Fluid dynamics and passive scalar transport driven by non-uniform tumbling of a prolate spheroid in simple shear flow

Using high-fidelity numerical simulations based on a lattice Boltzmann framework, the advection-enhanced transport of a passive scalar from a prolate spheroid in simple shear flow has been thoroughly investigated across various parameters, including the spheroid’s aspect ratio, particle-to-fluid density ratio, Reynolds number (defined as ${\textit{Re}}=\textit{GR}^{2}/\nu$ , where $G$ is the flow shear rate, $R$ is the radius of a sphere of the same volume as the spheroid and $\nu$ is the kinematic viscosity of the fluid) and Schmidt number (defined as $\textit{Sc}=\nu /D$ , where $D$ is the diffusivity of passive scalar transport). The Reynolds number is constrained to the range of 0 ≤ Re ≤ 1, where the prolate spheroid tumbles around its minor axis, aligned with the vorticity axis, in an equilibrium state. Several key findings have emerged: (i) particle inertia significantly influences the uniformity of the spheroid’s tumbling, affecting flow patterns around the spheroid and, consequently, the modes of scalar transport; (ii) both uniform and non-uniform tumbling generate a scalar line in the fluid with elevated scalar concentration, which sweeps through the wake region and merges with clusters of previously formed scalar lines; (iii) fluid passing over the spheroid carries the passive scalar downstream along these scalar lines; (iv) variations in the uniformity of spheroid tumbling result in distinct flow patterns and scalar transport modes, leading to different transport rates; (v) within the studied parameter ranges, increased particle inertia enhances the scalar transport rate; (vi) when particle inertia is minimal, the dimensionless scalar transport rate for different aspect ratios converges to a common dependence on the Péclet number. These phenomena are analysed in detail.