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Optimum stacking sequence design of composite sandwich panel using genetic algorithms

Experimental evaluation of erosive burning in composite propellants - effect of binder

<p>In recent years there has been a significant increase in interest in smaller satellites as lower cost alternatives to traditional satellites, particularly with the rise in popularity of the CubeSat. Due to stringent mass, size, and often budget constraints, these small satellites rely on making the most of inexpensive hardware components and sensors, such as coarse sun sensors (CSS) and magnetometers. More expensive high-accuracy sun sensors often combine multiple measurements, and use specialized electronics, to deterministically solve for the direction of the Sun. Alternatively, cosine-type CSS output a voltage relative to the input light and are attractive due to their very low cost, simplicity to manufacture, small size, and minimal power consumption. This research investigates using coarse sun sensors for performing robust attitude estimation in order to point a spacecraft at the Sun after deployment from a launch vehicle, or following a system fault. As an alternative to using a large number of sensors, this thesis explores sun-direction estimation techniques with low computational costs that function well with underdetermined sets of CSS. Single-point estimators are coupled with simultaneous nonlinear control to achieve sun-pointing within a small percentage of a single orbit despite the partially underdetermined nature of the sensor suite. Leveraging an extensive analysis of the sensor models involved, sequential filtering techniques are shown to be capable of estimating the sun-direction to within a few degrees, with no a priori attitude information and using only CSS, despite the significant noise and biases present in the system. Detailed numerical simulations are used to compare and contrast the performance of the five different estimation techniques, with and without rate gyro measurements, their sensitivity to rate gyro accuracy, and their computation time. One of the key concerns with reducing the number of CSS is sensor degradation and failure. In this thesis, a Modified Rodrigues Parameter based CSS calibration filter suitable for autonomous on-board operation is developed. The sensitivity of this method's accuracy to the available Earth albedo data is evaluated and compared to the required computational effort. The calibration filter is expanded to perform sensor fault detection, and promising results are shown for reduced resolution albedo models. All of the methods discussed provide alternative attitude, determination, and control system algorithms for small satellite missions looking to use inexpensive, small sensors due to size, power, or budget limitations.

<p> Since its invention at the University of Stuttgart, Germany in the mid-1960, scientists have been trying to understand and explain the mechanism of the plasma interaction inside the magnetoplasmadynamics (MPD) thruster. Because this thruster creates a larger level of efficiency than combustion thrusters, this MPD thruster is the primary cadidate thruster for a long duration (planetary) spacecraft. However, the complexity of this thruster make it difficult to fully understand the plasma interaction in an MPD thruster while operating the device. That is, there is a great deal of physics involved: the fluid dynamics, the electromagnetics, the plasma dynamics, and the thermodynamics. All of these physics must be included when an MPD thruster operates. </p><p> In recent years, a computer simulation helped scientists to simulate the experiments by programing the physics theories and comparing the simulation results with the experimental data. Many MPD thruster simulations have been conducted: E. Niewood et al.[5], C. K. J. Hulston et al.[6], K. D. Goodfellow[3], J Rossignol et al.[7]. All of these MPD computer simulations helped the scientists to see how quickly the system responds to the new design parameters. </p><p> For this work, a 1D MPD thruster simulation was developed to find the voltage drop between the cathode and the plasma regions. Also, the properties such as thermal conductivity, electrical conductivity and heat capacity are temperature and pressure dependent. These two conductivity and heat capacity are usually definded as constant values in many other models. However, this 1D and 2D cylindrical symmetry MPD thruster simulations include both temperature and pressure effects to the electrical, thermal conductivities and heat capacity values interpolated from W. F. Ahtye [4]. Eventhough, the pressure effect is also significant; however, in this study the pressure at 66 Pa was set as a baseline. </p><p> The 1D MPD thruster simulation includes the sheath region, which is the interface between the plasma and the cathode regions. This sheath model [3] has been fully combined in the 1D simulation. That is, the sheath model calculates the heat flux and the sheath voltage by giving the temperature and the current density. This sheath model must be included in the simulation, as the sheath region is treated differently from the main plasma region. </p><p> For our 2D cylindrical symmetry simulation, the dimensions of the cathode, the anode, the total current, the pressure, the type of gases, the work function can be changed in the input process as needed for particular interested. Also, the sheath model is still included and fully integrated in this 2D cylindrical symmetry simulation at the cathode surface grids. In addition, the focus of the 2D cylindrical symmetry simulation is to connect the properties on the plasma and the cathode regions on the cathode surface until the MPD thruster reach steady state and estimate the plasma arc attachement edge, electroarc edge, on the cathode surface. Finally, we can understand more about the behavior of an MPD thruster under many different conditions of 2D cylindrical symmetry MPD thruster simulations.</p>

