Double diffusion in oceanography : proceedings of a meeting September 26-29, 1989

A meeting to review research progress on double-diffusive phenomena in the ocean was held September 26-29, 1989, at the Woods Hole Oceanographic Institution. Twenty-five oral presentations were made and a number of discussion sessions were held. This report contains manuscripts provided by meeting participants, summaries of the discussion sessions and an extensive bibliography on oceanic double-diffusion. Since double-diffusive processes appear to play an important role in ocean mixing, further research in this field should have high priority. It is hoped that this update on the status of our current understanding will facilitate planning of additional research.Funding was provided by the National Science Foundation under grant No. OCE 88-13060

Adaptive Sampling For Efficient Online Modelling

This thesis examines methods enabling autonomous systems to make active sampling and planning decisions in real time. Gaussian Process (GP) regression is chosen as a framework for its non-parametric approach allowing flexibility in unknown environments. The first part of the thesis focuses on depth constrained full coverage bathymetric surveys in unknown environments. Algorithms are developed to find and follow a depth contour, modelled with a GP, and produce a depth constrained boundary. An extension to the Boustrophedon Cellular Decomposition, Discrete Monotone Polygonal Partitioning is developed allowing efficient planning for coverage within this boundary. Efficient computational methods such as incremental Cholesky updates are implemented to allow online Hyper Parameter optimisation and fitting of the GP's. This is demonstrated in simulation and the field on a platform built for the purpose. The second part of this thesis focuses on modelling the surface salinity profiles of estuarine tidal fronts. The standard GP model assumes evenly distributed noise, which does not always hold. This can be handled with Heteroscedastic noise. An efficient new method, Parametric Heteroscedastic Gaussian Process regression, is proposed. This is applied to active sample selection on stationary fronts and adaptive planning on moving fronts where a number of information theoretic methods are compared. The use of a mean function is shown to increase the accuracy of predictions whilst reducing optimisation time. These algorithms are validated in simulation. Algorithmic development is focused on efficient methods allowing deployment on platforms with constrained computational resources. Whilst the application of this thesis is Autonomous Surface Vessels, it is hoped the issues discussed and solutions provided have relevance to other applications in robotics and wider fields such as spatial statistics and machine learning in general

A GPU-ACCELERATED, HYBRID FVM-RANS METHODOLOGY FOR MODELING ROTORCRAFT BROWNOUT

A numerically effecient, hybrid Eulerian- Lagrangian methodology has been developed to help better understand the complicated two- phase flowfield encountered in rotorcraft brownout environments. The problem of brownout occurs when rotorcraft operate close to surfaces covered with loose particles such as sand, dust or snow. These particles can get entrained, in large quantities, into the rotor wake leading to a potentially hazardous degradation of the pilots visibility. It is believed that a computationally efficient model of this phenomena, validated against available experimental measurements, can be a used as a valuable tool to reveal the underlying physics of rotorcraft brownout. The present work involved the design, development and validation of a hybrid solver for the purpose of modeling brownout-like environments. The proposed methodology combines the numerical efficiency of a free-vortex method with the relatively high-fidelity of a 3D, time-accurate, Reynolds- averaged, Navier-Stokes (RANS) solver. For dual-phase simulations, this hybrid method can be unidirectionally coupled with a sediment tracking algorithm to study cloud development. In the past, large clusters of CPUs have been the standard approach for large simulations involving the numerical solution of PDEs. In recent years, however, an emerging trend is the use of Graphics Processing Units (GPUs), once used only for graphics rendering, to perform scientific computing. These platforms deliver superior computing power and memory bandwidth compared to traditional CPUs and their prowess continues to grow rapidly with each passing generation. CFD simulations have been ported successfully onto GPU platforms in the past. However, the nature of GPU architecture has restricted the set of algorithms that exhibit significant speedups on these platforms - GPUs are optimized for operations where a massively large number of threads, relative to the problem size, are working in parallel, executing identical instructions on disparate datasets. For this reason, most implementations in the scientific literature involve the use of explicit algorithms for time-stepping, reconstruction, etc. To overcome the difficulty associated with implicit methods, the current work proposes a multi-granular approach to reduce performance penalties typically encountered with such schemes. To explore the use of GPUs for RANS simulations, a 3D, time- accurate, implicit, structured, compressible, viscous, turbulent, finite-volume RANS solver was designed and developed in CUDA-C. During the development phase, various strategies for performance optimization were used to make the implementation better suited to the GPU architecture. Validation and verification of the GPU-based solver was performed for both canonical and realistic bench-mark problems on a variety of GPU platforms. In these test- cases, a performance assessment of the GPU-RANS solver indicated that it was between one and two orders of magnitude faster than equivalent single CPU core computations ( as high as 50X for fine-grain computations on the latest platforms). For simulations involving implicit methods, a multi-granular technique was used that sought to exploit the intermediate coarse- grain parallelism inherent in families of line- parallel methods like Alternating Direction Implicit (ADI) schemes coupled with con- servative variable parallelism. This approach had the dual effect of reducing memory bandwidth usage as well as increasing GPU occupancy leading to significant performance gains. The multi-granular approach for implicit methods used in this work has demonstrated speedups that are close to 50% of those expected with purely explicit methods. The validated GPU-RANS solver was then coupled with GPU-based free-vortex and sediment tracking methods to model single and dual-phase, model- scale brownout environments. A qualitative and quantitative validation of the methodology was performed by comparing predictions with available measurements, including flowfield measurements and observations of particle transport mechanisms that have been made with laboratory-scale rotor/jet configurations in ground effect. In particular, dual-phase simulations were able to resolve key transport phenomena in the dispersed phase such as creep, vortex trapping and sediment wave formation. Furthermore, these simulations were demonstrated to be computationally more efficient than equivalent computations on a cluster of traditional CPUs - a model-scale brownout simulation using the hybrid approach on a single GTX Titan now takes 1.25 hours per revolution compared to 6 hours per revolution on 32 Intel Xeon cores

