A drag issue : to bungee wings or not to bungee

[skankpile]

it ain't DIR, and I'm no a pro/master, but when they speak I owe it to myself to listen. Not that you have to be DIR, but its good practice in many aspects.


DIR-diver.com - DIR III (from video)

 

[Blackwood]

..... why are they both called fluid dynamics?

All squares are rectangles, not all rectangles are square.

Liquids (like water) are a special case of fluid.

 

[UCFKnightDiver]

So a bunch of engineers took considerable time and resources to perform this "test" but all we have as a result is this silly letter.

Where is the data? Where are the pictures?

Sounds like bull****.

http://www.scubaboard.com/forums/hogarthian-diving/327938-wing-oms.html

Still arguing this point in one way or another I see even though you were proved wrong. LOL

 

[Blackwood]

This is an extract from an article I read recently,

BAUE Wing Size FAQ

Bungied Wings

...Hydrodynamics dictates that rough surfaces create increased turbulence which consequently increases drag.

What a load of unqualified and demonstrably untrue crap.

-1 to BAUE.

 

[DevonDiver]

Those reports seem likely and believable.

It doesn't take a great leap of faith to accept that
this...

....is more streamlined than this....

Is it just me....or does that just seem like a whole heap of common sense??

 

[DevonDiver]

What a load of unqualified and demonstrably untrue crap.

-1 to BAUE.

Golf Balls have dimples to increase friction.

Taken from: Aerodynamics in Sports Equipment - Golf

The Dimples Why, then, does a golf ball have dimples? The answer to this question can be found by looking at the aerodynamic drag on a sphere. There are two types of drag experienced by a sphere. The first is the obvious drag due to friction. This only accounts for a small part of the drag experienced by a ball. The majority of the drag comes from the separation of the flow behind the ball and is known as pressure drag due to separation. For laminar flow past a sphere, the flow separates very early as shown in Figure 1. However, for a turbulent flow, separation is delayed as can be seen in Figure 2. Notice the difference in the size of the separation region behind the spheres. The separation region in the turbulent case is much smaller than in the laminar case. The larger separation region of the laminar case implies a larger pressure drag on the sphere. This is why the professor experienced a longer drive with the marked ball. The surface roughness caused the flow to transition from laminar to turbulent. The turbulent flow has more energy than the laminar flow and thus, the flow stays attached longer.

 

[The Chairman]

Recently, myself and several engineering colleagues were performing wind tunnel tests on a new airfoil, under design within our division at
Grumman Aerospace.

I call Shenanigans #1. No data = no test. Let's see some coefficients and at diver's speed (we don't do 100+mph).

I call Shenanigans #2. Drag is a function of speed, viscosity and the drag coefficient. We aren't racing, so speed is not that big of a factor. Saying that reducing drag is the reasoning behind the bungees misses the entire reason for them which is a lower profile.

We also have several known deaths attributable to the bungeed wings...

I call Shenanigans#3. I have yet to see any real deaths attributed to the bungees, and I have dove them with a HOLE without perishing or even struggling. Considering your source, I am not surprised.

Look, if you don't like bungees: don't use them. Calling them "BWOD" is simply stupid on several levels. The "if you don't dive my way, you will surely die" line of argument is as vapid as is it is egotistical. They are specious at best.

Disclaimer... I have dove OMS bungeed wings as well as many unbungeed wings. My current sidemount rigs (both Hollis and Zeagle) use bungees to help with my profile.

 

[Blackwood]

Golf Balls have dimples to increase friction.

Taken from: Aerodynamics in Sports Equipment - Golf

The Dimples Why, then, does a golf ball have dimples? The answer to this question can be found by looking at the aerodynamic drag on a sphere. There are two types of drag experienced by a sphere. The first is the obvious drag due to friction. This only accounts for a small part of the drag experienced by a ball. The majority of the drag comes from the separation of the flow behind the ball and is known as pressure drag due to separation. For laminar flow past a sphere, the flow separates very early as shown in Figure 1. However, for a turbulent flow, separation is delayed as can be seen in Figure 2. Notice the difference in the size of the separation region behind the spheres. The separation region in the turbulent case is much smaller than in the laminar case. The larger separation region of the laminar case implies a larger pressure drag on the sphere. This is why the professor experienced a longer drive with the marked ball. The surface roughness caused the flow to transition from laminar to turbulent. The turbulent flow has more energy than the laminar flow and thus, the flow stays attached longer.

The appropriate conclusion from that blurb is: golf balls have dimples to decrease drag.

Really, they have dimples to increase range, but they do it by increasing turbulence in the boundary layer and thus decreasing form drag.

 

[cool_hardware52]

My perspective on bungeed wings:

Do the bungees cause a failed wing to immediately and completely empty?

No, at worst some volume might be lost, but the bungees can't force the wing dead flat.

Do the bungees increase or decrease hydrodynamic drag?

My *guess* is they increase it, but there is a trade off against smaller profile of a restrained wing. My guess is the net added drag is likely pretty small at swimming speeds, and maybe more significant when scootering.

My main objection to bungeed wings is the convoluted shape they force the wing into as it vented.

I am referring to physically convoluted, not convoluted in the GI# sense.

con·vo·lut·ed (knv-ltd)
adj.
1. Having numerous overlapping coils or folds:

These folds trap gas, making it very difficult to fully vent many bungeed wings. This trapped gas requires the diver to carry more ballast. More ballast means more gas in the wing during all phases of the dive. Of course with 90 +lbs wing being grossly over weighted seems like a good idea to some.

The bungees may assist with venting the wing from say 100% of capacity down to 30-40% of capacity, but they actually interfere with venting the remaining gas.

Just look at the typical bungeed wing in the collapsed state.

Any resemblance?


Tobin

 

[DevonDiver]

The appropriate conclusion from that blurb is: golf balls have dimples to decrease drag.

Really, they have dimples to increase range, but they do it by increasing turbulence in the boundary layer and thus decreasing form drag.

I didn't want to quote an entire article on golf balls, here on a diving forum... but that effect is evidently not appropriate as a metaphor to what happens with water resistance and a diver.

The dimples reduce the speed needed to cause turbulence. This turbulance, at speed, creates a 'pocket' behind the ball... a 'turbulent flow'.

In addition, the dimples are able to better 'grip' the air. This improves backspin on the ball in flight. That backspin increases pressure below the ball... creating lift. More lift equals more flight distance.

The same 'grip' caused by the dimples give golfers control...to hook or slice the ball more effectively.

A diver never reaches a speed where the turbulance creates a pocket around him. The slow speed turbulance is just resistance... more akin to the flaps on a plane causing 'stall effect'.