Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D27: Wall-Bounded Turbulent Flows |
Hide Abstracts |
Chair: Xiaohua Wu, The Royal Military College of Canada Room: 2009 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D27.00001: Direct simulation of flat-plate boundary layer with mild free-stream turbulence Xiaohua Wu, Parviz Moin Spatially evolving direct numerical simulation of the flat-plate boundary layer has been performed. The momentum thickness Reynolds number develops from 80 to 3000 with a free-stream turbulence intensity decaying from 3 percent to 0.8 percent. Predicted skin-friction is in agreement with the Blasius solution prior to breakdown, follows the well-known T3A bypass transition data during transition, and agrees with the Erm and Joubert Melbourne wind-tunnel data after the completion of transition. We introduce the concept of bypass transition in the narrow sense. Streaks, although present, do not appear to be dynamically important during the present bypass transition as they occur downstream of infant turbulent spots. For the turbulent boundary layer, viscous scaling collapses the rate of dissipation profiles in the logarithmic region at different Reynolds numbers. The ratio of Taylor microscale and the Kolmogorov length scale is nearly constant over a large portion of the outer layer. The ratio of large-eddy characteristic length and the boundary layer thickness scales very well with Reynolds number. The turbulent boundary layer is also statistically analyzed using frequency spectra, conditional-sampling, and two-point correlations. Near momentum thickness Reynolds number of 2900, three layers of coherent vortices are observed: the upper and lower layers are distinct hairpin forests of large and small sizes respectively; the middle layer consists of mostly fragmented hairpin elements. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D27.00002: Taylor's hypothesis in turbulent channel flow considered using a transport equation analysis James Wallace, Chenhui Geng, Guowei He, Yinshan Wang, Chunxiao Xu A DNS of turbulent channel flow was carried out to examine Taylor's ``frozen turbulence'' hypothesis, i.e. the simple time-space transformation that allows $(1/ \overline{U}) \partial / \partial t$ to approximate streamwise derivatives, $\partial / \partial x$, of velocity fluctuations. These terms in Taylor's hypothesis appear in the transport equation for instantaneous momentum for this flow. The additional terms, i.e. the additional convective acceleration and the pressure gradient and viscous force terms, act to diminish the validity of Taylor's hypothesis when they are relatively large compared to the Taylor's hypothesis terms and are not in balance. A similar analysis also has been applied to the transport equation for instantaneous vorticity. There the additional terms, namely the additional convective rates of change, the stretching/compression/rotation and the viscous diffusion of vorticity terms, similarly act to diminish the validity of Taylor's hypothesis when they also are relatively large compared to the terms in the hypothesis and are not in balance. Where in the channel flow this diminishment occurs, and to what degree, and which of the non-Taylor's hypothesis terms in the momentum and vorticity equations contribute most to this diminishment will be presented. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D27.00003: Large-scale motions for a high-Reynolds-number turbulent pipe flow at \textit{Re}$_{\tau } = $ 3008 Junsun Ahn, Jae Hwa Lee, Jin Lee, Hyung Jin Sung Direct numerical simulation (DNS) of turbulent pipe flow at \textit{Re}$_{\tau } =$ 3008 with a very long streamwise domain length ($L_{x} =$ 30$R$, $R$ is a pipe radius) was performed to explore the wall scaling laws in the overlap region. The high-\textit{Re} turbulent pipe flow was found not to follow a log law, but rather to follow a power law. By contrast, the high-\textit{Re} turbulent channel flow (\textit{Re}$_{\tau } \ge $ 2006) followed a log law. A mesolayer was observed in both the pipe and channel flows, in agreement with the power law. The retarded log law in the turbulent pipe flow was attributed to the presence of large-scale structures in the outer region of the pipe flow. These large-scale structures were more dominant in the turbulent channel flows than in the turbulent pipe flows. As the Reynolds number increased, the fluids transitioned from a power law to a log law. The development of large-scale structures in the pipe flow was slower than the corresponding development in the channel flow The proportion of large-scale and very-large-scale motions (LSMs and VLSMs) was obtained in comparison with the low-Reynolds-number pipe flow at \textit{Re}$_{\tau } =$ 934. [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D27.00004: Convection of momentum