Font size: +

OpenFOAM Baby Steps - Chapter 2: Cavity Flow, Part 3

Keeping track of the physical properties, so that results can be compared to other calculations or real-world data, is difficult for me.  So this section dealing with changes to the Reynolds number needed to be its own section.

DISCLAIMER:
This is not intended as a tutorial. This is my process for working through the available documentation in a manner that makes sense to me. Feel free to follow along, but realize that I am interpreting documentation that already exists.

Paragraph 2.1.7.1 takes only one sentence to change the Reynolds number,

"Since the Reynolds number is required to be increased by a factor of 10, decrease the kinematic viscosity by a factor of 10, i.e. to 1x10^-1 m^2/s."

Keeping track of the physical property, along with the appropriate units, is difficult for me.  It probably gets easier the more frequently one works with these cases.  In the meantime, I will probably add my own comment header to the physicalProperties file in the constant directory.  This is my version 1.0:

/*-------------------------- Properties Cheat Sheet-------------------------*\
Re = d|U| / ν
Re = Reynolds Number = 100
d = characteristic length = 0.1 m
|U|= characteristic velocity = 1 m/s
ν = nu (kinematic viscosity) = 0.0001 m^2/s

properties in order:
[
NA
Length
Time
NA
NA
NA
NA
]
\*---------------------------------------------------------------------------*/

With that in place, the last part of part of paragraph 2.1.7.1 talks about setting the controlDict startFrom value to latestTime.  Although not specified either way, I assumed the value for startTime was irrelevant when using latestTime.   I assumed it would look for the folder containing the latest time.  That was incorrect; I still needed to st the startTime value to 0.5.

Cavity Flow - High Reynolds Number (Re=10)

I was interested to see what a higher Reynolds number would look like with a finer grid.  So, I increased the Reynolds number to 100 (by decreasing nu to 0.001 in the constant file.  I then used the graded cube from that portion of the tutorial and increased the number of cells in each sub-block to 30.  The solver was unable to find a solution like that because I failed to change the timeStep after changing the cell size.  I added a memory jogger, similar to the physical properties one, to place in the controlDict file.  I also had the solver start at 0, rather than mess with re-mapping.

/*---------------------------- Reminders ------------------------------------*\
Co = δt|U|/δx; Co < 1

so δt = Co δx/|U|

if the cell size is graded, the smallest cell size, δxs, is:

δxs = l*(r-1)/(αr-1)
r = R^(1/(n-1))
α = R for R>1 and α = 1 - r^-2 + r^-1 for R<1

l = the length along which the cells are graded
n = number of cells
r = ratio between one cell size and the next
R = Ration between the last and first cells

For this case:
l = 0.05m (0.1m cube divided into quarters)
R = 4
α = 4
n = 20
r = 1.0757

so δxs = 0.001146

and δt = 0.001s (rounded) since |U| = 1 m/s

\*---------------------------------------------------------------------------*/

The initial run entTime was 2.0 seconds, which wasn't long enough.  I changed that value to 10.0 seconds, but the solver converged just before 5 seconds.  As noted in the User Guide, the velocity converged before the pressures, somewhere around 2.5 seconds.

Cavity Flow - Graded Mesh with Reynolds number = 100
Stay Informed

When you subscribe to the blog, we will send you an e-mail when there are new updates on the site so you wouldn't miss them.

Related Posts

Thursday, 25 April 2024

Font size: +

OpenFOAM Baby Steps - Chapter 2: Cavity Flow, Part 3

Keeping track of the physical properties, so that results can be compared to other calculations or real-world data, is difficult for me.  So this section dealing with changes to the Reynolds number needed to be its own section.

DISCLAIMER:
This is not intended as a tutorial. This is my process for working through the available documentation in a manner that makes sense to me. Feel free to follow along, but realize that I am interpreting documentation that already exists.

Paragraph 2.1.7.1 takes only one sentence to change the Reynolds number,

"Since the Reynolds number is required to be increased by a factor of 10, decrease the kinematic viscosity by a factor of 10, i.e. to 1x10^-1 m^2/s."

Keeping track of the physical property, along with the appropriate units, is difficult for me.  It probably gets easier the more frequently one works with these cases.  In the meantime, I will probably add my own comment header to the physicalProperties file in the constant directory.  This is my version 1.0:

/*-------------------------- Properties Cheat Sheet-------------------------*\
Re = d|U| / ν
Re = Reynolds Number = 100
d = characteristic length = 0.1 m
|U|= characteristic velocity = 1 m/s
ν = nu (kinematic viscosity) = 0.0001 m^2/s

properties in order:
[
NA
Length
Time
NA
NA
NA
NA
]
\*---------------------------------------------------------------------------*/

With that in place, the last part of part of paragraph 2.1.7.1 talks about setting the controlDict startFrom value to latestTime.  Although not specified either way, I assumed the value for startTime was irrelevant when using latestTime.   I assumed it would look for the folder containing the latest time.  That was incorrect; I still needed to st the startTime value to 0.5.

Cavity Flow - High Reynolds Number (Re=10)

I was interested to see what a higher Reynolds number would look like with a finer grid.  So, I increased the Reynolds number to 100 (by decreasing nu to 0.001 in the constant file.  I then used the graded cube from that portion of the tutorial and increased the number of cells in each sub-block to 30.  The solver was unable to find a solution like that because I failed to change the timeStep after changing the cell size.  I added a memory jogger, similar to the physical properties one, to place in the controlDict file.  I also had the solver start at 0, rather than mess with re-mapping.

/*---------------------------- Reminders ------------------------------------*\
Co = δt|U|/δx; Co < 1

so δt = Co δx/|U|

if the cell size is graded, the smallest cell size, δxs, is:

δxs = l*(r-1)/(αr-1)
r = R^(1/(n-1))
α = R for R>1 and α = 1 - r^-2 + r^-1 for R<1

l = the length along which the cells are graded
n = number of cells
r = ratio between one cell size and the next
R = Ration between the last and first cells

For this case:
l = 0.05m (0.1m cube divided into quarters)
R = 4
α = 4
n = 20
r = 1.0757

so δxs = 0.001146

and δt = 0.001s (rounded) since |U| = 1 m/s

\*---------------------------------------------------------------------------*/

The initial run entTime was 2.0 seconds, which wasn't long enough.  I changed that value to 10.0 seconds, but the solver converged just before 5 seconds.  As noted in the User Guide, the velocity converged before the pressures, somewhere around 2.5 seconds.

Cavity Flow - Graded Mesh with Reynolds number = 100
Stay Informed

When you subscribe to the blog, we will send you an e-mail when there are new updates on the site so you wouldn't miss them.