Robert S. asked 1 month ago
Hello. I have a question.
The first question,
Which of STEP, NURBS, or STL represents the most accurate figure when creating a fuselage in Flow5?
The way I experimented is as follows:
1. NURBS make fuselage and wing punch, make fuselage, check panel, free edge check
2. OPENVSP make fuselage – CFD mesh output(STL) – FREECAD program trans to STEP – Flow5 open STEP – wing punch, make fuselage, check panel, free edge check
For the NRUBS case, the wing punch and connect is automatically very good.
However, in the case of STEP, the connect is not working well, so I have to modify the node a lot.
In addition, as a result of the plane analysis, STEP shows a CL curve more similar to LLT than NURBS, and interpolates better.
The interpretation was conducted by selecting Triuniform Thick, Friction Drag, and Visible loop for both NURBS and STEP.
[Note: It shows the pitch angle of 2 degrees when the flight speed is 20 m/s, 0 degrees when it is 25 m/s, and -2 degrees when it is 30 m/s,
As a result of calculating the lift coefficient according to the CL curve, LLT is the most accurate, so I am comparing CL based on LLT.]
The second question,
Cp In 3D, the pressure on the wing tracking edge and fuselage contact surface is relatively low compared to other places, so the whole always comes out red.
Of course, if I release AUTO on the 3D scale and change it to +1, -1, the color change will normally appear according to the angle.
However, very low pressure areas are indicated in black.
Is there any way to solve this problem?
above note comment is actual flight date(m/s, pitch angle)
Hello,
1. From the results point of view, only the quality of the mesh matters. Therefore the ‘best’ fuselage is the one that produces the mesh of highest quality.
There is no simple measure of this quality. Typically, small elements should be avoided and triangles shouldn’t be skinny, i.e. should have sides with comparable lengths. Also mesh density should be higher in areas of local geometry changes and of high pressure gradients.
With hindsight and experience it seems that the viscous loop is an unnecessary complication. Given the comparison to the jibe2 experimental results, it does not seem to bring any additional accuracy. The option may well be disabled in a future version to simplify the code and allow the implementation of more useful options.
2. The connection of wing to fuselage is an area of high pressure gradients and high local velocities. In reality, those velocities would be dampedp by viscosity, therefore it is in such areas that the assumptions implicit in the potential methods are likely to be invalid, leading to unprecise evaluation of Cp coefficients. There’s not a lot that can be done about it. The same consideration applies to the wing tips. In addition, the proximity of the wing’s wake panels with the fuselage creates parasite numerical interactions which tend to make the local evaluation of Cp coefficients incorrect. This is why there is an option in the analysis to exclude those pressure forces from the evaluation of moments.
Hope this helps,
André
Thank you for your answer.
have a good day.