'GE - 5deg' simulation project by Maciek

#1

I created a new simulation project called 'GE - 5deg':

A short investigation: ground proximity influence on lift coefficient.

More of my public projects can be found here.

#2

Hi everyone!

With this topic Iâ€™d like to start a series of simple simulations dealing with principles of aerodynamics. I need to check a few things and theses simulations are/will be a kind of collateral effect of my investigations. However, because of big number of students, I thought to myself it could be useful and interesting for some of you.

So, we start with basic wing shape (without camber) and influence of ground proximity on its two parameters: lift and drag. The airfoil shape is presented below.

As you can see curvature comb looks quite nice. Itâ€™s not perfect, but the upper surfaceâ€™s spline is tangent to the leading edge curve. The wingâ€™s span (the domain width) is 0.1 [m]. â€“ I just wanted to avoid pseudo-2D case. â€“ Further, wingâ€™s angle of attack was set at 5 [deg].

The flow velocity was set at 40 [m/s].

The â€śheightâ€ť in this virtual experiment was measured from the red line. Therefore the values in meshes/cases and tableâ€™s descriptions donâ€™t reflect the exact distance between wing and wall. I did it to make my life easier. I also wanted to have constant starting point.

In the results Iâ€™ve included the pressure forces only. The viscous ones, because of their very low values, were ignored. If we have a â€śpharmacy shop assistantâ€ť here, he or she can rerun the cases and up-date the table. - Everything is prepared; all you need to do is to run them on your own.

The domain size check:

Predicted velocity differences:

• at the bottom: v=39.895 [m/s] and it gives -0.26%
• at the top: v=40.255 [m/s] and it gives +0.64%.

I think thatâ€™s it in terms of basic information. Now, letâ€™s take a look at the results.

As you can see in the picture above I started my investigation from the point where there was no influence of upper and lower wall. Then I gradually decreased the space between the wing and the wall â€“ Iâ€™ve analysed two situations:

• standard, classic wing where lift was generated (bottom wall was moved up)
• inverted wing where downforce was generated (top wall was moved down); in this case I described the downforce as the negative lift (hence â€śâ€“Lâ€ť in the notations) to avoid misunderstandings with drag

Also, to be able to draw some general conclusion, I plotted particular curves in function of height (h) to chord Â© ratio. â€“ Chord length was 0.4 [m] and is presented in the picture showing airfoil shape.

So, what do we have here? First and foremost, the wall influence is clearly visible right away from the start. Itâ€™s very important information to remember for all the beginners here (!). Secondly, classic and inverted wing differ from each other.

Standard wing
The lift coefficient increases slowly as we close to 1:1 mark of h/c ratio. If we add some accuracy margin to our simulation, we can even assume it is stable until this point. Then, suddenly, it speeds up and reaches over 70% of the reference coefficient value. It means we can extract up to 70% lift more from this particular wing just limiting the space below. Whatâ€™s very important too is that at the same time wingâ€™s drag stays at the same level. Of course there are some fluctuations at one point, but it doesnâ€™t seem to be significant in comparison to the overall gains in lift force. â€“ Do we have a specialist in wingâ€™s aerodynamics here? Did I hit some exception here? Or maybe is it related with airfoil shape?

Inverted wing
Here situation looks a bit different. The lift coefficient (and automatically lift force) increases slightly faster. However, itâ€™s compromised by considerable drag rise. The good news is that itâ€™s still in positives as the net percentage value is just below 30% for the smallest gap.

Table with outcomes:

Plots:

I know it was short and simple investigation, but still I hope itâ€™s interesting for some of you. Next step I plan is ground effect on carâ€™s floor. Iâ€™m having a few days off now, so I hope to start working on it tomorrow.

Also, if you have any comments / suggestions donâ€™t hesitateâ€¦

SPOTTED:
I have a question to SimScale guys: Iâ€™ve spotted something strange here. While running simulations some of them stopped despite they didnâ€™t reach convergence threshold. In this particular case itâ€™s not a big deal and can be ignored, but, Iâ€™m sure, Iâ€™d seen something like this earlier too. Then I thought it was incidental. Any idea why it happens?

Please check Convergence plots. Here is an example for h_0.075_inverted:

Suspicious cases:
h_0.050
h_1.25_inverted
h_0.10_inverted
h_0.075_inverted

#3

Very impressive @Maciek! very well explained! I am sure this will help lot of students