Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G21: Boundary Layers: Rough or Compliant Walls |
Hide Abstracts |
Chair: Nikolaos Beratlis, Arizona State University Room: 209 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G21.00001: A numerical investigation of the impact of surface topology on laminar boundary layers Nikolaos Beratlis, Kyle Squires, Elias Balaras Surface topology, such as dimples or trip wires, has been utilized in the past for passive separation control over bluff bodies. The majority of the work, however, has focused on the indirect effects on the drag and lift forces, while the details of the impact on the boundary layer evolution are not well understood. Here we report a series of DNS of flow over a single row of spherical and hexagonal dimples, as well as, circular grooves. The Reynolds number and the thickness of the incoming laminar boundary layer is carefully controlled. In all cases transition to turbulence downstream of the elements comes with reorientation of the spanwise vorticity into hairpin like vortices. Although qualitatively the transition mechanism amongst different dimples and grooves is similar, important quantitative differences exist: two-dimensional geometries such as the groove, are more stable than three-dimensional geometries. In addition, it was found that the cavity geometry controls the initial thickness of the boundary layer and practically results in a shift of the virtual origin of the turbulent boundary layer. Important differences in the momentum transport downstream of the dimples exist, but in all cases the boundary layer evolves in a self-similar manner. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G21.00002: Near surface flow structure over a dimpled surface with blowing Colby Borchetta, Alexandre Martin, Sean Bailey The combined effects of surface roughness with flow injection are of particular interest in understanding the flow over ablative heat-shields, a common form of thermal protection system (TPS) used for atmospheric entry. Stereoscopic, time-resolved particle image velocimetry was used to investigate the near-surface flow over a surface geometry consisting of hexagonal dimples, typical of a TPS. Of particular interest are the modifications made to the flow structures generated by the dimpled elements caused by flow injection through the surface. Without flow injection, inclined flow structures are generated periodically at the upstream edge of the dimples and convected downstream. This behavior is coupled with fluid becoming entrained inside the dimples, recirculating and ejecting away from the surface. When flow injection occurs through the surface, this process occupies a larger region of the flow, extending further from the surface, with a corresponding increase in the size of the convecting structures and increase in turbulent kinetic energy. These features persist over the range of Reynolds numbers investigated, with increasing Reynolds number resulting in increased turbulence and a corresponding broadening of the region of the flow influenced by the surface. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G21.00003: Abstract Withdrawn
|
Monday, November 23, 2015 8:39AM - 8:52AM |
G21.00004: Bypass transition of low-speed boundary layers using realistic sandpaper roughness Jesse Capecelatro, Wentao Zhang, Ryan Fontaine, Gregory Elliot, Daniel Bodony, Jonathan Freund The transition process from laminar to turbulent flow over three-dimensional irregular surfaces is known to affect downstream quantities of interest. Properly simulating this transition numerically is therefore potentially critical for accurate predictions of turbulent flows. In this work, a numerical study of bypass transition of a Blasius boundary layer subjected to realistic sandgrain roughness is presented to analyze the effects of surface topography on the characteristics of the downstream turbulence. Direct numerical simulations are performed using a high-order finite difference scheme in general curvilinear coordinates that is fitted to the roughness. A main goal is to model sandpaper-induced transition and quantify its impact on downstream boundary layer turbulence. A simple but effective model for the sandpaper roughness is presented, and parameters for the roughness profile are chosen to match low-order moments of the surface height, in addition to the spectral content measured from real sandpaper. The effects of roughness Reynolds number and ratio of upstream laminar boundary layer thickness to the roughness height are also investigated. [Preview Abstract] |
(Author Not Attending)
|
G21.00005: Turbulence in soft-walled micro-channels S.S. Srinivas, V. Kumaran Transition to turbulence in soft-walled microchannels channels occurs at a much lower Reynolds number (Re) than that when the walls are rigid. To gain insights into the transition, we have studied the fluid flow in these channels using Particle Imaging Velocimetry, along the streamwise and wall-normal directions. The dimensions of the microchannels studied are approximately 4 cm $\times$ 160 $\mu$m $\times$ 1.5 mm. We see qualitative agreement between experiments and simulations, considering channel deformation, for laminar flows. Significant departure from the laminar flow profile is seen after transition. The root mean square of velocity fluctuations along streamwise and wall-normal directions is unsymmetric and is non-zero at the wall. This gives rise to non-zero Reynolds stress at the surface, indicating the coupling between soft surface and the fluid. The turbulence production term too is nonzero at the surface, as opposed to that for a rigid channel flow. This implies there is transfer of energy from the surface to the fluid. The scaled maximum of the velocity fluctuations and the Reynolds stress (divided by the fluid density) in the soft-walled microchannel for Re in the range 250-400 are comparable to those in a rigid channel at Re in the range 5000-20000. