Article
Mechanics
Zisong Zhou, Chun-Xiao Xu, Javier Jimenez
Summary: The study found that the large-scale motions in the outer region of wall-bounded flows have a top-down influence on the spanwise motion of near-wall streaks. However, the density of streaks is weakly related to LSMs. Numerical experiments also showed that near-wall streaks merging is not strongly correlated with the generation of LSMs.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
J. G. Chen, S. L. Tang, T. M. Zhou, Y. Zhou
Summary: This study investigated the very-large-scale motions (VLSMs) observed in the wake of a turbulent cylinder, using two experimental datasets. The results revealed a spectral peak at a low wavenumber in the premultiplied spectra of streamwise velocity and temperature fluctuations, indicating the presence of VLSMs. Further analysis using a proper orthogonal decomposition method confirmed the existence of prolonged coherent structures with the same frequency as the spectral peak.
Article
Mechanics
Wenkang Wang, Chong Pan, Jinjun Wang
Summary: The study investigates the role of large-scale motions (LSMs) in energy transfer using wall-parallel velocity fields at low-to-moderate Reynolds number. An anisotropic filter is designed to explore energy flux, revealing a strong connection between large-scale energy flux events and LSMs. The findings suggest that the meandering nature of LSMs could play a determining role in the process of large-scale energy transfer.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Milad Samie, Philippe Lavoie, Andrew Pollard
Summary: Through experiments and numerical simulation, coherent structures in turbulent round jets are found to consist of hierarchical eddy structures and very-large-scale motions, with the latter being dominated by helical structures.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Yanguang Long, Jinjun Wang, Chong Pan
Summary: The study reveals the significant impact of large-scale motions on the turbulent/nonturbulent interface and turbulent entrainment in turbulent boundary layers. High-speed large-scale motions enhance the engulfment process and result in a more distorted TNTI. These mechanisms are universal for turbulent boundary layers with different wall shapes.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Marco Zecchetto, Carlos B. da Silva
Summary: The study reveals that the statistics of small-scale motions within the turbulent/non-turbulent interface layer are universal, with minor deviations near the outer surface of the layer. The proposed normalization method allows for a clearer identification of the viscous superlayer and the turbulent sublayer within the TNTI layer.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Carlo Cossu
Summary: In this study, the genesis of large-scale coherent rolls in turbulent wall-bounded flows was investigated through linear stability analysis, revealing the importance of modeling turbulent Reynolds stresses for consistent predictions. The onset of large-scale convection was found to be associated with a critical friction Richardson number.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Alexandros Tsolovikos, Akshit Jariwala, Saikishan Suryanarayanan, Efstathios Bakolas, David Goldstein
Summary: This study explores the effect of selectively manipulating large-scale motions (LSMs) on the delay of separation in a moderate Reynolds number turbulent boundary layer. The performance improvement is achieved through a model predictive control scheme based on a simplified model of the flow.
Article
Mechanics
C. Chan, R. C. Chin
Summary: The study reveals that MVGs can create high-speed and low-speed regions through PVPs in a turbulent boundary layer, with varying skin friction in different areas. In addition, pre-processed energy spectra analysis shows that the velocity fluctuations induced by MVGs are mainly influenced by large-scale modes that span wavelengths and MVG lengths, but the energy peak eventually repositions to the near-wall streak scale in the streamwise direction.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
K. R. Maryada, S. W. Armfield, P. Dhopade, S. E. Norris
Summary: This study investigates the coherence of turbulent fluctuations in a turbulent vertical natural convection boundary layer immersed in a stably stratified medium. The results show that streamwise velocity fluctuations exhibit coherence over large streamwise distances, thanks to large-scale motions (LSMs) observed in the flow. The LSMs have significant meandering and contribute significantly to turbulence production in the outer layer.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Limin Wang, Ruifeng Hu, Xiaojing Zheng
Summary: The study decomposed turbulent velocities in turbulent channel flows into small-scale and large-scale components by improving the predictive inner-outer model proposed by Baars et al. The small-scale motions were found to be Reynolds-number invariant, while the large-scale ones could be well scaled by the viscous units.
