JP6366172B2 - Flow field measurement method using microbubbles and flow field measurement device for aquarium - Google Patents

Description

  The present invention relates to a flow field measuring method and a water tank flow field measuring apparatus using microbubbles.

There are many tests to predict the resistance and propulsion performance of an actual ship by resistance test and self-propulsion test, but in recent years PIV (Particle Image) has been used for more accurate propulsion performance evaluation and ship shape improvement. Flow field measurement by velocimetry) is actively performed. Application of PIV to towed water tanks and circulating water tanks is already common, and various measurement cases have been reported. The stern flow (flow field) of a ship has many complicated three-dimensional flows. In order to measure this flow field, SPIV (stereo PIV) or 3DPTV (3 Dimensional Particle Tracking) using two or more cameras using two or more cameras. Measurement using Velocity) is performed and used as a flow field measurement method in a water tank test.
A tracer is indispensable for the measurement of PIV and PTV. In general, solid particles close to the specific gravity of water such as silver-coated hollow glass beads and nylon powder are often used. However, it is almost impossible to recover the supplied solid particles, and there is a problem in spraying the solid particles into the aquarium water from the viewpoint of maintenance and accuracy control of the aquarium facility.
On the other hand, unlike solid particles, flow field measurement using microbubbles that do not remain in the water tank as a tracer is also performed, but these are intended for flow fields that do not have a free surface.
Therefore, by using microbubbles that do not remain in the tank as a tracer, it is possible to measure the flow field with a free surface, especially the stern flow of a model ship in plain water and waves in a ship test tank. Observation is required.
Here, Patent Document 1 relates to a visualization particle generator that can analyze a flow field inside a device and measure a flow velocity using visible particles, and uses bubbles as visible particles.
Patent Document 2 proposes an apparatus for visualizing a flow using bubbles.
Patent Document 3 describes using PIV measurement and laser sheet light.
Patent Literature 4 and Patent Literature 5 describe that fluid flow in a complicated flow field is measured using PIV or laser sheet light.
Further, Patent Document 6 supplies a first fluid as a tracer from a nozzle hole into a flow field of a second fluid, irradiates laser light so as to cross the flow field, and receives the laser light that has passed through the flow field, An apparatus is disclosed that can visualize the flow of the second fluid by determining the position at which the first fluid crosses the laser beam using the scan intensity signal of the received laser beam.

JP-A-6-148216 JP-A-9-288122 JP 2013-29423 A JP 2002-22759 A JP 2004-20385 A JP2012-103041A

However, Patent Document 1 does not measure the flow field around an object moving in water, but measures the flow field itself, and the initial velocity when the visible particles are injected into the flow field is the flow field. In order to solve this problem, a visible particle initial velocity reduction mechanism is provided in the preceding injection path so that the visible particles are decelerated so as not to have the initial velocity and injected into the flow field by negative pressure.
Patent Document 2 visualizes a flow by causing bubbles to exist in mineral oil and collapsing, and depositing or generating a smoke-like substance or the like in the mineral oil as a substance identifiable from the outside. It does not measure the flow field around the moving object.
Moreover, patent document 3 calculates the flow velocity and particle size of the particle | grains which flow through the inside of a measuring object.
Patent Document 4 measures the flow velocity and flow direction of a fluid in a closed space that is different from the environment from the outside, and there is no description regarding the movement of an object in the flow field.
Patent Document 5 captures particles in a pipe in a test section, calculates a velocity vector from two images, and measures a time-series fluid velocity in space. It does not measure the field.
In Patent Document 6, an air flow field in which air flows at a constant speed is formed in a wind tunnel, an object is placed in the wind tunnel, and carbon dioxide gas is supplied as a tracer from a plurality of nozzle holes to a fluid flow field. The object that is the object is fixed and does not measure the surrounding flow field when the object moves.

  The present invention uses, as a tracer, microbubbles that do not remain in water, for example, a flow field around an object moving in water, such as a stern flow of a model ship in a flat water or a wave in a ship test tank. Thus, an object of the present invention is to provide a flow field measurement method and a water tank flow field measurement device using microbubbles that can be measured with high accuracy.

The flow field measurement method using microbubbles corresponding to the present invention according to claim 1 is a flow field measurement method for measuring a flow field around an object moving in water, wherein the microbubbles are placed in front of the object. The bubble blowing means is provided to move the bubble blowing means in the same direction as the object at the same speed, and the microbubbles blown from the bubble blowing means are blown out with the initial velocity relative to the bubble blowing means, and the bubble blowing The flow field is measured based on the movement of microbubbles blown out from the means and flowing around the object , and the measurement result of the flow field is corrected based on the initial velocity . According to the first aspect of the present invention, since the microbubbles are blown out from the bubble blowing means that moves in the same direction at the same speed as the target provided in front of the target, the microbubbles are only timely around the target. The flow field around the object can be accurately measured. In addition, since the microbubbles are not lost before the measurement is completed, a highly accurate flow field measurement result can be obtained. In addition, since microbubbles that do not remain in the measurement facility are used as a tracer, the maintenance and accuracy management of the measurement facility is facilitated as compared with the case where solid particles are used as a tracer. Further, a means for preventing the microbubbles from having the initial velocity is not necessary, and the flow field measurement result is corrected based on the initial velocity, so that the result of removing the influence of the initial velocity can be obtained.

