Article
Mechanics
Pablo Trefftz-Posada, Antonino Ferrante
Summary: The study explains the physical mechanisms occurring in droplet-laden homogeneous shear turbulence and focuses on the modulation of turbulence kinetic energy caused by the droplets. Direct numerical simulations and equation derivations were conducted to investigate the effects of shear number and Weber number on the modulation of TKE, as well as the relationship between the power of surface tension and the rate of change of total droplet surface area.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Antoine Michel, Boris Arcen
Summary: Direct numerical simulations are performed to investigate the influence of friction Reynolds number (Re_t) on the translational and angular velocities of inertial, prolate ellipsoids in turbulent channel flow. The quadrant distribution of the turbulent events experienced by the particles remains unchanged with varying Re_t, while subtle modifications occur depending on channel position and particle relaxation time. The statistical moments of the ellipsoids' translational velocity display the same dependence on Re_t as the fluid velocity, and the weak dependence on particle shape observed at low Reynolds numbers persists at higher Re_t. Similarly, the mean and r.m.s. of the particles' angular velocity weakly depend on particle shape and show the same dependence on Re_t as the fluid angular velocity statistics.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
A. K. Aiyer, C. Meneveau
Summary: In this study, the dispersion characteristics of slightly buoyant droplets in a turbulent jet were investigated using large eddy simulations (LES). The model accurately captured the differential, size-based dispersion characteristics of the droplets with finer grid resolutions. Moreover, similarity solution and subgrid-scale models were used to predict the concentration profiles and turbulent concentration flux of the droplets, and the results showed good agreement with experimental data and high-resolution LES simulations.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Xin Lai, Shaofan Li, Jiale Yan, Lisheng Liu, A-Man Zhang
Summary: This study investigates the dispersion of virus-laden droplets in coughing jets using large-eddy simulations. The turbulence of the jet determines the droplets' dispersion, and the droplet particles increase the complexity of the turbulence. The spreading distance of droplets often exceeds the recommended social distancing rules, highlighting the importance of protective measures.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Marco Crialesi-Esposito, Marco Edoardo Rosti, Sergio Chibbaro, Luca Brandt
Summary: A numerical study was conducted on emulsions in homogeneous and isotropic turbulence at Re-lambda = 137, revealing the energy transport behavior and factors influencing it. The analysis showed that energy is consistently transported from large to small scales by the interface, with droplet size distributions following specific scaling laws.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
S. Beetham, R. O. Fox, J. Capecelatro
Summary: In this study, model closures for multiphase Reynolds-averaged Navier-Stokes (RANS) equations are developed using sparse regression and Eulerian-Lagrangian simulations to ensure accuracy and robustness of the models across different flow conditions. The focus is on capturing the dynamics of gas-particle flows, particularly the generation of particle clusters and interphase momentum exchange, in a compact and algebraic manner.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Sourabh Apte, Thibault Oujia, Keigo Matsuda, Benjamin Kadoch, Xiaoliang He, Kai Schneider
Summary: Direct numerical simulation is used to investigate the effects of turbulent flow on the transport, clustering, and deposition of fine particles in a face-centred cubic porous unit cell. The results show that particle clustering occurs at large volumes and is enhanced with increasing Stokes number.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Linfeng Jiang, Cheng Wang, Shuang Liu, Chao Sun, Enrico Calzavarini
Summary: In this study, we investigate the translational and rotational dynamics of neutrally buoyant finite-size spheroids in hydrodynamic turbulence using numerical simulations. Our results show that the accelerations and rotations of non-spherical particles are influenced by filtered fluid forces and torques, similar to spherical particles. However, the orientations of the particles exhibit preferential alignments with the surrounding flow structures. We also demonstrate the significance of inertial-scale flow structures in the transport of neutrally buoyant bodies within the inertial range.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Gautier Verhille
Summary: The aim of this study is to experimentally investigate the transition from a rigid regime to a deformed regime for flexible discs in turbulent flows. The experimental results show that the amplitude of deformation of the discs depends on the size, density, flexibility, and turbulent kinetic energy.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Hao-Ran Liu, Kai Leong Chong, Rui Yang, Roberto Verzicco, Detlef Lohse
Summary: Turbulent Rayleigh-Benard convection was numerically investigated in two immiscible fluid layers, revealing two flow regimes with varying sensitivity to the alpha parameter. The study provided insights into heat transfer characteristics in two-layer systems.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Ashik Ullah Mohammad Masuk, Ashwanth K. R. Salibindla, Rui Ni
Summary: The orientation of bubbles is strongly influenced by the slip velocity, especially during severe deformation. As bubbles begin to break, their relative orientations change significantly, causing the slip velocity to align with the bubble's semi-major axis.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Ashwanth K. R. Salibindla, Ashik Ullah Mohammad Masuk, Rui Ni
