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Search Results: 1 - 10 of 334522 matches for " Michael J. Thompson "
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Grand challenges in the physics of the sun and sun-like stars
Michael J. Thompson
Frontiers in Astronomy and Space Sciences , 2014, DOI: 10.3389/fspas.2014.00001
Advances in Shell Buckling: Theory and Experiments
J. Michael T. Thompson
Physics , 2014, DOI: 10.1142/S0218127415300013
Abstract: In a recent feature article in this journal, co-authored by Gert van der Heijden, I described the static-dynamic analogy and its role in understanding the localized post-buckling of shell-like structures, looking exclusively at integrable systems. We showed the true significance of the Maxwell energy criterion load in predicting the sudden onset of 'shock sensitivity' to lateral disturbances. The present paper extends the survey to cover non-integrable systems, such as thin compressed shells. These exhibit spatial chaos, generating a multiplicity of localized paths (and escape routes) with complex snaking and laddering phenomena. The final theoretical contribution shows how these concepts relate to the response and energy barriers of an axially compressed cylindrical shell. After surveying NASA's current shell-testing programme, a new non-destructive technique is proposed to estimate the 'shock sensitivity' of a laboratory specimen that is in a compressed meta-stable state before buckling. A probe is used to measure the nonlinear load-deflection characteristic under a rigidly applied lateral displacement. Sensing the passive resisting force, it can be plotted in real time against the displacement, displaying an equilibrium path along which the force rises to a maximum and then decreases to zero: having reached the free state of the shell that forms a mountain-pass in the potential energy. The area under this graph gives the energy barrier against lateral shocks. The test is repeated at different levels of the overall compression. If a symmetry-breaking bifurcation is encountered on the path, computer simulations show how this can be supressed by a controlled secondary probe tuned to deliver zero force on the shell.
Grand Challenges in the Physics of the Sun and Sun-like Stars
Michael J. Thompson
Physics , 2014,
Abstract: The study of stellar structure and evolution is one of the main building blocks of astrophysics, and the Sun has an importance both as the star that is most amenable to detailed study and as the star that has by far the biggest impact on the Earth and near-Earth environment through its radiative and particulate outputs. Over the past decades, studies of stars and of the Sun have become somewhat separate. But in recent years, the rapid advances in asteroseismology, as well as the quest to better understand solar and stellar dynamos, have emphasized once again the synergy between studies of the stars and the Sun. In this article I have selected two "grand challenges" both for their crucial importance and because I thnk that these two problems are tractable to significant progress in the next decade. They are (i) understanding how solar and stellar dynamos generate magnetic field, and (ii) improving the predictability of geo-effective space weather.
Nonlinear softening as a predictive precursor to climate tipping
Jan Sieber,J. Michael T. Thompson
Physics , 2011, DOI: 10.1098/rsta.2011.0372
Abstract: Approaching a dangerous bifurcation, from which a dynamical system such as the Earth's climate will jump (tip) to a different state, the current stable state lies within a shrinking basin of attraction. Persistence of the state becomes increasingly precarious in the presence of noisy disturbances. We consider an underlying potential, as defined theoretically for a saddle-node fold and (via averaging) for a Hopf bifurcation. Close to a stable state, this potential has a parabolic form; but approaching a jump it becomes increasingly dominated by softening nonlinearities. If we have already detected a decrease in the linear decay rate, nonlinear information allows us to estimate the propensity for early tipping due to noise. We argue that one needs to extract information about the nonlinear features (a "softening") of the underlying potential from the time series to judge the probability and timing of tipping. This analysis is the logical next step if one has detected a decrease of the linear decay rate. If there is no discernable trend in the linear analysis, nonlinear softening is even more important in showing the proximity to tipping. After extensive normal form calibration studies, we check two geological time series from paleo-climate tipping events for softening of the underlying well. For the ending of the last ice age, where we find no convincing linear precursor, we identify a statistically significant nonlinear softening towards increasing temperature. The analysis has thus successfully detected a warning of the imminent tipping event.
