Resistance Error

pooyazr · January 25, 2021, 13:08

Hi
I working on simulation semi displacement. for present simulation i do mesh steady in froude number 0.35 and my results good. but in Fr = 0.2 The resistance results are high error.
Summary of simulation settings:
Number of prism layer mesh = 10
time step = 0.001
y+ in Fr = 0.35 = 50
Do you have a suggestion?

ping · January 27, 2021, 01:48

pressure resistance is just as important as skin friction so the mesh of the free surface, kelvin wake and rear wake are very important.

you might have a good boundary layer modelled but also ensure the mesh from the last prism layer to the volume mesh is is not growing too fast.

other ideas:
how good is your geometry compared to that used in testing?
run for long enough based on monitored reports?
how reliable is the measured data - these are difficult measurements to make and it is common to find poor data?

pooyazr · January 27, 2021, 10:06

Quote:

Originally Posted by ping

pressure resistance is just as important as skin friction so the mesh of the free surface, kelvin wake and rear wake are very important.

you might have a good boundary layer modelled but also ensure the mesh from the last prism layer to the volume mesh is is not growing too fast.

other ideas:
how good is your geometry compared to that used in testing?
run for long enough based on monitored reports?
how reliable is the measured data - these are difficult measurements to make and it is common to find poor data?

1.Does the thickness of near wall prism layer affect the resistance results?
2. Does the prism layer thickness the resistance results?

ping · January 28, 2021, 20:56

yes the 1st prism layer is very important and you need to follow the normal guidelines for its thickness dependent the wall physics you choose. if using a high y+ wall treatment then the cells after the 1st simply need to grow gradually eg < 50% bigger each step out to the core mesh and sometimes you only might need 2 prism layers to achieve this.
cd-adapco before siemens had some wonderful guidelines and papers about this done by prof peric and you should be able to find this or ask a support engineer. i remember one that gave guidelines for doing both model scale and full scale ship hulls. but one important point i remember was that to achieve accurate results you did not need to use low y+ models and the crazy mesh numbers that it requires to fully integrate the flow across the boundary layer.

January 25, 2021, 13:08	Resistance Error	#1
pooyazr New Member pooya Join Date: Jan 2021 Posts: 3 Rep Power: 5	Hi I working on simulation semi displacement. for present simulation i do mesh steady in froude number 0.35 and my results good. but in Fr = 0.2 The resistance results are high error. Summary of simulation settings: Number of prism layer mesh = 10 time step = 0.001 y+ in Fr = 0.35 = 50 Do you have a suggestion?

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January 27, 2021, 01:48		#2
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	pressure resistance is just as important as skin friction so the mesh of the free surface, kelvin wake and rear wake are very important. you might have a good boundary layer modelled but also ensure the mesh from the last prism layer to the volume mesh is is not growing too fast. other ideas: how good is your geometry compared to that used in testing? run for long enough based on monitored reports? how reliable is the measured data - these are difficult measurements to make and it is common to find poor data?

January 28, 2021, 20:56		#4
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	yes the 1st prism layer is very important and you need to follow the normal guidelines for its thickness dependent the wall physics you choose. if using a high y+ wall treatment then the cells after the 1st simply need to grow gradually eg < 50% bigger each step out to the core mesh and sometimes you only might need 2 prism layers to achieve this. cd-adapco before siemens had some wonderful guidelines and papers about this done by prof peric and you should be able to find this or ask a support engineer. i remember one that gave guidelines for doing both model scale and full scale ship hulls. but one important point i remember was that to achieve accurate results you did not need to use low y+ models and the crazy mesh numbers that it requires to fully integrate the flow across the boundary layer.