Drag Coefficient of Water in Relation to Air Question??

[dragforcequeen]

I've recently been doing studies on reduction of drag and skin friction for a project and I'm curious about a couple of things.

First, I found an academic paper on the reduction of skin friction for sharks. Their incredibly complex skin seems to disperse water in a way that allows sharp maneuverability and reduces skin friction as much as 8%.

 

This reduction however, can't go said without considering the speed they function at which is usually 50 km/h. (pretty slow in comparison to a sailfish of over 100 km/h) I'm sure at higher speeds the skin friction could vary... You would wonder why I'm not asking about the sailfish but actually the V-shaped protrusions on their skin have been discovered to increase skin friction.

 

With that aside, I wonder how comparable hydrodynamic properties are to aerodynamics properties in fluid mechanics. If they are similar, would you think the drag coefficient and/or Reynold's Number as well as the skin friction would also be reduced in an aerodynamic application? Thanks a ton in advance.

[studiot]

I was trying to discern a question in that first post.

 

The main difference between air and water is that air is compressible in a big way.

 

The other big difference is that water can flow in open channels and often has a free surface. Gases 'expand to fill theor container'.

 

Obviously air is also less dense, and has lower viscosity so any formulae that depend upon these properties will also change., that of course is partly taken care of in dimensionless number modelling, but it doesn't always work.

 

So where do you want to enter the arena?

[dragforcequeen]

Yes, absolutely. The mass density will vary as well as their physical and chemical properties. I was hoping people would assume that I was considering that already but that was my mistake. I suppose my question is does anyone have any base datum as to how an object's drag coefficient would change in water and air. I know the shape, size, velocity and other factors would affect the results but lets assume an ideal situation with all variables being the same. Thanks again sir.

Edited August 18, 2014 by dragforcequeen

[studiot]

What I am really saying is you need to give more detail of the situations you wish to compare.

 

One important variable is the 'Characteristic Dimension'.

 

What are these for the situations in air an water?

 

Once you have settled this that will fix some of your other parameters/boundaries.

Edited August 18, 2014 by studiot

[Bignose]

There are relations between the unitless numbers CD (drag coefficient) and Re (Reynolds number).

 

What you are asking has been done countless times over. Well before CFD became (relatively) cheap, it was not uncommon at all to make a scale model of an airfoil and put it in a water box. Because through the use of the CD to Re relationships, they could take the corresponding drag measurements in the water box and turn them into an equivalent drag in the air.

 

Really this is all encompassed in the fact that the branch of study is 'fluid mechanics' and both water and air are fluids, despite one being a liquid and the other a gas.

Edited August 18, 2014 by Bignose

[dragforcequeen]

Thanks for the info Bignose! Yeah, three of the abstracts I've pulled use both CFD's and structural models. Right now I'm just looking for puzzles pieces for my puzzle. Would you know of any articles that have done that conversion from hydrodynamics to aerodynamics?

Edited August 18, 2014 by dragforcequeen

[Bignose]

Buckingham Pi Theorem is a good place to start on the study of dimensionless numbers. Any good introductory level fluid mechanics text should cover this.

[Enthalpy]

Shark skin has been the topic of many wrong publications. Initial papers miscalculated the available propulsive force - by much more than 8% then - which may still be the case today.

 

To my knowledge, attempts to reduce drag on man-made items by extra corrugations have consistently failed - and there have been many such attempts. The only success is not against drag but to favour lift, by hindering stalling.

 

Air vs water: under a fuzzy limit of Mach 0.7, air is not compressible except for a proportionality correction between dP/P and dV/V, so that it behaves just like a liquid. The real difference is in the Reynolds number Re; if Re matches between the gas and the liquid, the analogy is accurate, and drag coefficients can be transposed accurately.

 

The present situation is not to try stiff corrugated shapes, but skins (smooth or not) that dampen the movements to reduce the drag, possibly by retarding the turbulences. Ferroelectric Pvdf with an additional damping electric circuit has been tried; I have read no victory report up to now.

[MigL]

I know of a Russian sub launched torpedo which uses decomposing hydrogen peroxide to encase its outer shell in a layer of oxygen/foam so as to reduce its drag and allow it to travel at higher speeds.

