GB2576747A

GB2576747A - Method and device to determine acoustic gas particle velocity

Info

Publication number: GB2576747A
Application number: GB1814113.5A
Authority: GB
Inventors: Nijman Eugene
Original assignee: Kompetenzzentrum das Virtuelle Fahrzeug Forchungs GmbH
Current assignee: Kompetenzzentrum das Virtuelle Fahrzeug Forchungs GmbH
Priority date: 2018-08-30
Filing date: 2018-08-30
Publication date: 2020-03-04
Also published as: GB201814113D0

Abstract

Plasma generator 1 generates plasma 2 in a measurement volume. Coils or permanent magnets generate magnetic field 5 in the measurement volume. The plasma is exposed to sound wave 3 in a gas, causing time-dependent deviation in the plasma which induces a voltage which is determined using electrodes 6a, 6b and voltage measurement unit 7. From this voltage, a gas particle velocity calculation unit calculates particle velocity over time. If the size of the measurement volume is not much smaller than the acoustic wavelength, a compensation to the velocity is calculated.

Description

Method and Device to Determine Acoustic Gas Particle Velocity

Background ofthe invention

Acoustic particle velocity measurements in air are of great importance in the field of acoustics. Combined with the sound pressure the acoustic particle velocity vector provides the acoustic intensity vector allowing for the measurement of energy flow maps in acoustic fields. Such maps are commonly used for acoustic source characterisation and ranking. Acoustic particle velocity measurements are also required when data is needed about the acoustic surface impedance of objects such as cavities or absorbing materials.

State of the Art

Until today the most widely applied experimental technique for the estimation of acoustic particle velocity in air is based on the Euler equation, which states that the particle velocity vector components can be obtained via the spatial derivative of the pressure field. In practice this spatial derivative is obtained by means of a finite difference approximation using commercially available dual (or triple) microphone probes. Apart from the fact that this indirect technique is limited to fields which meet the Euler equation, some other drawbacks may be listed: the finite difference error, the need for carefully matched measurement chains, the need for a frequency dependent microphone spacing, the errors due to diffraction around the probe, the sensitivity of the probe to reactive fields.

The pressure gradient can also be obtained with a so called pressure gradient microphone. In pressure gradient microphones both sides of the diaphragm are directly exposed to the air. These microphones are of little practical interest since they lack of sensitivity in the low frequency range and consequently tend to have a very large size. The calibration of these microphones is difficult.

Since 1997 the microflown® particle velocity probe has gained success (compare W0001999035470A1). This probe uses hot wire anemometry to measure particle velocity. The probe uses two hot wires and monitors the temperature difference between the wires in order to estimate the particle velocity. Accurate calibration of microflown® probes is difficult and the probes tend to loose sensitivity in the high frequency range, nonetheless, the measurement principle is valid and has proven to be useful for practical applications.

The remaining two measurement techniques are the Particle Image Velocimetry (PIV) and the Laser Doppler Velocimetry (LDV), compare e.g. (K. J. Taylor, Absolute measurement of acoustic particle velocity, J. Acoust. Soc. Am., Vol. 59, No. 3, March 1976). PIV uses digital optical sensors to capture the light scattered by so-called tracer particles. The measurement area is illuminated by a pulsed laser and monitored with a digital camera. The movement of the particles between two laser pulses is compared and employing image analysis techniques instantaneous velocity vector fields are obtained in high spatial resolution. LDV also uses tracer particles suspended in the measurement volume and uses the Doppler frequency of the laser light scattered from the small particles in the medium to estimate the acoustical particle velocity. These last two measurement methodologies are only suited for specific laboratory experiments since they require a complex measurement set-up including tracer particles to be suspended in the measurement area.

In conclusion: the principles underlying todays existing acoustic air particle velocity measurements are:

• Spatial derivative of the pressure field (either directly measured or through finite difference) • Hot wire anemometry • Optical particle tracing • Laser Doppler anemometry

Flow velocity measurement principles also comprise (electro-)magnetic flowmeters, where a moving conductive fluid or gas is exposed to a magnetic field inducing a voltage proportional to the flow velocity, compare e.g. (E.G. Strangas et al., Design of a magnetic flowmeter for conductive fluids, IEEE Transactions on Instrumentation and Measurement, vol. 37 (1), 1988).