The present invention described in claim 2 is characterized in that the initial velocity is varied in the direction of water depth, and the initial velocity at a deep water depth is made slower than the initial velocity at a shallow water depth. According to the second aspect of the present invention, the bubble blowing means can be realized with a simple configuration.

The invention according to claim 3 is characterized in that the initial speed is controlled by the initial speed adjusting means for adjusting the initial speed. According to the third aspect of the present invention, the microbubbles can be supplied in an optimum state for measurement by arbitrarily adjusting the initial velocity of the microbubbles.

The present invention according to claim 4 is characterized in that the bubble diameter of the microbubbles is in the range of 5 μm to 100 μm. According to the fourth aspect of the present invention, since the microbubbles in the bubble diameter range are not coupled to each other, the rising speed can be suppressed, and the flow field around the object is completed before the measurement is completed. Therefore, it is possible to measure the flow field with high accuracy without causing microbubbles to rise and disappear.

The present invention according to claim 5 is characterized in that micro bubbles are mixed in advance with water and supplied to the bubble blowing means, and the fine bubbles are blown out together with water from the bubble blowing means. According to the fifth aspect of the present invention, the microbubbles can be continuously supplied to the bubble blowing means together with water, and the microbubble diffusion into the flow field can be made uniform.

The present invention according to claim 6 is characterized in that microbubbles are also blown out from the bubble blowing means in the width direction of the object. According to the sixth aspect of the present invention, it is possible to diffuse the microbubbles in the width direction, further widen the measurement range of the flow field, and to measure the three-dimensional flow field.

The present invention according to claim 7 is characterized in that the flow field is measured by irradiating the vicinity of the object from the sheet light irradiation means with the sheet light moving in the same direction as the object at the same speed. According to the present invention described in claim 7 , it is possible to satisfactorily measure the flow field by capturing microbubbles moving around the object.

The present invention according to claim 8 is characterized in that the sheet light is irradiated in parallel with the moving direction of the object. According to the present invention described in claim 8 , the flow field in the front-rear direction of the object can be measured from the movement of the microbubbles in the same direction as the object, and the flow field is measured with a certain width in the front-rear direction. Can do.

In the water tank flow field measuring apparatus using microbubbles corresponding to the present invention according to claim 9, a pulling means for pulling the object in the water tank, and a bubble blowing means for blowing out the microbubbles in front of the object A bubble supplying means for supplying microbubbles to the bubble blowing means, a measuring means for measuring the flow field based on the movement of the microbubbles blown from the bubble blowing means and flowing around the object, and blown out from the bubble blowing means The microbubbles are blown out with an initial velocity relative to the bubble blowing means, and the initial velocity adjusting means for correcting the flow field measurement result based on the initial velocity is provided, the bubble blowing means and the measuring means, Is attached to the pulling means. According to the tenth aspect of the present invention, since the microbubbles are blown out from the bubble blowing means that moves in the same direction at the same speed as the target provided in front of the target, the microbubbles are only timely around the target. The flow field around the object can be accurately measured, and the bubble blowing means can be small. In addition, since the microbubbles are not lost before the measurement is completed, a highly accurate flow field measurement result can be obtained. In addition, since microbubbles that do not remain in the water tank are used as the tracer, maintenance and accuracy control of the water tank facilities are facilitated as compared with the case where solid particles are used as the tracer. Further, since the bubble blowing means and the measuring means are attached to the pulling means, it is not necessary to provide the bubble blowing means and the measuring means separately from the pulling means such as the bottom of the water tank. Further, a means for preventing the microbubbles from having the initial velocity is not necessary, and the flow field measurement result is corrected based on the initial velocity, so that the result of removing the influence of the initial velocity can be obtained.

The present invention according to claim 10 is characterized in that the bubble blowing means has a tubular configuration extending in the water depth direction and has a plurality of bubble blowing holes in the water depth direction. According to the tenth aspect of the present invention, a large number of microbubbles can be blown out with a simple configuration with respect to an object existing in the depth direction.

The present invention described in claim 11 is characterized by comprising a bubble amount adjusting means for adjusting the amount of fine bubbles supplied from the bubble supplying means. According to the eleventh aspect of the present invention, an optimum flow field for measurement can be obtained by arbitrarily adjusting the amount of microbubbles.

