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how to justify if a flow is turbulent in the ocean modeling

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Old   June 23, 2011, 04:41
Default how to justify if a flow is turbulent in the ocean modeling
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Robert Ong
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Hi All,

My apology for asking such a fundamental question, but I've been struggling to understand on what basis should we say that the flow is turbulent (apart from the experimental results)? For ideal cases such as duct or pipe, then we normally use Reynolds number to justify the flow characterisation, however, for open channel I don't think this is quite the case. How about using Froude number as opposed to Reynolds number?

Thanks very much for your time and attention.

Kind Regards,
Robert
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Old   June 23, 2011, 04:48
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Bernhard
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You should not focus to much on a Reynolds number. Even for pipe flow it is possible to have laminar flow for Re=10 000 in special occasions. The Froude number cannot be a measure, since the molecular viscosity plays an important role in the dissipation and break up of eddies at small scales.
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Old   June 23, 2011, 05:08
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Robert Ong
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Hi Bernhard,

Thanks for your reply. So to go back to the question, how do I know if a flow is turbulent prior to choose solvers, etc?

Kind Regards,
Robert
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Old   June 23, 2011, 05:34
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Bernhard
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In general you know what your simulating, so you know what to expect. This is the only option to tell it on beforehand, but I think it is really essential to understand what you are modelling when you start your simulations. And how can you judge the results if you don't really understand the case?

Of course you can look at your results, and check in what regime the turbulent viscosity is, but that doesn't make sense for me.
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Old   June 23, 2011, 05:48
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Robert Ong
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Thanks Bernhard.

I judge my results based on comparison with the experimental observations. The way I do it now is similar to your second suggestion (I know this isn't the right way - which is the reason why I post this discussion on the board) which is to cross reference the turbulent viscosity ratio based on the Reynolds number and match the results with the experimental data.

Thanks and Regards,
Robert
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Old   June 24, 2011, 12:22
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Paulo Vatavuk
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Hi Robert,
You can use the results for channel flows. Re=V B/nu
were V=medium velocity, B=channel depth, nu=kinematic viscosity. Transition from laminar to turbulent flow is expected for appproximately Re=1300.
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