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Lorentz force velocimetry

Lorentz force velocimetry is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Lorentz force velocimetry rather than just read about it. In short: Lorentz force velocimetry (LFV) is a noncontact electromagnetic flow measurement technique. LFV is particularly suited for the measurement of velocities in liquid metals like steel or aluminium and is currently under development for metallurgical applications.

Lorentz force velocimetry — main illustration
Lorentz force velocimetry — illustration

Key takeaways

  • Lorentz force velocimetry belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Lorentz force velocimetry to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Lorentz force velocimetry from memory before moving on to harder problems.

Reference excerpt

Lorentz force velocimetry (LFV) is a noncontact electromagnetic flow measurement technique. LFV is particularly suited for the measurement of velocities in liquid metals like steel or aluminium and is currently under development for metallurgical applications. The measurement of flow velocities in hot and aggressive liquids such as liquid aluminium and molten glass constitutes one of the grand challenges of industrial fluid mechanics. Apart from liquids, LFV can also be used to measure the velocity of solid materials as well as for detection of micro-defects in their structures. A Lorentz force velocimetry system is called Lorentz force flowmeter (LFF). A LFF measures the integrated or bulk Lorentz force resulting from the interaction between a liquid metal in motion and an applied magnetic field. In this case the characteristic length of the magnetic field is of the same order of magnitude as the dimensions of the channel. It must be addressed that in the case where localized magnetic fields are used, it is possible to perform local velocity measurements and thus the term Lorentz force velocimeter is used.

Introduction The use of magnetic fields in flow measurement date back to the 19th century, when in 1832 Michael Faraday attempted to determine the velocity of the River Thames. Faraday applied a method in which a flow (the river flow) is exposed to a magnetic field (earth magnetic field) and the induced voltage is measured using two electrodes across the same flow. This method is the basis of one of the most successful commercial applications in flow metering known as the inductive flowmeter. The theory of such devices has been developed and comprehensively summarized by Prof. J. A. Shercliff in the early 1950s. While inductive flowmeters are widely used for flow measurement in fluids at room temperatures such as beverages, chemicals and waste water, they are not suited for flow measurement of media such as hot, aggressive or for local measurements where surrounding obstacles limit access to the channel or pipe. Since they require electrodes to be inserted into the fluid, their use is limited to applications at temperatures far below the melting points of practically relevant metals. The Lorentz force velocimetry was invented by the A. Shercliff. However, it did not find practical application in these early years up until recent technical advances; in manufacturing of rare earth and non rare-earth strong permanent magnets, accurate force measurement techniques, multiphysical process simulation software for magnetohydrodynamic (MHD) problems that this principle could be turned into a feasible working flow measurement technique. LFV is currently being developed for applications in metallurgy as well as in other areas. Based on theory introduced by Shercliff there have been several attempts to develop flow measurement methods which do not require any mechanical contact with the fluid,. Among them is the eddy current flowmeter which measures flow-induced changes in the electric impedance of coils interacting with the flow. More recently, a non-contact method was proposed in which a magnetic field is applied to the flow and the velocity is determined from measurements of flow-induced deformations of the applied magnetic field,.

Principle and physical interpretation The principle of Lorentz force velocimetry is based on measurements of the Lorentz force that occurs due to the flow of a conductive fluid under the influence of a variable magnetic field. According to Faraday's law, when a metal or conductive fluid moves through a magnetic field, eddy currents generate there by electromotive force in zones of maximal magnetic field gradient (in the present case in the inlet and outlet zones). Eddy current in its turn creates induced magnetic field according to Ampère's law. The interaction between eddy currents and total magnetic field gives rise to Lorentz force that breaks the flow. By virtue of Newton's third law "actio=reactio" a force with the same magnitude but opposite direction acts upon its source - permanent magnet. Direct measurement of the magnet's reaction force allows to determine fluid's velocity, since this force is proportional to flow rate. The Lorentz force used in LFV has nothing to do with magnetic attraction or repulsion. It is only due to the eddy currents whose strength depends on the electrical conductivity, the relative velocity between the liquid and the permanent magnet as well as the magnitude of the magnetic field. So, when a liquid metal moves across magnetic field lines, the interaction of the magnetic field (which are either produced by a current-carrying coil or by a permanent magnet) with the induced eddy currents leads to a Lorentz force (with density f → = j → × B → {\displaystyle {\vec {f}}={\vec {j}}\times {\vec {B}}} ) which brakes the flow. The Lorentz force density is roughly