The present invention according to claim 12 is characterized in that the bubble supply means is controlled so that the bubble diameter of the microbubbles is in the range of 5 μm to 100 μm. According to the present invention described in claim 12 , the microbubbles in the bubble diameter range can suppress the rising speed because the bubbles are not coupled to each other, and the flow field around the object before the measurement is completed. Therefore, it is possible to measure the flow field with high accuracy without causing microbubbles to rise and disappear.

The present invention as set forth in claim 13 is characterized in that the bubble supply means supplies fine bubbles mixed with water. According to the present invention as set forth in claim 13 , microbubbles can be continuously supplied to the bubble blowing means together with water, and the diffusion of the microbubbles into the flow field can be made uniform.

The present invention according to claim 14 is characterized in that the bubble blowing means blows out micro bubbles in the width direction of the object. According to the present invention of the fourteenth aspect , it is possible to diffuse the microbubbles in the width direction, further widen the flow field measurement range, and to measure the three-dimensional flow field.

The present invention according to claim 15 is characterized in that sheet light irradiation means for irradiating the vicinity of the object with sheet light is attached to the pulling means. According to the present invention of the fifteenth aspect , the sheet light irradiating means can be moved at the same speed as the object and the flow field can be measured satisfactorily.

The present invention according to claim 16 is characterized in that the sheet light is irradiated in parallel with the moving direction of the object. According to the present invention described in claim 16, it can measure the longitudinal direction of the flow field of the object from the movement of microbubbles of the object in the same direction and by performing the measurement of the flow field with a constant width in the longitudinal direction Can do.

According to the flow field measurement method using microbubbles of the present invention, the microbubbles are blown out from the bubble blowing means that moves in the same direction at the same speed as the target provided in front of the target. Microbubbles can be supplied well, and the flow field around the object can be measured accurately. In addition, since the microbubbles are not lost before the measurement is completed, a highly accurate flow field measurement result can be obtained. In addition, since microbubbles that do not remain in the measurement facility are used as a tracer, the maintenance and accuracy management of the measurement facility is facilitated as compared with the case where solid particles are used as a tracer. Further, a means for preventing the microbubbles from having the initial velocity is not necessary, and the flow field measurement result is corrected based on the initial velocity, so that the result of removing the influence of the initial velocity can be obtained.

  Further, when the initial velocity is varied in the water depth direction and the initial velocity at a deep water depth is made slower than the initial velocity at a shallow water depth, the bubble blowing means can be realized with a simple configuration.

  Further, when the initial speed is controlled by the initial speed adjusting means for adjusting the initial speed, the microbubbles can be supplied in an optimum state for measurement by arbitrarily adjusting the initial speed of the microbubbles.

  In addition, when the bubble diameter of the microbubbles is in the range of 5 μm to 100 μm, the rising speed can be suppressed by preventing the coupling of the microbubbles, and the flow field around the object before the measurement is completed. Therefore, it is possible to measure the flow field with high accuracy without causing microbubbles to rise and disappear.

  In addition, when microbubbles are premixed in water and supplied to the bubble blowing means, and the microbubbles are blown out together with water from the bubble blowing means, the microbubbles can be continuously supplied to the bubble blowing means together with water. Diffusion of microbubbles into the field can be made uniform.

  In addition, when blowing out microbubbles from the bubble blowing means in the width direction of the object, it is possible to diffuse the microbubbles in the width direction to further expand the flow field measurement range and to measure three-dimensional flow fields. can do.

  In addition, when measuring the flow field by irradiating a sheet light moving in the same direction at the same speed as the object from the sheet light irradiating means in the vicinity of the object, the microbubbles moving around the object are captured and flown. The field can be measured well.

  In addition, when the sheet light is irradiated in parallel with the moving direction of the object, the flow field in the front-rear direction of the object can be measured from the movement of the microbubbles in the same direction as the object, and with a certain width in the front-rear direction. The flow field can be measured.

Further, according to the flow field measuring apparatus for aquarium using the microbubbles of the present invention, since the microbubbles are blown out from the bubble blowing means moving in the same direction as the target provided in front of the target, Microbubbles can be supplied to only the surroundings in a timely manner, the flow field around the object can be accurately measured, and the bubble blowing means can be small. In addition, since the microbubbles are not lost before the measurement is completed, a highly accurate flow field measurement result can be obtained. In addition, since microbubbles that do not remain in the water tank are used as the tracer, maintenance and accuracy control of the water tank facilities are facilitated as compared with the case where solid particles are used as the tracer. Further, since the bubble blowing means and the measuring means are attached to the pulling means, it is not necessary to provide the bubble blowing means and the measuring means separately from the pulling means such as the bottom of the water tank. Further, a means for preventing the microbubbles from having the initial velocity is not necessary, and the flow field measurement result is corrected based on the initial velocity, so that the result of removing the influence of the initial velocity can be obtained.

  In addition, when the bubble blowing means has a tubular configuration extending in the water depth direction and has a plurality of bubble blowing holes in the water depth direction, a large number of micro bubbles are blown out with a simple configuration with respect to an object existing in the water depth direction. Can do.

