RU2605643C1 - Subsonic wind tunnel with low level of flow pulsations of infrasound range - Google Patents

Abstract

FIELD: test equipment.

SUBSTANCE: invention relates to wind tunnels and can be used for various tests of aircraft models, ground transport, buildings, structures, bridges. Wind tunnel comprises prechamber, collector, damping plate at collector outlet, open operation part, diffuser with through damping holes with rows of holes, located at a distance from diffuser inlet section, circular funnel above diffuser, rotary sections with rotating blades, one return channel, blade fan, placed behind diffuser. Wherein diffuser has additional through damping holes, located at a certain distance in relation to existing holes, as well as holes, located with gap between blade fan and diffuser and at a distance relative to existing ones.

EFFECT: technical result consists in reduction of flow pulsations in infsound range, eliminating vibrations of wind tunnel and building, elimination of harmful effects on health of personnel.

3 cl, 13 dwg

Description

The invention relates to wind tunnels and can be used for various tests of models of aircraft, land vehicles, buildings, structures, bridges. The invention can be used to upgrade existing wind tunnels.

Information on the contours and parameters of subsonic wind tunnels is widely presented in the literature [1, 2].

Subsonic wind tunnels can be divided into three main classes: flow type, closed type and with the Eiffel chamber.

In Russia, wind tunnels have traditionally become widespread, see [1, 2], of a closed type with an open working part, containing a prechamber, a collector, an open working part, a diffuser, an annular bell above a diffuser, rotary sections with rotary blades, one return channel, a blade a fan located directly behind the diffuser.

Also known, for example, is a wind tunnel of a closed type, see [4], which contains a collector, a working part, a diffuser, a return duct, a fan and rotary sections, of which at least the first and second sections have different angles of flow rotation, moreover a section with a smaller flow angle is located in a section where the flow rate is greater, and a section with a larger flow angle is located in a section where the flow rate is less.

A significant drawback of subsonic wind tunnels of a closed type with an open working part, with a diffuser made with a smooth bell at its inlet or supplemented by an annular bell, is:

- the tendency of such pipes, with large linear dimensions of wind tunnels, to pulsations of the infrasonic range flow;

- the appearance of significant vibrations of wind tunnels and buildings, which can lead to the destruction of the latter;

- in the influence of infrasonic vibration of significant amplitude on the staff, which may result in a disease of the staff.

Many experimental works carried out at TsAGI, including work [5], are devoted to the elimination of a significant drawback of closed-type wind tunnels with an open working part.

Closest to the claimed invention according to the technical result and the technical solution of the problem and adopted as a prototype is a wind tunnel of the closed type T-103 TsAGI, [5]. The T-103 wind tunnel contains a pre-chamber (1), a collector (2), damping plates (3) at the outlet of the collector (2), an open working part (4), a diffuser (5), through damping holes (6) with a total area of 0 , 4 to 0.5 of the area S of the outlet cross section of the collector (2) with the arrangement of rows of holes at a distance from 0.6-0.9 of the diameter D of the collector (2) to L / 3 of the length of the diffuser (5) from its input section, an annular bell (7) above the diffuser (5), rotary sections (8) with rotary blades (9), one return duct (10), paddle fan (11) located directly continuously for diffuser (5).

The listed features of the prototype (T-103 pipe) are common with the claimed device of the wind tunnel.

The disadvantages of the known device (wind tunnel T-103) are:

- intense infrasonic flow pulsations at speeds above 40 m / s;

- significant vibrations of the wind tunnel and buildings;

- the negative effect of infrasonic vibration of significant amplitude on the staff.

The claimed invention is free from these disadvantages.

The technical result of the proposed device is to reduce the intense infrasonic pulsations of the flow at speeds above 40 m / s, to eliminate the vibrations of wind tunnels and buildings, to reduce the negative impact of infrasonic vibration on staff.

