Abstract
This work is aimed at experimental investigation of the single rib effect on the recirculation region behind the backfacing step. Measurements were carried out by the PIV method. The experiments were performed at Reynolds number Re = 15000 in the rectangular working channel of 21 X 150 mm and length of 1 m. At the distance of 600 mm from the channel inlet there was the back-facing step of the constant height H=9 mm. Rib height d and distance to the back-facing step S were varied (d from 3 to 6 mm and S from 0 to 77 mm). According to measurements, in this case the following interaction mechanism between separated flows occurs: if there is a rib in the channel in front of the step, the boundary layer separates. Depending on the distance between the rib and the step in the channel the flow will attach in front of the step or behind it. In our experiments the flow attached at S/D >7.5 and Δ=3mm and S/Δ > 10 at Δ= 6mm. In the case when the flow attaches in front of the step, the recirculation zone behind the step decreases to 35%. This relates to the fact that the flow separated behind a rib influences the mixing layer behind the step and intensifies the mixing processes, what finally leads to reduction in the length of recirculation zone. When S/Δ < 7.5, the recirculation zone behind the back-facing step increases, and in the limit case of S/Δ = 0 (the rib is at the step edge) becomes 60% larger for Δ= 3mm. We should note the general tendency of interaction between the separated flows of different scales. A rib changes the typical profile of longitudinal average velocities and mean-root velocity pulsations behind the back-facing step. For the studied configurations it was not enough time for velocity profiles and profiles of velocity pulsations to be restored to the values, corresponding to the separated flow behind the step without a rib. Maximal perturbations within the initial region of separated flow development behind the step were observed at the distance from the wall, corresponding to the coordinate of rib top. In general, the experimental data agree with experimental results of Miau et al., 1991 for S/Δ=12 and calculation results of Neumann, Wengle, 2004. The mechanism of interference for the separated flows of different scales is discussed in detail in the current study.
