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Introduction to hydrodymanics?

  1. Oct 16, 2011 #1
    Hello World
    I can't introduce the concepts of hydrodynamics into my mind,such as the streamline.
    I can't imagine what is a streamline?
    What makes us say that the flow is turblant or the flow is steady?
    "the direction of the tangent drawn at any point on the streamline represents the direction of tangential velocity" so what is tangential velocity?
    [PLAIN]http://img265.imageshack.us/img265/9021/unled11e.jpg [Broken]
    I imagine that,this means ,if the fluid particle was to leave the streamline,it will move in the direction and with this tangential velocity.Is that true?
    "the rate of flow is constant along the path of the flow,since fluids are incompressible"
    I don't understand.What is the rate of flow?
    why is the blood flow in aorta faster than the other arteries?
    I know how to explain this biologically,it is because aorta is the nearest to the heart
    but if we look at this in a physical point of view,We will find that the cross sectional area of aorta is bigger,so the speed of blood flow in the aorta should be smaller.then how could we explain this physically?may be because the speed of blood flow in Aorta is distributed among those arteries so the speed becomes less in smaller arteries.
    I can't imagine at all,what is viscosity?
    [PLAIN]http://img36.imageshack.us/img36/4862/unled123j.png [Broken]
    If we imagine a liquid between two plates ,plate A is moving while plate B is not moving
    Why does the speed of the fluid decreases as we go down from A to B?

    what is the force of viscosity?and why is this force inversely proportional to the vertical distance between the the moving layer and the layer that is not moving?

    After answering my questions,could you recommend some good websites about fluids?
    my favourite website "the physicsclassrooms" usually uses a very simple language
    I used this website to understand many concepts about waves but misfortunately,there's no tutorials about fluids in this website.

    P.S I'm really serious in learning and understanding new ideas in the correct way,so please don't let me down..
    another P.S my physical knowledge is very basic

    Thanks in advance..
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 16, 2011 #2
    How good is your maths?
  4. Oct 17, 2011 #3
    Not good.I study biology this year instead of maths
  5. Oct 17, 2011 #4


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    Staff: Mentor

    In non-turbulent flow, it is the path traced out by a particle in the flow.

    If flow becomes turbulent, streamlines get scrambled up and destroyed.

    The instantaneous velocity of a particle in the stream.

    measured in litres per minute.

    Compare honey with water.
  6. Oct 17, 2011 #5
    Only one particle?or particles of a certain layer in the fluid?

    Yeah,that's good but why does the direction of the instanoeus velocity different from the actual direction of the particle?
    could you read this?:
    tell me if I'm thinking of this in the right way or not
    Great got it now
    Yeah I can imagine this but I need some detail:

    when we say that the viscosity of honey is more than that of water , we probably mean that the friction forces between the layers of the honey is greater than that of water
  7. Oct 17, 2011 #6


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    Staff: Mentor

    Not layers. Streamlines are lines; pencil-thin channels of tiny dimension. Picture millions of parallel transparent pipes each channelling a stream of molecules. Strings of molecules.
    It doesn't. Because what you understand by the direction of the particle is in fact its instantaneous velocity. :smile:
    Well, you are going to have to research some of your assignment yourself. I'm just giving you a start.
    Viscous friction, yes.
  8. Oct 17, 2011 #7
    So streamlines are strings of molecules moving together on a line?

    I don't understand what you mean
    The particle is moving in the direction of streamline right?
    and the instanoeus velocity is tangent to the streamline
    so the actual path of the particle is different from the direction of tangential velocity
    how could you explain this??
    Another point is that I don't know wheather this idea is correct or not:

    and that's what I'm doing now.I'm already researching on the topic and there are some unanswered questions about viscosity in this thread
    Actually,I don't understand what kind of research do you mean

  9. Oct 17, 2011 #8


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    Science Advisor
    Gold Member

    Not, streamlines are not actually a physical thing, but more of a mathematical construct along which certain properties are constant. There is no line of particles, though if there was, they would all share the same time history of their properties.

    No. The tangent vector of any line is the direction of the line at that point. If you froze time, a particle at that point would have a velocity vector pointing tangent to its associated streamline.

    If fluid particles were attached to streamlines then yes, that would make sense. As it is, it is an acceptable way to visualize the situation.
  10. Oct 17, 2011 #9


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    Staff: Mentor

    That's how you can picture them. Though of course they are not bonded together; conditions they experience just happen to be identical so they follow the same path as the particles before and after them. Like a multi-line highway of well-behaved cars all headed for the same destination, and threading their way along the arterial freeways of a city.
    So that if you consider a sufficiently short length of the streamline, then it is its own tangent. So the velocity vector lies along the path.
    No. The tangent changes as the path changes, so that the tangent is always along the path (provided you consider a sufficiently short piece of the path).
    Library or search engine research.
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