 5 participants' behaviors were reinforced during a baseline phase on a mult VI 30 s VI 30 s schedule with either a conditioned (i.e., token) or unconditioned reinforcer (i.e., food). Following equal reinforcement rates across components, extinction was introduced as a disruptor. All participants showed greater resistance to extinction in the component associated with conditioned reinforcers than unconditioned reinforcers. In Experiment 2 (Varied Distractors Assessment), 4 participants experienced a baseline phase the same as Experiment 1 (i.e., mult VI 30 s VI 30 s). Each participant was then exposed to prefeeding and distraction as disruptors in separate analyses. Results showed that behaviors were more resistant to distraction with conditioned than unconditioned reinforcers, similar to Experiment 1. However, when prefeeding disrupted responding, greater resistance to change was observed with unconditioned reinforcers than conditioned reinforcers. Implications of the results are discussed.

Accretion discs efficiently transport angular momentum by a wide variety of as yet imperfectly understood mechanisms, with profound implications for the disc lifetime and planet formation. We discuss two different methods of angular momentum transport: first, generation of acoustic waves by mixing of inertial waves, and second, the generation of a self-sustaining magnetic field via the magnetorotational instability (MRI) which would be a source of dissipative turbulence. Previous local simulations of the MRI have shown that the dynamo changes character on addition of vertical stratification. We investigate numerically 3D hydrodynamic shearing waves with a conserved Hermitian form in an isothermal disc with vertical gravity, and describe the associated symplectic structure. We continue with a numerical investigation into the linear evolution of the MRI and the undular magnetic buoyancy instability in isolated flux regions and characterise the resultant quasi-linear EMFs as a function of height above the midplane. We combine this with an analytic description of the linear modes under an assumption of a poloidal-toroidal scale separation. Finally, we use RAMSES to perform full MHD simulations in a zero net flux shearing box, followed by spatial and a novel temporal averaging to reveal the essential structure of the dynamo. We find that inertial modes may be efficiently converted into acoustic modes for "bending waves", despite a fundamental ambiguity in the inertial mode structure. With our linear MRI and the undular magnetic buoyancy modes we find the localisation of the instability high in the atmosphere becomes determined by magnetic buoyancy rather than field strength for small enough azimuthal wavenumber, and that the critical Alfven speed below which the dynamo can operate increases with increasing distance from the midplane. We calculate analytically quasi-linear EMFs which predict both a vertical propagation of toroidal field and a method for creation of radial field. From our fully nonlinear calculations we find an electromotive force in phase with the toroidal field, which is itself 3π/2 out of phase with the radial (sheared) field at the midplane, and good agreement with our quasi-linear analytics. We have identified an efficient mechanism for generating acoustic waves in a disc. In our investigation of the accretion disc dynamo, we have reproduced analytically the EMFs calculated in our simulations, given arguments based on the phase of relevant quantities, several correlation integrals and the scalings suggested by our analytic work. Our analysis contributes significantly to an explanation for the dynamo in an accretion disc.