Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session M21: Boundary Layer Flows over Rough Surfaces II 
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Chair: JeanPierre Hickey, University of Waterloo Room: Georgia World Congress Center B309 
Tuesday, November 20, 2018 8:00AM  8:13AM 
M21.00001: Low Re flow in butterfly inspired grooves, a potential passive drag reduction method Sashank Gautam, Amy W Lang Butterfly wings are covered with minute inclined scales (for the Monarch these measure around 100 µm with angle of inclination 22°) that align to form rows of rooflike shingles with cavities between them. Flow in the longitudinal direction to the scales encounters increased surface area which increases the laminar drag. However, when the flow is transverse to the scales at low Re = 525, a single vortex is formed inside each of the cavities. These embedded vortices act as a “roller bearing effect” to the outer boundary layer. Therefore, when compared to a flat plate a sublaminar drag is possible. As the flow Re is increased beyond a critical number, the vortex ejects out of the groove causing the vortex to mix with outer boundary layer flow. Since the low momentum vortex mixes with the higher momentum flow, this exchange of momentum results in an increase in drag. Experiments are performed using DPIV in a tow tank facility to determine the critical Re where the embedded vortex ejects out of the cavity to mix with the outer boundary layer flow. For flow below critical Re, comparison of skin friction drag between the grooved and flat plate demonstrates that the butterfly scales can lead to drag reduction. 
Tuesday, November 20, 2018 8:13AM  8:26AM 
M21.00002: Turbulent boundary layers developing over spanwise heterogeneous surfaces Dea Daniella Wangsawijaya, Charitha M De Silva, Rio Baidya, Daniel Chung, Ivan Marusic, Nicholas Hutchins Turbulent flow over a surface with spanwise heterogeneity induces secondary flow in the form of counterrotating streamwise roll modes embedded within the boundary layer. In this study, we introduce the secondary flow using surfaces comprised of spanwise alternating smooth and rough strips and investigate the behavior of the secondary flow for various characteristic spanwise wavelengths. Hotwire anemometry and stereoscopic particle image velocimetry measurements are performed over surfaces with various halfwavelengths Λ (0.32 ≤ Λ/δ ≤ 6.81), where Λ corresponds to the roughness strip width and δ is the spanwiseaveraged boundary layer thickness. Preliminarily results indicate that when Λ/δ >> 1, the secondary flow is confined within the interface between smooth and rough strips, with regions far removed from this interface behaving as either homogeneous rough or smooth wall flow. On the other extreme when Λ/δ << 1, the secondary flow appears to be restricted to the nearwall region, while away from the wall, the surface becomes spanwise homogeneous. Our findings further suggest that the roll modes are not a timeinvariant feature. In cases where Λ/δ ≈1, we observe that the roll modes strongly meander with an apparent sinous instability. 
Tuesday, November 20, 2018 8:26AM  8:39AM 
M21.00003: Turbulent boundary layer flow over multiscale regular roughness Takfarinas Medjnoun, Manuel Ferreira, Eduardo Rodriguez Lopez, Johan Meyers, Bharathram Ganapathisubramani Multiscale rough surfaces have received substantial attention due to their presence in numerous research topics such as atmospheric boundary layers and hydrology. The presence of multiple scales affects both the mean flow and the turbulence when the characteristic length scales of the surfaces are significant compared to the length scales of the flow. Despite the burgeoning interest in this area, little is known about the effect hierarchy of scales on the flow. To this end, a comprehensive experimental study is performed over surfaces where multiscale roughness with selfsimilar cuboids is used. Starting with largescale cuboids, their sizes decrease with iterations while their number increases with a power law, with subsequent smaller cuboids being distributed on top as well as in between previous larger cuboids. Windtunnel testing using a floating element balance along with crossplane StereoPIV were performed to assess the drag and the flow field over three test cases. Results show that drag increases substantially from two iterations to three iterations of cuboids indicating that the small scales of roughness make a significant contribution to the drag. The effects of scales on the mean flow, turbulence statistics as well as other flow characteristics will be presented. 
Tuesday, November 20, 2018 8:39AM  8:52AM 
M21.00004: Effect of surface skewness on roughwall turbulent boundary layer structure Michael Schultz, Ralph Volino, Karen Flack Previous work by the authors and others has shown that the frictional drag of many rough surfaces correlates well with two primary roughness parameters. These include some measure of the surface amplitude (i.e. rootmeansquare height, mean height, etc.) and the skewness of the surface elevation distribution. In the present work, the results of an experimental investigation are reported in which turbulent boundary layer measurements were obtained over surfaces in which the rootmeansquare height (k_{rms}) was held fixed but the skewness of the surface elevation distribution (Sk) was systematically varied. The boundary layer measurements were made at a friction Reynolds number (Re_{τ}) ~2000 using both LDV and PIV. Specific effects of Sk on the skinfriction and the structure of the boundary layer are discussed. 
(Author Not Attending)

M21.00005: Influence of surface geometry on secondary flows in turbulent boundary layers Christina Vanderwel, Takfarinas Medjnoun, Bharathram Ganapathisubramani Secondary flows have recently earned an increased consideration due to their observations in a wide range of roughwall turbulent flows. While the mean flow universality was previously shown to be violated, the effect of surface geometry on the secondary flows has not been documented yet. This experimental study is designed to look at the effect of smooth ridges of different shapes and widths, with StereoPIV used to investigate the flow field and oilfilm interferometry to assess the drag. Their maximum heights and spanwise spacings were fixed to match 0.1 and 1 times the boundary layers thickness δ, in order to maximize potential secondary flows. Largescale modifications in the mean and turbulent flow are observed in each case and are characterized by the circulation of the secondary motions, local correlation maps and dispersive stresses. These modifications are shown to highly depend on the geometrical characteristics of the surface heterogeneity, which in turn impact the wall drag owing to the mean flow heterogeneity. 
