Measurement Method of Electromagnetic Scattering Characteristics of Plasma Sheath and Wake in Shock Tunnel Model

Abstract

This specification provides a method for measuring the electromagnetic scattering characteristics of a plasma sheath and wake of a shock tunnel model. This method relates to the field of high-enthalpy shock tunnel testing. High-enthalpy shock tunnel test sections are confined, leading to wall scattering clutter interference during testing. Additionally, flow interference from the model's tail support zone affects the accuracy of electromagnetic scattering measurements of the model and its surrounding flow field. This specification utilizes a large-size free-piston high-enthalpy shock tunnel, combined with heavy-model free-flight technology, and employs time-frequency and spatial filtering and high-resolution scattering center techniques to measure the electromagnetic scattering characteristics of the model, its surrounding flow field, and its wake. This invention not only reduces or even eliminates the influence of confined space walls on electromagnetic scattering but also eliminates the reflection of electromagnetic scattering by the model's support rods, enabling precise measurement of electromagnetic scattering from high-speed targets.

Description

Technical Field

[0001] This document relates to the field of hypersonic aerodynamics testing technology, and in particular to a method for measuring the electromagnetic scattering characteristics of a plasma sheath and wake of a shock tunnel model. Background Technology

[0002] When a high-speed aircraft travels at hypersonic speeds in near-space, the surrounding gas is heated by shock waves or slowed down by viscous drag, causing the temperature around the aircraft to rise rapidly. Under high temperatures, different air components around the aircraft undergo chemical reactions, resulting in ionization and forming a plasma flow containing free electrons, ions, and neutral particles that surround the aircraft. Additionally, the high temperature causes ablation of the aircraft's surface materials, producing ablation particles. These ablation particles also undergo partial ionization due to the high temperature, and the resulting electrons, ions, and neutral particles mix with the plasma flow, ultimately forming a plasma sheath. This plasma sheath can interfere with radar monitoring and early warning of flying targets and affect normal communication between monitoring stations and the aircraft, causing a communication "blackout." Limitations in our understanding of hypersonic flow physics lead to inappropriate physical modeling, resulting in uncertainties in numerical calculations. Therefore, experimental research into the laws governing hypersonic flow is crucial. Due to the high cost of flight testing, much of the experimental research needs to be conducted in ground-based simulation facilities.

[0003] To study the electromagnetic scattering characteristics of plasma sheaths and wakes of uninterrupted aircraft in clean air, ground simulation using a high-enthalpy shock tunnel is required. This invention utilizes a large-size free-piston high-enthalpy shock tunnel, employing techniques such as free-flight of a re-model without irritation, re-model heating, and transient electromagnetic scattering characteristic testing to measure the electromagnetic scattering characteristics of the high-enthalpy shock tunnel model and its surrounding flow field. Summary of the Invention

[0004] This specification provides one or more embodiments of a method for measuring the electromagnetic scattering characteristics of a plasma sheath and wake in a shock tunnel model, including:

[0005] S1. Modify the wind tunnel test section, build a microwave anechoic chamber, and perform static calibration on the microwave anechoic chamber;

[0006] S2. Equipped with a suspended heavy model hanging wire release system, the test model is suspended in the wind tunnel, and a heavy model recovery device is built at the same time.

[0007] S3. Establish a high-frequency transient laser schlieren system to monitor the motion state of the experimental model;

[0008] S4. Build a high-frequency electromagnetic scattering test system to measure the electromagnetic scattering signal when a high-enthalpy gas flows through the test model;

[0009] S5. Establish a synchronous timing control system. The synchronous timing control system is connected to the heavy model hanging wire release system, the high-frequency transient shock wave schlieren system, the high-frequency electromagnetic scattering test system and the data processing system. When the wind tunnel test section is started, the high-frequency transient laser schlieren system and the high-frequency electromagnetic scattering test system are controlled to work by the airflow pressure change of the compression tube or shock tube as the trigger signal.

[0010] S6. The static RCS measurement system is controlled by the test control software to quickly and accurately capture the scattered echo of the test model target and accurately extract the radar cross section (RCS) of the target.

[0011] S7. When measuring the electromagnetic scattering characteristics of the ablation model, the test model is preheated. After heating, the measurement is performed to obtain the electromagnetic scattering information of the ablation model.

