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 A20: Turbulent Boundary Layers I |
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Chair: Corey Markfort, University of Iowa Room: Georgia World Congress Center B308 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A20.00001: Turbulent boundary layer over a compliant surface tuned to deform by several wall units Subhra Shankha Koley, Jin Wang, Joseph Katz Previous simultaneous time-resolved measurement of the 3D flow structure and deformation of a compliant wall by a turbulent channel flow involved wall stiffness too high to affect the flow, resulting in one-way coupling between flow and deformation. The current experiments focus on cases with deformations extending to several wall units aimed at generating two-way coupling. Guided by theoretical analysis, the required Young's Modulus (0.15Mpa), shear speed (6m/s), and thickness (5mm) of the compliant surface is achieved. Measurements performed in a developing boundary layer using Mach Zehnder Interferometry indeed confirm that the deformation amplitude increase from submicron to four wall units as the free stream velocity increases from 1 to 6 m/s. An initial series of stereo-PIV measurements along a streamwise plane compare the mean velocity profiles in the boundary layer to those developing over a rigid wall. Based on a log fit, the friction velocity remains unchanged compared to a rigid wall at low speeds, when the deformations are smaller than a wall unit. However, they begin to increase as the deformations exceed a wall unit. On-going measurements with increasing resolution are aimed at resolving the near-wall flow and distributions of Reynolds stresses. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A20.00002: Experimental Investigation of Compliant Surface Deformation under a Turbulent Boundary Layer Jin Wang, Subhra Shankha Koley, Joseph Katz Previous experiments examined the correlations between a turbulent channel flow and deformation of a compliant wall with shear speed much higher than the flow speed, resulting in submicron deformations. Aiming to extend the scope to two-way coupling, where the deformation amplitude is several wall units, theoretical analysis is used for selecting a compliant material (PDMS + silicone gel) with Young’s modulus (0.15 MPa), coating thickness (5mm), and shear speed (~6 m/s) comparable to the flow velocity. Time-resolved (2 kHz) Mach-Zehnder Interferometry (MZI) with a field of view of 70×35mm^{2} is used for measuring the spatial distribution of surface deformation of the transparent compliant wall. The experiments have been performed in a water tunnel extension to the JHU refractive index matched facility in boundary layers having friction velocity Reynolds numbers in the 1000-7000 range. Data analysis shows that the amplitude of the deformations increases from less than 1μm (rms – 0.2 μm) at a freestream velocity of 1 m/s to well above 20 μm (rms - 6 μm) at 6 m/s, the latter corresponding to several (~4) wall units. Wavenumber-frequency spectra show that at all speeds, the typical advection speeds of surface deformation are consistently about 66% of the freestream velocity. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A20.00003: Response of an elastic plate to turbulent boundary layer wall-pressure fluctuations Krishnan Mahesh, Sreevatsa Anantharamu Understanding the excitation of elastic structures by wall pressure fluctuations from turbulent boundary layers is essential to investigate the radiated far-field sound. The response of an elastic plate to wall-pressure fluctuations is computed in the time domain using an in-house parallel unsteady finite element structural solver, and one-way coupled FSI simulations. The wall-pressure fluctuations are generated from DNS of turbulent channel flow at Re_{τ} = 180, 400 and 590. The nature of pressure fluctuations, response of the elastic plate and their coupling are investigated and compared. The resulting far-field acoustic radiation due to the plate vibration is discussed. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A20.00004: Measurements of a forced, turbulent boundary layer over an elastic surface David Huynh, Beverley J McKeon Since the seminal work of Kramer (1957), there has been scientific interest in surfaces that deform under fluid motion. Such compliant surfaces have potential in passive flow control, e.g. vibration damping or skin-friction drag reduction, through careful design of the surface properties. This requires an understanding of the fluid-structural interaction. However, for a turbulent boundary layer (TBL), the inherently broadband spectrum makes it difficult to isolate critical relationships between surface and flow. Here, an elastic, gelatin surface is embedded in a submerged flat plate, over which a TBL is developed. Addressing the aforementioned broadband complexity, a 2D, traveling-wave-like disturbance is forced into the flow just upstream of the compliant surface through a dynamic roughness element (Jacobi & McKeon, 2011). This wave isolates a spatio-temporal scale and dominates the responses of both the fluid and surface, supporting a reduced input-output analysis. Flow and surface measurements are made by 2D PIV and 3D DIC, phase-locked to the roughness motion. We describe the experiment and the interaction of the forced TBL and compliant surface, with an ultimate view to extend to a broadband spectrum. