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Thread ID: 83777	2007-10-12 21:13:00	Paging all Physics experts: A good analogy for blood flow speed?	Renmoo (66)	PC World Chat

Post ID	Timestamp	Content	User
600984	2007-10-14 07:36:00	Since the pressure remains a constant until the terminus or discharge of the flow, and the volume is still the same, the only available change is the velocity of discharge...we perceive it as increased pressure...but that's a misnomer. Constriction at the discharge end of the vessel (like a hose nozzle) is not a reason for increased velocity in the vessel. It is a PERCEIVED increase in velocity that is really a decrease in the cross-sectional flow out of the decreased end of the vessel that does it. So err.... which is which? If you can't be stuffed to explain, think I will just stick with Poiseulle's law, which is more relevant to the blood context. :o	Renmoo (66)
600985	2007-10-14 07:57:00	Combining the two equations gives for the mean flow velocity: v = P.a²/8pnL Thus you can see that the flow velocity is directly proportional to pressure difference across the ends of the tube times area of the tube. So for constant applied pressure, if the tube is made larger (lower flow resistance) the flow speed will increase. If the pressure (voltage analogy) is increased the flow speed will increase. I don't quite get this point, although the formula has proven it. How would a decrease in radius decreases the velocity of flow? :stare: [edit] Thinking...	Renmoo (66)
600986	2007-10-14 08:26:00	I don't quite get this point, although the formula has proven it. How would a decrease in radius decreases the velocity of flow? :stare: [edit] Thinking... Imagine the tube getting narrower and narrower, with the pressure being set at a constant value, then flow will get smaller and smaller as will the velocity......until when the tube has almost zero diameter the velocity and the flow will be almost zero. In laminar 'Poiseuille' flow, the theory is based upon the assumption that fluid next to the walls of the tube (or plate surfaces if we are considering flow between plates) has zero velocity. That is, there is no slippage of fluid at the walls. So if the diameter of the tube is almost zero, then the fluid in the tube is also near zero velocity. The maths does not lie :)	Terry Porritt (14)
600987	2007-10-14 10:43:00	Imagine the tube getting narrower and narrower, with the pressure being set at a constant value, then flow will get smaller and smaller as will the velocity......until when the tube has almost zero diameter the velocity and the flow will be almost zero. In laminar 'Poiseuille' flow, the theory is based upon the assumption that fluid next to the walls of the tube (or plate surfaces if we are considering flow between plates) has zero velocity. That is, there is no slippage of fluid at the walls. So if the diameter of the tube is almost zero, then the fluid in the tube is also near zero velocity. The maths does not lie :) Ah, is this then linked to SurferJoe's comment on perceived increase in velocity of the water at the output? (I know I shouldn't mix these two together... :o )	Renmoo (66)
600988	2007-10-14 18:13:00	Ah, is this then linked to SurferJoe's comment on perceived increase in velocity of the water at the output? (I know I shouldn't mix these two together... :o ) I don't know, but with respect to Joe, I couldn't understand what he was saying :) But there is a 'whoops' in what I said about flow Q=pAv, there shouldn't be a density term p in there. That is the expression for mass flow, whereas Q is volume flow. There is the concept of continuity of mass flow (for obvious reasons), whereas if the fluid is compressible, volume flow along a pipe will vary with pressure variation along the pipe, but the mass flow wont.	Terry Porritt (14)
600989	2007-10-14 19:06:00	OK . . . Joe-simple . . . maybe! Liquid flow = x pipe/tube diameter = y reduction in pipe diameter at "nozzle" or exit = z where: x is the volume of fluid past a fixed point in space . . . in this case the inside of the tube/vessel in question . Expressed in whatever units of measure you like . . . . and . . . . y is the diameter of the vessel/tube for the fluid to flow through . Fluid resistance, variable compressibility, vessel wall stability and laminar flow are discounted for simplicity . . . . and . . . . z is an expression of the total diminished diameter or reduction of cross sectional area just at the nozzle's or restriction's smallest point . . again use whatever algebraic enumeration you want . Since x and y are here constant, and in this experiment the volume of fluid discharged must exit via through the smaller diameter of the tube/vessel (nozzle) (factor z) it will at that point (z) achieve greater velocity but the volume and pressure remains the same . Doppler compression effect and laminar flow are not being considered in this simplification . . . this isn't launching rockets . If the flow is stopped by blockage at the nozzle, the value for x will drop to zero (0) Solids in suspension, fluidities, ionic bonds, valence, periodic charts, variable compressibility, harmonics, pulsations, stratified wave forms, lunar phases and heliotropic eccentricities are not being factored into this either . . . . . You are therefor trying to shove the same amount of liquid at the same pressure through a decreased discharge device (nozzle) and the velocity must increase to accommodate the reduction in the stream's size . The "observed" effect is increased pressure and velocity however the energy flow measurements remain the same .	SurferJoe46 (51)
600990	2007-10-14 20:15:00	You do make life difficult for yourself Joe . Your simple explanation is not simple at all, and your statements about pressure are erroneous :) >>"You are therefor trying to shove the same amount of liquid at the same pressure through a decreased discharge device (nozzle) and the velocity must increase to accommodate the reduction in the stream's size . The "observed" effect is increased pressure and velocity however the energy flow measurements remain the same . "<< This is how to keep it simple if you want to include an area reduction, like a venturi or nozzle, though James didn't originally talk at all about that: 1) Take a tube of area A1, a reduction in area down to A2 part way along, then the tube comes back to original size A1 . The fluid is water, but assume no viscosity, hence no losses (called potential flow) 2) A fixed pressure differential across the tube, hence a constant flow . 3) The flow through the different areas is the same, given by Q = A1 . v1 = A2 . v2 , where v1 and v2 are the velocities at the areas A1 and A2 . 4) Therefore v2=A1 . v1/A2, since A2 is smaller than A1 the velocity through the restriction increases . As you rightly said in a roundabout way . 5) BUT the pressures in the tube are given by Bernouillis theorem . See reference given previously . So at the restriction, the static pressure reduces (say as measured by a pressure gauge tapped into the restriction), it is the dynamic pressure, the pressure measured facing into the flow that increases . Since James was talking about blood flow, then we are very much into effects of viscosity, and the one does have to define the conditions and assumptions . To go on about : "Solids in suspension, fluidities, ionic bonds, valence, periodic charts, variable compressibility, harmonics, pulsations, stratified wave forms, lunar phases and heliotropic eccentricities are not being factored into this either . . . . . " is just plain silly Blood flow is not like flow through a hose pipe . . . so there . . . . . :)	Terry Porritt (14)
600991	2007-10-15 00:46:00	You're right of course...I was being silly. I just thought to spoof the original question about how to correlate the blood/electrons and show that there is really no real zone of reference that makes sense there either.	SurferJoe46 (51)
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