Three Dimensionality Of The Low Frequency Flapping Motion In A Turbulent Separated Flow Over A Backward Facing Step

Boundary layer separations and re-attachments can be found in various applications and were studied extensively.The large scale flow structures rolled up naturally in the initial part of the shear layer after separation due to K-H instability,similar to a free shear layer with little effect from the bottom wall,was usually referred to as the shear layer mode.The shear layer structures continued to pair and amalgamate along the shear layer,while the characteristic frequency continued to decrease until the shear layer reattached to the wall.The motion of the merged structures in the attachment region was referred to as the step mode or the preferred mode.In the central part of the separation region,there was a large scale structure grew periodically and then shed downstream.The quasi-periodic passage of large scale structures were also referred to as the shedding mode or the wake mode.Some studies speculated that the shedding of large structures to a low frequency flapping motion of the whole shear layer,i.e.the shedding was induced by the flapping motion,but did not give any proof.The relations between the low frequency flapping instability of the whole shear layer over a backward facing step and shedding instability were investigated using experimental methods.The fluctuating pressure were measured using a wall mount streamwise microphone array simultaneously with a spanwise single hotwire array positioned at different locations in the flow field.POD of the fluctuating wall pressure suggested that the passage of the large scale shear layer structures contributed more than 70% of the mean square value of the pressure fluctuations at the locations near the mean reattachment point,and the low frequency flapping motion contributed less than 15%,mostly at x/H ≤ 3.0.POD results of the spanwise velocity array suggested that the motion along most part of shear layer were three dimensional with a spanwise length scale less than H.The motion with large spanwise length scale started to emerge at 2/3 of the recirculation bubble along the shear layer,and in the upstream travelling recirculating flow.The temporal coefficient of pressure mode associated the flapping motion correlated well with the velocity modes with large spanwise wavelength both inside of the recirculation region and near the reattachment location,suggesting the two dimensional motion in this flow was likely from a same source,and the shedding motion was likely a direct consequence of the flapping motion of the whole shear layer,resulting from a global instability.