… excerpt ends here. Continue reading the full article.

Illustrations

Lorentz force velocimetry: Fig. 3 : Principle sketch of Lorentz force velocimetry: arrangement of the coil (a) and structure of the primary magnetic field (b) for a longitudinal flux flowmeter. (c, d) Same for a transverse flux flowmeter. Adapted from.[6]
Fig. 3 : Principle sketch of Lorentz force velocimetry: arrangement of the coil (a) and structure of the primary magnetic field (b) for a longitudinal flux flowmeter. (c, d) Same for a transverse flux flowmeter. Adapted from.[6]
Lorentz force velocimetry: Fig. 4 : A simplified sketch of the rotary LFV. Adapted from.[1]
Fig. 4 : A simplified sketch of the rotary LFV. Adapted from.[1]
Lorentz force velocimetry: Fig. 6 : Measurement principle, state-of-art weighting balance system : F M {\displaystyle F_{M}} -measurement force, F G {\displaystyle F_{G}} -gravity, F C {\displaystyle F_{C}} -force due to spring constant, c s {\displaystyle c_{s}} -spring constant, l p {\displaystyle l_{p}} -length of beams, a -deflection of pan carrier, a a b {\displaystyle a_{ab}} -deflection of lever, α {\displaystyle \alpha } -deflection angle, m 0 {\displaystyle m_{0}} -dead load, g {\displaystyle g} -gravitational acceleration. Adapted from [17]
Fig. 6 : Measurement principle, state-of-art weighting balance system : F M {\displaystyle F_{M}} -measurement force, F G {\displaystyle F_{G}} -gravity, F C {\displaystyle F_{C}} -force due to spring constant, c s {\displaystyle c_{s}} -spring constant, l p {\displaystyle l_{p}} -length of beams, a -deflection of pan carrier, a a b {\displaystyle a_{ab}} -deflection of lever, α {\displaystyle \alpha } -deflection angle, m 0 {\displaystyle m_{0}} -dead load, g {\displaystyle g} -gravitational acceleration. Adapted from [17]
Lorentz force velocimetry: Fig. 8 : LOFOS working principle.
Fig. 8 : LOFOS working principle.
Lorentz force velocimetry: Fig. 9 : Time-of-flight working principle. Taken from [20]
Fig. 9 : Time-of-flight working principle. Taken from [20]

Worked examples

Example 1 — a first encounter with Lorentz force velocimetry

Start with the simplest possible case. Write down what Lorentz force velocimetry claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lorentz force velocimetry before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lorentz force velocimetry ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lorentz force velocimetry

In research
Lorentz force velocimetry appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Lorentz force velocimetry in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Lorentz force velocimetry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Fluid dynamics, so understanding it makes those chapters shorter.
In everyday life
Look for Lorentz force velocimetry outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Lorentz force velocimetry in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Lorentz force velocimetry means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Lorentz force velocimetry out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lorentz force velocimetry in simple terms?

Lorentz force velocimetry (LFV) is a noncontact electromagnetic flow measurement technique. LFV is particularly suited for the measurement of velocities in liquid metals like steel or aluminium and is currently under development for metallurgical applications.

Why does Lorentz force velocimetry matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lorentz force velocimetry?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lorentz force velocimetry.

Tags

  • Fluid dynamics

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