Investigation of Ultrasonic Wave Scattering Effects using Computational Methods

Advances in computational power and expanded access to computing clusters has made mathematical modeling of complex wave effects possible. We have used multi-core and cluster computing to implement analytical and numerical models of ultrasonic wave scattering in fluid and solid media (acoustic and elastic waves). We begin by implementing complicated analytical equations that describe the force upon spheres immersed in inviscid and viscous fluids due to an incident plane wave. Two real-world applications of acoustic force upon spheres are investigated using the mathematical formulations: emboli removal from cardiopulmonary bypass circuits using traveling waves and the micromanipulation of algal cells with standing waves to aid in biomass processing for algae biofuels. We then move on to consider wave scattering situations where analytical models do not exist: scattering of acoustic waves from multiple scatterers in fluids and Lamb wave scattering in solids. We use a numerical method called finite integration technique (FIT) to simulate wave behavior in three dimensions. The 3D simulations provide insight into experimental results for situations where 2D simulations would not be sufficient. The diverse set of scattering situations explored in this work show the broad applicability of the underlying principles and the computational tools that we have developed. Overall, our work shows that the movement towards better availability of large computational resources is opening up new ways to investigate complicated physics phenomena

The fluid-structure interaction of an axisymmetric underexpanded jet flowing across an adjacent compliant surface

The current work characterizes, via detailed experiments, the flowfield and surface states of the relatively little-explored problem of an underexpanded axisymmetric jet flowing across an adjacent parallel surface, in both rigid-surface and compliant-surface configurations. To date, the vast majority of the literature for both the rigid- and compliant-surface cases has been generated by the acoustics community, and so most of the results obtained in those studies are rooted in pointwise measurements. This has resulted in little interest in the many relevant fluid-mechanical phenomena present in this turbulent flow. Meanwhile, there has been a growing interest in the area of fluid-structure interactions (FSIs) as we continue to push technological, engineering, and operational boundaries towards lighter and stronger structures at increasingly high speeds, pressures, and temperatures. The simultaneous acquisition of flowfield and structural conditions in fullfield has recently become a priority for the FSI community, but it presents a significant experimental challenge, and only a few studies to-date have successfully done so, and none (to the author’s knowledge) have done so for high-speed flows. It is the purpose of this work to document and experimentally characterize the rigid- and compliant-surface flowfields, and their corresponding surface states, towards the goals of improving the measurement quality and better understanding of the fluid-structure interactions involved. The study takes a multi-step approach. First, the fundamental flow - that of an isolated jet emanating from a 12.7-mm exit-diameter converging nozzle - is characterized in both a flowfield-velocimetry and unsteady manner at a nozzle pressure ratio, NPR, of 5.0. Next, the rigid-surface case is considered at three jet/surface separation distances, h/Dj (0.50, 0.55, and 0.60), and the flowfield and accompanying surface conditions are documented in detail, so as to provide a baseline for comparison. Then, the simultaneous acquisition of fullfield instantaneous velocity fields and surface deflection data are demonstrated and rigorously validated using particle image velocimetry (PIV) and stereo/3D digital image correlation (sDIC), respectively, for the compliantsurface case. These data provide insights into the relevant fluid-structure interaction involved via a comparison to the rigid-surface case. The results obtained for the isolated jet case were consistent with the theoretical expectations and previous findings. Compressible waves visualized using high-speed schlieren photography aligned well with the PIV velocity-vector fields. A shock-position oscillation timehistory analysis conducted using the high-speed schlieren imagery identified distinct narrowband spectral peaks. These peaks were found to agree well with those observed in the acoustic spectra, as well as semi-empirical screech tone prediction models. Good agreement was observed for all tested operating conditions, leading to the conclusion that high-speed schlieren can be used to identify jet screech in the absence of acoustic spectra. Tomographic PIV data revealed the three-dimensional nature of the jet. High-speed schlieren, planar PIV, steady pressure-sensitive paint (PSP), and surface oil flow visualization (SOFV) data obtained for the rigid-surface case were mutually consistent with each other, providing insight into the structure of the flowfield. For small h/Dj, a “plate-induced shock”, formed in response to the restricted near-wall flow expansion, was observed in the schlieren imagery. This shock wave interacts with the barrel shock to cause the formation of a prominent shock/boundary-layer interaction (SBLI) near the wall, replacing the traditional first-cell Mach disk structure. Surface pressure was found to decrease within the initial jet expansion region, and to be approximately constant within the separation region, and to increase near the reattachment point. The pressure ratio across the plate surface was observed to be less than unity everywhere within the first shock cell and SBLI, imposing a loading that would lead to a jetward deflection in the compliant-surface case. PIV data reveal the presence of a small normal shock above the SBLI separation region, inducing large losses. Farther downstream, the schlieren, PIV, PSP, and SOFV data show the shock-cell structure of the jet, typified by a crossing-type shock structure. Increasing h/Dj resulted in a reduction in SBLI size and strength, eventually restoring the isolated jet structure. Detailed PIV measurement uncertainties were calculated in a thorough manner using an in-house developed code, and surface temperature and unsteady PSP measurements (limited success) were obtained. The compliant-surface model was constructed using a frame and steel sheet shim stock (0.003” thick) approach. The compliant surface was 125-mm square, and all four edges were clamped. Surface deflection data were obtained on the backside of the surface using sDIC, while PIV data were obtained simultaneously on the other side. sDIC out-of-plane measurement resolution and accuracies were quantified and both experiments were rigorously validated individually, and a cross-validation was performed by plotting the sDIC results on the raw PIV images. Support frame out-of-plane deflections were accounted for via sDIC. The compliant surface was found to take a quasi-static/steady response characterized by a standing wave pattern with a large jetward deflection in the first shock cell (~0.7 mm), and smaller oscillations farther downstream primarily oriented away from the jet. The mean deflected shape was reminiscent of the anticipated (3,1) vibrational mode shape. Surface strains were below the material’s elastic strain limit. The large first-cell jetward deflection was found to impose a supersonic compression-expansion effect on the flow, transitioning near the deflected-surface inflection point. This altered the SBLI in-flow conditions, leading to a weaker SBLI shock system, and slightly elevated velocities and Mach number upon exiting the SBLI region. The compliant-surface case separation region was found to be longer and relatively thinner, and the boundary layer thickness throughout the measurement domain was increased, as compared to the rigid-surface case. In addition, the downstream shock-cell structure was different, where one of the shock-trains was attenuated, resulting in a zig-zag pattern. The jet primary shear layer position and size were found to be equivalent, and velocity and turbulence statistics profiles near the end of the measurement domain were found to be similar