The specified technical result is achieved by the fact that in a wind tunnel of a closed type with an open working part in accordance with the claimed invention:

до

, а также отверстия (13) суммарной площадью до 0,15 площади S выходного сечения коллектора (2) и продольным размером l, расположенные с зазором Δ между лопастным вентилятором (11) и диффузором (5) и по отношению к имеющимся на расстоянии [L-(l/2+Δ)] от входного сечения диффузора.1. The diffuser (5) of length L has additional through-damping holes (12) with a total area of up to 0.1 of the area S of the outlet cross section of the collector (2) and a longitudinal size l, located in relation to those available at a distance from

before

and also openings (13) with a total area of up to 0.15 of the area S of the outlet cross section of the collector (2) and a longitudinal size l, located with a gap Δ between the blade fan (11) and the diffuser (5) and with respect to the available at a distance [L - (l / 2 + Δ)] from the input section of the diffuser.

2. Damping holes (6), (12) and (13) in the diffuser (5) are made of thin rubber sewn from the outer surface of the diffuser (5);

3. The annular bell (7) above the diffuser (5) is made with a mechanism (14) for the longitudinal movement of the bell (7) parallel to the axis of the wind tunnel.

The essence of the claimed invention is illustrated in FIG. 1, which shows a diagram of the device (circuit) of a wind tunnel of a closed type with an open working part.

до

, а также отверстия (13) суммарной площадью до 0,15 площади S выходного сечения коллектора (2) и продольным размером l, расположенные с зазором Δ между лопастным вентилятором (11) и диффузором (5) и по отношению к имеющимся на расстоянии [L-(l/2+Δ)] от входного сечения коллектора. Демпфирующие отверстия (6), (12) и (13) в диффузоре (5) выполнены зашитыми тонкой резиной с наружной поверхности диффузора (5). Кольцевой раструб (7) над диффузором (5) выполнен с механизмом (14) продольного перемещения раструба (7) параллельно оси аэродинамической трубы.The wind tunnel is of a closed type, as can be seen from the one shown in FIG. 1 of the circuit, contains a pre-chamber (1), a collector (2), damping plates (3) at the outlet of the collector (2), an open working part (4), a diffuser (5), through damping holes (6) with a total area of 0.4 up to 0.5 of the area S of the outlet cross section of the collector (2) with the arrangement of rows of holes at a distance of 0.6-0.9 of the diameter D of the collector (2) to L / 3 of the length of the diffuser (5) from its inlet section, an annular socket (7 ), rotary sections (8) with rotary blades (9), a return duct (10), a blade fan (11) located directly behind the diffuser (5). In FIG. 1 shows additional through damping holes (12) with a total area of up to 0.1 of the area S of the outlet cross section of the collector (2) and a longitudinal dimension l, located in relation to those available at a distance from

before

and also openings (13) with a total area of up to 0.15 of the area S of the outlet cross section of the collector (2) and a longitudinal size l, located with a gap Δ between the blade fan (11) and the diffuser (5) and with respect to the available at a distance [L - (l / 2 + Δ)] from the input section of the collector. The damping holes (6), (12) and (13) in the diffuser (5) are made of thin rubber sewn from the outer surface of the diffuser (5). An annular bell (7) above the diffuser (5) is made with a mechanism (14) for the longitudinal movement of the bell (7) parallel to the axis of the wind tunnel.

The work of the proposed wind tunnel closed type with an open working part is as follows. When the drive motor of the ten-blade fan is turned on, the air flow is sucked into the diffuser of the wind tunnel. The pressure developed by the fan is enough to overcome the resistance of the entire contour of the wind tunnel. The air flow in its movement passes the diffuser, the channel after the fan, four rotary sections, the return channel and is turned 360 °. Then the air stream enters the prechamber and flows through the collector into the open working part, and then it is sucked into the diffuser. The change in the flow rate in the open working part is achieved by adjusting the speed of the electric motor.

When the proposed wind tunnel of the closed type with an open working part, intense pulsations of the infrasonic range flow can take place. These pulsations are eliminated by a movable annular bell, damping plates at the outlet of the collector and damping holes in the diffuser, made in accordance with the claimed invention.

