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The chapter begins with the basic thermodynamic concepts that form the basis of high-speed flow theory, including a basic physical understanding of the second law of thermodynamics. This results in the ability to use the isentropic flow relationships in analyzing the properties of a compressible flow field, which results in the ability to analyze flow in a stream tube, and understand how a converging–diverging nozzle works. The basic relations for determining the change in flow properties across shock waves and expansion fans are developed, which make it possible to analyze flow fields using shock and expansion calculation methods. The basic relations for viscous flow are developed, leading to the relations for calculating the local skin-friction coefficient for a compressible boundary layer. The reader will then be able to understand the cause and effect of shock–boundary layer and shock–shock interactions. Finally, concepts for how flight vehicles are tested in wind tunnels are developed, which explains why it is difficult to fully model full-scale flight characteristics.
Readers will learn why aerodynamics is important in determining the performance characteristics of airplanes. This will begin with a development of a basic understanding of fluid properties such as density, temperature, pressure, and viscosity and how to calculate these properties for a perfect gas. Basic details about the atmosphere are presented and why we use a “standard atmosphere” model to perform aerodynamic calculations; learn how to perform calculations of fluid properties in the atmosphere. Basic components of an airplane are presented and descriptions are included to describe what the components are used for.
Readers will understand the physical laws that form the basis of the fluid equations of motion, and will learn how to obtain the equations of fluid motion in both derivative and integral form. Presentations are included to show how to apply the equations of motion to calculate properties of fluid flows. Readers will understand dynamic similarity and how to calculate Mach number and Reynolds number, including descriptions of the various Mach and Reynolds number regimes and their distinguishing characteristics.
The concept of circulation is presented, including the physical and mathematical concepts of circulation and lift. A description of how potential flow theory is used to model flow for airfoils, including the predictions of lift. Readers are presented with the concept of the Kutta condition, including how it impacts the development of airfoil theory. Thin-airfoil theory is developed for symmetric and cambered airfoils and methods for prediction lift and pitching moment are presented. The accuracy and limitations of thin-airfoil theory is also presented. Descriptions are presented for why laminar flow airfoils have different geometries than airfoils used at higher Reynolds numbers. Finally, high-lift systems are discussed, including why they are important for aircraft design.
The chapter will begin with the five characteristics that distinguish hypersonic flow from supersonic flow and then discuss each of the characteristics. Analysis methods will then be discussed, including Newtonian and Modified Newtonian methods, as well as tangent wedge and tangent cone methods. Analysis techniques are developed to determine the flow characteristics in the region of the stagnation point of a hypersonic vehicle, as well as the lift, drag, and pitch moment for simple geometries at hypersonic speeds. Information on the importance of heating at hypersonic speeds will be presented, followed by analysis approaches for estimating heating rates on blunt bodies. Finally, the complexities of hypersonic boundary-layer transition are introduced, including details about why transition is so challenging to predict.
Basic concepts are presented to show the difference between airfoils and wings, as well as the physical processes that cause those differences, such as wing-tip vortices. A physical description is presented for the impact of wing-tip vortices on the flow around the airfoil sections that make up a wing, and lift-line theory is developed to predict the effects of wing-tip vortices. A general description and calculation methods are presented for the basic approach and usefulness of panel methods and vortex lattice methods. A physical description for how delta wings produce lift and drag is also presented, including the importance of strakes and leading-edge extensions. High angle of attack aerodynamics is discussed, including the physical mechanisms that cause vortex asymmetry. Unmanned aerial vehicles and aerodynamic design issues are discussed. Finally, basic propeller theory and analysis approaches are introduced, including the use of propeller data to design low-speed propellers.
Readers will understand what is meant by inviscid flow, and why it is useful in aerodynamics, including how to use Bernoulli’s equation and how static and dynamic pressure relate to each other for incompressible flow. Concepts are presented to describe the basic process in measuring (and correcting) air speed in an airplane. A physical understanding of circulation is presented and how it relates to predicting lift and drag. Readers will be presented with potential flow concepts and be able to use potential flow functions to analyze the velocities and pressures for various flow fields, including how potential flow theory can be applied to an airplane.
Designed for a single-semester course on strength of materials, this textbook offers detailed discussion of fundamental and advanced concepts. The textbook is written with a distinct approach of explaining concepts with the help of solved problems. The study of flexural shear stress, conjugate beam method, method of sections and joints, statically determinate trusses and thin cylinders is presented in detail. The text discusses advanced concepts of strength of materials such as torsion of non-circular sections, shear center, rotating discs, unsymmetrical bending and deflection of trusses. The textbook is primarily written for undergraduate mechanical and civil engineering students in India. Numerous review questions, unsolved numerical problems and solved problems are included throughout the text to develop clear understanding of fundamental concepts.
