Article
Mechanics
A. A. Chesnokov, S. L. Gavrilyuk, V. Yu. Liapidevskii
Summary: The study proposes a mathematical model to describe the mixing and propagation of nonlinear long waves in a shear three-layer flow of a stratified fluid under a lid. The model defines the concept of supercritical and subcritical three-layer flow and investigates both stationary and oscillating solutions. The model accurately represents the intense mixing region and characteristic features of the flow, as validated against experimental data and numerical results.
Article
Mechanics
Qi Dai, Xiang Wang, Wenbin Feng, Zijie Zhao, Zeqing Guo, Zhihua Chen
Summary: Non-isothermal effects on turbulence anisotropy and growth rate are investigated in high convective Mach number supersonic mixing layers. The non-isothermal effects enhance energy transfer and attenuate turbulence anisotropy, accelerating the growth of the shear layer and destabilizing the supersonic mixing layer. However, the fluid viscosity and viscous dissipation are enhanced, leading to a stronger three-dimensionality and a decrease in the mixing layer growth rate.
Article
Mechanics
Xiaoning Wang, Jianchun Wang, Shiyi Chen
Summary: This study investigates the effects of compressibility on the statistics and coherent structures of a temporally developing mixing layer through numerical simulations. The results show that as the convective Mach number increases, the streamwise dissipation becomes more effective in suppressing turbulent kinetic energy. At low convective Mach numbers, the mixing layer is accompanied by spanwise Kelvin-Helmholtz rollers, while at higher convective Mach numbers, large-scale high- and low-speed structures dominate. The study also reveals a correlation between high-shearing motions on top of low-speed structures and the clustering of small-scale vortical structures.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Physics, Fluids & Plasmas
Peng Zhang, Yubin Song, Yilang Liu, Zhenhua Xia
Summary: This study investigates the equivalence of different thermal boundary conditions in compressible turbulent channel flows. The results show that the three thermal boundary conditions have almost negligible effect on the velocity field, whereas some discernible deviations can be observed for the temperature field in the near-wall region.
Article
Mechanics
Maria Rita Maggi, M. Eletta Negretti, Emil J. Hopfinger, Claudia Adduce
Summary: Understanding gravity currents developing on complex topography is crucial for estimating near ground fluxes in oceanic and atmospheric circulation. Experimental results show the dependence of mean flow, turbulence characteristics, and mixing properties of gravity currents on topography. These characteristics are compared with field measurements of katabatic winds, revealing differences in mean flow structure but comparable mixing lengths and eddy coefficients.
Article
Mechanics
Qi Dai, Xin Zhang, Xin Yuan, Wenbin Feng
Summary: This study investigates the non-isothermal effects on turbulent structures and asymmetric properties in spatially developing supersonic mixing layers with high convective Mach numbers through direct numerical simulations. It is found that temperature gradients enhance flow instability and accelerate the growth of vortices and shocklets in the initial mixing layer. However, in the fully developed region, turbulent structures are weakened due to increased viscous dissipation and stronger turbulence decay. Additionally, the flow momentum gradient is reduced in the non-isothermal mixing layers, leading to reduced asymmetry of the mixing layer.
Article
Mechanics
Yuping Bai, Peiwen Yan, Yifei Yu, Quanbin Zhao, Daotong Chong, Junjie Yan
Summary: In this study, compressible mixing layers are simulated using direct numerical simulation. The origin of spanwise vortex deformations in turbulence is investigated, and it is found that these deformations are a result of a linear superposition of various disturbance modes. The instability mechanism of the simplified vortices is explored, and it is determined that inviscid inflectional instability is responsible for the formation of spanwise and oblique vortices. The findings suggest that the spanwise vortex deformations are a result of the combined effects of a parametric resonant mechanism and inviscid inflectional instability.
Article
Meteorology & Atmospheric Sciences
Neel Desai, Yangang Liu, Susanne Glienke, Raymond A. Shaw, Chunsong Lu, Jian Wang, Sinan Gao
Summary: Marine stratocumulus clouds have a significant impact on Earth's radiation budget, but subgrid variability in cloud properties can cause errors in global climate models. This study analyzes turbulent entrainment-mixing processes and cloud microphysical properties at different heights within a warm marine stratocumulus cloud layer over the Eastern North Atlantic. The findings emphasize the importance of turbulent mixing and appropriate microphysical time scales in understanding cloud microphysical processes.