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A20.00005: Experimental study of modulating effect in permeable-wall turbulence Taehoon Kim, Gianluca Blois, Jim Best, Kenneth Thor Christensen In natural and engineering flow systems, turbulent flow overlying permeable walls are encountered across a broad range of length scales. Understanding the coupling between the free and the pore flows is key to accurately predicting many biochemical processes occurring in such systems. In an intermediate region between these two distinct flows, non-linear interactions take place across the interface and render permeable-wall turbulence unique compared to more canonical turbulent boundary layers over impermeable walls. The aim of this study is to explore the unique nature of these flow interactions, particularly modulation of the near-wall and pore flow by larger-scale motions in the outer layer in flow over cubically-packed uniform spheres. Low- and high-frame PIV measurements were in a refractive-index matching (RIM) environment that allows almost full optical access in the vicinity of the permeable interface for the current wall models. Conditional averaging with a proper orthogonal decomposition (POD) filter and two-point amplitude modulation (AM) correlation coefficients revealed the prevalent AM effect in permeable-wall turbulence. Furthermore, these results also showed that the AM phenomenon propagates into the pore space along the penetrating flow path. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A20.00006: Mechanism for Increased Viscous Drag over Porous Sheet Acoustic Liners Thomas Charles Corke, Christopher M Jasinski High tonal sound levels have been observed to produce a dramatic increase in the viscous drag of turbulent boundary layers over porous face-sheet acoustic liners. The mechanism for this increase in drag is the topic of this presentation. This involves a series of acoustic liners with fabricated porous sheets having a range of hole diameters, spacing and patterns. These spanned a range of viscous length scales from 22 < z^{+} < 178. Boundary layer measurements include mean velocity and turbulence intensity profiles, velocity spectra, and ``burst" statistics based on VITA and uv-quadrant methods. Coupled with this are direct drag measurements. The results show a dramatic increase in drag at critical sound levels and frequencies. This is presumed to be driven by net zero mass jets emanating from the porous sheets, but only occurs when the hole spacing is comparable to the spanwise wavelength of the wall ``streak" structure. The increase in drag is also correlated with an increase in the turbulent ``burst" events. The details of this are presented. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A20.00007: Spatio-temporal spectra in a strongly decelerated turbulent boundary layer Taygun Recep Güngör, Ayse G Gungor, Yvan Maciel, Mark Phil Simens Spatio-temporal spectra are analysed from a direct numerical simulation of a non-equilibrium adverse pressure gradient boundary layer spanning Re$_\theta$=1500-8200. The shape factor increases from H=1.41 to 3.00. Streamwise and spanwise energy spectra are obtained in order to investigate the energy carrying structures in the inner and outer layers. Twenty probe data points at four streamwise locations (H=1.6, 2.0, 2.5 and 2.8) and 5 wall-normal locations (y/$\delta$=0.1, 0.12-0.18, 0.2, 0.5 and 0.8) are used for the temporal spectra. The spanwise spectra is available in the whole domain. Taylor's frozen turbulence hypothesis based on the local mean velocity is used to obtain the streamwise spectra. Shape, wavelength and wall-normal distance of the maximum value of the energy spectra are compared with other DNS databases based on shape factors, Re$_\theta$ and pressure gradient parameters. When the boundary layer attains a large mean velocity defect (H$>$2), the outer peak in the spectra becomes spatially localized (y/$\delta$ $\sim$ 0.40-0.60 at a spanwise wavelength of $\lambda_z/\delta$ $\sim$ 0.55-0.80 for the streamwise and spanwise velocity, and $\lambda_z/$$\delta$ $\sim$ 0.25-0.40 for the wall normal velocity. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A20.00008: Axisymmetric boundary layers evolving under pressure gradients Praveen Kumar, Krishnan Mahesh Axisymmetric boundary layers are studied using integral analysis of the governing equations for axial flow over a circular cylinder. The analysis includes the effect of pressure gradient and focuses on the effect of transverse curvature on boundary layer parameters such as shape factor and skin-friction coefficient. Relations are obtained relating the mean wall-normal velocity at the edge of the boundary layer and skin-friction coefficient to the boundary layer and pressure gradient parameters. The analytical relations reduce to established results for planar boundary layers in the limit of infinite radius of curvature. The relations are used to obtain skin-friction coefficient which shows good agreement with the data reported in the literature. The analytical results are used to discuss different flow regimes of axisymmetric boundary layers in the presence of pressure gradients. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A20.00009: Investigation of the factors contributing to skin friction coefficient in a self-similar adverse pressure gradient (APG) turbulent boundary layer (TBL) flow using direct numerical simulation (DNS) Shevarjun Senthil, Callum Atkinson, Vassili Kitsios, Atsushi Sekimoto, Julio Soria DNS is performed to solve the incompressible Navier-Stokes equation for pressure and velocity in a self-similar APG-TBL.^{2} We study the contributions to the skin friction coefficient (C_{f}) from turbulent fluctuations, viscous effects and pressure gradient. C_{f }is calculated based on the formulation given by Renard and Deck for boundary layer (BL) flows.^{3 }The non-dimensional pressure gradient (β) is defined as β = δ_{1} P_{e,x }/ τ_{w }where δ_{1} is the displacement thickness, P_{e,x }is the far-field pressure gradient and τ_{w} is the mean wall shear stress. The C_{f} decomposition is studied for three different BL flows with β = 0, 1 and 39. With increasing β, C_{f }approaches zero and the flow becomes more like a free shear layer. It is found that the contribution from viscous effects reduces as β changes from 0 to 39. The Reynolds stress remains the dominant contributor for all β and its contribution to C_{f} has a peak at y = δ_{1 }for β = 39 where y is the wall normal direction. |
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