Tuesday, November 20, 2018 9:05AM  9:18AM 
M21.00006: Modeling Nonlinear Interactions in a RoughWall Turbulent Boundary Layer Jonathan Morgan, Beverley McKeon We model a wall with smallamplitude sinusoidal roughness as a small perturbation to a turbulent boundary layer. By altering the linearized boundary condition of the NavierStokes equations for a single spatial wavenumber, the roughness introduces a stationary, periodic velocity perturbation that interacts nonlinearly with pairs of triadicallyconsistent convecting modes which make up the fluctuating velocity field. This study models these processes in a loworder fashion by approximating the stationary and convecting modes as the mostamplified resolvent modes for a given wavenumber and frequency. The model’s predictions are found to qualitatively reproduce the effects of two distinct sinusoidal roughnesses on the power spectrum of a turbulent boundary layer. Implications to more general roughness are discussed. 
Tuesday, November 20, 2018 9:18AM  9:31AM 
M21.00007: Effects of irregular roughness on roughnesssublayer turbulence statistics and coherent motions Mostafa Aghaei Jouybari, Junlin Yuan, Giles J Brereton Nearwall turbulence statistics and structure are compared using DNS data from openchannel flows at Re_{τ} = 1000, over four different surfaces. The first two surfaces have irregular roughness geometries with height distributions of very similar rootmeansquare and skewness, which fall within the fullyrough regime. One is a random sandgrain roughness with distinct peaks in its height power spectrum; the other is a fractallike representation of the roughness of a turbine blade. The other two surfaces are a regular, cuberoughened (ktype) surface and a smooth wall. It is shown that relatively peaky surface structures contribute much of the pressure drag and are strongly correlated with intermittent, instantaneous generation of wallnormal turbulent fluctuations. Thus, surfaces with a higher density of peaky structures lead to higher friction coefficients and more isotropic Reynolds stresses. Integral length scales reveal a significantly shortened streamwise coherence near sand grains and cubes, but not over the turbineblade surface. Shape modification of conditional eddies in the roughness sublayers will be discussed. 
Tuesday, November 20, 2018 9:31AM  9:44AM 
M21.00008: Singlepoint structure tensors in wall turbulence Junlin Yuan, Giles J Brereton, Gianluca Iaccarino, Aashwin Mishra, Magnus Vartdal A longstanding problem in turbulence modeling is that the Reynolds stress tensor alone is not necessarily sufficient to characterize the transient and nonequilibrium behaviors of turbulence under arbitrary mean deformation or frame rotation. More complete flow characterizations include additional singlepoint structure tensors (e.g. Kassinos, Reynolds and Rogers, JFM 2001), such as the dimensionality, circulicity and inhomogeneity tensors. These tensors are onepoint correlations of local streamfunction vector and can carry nonlocal information on turbulence structure. A seminal question involves the information supplied by these tensors and the potential utility of this additional information to improve the predictive fidelity of a model. In this talk, we explore the role of these tensors in smooth and roughwall channel flows using existing DNS data. Roughness effects on the inclination and aspect ratio of twopoint velocity correlations inside the roughness sublayer are qualitatively reproduced by their corresponding singlepoint tensor representations; in the outer layer, such representations are almost exact. Other observations on turbulencestructure changes on account of wall roughness and their relevance to turbulence modeling will be discussed. 
Tuesday, November 20, 2018 9:44AM  9:57AM 
M21.00009: Mixing Enhancement by an Actively Controlled Bristled Shark Scale Model in a Turbulent Boundary Layer Adam Cross, Amy W Lang, Leonardo Santos The scales of the shortfin mako shark are capable of bristling up to 50 degrees. This bristling impedes flow reversal, which then leads to separation control and less drag. Previous research has confirmed that the actuation of the scales is a passive mechanism. Although the scales are actuated by the flow and not the shark, being able to actively actuate a shark skin model can help to isolate the momentum mixing that occurs between the turbulent boundary layer and cavities that form during scale actuation. To closely match the cavity Reynolds number of the actual shark scales we have 3D printed a larger sized model. We tested the model at three free stream velocities, and at three different actuation timescales for each free stream velocity. The effect on the turbulent boundary layer, with emphasis on changes in momentum of the flow near the wall, was studied using DPIV. 
Tuesday, November 20, 2018 9:57AM  10:10AM 
M21.00010: Effect of surface morphology on hydrodynamic drag and outerlayer similarity Kristofer Michal Womack, Michael Paul Schultz, Charles Vivant Meneveau An experimental study was conducted on roughwall, turbulent boundary layer (TBL) flow with regular and random roughness element arrangements. Varying planform densities of truncated cone roughness elements in a square staggered pattern were investigated. The same planform densities were also investigated in random arrangements. Element densities studied ranged from 10% to 79%. Detailed TBL velocity statistics were recorded with a twocomponent LDV system on a threeaxis traverse. Important profile parameters were determined for all surfaces. In particular, hydrodynamic roughness length (z_{0}) was determined and compared with other studies. Additionally, a breakdown in outerlayer similarity was observed on the staggered element array similar to that reported by Placidi and Ganapathisubramani (2017) over regular arrays with varying planform density. However, outerlayer similarity was observed in spanwiseaveraged profiles over the random arrangements even though individual profiles across the span varied significantly in both the inner and outer layers. This difference in outerlayer similarity over statistically similar surfaces adds interesting new evidence to the salient surface morphology parameters which generate or disrupt outerlayer similarity. 
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