Why is this airfoil curved upwards on the bottom part of the airfoil?
Downwash is simply the deflection of air due to airfoil (or wing etc.)

Symmetric airfoils don't produce downwash (or lift) at zero angle of attack.



Source: oppositelock.kinja.com

However, as the angle of attack is increased, the (symmetric) airfoil produces both.



Source: Effect of angle of attack on flow field and body forces by Paul Nathan from youtube.com

Symmetrical airfoils are used in rotary wing applications, vertical tail (and rudder) etc.