abstract: Production from a high pressure gas well at a high production-rate encounters the risk of operating near the choking condition for a compressible flow in porous media. The unbounded gas pressure gradient near the point of choking, which is located near the wellbore, generates an effective tensile stress on the porous rock frame. This tensile stress almost always exceeds the tensile strength of the rock and it causes a tensile failure of the rock, leading to wellbore instability. In a porous rock, not all pores are choked at the same flow rate, and when just one pore is choked, the flow through the entire porous medium should be considered choked as the gas pressure gradient at the point of choking becomes singular. This thesis investigates the choking condition for compressible gas flow in a single microscopic pore. Quasi-one-dimensional analysis and axisymmetric numerical simulations of compressible gas flow in a pore scale varicose tube with a number of bumps are carried out, and the local Mach number and pressure along the tube are computed for the flow near choking condition. The effects of tube length, inlet-to-outlet pressure ratio, the number of bumps and the amplitude of the bumps on the choking condition are obtained. These critical values provide guidance for avoiding the choking condition in practice. / Dissertation/Thesis / M.S. Mechanical Engineering 2013

Made available in DSpace on 2014-10-09T12:53:37Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:27Z (GMT). No. of bitstreams: 1 12226.pdf: 2981863 bytes, checksum: f04d559e0b2d5d5ba05718e2738e9989 (MD5) / Tese (Doutoramento) / IPEN/T / Escola Politécnica, Universidade de Sao Paulo - POLI/USP

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The difficulties regarding the control of high velocity flying vehicles in supersonic/hypersonic flight regime are still prevailing. Whether it is mixing enhancement,side force generation or aerodynamic steering, wall cooling or any otherfavourable method to control the flow, the resultant effects of different flow controltechniques on the associated flowfield demands careful experimental and numericalinvestigations. Traditional aerodynamic control surfaces are subjected tosevere flight conditions and loadings in different flight regimes resulting in impairedthe control effectiveness. Active flow control methods serve strong alternativeto achieve separation postponement, transition control, lift enhancement,mixing enhancement, drag reduction, turbulence modification and/or noise suppression,etc. This thesis deals with two main active flow control techniques;transverse jets at Mach 5 cross flow and energy deposition using arc discharge atMach 5 flow. The influence of roughness on the control effectiveness of transversejet interactions is also examined. The first objective of this thesis is to investigate experimentally the flowphysics of the sonic transverse jets at Mach 5 laminar cross flow both in timeaveraged and time resolved manner to provide reliable experimental data andbetter understanding at high Mach numbers. The parameters such as momentumflux ratio, incoming Reynolds number, type of the gas and the surface roughnessare studied. The size and structures of the upstream and downstream separationregions and jet penetration characteristics together with jet shear layer behaviourare examined. Moreover CFD simulations are conducted on a two dimensionalcase of Spaid and Zukoski and the numerical solver/procedure is validated. Thena three dimensional experimental case is simulated to provide greater understandingon the flow physics as well as to cross check measurements. As the main finding; jet interaction flow field can not be oversimplified andrepresented with only one parameter that is momentum flux ratio, J, as suggested by the literature; the incoming Reynolds number, type of injectant and roughnessare clearly affecting the interaction resulting in advantages or drawbacks for flowcontrol point of view. The second objective of this thesis is to investigate experimentally the dynamicsbetween the localised energy spot and the blunt body shock for dragreduction at Mach 5 flow. The localised energy spot is created firstly via steadyelectric arc struck between two electrodes using a small amount of energy andsecondly via pulsed laser focusing with a significant amount of energy. In caseof electric discharge, the effects of discharge are evaluated in comparison to nodischarge case with the electrodes. The unsteady wake/compression structuresare examined between the steadily deposited energy spot and the modified bowshock wave. And for the laser focussing unsteady interaction that is happeningin a short duration of time is investigated. The effect of the truncation, the distancebetween the electrodes and the model as well as the type and amount ofthe energy input on this phenomenon are examined. Moreover CFD simulationsare conducted on the baseline cases to cross check measurements together withtheoretical estimates. As the main finding; the effectiveness of the arc discharge is increasing withincreased truncation or the frontal area and when the arc to nose distance isthe shortest. However an important thing to note is that energy deposition atshorter distances might result higher stagnation point heating rates which aredetrimental. The test campaign clearly renders that the use of small amount ofonboard energy to create a local focused thermal spot in front of a vehicle is anefficient way of reducing drag.

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The Air Inlet Method of Characteristics Analysis Tool (AIMCAT), a tool based in Python 3, is developed to model supersonic air inlet geometries during the early phases of design. The method of characteristics (MOC) is used to solve the governing equations for an inviscid, irrotational, isentropic, steady, supersonic flowfield. A comparison is made between modeling shock waves implicitly using Mach wave coalescence and modeling them explicitly using oblique shock relations. Multiple test cases are used to assess the accuracy of the tool by comparing to experimental wind tunnel data. Good general agreement was achieved over the majority of the supersonic portion of the flowfield for all test cases. The implicit shock mesh achieved better accuracy for shock wave positions compared to the explicit shock mesh. However, the explicit shock mesh captured total pressure losses across the shocks which is of value when assessing the efficiency of the inlet. Both approaches show their respective values and their suitability depends on the conditions being studied. AIMCAT has shown initial promise, however further development is need to improve its utility and robustness.

