A wake electromagnetic field enhancement detection method based on circulating water tank

Abstract

The present invention relates to the field of electromagnetic detection and provides a method for detecting wake electromagnetic field enhancement based on a circulating water flume. The method comprises: constructing a wake electromagnetic field test device based on a circulating water flume, fixing a model to be tested in the flow channel of the circulating water flume; placing a Helmholtz coil outside the circulating water flume to generate an enhanced background magnetic field; periodically changing the rotational speed of a drive device, thereby periodically changing the water flow velocity in the circulating water flume; measuring the electric and magnetic fields in the target area; and processing and analyzing the obtained signals.

Description

Technical Field

[0001] The present invention relates to the field of electromagnetic detection, and in particular to a wake electromagnetic field enhancement detection method based on a circulating water tank. Background Art

[0002] Seawater has a high electrical conductivity. When a submersible navigates underwater, it causes the conductive seawater to flow in the Earth's magnetic field. This movement cuts through the Earth's magnetic field lines. According to Faraday's law of electromagnetic induction, this process induces an electromotive force in the seawater, generating an electric current. These currents, in turn, generate an induced electromagnetic field, forming a wake electromagnetic field in the seawater. This electromagnetic field has a wide range and long duration. The development of a wake electromagnetic field test device and the corresponding detection methods are of great significance and potential application value for exploring the mechanism of submersible wake electromagnetic fields and the development of new detection technologies.

[0003] Compared to the electromagnetic field of the submersible itself, the wake electromagnetic field generated by a submersible during navigation is relatively weak. Research on this topic is only just beginning, both domestically and internationally, and there are currently few public reports on experimental setups and detection methods. The difficulty in constructing a test setup lies in measuring the weak wake electromagnetic field. Although electromagnetic sensing technology has made significant progress, directly applying these advanced sensors to a wake electromagnetic field test setup still faces several challenges. These challenges are primarily due to the following three factors: First, when a submersible is navigating underwater, the surrounding seawater moves at a low velocity, resulting in a very weak wake electromagnetic field. Second, because the Earth's magnetic field is a steady-state field and the velocity of seawater changes slowly, the wake electromagnetic field is primarily concentrated in the extremely low-frequency band, but electromagnetic sensors have high noise levels at low frequencies. Finally, the low-frequency ambient noise surrounding the test setup can also affect wake electromagnetic field measurements.

[0004] Under the geomagnetic background, the wake electromagnetic field generated by the submersible's navigation is not only very weak and low in frequency, but also easily interfered by the low-frequency electromagnetic noise of the environment. Therefore, there are many challenges when directly applying advanced electromagnetic sensors to the wake electromagnetic field test device. Summary of the Invention

[0005] In order to find a method for effectively detecting and testing the wake electromagnetic field of a submersible, the present invention proposes a wake electromagnetic field enhancement detection method based on a circulating water tank, which improves the testing capability of the device from the following three aspects: First, the main body of the test device is designed based on a circulating water tank. The test model is fixed in the water tank, and the water flow is circulated at a set speed with the help of a power system, so that the wake electromagnetic field can be monitored in real time and long-term testing can be carried out in the working section; second, a signal enhancement technology is designed for the wake electromagnetic field test device, which improves the sensitivity of signal detection and thus enhances the reliability of the test results; third, a signal processing algorithm is designed to extract weak wake electromagnetic field signals. The algorithm can effectively separate useful signal features from environmental noise, thereby improving the recognition capability of wake electromagnetic field signals.

[0006] In summary, the wake electromagnetic field enhancement detection method based on a circulating water tank proposed in the present invention can significantly improve the detection and testing capabilities of the wake electromagnetic field, provide a new technical means for the study of the wake electromagnetic field mechanism and detection methods of submersibles, and has broad application prospects in the field of electromagnetic field characteristic analysis and detection of marine targets.

