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arXiv · 2610.31417

Tandem jets in crossflow: the enclosed-recirculation momentum exchange and the unsteady shear-layer dynamics

Xunchen Liu
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Abstract

Twin tandem jets in crossflow are examined by 40\ kHz time-resolved particle image velocimetry (PIV) over nine spacing--velocity-ratio conditions ($L=2d$--$4d$, $r\approx3$--7; 68 twin cases, 11 single-jet records). The front jet is diluted: its trajectory lies below the single-jet reference, with a near-field segment up to 40\% shorter, and its velocity ratio falls to an effective value $r_{\mathrm{eff}}<r$, while the shielded rear jet rises above the reference. This paper identifies the carrier of that dilution and the unsteady dynamics that sustains it. The rear-jet blockage closes a gap recirculation whose reversed flow puts the two gap-side layers in the counter-current shear-layer configuration, the classical route to enhanced entrainment. The measured Reynolds stress and Prandtl mixing length show that the absolute exchange is nearly independent of the velocity ratio and exceeds the single-jet leeward zone like-for-like by up to $1.5\times$ (zone peak, across conditions) and $2.8\times$ (area-integrated) at $r\approx7$. The gain comes from an enlarged mixing scale rather than from a steeper shear, and the front-jet near-field shortening follows the absolute exchange transport rather than the twin/single enhancement ratio. The turbulent term exceeds the pressure term in eight of the nine conditions, and the flux budget reads the dilution as a limited mass addition plus a momentum loss to the crossflow; the evidence remains observational. The gap-side layers remain convectively unstable---DMD and SPOD find no growing mode within the resolvable band and record length---so the exchange is carried by convecting vortex trains rather than by a self-excited global mode.

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Xunchen Liu. 2026-10-22. Tandem jets in crossflow: the enclosed-recirculation momentum exchange and the unsteady shear-layer dynamics. https://arxiv.org/abs/2610.31417

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