  In addition, when a bubble amount adjusting unit that adjusts the amount of microbubbles supplied from the bubble supply unit is provided, an optimum flow field for measurement can be obtained by arbitrarily adjusting the amount of microbubbles. .

  In addition, when the bubble supply means is controlled so that the bubble diameter of the microbubbles is in the range of 5 μm to 100 μm, it is possible to prevent the microbubbles from rising and to suppress the rising speed of the microbubbles. The flow field can be measured with high accuracy without causing microbubbles to rise and disappear from the flow field around the object before completion.

  Further, when the bubble supply means supplies the microbubbles mixed with water, the microbubbles can be continuously supplied together with the water to the bubble blowing means, and the microbubble diffusion into the flow field can be made uniform.

  In addition, when the bubble blowing means blows out microbubbles in the width direction of the object, the microbubbles are diffused in the width direction, and the flow field measurement range can be expanded to measure three-dimensional flow fields. It can be.

  In addition, when the sheet light irradiating means for irradiating the vicinity of the object with the sheet light is attached to the pulling means, the sheet light irradiating means can be moved at the same speed as the object and the flow field can be measured well. it can.

  In addition, when the sheet light is irradiated in parallel with the moving direction of the object, the flow field in the front-rear direction of the object can be measured from the movement of the microbubbles in the same direction as the object, and with a certain width in the front-rear direction. The flow field can be measured.

The block diagram which shows the whole structure of the flow field measuring apparatus for water tanks using the microbubble by one Embodiment of this invention Configuration diagram showing the measuring means of the device Distribution diagram of bubble diameter generated by the same device Configuration diagram showing bubble blowing means of the apparatus The figure which shows the photographed image of the flow field measurement in the uniform flow with the same device The figure which shows the analysis result of the flow field measurement in the uniform flow by the same device The figure which shows the turbulence degree of the velocity of the main flow direction of the flow field measurement in the uniform flow by the same device The figure which shows the correlation of the average velocity (E (Vx)) of the main flow direction measured with PIV of the flow field measurement in the uniform flow by the same apparatus, and the average velocity (E (Vxp)) of the main flow direction measured with the Pitot tube. The figure which shows the photographed image of the flow field measurement in the regular wave with the same device The figure which shows the analysis result of the flow field measurement in the regular wave by the same device The figure which shows the velocity Vx of the main flow direction in z = -0.187m of the flow field measurement in the regular wave by the same apparatus, and the speed Vz of an up-down direction in time series. The figure which shows the velocity Vx of the main flow direction in z = -0.224m of the flow field measurement in the regular wave by the same apparatus, and the speed Vz of an up-down direction in time series. The figure which shows the velocity Vx of the main flow direction in z = -0.261m of the flow field measurement in the regular wave by the same apparatus, and the speed Vz of an up-down direction in time series.

  Below, the flow field measuring device for water tanks using the microbubble by embodiment of the present invention is explained.

FIG. 1 is a block diagram showing the overall configuration of a water tank flow field measuring apparatus using microbubbles according to an embodiment of the present invention, and FIG. 2 is a block diagram showing the measuring means of the apparatus.
The water tank flow field measuring device using microbubbles according to the present embodiment has a towing means 30 for towing an object (model ship) 20 in the aquarium 10, and the towing means 30 includes a bubble blowing means. 40 and measuring means 50 are attached.
Accordingly, the model ship 20, the bubble blowing means 40, and the measuring means 50 are moved in the water tank 10 in the same direction at a constant speed by the towing means 30.
The bubble blowing means 40 blows out micro bubbles in front of the model ship 20 in water. Microbubbles are supplied to the bubble blowing means 40 from the bubble supply means 60.
The measuring means 50 measures the flow field based on the movement of the fine bubbles that are blown out from the bubble blowing means 40 and flow around the model ship 20.

The bubble blowing means 40 includes a tubular tube member 41 extending in the water depth direction in the water tank 10, and the tube member 41 has a plurality of bubble blowing holes 42 in the water depth direction. By having the plurality of bubble blowing holes 42 in the water depth direction, many micro bubbles can be blown out with a simple configuration with respect to the hull of the model ship 20 existing in the water depth direction.
The bubble blowing means 40 also blows out micro bubbles in the width direction of the model ship 20. By arranging the plurality of pipe members 41 in the width direction of the model ship 20, microbubbles can be blown out in the width direction of the model ship 20. By blowing out in the width direction, it is possible to stably perform measurement and, depending on the purpose, three-dimensional measurement is also possible.
Further, the single tube member 41 may blow out microbubbles in the width direction by changing the blowing direction of the bubble blowing holes 42 respectively. Moreover, it is also possible to set the blowing direction of the bubble blowing holes 42 in an arbitrary direction such as a horizontal direction and a forward direction in a state where the tube member 41 is viewed in plan.
The bubble blowing means 40 blows out microbubbles also in the width direction of the model ship 20, so that the microbubbles can be diffused and the flow field measurement range can be further expanded.
The microbubbles blown out from the bubble blowing means 40, that is, blown out from the bubble blowing holes 42, are blown out with an initial velocity relative to the bubble blowing means 40. The micro bubbles blown out from the bubble blowing holes 42 have an initial velocity at a deep water depth lower than an initial velocity at a shallow water depth, and the initial velocity is varied in the depth direction. For this reason, the pipe member 41 can have a simple configuration without requiring a complicated mechanism or structure for aligning the initial blow-out speed from the bubble blowing hole 42.

