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 P42: Boundary Layers: Rough Wall Effects |
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Chair: Krishnan Mahesh, University of Minnesota Room: 6e |
Monday, November 25, 2019 5:16PM - 5:29PM |
P42.00001: Modeling Turbulent Rough-Wall Flow with Pseudo Body Forces Giles Brereton, Junlin Yuan, Mostapha Aghaei Jouybari In RANS representations of turbulent flow over rough walls, it is desirable to describe flow variables as their superficial averages, so that the flow can be solved on a smooth-wall grid. The superficially averaged `extra' terms in the $x$-momentum equation which describe the local effect of roughness on the flow, and which can be resolved by DNS with an immersed boundary method to enforce the precise local roughness boundary conditions, are $\langle f_p \rangle$ and $\langle f_v \rangle$. They describe averages per unit volume of the pressure dispersion and viscous stresses on account of roughness elements. As such, they are pseudo body forces distributed in the $y$ direction, from the troughs to crests of the roughness sublayer. The distributed body force model appears to offer a more fundamental way of modeling general surface roughness than other approaches. In this talk, we present results which illustrate the applicability of the model for different kinds of roughness in fully-developed and boundary-layer flows. We also present computed distributions of $\langle f_p \rangle+\langle f_v \rangle$ for flows over surfaces with different roughness textures, and describe the practical implementation of such a model within a wall function, for high Reynolds-number flow. [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P42.00002: Multi-parameter prediction of roughness function and drag profiles in turbulent channel flows over rough walls Mostafa Aghaei Jouybari, Giles J. Brereton, Junlin Yuan We report on numerical experiments in which DNS is carried out for turbulent fully-developed channel flows over rough walls, describing 42 different surfaces at two frictional Reynolds numbers of 500 and 1000. The surfaces differ in roughness area distribution, effective slope, average inclination, porosity and degree of randomness. The dependence of equivalent sand-grain-height $k_s$ on different texture parameters was explored by using dimensional analysis and a differential evolutionary algorithm to optimize the coefficients of a single low-order multidimensional fitting function for all surfaces. It shows a strong dependence of $k_s$ on mean roughness height, effective slope and skewness, and fits the $k_s$ data with an error of no more than 20\% in any of the 42 surfaces, even though the actual value of $k_s^+$ varies from 20 to 300 in these simulations. This corresponds to less than 10\% error in prediction of the roughness function $\Delta u^+ (k_s^+)$ for $k_s^+ \ge 25$. The same procedure is then used to model the total drag term in the Navier-Stokes equations when the Reynolds stress is described by the $v^2$-$f$ model of Durbin (1991). It indicates a strong dependence of the drag profile on the roughness area distribution and on the profiles of $uv$ and $v^2$. [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P42.00003: Determination of roughness scales that accurately predict frictional drag Karen Flack, Michael Schultz Significant progress has been made towards the understanding of rough-wall boundary layers and the subsequent drag penalty. Continued progress is promising since a larger range of parameter space can now be investigated. Recent advances in rapid prototyping techniques enables the generation of systematic variations of roughness scales and computationally efficient simulations with creative surface mapping techniques allows for experiments and computations to investigate similar complex roughness. While a universal drag prediction correlation is still elusive and may not be possible, predictive correlations for classes of surface roughness pertinent to engineering applications seem achievable. Three surface parameters based solely on surface statistics are showing promise in predictive correlations for a range of studies. These include a measure of surface elevation ($k_{rms}, k_{a}, k_{t})$ a slope parameter (\textit{ES}, solidity) and the skewness of the \textit{pdf}. Other candidate parameters that may be useful in a predictive correlation or a surface filter are the streamwise and spanwise correlation lengths. [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P42.00004: Turbulent boundary layer perturbation by two wall-mounted cylinders arranged in tandem at various spacings and height ratios Ali Hamed, Adam Peterlein The perturbation of a turbulent boundary layer by two cylindrical roughness elements in close proximity was experimentally investigated using particle image velocimetry (PIV). The two cylinders were arranged in tandem with center-to-center streamwise spacings of 2$d$, 4$d$, and 6$d$, where $d$ is the diameter of the cylinders. The downstream cylinder had a fixed height (approximately 20{\%} of the boundary layer thickness); the height of the upstream cylinder was varied to achieve upstream to downstream cylinder height ratios of 1, 0.75, and 0.5. The flow measurements were made at \textit{Re} $=$ 57000 (based on the boundary layer thickness and freestream velocity) and included measurements over an isolated cylinder as a baseline case. The results highlight the effects of sheltering by an upstream cylinder on the wake of the downstream cylinder. Flow features in the wake, including the downwash, upwash, recirculation zone, velocity deficit, Reynolds shear stress, and turbulent kinetic energy (TKE), are highly dependent on the degree of sheltering which is governed by both the streamwise spacing and height ratio. Additionally, proper orthogonal decomposition (POD) and quadrant analysis were used to examine the changes to the turbulence structure as a function of both the spacing and height ratio. [Preview Abstract] |
Monday, November 25, 2019 6:08PM - 6:21PM |
