Free-stream turbulence and tube spacing effects on surface pressure fluctuations for two tubes in an in-line arrangement

An experimental investigation of surface pressure distributions (mean and r.m.s.) on two tubes (circular cylinders) mounted in an in-line or tandem arrangement was carried out in a low-speed, closed-circuit wind tunnel. The measurements were mainly performed at a subcritical Reynolds number, Re = 2·0 × 10 4 , at three different turbulence intensities of the approaching cross-flow (0·1, 1·4 and 3·2%). The spacing between the tubes was in the range of 1·25 ≤ S/D ≤ 5·0. The fluctuating pressures were recorded by a microphone in a pinhole arrangement. End plates were used to minimize the influence of the boundary layers on the wind tunnel walls. The results indicate that the effect of an increased free-stream turbulence intensity is strongly dependent on the tube spacing S/D . It is suggested that this is due to the influence of the free-stream turbulence on the separated boundary layer from the upstream tube. The behaviour of this shear layer determines the type of flow occurring around the tubes and therefore also the magnitude of the forces acting on the tubes. The so-called critical spacing between the tubes was found to decrease as the turbulence intensity was increased. An overall increase in the r.m.s. pressure coefficient was also found as the turbulence intensity was increased.