Aerothermodynamics is an integral component in the design and implementation of hypersonic transport systems. Accurate estimates of the aerodynamic forces and heat transfer rates are critical in trajectory analysis and for payload weight considerations. The present work seeks to investigate the ability of an unstructured Cartesian grid framework in modeling hypersonic viscous flows. The effectiveness of modeling viscous phenomena in hypersonic flows using the immersed boundary ghost cell methodology of this solver is analyzed. The capacity of this framework to predict the surface physics in a hypersonic non-reacting environment is investigated. High velocity argon gas flows past a 2-D cylinder are simulated for a set of freestream conditions (Reynolds numbers), and impact of the grid cell sizes on the quality of the solution is evaluated. Additionally, the formulation is verified over a series of hypersonic Mach numbers for the flow past a hemisphere, and compared to experimental results and empirical estimates. Next, a test case that involves flow separation and the interaction between a hypersonic shock wave and a boundary layer, and a separation bubble is investigated using various adaptive mesh refinement strategies. The immersed boundary ghost cell approach is tested with two temperature clipping strategies, and their impact on the overall solution accuracy and smoothness of the surface property predictions are compared. Finally, species diffusion terms in the conservation equations, and collision cross-section based transport coefficients are installed, and hypersonic flows in thermochemical nonequilibrium environments are studied, and comparisons of the off-surface flow properties and the surface physics predictions are evaluated. First, a 2-D cylinder in a hypersonic reacting air flow is tested with an adiabatic wall boundary condition. Next, the same geometry is tested to evaluate the viscous chemistry prediction capability of the solver with an isothermal wall boundary condition, and to identify the strengths and weaknesses of the immersed boundary ghost cell methodology in computing convective heating rates in such an environment.

In the present study, circular cylinders in the cross-flow or the motions of circular cylinders in a fluid at rest are especially of interest in fields, such as offshore and civil engineering or heat exchanger. For last two decades, the researches of the force caused by the fluid on the cylinder surface are mainly studied by the ways of experiment and numerical methods. A time-independent finite different method is developed to solve the two-dimensional fixed or transversely oscillating cylinder passing by a cross flow. The present study focuses on the cylinder under a cross flow with only two kinds of conditions, which are Re = 100, KC = 5 and Re=200, KC=4. The benchmark tests of the present numerical results are made and validated by the reported numerical simulation and experimental results, for instant, the flow visualization of the vorticity contours and the in-line force for a flow across a moving circular cylinder. The developed numerical method can easily apply on the analyses of interactions between viscous flow and motion cylinder. Besides, we also consider the oscillatory flow passes a circular cylinder connecting with a spring. The spring -linking cylinder is released in the beginning on the position of zero deflection of the spring and stares moving due to the influence of the in-line force acting on the cylinder. We can find that the spring-linking cylinder under a oscillating flow produces restoring force and drag force due to considering the influence of the spring and damping effect, the developed numerical method can easily apply on the analyses of interactions between viscous flow and oscillating cylinder.

The hydrodynamic disturbances generated by two types of free-swimming, marine zooplankton were quantified experimentally in the laboratory with a novel, infrared Particle Image Velocimetry (PIV) system. The study consisted of three main parts: (1) the flow fields of free-swimming and tethered Euchaeta antarctica were compared to determine the effects of tethering, (2) three species of copepods (Euchaeta rimana, Euchaeta elongata, and Euchaeta antarctica) that live in seawater in a range of temperatures (23 ºC - 0 ºC) and a corresponding range of fluid viscosity (0.97 - 1.88 mm2 s-1) were analyzed experimentally and with a computational fluid dynamics model (FLUENT) to assess the effect of size and fluid viscosity on the flow fields, (3) the flow fields were collected for individuals of two species of euphausiids (Euphausia pacifica and Euphausia superba) to compare the effect of size and Reynolds number on propulsion and the spatial extent of the flow disturbance. In addition to the measured flow fields around solitary krill, flow fields were collected around small, coordinated groups of E. superba to examine group sensory cues through hydrodynamics. In the first part of this investigation, it was determined that tethering zooplankton during data collection resulted in flow fields with increased asymmetry and larger spatial extent due to the unbalanced force applied to the fluid by the tether. In response to these findings, only flow fields collected for free-swimming organisms were used in the subsequent studies. In the second part of the study, the increase in viscosity between subtropical and temperate fluid environments in conjunction with increased size and species-specific swimming speeds resulted in similar Reynolds numbers among E. elongata and E. rimana (in both cruising and escaping modes). During cruising (Re ~10), the spatial extent of the copepod hydrodynamic disturbances and propulsion costs were similar between species. In the case of fluid distrubances of escape (Re ~ 100), the spatial extent and energetic cost were larger for the larger species ( E. elongata). In the third part of the study, the hydrodynamic disturbance produced by E. superba (larger krill species) was found to be longer in horizontal spatial extent and at scales more appropriate for communication within schools than the hydrodynamic disturbance produced by E. pacifica. However, the sensory cue in coordinated groups of krill was complicated by the interaction of multiple flow disturbance fields, which suggests that hydrodynamic cues between krill in groups are restricted to small distances. The energetic cost of propulsion was ten times greater for the larger species of krill, and energetic expenditure did not appear to decrease for krill swimming in coordinated groups.