FIRE Science Results 1989

FIRE (First ISCCP Regional Experiment) is a U.S. cloud-radiation research program formed in 1984 to increase the basic understanding of cirrus and marine stratocumulus cloud systems, to develop realistic parameterizations for these systems, and to validate and improve ISCCP cloud product retrievals. Presentations of results culminating the first 5 years of FIRE research activities were highlighted. The 1986 Cirrus Intensive Field Observations (IFO), the 1987 Marine Stratocumulus IFO, the Extended Time Observations (ETO), and modeling activities are described. Collaborative efforts involving the comparison of multiple data sets, incorporation of data measurements into modeling activities, validation of ISCCP cloud parameters, and development of parameterization schemes for General Circulation Models (GCMs) are described

Modeling spatial and temporal variations of surface moisture content and groundwater table fluctuations on a fine-grained beach, Padre Island, Texas

The basic goals of this study are to document, represent and model beach surface moisture dynamics. Achieving these goals requires that the dynamics be understood within the context of the key associated processes including evaporation and groundwater table fluctuations. Atmospheric parameters including wind speed, air temperature and relative humidity, evaporation, beach surface moisture content, groundwater table fluctuations and tidal oscillations were directly monitored in an eight-day field experiment. Field measurements demonstrated that beach surface moisture content has a relatively high degree of variability in the cross-shore direction and a relatively low variability in the alongshore direction. The highest levels of variability were found in the middle beach, where daily fluctuations of up to 30% (volume) were common. Long-term variations in surface moisture content are controlled by water table fluctuations, while short-term variations are dominated by either evaporation or groundwater table fluctuations depending on the local water table depth. Two traditional methods to estimate potential evaporation rates were tested, the mass-transfer method and the combined (energy-budget and mass-transfer together) approach. Results showed that the mass-transfer method produces consistent large errors in simulations, even with recalibration of the constants. Simulations utilizing the Penman equation provide much better agreement with field data. It was found that groundwater table fluctuations at the studied beach are mainly forced by tidal oscillations. The numerical solution of the linealized Boussinesq equation provides an accurate approach to simulate tide-forced beach groundwater table fluctuations. These simulations were found to be significantly improved by modeling the system as a sloping beach rather than using the traditional vertical beach approach. Spatial and temporal variations in beach surface moisture were modeled using the numerically solved Richard’s equation and the Force-Restore method. The Force-Restore method underestimates surface moisture contents when the water table is relatively shallow owing to an inherent defect in the model itself. The simulations employing the numerically solved Richard’s equation agree closely surface moisture content from the field. This study represent perhaps the first, certainly the most comprehensive, attempt that has been made to date, to explain intermediate-scale variability in beach surface moisture content in light of the microscale process that drive these dynamics. The findings of this study should be applicable to longer time periods, and larger spatial areas with similar environmental settings. However, more investigations regarding hydraulic properties of local sediment are needed to enhance the model applicability

Eulerian-Lagrangian definition of coarse bed-load transport: Theory and verification with low-cost inertial measurement units

