The discussion revolves around the relationship between the properties of acoustic waves generated by a body moving through a fluid and various factors such as momentum, fluid density, and temperature. Participants explore concepts related to wave formation, turbulence, and the Doppler effect in acoustics.
Participants express differing views on the nature of wave formation behind moving bodies, with some supporting the idea of turbulence and others questioning the premise of vacuum formation. The discussion remains unresolved, with multiple competing perspectives on the topic.
Participants acknowledge limitations in their understanding of fluid dynamics and turbulence, indicating that further learning is necessary to refine their hypotheses.
Why would you expect a body moving at constant speed to generate a periodic wave form?mohamed_a said:I have read about doppler effect in acoustics so i searched for the relation ship between wavelength of wave produced by linear movement of body and its momentum along with other dependent variables such as density of fluid (leaving acoustics for a second) and temperature but souldn't find a source on the internet
at least it will form eddy currents and vortices with velocity-dependent frequency.I think that if the body moves in a linear motion it will displace a certain volume of fluid and then the surrounding medium will refill the vaccum-like space left behind .this process would form waves (since the pressure difference would produce a force -> compression -> followed by rarefaction) with properties depending on several factors such as fluid elasticity , temperature and velocity or momentum (this is my hypothesis)jbriggs444 said:Why would you expect a body moving at constant speed to generate a periodic wave form?
But, unless these waves are periodic, they will not have a "wavelength".mohamed_a said:at least it will form eddy currents and vortices with velocity-dependent frequency.I think that if the body moves in a linear motion it will displace a certain volume of fluid and then the surrounding medium will refill the vaccum-like space left behind .this process would form waves (since the pressure difference would produce a force -> compression -> followed by rarefaction) with properties depending on several factors such as fluid elasticity , temperature and velocity or momentum (this is my hypothesis)
I think i didn't make my point clear. I mean the speed of the body, if a body moves in a linear motion in air supposing the speed is very high (relative to speed of sound ) near 200 m/s, the collapse of vacuum bubble will depend upon the speed of the object and properties of the medium, if the object creates vaccum-like bubble. A wave is formed by strong collapse due to pressure difference and since the fluid we are talking about is elastic it will recoil and recompress periodically in the same way a spring will produce a harmonic wave when put to similar test the frequency of this wave will be determined by how much force (due to pressure difference) acted upon the air filling the vacuum bubble sice this force causes compression if it is very strong it will compress the medium fast and due to the developing repulsion forces in the medium it will recoil fast causing a shorter wavelength of the wave.jbriggs444 said:But, unless these waves are periodic, they will not have a "wavelength".
Gravity waves (surface waves in deep water) such as the bow wave on a ship can have a distinct wavelength based on their speed of propagation. The speed of propagation will match the speed of the ship and, since that speed depends on wavelength, there will be a characteristic wavelength for a particular hull speed.
However, you are discussing acoustic waves. The propagation speed (the speed of sound) is fixed, independent of wavelength, so there is no hope from that angle.
The idea of the "maximum capacity of air withholding a vacuum" sounds like word salad to me.mohamed_a said:I think i didn't make my point clear. I mean the speed of the body, if a body moves in a linear motion in air supposing the speed is very high (relative to speed of sound ) near 200 m/s, the collapse of vacuum bubble will depend upon the speed of the object and properties of the medium, if the object creates 40 ml/s of vaccum-like bubble and the maximum capcacity of the air withholding such a vaccum-like circumstance is 20 ml this means that 2 waves propagating at the speed of sound will be produced in 1 second . each wave is formed by strong collapse due to pressure difference and since the fluid we are talking about is elastic it will recoil and recompress periodically in the same way a spring will produce a harmonic wave when put to similar test the frequency of this wave will be determined by how much force (due to pressure difference) acted upon the air filling the vacuum bubble sice this force causes compression if it is very strong it will compress the medium fast and due to the developing repulsion forces in the medium it will recoil fast causing a shorter wavelength of the wave.
omit the sentence it is really a just conceptual mistakejbriggs444 said:The idea of the "maximum capacity of air withholding a vacuum" sounds like word salad to me.
That the effect of a high speed object is a turbulent wake rather than laminar flow is quite plausible. But turbulence is complicated. Quite far above my level of competence, I am afraid. Probably something for a CFD approach where details matter a lot.
I need to learn much more about fluid dynamics before putting any hypothesesArjan82 said:The premise is false. You will not get a vacuum (or anything near that) behind the object. You will get a flow region more or less attached to the object (a wake). You only get a distinct periodic behavior (i.e. a discrete frequency) when you get Von Karman Shedding. Otherwise it is mostly generated by turbulence and you get broadband noise (i.e. a wide spectrum). Possibly related to the turbulence spectrum (Kolmogorov or alike).
CFD and acoustics is a difficult topic by the way. You definitely cannot do standard CFD, you must resolve a large part of the turbulence, but then still, you need for the propagation something like Fowcs Williams Hawkins.