JP2647489B2 - Low noise circulation water tank - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a circulating water tank, and more particularly to a circulating water tank with reduced noise.

[Conventional technology]

FIG. 7 shows a typical example of a conventional circulating water tank. The water driven by the pump 1 passes through the upstream duct 2 through a honeycomb 31 provided across the flow path and the like, and flows into the measuring cylinder 5 via the rectifying cylinder 3 and the contraction cylinder 4. Further, the flow circulates from the diffusion cylinder 6 through the downstream duct 7 and enters the pump 1 by a route.

The test model 10 is installed on the measuring cylinder 5 to measure hydrodynamic performance and measure radiation noise such as flow noise and cavitation noise.

In the above example, usually, the upstream duct 2 and the downstream duct 7
The rectifying cylinder 3 to the diffusion cylinder 6 are made of a rigid steel plate or concrete, and only the measuring cylinder 5 is made of a transparent material such as an acrylic plate or glass, or provided with a window of a transparent material. I have.

It is common practice to submerge the upstream duct and the downstream duct so that the vibration of the pump 1 does not propagate through the duct wall and adversely affect the measuring cylinder.

[Problems to be solved by the invention]

The conventional circulating water tank has the following problems to be solved.

That is, since the duct wall is a rigid wall, the underwater sound generated by the pump is transmitted to the measuring cylinder without attenuating the inside of the duct.
Therefore, the sound generated by the test model was buried in the background noise (background noise; BGN), and there was a problem that it was difficult to measure.

It is also conceivable to install underwater sound absorbing material inside the duct or install a reservoir tank, learning the noise suppression measures in the wind tunnel, but there is a disadvantage that the measures are large, expensive and pressure loss increases, Practical application is difficult.

[Means for solving the problem]

The present invention provides a means for solving the above-mentioned problems, in which a pump is installed at an arbitrary point in a closed loop pipe, and another point in the closed loop pipe is used for fluid performance of a test model, flow noise and cavitation noise generated from the test model. In a circulating water tank configured with a measuring cylinder for measuring the like, each of the pipelines at predetermined positions on the upstream and downstream sides of the measuring cylinder is made of a material having an acoustic impedance substantially equal to water. It is intended to provide a low-noise circulating water tank.

(Operation)

The present invention has the following effects because it is configured as described above.

That is, since a part of the pipeline on the upstream side and the downstream side of the measuring cylinder is made of a material whose acoustic impedance is almost equal to water,
The outer circumference of the pipe is equivalent to the boundary between water and air (or an environmental substance surrounding the outer circumference), and the sound passes through the environmental material such as air, so that noise and the like that propagates in the pipe are reduced accordingly.

〔Example〕

Before describing the embodiment of the present invention, the principle of sound attenuation in the pipeline when the outer periphery of the pipeline is filled with water will be described with reference to FIG. 3 for easy understanding.

As shown in FIG. 3, when a part of a duct (pipe) 31 filled with water is submerged in water 32, a part of the resin is made of resin or rubber having the same acoustic impedance as water, like an acrylic duct 30, as shown in FIG. Is equivalent to the absence of the acrylic duct 30, and the sound that has propagated through the underwater 33 in the duct 31,
The light is reflected at the boundary between the rigid wall duct 34 and the acrylic duct 30, and another part leaks from the acrylic duct 30 into the external water 32.
Therefore, the sound wave propagating further in the duct 31 is greatly attenuated. In other words, if a part of the conduit between the noise source and the measuring cylinder is made of a material such as acrylic, which has almost the same acoustic impedance as water, the noise reaching the measuring cylinder remains a rigid conduit. It will be transmitted more attenuated than in the case of.

Next, a first embodiment of the present invention will be described with reference to FIG. The same reference numerals are given to the same components as those in the conventional example, and the description is omitted.

FIG. 1 is a side sectional view of the present embodiment.
Most of the upstream duct 2 and the downstream duct 7 are submerged, and acrylic ducts 8 and 9 are respectively installed at arbitrary locations of the upstream duct 2 and the downstream duct 7 as shown in the figure.

The structure of the acrylic ducts 8 and 9 may be arbitrary, and may be, for example, a rectangular cross section or a circular cross section. Further, it may be reinforced with a steel material or the like, or an acrylic plate window may be provided in a steel duct.

FIG. 4 is a perspective view of various embodiments of these ducts 8, 9.
(A)-(e) is each figure.

Further, instead of the acrylic plate, another plastic resin or rubber having the same acoustic impedance as water may be used.

Since the present embodiment is configured as described above, the sound that has passed through the acrylic ducts 8 and 9 to the outside is absorbed by water and attenuated.

Next, a second embodiment of the present invention will be described with reference to FIG. The same components as those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

In FIG. 2, annular containers 21 and 22 filled with water are provided around the acrylic ducts 8 and 9, and the insides of the containers 21 and 22 are subjected to sound absorbing treatment by a water absorbing material 23.

Since the present embodiment is configured as described above, the sound that has passed through the acrylic ducts 8 and 9 and exited to the containers 21 and 22 is underwater sound absorbing material 23.
It is absorbed and muted.

As described above, in the first and second embodiments, the environment outside the acrylic ducts 8 and 9 is filled with water. However, the environment is not limited to water, and the acrylic ducts 8 and 9 or other water are merely used. It may be a duct made of a material having almost the same acoustic impedance as that of the duct. That is, the outside of the acrylic ducts 8 and 9 may be left as air. Even in such a case, as described below, it has a corresponding effect.

The sound pressure in a water duct as shown in FIG. 5 usually propagates while forming a specific sound pressure mode in a cross section. Sound pressure P that forms (m, n) mode at any point (x, y, z) in the duct
_mn (x, y, z) is represented by the following equation.