An example of a specific implementation of a subsonic wind tunnel with a low level of pulsations of the infrasonic range flow is based on the AT-11 subsonic industrial wind tunnel of St. Petersburg State University. The outline of the AT-11 wind tunnel can be represented in FIG. 2, which shows practically the main geometric dimensions of the pipe.

Technical parameters of the AT-11 pipe:

 Collector diameter 2250 mm  The width (thickness) of the collector wall at the outlet 180 mm  Working length ~ 4000 mm  Diffuser diameter 2450 mm  The diameter of the annular socket 3500 mm

An annular bell is mounted on a trolley that can move relative to the inlet section of the diffuser in the longitudinal direction towards the collector from 0.1 to 0.29 of the diameter D of the collector.

In the wind tunnel AT-11, in accordance with the alleged invention, there were practically no necessary damping devices.

The setting of the experiment. The AT-11 wind tunnel model can be represented as shown in FIG. 3. Here the model of the wind tunnel is elongated along the longitudinal axis. The given length for it is given taking into account the length of the open working part.

In the model of the AT-11 pipe, three composite resonators can be distinguished:

- pipe AT-11 as a half-wave resonator;

- a prechamber as a Helmholtz resonator with injection of flow from the bottom;

- a diffuser from the side of the pipe inlet to the blade fan as a quarter-wave resonator.

The frequencies for the half-wave resonator are determined from the following relation:

Where: m = 1, 2, 3 ...; a is the speed of sound; L is the cavity length (L ~ 58250 mm).

Frequency for Helmholtz resonator:

Where: S - hole area; L is the length of the hole; V _O is the volume of the resonator.

In the geometry under consideration, it is problematic to consider the prechamber of the wind tunnel as a Helmholtz resonator.

Frequencies for a quarter-wave resonator:

,

,

Where: m = 0, 1, 2, 3, ...; L is the length of the cylindrical resonator.

It seems that the self-oscillation process developing in the diffuser or in the channel from the entrance to the diffuser to the fan blades is responsible for the existence of such powerful pressure pulsations in the wind tunnel during its operation. Rotating fan blades can be considered as the perforated bottom of a quarter-wave resonator. The existence of this process in a certain range of flow velocities with a certain frequency can lead, with coincidence with the natural frequency of the prechamber, to resonance in the prechamber. For the existence of a self-oscillating process in the diffuser, it is necessary to create heterogeneity in the flowing stream. Here, the heterogeneity in the flowing stream is created by the rotating fan blades.

The determined parameters of the oscillatory process in the wind tunnel: the amplitude and frequency of pressure pulsations. It is desirable to measure these parameters in the diffuser and in the chamber of the wind tunnel. In this case, it is possible to determine at least at what flow rates (Mach numbers M of the flow) this or that resonator “sounds” in the open working part.

Defining parameters: process similarity parameters Mach number M, Strouhal number St, Reynolds number Re and geometrical parameters of the wind tunnel at the inlet to the diffuser. Thus, in an experimental study of flow pulsations in a wind tunnel, one can vary the flow velocity and non-obvious additional structural devices in the diffuser, affecting the parameters being determined.

From here the goal of experimental research is formulated:

- determine the main resonator in the wind tunnel;

- determine the design devices that reduce or eliminate pressure pulsations of the infrasonic range in the wind tunnel.

In FIG. Figure 4 shows the arrangement of the differential small-sized inductive pressure sensors DMI 0.1 used to register pressure pulsations in the wind tunnel AT-11. The sensors are installed at the following points: D1 - on the wall of the prechamber; D2 - on the wall of the diffuser; D3 - on the flow axis (the pitot tube is placed on the flow axis, and already from the pitot tube the full pressure is transferred to the D3 sensor with silicone hoses); D4 - in the near field of the flow. The first two sensors record the pulsating pressure in the stream near the walls of the channel of the wind tunnel.

A block diagram of a pressure pulsation measurement system is shown in FIG. 5. Pressure sensors are connected to inductive high-frequency converters IVP-2. The outputs of the measuring channels IVP-2 are connected

- with the PULSE measuring and computing complex through the LAN-XI data acquisition system (Bruhl and Kjерr equipment) and

- with a digital oscilloscope (4-beam oscilloscope LECROY WaveSurfer 24Xs-A).

