Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session R36: Boundary Layer Roughness Effect II |
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Chair: Junlin Yuan, Michigan State University Room: 355 B |
Monday, November 25, 2024 1:50PM - 2:03PM |
R36.00001: Molecular-level simulations of turbulent Couette flow over a thermal-protection-system material Ryan M McMullen, Michael A Gallis, Arnaud Borner We report flow statistics and visualizations from molecular-gas-dynamics simulations using the direct simulation Monte Carlo (DSMC) method for turbulent Couette flow in which the lower wall is replaced by a rough and permeable substrate obtained from a three-dimensional image scan of the low-density carbon reinforcement material FiberForm, used to manufacture NASA’s PICA thermal protection material. We focus on near-continuum conditions corresponding to a modest global Reynolds number Re = 2800 and supersonic Mach number Ma = 1.5. Additionally, we present permeability calculations for differing levels of rarefaction and assess the potential for noncontinuum effects on the overlying turbulent flow. |
Monday, November 25, 2024 2:03PM - 2:16PM |
R36.00002: Rough-wall modelling on a low-dimensional manifold Shyam S Nair, Robert F Kunz, Xiang Yang Linking rough surface topography to the drag experienced by turbulent flow over it is a challenging problem, owing to the wide range of roughness possible. Conventional roughness parameters, which involve roughness height statistics, may be suboptimal in their ability to construct rough-wall models. A novel approach to rough-wall modelling using deep convolutional autoencoders is being proposed for drag prediction. A low-dimensional encoding of 3 latent space variables was derived from a dataset comprising 100 rough wall height distributions. These rough surfaces were obtained from rough walls used in various experiments and Direct Numerical Simulation (DNS) studies. A Feedforward Neural Network (FNN) was trained on these learned representations and the associated equivalent sand-grain roughness height (ks) values in developing a rough-wall model. Further, new surfaces are constructed by the decoder in latent space regions not occupied by the training dataset. We observe that the formulated FNN rough-wall model predicts ks values for these previously unseen surfaces to a reasonable accuracy. The present findings suggest that these newly discovered roughness parameters could not only pave the way for rough-wall modelling but also offer an interpolative basis that could cover a wide range of rough surfaces. |
Monday, November 25, 2024 2:16PM - 2:29PM |
R36.00003: Interaction of incompressible hyperelastic structures with turbulent flows Soham Prajapati, Ali Fakhreddine, Krishnan Mahesh We discuss an Eulerian approach to simulate fluid-structure interaction problems for hyperelastic solids. To solve the momentum conservation equations in the combined domain, we use a one-continuum formulation, and the simulations are performed using our in-house flow solver which uses a finite volume method that is second-order accurate in space and time. In both domains, incompressibility constraints are enforced, and in the solid domain, we use neo-Hookean constitutive equations to account for the hyperelastic behavior. To convect the solid volume fraction, we consider two methodologies - geometric volume-of-fluid and fifth-order weighted essentially non-oscillatory (WENO) scheme. To convect the left Cauchy-Green deformation tensor, we use the fifth-order WENO scheme. Our approach is validated using problems such as a solid disk and a compliant wall in a lid-driven cavity. We apply this methodology to simulate the interaction of wall-bounded turbulent flow with dynamic complex surfaces and investigate the coupling. The results will be discussed. |
Monday, November 25, 2024 2:29PM - 2:42PM |
R36.00004: Efficient and accurate simulations of square bar roughness using adaptive and overlapping meshes Venkatesh Pulletikurthi, Aditya Parik, Daniele Massaro, Som Dutta, Philipp Schlatter Surface morphology determines the type of turbulence structures that lead to form drag and skin-friction drag. Several experimental studies have focused on understanding the effect of surface modifications on turbulence structures. Flow over rough walls can be simulated using body-fitted grids, adaptive mesh refinement, immersed boundary methods (IBM), or overlapping grid methods. Each method has its advantages and disadvantages. Here, we carried out direct numerical simulations of square bar roughness at Reτ = 300, 544 and 900 using adaptive mesh refinement technique, overlapping method and immersed boundary method using the high-order spectral element code Nek5000, and discussed the effectiveness of these methods in resolving all the scales present in the flow. In particular, we analyze the capability of the methods to resolve both small-scale and larger-scale turbulent structures. The latter are extracted using proper orthogonal decomposition to identify the effectiveness of the various methods in capturing the dominant modes over the bars. |
Monday, November 25, 2024 2:42PM - 2:55PM |
