Research on flow field quality measurement of super high speed circulating water channel based on LDV

The flow field quality of the super high speed circulating water channel (SHSCWC) test section in the open (with a free liquid surface state) and closed (with a cavitation tunnel state) was studied. An array of flow field measuring points was established to measure the axial velocity using a laser doppler velocimeter (LDV) for both states. Meanwhile, ultrasonic sensors were used to measure the levelness of the free liquid surface in the test section. The measurement results indicated that in the open state, the non-uniformity and instability of 8 kinds of flow velocity in the flow field section are less than 1.0%, respectively, while the free liquid surface level is less than 25mm at the flow rate of 8m/s. In the closed state, the non-uniformity and instability of eight types of flow velocity in the flow field section are all below 1.0%, with turbulence being less than 0.5%. The measurement results in both states meet the requirements of technical indicators, and provide data support for the experimental research and optimal design of the SHSCWC.


Introduction
The circulating water channel is an important equipment of ship hydrodynamics testing.The hull under test is secured in the testing section of a circulating water channel, where water is then circulated by a pump.Consequently, the relative motion between the hull and the current is generated, breaking the limitations of the test space and time [1].A variety of measuring instruments can be arranged in the water channel, such as flowmeter, current meter, hydrophone, etc.With the aid of these flow field testing instruments and the transparent observation window installed on the test section, a multi-objective and automatic comprehensive measurement of hydrodynamic characteristics can be easily achieved.
Due to the numerous irreplaceable advantages of circulating water channels, universities and scientific research institutions worldwide have established their own facilities and conducted extensive research on circulating water channels and related experiments [2,3].In China, Zhonggu Tang et al. conducted a detailed observation and testing of water flow in a 1/10 scale horizontal model channel test section, and measured the vertical velocity distribution on the centerline of the section located 1/3 away from the front end [4].Gucheng Zhu et al. conducted a comprehensive investigation into the hydrodynamic performance of the circulating water channel, utilizing both numerical methods and experimental verification techniques [5].Their findings provide a solid foundation for subsequent optimization efforts of circulating water channels.Junpei Xia et al. conducted experiments on the velocity distribution in a vertical circulating water channels test section under three flow rates, with and without a wave suppression plate [6].The results demonstrated that the use of a wave suppression plate led to a more uniform velocity distribution compared to its absence.Chen Songge analyzed the influence factors of wall vibration in the power section of circulating water channel [7].Chen Jianping evaluated the quality of the flow field in the test section of the CWC high-speed free-surface circulating water channel [8].Abroad, A M Santos used PIV to evaluate the scope of application of circulating water channel [9].FD Felice used PIV to measure and analyze the wake in the longitudinal plane under the rotation angle of the propeller based on the INSEAN circulation water channel [10].At present, the research literature on the quality of open/closed flow field in the test section of SHSCWC is relatively scarce, and the relevant test and measurement data are lacking.
This study is based on the SHSCWC in the Key Laboratory of Water jet Propulsion Technology, and uses LDV to measure the flow field velocity in the test section, which provides test data support for the water channel flow field quality evaluation and experimental research.

SHSCWC's structure and parameters
As shown in Figure 1, the body of the SHSCWC applied to the key Laboratory of water jet propulsion Technology was mainly composed of the test section, diffusion section, contraction section, corner section, auxiliary pump, and axial flow pump power system group.The common test flow rate of 2 m/s ~10 m/s could be formed in the test section, and the maximum flow rate of the test section could reach 12 m/s.The partial design parameters and technical indicators of the SHSCWC in both open and closed states are presented in Tables 1 and 2, respectively.The measurement of the state of the closed cavitation cylinder in the circulating water channel was achieved by installing a sealed hatch cover in the test section.8 plexiglass observation windows were installed on both sides of the test section to facilitate flow field measurement, as demonstrated in Figure 2. The SHSCWC has the functions of a cavitation water cylinder and free surface circulating water channel, which can be used for the measurement of hydrodynamic characteristics, cavitation, pulsating pressure and noise of the whole ship model with the propeller, such as various surface ships, underwater vehicles and civil ships.It is an important experimental equipment for the study of the hydrodynamic force, cavitation, noise and vibration of ship thrusters.

Principle and measurement system
In this study, the axial flow field velocity of the SHSCWC test section was measured by laser Doppler velodometer (LDV).As shown in Figure 3, the LDV measurement system mainly consists of a data processing system, signal processor, laser, transmitting/receiving probe and splitter.When the size of moving particles in a fluid is small enough and follows well, the velocity of moving particles can represent and reflect the velocity of the fluid.When measuring the velocity of flow field, LDV emits two laser beams with identical optical properties and intersects to form the measuring body.When the moving particle passes through the interference fringe region in the control body with the speed Uy, the separated light and dark light signals will be scattered to any direction in space, so that the Doppler signal that is proportional to the velocity of the moving particle is observed.According to the relationship between Doppler shift frequency and velocity acquired, the velocity of moving particles can be obtained, that is the velocity of the flow field.The relationship between Doppler shift frequency and velocity [11,12] is as follows: 2sin Where fD is the Doppler shift frequency, λ is the laser wavelength, ĸ is the 1/2 intersection Angle of the beam, Uy is the velocity in the beam plane perpendicular to the intersection bisector.The levelness of the free liquid surface is measured by an ultrasonic sensor.As Figure .4 displayed, the ultrasonic sensor will emit a set of high-frequency sound waves, and when the sound waves touch the liquid surface, they will bounce back and be received.The distance h between the liquid surface and the sensor can be obtained by calculating the time from launching to returning of the sound wave and multiplying the propagation speed of the sound wave.

