Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session L11: Boundary Layers: Turbulent I |
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Chair: Ricardo Garcia-Mayoral, University of Cambridge Room: 143A |
Monday, November 20, 2023 8:00AM - 8:13AM |
L11.00001: Impact of Propeller Tip Vortices on Turbulent Boundary Layers Marco Virgilio, Hulya Biler, Bharathram Ganapathisubramani Hybrid electric propulsion systems featuring Over-The-Wing (OTW) propellers offer substantial advantages, providing an increased lift-to-drug ratio. A critical issue arises from the intricate interaction between tip vortices and the turbulent boundary layer, leading potentially to flow separation. Understanding the dynamics of the intricate interactions between tip vortices and the boundary layer and the underlying causes of flow separation is crucial. To address these challenges, an experimental investigation using a 3 cameras planar Particle Image Velocimetry (PIV) system was conducted. The study focused on the propeller wake structure's effects on the turbulent boundary layer at high Reynolds numbers. The experimental analysis evaluated the propeller's impact on boundary layer development, encompassing flow statistics, as well as coherent and vortical structures. By comparing two different tip clearances, the influence of the propeller's distance from the wall on the boundary layer was assessed. Through a phase-locked and conditional analysis of vorticity and turbulent energy production mechanisms, the major sources of viscous losses were identified. The effects of periodic vortical structures, random fluctuations and non-linear interactions are separately analysed. The knowledge gained can contribute to the design and optimization of future propulsion systems for improved efficiency and performance, addressing the challenges associated with OTW propellers and advancing hybrid electric propulsion technology. |
Monday, November 20, 2023 8:13AM - 8:26AM |
L11.00002: Response of plates and compliant walls in turbulent channel flow Soham Prajapati, Sreevatsa Anantharamu, Krishnan Mahesh We simulate fluid-structure interaction of elastic plates and viscoelastic compliant walls in a turbulent channel using Direct Numerical Simulation (DNS). The impact of solid boundary conditions and material properties on the structural response is studied. The boundary conditions considered are - reflective, fixed, free, and simply supported. Intermittent large-amplitude fluctuations are present in the turbulent wall-pressure that are related to the near-wall burst-sweep events. The effect of these fluctuations on the plate response depends on the separation of the plate vibration and near-wall eddies time scales. As a result, the stiff plate vibration signal consists of similar intermittent events because of small time scale separation. However, the soft plate vibration signal consists of similar events only near the fixed or simply supported boundaries due to large time scale separation, and away from the plate boundaries, the plate vibration signal resembles an amplitude-modulated wave. |
Monday, November 20, 2023 8:26AM - 8:39AM |
L11.00003: Wavelet-based resolvent analysis of intermittently turbulent Stokes boundary layer Micah K Nishimoto, Eric Ballouz, H. 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 Stokes boundary layer systematically to study the time (phase)-localized forcing and response modes that inform the physical mechanism of this oscillatory flow with emphasis on the mechanism of temporal laminar-to-turbulent transition in the intermittently turbulent regime. |
Monday, November 20, 2023 8:39AM - 8:52AM |
L11.00004: Model-based spectral coherence analysis enhanced by scale-dependent eddy dissipation and anisotropic forcing terms Seyedalireza Abootorabi, Armin Zare The eddy-viscosity enhanced linearized Navier-Stokes equations have been shown to capture the geometric scaling of wall-coherent structures in the logarithmic layer of turbulent wall-bounded flows. Together with a source of white- or colored-in-time stochastic excitation, the eddy-viscosity term is used to compensate for the absence of nonlinear terms and their role in the production and dissipation of energy among various scales of motion. In this work, we explore the efficacy of two recently proposed enhancements to theses classes of low-complexity models, namely, the entrainment of scale-dependence in the magnitude of the dissipation term and anisotropy in the stochastic forcing term. We show the benefits of using such models in capturing the energy spectrum of turbulent channel flow at different Reynolds numbers. We also show that while their resulting linear coherence spectrum fails to demonstrate the anticipated wall-distance scaling, it excels in extracting components of the energy spectra that are attributed to active self-similar motions. Our results support the utility of such enhancements as a computationally efficient alternative to the entrainment of colored-in-time stochastic forcing models in capturing the structural features of high-Reynolds number wall-bounded flows. |
Monday, November 20, 2023 8:52AM - 9:05AM |