R36.00005: Assessment and Prediction of Friction in ZPG Boundary Layers Over Sand Grain Roughness Neetesh S Raghuvanshi, Abhishek Gupta, Shibani Bhatt, Harish Mangilal Choudhary, Pranav Sood, Prajyot Sapkal, Thara Prabhakaran, Shivsai A Dixit Prediction of friction in rough wall boundary layers is of great practical importance. For the same overall Reynolds number, a boundary layer over a rough wall experiences higher friction compared to that over a smooth wall. The degree of departure from the smooth-wall value is expected to depend on the amount of roughness present. We use our previously published smooth wall friction model to assess the departures of dimensionless friction from the corresponding smooth-wall values as a function of roughness Reynolds number. It is found that these percentage departures scale on the sand grain roughness Reynolds number and can be modelled empirically. Using this, a new methodology is presented to estimate friction in a rough wall boundary layer provided the sand grain roughness value is known. Predictions of friction velocity with ±5% accuracy are obtained over three decades of roughness Reynolds number range. The accuracy and limitations of the proposed method are discussed. |
Monday, November 25, 2024 2:55PM - 3:08PM |
R36.00006: Rearrangement of Secondary Flows over Multi-Column Roughness Configurations Atharva Sunil Sathe, William Anderson, Marc Calaf, Marco G Giometto Turbulent flows over spatially inhomogeneous rough surfaces are crucial in various applications, such as wind farms, urban landscapes, and fluvial processes. When these rough surfaces exhibit cross-stream heterogeneity comparable to the boundary layer height, they generate secondary flows. Over the past decade, significant efforts have focused on understanding secondary flows due to their critical role in modulating flow statistics. A key interest in these flows is identifying the parameters that govern their polarity, which dictates whether fast-moving or slow-moving fluid aligns with the roughness elements. In this study, we investigate how the cross-stream gap between the roughness elements in localized high-roughness patches affects the polarity of secondary flows. We will demonstrate how this parameter alters the preferential fluid pathways in these patches, thereby affecting the polarity globally. Our analysis includes three simulations with varying cross-stream gaps: two showing clear polarity reversal and one intermediate case. The insights will be based on first and second-order velocity statistics and TKE budget analysis. Overall, this research aims to deepen our understanding of the characteristics of secondary flows over complex arrangements of roughness elements. |
Monday, November 25, 2024 3:08PM - 3:21PM |
R36.00007: Experimental Characterization of the 3D Flow Field around a Roughness Element Embedded in the Inner Part of a Rough-Wall Turbulent Boundary Layer Deepan Sharma, Spencer J Zimmerman, Joseph Katz Knowledge of detailed flow structure around roughness elements in a high Reynolds number rough wall boundary layer (BL) is essential for understanding the transport of momentum and turbulence away from the wall. However, there is very little 3D experimental data in the flow in the roughness sublayer due to technical challenges in accessing this region. To address this issue, Microscopic Dual-View Tomographic Holography (M-DTH) is applied in a refractive index-matched water tunnel to fully resolve the 3D flow in the inner part of the rough wall turbulent boundary layer (Reτ~ 3300). The array of 0.5 mm high (k), 0.79 mm diameter cylindrical roughness elements correspond to k/δν~35 and δ/k~90. 3D velocity field is obtained by interpolating particle tracks onto a structured grid. The time-averaged distributions of velocity, vorticity, wall shear stress, and Reynolds stresses reveal many phenomena, e.g. flow channeling between elements, which causes high wall shear stresses, flip-flopping wakes behind the cylinders, roll-up of a horseshoe vortex, which entrains most of the BL vorticity, 3D open separation on the sides and behind the cylinder, formation of a very turbulent vortical canopy on top and in the near wake behind the cylinder, formation of multiple axial vortices originating from the horseshoe vortices and tilting of the canopy legs. |
Monday, November 25, 2024 3:21PM - 3:34PM |
R36.00008: Impacts of system yaw on secondary and tertiary flow structures generated over E-type roughness elements. Katie N Taylor, Emma Rocio Compton, Sarah E Smith, Zein Ahmad Sadek, Ondrej Fercak, Abdelhalim Abdeldayem, Raúl Bayoán B Cal, Marc Calaf Growing demand for photovoltaic (PV) solar energy has caused many open landscape surfaces to change with the adoption of PV systems. It is known that flow over natural and manmade objects, such as mountains and urban centers, create secondary vortices which can extend to the atmospheric boundary layer (ABL). These vortices are widely studied over blunt roughness types labelled only by height and separation (e.g. K-type or D-type). However, elevated profiles of PV systems are unique, even from vegetative canopies, since common variations such as spacing, angle, and flow direction are known to greatly effect local turbulence. This study proposes PV canopies as a novel type of elevated roughness (E-type) and explores how PV system configuration impacts secondary and tertiary vortices for outer flow approaching the ABL. Wind tunnel experiments were performed on a scaled PV array subjected to varied inflow conditions, inter-panel spacing, and system yaw (a). Stereoscopic particle image velocimetry data show modified secondary and tertiary motions in the reset turbulent boundary layer dependent on system configuration. This study informs not only design of future PV systems, but also introduces the complexity of E-type roughness applicable to natural and industrial environments. |
Monday, November 25, 2024 3:34PM - 3:47PM |
R36.00009: Abstract Withdrawn |
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