This answer explains why a thicker boundary layer will cause a type of form drag. It uses inviscid flow to explain this.

It is a great answer, but I can't seem to see how this applies in a real world scenario. Because flow is not inviscid in actuality, how does this work? (Ex. real flow has viscosity and drag, the BL doesn't extend forever)

I'm not very experienced with inviscid flow, so that answer is a little hard to understand, but this is what I've gathered so far:

The slower moving BL on top of the wing will create less skin friction drag than if it was moving faster. Pretty simple.

If a faster moving parcel of air 'speeds up' the boundary layer, then that would make more drag, because it causes more skin friction drag.

I'm not sure if this is what the linked answer was essentially saying, but it's what I've understood.

The linked answer also says:

From all this, we see that form drag is a pressure drag -- but it is a pressure drag that only exists because of viscous effects (the presence of the boundary layer).

Which also helped me come to that conclusion. (Not sure if it's the right conclusion or not)

To summarize, real flow isn't inviscid, so how does this work in a real situation?


2 Answers 2


Bottom line...

The pressure distribution around an inviscid closed 2d curve (airfoil) is such that all the pressures cancel and the pressure drag is zero.

As you point out, that is not the real world.

When you consider a real viscous flow, the pressure distribution is changed and they no longer cancel and the pressure drag is non-zero.

All the rest about control volumes and boundary layer thickness and inviscid layers on the outside of boundary layers, etc. All of those things are mathematical abstractions and approximations that we use so we can analyze and calculate the flow using simplified tools.

In the early 1900's aerodynamicists developed powerful techniques for analyzing 2d inviscid flows. They were also developing techniques for understanding boundary layers.

These techniques are powerful, but they are also very much simplifications (inviscid incompressible flow for example).

In reality, all of the different kinds of drag (form, induced, wave, trim, cooling, interference, etc). Are all just mathematical abstractions that are related to the mathematical tools we've developed to analyze these systems. In the real world, all aerodynamic forces are either pressure or shear. That is it. Pressure or shear.


My answer above may have missed your question a bit (I found your question hard to identify...). Here is another answer that may help.

Imagine a super-thin flat plate airfoil at zero angle of attack. The top and bottom surfaces are in contact with the flow. We call these surfaces 'wetted'. The wetted area is twice the chord times the span.

If the flow were inviscid, this thin flat plate at zero alpha would be 'invisible' to the flow. It would not cause any accelerations. It would not cause any turning. It would just sit there having no effect.

If we measure the drag on this two-sided (top and bottom) thin flat plate at zero angle of attack, we get pure skin friction drag. Sometimes we even call this the 'wetted area drag'.

If we could measure pressures along the plate, we would find that pressure is equal to the freestream static pressure everywhere along the plate ($P=P_\infty$) -- pressure coefficient is zero everywhere along the plate.



Next, we 'inflate' the flat plate to make it into a reasonable symmetrical airfoil shape. It is still at zero alpha (zero lift). It has the same wetted area as our flat plate.

Now, as we know, the airfoil causes acceleration in the flow. It causes a pressure distribution. At most locations around the airfoil, the pressure coefficient will be non-zero.

The drag on the thick airfoil (non-lifting, still at zero alpha) will be greater than the drag on the flat plate (of the same wetted area).

This difference is the form drag.

If you make the airfoil thicker, the flow will accelerate more, the form drag will increase. If the airfoil is thinner, the flow will accelerate less around it, and the form drag will be less.

The non-lifting flat plate only has skin friction drag -- also called wetted area drag. We call it wetted area drag because we can calculate it by calculating the wetted area and the Reynolds number.

Some might say that the thin flat plate airfoil has no shape to it. It has no 'form'.

When we thicken the airfoil, we give it shape. We give it 'form'. The increase in drag (over the drag of a flat plate with equal area) is the form drag. The form drag is caused by the super-velocities (as compared to a flat plate) around the airfoil caused by its thickness.


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