Thermohydraulic analyses for a fundamental water experiment simulating thermal striping phenomena at T-junction piping systems were carried out using a quasi-direct numerical simulation code DINUS-3. Calculated results were compared with the experimental results on the flow patterns in the downstream region of the systems, the arched vortex structures under the deflection jet condition, the generation frequency of the arched vortex in the various conditions ; i.e., diameter ratio α, flow velocity ratio β and Reynolds number. From the comparisons, it was confirmed that (1) the DINUS-3 code is applicable to the flow pattern classifications in the downstream region of the T-junction piping systems, (2) the arched vortex characteristics with lower frequency components can be estimated numerically by the DINUS-3 code, and (3) special attentions should be paid to the arched vortex generations with lower frequency components of fluid temperature fluctuations, which might induced high-cycle thermal fatigue for the design of T-junction systems.