boring friction thread

Discussion in 'Off Topic' started by beavis, Sep 28, 2008.

  1. beavis

    beavis Well-Known Member

    893
    0
    391
    back where we were



    less filling [smiley=1-boxing2.gif]


    tastes great [smiley=smashfreakB.gif]



    To the airplane wing,

    it doesn't matter if the wing is moving thru the air or the plane is sitting on the ground in 500 mph wind. The airfoil on the wing creates lift. How? The air travels at 2 different speeds. The slower air goes under the wing, less distance to travel. The air that goes over the top of the wing is accelerated because it has to travel a further distance in the same time period. This faster air has now dropped in pressure. So the higher pressure section that is under the wing can direct itself towards the lower pressure on top of the wing and give it lift.

    And yes the molecules next to the wing are moving. An extreme example is the sr-71 blackbird. That plane was designed to take into account the friction of that plane flying so fast. On the ground it drips fuel like a leaky faucet and has to be refueled as soon as it is airborne. Then it hauls assssszzzzzzzzzzzzzzzzzzzzzzzzzzz and heats up and the body panels expand to seal itself.

    What you are calling form drag is basicallt what causes eddy currents as a fluid moves by a stationery object or an object moves thru a fluid. Off the top of my head if I can remember, wind coming around a building or a tree takes 10times the height of the object in distance downstream of the object to straighten back out into a smooth or close to a laminar type flow. I can't remember the water and piling example I learned. But look at a dock post in the current and see long it takes for the water to straighten out. The ship's bow bulkhead does what you explained "So by mixing the layers of flow together it helps speed up the molecules closer to the hull and they dont get "left behind" this gets the water moving around the hull of the ship to cut down on drag. This is done on just about all ships in over to increase effiecency and save on fuel costs.

    The shark's skin is the perfect example and they have been around since the beginning of time with very little evolution. Which now the olympic swimmers have somewhat tried to duplicate and broken just about every swimming record there is in a short span of time since the introduction of thesesuits. Basically fine sand bar type surface that breaks that laminar flow barrier and makes the water next their bodies turbulent and reduces friction allowing them to cut thru the water faster.

    What's a remolds number? Do you mean reynolds number?
     
  2. aaronshore

    aaronshore Well-Known Member

    Round two goes to RJ. Very well explained sir. Even I understood that.
     

  3. beavis

    beavis Well-Known Member

    893
    0
    391
    you guys are boneheads. no animosity here. we are just talking and to a lot of people this boring chit.
     
  4. deerfly

    deerfly Opinicus iracibilus

    airfoil dynamics are high school level physics  ::)

    now what would be potentially more interesting to know here is if a pocket tunnel provides any lift from the longer running surface of the top of the tunnel. I think its safe to say the downward thrust of the water through the exit might contribute to some lift, but my guess is the drag created by the water on the rest of the whetted surface cancels out any lift associated with the faster moving water at the top of the tunnel. Put another way the decrease in pressure at the top of the tunnel isn't sufficient to overcome the drag on the rest of the hull, at least until the forward speeds approach that of Curtis levels. :)  
     
  5. Brett

    Brett > PRO STAFF <

    Only one bad assumption I see in your theory DF.
    The only water moving in this situation is that water disturbed
    by the forward motion of the hull. The water in contact
    with the hull is forced into a circular motion along the
    hull water boundary. The rest of the water is basically
    a static force upwards that the hull is being forced to climb over.
    The faster you go, the more a fluid acts like a solid.
    Spray is water acting like a solid due to high velocity impact.
    Hull running angle is the result of a low angle thrust from the prop,
    the inertial resistance of water to being moved out of the way,
    bouyancy due to displacement, all relative about the center of gravity
    of the hull. Almost all the lift is coming from the undisturbed
    water. So very little lift would be coming off the aerated water
    inside the tunnel.
     
  6. snooknreds2

    snooknreds2 Well-Known Member

    558
    0
    391
    To the airplane wing,

    it doesn't matter if the wing is moving thru the air or the plane is sitting on the ground in 500 mph wind.  You are absolutly correct it is either lagrangian or Eulerian approach.  Either way the same results are achieved

    The airfoil on the wing creates lift.  How?  The air travels at 2 different speeds.  The slower air goes under the wing, less distance to travel.  The air that goes over the top of the wing is accelerated because it has to travel a further distance in the same time period.  This faster air has now dropped in pressure. So the higher pressure section that is under the wing can direct itself towards the lower pressure on top of the wing  and give it lift.Umm Kinda.  First off pressures dont have direction like a force.  Pressure acts equally in all directions.  But i think I know what you mean since the upwards pressure is the only pressure that has an area to act upon and become a force.  Moving on, I think you and I are on the same page.  First off you are correct that in the case of a airplane wing the flow is faster across the top of the wing, and as a result of the faster flow the pressure is dropped.  in fact, and I am quoting, "Because the velocity of the flow over the top of the airfoil is greater than the free stream velocity, the pressure over the top of the air foil is negative, or less than the free-stream pressure.  this flows directly from the application of the Bernoulli equation."   Although this was brought up due to the difference in laminar Vs turbulent flow it is only relavint when applied to air foils.  When you have a symmetrical shape and the flow is uniform around the body.  The following is an exceprt from my fluid mechanics book that deals with 2-dimensional bodies.  Three sections latter it discuss 3-dimensional bodies and begins by saying that the same principles are applied.


