Measurement base for mine ventilation flow rate control-acoustic choice.

S.Z. Shkundin
Moscow State Mining University

S.L Klimov
Russian Coal Cometee

V.V. Sobolev DBT,
representative in Russia

ABSTRACT: The selection of the measurement base for underground mines Ventilation regimes control includes the choice of the devices for measuring of the velocities and airgas flow rates. The resolution of this task is rather difficult. The tachometric sensors, traditionally used in many countries (Grate Leonardo's invention) have big number of disadvantages: low sensitivity , high inertia, impossibility to cover necessary measurement range by one device, presence of movable accumulating error parts, vulnerability to dust affect and others. The usage of the tachometric anemometers in spite of the named above disadvantages confirms the difficulty of the modern technical level anemometer creation.

About hundred years the attempts to create thermoanemometers continue, somewhere successfully, but not in mining. There are some very deep physical reasons of very slow progress in this two directions. The present report is devoted to the number of new acoustic devices elaborated in Moscow State Mining University. Acoustics anemometers are free of all named above disadvantages: have unsurpassed sensitivity, time lag freedom, big range for velocity and flow rate measurements. They do not contain movable parts, sensitive to the direction of the stream and not sensitive to the coal dust.


The solution of mine safety delivery problem demands the creation of modern and reliable technical means and tools for mine workings ventilation control. The increasing demands to the quality of airgas dynamics process monitoring result in the necessity of more and more perfect devices usage for airgas flow and velocities measurement.

The anemometers which are being used now for occasional measurements of airgas flow characteristics, as well as anemometer sensors involved into the systems of mine ventilation monitoring and also their metrology support are not capable to satisfy completely modern safety and metrology demands and regulations and to provide appropriate level of mine atmosphere control.

The main means of airgas velocity measurement in coal mines of Russia are still tachometric anemometers, some of which had been elaborated more than 40 years ago ASO-3, MS-13, some of them are latest elaborations AP-1, APR-2.

As has been already mentioned tachometric devices such as wind wheeled anemometer ASO3, cup anemometer MS13 and a new tachometer APR1 (all-russian) are the most widely used means of measurement for gas and air consumption at mining enterprises. They operate on the principle of all turbine type anemometers (with a small rotating turbine). Under the dynamic thrust of the flow the small turbine develops torque the quantity of which is a function of the speed of flow. The value of rotation frequency of the small turbine can be used as a base to assess the speed of the monitored flow. ASO3 anemometer is used to measure the speed of the air flow ranging from 0.3 5 m. per sec.

The air flow acts on the blades, causing their rotation through the stringed axis. In a cup anemometer MS13 unlike anemometer ASO3 speedy air flow puts pressure on the inside surface of four semispheric cups placed symmetrically in a circle. A cup anemometer is used to measure the speed of the air flow ranging from 1 20 m. per. sec.

It takes a stop watch to time the number of rotation of the impeller and then in order to calculate the value of frequency the speed of the air flow is determined through grade characteristics (data) given in anemometer manual. The period of measurement, using ASO3 anemometer, to identify the average speed of the air flow in a mine working must not be less than 100sec. With no less than three measurement taken. Thus, one measurement, preparation including, takes about 8 10 minutes. Tachometric anemometers have their disadvantages affecting measurement error. On the one hand the impeller has to be as light as possible to make sure friction threshold and its sensitivity are adequate enough, on the other hand it has to be as rigid as possible not to be deformed by turbulence flow. On the one hand such axis has to have the least possible diameter ,but on the other hand such axis is quicker to wear, accumulates error and is subject to dust. The impeller axis deviation from a vector parallel or perpendicular to the flow is also caused by occurrence of inlet turbulent vortex. Mechanical sensitive parts of the devices are subject to unfavorable mining environment, in particular dust subsidence and penetrartion. At the same time their metrological support, for a number of reasons is not adequate enough (in fact of all anemometers used in mines not more than 15% are tested annually).

Most mining anemometers designed and produced currently are devices with a small movable turbine, the rotation speed of which is measured not by mechanical meters but with the help of inductive AE2, optic AFA1, electromagnetic ESNV1, capacitive and other means. They have the same disadvantages of having an impeller and movable axis. Apart from that, error for induction converters of the rotation speed of the impeller increases due to permanent magnet on the blades creating antitorque. Mine dust having magnetic properties and existing in the flow monitored also contributes to the measurement error increase. One disadvantage of the photoelectric converter is an increase in error as a result of less detectable light flow in dusty mine environment. Turbine type anemometers because of their design peculiarities are more suitable for measurements of high speeds and as far as low speeds are concerned these devices have unlinear characteristics determined by considerable influence of force moments of viscous and mechanical friction as compared to the torque.

