scholarly journals SCHMIDT NUMBER OF SAND SUSPENSIONS UNDER OSCILLATING GRID TURBULENCE

2012 ◽  
Vol 1 (33) ◽  
pp. 20 ◽  
Author(s):  
Daniel Buscombe ◽  
Daniel Conley
Keyword(s):  
Schmidt Number ◽  
Ad Hoc ◽  
Isotropic Turbulence ◽  
Grid Turbulence ◽  
Flow Conditions ◽  
Velocity Measurements ◽  
Ongoing Work ◽  
Turbulent Schmidt Number ◽  
Initial Results ◽  
Oscillating Grid

In many models of sand suspension under waves, the diffusivity of sediment is related to the diffusivity of momentum by the inverse of the turbulent Schmidt number. The value and parameterization of this number has been the topic of much research, yet a lack of consensus has led to ad hoc adjustments in models of turbulent sediment suspensions, with apparently little physical justification. In order to study sediment diffusivity we conducted laboratory experiments to generate gradient-only sediment diffusion. Concentrations of sand suspended by near-isotropic turbulence generated by an oscillating grid, together with detailed velocity measurements, were used to calculate vertical profiles of the Schmidt number with a range of grain sizes and flow conditions. Initial results suggest that momentum diffusivity is greater than sediment diffusivity, and that the ratio of the two scales with grid Reynolds number. Ongoing work will ascertain whether an apparent grain size dependence could instead be explained by two-way feedbacks between sediment and turbulence.

scholarly journals Experimental study on two oscillating grid turbulence with viscoelastic fluids based on PIV

2017 ◽  
Vol 95 (12) ◽  
pp. 1271-1277 ◽  
Author(s):  
Yue Wang ◽  
Wei-Hua Cai ◽  
Xin Zheng ◽  
Hong-Na Zhang ◽  
Feng-Chen Li
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Turbulent Flows ◽  
Isotropic Turbulence ◽  
Critical Concentration ◽  
Reduction Rate ◽  
Small Scale ◽  
Grid Turbulence ◽  
Viscoelastic Effect ◽  
Oscillating Grid

In this paper, to study the viscoelastic effect on isotropic turbulence without wall effects, a two oscillating grid turbulence is built to investigate this phenomenon using particle image velocimetry. In the experiments, the classical drag-reducing additives are chosen: polyacrylamide (PAM) and cetyltrimethyl ammonium chloride (CTAC), which have shown remarkable drag-reducing effect in wall-bounded turbulent flows. The results show that the existence of drag-reducing additives makes velocity field more anisotropic and reduces turbulent kinetic energy. We propose an intuitive and natural definition for a reduction rate of turbulent kinetic energy to show viscoelastic effect. It suggests that there exists a critical concentration for the reduction rate of turbulent kinetic energy in the CTAC solution case. Also, the small-scale vortex structures are inhibited, which suggests the drag-reducing mechanism in grid turbulence without wall effect.

The Turbulent Schmidt Number

2014 ◽  
Vol 136 (6) ◽  
Author(s):  
Diego A. Donzis ◽  
Konduri Aditya ◽  
K. R. Sreenivasan ◽  
P. K. Yeung
Keyword(s):  
Schmidt Number ◽  
Isotropic Turbulence ◽  
Functional Dependence ◽  
Three Dimensional ◽  
Passive Scalars ◽  
Turbulent Schmidt Number ◽  
Schmidt Numbers ◽  
Homogeneous And Isotropic Turbulence ◽  
Homogeneous Mean ◽  
Parameter Ranges

We analyze a large database generated from recent direct numerical simulations (DNS) of passive scalars sustained by a homogeneous mean gradient and mixed by homogeneous and isotropic turbulence on grid resolutions of up to 40963 and extract the turbulent Schmidt number over large parameter ranges: the Taylor microscale Reynolds number between 8 and 650 and the molecular Schmidt number between 1/2048 and 1024. While the turbulent Schmidt number shows considerable scatter with respect to the Reynolds and molecular Schmidt numbers separately, it exhibits a sensibly unique functional dependence with respect to the molecular Péclet number. The observed functional dependence is motivated by a scaling argument that is standard in the phenomenology of three-dimensional turbulence.

