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 Q16: Turbulent Boundary Layers III |
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Chair: Ivan Marusic, Melbourne Room: 4c3 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q16.00001: Turbulent Boundary Layer Response to Sinusoidal Spanwise Perturbation Yuan Wang, Rio Baidya, Charitha M. de Silva, Matthew Fu, Ivan Marusic, Nicholas Hutchins In this study, a developing turbulent boundary layer is perturbed with a sinusoidal spanwise mode. The downstream persistence of these modes, and response of the underlying turbulent structure, is measured using hotwire anemometry. The modes are introduced using spanwise fences with a sinusoidally varying height $h$ of a given spanwise wavelength $\Lambda$. The motivation here is to test Townsendâ€™s analysis which suggests that the turbulent boundary layer is receptive to spanwise periodic modes of certain wavelengths $\Lambda$. Hot-wire measurements are performed at different downstream locations and across the span of the introduced perturbation. Preliminary results indicate that high/low momentum pathways will appear behind the peaks/troughs of the perturbations. The persistence of these spanwise heterogeneous patterns are correlated with the ratio $\Lambda/\delta$, where $\delta$ is the boundary layer thickness at the perturbation station. Particularly, the perturbed flow with $\Lambda/\delta\sim2$, exhibits persistent spanwise periodicity up to 70$\delta$ downstream from the perturbation location, whereas the cases with smaller $\Lambda/\delta\sim1$ seem to recover to canonical spanwise homogeneous conditions over shorter downstream distances. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q16.00002: Spanwise extent and time persistence of uniform momentum zones in zero-pressure gradient turbulent boundary layers Shevarjun Senthil, Callum Atkinson, Javier Jimenez, Julio Soria A time-resolved zero-pressure gradient turbulent boundary layer direct numerical simulation at $Re_\tau = 1,176$ has been used in this study. Uniform momentum zones (UMZs) have been investigated from the following points of view: (i) streamwise extent up to 6 boundary layer thickness, (ii) spanwise extend up to 6.5 boundary layer thickness and (iii) time persistence up to 3 convective time scales. Probability density functions (PDFs) of the number of UMZs have been investigated using 200 time-resolved velocity fields. Using these PDFs, PDFs of the number of UMZs have been computed as a function of extent in the streamwise direction, confirming the previous result that a domain of 2 boundary layer thickness in the streamwise extent is the appropriate length scale to determine UMZs. In the spanwise direction, this is found to be shorter being only one-tenth of a boundary layer thickness, yielding an equal probability of finding 1 or 2 UMZ with a width of 0.1 boundary layer thickness. It is found that some UMZs with a streamwise extent of 2 boundary layer thickness, a spanwise extent of 0.1 boundary layer thickness will have a time persistence of 1 convective time scale. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q16.00003: Universality of uniform momentum zones in high-Reynolds-number boundary layers Michael Heisel, Charitha De Silva, Nicholas Hutchins, Ivan Marusic, Michele Guala Regions of coherent streamwise velocity known as uniform momentum zones (UMZs) are evaluated using eleven zero-pressure-gradient boundary layer datasets including a direct numerical simulation and atmospheric field measurements. UMZ properties are compared across a wide range of friction Reynolds number $Re_\tau \sim O(10^3 - 10^6)$ and surface conditions from hydraulically smooth to fully rough. In the logarithmic region, the UMZs exhibit universal behavior irrespective of Reynolds number and surface conditions. The velocity difference across the shear interfaces between UMZs scales with the friction velocity $u_\tau$ and the wall-normal thickness of UMZs scales with the wall-normal distance. Further, the UMZ statistics provide a direct link between the spatial organization of boundary layer turbulence and the hypothetical attached eddies used to derive velocity statistics in the logarithmic region. The observed universal behavior of the UMZs can also be used to develop and refine the representative eddies used in reduced-order models of high-Reynolds-number boundary layers. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q16.00004: Observing Reversing Flow in Low Speed Streaks of a Separating Turbulent Boundary Layer Sarah Foley, Amy Lang, Leo Santos, Andrew Bonnaci, Chase Parsons For the improvement of flow control and drag reduction, a better fundamental understanding of the flow physics is essential. A separating turbulent boundary layer over a flat plate was studied in search of boundary layer structures such as low speed streaks and vortices. A V3V system was used to capture the 3D3C velocity measurements within a boundary layer in a water tunnel study at three different Re, while a rotating cylinder placed in the flow induced an adverse pressure gradient (APG). After identifying low speed streaks, a statistical characterization was conducted, including length and width, peak reversing velocity, location of peak velocity, and duration. It was found that the length and peak reversing velocity both increase with increasing Re, while the duration of the streaks and width show little to no correlation to Re. It was also found that the location of peak reversing velocity tended to move upstream towards the rotating cylinder with increasing Re and strength of APG. Evidence was also seen of streamwise vortices which may be the legs of hairpin vortical structures that are thought to be the mechanism of low speed streak formation. These vortices may be connected to the formation of low speed streaks and subsequent reversing flow leading to separation of the boundary layer. