Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session A32: Rough Wall Turbulent Boundary Layers - I |
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Chair: Mostafa Toloui, University of Minnesota Room: Oregon Ballroom 201 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A32.00001: Comparison of turbulent flows over surfaces of rigid and flexible roughness Mostafa Toloui, Jiarong Hong The work aims at examining the influence of flexible surface roughness on wall-bounded turbulent flows. The experiments are conducted in a refractive-index-matched turbulent channel (using NaI solution) with a test section of 1.2 m in length and 50 mm square cross section. The rough samples consist of tapered cylinders of 0.35 mm in base diameter with 3 mm in height and 4 mm spacing in a 25 cm stretch. Two types of transparent polydimethylsiloxane (Sylgard 184 and Solaris) are used to generate roughness with about an order of magnitude difference in compliance (i.e. bulk elastic modulus \quad of \quad 1.8 Mpa vs. 0.2 Mpa). The dimension and the elastic modulus of roughness elements are designed such that the rough surface with higher modulus shows no deformation (namely rigid roughness) while the one with lower value deforms appreciably under the present flow conditions. Flow measurements are conducted using digital inline holographic PTV to obtain 3D velocity fields in a volume of 10 mm (streamwise) x 50 mm (wall-normal) x 10 mm (spanwise) with a temporal resolution of 0.16 ms and a spatial resolution of 1.1 mm/vector, above both rigid and deformable rough surfaces under two Reynolds numbers. A selection of instantaneous samples, mean velocity profile, turbulent fluctuation, and energy spectra as well as conditionally-sampled flow fields are analyzed to quantify the effect of roughness compliance on general turbulent statistics and coherent flow structures. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A32.00002: Effects of spanwise topographic heterogeneity on amplitude and frequency modulation of streamwise velocity fluctuations. Ankit Awasthi, Matthew Subdberg, William Anderson We present results on the effects of topographic height, and spanwise heterogeneity, on amplitude and frequency modulation of small-scale structures in the roughness sublayer due to large-scale structures in the logarithmic region of turbulent channel flows. This work follows preceding contributions on amplitude and frequency modulation in smooth wall turbulent boundary layers (Mathis et al., J. Fluid Mech.$, $\textbf{628}, 2009a, 311-337 and Ganapathisubramani et al., J. Fluid Mech., \textbf{712}, 2012, 61-91). We have considered three topographic cases with different characteristics from homogeneous (sandpaper), to two spanwise heterogeneous cases where the height amplitude is increased (this topographic configuration induces turbulent secondary flows, which are known to alter the outer-layer flow characteristics). Indeed, pre-multiplied energy spectra across wavelength and elevation (so-called spectrograms) illustrate how turbulent energy is redistributed with systematic modification to the underlying topography. We have determined how the large-scale (low-pass filtered) streamwise velocity modulates the amplitude and frequency response of small-scale (high-pass filtered) signal. We find that outer-layer topographic-induced perturbations completely alter the intensity of amplitude and frequency modulation. This highlights the passive-actuator-like role of roughness heterogeneity, and underpins the need to incorporate such functional dependence in the development of wall models for LES. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A32.00003: Effects of bio-inspired microscale roughness on macroscale flow structures Humberto Bocanegra Evans, Ali M. Hamed, Serdar Gorumlu, Ali Doosttalab, Burak Aksak, Leonardo P. Chamorro, Luciano Castillo The interaction between rough surfaces and flows is a complex physical situation that produces rich flow phenomena. While random roughness typically increases drag, properly engineered roughness patterns may produce positive results, e.g. dimples in a golf ball. Here we present a set of PIV measurements in an index matched facility of the effect of a bio-inspired surface that consists of an array of mushroom-shaped micro-pillars. The experiments are carried out---under fully wetted conditions---in a flow with adverse pressure gradient, triggering flow separation. The introduction of the micro-pillars dramatically decreases the size of the recirculation bubble; the area with backflow is reduced by approximately 60\%. This suggests a positive impact on the form drag generated by the fluid. Furthermore, a negligible effect is seen on the turbulence production terms. The micro-pillars affect the flow by generating low and high pressure perturbations at the interface between the bulk and roughness layer, in a fashion comparable to that of synthetic jets. The passive approach, however, facilitates the implementation of this coating. As the mechanism does not rely on surface hydrophobicity, it is well suited for underwater applications and its functionality should not degrade over time. