Two heads - drag forces compared

[gordonjudd]

Both Gaddis findings and the data I presented are for turbulent freestream flow.

Vince,
Since fluid dynamics effects are so nonlinear I would tend to think that the superposition of skin drag and form drag effects might be a bit chancy but is seems that is what is done in most of the papers that I see that are describing drag effects on fly lines.

So are you saying that if the skin drag is producing turbulent flow on the upstream side of the line then that would reduce the form drag to the Cd value expected for the critical flow Regime regardless of the much lower Reynolds number you would get from using the diameter of the line and the normal velocity?

Do you have any papers dealing with the form drag and skin drag effects on airborne cables? There has to be a lot of work done to study that problem.

Gordy

[VGB]

So are you saying that if the skin drag is producing turbulent flow on the upstream side

No Gordy, I am not saying drag causes upstream turbulent flow, real world conditions at ground level are generally turbulent. Turbulent air changes the behaviour of the boundary layer, if it is more turbulent, it stays attached to the cylinder longer, reducing the size of the shed vortices.

The data I provided at provided at Post 15 was for reproduced for 8mm, 15mm and 28 mm smooth cylinders.

Vince

[gordonjudd]

I am not saying drag causes upstream turbulent flow, real world conditions at ground level are generally turbulent.

Vince,
I don't understand why the ambient air itself would generally be turbulent. Is that the case when the air is still (i.e no wind) as well?
This quick search on turbulent ground air seems to say it takes relatively high winds for the air to become turbulent.

How much wind causes turbulence?
The stronger the wind speed (generally, a surface wind of 20 knots or higher is required for significant turbulence), the rougher the terrain and the more unstable the air, the greater will be the turbulence.

Since fluid dynamics effects are so nonlinear I would tend to think that the superposition of skin drag and form drag effects might be a bit chancy but is seems that is what is done in most of the papers that I see that are describing drag effects on fly lines.

Evidently that is also an accepted approach in studying drag effects on inclined cylinders in air or water.

Gordy

[Torsten]

Thanks Gordy for finding the typo in my script.

The viscosity value was too low, I've corrected it, also I've changed the velocity for the Reynolds computations to the projected velocities.

I'll update the results in the first posting (only the Reynolds Numbers are affected).

Greetings,
Torsten

[VGB]

Torsten

What line have you taken this diameter from?

Line diameter = 1.72 mm

And why are the 2 lines now different diameters?

Regards

Vince

[gordonjudd]

I'll update the results in the first posting (only the Reynolds Numbers are affected).

Torsten,
I too think it makes sense to use the normal component (v*sin(deg)) for calculating the Reynolds number for the form drag case and to use the tangential component velocity (v*cos(deg)) in computing the Reynolds number for the skin drag case.

The tilt angle on the fly leg for Paul's and Lasse's distance cast seems to be around 9 degrees so using a normal velocity of 40*sind(9)=6.2 m/s would give a calculated Reynolds number of 500 to 700 using the other estimates given in your original post.

Air density = 1.204 kg/m³
Tangential velocity = 39.51 m/s
Normal velocity = 6.26 m/s
Skin friction coefficent = 0.005
Form drag coefficent = 1.0
Line inclination = 9.0°
--
Line length = 10.0 m
Line diameter = 1.72 mm
Reference mass = 2.00e-02 Kg
Reference area (skin friction) = 0.054 m²
Reference area (form drag) = 0.017 m²
Reference volume = 2.33e-05 m.^3
Reynolds Number Re (form drag) = 7.10e+02
Reynolds Number Re (skin friction) = 2.61e+07
Skin friction force Fdt = 0.2540 N
Form drag force Fdn = 0.4056 N
--
Line length = 20.0 m
Line diameter = 1.22 mm
Reference mass = 2.00e-02 Kg
Reference area (skin friction) = 0.076 m²
Reference area (form drag) = 0.024 m²
Reference volume = 2.33e-05 m.^3
Reynolds Number Re (form drag) = 5.02e+02
Reynolds Number Re (skin friction) = 5.21e+07
Skin friction force Fdt = 0.3592 N
Form drag force Fdn = 0.5736 N
--
Force difference = 0.2732 N

Those Reynolds numbers would still produce an expected form drag coefficient around 1.0 but the drag forces are larger by a factor of (sind(6)/sind(3)).^2=4 because of the normal velocity differences.

There would be another sind(deg) factor to get the drag force in the x direction. Thus the relative drag in the x direction is somewhat sensitive to the tilt angle as (sind(6)/sind(3)).^3 is a factor of around 8.

As Grunde pointed out years ago, the form drag on an inclined section of line is not negligible because its drag coefficient of Cd_n=1
1 is so much larger than the nominal skin drag coeffiecent of Cd_t=.005. You would expect the x directed drag from form drag would equal the x directed drag from skin drag for a tilt angle of around 14 degrees.

Gordy

[VGB]

Reynolds number for a 1.72mm line at 10 degrees C

[Graeme H]

What was the surprise Vince?

[Lasse Karlsson]

Torsten

What line have you taken this diameter from?

Line diameter = 1.72 mm

And why are the 2 lines now different diameters?

Regards

Vince

Hi Vince

Same weight and density, but different lengths. So they need to be differemt thickness.

Cheers
Lasse

[VGB]

What was the surprise Vince?

It wouldn’t be a surprise if I told you

Thanks Lasse, I wasn’t entirely sure which scenario was being tested as a few had been suggested.

Regards

Vince