Wake flow electromagnetic field enhancement detection method based on circulating water tank

Abstract

The invention relates to the field of electromagnetic detection, and provides a wake flow electromagnetic field enhancement detection method based on a circulating water tank. The method comprises the following steps: constructing a wake flow electromagnetic field test device based on a circulating water tank, and fixing a to-be-tested model in a flow channel of the circulating water tank; a Helmholtz coil is arranged on the outer side of the circulating water tank to generate an enhanced background magnetic field; the rotating speed of the driving device is periodically changed, so that the water flow speed in the circulating water tank is periodically changed; measuring an electric field and a magnetic field of the target area; and the obtained signals are processed and analyzed.

Description

Technical Field

[0001] The 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 high conductivity. When a submersible travels underwater, it causes the conductive seawater to flow in the geomagnetic field. This movement cuts the magnetic lines of the geomagnetic field. According to Faraday's law of electromagnetic induction, this process induces electromotive force in the seawater, thereby generating current. These currents will in turn generate an induced electromagnetic field, forming a wake electromagnetic field in the seawater. This electromagnetic field has a wide range of action and a long duration. The development of a wake electromagnetic field test device and the study of corresponding detection methods are of great significance and potential application value for exploring the mechanism of the wake electromagnetic field of a submersible and the development of new detection technologies.

[0003] Compared with the electromagnetic field of the submersible body, the wake electromagnetic field generated by the submersible is relatively weak. Research on this has just started at home and abroad, and there are few public reports on test equipment and detection methods. The difficulty in building a test device lies in how to measure the weak wake electromagnetic field. Although electromagnetic sensing technology has made great progress, there are still some challenges in directly applying these advanced sensors to the wake electromagnetic field test device. There are mainly three reasons: First, when the submersible is sailing underwater, the movement speed of the surrounding seawater is relatively low, resulting in a very weak wake electromagnetic field; second, because the geomagnetic field is a steady-state magnetic field and the movement speed of seawater changes slowly, the wake electromagnetic field is mainly concentrated in the extremely low frequency band, but the electromagnetic sensor has high noise at low frequencies; finally, the low-frequency environmental noise around the test device will also affect the measurement of the wake electromagnetic field.

[0004] Under the geomagnetic background, the wake electromagnetic field generated by the navigation of a submersible is not only very weak and of low frequency, but is also easily interfered by the low-frequency electromagnetic noise of the environment. Therefore, there are many challenges when directly applying advanced electromagnetic sensors to wake electromagnetic field test devices. 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, and the test model is fixed in the water tank. 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 the 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 wake electromagnetic field enhancement detection method 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 the magnetic field of the target area respectively;

[0013] Step (6) extracts relevant characteristic signals from the measurement data by 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 rotation 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 also includes: selecting the power of the driving 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 driving motor, determining the steady-state maximum value of the water flow velocity, and the maximum speed of the driving 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 driving 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 low-frequency signals in the signal.

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

[0022] In another preferred implementation, the filtering process includes: for a discrete time series x of an electromagnetic field L , assuming that the filtering window length is 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 ), assuming that a k-1 degree polynomial is used to fit the electromagnetic field data points in 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 thus the filtered value of Y for:

[0031]

[0032] Principle

[0033] The inventors of this application discovered two ways to enhance the wake signal strength during the study of the wake of a submersible; (i) The inventors found 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 allows the fluid to flow at a variable speed in a fixed period, thereby achieving a spectrum shift of the wake electromagnetic field and reducing the measurement difficulty by increasing its frequency. (ii) The inventors found 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, that is, to increase the background magnetic field in the wake area through 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 measurement accuracy.

[0034] The present invention has the following advantages:

[0035] First: The present invention adopts a circulating water tank structure, which has significant advantages over the towing water tank. Due to track restrictions, the test time of the towing water tank is limited, while the circulating water tank allows water to circulate at a certain speed through the power system, so that the test model can be measured in flowing water. It can provide the required simulation environment for a long time without causing towing damage to the model, and the simulation environment is more stable. The feature of the circulating water tank that can be tested for a long time not only facilitates the measurement of the wake electromagnetic field, but also allows observation and photography through the observation window of the tank body. In addition, the circulating water tank also has the advantages of small investment, small footprint and quick 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 influence 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, while such a weak wake electromagnetic field cannot be detected at all in the case of no background enhanced magnetic field and non-periodic water flow.

