Preferential concentration in a periodically forced particle-laden turbulent planar jet

Abstract

Periodic forcing is common in particle-laden jets, yet its influence on turbulence and inertial-particle clustering remains poorly understood. We investigate a turbulent planar jet at two Reynolds numbers, approximately 4950 and 9900 based on nozzle height, subjected to sinusoidal forcing at a frequency three orders of magnitude below the preferred jet instability. Hot-wire anemometry and high-speed particle imaging characterise the carrier turbulence, while Voronoi tessellations quantify preferential concentration of dilute inertial droplets. Despite its low frequency and moderate amplitude, forcing substantially modifies turbulence while leaving the mean-flow development nearly unchanged. The integral scale decreases by up to $40\%$, while forced and unforced jets retain the same time-averaged relationship between the dimensionless dissipation coefficient and Taylor-scale Reynolds number. Within the forcing cycle, velocity fluctuations and the Taylor microscale vary approximately in opposite phase, keeping the Taylor-scale Reynolds number within $\pm10\%$ of its cycle mean despite substantial variations in dissipation and Kolmogorov time scale. Particle clustering is even more sensitive, with forcing increasing mean preferential concentration and producing phase-dependent variations of $21$--$49\%$ of the cycle mean. Clustering peaks when the Kolmogorov time scale is largest, corresponding to the smallest particle Stokes number. Thus, markedly different clustering states occur at similar instantaneous Taylor-scale Reynolds numbers. These findings demonstrate that weak temporal modulation can control particle organisation without substantially altering the mean transporting flow.