A method for multiple high-frequency shooting of single phase-locked PIV signals on a rotor test bench

Abstract

The present invention belongs to the technical field of flow field velocity testing and relates to a device for measuring flow field velocity parameters. The present invention discloses a PIV system that uses a single phase-locked signal to trigger camera shooting at high frequency multiple times, and can achieve high-frequency continuous shooting in a rotor channel of a rotating machine. The device comprises a compressor, a camera, rotor blades, a rotor shaft system, an optical fiber sensor, a voltage reduction module, a synchronizer, a laser, a camera, and a laser sheet. The optical fiber sensor outputs a phase-locked signal. The voltage reduction module can reduce the voltage of a 24V high-level signal to a standard control signal voltage of 5V. A signal generator can generate a control signal with a specific frequency and rising edge time under external triggering. The synchronizer is responsible for receiving the control signal transmitted by the signal generator. The high-frequency laser is a dual-cavity Nd:YAG high-frequency laser.

Description

Technical Field

[0001] The present invention belongs to the technical field of flow field velocity testing, relates to a device for measuring flow field velocity parameters, and specifically to a method for triggering a PIV system camera to shoot with a single phase-locked signal at high frequency and multiple times. Background Art

[0002] Currently, in aircraft engine internal flow experimental tests, such as the measurement of three-dimensional flow field parameters in compressor and turbine blade channels, including radial velocity and vortex identification, hot wire and PIV (Particle Image Velocimetry) systems are generally used.

[0003] The PIV system is widely used because it can capture and reproduce the fluid velocity field on the cross section, which is better than the capture of point velocity by hot lines, and can obtain instantaneous or time-averaged velocity field parameters in a wider area.

[0004] The technical principle of the PIV system is to spread tracer particles in the flow field, illuminate the measured section with a laser light source, and record the motion trajectory of the particles on the section through a camera in a relatively short time. According to the image processing algorithm, the length of the particle's motion trajectory in the section is calculated, and then divided by the interval between two photos to calculate the particle's motion speed. The particle velocity measurement of the entire section is completed in a very short time, and the result is a velocity field. The PIV system can be divided into two-dimensional PIV systems according to the dimension of the velocity field it can obtain, as shown in the attached figure. Figure 1 As shown in the attached Figure 2 The two PIV systems share the same principle for acquiring velocity fields, but differ in the number of cameras required to acquire the velocity in the third dimension and their placement.

[0005] The most critical aspect of PIV flow field measurement is the acquisition of velocity field parameters. Besides dimensional differences, the overall operating frequency of the PIV system—the number of images captured in a given timeframe—is also a crucial parameter. In the field of aeroengine internal flow testing, the demand for highly three-dimensional and highly unsteady flows is increasingly pressing for high-frequency SPIV systems. The technological innovations enabled by high acquisition frequencies can lead to breakthroughs in the study of the unsteady characteristics of flow fields within the blade passages of rotating test pieces.

[0006] In the field of aero-engine research, cascade experiments and rotor experiments are widely used. The cascade experiment is to transform the engine blades into stationary blades. The plane cascade experiment and the annular cascade experiment are carried out in the wind tunnel, as shown in the attached figure. Figure 3As shown in the figure. The rotor experiment simulates the flow field of the real compressor and turbine under rotating state and conducts experimental research on it. Figure 3 As shown in the figure, since there are no rotating parts involved, there is no need for triggering of a phase-locked signal, and the maximum shooting frequency of the PIV system can be used for the experiment.

[0007] For the rotor test bench, due to the high-speed rotation of the rotor blades, how to control PIV shooting requires the introduction of an external trigger device. The commonly used external trigger is the phase-locked signal, that is, the rotor outputs a signal for one rotation, or it can output multiple signals for one rotation. The output signal is transmitted to the PIV system to control the synchronous action of the camera and laser to achieve the capture and analysis of the velocity field. The specific process is shown in the attached figure. Figure 4 Currently, the most popular rotor PIV experiment methods mostly use a rotary encoder mounted on the end of the drive motor shaft. This encoder picks up a high-level TTL signal per revolution and transmits it to a synchronizer. The synchronizer, through its external trigger signal acquisition function, triggers its own signal to generate three TTL signals to two cameras and a laser, controlling the laser's pulsed laser emission and the camera's coordinated synchronization for photographic capture. The high-frequency imaging in current PIV systems utilizes a dual-cavity laser xenon lamp to generate high-frequency laser pulses. This is combined with a high-speed camera, combining hardware and software to achieve high-frequency imaging results.