[0037] Third: The present invention designs a data analysis method for extracting weak wake electromagnetic field characteristics. This method combines empirical mode decomposition, SG filtering and fast Fourier transform technology. First, the relevant characteristic signals are extracted from the measurement data through empirical mode decomposition, and then the SG filter is used to further reduce the noise and improve the purity of the signal. Finally, the fast Fourier transform is applied to perform spectrum analysis on the signal, so as to deeply analyze and identify the characteristics of the wake electromagnetic field. 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 the embodiment of the present invention, including from left to right: the water tank body, the paddle 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 a water tank impeller in an embodiment of the present invention;

[0042] Figure 5 It 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 arrangement 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] Fig. 9 This is a spectrum diagram of the wake electric field after data processing when the water flow velocity change frequency is 0.2 Hz in an embodiment of the present invention;

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

[0048] The present invention is further described in detail below in conjunction with 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 during the navigation of a submersible, measure the wake electromagnetic field and perform data analysis, so as to provide 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 this embodiment, the wake electromagnetic field test device includes three parts, namely, a circulating water tank, a magnetic field generator, and a wake electromagnetic field measurement system. The three parts are all powered by a current source. The circulating water tank includes a circulating water tank body, a blade, a transmission shaft, and a bearing drive motor.

[0051] The main body of the circulating water tank is the core structure of the entire device, and is made of corrosion-resistant, non-conductive and non-magnetic materials such as fiberglass. This material can not only resist salt water 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, paddles, transmission shaft and bearings. Figure 3 The overall connection diagram of the circulating water tank is shown. To ensure smooth flow of the fluid, the interior of the water tank is designed with a streamlined geometry to reduce fluid resistance and enhance water flow intensity. The test model is fixed in the main body of the circulating water tank, and the wake is generated behind the model through the circulating motion of the fluid, and the electromagnetic field generated is measured.

[0053] The propeller is another key component that causes the fluid to circulate by rotating. Its design must follow the principles of fluid dynamics to maximize efficiency and minimize energy consumption, and is designed to be spiral. In addition to the 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 that 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 transmission shaft, which is the core component of the system's power transmission. One end of the transmission 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 transmission shaft not only needs high strength to meet the requirements of high-speed rotation, but also needs to have excellent corrosion resistance. In terms of materials, high-quality engineering plastics are selected to provide sufficient corrosion resistance and mechanical strength. In addition, the design of the transmission shaft must consider 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 transmission 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, and the propeller shaft passes through the sealed bearing and is fixed on the outer wall. The bearing is a key component connecting the propeller shaft and the water tank, and its quality directly affects the running stability and service life of the entire device. The bearing is not only responsible for supporting the rotating parts in the circulating water tank, but also needs to withstand high loads and resist corrosion and damage. In order to adapt to the use environment, ceramic bearings are selected.

[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 propeller blade 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 and consider 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 blades are perpendicular to the flow velocity. Figure 4 shown.

[0062] The process of water flow contacting with the impeller is shown in the following figure. Figure 4 and attached Figure 5As shown in the figure, 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, where the density of seawater is taken as ρ w =1.025×10 3 kg / m 3 , b is the width of the impeller, b = 0.14m, d2, v1, v2 as shown in the attached Figure 5 As shown, d2 is approximately 0.2m. The motor power is 750W, that is, P = 750W. d1 and d2 represent the water level of the low water level and the high water level during the blade pushing process, respectively, and v1 and v2 represent the water flow velocity of the low water level and the high water level (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 transmission shaft. The servo motor drives the blades to rotate through software control circuit input. The motor power is 750W, and the speed is controlled through the control circuit. The control UI interface is as shown in the attached figure. Figure 6 The motor can realize variable speed rotation (the frequency of the variable speed can be set), and this control method can realize the variable flow of water flow, providing support for the test.

