Wind Boundary Layer & Effective Reynolds Number?

[Erwan]

Happy New Year Everybody,

It is common knowledge that under 6 knots true wind speed, the wind is laminar above the water and above this wind speed, the boundary layer of the wind above the sea is turbulent.

The question is: With wind speed above 6knots, is the actual Reynolds number on a sail different (higher) from theorical Reynolds number computed with the classic equation ?

Thanks in advance

Regards

EK

[chabrenas]

This is outside my domain. If no-one here can answer, how about trying to contact the Wolfson Unit at the University of Southampton, UK.

[daiquiri]

I didn't quite understand what do you mean when you make a distinction between the "actual" and "theoretical" Reynolds number. And what is the "classical equation" you mention?
If it is the equation which defines the Reynolds number:
Re = (rho*V*L)/viscosity
then the answer is NO. The velocity "V" which appears in the equation is the mean (time-averaged) far-field velocity.
In case of steady laminar wind it is the actual wind velocity.
In case of turbulent wind it is a statistic mean velocity, stripped off the fluctuating velocity components u, v, w (in x, y, and z direction respectively).
So when you say "6 knots", you are talking about the mean flow velocity, which is the one used to define the Reynolds number.

The problem arises if you have a test data for your sail, obtained through measurements in a flow with some given level of turbulence. If you try to use it to calculate the sail performance in an airflow having a different turbulence scale, then you will introduce some (a priori unknown) degree of error. It happens because the different turbulence levels in the airflow ahead of the sail (far-field flow) will modify the laminar-to-tubulent transition characteristics of the flow around the sail surface and also will modify the point of boundary layer separation (wake formation).
If it all sounds tricky, that's because it is tricky.

[Erwan]

It is exactly the answer I was looking for: In other words I was suspecting
that the turbulent flow of the wind will modify the point where the transition occurs. (earlier with turbulent airflow)
By the way, I was guessing that for higher speeds above transition, the turbulence of the airflow would increase the "effective Reynolds Number, decreasing the friction coefficient accordingly.

My idea was to achieve an appraisal of what you have defined as a " degree of error", I didn't know it was so tricky .

Thank you again and good winds for the new Year

Erwan

[daiquiri]

Originally Posted by Erwan

It is exactly the answer I was looking for: In other words I was suspecting that the turbulent flow of the wind will modify the point where the transition occurs. (earlier with turbulent airflow)

Actually, if we are talking about a traditional sails working in the wake of its' mast, it is very doubtful that you will have any laminar flow over the sail surface. You can put aside considerations about laminar to turbulent transition and consider the flow over the sail as fully tubulent (unless we're talking about some very low wind speeds).
Remains the problem of the boundary layer separation and stall angle, which will also be modified with wind turbulence.
Unfortunately, there is very little freely available test data about sail aerodynamics. If you are not involved in some America Cup design team or in some specific academic research programme, you have to fight your way through a stringent literature found in internet or in some of very rare (and mostly obsolete) books dealing with this topic.
By the way, even people who work in A. Cup (with all the money and facilities thay can rely upon) have a nice amount of problems and uncertainities when they have to translate the wind-tunnel measurements data to a real-size boat...

[chabrenas]

Thank you both for tempting me to start digging around for free papers currently available on sail aerodynamics, and even for the abstracts of subscription material. I may even be tempted to fork out the odd 10 Euros to buy some of it.

What is the purpose of your research, Erwan?

[Erwan]

Thank You Daiquiri for taking time to answer.
I forgot to mention that the subject of my "research" is the A-Cat rig, any kind of ring, including wingsail. So that is why I though it was important to have an idea about what I improperly defined as "Effective Reynolds Number".

[daiquiri]

Originally Posted by Erwan

I forgot to mention that the subject of my "research" is the A-Cat rig, any kind of ring, including wingsail. So that is why I though it was important to have an idea about what I improperly defined as "Effective Reynolds Number".

Yes, wingsails are affected by laminar to turbulent boundary layer transitions. They are very akin to airplane wings, and that's where you can find a lots of literature and airfoil test data around.
Sails with sleeve-luffs, like windsurf sails, probably have a minor region of laminar flow at the leading edge too, but it depends very much on how the sleeves are made (sleeve material, shape and extension, mast shape etc.).