[Moreza7]

Hello,

I'm simulating an airfoil flow using LES. I export the drag and lift coefficient at every iteration, but in some iterations(for example 1 in every 20 iterations), the drag coefficient becomes negative while the average value is OK.

It oscillates from 0.0300 to -0.0150 and the average value is about 0.0200.

Why is this happening?

[sbaffini]

While I cannot know why this is happening to you, a negative drag coefficient just means a negative force in the wind direction, which means an actual thrust, so to speak.

This is not impossible, however, especially for limited amount of times in unsteady cases or for limited portions of some larger manufacts. On average, however, the whole manufact is expected to have positive drag.

Of course, besides this, there is the non negligible aspect regarding the very difficult nature of the numerical drag computation. Which means that the effect of numerical errors might be relevant if the computation is not performed carefully... and even more so in LES.