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

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August 4, 2022, 07:45
Divergence when surpassing Mach 1 using simple grid (studying compressible effects)
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Federico Echeverrz Vega
Join Date: Jul 2022
Location: Münster, Deutschland
Posts: 5
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Hi Everyone!

I´m 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). I´ve 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 can´t 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.

I´m 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
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
 Photo 2.jpg (27.1 KB, 4 views) Photo 6.jpg (46.2 KB, 4 views) Photo 8.jpg (63.5 KB, 5 views) Photo 3.jpg (37.0 KB, 5 views) Photo 4.jpg (35.6 KB, 3 views)

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