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 H42: Boundary Layers: Multi-scale Roughness and Secondary Flows |
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Chair: Joseph Katz, Johns Hopkins University Room: 6e |
Monday, November 25, 2019 8:00AM - 8:13AM |
H42.00001: Time-varying secondary flows in turbulent boundary layers over surfaces with spanwise heterogeneity Dea Wangsawijaya, Kevin Kevin, Daniel Chung, Ivan Marusic, Nicholas Hutchins Secondary flows form in turbulent flow over surfaces with spanwise heterogeneity. In this study, we use surfaces comprised of spanwise alternating smooth and rough (P-36 grit sandpaper) strips to investigate the behaviour of these secondary flows for various strip widths. PIV measurements are performed on the wall-parallel plane above surfaces with various strip widths $S$ ($0.3 \leq S/\overline{\delta} \leq 3.6$), where $\overline{\delta}$ is the spanwise-averaged boundary layer thickness. We find that, when $S/\overline{\delta} \approx 1$, these secondary flows not only strengthen but also exhibit a pronounced unsteadiness. This unsteadiness is consistent with a flapping from side-to-side of large-scale streaks with a streamwise wavelength of 3-4$\overline{\delta}$. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H42.00002: Turbulent wall flows over spanwise-heterogeneous surfaces: non-periodic deviation from Reynolds-averaged flow patterns. William Anderson Turbulent flows respond to bounding walls with a predominant spanwise heterogeneity -- that is, a heterogeneity parallel to the prevailing transport direction -- with formation of Reynolds-averaged turbulent secondary flows. These secondary rolls constitute manifestation of Prandtl's secondary flow of the second kind: driven and sustained by spatial heterogeneities in the Reynolds (turbulent) stresses. Results from large-eddy simulations and complementary experimental measurements of flow over spanwise-heterogeneous surfaces are shown: the resultant secondary cell location is clearly correlated with the surface characteristics, which ultimately dictates the Reynolds-averaged flow patterns. However, results also show the potential for instantaneous sign reversals in the rotational sense of the secondary cells. This is accomplished with probability density functions and conditional sampling. In order to further this, a base flow representing the streamwise rolls is introduced. Upon substitution of the base flow into the streamwise momentum and streamwise vorticity transport equations, and via use of a vortex forcing model, we assess phase-space evolution (orbit) of the resulting system of ordinary differential equations. The system resembles the Lorenz system, but the forcing conditions differ intrinsically. Nevertheless, the system reveals that chaotic, non-periodic trajectories are possible for sufficient inertial conditions. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H42.00003: Extending the restricted nonlinear model to flow over rough surfaces with spanwise heterogeneities Xiaowei Zhu, Benjamin Minnick, Dennice Gayme The restricted nonlinear (RNL) model has shown success in accurately predicting statistical features of smooth wall-turbulence. The RNL equations are obtained by decomposing the Navier-Stokes equations into a streamwise-averaged component and streamwise-varying perturbations, and then restricting the nonlinearity in the perturbations. The resulting dynamics are supported by a small number of streamwise varying modes (kx wave numbers) interacting with the streamwise constant mean flow. In this work, we extend the RNL modeling paradigm to rough walls by first determining the streamwise wave number support that correctly predicts the log-law behavior associated with rough wall flows. We demonstrate that the parametrization can be obtained analogously to the smooth-wall case, where it was shown that the particular streamwise varying modes required to correctly reproduce low-order statistics and spectra at low and moderate Reynolds numbers are associated with the outer-layer peak of the surrogate dissipation spectra. Comparisons of DNS and RNL simulations over riblets at various spanwise spacing using an immersed boundary method indicate that the properly parameterized rough-walled RNL model reproduces low-order statistics and spanwise energy spectra reminiscent of the DNS data. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H42.00004: Effect of roughness texture on transient, accelerating~channel flows Sai Chaitanya Mangavelli, Junlin Yuan, Giles Brereton The effect of surface~roughness texture on non-equilibrium, accelerating wall turbulence is studied~using direct numerical~simulation of transient periodic channels. The~smooth-wall base case is compared with two irregular rough~surfaces: i) a small-wavelength sand-grain roughness and ii) a~multiscale turbine-blade roughness. The flow is~accelerated from a bulk~Reynolds number of 3000 to 12000 in a short time interval, rendering the~rough-wall flows~transitionally-rough to fully-rough. The smooth wall undergoes~reverse transition towards a laminar-like state with quasi-1D~turbulence, before~re-transition into a new equilibrium state. In contrast, near a rough wall a more~isotropic Reynolds stress~tensor and a higher friction coefficient are observed.