A digital oscilloscope allows recording temporary implementations of sensor signals at the open or closed inputs of Y-amplifiers, as well as working in the registration mode of X-Y signals. In the X-Y signal registration mode, it is possible to obtain signal responses to pressure pulsations at the points of installation of the sensors as a function of the pressure head recorded by the D3 sensor.

Pressure pulsations are observed in the pitot tube and in the connecting hose from the tube to the D3 sensor, caused by the vibration of the pitot tube in the flow and the pulsations of the flow itself. For satisfactory damping of pulsations, it is enough to firmly fix the Pitot tube to the design of the wind tunnel and make a long connecting hose from the Pitot tube to the D3 sensor.

Additional difficulties in registering pressure pulsations with DMI 0.1 sensors are manifested in the fact that the membrane of a sensitive differential DMI 0.1 sensor in an experimental study of flow pulsations in a wind tunnel can be subjected to proportional pressure on both sides. To increase the accuracy of measuring the working pressure and eliminate phase-frequency distortions of the sensor signal from the non-working side, connect a silicone hose up to 100 mm long to the nozzle-receiver of the sensor, plug it and shield it.

Difficulties arise with the dynamic calibration of inductive sensors DMI 0.1. Here you can use the results of static calibration of sensors on a pneumohydraulic press, see [6], and perform dynamic calibration of sensors using a model 4228 pistonphone from Bruhl and Kjерr. The 4228 Pistonphone is an accurate reference reference source for calibrating sound measurement equipment in laboratory and field conditions (the sound pressure level of the reference signal is 124 dB, the frequency of the reference signal is 250 Hz).

раза выше чувствительности по СКЗ. Таким образом, при отсутствии эталонных генератора давления и преобразователя давления появляется реальная возможность измерить амплитуды пульсаций давления в канале аэродинамической трубы.The sensitivity determined by the RMS and introduced into the programs of the Bruhl and Kjерr measuring and computing complex, as a parameter of the sensor, leads to a pressure level of 124 dB when processing the reference signal. A comparison of the sensitivity values of the sensors determined by the RMS and the results of the static calibration of the sensors shows that the sensitivity during static calibration in

times higher sensitivity for VHF. Thus, in the absence of a reference pressure generator and pressure transducer, there is a real opportunity to measure the amplitude of the pressure pulsations in the channel of the wind tunnel.

When measuring pressure pulsations in the AT-11 wind tunnel, it is necessary to verify the correct installation of the sensors. The check consists in the fact that there were no points where the sensors were placed in the diffuser and in the prechamber in the node or in the antinode of the acoustic wave in the closed channel of the wind tunnel.

The verification of the correct placement of the sensors in the diffuser and in the prechamber is illustrated by the following drawing, see Fig. 6.

In FIG. 6 axis X - velocity head in the stream at the outlet of the collector. Y axis - dynamic pulsations of static pressure at the points of installation of the sensors. As can be seen from the presented oscillograms, an oscillatory process develops in the diffuser (sensor D2) and in the prechamber (sensor D1), starting from low flow rates at the outlet of the collector, in the closed channel of the wind tunnel. It is not necessary to speak about any characteristic points or zones (modes) in the oscillatory process according to these results. The process lives in the entire range of flow rates.

In the near field of the jet stream (sensor D3) outside the open working part of the wind tunnel there are no intense pressure pulsations.

In the range of speeds over 42 m / s (high-pressure head up to 940 Pa) during experimental research, an unsafe operating mode was revealed, which manifests itself in significant vibration of the building and the construction of the wind tunnel.

