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 X36: Boundary Layer Pressure Gradient Effect II |
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Chair: Chang Liu, University of Connecticut Room: 355 B |
Tuesday, November 26, 2024 8:00AM - 8:13AM |
X36.00001: Resolvent analysis of high Reynolds number turbulent boundary layers with streamwise pressure gradient histories Prateek jaiswal, Tomek M Jaroslawski, Thomas Preskett, Salvador Rey Gomez, Shilpa Vijay, Beverley J McKeon, Bharathram Ganapathisubramani Mean pressure gradient histories and associated wall-shear stress play an overarching role in turbulent flows relevant to engineering applications. The relative importance of these two parameters is captured through β and a series of wind tunnel experiments were carried out at Southampton to explore the effects of varied β histories on the structure of boundary layer. Measurements were obtained over a large domain at Reζ ≈10,000, on flow over smooth and rough walls subjected to a family of favorable and adverse pressure gradients. This study extends the current state-of-the-art by exploring flows at high Reynolds numbers with novel sets of measurements for non-equilibrium boundary layers with and without surface roughness. A low-order representation of this problem is obtained by applying resolvent analysis to experimental mean-flow data. In particular, the biglobal resolvent analysis is used to capture nonparallel flow effects, while a local resolvent analysis is performed at a downstream location where β≈0. The rank-1 model of resolvent modes is shown to predict the shape of the energy spectrum, and thus accounts for history effects. The framework shows that the convective non-normality leads to the amplification of large-scale motions in the outer region and universality of very large-scale motions for non-equilibrium boundary layers at high Reynolds number. |
Tuesday, November 26, 2024 8:13AM - 8:26AM |
X36.00002: An Integral Boundary Layer Model for Superhydrophobic Drag Reduction in General Flows Paolo Luzzatto Fegiz, Chang-Jin Kim There have been major recent advances in understanding and achieving superhydrophobic drag reduction. For practical applications (e.g. watercraft), a low-order model valid for flow over curved surfaces and with pressure gradients is still needed. Although no-slip versions of these flows can be handled by integral boundary layer (IBL) techniques, such methods rely on closures that apply only to no-slip flows. To extend the applicability of existing closures, we introduce a mapping between the properties of a "slip" boundary layer and those of an "equivalent" no-slip flow. In laminar flow, we test our approach using Falkner-Skan solutions with slip, and find that slip-flow properties are correctly mapped onto the no-slip parametrization, for any pressure gradient. For turbulent flow, model predictions with slip are consistent with published DNS and with prior log-law models (e.g. Tomlinson et al 2023). To examine general geometries, the mapping is incorporated into a code that hierarchically considers the outer flow, the IBL, and the sublayer, inclusive of air viscosity and of surfactant-induced stresses. The model's predictions compare favorably to turbulent experiments. The resulting code can predict superhydrophobic flow past general slender bodies. |
Tuesday, November 26, 2024 8:26AM - 8:39AM |
X36.00003: Evolution of a turbulent boundary layer over an axisymmetric body of revolution Peter Manovski, Dominic Loveday, Matteo Giacobello, Charitha M De Silva, Nicholas Hutchins, Ivan Marusic Axisymmetric bodies of revolution (ABOR) are representative of many streamlined or aerodynamic platforms. The turbulent boundary layer (TBL) over such geometries is important as it contributes significantly to drag, structural vibrations and flow noise. In this work, measurements of a TBL over an ABOR were made to study the effects of pressure gradient and curvature. Experiments were conducted in a wind tunnel with a friction Reynolds number range of Reτ = 1000 – 2600 over the geometry. A seeding rake with 100 nozzles dispersed helium-filled soap bubbles as tracers, which were illuminated by high-powered LED arrays for tracking by three high-speed cameras with acquisition frequencies up to 20 kHz. Three-dimensional Lagrangian Particle Tracking (LPT) was obtained using the shake-the-box algorithm, and ensemble bin-averaging of particle tracks provided turbulence statistics on a regular grid. |
Tuesday, November 26, 2024 8:39AM - 8:52AM |
X36.00004: Direct Measurement of Wall Shear Stress in Turbulent Boundary: Analyzing the Impact of Pressure Gradient Sequences Marco Mattei, Theresa A Saxton-Fox This study focuses on measuring mean and fluctuating wall shear stress in a turbulent boundary layer over a flat plate using a streamwise array of four capacitive shear stress sensors. Measurements are conducted under zero-pressure gradient (ZPG) conditions, as well as under sequences of steady and spatially varying favorable and adverse pressure gradients (FAPGs). The pressure gradients are imposed on the boundary layer through a deformable false ceiling (Parthasarathy & Saxton-Fox, 2022). The sensors are flush-mounted in the adverse pressure gradient region, and the total extent covered by them spans from 1.2δ to 0.6δ at friction Reynolds numbers 660 and 2600, respectively. |
