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New Am I the dumb?

Hopefully that link takes you to the right comment. The parent is from "Jasper DM" starting "It seems logical to me ..."

Then "Interfearence" jumps in to explain that aerodynamics is bullshit. In particular, "Also, every bro science equation used by these guys takes the coefficient of drag to be a constant value that does not change which is basically like lying."

I disagreed and we went back and forth. I'm pretty sure he's the dumb one here. Is he?

New No.
He's making a stupid argument.

Turbulent flow is chaotic and extremely difficult to accurately model. Laminar flow is much easier to model and understand and close enough to reality in most important cases. (Bikers aren't bumblebees.) Wind tunnels are designed to test and improve models so have to be carefully controlled and reproducible. That's why they (generally) have laminar flow.

If one wants to study turbulent flow (high Reynolds numbers), one can certainly do so with various tricks.



The aerodynamic properties of an object can not all remain the same for a scaled model.[3] However, by observing certain similarity rules, a very satisfactory correspondence between the aerodynamic properties of a scaled model and a full-size object can be achieved. The choice of similarity parameters depends on the purpose of the test, but the most important conditions to satisfy are usually:

* Geometric similarity: all dimensions of the object must be proportionally scaled;
* Mach number: the ratio of the airspeed to the speed of sound should be identical for the scaled model and the actual object (having identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds)
* Reynolds number: the ratio of inertial forces to viscous forces should be kept. This parameter is difficult to satisfy with a scaled model and has led to development of pressurized and cryogenic wind tunnels in which the viscosity of the working fluid can be greatly changed to compensate for the reduced scale of the model.

In certain particular test cases, other similarity parameters must be satisfied, such as e.g. Froude number.

The Coefficient of Drag has a definition. If he wants to redefine the term, then he's talking about something else.


tl;dr - Everything's complicated when you look at the details. But it seems to me that he's throwing up smoke.

HTH a little. Good luck!

New He's hanging his hat on this line
In general, Cd is not an absolute constant for a given body shape. It varies with the speed of airflow (or more generally with Reynolds number).

Now that I'm on my laptop instead of my phone, I'm seeing the much better comeback, from the previous section:
Cd is not a constant but varies as a function of flow speed, flow direction, object position, object size, fluid density and fluid viscosity. Speed, kinematic viscosity and a characteristic length scale of the object are incorporated into a dimensionless quantity called the Reynolds number Re. Cd is thus a function of Re. In a compressible flow, the speed of sound is relevant, and Cd is also a function of Mach number Ma.

For certain body shapes, the drag coefficient Cd only depends on the Reynolds number Re, Mach number Ma and the direction of the flow. For low Mach number Ma, the drag coefficient is independent of Mach number. Also, the variation with Reynolds number Re within a practical range of interest is usually small, while for cars at highway speed and aircraft at cruising speed, the incoming flow direction is also more-or-less the same. Therefore, the drag coefficient Cd can often be treated as a constant.

New he is a stupid bassar, the answer is put a motor on it then sit down and relax while biking uphill
"Science is the belief in the ignorance of the experts" – Richard Feynman
New aka a Whizzer bicycle-cum-engine was my first Freedom-is-moi! escape :-)
     Am I the dumb? - (drook) - (4)
         No. - (Another Scott) - (1)
             He's hanging his hat on this line - (drook)
         he is a stupid bassar, the answer is put a motor on it then sit down and relax while biking uphill -NT - (boxley) - (1)
             aka a Whizzer bicycle-cum-engine was my first Freedom-is-moi! escape :-) -NT - (Ashton)

It's hard to be religious when certain people are never incinerated by bolts of lightning.
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