Energy Spectral Properties Due to Bubble-Liquid Interaction in Oscillating-Grid Turbulence

2008 ◽  
Author(s):  
Koichi Morikawa ◽  
Shigeyuki Urano ◽  
Toshiyuki Sanada ◽  
Takayuki Saito
Keyword(s):  
Energy Spectrum ◽  
Liquid Phase ◽  
High Speed ◽  
Isotropic Turbulence ◽  
Video Camera ◽  
Grid Turbulence ◽  
Integral Scale ◽  
Bubble Liquid ◽  
Correlation Integral ◽  
Oscillating Grid

In the present study, liquid-phase turbulence modulation induced by bubble swarms ascending in arbitrary turbulence is experimentally investigated. Liquid-phase homogeneous isotropic turbulence is formed using an oscillating grid in a cylindrical acrylic vessel of 149 mm in inner diameter. A bubble swarm of 19 bubbles is examined. The bubble size and generating time are completely controlled applying a bubble generating device using audio speakers. The bubble swarms are generated at a frequency of 4 Hz. The motion of individual bubbles of the bubble swarm is captured by a high-speed video camera. The liquid phase motion is also measured by PIV. The turbulence intensity, spatial correlation, integral scale and energy spectrum are calculated from the PIV data. It is confirmed that the oscillating-grid turbulence is an isotropic turbulence from the PIV result. When the bubble swarm is added, the original isotropic turbulence is modulated to an anisotropic turbulence by the mutual interaction between the bubble swarm and ambient isotropic turbulence. Moreover the bubble motion is notably changed. The bubble swarm contributes an increase of the energy spectrum to the ambient oscillating-grid turbulence. Moreover the suppression of energy spectrum occurred by interaction between bubble swarm and oscillating-grid turbulence compared with the energy spectrum in the flow induced by bubble swarms.

scholarly journals Optimization of the flow conditions in the spawning ground of the Chinese sturgeon (Acipenser sinensis) through Gezhouba Dam generating units

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Anyang Huang ◽  
Jinzhong Yao ◽  
Jiazhi Zhu ◽  
Xingchen Gao ◽  
Wei Jiang
Keyword(s):  
Spawning Ground ◽  
Chinese Sturgeon ◽  
Flow Conditions ◽  
Velocity Measurements ◽  
Acipenser Sinensis ◽  
Critically Endangered Species ◽  
Gezhouba Dam ◽  
Opening Mode ◽  
Suitable Area ◽  
Good Agreement

AbstractChinese sturgeon (Acipenser sinensis) is a critically endangered species, and waters downstream from Gezhouba Dam are the only known spawning ground. To optimize the velocity conditions in the spawning ground by controlling the opening mode of Gezhouba Dam generator units, a mathematical model of Chinese sturgeon spawning ground was established in FLOW-3D. The model was evaluated with velocity measurements, and the results were determined to be in good agreement. By inverting the 2016–2019 field monitoring results, the model shows that the preferred velocity range for Chinese sturgeon spawning is 0.6–1.5 m/s. Velocity fields of different opening modes of the generator units were simulated with identical discharge. The suitable-velocity area was maximal when all units of Dajiang Plant of Gezhouba Dam were open. For discharges below 12,000 m3/s, most of the area was suitable; for discharges above 12,000 m3/s, the suitable area rapidly decreased with increasing discharge. A comparison of suitable areas under high-flow showed that at discharges of 12,000–15,000 m3/s, opening 11–13 units on the left side was optimal. For discharges above 15,000 m3/s, all units should be open. We used these results to recommend a new operation scheme to support the conservation of Chinese sturgeon.

