Abstract
A sediment particle in a flow not only follows the flow direction but also randomly diffuses through the surrounding fluid, because of the turbulence in the flow. In this study, particle movement is regarded as a stochastic process. The stochastic-diffusion particle tracking model is able to simulate the random characteristics of particle trajectories based on stochastic methodologies and physical mechanisms. This study proposes a state-of-the-art two-particle stochastic-diffusion particle tracking model, which considers particle correlation. The spatial correlation depends on the distance between paired particles. Particles are highly correlated and have similar random movement because of large-scale eddies when paired particles are located in the immediate vicinity of each other. The model is applied to open-channel flows, and simulation results of ensemble means and variances of particle positions are examined. Concentration results can be expressed as time-varying probability density functions. The proposed models are validated against the experimental data through ensemble mean velocities and sediment concentrations. This study also examines whether movements of sediment particles in open-channel flow follow the Fickian law. Simulation results of ensemble variances of particle displacements in longitudinal and vertical directions reveal a deviation from the Fickian hypothesis. The influence of the resuspension mechanism on the observed anomalous diffusions is discussed.
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Abbreviations
- \(\varvec{A}\left( {\varvec{X}_{t} ,t} \right)\) :
-
The drift terms in the SDE
- B t :
-
The standard Brownian motion modeled by the Wiener process
- \(\varvec{B}_{t}^{\prime }\) :
-
A correlated Brownian motion independent of B
- C μ :
-
Empirical coefficients given as 0.09 (Toorman 2008)
- DX :
-
A constant related to the numerical grid size in f(r)
- d :
-
Sediment particle diameter
- E[·]:
-
Expected value
- f(r):
-
A correlated coefficient related to the distance between paired particles r
- g :
-
Gravity acceleration
- H :
-
Water depth
- I :
-
Identity matrix
- k :
-
The turbulent kinetic energy
- k′:
-
The coefficient of experimental investigation, Cai (1965) gives a value 0.75 for k by considering the force conservation
- k s :
-
The roughnees high
- l :
-
The Prandtl’s mixing length
- \({\mathcal{N}}\left( {0,1} \right)\) :
-
The standard normal distribution with a zero mean and a unit standard deviation
- r :
-
The distance between paired particles \(r = \left\| {\varvec{X}^{\left( i \right)} - \varvec{X}^{\left( j \right)} } \right\|\)
- \(\text{Re}_{{{\text{k}}_{\text{s}} }}\) :
-
The roughness Reynolds number, \(\text{Re}_{{k_{s} }} = k_{s} u_{*} /\nu\)
- Reτ :
-
The friction Reynolds number
- Sc t :
-
The Schmidt number
- St :
-
The Stokes number
- S v :
-
The sediment volume concentration
- \(\bar{U},\bar{V},\bar{W}\) :
-
The mean flow velocities of direction x, y and z, respectively
- u * :
-
Shear velocity
- \(\bar{u}\) :
-
Mean velocity in x direction
- u’, v’ :
-
Velocity fluctuations caused by turbulent eddies in longitudinal and vertical directions
- \(\sqrt {\overline{{w^{{{\prime }2}} }} }\) :
-
Turbulent intensity
- w′:
-
Velocity fluctuations in vertical direction
- w s :
-
The settling velocity of sediment particle at concentration Sv
- \(w_{{s_{0} }}\) :
-
The settling velocity of sediment particle at zero concentration
- \(\hat{w}\) :
-
\(w^{\prime } /\upsigma_{2}\)
- \(\left\langle x \right\rangle\) :
-
The mean of the particle displacement xi over time t
- X t :
-
\(\varvec{X}_{t} = \left\{ {X\left( t \right),Z\left( t \right)} \right\}\) particle position at time t
- α 0 :
-
A coefficient for two-equation (k–ε model) which is defined as α/Cμ
- β :
-
The constant diffusion effect which can be chosen between 0 and 1
- \(\beta \varvec{B}_{t}^{{\prime }}\) :
-
The diffusion due to large scale turbulence
- \(\sqrt {1 - \beta^{2} } \Delta B_{t}\) :
-
The diffusion due to molecular diffusion and small scale turbulence
- γx, γz :
-
The scaling diffusion exponent in x and z directions
- ε :
-
Diffusion coefficient
- ε m :
-
The momentum exchange coefficient
- ε s :
-
The turbulent diffusion coefficients of suspended particles
- εx, εy, εz :
-
The sediment diffusion coefficient in x, y and z direction
- κ :
-
The von Karman constant
- ν :
-
The kinematic viscosity
- ν t :
-
The eddy viscosity or the diffusivity of momentum
- ρf, ρs :
-
The density of fluid and solid, respectively
- \(\varvec{\sigma}\left( {\varvec{X}_{t} ,t} \right)\) :
-
The diffusion terms in the SDE
- σ 2 :
-
The root-mean square of w′
- σx, σz :
-
The ensemble averaging mean-square particle displacement in x and z directions
- τ p :
-
The particle timescale
- τ t :
-
The turbulent shear stress
- τ w :
-
The wall shear stress
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Acknowledgements
This research was supported by the Taiwan Ministry of Science and Technology under Grant Contract Number 107-2628-E-002-002-MY3, and 108-2221-E-002-011-MY3. The research fund awarded to the first author by the US National Science Foundation under Grant Contract Number EAR-0748787 is appreciated. The authors state that data used are available in the tables and figures in the manuscript.
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Tsai, C.W., Hung, S.Y. & Wu, TH. Stochastic sediment transport: anomalous diffusions and random movement. Stoch Environ Res Risk Assess 34, 397–413 (2020). https://doi.org/10.1007/s00477-020-01775-3
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DOI: https://doi.org/10.1007/s00477-020-01775-3