In order to reduce friction threshold (friction between the axis of the impeller and the bearings) special devices are used. For instance, –1 uses the impeller mounted on a special vibrating frame. uses axes fixed with the help of hard stones. However it complicates design and makes it more costly. Modern designs of portable turbine type anemometers allow for temporary automatic averaging of measurement results. They also allow for digital indicators and for the sensor element being fixed to a telescopic rod. However neither this anemometer nor any other tachometric devices that have been developed recently including those built on modern technology of the watch making factories can not meet the challenge of measuring speeds ranging from 0.05 m. per. sec. That has been confirmed by aerodynamics tube tests at the Moscow Mining University.

Another method of measuring the speed of the air flow is a thermo one. Here two main trends exist. They are thermoanemometric and thermocatalitic. The devices where signal to be measured is a function of the heat dissipated throughout the monitored environment by the electrically heated body by the source of energy are recognized as a group of thermoanemometers. There are two methods to measure the speed of the flow by thermoanemometers. The first one is based on maintaining constant current heating thermoelement, the measurement of the speed of the flow is associated with the measurement of temperature of the thermoelement.

The other method presupposes that the heating current keeps the temperature of the filament constant as a result, the measurement of the speed of the flow is associated with the measurement of compensating electric current. Most thermoanemometers operate on the principle that the filament heated by an electric current is included in Wheenstone bridge circuit. The passing flow cools the filament, its temperature decreases, as a result its electrical resistance also decreases which causes disbalance of the bridge and is monitored by electric device. Themoanemometers TA8 of the same type measures the speed of the flow in the range of 0.1 5 m. per. sec. The advantages of thermoanemometers are the possibility of measurement of low speeds (0.1 0.5 m. per. sec.) but their disadvantage- thermal lag. However unstable results of the graduation, dependence of the readings on temperature flow, construction frugality as well as influence of subsiding coal dust while taking readings limit the range of their application. The thermoanemometers have not found any wide spread application in mines. The device in which head is transferred from a heater to a measuring converting element with the help of controlled gas flow belongs to the constant type coloremetric anemometers. The greater the speed of the controlled flow, the lower the sensitivity of the devices become, that is why they are suitable for speeds measurement. Thermoelectric anemometer ݖ2 ensures the desired accuracy of measurements with the flow speed of 0.5 m. per. sec. As the air within thermoelectric anemometer moves the heat off the thermofilament is transferred on to a number of thermocouples, that being registered by millivoltmeter. In this way the speed and direction of the flow are determined. The sensitivity of the thermofilament to environmental temperature and pressure is a disadvantage of any thermoanemometer. The sensitive element of a thermoanemometer is made of platinum, sometimes, nickel filament with a diameter of several micrometers and 2 10 mm long. The less the diameter of the thermofilament becomes, the less durable and more susceptible to wear it becomes. The thermofilament is also subject to aerodynamic stress. As a result of pulsation while measurements are taken in turbulent flows vibrations of the thermofilament occur which can either cause damage to it or cause fluctuations of resistance, which bring about considerable error in measurements. One disadvantage of colorimetric flow meters is a negative influence of moisture and corrosive admixture of gassy environment on the work of the thermoconverter and the heater which are in immediate contact with the environment.

The way to widen the upper limit of the speed measurement possibility range in colorimetric anemometers is to slow down part of the flow so much so as its speed does not exceed the maximum speed for the given thermoconverter. Colorimetric sensor of the speed of the air flow meter as part of the complex system for continuous automatic air monitoring in mining workings provides measurement in the range of 0 2.5, 0 5, 0 10 m. per. sec. And the change from one range limit to another is carried out by changing diaphragms placed in conical extensions of the sensor. To the point I would remark that the noted ranges , beginning from zero is nonsense- no meter can measure zero due to quite definite sensitivity.