I believe the peroxide reacted with the stainless steel containment to overpressurize and explode on the Kirsk ( I believe it was called ) about 15 yrs ago sinking it and its crew. Should be easy enough to goggle the details.

[Acme]

Reducing Skin-Friction Drag by Laminar Flow

 

 

...Tests on an experimental F-16XL aircraft were used in a NASA programme to assess laminar flow on aircraft flying at supersonic speeds. The main aim was to assess the merit of swept-wings for future high speed civil aircraft. The swept delta-wings used active perforated titanium gloves attached to the surface featuring tiny holes through which most of the boundary layer was drained-off by an internal suction system. The panels covered 60% of the wings leading edge perforated with about 10 million microscopic size laser-cut holes. Through these holes the suction system in the wing drew away a significant portion of the slower fluid in the boundary layer close to the surface, thereby expanding the extent of laminar flow across the wing. The Supersonic Laminar Flow Control (SLFC) successfully achieved laminar flow over large portions of the wing up to supersonic speeds of Mach 1.6 [2]. ...

.

Addendum: Shark week.

Shark-skin surfaces for fluid-drag reduction in turbulent flow: a review

Abstract

The skin of fast-swimming sharks exhibits riblet structures aligned in the direction of flow that are known to reduce skin friction drag in the turbulent-flow regime. Structures have been fabricated for study and application that replicate and improve upon the natural shape of the shark-skin riblets, providing a maximum drag reduction of nearly 10 per cent. Mechanisms of fluid drag in turbulent flow and riblet-drag reduction theories from experiment and simulation are discussed. A review of riblet-performance studies is given, and optimal riblet geometries are defined. A survey of studies experimenting with riblet-topped shark-scale replicas is also given. A method for selecting optimal riblet dimensions based on fluid-flow characteristics is detailed, and current manufacturing techniques are outlined. Due to the presence of small amounts of mucus on the skin of a shark, it is expected that the localized application of hydrophobic materials will alter the flow field around the riblets in some way beneficial to the goals of increased drag reduction. ...

[Bignose]

To my knowledge, attempts to reduce drag on man-made items by extra corrugations have consistently failed - and there have been many such attempts. The only success is not against drag but to favour lift, by hindering stalling.

The design of every golf ball made in the last 150 years would disagree with this statement.

 

The roughness of a surface does increase the surface drag, but reduces the form drag because it helps trip the boundary layer from laminar to turbulent and a turbulent boundary layer separates much later along a bluff body.

 

A smooth sphere with a laminar boundary layer will exhibit its wake separating around 135*, leaving a very large low pressure void in the wake, significantly increasing form drag. http://authors.library.caltech.edu/25017/4/figs/fig502a.jpg

 

A dimpled sphere where the boundary layer is turbulent will separate around 160-170*, leaving a much smaller low pressure void. The decrease in form drag more than makes up for the small increase in skin friction. http://www.simscience.org/fluid/red/image/golfball_d5.jpg Note how much smaller the wake is behind the dimpled golf ball.

 

Mythbusters found an improvement from dimpling a car, too. http://www.discovery.com/tv-shows/mythbusters/videos/dimpled-car-minimyth.htm

 

I think a main reason this isn't done today is that the manufacturers think it looks ugly. But I predict it won't be too much longer before dimples and similar features are on our cars. I, for one, would buy a dimpled car for 11% fuel consumption savings.

 

Your comment about reducing stalling is relevant however, because you will see very similar dimples on airplane wings, typically called turbulators, and they are there for the exact same purpose -- to trip the boundary layer from laminar to turbulent to result in better wake separation.

 

Edited to add: one of many graphs that can be found of CD as a function of Re. http://www.aerospaceweb.org/question/aerodynamics/drag/drag-disk.jpg Note the change in the graph for a rough sphere vs. a smooth sphere.

Edited August 20, 2014 by Bignose

[Enthalpy]

I agree with the golf ball. I was thinking at shapes more adequate for aeroplanes and boats, where the stern would not be a hemisphere.

 

The F16L experiment isn't relevant here. It sucks actively the boundary layer, a known technique that doesn't relate with the shark effect.

 

Yes, many papers reported to observe an improvement with corrugated skins, but many others reported no improvement, over many decades. The net result is that air gliders still thrive to have a skin as smooth as possible, and so do airliners. You can trust gliders to adopt new ideas if they're better, even if unexplained or controversial: competition drives this activity, where neither designers nor pilots are conventional.

