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 R40: Turbulent Wall Bounded Flow I |
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Chair: Joseph Klewicki, University of Melbourne Room: 355 F |
Monday, November 25, 2024 1:50PM - 2:03PM |
R40.00001: Identifying regions of importance in wall-bounded turbulence through explainable deep learning. Andrés Cremades Botella, Sergio Hoyas, Rahul Deshpande, Pedro Quintero Igeño, Martin Lellep, Will Junghoon Lee, Jason P Monty, Nicholas Hutchins, Moritz F Linkmann, Ivan Marusic, Ricardo Vinuesa Understanding the mechanisms underlying turbulent flows has been a challenge for more than a century. Traditionally, the study of these flows has relied on analyzing so-called coherent structures. One of the most common definitions of coherent structures is the intense Reynolds-stress events or Q events. This work uses an explainable-deep-learning methodology to assess the importance of the different Q events for two different databases. The first one is a simulated three-dimensional turbulent channel with a friction Reynolds number of 125. The second is an experimental turbulent boundary layer at a friction Reynolds number of 1377. The analysis reinforces that ejections (low-speed flow moving upstream and away from the wall) and sweeps (high-speed moving downstream and towards the wall) dominate the evolution of the flow, containing, those structures of higher importance, a higher Reynolds stress. However, when both magnitudes are divided by the volume of the structure, the direct relationship disappears, evidencing that the regions of higher importance in the domain are different from those regions containing higher values of the Reynolds stress. |
Monday, November 25, 2024 2:03PM - 2:16PM |
R40.00002: The lifetimes of velocity structures in turbulent channels Adal D Galvan-Castro, Miguel P Encinar, Javier Jimenez As a continuation of previous work on the dynamics of the very large velocity structures in wall-bounded flows, we compute correlation lifetimes in two channels at Reτ=950 and 5200, in a simulation box 8πh×3πh. The longest lifetimes collapse reasonably well across Reynolds numbers in terms of eddy turnovers h/uτ, although h/Ub cannot be excluded. The longest living structures are those at the longest streamwise wavelength compatible with the computational domain and λz=O(2h), centered at y≈0.5h. The three velocity components live similar amounts at the same scales, even if the energy contents of those scales can be very different. Shorter wavelengths recover the known relation for the momentum-carrying structure uτT∼λz, which also applies to the three velocity components. |
Monday, November 25, 2024 2:16PM - 2:29PM |
R40.00003: A Statistical investigation of near-wall quasi-streamwise vortices with extreme events of turbulent kinetic energy dissipation in turbulent channel flow Yedam Lee, Sang Lee The investigation of near-wall vortical structures with extreme Turbulent Kinetic Energy (TKE) dissipation rate events using Direct Numerical Simulation (DNS) of turbulent channel flow across various Reynolds number conditions, based on friction velocity, is presented. The simulation results are validated against previous DNS studies and experimental data by comparing turbulent statistics. Statistical evidence confirms the presence of quasi-streamwise (QS) vortices near regions with extreme TKE dissipation rates, consistent with earlier analyses from Zaripov's experimental studies, including joint probability density functions of velocity fluctuation components and TKE dissipation rates. The third-generation vortex identification method, Rortex, is employed to elucidate the statistical morphological characteristics of QS vortices, with a conditional average on spatial subdomains where the TKE dissipation rate exceeds tenfold the standard deviation. The temporal behavior of QS vortices during extreme TKE dissipation events is investigated using Connected Component Labeling and a temporal tracking algorithm based on the velocity of labeled regions. The lifecycle of QS vortices, including the stages of coupled counter-rotating streamwise vortices, tilting, and dissipation, is analyzed. A roll-up phenomenon is observed at the tail of QS vortices when the TKE dissipation rate peaks during the temporal evolution. |
Monday, November 25, 2024 2:29PM - 2:42PM |
R40.00004: Sensitivity study of resolution and convergence requirements for the extended overlap region in wall-bounded turbulence Sergio Hoyas, Ricardo Vinuesa, Peter J Schmid, Hassan M Nagib Even though the achievable Reynolds numbers computed by Direct numerical simulations (DNSs) are lower than those obtained through experimental means, DNS offers a clear advantage: Any desired quantity can be evaluated. This capability includes the computation of derivatives of all relevant terms. One such derivative provides the indicator function, which is crucial for understanding inner and outer interactions. It is defined as the product of the wall distance and the wall-normal derivative of the mean streamwise velocity. This derivative may depend on mesh spacing and distribution, but it is extremely affected by the convergence of the simulation. We find a clear dependence of this indicator function on the mesh distributions we examine, raising questions about classical mesh and convergence requirements for DNS and achievable accuracy. Within the framework of the logarithmic plus linear overlap region, coupled with a parametric study of channel flows and some pipe flows, sensitivities of extracted overlap parameters are examined. This study reveals a path to establishing their high-Reτ or nearly asymptotic values at modest Reynolds numbers, but larger than the ones used in this work, accessible by high-quality DNS with reasonable cost. |
Monday, November 25, 2024 2:42PM - 2:55PM |
