asking about perturbation

[arjohar19]

Simulation-and-modelling-of-natural-processes/lbmFlowAroundCylinder.py

Line 59 in aaabf1a

return (1-d) * uLB * (1 + 1e-4*sin(y/ly*2*pi))

Hello sir, im a newbie in lattice boltzmann, and i want to know about perturbation in lattice boltzmann method, i want to know about your 59th line. where you find the equation for the perturbation? can you please explain it to me? thanks