## Fluids

#### Reynolds number – fluid between spheres

• shehab Gamrah
Subscriber

I have a vibrating inner sphere 14 cm diameter (blue) that reaches a maximum velocity of 5 m/s and an outer sphere (grey) of 16 cm diameter. The gap between the spheres is 1 cm containing a fluid of density 1000.567 kg/m3 and viscosity 0.0009 kg/m. s. I used the viscous laminar model and the solution converged, however I was wondering should I use a turbulence model, and also how can I find Reynolds number for this case. Also if viscous laminar model worked properly, does it mean the flow is laminar.

• YasserSelima
Subscriber
I think using the gab between the two cylinders as a characteristic length would be reasonable. nIf the results you got is similar or close to experimental data, the model you used is valid. If you don't have experimental data and you don't know if the flow is turbulent or laminar, use a transition model like k-omega ... but in this case you will need to refine the boundary layer n
• shehab Gamrah
Subscriber
Thanks for your help. I have a question regarding y+ value. when I calculate y+ value do I use the value of the sphere's vibrating velocity, or an approximate value of the fluid's maximum velocity as the fluid's velocity would be much higher than the inner sphere as a result of the vibration n
• YasserSelima
Subscriber
I don't think the fluid velocity would be much higher than the sphere. n
• shehab Gamrah
Subscriber
I miswrote a number in the first comment. The sphere's maximum velocity is 1.5m/s, however the fluid's velocity reaches a maximum value in the range of 5-5.3 m/s.n
• DrAmine
Ansys Employee
You might assume the fluid to be weak compressible to avoid any velocity overshoot.n
• DrAmine
Ansys Employee
How are u modeling the vibration or motion of inner sphere?n
• shehab Gamrah
Subscriber
Through a udf code of inner sphere and solid deforming. And turbulence model kepsilon realizable. I've done mesh convergence and the residuals converge and the results don't vary as mesh is reduced.n
• Rob
Ansys Employee
Then you've probably got as good an answer as you can, assuming the various settings (materials, bc's etc) are correct. n