The present invention provides a new, contactless approach for measuring particle velocity in the fields of acoustics by providing a conductive medium (plasma) exposed to the flow. The deformation of the plasma due to the particle velocity is determined by means of a magnetic-inductive flow measurement principle.

Description of the Invention

The present invention presents a device to determine the particle velocity of gas over time. Unlike the existing sensors, the new acoustic gas particle velocity sensor proposed is based on the magnetoinductive principle, similar to magnetic flow meters currently used in industry. Assume a Cartesian coordinate system with a fluid flowing in the x direction. A magnetic field is applied in the y-direction. If the fluid is electrically conductive this will result in an induced potential in the z-direction which is proportional with the instantaneous fluid velocity. The physical principle at work is Faraday's law of electromagnetic induction, and the induced potential difference can be sensed by electrodes aligned perpendicular to the flow and the applied magnetic field.

As stated above the magnetic flow meter requires a conductive fluid. The novelty of this invention consists in making the air conductive by generating a cold plasma within the air of a small measurement volume. Small hand-held cold, atmospheric pressure, air plasma sources are commercially available. If needed constantly reversing magnetic fields may be applied. As long as the size of the measurement volume is much smaller than the acoustical wavelength, the influence of plasma on the acoustical field are expected to be negligible (same density, slightly different compressibility). The plasma oscillations, and consequently the induced voltage will consequently be representative for the particle velocity.

When the size of the measurement volume is larger than the acoustical wavelength, modal behavior of the plasma (i.e. no rigid body behavior) is expected and hence a compensation of this effect on the speed of sound has to be considered. This compensation depends on the geometry of the plasma and hence of the plasma generator and electrode assembly and calculated for the given geometry by means of simulation.

The method of the invention comprises the following steps:

In a first step, a plasma generator creates a conductive medium (plasma)

In a second step, a magnetic field is created by means of coils (AC or DC) or by means of permanent magnets (DC) covering the entire measurement section.

In a third step, the plasma is exposed to the gas in which the sound wave is propagating so that the prevailing particle velocity in the gas causes a time-variant deviation of the plasma.

In a forth step, the voltage induced due to the time-dependent deviation of the conductive medium inside the magnetic field is determined by an arrangement of electrodes and a voltage measurement unit. The polarity of the measurement signal indicates the orientation of the particle velocity vectors.

In a fifth step, the sound velocity is compensated if required (i.e. if the size of the measurement volume is not much smaller than the acoustical wavelength).

In a sixth step, the particle velocity is calculated based on the determined voltage, making use of calculation procedures well known from existing magnetic-inductive flow measurement principles.

Ina seventh step, the calculated gas particle velocity is provided as output value over time.

For better understanding, figures are provided:

Fig. 1 shows the setup of the device with

Plasma generator (1)

Plasma generated (2)

Acoustic sound wave (gas particle velocity to be measured) (3)

Coil or permanent magnet to generate magnetic field (4a) and (4b)

Magnetic field (5)

Measurement electrodes (6a) and (6b)

Measurement unit for voltage determination (7)

Speed of sound compensation unit (8)

Gas particle velocity calculation unit (9)

Fig. 2 shows the flow chart describing the methods.

Claims

1. Device to determine acoustic gas particle velocity comprising a plasma generator (1) to generate conductive plasma (2) in the measurement section a set of coils or permanent magnets (4a) and (4b) to generate a magnetic field a set of measurement electrodes (6a) and (6b) a measurement unit for voltage determination (7) a speed of sound compensation unit (8) a gas particle velocity calculation unit (9).

2. Method to determine acoustic gas particle velocity comprising in a first step the creation of conductive plasma (2) by means of a plasma generator (1), in a second step the creation of a magnetic field (5) by means of coils or permanent magnets (5a), (5b)(AC or DC) covering the entire measurement section, in a third step the exposure of the plasma (2) to the gas in which the sound wave (3) is propagating so that the prevailing particle velocity in the gas causes a time-variant deviation of the plasma (2), in a forth step the induction of a voltage due to the time-dependent deviation of the plasma (2) inside the magnetic field (5) and the determination of this voltage by an arrangement of electrodes (6a) and (6b) and a voltage measurement unit (7), in a fifth step the compensation of the sound velocity if the size of the measurement volume is not much smaller than the acoustical wavelength, in a sixth step the calculation of the particle velocity based on the determined voltage, making use of calculation procedures well known from existing magnetic-inductive flow measurement principles, and in a seventh step the calculation of the gas particle velocity as output value over time.