# Browsing by Subject "Numerical simulation"

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Item Advances in saccular aneurysm biomechanics : enlargement via rate-sensitive inelastic growth, bio-mathematical stages of aneurysm disease, and initiation profiles.(2016-12) Nugen, Frederick Theodore; Hughes, Thomas J. R.; Moser, Robert deLancey; Sacks, Michael S; Barr, Ronald; Gonzalez, Oscar; Kemper, Craig; Beasley, Haley KShow more I have created the first simulation of saccular aneurysm initiation and development from a healthy artery geometry. It is capable of growing saccular aneurysm geometries from patient-specific data. My model describes aneurysm behavior in a way that bridges fields. I assume arteries are made of a rate-sensitive inelastic material which produces irreversible deformation when it is overstressed. The material is assumed to consist of a 3D hyperelastic background material embedded with 1D transversely-isotropic fibers. I optionally use a Winkler foundation term to model support of external organs and distinguish healthy tissue from diseased tissue. Lesions are defined as a local degradation of artery wall structure. My work suggests passive mechanisms of growth are insufficient for predicting saccular aneurysms. Furthermore, I identify a new concept of stages of aneurysm disease. The stages connect mathematical descriptions of the simulation with clinically-relevant changes in the modeled aneurysm. They provide an evocative framework through which clinical descriptions of arteries can be neatly matched with mathematical features of the model. The framework gives a common language of concepts---e.g., collagen fiber, pseudoelastic limit, inelastic strain, and subclinical lesion---through which researchers in different fields, with different terminologies, can engage in an ongoing dialog: under the model, questions in medicine can be translated into equivalent questions in mathematics. A new stage of “subclinical lesion” has been identified, with a suggested direction for future biomechanics research into early detection and treatment of aneurysms. This stage defines a preclinical aneurysm-producing lesion which occurs before any artery dilatation. It is a stage of aneurysm development involving microstructural changes in artery wall makeup. Under the model, this stage can be identified by its reduced strength: its structural support is still within normal limits, but presumably would perform more poorly in ex vivo failure testing than healthy tissue from the same individual. I encourage clinicians and biomechanicians to measure elastin degradation, and to build detailed multiscale models of elastin degradation profiles as functions of aging and tortuosity; and similarly for basal tone. I hope such measurements will to lead to early detection and treatment of aneurysms. I give specific suggestions of biological tissue experiments to be performed for improving and reinforming constitutive modeling techniques.Show more Item Interpretation of multi-component induction and sonic measurements acquired in high-angle wells and joint 1D radial inversion of resistivity and sonic logs(2010-05) Mallan, Robert Keays; Torres-Verdín, Carlos; Sepehrnoori, Kamy; Olson, Jon E.; Fomel, Sergey B.; Day, PeterShow more Multi-component induction resistivity and sonic measurements acquired in high-angle wells can be strongly influenced by shoulder-bed effects, anisotropy resulting from sand-shale laminations, and presence of mud-filtrate invasion. Understanding the corresponding biasing effects aids in the interpretation of resistivity and sonic measurements and subsequently leads to more accurate and reliable formation evaluation. This dissertation describes numerical simulation studies examining the effects on multi-component induction and sonic measurements in a variety of complex formation models. Subsequently, a joint inversion scheme is presented that combines resistivity and sonic measurements to estimate in situ petrophysical and elastic properties in the presence of mud-filtrate invasion. To facilitate the simulation study of multi-component induction logs, I develop a new finite-difference algorithm for the numerical simulation of frequency-domain electromagnetic borehole measurements. The algorithm~uses a coupled scalar-vector potential formulation for arbitrary three-dimensional inhomogeneous and electrically anisotropic media. Simulations show that shoulder-bed anisotropy: enhances shoulder-bed effects across sand layers; and impacts invasion sensitivities to significantly alter the assessment of invasion in terms of invaded- and virgin-zone resistivities, radial length, and front shape. For the simulation study of sonic logs, I develop a three-dimensional, finite-difference time-domain algorithm that models elastic wave propagation in a fluid-filled borehole. Simulations show that presence of anisotropy not only alters the degree of dispersion observed in flexural and Stoneley waves, but also alters their responses to invasion. In addition, presence of a dipping shoulder