- Hagen-Poiseuille equation
The

**Hagen-Poiseuille equation**is aphysical law that describes slowviscous incompressible flow through a constant circular cross-section. It is also known as the**Hagen-Poiseuille law**,**Poiseuille law**and**Poiseuille equation.****Equation****tandard fluid dynamics notation**In standard fluid dynamics notation::$Delta\; P\; =\; frac\{8\; mu\; L\; Q\}\{\; pi\; r^4\}$ or :$Delta\; P\; =\; frac\{128\; mu\; L\; Q\}\{\; pi\; d^4\}$

Where::$Delta\; P$ is the pressure drop:L is the length of pipe:$mu$ is the

dynamic viscosity :Q is thevolumetric flow rate :r is theradius :d is thediameter :$pi$ is the mathematical constant (approximately 3.1416).**Physics notation**:$Phi\; =\; frac\{dV\}\{dt\}\; =\; v\; pi\; R^\{2\}\; =\; frac\{pi\; R^\{4\{8\; eta\}\; left(\; frac\{-\; Delta\; P\}\{Delta\; x\}\; ight)\; =\; frac\{pi\; R^\{4\{8\; eta\}\; frac\{\; |Delta\; P\{L\}$

Where: :"V" is a volume of the liquid poured (cubic meters):"t" is the time (seconds):"v" is mean fluid

velocity along the length of the tube (meters/second):"x" is a distance in direction of flow (meters):"R" is the internal radius of the tube (meters):"ΔP" is the pressure difference between the two ends (pascals):"η" is the dynamic fluidviscosity (pascal-second (Pa·s)), :"L" is the total length of the tube in the "x" direction (meters).**Relation to Darcy-Weisbach**This result is also a solution to the phenomenological

Darcy-Weisbach equation in the field ofhydraulics , given a relationship for the friction factor in terms of the Reynolds number::$Lambda\; =\; \{64over\; \{it\; Re\; ;\; ,\; quadquad\; Re\; =\; \{2\; ho\; v\; rover\; eta\}\; ;\; ,$

where "Re" is the

Reynolds number and "ρ" fluid density. In this form the law approximates the "Darcy friction factor ", the "energy (head) loss factor", "friction loss factor" or "Darcy (friction) factor" Λ in the laminar flow at very low velocities in cylindrical tube. The theoretical derivation of a slightly different form of the law was made independently by Wiedman in1856 and Neumann and E. Hagenbach in1858 (1859 ,1860 ). Hagenbach was the first who called this law the Poiseuille's law.The law is also very important specially in

hemorheology and hemodynamics, both fields ofphysiology . [*[*]*http://www.cvphysiology.com/Hemodynamics/H003.htm Determinants of blood vessel resistance*]The Poiseuilles' law was later in

1891 extended toturbulent flow by L. R. Wilberforce, based on Hagenbach's work.**Derivation**The Hagen Poiseuille equation can be derived from the Navier-Stokes equations.

**Viscosity**The derivation of Poiseuille's Law is surprisingly simple, but it requires an understanding of

Viscosity . When two layers of liquid in contact with each other move at different speeds, there will be aforce between them. This force is proportional to thearea of contact "A", the velocity difference in the direction of flow "Δv_{x}"/Δ"y", and a proportionality constant "η" and is given by:$F\_\{\; ext\{viscosity,\; top\; =\; -\; eta\; A\; frac\{Delta\; v\_x\}\{Delta\; y\}$

The negative sign is in there because we are concerned with the faster moving liquid (top in figure), which is being slowed by the slower liquid (bottom in figure). By Newton's third law of motion, the force on the slower liquid is equal and opposite (no negative sign) to the force on the faster liquid. This equation assumes that the area of contact is so large that we can ignore any effects from the edges and that the fluids behave as

Newtonian fluid s.**Liquid flow through a pipe**In a tube we make a basic assumption: the liquid in the center is moving fastest while the liquid touching the walls of the tube is stationary (due to

friction ). To simplify the situation, let's assume that there are a bunch of circular layers (lamina) of liquid, each having a velocity determined only by their radial distance from the center of the tube.To figure out the motion of the liquid, we need to know all forces acting on each lamina:

# The force pushing the liquid through the tube is the change in pressure multiplied by the area: "F" = -"ΔPA". This force is in the direction of the motion of the liquid - the negative sign comes from the conventional way we define $Delta\; P\; =\; P\_\{end\}-P\_\{top\}\; <\; 0$.