- 0.315. В работе [5] дана только качественная оценка влияния раструба на пульсации потока в аэродинамической трубе: раструб сужает области существования пульсационных режимов в аэродинамической трубе по скорости потока.1. Damping of pulsations by a circular bell. On the wind tunnel T-103 TsAGI (prototype) the annular bell is fixed motionless with respect to the diffuser. The distance from the collector to the annular socket is 4000 mm with a working part length of 4740 mm. Consequently, the extension of the bell l with respect to the diffuser is equal to 740 mm or, with one of the dimensions of the ellipsoidal collector 2350 mm,

- 0.315. In [5], only a qualitative assessment of the effect of the bell on the flow pulsations in the wind tunnel was given: the bell narrows the regions of existence of the pulsation regimes in the wind tunnel by the flow velocity.

- от 0,1 до 0,3. Влияние положения раструба на пульсации потока в трубе АТ-11 показано на приведенных осциллограммах пульсаций давления в форкамере и диффузоре в функции скоростного напора потока на выходе коллектора, см. Фиг. 7.On the AT-11 wind tunnel, the annular bell is mounted on the trolley, and it can be moved towards the collector at a distance

- from 0.1 to 0.3. The influence of the position of the socket on the flow pulsations in the AT-11 pipe is shown in the given oscillograms of pressure pulsations in the prechamber and diffuser as a function of the flow velocity head at the collector outlet, see FIG. 7.

The influence of the position of the annular bell is obvious. If it is possible to advance the annular bell in the existing wind tunnels towards the collector, then this must be done.

2. Through damping holes in the diffuser with a total area from 0.4 to 0.5 of the area S of the outlet cross section of the collector with the arrangement of rows of holes at a distance from (0.6-0.9) of the diameter D of the collector (2) to L / 3 of the length of the diffuser (5) from its inlet section.

In the wind tunnel T-103 TsAGI, see [5], two rows of damping holes with an area of 0.44 S at a distance of (0.57-0.78) D from the inlet section of the diffuser and one row of the same holes are sewn tarpaulin, an area of 0.22 S at a distance of (0.87-0.93) D. D - diameter of the outlet cross section of the collector; S is the area of the outlet section of the collector. Two rows of these holes are made within 1/3 of the length of the diffuser.

In the AT-11 wind tunnel diffuser, one row of ten damping rectangular holes of 700 × 300 mm is made in the cross section of the diffuser at a distance of 1/3 of the diffuser length L, with rectangular vacuum sewing up with thin rubber rubber.

In FIG. Figure 8 shows photographs with views of the damping holes in the diffuser from the side of the working part and from the outside of the wind tunnel. Ten holes are visible in the walls of the diffuser, with rectangular holes sewn in with thin vacuum rubber outside the diffuser and inside with metal mesh sewing.

The influence of the holes in the diffuser on the pressure pulsations was estimated based on the results of spectral analysis of the signals of sensors D1 and D2 installed in the chamber and in the diffuser of the wind tunnel. The measurements were carried out step by step, with a change in the speed of rotation of the fan shaft.

In FIG. Figure 9 shows the results of measurements of the amplitudes and frequencies of pressure pulsations in the prechamber and in the diffuser of the AT-11 wind tunnel (Fig. 9: a - pressure pulsations in the prechamber; b - pressure pulsations in the diffuser; c - pressure pulsation frequencies).

The results of pressure pulsation measurements for a given series of damping holes are shown in FIG. 9 to the boundary of the rotation of the fan shaft n = 420 rpm Above this boundary, the operation of the AT-11 wind tunnel became unsafe due to the developing intense vibration of the pipe and the building.

The upper curves, marked with the symbol "b / o", are pressure pulsations in the absence of holes in the diffuser and at the extreme position of the annular socket with a reach of 230 mm with respect to the slice of the diffuser. In this embodiment, flow pulsations are noted in the entire range of changes in the fan shaft rotation speed from 60 to 420 rpm, or in the Mach number M of the flow from 0.012 to 0.112.

The pressure pulsation frequencies in the prechamber and in the diffuser coincide and grow with increasing fan shaft speed. In FIG. 9, a line is constructed for the rotational speed of the fan shaft. The fan is ten-bladed. The rotational speed of the blades is significantly higher than the observed frequency of pressure pulsations in the chamber and in the diffuser of the wind tunnel.