L11.00005: Characteristics of active and inactive motions in high Reynolds number turbulent boundary layers Rahul Deshpande, Ricardo Vinuesa, Ivan Marusic Turbulent boundary layers (TBL) at high friction Reynolds numbers (Re_{τ}) are characterized by an extended region and high level of Reynolds shear stresses in their log region. This makes the associated coherent motions of this region (i.e., the attached eddies) a significant contributor to the skin-friction drag produced by the high Re_{τ} TBL. The present study investigates the underlying mechanism behind their drag contribution by analysing the active and inactive components of the attached eddies postulated by Townsend^{1}. A recently proposed energy decomposition scheme^{2} is implemented on TBL datasets spanning a large Re_{τ} range of ∼ Ο(10^{3}) - Ο(10^{6}), to reveal the individual role played by these two components in drag generation. We find that while the active motions are solely responsible for turbulence energy production, the inactive motions transport a part of this turbulent kinetic energy from the log region to the wall, yielding empirical evidence to the hypothesis of Bradshaw^{3} for the first time. |
Monday, November 20, 2023 9:05AM - 9:18AM |
L11.00006: Interscale causality in near-wall turbulence James de Salis Young, Zengrong Hao, Ricardo Garcia-Mayoral This work aims to map how information is transferred between different lengthscales through the momentum equations in near-wall turbulence using periodic channels, with a particular focus on streamwise (x) and spanwise (z) lengthscales. The flow can then be viewed as a dynamical system where the state space variables Φ= {u,w,v,p} are the three velocity components and the pressure, which depend on the x- and z-wavenumbers k = (k_{x},k_{z}), y, and t. The evolution equations for Φ are the Navier-Stokes equations plus the continuity equation, which can then be written as F(Φ_{k,y,t}) = N(Φ_{k′,y,t},Φ_{k′′,y,t}), where k, k′, k′′ and satisfy a triadic relationship, F represents the linear part of the equations and N the nonlinear terms, which are responsible for the transfer of information from other lengthscales into lengthscale k. The objective of this work is to characterize and quantify the relative importance of the different contributions to N. While intimately connected to triadic interactions, which are derived from energy considerations, this analysis, based instead on the momentum equations, aims to provide additional cause-effect information to the inter-scale interactions. For the near-wall cycle, for instance, we observe that the most significant interactions are of w structures of wavelengths λ_{x}^{+}≈100, λ_{z}^{+}≈1000 advecting u structures of wavelengths λ_{x}^{+}≈1000, λ_{z}^{+}≈100, to produce a signature in u at wavelengths λ_{x}^{+}≈100, λ_{z}^{+}≈100. This interaction is essentially the generation of meandering in streamwise streaks. |
Monday, November 20, 2023 9:18AM - 9:31AM |
L11.00007: Measurements of coherent structures during low-drag hibernation events in turbulent boundary layer flows TAO LIU, Michael Wilkes, David Swailes, Richard D Whalley Recent experimental investigations of intrinsic low-drag states called "hibernating turbulence" revealed time intervals where the instantaneous streamwise velocity profiles approached the MDR asymptote in channel flow, even with Newtonian fluids. Here we present the detection and characterisation of this hibernating turbulence phenomenon in a turbulent boundary-layer flow. Stereo particle image velocimetry (SPIV) was conducted in a water flume to capture the intrinsic low-drag states associated with hibernating turbulence. During states of hibernating turbulence, the ensemble-averaged wall-shear stress, or skin-friction drag is up to 10% lower than the time-averaged skin-friction drag value. To measure these states, the instantaneous wall-shear stress was measured using a hot-film probe whilst, simultaneously, the three velocity components in the cross-section of the water flume were measured using stereo-PIV system. Conditionally sampling the wall-shear stress data when the instantaneous signal drops 10% below the mean value for a duration of t+ > 200 reveals intervals of hibernating turbulence. 3D coherent structures have been constructed from the stereo PIV data using Taylor hypothesis. These structures show a long low speed steaks passing through the hot-film during the low-drag states. Ensemble averaged coherent structures show pairs of long straight low-high speed streaks together with longitudinal vortices in the hibernation intervals. |
Monday, November 20, 2023 9:31AM - 9:44AM |
L11.00008: Uncovering the Nature of Large-Scale Flow Events in Turbulent Boundary Layers Using Data Driven Methodologies Surabhi Singh, Anubhav Dasgupta, Jacopo Magnani, Rahul Sengupta Turbulent boundary layers (TBLs) are complicated flows characterized by a hierarchy of flow scales. Historical research has identified large-scale spatio-temporal events in such flows. From a control standpoint, it is important to model these larger flow scales so they can be used as inputs to flow actuation. To this end, the current work attempts to isolate spatio-temporal large-scale flow events in a zero-pressure-gradient TBL flow using data driven methodologies including proper orthogonal decomposition and a U-Net based autoencoder. Such methods can help extract spatio-temporal signatures of large-scale events in the flow utilizing datasets that are readily available from Particle Image Velocimetry and Direct Numerical Simulations. The results obtained will provide insights into the non-linear and multi-scalar nature of these large-scale events, thereby better informing modeling efforts for physics-based flow control. |
Monday, November 20, 2023 9:44AM - 9:57AM |