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A32.00004: Turbulent boundary layer measurements over high-porosity surfaces Christoph Efstathiou, Mitul Luhar Porous surfaces are ubiquitous across a variety of turbulent boundary layer flows of scientific and engineering interest. While turbulent flows over smooth and rough walls have been studied extensively, experimental measurements over porous walls have thus far focused on packed beds, which are limited in porosity ($\Phi = 0.3-0.5$) by their geometry. The current project seeks to address this limitation. A two-component laser doppler velocimeter (LDV) is used to generate velocity measurements in turbulent boundary layer flows over commercially available reticulated foams and 3D-printed porous media at Reynolds number $Re_\theta \approx 3000-4000$. Smooth wall profiles for mean and turbulent quantities are compared to data over substrates with porosity $\Phi >0.8$ and average pore sizes in the range 0.4-2.5mm (corresponding to $\approx 8-50$ viscous units). Previous analytical and simulation efforts indicate that the effects of porous substrates on boundary layer flows depend on a modified Reynolds number defined using the length scale $\sqrt{ \kappa}$ , where $\kappa$ is substrate permeability. A custom permeameter is currently being developed to estimate $\kappa$ for the substrates tested in the boundary layer experiments. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A32.00005: Turbulent flow past three-dimensional patches of roughness Manuel Ferreira, Bharathram Ganapathisubramani Generally, investigations of flows over rough surfaces tend to focus on regular arrays of uniform obstacles (such as cubes or cylinders) or irregular distributions. This approach has led to significant progress in this field of research but so far has been unable to provide an accurate representation of flows past more complex topologies that are of a finite size. In this context, wind tunnel experiments are conducted of flows over three-dimensional patches of roughness. Randomly generated rough patches with large relative height ($h/\delta \approx$ 0.1) are placed within a turbulent boundary layer. The characteristics of the finite patch of roughness are systematically varied by altering both frontal solidity ($\lambda_F$) and plan solidity ($\lambda_P$) over a large range ($\lambda_F \approx$ 0.05-0.25 and $\lambda_P$ = 0.10-0.38) from sparse to dense. Measurements are made using a floating-element force balance for all cases to study the behaviour of the drag with varying $\lambda_F$ and $\lambda_P$. Additionally, high-resolution planar Particle Image Velocimetry (PIV) are carried out for selected cases in two different planes, streamwise wall-normal plane at the spanwise centerline of the patch as well as wall-parallel plane at $y/h \approx 3$. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A32.00006: Roughness induced flow separation in adverse pressure gradient Jongwook Joo, Mike Emory, Sanjeeb Bose, Gorazd Medic, Om Sharma Surface roughness does not only increase turbulent mixing, but also thickens boundary-layers, making flows more susceptible to separation. Detailed flow physics related to the separation is not understood well. Bammert and Milsch (1972) demonstrates a clear example of surface roughness induced separation under adverse pressure gradient. In the study, compressor cascades with NACA 65 airfoils are systematically roughened and the flow over suction surface gradually separates early as roughness increases. A set of Large-Eddy Simulations (LES) over the Bammert's case is investigated, since RANS simulations using roughness models suffer from capturing the separation. In the current study, surface roughness is represented in two different approaches; 1) Realistic rough surface represented by stochastically distributed hills and valleys are gridded and solved with unstructured finite volume method, 2) Using block-structured grid, surface roughness is gridded as a staggered array of 3D rectangles, in a similar way of the previous study for roughened low pressure turbine (GT2016-57912). The current LES's capture rich features of the flow phenomena, which will bring comprehensive understanding of the roughness induced separation. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A32.00007: ABSTRACT WITHDRAWN |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A32.00008: Investigation of inner-outer interactions in a rough-wall turbulent boundary layer using time-resolved PIV Gokul Pathikonda, Kenneth T. Christensen A turbulent boundary layer over hexagonally packed hemispherical roughness is investigated using time resolved PIV in a Refractive Index Matched (RIM) facility. Two cameras, with different fields of view and spatial resolutions, have been used to view the streamwise -- wall normal plane. Matching the refractive index of the working fluid with that of the roughness model eliminates near-wall reflections of the light sheet, and enables measurements very close to the wall. The high-sampling rate ($dt \approx 3t^*$) and near-wall spatial resolution of first camera ($dx \times dy \approx 10^2 y^*$) capture small scales within the roughness sublayer. The large field of view ($X \times Y \approx 2\delta \times 1.2\delta$) of the second camera simultaneously captures the large scales away from the wall. The modulation influences of the outer large scales on the small scales within the roughness sublayer is explored, and compared with the literature investigating the same using hot-wire measurements. The rich spatio-temporal data available provides a new perspective of the inner-outer interactions, and provides a more direct way of observing the existing hypotheses (for e.g., Baars et al., 2015, Exp. Fluids, 56 : p188). [Preview Abstract] |
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