Programming for generating waves in towing tank

Discussion in 'Hydrodynamics and Aerodynamics' started by conceptia, Oct 4, 2012.

  1. conceptia
    Joined: Nov 2009
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    conceptia Naval Architect

    Hey guys, I was looking ahead to start my hands on experience on the towing tanks and model testing. Do anyone have any idea on the programming usually done to model a wave in the tank? Please do share any relevant experience and documents to me at the earliest. It is an urgent matter. Im a student, and need to start with the experiments.
     
  2. BMcF
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    BMcF Senior Member

    There are many documented math models for wave spectra (ITTC-2, Pierson-Moskowitz, JONSWAP..) A google search should be sufficient to locate any of those and more.

    To generate those waves in a model basin is quite a different matter altogether and is vastly more than "programming". Someone(s) with a pretty solid controls, mechanical design and fluid dynamics background need to be involved and the resulting end-to end system will be both complex and unique to the specific tank or basin in question.

    Tank, length, width and depth...beaches or other damping devices or not?..type (hydraulic, pneuamatic, mechanical) of servo control of the wave-producing mechanism and its frequency resonse to input commands (gain and phase)...and the actual transfer function (gain and phas) between wave generator and the tank water; something that has to be characterized by actual test after everything is built.) There is also the duration of time for which the generated waves still conform reasonably well to the intended spectral model; something varies as a function of actual tank size, wave type, size of model under test relative to tanbk size, etc. Another bunch of fairly tank-unique parameters, in other words.


    So, end-to-end, you have: Wave Spectral Definition--->Wave Maker Servo Commands--->Wave Maker Response--->Tank Water Surface Response. Of those four, only the first is..and even can be, in most cases..characterized in a general sense and available in the literature.


    Now, if you are going to be testing in an existing tow tank, the latter three "models" I show above have already been characterized and you can rest assured that most accepted wave spectral definitions have been run through it. All you would do is pick one..and check with the technicians that operate the tank to verify that your selection of spectra is compatible with their ability to produce it; some tanks have difficulty with the more peaked and short-period wave models, like the JONSWAP, for example.
     
  3. CWTeebs
    Joined: Apr 2011
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    CWTeebs AnomalyGenerator

    Are you looking to generate regular or irregular waves?
     
  4. conceptia
    Joined: Nov 2009
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    conceptia Naval Architect

    thank you BMcF, it was useful. Let me explain you the complete scenario. We are going to upgrade our control system from DOS to Labview. So, inorder to calculate the inverse transfer functions for the wave maker system we have to do wave runs. Moreover the beach is also to be modified, as most of the spectral cases we get returning waves coinciding with the generated waves.
    CWTeebs - at present, we are more interested in Transient waves, which will much reduce the returning wave effect.
    Thank you both for your responses.
     
  5. BMcF
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    BMcF Senior Member

    Ahh. Mr. McKesson should be able to offer a few pointers, I would think. :)

    It sounds like you may have to characterize the transfer functions for each block as I outlined in an overly simplified fashion above. The simplest way to do that (IMHO..without resorting to random signla generators with shaping filters and more sophisticated and difficult analysis methods) is using sine wave inputs to the wave-maker servo and measuring responses of both the mechanism and the resulting surface wave. Do that for a range of discrete frequencies that cover the spectral window appropriate for that tank.

    Use a simple sine-fit routine to the measured responses and then from there you can easily determine the gain and phase relationship between input and responses.

    As long as your individual regular wave tests are fairly close together in frequency (say, 0.2 HZ or less..or use a logarithmic increment, increasing spacing as the frequency goes up), you can combine the results to create a continuous wide-band gain and phase function....the transfer function you are looking for.
     

  6. conceptia
    Joined: Nov 2009
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    conceptia Naval Architect

    Alright. Thank you. The information you gave me is very helpful. Once again thank you.
     
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