Inviscid flow

Inviscid flow is the flow of an inviscid fluid, in which the viscosity of the fluid is equal to zero. Though there are limited examples of invisicd fluids, known as superfluids, inviscid flow has many applications in fluid dynamics. The Reynolds number of inviscid flow approaches infinity, as the viscosity approaches zero. When viscous forces are neglected, such as the case of inviscid flow, the Navier-Stokes equation can be simplified to a form known as the Euler equation. This simplified equation is applicable to inviscid flow as well as flow with low viscosity and a Reynolds number much greater than one. Using the Euler equation, many fluid dynamics problems involving low viscosity are easily solved, however, the assumed negligible viscosity is no longer valid in the region of fluid near a solid boundary.

Superfluid is the state of matter that exhibits frictionless flow, zero viscosity, also known as inviscid flow.

To date, Helium is the only fluid to exhibit superfluidity, that has been discovered. Helium becomes a superfluid once it is cooled to below 2.2k, this point is known as the lambda point. Temperatures above the lambda point, Helium exists as a liquid exhibiting normal fluid dynamic behavior. Once it is cooled to below 2.2k it begins to exhibit quantum behavior. For example, at the lambda point there is a sharp increase in heat capacity, as it is continued to be cooled, the heat capacity begins to decrease with temperature. In addition, the thermal conductivity is very large, contributing to the excellent coolant properties of superfluid Helium.

Spectrometers are kept at a very low temperature using Helium as the coolant. This allows for minimal background flux in far-infrared readings. Some of the designs for the spectrometers may be simple, but even the frame is at its warmest less than 20 Kelvin. These devices are not commonly used as it is very expensive to use superfluid helium over other coolants.

Superfluid Helium has a very high thermal conductivity which makes it very useful for cooling superconductors. Superconductors such as the ones used at the LHC (Large Hadron Collidor) are cooled to temperatures approximately 1.9 Kelvin. This temperature allows the niobium-titanium magnets to reach a superconductor state. Without the use of the superfluid Helium this temperature would not be possible. Cooling to these temperatures, with this fluid, is a very expensive system and there are few compared to other cooling systems.

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