Lift and Drag Force on a planing surface (hull)

josch · April 28, 2016, 10:53

Hey guys,
Im working on a simulation to calculate lift- and drag forces on simple planing surface.

dimensions: 1.691m (lenght) x 0.49m (beam) x approx. 0.06m with a small deadrise angle.

draft: 0.04m
trim: -2 deg
velocity: 4 m/s

Therfore I worked with the EHP add on form Star CCM+. That includes VOF between air und water and the DFBI motion for the planing surface. At what I already disabled heave and pitch motion to shorten the calculation time. The mesh looks actually really good ....as far as i am concerned :-)

Despite everything I still get very illogical results. A negative lift force of -5 N and a total drag force of 8 N. I already check the directions of the force reports. Furthermore Im not sure if it is normal for a simulation like this that the residuals are oscillating (as you can see in the pic.)
drag.jpg

lift.jpg

residuals.jpg
Im pleased about very help or hint I can get from you. I could also send you the sim. file or other information if that helps

fluid23 · April 28, 2016, 13:47

I assume since you are using DFBI that this is an implicit unsteady analysis, no?

It is totally normal for an unsteady analysis to show oscillations in the residuals. This is a byproduct of how each time step is resolved before moving on to the next instant in time.

As for your results, there are any number of issues that could cause this. Unfortunately, you don't give enough info to tell. However, given your residual plot I would start with your initialization and stopping criteria. I would initialize your unsteady analysis by first running a steady state analysis. This will get you a better starting point than default initialization on an unsteady analysis. Then I would also consider loosening your inner iteration stopping criteria. It looks like you kept the default of 5 iterations per time step. This is almost never enough. I would start out with something more on the order of 100 or 200 then reduce it once you see how well your residuals converge. Right now it would appear that you don't even give them a chance to drop so you could very easily be getting bad results just from that. As the solution progresses, you can further reduce the number of inner iterations as appropriate. Just monitor your plot and when they either get below some theshold (my typical goal 0.001) or they flatten out simply adjust based on how many iterations it took to get there. Now, if your residuals are flattening out at a high value then you probably need to think about also adjusting your time step. Rarely will the default value of 0.001 seconds be the best choice.

josch · May 19, 2016, 17:42

Hey thanks for these hints I tried to adapted the sim. a bit like you said it. But even with 100 inner iterations (time step of 0.002) the residuals look strange .....as a comparison I changed the inner iterations back to 10 (circled in red in the pic). Also it is strange that lift and drag forces are so small.....

The mesh should be ok with 1.6 mio cells and lots of volumen and surface refinement. Also I unabled the pitch so that the surfboard has only 1 DOF (heave in z-axis)

So how can I adapte the timestep? and by which criteria?

Im thankful for any help

josch · May 19, 2016, 17:51

Here are few more pics

avciahm · May 29, 2016, 11:24

how can i get "EHP" tool ?

Quote:

Originally Posted by josch

Hey guys,
Im working on a simulation to calculate lift- and drag forces on simple planing surface.

dimensions: 1.691m (lenght) x 0.49m (beam) x approx. 0.06m with a small deadrise angle.

draft: 0.04m
trim: -2 deg
velocity: 4 m/s

Therfore I worked with the EHP add on form Star CCM+. That includes VOF between air und water and the DFBI motion for the planing surface. At what I already disabled heave and pitch motion to shorten the calculation time. The mesh looks actually really good ....as far as i am concerned :-)

Despite everything I still get very illogical results. A negative lift force of -5 N and a total drag force of 8 N. I already check the directions of the force reports. Furthermore Im not sure if it is normal for a simulation like this that the residuals are oscillating (as you can see in the pic.)
Attachment 47053

Attachment 47054

Attachment 47055
Im pleased about very help or hint I can get from you. I could also send you the sim. file or other information if that helps

April 28, 2016, 10:53	Lift and Drag Force on a planing surface (hull)	#1
josch New Member Jochen Join Date: Apr 2016 Posts: 4 Rep Power: 10	Hey guys, Im working on a simulation to calculate lift- and drag forces on simple planing surface. dimensions: 1.691m (lenght) x 0.49m (beam) x approx. 0.06m with a small deadrise angle. draft: 0.04m trim: -2 deg velocity: 4 m/s Therfore I worked with the EHP add on form Star CCM+. That includes VOF between air und water and the DFBI motion for the planing surface. At what I already disabled heave and pitch motion to shorten the calculation time. The mesh looks actually really good ....as far as i am concerned :-) Despite everything I still get very illogical results. A negative lift force of -5 N and a total drag force of 8 N. I already check the directions of the force reports. Furthermore Im not sure if it is normal for a simulation like this that the residuals are oscillating (as you can see in the pic.) drag.jpg lift.jpg residuals.jpg Im pleased about very help or hint I can get from you. I could also send you the sim. file or other information if that helps

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April 28, 2016, 13:47		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	I assume since you are using DFBI that this is an implicit unsteady analysis, no? It is totally normal for an unsteady analysis to show oscillations in the residuals. This is a byproduct of how each time step is resolved before moving on to the next instant in time. As for your results, there are any number of issues that could cause this. Unfortunately, you don't give enough info to tell. However, given your residual plot I would start with your initialization and stopping criteria. I would initialize your unsteady analysis by first running a steady state analysis. This will get you a better starting point than default initialization on an unsteady analysis. Then I would also consider loosening your inner iteration stopping criteria. It looks like you kept the default of 5 iterations per time step. This is almost never enough. I would start out with something more on the order of 100 or 200 then reduce it once you see how well your residuals converge. Right now it would appear that you don't even give them a chance to drop so you could very easily be getting bad results just from that. As the solution progresses, you can further reduce the number of inner iterations as appropriate. Just monitor your plot and when they either get below some theshold (my typical goal 0.001) or they flatten out simply adjust based on how many iterations it took to get there. Now, if your residuals are flattening out at a high value then you probably need to think about also adjusting your time step. Rarely will the default value of 0.001 seconds be the best choice.