Vortex shedding in sphere

Far · July 5, 2016, 03:58

Quote:

Time step size = 0.02s
Max Itr per Time step = 30

What is vortex shedding period?

No benefit of increasing max itr per time step. Should not be more than 20. If you feel convergence problem then reduce time step by 50% which will have similar effect but with better convergence and flow features

scyllakeeper · July 5, 2016, 07:13

That is what I am trying to get.
I'm trying to simulate Vortex Shedding, for flow around sphere at Re 300.

Far · July 5, 2016, 07:59

vortex shedding periodic = 1/frequency. Frequency is calculated from strouhal number

scyllakeeper · July 5, 2016, 13:21

Pardon me for misunderstanding. I do have that in my calculations.

Strouhal Number for flow past Sphere for Re 300 would be around 0.15. (rectify me if I'm wrong with this, source: http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf)

My U=300e-3m/s, and D=1e-3m

Sr = 0.15, so f = 45Hz
T = 1/f, = 0.0222s

So my cycle time is 0.022s
If I plan on having 20 to 25 time steps in one shedding cycle,
time step size = 0.022/22, = 0.001s.

Now I would use max iterations per time steps as 25.

Please do tell if any changes can be incorporated.

Far · July 5, 2016, 13:23

thats correct.

Far · July 5, 2016, 13:39

why are taking too low dia and velocity?

For Re = 300, why not put vel = 1 m, Dia = 1 m, density = 1000, so viscosity = 1000*1*1/300 = 3.3333

Time period = 1 / f ,

since strouhal no = 0.15 = f D / V

so frequency = 0.15 * V / D = 0.15 * 1 / 1 = 0.15

Therefore time period = 1/0.15 = 6.6667 s

For two steps , delta t = 6.6667 / 20 = 0.33335 s

scyllakeeper · July 5, 2016, 13:56

My analysis is for a Newtonian Fluid with density=1000kg/m3 and viscosity 0.001kg/s, so I'll have to stick to these values.

Anyway, keeping the simulation running.
Let's see how it goes. Would post the progress.
Thank you so much for this help.

scyllakeeper · July 5, 2016, 13:57

Do tell me if you have any additional suggestions/advice I could use.

Far · July 5, 2016, 14:01

why to stick to these values of density and viscosity? Reynolds number should be matched thats what we know from very basics.

scyllakeeper · July 5, 2016, 14:45

This is a project I am working on, where I need to study critical Reynolds number (the Re at which vortex shedding starts) for 6 given non-Newtonian power law fluids (for which consistency index and power index are given).
Results are to be analysed and compared.

In case of first test fluid, k = 0.001 and n = 1, which basically makes it a Newtonian fluid.
So I cannot really vary the fluid properties.

Far · July 5, 2016, 18:52

Is it hexa mesh?

scyllakeeper · July 6, 2016, 03:38

Yes.
Working on a fine mesh, as fine as it could've been with hexa cells.

shashanktiwari619 · January 19, 2017, 13:18

Did you get the results for drag and lift??Also do you get a proper vortex street?
I am doing a simulation for similar case. When I do simulations for a 2D cylinder with a very fine mesh for Re=100, I get a proper vortex shedding, however I don't get proper values of drag coefficient, lift coefficient and Strouhal number. In case of 3d flow past sphere for Re=300, I simply get a wake without any shedding?? I use the laminar-viscous model.
Please reply if you had any success with the simulations.

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July 5, 2016, 07:13		#22
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	That is what I am trying to get. I'm trying to simulate Vortex Shedding, for flow around sphere at Re 300.

July 5, 2016, 07:59		#23
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,558 Blog Entries: 6 Rep Power: 54	vortex shedding periodic = 1/frequency. Frequency is calculated from strouhal number

July 5, 2016, 13:21		#24
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	Pardon me for misunderstanding. I do have that in my calculations. Strouhal Number for flow past Sphere for Re 300 would be around 0.15. (rectify me if I'm wrong with this, source: http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf) My U=300e-3m/s, and D=1e-3m Sr = 0.15, so f = 45Hz T = 1/f, = 0.0222s So my cycle time is 0.022s If I plan on having 20 to 25 time steps in one shedding cycle, time step size = 0.022/22, = 0.001s. Now I would use max iterations per time steps as 25. Please do tell if any changes can be incorporated.

July 5, 2016, 13:23		#25
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,558 Blog Entries: 6 Rep Power: 54	thats correct.

July 5, 2016, 13:39		#26
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,558 Blog Entries: 6 Rep Power: 54	why are taking too low dia and velocity? For Re = 300, why not put vel = 1 m, Dia = 1 m, density = 1000, so viscosity = 100011/300 = 3.3333 Time period = 1 / f , since strouhal no = 0.15 = f D / V so frequency = 0.15 * V / D = 0.15 * 1 / 1 = 0.15 Therefore time period = 1/0.15 = 6.6667 s For two steps , delta t = 6.6667 / 20 = 0.33335 s

July 5, 2016, 13:56		#27
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	My analysis is for a Newtonian Fluid with density=1000kg/m3 and viscosity 0.001kg/s, so I'll have to stick to these values. Anyway, keeping the simulation running. Let's see how it goes. Would post the progress. Thank you so much for this help.

July 5, 2016, 13:57		#28
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	Do tell me if you have any additional suggestions/advice I could use.

July 5, 2016, 14:01		#29
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,558 Blog Entries: 6 Rep Power: 54	why to stick to these values of density and viscosity? Reynolds number should be matched thats what we know from very basics.

July 5, 2016, 14:45		#30
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	This is a project I am working on, where I need to study critical Reynolds number (the Re at which vortex shedding starts) for 6 given non-Newtonian power law fluids (for which consistency index and power index are given). Results are to be analysed and compared. In case of first test fluid, k = 0.001 and n = 1, which basically makes it a Newtonian fluid. So I cannot really vary the fluid properties.

July 5, 2016, 18:52		#31
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,558 Blog Entries: 6 Rep Power: 54	Is it hexa mesh?

July 6, 2016, 03:38		#32
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	Yes. Working on a fine mesh, as fine as it could've been with hexa cells.

January 19, 2017, 13:18		#33
shashanktiwari619 New Member Shashank Join Date: Sep 2014 Posts: 28 Rep Power: 12	Did you get the results for drag and lift??Also do you get a proper vortex street? I am doing a simulation for similar case. When I do simulations for a 2D cylinder with a very fine mesh for Re=100, I get a proper vortex shedding, however I don't get proper values of drag coefficient, lift coefficient and Strouhal number. In case of 3d flow past sphere for Re=300, I simply get a wake without any shedding?? I use the laminar-viscous model. Please reply if you had any success with the simulations.