Effects of gap ratio on aerodynamic performance and flow regimes of two tandem flat plates with 5:1 aspect ratio

This paper investigates the effects of gap ratio on the aerodynamic performance and flow regimes of two tandem flat plates with a 5:1 aspect ratio through wind-tunnel experiments. A total of 24 finite tandem gap-ratio cases were tested over G/D = 0–24 at Re = U_inD/ν = 1.06 × 10⁴–2.12 × 10⁴, with an isolated single-plate case (G/D = ∞) used as the reference condition. Surface pressure measurements and hot-wire anemometry were used to quantify the pressure distributions, aerodynamic force coefficients, and vortex-shedding frequencies, while smoke-wire visualization was employed to identify the corresponding flow patterns. The upstream plate remains aerodynamically insensitive to the downstream plate across the entire gap range, whereas the downstream plate exhibits strong gap-ratio dependence in its pressure distributions, drag, and unsteady lift. Four distinct flow regimes are identified: (I) merged wake regime (G/D ≤ 1.0), in which the two plates shed vortices at a common frequency, resembling a single plate; (II) transitional regime (1.0 < G/D ≤ 4.0), characterized by progressive gap flow development and the onset of inter-plate frequency locking; (III) gap reattachment regime (4.0 < G/D ≤ 9.0), in which a coherent gap jet forms, impinges on the downstream plate, and induces peak unsteady aerodynamic loading; and (IV) wake recovery regime (G/D > 9.0), in which the downstream plate progressively recovers toward independent bluff-body behavior. These findings provide physical insight for the aerodynamic design of tandem bluff-body structures.