There is a tendency abroad to combine a colorimetric converter and a pressure mechanism creating pressure differential depending on dynamic pressure of the flow. ? Ventor ? made by ? Maichak ? (Germany) is a mechanism for monitoring the speed of the flow in mining workings has a speed sensor consisting of differential Pitot tube and colorimetric speed converter. Within the sensor, the flow passing through the channels of the colorimetric converter is formed as a result of impact and static pressure difference at various points of the monitored flow. The sensor is capable of measurements ranging from 0.15 0.75, 0.5 2.5, 1.5 7.5 m. per. sec, the sensitivity of the mechanism to the speed can by regulated in the range of 15 % of the upper measurement limit.

The difference in range of the speeds measured among turbine type anemometers and thermoanemometres suggests the idea of combining the two sensors based on different principles into a single device. UBM 1 is a domestic device designed for methane consentration and air flow measurements during mine air and gas dynamics research, and in solving engineering problems of coal mine ventilation with dust and gas hazards has a unit responsible for the speed of the flow measurements which combines thermoanemometric and tachometric primary converters. The thermoanemometer is responsible for measurements of low speeds of the flow ranging from 0.05 0.5 m. per. sec., while with the speeds of the flow which are within 0.5 10 m. per. sec. range the measurements are taken by the tachometric sensor. However such engineering solution is apparently possible only for laboratory research instruments as the device combining two different sensors costs twice as much and is bulky and the whole structure turns out to be too complicated.

From all the above mentioned, one can make a conclusion that mining enterprises in Russia have no anemometers based on the principles other that those employed in turbine type and thermo type anemometers.

However, as we can see commercially available turbine type and thermoanemometers and sensors to control the speed of gas and air flows in mining workings have a host of disadvantage already mentioned, in particular, they can not ensure the speed range defined by the safety regulation and the are bulky and not reliable and cheap. As things stand now there is no hope that further improvements of engineering solutions based on traditional principles of thermo and tachometric anemometry used to measure speeds of the flow in mines will lead to creation of the device with the desired characteristics. As the analysis of modern anemometric sensors shows, further improvement in quality of measurements is connected with the application of more expensive materials and more complex technologies. One can say that thermo and turbine type methods for the measurement of the speed flow have reached their limit in as far as being a base for creation of mine speed monitoring devices. But still they can not meet the needs of mining anemometry.

Apparently, the way of solving this problem should be found not by improving traditional principles used in these constructions, but by employment of new methods of anemometry.

Modern complicated mining technology comprises usage of appropriate means of automation, hard and software for mine ventilation control. Such apparatus and software means would allow to increase the permissible level of methane concentration, i.e to increase the mining. The effectiveness of such systems operation depends very mach on the reliability and quality of the anemometers involved, their sensitivity, precision and inertia.


The careful analysis of publications and preliminary laboratory estimation have showed that on the one hand acoustical methods in flow measurements have not realized their potential possibilities, and on the other hand none of the existing acoustics means of flow measurement allows to create anemometer, which should be able:

- to measure speeds of flow in the range of (0.05 ... 30) m/s;

- not to have moving parts;

- to measure high frequency pulsations;

- to measure average flow speed in the working cross section;

- to have stabile characteristics, allowing to decrease error.

The suggested measurement method satisfies all mentioned here demands [1]. The method is based on the air-acoustics interaction and involves vibrations excitation in the cylindrical wave guide-air duct, their reception at some distance from the excitation point and a comparative analysis of radiated and received vibrations as a result of which an informative signal (e.g., vibration phase or time difference) is singled out, which serves as a flow rate measure. It differs from others known in that definite mode waves are being radiated and received by excitement of the air conduit elements, acoustically isolated from each other. This method provides accuracy and exception of air conduit effect upon the aerodynamics field of air-gas flow.

The description of wave propagation process in the tubes without flow was suggested by E.Scuchic [2]. From this description we made the conclusion which waves can spread in the round channel of the given diameter. The spread rate of the zero mode wave fronts of these vibrations is equal to one of sound velocity in open space with the same medium

For the analytic description of the air-acoustic interaction, which is the base of acoustical anemometry, it is necessary to solve the boundary problem for the equation with partial derivatives. For the first approach the anemometer channel may be performed of unlimited length. For better correspondence with real physical phenomenon in the mathematics model it is necessary to take into consideration the error, coursed by the acoustical waves reflection from the open ends of the anemometer [3].

The means of the correction determination, corresponding to the reflected waves field with the help of normal modes reflection coefficient calculation is offered. The analytic dependencies of the velocities upon the dimensions, channel walls material and air-gas medium characteristics have been got using G. Gohnson's and K. Ogimoto's work [4].

