Article
Mechanics
Mohammad Umair, Sedat Tardu
Summary: Direct numerical simulations were conducted to investigate the drag reduction mechanism in turbulent channel flow with spanwise wall oscillations in the form of streamwise travelling waves (STW). The study revealed that the primary effect of the STW forcing is to attenuate the spanwise turbulent enstrophy at the wall, which is linked to the fluctuating wall shear stress. The suppression of the wall-normal turbulent enstrophy is considered to be subordinate.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Kushal U. Kempaiah, Fulvio Scarano
Summary: This study quantifies the distortions of turbulent structures induced by spanwise wall oscillations through feature analysis and compares the results with statistical analysis to understand the mechanism of drag reduction. The findings suggest that the rear region close to the wall is primarily affected by the wall motion, inhibiting hairpin auto-generation and resulting in different organizations of turbulent structures in the near-wall region. The reduction of low-speed streaks and ejections further supports the hypothesis of rapid lateral distortion being responsible for drag reduction.
Article
Mechanics
Esther Mateling, Marian Albers, Wolfgang Schroder
Summary: Investigation of the alteration of the near-wall flow field in a turbulent boundary layer flow subjected to spanwise travelling transversal surface waves at a friction Reynolds number Re tau asymptotic to 1525 reveals that this flow control method induces periodic large-scale bursts near the wall and reduces the energetic content of small-scale features. The occurrence of intense large-scale ejections near the wall is crucial for reducing wall-shear stress, as they balance large-scale sweeps from the outer layer, thus attenuating overall fluctuation intensity. Additionally, the periodic secondary flow field interferes with the quasi-streamwise vortices near the wall, deforming their cross-section into an elliptic shape and resulting in vortex disintegration, which, combined with the effect of large-scale ejections, leads to a decrease in wall-normal momentum exchange and the weakening of near-wall streaks.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
A. Rouhi, M. K. Fu, D. Chandran, A. Zampiron, A. J. Smits, I. Marusic
Summary: Turbulent drag reduction through streamwise travelling waves is investigated over a wide range of Reynolds numbers. Wall-resolved large-eddy simulations are conducted to examine how the frequency and wavenumber of the travelling wave influence the drag reduction. The study finds that the level of turbulence attenuation, and hence drag reduction, changes with the near-wall Stokes layer protrusion height. A range of frequencies is identified where the Stokes layer attenuates turbulence and increases the drag reduction, while outside this range, the strong Stokes shear strain leads to a drop in drag reduction.
JOURNAL OF FLUID MECHANICS
(2023)
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
D. Chandran, A. Zampiron, A. Rouhi, M. . K. Fu, D. Wine, B. Holloway, A. J. Smits, I. Marusic
Summary: This paper presents measurements of turbulent drag reduction in boundary layers at high friction Reynolds numbers. The study finds that using streamwise travelling waves of spanwise wall oscillations can effectively reduce turbulence drag, especially with lower frequency actuation that saves energy.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Thermodynamics
Qiang Yang, Yongmann M. Chung
Summary: Travelling waves induced by spanwise Lorentz force for skin-friction drag reduction were studied using direct numerical simulations. Both the streamwise and spanwise travelling waves showed regions of drag reduction (DR) and drag increase (DI). The streamwise travelling wave of spanwise Lorentz force had the same DR and DI regions as the streamwise travelling wave of spanwise wall velocity, while the spanwise travelling wave had a DR pattern similar to that of spanwise oscillating Lorentz force. The study also explored oblique travelling waves and found that optimal drag reduction occurred when the wave travelled backward relative to the flow direction.
FLOW TURBULENCE AND COMBUSTION
(2023)
Article
Thermodynamics
Yusuke Nabae, Koji Fukagata
Summary: Direct numerical simulation of a fully developed turbulent channel flow controlled using a streamwise traveling wave reveals that the spanwise variation of the wave affects the drag reduction effect, with larger wavelengths resulting in more significant reductions. The flow field becomes more uniform in the streamwise direction but less uniform in the spanwise direction with the wave-machine-like traveling wave compared to the spanwise-uniform traveling wave.
FLOW TURBULENCE AND COMBUSTION
(2022)
Article
Mechanics
Xu Liu, Hongbo Zhu, Yan Bao, Dai Zhou, Zhaolong Han
Summary: In this study, direct numerical simulations are conducted to investigate the control method of turbulent pipe flow under streamwise-varying wall rotation. The results show that increasing the wavelength is a better choice for improving control efficiency than increasing the amplitude. The rotational effect leads to the formation of a spatial Stokes layer, which has different effects on drag reduction depending on its thickness. Furthermore, it is found that achieving turbulence relaminarization requires a sufficient thickness of the Stokes layer and velocity amplitude.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Engineering, Aerospace
Qiang Liu, Zhenbing Luo, Lin Wang, Guohua Tu, Xiong Deng, Yan Zhou
Summary: Direct numerical simulations were conducted on a spatially developing Ma 2.25 supersonic turbulent boundary layer with streamwise-striped wall blowing for turbulence drag reduction. It was found that despite weak control amplitudes, SSB can result in drag reduction effects. Analysis using compressible Renard-Deck decomposition revealed that the spatial growth term is mainly responsible for turbulence drag reduction.
