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Search Results: 1 - 10 of 31409 matches for " Thomas Seifert "
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Estimation of Tree Biomass, Carbon Stocks, and Error Propagation in Mecrusse Woodlands  [PDF]
Tarquinio Mateus Magalh?es, Thomas Seifert
Open Journal of Forestry (OJF) , 2015, DOI: 10.4236/ojf.2015.54041
Abstract: We performed a biomass inventory using two-phase sampling to estimate biomass and carbon stocks for mecrusse woodlands and to quantify errors in the estimates. The first sampling phase involved measurement of auxiliary variables of living Androstachys johnsonii trees; in the second phase, we performed destructive biomass measurements on a randomly selected subset of trees from the first phase. The second-phase data were used to fit regression models to estimate below and aboveground biomass. These models were then applied to the first-phase data to estimate biomass stock. The estimated forest biomass and carbon stocks were 167.05 and 82.73 Mg·ha-1, respectively. The percent error resulting from plot selection and allometric equations for whole tree biomass stock was 4.55% and 1.53%, respectively, yielding a total error of 4.80%. Among individual variables in the first sampling phase, diameter at breast height (DBH) measurement was the largest source of error, and tree-height estimates contributed substantially to the error. Almost none of the error was attributable to plot variability. For the second sampling phase, DBH measurements were the largest source of error, followed by height measurements and stem-wood density estimates. Of the total error (as total variance) of the sampling process, 90% was attributed to plot selection and 10% to the allometric biomass model. The total error of our measurements was very low, which indicated that the two-phase sampling approach and sample size were effective for capturing and predicting biomass of this forest type.
Fluctuation-Dissipation Theorem in Nonequilibrium Steady States
Udo Seifert,Thomas Speck
Physics , 2009, DOI: 10.1209/0295-5075/89/10007
Abstract: In equilibrium, the fluctuation-dissipation theorem (FDT) expresses the response of an observable to a small perturbation by a correlation function of this variable with another one that is conjugate to the perturbation with respect to \emph{energy}. For a nonequilibrium steady state (NESS), the corresponding FDT is shown to involve in the correlation function a variable that is conjugate with respect to \emph{entropy}. By splitting up entropy production into one of the system and one of the medium, it is shown that for systems with a genuine equilibrium state the FDT of the NESS differs from its equilibrium form by an additive term involving \emph{total} entropy production. A related variant of the FDT not requiring explicit knowledge of the stationary state is particularly useful for coupled Langevin systems. The \emph{a priori} surprising freedom apparently involved in different forms of the FDT in a NESS is clarified.
Distribution of work in isothermal non-equilibrium processes
Thomas Speck,Udo Seifert
Physics , 2004, DOI: 10.1103/PhysRevE.70.066112
Abstract: Diffusive motion in an externally driven potential is considered. It is shown that the distribution of work required to drive the system from an initial equilibrium state to another is Gaussian for slow but finite driving. Our result is obtained by projection method techniques exploiting a small parameter defined as the switching rate between the two states of the system. The exact solution for a simple model system shows that such an expansion may fail in higher orders, since the mean and the variance following from the exact distribution show non-analytic behavior.
Extended Fluctuation-Dissipation Theorem for Soft Matter in Stationary Flow
Thomas Speck,Udo Seifert
Physics , 2009, DOI: 10.1103/PhysRevE.79.040102
Abstract: For soft matter systems strongly driven by stationary flow, we discuss an extended fluctuation-dissipation theorem (FDT). Beyond the linear response regime, the FDT for the stress acquires an additional contribution involving the observable that is conjugate to the strain rate with respect to the dissipation function. This extended FDT is evaluated both analytically for Rouse polymers and in numerical simulations for colloidal suspensions. More generally, our results suggest an extension of Onsager's regression principle to nonequilibrium steady states.
Jarzynski Relation, Fluctuation Theorems, and Stochastic Thermodynamics for Non-Markovian Processes
Thomas Speck,Udo Seifert
Statistics , 2007, DOI: 10.1088/1742-5468/2007/09/L09002
Abstract: We prove the Jarzynski relation for general stochastic processes including non-Markovian systems with memory. The only requirement for our proof is the existence of a stationary state, therefore excluding non-ergodic systems. We then show how the concepts of stochastic thermodynamics can be used to prove further exact non-equilibrium relations like the Crooks relation and the fluctuation theorem on entropy production for non-Markovian dynamics.
