Delft Hull Series

I assume that this is a result of your LES. In this case the indicated turbulence level depends on the grid size. How confident are you that the grid is fine enough to resolve the small eddies created by the bow wave? The break up of the eddies down stream along the model normally leads to small scale motions that are not resolved in LES. What is promising, is the comparison of your results to the measurements by Roth et al. which I have cited in my report on turbulence. He measured 3% to 6% directly in the bow wave. Your simulation increases my confidence in the assumption that the flow around the fin keel is not influenced by the bow wave. Again, thanks for sharing!

A different story is the turbulence in the oncoming flow far in front of the model. In real life this is caused by the wind waves and turbulent mixing in the surface layer.

 

Natural transition, yes. I could simulate the strips but i don't know their location, so i used the turbulence instead. At zero turbulence half the bottom is laminar, and Cf 0,0024.

The bow in the excel is right and the graf is stretched approximately the length of the WL. Until about 1,5 m the flow is laminar Cf around 0,002. Cf in th graf is on the centerline... The average is for the whole bottom - what do you mean by averaging in Y?

 

It is LES, yes, but close to model there's a wall function. It is coarse, if I recall the distance between particles (resolved resolution by LES) is 1,5 mm (I would have to check that), so I don't know how reliable this is.

Not sure the turbulence would not influence the keel - there would appear to be 0,5% little under the keel root, and maybe 0,1% all the way down. The scale starts at 0,1%, not zero.

Yes, with heel, leeway and waves there would be much more turbulence. I may have some Star simulations that would cast light on that.

 

For Sysser 26 the location of the strips, measured from the forward end of the Lwl are:
1. strip 0 cm to 2 cm
2. strip 33 cm to 35 cm
3. strip 66 cm to 68 cm
the strips are 2 cm wide, hence the two values.
At FN=0.4 the first strip is active for the center part of the hull. Some streamlines may pass sideways from the trip and the transition will occur for these streamlines later, downstream.

For each position along the x-axis I sum up all Cf-values around the girth (of the section at right angle to the x-axis). Then I take the average of these values with the same x-position and plot the averages as a function of X

 

OK, clever. No easy way for me to take those averages, but here's a plot of the curves (rather messy), and the xls-file, you can figure out yourself. The X-position is the whole model divided by 10, not the waterline, unfortunately. I believe the first curve is from the bow.

 

The sand strips would be fairly simple to simulate, too, would have to know the grain size in microns. But here's the run as fully turbulent, the average Cf is now 0,0042, so we are well in the ballpark with your integral calculations.

 

Mikko, you have made my Sunday and are keeping me busy.
It seems to me, that the "Length" in the excel-file is measured from the outboard towards the centerline, right?
This "Length", is it the distance along the y-axis, or is it the girth-length along the surface?

 

Yes, that would seem to be so, and length would seem along the girth, not the axis. Note that the model could be floating a bit too high, I get a WL length 1,975, while it should probably be 2,0. But that would not change much.

 

Lovely work Mikko and Uli, very interesting shots of the turbulence. Practical experience bears out the best thing is to sail in 'clean' water.... Most absorbing discussion, I hope to learn a little more from it, thanks.

 

I re-ran my boundary-layer calculation for your sinkage and zero trim to get the Lwl of 1.975, results for the average cf are attached.
The small sandstrip at the bow creates turbulent flow only on the centerline. The second sandstrip induces fully turbulent flow. This is an explanation of the difference at the bow between your simulation and mine.
Concerning your simulation, it is hard to believe that cf would increase towards the stern.
On your picture there is a sharp elliptical line visible from yellow to red, close to the rear end of the waterplane. Could this be the line of flow separation? If there is a region of recirculation aft of this line, cf could be high but negative in this region. Does the program differ between positive and negative cf, or is the color an indication of the absolute value?

 

Software available

There is a software available that makes use of the regression analysis, as discussed in this thread.
See http://www.boatdesign.net/forums/design-software/ulitank-available-52362.html

 

Admittedly it would seem strange Cf would increase towards the stern. I wouldn't know what orange elliptical is, but definitely the flow is not near separation. This hull is almost a flat plate, so little depth there is. Maybe it's not a good idea to define symmetry at the water surface.

I plotted shear stress on the bottom - at separation it would go to zero, it's nowhere near that here.

 

Very strange. Since cf = shear stress /(0.5*rho*V^2) the two pictures don't go together. If you can post an excel-file with values of the shear stress at the sections (like you did it for cf), I can calculate cf manually. It would also be helpful to have a vector plot of the shear stress. The directions of the shear forces are also important, because only the component in x-direction adds up to the resistance.
You are right, if one would test the double body model in the wind-tunnel, it would almost be a flat plate, at least in the rear part. It should pose no problem for a RANSE-solver. The results are therefore even more puzzling.

 

Here's the shear stress plots. Funny the "waterlines" don't show in it - I think it's the very low resolution and partial cells on the aft run, projecting the values on the surface-problem. My run only took some 10 minutes, so it's lores. I can run at a higher res to see if it makes a difference.

In your integral calc, cf goes to zero in the aft part, as your cross sections go to zero at the aft PP. Yet, at Fn 0,4 the whole transom would be wet, so it's not very realistic.

 

Oops.. forgot the excel