How to read representations of pressure around an airfoil?

Question

Actual representations found online

I selected 6 online representations of pressure around an airfoil, mostly using inconsistent symbolization of the pressure field.

^{Source of the individual pictures: Fig 1, fig 2, fig 3, fig 4, fig 5, fig 6}

As visible, arrows can be found in nearly every combination of directions. On figure 4, pressures are about 2 hPa (200 Pa) and 5 hPa, which is not really understandable when pressure at sea level is about 1,000 hPa.

Last figure shows the pressure field in the cross-section, not only at the surface of the airfoil (which I believe is the case for the other figures).

Assumptions made to read the figures

By following the answers on this site, I know that the pressure on the lower side of the airfoil is greater than the pressure on the upper side. I also assume that the pressure on the upper side is lower than the ambient pressure, and the pressure on the lower side is greater than it. I believe the drawings are trying to depict these two aspects, but using different (and unspecified) conventions.

Probably the clearer convention is the one used on fig 6: Yellow is likely the ambient pressure, orange would be greater, green lower, blue even lower.

Questions

• What are the usual pressures found around an airfoil, e.g. in cruise, at mid-span of a A320 wing (compared to the ambient pressure).

• What is the most correct representation of the pressure field in the figures above?

• Is there some conventional way used by engineers to depict the pressure field without ambiguity?

Answer 1

The arrows are more for the "gee wiz" factor - while they help to show in which direction which pressure is acting, the regular way of depicting pressures is using the pressure coefficient.

Now to your figures:

1. This probably tries to illustrate the fact that there is air pressure all around the airfoil, and less on top than on the bottom. But the length of the arrows does not correspond to any real pressure distribution. Please do not use this plot to learn aerodynamics.
2. This one tries to be more realistic, but gets the pressure distribution around the nose totally wrong. While better than 1, it is still not recommended.
3. This one is clearly the best of the bunch. Here you have a small but intense overpressure region at and near the stagnation point and suction on both sides, as would be the case at small and medium angle of attack if the airfoil has some thickness. The displacement effect will cause the flow on both sides to accelerate to a speed above flight speed, but camber and angle of attack make this effect much more pronounced on top. At the trailing edge you will find a small overpressure on top and a little suction at the bottom when the flow negotiates around the trailing edge because the boundary layer on the upper surface is much thicker than on the lower surface.
4. Here you see absolute pressure difference numbers, which is unusual. While the figures look realistic, this plot is also not really helpful because it omits the pressure distribution in the forward part of the airfoil where most of the action takes place. I think this is another special plot to illustrate one specific aspect. The numbers are the pressure difference to ambient pressure.
5. This one just tries to show how the pressure varies along one streamline. But the streamline has a constant distance from the airfoil, which is wrong, because it would normally come closer to the airfoil surface in proportion to the local speed increase. Also, the stagnation area is stretched out too far backwards.
6. This is a typical plot from a Euler or Navier-Stokes code where the space around the airfoil is panelled instead of only the surface as is customary for potential flow. The plot looks credible, but without a legend you do not learn much about the pressures around the airfoil. Your assumption about the colours is correct.

Please note that 1 tries to show absolute pressure while the others show the pressure difference to ambient.

To answer your question: Most of the plots suck and it would be a waste of time to "read" them because of their limited truthfulness. Maybe this would improve when you read the accompanying text as well, but it would be even better if all plots were like number 3.

What helps most when reading pressure plots is consistency - here all 6 plots fail. The proper way is to plot the pressure coefficient as explained in the linked answer. Only then can you gain some familiarity with the representation and make meaningful comparisons among different airfoils or flow conditions. Negative values of the pressure coefficient $c_p$ denote suction and positive ones overpressure, all relative to ambient static pressure.