Measurement scheme
The axial velocity measurement of the flow field is carried out in two states: open (with free liquid level state) and closed (cavitation cylinder state).8 kinds of flow speeds (3m/s, 4m/s, 5m/s, 6m/s, 7m/s, 8m/s, 9m/s and 10m/s) are realized in the test section by controlling the speed of the axial flow pump power system and the auxiliary pump.
In both states (open and closed), rectangular coordinate system uov was established on the test section of the circulating water channel, as shown in Figure .5 and Figure .6. Three velocity measurement cross sections (measurement section Ⅰ, measurement section Ⅱ and measurement section Ⅲ) were selected, and the axial (u) distance of the three measurement cross sections was 2100mm, 5300mm and 8500mm respectively.Due to the symmetry of the circulating water channel test section, the actual measurement area is rectangular ABCD.8 velocity measurement points are uniformly selected in the x direction and 7 velocity measurement points are uniformly selected in the z direction.An 8×7 array of measuring points (with a spacing of 100mm) is formed on the measuring area ABCD.The axial velocity of the measuring point array was measured by moving the LDV laser measuring body through the three-dimensional coordinate frame.The measuring time of each measuring point was 2min and the acquisition rate was 667Hz.Non-uniformity Cv is used to evaluate the uniformity of flow field velocity distribution.The expression of non-uniformity Cv [13] is as follows: Where S is the standard deviation of the flow rate of the measuring point array,  V is the average velocity of the measurement point array, Cv is a measure representing the relative variation of flow velocity, the smaller the Cv value, the higher the uniformity of flow field.The instability ẟ of the incoming flow in the test section under two conditions (open and closed) is equal to the ratio of the standard deviation of axial velocity measured at the velocity measuring point (700,600) to the mean value.The expression for the instability ẟ is as follows: Where vi is the measurement value of axial flow velocity at measuring point (700,600), unit is m/s; v is the mean value of 8 measurements of axial flow velocity at the measuring point (700,600), unit is m/s.In the open state, the levelness of the free liquid surface of the test section is the water surface drop at the entrance and exit of the test section.As shown in Figure 7, ultrasonic sensors are arranged at the six monitoring points above the free liquid surface of the circulating flume, respectively.The liquid level height of each monitoring point is obtained through the ultrasonic sensors, and the levelness of the free liquid surface is obtained.Free liquid level β is expressed as follows: Where H1 , H3, H4 and H6 are the liquid level heights of monitoring points 1, 3, 4 and 6 respectively, unit is mm.The turbulence ε of each measurement point in the three cross section measurement areas is calculated in the closed state.Turbulence ε is equal to the ratio of the standard deviation of the measured velocity value to the mean value.The expression of turbulence ε [14] is as follows: Where Ui is a single measurement value of a single point axial flow rate, unit is m/s; U is the average measurement value of single-point axial flow rate (unit: m/s); n is the number of measurements.

Measuring device
The measurement test was carried out on the SHSCWC of the Science and Technology on Water Jet Propulsion Laboratory.The flow velocity of the flow field in the test section was measured by LDV, and the measuring device is illustrated in Figure 8.In the flow field velocity measurement, as shown in Figure .8 (d) and Figure .8 (e), the laser of LDV emited a laser beam with high brightness, high directionality and simple wavelength.The beam splitter divided the laser beam into a pair of monochromatic beams, and used the laser probe to intersect the pair of monochromatic beams into the measurement area of the flow field to form a measuring body.When the particles in the flow field passed through the control body, scattered light was generated, and the scattered light was received by the receiving optical path.The photoelectric conversion module converted the received optical signal into electrical signal, and the electrical signal was processed by the digital signal processor and the Doppler signal was separated from the noise for measurement.The FlowSizer analysis and processing software was connected to the digital signal processor through the firewire, and the flow rate value of the flow field was obtained after the digital signal was processed by the software.The interface of the analysis and processing software is displayed in Figure 8 (f).In order to ensure the measurement accuracy of flow field velocity, sampling parameters of LDV software were set according to the actual operating conditions of circulating water channel, and specific parameters were presented in Table 3.  Ultrasonic sensors were used to measure the levelness of the free liquid surface in the test section.The measuring device is shown in Figure 9.According to the test measurement scheme, 6 ultrasonic sensors are installed above the free liquid surface in the test section of the circulating water channel perpendicular to the liquid surface, and sensor parameters are displayed in Table 4.The measuring surfaces of the six sensors were adjusted to the same height, and the distance between the liquid level and the sensor was ensured to be within the effective detection range.

