Fossati's Aero-Hydrodyamics

I have been working through Fabio Fossati's "Aero-Hydrodynamics and the Performance of Sailing Yachts" and am encountering some challenges with the some of the concepts presented in Appendix 1.
I was wondering if there's anyone familiar with this book who might be able to offer some explanations of specific details within that appendix.

 

I might not be able to help with any explanation but I would hope somebody familiar with the Italian original text and able to translate would come forward as I found some pretty poor translations in my copy.The definition of Prismatic Coefficient being a glaring example.

 

Could I ask you to look at just the two pages pp. 315-6 (attached) where he discusses the use of a wind tunnel to identify the pressure distribution around an airfoil?

Unfortunately Fossati passed away in 2015 at the early age of 53 but from 2004 he was scientific coordinator for the wind tunnel testing of sailing yachts at Milan Polytechnic under whose auspices the book was published. With those credentials, we should acknowledge his authority on the subject of wind tunnel testing as it applies to yacht racing.

As you will be aware, Fig A 1.12 or similar images are widely used in sailing literature explanations of how sails work, but I don't understand how he used the manometer readings that can be discerned in Fig A.1.11 to arrive at the image in Fig. A.1.12. Maybe you can shine some light?

 

The manometer bank will give give air pressure readings at various points of the sail, and for this to work I would imagine the direction of the force creating the air pressure also. From this he has drawn a simple set of vectors indicating how a sail propels a boat upwind.

I would suggest, without reading the text, that the first picture could be derived by introducing some reflective component to the airstream to make it visible.

 

Having looked at the pages in question my view is that the scales shown are the connections to the pressure tappings and the ends of the manometers that are open to the atmosphere are not visible.Which would be why the tubes at the left show a high fluid level.My theory is supported by the value of the numbers by the tubes decreasing in value as they rise up the backing board.It would be clearer if the numbers assigned to each manometer tube were visible and I have a feeling that each establishment with a wind tunnel will have their own method of assigning those numbers.The diagram showing the locations for the specimen section shows 1-8 on the top surface and 9-16 on the lower surface but with both sets of numbers beginning at the leading edge.It doesn't actually matter as the plots are shown with the vectors representing the pressure readings in the correct locations but it might be more easily visualised if the number sequence was continuous around the surface.perhaps in the manner of the coordinates of a .dat file.

 

Agreed with @DCockey. And as much I am a fan of physical models helping folks correlate observation to analytic principles, this rig makes little sense without DCokeys' explanation unless you already understand the analysis.

A copy of XFLR5 will do everything you want in software. It's based on Mark Drela's (MIT) XFOIL code, but with a user interface that is actually usable! Adjust your fluid density and viscosity as required...

 

But that's where I'm confused.
If the higher water level in the tube represented a higher pressure at the pressure tap on the foil, then tube #1 would represent a high pressure, but the arrow #1 on the diagram is pointing away from the surface, indicating a lower pressure.
It all depends whether the tubes from the experiment are connected to the top of the manometer or the bottom!

 

A manometer indicates a local pressure reading subject to the force of gravity at a point. That reading is, for all practical purposes, at right angles to the X-axis of Cartesian coordinates. This is not typically perpendicular to the surface of the foil. Fluid flow (gas or liquid) around the form creates a local pressure (and velocity) change. The liquid is higher in the manometer tube where the local pressure is lower (and the fluid velocity across the tap is higher). The fluid height at the tap must be resolved, which can be done graphically or formulaically, into a vector normal to the surface at the tap point. The Y coordinate will be represented by the manometer reading. The x coordinate won't be captured by the manometer ( because the manometer only measures the vertical component 'due to gravity').

A manometer will have one end at surface of the test form. The other end will be outside the flow field referring to 'static pressure'. The difference is 'dynamic pressure.'

 

And that's exactly the opposite of

It has to be one or the other, and it's really quite important to the interpretation of the results.

 

The liquid is higher in a manometer tube at a low pressure point because the static pressure outside the 'field of view' of the instrument is higher than the local pressure at the surface tap. Thus, the liquid in the tube rises higher at areas of low pressure relative to the surface of the test form. Don't believe it? get a piece of vinyl tubing. Make an inverted 'U'. Run a stream of compressed air at high velocity across one end of the tube - that is your 'test tap.' The liquid will rise toward that end of the tube.

 

You misunderstand my confusion. It's to do with the interpretation of the readings in the photograph of the manometer.
@DCockey suggests a higher the level in the tube indicates a higher pressure at the surface tap. You say the higher the level in the tube the lower the pressure at the surface tap.
It has to be one or the other, it can't be both.
I have never worked with this kind of setup and so I don't know how to interpret what is being displayed, and I'm getting two contradictory explanations.

 

A higher level in a manometer tube indicates a local lower pressure. I think DCockey misspoke, but am sure he understands the physics. Any manometer reading will need to be read as the difference between static pressure outside the area of concern ( call it 'local static pressure' due to water depth/ elevation above sea level/whatever) and the measured pressure at the tap point on the test model. Higher fluid flow at the tap point = lower local pressure = higher fluid level in the tube. All of the tubes in the manifold have a common 'hidden' end outside the test area.

What you are looking at is 2D flow in a fluid medium - water, air, doesn't really matter. It all assumes 2D flow not subject to pressure differentials caused by elevation changes in a a single medium, or interference due to differences between two mediums with quite different densities and viscosities (such as an air/water interface).

Does this help?

 

Yes, I'm sure we all understand the physics of manometers - they've been around since C17.
It's their use in measuring air pressures around foils that I'm interested in: a study has been around since Gustave Eiffel, who, in his Paris lab in 1913, measured the over- and under- pressure on plates in a wind tunnel using water-based manometers. (The Wright brothers measured the lift and drag forces, not air pressure)

Since Senior Member @DCockey has suggested the opposite of your assertion, I would like to get some confirmation of your interpretation. He may have "misspoke" but he seemed pretty confident of his statement. As I said, I have not used this type of equipment, so am not in a position to adjudicate.

 

It's Bernoulli, long before any of us were born and then some. Higher fluid flow velocity = local pressure reduction considering flow tangential to the pressure tap. If that flow were normal to the tap (pushing straight into it, which it is not) there would be a local pressure increase.

I have a paperback copy of the Fossati book. I looked at the appendix with your referenced pages.

Local increase in velocity and pressure reduction on top surface of the foil and local pressure increase and velocity reduction on the bottom of the foil = net mass airflow with a downward component and a commensurate lift component, provided we are not in a stall regime. Check any of your resources. Done.

 

I deleted my post because I was backwards about what the manometers were showing.

Correct.