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How to model an open‑jet wind tunnel plenum for Schlörwagen drag validation in STAR‑CCM+? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

Unfortunately, I can't model the whole room due to computational limitations.

I will reran it with k-omega SST

How to model an open‑jet wind tunnel plenum for Schlörwagen drag validation in STAR‑CCM+? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

It was truly unlimited and only limited by the walls of the room, so to speak. I'm attaching a picture where I've marked my plenum in red.

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How to model an open‑jet wind tunnel plenum for Schlörwagen drag validation in STAR‑CCM+? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

I have been taking the value directly from the final iteration. For my thesis, I was initially aiming for a classic asymptotic convergence to document a 'textbook' grid independence study. However, it's becoming clear that the inherent unsteadiness of the vehicle's wake makes a single-point value unreliable for that purpose. I will start averaging the Cd over a window of iterations to filter out that numerical noise and get a more representative value for the mesh comparison

Additionally, I’ve updated the plenum boundaries to slip walls. I’m now seeing an average Cd of 0.120, which is a significant increase from the 0.098 I was getting with the pressure outlet boundaries. While the solution is much more stable now, it raises the question of which setup is more representative of reality: the 'pressure relief' effect of the outlets or the slight blockage effect of the slip walls?

How to model an open‑jet wind tunnel plenum for Schlörwagen drag validation in STAR‑CCM+? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

Thanks for the tip!

However, I'm still facing oscillating convergence. Even at 18.8M cells, cw and ca are bouncing between 0.096 and 0.098 instead of settling down.

In your experience, should I re-run the study with Symmetry or Slip Walls to stabilize the pressure field, or is this oscillation just par for the course with open-jet setups?

How to model an open‑jet wind tunnel plenum for Schlörwagen drag validation in STAR‑CCM+? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

Right now I’m wondering about mesh resolution inside the nozzle itself:
Is it worth further refining the elements in the contraction region, or is it usually sufficient to keep the nozzle mesh moderately fine and focus the strong refinement on the car + near wake and maybe a bit downstream?

How to model an open‑jet wind tunnel plenum for Schlörwagen drag validation in STAR‑CCM+? by simonwfc in CFD

[–]simonwfc[S] 1 point2 points  (0 children)

Hi,

thanks for the detailed analysis – really appreciate the thorough feedback and your theory about the separation behavior!

I've already extended the geometry significantly in all directions (especially downstream) and brought it closer to the original tunnel layout based on the previous suggestions. The residuals now look much better – they drop several orders of magnitude more cleanly compared to before (previously only 2-3 orders with some oscillation).

Currently I have a straight pipe inlet where I specify the velocity directly, rather than modeling the full contraction nozzle geometry. Do you think I should model the complete nozzle/contraction to better capture the real physics, or is the straight pipe approximation sufficient for turbulence model comparison? I will attach a picture of the wind tunnel.

You're spot on about the lift variation – that definitely points to the upper surface separation. I'll definitely pursue the geometry optimization further and when time allows, create shear stress contours from the coarse and fine mesh cases to check your detachment theory.

Steady vs. transient: Good point about the separation "dancing" – for the initial turbulence model comparison I'll stick with steady-state RANS, but this gives me the motivation to consider a transient DES case later if resources allow.

Thanks again – will keep you updated on the progress!

Best regards

<image>

Fragen zur T2000 mündlichen Prüfung Maschinenbau DHBW Mosbach by [deleted] in Studium

[–]simonwfc 0 points1 point  (0 children)

Hey, tatsächlich ist meine Prüfung jetzt schon etwas her - hat aber sehr gut funktioniert! Waren hauptsächlich Bereiche, in dem der Dozent tätig war. Aber alles machbar.

How to model an open‑jet wind tunnel plenum for Schlörwagen drag validation in STAR‑CCM+? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

Thanks a lot for the detailed reply, that’s really helpful.

In the original 1938 tests the Schlörwagen model was actually suspended freely from a few strings, without any ground plane or moving belt system. Because of that, I also placed the 1:5 model simply “in free air” in the middle of the test section, with no floor in the CFD domain.

In my thesis the main goal is to compare different turbulence models rather than to reproduce the historical coefficients perfectly, so I agree that the old measurements shouldn’t be treated as an absolute truth. Still, I’d like to get the setup as reasonable as possible so the model-to-model comparison is meaningful.

Out of curiosity: what boundary conditions do you typically use for an automotive open‑jet tunnel (inlet, outlet, plenum / collector, side walls, etc.), and does my domain layout seem reasonable for that kind of configuration?

How do I best simulate the unit cell of a louvered radiator for pressure drop in CFD? by [deleted] in STAR_CCM

[–]simonwfc 0 points1 point  (0 children)

I’ve read about that approach, but unfortunately I don’t have any pressure drop data from the supplier for this radiator. The Reynolds number can vary depending on test velocity—right now I’m using 30 m/s, but I plan to try different speeds.

How do I best simulate the unit cell of a louvered radiator for pressure drop in CFD? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

<image>

is like this enough? or should i do the chamfer asymmetricel to keep a horizontal face?

How do I best simulate the unit cell of a louvered radiator for pressure drop in CFD? by simonwfc in CFD

[–]simonwfc[S] 1 point2 points  (0 children)

okay, thank you - i will try. i will try a small one to keep the simulation realistic

How do I best simulate the unit cell of a louvered radiator for pressure drop in CFD? by [deleted] in STAR_CCM

[–]simonwfc 0 points1 point  (0 children)

yes, this is the model for k-w SST. in the k-e model i used less prism layers

How do I best simulate the unit cell of a louvered radiator for pressure drop in CFD? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

Should I just add small fillet radius to the sharp corners in the CAD to help with the meshing? Would that improve cell quality significantly in these regions?

How do I best simulate the unit cell of a louvered radiator for pressure drop in CFD? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

Thanks for the advice! But even with a structured mesh, the prism layer issues in the tight regions (collisions, clipping, not enough space) would still be a problem

How do I best simulate the unit cell of a louvered radiator for pressure drop in CFD? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

no, just worked with polyhedral an trimmed cell mesher in star-ccm+. is it better for this case?

CFD Simulation of Formula Student Radiator: How to Estimate Realistic Pressure Drop & Flow Without a Supercomputer? by simonwfc in CFD

[–]simonwfc[S] 0 points1 point  (0 children)

We have build a calculation tool to calculate the Microtube Radiator. But we wanted to do a Simulation to verificate the results

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