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
Mechanics
Hao Wang, Yitong Fan, Zhenguo Yan, Weipeng Li
Summary: Different from conventional streamwise-aligned riblets, converging and diverging (C-D) riblets arranged obliquely along the main-flow direction induce a large-scale secondary flow to modulate or control existing large-scale vortical structures in wall turbulence. Direct numerical simulations show that C-D riblets can reduce skin-friction drag but increase pressure drag, resulting in an overall increase in total drag. Detailed flow-field analysis reveals modifications in velocity, pressure, Reynolds stresses, and turbulence kinetic energy distributions due to the presence of C-D riblets. Examination of turbulence structures over the C-D riblets is conducted through instantaneous flow-fields and two-point correlations.
Article
Energy & Fuels
Hoai Thanh Nguyen, Kyoungsik Chang, Sang-Wook Lee, Jaiyoung Ryu, Minjae Kim
Summary: In this study, the effect of parameters on drag-reducing performance of superhydrophobic surfaces was investigated using direct numerical simulation (DNS). The results reveal the drag reduction mechanism through turbulent kinetic energy budget and observation of coherent structures. The study demonstrates that significant drag reduction can be achieved under specific solid fraction and distribution conditions.
Article
Thermodynamics
Amin Moosaie, Hamed Panahi-Kalus
Summary: The study focuses on fully developed turbulent flow and heat transfer in a plane channel with different velocity slip and no-slip conditions. It is found that velocity and temperature fluctuations behave differently, with temperature jump leading to reduced turbulent heat flux under velocity slip condition, enhancing heat transfer.
JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER
(2021)
Article
Thermodynamics
H. Mamori, K. Fukudome, K. Tanaka, T. Miyazaki
Summary: This paper investigates the control of skin-friction drag reduction in a fully developed turbulent pipe flow through direct numerical simulations. The heating and cooling of the pipe wall are uniform in the streamwise direction and vary in the azimuthal direction, leading to a decrease in skin-friction drag due to buoyancy force. The study identifies two different mechanisms for skin-friction drag reduction: stable stratification and large-scale vortical structures. The former directly attenuates turbulence through buoyancy force, while the latter creates wall jets and forms large-scale streamwise vortices that increase the coherent contribution to the skin-friction coefficient while indirectly reducing the random contribution.
INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW
(2023)
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
Thermodynamics
Wentao Guo, Horst-Michael Prasser
Summary: This article investigates the impact of buoyancy force in a vertical channel with combined natural and forced convection liquid metal flow using direct numerical simulation (DNS). The influence of buoyancy on turbulent heat transfer and mean flow are analyzed. The DNS results can shed light on the heat transfer mechanism of turbulence in liquid metal flows and be used for validating and improving turbulent models for low-Prandtl-number fluids.
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER
(2022)
Article
Thermodynamics
S. Endrikat, D. Modesti, M. MacDonald, R. Garcia-Mayoral, N. Hutchins, D. Chung
Summary: The study investigates the breakdown of drag-reduction mechanisms in riblets as their viscous-scaled size becomes large enough for turbulence to approach the wall. Using the minimal-span channel concept, cost-efficient direct numerical simulation of rough-wall flows was conducted to study the inertial-flow mechanisms. Different shapes and sizes of riblets were examined, with findings indicating varying influences on the Kelvin-Helmholtz instability.
FLOW TURBULENCE AND COMBUSTION
(2021)
Article
Mechanics
Xiao-Ping Chen, Deng-Song Huang, Shuo Zhao, Chen-Shao Zhu
Summary: This study conducts direct numerical simulations to investigate the influence of compressibility effects on turbulent statistics and large-scale structures in temporally evolving turbulent channel flows with a weak spanwise rotation. The results show that compressibility has some impact on the turbulent statistics distributions, but many asymmetric characteristics still exist.
Article
Mechanics
Marwa Fakhfekh, Wael Ben Amira, Malek Abid, Aref Maalej
Summary: In this study, the mean deflection evolution of a flexible rotor blade is examined using a coupled model based on Navier-Stokes equations for fluid flow and linear elasticity equations for blade deformation. The elastic behavior of the flexible rotor blade is characterized under different parameter ranges, and the vorticity field is compared with that of a rigid rotor. The effect of pitch, tip speed ratio (or frequency), and its sign on blade deformation is investigated, revealing the non-monotonic relationship between deformation and rotation frequency and pitch angle. Additionally, the flexible rotor demonstrates the ability to develop a vortex ring state under specific conditions, which is not observed in the rigid rotor.
Article
Engineering, Chemical
Yan Xia, Zhaowu Lin, Yu Guo, Zhaosheng Yu
Summary: Data from IR-DNSs of upward turbulent channel flow laden with finite-size heavy particles are used to develop improved drag correlations accounting for turbulence effects. The contribution of drag nonlinearity to drag enhancement is analyzed, and several turbulence correction models are presented. Turbulent intensity, particle size, and slip velocity are found to be the most important factors in modeling the effect of turbulence on particle drag.
Article
Mechanics
Azadeh Jafari, Benjamin Cazzolato, Maziar Arjomandi
Summary: This study investigates the potential of finite-length porous surfaces with a subsurface chamber for controlling the turbulent boundary layer. Different chamber configurations were tested, and it was found that a short porous surface with individual cavities underneath each perforation was the most effective in reducing turbulence.
Article
Thermodynamics
Victor Coulon, Jessica Gaucherand, Victor Xing, Davide Laera, Corentin Lapeyre, Thierry Poinsot
Summary: Three turbulent premixed flames, including methane/air, ammonia-hydrogen/air, and hydrogen/air flames, were analyzed and compared using 3D Direct Numerical Simulation. The results showed that the hydrogen flame exhibited significant alteration of its local flame structure, with decreased flame length and surface area, and increased burning rates. In contrast, the methane and ammonia-hydrogen flames behaved similarly.
COMBUSTION AND FLAME
(2023)
Article
Mechanics
Yixiang Wang, Kit Ming Lam, Kam Tim Tse
Summary: This study investigates the settling velocity and clustering behavior of bidisperse inertial particles in turbulent channel flow through direct numerical simulation. The results show a significant turbophoresis effect on smaller diameter particles in bidisperse cases, influencing clustering and turbophoresis. It is also found that terminal settling velocities in bidisperse cases are affected by final volume fractions at dynamic equilibrium state, with a strengthened preferential sweeping mechanism observed with decreasing Stokes number.
Article
Mechanics
Abhishek Sharma, Rajaram Lakkaraju, Arnab Atta
Summary: This study investigates the behavior of wall-bounded turbulent flow on sharply bending channels. The time averaged velocity profiles at different sections of the channel show that the well-known logarithmic behavior is retained, but the values of the von Karman and additive constants change with increasing bend inclination. The near-wall fluctuations are enhanced at the bend due to diffusion of counter-rotating vortices. The spatial structure of the random fluctuating field exhibits the generation of multiple high speed and low speed streak pairs, as well as multiple streamwise vortices of different sizes when the bend inclination is increased. Turbulent kinetic energy is modulated at the bend as depicted by the budgets presented for various inclinations of the bend at different sections of the channel.