Here, l _y and l _z are the cross-sectional dimensions of the duct in the y and z directions, respectively, and A _mn is the amplitude g _ym g _zn of the cross section at x = 0.
The m-th and n-th eigenvalues in the direction z, which are determined by the boundary conditions of the duct wall. ω is an angular frequency, t is time, and c is sound speed. τ _mn is a transmission coefficient of a sound wave propagating in the x direction in the (m, n) mode, and is given by the following equation.

The symbol-indicates downstream propagation, and + indicates upstream propagation.
λ is the wavelength.

Here, when the root of τ _mn is positive, there is no attenuation of the sound pressure in the x direction, but when it is a complex number or a pure imaginary number, the sound pressure attenuates in the x direction. Volume reduction per unit length Is represented by the following equation.

Here, I _m () denotes the imaginary part of the argument.

on the other hand Is a solution of the following equation with the surface impedance of the duct wall as a boundary condition.

Here, π is the pi, i is the imaginary unit, and ζ _y and _{ｚ z} are the surface impedance of the duct wall surface perpendicular to the y and z directions, respectively.

For regular steel or concrete rigid wall ducts, ζ _y ,
_{ｚ z} → ∞, m, n = 0, 1, 2, 3... That is, there exists a solution of m = n = 0, and from equation (2), for every wavelength (frequency), Made solutions exist, the equation (3) becomes I _m (τ _oo) = 0 There exists a solution. That is, for any frequency, the wave propagates as a plane wave (sound pressure G _ain , a sound wave mode with a constant phase in the cross section) within the duct cross section without attenuation.

On the other hand, if the duct wall is made of a material with almost the same acoustic impedance as water, such as an acrylic plate, and the outside is air, the duct is acoustically regarded as having the same physical properties as water,
The outside of the duct (point of contact with air) is the boundary. The acoustic characteristic impedance of air is much smaller than the acoustic characteristic impedance of water, in which case ζ _y , _{ｚ z} ≒ 0. Substituting this relationship into equations (4) and (5) gives m, n = 1,2,3..., and there is no solution where m = 0 or n = 0. In this case, the transmission coefficient is as follows.

As described above, if the value within the root is ≧ 0, I _m (τ _mn ) = 0 and the sound reduction is 0, but if the value within the root is <0, I _m (τ _mn )> 0
And noise reduction occurs. I mean Then And the (m, n) mode propagates, Then the (m, n) mode is attenuated.

If a material with the same acoustic impedance as water as the acrylic plate is used for the duct wall, The minimum value of Becomes That is, The following frequencies cannot be formed in any mode and are attenuated.

In the case of a rigid wall, m = n = 0 exists The minimum value of And all frequencies can form a propagation mode.

By the way, if it is assumed that l _y = l _z = l _m , C ≒ 1500m / se
c , And the lower frequencies are attenuated. In this case, for f = 500 Hz, equations (8) and (3)
Is used to determine the amount of attenuation. Thus, a very large noise reduction effect can be obtained theoretically.

For reference, FIG. 6 shows two-dimensional internal sound pressure modes in the case of a rigid wall and the case of a duct having a wall having the same acoustic impedance as water as an acrylic plate. (A)
Is a case of a rigid wall, and (b) is a case of a wall made of a material having the same acoustic impedance as water. In the former, the sound pressure antinode mode is set on the wall surface, whereas in the latter, the sound pressure node mode needs to be set on the outer surface of the wall. From this, it is easily presumed that m = 0 does not hold. That is, a sound reduction effect is produced.

Although the rectangular section duct has been described here, the same applies to a circular section duct and other arbitrary section ducts.

As described above, even when the outside of the acrylic ducts 8 and 9 is air, a sufficient sound reduction effect is produced.

〔The invention's effect〕

The present invention has the following effects because it is configured as described above. That is, the noise generated by the pump propagates in the ducts on the upstream and downstream sides of the measuring cylinder, but is attenuated when passing through a duct made of a material having almost the same acoustic impedance as water.
Noise propagating to the measuring cylinder is greatly attenuated. As a result,
Accurate measurement data on the test model can be obtained.

[Brief description of the drawings]

FIG. 1 is a side sectional view of a low noise circulating water tank according to a first embodiment of the present invention, FIG. 2 is a side sectional view of a low noise circulating water tank according to a second embodiment of the present invention, and FIG. Explanatory drawing of the sound reduction principle in the case where the outside of the pipe is water, among the functions of the first and second embodiments,
FIG. 4 is a perspective view of various examples of an acrylic duct forming a part of the embodiment of the present invention, and FIG. 5 is a diagram showing the principle of sound reduction when the outside of the pipe is air. FIG. 6 is an explanatory view showing the operation of another embodiment, etc., wherein (a) shows a case where the pipe is a rigid wall, and (b) shows a case where the pipe is made of a material having the same acoustic impedance as water. FIG. 7 is a side sectional view of a conventional example. 1 ... Pump, 2 ... Upstream duct, 3 ... Rectifying cylinder, 4 ... Contraction cylinder, 5 ... Measurement cylinder, 6 ... Diffusion cylinder, 7 ... Downstream duct, 8, 9 ... Acrylic duct, 10… Test sample, 21,22… Container, 23… Underwater sound absorbing material, 31… Honeycomb.

Claims (1)

(57) [Claims] 1. A pump is installed at an arbitrary point in a closed loop pipe, and another point in the closed loop pipe is used to measure fluid performance of a test model, flow noise generated from the test model, cavitation noise, and the like. A low-noise circulating water tank comprising: a circulating water tank constituted by a measuring cylinder according to any one of the preceding claims, wherein each pipe at predetermined positions on the upstream and downstream sides of the measuring cylinder is made of a material having an acoustic impedance substantially equal to that of water.