CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > Siemens > STAR-CCM+

Divergence when surpassing Mach 1 using simple grid (studying compressible effects)

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Search this Thread Display Modes
Old   August 4, 2022, 07:45
Default Divergence when surpassing Mach 1 using simple grid (studying compressible effects)
New Member
Federico Echeverrz Vega
Join Date: Jul 2022
Location: Mnster, Deutschland
Posts: 5
Rep Power: 3
fede0122 is on a distinguished road
Hi Everyone!

Im currently writing my master's thesis about the behavior of a coolant (Novec 649) when passing through a passive grid inside a rectangular tunnel (3D sim). Bit of background:

I designed 2 Domains:
The first one is 4mmx4mmx215mm, a simple square grid is placed on z=15mm, its 1.5mm thick. The grid is composed of one horizontal bar 1.5mm thick, and another vertically with the same thickness. (See photo1 & 2). Ive established that the velocity inlet is the plane at z=0. The walls are established as symmetry planes, the cross as a wall.. and the plane a z=215mm is an outlet.

The second domain has the same attributes but now it has a full grid. The size is 50mmx25mmx215mm, the grid at z=15mm and 1.5mm thickness. (See photo2)

Ive established the boundary conditions according to the experimental data we have. Pressure, Temperature, etc.

My problem and I cant seem to find a reason why is...
why is it that my simulation using a single cross (domain1) converges (up to Initial of V=25m/s, beyond that we reach Mach 1 and the sim diverges) and the data is validated by our experiments... but for domain 2 using the *same* initial velocity the simulation diverges.

For Domain 1 the max v_in that I can establish is 25m/s, however, for domain 2 the max that I can establish is V_in=13 m/s?

Both of those numbers produce flows that reach Mach 0.9xxxxxx ... but the moment I input 1m/s more, we breach Mach 1 and the simulation automatically diverges.

Im using these settings:
Mesh Type: Trimmed Cells
# of Cells 30k+ for Full domain and
Space: 3D
Material: Gas -> NOVEC 649
Flow: Coupled
Equation of State: Real Gas
Real Gas Equation of State: Peng-Robinson
Time: Steady
Viscous Regime: Turbulent
Reynolds Averaged turbulence: (K-Omega or Spalart Allmaras)

For Boundary conditions I have:
Static Temperature 375K
Pressure: 281MPA

Photos 2 & 6 correspond just to a preview of the geometry of both domains
Photos 3&4 show an example of how 1m/s makes the sim Break the Mach 1 sim and thus creating divergnce
Photo 8 is domain 2 (full size) cross view of the Mesh.

Thanks a lot!
Attached Images
File Type: jpg Photo 2.jpg (27.1 KB, 4 views)
File Type: jpg Photo 6.jpg (46.2 KB, 4 views)
File Type: jpg Photo 8.jpg (63.5 KB, 5 views)
File Type: jpg Photo 3.jpg (37.0 KB, 5 views)
File Type: jpg Photo 4.jpg (35.6 KB, 3 views)
fede0122 is offline   Reply With Quote


compressible flow problem, compressible model, mach number

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
On grid independent solution for pulsatile flow David FLUENT 5 March 25, 2022 03:33
Grid Independence problem for simple pipe geometry chanchala FLUENT 13 November 20, 2003 08:24
Combustion Convergence problems Art Stretton Phoenics 5 April 2, 2002 05:59
Grid Independent Solution Chuck Leakeas Main CFD Forum 2 May 26, 2000 11:18
momentum under-relaxation for compressible flow with SIMPLE Mihai ARGHIR Main CFD Forum 0 April 7, 2000 04:58

All times are GMT -4. The time now is 17:34.