Velocity measurements close to the bed in a wave tank

1970 ◽  
Vol 42 (1) ◽  
pp. 111-123 ◽  
Author(s):  
J. F. A. Sleath
Keyword(s):  
Velocity Distribution ◽  
Vortex Formation ◽  
Reynolds Numbers ◽  
Flow Conditions ◽  
Velocity Measurements ◽  
Dominant Effect ◽  
Wave Tank ◽  
High Reynolds Numbers ◽  
High Reynolds ◽  
Laminar Flow Conditions

Measurements of the velocity distribution close to the bed have been made under laminar flow conditions in a wave tank. The classical solution for the velocity distribution was found to be valid when the bed was smooth, but considerable deviations between theory and experiment were observed with beds of sand. It is suggested that these deviations were caused by vortex formation around the grains of sand. The similarity between the velocity profiles obtained in these tests and those reported by other writers under supposedly turbulent conditions suggests that even at high Reynolds numbers vortex formation may continue to be the dominant effect in oscillatory boundary layers of this sort.

Analysis of the Mixing and Emission Characteristics in a Model Combustor

2018 ◽  
Author(s):  
Shan Li ◽  
Shanshan Zhang ◽  
Lingyun Hou ◽  
Zhuyin Ren
Keyword(s):  
Gas Turbines ◽  
Schmidt Number ◽  
Numerical Study ◽  
Flame Temperature ◽  
Scalar Dissipation ◽  
Mixing Process ◽  
Nox Formation ◽  
Turbulent Schmidt Number ◽  
Wide Range ◽  
Air Mixing

Modern gas turbines in power systems employ lean premixed combustion to lower flame temperature and thus achieve low NOx emissions. The fuel/air mixing process and its impacts on emissions are of paramount importance to combustor performance. In this study, the mixing process in a methane-fired model combustor was studied through an integrated experimental and numerical study. The experimental results show that at the dump location, the time-averaged fuel/air unmixedness is less than 10% over a wide range of testing conditions, demonstrating the good mixing performance of the specific premixer on the time-averaged level. A study of the effects of turbulent Schmidt number on the unmixedness prediction shows that for the complex flow field involved, it is challenging for Reynolds-Averaged Navier-Stokes (RANS) simulations with constant turbulent Schmidt number to accurately predict the mixing process throughout the combustor. Further analysis reveals that the production and scalar dissipation are the key physical processes controlling the fuel/air mixing. Finally, the NOx formation in this model combustor was analyzed and modelled through a flamelet-based approach, in which NOx formation is characterized through flame-front NOx and its post-flame formation rate obtained from one-dimensional laminar premixed flames. The effect of fuel/air unmixedness on NOx formation is accounted for through the presumed probability density functions (PDF) of mixture fraction. Results show that the measured NOx in the model combustor are bounded by the model predictions with the fuel/air unmixedness being 3% and 5% of the maximum unmixedness. In the context of RANS, the accuracy in NOx prediction depends on the unmixedness prediction which is sensitive to turbulent Schmidt number.

Lagrangian D’Alembert Modeled Maneuverable Autonomous Non-Holonomic Mobile Robot Using a Closed Loop PD Controller

2009 ◽  
Author(s):  
E. Georgiou ◽  
J. Dai
Keyword(s):  
Mobile Robot ◽  
Closed Loop ◽  
Ad Hoc ◽  
Search And Rescue ◽  
Pd Controller ◽  
Modeling And Control ◽  
Holonomic Constraints ◽  
And Control ◽  
Holonomic Mobile Robot ◽  
Initial Results

The motivation for this work is to develop a platform for a self-localization device. Such a platform has many applications for the autonomous maneuverable non-holonomic mobile robot classification, which can be used for search and rescue or for inspection devices where the robot has a desired path to follow but because of an unknown terrain, the device requires the ability to make ad-hoc corrections to its movement to reach its desire path. The mobile robot is modeled using Lagrangian d’Alembert’s principle considering all the possible inertias and forces generated, and are controlled by restraining movement based on the holonomic and non-holonomic constraints of the modeled vehicle. The device is controlled by a PD controller based on the vehicle’s holonomic and non-holonomic constraints. An experiment was setup to verify the modeling and control structure’s functionality and the initial results are promising.

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