The water tank flow field measuring apparatus using microbubbles according to the present embodiment includes an initial speed adjusting means 71 for adjusting the initial speed and a bubble amount adjusting means 72 for adjusting the amount of microbubbles.
The initial speed adjusting means 71 controls the initial speed of the micro bubbles blown out from the bubble blowing hole 42. The bubble amount adjusting unit 72 adjusts the amount of micro bubbles supplied from the bubble supplying unit 60.
In the present embodiment, the first speed adjustment means 71 is the first control valve that restricts the flow path of the supply pipe 81 that connects the bubble supply means 60 and the bubble blowing means 40. By narrowing the first control valve that is the initial speed adjusting means 71, the initial speed can be slowed down.

In the present embodiment, the second control valve provided on the lower end side of the pipe member 41 is used as the bubble amount adjusting means 72. A bubble escape portion 43 is provided below the second control valve, which is the bubble amount adjusting means 72. By opening the second control valve, the amount of bubble discharge from the bubble escape portion 43 is increased and blown out from the bubble blowing hole 42. The amount of microbubbles is reduced.
Note that the amount of microbubbles can also be adjusted by the first control valve, and the initial speed can be controlled by the second control valve. The amount of microbubbles can also be controlled by the bubble supply means 60.

By adjusting the initial velocity and amount of microbubbles arbitrarily, it is possible to obtain an optimum flow field for measurement.
The micro bubbles blown out from the bubble blowing holes 42 have a bubble diameter in the range of 5 μm to 100 μm, preferably in the range of 20 μm to 60 μm, more preferably in the range of 30 μm to 40 μm. By setting such a bubble diameter, it is possible to suppress or prevent the bonding of the microbubbles, and to suppress the rising speed of the microbubbles, and the flow field around the model ship 20 before the measurement is completed. Therefore, microbubbles will not rise and disappear.
The bubble supply means 60 sucks air from the atmosphere and sucks water from the water tank 10. The suction pipe 82 is provided at a distance behind the model ship 20 so as not to disturb the flow field around the model ship 20, and sends the water sucked from the water tank 10 to the bubble supply means 60. In the bubble supply means 60, the air sucked from the atmosphere is micronized, and microbubbles are mixed with water and supplied. By supplying microbubbles mixed with water and blowing the microbubbles together with water from the bubble blowing means 40, the microbubbles can be continuously supplied to the bubble blowing means 40, and the microbubbles can be diffused into the flow field. It can be made uniform. In the bubble supply means 60, the bubbles can be mixed with water and supplied, and the bubbles can be made fine by the bubble blowing means 40 and blown out as fine bubbles. It is also possible to equip a tank for storing air, supply air under pressure, and eject the fine bubbles by the bubble blowing means 40.

As shown in FIG. 2, the measuring unit 50 is attached to the pulling unit 30 and includes a sheet light irradiating unit 51 that irradiates the sheet light and an imaging unit 52 that images the irradiated microbubbles. By attaching the sheet light irradiation means 51 and the imaging means 52 to the towing means 30, the sheet light moving in the same direction as the model ship 20 is irradiated from the sheet light irradiation means 51 to the vicinity of the model ship 20. Therefore, the flow field can be measured satisfactorily.
The sheet light irradiation means 51 irradiates the vicinity of the model ship 20 with sheet light. By irradiating the sheet light in parallel with the moving direction of the model ship 20, the flow field in the front-rear direction of the model ship 20 can be measured from the movement of the microbubbles moving in the same direction as the model ship 20, and a constant width in the front-rear direction. Can be used to measure the flow field.
The sheet light irradiation means 51 according to the present embodiment irradiates the vicinity of the model ship 20 from underwater using the mirror 53. Since the sheet light irradiation means 51 is pulled by the pulling means 30 as part of the measuring means 50 and moves underwater with the model ship 20, it can be realized as a simple means.
In addition, you may irradiate the vicinity of the model ship 20 from underwater by making optical fiber and the sheet light irradiation means 51 into a waterproof type.