[0007] Specifically, the present invention provides a method for detecting wake electromagnetic field enhancement based on a circulating water tank, the method comprising:

[0008] Step (1), constructing a wake electromagnetic field test device based on a circulating water tank, wherein the test device at least comprises an annular circulating water tank and a driving mechanism for driving water flow in the water tank;

[0009] Step (2), placing the model to be tested in a circulating water tank, and fixing the model to be tested in the flow channel of the circulating water tank;

[0010] Step (3), setting Helmholtz coils on both sides of the model to be tested outside the circulating water tank, and using the Helmholtz coils to generate an enhanced background magnetic field;

[0011] Step (4), periodically changing the rotation speed of the driving device, thereby periodically changing the water flow speed in the circulating water tank;

[0012] Step (5), using an electric field sensor and a magnetic field sensor to measure the electric field and magnetic field of the target area respectively;

[0013] Step (6) extracts relevant characteristic signals from the measurement data through empirical mode decomposition, filters the obtained relevant characteristic signals, applies fast Fourier transform to perform spectrum conversion on the signals, and obtains the wake magnetic field and wake electric field corresponding to the frequency of water velocity change from the time domain spectrum.

[0014] In a preferred implementation, the rotational speed of the driving device has a variation period between 0.1 Hz and 2 Hz.

[0015] In another preferred implementation, an electric field sensor and a magnetic field sensor are arranged near the central axis of the Helmholtz coil so that a uniform area with a magnetic contour uniformity of more than 95% covers the measuring parts of the electric field sensor and the magnetic field sensor.

[0016] In another preferred implementation, step (5) includes: using a silver / silver chloride type electric field sensor and a fluxgate type magnetic sensor to measure the wake electric field and the wake magnetic field respectively, and converting the analog signals of the sensors into digital signals through a high-resolution data acquisition device.

[0017] In another preferred implementation, the empirical mode decomposition in step (1) is performed using the following formula:

[0018] Among them, c i (t) is the i-th IMF, r n (t) is the final residual term.

[0019] In another preferred implementation, the method further includes: selecting the power of the drive motor to ensure that the upper and lower limits of the water flow velocity are not lower than the set value, using a speed sensor to measure the water flow velocity in the water tank, adjusting the power of the drive motor, determining the steady-state maximum value of the water flow velocity, and the maximum speed of the drive motor when the water flow velocity reaches the steady-state maximum value, removing the speed sensor, inserting an electric field sensor and a magnetic field sensor, and periodically adjusting the speed of the drive motor within a range not exceeding the maximum speed.

[0020] In another preferred implementation, step (6) includes decomposing the obtained electromagnetic signal using an empirical mode decomposition method to retain only the low-frequency signal in the signal.

[0021] In another preferred implementation, the method further comprises preparing the circulating water tank using fiberglass reinforced plastic material.

[0022] In another preferred implementation, the filtering process includes: for a discrete time series x of an electromagnetic field L , let the filtering window length be n=2m+1, that is, the index of the selected electromagnetic field data point is (x -m ,x -m+1 ,...x0,x1,...x m-1 ,x m ), assume that a k-1 degree polynomial is used to fit the electromagnetic field data points within the window:

[0023] y=a0+a1x+a2x 2 +...+a k-1 x k-1

[0024] Then, the n electromagnetic field data points in the window can form a k-order linear equation system:

[0025]

[0026] Where e is the random error of matrix fitting, which can be expressed as:

[0027] Y (2m+1)×1 =X (2m+1)×k ·A k×1 +E (2m+1)×1

[0028] n≥k, the parameter matrix A is calculated by the least squares method, and then the electromagnetic field data point Y is fitted to obtain A as:

[0029]

[0030] is the least squares solution of the parameter matrix A, and then the filtered value of Y for:

[0031]