The execution of this series of damping holes both in the version of open holes (see the curve with the symbol "5 holes".), And in the variants sewn with rubber holes (curved lines with the symbol "5 holes." And "10 holes.") Leads to a decrease in the amplitude of pressure pulsations in the prechamber and in the diffuser, which results in the safe operation of the wind tunnel up to a fan shaft speed of n = 420 rpm (to the Mach number of the flow M = 0.112).

Note that in experiments with holes, the annular bell is already maximally extended towards the wind tunnel collector.

Its departure with respect to the inlet of the diffuser is 600 mm. The holes in the diffuser have a positive moment and the fact that in the range of flow rates from M = 0.1 to M = 0.112 (n from 330 rpm to 420 rpm) the pulsation process disappears. The discrete process frequency falls apart in a frequency band.

The execution of this series of damping holes in the diffuser did not allow the pipe to go beyond the fan rotation speed of 420 rpm, (M = 0.112).

3. A series of damping holes with a total area of up to 0.15 S of the outlet cross-sectional area of the collector and a longitudinal dimension l, located with a gap Δ between the blade fan and the diffuser and with respect to those available at a distance of [L- (l / 2 + Δ)] from the input cross-section of the diffuser.

Presumably, the oscillatory process in the wind tunnel is determined by the process in the diffuser. Then it is possible to influence the amplitude of pressure pulsations by placing additional holes in the walls of the diffuser near the fan blades, i.e. to perforate the walls of the quarter-wave resonator near its bottom with a certain gap Δ.

The implementation of such a solution in the AT-11 wind tunnel contributed to a decrease in the pressure pulsation amplitudes in the range of the fan shaft rotation speed abroad of 420 rpm (M> 0.112).

The holes are illustrated in the photograph in FIG. 10. Here, one can see rectangular holes of the first row made earlier in the walls of the diffuser, sewn from the inside of the diffuser with metal nets and, on the outside, with vacuum rubber. Further, behind the fan cowl in front of its blades, damping round holes of the 2nd row are visible. For round holes, size l is the diameter of the holes.

Round holes with a diameter of 200 mm were performed step by step: 2 holes; 5 holes; 8 holes; 12 holes; 18 holes Round holes went, during the experiments, in combination with rectangular holes.

In FIG. Figure 9 shows only the results with some hole combinations. On the given curves with designated symbols "5 + 2", "5 + 5", etc. the first number is the number of rectangular holes, the second number is the number of round holes. The notation "+18" omits the first number 10.

The experiment with making step-by-step openings of the second row led to step-by-step success in advancing extremely high pressure pulsations in the diffuser and in the wind chamber prechamber into the zone. As can be seen from the experimental results in FIG. 9, a combination of 5 rectangular and 5 round holes already led to a significant decrease in the amplitude of pressure pulsations in the chamber of the wind tunnel. Here, in FIG. 9, the best result is shown (black line with the symbol [5]) for reducing pressure pulsations in the TsAGI T-103 pipe. Note that in [5], pressure pulsations were measured in the middle of the mixing layer of a free jet.

Subsequent variants with combinations of rectangular and round holes had better results in reducing pressure pulsations in the wind chamber prechamber.

At the same time, the amplitudes of pressure pulsations in the pipe diffuser remained fairly high. Only the option with 10 rectangular and 18 round holes allowed to reduce the amplitude of pressure pulsations in the diffuser to ~ 60 Pa, with small values of the amplitudes of pressure pulsations (~ 20 Pa or 120 dB) in the chamber of the wind tunnel. The total passage area of the round holes was ~ 14% of the collector area S.

Thus, we can come to the following conclusions:

- execution of additional damping holes with a total area of up to 0.15 S of the outlet cross-sectional area of the collector and a longitudinal dimension l located with a gap Δ between the blade fan and the diffuser and with respect to those available at a distance of [L- (l / 2 + Δ)] from the input cross-section of the diffuser, helps to reduce infrasonic pulsations of the flow in a certain range of flow velocities over 40 m / s;

- sewing through holes in the walls of the diffuser with thin rubber does not violate the function of the holes as damping and eliminates mass transfer between the air flow inside the wind tunnel path and the outside air of the surrounding atmosphere.