bed can significantly distort flexural dispersion, making it difficult to identify the low frequency asymptote corresponding to formation shear wave velocity. Lastly, I consider a radial one-dimensional model in the development of a joint resistivity and sonic inversion algorithm. This scheme simultaneously inverts array-induction apparent conductivities and sonic flexural and Stoneley dispersions for the rock's elastic moduli and water saturation in the presence of mud-filtrate invasion. Inversions are performed on numerically simulated data for a variety of models reflecting soft and hard rock formations with presence of water- and oil-based mud-filtrate invasion. Results show the estimated invasion profiles display excellent agreement with the true models, and the elastic moduli are estimated to within a few percent of the true values.Show more Item Investigation of the effects of buoyancy and heterogeneity on the performance of surfactant floods(2014-12) Tavassoli, Shayan; Pope, G. A.; Sepehrnoori, Kamy, 1951-Show more The primary objectives of this research were to understand the potential for gravity-stable surfactant floods for enhanced oil recovery without the need for mobility control agents and to optimize the performance of such floods. Surfactants are added to injected water to mobilize the residual oil and increase the oil production. Surfactants reduce the interfacial tension (IFT) between oil and water. This reduction in IFT reduces the capillary pressure and thus the residual oil saturation, which then results in an increase in the water relative permeability. The mobility of the surfactant solution is then greater than the mobility of the oil bank it is displacing. This unfavorable mobility ratio can lead to hydrodynamic instabilities (fingering). The presence of these instabilities results in low reservoir sweep efficiency. Fingering can be prevented by increasing the viscosity of the surfactant solution or by using gravity to stabilize the displacement below a critical velocity. The former can be accomplished by using mobility control agents such as polymer or foam. The latter is called gravity-stable surfactant flooding, which is the subject of this study. Gravity-stable surfactant flooding is an attractive alternative to surfactant polymer flooding under certain favorable reservoir conditions. However, a gravity-stable flood requires a low velocity less than the critical velocity. Classical stability theory predicts the critical velocity needed to stabilize a miscible flood by gravity forces. This theory was tested for surfactant floods with ultralow interfacial tension and found to over-estimate the critical velocity compared to both laboratory displacement experiments and fine-grid simulations. Predictions using classical stability theory for miscible floods were not accurate because this theory did not take into account the specific physics of surfactant flooding. Stability criteria for gravity-stable surfactant flooding were developed and validated by comparison with both experiments and fine-grid numerical simulations. The effects of vertical permeability, oil viscosity and heterogeneity were investigated. Reasonable values of critical velocity require a high vertical permeability without any continuous barriers to vertical flow in the reservoir. This capability to predict when and under what reservoir conditions a gravity-stable surfactant flood can be performed at a reasonable velocity is highly significant. Numerical simulations were also used to show how gravity-stable surfactant flooding can be optimized to increase critical velocity, which shortens the project life and improves the economics of the process. The critical velocity for a stable surfactant flood is a function of the microemulsion viscosity and it turns out there is an optimum value that can be used to significantly increase the velocity and maintain stability. For example, the salinity gradient can be optimized to gradually decrease the microemulsion viscosity. Another alternative is to inject a polymer drive following the surfactant solution, but using polymer complicates the process and adds to its cost without significant benefit in most gravity-stable surfactant floods. A systematic approach was introduced to make decisions on using polymer in applications based on stability criteria and cost. Also, the effect of an aquifer on gravity-stable surfactant floods was investigated as part of a field-scale study and strategies were developed to minimize its effect on the process. This study has provided new insights into the design of an optimized gravity-stable surfactant flood. The results of the numerical simulations show the potential for high oil recovery from gravity-stable surfactant floods using horizontal wells. Application of gravity-stable surfactant floods reduces the cost and complexity of the process. The widespread use of horizontal wells has greatly increased the attractiveness and potential for conducting surfactant floods in a gravity-stable mode. This research has provided