[0032] Principle Description

[0033] The inventors of this application discovered two ways to enhance the strength of the wake signal during their research on the wake of a submersible: (1) The inventors discovered that the frequency of the wake electromagnetic field is relatively low and is easily affected by the 1 / f noise of the sensor. Therefore, the inventors proposed a signal enhancement technology that causes the fluid to flow at a variable speed in a fixed period, thereby achieving spectrum shifting of the wake electromagnetic field and reducing the measurement difficulty by increasing its frequency. (2) The inventors discovered that the wake electromagnetic field is not only proportional to the fluid velocity, but also to the background magnetic field. In order to reduce the measurement difficulty, another signal enhancement technology is used, namely, increasing the background magnetic field in the wake area by using a magnetic field generator. The magnetic field generator consists of a Helmholtz coil and a current source. The Helmholtz coil is energized by the current source to adjust the background magnetic field around the wake, further improving the accuracy of the measurement.

[0034] The present invention has the following advantages:

[0035] First, this invention utilizes a circulating water tank structure, which offers significant advantages over towing tanks. While towing tanks have limited testing time due to track constraints, the circulating water tank utilizes a power system to circulate water at a constant rate, allowing the test model to be measured in flowing water. This provides the required simulation environment for extended periods without causing towing damage to the model, and the simulation environment is more stable. The ability to conduct long-term testing in the circulating water tank not only facilitates the measurement of the wake electromagnetic field, but also allows for observation and video recording through the tank's observation windows. Furthermore, the circulating water tank offers the advantages of low investment, minimal footprint, and rapid results.

[0036] Second: The present invention designs a signal enhancement technology for a wake electromagnetic field test device. On the one hand, by periodically changing the fluid velocity, spectrum shifting is achieved, and the signal frequency of the wake electromagnetic field is increased, thereby reducing the 1 / f noise impact of the electric field sensor and the magnetic field sensor. On the other hand, a magnetic field generator is used to enhance the background magnetic field in the wake area, increase the intensity of the wake electromagnetic field signal, and reduce its measurement difficulty. The present invention can detect wake electric fields less than 1μV / m and wake magnetic fields less than 1nT. Such weak wake electromagnetic fields cannot be detected at all in the absence of background enhanced magnetic fields and non-periodic changes in water flow.

[0037] Third: This paper designs a data analysis method for extracting weak wake electromagnetic field characteristics. This method combines empirical mode decomposition (EMD), SG filtering, and fast Fourier transform (FFT) techniques. First, EMD extracts relevant characteristic signals from the measured data. SG filtering then further reduces noise and improves signal purity. Finally, FFT performs spectral analysis on the signal, enabling in-depth analysis and identification of the wake electromagnetic field characteristics. This method improves signal recognition and measurement reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Schematic diagram of a wake electromagnetic field test device used in an embodiment of the present invention;

[0039] Figure 2 Schematic diagram of various local structures of the circulating water tank in an embodiment of the present invention, including from left to right: the water tank body, the blades and the transmission shaft, and the bearing;

[0040] Figure 3 This is a schematic diagram of the overall connection of the circulating water tank in an embodiment of the present invention;

[0041] Figure 4 Schematic diagram of the water tank impeller in an embodiment of the present invention;

[0042] Figure 5 This is the subcritical flow in the channel in the embodiment of the present invention;

[0043] Figure 6 This is the speed control UI interface in the embodiment of the present invention;

[0044] Figure 7 This is the placement of the circular Helmholtz coil in the embodiment of the present invention;

[0045] Figure 8 The spatial magnetic field uniform distribution area of ​​the Helmholtz coil in the embodiment of the present invention is divided;

[0046] Figure 9 This is a diagram of the wake electric field spectrum after data processing when the water velocity variation frequency is 0.2 Hz in an embodiment of the present invention;

[0047] Figure 10 This is a spectrum diagram of the wake magnetic field after data processing when the water flow velocity variation frequency is 0.12 Hz in an embodiment of the present invention. DETAILED DESCRIPTION

[0048] The present invention will be described in further detail below with reference to the embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0049] The main idea of ​​the wake electromagnetic field enhancement detection method based on a circulating water tank of the present invention is to design a corresponding test device for the wake electromagnetic field derived from the navigation of a submersible, measure the wake electromagnetic field and perform data analysis, providing a new technical means for the study of the wake electromagnetic field mechanism of the submersible and long-distance detection.

