Experimental Investigation of Near-Field Surface Pressure Characteristics of Two Side-by-Side Cylinders

The near-field surface pressure characteristics of two identical circular cylinders in a side-by-side configuration were experimentally investigated at Re = 3.0 × 10^4. Surface pressure fluctuations were measured along the spanwise and azimuthal directions using miniature transducers for center-to-center spacings (T) of 1.2, 2, and 3 times the cylinder diameter (D). At the smallest spacing (T/D = 1.2), the front stagnation point and other critical angles around the lower cylinder shifted approximately 20○ clockwise, with a symmetric counterclockwise shift around the upper cylinder, indicating gap flow deflection toward and impingement on the lower side of the upper cylinder. This resulted in a wider wake and potentially reduced drag on the lower cylinder compared to the upper one. The deflected flow also produced an elliptical distribution of the fundamental vortex shedding peak, tilted clockwise for the lower cylinder and counterclockwise for the upper one. At T/D = 2, the gap flow remained deflected—this time toward the lower cylinder without direct impingement—producing a dipole-like distribution similar to the single-cylinder case but with asymmetric lobes. At T/D = 3, the flow symmetry was restored, and both cylinders exhibited behavior resembling that of an isolated cylinder. Spanwise coherence analysis showed that the flow over the lower cylinder retained a two-dimensional character similar to the single-cylinder case at larger spacings, but with a reduced coherence length at T/D = 1.2. In contrast, the upper cylinder exhibited increased coherence length at T/D = 2, which then decreased again at T/D = 3, though it remained longer than that at the smallest spacing.