In FIG. 9 c) a graph of the dependence of the frequency of pressure pulsations in the diffuser and in the prechamber in the range of rotational speeds in excess of 420 rpm is shown. With various combinations of holes, the graph is almost straightforward. The ripple frequency increases in proportion to the flow rate.

We can approximately estimate the pulsation frequencies in the diffuser as in a quarter-wave resonator for the speed of sound 330 m / s and for the reduced length of the diffuser, taking into account the length of the working part:

.

.

L = l _ol ~ 8.4 + 4 = 12.4 m; k ~ 0.7-0.8;

;

.

;

.

) график изменения частоты пульсаций в аэродинамической трубе АТ-11. Приведенный результат свидетельствует о существовании колебательного процесса в полости аэродинамической трубы на дискретных частотах вблизи первой, при малых скоростях потока, и третьей, при больших скоростях потока, акустических частот диффузора.In FIG. 11 is shown in dimensionless coordinates (Mach number M of the flow is the dimensionless frequency

) a graph of changes in the frequency of pulsations in the wind tunnel AT-11. The above result indicates the existence of an oscillatory process in the cavity of the wind tunnel at discrete frequencies near the first, at low flow rates, and the third, at high flow rates, acoustic frequencies of the diffuser.

Here we can turn to the results of an experimental study of longitudinal modes in a quarter-wave resonator in [7], in which flow models are constructed for these modes (odd longitudinal modes). The assumption that in the diffuser of the wind tunnel an odd longitudinal oscillation mode is realized during its operation allows one to correctly approach the damping of flow pulsations in a subsonic wind tunnel with an open working part.

Higher longitudinal (third, fifth, etc.) modes in a quarter-wave cavity, see [7], are critical to a change in the determining geometric and regime parameters of the interaction of gas jets with tube cavities. Changes in the determining parameters can be obtained, for example, by structural elements in the output section of the collector, such as damping plates.

4. Damping plates at the outlet of the collector. The technical solution for installing damping plates is illustrated by the photograph in FIG. 12. In FIG. 12 thin wedges (20 pcs.) Are installed, installed evenly around the perimeter of the collector. The wedges are made of plywood with a thickness of 14 mm with a base of 100 mm and a wedge height of 150 mm.

The experimental results on the influence of damping plates on the amplitudes of pressure pulsations in the diffuser and in the chamber of the wind tunnel are shown in FIG. 9 (curve with the symbol "+ 18 + wedges.", 10 rectangular holes in the diffuser). Damping plates in the entire investigated range of the fan shaft rotation speed contributed to:

- reduction of amplitudes of pressure pulsations in the prechamber to quite acceptable values (~ 16 Pa);

- “collapse” of the discrete pulsation frequency in the prechamber into components;

- reducing the amplitude of the pressure pulsations in the diffuser to values that allow the safe operation of the AT-11 wind tunnel.

At the same time, the amplitudes of the pressure pulsations in the wind tunnel diffuser are still significant.

от входного сечения,

от входного сечения диффузора и глубина диффузора L - могут быть расстояниями, характерными расстояниям узловых точек волновой диаграммы третьей продольной моды, если проходит аналогия процессов в диффузоре и четвертьволновом резонаторе.Here one can turn to flow models (wave diagrams) of longitudinal modes in the interaction of gas jets with a quarter-wave resonator, see [7]. Characteristic distances -

from the inlet section

from the inlet section of the diffuser and the depth of the diffuser L - can be distances characteristic of the distances of the nodal points of the wave diagram of the third longitudinal mode, if the analogy of the processes in the diffuser and the quarter-wave resonator passes.

от входного сечения диффузора, с зашивкой отверстий снаружи тонкой резиной.Therefore, it is possible to perform a series of damping holes extended in parallel along the axis of the wind tunnel in a section along the length

from the inlet section of the diffuser, with the sewing holes outside with thin rubber.