L11.00009: Directly Characterising the Interplay between Entrainment and the TNTI in a Turbulent Boundary Layer Agastya Parikh, Christian J Kähler A plethora of natural and engineered processes are influenced or driven by turbulent flows. Since the advent of fluid mechanical research, turbulent boundary layers (TBLs) have been studied as a fundamental model through which the characteristics of turbulent transport can be understood. In recent years, a large body of research has focussed on the role of intermittency and the turbulent/non-turbulent interface (TNTI). The entrainment of external flow into the boundary layer is understood to be responsible for shaping the TNTI and thus the transfer of momentum between the near-wall and external flow. However, it is non-trivial to experimentally characterise entrainment and the TNTI itself using conventional PIV while retaining information about the outer flow. To this end, locally seeding the TBL with particles that fluoresce under laser emission while globally seeding the flow with particles that generate only Mie scattering has shown promise in directly characterising entrainment and the TNTI without compromising information about the external flow. Furthermore, it is possible to measure and characterise the flow properties of entrained regions to provide greater insights into the interaction between them and the flow within the boundary layer. |
Monday, November 20, 2023 9:57AM - 10:10AM |
L11.00010: Extension of the law of the wall exploiting the concepts of strong and weak universality of velocity fluctuations in a channel Christoffer Hansen, Jens N Sørensen, Ph.D., Xiang Yang, Mahdi Abkar, Ph.D. This work explores the universality of the instantaneous velocity profiles in a channel. In the analysis, we employ a one-dimensional scalar variant of the proper orthogonal decomposition (POD) and exploit the concepts of strong and weak universalities. Strong universality requires that all POD modes are universal with respect to the Reynolds number, while weak universality only requires that the first few POD modes are universal. As POD analysis concerns information at more than one location, these universalities are more general than various similarities and universality concerning single-point flow statistics, e.g., outer layer similarity or universality of the log law. We examine flows at Re_{τ }= 180, 540, 1000, and 5200. Strong universality is observed in the outer layer, and weak universality is found in both the inner layer and the outer part of the logarithmic layer. The presence of weak universality suggests the existence of an extension to the law of the wall (LoW). Here, we propose such an extension based on the results from one-dimensional POD analysis. The usefulness of the LoW extension is assessed by comparing flow reconstructions according to the extended LoW and the equilibrium LoW. We show that the extended LoW provides strikingly accurate flow reconstruction in the wall layer across a wide range of Reynolds numbers, capturing fine-scale motions that are entirely missed by the equilibrium LoW. |
Monday, November 20, 2023 10:10AM - 10:23AM |
L11.00011: Log-Linear Overlap Parameters Extracted from Pressure Gradient Boundary Layers Hassan M Nagib, Victor Baxerres, Ricardo Vinuesa Following on the recent paper "The hunt for the Kármán ‘constant’ revisited", by Peter Monkewitz and Hassan Nagib, in Journal of Fluid Mechanics 967, A15, 2023, and utilizing experimental data obtained under various favorable and adverse pressure gradients in the NDF wind tunnel at IIT and the MTL wind tunnel at KTH, we extracted the parameters of the "Log-Linear" overlap region. Different schemes to extract the parameters from the mean velocity profiles were evaluated on hot-wire meassured profiles over a wide range of pressure gradients, including the Zero Pressure Gradient (ZPG) boundary layers from the NDF. The Kármán “coefficient” κ, the slope of the linear component of the overlap region S_{0} and the constant B_{0} are presented as functions of the pressure-gradient parameter β, and compared to trends from ZPG boundary layers, and fully-developed channel and pipe flows. The Log-Linear overlap parameters are also contrasted with the trends of the traditional "Pure-Log" overlap parameters κ and B from "Variations of von Kármán coefficient in canonical flows" by Hassan Nagib and Kapil Chauhan, in Physics of Fluids, 20, 101518, 2008. The results and their trends are valuable not only for our basic understanding of a wide range of wall-bounded turbulent flows, but also are proposed as input to turbulence models and computational turbulence in general. |
Monday, November 20, 2023 10:23AM - 10:36AM |
L11.00012: Coles "wake function" revisited and modeled using an offset from the wall Gregoire Winckelmans, Matthieu Duponcheel We examine the wake function F(Y) with Y = y/δ: the excess velocity above the logarithmic profile in wall-bounded turbulence at high Reynolds number, as proposed by Coles. It is first measured using DNS data of channel flow at Re_{τ} ≈ 5200, which has a distinguishable overlap layer with 1/κ ≈ 2.61. The "generating function" Q(Y) = Y F'(Y) is seen to be significant only beyond Y_{c} ≈ 0.16, and also linear (with a slope α ≈ 1.15) up to Y ≈ 0.5 . Our Q(Y) model for that part is taken as a linear ramp function that starts at Y_{c}. The model is also extended for the second part: by subtracting a quadratic term which satisfies the boundary condition at Y=1. The analytical integration of the Q(Y) model finally provides the complete F(Y) wake function model with offset Y_{c}. It is seen to fit very well the DNS data over the full range. The first part of the model is also usefully compared to the "extended law of the wall model" of Bernardini et al. (a model without offset). |
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