[Bignose]

The net result is that air gliders still thrive to have a skin as smooth as possible, and so do airliners.

No. Turbulators (also known as vortex generators) are very common in commercial aircraft. While large swaths of the wings are kept smooth, the entire thing is not 'as smooth as possible'. Maybe this is just being pedantic, but I think that it is important to get it right. I do not know about gliders specifically, but would be a little surprised if they didn't have something that performed the same function. Because, again, tripping the boundary layer to turbulent is overall considered very helpful.

[Acme]

...

The F16L experiment isn't relevant here. It sucks actively the boundary layer, a known technique that doesn't relate with the shark effect. ...

Since the original poster did not specify the project, the active system is as relevant as any other.

 

I've recently been doing studies on reduction of drag and skin friction for a project and I'm curious about a couple of things. ...

[MigL]

I would have to agree with Bignose and Acme also, Enthalpy.

The smooth back end of a car experiences large angle flow separation also, but, add a spoiler to induce turbulence, and drag is reduced.

Vortex generators such as LERXs ( F-16 and F-18 ) or chines ( F-22, F-35 or even SR-71 ) serve to re-energize the flow over a wing at angles of incidence so as to discurage flow separation. The vortex flow is, in effect, a controlled turbulence.

[Enthalpy]

The car's spoiler does improve the drag, yes - this is a for the golf ball a case where the stern has an unfavourable shape. On a glider, designers avoid that carefully. In addition, the spoiler reduces the noise and the rear uplift that would destabilize the car at high speed.

 

The turbulators have a completely different function. Through turbulence, they introduce fast-moving air in the boundary layer, (books say "energize" the boundary layer without details) which retards stalling. Though, the effect on drag is bad, and this is why gliders have none and don't want any. Most planes rely mainly on the more complicated leading-edge slats rather than turbulators because of drag

http://en.wikipedia.org/wiki/Leading-edge_slats

 

Gliders are designed and produced as smooth as possible, and pilots wash all smashed mosquitos away before taking off, as they would increase the drag perceivably.

 

Few years ago a new design emerged where elements protrude before the wing (non-swept wing, so that's not a chine). They're parallel to the flow at moderate angle of attack, hence introduce little drag, but make vortices as the angle of attack increases, bringing the same advantage.

Edited August 21, 2014 by Enthalpy

[Bignose]

There seems to be a lot of talk about adding turbulators to gliders:

 

http://www.standardcirrus.org/Turbulators.php

http://www.mh-aerotools.de/airfoils/turbulat.htm

 

http://www.hanggliding.org/viewtopic.php?t=18126&view=previous

 

Enthalpy I think you misinterpreting the function of these.

 

Total drag = skin drag + form drag. Skin drag is a function of the roughness of the surface. Form drag is a function of the shape and how the vortexes shed from the shape. Vortexes shed very differently if the boundary layer is turbulent or laminar. Laminar boundary layers shed much more easily, leading to a much large wake and low pressure zone behind the body, leading to high form drag. Turbulent boundary layers lead to less form drag. But, laminar flow does have less skin drag because the velocity gradients in a laminar flow are less than in a turbulent flow.

 

However, studies have shown, time and time again, that because a turbulent boundary layer lead to better vortex shedding that the overall drag is reduced.

 

For a turbulent BL skin drag goes up, form drag goes way down. Net result is that total drag is less.

 

It is not just a question of stalling (though that is related). The turbulence helps. Not just golf balls, but also airfoils. There is a reason they are put on everything. And you claims that they aren't are not backed up by what I see with just a few quick searches.

[Enthalpy]

Gliders (I am a glider pilot) don't use turbulators because they worsen the drag. A quick Internet search won't change that. By the way, the first link you give describes an airfoil profile from >50 years ago that doesn't resemble at all one of present day, the second link gives computed curves only, and the third link talks of turbulators on cylindrical rods, not on the wing.

 

So we go back to the idea that on a well-designed shape, for instance a glider, the turbulators are bad. They help at cylinders and spheres, which one won't find on a good aeroplane.

 

And then, they help to avoid stalling, but at the cost of increased drag. Most aeroplanes afford the more expensive leading-edge slats instead, to avoid this drag.

