Turbulence modulation by Taylor-scale settling particles

In this study, we investigate the modulation of sustained turbulence by settling spherical particles using particle-resolved direct numerical simulations. Gravity effects are studied by varying the Galileo number, ${\textit{Ga}}$ , through the particle–fluid density ratio for particles of Taylor-scale size. Particle sedimentation causes enhanced viscous dissipation and anisotropy of the fluid velocity fluctuations, which increase with ${\textit{Ga}}$ . More significantly, the energy spectra exhibit a $\kappa ^{-3}$ scaling, which coexists with the classic $\kappa ^{-5/3}$ law for particle-laden turbulence with weak sedimentation ( ${\textit{Ga}} \lesssim 40$ ); the $\kappa ^{-3}$ scaling widens its wavenumber range with ${\textit{Ga}}$ and dominates the energy spectrum at the highest ${\textit{Ga}}$ under study. The scale-by-scale energy budgets demonstrate that the particle–fluid interactions mainly transfer energy from large to small scales for small settling speeds, whereas the particle sedimentation injects energy into the carrier flow and breaks the isotropic energy distribution for a high ${\textit{Ga}}$ . In particular, the gravity-induced forcing originates vertically elongated structures while disrupting horizontal flow correlations, thereby altering the nonlinear interscale energy transfer and reshaping the energy spectra. For the solid phase, it is found that the particle vertical velocity fluctuations decrease with ${\textit{Ga}}$ , and turbulence has a weaker influence on the mean settling velocity for particles with a higher ${\textit{Ga}}$ . Moderate particle clustering is identified, an observation not changing significantly with the particle settling speed. The collision rate among particles is highest at the strongest particle sedimentation, due to the enhanced relative radial velocity of particle pairs which are preferentially horizontally aligned.