[0050] like Figure 1 As shown in the figure, in this embodiment, the wake electromagnetic field test apparatus consists of three main components: a circulating water tank, a magnetic field generator, and a wake electromagnetic field measurement system. All three components are powered by a current source. The circulating water tank includes the main body, propeller blades, a drive shaft, and a bearing drive motor.

[0051] The circulating water tank, the core structure of the entire device, is constructed from corrosion-resistant, non-conductive, and non-magnetic materials, such as fiberglass. This material not only resists saltwater corrosion but also does not interfere with the measurement of the wake electromagnetic field.

[0052] As attached Figure 2 As shown in the figure, various parts of the circulating water tank are shown, including the circulating water tank body, impellers, transmission shaft and bearings. Figure 3 A schematic diagram of the overall connections for the circulating water tank is shown. To ensure smooth fluid flow, the tank's interior features a streamlined geometry, minimizing fluid resistance and enhancing flow intensity. A test model is secured within the tank's main body, and the circulating fluid motion generates a wake behind the model, generating an electromagnetic field.

[0053] The propeller is another key component, rotating to circulate the fluid. Its design follows the principles of fluid dynamics to maximize efficiency and minimize energy consumption, resulting in a spiral shape. In addition to its shape, the material of the propeller is equally important. High-strength and corrosion-resistant materials, such as high-quality engineering plastics, should be selected to ensure it does not deform or corrode during long-term operation.

[0054] The propeller blades are connected to the external drive device (servo motor) through the drive shaft, which is the core component of the system's power transmission. One end of the drive shaft passes through the outer wall of the circulating water tank and is fixedly connected to the propeller blades, and the other end is connected to the output shaft of the servo motor. If a reducer is used, it is connected to the output shaft of the reducer. The drive shaft not only needs high strength to meet the requirements of high-speed rotation, but also needs to have excellent corrosion resistance. High-quality engineering plastics are selected for materials to provide sufficient corrosion resistance and mechanical strength. In addition, the design of the drive shaft must take into account the dynamic balance of the system to prevent vibration and instability caused by high-speed rotation. In order to improve the flexibility of the system, the drive shaft is designed to be detachable or equipped with a quick connection device to facilitate the installation and maintenance of the equipment.

[0055] Sealed bearings are installed on one or both sides of the circulating water tank. The propeller shaft passes through the sealed bearings and is fixed to the outer wall. The bearings are a key component connecting the drive shaft to the water tank, and their quality directly affects the smooth operation and service life of the entire device. Bearings in the circulating water tank not only support rotating components but also must withstand high loads and resist corrosion and damage. Ceramic bearings are used to adapt to the operating environment.

[0056] In addition to the above components, in order to further improve the overall performance and applicability of the circulating water tank, an automated control system can also be used to accurately control and adjust the flow rate of the brine through a PLC (programmable logic controller).

[0057] The working process is as follows: the speed of the servo motor on the far right is input, and the motor speed is converted into the speed of the blade rotation through a 10:1 reducer, thereby realizing the flow of water in the circulating water tank.

[0058] Perform the power calculation for the circulating water tank input size, taking into account the following assumptions:

[0059] (1) The water flow direction is one-dimensional, and only the steady-state flow velocity is considered;

[0060] (2) Only one blade interacts with the water flow;

[0061] (3) The blade is perpendicular to the flow velocity, as shown in the following figure: Figure 4 shown.

