Lock Release Research Articles

Gravity currents

We review various aspects of buoyancy-driven flows arising from the sudden release of a fixed volume of heavy fluid into a larger body of less dense ambient fluid. The driving density contrasts for these flows may be due to temperature or salinity differences in the case of compositionally driven flows or to the presence of suspended material for their particle-driven counterpart. These so-called gravity currents play an important role in a vast array of geophysical and engineering applications and have been the subject of extensive theoretical, numerical, and experimental investigations over the past few decades. We shall first explore some of what we feel are the pertinent issues related to these flows in the context of their compositionally driven manifestation. In particular, we examine the efficacy of hydraulic theory in this context and explore its limits. We also discuss the notion of hyperbolicity as it relates to the governing hydraulic equations and the role of the stability Froude number in this concept. For the particle-driven flows we demonstrate the role played by the particles in the production of nonhydraulic effects. The roles of settling velocity and volume fraction of suspended particles in the generation of velocity shear will also be explored. The notion that particles give rise to nonhydraulic effects in the form of velocity shear has not been included in any published studies to date that we are aware of. We shall also present a means of accounting for initial turbulent energy of mixing in the release volume. This energy is that which would be injected into the system in the classic lock release experiment involving a well-mixed fixed-volume suspension maintained behind the lock gate prior to release. Results will be related to experimental data where possible.

Nonhydraulic Effects in Particle‐Driven Gravity Currents in Deep Surroundings

In this article, we present an approach to modeling the flow of particle‐driven gravity currents produced by the sudden release of well‐mixed, fixed‐volume suspensions into deep surroundings. Our model accounts for the initial turbulent energy of mixing in the release volume, characteristic of the classical lock–release experiments, as well as the spatiotemporal variability in the driving buoyancy forces attributable to particle settling. We show that, in contrast to compositionally driven flows, particle‐driven flows cannot be described consistently in terms of shallow water theory. Specifically, we show that the presence of particles in the flow dynamics produces significant horizontal velocity shear, thereby changing the flow configuration in important ways from flows assumed to be governed by the shallow water equations. These new flow properties are calculated and contrasted with flow properties derived on the basis of the shallow water equations to show that the shallow water analysis misses dynamical features of the flow. We also show that our model provides significant improvement over the previous shallow water‐based models in predicting the experimentally determined deposition patterns associated with the lock–release experiments.

Turbulent lock release gravity current

The time evolution of a turbulent lock release gravity current, formed by a finite volume of homogeneous fluid released instantaneously into another fluid of slightly lower density, was studied by experimental measurements of the density structure via elaborate digital image processing and by a numerical simulation of the flow and mixing using a two-equation turbulence model. The essential fact that the gravity current passes through an initial slumping phase in which the current head advances steadily and a second self-similar phase in which the front velocity decreases like the negative third power of the time after release is satisfactorily presented by the laboratory observation. An overall entrainment ratio proportional to the distance from the release point is found by the numerical simulation. The renormalization group (RNG)k-e model for Reynolds-stress closure is validated to characterize the gravity current with transitional and localized turbulence.

Activation of the β2-Adrenergic Receptor Involves Disruption of an Ionic Lock between the Cytoplasmic Ends of Transmembrane Segments 3 and 6

The movements of transmembrane segments (TMs) 3 and 6 at the cytoplasmic side of the membrane play an important role in the activation of G-protein-coupled receptors. Here we provide evidence for the existence of an ionic lock that constrains the relative mobility of the cytoplasmic ends of TM3 and TM6 in the inactive state of the beta(2)-adrenergic receptor. We propose that the highly conserved Arg-131(3.50) at the cytoplasmic end of TM3 interacts both with the adjacent Asp-130(3.49) and with Glu-268(6.30) at the cytoplasmic end of TM6. Such a network of ionic interactions has now been directly supported by the high-resolution structure of the inactive state of rhodopsin. We hypothesized that the network of interactions would serve to constrain the receptor in the inactive state, and the release of this ionic lock could be a key step in receptor activation. To test this hypothesis, we made charge-neutralizing mutations of Glu-268(6.30) and of Asp-130(3.49) in the beta(2)-adrenergic receptor. Alone and in combination, we observed a significant increase in basal and pindolol-stimulated cAMP accumulation in COS-7 cells transiently transfected with the mutant receptors. Moreover, based on the increased accessibility of Cys-285(6.47) in TM6, we provide evidence for a conformational rearrangement of TM6 that is highly correlated with the extent of constitutive activity of the different mutants. The present experimental data together with the recent high-resolution structure of rhodopsin suggest that ionic interactions between Asp/Glu(3.49), Arg(3.50), and Glu(6.30) may constitute a common switch governing the activation of many rhodopsin-like G-protein-coupled receptors.

