OpenFOAM with a GUI: two BARAM tutorials on cloudHPC (pipe flow and Ahmed body)

Why these two tutorials

Every year we run a CFD training for engineering students at the University of Padova. Most of them have never run a simulation, so we want them to see the whole chain (geometry → mesh → solver → results → validation) in one afternoon. Writing OpenFOAM dictionaries by hand is the wrong first step for that audience.

BARAM, developed by NEXTfoam, covers exactly this gap: BARAM Mesh is a step-by-step front-end for snappyHexMesh, BARAM Flow sets up and runs the solver, and both write standard OpenFOAM case files that students can open later. We picked two cases with a known answer, so the class ends with a validation and not just with a nice picture:

• a straight pipe, checked against the Moody diagram (Colebrook equation);
• the Ahmed body, the simplified car used as a reference for automotive aerodynamics since 1984.

Results at a glance

Case	Cells	Model	Quantity	BARAM	Reference	Difference
Pipe, D 50 mm, L 1 m, 2 m/s water	78,229	steady k-ω SST	Δp over 1 m	841 Pa	740 Pa (Colebrook, smooth)	+14 %
Ahmed body, 25° slant, 40 m/s air	207,022	steady k-ω SST	Cd	0.303	0.285 (Ahmed et al. 1984)	+6 %

How the runs were set up

• Software: Salome for the geometry, BARAM Mesh and BARAM Flow for mesh and solver, all pre-installed on the cloudHPC remote desktop.
• Machine: cloudHPC remote desktop started with the ubuntu-static app, the one that gives a browser-based desktop; both solver runs in parallel on 8 MPI processes (BARAM runs the decomposition for you: Parallel › Environment).
• Turbulence: k-ω SST in both cases; pressure-based steady solver (SIMPLE) with the BARAM default schemes and under-relaxation, convergence criteria lowered to 1·10⁻⁵.

Tutorial 1: turbulent flow in a straight pipe

Geometry in Salome

A cylinder of radius 25 mm and length 1 m (Geometry › New Entity › Primitives › Cylinder), exploded into its three faces, each exported as a separate STL: inlet.stl, wall.stl, outlet.stl. One STL per boundary means the OpenFOAM patches are already named when the geometry reaches the mesher. The same steps as a Python script are in the appendix at the end of the post.

Mesh in BARAM Mesh

BARAM Mesh follows the snappyHexMesh steps from left to right: Geometry → Region → Base Grid → Castellation → Snap → Boundary Layer → Export.

Step	What we set	Why
Geometry	import the 3 STL files, rename the surfaces	names become the OpenFOAM patches
Region	point (0.003, 0.002, 0.7)	must lie inside the fluid and not on a cell face
Base grid	10 × 10 × 200 → 5 mm cubes	keep base cells as close to cubes as possible
Castellation	level 1 on all surfaces	2.5 mm cells near the boundary
Snap	defaults	projects the staircase onto the round wall
Boundary layer	3 layers on wall, final layer 0.4, expansion 1.2	near-wall resolution

Result: 78,229 cells, maximum non-orthogonality 30.5°.

Solver in BARAM Flow

Export as BaramFlow project and tick Run BaramFlow on Exported Project. Then:

• Models: turbulence k-ω SST, energy off;
• Materials: add waterLiquid from the database and assign it in Cell Zone Conditions (the default is air, a classic beginner’s mistake);
• Boundary Conditions: Velocity Inlet 2 m/s with 5 % intensity, Pressure Outlet 0 Pa, wall no slip;
• Monitors: area-weighted average pressure on the inlet, which is directly the pressure drop of the pipe;
• Initialization: Uz = 2 m/s.

Results

The run converges in 239 iterations. The section at z = 0.9 m shows the flat turbulent profile, with a peak of 2.35 m/s (1.17 × the mean velocity).

The inlet pressure settles at 841 Pa. The Colebrook equation for a smooth pipe at Re = 87,600 gives f = 0.0185 and Δp = 740 Pa: BARAM is 14 % higher.

What we learned. The gap is expected and it is a good classroom discussion:

• with L/D = 20 the flow is not fully developed (entrance length about 30 D for this Re), and the developing region has higher wall friction than the fully developed value used by Colebrook;
• wall y+ is 28–43, so the wall treatment relies on wall functions instead of resolving the viscous sublayer.

