Pressure forces on sediment particles in turbulent open-channel flow : a laboratory study

Acknowledgements This research was sponsored by EPSRC grant EP/G056404/1 and their financial support is greatly appreciated. We also acknowledge Dr S. Cameron, who developed the PIV system and its algorithms. The design and construction of pressure sensors was carried out at the workshop and the experiments were conducted in the fluids laboratory at the University of Aberdeen. We therefore express our gratitude to the workshop and laboratory technicians and also to Mr M. Witz and Mr S. Gretland for their assistance in carrying out these experiments. The authors would also like to thank Professor J. Frohlich, Professor M. Uhlmann, Dr C.-B. Clemens and Mr B. Vowinckel for their useful suggestions and discussions throughout the course of this project. The Associate Editor Professor I. Marusic and four anonymous reviewers provided many useful and insightful comments and suggestions that have been gratefully incorporated into the final version.Peer reviewedPublisher PD

Reflection measurement of building materials at microwaves

Abstract. Radio waves interact differently with different materials. The knowledge of reflection and transmission characteristics of the electromagnetic waves through and from the building walls is the key in designing a radio propagation model. The dielectric properties of the material determine the behavior of reflection and transmission of the electromagnetic waves. Therefore, an oblique reflection model is implemented in this thesis to estimate the dielectric properties of various walls at frequency range of 0.7–7 GHz (6.3 GHz bandwidth). The measurement setup consists of a four-port vector network analyzer, two wideband dual-polarized cross-shaped Vivaldi antennas and two 8 m long coaxial cables. Measurements for parallel and perpendicular polarizations are achieved simultaneously by using the dual-polarized antennas. Time-domain gating is applied to separate the desired reflection and eliminate all other multiple reflections from the environment and to suppress the Line-of-Sight component from the delayed response. The estimation of dielectric property of a material is an optimization problem where a suitable objective function is minimized to get the appropriate value. A theoretical model is implemented, so that the minimum difference between the theoretical and measured absolute value of reflection coefficient gives an estimated value of complex relative permittivity. The non-linear least squares algorithm is used for optimization purpose. The real and imaginary part of complex relative permittivity is investigated in this thesis. The real part signifies the amount of electric energy stored in a material, and is called dielectric constant whereas the imaginary part is called the loss factor, which signifies the dissipation of the radiated energy. The estimated values are in good agreement with the values found in the literature. The estimated dielectric properties in this study, such as dielectric constant, loss tangent and Brewster angle of the various materials can be utilized further in designing radio propagation models for similar environments

Full Wave 2D Modeling of Scattering and Inverse Scattering for Layered Rough Surfaces with Buried Objects.

Efficient and accurate modeling of electromagnetic scattering from layered rough surfaces with buried objects finds applications ranging from detection of landmines to remote sensing of subsurface soil moisture. In this dissertation, the formulation of a hybrid numerical/analytical solution to electromagnetic scattering from layered rough surfaces is first developed. The solution to scattering from each rough interface is sought independently based on the extended boundary condition method (EBCM), where the scattered fields of each rough interface are expressed as a summation of plane waves and then cast into reflection/transmission matrices. To account for interactions between multiple rough boundaries, the scattering matrix method (SMM) is applied to recursively cascade reflection and transmission matrices of each rough interface and obtain the composite reflection matrix from the overall scattering medium. The validation of this method against the Method of Moments (MoM) and Small Perturbation Method (SPM) will be addressed and the numerical results which investigate the potential of low frequency radar systems in estimating deep soil moisture will be presented. Computational efficiency of the proposed method is also addressed. In order to demonstrate the capability of this method in modeling coherent multiple scattering phenomena, the proposed method has been employed to analyze backscattering enhancement and satellite peaks due to surface plasmon waves from layered rough surfaces. Numerical results which show the appearance of enhanced backscattered peaks and satellite peaks are presented. Following the development of the EBCM/SMM technique, a technique which incorporates a buried object in layered rough surfaces is proposed by employing the T-matrix method and the cylindrical-to-spatial harmonics transformation. Validation and numerical results are provided. Finally, a multi-frequency polarimetric inversion algorithm for the retrieval of subsurface soil properties using VHF/UHF band radar measurements is developed. The top soil dielectric constant is first determined using an L-band inversion algorithm. For the retrieval of subsurface properties, a time-domain inversion technique is employed together with a parameter optimization for the pulse shape of time delay echoes from VHF/UHF band radar observations. Some numerical studies to investigate the accuracy of the proposed inversion technique in presence of errors are shown.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/58459/1/kuoch_1.pd

The effect of confinement on the development of an axisymmetric wall jet in confined jet impingement

