Abstract
The physics of flow around straight and curved cylinders in tandem is investigated by means of direct numerical simulations (DNS), for transitional and turbulent flow regimes. For straight tandem cylinders in the reattachment regime, the effect on the gap and wake flow of transition to turbulence in the shear layers is scrutinized, along with the interaction between large-scale recirculating gap vortices and the development of the gap shear layers. The role of flow three-dimensionality in the gap and wake is studied. It is discovered that tandem cylinder flow regimes may be spanwise cellular and asymmetric in the gap region. The physical mechanism behind such cellular flow regime variations is discussed. Tandem cylinders with axial curvature are studied for the first time. The convex configuration, where the incoming flow is directed towards the outside of the cylinder curvature, is chosen. The effect of axial flow on the forces on the cylinders and their characteristic frequencies is addressed, as well as spanwise variation of tandem cylinder flow regimes. Several flow regimes co-exist along the span, and some of these are unstable, which has a profound effect on the development of the wake. The flow topology of the gap and wake, vortex shedding modes and the complex interaction between the spanwise localized flow regimes are discussed in detail. The effect of transition to turbulence either in the wake or in the shear layers is addressed, as is the interaction between the primary instability and the shear layer instability, under influence of axial flow.