<p>In the present study, two-dimensional transient flow over circular cross-sectional smooth and grooved cylinders in tandem is investigated comparatively using a delayed detached eddy simulation (DDES) based on the shear stress transport (SST) k-omega turbulence model for various Reynolds numbers (10 ≤ Re_D ≤ 65×10³). The flow and heat transfer characteristics are presented for a wide range of gap ratios (1 ≤ <i>L</i>/<i>D</i> ≤ 8) between two cylinders, where <i>L</i> and <i>D</i> are the distance between the cylinders and the cylinder diameter, respectively. Following successful validation, the study is extended to the flow past tandem circumferentially grooved heated cylinders. It is shown that texturing the surfaces of both cylinders reduces heat transfer compared with tandem smooth cylinders, while texturing with circumferentially grooved surfaces increases the drag coefficients of the upstream and downstream cylinders. Regardless of the Reynolds number, the critical gap ratio for the grooved cylinders in tandem is <i>L</i>/<i>D</i> = 4 since the Nusselt number becomes minimum at this gap ratio, while both the drag coefficient and the Strouhal number become independent of the gap ratio after that.</p>