published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Η περιγραφή των αλληλεπιδράσεων που αναπτύσσονται μεταξύ ενός συνόλου σωμάτων που είτε είναι ακίνητα είτε κινούνται και ενός ρευστού, ρέοντος ή ηρεμούντος, βρίσκεται στον πυρήνα ενός μεγάλου αριθμού επιστημονικών προσπαθειών. Η μοντελοποίηση των αλληλεπιδράσεων αυτών μπορεί να γίνει με πολλούς τρόπους, καθένας από τους οποίους εμφανίζει πλεονεκτήματα και μειονεκτήματα, ανάλογα με το είδος του συστήματος στο οποίο επιχειρείται να εφαρμοστεί. Σκοπός της εργασίας αυτής είναι η ανάδειξη ενός εξ αυτών που είναι η Μέθοδος των Προτύπων Ροών. / The description of interactions between a number of solid particles, which may be in motion or fixed in space and a fluid which in turn may either be flowing or resting, is placed at the core of a large number of scientific publications. Modelling such interactions may follow a variety of approaches. Each approach has its own pros and cons, regarding the nature and the specific features of the system which is aimed to describe. Current thesis goal is to describe one of these approaches, which is the approach of Singularities or the Method of Elementary Flows.

Crystallization is done in the sugar industry using natural circulation vacuum evaporative crystallizers known as vacuum pans. the fluid which is known as massecuite consists of a suspension of crystals in concentrated molasses. It is highly viscous and slightly non-Newtonian, and laminar conditions prevail in the apparatus. Research on forced convection boiling heat transfer, pressure drop and vapour holdup has been done mostly in turbulent flow under pressures higher than atmospheric, but no studies have been made when boiling viscous fluids under vacuum. This thesis describes a series of experiments which were undertaken with the following objectives: (a) to determine the influence of the pertinent variables on heat transfer, friction losses and vapour holdup while boiling under laminar conditions (b) to produce a method for the calculation of the evaporation and circulation rates in vacuum pans, as this would make possible the optimization of this type of equipment. The apparatus used consisted of a single tube steam heated forced circulation evaporator. The void fraction, pressure and centerline temperature were measured along the tube. The fluids used were syrup, molasses and massecuite covering a thousandfold change in viscosity. The tests were conducted under different conditions of vacuum and steam pressures with varying tube inlet velocities. The experimental results show that the boiling heat transfer coefficient can be correlated as a function of the two phase Reynolds number and dimensionless density ratio and that it is inversely proportional to the tube length to the power of one third. The pressure drop can be estimated using the equation of Oliver and Wright (1964) for bubbly flow. Equations are proposed for calculating the void fraction in the highly subcooled region and point of bubble departure. These equations form the basis of a computer program which by a stepwise and iterative method simulates the boiling process along the tube. Measurements taken on a natural circulation pan with tubes of different length show that this method predicts the effect of the tube length with reasonable accuracy. The limitations of this study are that the experiments were done with a single diameter tube so that the effect of diameter has not been established with certainty. Only sugar products were used in the experiments, and caution is necessary if this method is applied to other fluids. / Thesis (Ph.D.)-University of Natal, Durban, 1985.

The results of an experimental study of phenomena which occur in the flow of a viscous fluid in closed domains with discrete symmetries are presented. The purpose is to investigate the role which ideas from low-dimensional dynamical systems have to play in describing qualitative changes that take place with variation of the governing parameters. Such a descriptive framework already exists for the case of the Taylor-Couette system, where the domain possesses a continuous azimuthal symmetry group. The present investigation is aimed at establishing the typicality of previously reported behaviour under progressive reductions of azimuthal symmetry. In the first investigations, the fixed outer circular cylinder of the standard system is replaced with one of square cross-section. Thus there is now discrete Ζ₄ symmetry in the azimuthal direction. Knowledge of the two-dimensional flow field is used to establish the nature of the steady three-dimensional motion equivalent to Taylor vortex flow. It is shown that similar bifurcation sequences exist in both standard and square systems for the case of very small aspect ratio where a single Taylor cell is formed. This flow develops as the result of a bifurcation which breaks the Ζ₂ symmetry that is imposed on the annulus by two solid stationary ends. The study is then extended to consider time-dependent effects in the square system. Two different oscillatory single-cell flows are identified, and it is shown that each is the result of a Hopf bifurcation. Selection of a particular dynamic mode is found to depend on the aspect ratio of the system. A low-dimensional bifurcation structure is uncovered which connects the two modes in parameter space, and involves a novel type of steady single-cell flow. Finally, observations are reported of a nontrivial type of dynamical behaviour which bears strong resemblance to motion found in a circularly symmetric Taylor-Couette system that is related to the Šilnikov mechanism for finite-dimensional chaos. A second variant on the Taylor-Couette system is considered where the outer cylinder is shaped like a stadium. The effect is to reduce further the overall symmetry of the domain to a Ζ₂ × Ζ₂ group. The two-dimensional flow field is investigated using both numerical and experimental techniques. Time-dependent phenomena are then investigated in the three-dimensional flow over a relatively wide range of aspect ratio. It is found that a sequence of a Hopf bifurcation followed by period-doubling bifurcations exists up to a certain aspect ratio, beyond which there is an apparently sudden and reversible transition between regular and irregular dynamical behaviour. Although this transition is not of a low-dimensional nature, the experimental results suggest that it exists as the result of a coalescence of the bifurcations which are found at lower values of aspect ratio.

Thesis (Ph.D.) -- Mississippi State University. Computational Engineering. / Title from title screen. Includes bibliographical references.

Thesis (M.Eng. (Mechanical )) -- University of Pretoria, 2007. / Includes bibliographical references (leaves 73-81)