The Anemometer Operating

The measuring channel of the referred anemometer presents the cylindrical air conduit, containing two semi-channels Fig. 1.

Figure 1.Scheme of anemometric channel

There is a radiating element in the center of air conduit, on each side of which receiving elements are found at a distance L. The difference of phases in the right semi-channel D j 1

( down the flow ) is expressed:



C = sound velocity;

U = flow velocity;

j 1 = initial phase difference;

Analogous for the second, left semi-channel (against the flow):


j 2 = initial phase difference in the second semi-channel.

The anemometer has to operate in accordance with technical task at temperatures from 5 to 25 0 C that corresponds to the regime of metrological tests. In this range of temperature changes, as it is easy to count, the sound speed does not exceed 345 m/s.

In the right semi-channel minimum value of phase difference corresponds to the maximum of (C + U), i.e. C = 345 m/s and U = 20 m/s; that is, in assumption j 1 = 0:

( 3 )

In the left semi-channel D j 2 min corresponds to the case when C = 345 m/s and U = 0. That is, in assumption j 2 =0 will be:

Now formulas (2) and (3) will give the following:



Before measurement the phase zero in the right semi-channel is being adjusted to C + U = 365 m/s; in the left semi-channel the phase zero corresponds to C - U = 345 m/s. The phase zero schematically may be fixed precisely, therefore anemometer phasing does not generate measurement error. We can describe different basic phases of the measurement algorithm from the above formulas in the next way:

1. Automatic amplification and adjustment of the received vibrations.

2. Transformation of sinusoidal vibrations into meander.

3. Correcting phase change.

4. Forming of binary codes of the analog signals in each semi-channel.

5. Time averaging of binary codes.

6. Calculation of binary codes corresponding to the flow speed.

Automatic amplification adjustment is necessary for modification of radiator and receiver signals to standard level. The inertia of this adjustment does not limit the measuring possibilities because the main task isn't to measure pulls speed but averaged one during some interval ( for example 1s). Just because of this automatic amplification adjustment does not bring the errors into the process of measurements. The transformation of sinusoidal vibrations into meander, in fact, can cause the bias of pulse front relatively to zeroes of harmonic. However the correction of phases allows make it equal to zero. The transformation of informative impulses into the binary code is made by filling out their duration by the impulses of higher tact frequency. The choice of this frequency is made from the accuracy and sensitivity requirements. We'll determine the value of this frequency in case when it is necessary to measure U = 0.05 m/s. For this purpose we differentiate the last expressions by the speed:



The expression (7) takes smallest value at C = 345 m/s and U =20 m/s. We'll find the value of informative parameter (phase difference), corresponding to U = 0.05 m/s at L = 0.1 m and f =30 kHz (the resonance of piezoceramics transformers).

This means that frequency of filling (tact pulses) has to be not less than 3 MHz. The expression (8) takes minimum value at C = 345 m/s and U = 0. For increment = 0.05 m/s phase difference will be equal:

( 9 )

Thus when filling frequency is equal to 3 MHz and more, it is possible to follow the changing of flow speed, not exceeding 0.05 m/s in both channels. During the filling of informative pulses by tact ones it can happen discrepancy of leading and (or) back fronts of informative and tact pulses. Error is resulted (plus or minus one tact pulse), maximal estimation of which corresponds to mistake 0.05 m/s. Mistake is not increased in the following step of logical division because the division is done in number view. As a result of this operation we have got sum ( C + U ) and difference ( C - U ) with accuracy of constant multiplier.

Finally, as a result of logical subtraction we get value, proportional to flow speed.

After the fulfillment of this operation maximum error value can be g = 0.05 + 0.05 = 0.1 m/s. Thus the resulting absolute error of calculation of measured speed does not exceed 0.1 m/s. The described algorithm has been realized in the anemometer for mine workings. Multiple increasing of filling frequency allows to reduce application error into corresponding number of times.


The hardware part of metrology installation functionally includes two fundamental blocks - measuring (measurer) and calculating (calculator). In the measuring block informative parameter (phase shift) is extracted, and analog-digital transformation is proceeded. The measuring block includes: radiator, high - stabile 20 kHz generator, controlled divider, transformer meander into sinus, giving level scheme, power amplifier, two receiving transformers (one in each semichannel), two schemes of automatic amplifier adjustment; two transformers sinus into meander, two controlled phase shifters, scheme of selections frequency control.