The flow field measurement method using microbubbles according to the present embodiment blows out microbubbles blown out from the bubble blowing means 40 with an initial velocity relative to the bubble blowing means 40 and the flow field measurement results for the first time. Correct based on speed. By correcting the measurement result of the flow field based on the initial velocity, a result obtained by removing the influence of the initial velocity can be obtained.
When V0 is a true velocity, VPIV is a velocity analyzed by PIV (Particle Image Velocity), ΔVw is a change in flow velocity by the bubble blowing means 40, Vs is an initial velocity, and ΔVerr is a measurement error, the true velocity V0 is expressed by the following equation: It is represented by
V0 = VPIV−ΔVw−Vs−ΔVerr

ΔVw is obtained by the following equation using another measuring device such as a Pitot tube, comparing the state without the bubble blowing means 40 and the state with the bubble blowing means 40.
ΔVw = Vw−Vw0
Here, Vw is the speed of the bubble blowing means 40, and Vw0 is the speed without the bubble blowing means 40.
Vs is measured by using another measuring device such as a Pitot tube in plain water, and in waves, wave making is performed in a stopped state, and an average value is obtained from time-series data of flow velocity changes.
By correcting the measurement result of the flow field based on the initial velocity, means for preventing the microbubbles from having the initial velocity becomes unnecessary, and measurement can be performed with a simple configuration such as the tube member 41 extending in the water depth direction. Become.

The experimental results of the flow field measurement method using microbubbles according to the present invention are shown below.
In the same flow field measurement method, the apparatus shown in FIGS. 1 and 2 is used to blow out microbubbles from the bubble blowing holes 42 in front of the laser sheet light, and the microbubbles are illuminated by the laser sheet light behind the microbubbles. The two movements were continuously photographed in stereo by two imaging means (high speed cameras) 52.

In this experiment, a combination of a gas-liquid shearing method and a cavitation method was used as the bubble supplying means (bubble generating device) 60.
In order to investigate the basic properties of bubbles used as a tracer, the bubble diameter generated by the bubble generator 60 was measured, and the bubble rising speed was examined. The bubble diameter was measured by irradiating a transparent container mixed with microbubbles with laser sheet light and using a microscope. The resolution of the microscope is 720 × 480 pixels, and the imaging range is 6.5 × 4.3 mm, and 9.1 μm per pixel.

FIG. 3 shows the results obtained by measuring the number of pixels of bubbles from the photographed image and obtaining the bubble diameter distribution.
It can be seen from FIG. 3 that the bubble diameter generated by the bubble generator 60 is in the range of 10 to 60 μm (average 31 μm). The rising speed (Vr) of the microbubbles is proportional to the square of the bubble diameter according to the following Stokes' law.
Vr = gd ² / 18ν
Here, g is the acceleration of gravity, d is the bubble diameter, and ν is the kinematic viscosity coefficient of water.
The kinematic viscosity coefficient of water varies with temperature. The rising speed at the average bubble diameter at the water temperature (15 ° C.) during the experiment is determined to be 0.47 mm / s.

  Further, this time, when blowing out these micro bubbles as a tracer, as shown in FIG. 4, a bubble blowing means 40 having a plurality of bubble blowing holes 42 formed in a pipe member (vinyl chloride pipe) 41 was used. Since the final purpose is to measure the flow field at the stern, two bubble blowing means 40 are used and diffused over a wide range. The pipe 41 is formed into an airfoil for rectification.

As the bubble generating device 60, the measuring means (PIV system) 50 shown in FIG. 2 was used.
The high-speed camera 52 used in the PIV system 50 can capture a VGA size (640 × 480 pix) image at 200 frames per second, and the minimum subject illuminance is 10 lux (F value: 1.4). A lens having a focal length of 12 mm and an F value of 1.2 was used. The shooting range of the high-speed camera 52 is about 250 mm × 190 mm, and is about 0.4 mm per pix.
As the light source of the laser optical system, a continuous wave laser of green light (532 nm) with an output of 4 W was used. The laser light source is guided to the water through the two mirrors 53, the laser light source is diffused into a sheet shape by the cylindrical lens 54 and formed into a sheet shape, and finally the laser sheet light is irradiated vertically upward through the mirror 55. The water tank flow field measuring device has a traverse device 90 for moving in the width direction of the ship in consideration of measurement with the model ship 20. As a result, the cross section to be measured can be changed to perform three-dimensional measurement.
The inspection region was analyzed by the FFT cross-correlation method with 64 × 64 pix and 50% overlap.

In order to verify the accuracy of PIV measurement using microbubbles as a tracer, the flow field of uniform and regular waves in the shaking test water tank (length = 50m, width = 8m, depth = 4.5m) of the National Maritime Research Institute Measurement was performed. It was installed in the center line of the water tank 10 when measuring the uniform flow. The coordinate system used below is a right-handed system in which the x-axis is the mainstream direction, the y-axis is from the far side to the near side of the measurement screen, and the z-axis is vertically upward.
In order to verify the accuracy of this device, we measured the flow field in a uniform flow in this study. The towing speed (V) was set to three types of 0.4 m / s, 0.6 m / s, and 0.8 m / s, and measurement was performed 5 times each. The measurement cross section of the PIV system 50 and the Pitot tube were installed behind the bubble blowing means 40 composed of two vinyl chloride pipes 41 and measured simultaneously, and the measured values were compared.
5 and 6 show an example of a captured image and an analysis result at V = 0.4 m / s.