Spatial variability of recent sedimentation rates in the Eastern Gotland Basin (Baltic Sea)
Sven Hille,Thomas Leipe,Torsten Seifert
Oceanologia , 2006,
Abstract: In order to study recent sedimentation rates in the Eastern GotlandBasin, 52 short sediment corescollected fromthe deepest part (< 150 m) of the Basin in 2003were investigated.The upperparts of all the cores were distinctly laminated and dark incolour, followed by a homogeneous, greyish lower part. The thicknessof the laminated sequences varied from 17 to 300 mm. 210Pb datinganalyses of selected cores revealed that the change from non-laminatedto laminated sediments happened about 100 years ago, indicatinga shift from predominantly oxic bottom water conditions to anoxicconditions. Used as a time marker, this shift in the sedimenttexture enabled sediment accumulation rates to be estimated forall sediment cores. The observed mean linear sedimentation ratefor the whole basin was 0.93 ± 0.67 mm y-1. The respectivebulk sediment accumulation rates ranged from 10.5 to 527 g m-2 yr-1with an average of 129 ± 112 g m-2 yr-1, indicatinga high spatial variability of sedimentation rates within thebasin. This agrees very well with the long-term sedimentationpattern since the Litorina transgression. The observed patternclearly reflects the hydrographic conditions at the seaflooras studied by modelled near-bottom current velocities.
Large deviation function for entropy production in driven one-dimensional systems
Jakob Mehl,Thomas Speck,Udo Seifert
Physics , 2008, DOI: 10.1103/PhysRevE.78.011123
Abstract: The large deviation function for entropy production is calculated for a particle driven along a periodic potential by solving a time-independent eigenvalue problem. In an intermediate force regime, the large deviation function shows pronounced deviations from a Gaussian behavior with a characteristic ``kink'' at zero entropy production. Such a feature can also be extracted from the analytical solution of the asymmetric random walk to which the driven particle can be mapped in a certain parameter range.
Specific Adhesion of Membranes: Mapping to an Effective Bond Lattice Gas
Thomas Speck,Ellen Reister,Udo Seifert
Physics , 2010, DOI: 10.1103/PhysRevE.82.021923
Abstract: We theoretically consider specific adhesion of a fluctuating membrane to a hard substrate via the formation of bonds between receptors attached to the substrate and ligands in the membrane. By integrating out the degrees of freedom of the membrane shape, we show that in the biologically relevant limit specific adhesion is well described by a lattice gas model, where lattice sites correspond to bond sites. We derive an explicit expression for the effective bond interactions induced by the thermal undulations of the membrane. Furthermore, we compare kinetic Monte Carlo simulations for our lattice gas model with full dynamic simulations that take into account both the shape fluctuations of the membrane and reactions between receptors and ligands at bond sites. We demonstrate that an appropriate mapping of the height dependent binding and unbinding rates in the full scheme to rates in the lattice gas model leads to good agreement.
Role of External Flow and Frame Invariance in Stochastic Thermodynamics
Thomas Speck,Jakob Mehl,Udo Seifert
Physics , 2007, DOI: 10.1103/PhysRevLett.100.178302
Abstract: For configurational changes of soft matter systems affected or caused by external hydrodynamic flow, we identify applied work, exchanged heat, and entropy change on the level of a single trajectory. These expressions guarantee invariance of stochastic thermodynamics under a change of frame of reference. As criterion for equilibrium \textit{vs.} nonequilibrium, zero \textit{vs.} nonzero applied work replaces detailed balance \textit{vs.} nonvanishing currents, since both latter criteria are shown to depend on the frame of reference. Our results are illustrated quantitatively by calculating the large deviation function for the entropy production of a dumbbell in shear flow.
Dynamic Theory of Pearling Instability in Cylindrical Vesicles
Philip Nelson,Thomas Powers,Udo Seifert
Physics , 1994, DOI: 10.1103/PhysRevLett.74.3384
Abstract: We give a simple theory for recent experiments of Bar-Ziv and Moses% Phys. Rev. Lett. {\bf73} (1994) 1392, in which tubular vesicles are excited using laser tweezers to a ``peristaltic'' state. Considering the hydrodynamics of a bilayer membrane under tension, we reproduce some of the qualitative behavior seen and find a value for the wavelength of the instability in terms of independently measured material parameters, in rough agreement with the experimental values.
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