The particle azimuthal distribution measured with respect to the reaction plane is not isotropic; so it is customary to expand it in a Fourier series: $$ E \frac{d^3N}{d^3p} = \frac{1}{2\pi} \frac{d^2N}{p_T dp_T dy } \left( 1 + \sum 2 v_n \cos( n(\phi - \Psi) ) \right) $$
[1] Sergei A. Voloshin, Arthur M. Poskanzer, and Raimond Snellings "Collective phenomena in non-central nuclear collisions"

Explore the Fourier Expansion:

The parametric curves are given by: $$ x(\phi) = \bar{x} + R * \cos(\phi) + 2 R v_n \cos( n \phi ) $$ $$ y(\phi) = \bar{y} + R * \sin(\phi) + 2 R v_n \cos( n \phi ) $$