Tuesday, November 26, 2024 8:52AM - 9:05AM |
X36.00005: Validation and Improvement of RANS models for Turbulent Flow over a Three-Dimensional Bump Vishal A Wadhai, Shyam S Nair, Abdullah Geduk, Xiang Yang, Robert F Kunz
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Tuesday, November 26, 2024 9:05AM - 9:18AM |
X36.00006: New momentum integral equation applicable to boundary layer flows under arbitrary pressure gradients Tie Wei, Zhaorui Li, Yanxing Wang By incorporating the traditionally overlooked advective term in the wall-normal momentum equation, a new momentum integral equation is developed for two-dimensional incompressible turbulent boundary layers under arbitrary pressure gradients. The classical K\'arm\'an's integral arises as a special instance of the new momentum integral equation when the pressure gradient is weak. The new momentum integral equation's validity is substantiated by direct numerical simulation data. Unlike the classical K\'arm\'an's integral, which is limited to predicting wall shear stress within mild pressure gradients, the new momentum integral equation accurately computes wall shear stress across a broad range of pressure gradients, even in the presence of strong adverse pressure gradients (APG) that lead to flow separation. Moreover, a new pressure parameter $\beta_\kappa$ is introduced through examining terms in the new momentum integral equation. This parameter naturally quantifies the pressure gradient's influence on turbulent boundary layers and offers guidance for applying the classical K\'arm\'an's integral. Additionally, to facilitate experimental determination of wall shear stress under strong pressure gradients, an approximate integral equation is proposed that relies solely on easily measurable variables. Validation against DNS data demonstrates that this simplified equation provides reasonably accurate estimates of wall shear stress in turbulent boundary layers experiencing strong pressure gradients. |
Tuesday, November 26, 2024 9:18AM - 9:31AM |
X36.00007: Sensor-based blended wall-modeled LES of the flow over a NASA hump Naili Xu, Ivan Bermejo-Moreno Wall-bounded flows subjected to pressure-driven relaminarization present significant challenges for wall-modeled large-eddy simulations with equilibrium wall models, which assume the boundary layer to be fully turbulent, leading to inaccurate predictions of skin friction. We present a numerical study of the relaminarizing flow over the NASA wall-mounted hump that applies a recently introduced blended wall-modeling approach that integrates a local relaminarization sensor. This sensor-based blended wall model, previously validated on the relaminarizing flow over a Gaussian-shaped bump geometry, blends an equilibrium shear-stress wall model with a laminar wall model, incorporating hysteresis time along fluid pathlines. We analyze the configuration with a freestream Mach number of 0.2 and a Reynolds number Reθ≈7000 based on the momentum thickness of the flow upstream of the hump. Results will be compared with experimental data from Greenblatt et al. (2006) and Naughton et al. (2006), as well as WRLES results from Uzun & Malik (2017). The mean velocity and stresses in the target region will be analyzed to characterize the flow features. A systematic study regarding the effects of the wall-model exchange height and the grid resolution will be presented. |
Tuesday, November 26, 2024 9:31AM - 9:44AM |
X36.00008: Pressure Gradient and Surface Curvature Effect on the Turbulent/Non-Turbulent Interface Nicholas Zhu, Zheng Zhang, Robert J Minniti, Ebenezer P Gnanamanickam, John G Leishman The turbulent boundary layer (Reτ=2700) over a generic submarine shape (Suboff) was measured in a wind tunnel using particle image velocimetry. The study focused on the turbulent/non-turbulent interface from 70% to 95% of the body length, where pressure gradient and wall curvature effects were significant. To this end, a local kinetic energy criterion was modified to better demarcate the irrotational flow in boundary layers with longitudinal streamline curvature. A study of the interface location revealed that the mean interface was shifted closer to and further from the wall by favorable and adverse pressure gradients (FPG, APG), respectively. In contrast, the variance of the interface location exhibited the opposite trend, showing an increase with the FPG and a decrease with the APG. Analysis of the interface geometry suggested that small-scale “nibbling” on the order of the Taylor microscale was invariant, whereas the large-scale motions on the order of the boundary layer thickness gradually increased with downstream distance. |
Tuesday, November 26, 2024 9:44AM - 9:57AM |
