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I am running a simulation with LAMMPS involving a colloid suspended in a fluid. Simple shear is applied creating flow. My question is, how can I calculate the Reynolds number of the fluid given I don't know it's density? The fluid is modeled through LAMMPS only by specifying it's viscosity.

To get the Reynolds number I need to calculate Re = $\rho u L / \mu $. $\rho$ is the fluid density (unknown), $u$ is the flow speed (I'll use the value at the top of the simulation box since this varies), $L$ is a characteristic length (the diameter of the colloid particles), and $\mu$ is the viscosity of the fluid (which I know).

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  • $\begingroup$ Consider to spell out acronyms. $\endgroup$
    – Qmechanic ♦
    Commented Jan 12, 2023 at 6:59
  • $\begingroup$ What command are you using for it? For example fix lb/fluid need viscosity and density as inputs (at least in the current documentation), so you should be able to extract the values that you need, isn't it? $\endgroup$
    – user239504
    Commented Jan 12, 2023 at 8:28

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The evaluation of the Reynolds number does not require a precise determination of its ingredients. The order of magnitude $\pm 1$ is enough. Looking at typical liquid densities under normal conditions, you'll realize that most liquid solvents have a mass density of the same order of magnitude as water density. Therefore, using water mass density should not be critical for evaluating the flux regime through the Reynolds number.

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Kinematic viscosity $\nu=\mu/\rho$ is often tabulated separately from dynamic or "regular" viscosity $\mu$.

Also, if it's a simulation, can you not simply choose a Reynolds number as an input parameter? You can pick an arbitrary system of units where $\rho=1$, and it becomes irrelevant.

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  • $\begingroup$ I suspect the Reynolds number is undetermined/ irrelevant since I don't have a density. But I'm surprised I can't calculate it some other way (e.g. by stopping the shear and seeing how quickly the particles come to rest or something.) $\endgroup$
    – Zesty Physicist
    Commented Jan 12, 2023 at 4:13
  • $\begingroup$ It's hard to say without knowing what you are inputting and what output you are attempting to get out. You can always assume it is water, as most common fluids are not too different in density $\endgroup$
    – RC_23
    Commented Jan 12, 2023 at 4:42

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