Summary: The study experimentally determined the acceleration statistics and added mass tensor of deformable gas bubbles in turbulence, finding two independent ways of estimating bubble acceleration. The research showed that the orientation of bubble major axis in turbulence is not random but preferentially aligned with the slip acceleration.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Cheng Wang, Lei Yi, Linfeng Jiang, Chao Sun
Summary: This experiment investigates the influence of deformability of dispersed droplets on the global transport quantity of turbulent emulsion. The deformability leads to drag modification and affects the rheological properties of the emulsion.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Vikash Pandey, Dhrubaditya Mitra, Prasad Perlekar
Summary: A direct numerical simulation study of buoyancy-driven bubbly flows in the presence of large-scale driving that generates turbulence reveals that increasing turbulence intensity leads to more curved bubble trajectories and decreased average rise velocity. The energy spectrum of the flow exhibits pseudo-turbulence scaling for small length scales and Kolmogorov scaling for larger scales. A scale-by-scale energy budget analysis helps understand the scaling behavior seen in the spectrum, with statistical properties consistent with experiments on turbulence modulation by air bubbles in water despite the weak buoyancy of the bubbles.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Tim Berk, Filippo Coletti
Summary: The study investigates the settling behavior of microscopic heavy particles in homogeneous air turbulence. It finds that the particle settling velocity is enhanced proportionally to the turbulence velocity scale, gravity and inertia reduce particle velocity fluctuations and have competing effects on particle acceleration. Additionally, preferential sampling of high-strain/low-vorticity regions increases particle pair dispersion, but gravity offsets this effect.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
G. Zampino, D. Lasagna, B. Ganapathisubramani
Summary: A rapid predictive tool based on linearized Reynolds-averaged Navier-Stokes equations is proposed to investigate secondary currents generated by surface topography modulations in turbulent channel flow. The tool allows for the exploration of large parameter spaces and the analysis of the flow response induced by arbitrary modulations. The results suggest the existence of amplification mechanisms and propose a more convenient method for interpreting secondary structures.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
M. Gul, B. Ganapathisubramani
Summary: Based on experimental data, this study examines the turbulent boundary layers subjected to a sudden change in wall roughness in the streamwise direction. The results show that the thickness of the internal boundary layer (IBL) is proportional to the strength of the surface transition, and the IBLs are thicker for S->R cases compared to R->S cases. The growth rates of the IBLs can be represented by a power law, with an average exponent value of 0.75. The study also analyzes the variations in velocity defect, diagnostic plots, and spatial correlations.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Physics, Fluids & Plasmas
L. B. Esteban, E. Rodriguez-Lopez, M. A. Ferreira, B. Ganapathisubramani
Summary: This study presents mean-flow measurements of turbulent boundary layers over porous walls with varying pore size, permeability, and thickness. The results show that substrate permeability increases the magnitude of the mean velocity deficit. A modified indicator function is used to estimate important parameters, such as the roughness function and equivalent sandgrain roughness. The study also suggests that the porous wall can be modeled as a combination of rough and permeable wall.
PHYSICAL REVIEW FLUIDS
(2022)
Article
Engineering, Mechanical
Girish K. Jankee, Bharathram Ganapathisubramani
Summary: This investigation explores the flapping behavior of a flexible flag under confinement, focusing on the least understood aspect of hysteresis and its sensitivity to flexural rigidity and confinement ratio. The results show that while confinement ratio does not contribute to hysteresis existence, the relationship between critical reduced velocities and mass ratio is sensitive to the level of confinement.
EXPERIMENTS IN FLUIDS
(2022)
Article
Mechanics
John M. Lawson, Bharathram Ganapathisubramani
Summary: Using laboratory experiments and numerical simulations, this study examines the transfer of soluble material from sinking particles in turbulence. The analysis predicts two mechanisms of convective mass transfer and confirms their existence through experiments and simulations. The study also reveals a transition in the transfer mechanism under specific conditions.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Engineering, Mechanical
Douglas W. Carter, Bharathram Ganapathisubramani
Summary: This study presents an aeroacoustic investigation of planar time-resolved Particle Image Velocimetry (PIV) measurements on a NACA 0012 airfoil in static stall at a chord-based Reynolds number of 7.1 x 10(4). Instantaneous planar pressure reconstructions are obtained using a Poisson solver, and the dipole noise emanating from the airfoil surface is estimated using Curle's acoustic analogy. By applying a data-driven framework based on Proper Orthogonal Decomposition (POD) and Spectral Linear Stochastic Estimation (sLSE), the flow structures responsible for noise generation are identified, primarily concentrated near the trailing edge. Furthermore, a conditional analysis of extreme noise events reveals correlations between downwash and upwash events near the trailing edge, and local maxima and minima in acoustic fluctuations associated with slow and fast-moving fluid in the incipient shear layer.
