(a) Schematic illustration of the anisotropic TDTR method with a flowing fluid (not-to-scale), where $Δx$ is the pump-probe offset, $w$ is the pump beam waist, $vavg$ is the average flow field velocity, and $ℓth$ is the thermal penetration depth. (b) Probing upstream the pump-induced thermal BL. (c) Probing downstream (or within) the pump-induced thermal BL.