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 A36: Turbulent Boundary Layers I |
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Chair: Ivana Stiperski, University of Innsbruck Room: 355 B |
Sunday, November 24, 2024 8:00AM - 8:13AM |
A36.00001: Optimal estimation of log-layer turbulence using surface pressure measurements Seyedalireza Abootorabi, Miguel P Encinar, Armin Zare The estimation and control of wall-bounded flows often relies on practical sensing capabilities that are physically bound to the wall. Wall-pressure sensors provide such capabilities while maintaining a robust measurement signal in detecting flow variations away from the wall. Prior studies have investigated the efficacy of data-driven projection schemes such as linear stochastic estimation in tracking the evolution of attached eddies using wall-pressure sensing. This approach, however, relies on strong correlations between wall pressure and the velocity field in the log layer. We present an alternative framework that leverages persistently strong two-point correlations between the pressure field in the measurement and estimation planes. In this approach, said projection schemes are first used to infer pressure fluctuations in the log layer from the wall pressure. The target velocity field is subsequently obtained as the solution of the pressure Poisson equations. We demonstrate the efficacy of this approach in the context of Kalman filtering for real-time estimation of log-layer turbulence whereby wall-pressure measurements are used to correct the predictions of the stochastically forced linearized Navier-Stokes equations. The spatio-temporal coloring of the stochastic forcing is identified using the noise-modeling approach of Zare et al. (J. Fluid Mech., vol. 812, 2017) to ensure statistical consistency of the model with the results of direct numerical simulations. |
Sunday, November 24, 2024 8:13AM - 8:26AM |
A36.00002: The role of friction on the annihilation of self-similarity in a scalar field Asghar Noormohammadi, Ronald Barron, Ram Balachandar The self-similarity nature of a scalar field released from a wall source located at the bottom wall of a fully-developed turbulent channel flow is investigated. In pursuit of this objective, direct numerical simulation (DNS) was carried out for two distinct cases involving weak and strong source strengths. The findings show the absence of self-similarity in the distribution of mean concentration fields. Analysis carried out in both physical and spectral spaces revealed that the convection-diffusion mechanism and the development of short- and long- range dispersion play only a minor role in influencing the nature of self-similarity. Friction is shown to be the key factor in determining the emergence of self-similarity. |
Sunday, November 24, 2024 8:26AM - 8:39AM |
A36.00003: Dolphin-skin-inspired anisotropic flexible surfaces for turbulent drag reduction Rutvij Bhagwat, Al Shahriar, Ana de Leon, Mitesh Patadia, Burak A Tuna, Rebekah Sweat, Kourosh Shoele Skin-friction drag reduction (DR) in turbulent boundary layers (TBL) has significant economic and environmental benefits for both civil and military transportation. The study aims to investigate the potential of dolphin-skin-inspired compliant surfaces for achieving DR in TBLs. Dolphins, being among the most efficient swimmers in nature, have a skin that is a naturally anisotropic compliant material, which may offer valuable insights for designing optimal compliant surfaces for DR. Our framework integrates multi-disciplinary techniques to tackle this problem, including a resolvent-based model to determine optimal design parameters, fully coupled, high-fidelity, fluid-structure-interaction (FSI) simulations, and experimental tests using 3D-printed models and a water towing tank facility. We present an overview of our investigative framework and our progress towards the final goal, showing how all these different components are brought together to understand the causal relationships between different features of dolphin skin and drag reduction performance in high-Reynolds number flows. We further discuss the main modifications to near-wall turbulence over a compliant skin model that are associated with large skin-friction drag changes. Finally, we provide physical insights to design and manufacture optimal compliant surfaces for TBL DR. |
Sunday, November 24, 2024 8:39AM - 8:52AM |