# The pull from the faster lamina immediately closer to the center of the tube

# The drag from the slower lamina immediately closer to the walls of the tubeThe first of these forces comes from the definition of

pressure . The other two forces require us to modify the equations above that we have forviscosity . In fact, we are not modifying the equations, instead merely plugging in values specific to our problem. Let's focus on the pull from the faster lamina (#2) first.**Faster lamina**Assume that we are figuring out the force on the lamina with

radius "s". From the equation above, we need to know thearea of contact and the velocitygradient . Think of the lamina as a cylinder of radius "s" and thickness "ds". The area of contact between the lamina and the faster one is simply the area of the inside of the cylinder: "A" = "2πsΔx". We don't know the exact form for the velocity of the liquid within the tube yet, but we do know (from our assumption above) that it is dependent on the radius. Therefore, the velocity gradient is the change of the velocity with respect to the change in the radius at the intersection of these two laminae. That intersection is at a radius of "s". So, considering that this force will be positive with respect to the movement of the liquid (but the derivative of the velocity is negative), the final form of the equation becomes:$F\_\{\; ext\{viscosity,\; fast\; =\; -\; eta\; 2\; pi\; s\; Delta\; x\; left\; .\; frac\{dv\}\{dr\}\; ight\; vert\_s$

where the vertical bar and subscript "s" following the

derivative indicates that it should be taken at a radius of "s".**Slower lamina**Next let's find the force of drag from the slower lamina. We need to calculate the same values that we did for the force from the faster lamina. In this case, the area of contact is at "s"+"ds" instead of "s". Also, we need to remember that this force opposes the direction of movement of the liquid and will therefore be negative (and that the derivative of the velocity is negative).

:$F\_\{\; ext\{viscosity,\; slow\; =\; eta\; 2\; pi\; (s+ds)\; Delta\; x\; left\; .\; frac\{dv\}\{dr\}\; ight\; vert\_\{s+ds\}$

**Putting it all together**To find the solution for the flow of liquid through a tube, we need to make one last assumption. There is no

acceleration of liquid in the pipe, and by Newton's first law, there is no net force. If there is no net force then we can add all of the forces together to get zero:$0\; =\; F\_\{\; ext\{pressure\; +\; F\_\{\; ext\{viscosity,\; fast\; +\; F\_\{\; ext\{viscosity,\; slow$

or

:$0\; =\; -\; Delta\; P2\; pi\; sds\; -\; eta\; 2\; pi\; s\; Delta\; x\; left\; .\; frac\{dv\}\{dr\}\; ight\; vert\_s\; +\; eta\; 2\; pi\; (s+ds)\; Delta\; x\; left\; .\; frac\{dv\}\{dr\}\; ight\; vert\_\{s+ds\}$

Before we move further, we need to simplify this ugly equation. First, to get everything happening at the same point, we need to do a Taylor series expansion of the velocity gradient, keeping only the

linear and quadratic terms (a standard mathematical trick).:$left\; .\; frac\{dv\}\{dr\}\; ight\; vert\_\{r+dr\}\; =\; left\; .\; frac\{dv\}\{dr\}\; ight\; vert\_r\; +\; left\; .\; frac\{d^2\; v\}\{dr^2\}\; ight\; vert\_r\; dr$

Let's use this relation in our equation. Also, let's use "r" instead of "s" since the lamina we chose was arbitrary and we want our expression to be valid for all laminae. Grouping like terms and dropping the vertical bar since all derivatives are assumed to be at radius "r",

:$0\; =\; -\; Delta\; P2\; pi\; rdr\; +\; eta\; 2\; pi\; dr\; Delta\; x\; frac\{dv\}\{dr\}\; +\; eta\; 2\; pi\; r\; dr\; Delta\; x\; frac\{d^2\; v\}\{dr^2\}\; +\; eta\; 2\; pi\; (dr)^2\; Delta\; x\; frac\{d^2\; v\}\{dr^2\}$

Finally, let's get this in the form of a

differential equation , moving some terms around to make it easier to solve later, and neglecting the term quadratic in "dr" since this will be really small compared to the rest (another standard mathematical trick).:$frac\{1\}\{eta\}\; frac\{Delta\; P\}\{Delta\; x\}\; =\; frac\{d^2\; v\}\{dr^2\}\; +\; frac\{1\}\{r\}\; frac\{dv\}\{dr\}$

It can be seen that both sides of the equations are negative: there is a drop of pressure along the tube (left side) and both first and second derivatives of the velocity are negative (velocity has a maximum value of the center of the tube). The equation may be re-arranged to:

:$frac\{1\}\{eta\}\; frac\{Delta\; P\}\{Delta\; x\}\; =\; frac\{1\}\{r\}\; frac\{d\}\{dr\}\; r\; frac\{dv\}\; \{dr\}$

This differential equation is subject to the following boundary conditions: :v(r) = 0 at r = R -- "No-slip" Boundary Condition at the Wall :$frac\{dv\}\; \{dr\}\; =\; 0$ at r = 0 -- Axial symmetry

Axial symmetry means that the velocity v(r) is maximum at the center of the tube, therefore the first derivative $frac\{dv\}\{dr\}$ is zero at r = 0.

The differential equation can be integrated to:

:$v(r)\; =\; frac\{1\}\{4\; eta\}r^2frac\{Delta\; P\}\{Delta\; x\}\; +\; A\; ln(r)\; +\; B$

To find A and B, we use the boundary conditions.

