Laminar forced convection heat transfer around wavy elliptic cylinders with different aspect ratios

The present study investigates the forced convection heat transfer around the wavy elliptic cylinders by considering wide range of wavelength (λ) for different aspect ratios (AR). The wake thermal structures were classified into four modes of the quasi-2D unsteady, quasi-2D steady, complex 3D unsteady and 3D steady structures, which is mapped in AR-λ plane. The short wavelengths formed the quasi-2D thermal structure for both steady and unsteady forced convections. The quasi-2D unsteady structure is characterized by the spanwise nearly invariant isothermal structures and the periodical time evolution of the Nusselt number. The long wavelengths formed the 3D thermal structure in the present range of AR. The complex 3D unsteady thermal structure is featured by the hole and spanwise separated formation of the temperature iso-surface owing to the vortex dislocation and hairpin vortex formation. The 3D steady thermal structure also appeared in the steady regime and the spanwise dependence of the thermal structure became considerable with decreasing AR. The surface distribution of the mean Nusselt number depended on the regime of the thermal structures. In the long wavelength region, the additional appearance of two types of the streamwise vorticities locally attenuated the heat transfer by weakening the reverse flow in the near wake. As the cylinder becomes more elliptic, the two types of the streamwise vorticities becomes weak and finally disappears, resulting in weakening the effect of the local attenuation of the heat transfer. Therefore, it can be concluded that the streamlined shape is predominant to the stabilization of the forced convection with decreasing AR, regardless of the wavelength. © 2022