transport events in a turbulent boundary layer Roeland de Kat, Bharathram Ganapathisubramani Momentum transport in turbulent boundary layers increases drag. Understanding how momentum transport events interact and evolve will allow us to find ways to control them. In this study, we determine the convection of momentum transport events from time-resolved particle image velocimetry measurements in a stream-wise wall-normal plane of a turbulent boundary layer at $Re_\tau\approx$ 2700. A field-of-view covering approximately $2\times0.5\delta$ with high spatial, $l^+=$ 20, and temporal resolution, $\Delta t^+=$ 0.7, allows us to determine convection velocities of momentum transport events of range of different sizes for wall-normal locations $y/\delta=$ 0.02 to 0.47 ($y^+=$ 60 to 1260). In the talk, a detailed description of convection of momentum transport events in different quadrants will be presented. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D27.00005: Townsend's similarity hypothesis applies to the intermittent region of a boundary layer Guillem Borrell, Javier Jimenez The intermittent region of two boundary layers with different entrainment rates obtained by direct numerical simulation are compared at $\delta_{99}^+=1500$, one with the natural friction coefficient, and a second where the spreading rate is increased by 70\% by a smooth volumetric force. The two flows are compared by thresholding the vorticity magnitude field, using a vorticity isosurface as a reference frame. Three regions can be observed in the conditional analysis. The two that are associated with the turbulent-nonturbulent interface match if $u_\tau^2/\nu$ is used as the unit for vorticity, where $u_\tau$ takes into account the additional friction caused by the forcing. The third one, where the two flows are not comparable, corresponds to the near-wall region where the force is applied. This result suggests that Townsend's similarity hypothesis is also valid for the intermittent region of the boundary layer. [Preview Abstract] |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D27.00006: Effects of external disturbances on turbulent boundary layers Eda Dogan, Ronald Hanson, Bharathram Ganapathisubramani The state of a turbulent boundary layer that develops under the influence of different types of freestream turbulence is examined. The freestream turbulence conditions with different length-scale and turbulence intensity are generated using active and passive grids. Downstream of the grid, a flat plate is placed to establish a zero-pressure gradient turbulent boundary layer. The interaction between the freestream and the turbulent boundary layer is investigated using simultaneous measurements of the boundary layer and freestream using single component hot-wire anemometry and multi-camera Particle Image Velocimetry (PIV). Results from the hot-wire measurements of different cases show that the near-wall peak turbulence intensity increases with increasing levels of free stream turbulence indicating the level and extent of penetration by free stream turbulence into the boundary layer. It is also observed that for different level of freestream perturbations to the flow, the momentum loss in the turbulent boundary layer could be similar. The data from these cases will be investigated further using spectral analysis to examine the energetic scales of the flow. The PIV data will be analysed to elucidate the coherent structures associated with these interactions. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D27.00007: Reynolds shear stress near its maxima, turbulent bursting process and associated velocity profle in a turbulent boundary layer Noor Afzal The Reynolds shear stress around maxima, turbulent bursting process and associate velocity profile in ZGP turbulent boundary layer is considered in the intermediate layer/mesolayer proposed by Afzal (1982 Ing. Arch 53, 355-277), in addition to inner and outer layers. The intermediate length scale $\delta_m = \delta R_\tau^{-1/2}$ having velocity $U_m = m \; U_e$ with $1/2 \leq m \leq 2/3$ where $U_e$ is velocity at boundary layer edge. Long \& Chen (1981 JFM) intermediate layer/ mesolayer scale $\delta_m = \delta R_\tau^{-1/2}$ with velocity $U_m$ the friction velocity $u_\tau$, is untenable assumption (Afzal 1984 AIAA J). For channel/pipe flow, Sreenivasan et al (1981989, 1997, 2006a,b) proposed critical layer / mesolayer, cited/adopted work Long and Chen and McKeon, B.J. \& Sharma, A. 2010 JFM 658, page 370 stated ``retaining the assumption that the critical layer occurs when $U(y)=(2/3)\; U_{CL}$ (i.e. that the critical layer scales with $y^+ \sim R_\tau{^{+2/3}}$),'' both untenable assumptions, but ignored citation of papers Afzal 1982 onwards on pipe flow. The present turbulent boundary layer work shows that Reynolds shear maxima, shape factor and turbulent bursting time scale with mesolayer variables and Taylor length/time scale. