Flow around tandem rough cylinders: Effects of spacing and flow regimes

The aerodynamic features of the flow around static tandem cylinders are investigated in different flow regimes with the Reynolds number ranging from 21k to 395k. Adapted surface roughness (k/D = 7.2 × 10−3) is used to promote the transition from laminar to turbulent conditions in a low-subsonic wind tunnel. The effect of roughness on the mean force coefficients is investigated and a special attention is given to the uniformity of the roughness level along the span-wise dimension of the cylinders. Different spacing ratios are tested (L/D = 1.2, 1.4, 1.56 and 1.8). The measured unsteady pressure distributions are analysed and discussed for the different flow regimes as a function of the spacing ratio. The frequency content of the pressure fields is analysed in the range 0 ≤ St ≤ 0.75 and eddy shedding phenomenon is identified for 0.13 < St < 0.32 (including first harmonics) depending on the Reynolds number and the spacing ratio. In the sub- and critical regimes, it is shown that the flow belongs to the re-attachment pattern in which the shear layers re-attach either alternately or steadily, respectively. In the post-critical regime, the flow pattern is unstable for 1.2 ≤ L/D ≤ 1.56 because of the intermittent re-attachment of the shear layers. For L/D = 1.8, the shear layers re-attach steadily.