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES
(2021)
Article
Mechanics
Yuping Bai, Peiwen Yan, Quanbin Zhao, Daotong Chong, Yifei Yu, Zhihui Zhang, Junjie Yan
Summary: The origin and relationship between initial streamwise vortices in braid regions and deformed spanwise vortices are studied using direct numerical simulation (DNS) in a compressible mixing layer. Through analysis of DNS data using fast Fourier transform, two low-rank velocity models (v(d) and v(s)) are developed to show that both deformed spanwise vortices and streamwise vortices originate from the linear superposition of several modes. Further investigations reveal that the increase in amplitude ratio between modes leads to the formation of deformed spanwise vortices in vortex regions and the generation of streamwise vortices in braid regions.
Article
Mechanics
Jonas Buchmeier, Alexander Bussmann, Xiangyu Gao, Ivan Bermejo-Moreno
Summary: This study presents a structure-based numerical analysis on passive scalar mixing in decaying homogeneous isotropic turbulence and shock-turbulence interaction canonical configurations, focusing on the temporal evolution of scalar structures and their interactions. The results show that the initial size of structures influences the mixing process and structure breakup, with larger structures leading to increased surface convolution and higher probability of locally hyperbolic geometries. Additionally, shock-induced deformation of structures enhances mixing processes, particularly for larger structures, leading to an amplification of surface-averaged scalar gradient and alignment between scalar gradient and strain-rate eigendirections.
Article
Mechanics
J. G. Wang, J. L. Yu
Summary: The interactions between large and small scales in compressible turbulent mixing layers were investigated using numerical simulations. The study found that large-scale compressive velocity fluctuations modulate the small scales with different properties on each side of the mixing layer, and this modulation decreases with increasing convective Mach number.
Article
Mechanics
T. Watanabe, K. Tanaka, K. Nagata
Summary: This study investigates a solenoidal linear forcing scheme with reduced oscillation of a turbulent Mach number M-T for direct numerical simulations of homogeneous isotropic turbulence. Adjusting the forcing coefficient effectively reduces the temporal oscillation of M-T and increases the power input to kinetic energy. The solenoidal and dilatational kinetic energy dissipation rates increase with M-T, and the shocklet thickness normalized by the Kolmogorov scale hardly depends on the Reynolds number.
Article
Physics, Fluids & Plasmas
Vishnu Mohan, A. Sameen, Balaji Srinivasan, Sharath S. Girimaji
Summary: Gas-kinetic simulations were used to study the continuum breakdown and its effect on the Kelvin-Helmholtz instability in rarefied and compressible mixing layers. Two distinct continuum breakdown regimes were identified, one at low convective Mach numbers and the other at high convective Mach numbers. The deviations from continuum behavior were found to scale with the Knudsen number at low Mach numbers and with the product of the Mach and Knudsen numbers at high Mach numbers. A local parameter called the gradient Knudsen number was proposed to characterize the rarefaction effects.
Article
Mechanics
Ethirajan Rathakrishnan
Summary: An experimental investigation was conducted to evaluate the effectiveness of rectangular tabs with different aspect ratios on promoting the mixing of a Mach 1.73 axisymmetric free jet. The results show that the tab with an aspect ratio of 1.5 can significantly reduce the core length, although it leads to some loss of momentum thrust and drag caused by the tabs.
Article
Thermodynamics
Xuming Li, Guoqing Zhu, Lu He, Min Peng
Summary: This study investigates the characteristics and thickness of smoke flow under a curved roof in natural ventilation tunnels. It is found that the smoke mass flow rate is positively correlated with the heat release rate and section radius, while the smoke layer thickness is not significantly affected by the heat release rate and is positively related to the section radius. A practical prediction model for smoke layer thickness and an empirical equation for estimating the total smoke mass flow rate are proposed, and the results are in good agreement with previous simulations and experimental data.
CASE STUDIES IN THERMAL ENGINEERING
(2023)
