# What is an MHD submarine

The Magnetohydrodynamics (MHD) is a branch of physics and describes the interaction of an electrically conductive fluid with electrical and magnetic fields. Typical examples of such fluids are plasmas, liquid metals, electrolytes and semiconductor melts. The basic equations of MHD are the Navier-Stokes equations of hydrodynamics and the Maxwell equations of electrodynamics.

The most widespread technical application of magnetohydrodynamics is electromagnetic flow measurement, which is also referred to as magneto-inductive flow measurement. This method measures the flows of beer, milk, yogurt, drinking water, tomato ketchup, chemicals and sewage.

Another application of magnetohydrodynamics is magnetohydrodynamic propulsion. This application was originally developed for military purposes and was tested in the civilian sector in the 1990s. However, due to their low level of efficiency, MHD drives have gained neither military nor civil significance.

The earth's magnetic field is generated by magnetohydrodynamic phenomena in the liquid core of the earth. Furthermore, the development of future facilities for nuclear fusion requires the technical mastery of magnetohydrodynamic instabilities in plasmas.

### Ideal best before date

The ideal MHD is a simplified theory compared to the real MHD, with which magnetohydrodynamic processes in plasmas can be approximately described. The equations of the ideal MHD are used, for example, in the calculation of nuclear fusion and in astrophysical problems. The basic assumptions of the ideal MHD are:

1. The plasma is considered to be a homogeneous fluid.
2. The plasma has an infinitely high electrical conductivity.
3. The plasma has a negligible viscosity.

The resulting equations result from the Navier-Stokes equations, the Maxwell equations and Ohm's law. Furthermore, an equation of state is necessary, which is not given here. In the third equation, to the right of the pressure, there is a term called magnetic pressure. It also contains an additional term that describes the magnetic tension.

The symbols have their usual meanings. Φ denotes the potential of an external force, e.g. due to gravity; denotes the dyadic product. The hydrostatic pressureP is made by the magnetic pressureadded, which can have a significant influence on the dynamics under certain circumstances.

### Frozen magnetic field

When a plasma moves in a magnetic field, electrical currents are generated inside. These currents in turn contribute to a further magnetic field that is superimposed on the externally applied fields and deforms them. If this state can be maintained in a stable manner, the plasma and the resulting magnetic field are connected to one another, the magnetic field appears as if frozen in the plasma. This happens ideally (i.e. completely) when the electrical resistance becomes 0, i.e. disappears. This is the case in very hot plasma. Such phenomena can be observed in nature, e.g. in the so-called loop protuberances of the sun.

### Alfven waves

In the interior of the fluid, transverse magnetosonic waves can occur, the so-called Alfven waves (see plasma waves).

### Geodynamo

The earth's magnetic field is generated by the so-called geodynamo, which is described by the equations of magnetohydrodynamics. The earth's magnetic field is created in the outer core of the earth, which mainly consists of liquid iron (see creation and maintenance of the earth's magnetic field (geodynamo)).

The partial differential equations occurring in connection with a geodynamo can only be solved analytically in highly simplified cases. Since the mid-1990s, numerical methods have provided the first approaches to understanding the dynamics of the earth's magnetic field.

### Electromagnetic flow measurement

If an electrically conductive fluid moves under the influence of a magnetic field, a voltage is generated in the fluid which is proportional to the strength of the magnetic field and the flow velocity. This principle was discovered in 1832 by Michael Faraday, who tried (unsuccessfully) in this way to determine the flow velocity of the Thames. The principle is found today in the form of electromagnetic flow meters (also called inductive flow meters), broad industrial applications in the food industry, chemical industry and in waterworks.

### Best before generator

A technical application of magnetohydrodynamics is the magnetohydrodynamic generator (MHD generator). A plasma is shot through between two conductive electrodes. A magnetic field is applied perpendicular to the electrodes, which then separates the electrons and positive ions due to the different charges. This creates a voltage difference between the plates. In this way, kinetic energy can be converted directly into electrical energy without having to use mechanical components (turbines or generators). Despite intensive efforts, especially in the 1960s, it has not been possible to develop electrodes that are stable over the long term. For this reason, the MHD generator is currently of no practical importance.

### MHD drive

An electrically conductive substance such as a plasma can be accelerated in an electromagnetic field (magnetoplasmic dynamic drive). Since sea water and ionized air are also conductive as ion mixtures, they can also be accelerated in an electromagnetic field (magnetohydrodynamic drive). In principle, these properties can be used to propel ships, submarines and aircraft. However, the efficiency of such systems is low. For this reason, the MHD drive has no practical significance today.

### Best before sensor

Magnetohydrodynamic sensors are used to measure angular velocities. Accuracy increases with size. One area of ​​application is the aerospace industry. The principle of an MHD sensor, which also helps to understand the basic idea of ​​magnetohydrodynamics (MHD) as a whole, is shown in this sketch:

Categories: Magnetohydrodynamics | magnetism