r/CFD

Hey everyone, I’m running into a massive roadblock with my glider design and could really use a sanity check.

I’ve been designing a glider in XFLR5 and was honestly pretty happy with the numbers. Today, I decided it was time to do a "proper" 3D CFD analysis using ANSYS Fluent. After about 4 hours of intense struggling (this is my first time doing any CFD), the simulation finally converged.

However, under the exact same operating conditions (same velocity, density, and angle of attack), my lift force in Newtons is literally three times smaller in ANSYS than it is in XFLR5.

Is it possible that I screwed something up during the setup or are results like these expectable?

Hello guys I have 6 months left for placement and I want to learn CFD, and have no prior experience, I do have plenty enough experience with Solidworks geometry only I can dedicate 4 hours per day can anyone suggest me a learning path so I will get placements

The classical bulk-flow model of CSF — continuous secretion by choroid plexus, flow through ventricles, absorption via arachnoid granulations — has faced mounting challenges over the past two decades (Orešković & Klarica 2010, 2011; Greitz 2004). Yet an alternative coherent framework for ventricular CSF dynamics within the cardiac cycle has not been fully articulated.

I am an independent researcher (physician, Slovenia) with a long-standing interest in intracraniovertebral volume homeostasis. My preprint proposes a push-and-refill mechanism: freshly actively formed (FAF) CSF is released passively into the cerebral ventricles during diastole: https://doi.org/10.5281/zenodo.18783880.

This model predicts a specific, testable signature: ventricular volume should increase during diastole, coinciding with net aqueductal flow directed caudally. To my knowledge, no published study has combined simultaneous cine-volumetric ventricular MRI with directional PC-MRI at the aqueduct of Sylvius within a single cardiac cycle protocol — which represents the critical verification gap.

I am looking for:

• Critical engagement with the volumetric/flow logic
• Awareness of any existing datasets that might bear on this (cine MRI + aqueductal PC-MRI in the same subjects)
• Potential collaborators with access to cardiac-gated 4D flow or PC-MRI protocols

Preprint: https://doi.org/10.5281/zenodo.18783880

https://darko.med-lavrencic.si/#csf-cardiac

Trying to fill a pool as quickly as possible. The house has two spigots. The pool is 70’ from each spigot and I have two 3/4” hoses that are 50’ each and three 5/8” hoses that are 25’ each. To maximize fill speed would it be better to run a 50’ hose to each spigot then add a 25’ long 5/8” hose on the end of each to reach the pool or run the 3/4” hoses end to end from the same spigot and then run the three 5/8” hoses end to end from the other spigot?

https://preview.redd.it/tof2zuicm7bh1.png?width=498&format=png&auto=webp&s=7434e96e240fbcc0422c54c1d5f0c23d4b58addd

this is first time it happened to me but i believe i will need to change geometry anyways (student) 🤣

I have a problem statement: “ A closed tube, water bubble into it, what happens to its droplet dynamics if we vary internal pressure conditions in a fixed temp.” how to approach and model this using ANSYS Fluent. Can anyone suggest tutorial video or help me with the CFD modelling approach?

The opening is preferablely for female candidates only because our company needs to comply with 30% female workforce internal guidelines. DM if anyone is interested.

Edit 1: Fellow Redditors look, I don't want to be seen like I am promoting gender biased hiring & get caught so, I will refrain from naming my company for my safety until I am convinced you are a genuinely interested & good a candidate I can recommend.

Edit 2: We are looking for people with 3-4 years experience, the role involves vibration & fatigue simulations on automotive components.

Perks & benefits: WFH & full ownership of your design recommendations (no boss or other entity taking decisions for you)

I can't reveal anymore unless I've seen the resumes.

Regards

The errors that occur are: divergence, floating point exception, invalid cp, reversed flow, etc. but these are the most common

The solutions I have tried now are

In the Setup
- Changed discretization
- Changed from real-gas-NIST to polynomial
- Changed courant number and relaxation factor

In the mesh
- Changed element size from very fine to coarse
- Tried refining the throat area, tried other refinement methods

In the geometry
- Tried various different physical parameters that are allowed for the ejector study.

In the picture, only half of it is done as it is a 2D-axisymmetric instead
I also attached the allowed parameters

Please send help, I have been stuck on these for weeks now. I still cant get those shockwaves to form. Thank you!

https://preview.redd.it/8wveeuszf7bh1.png?width=533&format=png&auto=webp&s=d919d4c5699c155b6bfac67de9266073e1c18beb

https://preview.redd.it/j8j0kj8xf7bh1.png?width=857&format=png&auto=webp&s=2d188681d542d85347a70824ecef844b60d47272

