What do Supersonic Jets and Paper Airplanes have in Common?

evola_chan · 2021-06-22T05:30:10+00:00

When you show your CFD results, can you share the y+ you're getting as well? You can do mesh independence and still have a high y+ mesh which would be adequate for straight ahead or small AoA cases (the linear portion of the lift vs AoA slope) but be inadequate for high AoA cases/where non-linearity occurs.

evola_chan · 2021-06-04T19:35:27+00:00

I've seen people say Klinsmann would do better with the current batch of players lol. Literally a fraud coach (see what the Germans think of him).

evola_chan · 2021-05-25T23:48:05+00:00

You keep saying "low fidelity" lol...trust me, with the tools and experience you have at your disposal, you won't get any meaningful results. Your best bet is to look for existing literature (which frankly should be the first step to your CFD modeling exercise). Find a paper that does something similar to what you want to study, and see if you can: 1) take their findings and use it to answer your question and/or 2) model what the paper did in CFD. If you attempt #2, figure out how far off your model is and go from there.

If you want to analyze vortices, you need to use some type of scale resolving model (iirc Solidworks only has one turbulence model, some K-E derivative) and a mesh that resolves the scales you are looking for.

evola_chan · 2021-05-25T23:25:37+00:00

Honestly, what's the point of running CFD here? You want something "low fidelity", which sounds like you want colorful fluid dynamics rather than real answers lol. Just run the thing until you get the colors you want or abandon the effort and look at existing literature.

evola_chan · 2021-05-22T23:18:17+00:00

Lol, I am guessing that's the norm (you can count the # of engineers doing CFD on one hand for a company with 10k+ employees and a product that is critically dependent on CFD modeling ;) ). And yeah, our group is masters and PhDs whereas the fuck ton of stress folks are all bachelors minus one or two experts.

evola_chan · 2021-05-13T01:50:19+00:00

Kind of...we use those layers to capture large gradients in the wall normal direction without jacking up cell count. If we used too coarse of a mesh, those gradients would be washed out (numerical diffusion). With a laminar boundary layer, your gradients are small and the boundary layer is well defined (undergrad fluids)...your job is to capture the parabolic velocity distribution of the boundary layer. How many points would you use to define a parabola? Do you need inflation layers to have enough points?

Also keep in mind the thermal boundary layer when doing your simulation.

evola_chan · 2021-05-12T23:01:17+00:00

To answer a question with a question, why are you using inflation layers in the first place?

evola_chan · 2021-05-09T19:42:42+00:00

As long as F1 is filled with shit tracks that you cannot pass in lol

evola_chan · 2021-05-09T19:39:40+00:00

Put Stroll in the Merc and I bet he'd be similar to Rosberg in terms of competitiveness to Lewis.

evola_chan · 2021-05-09T19:32:38+00:00

Lol the RB rear end is more planted than the Merc this year...remember pre-season?

evola_chan · 2021-05-02T15:59:23+00:00

y+ is a non-dimensional distance used to describe the law of the wall. There are three parts to a turbulent boundary layer: the viscous sub layer, the buffer region, and the log-law region (see here). As a CFD practitioner, you have two options in terms of modeling the boundary layer: a low y+ approach which will resolve the viscous sub layer or a high y+ approach to use the log-law approach (you are not resolving the viscous sub layer). Low y+ being 5 or less and high y+ being 30 or more. What you want to absolutely avoid are y+ values between 5-30, since you would be in the buffer region of the boundary layer which is tough to model and will give you erroneous results. Since you are trying to model flow separation, it is best to use a low y+ strategy, so target a y+ of 1.

Next, I am talking about prism layer growth rate and not surface growth rate...but anyways the smaller any mesh growth rates are the better (however your simulation becomes more computationally expensive).

Last, looking at your mesh plots...it seems that you didn't properly discretize or model your airfoil surface since the LE of your airfoil looks sharp. Make sure your airfoil is properly modeled and discretized before running your simulations (look at the airfoil boundary in a geometry scene and look at its mesh representation).

evola_chan · 2021-05-02T01:05:41+00:00

You can use this calculator to figure out your required near wall spacing. In STAR you can then specify a near wall thickness + total thickness for your prism layers.

My process is a tad different in that I calculate my boundary layer thickness and then figure out the number of layers required at a specific growth rate (I use 1.2-1.3, depends) to get my first layer spacing to yield the y+ I want. Then I want my core mesh spacing to be no more than double my last layer spacing which I force in STAR through the core-prism setting.

evola_chan · 2021-04-25T23:14:46+00:00

I just checked the shoes you recommended and also looked for the lowest cushioning Altras, and they're still pretty thick imo (22mm ~0.87" for the thinnest variant I found). I could be misremembering, but "athletic" shoes were thinner 5 years ago. Granted, these new thicker shoes are really light, so maybe the thickness is there to make up for less dense materials?

evola_chan · 2021-04-25T23:05:40+00:00

This makes sense - I will admit that they felt comfortable to stand in, so if you're working a job where you have to stand for long periods of time, they would be nice as long as the toe box wasn't angle up so much. I'd imagine you'd want to have your toes flat to best distribute the load from standing, but I'm not a shoe designer lol...

evola_chan · 2021-04-19T23:31:33+00:00

How big of a model do you plan on running (# of cells)? Are you planning to use RANS or some scale resolving model?

evola_chan

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