The calculator includes: two binary code formers, two averaging schemes, converter, substracting binary numbers, got in the two semichannels and transforming the binary code into binary-decimal one, indicator.

The signals are synchrotreated in both semichannels. The temperature instability coefficient for quartz generator is 10 -6 , that supplies sufficient accuracy of filling the informative pulse by the tact ones, and also the stability of the radiated vibrations frequency. In the controlled divider the assumed frequency is divided to the value, corresponding to the radiator resonance - 10.3 kHz with possible maximum deviation - 1 Hz. Such instability allows to have stabile amplification in the receiving transducer. But really the frequency 20 kHz supplies sensitivity of the anemometer to the flow speed 0.01 m/s, corresponding to the time interval of 50 nanoseconds. The changing of divider coefficient by the unit causes the changing of frequency at the exit for 10 Hz in the range of 10-12 kHz.

Thus, generator and programmed divider has the following functions:

- create the possibility of radiator frequency adjusting;

- supply stabile high-frequency filling of informative time interval with necessary step;

- define support frequencies for control signals forming.

On the radiating ceramics element periodical voltage (4-10) V must be applied. To form such signal, sequence of rectangular pulses of resonance or near to it frequency has to be transformed into sinosoidal and strengthen. The level giving scheme matches the low frequencies filter exit with the entrance of the power amplifier and semultaneously smooth tuning of signal level. The choice of vibrations amplitude of the radiator is being produced with the reference of two conflicting to each other considerations. On the one hand, increasing of the transformer signal causes warming of it and, consequently, changing of its characteristics. Besides, appearing warm asymmetrical field in the anemometer channel results to the nonidentical sound speed value in semichannels. On the other hand, decreasing of vibration signal amplitude on the radiating element can cause reduction of measurement accuracy. In the aerometric installation anemometer the radiated through base L = 0.5 m and for it signal level on the receiver may be (1-10) mV when amplitude on the radiator - 2V. Power amplifier implemented with the use of micro cheep, designed to work with the load of about units of Om. The received signal is reinforced to the value, necessary for normal work of scheme of the automatic amplification adjustment (5V). Besides the characteristics of the controlled medium - temperature, moisture, pressure, which influence on the sound speed is compensated by the existence of two semichannals, the informative parameter - phase shift is subjected to the influence of the electron schemes characteristics. So receiving amplifier has not to bring phase shift more than 40 nanoseconds. To broaden the dynamic range of the input pulses in the receiving tract non linear amplifier providing the ratio of maximum to minimum signals equal to 40 is used.

The scheme of automatic amplification adjustment with phase tuning allows to change the input voltage from 0.5 to 20 mV and excludes one of the basic errors of measurement - dependence of phase shift upon the amplitude in the receiver. The harmonic signal of several volts amplitute is removed from the automatic amplification adjustment and transformed into meander by the comparator, which follows to indicate the buckhead voltage - 0.15 V. Controlled phase shifter permits to make zero phase difference between the transmited and received signals during the flow absence. The informative meander in the same block is being converted into the sequence of short pulses, which open the count of each given measurement of the flow speed. This count is being produced with the frequency 10 kHz. The shift of the short pulses relatively support signal may be changed by phase turning and, in particular, made to zero before measurement. Time dynamics range consists of 100 ns, that permits working at frequency (10-12) kHz, to overlap 2 p dynamics range. Decimal counters are used as counting elements.

Any measurement of flow speed is the average one during certain period of time. Acoustic method of measurement, being inertialess, permits to produce thousands measurements of speed per second. Because of the turbulent structure of the stream these measurements, being momentary, are confirmed to the probability distribution, i.e. have the value spread.

The control scheme of samplings frequency permits to vary the average interval from 1 to 20 seconds. For the following calculations concerned with algorithm of flow speed display, it is necessary to perform the informative parameter as the digital code. This is carried out for each semichannel in the cheep . The transformation of pulse duration into the digital number is being made with the help of synchronized counters, which work with frequency 20 kHz. During each interval of filling it is possible to get mistake which value is equal to 1 tact of frequency 20 kHz, i.e. in 50 nanosecond. But this mistake, if appeared, has equal probability in both channels, that is on the stage of substraction of codes corresponding to the phase climbs in the right and left semichannels, and is being destroyed. Thus, at the exit of digital code former, built on the synchronized digital counters and controlling trigger, we get numbers in the digital code, which must be averaged.

