Open pipe: a pipe open at 'both the ends' is called  open pipe

Formation of Stationary wave: When a sound wave is sent to an open pipe, the wave reflects back at the other end of the pipe. The incident wave and reflected wave 'travelling in opposite directions' super impose each other to produce Stationary waves.

Antinodes are formed at the open ends.

Notation: L= length of air column, V= Velocity of sound in air, λ₁, λ₂, λ₃ are the wave lengths of waves in respective harmonics.

First Harmonic:

Here 1 node and 2 antinodes are formed.
A = antinode N = node
Length of the air column = Half of the wavelength