Exponentially slow transitions on a Markov chain: the frequency of Calcium Sparks

Calcium sparks in cardiac muscle cells occur when a cluster of Ca2+ channels open and release Ca2+ from an internal store. A simplified model of Ca2+ sparks has been developed to describe the dynamics of a cluster of channels, which is of the form of a continuous time Markov chain with nearest neighbour transitions and slowly varying jump functions. The chain displays metastability, whereby the probability distribution of the state of the system evolves exponentially slowly, with one of the metastable states occurring at the boundary. An asymptotic technique for analysing the Master equation (a differential-difference equation) associated with these Markov chains is developed using the WKB and projection methods. The method is used to re-derive a known result for a standard class of Markov chains displaying metastability, before being applied to the new class of Markov chains associated with the spark model. The mean first passage time between metastable states is calculated and an expression for the frequency of calcium sparks is derived. All asymptotic results are compared with Monte Carlo simulations

Concurrent Sessions B: Fish Physiology and Fishway Passage Success - Olfactory Gene Regulation in a Regulated River: Understanding the Effects of Altered Flow Patterns on Sockeye Salmon Homing

Pacific salmon use olfactory cues to locate their spawning grounds. Past homing research has explored various aspects of the olfactory system, such as the imprinting process, but the mechanisms through which olfaction drives homing remain largely unknown. Genomics studies provide a novel approach to homing research, allowing us to investigate how olfaction and alterations to flow regimes that result from hydroelectric development mediate homing from a molecular level. Olfactory receptors (ORs) located in the olfactory epithelia detect odorants in the external environment. These receptors initiate the olfactory process, and expression of OR genes therefore strongly influences route selection during the spawning migration. We examined the expression of OR genes in a population of early summer Fraser River sockeye at the site of a hydroelectric dam. Due to diversions caused by the dam, chemical cues originating from the natal tributary are diluted by water that enters the system from a different watershed. We held sockeye in the river containing their home stream water and in the river that originates from the different watershed. We then used quantitative PCR to determine whether the absence of natal chemical cues alters olfactory gene expression. In addition, we analyzed olfactory gene expression of sockeye exposed to an artificial stressor event, to determine whether stressful events such as fishway passage or blocked waterways affect olfactory gene expression

Additive Equivalence in Turbulent Drag Reduction by Flexible and Rodlike Polymers

We address the "Additive Equivalence" discovered by Virk and coworkers: drag reduction affected by flexible and rigid rodlike polymers added to turbulent wall-bounded flows is limited from above by a very similar Maximum Drag Reduction (MDR) asymptote. Considering the equations of motion of rodlike polymers in wall-bounded turbulent ensembles, we show that although the microscopic mechanism of attaining the MDR is very different, the macroscopic theory is isomorphic, rationalizing the interesting experimental observations.Comment: 8 pages, PRE, submitte

Particle size segregation in granular flow in silos

Segregation and layering of alumina in storage silos are investigated, with a view to predicting output quality versus time, given known variations in input quality on emplacement. A variety of experiments were conducted, existing relevant publications were reviewed, and the basis for an algorithm for predicting the effect of withdrawing from a central flowing region, in combination with variations in quality due to geometric, layering and segregation effects, is described in this report

Orientation dynamics of weakly Brownian particles in periodic viscous flows

Evolution equations for the orientation distribution of axisymmetric particles in periodic flows are derived in the regime of small but non-zero Brownian rotations. The equations are based on a multiple time scale approach that allows fast computation of the relaxation processes leading to statistical equilibrium. The approach has been applied to the calculation of the effective viscosity of a thin disk suspension in gravity waves.Comment: 16 pages, 7 eps figures include

Theory of the collapsing axisymmetric cavity

We investigate the collapse of an axisymmetric cavity or bubble inside a fluid of small viscosity, like water. Any effects of the gas inside the cavity as well as of the fluid viscosity are neglected. Using a slender-body description, we show that the minimum radius of the cavity scales like $h_0 \propto t'^{\alpha}$, where $t'$ is the time from collapse. The exponent $\alpha$ very slowly approaches a universal value according to $\alpha=1/2 + 1/(4\sqrt{-\ln(t')})$. Thus, as observed in a number of recent experiments, the scaling can easily be interpreted as evidence of a single non-trivial scaling exponent. Our predictions are confirmed by numerical simulations

Revisiting the positive DC corona discharge theory: Beyond Peek's and Townsend's law

The classical positive Corona Discharge (CD) theory in cylindrical axisymmetric configuration is revisited in order to find analytically the influence of gas properties and thermodynamic conditions on the corona current. The matched asymptotic expansion of Durbin \& Turyn of a simplified but self-consitent problem is performed and explicit analytical solutions are derived. The mathematical derivation permits to express a new positive DC corona current-voltage charachteristic, either chosing dimensionless or dimensional formulation. In dimensional variables the current voltage law and the corona inception voltage explicitly depends on electrodes size and on physical gas properties such as ionization and photoionization parameters. The analytical predictions are successfully confronted with experiments and with Peek's and Townsend's laws. An analytical expression of the corona inception voltage $\varphi_{on}$ is proposed, which depends on known values of the physical parameters without adjustable parameters. As a proof of consistency, the classical Townsend current-voltage law $I=C\varphi(\varphi-\varphi_{on})$ is retrieved by linearizing the non-dimensional analytical solution. A brief parametric study showcases the interest of this analytical current model especially for exploring small corona wires or considering various thermodynamic conditions