Unpublished

We want to start this blog post with two images and a question.  We have placed a ball and an airfoil, in a tube. The tube's walls do not interfere with the objects flow field. We are flowing air at a speed that has turbulence of Re20. This is very much laminar flow, which is defined as anything under Re2000. We then increase the airspeed so that the turbulence increases to Re40. Double that of the initial case, yet still very much laminar.

QUESTION: What happens to the wake region when we double the Reynold's number in each of these cases? We have options laid out below, please choose one for each case.

 

Option A:  The object’s wake increases in length, decreases in height.

Option B:  The object’s wake does not change.

Option C:  The objects wake decreases in length, increases in height.

 

Ball Image

Verus Engineering Why CFD Analysis is Important

Airfoil Image

Verus Engineering Why CFD Analysis is Important

Ball Answer:  The correct answer is A, the wake will elongate and thin out at Re40.

Verus Engineering Why CFD Analysis is Important RE40

Airfoil Answer:  The correct answer is B, the wake will not change.

Verus Engineering Why CFD Analysis is Important RE40

 

Let’s take it a step further, to Re680.  Still laminar, but 32x's the original turbulence.  What will happen?

 

The ball’s flow field is unsteady and waving around like a flag!  Pretty radical.

The airfoil’s flow field barely changes even at 32x’s the original turbulence, pretty impressive!

 

Typical Answers:

We asked these questions to educated individuals and engineers.  On average, most got the ball answer correct, the airfoil answer incorrect, and no one could guess what happened at Re680.

 

We get asked often, why is CFD Analysis Important?  What are you accomplishing? 

This is a great example of why CFD analysis can be extremely important!  Intuition can only get us so far.  Even as engineers, intuition can sometimes lead us astray, in the opposite direction even!  Air is invisible and as humans we typically have a difficult time understanding what we cannot visualize.  This is one of the many reasons aerodynamics is so difficult.  This is why CFD is so important!

 

Conclusion:

Our main point of this, hopefully, short and informative, blog post is that you cannot always trust your intuition with certain things, and aerodynamics is one of them.  We would be lying through our teeth if we said we always understood aero parts and how they affect vehicles without performing an analysis.  That is why we typically do not make assumptions without running an analysis, and why we heavily rely on CFD analysis for improving our understanding, improving our own products, ensuring they function as designed, and finally supplying some useful information to the end user.