The generation and development of large 2D vortical disturbances (coherent structures) in forced transitional and turbulent wall jets is investigated using several numerical techniques. For the early and late transition stages, 2D Numerical Simulation (2D-NS) and Direct Numerical Simulation (DNS) are employed, while for the forced turbulent flow Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculations are used including a new, simplified approach called "Stability" RANS (SRANS) which substantially reduces the computational effort when compared to URANS. As base flows for the investigations, three prototypical wall jets are considered: Low and high Reynolds number laminar wall jets, represented by the Glauert similarity solution, and a turbulent wall jet (Rej = 10,000), modeled using a nearly self-preserving RANS solution starting at a virtual nozzle. The investigations of 2D vortical disturbances in both the transitional and the turbulent wall jet follow the 2D stages of shear flow transition, beginning with receptivity to harmonic forcing, followed by linear and nonlinear disturbance development, and 2D secondary instability. It is shown that the disturbance development in the turbulent flow parallels the one in the transitional flow in many respects. In particular, a 2D subharmonic resonance is found in both flows leading to a subharmonic resonance cascade with repeated vortex merging. Competing 3D fundamental and subharmonic resonances in the transitional wall jet are studied using a linearized Navier-Stokes code and 3D DNS. These 3D secondary instabilities weaken or diminish the 2D disturbances and lead to turbulent breakdown. Yet, for large amplitude forcing, the 3D resonances are surpassed by the 2D subharmonic resonance which leads to vortex merging upstream of the breakdown. With a 3D DNS of bypass transition, where a high Reynolds number laminar wall jet is tripped with large amplitude 3D forcing, it is demonstrated that 2D vortical structures persist in the presence of 3D turbulent fluctuations. In this simulation, 2D vortical structures emerge during transition and undergo repeated merging in the turbulent flow downstream.

The effect of high amplitude forcing on laminar and turbulent wall jets over a heated flat plate is analyzed. Highly accurate Direct Numerical Simulations (DNS) are used in the laminar case to investigate the dominant transport mechanisms. When forcing is applied, the skin friction is reduced markedly and the wall heat transfer is increased, in contrast to the prediction of the Reynolds analogy, which states proportionality between both quantities. Detailed examination of the unsteady flow field showed that the concepts of eddy viscosity and eddy thermal diffusivity can be applied to analyze unsteady laminar flows and to explain the effect of highly unsteady phenomena. For the investigation of the turbulent wall jet, a new Flow Simulation Methodology (FSM) is employed in the limit of unsteady BANS (Reynolds averaged Navier-Stokes) simulations. With this novel approach, the simulation of large, coherent structures in the turbulent flow field very closely parallels the laminar simulations. Following the idea of Large Eddy Simulation (LES), the large coherent motion is computed directly, while the effect of the small scale, random motion is modelled. In FSM, a state-of-the-art two-equation turbulence model is used. Forcing the turbulent wall jet results in a reduction of the skin friction and an increase in wall heat transfer. The mechanisms responsible for these mean flow changes show a remarkable similarity to the mechanisms found in the laminar case. This is confirmed by close examination of the large coherent motion and its effect on the turbulent mean flow. Using this approach, several questions regarding the character of the turbulent wall jet could be answered.