Results and analysis
In the open measurement, the distribution of axial flow velocity in sections Ⅰ, Ⅱ and Ⅲ were measured by LDV under 8 incoming flow velocities.Then the mean deviation A.D., non-uniformity Cv and free liquid surface levelness β of axial flow velocity on the 3 sections were calculated, results shown in Figure .10 (a, b, d).The instability δ of measuring points (700,600) on section Ⅰ, section Ⅱ and section Ⅲ was measured and calculated respectively under 8 incoming flow velocities, results displayed in Figure 10 (c).By observing Figure 10 (a), it is found that the average deviation of axial flow velocity in the three measurement areas shows a trend of increasing with the increase of incoming flow velocity.As can be seen from Figure .10 (b), the non-uniformity of velocity in the measurement area of section Ⅰ is larger in the velocity zone of 3m/s~4m/s, smaller in the velocity zone of 6m/s~8m/s, and presents a steady trend in the velocity zone of 9m/s~10m/s.The non-uniformity of velocity in section II is larger in 3m/s~4m/s and smaller in 5m/s~10m/s, among which the non-uniformity of velocity 9m/s is the smallest.The non-uniformity of flow velocity in section Ⅲ is relatively stable under 8 different flow velocities, and shows a slightly decreasing trend with the increase of flow velocity.Under 8 flow velocities, the velocity non-uniformity of the three cross section measurement areas is less than 1.0%, which meets the design requirements of the flow velocity non-uniformity of the test section in the open state of the circulating water channel.
As depicted in Figure .10 (c), the instability of flow velocity in the three cross section measurement areas is greater at the flow velocity of 3m/s, and smaller at the flow velocity range of 4m/s to 10m/s, all of which are less than 1.0%, meeting the design requirements for instability of water channel flow field quality.Figure.10 (d) shows that the levelness of the free liquid surface of the circulating water channel is all less than 25mm under the 8 incoming flow speeds, which meets the design requirements for the free liquid surface levelness of the water channel flow field quality.It can be seen from Figure .11 (a) that the average deviation of axial velocity in the measured areas of the three cross sections all presents a trend of increasing with the increase of velocity.Figure .11 (b) demonstrates that the velocity non-uniformity in the measurement area of section Ⅰ is larger in the velocity zone of 3m/s~4m/s, and smaller in the velocity zone of 5m/s~10m/s.The non-uniformity of flow velocity in section II is larger in 3m/s~4m flow velocity zone, smaller in 8m/s~10m flow velocity zone, and shows a decreasing trend in 5m/s~7m/s flow velocity zone.The non-uniformity of the flow velocity in the measurement area of section Ⅲ is larger at the flow velocity 4m/s and 7m/s, and the overall uniformity fluctuation is relatively obvious.Under 8 different flow velocities, the velocity inhomogeneity of the three cross section measurement areas is less than 1.0%, which meets the design requirements of the flow velocity non-uniformity of the test section in the closed state of the circulating water channel.
As can be seen from Figure .11 (c), the instability of the velocity in the measurement area of section Ⅰ is larger in the velocity zone of 3m/s~5m/s, and smaller in the velocity zone of 6m/s~10m/s.The velocity instability of section Ⅱ and section Ⅲ is larger in the velocity range of 3m/s~4m/s, and smaller in the velocity range of 5m/s~10m/s.The instability of the three measurement sections is less than 1.0%, which meets the design requirements of the instability of the water channel flow field quality.Figure. 11 (d) indicates that the maximum turbulence of the three measurement sections is less than 0.5%, and the measurement results are in line with the design index.

Conclusion
Using LDV, this study evaluated the flow field quality of SHSCWC test section in both open and closed states by constructing an array of flow field measuring points, and reached the following conclusions: 1) In the open measurement, under 8 incoming flow speeds, the non-uniformity of axial flow velocity in the three cross section measurement areas is less than 1.0%, the instability is less than 1.0%, and the levelness of the free liquid surface is less than 25mm.The measured results all meet the design requirements of the flow field quality of the SHSCWC in the open state.
2) In the closed measurement, the non-uniformity of axial flow velocity in the three cross section measurement areas is less than 1.0%, the instability is less than 1.0%, and the maximum turbulence is less than 0.5% under the 8 incoming flow speeds.The measured results all meet the design requirements of the flow field quality of the SHSCWC in the closed state.

Figure 2 .
Figure 2. Schematic diagram of the test section.

Figure 3 .
Figure 3. Schematic diagram of LDV measuring system and measuring principle.

Figure 4 .
Figure 4. Schematic diagram of LDV ultrasonic sensor measuring liquid level distance.

Figure 6 .
Figure 6.Schematic diagram of flow rate measuring point array on measuring section of test section.

Figure 8 .
Figure 8. Diagram of measuring device for flow rate test in test section , where (a) Laser, (b) Optical splitter, (c) Transmit/Receive probe, (d) Coordinate control frame, (e) Laser beam, (f) Analysis and processing software.

Figure 9 .
Figure 9. Free liquid level measuring device.

Table 1 .
Partial design parameters and technical indicators of the SHSCWC in the open state.

Table 2 .
Partial design parameters and technical indicators of the SHSCWC in the closed state.