We show that passively mode-locked lasers, subject to feedback from a single external cavity can exhibit large timing fluctuations on short time scales, despite having a relatively small long-term timing jitter. This means that the commonly used von Linde and Kéfélian techniques of experimentally estimating the timing jitter can lead to large errors in the estimation of the arrival time of pulses. We also show that adding a second feedback cavity of the appropriate length can significantly suppress noise-induced modulations that are present in the single feedback system. This reduces the short time-scale fluctuations of the interspike interval time and, at the same time, improves the variance of the fluctuation of the pulse arrival times on long time scales.
The Jansen–Rit model of a cortical column in the cerebral cortex is widely used to simulate spontaneous brain activity (electroencephalogram, EEG) and event-related potentials. It couples a pyramidal cell population with two interneuron populations, of which one is fast and excitatory, and the other slow and inhibitory.
Our paper studies the transition between alpha and delta oscillations produced by the model. Delta oscillations are slower than alpha oscillations and have a more complex relaxation-type time profile. In the context of neuronal population activation dynamics, a small threshold means that neurons begin to activate with small input or stimulus, indicating high sensitivity to incoming signals. A steep slope signifies that activation increases sharply as input crosses the threshold. Accordingly, in the model, the excitatory activation thresholds are small and the slopes are steep. Hence, we replace the excitatory activation function with its singular limit, which is an all-or-nothing switch (a Heaviside function). In this limit, we identify the transition between alpha and delta oscillations as a discontinuity-induced grazing bifurcation. At the grazing, the minimum of the pyramidal-cell output equals the threshold for switching off the excitatory interneuron population, leading to a collapse in excitatory feedback.
The shimmy oscillations of a truck’s front wheels with dependent suspension are studied to investigate how shimmy depends on changes in inflation pressure, with emphasis on the inclusion of four nonlinear tyre characteristics to improve the accuracy of the results. To this end, a three degree-of-freedom shimmy model is created which reflects pressure dependency initially only through tyre lateral force. Bifurcation analysis of the model reveals that four Hopf bifurcations are found with decreased pressures, corresponding to two shimmy modes: the yaw and the tramp modes, and there is no intersection between them. Hopf bifurcations disappear at pressures slightly above nominal value, resulting in a system free of shimmy. Further, two-parameter continuations illustrate that there are two competitive mechanisms between the four pressure-dependent tyre properties, suggesting that the shimmy model should balance these competing factors to accurately capture the effects of pressure. Therefore, the mathematical relations between these properties and inflation pressure are introduced to extend the initial model. Bifurcation diagrams computed on the initial and extended models are compared, showing that for pressures below nominal value, shimmy is aggravated as the two modes merge and the shimmy region expands, but for higher pressures, shimmy is mitigated and disappears early.
For multi-scale differential equations (or fast–slow equations), one often encounters problems in which a key system parameter slowly passes through a bifurcation. In this article, we show that a pair of prototypical reaction–diffusion equations in two space dimensions can exhibit delayed Hopf bifurcations. Solutions that approach attracting/stable states before the instantaneous Hopf point stay near these states for long, spatially dependent times after these states have become repelling/unstable. We use the complex Ginzburg–Landau equation and the Brusselator models as prototypes. We show that there exist two-dimensional spatio-temporal buffer surfaces and memory surfaces in the three-dimensional space-time. We derive asymptotic formulas for them for the complex Ginzburg–Landau equation and show numerically that they exist also for the Brusselator model. At each point in the domain, these surfaces determine how long the delay in the loss of stability lasts, that is, to leading order when the spatially dependent onset of the post-Hopf oscillations occurs. Also, the onset of the oscillations in these partial differential equations is a hard onset.
Quorum sensing governs bacterial communication, playing a crucial role in regulating population behaviour. We propose a mathematical model that uncovers chaotic dynamics within quorum sensing networks, highlighting challenges to predictability. The model explores interactions between autoinducers and two bacterial subtypes, revealing oscillatory dynamics in both a constant autoinducer submodel and the full three-component model. In the latter case, we find that the complicated dynamics can be explained by the presence of homoclinic Shilnikov bifurcations. We employ a combination of normal-form analysis and numerical continuation methods to analyse the system.