[0012] Furthermore, the microwave anechoic chamber is constructed using modular pyramidal absorbing materials.

[0013] Furthermore, the establishment of a high-frequency transient laser schlieren system to monitor the motion state of the test model specifically involves: measuring the motion state of the model, the distance of the detached shock wave in the surrounding flow field, and the wake using laser schlieren technology, and determining the effective test time based on the laser schlieren technology.

[0014] Furthermore, the static RCS measurement system mainly includes a radio frequency (RF) subsystem, a turntable control subsystem, and an instrument automatic control subsystem. The RF subsystem mainly includes a vector network analyzer, a power amplifier, a directional coupler, and an antenna. The vector network analyzer has a built-in signal source, and its RF output signal is amplified by the power amplifier and then connected to the directional coupler. The antenna includes a transmitting antenna and a receiving antenna. The signal at the coupling end of the directional coupler is used as a reference signal and connected to the reference port of the vector network analyzer. The direct signal of the directional coupler is transmitted through the transmitting antenna to irradiate the target. The receiving antenna collects the echo signal of the target and sends it to the test port of the network analyzer. The test signal is compared with the reference signal in amplitude and phase. Based on time, frequency, and spatial filtering and high-resolution scattering center technology, the amplitude and phase information of the target echo signal is obtained.

[0015] Furthermore, the specific method for rapidly and accurately capturing the scattered echo of the experimental model target and precisely extracting the target's radar cross section (RCS) is as follows:

[0016] Quasi-single-base RCS measurement is employed, using a relative comparison method to obtain the RCS. This involves measuring the maximum echo power of a standard body (Lumberjack sphere) under the same test conditions and comparing it with the echo power of the test target. Since the RCS of the standard body is known, the RCS of the target at various locations and frequencies can be calculated. The calculation formula is as follows:

[0017]

[0018] Where: σ T For target RCS; P T P represents the target echo power. S σ represents the maximum target echo power. S The RCS of the standard body.

[0019] Furthermore, the calculated RCS of the target at each direction and frequency point is specifically as follows:

[0020] The RCS of the target in different azimuth regions is measured in the angular domain. Specifically, the target is placed on a turntable and rotated at a set azimuth angle. The echo signal of the target at each azimuth angle under the set frequency is tested synchronously by the receiver. The angular domain RCS of the target is obtained through calibration.

[0021] For frequency domain RCS measurement, specifically, a turntable is used to control the target azimuth angle. Frequency sweep measurements are performed on the target at the set azimuth angle, and the target's frequency domain RCS is obtained through calibration.

[0022] Furthermore, the specific method for preheating the test model is as follows: if the test model is a metal model, direct resistance heating is used; if it is a non-metallic model, indirect resistance heating is used. A heating element is made of a special alloy material or non-metallic material, and the heating element generates heat energy to transfer the heat to the object being heated.

[0023] Furthermore, the surface temperature of the ablation model is measured using thermal environment infrared thermography. A blackbody is used to calibrate the infrared thermal imaging system on-site, converting the given level value of the infrared thermal imager into the corresponding temperature value. During calibration, the blackbody is installed on the center line of the wind tunnel test section, while its radiation surface is located at the center of the infrared thermal imager's field of view.

[0024] Furthermore, the transmitting antenna is fixed inside the wind tunnel test section and illuminates the model head. The antenna cable is led out from the optical window through a small hole along the wind tunnel wall. The receiving antenna, vector network instrument, computer and other equipment are placed on the optical platform outside the wind tunnel test section. The wind tunnel provides a synchronous trigger signal to control the test equipment to start working. The receiving antenna receives the echo signal through the optical window and stores and processes it through the computer.

[0025] Furthermore, the effective test time for the flow field is 10ms, the test time is set to 1s, and the trigger signal is set to a delay of 150-250ms.

[0026] This invention utilizes a large-size high-enthalpy shock tunnel, combined with interference-free wake re-model free-flight technology, re-model heating technology, and transient electromagnetic scattering characteristic testing technology, to measure the electromagnetic properties of the plasma sheath and its wake. This invention can not only study the electromagnetic scattering characteristics of the plasma sheath in the leading edge region of the model, but also conduct research on the electromagnetic scattering characteristics of the model wake. At the same time, it can accurately measure the electromagnetic scattering characteristics of the plasma sheath and wake of the ablation model, solving the problem of accurate measurement of the model, plasma sheath, and wake.

