Predicting sonic pulse shapes of underwater spark discharges

Measurements of the acoustic pressure of spark discharges were made at a shallow depth (10 feet) for various voltages, stored energies, inductances and capacitances of the system, and electrode areas. The voltages ranged from 1500 V to 11 KV, and the energy storing capacitances from 8 to 800 ufd. In this range the peak pressure observed was proportional to peak current and the decay constant of the pressure-time curve was essentially the same as the electrical discharge decay constant.Office of Naval Research under Contract Nonr 1 367(00) NR 261-10

A Chemical Kinetics Approach to The Duration-of-Load Problem in Wood

The theory of absolute rates of chemical processes is presented as an appropriate conceptual framework for understanding the creep-rupture phenomena of duration of load (DOL) and rate of loading (ROL). The theory predicts the following experimentally observed phenomena:(1) The logarithm of the time to failure under constant deadload stress increases linearly as the stress level is decreased.(2) The rupture strength in a linear-ramp ROL experiment increases with the logarithm of the rate of stressing.Moreover, the equations derived to describe these phenomena contain the same parameters. These parameters are denned physical quantities that describe the creep characteristics of the material. It is possible to predict how long a material will support a constant deadload stress (DOL behavior) from measurements of apparent rupture strength as a function of the rate of stressing in a linear-ramp loading experiment (ROL behavior).Rupture of Douglas-fir in bending is selected as an example, and the experimental results from ROL-behavior experiments are used to predict DOL behavior. The theory adequately describes the experimentally observed results

A variational framework for flow optimization using semi-norm constraints

When considering a general system of equations describing the space-time evolution (flow) of one or several variables, the problem of the optimization over a finite period of time of a measure of the state variable at the final time is a problem of great interest in many fields. Methods already exist in order to solve this kind of optimization problem, but sometimes fail when the constraint bounding the state vector at the initial time is not a norm, meaning that some part of the state vector remains unbounded and might cause the optimization procedure to diverge. In order to regularize this problem, we propose a general method which extends the existing optimization framework in a self-consistent manner. We first derive this framework extension, and then apply it to a problem of interest. Our demonstration problem considers the transient stability properties of a one-dimensional (in space) averaged turbulent model with a space- and time-dependent model "turbulent viscosity". We believe this work has a lot of potential applications in the fluid dynamics domain for problems in which we want to control the influence of separate components of the state vector in the optimization process.Comment: 30 page

Discrete Choice, Social Interaction, and Policy in Encryption Technology Adoption

We introduce a model for examining the factors that lead to the adoption of new encryption technologies. Building on the work of Brock and Durlauf, the model describes how agents make choices, in the presence of social interaction, between competing technologies given their relative cost, functionality, and usability. We apply the model to examples about the adoption of encryption in communication (email and messaging) and storage technologies (self-encrypting drives) and also consider our model’s predictions for the evolution of technology adoption over time

Layer formation and relaminarisation in plane Couette flow with spanwise stratification

Recent research has shed light on the role of coherent structures in forming layers when stably stratified turbulence is forced with horizontal shear (Lucas, Caulfield & Kerswell, J. Fluid Mech., vol. 832, 2017, pp. 409-437). Here we extend our previous work to investigate the effect of rigid boundaries on the dynamics by studying stably-stratified plane Couette flow with gravity oriented in the spanwise direction. We observe near-wall layering and associated new mean flows in the form of large scale spanwise-flattened streamwise rolls. The layers exhibit the expected buoyancy scaling $l_z\sim U/N$ where $U$ is a typical horizontal velocity scale and $N$ the buoyancy frequency. We associate the new coherent structures with a stratified modification of the well-known large scale secondary flow in plane Couette and find that the possibility of the transition to sustained turbulence is controlled by the relative size of this buoyancy scale to the spanwise spacing of the streaks. We also investigate the influence on the transition to turbulence of the newly discovered linear instability in this system (Facchini et. al. 2018 arXiv:1711.11312).EPSR

Found in Translation: Co-design for Security Modelling

Background. In increasingly complex and dynamic environments, it is difficult to predict potential outcomes of security policies. Therefore, security managers (or other stakeholders) are often challenged with designing and implementing security policies without knowing the consequences for the organization. Aim. Modelling, as a tool for thinking, can help identify those consequences in advance as a way of managing decision-making risks and uncertainties. Our co-design approach aims to tackle the challenges of problem definition, data availability, and data collection associated with modelling behavioural and cultural aspects of security. Method. Our process of modelling co-design is a proposed solution to these challenges, in particular for models aiming to incorporate organizational security culture. We present a case study of a long-term study at Company A, where using the methods of participatory action research, humble inquiry, and thematic analysis, largely shaped our understanding of co-design. We reflect on the methodological advantages of co-design, as well as shortcomings. Result. Our methodology engages modellers and system stakeholders through a four-stage co-design process consisting of (1) observation and candidate data availability, (2) candidate model design, (3) interpretation of model consequences, and (4) interpretation of domain consequences. Conclusion. We have proposed a new methodology by integrating the concept of co-design into the classical modelling cycle and providing a rigorous methodology for the construction of models that captures the system and its behaviours accurately. We have also demonstrated what an attempt at co-design looks like in the real-world, and reflected upon necessary improvements

Localization of flow structures using infinity-norm optimization

International audienceStability theory based on a variational principle and finite-time direct-adjoint optimization commonly relies on the kinetic perturbation energy density E-1(t ) = (1/V-Omega) integral(Omega) e(x, t) d Omega (where e(x, t) = vertical bar u vertical bar(2)/2) as a measure of disturbance size. This type of optimization typically yields optimal perturbations that are global in the fluid domain Omega of volume V-Omega. This paper explores the use of p-norms in determining optimal perturbations for 'energy' growth over prescribed time intervals of length T. For p = 1 the traditional energy-based stability analysis is recovered, while for large p >> 1, localization of the optimal perturbations is observed which identifies confined regions, or 'hotspots', in the domain where significant energy growth can be expected. In addition, the p-norm optimization yields insight into the role and significance of various regions of the flow regarding the overall energy dynamics. As a canonical example, we choose to solve the infinity-norm optimal perturbation problem for the simple case of two-dimensional channel flow. For such a configuration, several solutions branches emerge, each of them identifying a different energy production zone in the flow: either the centre or the walls of the domain. We study several scenarios (involving centre or wall perturbations) leading to localized energy production for different optimization time intervals. Our investigation reveals that even for this simple two-dimensional channel flow, the mechanism for the production of a highly energetic and localized perturbation is not unique in time. We show that wall perturbations are optimal (with respect to the infinity-norm) for relatively short and long times, while the centre perturbations are preferred for very short and intermediate times. The developed p-norm framework is intended to facilitate worst-case analysis of shear flows and to identify localized regions supporting dominant energy growth