P42.00005: Random roughness effects on the near-wall flow in the transitionally rough regime Rong Ma, Karim Alame, Krishnan Mahesh Direct numerical simulation of turbulent channel flow over a random rough wall is performed at $Re_{\tau}=400$ and $600$. The rough surface corresponds to the experiments of Flack and Schultz (personal communication). The skin friction coefficient of the random-rough channel matches with the experimental results of Flack and Schultz. The roughness effects on the near-wall regions of mean velocity, Reynolds stresses, pressure fluctuations and streamwise mean momentum balance are investigated. The statistics of wall-shear stress fluctuations in the peak (above the mean height location) and valley (below the mean height location) regions are examined. The probability distribution function of wall-shear stress shows a better collapse after subtracting the mean and normalizing by the root-mean-squared value. The distribution tail is widened by the random roughness, implying that the probability of extreme events is increased. The probability of extreme events in the random-rough channel increases with increasing $Re_{\tau}$, in accordance with previous studies on smooth-wall flows. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P42.00006: Towards Modelling the Downstream Development of a Turbulent Boundary Layer Following a Rough-to-Smooth Step Change in Surface Condition Mogeng Li, Charitha M. de Silva, Daniel Chung, Dale I. Pullin, Ivan Marusic, Nicholas Hutchins In this study we examine the effect of both the friction Reynolds number $Re_{\tau}$ and the roughness Reynolds number $k_s^+$ on a turbulent boundary layer following a rough-to-smooth step change in surface condition along the flow direction. To investigate the effect of $Re_{\tau}$, a set of wind-tunnel experiments is conducted at $k_s^+=160$ while $Re_{\tau}$ is varied from 7100 to 21000. Similarly, to examine the dependence on $k_s^+$, a set of measurements is conducted at $Re_{\tau}=14000$ with $k_s^+$ ranging from 110 to 230. Hot-wire profiles are obtained on a logarithmically spaced grid up to 120 boundary-layer thicknesses downstream of the step change, and the local wall-shear stress is measured directly using oil-film interferometry. Using these data, we propose a new model of the recovering mean velocity profile which accounts for the well-known non-equilibrium behaviour of the internal layer. This mean velocity distribution is then evolved downstream of the step change using the integrated streamwise momentum equation to achieve a full prediction of the mean flow recovery. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P42.00007: Impact of spanwise effective slope on near-wall turbulence Thomas Jelly, Nicholas Hutchins, Angela Busse Whereas streamwise effective slope (ES) is widely accepted as a key scaling parameter in the context of rough-wall turbulent flows (Napoli et al., {\it J. Fluid Mech}., {\bf 613}:385-394, 2008), spanwise ES has received far less attention. Here, the statistical response of near-wall turbulence to systematic changes in spanwise ES is investigated by performing direct numerical simulations of rough-walled turbulent channel flow at a friction Reynolds number of 395. For the seven irregular surfaces considered in this study, the spanwise ES ranges from 0.35 to 0.10. All surfaces were synthesised with a near-Gaussian height distribution, i.e. negligible skewness and excess kurtosis, and a streamwise ES equal to 0.35. This allows the current study to focus on the impact of spanwise ES, since skewness, kurtosis and streamwise ES have been all effectively been eliminated as parameters. Starting from a baseline isotropic case, the hydraulic and hydrodynamic properties of each surface are compared as a function of spanwise ES. Details related to the Hama roughness function, turbulence intensities and dispersive stresses will be presented and discussed. Preliminary results from a complementary experimental campaign will also be presented. [Preview Abstract] |
Monday, November 25, 2019 6:47PM - 7:00PM |
P42.00008: Direct numerical simulation for irregular roughness on a curved surface Haoliang Yu, Umberto Ciri, Arif Malik, Stefano Leonardi Surface roughness is critical to aerodynamic performance. Ice accretion, insect contamination, dust accumulation, and surface erosion represent major sources affecting surface perturbations in atmospheric applications. In wind energy, for example, insect contamination on turbine surfaces can significantly increase skin drag and reduce power production. Although some experiments and numerical simulations have been carried out to study roughness effects, the contributions of different roughness parameters are not well understood, particularly when over curved surfaces. Commonly used engineering models for roughness penalty prediction also demand more insights and validation. Therefore, this study includes direct numerical simulations for irregular roughness over a curved (airfoil) surface. A precursor simulation is conducted to generate fully developed turbulent inflow. The blade geometry and roughness are defined by the ray triangle intersection test and modeled using immersed boundary method. Resulting flow fields are analyzed and compared for the various parametric cases. [Preview Abstract] |
Monday, November 25, 2019 7:00PM - 7:13PM |
P42.00009: The effect of surface-obstacle geometry on maximizing air exchange over rough surfaces for use in direct air carbon capture assemblies Simone Stewart, Paolo Luzzatto-Fegiz Climate mitigation scenarios from integrated assessment models show it will likely be necessary to remove previously-released atmospheric CO$_{\mathrm{2}}$. A proposed implementation of negative emission technologies is direct air capture (DAC), which consists of box-like absorbers, through which air is passed and scrubbed of CO$_{\mathrm{2}}$. While the objective has been to scale up DAC to large arrays, the flow over these arrays has never been examined. The details of this flow are crucial; once air flows through the first row of absorbers, it must mix quickly with the surrounding atmosphere before encountering the next row, or performance will be seriously degraded. To improve CO$_{\mathrm{2}}$ extraction from DACs, approaches to maximize the mass exchanges are investigated; exploiting the concept that flow over the DAC array is equivalent to flow over obstacles that comprise a rough surface. We perform reduced-scale experiments using model arrays in a water channel to study how the array geometry influences mass transport. We find that critical roughness spacings needed to avoid the development of recirculation regions are dependent on the aspect ratio of the obstacles. Thin obstacles have smaller critical spacings and result in stronger mixing, which supports the design of DAC arrays that minimize cost and land usage. [Preview Abstract] |
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