Director's discretionary fund

This technical memorandum contains brief technical papers describing research and technology development programs sponsored by the ARC Director's Discretionary Fund during fiscal year 1992 (Oct. 1991 through Sep. 1992). An appendix provides administrative information for each of the 45 sponsored research programs

Qualitative measurements of pressure-atomized sprays through simultaneous collection of planar fluorescence, phosphorescence, and Mie scattering data

A laser diagnostic technique useful for qualitatively locating and describing regions of vapor and liquid structures of a pressure atomized fuel spray is examined. While Mie scattering is sensitive to the liquid phase within a spray, planar laser-induced fluorescence is sensitive to both the liquid and vapor phases. Hence, a comparison of images utilizing these two techniques could be used to qualitatively distinguish regions of vapor from regions dominated by droplets. Quantitative subtraction of the two signals is subject to significant error in polydisperse sprays, however, due to the fact that scattering is sensitive to droplet surface area (diameter squared) while fluorescence is sensitive to droplet volume (diameter cubed). Moreover, even qualitative comparison of the two signals may yield false identification of fuel vapor because of possible differences in signal behavior within dense regions of the spray. By simultaneously capturing phosphorescence in addition to fluorescence and Mie scattering, it is possible to gain further insight because phosphorescence is proportional to droplet volume, like fluorescence, but is sensitive only to droplets, like Mie scattering. Hence, phosphorescence can be used to determine whether differences between fluorescence and Mie scattering signals are due to the presence of fuel vapor or due simply to the different photophysics between the two techniques. The current work shows the utility of using phosphorescence for added information and advances the state of the art by (1) testing the use of fluorescence, phosphorescence, and Mie scattering (FPM) in a dense spray, (2) testing FPM in a multi-component fuel, (3) implementing FPM in a practical device, and (4) conducting tests with FPM under elevated temperatures. Signal collection techniques and data conditioning methods are presented and discussed for both laboratory and test cell applications. Results show that the measurement of fuel vapor from differences in fluorescence and Mie scattering data can be misleading due to variations in multiple scattering with these two techniques. By adding phosphorescence, it is possible to show that regions that appear to consist of fuel vapor from fluorescence are more likely attributable to diffuse scattering from a dense field of droplets within the spray. This is an important result that shows the significance of simultaneous collection of FPM signals in practical fuel sprays. Suggestions to improve and advance the technique are also presented

Experimental and theoretical investigations of charge generation and transport in thin film photovoltaics

