Turbulence and reynolds number effects on the flow and fluid forces on a single cylinder in cross flow

The flow around a smooth and rigid tube (circular cylinder) in cross flow has been investigated experimentally. The experiments were carried out in a closed-circuit low-speed wind tunnel. Two test tubes (diameters 41 and 120 mm, respectively) fitted with end plates were used. The fluctuating pressures on the tube surface were sensed by microphones in pinhole arrangements while the modulus of the surface shear stress was measured using a surface hot film. The flow field was investigated using single hot wires. Effects of a low intensity freestream turbulence was studied by placing a grid at the entrance of the working section of the wind tunnel. The Reynolds number ranged from 2 x 104 to 3 x 105 and the turbulence intensities in the freestream were 0·1% and 1·4%, respectively. The results distinguished different flow regimes depending on the Reynolds number and turbulence intensity. For Reynolds numbers less than about 105, an increase in the turbulence intensity gave an increase in the pressure forces, while the opposite effect was observed at higher Reynolds numbers. A conditional sampling technique using the different flow gauges provided information on the relation between the flow field in the near wake and the forces on the tube. Distributions of mean- and r.m.s.-coefficients, spectra of fluctuating pressure and the alteration of various coefficients (e.g. Strouhal number) when entering the critical regime are presented. The separation from the tube was found to be delayed in the precritical regime. Also, the build-up of a turbulent zone towards reattachment, coupled to a narrowing of the wake, was observed.