до

от входного сечения диффузора.5. Damping holes with a longitudinal dimension l and a total area of up to 0.1 square S of the outlet cross section of the collector, located in relation to the available at a distance from

before

from the inlet section of the diffuser.

The technical solution for the execution of this series of damping holes is not difficult. The results of the experimental study are illustrated in FIG. 13 spectrograms of amplitudes of pressure pulsations in the chamber and in the diffuser of the AT-11 wind tunnel. In the spectrograms: the X axis is the frequency axis, the Y axis is the rms values of the pressure pulsation amplitudes, the Z axis is the step-by-step spectral estimates as functions of the AT-11 fan shaft speed. It is enough to make holes with a total area of up to 0.1 area S of the nozzle exit section in order to reduce the pressure pulsation amplitudes to acceptable values.

Thus, the implementation of structural elements in the wind tunnel AT-11 in accordance with the alleged invention contributes to a significant reduction in flow pulsations in the infrasonic range, eliminating vibration of the pipe and building, eliminating the harmful effects on the health of staff.

Information sources

1. Wind tunnels of eastern hemisphere / a report prepared by the federal research division, library of congress, for the aeronautics research mission directorate, national aeronautics and space administration. 2008 .-- 245 p.

2. Wind tunnels of western hemisphere / a report prepared by the federal research division, library of congress, for the aeronautics research mission directorate, national aeronautics and space administration. 2008 .-- 646 p.

3. Laboratory workshop on aerodynamics: Textbook. allowance / Belova A.V., Buravtsev A.I., Kovalev M.A., Matveev S.K. L .: Publishing house of Leningrad State University, 1980.288 s.

4. Patent (RU) No. 2377525 C2 (IPC: G01M 9/12).

5. S.P. Strelkov, G.A. Bendrikov, N.A. Smirnov. "Pulsations in wind tunnels and methods for damping them." - Proceedings of TsAGI, 1946, c. 593, 56 p.

6. Patent (RU) No. 2504747 (IPC: G01L 27/00) “Device for calibration of measuring instruments for differential pressure” (copyright holder - St. Petersburg State University; authors - G.A. Leonov, A.I. Tsvetkov, B.A. Shchepaniuk) ; Registered in the Register of Inventions of the Russian Federation on January 20, 2014

7. A.K. Poluboyarinov, A.I. Of flowers. An experimental study of longitudinal modes during Hartmann. Book: Applied aerodynamics and thermal processes. Publishing House ITPM SB AS USSR, 1980, p. 99-112

Claims (3)

1. A subsonic wind tunnel with a low level of pulsations of the infrasonic range flow of a closed type, containing a prechamber (1), a collector (2), damping plates (3) at the outlet of the collector (2), an open working part (4), a diffuser (5) in length L with through damping holes (6) with a total area from 0.4 to 0.5 of the area S of the outlet cross section of the collector (2) with the arrangement of rows of holes (6) at a distance from 0.6-0.9 of the diameter D of the collector (2) to L / 3 the length of the diffuser (5) from its inlet section, the annular socket (7) above the diffuser (5), rotary sections (8) with rotary blades (9), one return duct (10), a blade fan (11) located behind the diffuser (5), characterized in that the diffuser (5) of length L has additional through-damping holes (12) with a longitudinal dimension l and a total area of up to 0.1 of the area S of the outlet cross section of the collector (2), located in relation to the available at a distance from

before

and also openings (13) with a total area of up to 0.15 of the area S of the outlet cross section of the collector (2) and a longitudinal size l, located with a gap Δ between the blade fan (11) and the diffuser (5) and with respect to the available at a distance [L - (l / 2 + Δ)]. 2. The closed-type subsonic wind tunnel according to claim 1, characterized in that the damping holes (6), (12) and (13) in the diffuser (5) are made of thin rubber wired with the outer surface of the diffuser (5). 3. The closed-type subsonic wind tunnel according to claim 1 or 2, characterized in that the annular bell (7) above the diffuser (5) is made with a mechanism (14) for longitudinal movement of the bell (7) parallel to the axis of the wind tunnel.

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