EXPERIMENTS IN FLUIDS
(2023)
Article
Mechanics
Stefano Gambuzza, Bharathram Ganapathisubramani
Summary: The study analyzes the wake of a model-scale wind turbine under different inflow conditions. It is found that high turbulence intensity and low integral time scale flows result in a rapid wake evolution closer to the turbine, while higher integral time scales lead to a slower wake evolution similar to that in low-turbulence inflow conditions. The delayed onset of the wake evolution is related to the stability of the near-wake shear layer and the helical vortex set surrounding the wake. The rate of velocity recovery in the wake is shown to depend on the Reynolds shear stress at the wake centerline, and it is connected to the power harvested by the turbine.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Engineering, Marine
N. Hutchins, B. Ganapathisubramani, M. P. Schultz, D. I. Pullin
Summary: A new method based on power mean is proposed to define an equivalent homogeneous roughness length that considers the heterogeneous distribution of roughness on ship hulls. The method can be easily incorporated into existing drag prediction methods. The study shows that the relationship between drag coefficient and roughness length is non-linear, indicating a source of error in current approaches. The power-mean approach has low errors for both skewed beta distributions and fully-rough models, and retains accuracy in transitional regimes and with different roughness models.
Article
Engineering, Mechanical
Takfarinas Medjnoun, Manuel Aguiar Ferreira, Ralf Reinartz, Bagus Nugroho, Jason Monty, Nicholas Hutchins, Bharathram Ganapathisubramani
Summary: Full-scale drag penalty predictions of flows over rough walls require surface roughness characterisation. The impact of different methods on full-scale drag predictions, particularly the outer-layer similarity hypothesis, is evaluated through wind tunnel measurements. The predicted drag penalty can vary by over 15% among the different methods, highlighting the need for caution when employing such methods.
EXPERIMENTS IN FLUIDS
(2023)
Article
Engineering, Mechanical
Bradley McLaughlin, John Lawson, Bharathram Ganapathisubramani
Summary: This paper presents a floating element force balance design that uses optical measurement of force via photoelastic stress analysis. The force sensing element consists of pins embedded in photoelastic polyurethane pads, observed via a transmission polariscope. Calibration is derived through a polynomial model solved by least squares regression using known loads and their corresponding fringe patterns. Finite element analysis (FEA) simulation is carried out to validate the proposed method. The photoelastic balance is compared to a commercial, 6-axis strain-gauge load cell and shows differences of less than 6%. This optical approach provides accurate measurements with inexpensive and simple components inside the sensor, demonstrating that a photoelastic balance is a simple, inexpensive, and sensitive force transducer.
EXPERIMENTS IN FLUIDS
(2023)
Article
Mechanics
D. D. Wangsawijaya, P. Jaiswal, B. Ganapathisubramani
Summary: This study finds that the boundary-layer flow over a porous wall can be seen as a superposition of roughness effects and permeability effects. An empirical formulation is proposed based on independently obtained roughness and permeability length scales to predict the momentum deficit. The decoupling approach used in this study is consistent with recent research on heterogeneous rough surfaces and provides insights into the boundary-layer flow over porous walls.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
C. Thompson, H. Biler, S. Symon, B. Ganapathisubramani
Summary: The effect of variations in the integral length scale of incoming free-stream turbulence on a NACA0012 wing is investigated in this study. It is found that introducing turbulence can delay stall and increase the peak lift coefficient of the wing. The magnitude of fluctuations in forces and moments is larger for half-chord integral length scales compared to full-chord integral length scales.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
J. M. O. Massey, B. Ganapathisubramani, G. D. Weymouth
Summary: This study examines the effects of surface topography on the flow and performance of a self-propelled swimming body. The results show that roughness increases drag and decreases swimming efficiency, while increasing flow mixing. Comparison indicates that reducing roughness wavelength can reduce the required swimming power and the unsteady amplitude of the forces.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
N. S. Lagopoulos, G. D. Weymouth, B. Ganapathisubramani
Summary: This article focuses on three main aspects of transonic flow physics. Firstly, a practical implementation of an iterative resolvent algorithm is assessed in an industrial computational fluid dynamics code for computing optimal forcing and response modes. This relies on efficient solution of large sparse linear systems of equations. Secondly, the article showcases the application of this algorithm as a predictive tool to detect transonic buffet flow unsteadiness. Thirdly, it discusses the algorithm's ability to uncover modal physics not identifiable through global stability analysis.
Article
Mechanics
Francis De Voogt, Bharathram Ganapathisubramani
Summary: In this case study, the effect of airfoil shape/camber on the formation and existence of stall cells is examined. Through a series of experiments, the influence of angles of attack, flap angles, and chord-length-based Reynolds numbers on stall cell formation is explored. The results suggest that the airfoil shape has an impact on stall cell formation.