Because it is impossible to have a negative (absolute) pressure (not to be confused with overpressure) in an incompressible flow. The Hagen-Poiseuille equation is useful for determining vascular resistance and therefore intravenous (IV) fluid flow, which can be achieved with different sizes of peripheral and central cannulas. The equation indicates that the flow rate is proportional to the radius of the fourth power, which means that a slight increase in the inside diameter of the cannula results in a significant increase in the flow of fluids IV. The radius of IV cannulas is usually measured in « gauge », which is inversely proportional to the radius. Peripheral IV cannulas are usually available in 14G, 16G, 18G, 20G, 22G, 26G (large to small). For example, the flow of a 14G cannula is usually about twice that of a 16G cannula and ten times that of a 20G cannula. It also states that throughput is inversely proportional to length, meaning longer lines have lower throughputs. This is important to note because many doctors prefer shorter, larger catheters in an emergency over longer, narrower catheters. Although of lesser clinical importance, an increased pressure change (∆p) – such as pressurizing the bag of liquid, squeezing the bag, or hanging the bag higher (relative to the cannula level) – can be used to speed up the flow.
It is also useful to understand that viscous fluids flow more slowly (for example, in blood transfusions).  One of the simplest results that can be obtained in fluid mechanics is Poiseuille`s law. This basic relationship is also called the Hagen–Poiseuille equation by fluid mechanics purists. No matter what you call it, Poiseuille`s law is a simple relationship between the laminar flow in a capillary tube and the parameters of the material or system, namely the geometry of the pipe, the dynamic viscosity of the material, and the pressure gradient in the system. In fact, the relationship between these parameters is simple enough to be derived from a balance of forces equation. Electricity was originally understood as a type of liquid. This hydraulic analogy is always conceptually useful for understanding circuits. This analogy is also used to study the frequency response of fluid mechanics networks using circuit tools, in which case the fluid network is called the hydraulic circuit. Poiseuille`s law corresponds to Ohm`s law for electrical circuits, V = IR. Since the net force acting on the fluid is equal to ΔF = SΔp, where S = πr2, i.e. ΔF = πr2 ΔP, then it follows from Poiseuille`s law that the viscosity of the fluid: The flow is inversely proportional to the viscosity of the fluid.
Increasing viscosity reduces flow through a catheter. The viscosity of frequently infused intravenous solutions ranges from 1.0 centiPoise to 40.0 cP (reference: water viscosity is 1.002 cP). For a compressible fluid in a pipe, the volume flow Q(x) (but not the mass flow) and the axial velocity along the pipe are not constant. The flow rate is usually expressed at the outlet pressure. When the liquid is compressed or expanded, the work is done and the liquid is heated or cooled. This means that the flow rate depends on the transfer of heat to and from the fluid. For an ideal gas in the isothermal case, where the temperature of the fluid can be balanced with its environment, an approximate relationship for pressure drop can be derived.  Using the ideal gas equation of state for the constant temperature process, the relation Qp = Q1p1 = Q2p2 can be obtained. On a short section of the pipeline, it can be assumed that the gas passing through the pipe is incompressible, so Poiseuille`s law can be used locally. To ensure that fluid flow follows the Poiseuille equation in a general closed system, system designers can use the full set of CFD simulation capabilities in Cadence`s Omnis.
Advanced numerical approaches used in aerodynamic simulations, turbulent and laminar flow simulations, reduced flow models, etc. can be implemented in Cadence`s CFD simulation tools. To maximize flow, an ideal rapid infusion system would consist of the largest diameter and shortest length. The infused liquid must have the lowest possible viscosity and be released at maximum pressure. Perhaps the most popular field of application in which Poiseuille`s law is considered is hemodynamics. Because volume flow is so sensitive to changes in cross-section, Poiseuille`s law is used to explain why narrowed capillaries lead to higher blood pressure. A slight narrowing of the veins would reduce blood flow, but the heart compensates by working harder to increase blood pressure and keep blood flow constant. This explains why venous narrowing and high blood pressure are linked. Pipe diameter: An important and frequently cited relationship is the radius of the pipes. Doubling the diameter of the catheter increases the flow rate by 16x (r4).
The larger the IV catheter, the greater the flow. For simplicity, the two tanks are designed separately, but can be placed side by side with a connector that connects the rod valves of each. The device is placed on a bench with a drip tray inclined about 20° directly below the ends of the capillaries. In one corner of the compartment there is a hole that allows it to empty into a bucket underneath. Food dyes can be added to the water to make levels clear. Continuous flow: Poiseuille`s law only deals with continuous laminar flow, which means that the flow velocity profile and material properties do not change over time. where Re is the Reynolds number, ρ is the density of the fluid and v is the average flow velocity, which is half the maximum flow velocity for laminar flow. It is more useful to define the Reynolds number in terms of the average flow velocity, because this amount remains well defined even in turbulent flow, whereas the maximum flow velocity may not be close or may be difficult to close in any case.
In this form, the law is similar to the Darcy friction factor, the energy loss factor (head), the friction loss factor or the Darcy factor (friction) Λ in laminar flow at very low velocity in cylindrical tubes. The theoretical derivation of a somewhat different form of law was developed independently by Wiedman in 1856 and Neumann and E. Hagenbach in 1858 (1859, 1860). Hagenbach was the first to call this law Poiseuille. The Hagen–Poiseuille equation can be derived from the Navier–Stokes equation. Laminar flow through a pipe of uniform (circular) cross-section is called Hagen-Poiseuille flow. The Hagen-Poiseuille flow equations can be derived directly from the Navier-Stokes momentum equations in 3D cylindrical coordinates (r, θ, x) by making the following assumptions: The device consists of two 12-litre plexiglass tanks, one emptied by a single 6 mm bore capillary tube and the other by sixteen 3 mm bore tubes. All pipes are 60 cm long. For a direct comparison, all pipes must be opened at the same time as the tanks, and this is done with a valve consisting of a long steel bar with 17 drilled holes corresponding to the 17 pipes (see Figure 1b below).
The rod runs the length of the tanks and has a handle that rotates them to align the holes in the rod with those in the tank. where ber and at are the Kelvin functions and k2 = ρω/μ. The same considerations and relationships defined in Poiseuille`s law apply to microfluidics in general. Poiseuille`s law also applies to compressible liquids, although with rapid compression this becomes a more complex thermodynamic problem. If a fluid is compressible and the compression is slow enough to be treated as isothermal, then there is a modified version of Poiseuille`s law that relates the flow rate to the pressure values at each end of the flow region, as shown below: where J0(λnr/R) is the Bessel function of the first type of zero and λn are the positive roots of this function, and J1(λn) is the Bessel function of the first type of order one. Poiseuille`s solution is recovered as a → ∞.  Finally, you put this expression in the form of a differential equation and leave the quadratic term in dr. It follows that the resistance R is proportional to the length L of the resistance, which is true.
However, it also follows that the resistance R is inversely proportional to the fourth power of the radius r, that is, the resistance R is inversely proportional to the second power of the section S = πr2 of the resistance, which differs from the electric formula. The electrical relationship for resistance is flow tubes with an oscillating pressure gradient and finds application in the blood flow through the major arteries.     The imposed pressure gradient is given by Poiseuille`s law, which was later extended to turbulent flow by L. R. Wilberforce in 1891 on the basis of Hagenbach`s work. Now, this establishment of an orderly and law-abiding self seems to me to imply that there are impulses that ensure order.