[0008] Current high-frequency PIV systems can fully utilize their capabilities in cascade experiments, enabling the acquisition of flow field velocity data at high frame rates. However, experiments on flow field parameters within a specific rotor channel of a rotating machine require phase-locked technology to externally trigger the synchronizer. The encoder outputs only one high level per revolution, or 360 high levels per revolution. After transmitting this limited number of high levels to the synchronizer, the synchronizer can only output laser and camera control signals based on the number of high levels received. This severely limits the high-frequency capabilities of high-frequency PIV systems. In other words, a 10kHz PIV system operating in the encoder's one high level per revolution mode can capture one image per revolution, and the rotating machine's rotational frequency can remain within 10kHz. If the encoder operates in the 360 ​​high level per revolution mode, capturing 360 images per revolution, the rotational frequency must be increased by 360, based on the 10kHz value.

[0009] The limitation of this characteristic seriously restricts the high-frequency PIV system from testing the unsteady flow in the single rotor channel of rotating machinery. How to realize the function of continuous shooting of a certain channel in the blade experiment has become a difficult problem. Not limited by the number of times the encoder outputs a high level, the PIV system can continuously shoot a specific channel at the highest operating frequency of the PIV system. That is, the encoder or other optoelectronic device outputs a high level once, which enables the PIV system to achieve continuous shooting at a specific high frequency, rather than shooting once at a high level. The so-called limitation problem is that the current PIV system synchronizer external trigger mode cannot arbitrarily set the frequency, pulse width and number of cycles of the 5V TTL signal that controls the laser and camera. The most important limitation contradiction is that the number of cycles cannot be set. This limitation problem causes the entire PIV system to be unable to realize the function of continuously shooting N photos in the rotor channel when the phase-locked signal is input to the synchronizer. The number of flow field photos taken is directly related to the number of cycles of the input 5V TTL signal, and the transmission process of the control signal is attached. Figure 5 shown.

[0010] In summary, existing PIV systems for rotating machinery rotor flow field testing have significant shortcomings and limitations, which can be summarized as follows: 1. The shaft encoder and motor are mated via an interference fit, while the motor and rotor shaft are connected via a spline. Over time, this spline connection can loosen, leading to unstable phase-locked signals. 2. When conducting experiments on rotating machinery rotors, existing PIV systems can only capture a single image when triggered by the rotary encoder, resulting in only an ensemble average result, or the mean flow field, within the rotor channel. This makes it impossible to capture and analyze unsteady flow within the rotor channel. 3. With existing signal control transmission technology, using phase-locked PIV within the rotor channel of a rotating machinery only allows for single-shot capture and analysis of the phase-locked signal, making it difficult to fully utilize the capabilities of high-frequency PIV systems. 4. Due to the limitation of a single phase-locked trigger, existing high-frequency PIV systems require multiple acquisitions of flow field information for analysis, increasing the operating time of the PIV system. 5. Although existing high-frequency PIV systems have high-frequency capabilities, they can only capture the flow field within a single blade channel of a rotating machine, making it difficult to perform reduced-order modal analysis of the flow field, limiting the development of PIV technology and the promotion and application of data processing technology.

[0011] The limitations of the existing SPIV system described above will result in the need to select additional photos based on the phase-locked signal after the flow field information is captured, which is cumbersome and prolongs the data post-processing cycle of the PIV experiment. The limitations of the existing PIV system described above will result in the inability to accurately control the operation of key equipment with a certain lifespan of the PIV system, such as lasers and high-frequency cameras, resulting in the lasers and cameras running for a long time, and most of the photo data is discarded during post-processing, so there is a problem of excessive use of lasers and high-frequency cameras, which reduces the normal use time of the system and shortens the service life of the SPIV system. The reduced-order modal analysis technology of the POD (principal component mode decomposition) and DMD (dynamic mode decomposition) flow field occupies an important position in the flow field reconstruction analysis of impeller machinery and is applied to the post-processing of numerical simulation and experimental results. It is used to obtain the main flow vortex structure and main order modes of the flow field, and quantitatively analyze the dominant flow structure in the flow field, which is of great significance for the unsteady flow field in the tip area of ​​the impeller machinery.

[0012] However, the current SPIV system only takes one photo per week. When the DMD (dynamic mode decomposition) method is used to reduce the order of the dynamic characteristics of the flow field, the main mode is not the continuous flow field, but the flow field after one rotation. There is a phase difference, making it difficult to obtain the true unsteady spectral characteristics of the flow field. Summary of the Invention

[0013] The main technical problem solved by the present invention is: The present invention mainly focuses on solving the limitations of the above-mentioned existing PIV system. By using an external signal generator, the coordinated use of a laser, a high-frequency camera, and an external trigger signal source can be realized, which can achieve precise control of the laser and the high-frequency camera, reduce the workload of post-processing of PIV experimental data, and extend the service life of the PIV system. The configuration of the high-frequency PIV system of the present invention is shown in the attached figure. Figure 6 shown.