    And yes the molecules next to the wing are moving What about the no-slip condition.  Based on the formulas that govern these situations a basic assumption that is made to formulate themis the no-slip condition.  Also Be carefull when mixing fluid mechanics and aero-mechanics.  One is dictated largely by the Reynolds number and the other by the mach number.  I admit I dont know much about aero, so maybe they get around the no-slip condition some how.  Fluids can be controlled by the mach number but generally are controlled by the froud (sp) or Reynolds number.  An extreme example is the sr-71 blackbird.  That plane was designed to take into account the friction of that plane flying so fast.  On the ground it drips fuel like a leaky faucet and has to be refueled as soon as it is airborne.  Then it hauls assssszzzzzzzzzzzzzzzzzzzzzzzzzzz and heats up and the body panels expand to seal itself.  way beyond my area of expertise

    What you are calling form drag is basicallt what causes eddy currents as a fluid moves by a stationery object or an object moves thru a fluid.  Correct the form drag is caused by the shape of the object and is created due to the differences in pressure which come about due to the change in velocity.  Vicious drag, or the drag on the side of a flat body, is often called skin drag for obvious reasons.  The big ships have the bulb up front to create a turbulent flow with in the front 10% of the ship.  By doing this the skin drag is eliminated (I could have that backwards please let me know if you concurOff the top of my head if I can remember, wind coming around a building or a tree takes 10times the height of the object in distance downstream of the object to straighten back out into a smooth or close to a laminar type flow. I can't remember the water and piling example I learned.  But look at a dock post in the current and see long it takes for the water to straighten out.  The ship's bow bulkhead does what you explained "So by mixing the layers of flow together it helps speed up the molecules closer to the hull and they dont get "left behind" this gets the water moving around the hull of the ship to cut down on drag.   This is done on just about all ships in over to increase effiecency and save on fuel costs.again be careful you are discussing two different things.  With the airplane wing you say the molecules speed up and with the ship they get left behind....I agree with both statements...I think...LOL now I am starting to confuse my self

    The shark's skin is the perfect example and they have been around since the beginning of time with very little evolution.  Which now the olympic swimmers have somewhat tried to duplicate and broken just about every swimming record there is in a short span of time since the introduction of thesesuits.  Basically fine sand bar type surface that breaks that laminar flow barrier and makes the water next their bodies turbulent and reduces friction allowing them to cut thru the water faster.

    What's a remolds number?  Do you mean reynolds number?OK you got me there ;D



    also I am pretty sure that ALL of this discussion pertains to fully submerged objects and does not touch on partially submerged objects.  I am not sure if that is true so please inform me if you agree


    I will scan and post the pages from which I gathered this information latter, right now I have a meeting to discuss the dissipation of heat created by electronics located in the pressure vessel for the underwater turbine I am designing.  I admit it has been awhile since I looked over this stuff and by no means could I remember it all :D


    I must admit I really am enjoying this thread as it has been way too long since I have discussed something of this nature (fluid flows) in depth. I look forward to re-learning more as our discussion continues :)
     
  7. deerfly

    deerfly Opinicus iracibilus

    snooknreds2, I introduced the Bernoulli effects because I thought you were not understanding or were questioning whether the air flows faster over the top of the airfoil or not, which is what I thought you were contesting, might have to re-read your reply. Anyway, air flow across an airfoil as well as the resultant surface friction etc are most commonly discussed around Bernoulli, that's the only reason I mentioned it. I wasn't implying it was totally relevant to the fluid dynamic aspect.

    Brett, I haven't had time to give your response enough thought (way too busy trying to earn a living :)). I would think the shape or more specifically the pocket transition from the keel to the high point of the pocket would dictate a lot of that turbulence. Just like an airfoil generates lift only to a certain extent based on its shape, i.e. a blunt leading edge will effect lift as it changes the air flow as would too high of a profile given a certain length(leading edge to trailing). We see these effects at play when aircraft wings change shape during take off and landing, flaps and spoilerons, etc. So the airfoil cross section is only optimal at certain velocities, not all.

    My thought or reasoning with a boat hull tunnel is if the transition from the keel to the pocket were designed to minimize turbulence then there should be some acceleration of the water as it travels the greater distance across the top of the tunnel. Its still a fluid mass albeit more dense than air. Now there is no bottom surface with the tunnel, so I'm not sure how you calculate the pressure difference between the water flowing at the water line vs the top of the tunnel. This might be where ground effects would contribute. I could be all wet here and probably am, but that's the notion anyway. I'll try to dig around some after the kids go to bed and see if I can find any real science to support what I think I'm thinking.  :-?
     