Resuspension by saline and particle‐driven gravity currents

Saline and particle‐driven gravity currents play an important role in resuspension by environmental and industrial flows. We develop a model of resuspension by unsteady buoyancy‐driven flows by bringing together results from the large body of literature dealing with the dynamics of gravity current and resuspension in channel flows. The criterion for resuspending material of density ρp and diameter b is expressed in terms of a modified Shields parameter, Θ = ρfu2/(ρp − ρf)gb, where u is the depth‐averaged velocity within the current and ρf is the bulk fluid density. The critical modified Shields parameter, Θc, which corresponds to the condition when erosion is initiated, is a function of the particle Reynolds number, Rep = bυT/ν, expressed in terms of the terminal fall velocity of the sediment particles υT. Particles characterized by Rep ≫ 1 are resuspended when Θ > Θc ≈ 0.04/cD where cD is the bed drag coefficient. According to this resuspension criterion, a two‐dimensional saline gravity current generated by the release of a volume V per unit width of fluid density ρc in an ambient fluid ρa resuspends material over an erosive distance which scales as . These results are extended to describe resuspension by particle‐driven gravity currents, an analysis complicated by the change of bulk density of the suspension with time as particles sediment from the current. Here the erosive length scale is bounded by the maximum extent of the particle‐driven gravity current. A general form for the vertical mass flux qd is assumed, and the total mass resuspended is calculated in terms of the initial characteristics of the gravity current and shown to be a function of the volume V of dense fluid released, independent of the geometry of release, and independent of gravitational acceleration. Complementary laboratory experiments of resuspension by two‐dimensional saline and particle‐driven gravity currents are presented. These experiments consisted of a lock release of dense fluid running over a layer of particles. The critical conditions for resuspension were determined for different materials, and the variation of the erosive distance with gravity current characteristics was studied. These observations are discussed with reference to the theoretical model.

Simulation of a Vehicle in Longitudinal Motion with Clutch Lock and Clutch Release

A simple model for driver and vehicle in longitudinal motion is developed and simulated. The focus is on describing and handling simulation of clutch lock and clutch release which changes the model structure, both during start and gear shifts, in Simulink. Special attention is given the problem of simulating start and stop of a vehicle with rolling resistance at zero speed. Only principles for simulating the system with variable structure is of interest and therefore the models are maintained at lowest possible complexity. The system is successfully simulated and the validation is performed using three scenarios: one with only clutch lock and clutch release, one with start and stop of vehicle, and one full European drive-cycle.

Turbulent gravity current of lock release type: A numerical study

The time evolution of a turbulent gravity current of lock release type, formed by a finite volume of homogeneous fluid released instantaneously into another fluid of slightly lower density, is studied numerically via the renormalization group (RNG) κ-ϵ model for Reynolds-stress closure to characterize the flow with transitional and highly localized turbulence. Consistent with previous experimental observations, the numerical results show that the gravity current passes through two distinct phases, an initial slumping phase in which the current head advances steadily, and a second self-similar phase in which the front velocity decreases like the negative third power of the time after release. An overall entrainment ratio proportional to the distance from the release point is found and compares well with available experimental data for the slumping phase.

Implementation of transaction and concurrency control support in a temporal DBMS

Transactions and concurrency control are significant features in database systems, facilitating functions both at user and system level. However, the support of these features in a temporal DBMS has not yet received adequate research attention. In this paper, we describe the techniques developed in order to support transaction and concurrency control in a temporal DBMS that was implemented as an additional layer to a commercial DBMS. The proposed techniques make direct use of the transaction mechanisms of the DBMS. In addition, they overcome a number of limitations such as automatic commit points, lock release and log size increment, which are imposed by the underlying DBMS. Our measurements have shown that the overhead introduced by these techniques is negligible, less than 1% in all cases. The approach undertaken is of general interest, it can also be applied to non-temporal DBMS extensions.

Fast, contention-free combining tree barriers for shared-memory multiprocessors

In a previous article,(1) Gupta and Hill introduced anadaptive combining tree algorithm for busy-wait barrier synchronization on shared-memory multiprocessors. The intent of the algorithm was to achieve a barrier in logarithmic time when processes arrive simultaneously, and in constant time after the last arrival when arrival times are skewed. Afuzzy(2) version of the algorithm allows a process to perform useful work between the point at which it notifies other processes of its arrival and the point at which it waits for all other processes to arrive. Unfortunately, adaptive combining tree barriers as originally devised perform a large amount of work at each node of the tree, including the acquisition and release of locks. They also perform an unbounded number of accesses to nonlocal locations, inducing large amounts of memory and interconnect contention. We present new adaptive combining tree barriers that eliminate these problems. We compare the performance of the new algorithms to that of other fast barriers on a 64-node BBN Butterfly 1 multiprocessor, a 35-node BBN TC2000, and a 126-node KSR 1. The results reveal scenarios in which our algorithms outperform all known alternatives, and suggest that both adaptation and the combination of fuzziness with tree-style synchronization will be of increasing importance on future generations of shared-memory multiprocessors.