Unsteady wake characteristics of unequal-height tandem cylinders submerged in a turbulent boundary layer

The unsteady wake dynamics of two finite wall-mounted cylinders of unequal-height arranged in tandem are investigated using time-resolved particle image velocimetry. The cylinders were submerged in a water tunnel with a turbulent boundary layer (TBL) of thickness 8.7d (where d is the cylinder diameter) and Reynolds number based on d of 5540. The spacing ratio of the cylinders was fixed at 4d. Five height ratios (h/H = 0.1 – 1.0) were studied by varying the aspect ratio (AR) of the upstream cylinder, UC (h/d) while fixing the AR of the downstream cylinder, DC (H/d = 7.0). The height ratio introduced sheltering where part or the whole span of the DC was blocked from direct interaction with the TBL. As the height ratio increases, the downwash from UC impinges on the front of the DC and induces a stronger upwash on the opposite side of the DC. The induced upwash impedes the downwash behind the DC, enhances the Reynolds stresses and significantly modifies the wake topology for increasing height ratio. Spectral analysis and joint-probability density functions of the reverse flow area showed that the attachment of the downwash of the UC on the DC mutually affects the pumping motion of the reverse flow area behind each cylinder.



*Natural Sciences and Engineering Research Council.