The natural exercise is a 3 m pipe, measuring Δp only between z = 2 m and z = 3 m, and a refined boundary layer for y+ ≈ 1.

Tutorial 2: the Ahmed body

Geometry in Salome

The body is 1044 × 389 × 288 mm, with a 100 mm radius on the four front edges, a 25° rear slant and 50 mm ground clearance. A few lines of Salome Python build it (box, fillet on the front edges, cut with a rotated box) and export one STL. Change one number to get the 35° variant. The script is in the appendix.

Mesh in BARAM Mesh

• Wind tunnel: a Hex6 volume from x = −2 m to 6 m, y = 0 to 1.5 m, z = 0 to 1.5 m. It is a half model: the plane y = 0 is a symmetry plane, which halves the cell count.
• Wake box: a second Hex from x = −0.3 to 2 m with its surface type set to None, so it is used only for refinement and does not become a boundary.
• Base grid: Use Hex6 = domain, 80 × 15 × 15 cells (0.1 m).
• Refinement: level 4 on the body surface and feature edges (6.25 mm cells), level 2 in the wake box (25 mm).
• Boundary layer: 5 layers on the body, expansion 1.2.

Result: 207,022 cells.

Solver in BARAM Flow

Patch	Type	Settings
domain_xMin	Velocity Inlet	40 m/s, I = 0.5 %
domain_xMax	Pressure Outlet	0 Pa
domain_yMin (y = 0), domain_yMax, domain_zMax	Symmetry	–
domain_zMin	Wall	fixed floor
body	Wall	no slip

For the drag coefficient, Reference Values must match the half model: A = 0.056 m² (half of the 0.389 × 0.288 m frontal area), U = 40 m/s, ρ = 1.225 kg/m³. A Force Monitor on body (drag along x, lift along z) plots Cd and Cl during the run.

Results

800 iterations on 8 cores took about 2 minutes. Residuals stall between 10⁻³ and 10⁻⁵ and Cd oscillates between 0.302 and 0.304: the wake of a bluff body is unsteady, so a steady RANS solver never fully settles. On this type of case, judge convergence on the monitored force, not on the residuals.

Cd = 0.303 against 0.285 measured by Ahmed et al.: +6 %, a good number for a 2-minute run on a coarse mesh. To go further:

• a mesh study (level 5 on the body, level 3 in the wake) and a y+ check on the body;
• a transient run or a hybrid RANS-LES, because the 25° slant sits right at the edge of the separation regime, which steady RANS models struggle to predict;
• a moving floor at 40 m/s, as in a real automotive wind tunnel.

Run times

Case	Cells	Cores	Iterations	Wall time
Pipe	78,229	8	239 (converged)	under 1 min
Ahmed body	207,022	8	800	about 2 min

Both runs fit comfortably in an interactive remote desktop session; the meshing steps took a few seconds each. Larger meshes (mesh studies, transient runs) are better sent to dedicated compute nodes on cloudHPC.

How to run these tutorials on cloudHPC

1. Launch the ubuntu-static app from the cloudHPC dashboard: it starts a browser-based remote desktop with Salome, BARAM Mesh and BARAM Flow already installed.
2. Paste the scripts of the appendix in the Salome Python console: the STL files land in ~/Desktop/tutorial1_pipe and ~/Desktop/tutorial2_ahmed.
3. Follow the settings in the tables above in BARAM Mesh, export to BARAM Flow, set Parallel › Environment to the number of cores of your desktop and run.
4. Remember to stop the remote desktop when you are done: it is billed until it is stopped.

FAQ

Is BARAM free?

Yes. BARAM Mesh and BARAM Flow are open source (GPL-3.0) and developed by NEXTfoam; the source is on GitHub.

Does BARAM generate standard OpenFOAM cases?

Yes. BARAM Mesh writes a snappyHexMesh-based mesh in OpenFOAM format and BARAM Flow writes the usual 0/, constant/ and system/ folders, so you can open and edit the dictionaries later.

Why is the pipe pressure drop 14 % higher than the Moody diagram?

The pipe is only 20 diameters long, so the flow is still developing, and wall y+ is around 30. A longer pipe measured far from the inlet and a finer boundary layer bring the result closer to Colebrook.

Why don’t the residuals drop on the Ahmed body?

