Cecilia Ortiz-Duenas

Effects of Simple Wall-Mounted Cylinder Arrangements on a Turbulent Boundary Layer

I N MANYaerospace applications it is desirable to manipulate or control turbulent boundary layers in order, for example, to improve aerodynamic or combustion performance, locally increase or decrease skin friction or heat transfer, or reduce drag and noise. Turbulent boundary layers are known to contain coherent vortical structures or eddies, which have been characterized as hairpins, arches, horseshoes, and cane-shape vortices. These eddies are believed to be one of the main self-sustaining mechanisms in wallbounded turbulence [1]. Thus, a possible method to modify the behavior of turbulent boundary layers is to manipulate or alter the organization of these coherent structures. In particle image velocimetry (PIV) studies, Adrian et al. [2] and Tomkins and Adrian [3] demonstrated the preponderance of hairpin signatures in both the logarithmic and outer regions of turbulent boundary layers. The hairpin signature pattern consists of vortex cores with high values of swirling strength organized coherently in the streamwise direction in groups or packets above regions with high Reynolds shear stress (u0w0 < 0) and low speed (u0 < 0) [2,4]. These patterns have been shown to be both statistically relevant and important; i.e., hairpin packets make a significant contribution to the Reynolds stress in the logarithmic region, and therefore to the turbulence transport and production near thewall, while occupying a relatively small cross-sectional area within the boundary layer [4]. These recent studies may be used as guidelines for the scales and possible mechanisms that might potentially result in more efficient manipulation of turbulent boundary layers. For instance, based on these studies, the logarithmic region, where hairpins and hairpin packets signatures are prevalent, appears to be a location of particular interest to attempt an effective manipulation of the hairpin packet structure. In the current investigation, wall-mounted cylinders smaller than the boundary-layer thickness but large enough to protrude into the logarithmic region are used to modify a turbulent boundary layer in an attempt to affect the organization of the coherent vortical structures. The cylinders investigated herein have an aspect ratio, AR 1:5 (AR H=D, where H is the cylinder height and D is the cylinder diameter), low blockage ratio of no more than 1% (ratio of the frontal area of the cylinders to the tunnel cross-sectional area), and are fully immersed in a turbulent boundary layer with Re 1200 such that the ratio of cylinder height to boundary-layer thickness, H= , is approximately 0.13, corresponding to H Hu = 150 (where u is the wall friction velocity and is the kinematic viscosity). Therefore, the cylinders extend well into the logarithmic region or into the zone containing the bulk of the coherent eddies. The effects of individual cylinders and single spanwise arrays with three to six diameter spanwise spacing are evaluated by examining velocity statistics downstream. It is clear from previous studies that the flow structure downstream of wall-mounted cylinders (finite length cylinders with one free end) in crossflow is complex, due to the number of parameters that can affect the flow. In general, the flow downstream of wall-mounted cylinders can be characterized by tip vortices formed by the shear layer generated at the free end, Karmanvortices along themajority of the height (akin to those in the flow past cylinders of infinite length [5,6]), and a horseshoe vortex system near the base of the cylinder [7]. Clearly, the geometrical parameters of the cylinder and the characteristics of the incoming flow will have a profound effect on these downstream structures. Based on previous literature, three parameters that appear to affect the flow significantly are the aspect ratio, AR, the nature of the incoming flow: laminar or turbulent, and the height of the cylinder with respect to the boundary layer, characterized here byH= (where is the boundary-layer thickness). Since our goal is to perturb the organization of vortical structures within a turbulent boundary layer, we are interested in wall-mounted obstacles that reach into the logarithmic region but not beyond, i.e., with low H= values, and that do not obstruct the flow significantly, i.e., a sparse array arranged in a single spanwise row. Previous studies of the flow over canopies [8–10] and flow over general surface roughness, [11]wall-mounted obstacles, or roughness elements have H= values similar to those studied in this paper. However, the considerable streamwise extent of the canopy or roughness arrays yields perturbations and flowfields that are significantly different Received 14 October 2010; revision received 25 April 2011; accepted for publication 27 April 2011. Copyright