[0062] The process of water flow contacting with impeller is shown in the following figure. Figure 4 and attached Figure 5As shown in Figure 1, the water level before and after the impeller is inconsistent due to the power exchange between the water flow and the impeller. The power of the impeller in shallow flow can be estimated as:

[0063] P=ρ w bd2v2(v1-v2) 2 (1)

[0064] Among them, ρ w is the density of water, here we take the density of seawater ρ w =1.025×10 3 kg / m 3 , b is the width of the impeller, b = 0.14m, d2, v1, v2 are as shown in the attached Figure 5 As shown, d2 is approximately 0.2 m. The motor power is selected to be 750 W, i.e., P = 750 W. d1 and d2 represent the low and high water levels, respectively, during the propulsion process of the impeller. v1 and v2 represent the water flow velocity at the low and high water levels, respectively (also the water flow velocity before and after the impeller).

[0065] v2 and v1 meet a certain ratio, which is related to the specific shape of the water tank. Assuming v2 = C0·v1, C0 is the proportional coefficient of water flow velocity, the formula can be rewritten as:

[0066]

[0067] Here, v1 is the required water velocity, expressed as:

[0068]

[0069] The value of v1 is mainly affected by C0. According to experimental tests, C0 is approximately between 1.5 and 2, so the calculated speed v1 is between 2.35m / s and 4.115m / s. However, considering the power loss, assuming that there is only 400W of effective power, the calculated result is 1.91m / s to 3.337m / s. In addition, considering the kinetic energy loss of water flowing through the water tank, multiplied by the coefficient of 0.8, the upper and lower limits of the water flow speed are approximately between 1.528m / s and 2.669m / s, which meets the requirement that the water flow speed reaches the set value of 1.5m / s.

[0070] The motor control system consists of a control circuit, a servo motor, and a drive shaft. The servo motor drives the propellers to rotate through software control circuit input. The motor power is 750W, and the speed is controlled by the control circuit. The control UI interface is as shown in the attached figure. Figure 6 The motor can achieve variable speed rotation (the frequency of the speed change can be set). This control method can achieve variable flow of water and provide support for the test.

[0071] Preferably, before the test, the power of the driving motor is selected to ensure that the upper and lower limits of the water flow velocity are not lower than the set value, the water flow velocity in the water tank is measured using a speed sensor, the power of the driving motor is adjusted, the steady-state maximum value of the water flow velocity is determined (the steady-state maximum value here refers to the maximum value of the velocity that will not cause excessive splashing of water or large vibration of the equipment and can achieve a certain degree of required flow), and the maximum speed of the driving motor when the water flow velocity reaches the steady-state maximum value, the speed sensor is removed, and the electric field sensor and magnetic field sensor are inserted, and the speed of the driving motor is periodically adjusted within the range not exceeding the maximum speed.

[0072] In this embodiment, the main body of the circulating water tank is made of fiberglass, and the blade bearings and blade baffles are made of high-quality engineering plastics to ensure that no electric and magnetic field interference is introduced. The transmission shaft is fixed to the drive motor through a coupling and connected to the blades by a latch. A splash-proof cover can also be provided on the water tank, and the water tank and the splash-proof cover are fixed with snaps. The bearings on both sides of the blades rotate in conjunction with the bearing base using ceramic bearings. The bearing base is fixed to the water tank through openings, and a retaining ring is used to prevent the bearing from falling off. The bottom of the water tank and the bottom of the motor base are kept at the same level. The base needs to be fixed for shock absorption.

[0073] The effectiveness of this method is revealed through theoretical analysis below.

[0074] The wake motion can be characterized by the balance of mass and momentum, which is described by the continuity equation and the Navier-Stokes equations, as shown in Equation (1):

[0075]

[0076] Where u is the wake velocity, p is the seawater pressure, ρ is the seawater density, and μ is the seawater dynamic viscosity. The left side of the Navier-Stokes equation corresponds to the inertial force, while the terms on the right side correspond to the pressure, viscous force, and external forces acting on the seawater. Under the influence of the Earth's magnetic field, the wake electric field E and the wake magnetic field B satisfy Maxwell's electromagnetic theory:

[0077]

[0078] Among them, equations (2) and (3) are expressions of Maxwell's equations. The tail induced current conduction density is J = σ (E + v0 × B E ), σ is the conductivity of seawater, B E represents the background magnetic field, and v0 is the wake velocity.

