Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session E22: Turbulence: General II and Wakes I |
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Chair: Yaqing Jin, UT Dallas Room: North 222 AB |
Sunday, November 21, 2021 2:45PM - 2:58PM |
E22.00001: Self-attenuation of extreme events in Navier–Stokes turbulence Alain J Pumir, Dhawal Buaria, Eberhard Bodenschatz Turbulent fluid flows, ubiquitous in nature and technology, are mathematically described by the incompressible Navier-Stokes equations. A hallmark of turbulence is spontaneous generation of intense whirls, resulting from amplification of the fluid rotation-rate (vorticity) by its deformation-rate (strain). This interaction, encoded in the non-linearity of Navier-Stokes equations, is non-local, i.e., depends on the entire state of the flow, constituting a serious hindrance in turbulence theory and even establishing regularity of the equations. Here, we unveil a novel aspect of this interaction, by separating strain into local and non-local contributions utilizing the Biot-Savart integral of vorticity in a sphere of radius R. Analyzing highly-resolved numerical turbulent solutions to Navier-Stokes equations, we find that when vorticity becomes very large, the local strain over small R surprisingly counteracts further amplification. This uncovered self-attenuation mechanism is further shown to be connected to local Beltramization of the flow, and could provide a direction in establishing the regularity of Navier-Stokes equations. |
Sunday, November 21, 2021 2:58PM - 3:11PM |
E22.00002: The Incompressible Dissipation Transport Equation Simon L Ribergaard, Azur Hodzic, Clara M Velte A transport equation for dissipation has been derived from Navier-Stokes for incompressible flow making no assumptions of isotropy or homogeneity. This is then compared |
Sunday, November 21, 2021 3:11PM - 3:24PM |
E22.00003: Tomo-PIV study of turbulence structure through a rapid contraction Abdullah A Alhareth, Kenneth Langley, Vivek Mugundhan, Nathan B Speirs, Sigurdur T Thoroddsen Following the study of Mugundhan et al.1 in a 2.5:1 contraction, we perform Tomo-PIV and Shake-The-Box PTV measurements to investigate the 3-D vortical structures in the turbulent flow through a more rapid contraction. We use active grids in synchronous or random modes to reach turbulent Reynolds numbers Reλ ≈220. We study three separate 1-D contractions, all with the same contraction ratio of 4, but widely different streamwise lengths. We also pass the turbulence through a contraction in two directions with an extreme area ratio of 16. Most earlier studies of similar configurations have used hot-wire anemometry, to measure point-wise velocity component time-series, while herein we obtain time-resolved volumetric velocity and vorticity fields, with up to 50,000 consecutive measurement volumes. This allows us to compare the evolution of the rms fluctuations to earlier measurements and to test the alignment and enhancement of coherent vertical structures through the strong extensional strain of the various contractions. Our focus is on how two-dimensionality of the flow is promoted by the strong mean strain. We also track individual structures to follow their stretching/compression and changing orientation with time. |
Sunday, November 21, 2021 3:24PM - 3:37PM |
E22.00004: A new high aspect ratio water channel facility to study the interaction between turbulent flow and complex surfaces Sofia Saoncella, Shervin Bagheri, Fredrik Lundell A novel water facility capable of sustaining a turbulent flow with Retau up to 2000 and an aspect-ratio of 20:1 was designed and manufactured. The water channel structure is modular so that it can be adapted or modified depending on the problem under study. In particular, the test section presents two removable plugs to accommodate different types of complex walls and the boundary conditions can thus be varied. The high versatility of the channel allows multiple measurement techniques: the drag variation induced by the complex wall is evaluated through differential pressure measurements along the central axes and flow visualisation is obtained through PIV. Optical access to the full height of the channel is provided by side windows and transparent plugs, allowing the use of alternative imaging techniques such as micro-PIV and LDV. The main purpose of the study is to investigate the interaction between turbulent flow and a complex surface, specifically liquid-infused surfaces (LIS). On one hand, the ability of the LIS to reduce drag when subjected to a turbulent flow is assessed and different types of failure (oil drainage, surfactants effect) are investigated; on the other hand, modified turbulent structures near the wall are observed and described. |
Sunday, November 21, 2021 3:37PM - 3:50PM |
E22.00005: Adjoint-variational reconstruction of turbulence in isothermal and stratified channel flow Simon S Toedtli, Qi Wang, Tamer A Zaki In many canonical and engineering turbulent flows, measurements are challenging and limited in resolution. Accurate reconstruction of the full flow state, which can significantly enhance our scientific understanding and inform engineering objectives, is a challenging inverse problem. Recent work explored adjoint-variational data assimilation to reconstruct the velocity field in isothermal turbulent channel flow from sparse data (M. Wang and T. Zaki, 2021, J. Fluid Mech. 917, A9). The results demonstrated that accurate flow reconstruction at all scales is possible when the spatio-temporal resolution of measurements satisfies criteria set by the Taylor microscale and the Lyapunov timescale. Whether stratification simply modifies these scales or also alters the criteria for accurate reconstruction is not known, and will be examined using numerical experiments. Observations are extracted at various resolutions from independent, fully resolved simulations, and the accuracy of the adjoint-variational reconstruction will be assessed against the hidden truth. |
Sunday, November 21, 2021 3:50PM - 4:03PM |
