Learning Forum

[Anthony Irvine]

I have quite a complex geometry which vibrates due to a fsi I've been getting good results from this however I've only been looking at water as the working fluid so far but I wanted to investigate the impact of viscosity and density, so I changed the fluid to a variety of fluids such as ethanol mercury glycerine and a few others, but when I do this the results don't really seem to change other than the mass flow but this is due to a change in density. when looking at variables such as velocity, turbulent kinetic energy, temp, volume flow rate and others but when changing these fluids the results don't seem to change at all. I'm not sure what the issue is. my vibratory movement comes from a displacement at one end. any advice or ideas would be greatly appreciated 

[Rob]

Temperature will depend on calculated HTC, k & cp.density so you may not see much of a difference. 

How does the Reynolds Number compare across the fluids?

[Anthony Irvine]

thanks for the response however I'm unsure how to calculate the Reynolds number for my system due to its complex shape and changing velocity. how would I find this value best or what other ways could I assess it #

thanks

[Rob]

Same way as for an experiment. Pick a few positions and work it out. You're after a comparison, so as long as you're consistent you should be fine. Remember, the various dimensionless numbers and HTC are made up to help understand fluid flow, they don't control the flow. It's always taught backwards as cause & effect get muddled up: it's taken me years of looking at weird flowfields to really understand how things work! 

[Anthony Irvine]

so from my results, I've found each fluid gives me the same velocities so yes the Reynolds numbers are different but I don't think this tells me anything as they are only different due to the material properties, looking at water I get a value of 77000 and for mercury I get 70000. I'm not sure what this tells me, other than that my system is turbulent.

thanks

[Rob]

So similar levels of turbulence. Would that have an effect on the excitation frequencies?