Fossati's Aero-Hydrodyamics

I don't know if I should have used different words in my earlier post,but you all seem to have got the message now.I must admit to never seeing manometers in a wind tunnel,it used to be more common to use Scani valves connected to a pressure transducer but this required careful logging of the connections and took a while for the flow past each pressure tapping to stabilise when the port opened,leading to quite lengthy runs for each test.As pressure transducers became less expensive they were used in much greater numbers.

 

I have seen manometers used in small wind tunnels in college or university labs. Useful as a visual aid to students, but they were usually supplemented with one or more pressure transducers if the tunnel was set up for models with taps.

 

My only issue with that earlier post was that you didn't address the question of whether the readings were those on the open side or the connected side.
If a) the open side, then the height would increase with increased pressure, if b) on the side connected to the test rig, a higher level would indicate a lower pressure.
A close examination of the photograph shows the vertical scale decreasing with height,
suggesting that an increased height would indicate an reduced pressure corresponding to option b) which is where we have landed with @tropostudio and @DCockey , so I think that is further confirmation.
Also, a closer examination of the photograph will reveal a barely readable numbering of the tubes in from 1 to 16 left to right.

 

My words were:
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 is why the numbers on the scales were reducing as they went up the backboard.

On reflection,I suppose the use of manometers,as opposed to pressure transducers,does simplify the process as there is no need to apply an atmospheric pressure correction.

 

Depends on how the pressure transducers are built. The scanning electronic pressure transducers I used briefly had a reference pressure port which could be left open to the local atmosphere or connected to a reference pressure source, such as the reference pressure ring in a wind tunnel.

 

As I said in my OP, I am encountering some challenges in Appendix 1, and with your assistance, my first challenge has been resolved in how to read the manometer data.
My next challenge is to understand how fig. A.1. 12 : was deduced from the preceding material.
In particular:

1. How were the manometer readings converted into arrow lengths? Did the value in the rightmost manometer column represent the ambient air pressure with a value shown of about 250 on the manometer scale?
   
2. How do the arrows relate to the pressure taps shown in Fig A. 1.6? The numbering and positions don't appear to correlate with each other.
3. Where did the big value that I have identified in the purple come from?

 

Do you know what are the units being displayed by the numbers on the backboard?
If the 258 reading on the rightmost tube does indeed represent the ambient air pressure of say 101 kPa, then that would equate to a scale of around 0.4 kPa per scale unit.
I see the lowest pressure (in tube #1) at 164 and the highest pressure in tube#9 at 284 giving a range of under-pressure of (164-258 = ) -94 or -39 kPa, and highest overpressure of (284 -258 = ) 26 , or 10 kPa.
Do those numbers look right to you?

 

I think you need to take the translation from manometer tube readings and a sketch of pressure tap locations and local pressure vectors with a big grain of salt.

The wind tunnel in the picture from Fossati's book looks like a decent demonstration tunnel, from which inferences can be made or assumptions tested. Looking at the two illustrations, I see two BIG problems:

• The foils sections aren't the same
• The angle of attack isn't the same

If you want to map out a pressure distribution around a test section with a manometer, you'll need to:

• Resolve your local pressure readings into proper XY coordinates to get an accurate vector magnitude and direction
• be sure your tap points match in number and location to your drawn foil.
• Be sure you 'drawn foil' is at the same angle of attack as the test foil in the tunnel
• Connect the dots with a fair curve, making assumptions about points in between that may or may not be correct (Hmmm - what is REALLY going on at the stagnation point at the leading edge...)

Fossati's book is a good resource. It is broad and comprehensive. Some of the supporting images and illos are a bit of a grab-bag. Don't make too much of them.

 

@Sailor Al - check this guy's YouTube channel out:

https://www.youtube.com/@labratscientific1127

He is a very smart dude who clarifies physics concepts without making false or simplistic assertions. The video on 'Generating Lift' is quite good. If you want more, message me and I can recommend books or online/open source papers.