As has been said, averaging operation is necessary for exception of dependence of casual components measurement errors, and done in two stages: first, sum up 4096 measurements, second, division of result by 4096. Received digit number, equal to the average number for one measurement, rerecorded into register, where kept during one cycle of the averaging. Similar, a number - the rezult of measurement of the average speed in the second semichannel is being recorded. The process is repeated in the new cycle. So in the end of each cycle we have two numbers, corresponding to the speed of vibration propagation (i.e. sound speed together with flow speed) in the left and right semichannels. The final rezult, i.e. the number, corresponding to the speed value of air flow, is got as the function of two numbers. The indication represents the numbers from 00.00 to 20.00, that corresponds to the flow speed, expressed in m/s. The indication step 0.01 m/s. For receiving of the necessary metrology characteristics it is necessary to compensate the basic anemometer errors. Compensation of error, causing by electron tract and phase characteristics of converter, is produced in each of channels with the help of controlled phase shifters. Compensation of error, caused by asymmetry of anemometer semichannels as not changing in time, is being produced by device calibration, i. e. adjustment of phase shifts. Compensation of error, concerned with reflecting waves, is made on the stage of airduct constraction.

Error compensation, concerned with value difference of sound speed in both semichannels, is made, as has been described, by averaging of the enough amount of measurements. This averaging is effective because the air volume, passing through the left and the right semichannels are the same, so sound speed in the left semichannel at the next moment will be sound speed in the right one. Thus, the same changes of sound speed will be in both semichannels, but with time shift. The averaging permits to exclude momentary assimetry of channels.

Acoustic integrating anemometers

Acoustic integrating anemometers based on ultrasonic wave propagation through a tunnel cross-section are free from all the deficiencies referred to earlier. The working acoustic beam accumulates information about vectors of longitudinal velocities of the flow, which forms the aerodynamical curve, and as such is an integrating anemometer. Such an anemometer is able to determine the average velocity of air streams in cross-sections with consistancy. The principle of acoustic integrating anemometry is based on the use of two pairs of electric-acoustic converters. One of them works streamwise and the other in the opposite direction. In other words, acoustic signals used by the first pair of converters are accelerated by the stream and signals used by the second pair are decelerated by the stream.

Figure 2. The integrating acoustic anemometry principle:

1,2 = electroacoustics transformers;

D = cross dimension of airduct;

L = the length of through sounded base;

U(r) = the air velocity in plan cross section.

The value of this deceleration or acceleration is a measure of the controlled average velocity. Particular devices may consist of only two converters, each of them taking turns in working as an emitter and as a receiver.

Such a system based on two pairs of converters allows a reduction in error, concerned with variation of sound velocity.

Figure 2 illustrates the principle. Electric acoustic converters (if there are two of them) are switched in turn from emitting to receiving mode, so that the vector, characterising the

emitting direction, forms either an acute angle (emittance streamwise) or an obtuse angle (emittance against the stream) with the flow axis.

One more advantage of the integrating anemometers should be mentioned: its independence of the dust loading in the medium in which a device operates.

This stability is explained by the fact that the informative parameter, which is either oscillation frequency or phase, does not change with dust deposition on the converters. Besides, oscillations of the emitting converter prevent dry dust deposition on its surface. When used in an airduct or a tunnel, the acoustic rays are bent due to the aerodynamic field . Here two kinds of problems had been considered, those related to the laminar and turbulent flow regimes.


Acoustics is the most perspective way for mine ventilation control devices progress.

As had been proved in the Moscow State Mining University on the acoustic basis it's possible to develop different types of acoustic control hardware: portable anemometers for episodically control, stationary devices for permanent monitoring and also means for metrology support of the ventilation measurements. The Plant of Measurement apparatus partner of MSMU is ready to provide any request.


1. S. Shkundin, L. Puchkov, oth. : The method of airgas flow velocity.

Patent 16822590, 1991.

2. E. Scuchic. Basic acoustics. Moscow 1976. vol 2. pages 112-116.

3. O. Kremleva , S.Shkundin : Method for calculating Acoustic fields in a finite cylindrical channel with a flow . Acoustical Physics, Vol 44, No. 1, 1998, pp 68-72.

4. G. Gohnson and K.Ogimoto. Sound radiation from a finite length unfledged circular duct with uniform axial flow. Acoustic Society of America, papers. 68, 1980.

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