Momentum and heat transfer in a turbulent cylinder wake behind a streamwise oscillating cylinder

The work aims to understand the effect of an oscillating circular cylinder on momentum and heat transport in the turbulent wake of a downstream identical cylinder, which is slightly heated. The oscillating amplitude, A, was 0.79d (d is the cylinder diameter) and the frequency was 0.17fs for L/d = 2.5 and 0.24fs for L/d = 4, where L and fs are the cylinder centre-to-centre spacing and the frequency of vortex shedding from the downstream cylinder, respectively, at A = 0. The velocity and temperature fields were measured using a three-wire probe at 10d behind the stationary cylinder and a Reynolds number of 5920 based on d and the free-stream velocity. It is found that the upstream cylinder oscillation modifies the frequency of vortex shedding from the stationary cylinder, which is locked on with one harmonic of the oscillating frequency. This harmonic frequency is nearest to and below the natural vortex-shedding frequency. Furthermore, the wake response to the oscillation depends on L/d in terms of the cross-stream distributions of mean velocity, Reynolds stresses, temperature variance and heat fluxes; the turbulent Prandtl number decreases at L/d = 4 but increases at L/d = 2.5. The observations are linked to whether the flow regime experiences a change under the upstream cylinder oscillation.