[0014] In order to solve the above technical problems, the design scheme of the present invention is as follows:

[0015] 1. The phase-lock signal involved in this invention can be provided by a fiber optic sensor. The phase-lock signal provided by the fiber optic sensor can lock onto any position of a specific rotor blade tip. The phase-lock signal provided by a shaft encoder is the same as that provided by existing PIV systems. However, due to the quality of the encoder output signal, the shaft encoder cannot accurately lock onto the blade channel.

[0016] 2. The high-level signal output from the optical fiber sensor is converted into a 5V TTL signal through a step-down module and transmitted to the external trigger signal input channel of the signal generator.

[0017] 3. After receiving the 5V TTL signal from the step-down module, the signal generator sets the waveform, pulse width, and rising edge time of the external trigger signal to output a TTL signal with a set frequency of 5V. The single phase-locked signal triggering high-frequency multiple triggering to generate a high-frequency 5V TTL signal in the present invention does not have this functional condition in existing PIV systems.

[0018] 4. The signal output by the signal generator can control the synchronizer's synchronous output signal, achieving the purpose and function of aligning the synchronizer and signal generator output signals. The signal generator generates a high-frequency, multiple-trigger signal triggered by a single phase-locked signal, which is then transmitted to the PIV system's synchronization controller. The signal generator disclosed herein enables the PIV system's single phase-locked signal to trigger high-frequency, multiple-shot capture, a feature not available in existing PIV systems.

[0019] 5. The control signal of a specific frequency output from the signal generator is transmitted to the CMOS camera and high-frequency laser, realizing the coordination of the phase-locked signal to the camera and laser machine action, and realizing the function of a single phase-locked signal triggering the camera and high-frequency laser to continuously shoot and obtain the flow field velocity field.

[0020] 6. The continuity of the shooting results ensures the transient continuity of the flow field in the rotor blade channel, making high-quality flow field reduced-order modal analysis possible. The use of flow field reduced-order modal analysis can extract the pulsation frequency information of the flow field from the velocity field, greatly improving the data analysis capability of the PIV system and making it possible to analyze the dynamic frequency domain characteristics of the flow field in the blade channel from the perspective of the field. The transmission process of its control signal is shown in the attached figure. Figure 7 The key device for implementing this technology is a signal generator. This signal generator can be triggered externally to output a 5V TTL signal that can control the operation of the laser and high-frequency camera. The 5V TTL signal's cycle number, frequency, and pulse width can be set, enabling high-speed, controllable in-channel photography of the flow field within a specific rotor blade channel of a rotating fluid machinery.

[0021] Compared with the previous PIV technology, the key core of the present invention is: first, it solves the problems of the existing PIV system in the internal flow test of high-speed rotating machinery, and obtains a method that can get rid of the limitation of single output and single shooting of encoders or photoelectric signals, greatly broadening the application scenarios of high-frequency PIV systems and obtaining more information on the rotor flow field shooting; second, it realizes the conversion of the signal of a single phase-locked trigger synchronizer action into a signal with adjustable frequency, adjustable pulse width, and adjustable signal rise time; third, this trigger signal with controllable frequency, pulse width and rise time controls the synchronizer to act simultaneously, realizing the triggered continuous shooting of the PIV system in the rotor channel of the rotating machinery; fourth, this shooting method maximizes the use of the high-frequency characteristics of the PIV system, and maximizes the capabilities of the high-frequency PIV system in the field of rotating machinery testing; BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a simplified diagram of the two-dimensional particle image velocity measurement (PIV) system configuration.

[0023] Figure 2 This is a simplified diagram of the configuration of the Stereo Particle Image Velocimetry (SPIV) system.

[0024] Figure 3 It is the optical path layout and camera placement of the plane cascade volume particle velocimetry SPIV shooting;

[0025] Figure 4 This is the configuration of the existing high-frequency particle velocimetry PIV system, where 1 is the compressor inlet, 2 is the compressor, 3 is the rotor blade, 4 is the rotor shafting, 5 is the compressor outlet, 6 is the shaft encoder, 7 is the synchronizer, 8 is the laser, 9 is the camera, and 10 is the laser sheet light;

[0026] Figure 5 This is the transmission process of control signals during the operation of the existing high-frequency particle velocimetry (PIV) system.

