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Principles of Doppler ultrasound
Published in Peter R Hoskins, Kevin Martin, Abigail Thrush, Diagnostic Ultrasound, 2019
The Doppler effect is observed regularly in our daily lives. For example, it can be heard as the changing pitch of an ambulance siren as it passes by. The Doppler effect is the change in the observed frequency of the sound wave (fr) compared to the emitted frequency (ft) which occurs due to the relative motion between the observer and the source, as shown in Figure 7.1. In Figure 7.1a, both the source and the observer are stationary so the observed sound has the same frequency as the emitted sound. In Figure 7.1b, the source is moving towards the observer as it transmits the sound wave. This causes the wave fronts travelling towards the observer to be more closely packed, so that the observer witnesses a higher frequency wave than that emitted. If, however, the source is moving away from the observer, the wave fronts will be more spread out, and the frequency observed will be lower than that emitted (Figure 7.1c). The resulting change in the observed frequency from that transmitted is known as the Doppler shift, and the magnitude of the Doppler shift frequency is proportional to the relative velocity between the source and the observer.
Composition of Global Environment
Published in Takashiro Akitsu, Environmental Science, 2018
Redshift refers to a phenomenon in which, in astronomy, the spectrum of light from observation objects (including not only visible light but also all wavelength electromagnetic waves) shifts to the long wavelength side (closer to red in visible light). The Doppler effect is a phenomenon in which the frequency of a wave is observed differently due to the existence of the relative velocity between the source (sound source, light source, etc.) of the wave (sound source, electromagnetic wave, etc.) and the observer.
Fluid–Fluid Dispersions: Liquid–Liquid and Gas–Liquid Systems
Published in Wioletta Podgórska, Multiphase Particulate Systems in Turbulent Flows, 2019
The coalescence rate is determined by the collision frequency and coalescence efficiency. The collision of particles is caused by their relative velocity. In turbulent flows, the relative motion of particles depends on the particle size relative to the Kolmogorov scale. In the case of inertial collisions, the relative velocity between particles of diameter d is determined as a characteristic velocity variation in the flow over a distance d, urel∝(εd)1/3. In the case of viscous collisions in turbulent flow, the relative velocity is determined by the rate of strain characteristic of flow in the smallest eddies, urel∝d(ε/ν)1/2 (Chesters, 1991). Other sources of collisions in turbulent flow are also possible. The relative motion may occur due to mean velocity gradients. Different bubble rise velocities due to buoyancy can also lead to collisions. Yet another type of collision that may be of importance is the wake-induced collision. When a gas bubble rises through a liquid, an amount of liquid is accelerated behind the bubble, which is known as the wake. When bubbles enter the wake region of a leading bubble, they will accelerate and may collide with the preceding bubble (Bilicki and Kestin, 1987; Liao and Lucas, 2010).
FSI simulations for sailing yacht high performance appendages
Published in Ships and Offshore Structures, 2021
Olivia D’Ubaldo, Stefano Ghelardi, Cesare M. Rizzo
In the governing equation of an aero/elastic problem, the resultant force R acting on the structure is actually due to the motion of the lifting body itself in the flow. The term R is then expressed as an aero-elastic force, dependent on the relative displacement and on its two time derivatives. The relative velocity is defined as the difference between the velocity of the undisturbed flow and that of the body, induced by the velocity field of the flow itself.