The velocity disturbance (Ivx) in the mainstream direction is defined by the following equation. An example of V = 0.4 m / s is shown in FIG.
Ivx = σvx / E (Vx)
Here, E (Vx) is an average value of the velocity in the mainstream direction measured by the PIV system 50, and σvx is a standard deviation of the velocity in the mainstream direction measured by the PIV system 50.
It can be seen that the degree of disturbance is within 3% in most of the measurement range.
Also, the reason why the center of the range with a small degree of disturbance is shifted from the center of the measurement range is that the center of the captured image differs between the left and right due to the angle shift of the left and right high speed cameras 52. It is considered that the degree of disturbance increases toward the outside of the measurement range due to lens aberration.

FIG. 8 shows the correlation between the average velocity (E (Vx)) in the mainstream direction measured by the PIV system 50 and the average velocity (E (Vxp)) in the mainstream direction measured by the Pitot tube.
The correlation coefficient of both was 0.99 or more, and it was confirmed that there was a high correlation.

Following verification by flow field measurement in uniform flow, flow field measurement in regular waves was performed. The experiment was performed in a stopped state where the wave velocity Hw = 0.050 m, the wave period T = 1 s, and 2 s and the test speed V = 0 m / s.
FIG. 9 shows an example of a captured image at T = 2s.

  FIG. 10 shows the velocity vector of the PIV analysis result at T = 2s. It is a speed vector from time t = 0 to 1.5 s. It can be seen that the direction of the velocity vector changes with time because the water particle velocity in the waves is captured.

11, 12, and 13 show time-series data of the velocity Vx in the mainstream direction and the velocity Vz in the vertical direction at z = −0.187 m, −0.224 m, and −0.261 m, respectively.
The solid line in the figure is the calculated value obtained by the micro amplitude wave theory, and the point is the measured value.
It can be seen that at z = −0.224 m and −0.261 m, the theoretical value and the measured value are almost the same for both Vx and Vz. Note that when z = −0.187 m, Vz is almost the same, but Vx is shifted to the speed increasing side as a whole. This is thought to be due to the speed at which the microbubbles are blown into the water tank 10. The reason why the Vx coincides worse as the water depth becomes shallower is that the speed at which the microbubbles are blown out in the portion close to the water surface is high, and the speed decreases as the distance from the water surface increases.

  From this test result, it was confirmed that the uniform flow measurement had a higher correlation than the measured value of the Pitot tube, and the flow field measurement in the regular wave was compared with the theoretical value obtained by the micro amplitude wave theory. As a result, high consistency was shown.

As described above, in the flow field measurement method using microbubbles according to the present embodiment, the bubble blowing means 40 for blowing microbubbles is provided in front of the object (model ship) 20, and the bubble blowing means 40 is connected to the model ship 20. By moving in the same direction at a constant speed and measuring the flow field based on the movement of the microbubbles blown out from the bubble blowing means 40 and flowing around the model ship 20, microbubbles can be introduced only around the model ship 20 in a timely manner. The flow field around the object can be accurately measured.
In addition, the microbubbles do not rise and disappear before the measurement is completed. Moreover, since the microbubbles which do not remain in the water tank 10 are used as the tracer, the maintenance and accuracy management of the water tank facilities are facilitated as compared with the case where the solid particles are used as the tracer.
In particular, since the bubble blowing means 40 and the measuring means 50 attached to the towing means 30 move at a constant speed together with the model ship 20 towed by the towing means 30 and move underwater, the flow field around the model ship 20 It is possible to obtain a highly accurate measurement result by supplying the minute bubbles only to the surface of the model ship 20 and focusing on only the periphery of the model ship 20.
For this reason, the bubble blowing means 40 can be made small-scale and simple compared with a fixed method in which bubbles are continuously diffused in the entire water tank 10 or bubbles are blown out for each region when the model ship 20 approaches. . Further, since microbubbles can be supplied only to the flow field around the model ship 20 to be measured, the amount of microbubbles can be very small, and the bubble blowing means 40 should be small and simple. Can do. When the model ship 20 is not only moved straight, but also when a test involving planar motion such as meandering or zigzag operation is performed, the bubble blowing means 40 moves in the same direction as the model ship 20, so this effect becomes remarkable.
In addition, the number of sheet light irradiation means and imaging means is compared with a system in which sheet light irradiation means and imaging means are arranged in the entire water tank 10 or either the sheet light irradiation means or the imaging means is arranged in the entire water tank 10. Can be realized with simple equipment. Further, as in the case of the bubble blowing means 40, in the case of a test involving a planar motion rather than just moving the model ship 20 straight, the measuring means 50 moves in the same direction as the model ship 20, so this effect is remarkable. It becomes.
Note that both or one of the bubble blowing means 40 and the measuring means 50, or a part of the components can be moved by means other than the towing means 30 and moving at a constant speed.