This research aims at modifying, improving and calibrating the Stress Wave Factor Nondestructive Evaluation (SWF NDE) technique and applying it to a fiber epoxy composite material and other composite structures. In order to access the composite's integrity the Energy of SWF within a selected Sub Frequency Range (SFR) instead of the whole measured frequency range as of conventional SWF is used. This technique, introduced and examined herein and is termed the Sub Frequency Range Stress Wave Factors (SFR-SWF) and is tailored to improve the conventional SWF technique with respect to sensibility and accuracy. A series of controlled damage tests were performed, and relevant acousto-ultrasonic observations were conducted. The overall property of the composites subjected to hygrothermal degradation, the localized defects such as the surface crack and the historical damage were assessed with conventional SWF and SFR-SWF. The two methods are compared in detail. The hygrothermal degradation and surface crack experiments were also simulated using the finite element method. Dynamic numerical analysis was conducted to simulate the wave propagation process, both in time domain and frequency domain using the commercial finite element code ABAQUS. The numerical results were also evaluated via both SWF and SFR-SWF, and were compared with the results of experiments. Thus, the potential of SFR-SWF was evaluated. A general conclusion from this research is that the SFR-SWF has the better capability than that of the conventional SWF in assessing the composite's overall condition, localized defects and historical damage. Since there are still open questions regarding the physical understanding of the SWF and SFR-SWF, the finite element analysis provides confirmation for certain observed behaviors of the Acousto-Ultrasonic and SFR-SWF technique.

Transitional and turbulent flows over a backward-facing step are physically highly complex. Apart from vastly different mean flow regimes and the rapid generation of turbulence, additional complexities arise from the presence of large coherent structures. For the present study, the mean flow, turbulence statistics and the origin of large coherent structures were investigated using Direct Numerical Simulations and turbulence modeling approaches. The latter included Large Eddy Simulations (LES) and state-of-the-art Reynolds-Averaged Navier-Stokes (RANS) computations. Wall-distance independent forms of the RANS models were developed, validated and calibrated. The ability of computing the step flows investigated and the associated computational costs were evaluated, for both LES and RANS. By employing harmonic forcing of the shear layer and a Fourier analysis in time and in the lateral direction the generation of coherent structures was linked to specific hydrodynamic instabilities. Comparison with references in the literature, resolution and domain size studies, and variations of inflow conditions established an accurate description of the mean flow and turbulence quantities and the level of sensitivity of the flow field to boundary conditions. From the controlled environment of the simulations, a simplified scenario was proposed for the creation of large coherent structures in transitional and turbulent step flows. The scenario suggests that Kelvin-Helmholtz, elliptical and centrifugal instabilities may be the relevant physical mechanisms for the observed primary, secondary and tertiary instabilities of the shear layer, respectively. The onset of the elliptical instability can also be described as a fundamental resonance of two waves. A cascade of subharmonic resonances is regarded to be responsible for vortex mergings and the generation of low frequency waves in the flow field. Furthermore, the simulations indicate that a three-dimensional global instability of the time and spanwise averaged separation bubble may be present. It was observed that the range of all unstable lateral wavelengths has a short-wave cutoff depending on Reynolds number and an upper bound on the order of the reattachment length.

Transitional and turbulent supersonic axisymmetric wakes are investigated by conducting various numerical experiments. The main objective is to identify hydrodynamic instability mechanisms in the flow at M = 2.46 for several Reynolds numbers, and relating these to coherent structures that are found from various visualization techniques. The premise for this approach is the assumption that flow instabilities lead to the formation of coherent structures. The effect of these structures on the mean flow is of particular interest, as they strongly affect the base drag. Three high-order accurate compressible codes were developed in cylindrical coordinates for this research: A spatial Navier-Stokes (N-S) code to conduct Direct Numerical Simulations (DNS), a linearized N-S code for linear stability investigations using two-dimensional basic states, and a temporal N-S code for performing local stability analyses. The ability of numerical simulations to deliberately exclude physical effects is exploited. This includes intentionally eliminating certain azimuthal/helical modes by employing DNS for various circumferential domain-sizes. With this approach, the impact of structures associated with certain modes on the global wake-behavior can be scrutinized. It is concluded that azimuthal modes with low wavenumbers are responsible for a flat mean base-pressure distribution and that k = 2 and k = 4 are the dominant modes in the trailing wake, producing a four-lobe wake pattern. Complementary spatial and temporal calculations are carried out to investigate whether instabilities are of local or global nature. Circumstantial evidence is presented that absolutely unstable global modes within the recirculation region coexist with convectively unstable shear-layer modes. The flow is found to be absolutely unstable with respect to modes k > 0 for ReD > 5,000 and with respect to the axisymmetric mode for ReD > 100,000. Furthermore, it is investigated whether flow control measures designed to weaken the naturally most significant modes can decrease the base drag. Finally, the novel Flow Simulation Methodology (FSM), using state-of-the-art turbulence closures, is shown to reproduce DNS results at a fraction of the computational cost.