New Measurements of Inertial Particle Relative Velocity and Radial Distribution Function down to Near-contact Separations in Isotropic Turbulence Reveals Hydrodynamic Interaction Effects*

Understanding particle collision mechanisms in isotropic turbulence is important to applications such as droplet coalescence. It is known that particles with finite Stokes numbers (\textit{St}) experience enhanced clustering due to turbulence, measured by the radial distribution function $g(r)$, and increased inward relative velocities, measured by the mean inward radial relative velocity $\langle w_{r}(r)\rangle^{\mathrm{-}}$, compared to inertia-free particles. However, collision occurs when particles are near contact. When their separation distance $r$ approaches the collision radius, particle pairs begin to experience hydrodynamic interactions (HI). Yavuz et al. (2018) (Phys Rev Lett 120 244504) observed $g(r)$ enhancement by HI; however their data exhibited significant scatter at $r$/$a=O$(10), ($a$: particle radius). We used a new high-resolution particle tracking technique by LaVision$^{\mathrm{1}}$ and optimized it for small-$r$ measurements of particles in a fan-driven enclosed isotropic turbulence chamber. This enabled high-resolution measurements of $g(r)$ and $\langle w_{r}(r)\rangle^{\mathrm{-}}$ down to near contact ($r$/$a=$2.07) using particles of different radii (2.5$\mu $m\textless $a$\textless 22.5$\mu $m) and inertia (0.1\textless \textit{St}\textless 3.7). When $r$/$a$\textless $O$(10), we observe that $g(r)$ varies as $r^{\mathrm{-6}}$, and $\langle w_{r}(r)\rangle ^{\mathrm{-\thinspace }}$begins to increase drastically. In this talk we explore how inertia affects clustering and relative velocities of particle pairs near contact through HI as well as turbulence.