the necessary criteria and tools needed to determine when gravity-stable surfactant flooding is an attractive alternative to conventional surfactant-polymer flooding.Show more Item Numerical simualtion of mixed convection over a three-dimensional horizontal backward-facing step(Texas A&M University, 2005-08-29) Barbosa Saldana, Juan GabrielShow more A FORTRAN code was developed to numerically simulate the mixed convective flow over a three-dimensional horizontal backward-facing step. The momentum and energy equations under the assumption of the Boussinesq approximation were discretized by means of a finite volume technique. The SIMPLE algorithm scheme was applied to link the pressure and velocity fields inside the domain while an OpenMP parallel implementation was proposed to improve the numerical performance and to accelerate the numerical solution. The heating process corresponds to a channel heated from below at constant temperature keeping insulated all the other channel walls. In addition, the back-step was considered as a thermally conducting block and its influence in the heating process was explored by holding different solid to fluid thermal conductivity ratios. The effects over the velocity and temperature distribution of buoyancy forces, acting perpendicular to the mainstream flow, are studied for three different Richardson numbers Ri=3, 2, and 1 and the results are compared against those of pure forced convection Ri=0. In these simulations the Reynolds number is fixed at 200 while the bottom wall temperature is adjusted to fulfill the conditions for the different Ri. Under this assumption, as Ri increases the buoyancy effects are the dominant effects in the mixed convective process. The numerical results indicate that the velocity field and the temperature distribution for pure forced convection are highly distorted if compared with the mixed convective flow. If the Ri parameter is increased, then the primary re-circulation zone is reduced. Similarly, as the buoyancy forces become predominant in the flow, the convective rolls, in the form of spiral-flow structures, become curlier and then higher velocity components are found inside the domain. The temperature field distribution showed that as the Ri is increased a thicker layer of high temperature flow is located at the channel??s top wall as a result of the higher rates of low-density flow moving to the top wall. The flow is ascending by the channel sidewalls, while descending by the channel span-wise central plane. The parallel numerical strategy is presented and some results for the performance of the OpenMP implementation are included. In this sense, linear speedup was obtained when using 16 possessors in parallel.Show more Item Numerical Study On A SPAR Type Floating Offshore Wind Turbine Using COUPLE-FAST Code(2015-03-03) Peng, ChengShow more Floating offshore wind turbine (FOWT) attracts more and more attention for harnessing wind power over the surface of relatively deep ocean water, where steady and strong wind occurs. Although it has been shown that the knowledge gained from the development of floating platforms for oil and gas production is helpful for the development of FOWTs, it alone is insufficient for understanding dynamic interactions between the supporting platform and the wind turbine. Therefore, it is desirable to conduct numerical simulations of a FOWT under the impact of different combinations of winds, waves and currents. In this study, a numerical code named as COUPLE-FAST has been developed to investigate the motions of a selected FOWT and the tensions in its mooring lines. The selected FOWT mainly consists of a 5MW NREL wind turbine and OC3-Hywind Spar support platform. COUPLE-FAST is made based two existing codes COUPLE and FAST. The former is an in-house code developed and being continuously expanded for the simulation of an offshore floating platform positioned by a mooring-line/tendon system. FAST is an open-source code capable of predicting both the extreme and fatigue loads of two- and three- bladed horizontal-axis wind turbines [1]. In COUPLE-FAST, COUPLE module is used to calculate the external loads on the support floating platform, mooring line forces and its motions, and FAST module to calculate the aerodynamic loads and flexible responses of the wind turbine. The displacements, velocities and accelerations predicted in COUPLE are transferred to FAST. The forces at the tower bases calculated by FAST are transferred to COUPLE. Total 25 cases with different combination of winds, waves and currents are simulated for calculating the motions of the FOWT and tensions in its mooring lines. Among many interesting observations made based on these simulations, it is confirmed that when the mean wind speed is above the rated wind speed the blade pitch control system may induce resonant interaction (also known as ?negative damping?) between the surge of the FOWT and dynamic wind loads induced by the adjustment of blade pitch angle. However, the resonant effects on surge of the FOWT in the case of