Browsing by Subject "acceleration"
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Item Diffusion Preconditioner for Discontinuous Galerkin Transport Problems(2011-08-08) Barbu, Anthony PetruA simple Richardson iteration procedure converges slowly when applied to thick, diffusive problems with scattering ratios near unity. The current state of the art for overcoming this is to use a Krylov method with a diffusion preconditioner. However, the diffusion preconditioner must be tailored to the discretization of the transport operator to ensure effectiveness. We expand work from the bilinear discontinuous (BLD) finite element method (FEM) in two dimensions into a preconditioner applicable to all Discontinuous Galerkin FEMs in two and three dimensions. We demonstrate the effectiveness of our approach by applying it to the piecewise linear discontinuous (PWLD) FEM, which is notable for its flexibility with unstructured meshes. We employ a vertex-centered continuous FEM diffusion solution followed by local one-cell calculations to generate discontinuous solution corrections. Our goal is to achieve the same level of performance for PWLD and other methods, in two and three dimensions, as was previously achieved for BLD in two dimensions. We perform a Fourier analysis of this preconditioner applied to the PWLD FEM and we test the preconditioner on a variety of test problems. The preconditioned Richardson method is found to perform well in both ne and coarse mesh limits; however, it degrades for high-aspect ratio cells. These properties are typical for partially consistent diffusion synthetic acceleration (DSA) schemes, and in particular they are exactly the properties of the method that was previously developed for BLD in two dimensions. Thus, we have succeeded in our goal of generalizing the previous method to other Discontinuous Galerkin schemes. We also explore the effectiveness of our preconditioner when used within the GMRES iteration scheme. We find that with GMRES there is very little degradation for cells with high aspect ratios or for problems with strong heterogeneities. Thus we find that our preconditioned GMRES method is efficient and effective for all problems that we have tested. We have cast our diffusion operator entirely in terms of the single-cell matrices that are used by the discontinuous FEM transport method. This allows us to write our diffusion preconditioner without prior knowledge of the underlying FEM basis functions or cell shapes. As a result, a single software implementation of our preconditioner applies to a wide variety of transport options and there is no need to re-derive or re-implement a diffusion preconditioner when a new transport FEM is introduced.Item Measurement of accelerations experienced by rough stock riders(2008-04-15) Sharmila Devi Watkins; Robert Johnson, M.D., M.P.H., M.B.A.; Jonathan Clark, M.D., M.P.H.; A. Nelson Avery, M.D.Introduction: Head injury is common in many sports, but it is of particular concern in professional rodeo events. Rough stock events (bareback, saddle bronc, and bull riding) provide multiple opportunities for injury. Head injuries sustained during a rough stock event may be the result of whiplash effects or impact with the animal. Although there are a few recent studies investigating the incidence of head injury in rodeo events, little is known about the acceleration profile experienced by the riders.\r\n\r\nMethods: This study was conducted at the 2007 Houston Livestock Show and Rodeo. Two subjects were enrolled: one bull rider and one bareback rider. The subjects were fitted with custom-molded accelerometers and a waist mounted data recorder. The head accelerations experienced during the subjects’ scheduled rodeo events were then measured and recorded. The motions of the riders were also captured on video.\r\n\r\nResults: This study demonstrated the ability to record both the magnitude and direction of the head accelerations experienced. Data were obtained from both subjects and revealed significant accelerations in all axes, particularly the z-axis. The maximum resultant acceleration for the bull rider was 258 m/s2 (26 g’s), while the bareback rider experienced a greater magnitude acceleration of 450 m/s2 (46 g’s).\r\n\r\nConclusions: Head accelerations experienced by rough stock riders are high in magnitude and have the potential to result in injury. Further studies of accelerations experienced during actual rough stock events are needed.\r\nItem Measurement of accelerations experienced by rough stock riders: A model for examining acceleration-induced head injuries in astronauts(2009-07-10) Charles Hood Mathers; Richard Jennings, M.D., M.S.; Jonathan Clark, M.D., M.P.H.; James Vanderploeg, M.D., M.P.H.Head injuries result in significant morbidity in rough stock rodeo events. Concussions are the most common injury sustained by rough stock riders, representing 50% of all major injuries. A pilot study conducted in 2007 examined head acceleration experienced by two rough stock riders. Ear-mounted tri-axial accelerometers showed a maximum of 26 G experienced by the bull rider, while the bareback rider experienced 46 G. An aim of the current study was to expand upon this pilot study by examining head acceleration experienced by 10 bull riders and 10 bareback riders during the 2009 Houston Livestock Show and Rodeo. Riders were outfitted with two earplugs, one measuring tri-axial linear acceleration and the other tri-axial angular rates. On average, bareback riders experienced a statistically-significant increase in linear acceleration in the x and z axes compared with bull riders. Bareback riders also experienced a statistically-significant increase in angular acceleration in the x and y axes. There was no difference seen between bareback and bull riders in linear head acceleration in the y axis, and the difference in angular rates experienced in the z axis did not reach statistical significance. Another population at risk for injuries due to repetitive acceleration is astronauts. The Russian Soyuz spacecraft can expose astronauts to high accelerative forces during re-entry and up to 10 G during ballistic re-entry. Soyuz landing impact has reached 17 G. The new NASA Aries launch vehicle is predicted to experience thrust-oscillation problems that may affect crew health. A second aim of this study was to apply the test procedure, hardware, and knowledge gained at the rodeo toward the development of a protocol for measuring head acceleration experienced by astronauts. The first step will be implementation of the study hardware and protocol in centrifuge training. Once validated, the hardware and protocol can undergo flight certification for testing on Russian and U.S. spacecraft. This would provide invaluable insight into launch loads, vibration, reentry and impact loads to ensure crew health in the new vehicle design.Item Measurement of head acceleration and angular rate experienced by aerobatic pilots(2010-05-18) Leigh Ellen Lewis; Dr. Richard Jennings; Dr. Jonathan Clark; Dr. James VanderploegBackground: Aerobatic pilots are exposed to high levels of positive and negative G’s which can be associated with career-limiting neurovestibular effects including “the wobblies.” Extensive research has been conducted on the effects of positive G’s in centrifuge experiments. Gz tolerances have been quantified for gray-out, black-out, and G-LOC. G-induced vestibular dysfunction or “the wobblies,” though not yet well studied, is thought to affect many aerobatic pilots who are exposed to high levels of negative G’s. Neurovestibular symptoms induced during flight can increase the risk of loss of aircraft control. The actual G forces experienced at head-level in aerobatic pilots have never been characterized, and this study intends to solve this knowledge gap. Methods: Five volunteers at the 2009 US National Aerobatic Championships were fitted with tri-axial accelerometer and angular rate earplug sensors. A second tri-axial accelerometer and angular rate sensor package was fixed to the plane. For each subject, data were collected from the two synchronized sets of hardware during a 10-minute practice session. The recordings of the maximum and minimum G values were also obtained from the aircraft’s G-meter. Results: The maximum and minimum values obtained from the sensors measuring linear acceleration and angular rates from the pilot and the plane were well-correlated. Paired t-tests demonstrated no significant difference between head-level and plane mean linear acceleration. Angular velocity means were mixed. The Gz accelerometer values of the pilot and the plane correlated very closely with the plane’s G-meter. Conclusion: Aerobatic pilots experience a large range of positive and negative accelerations, which appear to correlate well to those of their aircraft. Data can be successfully collected and correlated using tri-axial accelerometers and angular rate sensors. Future work in this field may involve clinical modeling of G-effects based on head-level accelerations and angular rates.