Ceramic coatings

Flow will separate at a fixed distance depending on speed (assuming the same shape). This means that separation starts later, as a percentage of length, for smaller hulls. That is one of the difficult corrections to make when testing small models and then scaling the design up.

 

What are some specific ceramic coatings? Are we talking about actual ceramic coatings or ceramic particles in an organic matrix?

One spray coating mfr says "In the car care industry, ceramic coatings and nano technology have become buzzwords and industry standards and although sometimes effective, these are not true “ceramic coatings”. They are 'silica infused' products that behave like a coating." Silica infused sounds like marketing speak for finely ground crystalline silica or fumed silica powder in an organic lacquer. They were talking cosmetics, not drag reduction. Graphine oxide sounds promising, but carbon is not exactly a ceramic.

I have seen a number of claims (speculation anyway) that a coating that reduces friction between solids, or has a very low surface energy (water beads up) can reduce hydrodynamic drag, but I don't think the comparison is valid or borne out by the data. A Belzona sales rep back in the 90s told me about some preliminary tank testing they had done and thought that the lack of drag reduction was due to some flaw in the test procedure. As usual, when there is money to be made selling a shiny object, it is hard to accept facts that get in the way.

How smooth is smooth or how rough is rough? Manufacturing and machining standards are well established, but some of the units used in hydrodynamics are baffling. I suspect there are some here who would disagree, and perhaps they can translate the jargon into terms familiar to an engineer who thinks he understands Ra.

I found one reference relating surface roughness to hydrodynamic drag that actually used Ra and had this to say about measurement:
"• There is a need for all workers in the field to standardize on measuring the same parameters so that the results of one group can be more easily related to the work of other groups."
NBS TECHNICAL NOTE 1151 Surface Roughness Measurements of Circular Disks and Their Correlation with Hydrodynamic Drag
It deals with microroughness, which is where a coating would apply. Macroroughness , and structural roughness are additional complications.

Another reference used sandpaper as a roughness standard starting with 80 grit and moving up to 24 grit. Smooth was not defined in terms that I am accustomed to.

3M riblet film seems to have a significant effect when placed in the right area and in the correct orientation. Last time I looked into it for a hull there were more problems with maintenance than benefits from drag reduction, including that barnacles find it the ideal substrate to latch onto.

 

Riblets and roughness are used to induce early start of turbulence/separation. The vortices are then smaller so there is less overall energy creating drag.

 

The numerous small vortices produce a turbulent boundary layer that delays flow separation.

 

I always considered a turbulent boundary layer as flow separation.

 

There is a difference between a thin turbulent boundary layer and flow separation.

Short version:

Long version, click here. Mildly entertaining, but I only dabble in aerodynamics.


I only brought riblets up because some of the claims for ceramic coatings say that they reduce drag by improving surface finish. Both increasing and decreasing roughness can reduce drag. How, when and where and under what circumstances gets complicated.

 

I understand what you are saying. My opinion on this, is that the small vortices are separating the laminar flow from the surface. I see that as early separation.

 

Great discussion. Thanks to Dave G and for Gonzo's contributions. Interesting on the lack of a standard surface smoothness gauge, such as the Ra in the machinist world. I am a machinist; I would think that system would work just fine. And, yes, the slippery world of these psycho-babble nano particles gets more confusing as you try to research all the various manufacturers of these products. There is a big lack of information about their products. Carnival elixir? I will continue to watch for information on the subject, but I do not see anyone doing serious research that would help answer my questions. Please let me know if anyone knows of such.
Again, great information, more to ponder and study. Keep having fun!
Chaos