Oscillations, flow acoustic wave

The Presumed Probability Density Function method is developed and implemented to study turbulent flame stabilization and combustion control in subsonic combustors with flame holders. The method considers turbulence-chemistry interaction, multiple thermo-chemical variables, variable pressure, near-wall effects, and provides the efficient research tool for studying flame stabilization and blow-off in practical ramjet burners. Nonreflecting multidimensional boundary conditions at open boundaries are derived, and implemented into the current research. The boundary conditions provide transparency to acoustic waves generated in bluff-body stabilized combustion zones, thus avoiding numerically induced oscillations and instabilities. It is shown that predicted flow patterns in a combustor are essentially affected by the boundary conditions. The derived nonreflecting boundary conditions provide the solutions corresponding to experimental findings. 

An account of the behavior of acoustic wave propagation in a gas-solid suspension or particle movement in a turbulent eddy requires comprehensive knowledge of the dynamics of particle motion in an oscillating flow field. This oscillating flow can be analyzed in terms of one-dimensional simple harmonic oscillation represented by... 

The Stokes-layer excitation in a cylindrical duct is one of the effective techniques for dynamic calibration of shear stress sensors [8]. The flow inside the duct is driven by an oscillating pressure gradient generated by a loud speaker (see Fig. 10). The loud speaker driven by an amplifier generates an acoustic wave. The amplifier receives sinusoidal input from a function generator. The microphone and shear stress sensor are mounted at opposite locations of the tube. The data acquisition system records the signal from the microphone and shear stress sensor. 

The flow control is a mechanism by which the flow field is manipulated to obtain required behavior in comparison to the natural uncontrolled case. The flow control is broadly classified as (a) passive control, where no auxiliary power source is required and (b) active control, where there is expenditure of energy. In passive control, parameters like geometry, compliance, temperature and porosity etc. are varied. Boundary oscillation, acoustic waves, blowing, suction etc. are used for active control. The active control schemes use actuators for manipulating the flow behavior. The size of these actuators depends on the nature of the flow. When the Reynolds number is increased, the required size of the actuator is reduced. The availability of MEMS fabrication technique has contributed towards small scale actuators development. 

Several theories aimed at explaining the phenomena have been proposed, each of which is founded on completely different concepts. Sripaipan et al. [21] proposed a nematic layer with free ends, in which the interaction between the longitudinal oscillations (induced by the motion of the free ends of the layer in compression) and the traverse oscillations establishes steady flow of the liquid and, as a result, rotation of the molecules. However, these authors used incorrect dispersion relations and their calculations are not consistent with observed layer compression patterns. Nagai and coworkers [26,27] hypothesized that with normal incidence of an ultrasound beam on the layer the rotation of molecules is attributable to radiation fluxes. Radiation fluxes are the steady acoustic flows caused by radiation forces in a traveling acoustic wave, the only provision being that the width of the ultrasound beam is smaller then the dimensions of the cell. In reality, radiation fluxes can only occur near the boundaries of the beam and produce a compression effect that is smaller than the one that is actually ob-... 

N) E.W. Price, "Recent Advances in Solid Propellant Combustion Instability , Ibid, pp 101-113 O) G.A. Marxman C.E. Wooldridge, "Finite-Amplitude Axial Instability in Solid-Rocket Combustion , Ibid, pp 115-27 P) W.A. Sirignano, "A Theory of Axial-Mode Shock-Wave Oscillations in a Solid-Rocket Combustor ,Ibid, pp 129-37 Q) B.T.Zinn C.T. Saveli, "A Theoretical Study of Three-Dimensional Combustion Instability in Liquid-Propellant Rocket Engines , Ibid, pp 139-47 R) R.J. Priem E.J. Rice, "Combustion Instability with Finite Mach Number Flow and Acoustic Liners , Ibid, pp 149-59 S) M.W. Thring, "Combustion Oscillations in Industrial Combustion Chambers , Ibid, pp 163-68... 

On the other hand, researchers interested in mass transport in water waves have shown that a similar phenomenon occurs for incompressible flow (see [4] for an example). The salient feature is when an obstacle oscillates with high frequency in the quiescent viscous fluid. In this case the steady streaming emerges from both sides of the obstacle along the oscillating direction at small viscosity [5]. Increased importance of the acoustic streaming in small scales has been addressed in [6]. 

Ultrasonic pumps are pumps that use acoustic streaming to create a fluid motion inside microchannels. Acoustic streaming is the time-averaged flow induced hy an ultrasonic wave (periodic pressure oscillation). Attenuation of the acoustic energy (via reflection and other distortion) generates a body force within the fluid and converts acoustic energy into kinetic energy of the fluid [1]. 

To summarize, in the PCD process convective heat and vapor transfer between the material surface and the drying gas is strongly affected by the flow oscillations of the gas stream. Additionally, sound waves produced by the combustor can penetrate the solid materials, affecting the internal moisture transport rate. The local pressure difference induced by the acoustic field may cause an enhanced removal of water vapor from capillaries, and also promote the movement of liquid water towards the material surface. Scarboroug et al. (2006) observed a significant increase in the transfer coefficients when the ratio of the acoustic velocity amplitude to the mean velocity was greater than 1.8, which means that the acoustic oscillations of an appropriate amplitude could increase the drying rate of the pulse combustion process. 

Chapter 23 shows how modeling can propose mechanisms to explain experimentally observed vibrations in the cardiovascular system. A control system characterized by a slow and delayed change in resistance due to smooth muscle activity is presented. Experiments on this model show oscillations in the input impedance frequency spectrum, and flow and pressure transient responses to step inputs consistent with experimental observations. This autoregulation model supports the theory that low-frequency oscillations in heart rate and blood pressure variability spectra (Mayer waves) find their origin in the intrinsic delay of flow regulation. A second example presents acoustic classification of vascular... 

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