With concerns regarding climate change, pollution, and a limited supply of fossil fuels, photovoltaics are an attractive alternate energy source. Within the field of photovoltaics, thin film organic solar cells are alluring due to their potential low cost, mechanical flexibility, and ease of fabrication. However, there are many drawbacks that need to be overcome such as incomplete photon absorption, incomplete exciton dissociation, and carrier recombination. Three distinct projects addressing charge generation and collection in thin film photovoltaics are described. The first details the use of microlens arrays (MLAs) as a nonintrusive method to increase photon absorption in organic solar cells. Laser holography and soft lithography were used to produce the MLAs on the glass side of an indium tin oxide substrate. In PTB7-based devices, we saw improvements in short circuit current (Jsc) of more than 10%, and achieved a high average power conversion efficiencies of 8.5%. Additionally, we used simulations utilizing the scattering matrix method to corroborate our experimental results. These simulations revealed that, for a given pitch of a MLA, a taller height typically yields more enhancement. Second, the effects of using BaTiO3 nanoparticles as additives in polythiophene:fullerene solar cells are experimentally and theoretically investigated. BaTiO3 nanoparticles were chosen because of their high dielectric constant, which can increase exciton dissociation, and the potential for light scattering. To achieve stable suspensions for device fabrication, the nanoparticles were functionalized with organic ligands. Solar cells fabricated in air showed ~40% enhancement in the photocurrent primarily due to string-like aggregates of functionalized BaTiO3 particles, which increase light absorption without hindering charge collection. Solar cells fabricated in an inert atmosphere yielded overall more efficient devices, but the string-like aggregates were absent and enhancement in photocurrent was up to ~6%. Simulations with the excitonic drift-diffusion model demonstrate that a bare nanoparticle significantly increases exciton dissociation, whereas the functional group negates this effect. Simulations utilizing the scattering matrix method reveal that absorption enhancements caused by light scattering increase as the nanoparticles aggregate into string-like structures. Lastly, a computational study investigating correlations between morphological features in two dimensional bulk heterojunctions and relevant photo physical processes is reported. A set of morphological descriptors were evaluated for a large set of morphologies utilizing a graph-based method. A morphology aware excitonic drift-diffusion model was used to compute current-voltage curves, fill factors, efficiencies, as well as spatial distributions of exciton generation, dissociation, and charge collection for each morphology. We find that the device efficiency primarily depends on the short circuit current, and has almost no dependence on the fill factor. Interestingly, we find that the fill factor is largely insensitive to many of the investigated descriptors. It is only weakly dependent on the contact area mismatch – the difference between the fraction of anode in direct contact with donor and the fraction of cathode in direct contact with acceptor. The fill factor is maximized when this quantity is nearly balanced. Since morphologies with a higher fraction of the electrodes in contact with the desirable material show higher short circuit current, we conclude that designing morphologies for a high short circuit current will necessarily lead to reasonably high fill factors

Fluid-Structure-Jet Interaction Effects on High-Speed Vehicles

This dissertation is focused on two design considerations for supersonic intercept missiles: (i) increased structural slenderness and (ii) attitude control jets. The resulting new designs have the potential to increase vehicle performance, but will lead to a coupled fluid-structure-jet interaction that has yet to be studied. Numerical results of the vehicle response across the design space and flight envelope can be used as guidelines for assessment of improved control effectiveness, maneuverability and agility. First, vehicle models are developed that include slender structures and attitude control jets to conduct flight simulations. The numerical analysis of fluid-structure-jet interaction using these vehicle models deleted{helps to fill the gap in the literature and} provides insight into how this interaction can be leveraged during the design to improve performance. Next, approximate methods for including jet interaction effects are developed for slender high-speed vehicles. These methods allow for more complex geometry, a range of flight conditions, and varying control inputs. The jet interaction models are developed for flight simulation to maintain accuracy without significant computational cost. A detailed computational model of the maneuverable vehicle with fluid-structure-jet interaction is created to study the sensitivity to changes in flight conditions. These steady and dynamic results of the nonlinear system identify the conditions that may be difficult to model as well as those that can be exploited for improved performance. Next, modeling methods for the fluid-structure-jet interaction dynamics in flight are developed and evaluated using aggressive maneuvers throughout the flight envelope. Previous methods are evaluated to identify their effectiveness and a new method is developed specifically to model the nonlinear vehicle response to aggressive maneuvers. Finally, fluid-structure-jet interaction effects introduced by a slender missile body and attitude control jets are modeled during flight simulations. Multiple vehicle configurations are considered and the simulation results demonstrate the corresponding design modifications can impact vehicle maneuverability and agility. Overall, this dissertation explores a new topic in fluid-structure-jet interaction that arises due to new design trends that seek to improve intercept missile performance. New modeling methods were developed to analyze the problem and numerical simulation results identify regions where the fluid-structure-jet interaction significantly affects the vehicle response.PHDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147571/1/kitson_1.pd