R40.00005: Unraveling the structural dynamics of plane turbulent wall jets Harish Mangilal Choudhary, Abhishek Gupta, Shibani Bhatt, Pranav Sood, Prajyot Sapkal, Neetesh Singh Raghuvanshi, Thara Prabhakaran, Anandakumar Karipot, Shivsai A Dixit The plane turbulent wall jet is traditionally viewed as a turbulent boundary layer (TBL) below the velocity maximum, topped by a half-free jet. However, recent studies reveal that the region below the velocity maximum is far more complex than a conventional TBL, featuring a meso-layer overlap region and a counter-gradient momentum diffusion (CG) region (Gupta et al. 2020, JFM; Choudhary et al. 2024, JFM). To further investigate this complexity, the present study employs a long-field-of-view two-dimensional particle image velocimetry (2D-PIV) technique with four side-by-side cameras. Analysis of the obtained PIV fields using two independent approaches confirms the presence of an inner jet mode below the velocity maximum, which corresponds to jet-scaled structures. First, two-point correlation of binned PIV fields indicates that both outer and inner jet modes are coherent, with the inner jet mode exhibiting a different structural angle compared to typical TBLs. Second, conditional averages based on prograde swirl strength in the outer region show the presence of an inner jet mode attributed to clockwise eddies of the full-free jet, which move wall-ward with increasing Reynolds number. Finally, quadrant analysis reveals that the CG region exists due to the intrusion of outer jet-scaled structures into the region below the velocity maximum, which act as non-local events, leading to the production of negative Reynolds shear stress in the CG region. |
Monday, November 25, 2024 2:55PM - 3:08PM |
R40.00006: Wall-modeled large eddy simulation of high speed flow over a blunt ogive cone Aniruddh D Deshpande, Joel A McQuaid, Christoph Brehm, Johan Larsson The state-of-the-art in wall-modeled large eddy simulation (WMLES) has made significant advancements towards predicting the flow properties of canonical zero pressure gradient cases at high-speed conditions. Recent wall-models are capable of predicting wall quantities accurately to within 10% of direct numerical simulations (DNS) and hence hold promise towards application to more challenging test cases such as non-zero pressure gradients and cold wall conditions. In this work, we perform WMLES for high-speed flow over a blunt ogive cone with cold walls, using geometry and flow conditions from Morreale et al. (AIAA SCITECH 2023 Forum, 2023) with the objective of testing the applicability of these wall-models and to explore possibilities of further improvements in wall-modeling for more realistic cases. |
Monday, November 25, 2024 3:08PM - 3:21PM |
R40.00007: Experimental evidence of jet-mode motions below velocity maximum in plane turbulent wall jets Shivsai A Dixit, Harish Mangilal Choudhary, Abhishek Gupta, Shibani Bhatt, Pranav Sood, Neetesh Singh Raghuvanshi, Prajyot Sapkal, Anandakumar Karipot, Abhay Singh, Thara Prabhakaran Plane turbulent wall jets (PWJs) are complex wall-bounded flows formed when a plane jet is blown along a solid surface. A PWJ in a quiescent ambience is characterised by a non-monotonic mean velocity profile with maximum velocity located at a distance from the wall and velocity decreasing on its either side attaining zero value at the wall as well as far way from the wall. The part of the PWJ between the velocity maximum and the wall is traditionally considered to be akin to a turbulent boundary layer whereas the part above velocity maximum is treated as a half free jet. Our earlier work (Gupta et al. 2020, JFM) demonstrates that the mean velocity scaling does not support this framework and suggests two different scaling regions, namely full free jet region and wall region, where the former extends below the velocity maximum as well. In this work (see also Choudhary et al. 2024, JFM), we provide strong experimental evidence for the structure of a PWJ comprising of two modes - the full free jet mode and the wall mode. Novel long field-of-view PIV measurements reveal that the long-wavelength jet-mode motions are dominantly present in the region below velocity maximum and contribute significantly to the dynamics of that region. Directly computed spatial spectra (without Taylor's hypothesis) of various turbulence quantities strongly suggest this structure of PWJs. The region between the wall and the velocity maximum of a PWJ is thus far more complex than a turbulent boundary layer. |
Monday, November 25, 2024 3:21PM - 3:34PM |
R40.00008: ABSTRACT WITHDRAWN
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Monday, November 25, 2024 3:34PM - 3:47PM |
R40.00009: Modelling Pushover Transient Growth William Oxley, Rich R Kerswell The presence of wall-tangent shear, such as that set up by a streak field, gives rise to an additional source of transient growth in wall-bounded shear flows beyond the better-known `lift-up' and Orr processes. This additional, poorly studied mechanism, dubbed `pushover’ by Lozano-Duran et. al. (2021), has been found to play an important role in sustaining turbulence at least at low Reynolds numbers Re. To examine this, we consider a simple model of unbounded wall-normal constant shear to which a (spatially) sinusoidally-varying spanwise shear is added. Along with investigating the relative importance of each mechanism, of particular interest are the possible interactions between the pushover and Orr mechanisms, and the timescales that the growth processes act on. We hope to discuss whether the pushover mechanism acts on an O(Re) time scale (perhaps expected in analogy with lift-up) and if the lift-up timescale remains O(Re) or does some interaction with pushover speed that growth process up. |
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