Title:
STAR-CCM+ Francis Turbine CFD – Pressure-limited cells increasing despite good mesh. Should I keep iterating?
Body:
Hi everyone,
I’m simulating a Francis turbine in STAR-CCM+ and would appreciate some advice before I spend many more hours running the case.
Turbine (published design point)
Discharge: Q = 2.25 m³/s
Mass flow: 2250 kg/s
Net head: 38.335 m
Speed: 1000 rpm
Hydraulic efficiency: 0.93
Published shaft power: 786.9 kW
CFD setup
Steady-state
MRF (Multiple Reference Frame)
Water (incompressible)
Mass Flow Inlet = 2250 kg/s (Constant)
Pressure Outlet = 0 Pa gauge
MRF = +1000 rpm
Turbulence model: k-ω SST
Initial CFL was reduced (around 30–45) because higher CFL caused instability.
Pressure and torque monitored throughout the run.
Mesh
Total cells: ~7.3 million
Mesh diagnostics:
No negative volume cells.
Mesh is topologically valid.
Minimum face validity ≈ 0.784
99.96% of cells have face validity = 1.
Volume change statistics also look good.
So I don’t think the mesh quality itself is the issue.
Moment report
The moment report is calculated on the runner (cark) boundaries only, excluding the rotating interfaces.
Problem
Around 360 iterations:
Torque (moment) appears to be stabilizing around 2–4 kN·m.
TKE residual is now decreasing.
Outlet flow is becoming steadier.
However, STAR-CCM+ keeps reporting:
“Minimum absolute pressure limited to 1000 Pa on XXX,XXX cells”
The number of pressure-limited cells has actually been increasing over roughly the last 10 iterations.
Things I’ve already checked
Tried both +1000 rpm and −1000 rpm. The torque sign flipped with the rotation sign, so I’m currently using +1000 rpm.
Verified the rotating region contains only the runner.
Verified the interfaces.
Verified the outlet boundary.
Mesh quality appears good.
Using the published operating speed (1000 rpm) and published flow rate (2250 kg/s).
My questions
Is it normal for the pressure-limited cell count to keep increasing while the torque and TKE residual are stabilizing?
Would you continue running for another few hundred iterations, or would you stop and investigate before wasting more computation time?
If a steady solution converges but still reports a large number of pressure-limited cells every iteration, would you trust the resulting torque and efficiency?
What would you check next in this situation (boundary conditions, MRF setup, pressure reference, head calculation, something else)?
Any advice from people with STAR-CCM+ or hydraulic turbine CFD experience would be greatly appreciated.
Thanks!

Hi all. I am an Aerospace engineer with an interdisciplinary doctorate. I have worked on Shockwave, multiphase, lagrangian particle tracking simulations in Ansys, Openfoam and such. I went the entrepreneurial route into medical devices as soon as PhD was done. This was also very simulation oriented.

Now, after 5 years of working on this project, I have decided to find employment as the product development is taking lot of time and money.

My PhD was from UK but I moved to a developing country for business. I feel like I am clueless as how to go about finding employment now? Can someone talk to me about this? Do i need a recent portfolio? Are all jobs only through connections?

Tomorrow I've got an hour with Dr. Mohamed Aly Sayed. PhD, runs CFD at Audi's F1 team in Switzerland, and was also a Jr. CFD Engineer at Aston Martin F1 before. I make content on F1 and motorsport careers, but this one's aimed at the CFD side.

You all do this for a living, so I'd rather ask your questions than the surface ones. What would you actually want to know from someone running CFD at this level of motorsport?

Five of my questions so far:

1. How well does CFD correlate to the wind tunnel, and then to the track? Where does it still fool you?
2. With the cost cap capping core-hours and tunnel time, what did that force you to drop?
3. How do you decide between RANS and scale-resolving methods when a result has to land on a deadline?
4. What does a normal week look like? Mostly meshing and setup, or mostly waiting on the cluster?
5. Coming into a team being built up at Audi vs. an established one at Aston Martin. What's different in how the CFD side runs?

Curious to see what else you might want covered!

I'm new to CFD and battery thermal management, and I have a question.

If I'm using liquid cooling and phase change material (PCM) in a battery thermal management system, what type of hybridization is generally considered the most effective? For example, are there particular channel designs, PCM enhancements, or other hybrid approaches that consistently perform better?

I have a 2D pipe with pressure-driven flow. Inside the pipe there is a fluid blob of the same density as of semi molten metal but with infinite viscosity.

My objective is to observe how the surrounding metal flows past the blob and how the blob deforms under pressure.

The blob boundary should behave similar to a no-slip interface, while the pipe wall can be treated as a slip if required.

It is needed to be done in ANSYS FLUENT.

Any suggestions, example cases, or Fluent setup recommendations would be greatly appreciated.

Thank you!

Hi, I’ve been running some CFD sims for a student org for a good while now, although they are mostly CHT and aerodynamics simulations. Recently I’ve started looking at getting into combustion sims. So I’ve tried to have a look at the internet to see what is out there but I want a better background understanding of what I actually am doing and simulating.

I have taken some subjects about fluid dynamics so I have some baseline knowledge that I can base off of. So any guides on relevant videos or learning materials that gives a better understanding would be wonderful. The sims are on mostly just rocket engines with a simple oxidizer and fuel combination.

I have access to Ansys fluent so preferably simulations based on that would be nice to have. Thanks in advance :)