Article
Physics, Fluids & Plasmas
Cheng Cheng, Lin Fu
Summary: In this study, the scale characteristics of log-and outer-region motions and structures in subsonic and supersonic turbulence are investigated using direct numerical simulations. The results show that the energy distribution among multiscale structures in the outer region is dominated by the semilocal friction-Reynolds-number effects. Additionally, self-similar structures exist in compressible wall turbulence with specific geometric characteristics.
PHYSICAL REVIEW FLUIDS
(2022)
Article
Mechanics
Yong Ji, Jie Yao, Fazle Hussain, Xi Chen
Summary: Investigation on vorticity transport in turbulent channels under large-scale active drag control reveals the significant contribution of vorticity fluctuations to frictional drag, with random and coherent components playing different roles in drag reduction. Analysis suggests that suppressing random spanwise-vorticity transport is crucial for more effective drag reduction.
Article
Mechanics
B. Viggiano, J. Bossuyt, N. Ali, J. Meyers, R. B. Cal
Summary: Wind tunnel experiments were conducted to investigate the turbulent flow over an array of roughness elements. The results showed that the roughness elements created a concentration pathway along the surfaces and the secondary motion was most prominent at the ridges of the roughness.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Engineering, Civil
Tian-Li Bo, Fang Li
Summary: Analysis of observational data reveals that dust transportation events at different scales have varying contributions. Consequently, the multi-scale characteristic needs to be considered in predicting dust transport. Based on this idea, a multi-scale dust transport model is proposed, guided by turbulent motions at different scales. The prediction results are in good agreement with observations and provide insights on the spatial variation of dust concentration and vertical dust flux at different scales. It is also found that large-scale turbulent motions can form a channel for dust upward transportation, but not all events can transport dust to the upper atmospheric surface layer. Small-scale turbulent motions do not affect the location of the dust transport channel but mainly impact the size of the dust concentration, leading to an overestimation if ignored.
JOURNAL OF WIND ENGINEERING AND INDUSTRIAL AERODYNAMICS
(2022)
Article
Thermodynamics
Xiaobo Zheng, Gabriele Bellani, Lucia Mascotelli, Ramis Orlu, Andrea Ianiro, Carlos Sanmiguel Vila, Stefano Discetti, Jacopo Serpieri, Marco Raiola, Alessandro Talamelli, Ye Li, Nan Jiang
Summary: Hot-wire measurements were conducted in the large-scale pipe-flow facility CICLoPE, showing Re-tau-independent modulation features and geometric characteristics. The study revealed the coherence and phase relations of different scale motions, with AM effects and opposite effects observed in different regions of the pipe flow.
EXPERIMENTAL THERMAL AND FLUID SCIENCE
(2022)
Editorial Material
Engineering, Multidisciplinary
Andrea Sciacchitano, Stefano Discetti
MEASUREMENT SCIENCE AND TECHNOLOGY
(2022)
Article
Thermodynamics
Rodrigo Castellanos, Theodoros Michelis, Stefano Discetti, Andrea Ianiro, Marios Kotsonis
Summary: This study experimentally investigates the effect of streamwise plasma vortex generators on the convective heat transfer of a turbulent boundary layer. The results show that the plasma-induced vortices are stationary and confined across the spanwise direction due to the action of the plasma discharge. The flow-field measurements reveal a low-velocity region caused by a mass- and momentum-flux deficit within the boundary layer, leading to a reduction in convective heat transfer. Near the wall, the plasma-induced jets divert the main flow, further decreasing the convective heat transfer.
EXPERIMENTAL THERMAL AND FLUID SCIENCE
(2022)
Article
Mechanics
R. Castellanos, G. Y. Cornejo Maceda, I de la Fuente, B. R. Noack, A. Ianiro, S. Discetti
Summary: This paper presents a comparative assessment of machine-learning methods for active flow control. The study focuses on drag reduction of a two-dimensional Karman vortex street past a circular cylinder at a low Reynolds number. The results show that both Deep Reinforcement Learning (DRL) and Linear Genetic Programming Control (LGPC) successfully reduce the drag and stabilize the vortex alley. DRL demonstrates higher robustness, while LGPC identifies compact and interpretable control laws using only a subset of sensors.
