Physics Help Forum Finding new Reynold's Number (Re) with decrease and increases in percentages

 Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum

 Apr 26th 2019, 06:52 AM #1 Junior Member   Join Date: Apr 2019 Posts: 23 Finding new Reynold's Number (Re) with decrease and increases in percentages This is the OG question: "A small hose in a machine carries a viscous fluid. When the machine is first turned on the fluid flows through the hose and the Reynold's number is 1800. As it warms up, there is a reduction of the density of the fluid of 1.61%, a reduction in the viscosity of the fluid of 6.74%, and an increase in the diameter of the hose by 1.50%. Assuming the speed of the fluid is kept constant, what is the Reynold's number now?" I know what I did probably looks like the dumbest thing ever, but how would I go about figuring out the new Reynold's number? My working for this question is the section at the VERY bottom, with the Reynold's equation, ofc (ignore the work in progress above lolol) https://imgur.com/a/ecjv0eM Thanks for any and all help -I
 Apr 29th 2019, 06:06 AM #2 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,011 Reynolds Number is the ratio between the inertial forces (which act to keep the fluid moving exactly as it is now) and the viscous forces (which act to resist the fluids movement and slow it down). Re=Vflow.Rho.Lref/Visc. Vflow is the speed of the fluid Rho is the density Visc is the viscosity Lref is "a representative length" Lref is subject to interpretation depending on the practical application. (for a wing it is usually the distance between the leading a trailing edge of the wing) For a pipe, I am guessing that Lref might be the pipe diameter. If so, your problem becomes: Re'=Vflow.Rho*{(100-1.61)/100}.Lref*{(100+1.5)/100}/(Visc*{(100-6.74)/100}) Re'=Vflow.Rho*{0.9839}.Lref*{1.015}/(Visc*{0.9326}) Re'=(Vflow.Rho.LRef/Visc) * {0.9839*1.015/0.9326} but (Vflow.Rho.LRef/Visc) is what we originally started with which the question tells us is 1800. __________________ ~\o/~ Last edited by Woody; Apr 29th 2019 at 06:11 AM.

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