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q16.00005: Low speed streaks as triggers for passive bristling of shark scales for turbulent boundary layer separation control Leonardo Santos Flow separation control has been observed over samples of flank skin from a shortfin mako shark. It is hypothesized that this control is enabled by the passive bristling of mako denticles by reversing flow close to the surface. It is hypothesized that the reversing flow within the low speed streaks, of a turbulent boundary layer experiencing and adverse pressure gradient, is the main mechanism that induces scale actuation leading to flow control. Upon scale actuation, the reversed flow would be prevented from moving further upstream and maintaining attached flow over the surface. Since the lifespan of the low speed streaks is very short, the response time of the scales could also be important for the functioning of this mechanism. A water tunnel DPIV study has been carried out to analyze the geometry and behavior of the low speed streaks forming within a turbulent boundary layer (Re around 10$^{\mathrm{5}})$ under various strengths of adverse pressure gradient. Possible correlations between the low speed streak characteristics in terms of the viscous length scales and the geometry of shortfin mako scales from the flank region are explored. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q16.00006: Avalanche of drag-inducing near-wall vortices due to streak transient growth in turbulent channel flow Eric Stout, Xuerui Mao, Fazle Hussain A key question in turbulent boundary layers is the evolutionary dynamics of the coupling of near-wall streamwise vortices and streaks. A single low-speed streak in a large ($L_{x}^{+}=$900 by $L_{z}^{+}=$900) channel flow is triggered with the typical spanwise perturbation to excite streak transient growth (STG) at Re$_{\mathrm{\tau }}=$220, and reveals an avalanche of streamwise vortices, as well as very long structures. Development of hairpin vortices, hook vortices, arched vortices, and very long streamwise vortices (VLSM, much longer than $x^{+}$ of 300) reveal the avalanche dynamics involved in the spread and development of the near-wall structures. Lift up of near-wall fluid by the streamwise vortices generate new streaks and hence STG, revealing a sequence of dynamical events involving complex interactions between streaks and streamwise vortices, undiscovered in previous studies. The interaction between the growing streamwise vortices and the quiescent regions aid in understanding the growth of near-wall dynamics, even also of turbulent spots. Visualization reveals evolution of very long streamwise vortices -- a promising finding that may reveal the enigmatic genesis and dynamics of VLSM in turbulent boundary layers. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q16.00007: Dissipation events in wall turbulence M. J. Philipp Hack, Oliver T. Schmidt The intermittent nature of turbulent flows is characterized by periods of low intensity that are interrupted by brief extreme events during which quantities such as production or dissipation of fluctuation kinetic energy develop marked peaks. Our study examines the mechanisms of extreme dissipation events in turbulent boundary layers at moderate Reynolds numbers. Conditional sampling is applied to time-series data generated in direct numerical simulations. The results point to a connection between localized dissipation maxima and the formation of hairpin vortices via exponential instability. Specifically, time-resolved conditionally averaged velocity fields at a dissipation event show the characteristic spatial structure of an instability of varicose type, as predicted in linear analyses (\emph{M.J.P. Hack \& P. Moin, J. Fluid Mech., vol. 844, 2018}). Visualizations of vortex-identification criteria recover a hairpin-type structure which coincides with the region of highest dissipation. The analysis identifies the precursors of the dissipation events as perturbations in the streamwise velocity component which give rise to the varicose instability by locally augmenting the shear. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q16.00008: A DNS study of a shear-driven three-dimensional turbulent boundary layer with emphasis on momentum transport Hiroyuki Abe DNS is used to examine a non-equilibrium three-dimensional turbulent boundary layer (3DTBL) over a flat plate owing to sudden imposition of surface spanwise velocity $W_s$. Particular attention is given to the effects of crossflow and Reynolds number on momentum transport. In the simulations, three values of inlet momentum thickness Reynolds number (=300, 600 and 900) are used with several values of $W_s$. The present largest $W_s$ is twice the freestream velocity $U_0$, comparable to the maximum value in the spinning cylinder experiment by Lohmann (1976). After imposing $W_s$, the secondary Reynolds shear stress $\overline{vw}$ builds up and the mean spanwise velocity (crossflow) increasingly propagates towards the outer region where there is a mean streamwise velocity deficit due to the skewing. Near-wall Reynolds stresses (normalized by $U_0^2$) increase with crossflow due to the increased straining. As $Re$ increases, the inner region of a near equilibrium 3DTBL becomes increasingly enlarged where the structure parameter is smaller than 0.15. The mean velocity magnitude also exhibits a departure from the classical log law (i.e. a larger $\kappa$ than in a 2DTBL). After turning off $W_s$, the recovery to a 2DTBL is slow in the outer region since the 3D effect persists there. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q16.00009: Direct numerical simulation of a turbulent thermal boundary layer spatially evolving on an isothermal wall from a fully turbulent adiabatic flow Matteo Gelain, Olivier Gicquel, Alexandre Couilleaux, Ronan Vicquelin A direct numerical simulation of a spatially evolving turbulent thermal boundary layer is performed in a channel flow at Re$\tau \quad =$ 395. The domain is made of two parts in the streamwise direction. Upstream, the flow is turbulent, homogeneous in temperature and the channel walls are adiabatic. The inflow conditions are extracted from a recycling plane located further downstream so that a fully developed turbulent adiabatic flow reaches the second part. In the domain located downstream, isothermal boundary conditions are prescribed at the walls. The boundary layer, initially at equilibrium, is perturbed by the abrupt change of boundary conditions and a non-equilibrium transient phase is observed until, further downstream, the flow reaches a new equilibrium state presenting a fully developed thermal boundary layer. The study focuses on the spatial transient phase, identifies different zones in terms of active physical phenomena and contrasts these results with usual assumptions in wall-modelled large-eddy simulations. Mean and root-mean-square profiles of temperature and velocity are presented and discussed along with budgets of first- and second-order moments balance equations for the enthalpy and momentum turbulent fields. [Preview Abstract] |
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