Article
Thermodynamics
Rodrigo Castellanos, Gianfranco Salih, Marco Raiola, Andrea Ianiro, Stefano Discetti
Summary: The experiment investigates the convective heat transfer enhancement in a turbulent boundary layer using a pulsed, slot jet in crossflow. A parametric study on actuation frequencies and duty cycles is conducted. The results show that both jet penetration and overall Nusselt number increase with increasing duty cycle. The flow topology is significantly altered by the jet pulsation, with a wall-attached jet rising from the slot accompanied by counter-rotating vortices.
APPLIED THERMAL ENGINEERING
(2023)
Article
Thermodynamics
Iacopo Tirelli, Andrea Ianiro, Stefano Discetti
Summary: We present a novel end-to-end approach to enhance the resolution of Particle Image Velocimetry (PIV) measurements. Our method utilizes information from different snapshots to obtain high-resolution flow fields and uncertainty estimations with minimal user intervention.
EXPERIMENTAL THERMAL AND FLUID SCIENCE
(2023)
Article
Mechanics
E. Farzamnik, A. Ianiro, S. Discetti, N. Deng, K. Oberleithner, B. R. Noack, V. Guerrero
Summary: We propose a novel nonlinear manifold learning method using Isomap as encoder and K-nearest neighbours algorithm as decoder, and demonstrate its superiority over POD for shedding-dominated shear flows. The method is applied to numerical and experimental datasets including fluidic pinball, swirling jet and wake behind tandem cylinders, and is able to describe the bifurcation, chaotic regime and shedding phases of the flow. The reconstruction error of the manifold model is small, indicating that the low embedding dimensions contain the coherent structure dynamics.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Engineering, Multidisciplinary
Stefano Discetti, Yingzheng Liu
Summary: Advancements in machine-learning techniques are driving a paradigm shift in image processing, and optical techniques play an important role in flow diagnostics. This perspective reviews the recent advancements in machine learning methods for flow field measurements and highlights possible routes for further developments.
MEASUREMENT SCIENCE AND TECHNOLOGY
(2023)
Article
Thermodynamics
Rodrigo Castellanos, Andrea Ianiro, Stefano Discetti
Summary: The convective heat transfer in a turbulent boundary layer (TBL) on a flat plate is enhanced using an artificial intelligence approach based on linear genetic algorithms control (LGAC). The actuator is a set of six slot jets in crossflow aligned with the freestream. The optimal controller yields a slightly asymmetric flow field and the LGAC algorithm converges to the same frequency and duty cycle for all the actuators. The results pinpoint the potential of machine learning control in unravelling unexplored controllers within the actuation space.
APPLIED THERMAL ENGINEERING
(2023)
Article
Thermodynamics
Iacopo Tirelli, Andrea Ianiro, Stefano Discetti
Summary: Particle Image Velocimetry (PIV) estimates velocities through particle image correlations, which leads to a modulation effect. To exploit the scattered data from Particle Tracking Velocimetry (PTV), interpolation on a structured grid is necessary, causing spatial modulation bias. A technique called Ensemble Particle Tracking Velocimetry (EPTV) is introduced to reduce this systematic error by merging different instantaneous realizations and obtaining high-resolution mean flow. The methodology is validated against various datasets with increasing complexity, using PTV and PIV analysis.
EXPERIMENTAL THERMAL AND FLUID SCIENCE
(2023)
Article
Computer Science, Artificial Intelligence
Alejandro Guemes, Carlos Sanmiguel Vila, Stefano Discetti
Summary: In this paper, a super-resolution generative adversarial network framework is proposed to estimate field quantities from random sparse sensors. The algorithm utilizes random sampling to provide incomplete views of the high-resolution underlying distributions and has been tested on synthetic databases, showing excellent performance.
NATURE MACHINE INTELLIGENCE
(2022)