Solar and Stellar Activity: Diagnostics and Indices
Philip G. Judge,Michael J. Thompson
Physics , 2012, DOI: 10.1017/S1743921312004589
Abstract: We summarize the fifty-year concerted effort to place the "activity" of the Sun in the context of the stars. As a working definition of solar activity in the context of stars, we adopt those globally-observable variations on time scales below thermal time scales, of ~ 100,000 yr for the convection zone. So defined, activity is dominated by magnetic-field evolution, including the 22-year Hale cycle, the typical time it takes for the quasi-periodic reversal in which the global magnetic-field takes place. This is accompanied by sunspot variations with 11 year periods, known since the time of Schwabe, as well as faster variations due to rotation of active regions and flaring. "Diagnostics and indices" are terms given to the indirect signatures of varying magnetic- fields, including the photometric (broad-band) variations associated with the sunspot cycle, and variations of the accompanying heated plasma in higher layers of stellar atmospheres seen at special optical wavelengths, and UV and X-ray wavelengths. Our attention is also focussed on the theme of the Symposium by examining evidence for deep and extended minima of stars, and placing the 70-year long solar Maunder Minimum into a stellar context.
Shock-Sensitivity in Shell-Like Structures: with Simulations of Spherical Shell Buckling
J. Michael T. Thompson,Jan Sieber
Physics , 2015,
Abstract: Under increasing compression, an unbuckled shell is in a metastable state which becomes increasingly precarious as the buckling load is approached. So to induce premature buckling a lateral disturbance will have to overcome a decreasing energy barrier which reaches zero at buckling. Two archetypal problems that exhibit a severe form of this behaviour are the axially-compressed cylindrical shell and the externally pressurized spherical shell. Focussing on the cylinder, a non-destructive technique was recently proposed to estimate the 'shock sensitivity' of a laboratory specimen using a lateral probe to measure the nonlinear load-deflection characteristic. If a symmetry-breaking bifurcation is encountered on the path, computer simulations showed how this can be suppressed by a controlled secondary probe. Here, we extend our understanding by assessing in general terms how a single control can capture remote saddle solutions: in particular how a symmetric probe could locate an asymmetric solution. Then, more specifically, we analyse the spherical shell with point and ring probes, to test the procedure under challenging conditions to assess its range of applicability. Rather than a bifurcation, the spherical shell offers the challenge of a de-stabilizing fold (limit point) under the rigid control of the probe.
Stellar hydrodynamics caught in the act: Asteroseismology with CoRoT and Kepler
Jorgen Christensen-Dalsgaard,Michael J. Thompson
Physics , 2011, DOI: 10.1017/S1743921311017443
Abstract: Asteroseismic investigations, particularly based on data on stellar oscillations from the CoRoT and Kepler space missions, are providing unique possibilities for investigating the properties of stellar interiors. This constitutes entirely new ways to study the effects of dynamic phenomena on stellar structure and evolution. Important examples are the extent of convection zones and the associated mixing and the direct and indirect effects of stellar rotation. In addition, the stellar oscillations themselves show very interesting dynamic behaviour. Here we discuss examples of the results obtained from such investigations, across the Hertzsprung-Russell diagram.
Nonlinear dynamic interactions between flow-induced galloping and shell-like buckling
J. Michael T. Thompson,Jan Sieber
Mathematics , 2014, DOI: 10.1016/j.ijmecsci.2013.12.007
Abstract: For an elastic system that is non-conservative but autonomous, subjected for example to time-independent loading by a steadily flowing fluid (air or water), a dangerous bifurcation, such as a sub-critical bifurcation, or a cyclic fold, will trigger a dynamic jump to one or more remote stable attractors. When there is more than one candidate attractor, the one onto which the structure settles can then be indeterminate, being sensitive to infinitesimally small variations in starting conditions or parameters. In this paper we develop and study an archetypal model to explore the nonlinear dynamic interactions between galloping at an incipient sub-critical Hopf bifurcation of a structure with shell-like buckling behaviour that is gravity-loaded to approach a sub-critical pitch-fork bifurcation. For the fluid forces, we draw on the aerodynamic coefficients determined experimentally by Novak for the flow around a bluff body of rectangular cross-section. Meanwhile, for the structural component, we consider a variant of the propped-cantilever model that is widely used to illustrate the sub-critical pitchfork: within this model a symmetry-breaking imperfection makes the behaviour generic. The compound bifurcation corresponding to simultaneous galloping and buckling is the so-called Takens-Bodganov Cusp. We make a full unfolding of this codimension-3 bifurcation for our archetypal model to explore the adjacent phase-space topologies and their indeterminacies.
