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Bogdan Petrov
Bogdan Petrov

Lighthill Waves In Fluids Djvu Files


The propagation of sound in solids happens through small-amplitude elastic oscillations of the solid's shape and structure. These elastic waves are transmitted to surrounding fluids as ordinary sound waves.




lighthill waves in fluids djvu files



You can use the Acoustics Module to model the propagation of elastic waves in solids and porous materials, for single-physics or multiphysics applications, such as vibration control, nondestructive testing (NDT), or mechanical feedback. Application areas range from micromechanical devices to seismic wave propagation. Elastic wave propagation over large domains containing many wavelengths is solved using a higher-order dG-FEM time-explicit method, and is multiphysics enabled for couplings with fluids as well as piezoelectric materials. The full structural dynamics formulation accounts for the effects of shear waves as well as pressure waves. You can model the coupled propagation of elastic and pressure waves in porous materials solving Biot's equations.


Acoustic disturbances with frequencies that are not audible for humans are classified as ultrasound, which implies that ultrasonic waves have a short wavelength. For this, you can compute the transient propagation of acoustic waves in fluids over large distances in two ways: modeling wave propagation that includes a background flow or modeling the effects of high-power nonlinear acoustics.


In nature, it is not unusual to find stably stratified fluid adjacent to convectively unstable fluid. This can occur in the Earth's atmosphere, where the troposphere is convective and the stratosphere is stably stratified; in lakes, where surface solar heating can drive convection above stably stratified fresh water; in the oceans, where geothermal heating can drive convection near the ocean floor, but the water above is stably stratified due to salinity gradients; possible in the Earth's liquid core, where gradients in thermal conductivity and composition diffusivities maybe lead to different layers of stable or unstable liquid metal; and, in stars, as most stars contain at least one convective and at least one radiative (stably stratified) zone. Internal waves propagate in stably stratified fluids. The characterization of the internal waves generated by convection is an open problem in geophysical and astrophysical fluid dynamics.


Microactuation of free standing objects in fluids is currently dominated by the rotary propeller, giving rise to a range of potential applications in the military, aeronautic and biomedical fields. Previously, surface acoustic waves (SAWs) have been shown to be of increasing interest in the field of microfluidics, where the refraction of a SAW into a drop of fluid creates a convective flow, a phenomenon generally known as SAW streaming. We now show how SAWs, generated at microelectronic devices, can be used as an efficient method of propulsion actuated by localised fluid streaming. The direction of the force arising from such streaming is optimal when the devices are maintained at the Rayleigh angle. The technique provides propulsion without any moving parts, and, due to the inherent design of the SAW transducer, enables simple control of the direction of travel.


The Oobleck waves instability mechanism highlighted in this study is not limited to shear-thickening suspensions and could be extended to any other complex fluids having a rheology with a negatively sloped region (e.g., granular materials and geomaterials exhibiting velocity-weakening rheology43,44, concentrated polymers or surfactant solutions35,36, liquid crystals45, active self-propelled suspensions46). More generally, our analysis shows that gravity forces, which are usually stabilizing for gravity-driven free-surface flows, can become destabilizing in the presence of a non-monotonic rheology. Our result could thus be extended to other stabilizing forces such as capillary forces arising from the free-surface deformation. We thus anticipate that other interesting instabilities may be explained directly, or in the light of our study.


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