[0079] It is generally believed that the conduction current density of seawater is much greater than the displacement current density. Therefore, the second term on the right side of the equal sign in equation (3) can be ignored. The wake electromagnetic field can be expressed as:

[0080] E(r)=v0(r)×B(r)=v0(r)×(B E (r)+B i (r))≈v0(r)×B E (r)(4)

[0081]

[0082] According to equations (4) and (5), it can be seen that the wake electromagnetic field is proportional to the flow velocity. By controlling the periodic variation of the flow velocity, the measurement frequency of the wake electromagnetic field can be increased, thereby reducing the impact of the 1 / f noise of the electric and magnetic field sensors.

[0083] 2. Magnetic field generator

[0084] According to equations (4) and (5), it can also be seen that the wake electromagnetic field is proportional to the background magnetic field. The use of a magnetic field generator can enhance the background magnetic field in the wake area, increase the intensity of the wake electromagnetic field signal, and reduce its measurement difficulty. Figure 7 The arrangement of the Helmholtz coils shown is that the two coils have a diameter of 800 mm and a spacing of 40 mm. A current of 2.5 A is passed through the coils, and the number of turns is 200.

[0085] According to the symmetry of the magnetic field distribution in the Helmholtz coil space, the magnetic field distribution in the entire space can be calculated as long as the magnetic field distribution in the oxy plane is calculated. Figure 8 The left picture is the magnetic field distribution in the oxy plane direction, and the right picture is the magnetic field distribution in the oxy plane By direction. The magnetic contour lines divide the uniform areas of 99%, 95%, and 90% from the inside to the outside.

[0086] Taking the 95% uniformity zone as an example, its overall area resembles a spindle with a maximum width of 13 cm. If the maximum length of the 99% uniformity zone is used as the uniformity zone parameter, the overall area is a diamond-shaped area with a diagonal length of 13 cm × 25 cm. The area is 0.5 × 13 cm × 25 cm = 162.5 cm². This area can be used to determine the sensor size, or to select different Helmholtz coils based on the sensor size, so that the sensor measurement area is within the uniformity zone.

[0087] 3. Wake electromagnetic field measurement system

[0088] Silver / silver chloride electric field sensors and fluxgate magnetic sensors were used to measure the wake electric and magnetic fields, respectively. A high-resolution data logger converted the sensor analog signals into digital signals, facilitating data analysis to extract the wake electromagnetic field signals.

[0089] First, empirical mode decomposition (EMD) was used to remove electromagnetic interference during the test, allowing for a more focused analysis of the low-frequency domain. Next, the SG filter method was used to process the signal, extracting data and removing outliers. This process yielded optimized data. Finally, a fast Fourier transform (FFT) was used to analyze this data in the frequency domain, calculating the wake electromagnetic field intensity corresponding to the frequency of the water velocity changes. This processing method provides frequency-domain information about the wake electromagnetic field.

[0090] (1) Empirical Mode Decomposition (EMD) is an adaptive method for signal decomposition and analysis, suitable for nonlinear and non-stationary signals. This method decomposes a complex signal into a series of essential mode functions (IMFs), each of which represents a different frequency and amplitude feature in the signal, and its marginal frequency variation is relatively stable within a local range.

[0091] The basic idea of ​​EMD is to iteratively extract the intrinsic mode functions (IMFs) from the data, generating an IMF with each iteration. This process continues until the resulting IMFs satisfy two conditions for intrinsic mode functions: the number of extreme points is equal to or at most one more than the number of zero points over the entire signal range; and within each local interval, the maximum and minimum points alternate, and the sum of the absolute values ​​of the differences is minimized. Ultimately, the accumulated IMFs can be used to approximately reconstruct the original signal.

