Bulk flow methods for circumferentially grooved seals use simplified physics models to predict leakage and rotordynamic coefficients efficiently, but uncertainty in empirical quantities like friction factors and loss coefficients leads to limited accuracy. To develop a more fundamental understanding of incompressible grooved seal flow, this study utilizes computational fluid dynamics (CFD) and an effective film thickness, a physical boundary between the jet and recirculating flows, to investigate Reynolds number effects on flow fields and shear stresses. Simulations are run using ansyscfx for a single groove seal model where streamlined analysis produces the effective film thickness. Flow structures, film thicknesses, shear stresses, and net flow expansion into the groove are found to be described completely by the ratio of circumferential to axial Reynolds number and the total resultant Reynolds number. Decreases in leakage with rotor speed are found to be dictated by increased land shear stresses and a decreased role of the groove in inducing pressure drop. An optimal groove aspect ratio between 0.07 and 0.19 is presented based on maximizing expanded film area while retaining a main groove recirculation region. This is the first paper to analyze circumferentially grooved seal flow from an effective film thickness standpoint. The results highlight bulk flow analysis areas where an effective film thickness approach could lead to new, physics-motivated model development and the elimination of particular empirical coefficients, thus providing a foundation on which substantial improvements in bulk flow modeling accuracy can be achieved.