Acoustic sensing provides a possibility of measuring propulsion flow fields non-intrusively, and is of great interest because it may be applicable to cases that are difficult to measure with traditional methods. In this work, some of the successes and limitations of this technique are considered. In the first main result, the acoustic time of flight is shown to be usable along with a calibration curve in order to accurately estimate the thrust of two turbofan engines (1.0-1.5%). In the second, it is shown that acoustic tomography methods that only use the first ray paths to arrive cannot distinguish some relevant propulsion flow fields (i.e., different flow fields can have the same times of flight). In the third result we demonstrate, via the first validated acoustic tomography experiment on a turbofan engine, that a reasonable estimate of the flow can be produced despite this challenge. This is also the first successful use of acoustic tomography to reconstruct a compressible, multi-stream flow. / Doctor of Philosophy / Sound may be used to measure air flows, and has been used for this purpose in studies of the atmosphere for decades. In this work, the extension of the method to measure air flows in aircraft engines is considered. This is challenging for two main reasons. The first challenge is that aircraft engines are very loud, which makes it harder to accurately measure the sounds that are needed to determine the speeds and temperatures. In this work, we show that the thrust (the force made by an engine) may be accurately measured using sound despite this difficulty. The second challenge is that the temperatures and velocities involved are very large compared to those in the atmosphere. We show that these large variations in temperature and velocity can make it impossible to distinguish between two different air flows in certain circumstances. We also show that despite this limitation, sound can be used to produce a reasonable, though imperfect, estimate of the flow. In particular, the technique was successfully used to measure the varying temperatures and velocities in a jet engine, which has not been done successfully before.

In this study we highlight the influence of mean dilatation and mean density gradient on the Reynolds stress modeling of compressible, heat-releasing and supercritical turbulent flows.Firstly, the modeling of the rapid pressure-strain correlation has been extended to self-consistently account for the influence of mean dilatation.Secondly, an algebraic model for the turbulent density flux has been developed and coupled to the tensor equationfor Reynolds stress anisotropy via a 'local mean acceleration',a generalization of the buoyancy force. We applied the resulting differential Reynolds stress model (DRSM) and the corresponding explicit algebraic Reynolds stress model (EARSM) to homogeneously sheared and compressed or expanded two-dimensional mean flows. Both formulations have shown that our model preserves the realizability of the turbulence, meaning that the Reynolds stresses do not attain unphysical values, unlike earlier approaches. Comparison with rapid distortion theory (RDT) demonstrated that the DRSM captures the essentials of the transient behaviour of the diagonal anisotropies and gives good predictions of the turbulence kinetic energy. A general three-dimensional solution to the coupled EARSM  has been formulated. In the case of turbulent flow in de Laval nozzle we investigated the influence of compressibility effects and demonstrated that the different calibrations lead to different turbulence regimes but with retained realizability. We calibrated our EARSM against a DNS of combustion in a wall-jet flow. Correct predictions of turbulent density fluxes have been achieved and essential features of the anisotropy behaviour have been captured.The proposed calibration keeps the model free of singularities for the cases studied. In addition,  we have applied the EARSM to the investigation of supercritical carbon dioxide flow in an annulus. The model correctly captured mean enthalpy, temperature and density as well as the turbulence shear stress. Hence, we consider the model as a useful tool for the analysis of a wide range of compressible flows with large density variation. / <p>QC 20160314</p>

Simulating transient compressible flows involving shock waves presents challenges to the CFD practitioner in terms of the mesh quality required to resolve discontinuities and prevent smearing. This document discusses a novel two-dimensional Cartesian anisotropic mesh adaptation technique implemented for transient compressible flow. This technique, originally developed for laminar incompressible flow, is efficient because it refines and coarsens cells using criteria that consider the solution in each of the cardinal directions separately. In this document the method will be applied to compressible flow. The procedure shows promise in its ability to deliver good quality solutions while achieving computational savings. Transient shock wave diffraction over a backward step and shock reflection over a forward step are considered as test cases because they demonstrate that the quality of the solution can be maintained as the mesh is refined and coarsened in time. The data structure is explained in relation to the computational mesh, and the object-oriented design and implementation of the code is presented. Refinement and coarsening algorithms are outlined. Computational savings over uniform and isotropic mesh approaches are shown to be significant.

A Large Eddy-Simulation code, based on a mesh transparent algorithm, for hybrid unstructured meshes is presented to deal with complex geometries that are often found in engineering flow problems. While tetrahedral elements are very effective in dealing with complex geometry, excessive numerical diffusion often affects results. Thus, prismatic or hexahedral elements are preferable in regions where turbulence structures are important. A second order reconstruction methodology is used since an investigation of a higher order method based upon Lele's compact scheme has shown this to be impractical on general unstructured meshes. The convective fluxes are treated with the Roe scheme that has been modified by introducing a variable scaling to the dissipation matrix to obtain a nearly second order accurate centred scheme in statistically smooth flow, whilst retaining the high resolution TVD behaviour across a shock discontinuity. The code has been parallelised using MPI to ensure portability. The base numerical scheme has been validated for steady flow computations over complex geometries using inviscid and RANS forms of the governing equations. The extension of the numerical scheme to unsteady turbulent flows and the complete LES code have been validated for the interaction of a shock with a laminar mixing layer, a Mach 0.9 turbulent round jet and a fully developed turbulent pipe flow. The mixing layer and round jet computations indicate that, for similar mesh resolution of the shear layer, the present code exhibits results comparable to previously published work using a higher order scheme on a structured mesh. The unstructured meshes have a significantly smaller total number of nodes since tetrahedral elements are used to fill to the far field region. The pipe flow results show that the present code is capable of producing the correct flow features. Finally, the code has been applied to the LES computation of the impingement of a highly under-expanded jet that produces plate shock oscillation. Comparison with other workers' experiments indicates good qualitative agreement for the major features of the flow. However, in this preliminary computation the computed frequency is somewhat lower than that of experimental measurements.