Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session S40: Turbulent Boundary Layers: Control and Perturbations II |
Hide Abstracts |
Chair: Stavros Tavoularis, U Ottawa Room: 6b |
Tuesday, November 26, 2019 10:31AM - 10:44AM |
S40.00001: Wall-bounded turbulence control using a Monte-Carlo approach Oscar Flores, Roberto Pastor, Alberto Vela-Martin Wall-bounded turbulence is very important in engineering applications involving fluids. Indeed, whenever flow control is sought in engineering applications both sensing and actuation are restricted to devices placed at solid walls. In the present work we tackle the classical problem of skin friction control in wall-bounded turbulence for the case of localised actuation using a Monte Carlo approach. To that end, DNS of a minimal channel of the buffer layer ($Re_\tau=180$) are run using GPUs. The actuation is a volumetric vertical force applied close to the wall on a characteristic volume $L^3$ and with duration $T$. Several forcing are considered, with $L^+=[50, 100]$ and $T^+=[25, 50, 100]$. Their effect is evaluated for $O(10^4)$ episodes, directly comparing the instantaneous skin friction of forced and unforced simulations. The statistical analysis shows that drag increase and decrease are equally probable when the forcing is randomly applied. Episodes with drag decrease show a positive (negative) vertical force being applied to passing sweep (ejection) events, in agreement with opposition control strategies. Preliminary results also suggest that a skin friction sensor upstream of the forcing is a better trigger for the forcing than a wall-pressure sensor at the same location. [Preview Abstract] |
Tuesday, November 26, 2019 10:44AM - 10:57AM |
S40.00002: Selective opposition-like control of large-scale structures in wall-bounded turbulence Joseph Ibrahim, Anna Guseva, Ricardo Garcia-Mayoral We investigate the effect of controlling large-scale, logarithmic-layer turbulent structures, which have a characteristic size and aspect ratio that scale with the distance from the wall. The aim is to quantify the effect of suppressing these structures while leaving the near-wall turbulent dynamics unaltered. We conduct direct numerical simulations of turbulent channel flows at $Re_\tau \approx 500$--$1000$ and artificially remove certain streamwise and spanwise wavelengths of the wall-normal velocity across a range of heights. The wavelengths chosen depend on the target height (and size) of the structures that we wish to target. Our preliminary results suggest that the reduction in drag is observed as a positive, outward shift in the mean velocity profile above the target height that scales in outer units. [Preview Abstract] |
Tuesday, November 26, 2019 10:57AM - 11:10AM |
S40.00003: Direct Numerical Simulation of oscillatory boundary layers in the intermittently turbulent regime: coherent structures, laminarization and scaling Dimitrios K. Fytanidis, Jose M. Mier, Marcelo H. Garcia, Paul Fischer Experiments conducted at the Ven Te Chow Hydrosystems Laboratory (UIUC) in the transitional regime of oscillatory boundary layer flows with smooth bed, show changes in the phase shift diagram between bed shear stress and free-stream velocity maxima (Mier J.M., 2015). Nevertheless, limited by the point-wise measurements (Laser Doppler Velocimetry), it was not possible to relate this finding with the development of three-dimensional flow structures. In this work, Nek5000 is used to perform Direct Numerical Simulation (DNS) of oscillatory boundary layer flows in moderately high Re numbers. DNS results of mean flow and turbulent statistics compare well with experimental observations. Coherent structures and their effect on turbulence characteristics are examined. Vortex tubes have minimal effect on turbulent statistics and friction factor, while turbulent spots defined as sporadic, highly-energetic, lambda-shaped structures, have a significant effect. Analysis of phase shift between free-stream velocity and bed shear stress maxima agrees with the experimental observations. Theoretical analysis is performed for the prediction of laminarization during acceleration phase. A generalized logarithmic law is proposed for accelerating flows using a novel composite acceleration-shear velocity scale. [Preview Abstract] |
Tuesday, November 26, 2019 11:10AM - 11:23AM |
S40.00004: The interfaces with the freestream of a spatially developing boundary layer James Wallace, Xiaohua Wu, Jean-Pierre Hickey We have investigated the interfaces with the freestream turbulence of the laminar boundary-layer (LBFTI), turbulent spots (TSFTI) and the turbulent boundary-layer (BTFTI) using direct simulation of a zero pressure-gradient, smooth-wall boundary-layer flow developing from a laminar state, through transition, to a developed turbulent state. Probability density functions of temperature and its derivatives are used to select the interface identification thresholds. These interfaces are confirmed to be physical by the distinctive quasi-step-jump behavior in the swirling strength and temperature statistics along traverses normal to the BTFTI and TSFTI. No interface normal inflection is detected across the LBFTI for swirling strength, temperature, vorticity magnitude, Reynolds shear stress, streamwise and normal velocity or turbulent kinetic energy. This casts serious doubt on the shear-sheltering hypothesis/theory which asserts that freestream fluctuations are blocked by the LBFTI. In the early stage of transition, quasi-spanwise structures exist on the LBFTI. The TSFTI shape is dominated by head prints of concentrated hairpin vortices. Further downstream, the BTFTI geometry is strongly modulated by groves of hairpin vortices. [Preview Abstract] |
Tuesday, November 26, 2019 11:23AM - 11:36AM |
S40.00005: Multi-structure turbulence in a boundary layer interacting with a uniformly sheared flow. Stavros Tavoularis, Curtis Livingston Turbulence generated by two or more distinct production mechanisms and having two or more types of large-scale structure has been termed multi-structure. Depending on the conditions, such flows may retain a non-canonical character or relax to a canonical flow. This study investigates a case in which both production mechanisms persist as the flow evolves. It examines the multi-structure-turbulence region of a turbulent boundary layer (TBL) developing along a smooth wall in a water tunnel, while being adjacent to a nearly homogeneous, uniformly sheared flow (USF). Detailed measurements are collected with laser Doppler velocimetry, particle image velocimetry and hot film anemometry. The mean shear and the turbulent shear stress change direction from the TBL to the USF and vanish near the TBL edge. The statistical properties of the turbulence in the multi-structure region have been measured and compared to those in canonical TBL and USF. Particular interest focusses on the variation of the dissipation parameter across the flow and the shapes and orientations of the coherent structures in the multi-structure region. [Preview Abstract] |
Tuesday, November 26, 2019 11:36AM - 11:49AM |
S40.00006: Development of a turbulent boundary layer subjected to free-stream turbulence Yannick Jooss, Leon Li, Tania Bracchi, R. Jason Hearst Turbulent boundary layers are a fundamental flow that exists in a wide range of natural processes and technical applications. Over the past three decades the effect of free-stream turbulence on the features of a canonical zero-pressure-gradient turbulent boundary layer has been studied extensively, with particular focus on single downstream positions. Nonetheless, there has been little attention given to the influence of varying free-stream turbulence intensity u'$_{\mathrm{\infty }}$/U$_{\mathrm{\infty }}$ on the actual streamwise development of a boundary layer. This study addresses this gap with hot-film measurements in a water channel. Free-stream turbulence is created and varied with an active grid. Wall-normal boundary layer scans are performed along the centerline of the channel at multiple streamwise positions. The resulting mean velocity profiles and turbulent fluctuations in the boundary layer for four different free-stream turbulence conditions are analyzed. Further insight is gained by looking at the spectral distribution of energy at selected streamwise positions. [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