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Seakeeping and Manoeuvring Prof. Dr. Debabrata Sen Department of Ocean Engineering and Naval Architecture Indian Institute of Technology, Kharagpur Lecture No. # 12 Description of Irregular Waves by Spectrum See in the last class, we were talking about describing irregular waves. (Refer Slide Time: 00:33) What we mentioned is that you could take wave heights and you can plot a histogram, which looks like this. We also mentioned that you can take instead of height, you can take say period and you can plot p of T z versus T z or so. You can get all statistics about height – one-third significant height, etcetera. Probability of how much height occurring where; what is the chance of a given height to be exceeded, etcetera. However, the problem in this is that, although we are getting this information of height and period, there is no correlation between the two. Remember, height is a measure in vertical direction here. And, period, length, frequency, all are measured in the horizontal coordinate. And, these two are independent as such, because for the same height, you can have same frequency (( )) different heights. But, when I want to go to a sea, we actually
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require information of a description, where both are coupled. Now, how this is done is the interesting concept. It is done usually using the concept of energy spectrum. (Refer Slide Time: 02:19) Now, we come to this interesting thing. See take some irregular signal t versus eta. Now, you see I can represent an irregular signal by eta of t equals A naught plus sigma A i into cosine omega i t plus B i into sine omega i t or A naught plus sigma A i bar into cosine of omega i t minus epsilon i. See what I wrote here. You know everybody has heard of this Fourier analysis. Any irregular signal can be expressed as if it is sum of number of sine frequency and cos frequency or number of cos frequency with the phase angle. This is only a generic, may not be exact. The point is that in signal processing, you would have seen that any signal, which is of this nature, you can express them by means of a long series, sine series. Let us say look at that. Now, what this (Refer Slide Time: 03:43) represent? This represents here that as if I can conversely get an irregular spectrum, I can break it down to these components, that is, different so-called A i bar omega versus omega; or, conversely, if I add number of sine waves, I end up getting an irregular wave. Whichever way, because essentially, what is happening here, there is a way to get… See this is time series – time to frequency, frequency to time. Now, the question is like this – any signal, maybe the signal is also like that; you would have seen in mathematical physics book, any signal – I can always break it down to such Fourier analysis. That is from signal
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processing. The question is that is it possible for us? Can we really do that for seawave description? The answer is very interesting. (Refer Slide Time: 04:56) What we have seen in linear wave theory is that according to the linear waves, the individual waves eta t 1 is basically is a cos curve – A 1 cosine of K 1 x minus omega i t plus epsilon i; something like that. Now, eta 2 T – take another way, which is another cos curve – A 2 cosine of K 2 x minus omega I t plus epsilon i. Now, the point is that we have seen according to linear theory, sum of these two waves will simply be given by this sum of the two. Why? Because linear theory allows me to superpose waves. You see if for example, this is a solution the for problem phi 1, this is a solution for the problem phi 2, then we find out that basically, phi is a linear, the system is linear as well as… Therefore, phi 1 plus phi 2 can be added, superpose. In other words, what happened, if I add these two up (Refer Slide Time: 05:58), what I get becomes actually as per the linear theory of the wave, which is sum of the two. This is very important, because what it means that it allows me to superpose waves, which therefore, tells me that supposing I can break it down to this (Refer Slide Time: 06:21)? Yes, the every one of them individually would represent a typical regular wave. In other words, I can think therefore, that an irregular wave of this signal is nothing but a sum of regular waves.
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Conversely, I can add regular waves together to get irregular waves. Possible, because remember, regular gives the sine curves and also linear. See the two things are important; regular waves are not only sine curves, we have seen it is also linear. In other words, I should say other way round – as per linear theory goes, regular waves are sinusoidal curves. And, because it is linear theory, these curves can be added together to get a sum wave. So, super position is possible. So, you see there is physics involved and maths involved. Maths, because any signal can be broken down to this (Refer Slide Time: 07:16). I can always break it down, but the breaking down would have been meaningless provided this sum does not really represent a realistic physical wave. But, we find out that according to the linear wave theory; linear wave theory tells me a sine curve and this can be added together, superposed together; that means I can have plus. If I add it up, I will get a wave, whatever sum wave. This is actually also a possible wave. So, I can think now a converse process that I have begun with this wave, I can think I have this (Refer Slide Time: 08:01). So, I can break it down and I can think that it is nothing but sum of these regular waves. So, this is the concept behind that. (Refer Slide Time: 08:18) In other words, what is happening now, we will just now go to this picture part; it will be easier. What is happening, then, I can say that I have say in this all sine waves. Say this is omega 1; like that say omega n. This when I add it all up, I get an irregular wave. So, you see of course, here we have assumed that all the waves are travelling in the same direction, all of them are same direction and I am adding up. What is happened in reality