Hello everyone! 1. The problem statement, all variables and given/known data I want to model the irradiance on a fluid element while it is flowing, from a fixed(above or below) light source that has an irradiance that shows first order decay with time and from x-naught to x-infinity . here are the assumptions: -Laminar flow at fixed rate (in x-direction) -fluid element in fixed y-value from light source (above or below, doesnt matter. -reflection/scatter effects are insignificant -flow is in a tube (differential irradiance in y-direction) 2. Relevant equations first order decay. 3. The attempt at a solution I would like some direction regarding the best approaches to model such a situation, and am comfortable with solving differential equations. Thank you so much!