The wake behind a bluff body is unsteady. A steady RANS run keeps oscillating, so use the drag coefficient monitor (stable within ±0.001 here) to judge convergence.

How do I get a remote desktop with BARAM on cloudHPC?

Launch the ubuntu-static app: it opens a Linux desktop in the browser with Salome, BARAM Mesh and BARAM Flow pre-installed. Stop it when you finish, because it is billed until stopped.

What reference area should I use for a half model?

Half of the frontal area: 0.056 m² for the Ahmed body, otherwise Cd comes out half of the right value.

Appendix: Salome scripts

Pipe

# Tutorial 1 - straight pipe for BARAM Mesh. Run in the Salome Python console (File > Load Script).
import os, salome, GEOM
from salome.geom import geomBuilder
salome.salome_init()
geompy = geomBuilder.New()
out = os.path.expanduser('~/Desktop/tutorial1_pipe'); os.makedirs(out, exist_ok=True)
pipe = geompy.MakeCylinderRH(0.025, 1.0)          # R = 25 mm, L = 1 m, axis +Z
geompy.addToStudy(pipe, 'Pipe')
faces = geompy.ExtractShapes(pipe, geompy.ShapeType['FACE'], True)
# name faces by the Z of their centre of gravity: 0 -> inlet, 1 -> outlet, 0.5 -> wall
for f in faces:
    z = geompy.PointCoordinates(geompy.MakeCDG(f))[2]
    name = 'inlet' if z < 0.01 else 'outlet' if z > 0.99 else 'wall'
    geompy.ExportSTL(f, f'{out}/{name}.stl', True, 1e-4, False)
print(os.listdir(out))

Ahmed body

# Tutorial 2 - Ahmed body (25 deg slant) for BARAM Mesh. Run in the Salome Python console.
import os, math, salome, GEOM
from salome.geom import geomBuilder
salome.salome_init()
geompy = geomBuilder.New()
SLANT = 25.0                                       # change to 35.0 for the separated-flow variant
L, W, H, R, clr = 1.044, 0.389, 0.288, 0.100, 0.050
s = 0.222 * math.cos(math.radians(SLANT))          # slant length 222 mm (projected)
body = geompy.MakeTranslation(geompy.MakeBoxDXDYDZ(L, W, H), 0, -W/2, clr)
front = geompy.GetShapesOnPlaneWithLocationIDs(body, geompy.ShapeType['EDGE'],
        geompy.MakeVectorDXDYDZ(1, 0, 0), geompy.MakeVertex(0, 0, 0), GEOM.ST_ON)
body = geompy.MakeFillet(body, R, geompy.ShapeType['EDGE'], front)
axis = geompy.MakeLine(geompy.MakeVertex(L - s, 0, clr + H), geompy.MakeVectorDXDYDZ(0, 1, 0))
cutter = geompy.MakeRotation(geompy.MakeTranslation(geompy.MakeBoxDXDYDZ(1, 2, 1), L - s, -1, clr + H),
                             axis, math.radians(SLANT))
ahmed = geompy.MakeCut(body, cutter)
geompy.addToStudy(ahmed, 'AhmedBody')
out = os.path.expanduser('~/Desktop/tutorial2_ahmed'); os.makedirs(out, exist_ok=True)
geompy.ExportSTL(ahmed, f'{out}/ahmed.stl', True, 5e-4, False)
geompy.ExportSTEP(ahmed, f'{out}/ahmed.step')
print(geompy.BoundingBox(ahmed))

References

1. Ahmed, S. R., Ramm, G., Faltin, G. (1984). Some salient features of the time-averaged ground vehicle wake. SAE Technical Paper 840300. doi:10.4271/840300
2. Colebrook, C. F. (1939). Turbulent flow in pipes, with particular reference to the transition region between the smooth and rough pipe laws. Journal of the Institution of Civil Engineers 11(4), 133–156. doi:10.1680/ijoti.1939.13150
3. Moody, L. F. (1944). Friction factors for pipe flow. Transactions of the ASME 66, 671–684.
4. Menter, F. R. (1994). Two-equation eddy-viscosity turbulence models for engineering applications. AIAA Journal 32(8), 1598–1605. doi:10.2514/3.12149
5. BARAM, NEXTfoam: github.com/nextfoam/baram
6. SALOME platform: salome-platform.org

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