[0027] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in one or more embodiments of this specification or in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this specification. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 A flowchart illustrating a method for measuring the electromagnetic scattering characteristics of a plasma sheath and wake in a shock tunnel model, provided for one or more embodiments of this specification.

[0030] Figure 2 This is a schematic diagram of the wind tunnel test section structure for a method of measuring the electromagnetic scattering characteristics of a shock tunnel model plasma sheath and wake, provided for one or more embodiments of this specification. Detailed Implementation

[0031] To enable those skilled in the art to better understand the technical solutions in one or more embodiments of this specification, the technical solutions in one or more embodiments of this specification will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this specification, and not all of the embodiments. Based on one or more embodiments of this specification, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of this document.

[0032] Method Implementation Examples

[0033] According to an embodiment of the present invention, a method for measuring the electromagnetic scattering characteristics of a plasma sheath and wake in a shock tunnel model is provided. Figure 1This document presents a flowchart illustrating a method for measuring the electromagnetic scattering characteristics of a plasma sheath and wake in a shock tunnel model, provided for one or more embodiments of this specification. Figure 2 A schematic diagram of the wind tunnel test section structure for a method of measuring the electromagnetic scattering characteristics of a shock tunnel model plasma sheath and wake, provided for one or more embodiments of this specification, is shown below. Figure 1 , Figure 2 As shown, the method for measuring the electromagnetic scattering characteristics of the plasma sheath and wake of a shock tunnel model according to an embodiment of the present invention specifically includes:

[0034] S1. Modify the wind tunnel test section, build a microwave anechoic chamber, and perform static calibration on the microwave anechoic chamber.

[0035] A microwave anechoic chamber was built inside the wind tunnel test section, and modular pyramidal absorbing materials were used for bonding. The absorbing materials can withstand temperatures above 500K and will not fall off under strong scouring conditions, and their absorbing performance will not change.

[0036] S2. Equipped with a suspended heavy model release system, the test model is suspended in the wind tunnel, and a heavy model recovery device is built at the same time.

[0037] The suspension wire used in the suspended heavy model release system is prone to breakage under high temperature conditions. It is ensured that the wire can break when the high temperature airflow passes through, and the wire breaks with little or no smoke. The suspended heavy model release system is connected to the angle of attack mechanism and does not come into contact with the test section wall. The receiving material of the heavy model recovery system is an elastic, flexible, and high temperature resistant material.

[0038] S3. Establish a high-frequency transient laser schlieren system to monitor the motion state of the experimental model.

[0039] The model's motion state, the distance of the detached shock wave in the surrounding flow field, and the wake are measured using laser schlieren technology. The effective test time is determined based on the laser schlieren technology.

[0040] S4. Build a high-frequency electromagnetic scattering test system to measure the electromagnetic scattering signal when a high-enthalpy gas flows through the test model.

[0041] S5. Establish a synchronous timing control system. The synchronous timing control system is connected to the heavy model hanging wire release system, the high-frequency transient shock wave schlieren system, the high-frequency electromagnetic scattering test system and the data processing system. When the wind tunnel test section is started, the high-frequency transient laser schlieren system and the high-frequency electromagnetic scattering test system are controlled to work by using the airflow pressure change of the compression tube or shock tube as the trigger signal.

[0042] S6. The static RCS measurement system is controlled by the test control software to quickly and accurately capture the scattered echo of the test model target and accurately extract the radar cross section (RCS) of the target.

[0043] The static RCS measurement system mainly includes a radio frequency (RF) subsystem, a turntable control subsystem, and an instrument automatic control subsystem. The RF subsystem mainly includes a vector network analyzer, a power amplifier, a directional coupler, and an antenna. The vector network analyzer has a built-in signal source, and its RF output signal is amplified by the power amplifier and then connected to the directional coupler. The signal at the coupling end of the directional coupler is used as a reference signal and connected to the reference port of the vector network analyzer. The antenna includes a transmitting antenna and a receiving antenna, such as... Figure 2 As shown, the transmitting antenna is fixed inside the wind tunnel test section and illuminates the head of the model. The antenna cable is led out through a small hole in the optical window along the wind tunnel wall. The receiving antenna, vector network analyzer, computer, and other equipment are placed on the optical platform outside the wind tunnel test section. The wind tunnel provides a synchronous trigger signal to control the operation of the test equipment. The receiving antenna receives the echo signal through the optical window and stores and processes it through the computer. The direct signal of the directional coupler irradiates the target outward through the transmitting antenna. The receiving antenna collects the echo signal of the target under test and sends it to the test port of the network analyzer. The test signal is compared with the reference signal in amplitude and phase. Based on time, frequency, and spatial filtering and high-resolution scattering center technology, the amplitude and phase information of the target echo signal is obtained.