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G21.00006: Correlations of Surface Deformation and 3D Flow Field in a Compliant Wall Turbulent Channel Flow. Jin Wang, Cao Zhang, Joseph Katz This study focuses on the correlations between surface deformation and flow features, including velocity, vorticity and pressure, in a turbulent channel flow over a flat, compliant Polydimethylsiloxane (PDMS) wall. The channel centerline velocity is 2.5 m/s, and the friction Reynolds number is 2.3x10$^{\mathrm{3}}$. Analysis is based on simultaneous measurements of the time resolved 3D velocity and surface deformation using tomographic PIV and Mach-Zehnder Interferometry. The volumetric pressure distribution is calculated plane by plane by spatially integrating the material acceleration using virtual boundary, omni-directional method. Conditional sampling based on local high/low pressure and deformation events reveals the primary flow structures causing the deformation. High pressure peaks appear at the interface between sweep and ejection, whereas the negative deformations peaks (dent) appear upstream, under the sweeps. The persistent phase lag between flow and deformations are presumably caused by internal damping within the PDMS. Some of the low pressure peaks and strong ejections are located under the head of hairpin vortices, and accordingly, are associated with positive deformation (bump). Others bumps and dents are correlated with some spanwise offset large inclined quasi-streamwise vortices that are not necessarily associated with hairpins. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G21.00007: Experimental investigation of compliant wall deformation under a fully developed turbulent channel flow using tomographic PIV and Mach-Zehnder interferometry Cao Zhang, Jin Wang, Joseph Katz A fully developed turbulent channel flow bounded by a transparent, compliant wall made of polydimethylsiloxane is experimentally investigated at friction Reynolds number of 2.3x10$^{3}$. The time-resolved 3D flow field and 2D distribution of wall-normal deformation are measured simultaneously using tomographic PIV combined with Mach-Zehnder interferometry. A new interferogram filtration technique based on spatial correlations of small windows, followed by phase calculation from intensity arccosines, is introduced to capture submicron deformations. It has lower errors and sensitivity to fringe shape compared to spectral band-pass filtering. The measured wavenumber-frequency spectra show the deformation consists of patterns that are larger than the field-of-view, surface waves, and small-scale patterns. Some of the latter are advected at the channel centerline velocity, $U_{c}$, but most are advected at 0.7$U_{c}$, the mean speed at 10{\%} of the channel half height, $h$. Correlations between deformation and velocity conditioned on the sign of the deformation indicate the positive and negative deformations are related to the ejection and sweeping events, respectively. The correlation peaks also reside at about 0.1$h$, suggesting this is the elevation where relevant coherent structures are concentrated. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G21.00008: Experimental Investigation on Near-wall Turbulent Flow Structures over Deformable Roughness Mostafa Toloui, Nolan John, Jiarong Hong Wall-bounded turbulent flows over rough surfaces have been studied for almost a century. However, in most of the prior studies, little attention has been paid to the role of roughness mechanical properties, e.g. deformability, in altering the flow characteristics including both general turbulent statistics and near-wall flow structures. In this study, high resolution time-resolved digital in-line holographic PIV is employed to investigate the near-wall turbulent structures as well as turbulent statistics around and above deforming roughness structures. The rough wall samples consisting of tapered cylinders of size 0.5 mm in diameter and 3 mm in height are manufactured from transparent PDMS with similar geometrical features but various deformability levels. The experiments are conducted within an optically index-matched facility (using NaI solution) operating with different Reynolds numbers where roughness samples of different deformability are placed downstream of a 1.2 m long acrylic channel of 50 mm square cross section. The follow-up research envisions a large dataset including various Reynolds numbers and deformability to elucidate the role of roughness deformability on near-wall coherent structures and turbulent energy transport within and above the roughness sublayer. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G21.00009: Internal Structure and Interaction Within Turbulent Boundary Layers Following a Change in Surface Roughness Ronald Hanson, Bharathram Ganapathisubramani In this experimental study we consider the turbulent boundary layer developing past a surface that abruptly changes from rough to smooth. The change in surface condition leads to the formation of an internal layer. Above this layer the flow is characteristic of the upstream condition. Within the internal layer the near-wall turbulence establishes under the influence of the outer region remaining from the incoming rough-wall boundary layer. Wide-field Particle Image Velocimetry measurements were used to capture the development of the boundary layer over the smooth wall downstream of the rough surface. These measurement enable investigation of key features such as the structure inclination angle, which are considered to be invariant in equilibrium boundary layers. However, the structure within the internal layer resembles a smooth wall boundary layer and above the internal layer the structure resembles the upstream rough wall flow. Using the simultaneously monitored temporal streamwise velocity from two hotwires, one located within the peak energetic region of the near wall and the other within the outer region of the boundary layer, interaction occurring across the internal layer will be examined with respect to the development of the evolving boundary layer. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G21.00010: Analysis of velocity and thermal structures in a transitionally rough turbulent boundary layer Ali Doosttalab, Suranga Dharmarathne, Guillermo Araya, Murat Tutkun, Ronald Adrian, Luciano Castillo A zero pressure gradient turbulent boundary layer flowing over a transitionally rough surface (24-grit sandpaper) with $k^+=11$ and Reynolds numbers based on momentum thickness of around 2400 is studied using direct numerical simulation (DNS). Heat transfer between the isothermal rough surface and the turbulent flow with molecular Prandtl number $Pr=0.71$ is simulated. The dynamic multi-scale approach developed by Araya et al. (2011) is employed to prescribe realistic time-dependent thermal inflow boundary conditions. Above the roughness sub-layer (3 - 5$k$) it is found that statistics of the temperature field, including higher order moments and conditional averages, are the same for the smooth and rough surface flow, showing that the Townsend's Reynolds number similarity hypothesis applies for the thermal field as well as the velocity field for the Reynolds number and $k^+$ considered in this study. Also the velocity and thermal structures of the developing boundary layer were studied by means of multi-point correlations and POD analysis. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700