turbulent winds are not as significant as in the case of steady winds of the corresponding wind speed.Show more Item On understanding the physics and source conditions of the Enceladus South Polar Plume via numerical simulation(2015-05) Yeoh, Seng Keat; Goldstein, David Benjamin, doctor of aeronautics; Varghese, Philip L.Show more Enceladus, a tiny moon of Saturn, is found to be geologically active. In 2005, Cassini detected an anomalously warm region and a plume, consisting of mostly water vapor and ice grains, at its south pole. The plume has far-reaching effects on the Saturnian system and offers clues into the moon’s interior, particularly as to whether liquid water exists underground. Consequently, understanding the physics and source conditions of the plume is crucial, which is the focus of this work. The plume is not only two-phase but also multi-regime in nature and can be divided into three distinct regions: a subsurface region, a collisional near-field and a free-molecular far-field. To study it, a hybrid model of the plume, which treats each region separately, is constructed. Two subsurface models are considered. Using the resulting vent conditions from these models, the plume is propagated from the surface vents out to several Enceladus radii using the direct simulation Monte Carlo (DSMC) method in the near-field and a free-molecular model in the far-field. The model is used to examine the plume flow, with and without grains. Collisions are found to be important in various processes, including grain condensation and flow acceleration. Since collision rates drop away from the vent, they must be high enough at the vent to enable significant condensation to occur and the gas to accelerate to the maximum speed possible by allowing energy stored in internal modes to be converted into translational energy as the gas expands. When slower grains are present, however, they may decelerate and push the gas out more laterally. Moreover, grains may form a thick column and restrict the free expansion of the gas. Smaller grains have greater and more extensive effects on the gas, but are also more strongly affected by the gas. Their motions decouple from the gas motions higher above the vent. They are also more likely to spread with the gas and be accelerated to the gas speeds. By constraining the plume far-field using Cassini data, the H2O and grain production rates from the plume are estimated to be ~100–1000 kg/s and < 10 kg/s respectively, which agree with other estimates. Based on fit results, the gas jets appear to be narrow, suggesting high Mach numbers at the vents.Show more Item Studies on Dynamics of Suction Piles during Their Lowering Operations(2010-10-12) Huang, LiqingShow more Suction piles are used for anchoring the mooring lines at the seafloor. One of the challenges of their installing is the occurrence of the heave resonance of the pile-cable system and possibly the heave induced pitch resonance during the lowering process. When the heave and/or pitch frequency of the vessel which operates the lowering of the pile matches the heave natural frequency of the pile-cable system, the heave resonance may occur, resulting in large heave oscillations of the pile and thus significantly increasing loads on the lowering cable and lowering devices. Furthermore, the large heave may resonantly induce the pitch of a pile. To predict and possibly mitigate the heave/pitch resonance of the pile-cable system during the lowering process, it is crucial to under the mechanism of heave induced pitch resonance and estimate the added-mass and damping coefficients of the pile-cable system accurately. The model tests of the forced heave excitation of pile models were first conducted to investigate the added-mass coefficient for a pile model with different opening area ratios at its top cap at the Haynes Coastal Engineering Laboratory of Texas AandM University. In the model tests, it was observed that the resonant heave may occur if the heave excitation frequency matches the related heave natural frequency and the pitch resonance may be induced by the heave resonance. The results of the following theoretical analysis and numerical simulation of the heave excitation of the pile-cable system are found to be consistent with the related measurements, which is helpful to further understand the physics of lowering a pile-cable system. The results of this study may be used to determine the magnitudes of total heave added-mass and damping coefficient of a pile and the heave natural frequency of the pile-cable system based upon its main characteristics. The heave induced resonant pitch is found to occur when 1) the pitch natural frequency is roughly equal to one half of the heave natural frequency and 2) the heave excitation frequency is approximately equal to the heave natural frequency. If only one of the two conditions is satisfied, no significant pitch resonance will occur. These results may have important implications to the operation of lowering offshore equipment to the seafloor in deep water.Show more