[0044] The specific method for rapidly and accurately capturing the scattered echo of the test model target and precisely extracting the target's radar cross section (RCS) is as follows:

[0045] Quasi-single-base RCS measurement is employed, using a relative comparison method to obtain the RCS. This involves measuring the maximum echo power of a standard body (Lumberjack sphere) under the same test conditions and comparing it with the echo power of the test target. Since the RCS of the standard body is known, the RCS of the target at various locations and frequencies can be calculated. The calculation formula is as follows:

[0046]

[0047] Where: σ T For target RCS; P T P represents the target echo power. S σ represents the maximum target echo power. S The RCS of the standard body.

[0048] The calculated RCS of the target at each direction and frequency is as follows:

[0049] The RCS of the target in different azimuth regions is measured in the angular domain. Specifically, the target is placed on a turntable and rotated at a set azimuth angle. The echo signal of the target at each azimuth angle under the set frequency is tested synchronously by the receiver. The angular domain RCS of the target is obtained through calibration.

[0050] For frequency domain RCS measurement, specifically, a turntable is used to control the target azimuth angle. Frequency sweep measurements are performed on the target at the set azimuth angle, and the target's frequency domain RCS is obtained through calibration.

[0051] S7. When measuring the electromagnetic scattering characteristics of the ablation model, the test model is preheated. After heating, the measurement is performed to obtain the electromagnetic scattering information of the ablation model.

[0052] The specific method for preheating the experimental model is as follows:

[0053] If the test model is a metal model, direct resistance heating is used; if it is a non-metal model, indirect resistance heating is used. A heating element is made of a special alloy material or non-metal material, which generates heat energy and transfers the heat to the object being heated.

[0054] Furthermore, the surface temperature of the ablation model is measured using thermal environment infrared thermography. A blackbody is used to calibrate the infrared thermal imaging system on-site, converting the given level value of the infrared thermal imager into the corresponding temperature value. During calibration, the blackbody is installed on the center line of the wind tunnel test section, while its radiation surface is located at the center of the infrared thermal imager's field of view.

[0055] In this embodiment, the effective test time for the flow field is 10ms, the test time is set to 1s, and the trigger signal is set to a delay of 150-250ms.

[0056] The beneficial effects of this invention are as follows:

[0057] This invention utilizes a large-size high-enthalpy shock tunnel, combined with interference-free wake re-model free-flight technology, re-model heating technology, and transient electromagnetic scattering characteristic testing technology, to measure the electromagnetic properties of the plasma sheath and its wake. This invention can not only study the electromagnetic scattering characteristics of the plasma sheath in the leading edge region of the model, but also conduct research on the electromagnetic scattering characteristics of the model wake. At the same time, it can accurately measure the electromagnetic scattering characteristics of the plasma sheath and wake of the ablation model, solving the problem of accurate measurement of the model, plasma sheath, and wake.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for measuring electromagnetic scattering characteristics of a shock tunnel model plasma sheath and wake, characterized in that, The application relates to a wind tunnel test method for measuring the electromagnetic scattering characteristics of a test model. S1, reconstructing a test section of a wind tunnel and building a microwave anechoic chamber, and statically calibrating the microwave anechoic chamber; S2, loading a suspension type heavy model hanging line release system, suspending the test model in the wind tunnel, and building a heavy model recovery device; S3, establishing a high-frequency transient laser schlieren system to monitor the motion state of the test model; The model motion state, the surrounding flow field detached shock wave distance and the wake are measured through the laser schlieren technology, and the effective test time is determined according to the laser schlieren technology; S4, building a high-frequency electromagnetic scattering test system to measure the electromagnetic scattering signal when high-enthalpy airflow flows through the test model; S5, establishing a synchronous time sequence control system, which is connected with the heavy model hanging line release system, the high-frequency transient laser schlieren system, the high-frequency electromagnetic scattering test system and a data processing system, and when the test section of the wind tunnel is started, the airflow pressure change of the compression pipe or the shock tube is used as a trigger signal to control the high-frequency transient laser schlieren system and the high-frequency electromagnetic scattering test system to work; S6, controlling the static RCS measurement system through test control software to quickly and accurately capture the test model target scattering echo and accurately extract the radar scattering cross section (RCS) of the target; S7, when the electromagnetic scattering characteristics of the ablation model are measured, the test model is preheated, and after the heating is completed, the measurement is carried out to obtain the electromagnetic scattering information of the ablation model.

