Structure of turbulent channel flow perturbed by a wall-mounted cylindrical element

The current study reports structural modifications imposed in fully developed, turbulent channel flow by an isolated wall-mounted circular cylinder. The cylinder extended into the logarithmic layer of the flow, perturbing the larger flow scales that embody a significant fraction of the turbulent kinetic energy and Reynolds shear stress. Hotwire measurements were made in the wake of the wall-mounted circular cylinder at multiple wall-normal and stream wise positions. Besides observing the expected mean velocity deficit in the wake of the cylinder, a secondary peak in stream wise Reynolds normal stress away from the wall was observed, coupled with suppression of the near wall peak native to the incoming unperturbed, turbulent flow. These observations are similar to those made by Ryan et al. ("Effects of Simple Wall-Mounted Cylinder Arrangements on a Turbulent Boundary Layer," AIAA Journal, Vol. 49, No. 10, 2011, pp. 2210-2220) for a wall-mounted cylinder protruding into the logarithmic layer of a turbulent boundary layer. Pre-multiplied velocity spectra elaborated on these energy modifications, specifically the occurrence of two distinct energy peaks at two-thirds of the cylinder height and an attenuation of energy associated with larger flow scales close to the wall. All of these perturbations were found to decay with stream wise distance downstream as the flow relaxed toward the unperturbed state. A clear persistence of the structures at the aforementioned peak at two thirds cylinder height, similar in scale to the very large-scale motions in canonical wall turbulence, suggests an environment preferring structures of such scale. The influence of the cylinder aspect ratio on the characteristics of the perturbed flow is evaluated, and a distinction in wake structure is identified.