[0071] Preferably, before testing, 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), as well as 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 the 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 through 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 buckles. The bearings on both sides of the blades are rotated 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 to prevent shock.

[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] Among them, 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, and the terms on the right side of the equation correspond to the pressure, viscosity, and external forces acting on the seawater. Under the action of the geomagnetic 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 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, and 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 change of the flow velocity, the measurement frequency of the wake electromagnetic field can be increased, thereby reducing the influence 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 strength of the wake electromagnetic field signal, and reduce the difficulty of its measurement. Figure 7 The arrangement of the Helmholtz coils shown in the figure is that the diameter of the two coils is 800 mm, the spacing is 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 On direction, the right picture is the magnetic field distribution in the oxy plane By direction, and the magnetic contour lines divide the uniform areas of 99%, 95%, and 90% from the inside to the outside.

[0086] Taking the 95% uniform area as an example, its overall area is similar to a spindle shape with a maximum width of 13 cm. If the maximum length of the 99% uniform area is used as the definition of the uniform area parameter, the overall area is a diamond area with a diagonal length of 13 cm × 25 cm. The area is 0.5 × 13 cm × 25 cm = 162.5 cm2. The sensor size can be determined based on the area of ​​the area, or different Helmholtz coils can be selected based on the sensor size so that the sensor measurement area is within the uniform area.

[0087] 3. Wake electromagnetic field measurement system

[0088] Silver / silver chloride type electric field sensors and fluxgate type magnetic sensors are used to measure the wake electric field and wake magnetic field respectively. The analog signals of the sensors are converted into digital signals through a high-resolution data logger, which facilitates the extraction of wake electromagnetic field signals by data analysis methods.

[0089] First, the empirical mode decomposition method is used to remove electromagnetic interference during the test so as to focus more on the low-frequency domain. Then, the SG filtering method is used to process the signal, extract data and remove abnormal points. After the above processing, the optimized data is obtained. Finally, the fast Fourier transform is used to perform frequency domain analysis on these data to calculate the wake electromagnetic field intensity corresponding to the frequency of water flow velocity change. This processing method can provide frequency domain information of the wake electromagnetic field.

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

[0091] The basic idea of ​​EMD is to extract the essential mode function from the data through iteration, and each iteration will generate an IMF. This process continues until the obtained IMF meets the two conditions of the intrinsic mode function: in the entire signal range, the number of extreme points is equal to or at most one more than the number of zero points; in each local interval, the maximum and minimum points appear alternately, and the sum of the absolute values ​​of the differences is minimal. Finally, the accumulated IMF can 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 greatest feature is that it can keep the shape and width of the signal unchanged while filtering out noise. As an improvement to the moving 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 an electromagnetic field. L , assuming that the filtering window length is 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 point 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 thus 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. 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 Fig. 9 and attached Fig.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. Fig. 9 Left and attached Fig.10 As can be seen in the left figure, the weak wake electric field (less than 1μV / m) and wake magnetic field (less than 1nT) can be effectively detected through the periodic change of water flow velocity and the enhanced background magnetic field. Fig. 9 The right picture and attached Fig.10 As can be seen from the right figure, the weak wake electric field and wake magnetic field cannot be detected before signal enhancement technology and data processing.

[0113] Although the principles of the present invention are 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 explanations of the exemplary implementations of the present invention, and are not intended to limit the scope of the present invention. The details in the embodiments do not constitute limitations on the scope of the present invention, and any obvious changes such as equivalent transformations, simple replacements, etc. based on the technical solution of the present invention, without departing from the spirit and scope of the present invention, fall within the protection scope 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 the magnetic field of the target area respectively; Step (6) extracts relevant characteristic signals from the measurement data by 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 acquisition device.

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 driving 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 driving motor, determining the steady-state maximum value of the water flow speed, and the maximum speed of the driving 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 driving 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 an empirical mode decomposition method to retain only 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 also includes 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 , assuming that the filtering window length is 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 ), assuming that a k-1 degree polynomial is used to fit the electromagnetic field data points in 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 thus the filtered value of Y for:

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