 

You know, Ive been hearing about the shark skin for over 40 years. No use on gliders up to now. A few people try it again and again, imagine sometimes to observe something or often not (many more describe what they have read instead of experimenting), and nothing happens because the turbulators bring nothing good.

 

If a corrugated adhesive tape at mid-chord did improve the drag, be 100% sure that all glider pilots worldwide would have adopted it within two years. But we continue to wash the wing and body carefully, to keep the mirror finish so carefully manufactured, because smooth is better.

[dragforcequeen]

Wow guys this information is overwhelmingly great! Thank you! I plan to read more of it, I've been super busy with getting ready for college, but once I find time later today I definitely will consider all of what has been said! Again, thank you!

[Bignose]

Your thoughts here still aren't supported by what I see:

 

https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/glider_handbook/media/gfh_ch03.pdf

 

Look at page 3-4.

 

These bubbles can be reduced or even eliminated by shaping the

airfoil to move the separation point downstream or by adding

a turbulator. Turbulators are aerodynamically positioned in a

spanwise line along the wing and are used to trip laminar flow

air into turbulent flow air at a desired location on the wing. This

is beneficial because the turbulent boundary layer contains more

energy, which will delay separation until a greater magnitude

of negative pressure gradient is reached, effectively moving

the separation point further aft on the airfoil and possible

eliminating separation completely. A consequence of the

turbulent boundary layer is increased skin friction relative to a

laminar boundary layer, but this is very small compared to the

increase in drag associated with separation.

The FAA -- in their glider handbook -- published in 2013 (NOT 50 years old) -- is recommending adding turbulators.

 

I see what you're saying. And it may reflect your experiences. And I respect that. But it doesn't seem that generalization about the entire gliding community are valid. There seems to be plenty of recent discussions and recommendations about use of turbulators in gliders.

[Enthalpy]

OK, seen on the website of Alexander Schleicher's gliders turbulators meant to reduce the drag.

[Enthalpy]

I've Iearnt something about the turbulators, thanks, +.

 

I know of a Russian sub launched torpedo which uses decomposing hydrogen peroxide to encase its outer shell in a layer of oxygen/foam so as to reduce its drag and allow it to travel at higher speeds.

I believe the peroxide reacted with the stainless steel containment to overpressurize and explode on the Kirsk ( I believe it was called ) about 15 yrs ago sinking it and its crew. Should be easy enough to goggle the details.

 

I'm wary about public explanations of these torpedoes. The way I understand them:

 

Friction has about no importance. They swim at 300+ m/s, it's all bout water inertia and cavitation. The bubbles are not a lubricant layer.

 

Their tip is designed to produce a vacuum around the prow. At that speed, any enlarging section would produce a vacuum (or vapour, no importance); the design lets only the tip make one, so this bubble is but wider than the body instead of much wider, and may even reconnect with the body near the aft.

 

Ideally it reconnects where the body converges again, to regain some forward push from the power invested in pushing the water outwards. To the least, the bubble collapsing there must wet the stabilizer, because hydrodynamic forces at the tip only would make the torpedo unstable.

 

It must be very difficult (impossible?) to design a tip that works properly at every depth and speed. I suspect they adapt their length and shape actively.

 

No gas has to be ejected to achieve a bubble. It is the natural consequence of speed.

 

----------

 

The only engine I imagine for such a speed is a rocket. It could have blown at the tip, but I feel it disadvantageous as it would further hamper the collapse of the bubble before the aft. At least in a first attempt, I'd put the rocket at the aft.

 

Hydrogen peroxide is a rocket propellant that can also work alone. It's a quite dangerous one that often detonates without a known reason and may explain the Kursk disaster - or an other propellant. Other choices among liquids storable at room temperature aren't much better: hydrazine...

 

Peroxide is long known for torpedoes as its decomposition exploited in a turbine served to rotate the propeller. Do I remember that its concentration is <70% then, which avoids its detonation?

 

The choice of propellants would be the same dilemma as for launchers, made worse by the confined submarine room. I'd prefer to store cold liquid oxygen than explosive peroxide or toxic tetroxide and hydrazine - in a submarine even more. Solids might be an option as well, for instance nitrocellulose+nitroglycerine, if these torpedoes can have every time the same speed profile. I suppose bubble trails are no worry in such weapons.