A36.00004: Sub-Convective Pressure Fluctuations in Turbulent Boundary Layer Flows Shishir Damani, Humza Butt, William Devenport, Kevin T Lowe Interaction of turbulent boundary layers with surfaces generates vibrations due to coupling of the impinging wall pressure fluctuations with the surface modes. The coupling usually occurs at high phase speeds and sub-convective wavenumbers associated with large spatial scales as big as the surface dimensions. There are challenges in measuring these sub-convective pressure fluctuations as they are orders of magnitude weaker than the pressure fluctuations due to convective turbulence. This study characterizes sub-convective pressure fluctuations using a recently developed measurement technique. The technique uses an array of sub-resonant pressure sensors, comprising multi-neck Helmholtz resonators with microphones designed to selectively filter convective pressure fluctuations. The measured data reveals for the first time a continuous form of the wavenumber frequency spectrum especially at low wavenumbers for different flow conditions including varying pressure gradients. Comparisons of the data with existing wall pressure models reveal striking resemblance to the Chase model (first validation) with lower pressure levels suggesting the need for improved wall pressure models. PIV flow visualization technique performed in sync with the wall pressure measurements help reveal the correlation between large scale structures (velocity sources) and the sub-convective pressure fluctuations. Possible hypotheses on the sub-convective pressure fluctuations origin are presented. |
Sunday, November 24, 2024 8:52AM - 9:05AM |
A36.00005: Energy characteristics of large coherent structures in turbulent boundary layers Apratim Dasgupta, Daniel Foti While length scales in a turbulent boundary layer scale with distance from the wall, coherent structures with scales typically much larger than the integral length scale have a large modulating effect on the spectral characteristics and velocity distributions. Some highly probable motions, such as sweeps and ejections, are the result of packets of hairpin vortices. Furthermore, there are very long meandering structures: large coherent structures and very large coherent structures with scales of 2δ - 3δ and 3δ - 10δ, respectively, where δ is the boundary layer height. These large-scale features are embedded in the flow, modulate its surroundings, and affect smaller scales. The contributions of these coherent structures to the flow and kinetic energy can be quantified from turbulent fluctuations through a triple decomposition, where velocity is split into a mean, coherent, and random component. Using wall-modeled large-eddy simulations, spectral-based model decomposition of instantaneous velocity snapshots is used to filter the flow field and quantify the coherent kinetic energy. The characteristics of large structures such as the bispectrum are elucidated. The spatial organization of the large-scale flow structures is analyzed to understand the relationship between large-scale fields and smaller vortex-dominated scales. |
Sunday, November 24, 2024 9:05AM - 9:18AM |
A36.00006: Turbulent boundary layers under the influence of a sudden change in wall roughness in the streamwise direction Melika Gul, Bharathram Ganapathisubramani Turbulent boundary layers (TBLs) travelling over heterogeneous rough surfaces where surface roughness changes in the streamwise direction are commonly encountered in nature and in a wide range of engineering applications. Due to such surface transitions, a thin shear layer of disturbed flow is generated over the downstream wall. This new shear layer is called internal boundary layer (IBL), and it grows in the wall-normal direction, i.e. becomes thicker with the streamwise distance, as the flow within the IBL adjusts to the new wall condition. Although there has been considerable research on predicting the growth of IBLs based on mean flow properties since the seminal work of Elliott (1958), instantaneous behaviour of IBLs and the overall structure of TBLs under the influence of IBLs have remained under explored. Thus, this study aims at providing detailed analysis on the structure of a TBL that is subjected to an abrupt surface transition from a P60 to P24 grit sandpaper. We particularly focus on the behaviour of turbulent/non-turbulent interfaces, uniform momentum zones and their interfaces under such an abrupt surface transition, and compare these interfaces with the edges of instantaneously detected IBLs. |
Sunday, November 24, 2024 9:18AM - 9:31AM |
