Typical Decay Rates Research Articles

On the Validity of the Relationship 1/T = 1/TB+1/TS+1/TD in NMR Techniques With Regards to Permeability Estimation of Natural Porous Media

One of the most relevant feature of geophysical techniques based on nuclear magnetic resonance is their ability to estimate the permeability of natural porous media, since other geophysical techniques, as the use of the formation factor and neutron well-logs, allow to quantify the volume of water in the media. Permeability is conventionally obtained from decay time of the total resonance signal. However, the fluid in the pores of a medium normally has different mobility degree that can be differentiated by the NMR results. Therefore, a detailed estimation of permeability requires decomposing the total resonance signal as a function of the decay times corresponding to the three mechanisms that contribute to the signal: the intergranular free fluid, the surface layer, and the diffusion relaxation mechanism. The relationship currently used to make this decomposition states that the exponential decay rate attributed to the total resonance signal is the sum of the three existing decay rates. We demonstrate that this relationship is not generally applicable in porous media, showing the contradiction with the much more widely accepted relationships as well as computation examples from three typical decay rates in a single pore and from sandstone with bulk and surface relaxation mechanisms. Consequently, we conclude that the assertion whereby the permeability of any porous medium does not depend on the decay time of the free fluid is an overstatement, since it only applies to very small pore sizes.

Comparative analyses of microbial structures and gene copy numbers in the anaerobic digestion of various types of sewage sludge

Anaerobic co-digestion of various sewage sludges is a promising approach for greater recovery of energy, but the process is more complicated than mono-digestion of sewage sludge. The applicability of microbial structure analyses and gene quantification to understand microbial conditions was evaluated. The results show that information from gene analyses is useful in managing anaerobic co-digestion and damaged microbes in addition to conventional parameters like total solids, pH and biogas production. Total bacterial 16S rRNA gene copy numbers are the most useful tools for evaluating unstable anaerobic digestion of sewage sludge, rather than mcrA and total archaeal 16S rRNA gene copy numbers, and high-throughput sequencing. First order decay rates of gene copy numbers during pH failure were higher than typical decay rates of microbes in stable operation. The sequencing analyses, including multidimensional scaling, showed very different microbial structure shifts, but the results were not consistent.

Swell dissipation by induced atmospheric shear stress

AbstractObservations of swell dissipation across oceans reveal a significant loss of energy that can be the result of many of processes. Among these candidate mechanisms, this paper examines the properties of the viscous air‐sea boundary layer driven by swells in order to characterize the induced atmospheric flow regime and its associated viscous dissipation over swells. A series of 3‐D numerical experiments is carried out with a RANS model and appropriate turbulence closure. These experiments reveal a laminar to turbulent transition in the near free‐surface region for a common range of characteristic amplitudes and periods of swells under stationary conditions. At low Reynolds number, laminar conditions prevail and computed decay rates conform to the analytical formulation μν of the Stokes interfacial boundary layer for this problem. The turbulent regimes are characterized as well, and the new decay rates follow a nondimensional relation above (e.g., amplitude larger than 1.1 m for a 14 s monochromatic wave period). Typical decay rates are up to 4 times above the laminar values, which is a factor 10 less than the largest rates estimated for oceanic conditions. A sensitivity analysis is finally conducted to evaluate the influence of the stationary hypothesis. It demonstrates a short setup length and low relative variations of the unsteady decay rates for laminar, transitioning and developed turbulent conditions, which confirms the evaluation of steady decay rates.

The role of dissipation in the evolution of ocean swell

[1] Dissipation of ocean swell, inferred from published oceanographic data, is investigated to determine if laboratory results on the dissipative stabilization of narrow-banded wave trains are applicable to ocean swell. Three issues are addressed. (i) Dimensional decay rates of ocean swell are about a million times smaller than typical decay rates of laboratory waves. Nevertheless, when decay rates are nondimensionalized using scales of dispersive and nonlinear effects, the dimensionless decay rates of ocean swell are comparable to those of laboratory waves, indicating that dissipation and nonlinear effects can influence ocean swell on the same time scale. (ii) The stability of ocean swell to small perturbations is examined within the theoretical framework of nonlinear Schrödinger-type models that either do or do not include dissipation. As in laboratory experiments, for swell with small enough nonlinearity, dissipation can inhibit and eventually stop the growth of small perturbations before nonlinearity becomes important. And as in laboratory experiments, we document herein an example of ocean swell with stronger nonlinearity that exhibits frequency downshifting, which is not predicted by any nonlinear Schrödinger-type model, including higher-order models, with or without dissipation. (iii) Given that dissipation can influence the evolution of ocean swell, we compare the predicted decay rates of four (published) dissipative models with observed decay rates, both in the ocean and in a laboratory wave tank. The model that presupposes an inextensible film on the free surface agrees best with measured rates of dissipation of ocean swell.

Pulse Combustion: Tailpipe Exit Jet Characteristics*

Abstract A preliminary study of the convective transfer rates of scalar quantities in a strongly oscillating free jet (+70 m/s to -50 m/s at 100 Hz generated by a pulse combustor) has been performed. This work is described in two parts: This paper discusses the free jet character, whereas the companion paper discusses the impinging jet characteristics associated with the convective heat transfer rate to a plate normal to the jet axis. In this paper the essential differences between the steady and oscillating jets are described. Rayleigh temperature measurements and flow visualizations (schlieren photography and laser Mie scattering) were used to quantify the jet characteristics. The method of vortex dynamics was used to model the vorticity and velocity fields of the pulsating jet. The modeling results support the conclusions drawn from experimental data. The steady jet exhibited typical axial and radial temperature profiles for an axisymmetric jet and, hence, it had typical decay rates. The character of the oscillating jet was dominated by the formation of toroidal vortex elements during the positive velocity portion of the cycle. The entrainment of ambient fluid and, hence, the mixing of this fluid with that of the jet was an order of magnitude greater for the oscillating jet than for the steady jet. Thus, the temperature decay rate was also an order of magnitude higher than that of the steady jet. The high peak velocities, finer scale of turbulence, and the coherent toroidal vortex of the oscillating jet are believed to be responsible for the improved scalar transport.

Post-rotary nystagmus and optokinetic after-nystagmus in the rabbit linear rather than exponential decay.

The decay of the slow phase velocity of post-rotatory (PRN) and optokinetic (OKAN) after-nystagmus as a function of time was measured in Dutch rabbits after stimulation with velocity steps of 30, 60, and 150 degrees/s. The decays fitted linear functions very well, but only poorly exponential ones. Typical decay rates were 2-5 degrees/s2, with apparent time constants (defined by decay to 37% of initial velocity) in the order of 10-20 s. Within one animal, the decays of OKAN and PRN with similar initial velocities were indistinguishable. With sinusoidal oscillation, the time constant of the vestibulo-ocular reflex - estimated from phase lead - was only 2-3 s, and probably similar to the cupular time constant. In general, time constants increased when eye velocities increased. This indicates that the vestibulo-ocular reflex of the rabbit behaves as a non-linear system. A velocity storage system with a constant discharge rate is postulated as a main non-linear element. This would introduce a linear decay of velocity as well as a threshold for velocity. This storage system would be common to both vestibulo-ocular and optokinetic reflexes.