AEROSPACE SCIENCE AND TECHNOLOGY
(2021)
Article
Mechanics
Azadeh Jafari, Beverley J. McKeon, Maziar Arjomandi
Summary: The potential of frequency-tuned surfaces as a passive control strategy for reducing drag in wall-bounded turbulent flows is investigated using resolvent analysis. It is shown that wall impedance can suppress the modes resembling the near-wall cycle and the very-large-scale motions and the Reynolds stress contribution of these modes. Furthermore, a wall with only shear-driven impedance is found to suppress turbulent structures over a wider range in spectral space, leading to an overall turbulent drag reduction.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Engineering, Mechanical
Peter T. T. Nagy, Andras Szabo, Gyoergy Paal
Summary: Compliant coatings capable of delaying laminar-turbulent transition have been investigated. Previous research showed the benefit of streamwise motion of the coating in delaying the transition. In this study, a coating with purely in-plane motion is explored, and it is proven to delay laminar-turbulent transition without introducing new convective instabilities.
JOURNAL OF FLUIDS AND STRUCTURES
(2022)
Article
Mechanics
Xuebo Li, Guohua Wang, Xiaojing Zheng
Summary: This study investigates the logarithmic decay of turbulence intensity for wall-attached eddies in the spanwise direction, using linear coherence spectrum analysis to separate the coherent and incoherent portions. It identifies how the turbulence intensity scaling parameter increases with the Reynolds number, and provides a parameterization that can be used to enhance existing near-wall models.
Article
Mechanics
Rui Ma, Zheng-hong Gao, Shu-sheng Chen, Dong Li
Summary: In this paper, the authors numerically confirm the delay in the transition from laminar to turbulent flow in a compressible boundary layer induced by spanwise wall oscillation. They found that the Stokes boundary layer induced by the oscillation affects the streamwise vortices, weakening their instability and leading to a delayed transition. Furthermore, the oscillation enhances flow compressibility and stabilizes the streamwise vortices, also causing a delayed transition.
Article
Mechanics
Xuerui Mao, Bofu Wang
Summary: Active and passive controls for drag reduction in flow around a cylinder are obtained by computing the sensitivity of drag with respect to surface velocity perturbations and roughness, respectively. Both controls are concentrated around the separation line and localized in the spanwise direction, producing suction effects to the separating boundary layers. The control induces localized vertical displacements and streamwise stretches in the wake, delaying vortex shedding and pushing the local pressure minimum away from the cylinder, resulting in drag reduction of 20% at maximum control velocity 2% of the free-stream velocity.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Biotechnology & Applied Microbiology
Alessandro Chiarini, Maurizio Quadrio
Summary: A numerical study of algal cell motion in a thin-layer-cascade photobioreactor reveals that cells experience a light/dark cycle with a time scale ranging from 0.1-2 seconds during turbulent liquid culture. This time scale significantly benefits algae productivity compared to steady illumination.
JOURNAL OF APPLIED PHYCOLOGY
(2021)
Article
Computer Science, Interdisciplinary Applications
A. Chiarini, M. Quadrio, F. Auteri
Summary: The new finite-difference solver for the incompressible Navier-Stokes equations utilizes the direction-splitting method and a co-located grid approach, achieving high performance levels on thousands of processors through parallelization with the Schur-complement method. Several test cases were proposed to assess the accuracy and efficiency of the method.
JOURNAL OF COMPUTATIONAL PHYSICS
(2021)
Article
Mechanics
A. Chiarini, M. Quadrio, F. Auteri
Summary: The proposed scaling law predicts the onset of the primary Hopf instability in steady flow past two-dimensional symmetric bluff bodies by using the spatial extent of the separation bubble and the largest reverse-flow speed as scales. The Reynolds number collapses nicely for bodies of different shapes and aspect ratios, even with a small angle of attack perturbing the symmetry, and provides insight into the flow mechanism producing instability.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Alessandro Chiarini, Mariadebora Mauriello, Davide Gatti, Maurizio Quadrio
Summary: This study investigates the interaction between small- and large-scale structures in a turbulent plane Couette flow using the anisotropic generalized Kolmogorov equations. Both bottom-up and top-down processes are considered, with a particular focus on the wall-normal direction. The findings reveal the involvement of large-scale motions in the production and redistribution of turbulent fluctuations, with complex interactions and cascades occurring within different scale ranges.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
A. Chiarini, M. Quadrio, F. Auteri
Summary: This study investigates the shedding phenomenon and interaction between vortices in elongated rectangular cylinders at moderate Reynolds numbers. It is found that with increasing aspect ratio, two different flow behaviors are observed, and there is hysteresis in the Strouhal number near certain aspect ratios.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Biophysics
A. Schillaci, M. Quadrio
Summary: This paper presents a computational fluid dynamics study of the airflow in human nasal cavities and evaluates the influence of the main modeling assumptions. The choice of numerical scheme is found to be the most important aspect, with noticeable impacts on the solution. The study calls for an improved standard in describing and standardizing this type of numerical research.