X36.00009: Application of multi-timescale wall model to LES of flow over periodic hills including pressure gradient effects Ho Jun Kim, Tamer A Zaki, Charles Meneveau The multi-timescale (MTS) wall model has demonstrated successful predictions for various non-equilibrium flows (Fowler et al. JFM 2022, 2023). However, applications of the MTS wall model have been mostly limited to flows with simple geometries such as channel flows. To assess the effectiveness of MTS wall model in more complex geometries, the MTS and equilibrium wall models are applied to the flow over periodic hills, at a bulk Reynolds number Re= 10,595. The flow experiences the influence of separation, reattachment, wall curvature, and an additional small separation bubble on the windward side of the hill. Particularly, the flow is accelerated in the windward side of the hill with a strong favorable pressure gradient. Therefore, as expected the classical equilibrium wall model without pressure gradient contributions fails to accurately predict the wall shear stress peak value and its location. This underscores the importance of incorporating pressure gradient effects into wall modeling for this flow. Similarly, when the MTS wall model is applied to the periodic hill case, the wall shear stress peak value in the windward side is underpredicted. This underprediction is because the turbulent non-equilibrium part of MTS relies on an assumed near-wall velocity profile without pressure gradient. To address this deficiency, a velocity profile incorporating pressure gradient effect is used in the present study. With the velocity profile, the wall shear stress profile predicted by the MTS wall model shows good agreement with the reference DNS result. Reynolds number effects are explored by comparing MTS with data at Re=37,000. |
Tuesday, November 26, 2024 9:57AM - 10:10AM |
X36.00010: Asymptotic behavior of velocity spectra and variances in wall-bounded turbulence Sergio Pirozzoli We analyze the behavior of the streamwise velocity spectra in wall-bounded turbulent flow using DNS data from channel and pipe flows up to Reτ ≈12000. The data consistently indicate the presence of a near-wall layer where the velocity spectra exhibit a power-law behavior, distinctly different from the k-1 predicted from the attached-eddy model. This deviation is attributed to the formation of turbulent Stokes layers underneath the attached eddies. The key implication is that the near-wall footprint of superstructures gradually diminishes with increasing Reynolds number, and the streamwise velocity variance follows a defect power law, with universal wall scaling being restored in the infinite Reynolds number limit. |
Tuesday, November 26, 2024 10:10AM - 10:23AM |
X36.00011: Modal decomposition of high-Reynolds-number pipe flow Daniele Massaro, Philipp Schlatter, Jie Yao, Saleh Rezaeiravesh, Edgardo J Garcia, Fazle Hussain Large-scale coherent structures in incompressible turbulent pipe flow are studied for a wide range of Reynolds numbers (Reτ rangin from 180 to 10000), based on the recent data from high-order direct numerical simulations. Employing three-dimensional proper orthogonal decomposition (POD) and a novel weighting approach based on the Voronoi diagram, we identify and classify statistically coherent structures based on their location, dimensions, and Reynolds number. With increasing Reτ, two distinct classes of structures become most energetic, associated to wall-attached and detached eddies. We will discuss in detail the procedure of extraction of the modes, and the weighting based on energy, and the interaction between modes. The described two classes of structures are described in terms of their spatial characteristics, including radial size, helix angle, and azimuthal self-similarity. Eventually, the spatial distribution may help explain the differences in mean velocity between pipe and channel flows, as well as in modelling large and very-large-scale motions (LSM and VLSM), as it becomes clear that channel flows will have very different structures in the channel centre, as the geometrical confinement is very different. In addition, we will also discuss a new data set for pipe flow, obtained at the comparably high Ret of 10000, and make initial steps towards modal decomposition. |
Tuesday, November 26, 2024 10:23AM - 10:36AM |
X36.00012: Direct numerical simulation benchmarks for the prediction of boundary layer bypass transition in the narrow sense Xiaohua Wu, Carlos A Gonzalez, Rahul Agrawal, Parviz Moin We report a comprehensive set of direct numerical simulation benchmark statistics of bypass transition in the narrow sense with inlet freestream turbulent intensity levels 0.75%, 1.5%, 2.25%, 3.0% and 6.0%, respectively. Detailed descriptions of length scales and the rate of viscous dissipation are provided. Additionally, an intermittency correlation formula is proposed and compared with literature data, and the classical Abu-Ghannam and Shaw correlation (J. Mech. Eng. Sci. 1980) is found to describe the beginning, rather than the completion, of the late transitional stage. Further, accompanying RANS simulations were performed using OpenFOAM as a representative solver. Strong sensitivities are observed in the choice of the turbulence length scale used in the boundary condition for the k − ω SST and γ − Reθ models. Finally, we also found that the boundary layer freestream turbulence has a decay rate and length scales similar to those of its counterpart in a stand-alone spatially developing isotropic turbulence flow without the wall. |
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