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For Newtonian, ignore inertial terms the Drag Force is required in all domains in the selection of the physics interface.
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For Newtonian, ignore inertial terms the Drag law list is not shown. The Stokes drag law is always used.
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Stokes drag and the Oseen correction are applicable for particles that have a relative Reynolds number much less than one. If the particle Reynolds number is greater than one, then select Schiller-Naumann. If, in addition, the particles are nonspherical, select Haider-Levenspiel. If the particles are very pure gas bubbles or liquid droplets, select Hadamard-Rybczinski. The Standard drag correlations are a set of piecewise-continuous functions that are applicable over a wide range of relative Reynolds numbers.
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For all choices, enter coordinates for the Velocity field u (SI unit: m/s) based on space dimension. If another physics interface is present which computes the velocity field then this can be selected from the list.
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For all choices, the Dynamic viscosity μ (SI unit: Pa·s) is taken From material. For User defined enter another value or expression.
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For all choices, the fluid Density ρ (SI unit: kg/m3) is taken From material. For User defined enter another value or expression.
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For the Stokes drag law, the Include wall corrections check box is shown. This check box is cleared by default. Select it to apply corrections to the drag force for particles in a wall-bounded flow. If the Include wall corrections check box is selected, the Wall Corrections section will be shown (see below).
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Selecting the Include wall corrections check box can cause a significant increase in computation time in 3D models with a fine boundary mesh.
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For Haider-Levenspiel enter a value or expression for the particle Sphericity Sp (dimensionless). The default is 1.
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The Basset model is appropriate for relative Knudsen numbers much less than one, while the Epstein model is appropriate for free molecular flows. The Phillips model is usable at intermediate values of the relative Knudsen number and shares the same asymptotic behavior as the Epstein and Basset numbers at very large and small Knudsen numbers, respectively. The Cunningham-Millikan-Davies model includes three tunable parameters that can be used to fit the drag force correction factor to empirical data.
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If Basset, Epstein, or Phillips is selected from the Rarefaction effects list, enter an Accommodation coefficient σR (dimensionless). The default value is 1. The accommodation coefficient may be interpreted as the fraction of gas molecules that undergo diffuse reflection at the particle surface.
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If Cunningham-Millikan-Davies is selected from the Rarefaction effects list, enter the three dimensionless coefficients C1, C2, and C3. The default values are 2.514, 0.8, and 0.55, respectively. When entering a set of Cunningham coefficients, note that the COMSOL implementation of the Cunningham correction factor defines the relative Knudsen number using particle diameter, not radius.
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If None is selected, no turbulent dispersion term is applied.
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If Discrete random walk is selected, a random term is added to the background fluid velocity when computing the drag force at every time step taken by the solver. The random perturbation term is held constant for a time interval equal to the interaction time of the particle with an eddy in the flow.
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If Continuous random walk is selected, a random perturbation is applied to each particle by integrating a Langevin equation. Unlike Discrete random walk, the perturbation of the background velocity that is applied to each particle depends on the time history of all previously applied perturbations.
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The Turbulent kinetic energy k (SI unit: m2/s2) determines the magnitude of the turbulent dispersion term. If a physics interface is present that computes the turbulent kinetic energy, it can be selected directly from the list.
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The Turbulent dissipation rate ε (SI unit: m2/s3) is related to the lifetime of eddy currents in the flow. If a physics interface is present that computes the turbulent dissipation rate, it can be selected directly from the list.
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The Lagrangian time scale coefficient CL (dimensionless) is used to compute the Lagrangian time scale of the particle-eddy interactions. The default value is 0.2.
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Selecting the Include anisotropic turbulence in boundary layers check box can cause a significant increase in computation time in 3D models with a fine boundary mesh. See the Wall Corrections section.
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Use tolerance is a balanced option with better performance than Closest point, especially in finely meshed 3D geometries. As long as the Search radius is sufficiently large, this is a good option for channels or pipes with a large aspect ratio.
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Closest point is the most robust option, but also the slowest. Because it is not limited by a search radius, it is a fair choice when the model geometry includes both narrow channels and wide-open regions.
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Walk in connected component should only be used when the coordinates of the nearest point on a wall changes continuously over time for each particle, rather than jumping between different boundaries.
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