[0027] Figure 6 This is a schematic diagram of the configuration of a volumetric particle velocimetry (SPIV) system for capturing images with a camera using a single phase-locked signal with high frequency and multiple triggering. The system 1 represents the compressor inlet, 2 represents the compressor, 3 represents the rotor blades, 4 represents the rotor shafting, 5 represents the compressor outlet, 6 represents the shaft encoder, 7 represents the step-down module, 8 represents the signal generator, 9 represents the synchronizer, 10 represents the laser, 11 represents the light guide arm, 12 represents the camera, 13 represents the laser sheet, and 14 represents the optical fiber sensor.

[0028] Figure 7 This is a flow chart of control signals of a particle velocity measurement PIV system of the present invention that uses a single phase-locked signal to trigger a camera for multiple times at high frequency. DETAILED DESCRIPTION

[0029] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0030] Example

[0031] As attached Figure 6 As shown, this embodiment introduces a PIV system that uses a single phase-locked signal to trigger a camera multiple times at high frequency. Tracer particles are seeded into the internal flow channel from the inlet of a rotating machine. A fiber optic sensor installed on the top casing of a rotating blade with special light-absorbing or light-reflecting treatment senses the arrival of the blade and outputs a 24V TTL pulse signal. The pulse signal is reduced to 5V by a step-down module to meet the voltage limit of the signal generator for the external trigger signal. The stepped-down pulse signal is input to the external trigger input port of the signal generator. After receiving the externally triggered 5V TTL pulse signal inside the signal generator, a pulse signal is output at the signal output port according to the set pulse frequency, pulse time width, and pulse rise time. The pulse signal output by the signal generator is transmitted to the signal input terminal of the synchronizer. The synchronizer outputs a pulse signal that can control the operation of the camera and high-frequency laser according to the characteristic frequency of the signal at the input terminal. Under the action of the synchronizer output control signal, the camera and high-frequency laser operate according to the internally set program to capture and analyze the velocity field of the cross section illuminated by the light sheet.

[0032] In this embodiment, the function of the signal generator is to convert a single pulse signal into a pulse signal with settable frequency and period, so as to realize continuous shooting in the rotor channel of the rotating machinery.

[0033] Before using the signal generator, the frequency, period, and rise time of the pulse signal it outputs must be set. The thickness of the light sheet must be adjusted to the optimal range.

[0034] The PIV system needs to be calibrated before use. A calibration plate is used to collect the calibration image, capture the standard pixel position within the shooting area, and shift the camera to obtain the third velocity value perpendicular to the shooting section.

Claims

1. A rotor test bench PIV single phase-locked signal multiple high-frequency shooting device, characterized in that: Including, compressor, camera, rotor blade, rotor shaft system, fiber optic sensor, step-down module, synchronizer, laser, camera, laser sheet light, description of the coordinated use of the equipment, a fiber optic sensor that can realize an external trigger signal source, installed on the wall of the rotor blade tip casing, obtains the arrival information of a specific blade, the fiber optic sensor outputs a 24V high level as a phase-locked signal, and outputs a high level when the blade arrives, and the high level is reduced to 5V through the step-down module. After receiving the 5V TTL signal from the step-down module, the signal generator outputs a 5V TTL signal with a preset waveform, pulse width, rising edge time, and set frequency of the external trigger signal. The control signal of the specific frequency output from the signal generator is transmitted to the camera and high-frequency laser, realizing the coordination of the phase-locked signal to the camera and laser action, and realizing that a single phase-locked signal triggers the camera and high-frequency laser to continuously shoot and obtain the flow field velocity field.

2. The rotor test bench PIV single phase-locked signal multiple high-frequency shooting device according to claim 1 is characterized in that: The camera is a CMOS camera.

3. The rotor test bench PIV single phase-locked signal multiple high-frequency shooting device according to claim 1, characterized in that: The signal generator generates a multi-channel synchronous output of a 5V TTL signal with adjustable cycle number, frequency, and pulse width.

4. The rotor test bench PIV single phase-locked signal multiple high-frequency shooting device according to claim 1, characterized in that: Lasers can adjust the light intensity and thickness of the light sheet.

5. The rotor test bench PIV single phase-locked signal multiple high-frequency shooting device according to claim 1, characterized in that: The camera synchronizes with the high-frequency laser through a synchronizer to capture the instantaneous flow field characteristics of the flow field cross section.

6. A rotor test bench PIV single phase-locked signal multiple high-frequency shooting method, characterized in that: The method is realized by using a rotor test bench PIV single phase-locked signal multiple high-frequency shooting device as described in any one of claims 1 to 5.

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