Hello, The particle Reynolds number makes me confused, and I hope someone can help me with this, please! Reynolds number is calculated by Re = ρfUD/μ where U is particle velocity, D can be particle diameter, and ρf and μ are density and viscosity of the continuous fluid. How I can Reynolds number if the particle velocity U is unknow? Is it correct to calculate first the particle terminal velocity Ut = (gD2(ρp-ρf))/(18μ) , (where ρp is the density of the particle), then to use the calculated Ut to calculate the Reynolds number? I need to calculate the particle's terminal velocity but stuck with the Re number. Thank you in advance.