Calm water ship resistance why URANS?

flyingorc · June 24, 2015, 07:06

Hi,

I'm quite new to CFD. To my understanding, calm water resistance of ships is a steady wave problem. May I ask why people use unsteady RANS to model this problem? Is it possible to solve it as steady problem?

When I use the unsteady RANS to solve this problem, after the steady-state is reached, why the resistance from shear stress is still oscillating with a small amplitude?

Thank you guys.

lava12005 · July 16, 2015, 12:12

A rough picture:
The steady and unsteady is just a 'method' employed.. In both steady and unsteady the flow needs to be developed from initial condition to the final one (regardless steady state exist or not)..
In the steady method there is iteration which you can view is as advancing in 'time' though not physically possible..
The method is not designed for advancing with the steady state method

The convergence is rather slow for low Froude number (typical displacement ship) calculation even when the ship is stable..
Normally if everything is fine the oscillation is dampened and getting smaller..
If it is not, maybe your mesh?
And you can check the average value whether is it reasonable?

June 24, 2015, 07:06	Calm water ship resistance why URANS?	#1
flyingorc New Member Join Date: Jun 2015 Posts: 2 Rep Power: 0	Hi, I'm quite new to CFD. To my understanding, calm water resistance of ships is a steady wave problem. May I ask why people use unsteady RANS to model this problem? Is it possible to solve it as steady problem? When I use the unsteady RANS to solve this problem, after the steady-state is reached, why the resistance from shear stress is still oscillating with a small amplitude? Thank you guys.

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July 16, 2015, 12:12		#2
lava12005 Senior Member KHB Join Date: Aug 2010 Location: Singapore Posts: 118 Rep Power: 16	A rough picture: The steady and unsteady is just a 'method' employed.. In both steady and unsteady the flow needs to be developed from initial condition to the final one (regardless steady state exist or not).. In the steady method there is iteration which you can view is as advancing in 'time' though not physically possible.. The method is not designed for advancing with the steady state method The convergence is rather slow for low Froude number (typical displacement ship) calculation even when the ship is stable.. Normally if everything is fine the oscillation is dampened and getting smaller.. If it is not, maybe your mesh? And you can check the average value whether is it reasonable?