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When representing a wing airfoil, often the low pressure is on the top side and the lift is positive:
In this case upper and lower sides are enough to understand the role of the surfaces, But this is meaningless for a rudder or for a compressor blade.
Are there names to identify the faces in relation with the direction of the lift. (In French specific names, extrados and intrados, are used in relation with camber.)
You can call the two opposite sides suction side (for face on the convex side of the camber curve) and pressure side (for face on the concave side of the camber curve).
This has been used in a number of cases- for example in NASA Report CR-1767 Section Data for Thin, Highly Cambered Airfoils in Incompressible Flow:
.. flow separation on the pressure side at negative incidence angles and on the suction side at positive incidence angles.
for airfoils and in National Energy Technology Laboratory Handbook:
The airfoils are curved, convex on one side and concave on the other, with the rotor rotating toward the concave side. The concave side is called the pressure side of the blade, and the convex side is called the suction side of the blade.
for compressor blades.
In a very formal environment (like an engineering lecture), the upper side of a wing may also be referred as extrados and the lower part as intrados. This terminology comes from architecture. Archs in architecture have an intrados (inner concave part) being extrados the oposite side. Latin laguages, like french or spanish, use this names quite often in airfoil terminology.
