# Poiseuilles Law Assumptions And Critical Thinking

In nonideal fluid dynamics, the **Hagen–Poiseuille equation**, also known as the **Hagen–Poiseuille law**, **Poiseuille law** or **Poiseuille equation**, is a physical law that gives the pressure drop in an incompressible and Newtonian fluid in laminar flow flowing through a long cylindrical pipe of constant cross section. It can be successfully applied to air flow in lungalveoli, for the flow through a drinking straw or through a hypodermic needle. It was experimentally derived independently by Jean Léonard Marie Poiseuille in 1838^{[1]} and Gotthilf Heinrich Ludwig Hagen,^{[2]} and published by Poiseuille in 1840–41 and 1846.^{[1]}

The assumptions of the equation are that the fluid is incompressible and Newtonian; the flow is laminar through a pipe of constant circular cross-section that is substantially longer than its diameter; and there is no acceleration of fluid in the pipe. For velocities and pipe diameters above a threshold, actual fluid flow is not laminar but turbulent, leading to larger pressure drops than calculated by the Hagen–Poiseuille equation.

## Equation[edit]

In standard fluid-dynamics notation:^{[3]}^{[4]}^{[5]}

where:

- Δ
*P*is the pressure difference between the two ends, - L is the length of pipe,
- μ is the dynamic viscosity,
- Q is the volumetric flow rate,
- R is the pipe radius.

The equation does not hold close to the pipe entrance.^{[6]}^{:3}

The equation fails in the limit of low viscosity, wide and/or short pipe. Low viscosity or a wide pipe may result in turbulent flow, making it necessary to use more complex models, such as Darcy–Weisbach equation. If the pipe is too short, the Hagen–Poiseuille equation may result in unphysically high flow rates; the flow is bounded by Bernoulli's principle, under less restrictive conditions, by

## Relation to Darcy–Weisbach[edit]

Normally, Hagen-Poiseuille flow implies not just the relation for the pressure drop, above, but also the full solution for the laminar flow profile, which is parabolic. However, the result for the pressure drop can be extended to turbulent flow by inferring an effective turbulent viscosity in the case of turbulent flow, even though the flow profile in turbulent flow is strictly speaking not actually parabolic. In both cases, laminar or turbulent, the pressure drop is related to the stress at the wall, which determines the so-called friction factor. The wall stress can be determined phenomenologically by the Darcy–Weisbach equation in the field of hydraulics, given a relationship for the friction factor in terms of the Reynolds number. In the case of laminar flow:

where Re is the Reynolds number, ρ is the fluid density, v is the mean flow velocity, which is half the maximal flow velocity in the case of laminar flow. It proves more useful to define the Reynolds number in terms of the mean flow velocity because this quantity remains well defined even in the case of turbulent flow, whereas the maximal flow velocity may not be, or in any case, it may be difficult to infer. 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 in 1856 and Neumann and E. Hagenbach in 1858 (1859, 1860). Hagenbach was the first who called this law the Poiseuille's law.

The law is also very important in hemorheology and hemodynamics, both fields of physiology.^{[7]}

Poiseuille's law was later in 1891 extended to turbulent flow by L. R. Wilberforce, based on Hagenbach's work.

## Derivation[edit]

Main article: Hagen–Poiseuille flow from the Navier–Stokes equations

The Hagen–Poiseuille equation can be derived from the Navier–Stokes equations. Although more lengthy than directly using the Navier–Stokes equations, an alternative method of deriving the Hagen–Poiseuille equation is as follows.

### Liquid flow through a pipe[edit]

Assume the liquid exhibits laminar flow. Laminar flow in a round pipe prescribes 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. Also assume the center is moving fastest while the liquid touching the walls of the tube is stationary (due to the no-slip condition).

To figure out the motion of the liquid, all forces acting on each lamina must be known:

- The pressure force pushing the liquid through the tube is the change in pressure multiplied by the area:
*F*= −*A*Δ*P*. This force is in the direction of the motion of the liquid. The negative sign comes from the conventional way we define Δ*P*=*P*_{end}−*P*_{top}< 0. - Viscosity effects will pull from the faster lamina immediately closer to the center of the tube.
- Viscosity effects will drag from the slower lamina immediately closer to the walls of the tube.

### Viscosity[edit]

When two layers of liquid in contact with each other move at different speeds, there will be a shear force between them. This force is proportional to the area of contact A, the velocity gradient in the direction of flow Δ*v _{x}*/Δ

*y*, and a proportionality constant (viscosity) and is given by

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 fluids.

### Faster lamina[edit]

Assume that we are figuring out the force on the lamina with radiusr. From the equation above, we need to know the area of contact and the velocity gradient. Think of the lamina as a ring of radius r, thickness dr, and length Δ*x*. The area of contact between the lamina and the faster one is simply the area of the inside of the cylinder: *A* = 2π*r* Δ*x*. 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 r. 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

where the vertical bar and subscript r following the derivative indicates that it should be taken at a radius of r.

### Slower lamina[edit]

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 *r* + *dr* instead of r. 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).

### Putting it all together[edit]

To find the solution for the flow of a laminar layer 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

or

First, to get everything happening at the same point, use the first two terms of a Taylor series expansion of the velocity gradient:

The expression is valid for all laminae. Grouping like terms and dropping the vertical bar since all derivatives are assumed to be at radius r,

Finally, put this expression in the form of a differential equation, dropping the term quadratic in 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 at the center of the tube, where *r* = 0). Using the product rule, the equation may be rearranged to:

The right-hand side is the radial term of the Laplace operator ∇^{2}, so this differential equation is a special case of the Poisson equation. It is subject to the following boundary conditions:

— "no-slip" boundary condition at the wall — axial symmetry.

Axial symmetry means that the velocity *v*(*r*) is maximum at the center of the tube, therefore the first derivative *dv*/*dr* is zero at *r* = 0.

The differential equation can be integrated to:

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

First, the symmetry boundary condition indicates:

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

so therefore

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

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

### Poiseuille's law[edit]

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.

Finally, we integrate over all lamina via the radius variable r.

### Startup of Poiseuille flow in a pipe^{[8]}[edit]

When a constant pressure gradient is applied between two ends of a long pipe, the flow will not immediately obtain Poiseuille profile, rather it develops through time and reaches the Poiseuille profile at steady state. The Navier-Stokes equations reduce to

with initial and boundary conditions,

The velocity distribution is given by

where is the Bessel function of the first kind of order zero and are the positive roots of this function and is the Bessel function of the first kind of order one. As , Poiseuille solution is recovered.

## Poiseuille flow in annular section^{[9]}[edit]

**a)**A tube showing the imaginary lamina.

**b)**A cross section of the tube shows the lamina moving at different speeds. Those closest to the edge of the tube are moving slowly while those near the center are moving quickly.

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