Prevalence of Isolated “Pre-Malignant” Lesions on Prostate Biopsy in a Racially Diverse Community Screened Cohort  [PDF]
Michael A. Liss, Donna Ankerst, David Zapata, Javier Hernandez, Robin J. Leach, Ian M. Thompson
Open Journal of Urology (OJU) , 2015, DOI: 10.4236/oju.2015.512034
Abstract: Objective: We investigated rates of prostate cancer (PCa), high-grade prostatic intraepithelial neoplaisa (HGPIN) and atypical small acinar proliferation (ASAP) in a multiethnic cohort. Methods: We evaluated prostate biopsy outcomes in men enrolled in the San Antonio Center of Biomarkers of Risk for prostate cancer (SABOR) prospective, observational study. PCa-free men underwent annual PSA testing over nearly 14 years with biopsies based on community standards. We investigated biopsy outcomes with a special interest in rates of cancer, HGPIN, and ASAP. Results: We identified 975 prostate biopsies in 801 subjects from 3/1/2001 to 1/9/2014. PCa, HGPIN, or ASAP was encountered in 28.8% (281/975), 10.1% (98/975), and 5.2% (51/975) of prostate biopsy specimens, respectively. The most significant risk factor for a PCa diagnosis was African American race (OR 5.0, 95% CI: 2.2 - 11.4, p < 0.001). HGPIN and ASAP occurred more commonly in association with PCa (both p < 0.001). We identified 57% (24/42) of men diagnosed with a “pre-malignant” lesion on prostate biopsy and had a subsequent biopsy. Of those only 8% (2/24) were diagnosed with prostate cancer (both Gleason 3 + 3) within 1 year of the initial biopsy. Conclusion: We note a 5-fold increased risk of PCa for African American men. The incidence of HGPIN and ASAP are consistent with previously reported incidence. If diagnosed in isolation, repeat biopsy within one year could be delayed or eliminated as it may not change prostate cancer outcomes.
Outcomes of thromboprophylaxis with enoxaparin vs. unfractionated heparin in medical inpatients
Lisa J McGarry, Michael E Stokes, David Thompson
Thrombosis Journal , 2006, DOI: 10.1186/1477-9560-4-17
Abstract: To compare clinical and economic outcomes among acutely-ill medical inpatients receiving the LMWH enoxaparin versus UFH prophylaxis in clinical practice.Using a large, multi-hospital, US database, we identified persons aged ≥40 years hospitalized for ≥6 days for an acute medical condition (including circulatory disorders, respiratory disorders, infectious diseases, or neoplasms) from Q4 1999 to Q1 2002. From these patients, those who received thromboprophylaxis with either enoxaparin or UFH were identified. Surgical patients and those requiring or ineligible for anticoagulation were excluded. We compared the incidence of deep-vein thrombosis (DVT), pulmonary embolism (PE), and all VTE (i.e., DVT and/or PE). Secondary outcomes were occurrence of side-effects, length of hospital stay and total costs. RESULTS: 479 patients received enoxaparin prophylaxis and 2,837 received UFH. The incidence of VTE was 1.7% with enoxaparin prophylaxis versus 6.3% with UFH (RR = 0.26; p < 0.001). Occurrence of side effects, length of stay (10.00 days with enoxaparin vs. 10.26 days with UFH; p = 0.348) and total costs ($18,777 vs. $17,602; p = 0.463) were similar in the 2 groups.We observed a 74% lower risk of VTE among patients receiving enoxaparin prophylaxis versus UFH prophylaxis. There was no significant difference in side effects or economic outcomes. These results provide evidence that the LMWH enoxaparin is more effective than UFH in reducing the risk of VTE in current clinical practice.Acutely-ill medical inpatients – such as those hospitalized for congestive heart failure (CHF), chronic obstructive pulmonary disease (COPD), acute infections, or cancer – often have prolonged hospital stays with periods of immobility that place them at increased risk of venous thromboembolism (VTE) [1]. Because these patients frequently have additional risk factors (e.g., history of VTE, advanced age, obesity, varicose veins, estrogen use), the potential benefits of thromboprophylaxis in this pop
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