[0092] The core formula of EMD is as follows:

[0093] For the original electromagnetic field signal x(t), its EMD decomposition process can be expressed as:

[0094]

[0095] Among them, c i (t) is the i-th IMF, r n (t) is the final residual term.

[0096] (2) The SG filter is used for smoothing and denoising data streams. It is a time-domain filtering method based on local polynomial least squares fitting. Its most significant feature is that it can maintain the shape and width of the signal while filtering out noise. As an improvement to the motion smoothing algorithm, it is mainly used to eliminate burrs and has a significant effect on improving the signal-to-noise ratio.

[0097] SG filtering is an improvement of smoothing filtering. Suppose there is a discrete time series x of electromagnetic field. L , let the filtering window length be n=2m+1, that is, the index of the selected electromagnetic field data point is (x -m ,x -m+1 ,...x0,x1,...x m-1 ,x m), assume that a k-1 degree polynomial is used to fit the electromagnetic field data points in the window, that is, assume that these points are electromagnetic field data points on a k-1 degree polynomial:

[0098] y=a0+a1x+a2x 2 +...+a k-1 x k-1 (7)

[0099] Then the n electromagnetic field data points in the window can form a k-order linear equation system:

[0100]

[0101] Where e is the random error of matrix fitting, which can be expressed as:

[0102] Y (2m+1)×1 =X (2m+1)×k ·A k×1 +E (2m+1)×1 (9)

[0103] To make the equations have a solution, n ≥ k, and the parameter matrix A can be determined by least squares fitting, and then the electromagnetic field data points Y can be fitted. According to the least squares method, A can be obtained as:

[0104]

[0105] is the least squares solution of the parameter matrix A, and then the filtered value of Y for:

[0106]

[0107] By selecting the appropriate window length n and polynomial fitting order k according to the actual application, the smoothing effect of SG filtering can be achieved.

[0108] (3) Using fast Fourier transform, the processed wake electric field and wake magnetic field data are plotted into a spectrum diagram, and the wake magnetic field and wake electric field corresponding to the frequency of water velocity change are obtained from the time domain spectrum diagram. For the electromagnetic field time domain sequence T with a length of m X and frequency domain sequence F Y , its fast Fourier transform calculation formula is as follows:

[0109]

[0110] Among them, W m is one of the nth roots of unity:

[0111] W m =e (-2πi) / m (13)

[0112] As attached Figure 9 and attached Figure 10 As shown in the figure, the wake electric field intensity corresponding to the water velocity change frequency of 0.2Hz, and the wake magnetic induction intensity corresponding to the water velocity change frequency of 0.12Hz. Figure 9 Left picture and attached Figure 10 As can be seen from the left figure, by periodically changing the water velocity and combining it with the enhanced background magnetic field, the weak wake electric field (less than 1μV / m) and wake magnetic field (less than 1nT) can be effectively detected. Figure 9 The right picture and attached Figure 10 As can be seen from the right figure, the weak wake electric field and wake magnetic field cannot be detected without signal enhancement technology and data processing.

[0113] Although the principles of the present invention have been described in detail above in conjunction with the preferred embodiments of the present invention, those skilled in the art should understand that the above embodiments are merely illustrative of the present invention and are not intended to limit the scope of the present invention. The details in the embodiments do not constitute a limitation on the scope of the present invention. Without departing from the spirit and scope of the present invention, any obvious changes such as equivalent transformations and simple substitutions based on the technical solution of the present invention fall within the scope of protection of the present invention.