~This is mainly due to the fast responses of the form-induced~Reynolds-stress~production and pressure work to the increased shear, both contributing significantly~to higher Reynolds-stress isotropy. As the characteristics of the form-induced~fluctuations are important for these mechanisms, the roughness~texture determines~the initial rate of turbulence response. Results show that the roughness~geometry is important in a non-equilibrium turbulence over a rough wall. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H42.00005: Secondary flow structure development in multi-scale rough wall turbulent boundary layer Naseem Ali, Juliaan Bossuyt, Bianca Viggiano, Bharathram Ganapathisubramani, Johan Meyers, Raul Cal Turbulent flow over an array of heterogeneous multifractal roughness elements is experimentally investigated. The flow fields are captured covering one periodic cell of a multifractal roughness pattern. The roughness elements generate a large velocity deficit behind the roughness, induce flow-separation between low- and high-momentum pathways, and generate small scale turbulence with a length scale on the order of the roughness elements. The vertical velocity is observed to be increased directly prior to the elements indicating the upwards movement over the roughness element. Following the peak, the flow exhibits a negative wall-normal velocity as the localized wake recovers. The flow close to the valleys is perturbed by the wake generated around the small scale of the element, and the signatures of the flow interaction with the roughness scale of the geometry are shown. The formation of secondary motions that grows over the roughness elements is tested. The strength of the secondary flows is most accentuated at the ridges of the roughness. The features of the secondary motions are observed extending the location of high Reynolds stress above the roughness elements. The dispersive stresses reach up into the boundary layer past 0.4 of turbulent boundary layer thickness. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H42.00006: Turbulent boundary layer development over a step change in multiscale roughness John Lawson, Bharathram Ganapathisubramani We examine the development of the turbulent boundary layer across a step-change transition between four different three-dimensional, multiscale, cuboidal roughness surfaces. We collected wind-tunnel measurements of the flow above the canopy layer in spanwise stereo-PIV planes at 72 streamwise locations up- and downstream of the transition, as well as direct measurements of the skin-friction drag immediately downstream of the transition. This extensive dataset allows us to quantify the development of the resultant internal boundary layer in terms of its mean velocity profile, Reynolds and dispersive stresses, using true repeating-unit averaging. In addition, we compare measurements of surface drag to estimates from a recently-developed morphometric drag prediction model (Yang et al. 2016). Our findings are relevant to the prediction of drag and boundary layer development over urban surfaces and roughness transition in the presence of secondary flows. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H42.00007: On the interaction of streamwise vortices with surface textures Saikishan Suryanarayanan, David Goldstein, Garry Brown, Alexandre Berger, Edward White Recent studies (Suryanarayanan et al., AIAA 2017-4417, AIAA 2018-3077) have demonstrated the possibility of mitigating roughness induced transition using surface textures that can dissipate the streamwise vortices responsible for the lift-up effect. In order to identify the optimal surface texture geometry, a fundamental investigation into the effect of surface textures on the evolution of a streamwise vortex in a boundary layer is performed. This study involves a combination of scaling arguments (of the enstrophy flux transport equation) and direct numerical simulations of a single streamwise vortex in a laminar boundary layer with different (2D) surface textures. The results are analyzed from a vorticity dynamics point of view. The effects of local flow acceleration and enhanced dissipation are separately studied using appropriate synthetic simulations. Simple models are suggested and connections are made to previous work on the evolution of perturbations over wavy walls. Supporting wind tunnel experiments that study the development of streamwise vortices generated by micro vortex generators in the presence of surface textures will be presented. Applications to cross flow instability and other technologically relevant scenarios will be discussed. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H42.00008: Measurements in turbulent boundary layers over designed anisotropic porous materials Christoph Efstathiou, Mitul Luhar This talk describes the design and testing of anisotropic porous media with directional resistance as a means of passive flow control of turbulent boundary layers. A high-resolution stereolithographic 3D printer is used to manufacture a substrate with higher streamwise than wall-normal permeability ($\phi_{xy} =k_{xx}/k_{yy}>1$). Such streamwise-preferential materials have demonstrated potential for passive turbulence suppression and drag reduction in previous idealized simulations. The 3D-printed material is flush mounted into a flat plate boundary setup suspended in a large-scale water channel facility. The friction Reynolds number is varied between $Re_\tau \approx 200$ to $Re_\tau \approx 2000$. Measurements are made using a time-resolved Particle Image Velocimetry system and a 2-component Laser Doppler Velocimeter mounted on a precision traverse. These velocity measurements are used to characterize changes in the mean profile and turbulence statistics, and to test for the emergence of spanwise-coherent rollers triggered by a Kelvin-Helmholtz type instability. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H42.00009: Cube-roughened surfaces at high and extreme packing densities Haosen Xu, Xiang Yang We study turbulent channel flows with rough walls comprised of aligned arrays of cubical roughness elements at high surface coverage densities ($\lambda$). Specifically, we carry out direct numerical simulations (DNS) at $\lambda=0.5\sim 0.9$. Fluid motions in the narrow slits between the cubical roughness elements are resolved. We find pairs of relatively small-sized counter rotating vortices in between and above the cubes at moderately high packing densities, i.e. $\lambda=0.5\sim 0.7$. These vortices are replaced by large ones above each cube when surface coverage is high ($\lambda\leq 0.8$). Last, we will compare our DNS to wall-modeled LES, which is a more affordable tool than DNS at high Reynolds numbers. For the flows considered, WMLES compare reasonably well with the DNS, although noticeable differences in the roughness occupied layer is found. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H42.00010: Large-eddy simulation of turbulent flow within a regular array of cubes Takenobu Michioka, Ryo Funaki, Takumi Kawai Large-eddy simulation is implemented for turbulent flow within a regular array of cubes. The square cubes are arranged regularly on the bottom surface at equal intervals, and are normal to the flow. The cube array consists of twelve rows aligned in the streamwise direction. The approaching flow is uniform laminar flow to exclude the effect of inflow turbulence on the turbulent flow within the regular array. At the first row, the turbulent flow is generated by cubes, but large-scale turbulent flow is not observed. At the second row, the values of the energy spectra of the fluctuation in the velocity at low frequency becomes larger near the bottom surface, and large-scale turbulent flow is generated near the bottom surface. After the second row, the peak of the energy spectra shifts towards low frequency, and the size of the large-scale turbulent flow becomes larger as the fetch increases. After the seventh row, the turbulent flows eventually pass between the cubes at a regular interval, and the large-scale turbulent motions also affects the turbulent flow over the cubes. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H42.00011: Three-dimensional measurements of flow structure and turbulence around a pair of cubic roughness elements embedded in the inner part of a turbulent channel Jian Gao, Joseph Katz The 3D flow and turbulence around a pair of roughness cubes embedded in the inner part of a turbulent channel flow (\textit{Re}$_{\mathrm{\tau }}=$2500) are measured using microscopic tomographic holography accelerated using GPU-based algorithms. The cube height, $a=$1 mm, is 90 wall units. ${\rm T}$he cubes decrease the near-wall velocity and generate secondary flows as far as 3$a$ upstream. Horseshoe vortices form at the front surface, and propagate asymmetrically around the cubes, generating secondary vortices along the side surfaces. Each cube and its near wake are engulfed by a vortical canopy dominated by wall-normal vorticity along the sides and spanwise vorticity above the cube. Merging of the ``tip vortices'' developing along the cube upper edge, horseshoe and secondary vortices occurs downstream at locations that decrease with the spacing. They form a large streamwise vortex behind each cube, which rotates in the same direction as the inner leg of the horseshoe vortex. With decreasing spacing, the flow accelerates faster between cubes, but also decelerates faster in the near wake. Streamwise velocity fluctuations of 40{\%} of the freestream velocity and negative Reynolds shear stress develop near the front upper corner of the cube. The turbulence remains high around the entire surface and near wake. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H42.00012: Experimental flow characteristics over different area density roughness KwonHo Park, HeeChang Lim In this study, we aim to observe the flow and surface pressure characteristics over a variety of roughness pattern in a boundary layer wind tunnel. For each roughness element, the cuboid shape was chosen and size 50×50×50mm3. The atmospheric boundary layer (hereafter, ABL) has been an important role of exchange and transfer in momentum flux at ground level. As reported, the Roughness Sub-layer (RS), the lower part of ABL, is usually 2 times thicker than the building height, and the Urban Canopy Layer (UCL) the same as the building height. Therefore, this study performed experimentally the generation of boundary layer in a wind tunnel and observed the flow characteristics of boundary layer and the surface pressure distribution around each roughness. Besides, to observe the effect of the roughness pattern, changes in area density of surface roughness were made by 11, 17, and 25\%, and the arrangement was deployed in staggered and aligned. As a result, the RS height was estimated around 1.8H~2.2H, where H is roughness height. In addition, the surface pressure in front face become lower as area density increases, whereas the pressure on the back remains almost constant regardless of area density. In addition, the surface pressure at top face becomes higher as area density increases. [Preview Abstract] |
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