A36.00007: Influence of Small Gap Ratios on Wake-Induced Boundary Layer Transition Maziyar Hassanpour, Robert J Martinuzzi, Ugo Piomelli Wake-induced boundary layer transition due to a cylinder placed near a wall at ReD=3900 and Reθ=75 is investigated using direct numerical simulations. The ratio between the gap and the diameter is G=0.9. Thus, direct interaction between the vortices shed in the wake of the cylinder and the boundary layer results in flow transition. The transition mechanism is fundamentally different from that observed at high gap ratios (G>2.5), where the velocity forcing induced by the wake results in the standard bypass-transition scenarios. For G=0.9, a secondary coherent 2-D vortex forms on the wall due to the no-slip condition, which generates vorticity of the opposite sign at the wall, subsequently lifted by advection from the shed vortex. This vortex undergoes spanwise wavy deformations with lobes and troughs and eventually breaks down as it continues interacting with the wake vortices traveling close to the wall. This breakdown leads to the generation of hairpin vortices, resulting in the onset of turbulence. The wake-vortex dynamics vary significantly from one shedding cycle to the next, leading to different transition processes cycle-to-cycle. In some cases, the primary vortices in the wake directly interact with the secondary vortex, causing a faster transition to turbulence. |
Sunday, November 24, 2024 9:31AM - 9:44AM |
A36.00008: High-fidelity simulations of periodic turbulent boundary layers at high Reynolds numbers Sylvain Laizet, Saeed Parnav, Joseph O'Connor, Andrew Wynn Spatially evolving boundary layers are among the most significant canonical flows in fluid mechanics. Their behavior critically influences the aerodynamic efficiency of aircraft, road vehicles, ships, and even wind turbines through the atmospheric boundary layer. Due to the immense computational demands of simulating spatially-developing turbulent boundary layers, there has been increasing interest in periodic-based solutions. While these solutions are widely used for channel and pipe flows, they are less commonly applied to boundary layers. |
Sunday, November 24, 2024 9:44AM - 9:57AM |
A36.00009: Wavelet-based resolvent analysis of intermittently turbulent Stokes boundary layer Micah K Nishimoto, Eric Ballouz, Jane Bae In this work, we study turbulent flow over periodic oscillating walls in the intermittently turbulent regime using wavelet-based resolvent analysis. Wavelet-based resolvent analysis uses wavelet transforms, rather than Fourier transforms in time, to form the resolvent operator. This preserves both frequency and time information and allows identification of time-localized nonlinear forcing modes that are optimally amplified by the linearized Navier-Stokes operator about a turbulent mean profile. This method is applied to the intermittently turbulent Stokes boundary layer systematically to study the time-localized forcing, response, and amplification mechanisms. We first analyze the time-localized singular values at different wavelengths. Fast decay in the singular values at length scales corresponding to high intensity in the two-dimensional energy spectra is observed, indicative of low-rank behavior at energetic length scales. This alignment confirms the validity of the wavelet-based resolvent framework applied to length scales of interest, and identifying the time-localized forcing and response modes at these length scales illuminates the temporal laminar-to-turbulent transition mechanisms in the intermittently turbulent regime. |
Sunday, November 24, 2024 9:57AM - 10:10AM |
A36.00010: Stochastic generation and organization of uniform momentum zones and vortex cores in rough wall turbulence Roozbeh Ehsani, Michael Heisel, Matteo Puccioni, Jiarong Hong, Iungo Giacomo Valerio, Vaughan Voller, Michele Guala Leveraging on the spatial distribution and statistics of Uniform Momentum Zones (UMZs) attributes in rough wall turbulence, we were able to stochastically generate a 2D modal velocity field of the outer region (Ehsani et al., JFM 2024). The modal field results from the streamwise concatenation of correlated step-like velocity profiles, composed of vertically alternating UMZs separated by internal shear layers. First and second order statistics are used to compare the synthetic velocity field with the corresponding experimental datasets, along with the power spectrum of both streamwise and wall-normal velocity components, across a wide range of Reynolds numbers. One missing feature, which may be responsible for the underestimation of the Reynolds shear stress by our model, is the lack of swirling motions and vorticity outside of the shear layers. To implement this, we will first discuss the scaling and the distributions of both the size and azimuthal velocity of vortex cores extracted from our measurements in the outer region (Heisel et al. JFM 2021). Then, we will stochastically generate prograde and retrograde vortices and explore different strategies to stretch them and distribute them across the 2D generated modal velocity field, with increased spatial resolution. The overall contributions of UMZ and vortex cores to the Reynolds shear stress and turbulent kinetic energy in the logarithmic region of rough wall turbulent layers will be discussed. |
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