Article
Mechanics
J. M. F. Peter, M. J. Kloker
Summary: This article introduces the research on film cooling technology for the nozzle extension of rocket engines. By conducting basic experiments and numerical simulations, several new influencing factors have been identified, which are of great guidance for improving cooling modeling and simulation.
Article
Mechanics
Y. X. Wang, K. -S. Choi, M. Gaster, C. Atkin, V. Borodulin, Y. Kachanov
Summary: The experimental investigation in a low-turbulence wind tunnel revealed that artificially initiated turbulent spots in a laminar boundary layer over a flat plate quickly developed into hairpin-like structures, increasing in width, length and height downstream. Only disturbances greater than a threshold value evolved into turbulent spots, while others decayed. The rate of development was also influenced by the duration of initial disturbances. Additionally, the behavior of turbulence generation within a turbulent spot was found to be similar to burst events in a turbulent boundary layer.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Engineering, Aerospace
Andreas Gross, Jesse Little, Hermann F. Fasel
Summary: This study investigates turbulent shock-wave boundary layer interactions at a freestream Mach number of 2.3 using a hybrid turbulence model and large-eddy simulations. The research findings indicate that the sweep angle has a significant effect on the structures and pressure fluctuations.
AEROSPACE SCIENCE AND TECHNOLOGY
(2022)
Article
Mechanics
J. C. Klewicki
Summary: High resolution direct numerical simulation data are used to investigate the similarity solutions for mean velocity and Reynolds shear stress in turbulent channel flow. The analysis yields an invariant form of the mean momentum equation valid over a significant portion of the flow domain. The results provide insights into the development of wall-flow models and support conjectures regarding the behavior of similarity parameters at large Reynolds numbers.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Jie Yao, Fazle Hussain
Summary: Compressible turbulent plane Couette flows with high Reynolds and Mach numbers were studied through direct numerical simulation. Various turbulence statistics were compared with incompressible flows. The skin friction coefficient decreases with Reynolds number but weakly depends on Mach number. The thermodynamic properties show significant variations with Mach number. Proper scaling transformations collapse the mean velocity profiles for compressible and incompressible cases well. Semilocal units yield a better collapse for Reynolds stress profiles compared to wall units. The length scale of near-wall coherent structures and the strength of the superstructures increase with Reynolds number. The streamwise coherence of the superstructures degrades with increasing Mach number.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
S. Wu, M. Yaras
Summary: This study investigates the effect of a two-dimensional surface cavity on the structure and growth rate of a turbulent spot through experimental and numerical analysis. The interactions between the turbulent spot and its wake with the Kelvin-Helmholtz rollers of the cavity shear layer lead to significant increases in both lateral and streamwise growth rates. The underlying physics of these developments have been identified.
Article
Mechanics
Dinar Zaripov, Vladislav Ivashchenko, Rustam Mullyadzhanov, Renfu Li, Nikolay Mikheev, Christian J. Kaehler
Summary: In this study, the mechanism of near-wall reverse flow (NWRF) events in turbulent duct flow at a relatively low Reynolds number Re-tau around 200 was investigated using direct numerical simulations and particle image velocimetry. A conceptual model was proposed to explain the formation of NWRF events, suggesting they are caused by intense hairpin vortices at the interface between high- and low-momentum flow regions. The similarity of flow topologies associated with NWRF events at Re-tau around 200 with those at higher Reynolds numbers indicates a generality of the proposed mechanism.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Physics, Multidisciplinary
Takumi Matsuzawa, Noah P. Mitchell, Stephane Perrard, William T. M. Irvine
Summary: Researchers have generated and controlled a confined state of turbulence using vortex rings as building blocks. They have studied its three-dimensional structure, energy budget, and tunability. This approach opens up new possibilities for sculpting and harnessing turbulent flows.
Article
Mechanics
Rong Ma, Karim Alame, Krishnan Mahesh
Summary: The numerical simulation reveals that the influence of a rough bottom wall on flow mainly manifests in the roll-up of the shear layer in the wall-normal direction. There is a strong correlation between the height of random short wave roughness and wall shear stress, depending on the local roughness topography.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Sergio Pirozzoli
Summary: In this study, direct numerical simulations (DNS) of the Navier-Stokes equations were used to investigate the statistics of passive scalars in turbulent flow within a smooth straight pipe. The results showed that the organization of passive scalars was similar to the axial velocity field at moderate Prandtl numbers, but impaired at low and high Prandtl numbers. The mean scalar profiles exhibited logarithmic overlap layers and universal parabolic distributions in the core part of the flow at particular Prandtl numbers. Accurate predictive formulas for the mean scalar profiles were derived using the nearly universal eddy diffusivity.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Cheng Cheng, Wei Shyy, Lin Fu
Summary: A new methodology was developed to assess the distribution of wall-attached eddies, revealing that the SIAs depend on the Reynolds number at low and medium Reynolds numbers, saturating at 45 degrees as the Reynolds number increases. The average SIA reported in previous experimental studies was shown to be the result of the additive effect of multi-scale attached eddies.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Y. X. Wang, K-S Choi, M. Gaster, C. Atkin, V Borodulin, Y. Kachanov
Summary: This study conducted opposition control of artificially initiated turbulent spots in a low-turbulence wind tunnel in order to delay the transition to turbulence by modifying the turbulent structure within the spots. Through wall-normal jets, the high-speed region of the turbulent spots was cancelled and replaced by a carpet of low-speed fluid. The variable-interval time-averaging technique showed a decrease in burst duration and intensity within the spots, but an increase in burst frequency.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Xue Chen, Yongmann M. M. Chung, Minping Wan
Summary: The statistical characteristics and evolution of backflow structures in wall-bounded flows at Reynolds numbers up to R-et=1000 are explored. Backflow is induced through the merging of large-scale high- and low-speed structures near the wall, and is formed at the tail tip of the low-speed structure. The density and area of backflow structures increase with Reynolds number. The average lifespan of backflow structures is 8 wall units, slightly longer in pipes than channels, and they are convected downstream at an average velocity of approximately 10 wall units, similar to Cardesa et al. (J. Fluid Mech., vol. 880, 2019, R3). Backflow structures can occasionally split, merge, and detach from the wall. Evidence suggests that these phenomena are caused by near-wall structures. Split backflow structures are typically larger and more elongated spanwise due to the shearing of near-wall streaks. Wall-detached backflow structures form when the trailing end of the carrier low-speed structure sits on near-wall high-speed streaks. These structures tend to become wall-attached by approaching the wall, following a similar life cycle as normal backflow structures.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
R. Jason Hearst, Charitha M. de Silva, Eda Dogan, Bharathram Ganapathisubramani
Summary: The study on the instantaneous structure of a turbulent boundary layer affected by freestream turbulence revealed that the number and length of uniform-momentum zones decrease with increasing freestream turbulence intensity. The modal velocities associated with the zones are distributed similarly regardless of external flow conditions, indicating a certain resilience of the instantaneous structure to the freestream.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Ulrich Rist, Matthias Weinschenk, Christoph Wenzel
Summary: This paper analyzes the three-dimensional finite-time Lyapunov exponent (FTLE) field of a flat-plate turbulent boundary layer using direct numerical simulation. The study focuses on the regions of minimal stretching between the vortices and shear layers in the turbulent flow field, referred to as Lagrangian areas of minimal stretching (LAMS). The largest LAMS occur near the boundary-layer edge and exhibit increased relative velocity compared to the mean flow. Sweeping events dominate over ejection events above a certain threshold, and both occur in the layer with maximal vortical activity.
JOURNAL OF FLUID MECHANICS
(2023)