E22.00006: The effects of different RANS closures on modelling swirling flow in a macro-scale multi-inlet vortex reactor Thomas Nguyen, Alberto Passalacqua, Ehsan Madadi The swirling flow in a macro-scale multi-inlet vortex reactor (MIVR) is modeled using RANS (Reynolds-averaged Navier-Stokes) simulations. Typically, a linear eddy viscosity model such as k-ε or k-ω would be used. However, it is challenging to apply conventional RANS models to study the macro-scale MIVR because of the swirling flow, out-of-plane flow, and backflow at the center of the reactor. Furthermore, the high degrees of anisotropy, streamline curvature, rotational effects, and recirculating flow make the flow simulation difficult. Therefore, the Reynolds-Stress transport models such as the LRR [1] model and the SSG [2] model are applied to investigate the possibilities of improving flow prediction. In this study, CFD results were compared to the experimental data available in the literature [3] using different RANS models such as k-ε, RNG k-ε, Realizable k-ε, k-ω, LRR, and SSG at different Reynolds numbers. Each velocity component will be presented with respect to the radius for comparison. |
Sunday, November 21, 2021 4:03PM - 4:16PM |
E22.00007: Turbulent boundary layer flow over two side-by-side wall-mounted cylinders: Wake characteristics and aerodynamic loads. Dhanush Bhamitipadi Suresh, Emmanuvel J Aju, Yaqing Jin Experimental investigation of distinctive wake characteristics and aerodynamic loads was conducted for two wall mounted side-by-side cylinders with varying gap distances. The results indicate that with decreasing gap wake recovery can be effectively delayed as the upwash and downwash flows from top and bottom ends of cylinders are suppressed. In general, at smaller gaps, turbulence intensity levels increased at the top ends of cylinders due to strong local velocity shear. Integral time scales distribution along mid-span of the cylinder shows that local recirculation flows in the near wake region highly influence velocity fluctuations, whereas at the top end, mixing of boundary layer flows dominate them. Lift and drag force coefficients calculated by considering equivalent velocity along cylinder span show no deviation from “infinite length” cylinder case. However, unlike infinite cylinders, there was no clear intermittency of aerodynamic loads observed as three-dimensional flow effects and strong background turbulence suppress two-dimensional vortex shedding. Joint PDF distribution indicate that drag force coefficient remains unaffected by cylinder gaps but lift coefficients fluctuate stronger with growth of gap distances. |
Sunday, November 21, 2021 4:16PM - 4:29PM |
E22.00008: Decoupling wind-wave-wake interactions in a fixed-bottom offshore wind turbine Ondrej Fercak In the golden age of renewables, offshore wind-energy holds the most potential for growth, but the benefits are also entangled in dynamics not fully understood, such as the dynamic coupling of the atmospheric boundary layer, the turbine generated wake, and the surface waves. This study establishes the first experimental turbulent-interaction between the traditionally distinct fields of airflow-dynamics above the air-sea interface and turbine wakes. The experimental setup combined a wave tank, wind tunnel, and scaled fixed-bottom wind turbine. Particle image velocimetry (PIV) was performed to visualize wind-wake, wind-wave, and wave-wake interaction far downstream of the turbine. The wave phase-dependent dynamics of the turbine wake on the passing ocean-wave profile and location are outlined. The data were decomposed into incremental wave-phases revealing localized and predictable velocity maxima, stress maxima, and wake modulation that is normally obscured by a time-averaged mean. The results illustrate a more complete picture of offshore wind-energy dynamics, which have implications for insidious mechanical issues, design optimization and/or control strategies. |
Sunday, November 21, 2021 4:29PM - 4:42PM |
E22.00009: Downstream Evolution of Eddy Viscosity in the Wake of a Wind Turbine Ryan Scott, Luis Martinez-Tossas, Nicholas Hamilton, Raúl Bayoán B Cal We propose a two-scale model for the streamwise development of eddy viscosity in the wake of a wind turbine which augments the curled wake model by separating the wake flow from the background and assigning each region independent viscosities. Measurements of eddy viscosity were obtained by fitting data from wind tunnel experiments conducted at Portland State University (PSU) and SOWFA LES simulations performed at the National Renewable Energy Lab (NREL). A range of yaw and tilt misalignment angles were considered to discern the impact of wake asymmetry from the curled wake. The downstream evolution of eddy viscosity is presented as a function of distance with emphasis on differences between near and far wake behavior. Additionally, variations in eddy viscosity formation are detailed in response to nacelle misalignment. The model is implemented in the NREL FLORIS wake modeling framework and demonstrates agreement with experimental and simulation data in capturing the dynamics of both wake regions. |
Sunday, November 21, 2021 4:42PM - 4:55PM |
E22.00010: Steady waves and wake dynamics in pycnoclines Divyanshu Gola, Sheel Nidhan, Jose Luis Ortiz-Tarin, Hieu T Pham, Sutanu Sarkar Pycnoclines with nonlinear stratification occur commonly in the oceanic environment. In this study, we present the results of one of the first body-inclusive LES simulations of wakes in a pycnocline. A circular disk at diameter-based Reynolds number of 5000 is considered with three types of stratification profiles, two of which are pycnoclines and one has linear stratification. The minimum background Froude number is 1 for all the cases. The characteristics of steady lee waves are compared with the analytical results obtained from linear theory. A Kelvin ship wave pattern (absent for the linear case) is observed and is analysed by solving the Taylor-Goldstein Equation. The wakes in nonlinear stratification show an increase in fluctuation levels during the non-equilibrium regime. Simulation based diagnostics attribute the increased turbulence to enhanced lateral production and internal wave flux. Wakes evolving in pycnoclines also show diferent decay characteristics of mean defect velocity and turbulent kinetic energy compared to the linear stratification case. |
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