Design, Manufacture, and Structural Dynamic Analysis of a Biomimetic Insect-Sized Wing for Micro Air Vehicles

The exceptional flying characteristics of airborne insects motivates the design of biomimetic wing structures that can exhibit a similar structural dynamic behavior. For this purpose, this investigation describes a method for both manufacturing a biomimetic insect-sized wing using the photolithography technique and analyzing its structural dynamic response. The geometry of a crane fly forewing (family Tipulidae) is acquired using a micro-computed tomography scanner. A computer-aided design model is generated from the measurements of the reconstructed scanned model of the insect wing to design the photomasks of the membrane and the venation network required for the photolithography procedure. A composite material wing is manufactured by patterning the venation network using photoresist SU-8 on a Kapton film for the assembling of the wing. A single material artificial wing is fabricated using the photoresist SU-8 for both the membrane and the network of veins. Experiments are conducted using a modal shaker and a digital image correlation (DIC) system to determine the natural frequencies and the mode shapes of the artificial wing from the fast Fourier transform of the displacement response of the wing. The experimental results are compared with those from a finite element (FE) model of the wing. A numerical simulation of the fluid-structure interaction is conducted by coupling the FE model of the artificial wing with a computational fluid dynamics model of the surrounding airflow. From these simulations, the deformation response and the coefficients of drag and lift of the artificial wing are predicted for different freestream velocities and angles of attack. Wind-tunnel experiments are conducted using the DIC system to determine the structural deformation response of the artificial wing under different freestream velocities and angles of attack. The vibration modes are dominated by a bending and torsional deformation response. The deformation along the span of the wing increases nonlinearly from the root of the wing to the tip of the wing with Reynolds number. The aerodynamic performance, defined as the ratio of the coefficient of lift to the coefficient of drag, of the artificial wing increases with Reynolds number and angle of attack up to the critical angle of attack

Design, Manufacture, and Structural Dynamic Analysis of a Biomimetic Insect-Sized Wing for Micro Air Vehicles

The exceptional flying characteristics of airborne insects motivates the design of biomimetic wing structures that can exhibit a similar structural dynamic behavior. For this purpose, this investigation describes a method for both manufacturing a biomimetic insect-sized wing using the photolithography technique and analyzing its structural dynamic response. The geometry of a crane fly forewing (family Tipulidae) is acquired using a micro-computed tomography scanner. A computer-aided design model is generated from the measurements of the reconstructed scanned model of the insect wing to design the photomasks of the membrane and the venation network required for the photolithography procedure. A composite material wing is manufactured by patterning the venation network using photoresist SU-8 on a Kapton film for the assembling of the wing. A single material artificial wing is fabricated using the photoresist SU-8 for both the membrane and the network of veins. Experiments are conducted using a modal shaker and a digital image correlation (DIC) system to determine the natural frequencies and the mode shapes of the artificial wing from the fast Fourier transform of the displacement response of the wing. The experimental results are compared with those from a finite element (FE) model of the wing. A numerical simulation of the fluid-structure interaction is conducted by coupling the FE model of the artificial wing with a computational fluid dynamics model of the surrounding airflow. From these simulations, the deformation response and the coefficients of drag and lift of the artificial wing are predicted for different freestream velocities and angles of attack. Wind-tunnel experiments are conducted using the DIC system to determine the structural deformation response of the artificial wing under different freestream velocities and angles of attack. The vibration modes are dominated by a bending and torsional deformation response. The deformation along the span of the wing increases nonlinearly from the root of the wing to the tip of the wing with Reynolds number. The aerodynamic performance, defined as the ratio of the coefficient of lift to the coefficient of drag, of the artificial wing increases with Reynolds number and angle of attack up to the critical angle of attack

A framework for non-intrusive load monitoring and diagnostics

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.Includes bibliographical references (p. 259-260).The widespread use of electrical and electromechanical systems places increasing demands on monitoring and diagnostic techniques. The non-intrusive load monitor (NILM) provides a low-cost, low-maintenance way to perform this monitoring and diagnostics from a centralized location. This work critically evaluates the current state of the NILM hardware and software in order to develop new techniques and a new hardware and software framework in which to better apply the NILM to real-world systems. New diagnostic indicators are developed on the USCGC SENECA using an improved hardware and software platform. A database-driven framework with the flexibility to create and implement these and future diagnostic indicators is presented.by James Paris.M.Eng