2. The method of claim 1, wherein, The microwave anechoic chamber is pasted with modularized corner pyramid type wave absorbing materials.

3. The method of claim 1, wherein, The static RCS measurement system mainly comprises an RF subsystem, a turntable control subsystem and an instrument automatic control subsystem; the RF subsystem mainly comprises a vector network analyzer, a power amplifier, a directional coupler and an antenna; the vector network analyzer has a built-in signal source, the RF output signal of which is connected to the directional coupler after being amplified by the power amplifier, and the antenna comprises a transmitting antenna and a receiving antenna; the signal at the coupling end of the directional coupler is connected to the reference port of the vector network analyzer as a reference signal, the straight-through signal of the directional coupler is radiated to the target through the transmitting antenna, the receiving antenna collects the echo signal of the measured target and sends the echo signal into the test port of the network analyzer, the test signal and the reference signal are compared in amplitude and phase, the amplitude and phase information of the target echo signal is obtained based on time, frequency and space domain filtering and scattering center high resolution technology.

4. The method of claim 1, wherein, The specific method for quickly and accurately capturing the test model target scattering echo and accurately extracting the radar scattering cross section (RCS) of the target is as follows: Quasi-single-base RCS measurement is adopted, and the RCS is measured by using a relative comparison method, that is, a standard body under the same test state is measured to obtain the maximum value of the echo power, and the echo power of the test target is compared; since the RCS of the standard body is known, the RCS of the target at each azimuth and each frequency point can be calculated, and the calculation formula is as follows: (1) where: is the target RCS; is the target return power; is the target return power maximum; is the standard body RCS.

5. The method of claim 4, wherein, The specific method for calculating the RCS of the target at each azimuth and each frequency point is as follows: The RCS in different azimuth angle regions is measured in the angle domain, specifically, the target is placed on a turntable to rotate at a set azimuth angle, the echo signal of the target at each azimuth angle under a set frequency is synchronously tested through a receiver, and the angle domain RCS of the target is obtained through calibration. For the frequency domain RCS measurement, the target azimuth angle is controlled by a turntable, and the frequency sweeping measurement is performed at a set azimuth angle, and the target frequency domain RCS is obtained through calibration.

6. The method of claim 1, wherein, The specific method for preheating the test model is that if the test model is a metal model, direct resistance heating is used, and if the test model is a non-metal model, indirect resistance heating is used, that is, an alloy material or a non-metal material is made into a heating element, heat energy is generated by the heating element, and heat is transferred to the heated object.

7. The method of claim 1, wherein, The surface temperature of the ablation model is measured by using a thermal environment infrared thermal imaging technology, a surface source black body is used to calibrate the infrared thermal imaging system on site, and the level value given by the infrared thermal imager is converted into a corresponding temperature value. During calibration, the black body is installed on the center line of the wind tunnel test section, and the radiation surface is arranged at the center of the field of view of the infrared thermal imager.

8. The method of claim 3, wherein, The transmitting antenna is fixed in the wind tunnel test section and irradiates from the model head, and the antenna cable is led out from the optical window hole along the wind tunnel wall; the receiving antenna, the vector network analyzer and the computer are arranged on the optical platform outside the wind tunnel test section, and the test equipment is controlled to start working by a synchronous trigger signal provided by the wind tunnel; the receiving antenna receives the echo signal through the optical window, and the computer stores and processes the echo signal.

9. The method of claim 1, wherein, The effective test time of the flow field is 10 ms, the test time is set to 1 s, and the trigger signal is set to a delay of 150-250 ms.

Images