According to the flow field measurement method and the aquarium flow field measurement apparatus using microbubbles according to the present invention, for example, the stern flow of a model ship can be observed with high accuracy in a plain water or a wave in a ship test water tank.
In the above embodiment, the object is described as a model ship. However, the present invention can also be applied to a case where a floating body, a model of an underwater navigation body, or the like is an object. Further, the water tank can be applied to a sealed water tank in addition to the open-type water tank.

DESCRIPTION OF SYMBOLS 10 Water tank 20 Object 30 Pulling means 40 Bubble blowing means 41 Pipe member 42 Bubble blowing hole 43 Bubble escape part 50 Measuring means 51 Sheet light irradiation means 52 Imaging means 53 Mirror 54 Cylindrical lens 55 Mirror 60 Bubble supply means 71 Initial speed adjustment Means 72 Bubble amount adjustment means 81 Supply pipe 82 Suction pipe 90 Traverse device

Claims (16)

A flow field measurement method for measuring a flow field around an object moving in water, wherein a bubble blowing means for blowing out microbubbles is provided in front of the object, and the bubble blowing means is the same as the object at the same speed. The microbubbles that move in the direction and blow out from the bubble blowing means have an initial velocity relative to the bubble blowing means, and are blown out from the bubble blowing means and flow around the object. A flow field measurement method using microbubbles, wherein the flow field is measured based on the movement of the flow field, and the measurement result of the flow field is corrected based on the initial velocity . 2. The flow field measurement using microbubbles according to claim 1 , wherein the initial velocity is made different in a water depth direction, and the initial velocity at a deep water depth is made slower than the initial velocity at a shallow water depth. Method. The flow field measurement method using microbubbles according to claim 1 or 2 , wherein the initial speed is controlled by an initial speed adjusting means for adjusting the initial speed. The flow field measurement method using microbubbles according to one of claims 1 to 3 , wherein a bubble diameter of the microbubbles is set in a range of 5 µm to 100 µm. The water microbubbles previously mix was fed to the bubble blowing device, according to one of claims 4 the microbubbles from the bubble blowing means from claim 1, characterized in that blown together with the water Flow field measurement method using microbubbles. Flow field measurement method using microbubbles according to claims 1 to one of the claims 5, characterized in that blowing the microbubbles in the width direction of the object from said bubble blowing means. The light sheet moving in the same direction with the object and the constant velocity, the sheet light irradiation means of claims 1 to 6, characterized in that to measure the flow field by irradiating the vicinity of the object A flow field measurement method using the microbubbles according to item 1. The flow field measurement method using microbubbles according to claim 7 , wherein the sheet light is irradiated in parallel with a moving direction of the object. From the pulling means for pulling the object in the water tank, the bubble blowing means for blowing out the fine bubbles in front of the object, the bubble supplying means for supplying the fine bubbles to the bubble blowing means, and the bubble blowing means Measuring means for measuring a flow field based on the movement of the microbubbles that are blown out and flow around the object, and the microbubbles that are blown out from the bubble blowing means have an initial velocity relative to the bubble blowing means. And an initial speed adjusting means for correcting the measurement result of the flow field based on the initial speed , and the bubble blowing means and the measuring means are attached to the pulling means. An aquarium flow field measurement device using microbubbles. The water tank flow field measuring device using microbubbles according to claim 9 , wherein the bubble blowing means has a tubular configuration extending in a water depth direction and has a plurality of bubble blowing holes in the water depth direction. The flow field measuring device for a water tank using microbubbles according to claim 9 or 10 , further comprising a bubble amount adjusting means for adjusting the amount of the microbubbles supplied from the bubble supplying means. Tanks using microbubbles according to one of claims 11 claim 9, characterized in that the bubble diameter of the microbubbles is controlling the bubble supply means so that the range of 5μm to 100μm Flow field measuring device. The bubble supply means, a water tank for flow field measuring apparatus using microbubbles according to one of claims 12 wherein the microbubbles claim 9, characterized in that the feed mix into water. The said bubble blowing means blows out the said microbubble also in the width direction of the said target object, The flow field measuring apparatus for water tanks using the microbubble of any one of Claim 9 to 13 characterized by the above-mentioned. . The water tank using microbubbles according to any one of claims 9 to 14 , wherein sheet light irradiation means for irradiating sheet light in the vicinity of the object is attached to the pulling means. Flow field measuring device. The flow field measuring device for aquarium using microbubbles according to claim 15 , wherein the sheet light is irradiated in parallel with the moving direction of the object.