 

Yes, that's the conclusion I had come to, but it does seem strange that he went to the trouble of including two photographs of the manometer and details of the pressure tap locations and then threw in a totally unrelated image in figure A.1.12.
I found the exact same image in Marchaj Aero-Hydrodynamics of Sailing (similar book title too!), right down to the rogue arrow at the LE and the little curl over the TE: although Fossati omitted the rather crucial detail of the AoA!
I'm spending some time to see if Marchaj's data supports the image.

 

I have the Marchaj book too - picked it up in 1986 at a used bookstore. Another great resource, but think about it: he was probably interpolating data through measured points using a drawing template (French curve, spline, whatever) and perhaps wondering/hoping theory matched experiment. Not saying this is wrong - just that illo's don't mean much without evidence and understanding.

 

As tropostudio has posted,the illustrations are illustrative rather than test results of a particular aerofoil section.As with all graphical results the person drawing them can choose the scale at which the representations are drawn and the baseline value.I have no definite information about the manometer scales used in the book but it may be that the numbers are not unlike those on a tape rule where each metre is marked in one colour and the intermediate values are in another colour and we are seeing deviations from the baseline figure.

I would expect to see details of a foils performance expressed as a range of curves instead of a single curve and again I believe the illustrations are there to show the principle and not a particular data set.For examples of foils data you might look at these sites Airfoil database search http://airfoiltools.com/search/index https://m-selig.ae.illinois.edu/uiuc_lsat/Low-Speed-Airfoil-Data-V1.pdf .You will see that the data is normally expressed in the form of coefficients,angle of attack and Reynold's numbers.

I will mention in passing that the test equipment shown is some way from typical of that used to determine actual performance and again is a means of extracting data to give practical experience.This page gives more information about basic wind tunnel designs https://www.sciencebuddies.org/science-fair-projects/references/how-to-build-a-wind-tunnel .It clearly shows that he test section completely encloses the body being tested other,larger tunnels may work on the closed return principle as it takes a huge amount of energy to move the mass of air at high speed and conserving the momentum reduces the load Closed Return Wind Tunnel https://www.grc.nasa.gov/WWW/K-12/airplane/tuncret.html.

 

Wind tunnel test sections can be:

Closed with solid walls
Open with no walls
Open on 3 sides with a floor (commonly used for vehicles, buildings and other terrestial structures
Partially open with slotted walls (commonly used for transonic testing)​

Wind tunnels can have:

Closed return sections
Open with an upstream nozzle and a downstream diffuser
Open with an upstream nozzle but no diffuser (not common other than for small demonstration tunnels)​

​

 

That arrow represents stagnation pressure at the stagnation point on the nose, where the flow divides and the airspeed is zero at the surface.

 

1) In a couple of earlier posts I mentioned having to account for a horizontal distance 'x' in addition to the 'y' value read from a vertical manometer tube to draw the true length of the pressure vector normal to the surface of the test foil at the tap point.

I'm pretty sure this was a bonehead mistake. The manometer tube reading will be a direct (true) pressure measurement and can be laid out as the normal vector to the foil profile without resolving anything. If others confirm I was wrong, I'll edit or delete previous statements.

2) My guess the reason multi-tap test wing sections don't usually include a tap at the leading edge is because the stagnation point is close to the leading edge, but moves slightly with changes in angle of attack. Readings could be all over the place, although as @DCockey mentioned flow speed is always zero at the stagnation point.

3) There must be a good online video of a multi-tap manometer in action in a wind tunnel. I couldn't find one. Here is a video of someone setting a simple unit up:

Wind Tunnel Airfoil/Cylinder Pressure Distribution Model Set-Up

Inclining the tubes give you finer reading of fluid level between graduations, at the expense of limiting the usable differential pressure range. Fluids with densities different than water can be used in the manometer tubes. Lower density fluids in tall tubes gives better readings if you have the space for it. Higher density fluids let you fit the same pressure measurement range in a shorter rig.

4) For those of you more familiar with electronic pressure transducers: do they typically require calibration based on diaphragm orientation? A single axis accelerometer will measure 1G in a horizontal orientation, and 0G in a vertical orientation. Made me wonder if the mass of the diaphragm is a factor in orienting a pressure transducer.