Impinging jets have been widely used in the industry for cooling, heating, drying and many other purposes due to their excellent level of mass and heat transfer capacities. When issued into a confinement gap fully filled with working liquid, which is a typical configuration for the compact cooling devices designed to handle the extremely high heat fluxes generated by continuously working electronic components, they are classified as submerged confined impingement jets. Though the complicated flow field induced by the jet has attracted enormous amount of research efforts from heat transfer as well as fluid dynamics points of view, many key questions still remain unanswered. The present work reports a detailed experimental study of the flow field surrounding an axisymmetric, confined, impingement jet using stereo particle image velocimetry (SPIV). The experiments are conducted at three different orifice-to-plate spacings (2, 4 and 8 jet diameters) across Reynolds number ranging from 1000 to 9000. A maximum spatial resolution of 25 μm is achieved and the temporal resolution of the measurement remains 750 Hz. Special attention has been paid to the development of the triple-layered wall-jet with incomplete self-similarity. The jet core length and expansion angle for the vertical impingement jet has been calculated and presented. At small confinement height, the recirculating vortical structure has been found to strongly affect the wall-jet development. These flow field measurements and analysis will serve to inform a variety of practical applications in which impinging jets are used

Equivalent Capacitance Approach To Calculate Effective Roughness Dielectric Parameters For Copper Foils On Printed Circuit Boards

Effective roughness dielectric (ERD) is a homogeneous lossy dielectric layer of certain thickness with effective (averaged) dielectric parameters. The ERD layer is used to model copper foil roughness in printed circuit board interconnects by being placed on a smooth conductor surface to substitute an inhomogeneous transition layer between a conductor and laminate substrate dielectric. This work derives the ERD parameters based on the understanding that there is a gradual variation of concentration of metallic inclusions in the transition layer between the dielectric and foil. The gradual variation can be structured as thin layers that are obtained using the equivalent capacitance approach. The concentration profile is extracted from scanning electron microscopy or high-resolution optical microscopy. As the concentration of metallic particles increases along the axis normal to the laminate dielectric and foil boundary, two regions can be discerned: an insulating (prepercolation) region and a conducting (percolation) region. The rates of increase in effective loss (or corresponding conductivity) in these two regions differ significantly. The proposed model of equivalent capacitance with gradient dielectric is applied to a number of different types of copper foils. The frequency-dependent dielectric parameters of the homogenized ERD are calculated from the equivalent capacitance. The results are validated using 3D numerical electromagnetic simulations. There are two types of numerical models: with homogeneous ERD parameters and layered. Both models show excellent agreement with measurements

Picosecond time scale imaging of mechanical contacts

By means of an ultrafast opto-acoustic technique we study the nanoindentation of thin chromium films on sapphire substrates using a ceramic ball bearing. Acoustic pulses at 40 GHz returning from the film–indenter interface allow the film indentation profiles to be probed to sub-nanometer resolution over contact areas 25 lm in radius. The deformation of the films during loading is hereby revealed. Furthermore, thermal wave imaging of the contact at megahertz frequencies is simultaneously achieved

Randomizable phenology-dependent corn canopy for simulated remote sensing of agricultural scenes

Crop health assessment and yield prediction from multi-spectral remote sensing imagery are ongoing areas of interest in precision agriculture. It is in these contexts that simulation-based techniques are useful to investigate system parameters, perform preliminary experiments, etc., because remote sensing systems can be prohibitively expensive to design, deploy, and operate. However, such techniques require realistic and reliable models of the real world. We thus present a randomizable time-dependent model of corn (Zea mays L.) canopy, which is suitable for procedural generation of high-fidelity virtual corn fields at any time in the vegetative growth phase, with application to simulated remote sensing of agricultural scenes. This model unifies a physiological description of corn growth subject to environmental factors with a parametric description of corn canopy geometry, and prioritizes computational efficiency in the context of ray tracing for light transport simulation. We provide a reference implementation in C++, which includes a software plug-in for the 5th edition of the Digital Imaging and Remote Sensing Image Generation tool (DIRSIG5), in order to make simulation of agricultural scenes more readily accessible. For validation, we use our DIRSIG5 plug-in to simulate multi-spectral images of virtual corn plots that correspond to those of a United States Department of Agriculture (USDA) site at the Beltsville Agricultural Research Center (BARC), where reference data were collected in the summer of 2018. We show in particular that 1) the canopy geometry as a function of time is in agreement with field measurements, and 2) the radiance predicted by a DIRSIG5 simulation of the virtual corn plots is in agreement with radiance-calibrated imagery collected by a drone-mounted MicaSense RedEdge imaging system. We lastly remark that DIRSIG5 is able to simulate imagery directly as digital counts provided detailed knowledge of the detector array, e.g., quantum efficiency, read noise, and well capacity. That being the case, it is feasible to investigate the parameter space of a remote sensing system via “end-to-end” simulation