JOURNAL OF BIOMECHANICS
(2022)
Article
Mechanics
Maurizio Quadrio, Alessandro Chiarini, Jacopo Banchetti, Davide Gatti, Antonio Memmolo, Sergio Pirozzoli
Summary: In this study, flow control for reducing turbulent skin-friction drag is applied to a transonic airfoil, resulting in improved aerodynamic performance. The control method involves inducing spanwise forcing on a portion of the suction side of the airfoil. This not only reduces friction locally, but also modifies shock waves, leading to increased lift and decreased drag. The findings suggest that skin-friction drag reduction can be used as a tool to improve the overall aerodynamics of complex flows.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Chemistry, Multidisciplinary
Lorenzo Cacciatori, Carlo Brignoli, Benedetto Mele, Federica Gattere, Celeste Monti, Maurizio Quadrio
Summary: This study assesses for the first time the effects of riblets on the total aerodynamic drag of a low-speed UAV using RANS simulations. The results show that installing riblets with optimal size can significantly reduce the drag coefficient of the aircraft, and installing riblets on the wing can further decrease the drag, improving the cost-benefit ratio.
APPLIED SCIENCES-BASEL
(2022)
Article
Mechanics
Alessandro Chiarini, Davide Gatti, Andrea Cimarelli, Maurizio Quadrio
Summary: This study investigates the turbulent flow past a rectangular cylinder to understand the contribution of small and large scales to the sustaining mechanism of velocity fluctuations. The findings reveal forward and reverse energy transfers in the flow at the same time, with independent sources of velocity fluctuations acting at different scales.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Alessandro Chiarini, Maurizio Quadrio, Franco Auteri
Summary: The three-dimensional instability of the flow past a 5:1 rectangular cylinder is investigated using Floquet analysis and direct numerical simulations. A quasi-subharmonic unstable mode is found to become unstable at Reynolds number Re≈480, with a spanwise wavelength approximately three times the cylinder thickness. The structural sensitivity analysis reveals that the instability is triggered by the mutual inviscid interaction of vortices generated by the leading edge shear layer.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Physics, Fluids & Plasmas
Emanuele Gallorini, Maurizio Quadrio, Davide Gatti
Summary: This study investigates the effect of streamwise-traveling waves of spanwise wall velocity (StTW) on the quasistreamwise vortices (QSV) in the near-wall region of turbulent channels. The study finds that StTW significantly impact the wall-normal distribution of the vortex population and that the contributions of QSV to the quadrant Reynolds shear stresses change significantly during the control cycle.
PHYSICAL REVIEW FLUIDS
(2022)
Article
Mechanics
Federica Gattere, Alessandro Chiarini, Emanuele Gallorini, Maurizio Quadrio
Summary: Exact budget equations are derived for the coherent and stochastic contributions to the second-order structure function tensor. These equations extend the anisotropic generalised Kolmogorov equations by considering the coherent and stochastic parts of the Reynolds stress tensor. They are useful for statistically describing turbulent flows with periodic or quasi-periodic features and provide a detailed description of the scale-space interaction among mean, coherent and stochastic fields.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Giulio Foggi Rota, Alessandro Monti, Marco E. Rosti, Maurizio Quadrio
Summary: We demonstrate that the energy required for turbulent flow in a straight duct can be reduced by modulating the pumping power in time. Our control strategy utilizes a temporally periodic pumping pattern to alternate the flow between quasi-laminar and turbulent states. A computational study employing direct numerical simulations shows the significant drag reduction achieved by this technique. Further evaluation is needed to determine the best performance of this control method.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Physics, Fluids & Plasmas
Federica Gattere, Alessandro Chiarini, Maurizio Quadrio
Summary: In this paper, a comprehensive review of the available information on the effect of dimples on friction drag is provided, along with a discussion on the reasons for contradictory findings in the literature. Guidelines for future studies are also presented.
Article
Construction & Building Technology
Alessandro Chiarini, Maurizio Quadrio
Summary: The present study investigates the geometric representation of the leading-edge (LE) corners in BARC flow through Direct Numerical Simulation at a relatively low turbulent Reynolds number. The effects of small curvature radii on the mean flow fields are examined, as well as the impact on turbulent kinetic energy and its production, dissipation, and transport. Additionally, an analytical correction approach using the exact analytical solution of the Stokes problem is utilized to restore solution accuracy near the fully sharp LE corners. The findings contribute to a better understanding of existing numerical and experimental BARC data discrepancies and provide valuable insights for developing high-quality numerical simulations at high Reynolds numbers.
WIND AND STRUCTURES
(2022)