Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D20: Boundary Layers III: Flow over Roughness Elements |
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Chair: Luciano Castillo, Texas Tech University Room: 315 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D20.00001: Time Resolved Tomographic PIV Measurements of Rough-Wall Turbulent Channel Flow Rinaldo Miorini, Cao Zhang, Joseph Katz Time resolved tomographic PIV is used to study flow structures in the outer region of a rough-wall turbulent boundary layer, focusing on imprints of the roughness on the outer layer. Measurements are performed in a transparent channel installed in the JHU optically index matched facility. The roughness consists of pyramids with height, k$=$0.46 mm, and wavelength, $\lambda =$3.2 mm, satisfying h/k$=$55 (h$=$25.4 mm is the channel half-height), k$^{+}=$64 and Re$=$40000. The TPIV setup consists of four high-speed cameras operating at 3 kHz, which view the sample volume through acrylic prisms. The flow field is illuminated by an Nd:YLF laser. Following enhancement, calibration, and reconstruction, 64$^{3}$ voxels interrogation volumes with 0.75 overlap provide 3D velocity fields with spacing of 0.588$^{3}$ mm$^{3}$. Formation and transport of near-wall 3D U-shaped vortex structures, with base in front of the pyramids, and quasi-streamwise legs extending between pyramid crest lines are evident from the data. Extended streamwise regions of high wall-normal vorticity appear ``latched'' to the roughness elements close to the wall, but are transported downstream at higher elevations. Also evident are traveling streamwise low velocity streaks, which cover many roughness elements. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D20.00002: The rough-wall turbulent boundary layer revisited P. Henrik Alfredsson, Antonio Segalini, Ian P. Castro The turbulent boundary layer over a rough wall is a canonical flow case that, despite its long history, is still a subject of numerous debates regarding its appropriate scaling and the physical phenomena involved. In a recent paper\footnote{Castro, I.P., Segalini, A. \& Alfredsson, P.H. 2013 Outer-layer turbulence intensities in smooth- and rough-wall boundary layers, J. Fluid Mech. {\bf 727}, 119--13} a novel empirical scaling, based on the so called diagnostic plot, has been proposed to describe the streamwise velocity variance profile for rough-wall conditions. The scaling gives the turbulence intensity as a function of the local mean velocity and of the roughness function, $\Delta U^+$, regardless of the specific roughness morphology. The analysis is reviewed and extended to other components of the stress tensor, with the aim of providing a simple description of the turbulence statistics over rough surfaces and to establish a connection between rough and smooth surfaces beyond Townsend's hypothesis. Several independent experiments and numerical simulations are used for this purpose. Such an estimation of the turbulence statistical characteristics will be useful for model validation and for a variety of applications that range from wind energy to micrometeorology. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D20.00003: Atmospheric boundary layer flow over transverse roughness transitions: induced mixing and flow characterization David Willingham, William Anderson The response of turbulent atmospheric boundary layer flow to abrupt surface roughness heterogeneities has been discussed extensively in previous literature. However, many prior studies exclusively consider cases in which the streamwise flow is aligned perpendicular to the roughness heterogeneity, representing the noted smooth-to-rough or rough-to-smooth transition. This work seeks to identify the affects of parallel-aligned roughness transitions on turbulent boundary layer flow and determine whether these effects are non-negligible. To this end, a set of large eddy simulations have been performed over surfaces composed of parallel strips of high roughness adjacent to low roughness. The width of these strips, as well as the ratio of high to low roughness lengths were systematically varied between simulations. Close to the surface, there is a transverse gradient in streamwise velocity owing to the differing roughness lengths, and this gradient induces transverse mixing which serves to introduce an important secondary flow in the boundary layer. Low and high momentum pathways (Mejia-Alvarez et al.) are observed to form in the upper region of the boundary layer above the low and high roughness regions, respectively. Associated with this is the formation of boundary layer-scale counter-rotating vortices, adjacent to the high roughness strips. Interestingly, we find that even modest differences between the high and low roughness length is adequate to induce this process. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D20.00004: Numerical simulations of flow over realistic rough surfaces Junlin Yuan, Ugo Piomelli Large-eddy simulations are carried out on open-channel flows over multiple types of rough surfaces occurring in hydraulic turbine in both transitionally and fully rough regimes with the K\'arm\'an number ranging from 400 to 1000. The roughness imposed using an immersed boundary method is spatially resolved by the grid. The roughness functions are used to test several correlations proposed in the literature to relate surface parameters to the equivalent sand-grain height; agreement is obtained with experimental results on gas turbine roughness, despite slight differences in model coefficients. For relatively sparse distributions, the realistic roughness yields a higher drag compared to modeled roughness. The mean-flow ejecting and sweeping motions as part of the channeling phenomenon contribute to vertical momentum transports and correlate closely to regions of positive surface slope. It is observed that a stronger mean flow effect corresponds to higher frequency of relatively strong bursting events in the near-wall region, while the average size of these events is controlled by roughness length scales that are separated from the ones determining the event frequency. Further discussions will be given on possible indications of important surface parameters. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D20.00005: Characteristics of Larger-Scale Motions in Turbulent Flow Overlying Multi-Scale Roughness J.M. Barros, K.T. Christensen The energy and Reynolds-stress content of large-scale motions (LSM) and superstructures within turbulent flow overlying a multi-scale roughness topography are explored using high-frame-rate stereo particle image velocimetry measurements in the wall-normal--spanwise plane. The roughness under consideration was replicated from a turbine blade damaged by deposition of foreign materials and contains a broad range of topographical scales arranged in a highly irregular manner. Previous measurements revealed that this roughness induces significant spanwise heterogeneity in the turbulence statistics in the form of low- and high-momentum flow pathways marked by enhanced Reynolds stresses and turbulent kinetic energy. Frequency spectra of streamwise velocity at fixed wall-normal location also display strong dependence on spanwise position. In particular, the roughness promotes enhanced energy content of the LSMs and smaller-scale motions. Depending on spanwise location, pre-multiplied spectra highlight significant modification of the energy content of the superstructures due to roughness compared to smooth-wall flow. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D20.00006: ABSTRACT WITHDRAWN |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D20.00007: Parametric analysis of turbulent wall jet in still air over a transitional rough surface: Universal relations Noor Afzal The novel scalings for streamwise variations of the flow in a turbulent wall jet over a fully smooth, transitional and fully rough surfaces have been analyzed. The universal scaling for arbitrary wall roughness is condidered in terms of the roughness friction Reynolds number (that arises from the stream wise variations of roughness in the flow direction) and roughness Reynolds number at the nozzle jet exit. The transitional rough wall jet functional forms have been proposed, whose numerical constants power law index and prefactor are estimated from best fit to the data for several variables, like, maximum wall jet velocity, boundary layer thickness at maxima of wall jet velocity, the jet half width, the friction factor and momentum integral, which are supported by the experimental data. The data shows that the two asymptotes of fully rough and fully smooth surfaces are co-linear with transitional rough surface, predicting same constants for any variable of flow for full smooth, fully rough and transitional rough surfaces. There is no universality of scalings in terms of traditional variables as different expressions are needed for each stage of the transitional roughness. The experimental data provides very good support to our universal relations. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D20.00008: Direct numerical simulation of a turbulent rough-walled pipe Andrew Ooi, Leon Chan, Michael MacDonald, Nicholas Hutchins, Daniel Chung Direct Numerical Simulations (DNS) of turbulent pipe flow have been conducted at low to medium Reynolds numbers. The surface of the pipe is varied from a smooth-wall pipe to a rough-wall pipe, where the roughness is comprised of three-dimensional sinusoidal elements. Parametric tests were carried out to analyse the effects of the height and the wavelength of the sinusoidal surface. An analysis of the mean statistics convincingly supports Townsend's outer-layer hypothesis. Higher-order statistics such as skewness and flatness are also gathered and show reasonable collapse in the outer layer of the pipe for different roughness cases. Even at a mean-to-peak roughness height of 2.5 wall units, which is within the viscous sublayer of the pipe, the roughness effected a centerline velocity shift of 0.50 friction velocities. When the roughness height is increased to a mean-to-peak roughness height of 20 wall units, the flow can be considered to be in the fully rough regime, with a centerline velocity shift of 7.1 friction velocities. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D20.00009: Perturbation of turbulent channel flow structure by a cylindrical roughness element G. Pathikonda, K.T. Christensen The existence and dynamic significance of coherent structures in turbulent wall-bounded flows has led to substantial interest in both understanding and perhaps manipulating these structures. To this end, the current work investigates the perturbing influence of a single, wall-mounted cylindrical element in the log layer of a fully-developed turbulent channel flow ($\mathrm{Re}_\tau\sim 1250$), and the latter's response and subsequent evolution. Hot-wire measurements were made in the wake of these elements ($\sim 250$ viscous wall units tall; approximately 20\% of the channel half-height) at various streamwise positions. Mean and spectral statistics of the fluctuating streamwise velocity indicate preferential and inhomogeneous modifications to the incident flow by the cylinder. Subsequent recovery in pre-multiplied spectra of the perturbed flow downstream suggests a biased environment conductive to structures corresponding to the very-large-scale motions (superstructures). Though in an inhomogeneous manner, the flow was found to continuously recover towards the unperturbed incident flow with increasing downstream distance. [Preview Abstract] |
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