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Old   July 6, 2000, 12:10
Default Wave Theory
  #1
oliver.mulryan
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Hi all

I am hoping, that someone can give me a decent explanation of wave theory, any analogies than can explain it.

Thanks again for the help

Oliver
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Old   July 6, 2000, 12:38
Default Re: Wave Theory
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John C. Chien
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(1). Have you try the college physics text book? (2). There are two kinds of wave, one is longitudinal and the other is transverse wave. (3).It would be easier for you to read the text book. The former type is like the acoustic wave, it propagates by compression and expansion in the direction of propagation. The latter type is like the ocean wave, the fluid moves up and down at the same location, and the wave propagates in the transverse direction (normal to the motion of the fluid particles). (4). It is too hard to explain the physics to you here.
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Old   July 6, 2000, 12:46
Default Re: Wave Theory
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oliver.mulryan
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John As you probally know wave theory, is related to flow "transations", standing, upstream, downstarem directed travelling waves. However, I can't seem to grasp it at fully at present, so I would be delighted if you could shed light any light on the subject.

Thanks

Oliver
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Old   July 6, 2000, 13:12
Default Re: Wave Theory
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John C. Chien
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(1). Well, physics comes from the observation of the nature. So, I guess, you will have to accept it first. (2). But if you watch the ocean waves everyday, someday, you will become Newton the second. I got the feeling that you need to understand at least Newton's law first. Then, study the acoustic theory, and hydrodynamics. It's a lot of work.
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Old   July 6, 2000, 13:53
Default Re: Wave Theory
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oliver.mulryan
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John

So what your trying to say is that downstream standing waves carry the flow downstream, and the converse is true for upstream, when the wave is standing the flow is effectively stagnat. rtepresenting the transition bewteen the two. What rational basis has this concept in Fluid Mechanics if a flow is staionary and non periodic

Regards

oliver
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Old   July 6, 2000, 14:16
Default Re: Wave Theory
  #6
John C. Chien
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(1). Wave is the perturbation only. It is an effect on the local fluid, which is stationary on the average. (2). You can easily create a wave motion in a stadium covered with audience. They can sit there individually, while creating a wave pattern which actually moves from one side to the other. (3). So, wave is a small perturbation which does not require the large scale permanent motion of the local fluid particles.
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Old   July 6, 2000, 15:00
Default Re: Wave Theory
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oliver.mulryan
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John

Ok, your points is is credible. Therefore one can directly associate wave theory and hydrodynamic instabilities. (Direction and amplification of a flow disturbance). 1. It is well known the viscoity amplifies any disturbance, so what normally dampens it and keeeps it bound?. 2. If a wave is travelling downstream how can it spontaneously change direction ?.

regards oliver

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Old   July 6, 2000, 15:24
Default Re: Wave Theory
  #8
John C. Chien
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(1). The perturbation is normally so small that most theory on wave is inviscid. It will travel a long idstance. Under the inviscid theory, you can't stop it. (2). the wave simply propagate outward from the place where it is generated. just like the wave pattern generated when you throw a stone into a lake. (3). It does not change direction, (unless it goes through different medium, or non uniform medium). (4). But the wave can reflect from a boundary. A wall is a good example of a boundary. But it is not the only kind of boundary. It can be physical or man-made.
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Old   July 7, 2000, 11:26
Default Re: Wave Theory
  #9
oliver.mulryan
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John

First of all thanks for the help. John back to our discussion again; three basic wave types can be distinguished for fluid flow, being upstream travelling, stationary and downstream travelling longitudinal waveforms. these effectively originate from a localised flow distrubance within the flow.

My question is quite simple I think. If the "state" of the waveform changes, does it change at the perturbation in the flow field i.e. the origin of its disturbance?

If it doesn't change there and there are no wall boundary conditions in the flow what causes it to change?

This is all in relation to the Vortex breakdown phenomenon, in which averaging techniques associated with CFD destroy every asymmetry in the flow, so effectively CFD is, pardon my expression absolute crap in relation to swirling flows. All codes related to CFD are based on empirical methods from boundary layer data and therefore can't be applied with any level of confidence to swirling flows. Do you know of any turbulence model bar second order closure methods that give relatively decent results for swirling flows i.e. derived specifically for swirling flows as well as the empirical data that implements them.

Thanks for all the help sorry for being a "pain in the arse".

Regards

oliver
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Old   July 7, 2000, 11:58
Default Re: Wave Theory
  #10
John C. Chien
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(1). Well, the wave has to ride on something in order to propagate. (2). If a car is on a newly paved road section, you can feel the difference. The difference is the change. (3). That is why we have "wave propagation", ....through or on something. And that "something" will have effect on the wave. (4). I had worked on the cfd analysis and turbulence modeling of swirling flow through a dump combustor configuration. I was able to model it and match the prediction with the test results. (5). So, I think, you can always do the same thing with any turbulence models. (6). About the use of the words, I think, those are nothing but combinations of pixels on the screen. Even back in early 80's, I had written a program to print Chinese characters using dot matrix printer. Words are just dots, and in most cases a small pictures or bitmaps or icons. (7). If you can find a lake, why not throw two stones into the lake and watch the formation of the waves, collision of the waves, etc.....
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Old   July 9, 2000, 11:35
Default Re: Wave Theory
  #11
John C. Chien
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(1). Wave Theory, if one takes only very small amount from the environment, then he can move a small step. If one gives back the same very small amount to the environment, then the environment will remain untouched. Thus one can repeat the same process, on and on forever. That is the conservation of energy. (2). If one takes in only without giving it back, the gain will grow and the environment will change. Soon, the changed environment can no longer support the over-grown demand. The growth will stop, slow down and collapse. When a small wave decides to become a non-linear wave or a shock wave, entropy will be created in the shock wave, and only the loss can be realized. (3). If you study C++, you will realize that, even in the world of languages, you must remember to return the pointer and set it to zero, when you are through with the dynamically allocated variable through "NEW". You will be given an opportunity again, to create something "NEW", only when you remember to return the "OLD NEW" resources. (4). The nice thing is, there is always a "RESET" key somewhere, and that will tell you whether you are a "wave" or a "shock".
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