Claims

1. A wake electromagnetic field enhancement detection method based on a circulating water tank, characterized in that: The method comprises: Step (1), constructing a wake electromagnetic field test device based on a circulating water tank, wherein the test device at least comprises an annular circulating water tank and a driving mechanism for driving water flow in the water tank; Step (2), placing the model to be tested in a circulating water tank, and fixing the model to be tested in the flow channel of the circulating water tank; Step (3), setting Helmholtz coils on both sides of the model to be tested outside the circulating water tank, and using the Helmholtz coils to generate an enhanced background magnetic field; Step (4), periodically changing the rotation speed of the driving device, thereby periodically changing the water flow speed in the circulating water tank; Step (5), using an electric field sensor and a magnetic field sensor to measure the electric field and magnetic field of the target area respectively; Step (6) extracts relevant characteristic signals from the measurement data through empirical mode decomposition, filters the obtained relevant characteristic signals, applies fast Fourier transform to perform spectrum conversion on the signals, and obtains the wake magnetic field and wake electric field corresponding to the frequency of water velocity change from the time domain spectrum.

2. The wake electromagnetic field enhancement detection method based on a circulating water tank according to claim 1 is characterized in that: The rotation speed of the driving device has a variation period between 0.1 Hz and 2 Hz.

3. The wake electromagnetic field enhancement detection method based on a circulating water tank according to claim 1 is characterized in that: An electric field sensor and a magnetic field sensor are arranged near the central axis of the Helmholtz coil so that a uniform area with a magnetic contour uniformity of more than 95% covers the measuring parts of the electric field sensor and the magnetic field sensor.

4. The wake electromagnetic field enhancement detection method based on a circulating water tank according to claim 1 is characterized in that: The step (5) comprises: using a silver / silver chloride type electric field sensor and a fluxgate type magnetic sensor to measure the wake electric field and the wake magnetic field respectively, and converting the analog signals of the sensors into digital signals through a high-resolution data collector.

5. The wake electromagnetic field enhancement detection method based on a circulating water tank according to claim 1 is characterized in that: The empirical mode decomposition in step (1) is performed using the following formula: Among them, c i (t) is the i-th IMF, r n (t) is the final residual term.

6. The wake electromagnetic field enhancement detection method based on a circulating water tank according to claim 1 is characterized in that: The method also includes: selecting the power of the drive motor to ensure that the upper and lower limits of the water flow speed are not lower than the set value, using a speed sensor to measure the water flow speed in the water tank, adjusting the power of the drive motor, determining the steady-state maximum value of the water flow speed, and the maximum speed of the drive motor when the water flow speed reaches the steady-state maximum value, removing the speed sensor, inserting an electric field sensor and a magnetic field sensor, and periodically adjusting the speed of the drive motor within a range that does not exceed the maximum speed.

7. The wake electromagnetic field enhancement detection method based on a circulating water tank according to claim 1 is characterized in that: The step (6) includes decomposing the obtained electromagnetic signal using the empirical mode decomposition method, and only retaining the low-frequency signal in the signal.

8. The wake electromagnetic field enhancement detection method based on a circulating water tank according to claim 1 is characterized in that: The method further comprises preparing the circulating water tank using glass fiber reinforced plastic material.

9. The wake electromagnetic field enhancement detection method based on a circulating water tank according to claim 1 is characterized in that: The filtering process includes: for a discrete time series x of electromagnetic field L , let the filtering window length be n=2m+1, that is, the index of the selected electromagnetic field data point is (x -m ,x -m+1 ,...x0,x1,...x m-1 ,x m ), assume that a k-1 degree polynomial is used to fit the electromagnetic field data points within the window: y=a0+a1x+a2x 2 ++...+a k-1 x k-1 Then, the n electromagnetic field data points in the window can form a k-order linear equation system: Where e is the random error of matrix fitting, which can be expressed as: AND (2m+1)×1 =X (2m+1)×k ·TO k×1 +E (2m+1)×1 n≥k, the parameter matrix A is calculated by the least squares method, and then the electromagnetic field data point Y is fitted to obtain A as: is the least squares solution of the parameter matrix A, and then the filtered value of Y for:

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