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"Turbulence models"
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On the turbulence modelling of waves breaking on a vertical pile
Incipient wave breaking on a vertical circular pile is simulated with a Reynolds stress–$\\omega$ turbulence model. Comparison of results simulated with a stabilized two-equation turbulence model, as well as no turbulence model, demonstrates that the breaking point and the peak force on a vertical cylinder due to incipient breaking should not be affected by the turbulence closure model, provided that it is stable and the simulations are converged. Notably, the present results show that the build-up to peak force induced by incipient wave breaking can be accurately predicted without any turbulence closure model. However, for the prediction of the secondary load cycle (SLC), proper turbulence modelling is required, as this process involves both turbulence production and lee-side flow separation. The Reynolds stress–$\\omega$ model is demonstrated to predict the SLC more accurately than a stabilized two-equation $k$–$\\omega$ turbulence model, as the flow separation points and vorticity field are better predicted. Some existing studies indicate that the generation of the SLC does not necessarily result from flow separation, but is rather due to the suction force. The present work finds that the occurrence and point of flow separation significantly affect the magnitude of the suction force, which hence affects the SLC prediction significantly. For waves breaking on a vertical pile, proper turbulence modelling is therefore essential for accurate SLC predictions. (In the above, $k$ is the turbulent kinetic energy density and $\\omega$ is the specific dissipation rate.)
Journal Article
Transition to turbulence : a dynamical system approach to receptivity
\"Present understanding of transition to turbulence has now been studied over one hundred and fifty years. The path the studies have taken posed it as a modal eigenvalue problem. Some researchers have suggested alternative models without being specific. First-principle based approach of receptivity is the route to build bridges among ideas for solving the Navier-Stokes equation for specific canonical problems. This book highlights the mathematical physics, scientific computing, and new ideas and theories for nonlinear analyses of fluid flows, for which vorticity dynamics remain central. This book is a blend of classic with distinctly new ideas, which establish different dynamics of flows, from genesis to evolution of disturbance fields with rigorously developed methods to tracing coherent structures amidst the seemingly random and chaotic fluid dynamics of transitional and turbulent flows\"-- Provided by publisher.
Book
Study on the sensitivity of steam ejector simulation to wall treatment methods
Considering the importance of turbulence model and wall treatment to steam ejector simulation, the influence of five wall treatment methods on numerical results was studied based on the realizable k-ε turbulence model (RKE). Combined with experimental data and SST turbulence model, the prediction of complex flow phenomena inside the ejector, including shock wave and reflux, by different wall treatment methods is discussed. The results show that under the numerical simulation conditions, the enhanced wall treatment method (EWT) combined with the RKE model can better predict the injection coefficient of the steam ejector, and the relative error compared with the experimental data is 5.3%. The standard wall function method (STWF) ignores the influence of the viscous bottom layer, resulting in inaccurate simulation of the internal flow phenomenon of the injector, and the injection coefficient calculated by the simulation is 10.2% higher than the experimental value. The results of the non-equilibrium wall function (NWF) method are relatively indifferent to the experimental values, and are not recommended when simulating steam ejectors.
Journal Article
Study of hydraulic turbine flow-induced noise distribution law based on defective runner blade
In order to analyse flow-induced noise distribution law under the defective runner blade, this paper performs defective treatment on the runner blade according to the HLA855a-LJ-250 hydraulic turbine model. Two kinds of blade models, intact blade and outlet side abrasive blade, are established respectively, and the Realizable k-ε turbulence model and broadband noise source model are used to simulate the hydraulic turbine under rated operating conditions. The simulation results show: 1)Double-row leaf grilles and runner blade are the main dipole flow-induced noise sources; 2)Significant differences in the distribution of the acoustic power levels of the double-row leaf grilles’ characteristic blades and the runners’ characteristic blades under different Schemes; 3)Runner defective blades have less effect on the acoustic power level of spiral case, the rest of the overflow components acoustic power level has a tendency to increase, especially the runner has the greatest impact. The maximum increase in acoustic power level of the runner is almost 17 dB, followed by double-row leaf grilles with an increase of around 8 dB. Therefore, critical attention needs to be paid to the abnormal noise signals of runner and double-row leaf grilles.
Journal Article
Machinability evolution of gas–liquid-solid three-phase rotary abrasive flow finishing
Soft abrasive flow (SAF) finishing has advantages in precise processing for titaniferous alloy parts with large-area surfaces. However, the processing efficiency and surface quality of current SAF methods require to be improved. To address the issues, a novel gas–liquid-solid three-phase rotary abrasive flow polishing method (RGLSP) is proposed based on the microbubble cavitation effect. Referring to the energy conservation principles of bubbles group movement, an improved realizable
k
-
ε
turbulence model is set up. Based on the above model, a mechanical dynamic model of the machining apparatus is established to obtain the flow field distribution of different structural parameters. The simulated results show that the pitch-down angle determines the distribution of the rotary flow field, and an optimized processing apparatus is developed. The machining experiments show that the proposed RGLSP method can improve the machining efficiency by 60%, and the surface roughness can reach
R
a 0.1 μm with lower surface defects. The RGLSP can provide an important reference direction for low-carbon emission processing.
Journal Article
Marine turbulence : theories, observations, and models
This text gives a comprehensive overview of measurement techniques and theories for marine turbulence and mixing processes. Written by a team of 53 world experts, the book represents a rich source of data and methods for students, scientists, and engineers in oceanography, hydrology, limnology, and meteorology.
Book
Study on the Flow Characteristics of Piston Type Flow and Pressure Regulating Valve under Sand Containing Conditions
Based on numerical simulation, the flow characteristics of the valve under different opening degrees of sandy conditions are studied based on the Realizable k-ε turbulence model and Eulerian multiphase flow model, and the flow characteristic curves and flow coefficient-opening degree curves of the valve are obtained, and the influence of the sand rate on the valve flow is analyzed with the numerical calculation results. The results show that with the increase of sand content rate and valve opening, sediment deposition is easily formed in the throttle orifice area, which affects the normal flow inside the valve body and leads to a large deviation of the valve flow rate from the theoretical flow rate. In addition, the relationship between flow rate, sand content and flow coefficient is obtained by fitting the data to the numerical calculation results. The research results have certain reference value for the structural optimization and efficient operation of the valve.
Journal Article