[ody93]

Hi all! I am new to cfd and i have some issues with a university project i am working on for the past few months. I hope you could help me cause i am desperate, specially with my deadlines

The car geometry is very close to a real one so it has lot of surfaces and is highly complicated. I imported it as a solid body in Design Modeler (STEP file). I made an enclosure which is the flow domain and a boolean operation in order to substract the car body. I kept the half of the car. Then i made 3 volumes, one at the wake of the car, one at the underbody and one as a car box in order to make some volumetric meshes.

Here i have to mention that the car dimensions are 3101.5x1355.56x1227.7mm (Length x Width x Height) so the air domain is quite large. (40319x5329x10583)

Mesh
Size Function: Proximity and Curvature
Relevance Center: Fine
Initial Size Seed: Active Assembly
Transition: Slow
Span Angle: Fine
Min Size: 2mm
Min Proximity Size: 6.1350mm
Max Face Size: 613.5mm
Max Tet Size: 1227mm
Defeature Size: 1mm (Default)

I inserted 3 body sizings (all with body of influence and proximity and curvature) at each of the three "boxes" i made
Car Box: element size 100mm Local min size: 20mm
Wake Box: element size 15mm Local min size: 10mm
Underbody Box: element size 20mm Local min size: 4mm

and an Inflation method at the car body with the following characteristics:
First Layer Thickness
First Layer Height: 5mm
Max Layers: 5
Growth Rate: 1.2
Algorithm: Pre

At last, my mesh has 1.796.330 nodes and 9.249.126 elements with
orthogonal quality:
min: 0.21627
max: 0.99997
average: 0.87122

and Skewness:
min: 1.1693x10e-4
max: 0.90345
average: 0.21221

From what i have read, this is a very finite mesh and it also has 5 layers around the car for the boundary layer separation and in order to accurately obtain lift and drag.

For the Setup i used:
pressure based solver with steady time
the realizable k-e model with non equilibrium wall functions
materials: air (default fluent values) with velocity 27 m/s (100km/h) at -X direction
Boundary Conditions set as symmetry at the symmetry plane XY, car body and ground are stationary walls with no slip, velocity inlet has turbulent intensity 1% and viscosity ratio 10 and pressure outlet turbulent intensity 5% and viscosity ratio 10.

Reference values are computed from inlet and the frontal area is 0.6703165

The initialize was Hybrid. The Courant Number was set at 50.
I made the first 100 iterations with Coupled Scheme, Standard Pressure, First Order Upwind for Momentum, turbulent kinetic energy and dissipation rate and turbulent viscosity at 0.8. Then I made the next 500 iterations with the last 3 set at Second Order Upwind and turbulent viscosity at 0.95.

The solver was also set at double precision and 6 parallel processes.
The PC has:
CPU: Intel i7 7800X @ 4GHz (6 cores)
RAM: 32 GB (4x8)
SSD: 500gb (M2 type so speeds are very high)

The result was that after 600 iterations no convergence was reached. The residuals were at a steady number with small fluctuations (+/-). The Drag and specially the lift coefficients the same behavior. They didn't even managed to have a convergence at 2nd decimal.

Please help, i don't even know from where to start. I have read so much in internet that i am confused. Should i drop the number of elements? Make smaller cells at wake, underbody and car box? Test another model (and which one?) or just hope that they will converge at much more iterations? (which i think doesn't seem to happen)

Thank you all! (and sorry for the extended post)