Separated and reattached turbulent flows induced by two-dimensional forward-backward-facing steps with different streamwise lengths submerged in a thick turbulent boundary layer are investigated using a time-resolved particle image velocimetry. The examined aspect ratios (AR) of the step range from 1 to 8, and the Reynolds number based on free-stream velocity and step height is 13 200. The thickness of oncoming turbulent boundary layer is 6.5 times the step height. The dimensionless mean shear and turbulence intensity in the oncoming turbulent boundary layer at the step height are 0.25 and 15%, respectively. Three high speed cameras were arranged side-by-side to simultaneously measure turbulent separation bubbles upstream of, over and downstream of the steps with high spatial resolution. The effects of varying aspect ratio of the steps on the mean flow, Reynolds stresses, triple correlations and unsteadiness of turbulent separation bubbles are studied.

The results show that for the step of AR = 1, the mean flow separates from the leading edge and reattaches on the bottom wall in the wake region behind the step. In contrast, for the step of AR ≥ 2, mean flow reattaches over the step and the reattachment length remains constant with further increasing of AR. The mean reattachment length behind the step decreases with increasing AR up to 3, beyond which no further change in reattachment length was noticeable. By analyzing the vorticity thickness, it is observed that the shear layer emanating from the leading edge develops in the downstream location at a constant rate regardless of the aspect ratio. However, the shear layer initiating from the trailing edge of the step is strongest in the case of AR = 1. Reynolds stresses near the leading edge of the step are insensitive to AR, whereas those in the wake region is substantially altered by AR. For instance, the elevated streamwise and vertical Reynolds normal stresses extend longest in the downstream direction in the case of AR = 1. The triple correlations generally switch sign near the mean separating streamline from the leading of the step. This indicates that turbulent diffusion of turbulent kinetic energy is typically backwards and upwards upstream of the mean separation bubble over the step, but forwards and downwards near the top surface of the step.

In the final paper, the Reynolds stresses and triple correlations will be further analyzed in the curvilinear coordinate system along the mean streamlines. With the time variation of the reverse flow areas upstream of, over and downstream of the steps, the unsteadiness and mutual interaction of turbulent separation bubbles will be examined by exploring the frequency spectrum, temporal correlation and probability density function.


Effects of Streamwise Aspect Ratio on Turbulent Flows Over Forward-Backward Facing Steps

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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