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D27.00008: Completion of partially known second-order statistics of turbulent flows Armin Zare, Mihailo Jovanovic, Tryphon Georgiou Second-order statistics of turbulent flows can be obtained either experimentally or via direct numerical simulations. The statistics are relevant in understanding fundamentals of flow physics and for the development of low-complexity turbulence models. For example, such models can be used for control design in order to suppress or promote turbulence. Due to experimental or numerical limitations it is often the case that only partial flow statistics are reliably known. In other words, only certain correlations between a limited number of flow field components are available. Thus, it is of interest to complete the statistical signature of the flow field in a way that is consistent with the known dynamics. Our approach to this inverse problem relies on a model governed by stochastically forced linearized Navier-Stokes equations. In this, the statistics of forcing are unknown and sought to explain available velocity correlations. Identifying suitable stochastic forcing allows us to complete the correlation data of the velocity field. While the system dynamics impose a linear constraint on the admissible correlations, such an inverse problem admits many solutions. We use nuclear norm minimization to obtain correlation structures of low complexity. This complexity translates into dimensionality of filters that can be used to generate the identified forcing statistics. The ability of our approach to reproduce statistical features of a turbulent channel flow is demonstrated using stochastic simulations of the linearized dynamics. [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D27.00009: Nonlinear interactions and their scaling in the logarithmic region of turbulent channels Rashad Moarref, Ati S. Sharma, Joel A. Tropp, Beverley J. McKeon The nonlinear interactions in wall turbulence redistribute the turbulent kinetic energy across different scales and different wall-normal locations. To better understand these interactions in the logarithmic region of turbulent channels, we decompose the velocity into a weighted sum of resolvent modes (McKeon \& Sharma, J. Fluid Mech., 2010). The resolvent modes represent the linear amplification mechanisms in the Navier-Stokes equations (NSE) and the weights represent the scaling influence of the nonlinearity. An explicit equation for the unknown weights is obtained by projecting the NSE onto the known resolvent modes (McKeon et al., Phys. Fluids, 2013). The weights of triad modes -the modes that directly interact via the quadratic nonlinearity in the NSE- are coupled via interaction coefficients that depend solely on the resolvent modes. We use the hierarchies of self-similar modes in the logarithmic region (Moarref et al., J. Fluid Mech., 2013) to extend the notion of triad modes to triad hierarchies. It is shown that the interaction coefficients for the triad modes that belong to a triad hierarchy follow an exponential function. These scalings can be used to better understand the interaction of flow structures in the logarithmic region and develop analytical results therein. [Preview Abstract] |
Sunday, November 23, 2014 4:12PM - 4:25PM |
D27.00010: Time-evolving of very large-scale motions in a turbulent channel flow Jinyul Hwang, Jin Lee, Hyung Jin Sung, Tamer A. Zaki Direct numerical simulation (DNS) data of a turbulent channel flow at \textit{Re}$_{\tau} = $ 930 was scrutinized to investigate the formation of very large-scale motions (VLSMs) by merging of two large-scale motions (LSMs), aligned in the streamwise direction. We mainly focused on the supportive motions by the near-wall streaks during the merging of the outer LSMs. From visualization of the instantaneous flow fields, several low-speed streaks in the near-wall region were collected in the spanwise direction, when LSMs were concatenated in the outer region. The magnitude of the streamwise velocity fluctuations in the streaks was intensified during the spanwise merging of the near-wall streaks. Conditionally-averaged velocity fields around the merging of the outer LSMs showed that the intensified near-wall motions were induced by the outer LSMs and extended over the near-wall regions. The intense near-wall motions influence the formation of the outer low-speed regions as well as the reduction of the convection velocity of the downstream LSMs. The interaction between the near-wall and the outer motions is the essential origin of the different convection velocities of the upstream and downstream LSMs for the formation process of VLSMs by merging. [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