Fluvial sediment transport is controlled by hydraulics, sediment properties and arrangement, and flow history across a range of time scales. This physical complexity has led to ambiguous definition of the reference frame (Lagrangian or Eulerian) in which sediment transport is analysed. A general Eulerian-Lagrangian approach accounts for inertial characteristics of particles in a Lagrangian (particle fixed) frame, and for the hydrodynamics in an independent Eulerian frame. The necessary Eulerian-Lagrangian transformations are simplified under the assumption of an ideal Inertial Measurement Unit (IMU), rigidly attached at the centre of the mass of a sediment particle. Real, commercially available IMU sensors can provide high frequency data on accelerations and angular velocities (hence forces and energy) experienced by grains during entrainment and motion, if adequately customized. IMUs are subjected to significant error accu- mulation but they can be used for statistical parametrisation of an Eulerian-Lagrangian model, for coarse sediment particles and over the temporal scale of individual entrainment events. In this thesis an Eulerian-Lagrangian model is introduced and evaluated experimentally. Absolute inertial accelerations were recorded at a 4 Hz frequency from a spherical instrumented particle (111 mm diameter and 2383 kg/m3 density) in a series of entrainment threshold experiments on a fixed idealised bed. The grain-top inertial acceleration entrainment threshold was approximated at 44 and 51 mg for slopes 0.026 and 0.037 respectively. The saddle inertial acceleration entrainment threshold was at 32 and 25 mg for slopes 0.044 and 0.057 respectively. For the evaluation of the complete Eulerian-Lagrangian model two prototype sensors are presented: an idealised (spherical) with a diameter of 90 mm and an ellipsoidal with axes 100, 70 and 30 mm. Both are instrumented with a complete IMU, capable of sampling 3D inertial accelerations and 3D angular velocities at 50 Hz. After signal analysis, the results can be used to parametrize sediment movement but they do not contain positional information. The two sensors (spherical and ellipsoidal) were tested in a series of entrainment experiments, similar to the evaluation of the 111 mm prototype, for a slope of 0.02. The spherical sensor entrained at discharges of 24.8 ± 1.8 l/s while the same threshold for the ellipsoidal sensor was 45.2 ± 2.2 l/s. Kinetic energy calculations were used to quantify the particle-bed energy exchange under fluvial (discharge at 30 l/s) and non-fluvial conditions. All the experiments suggest that the effect of the inertial characteristics of coarse sediments on their motion is comparable to the effect hydrodynamic forces. The coupling of IMU sensors with advanced telemetric systems can lead to the tracking of Lagrangian particle trajectories, at a frequency and accuracy that will permit the testing of diffusion/dispersion models across the range of particle diameters

Sustainable design and durability of domestic micro combined heat and power scroll expander systems.

Research to understand the mechanisms of wear within the main components of three different micro-CHP scroll expander systems was conducted. This was performed in order to identify the possible tribo-mechanical effects (abrasion, adhesion, cavitation, fatigue) which occur on the substrate of these components during the operation of the scroll which can seriously affect the lifecycle of the micro-CHP unit. Three-dimensional interferometer, surface scanning and scanning electron rnicroscopy (SEM) were used for surface analyses. The critical components for durability were identified on the tip seal and the steel plate of the scroll expander. Abrasive wear derived from a two-body contact on the interface of the tip seal and the steel plate. Three-body wear was found across the steel plate of the scroll. Finally, cavitation pits were revealed. Interestingly, cavitation was generated by the increment of pressure. It was found that sufficiently high pressure can liquefy instantaneously part of the refrigerant close to the bottom boundary, creating conditions for the generation of cavitation bubbles within the liquefied refrigerant. This finding resolves the puzzle of how the refrigerant which enters the scroll in gas phase produces cavitation. The wear mechanisms identified can significantly reduce the performance of the scroll. Specimens made from the steel plate (high carbon steel) and the tip seal (high performance reinforced fluoroelastomer) of the scroll expander were used for bench tests. The parts were used to perform sliding tribological tests using a special purpose-built modified micro-friction machine TE 57 in order to clearly identify the sliding wear and friction mechanisms. These tests were performed under a specific load and lubrication regime. The experimental conditions were adjusted to those of the industrial applications. Furthermore an experimental study using an ultrasonic transducer (submerged into the fluids) was utilised to produce cavitation bubbles. Using high-speed camera techniques the bubbles were observed within the working fluids. A thorough investigation of the dynamic behaviour of the bubbles and their cavitation mechanisms was conducted using the two scroll fluids (lubricant/refrigerant). The experimental results were effectively correlated with the computational ones. Additionally, the impact of the scroll fluid cavities across the surface of various commercial steel grades, including the actual steel plate of the scroll, was determined. Finally, their cavitation performance and durability, over a prolonged period of time was investigated

Abstracts of papers presented at the Eleventh International Laser Radar Conference

Abstracts of 39 papers discuss measurements of properties from the Earth's ocean surface to the mesosphere, made with techniques ranging from elastic and inelastic scattering to Doppler shifts and differential absorption. Topics covered include: (1) middle atmospheric measurements; (2) meteorological parameters: temperature, density, humidity; (3) trace gases by Raman and DIAL techniques; (4) techniques and technology; (5) plume dispersion; (6) boundary layer dynamics; (7) wind measurements; visibility and aerosol properties; and (9) multiple scattering, clouds, and hydrometers