  8. Brett

    Brett > PRO STAFF <

    Anyone can bolt on more power.
    Roun' here, we make the effort to do more with less.
    In my case I think I've burned out a few brain cells,
    with the epoxy fumes and my lawnmower exhaust,
    so I definately have to do more with less!

    And guys, in your thinking, remember the water
    is the reactive force, the forward motion of the hull
    is the initiator. Otherwise the water would just sit there
    indulging in convection currents, tidal movement and brownian motion.

    Should we get into wave forms yet?
     
  9. deerfly

    deerfly Opinicus iracibilus

    the same could be said of the air, no? Its either some force that moves the air across the airfoil or the airfoil is propelled through the air by force. In either case the resultant pressures of lift and drag are determined by the shape of the airfoil and the speed of the interaction. I think water and a boat hull have more similarities than differences in that regard, although with a boat its more about overcoming drag than generating lift.

    For the extreme case consider race boats like a hydroplane being engineered for the effects of both water and air as the speeds approach Mach Curtis. :)  
     
  10. snooknreds2

    snooknreds2 Well-Known Member

    558
    0
    391
    Like I said all these cases are only so relevant since we are talking about a case where the body is completely submerged, where as the original question deals more with ship hydrodynamics dealing with a body partially submerged. Yes to talk about that we must first discuss wave forms, but lets keep it simple and only assume gravity waves are present and there are no wind waves. Now this is a subject that I did not pay so much attention too. reason i...well it is just not as interesting to me. there re many long and tedious equations that dictate ocean wave forms, which for the record I can not recall a single one. :(
     
  11. Brett

    Brett > PRO STAFF <

    I was referring to the bow wave which initiates the circular motion
    along the hull water boundary, and the stern wave, which has three
    components. Port, starboard, and upward, all pushing in to fill the void
    created by the hull passing through-over the water.
     
  12. Un-shore

    Un-shore Well-Known Member

    More power is doing more with less.. less slower.

    I may have missed it somewhere here, I have work to do, but howcome that jet that Chuck Yeager tried to fly into space had flat wings and still flew?

    Speed can cause lift too.
     
  13. deerfly

    deerfly Opinicus iracibilus

    Chuck wasn't "flying" the X-15, he was steering a rocket engine. :cool:
     
  14. beavis

    beavis Well-Known Member

    893
    0
    391
    This true but when you have 2 different pressures, the higher pressure wants to equalize by going toward the lower pressure. Thus giving it direction. Easy example of this would be 2 tanks of water connected by a tube to the bottom of each with an isolation valve in the middle and open to atmosphere. Fill one all the way up and one about a quarter. We can agree there is more pressure at the isolation valve on the side that connects to the full tank by virtue of the greater depth. Open the valve and the pressure will equalize across it by leveling the tanks to the same depth.
     
  15. beavis

    beavis Well-Known Member

    893
    0
    391
    The left behind thing, I was quoting you. You are admitting it now. Hte ship has to make turbulence to get through the water easier. ;D And i am aware about the differences between air and water in terms of mach numberes. But they are also similar as they are both mediums but of different densities.

    Don't worry about scanning pages and bringing references. This is all in good fun. I don't work in design, just fix all the problems they create. And no I am not takin a cheap shot at ya.

    Underwater turbine huh? That is a nightmare waiting to happen. I would think a sink heat sink problem wouldn't be a issue given the fact that it is underwater. I would definitely be interested in hearing about that system. Start a boring underwater turbine thread and let me know about it.
     
  16. beavis

    beavis Well-Known Member

    893
    0
    391

    Skirtus's boat ain't that fast. :eek:
     
  17. deerfly

    deerfly Opinicus iracibilus

    good stuff rj, I love this sort of thing. Sometimes I wish I would have gone into the physical sciences instead of software.  :( Anyway, here's a doc I ran across that discusses the main ideas behind computational fluid dynamics as it relates to displacement hulls, specifically racing sailboats. Its not at all technical, easy read for the math phobic, but does discuss the major concepts. It doesn't really get into the details at the boundary layer that spurred this thread either, but its still relevant and may be of interest to those following along.

    http://www.vacantisw.com/download/COMPUTATIONAL%20FLUID%20DYNAMICS.pdf
     
  18. snooknreds2

    snooknreds2 Well-Known Member

    558
    0
    391

    I will reply tot he rest latter when I am not at work....BUT as far as a boring underwater turbine thread......NO WAY I deal with that enough at work. I would be happy to discuss it with you but not have a whole thread on it....I just could not handle that :eek:
     
  19. beavis

    beavis Well-Known Member

    893
    0
    391
    no problem
     
Loading...
Similar Threads Forum Date
Stuart area, boring no-pics reports Fishing Reports Aug 19, 2010
Video Test (Boring !) Off Topic May 31, 2010
Friction Hinges General Discussion Feb 13, 2014