First, the symmetry boundary condition indicates:

:$frac\{dv\}\{dr\}\; =\; frac\{1\}\{2\; eta\}\; r\; frac\{Delta\; P\}\{Delta\; x\}\; +\; A\; frac\{1\}\{r\}\; =\; 0$ at r = 0

A solution possible only if A = 0. Next the no-slip boundary condition is applied to the remaining equation:

:$v(R)\; =\; frac\{1\}\{4\; eta\}\; R^2\; frac\{Delta\; P\}\{Delta\; x\}\; +\; B\; =\; 0$

so therefore

:$B\; =\; -\; frac\{1\}\{4\; eta\}\; R^2\; frac\{Delta\; P\}\{Delta\; x\}$

Now we have a formula for the velocity of liquid moving through the tube as a function of the distance from the center of the tube

:$v\; =\; -\; frac\{1\}\{4\; eta\}\; frac\{Delta\; P\}\{Delta\; x\}\; (R^2\; -\; r^2)$

or, at the center of the tube where the liquid is moving fastest ("r" = 0) with "R" being the radius of the tube,

:$v\_\{max\}\; =\; -\; frac\{1\}\{4\; eta\}\; frac\{Delta\; P\}\{Delta\; x\}R^2$

**Poiseuille's Law**To get the total volume that flows through the tube, we need to add up the contributions from each lamina. To calculate the flow through each lamina, we multiply the velocity (from above) and the area of the lamina.

:$Phi\; (r)\; =\; frac\{1\}\{4\; eta\}\; frac\{Delta\; x\}\; int\_\{0\}^\{R\}\; (rR^2\; -\; r^3),\; dr\; =\; frac\{|Delta\; P|\; pi\; R^4\}\{8\; eta\; Delta\; x\}$

**Poiseuille's equation for compressible fluids**For a compressible fluid in a tube the

volumetric flow rate and thelinear velocity is not constant along the tube. The flow is usually expressed at outlet pressure. As fluid is compressed or expands work is done and the fluid is heated and cooled. This meaning that the flow rate is dependant upon heat transfer to and from the fluid. For anideal gas in theisothermal case, where the temperature of the fluid is permitted to equilibrate with its surroundings, and when the pressure difference between ends of the pipe is small, the volumetric flow rate at the pipe outlet is given by:$Phi\; =\; frac\{dV\}\{dt\}\; =\; v\; pi\; R^\{2\}\; =\; frac\{pi\; R^\{4\}\; left(\; P\_\{i\}-P\_\{o\}\; ight)\}\{8\; eta\; L\}\; imes\; frac\{\; P\_\{i\}+P\_\{o\{2\; P\_\{o\; =\; frac\{pi\; R^\{4\{16\; eta\; L\}\; left(\; frac\{\; P\_\{i\}^\{2\}-P\_\{o\}^\{2\{P\_\{o\; ight)$

Where::$P\_\{i\}$ inlet pressure:$P\_\{o\}$ outlet pressure:$L$ is the length of tube:$eta$ is the

viscosity :$R$ is theradius :$V$ is thevolume of the fluid at outlet pressure:$v$ is thevelocity of the fluid at outlet pressureThis is usually a good approximation when the flow velocity is less than mach 0.3

This equation can be seen as Poiseuille's law with an extra correction factor $frac\{P\_\{i\}+P\_\{o\{2\}\; imes\; frac\{1\}\{P\_\{o$ expressing the average pressure relative to the outlet pressure.

= Electrical Circuits analogy =Electricity was originally understood to be a kind of fluid. This

hydraulic analogy is still conceptually useful.Poiseuille's law corresponds to

Ohm's law for electrical circuits ("V" = "IR"), where the pressure drop Δ"P " is analogous to thevoltage "V" and voluminal flow rate Φ is analogous to the current "I". Then the resistance :$R\; =\; frac\{\; 8\; eta\; Delta\; x\}\{pi\; r^4\}$This concept is useful because the effective resistance in a tube is inversely proportional to the fourth power of the radius. This means that halfing the size of the tube increases the resistance to fluid movement by 16 times.Both Ohm's law and Poiseuille's law illustrate

transport phenomena .**History**It was developed independently by

Gotthilf Heinrich Ludwig Hagen (1797 -1884 ) andJean Louis Marie Poiseuille .Poiseuille's law was experimentally derived in

1838 and formulated and published in1840 and1846 byJean Louis Marie Poiseuille (1797 -1869 ). Hagen did his experiments in1839 .**References***S. P. Sutera, R. Skalak, "The history of Poiseuille's law," "Annual Review of Fluid Mechanics", Vol. 25, 1993, pp. 1-19

*Citation

id =PMID :779509

url= http://www.ncbi.nlm.nih.gov/pubmed/779509

last=Pfitzner

first=J

publication-date=1976 Mar

year=1976

title=Poiseuille and his law.

volume=31

issue=2

periodical=Anaesthesia

pages=273-5**ee also***

Darcy's law

*Pulse

*Wave **External links*** [

*http://www.syvum.com/cgi/online/serve.cgi/eng/fluid/fluid802.html Poiseuille's law for power-law non-Newtonian fluid*]

* [*http://www.syvum.com/cgi/online/serve.cgi/eng/fluid/fluid203.html Poiseuille's law in a slightly tapered tube*]

* [*http://www.calctool.org/CALC/eng/fluid/hagen-poiseuille Web-based calculator of the Hagen-Poiseuille equation*]

*Wikimedia Foundation.
2010.*

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