ODS Analysis of Aero Thin-Walled Panel Structure
1. Experimental Overview
This experiment aims to study the dynamic response of hot-section components (such as thin-walled panel structures) in aero-engines under simulated real operating conditions (high temperature, high sound pressure level noise). Through Operational Deflection Shape (ODS) analysis, the vibration patterns of the structure at specific frequencies can be visually obtained, which is crucial for assessing its vibration fatigue life and optimizing its structural design.
2. Experimental System and Equipment
A non-contact vibration measurement system for high-temperature environments was established, with the following core equipment:
1. Excitation System:
High-Temperature Thin-Walled Panel Acoustic Fatigue Test Bench: The core facility. It generates a high sound pressure level acoustic field using high-power speakers or flow to simulate the noise environment inside an engine and excite the thin-walled panel specimen installed within. This test bench includes heating capabilities to provide a stable high-temperature environment for the specimen.
2. Measurement System:
VibroMicro Laser Doppler Vibrometers (2 units): The core measurement instruments. The advantages of laser vibrometers include:
Non-Contact Measurement: Does not affect the dynamic characteristics of the lightweight structure.
High Spatial Resolution: Precisely measures the vibration velocity/displacement at a specific point on the surface.
Suitable for High-Temperature Environments: Capable of remotely measuring high-temperature objects, avoiding sensor temperature limitations.
Use of Two Devices: Enables Operational Deflection Shape Analysis. By scanning or performing multi-point measurements, phase and amplitude information across the entire panel can be acquired synchronously, allowing for the reconstruction of three-dimensional vibration patterns.
3. Control and Scanning System:
XY Moving Platform Device: Used to fix and precisely move the laser vibrometer probes (or mirrors), enabling a grid-based point-by-point scan of the thin-walled panel surface.
4. Data Analysis System:
Online Signal Monitoring and Analysis Software: Responsible for equipment control, data acquisition, signal processing (e.g., FFT spectrum analysis), and generating ODS animations and contour plots.
3. Results and Analysis
1. Modal Frequency Identification
Through spectrum analysis of the vibration signals from measurement points, three main resonance peaks of the thin-walled panel were identified within the 1000 Hz frequency range:
Mode 1: 356.25 Hz
Mode 2: 545.16 Hz
Mode 3: 709.22 Hz
These frequencies represent the inherent characteristics of the structure under specific temperature and boundary conditions. High temperature can cause a reduction in the material's elastic modulus, potentially leading to lower frequencies compared to room temperature.
2. Operational Deflection Shape (ODS) Analysis
After identifying the peak frequencies, ODS analysis was further conducted.
ODS describes the steady-state vibration pattern of a structure at a specific frequency (especially resonant frequencies) and under specific loading (in this case, acoustic loading).
Analysis Results: Operational deflection shape contour plots were obtained for the three frequency points (356.25 Hz, 545.16 Hz, 709.22 Hz).
Interpretation of Contour Plots:
These plots visually display the distribution of nodal lines and anti-nodes on the thin-walled panel when vibrating at different frequencies, using color contours.
Typically, 356.25 Hz corresponds to a lower-order mode (likely first bending), while 545.16 Hz and 709.22 Hz correspond to higher-order bending or torsional modes.
By analyzing these deflection shapes, engineers can:
Locate High-Stress Areas: Areas with the largest deflection are often the initiation points for fatigue cracks.
Validate Simulation Models: Compare experimentally measured ODS with Finite Element Analysis (FEA) results to correct model boundary conditions and material parameters.
Optimize Structural Design: Modify unfavorable vibration patterns through means like stiffeners or damping materials to improve fatigue life.
4. Technical Extension and Application Examples
The related application examples you mentioned are highly typical, demonstrating the wide application of this technical framework in the aerospace field:
1. Natural Frequency Testing of High-Temperature Blades
Scenario: Aero-engine turbine/compressor blades operate under high-speed rotation and exposure to high-temperature gas.
Method: Using single or multiple laser vibrometers, blades are measured non-contact in a thermal vacuum chamber or on a dedicated heating stage, with excitation provided acoustically or via a shaker with a high-temperature rod.
Purpose: To obtain the true natural frequencies and mode shapes of the blades at operating temperatures, preventing resonance during operation and ensuring safety.
2. Combustor Vibration Testing
Scenario: The engine combustor liner is subjected to intense excitation from high-temperature, high-pressure unsteady combustion (combustion oscillations).
Method: High-temperature pressure sensors mounted on the combustor walls measure the excitation force, while laser vibrometers scan the vibration of the combustor liner or casing through observation windows.
Purpose: To analyze the coupling between combustion instability and structural vibration, identify dangerous modes that lead to High-Cycle Fatigue (HCF), and provide a basis for combustor stability design and life prediction.
5. Summary
This experiment successfully combined high-temperature environment simulation, acoustic excitation, and non-contact optical measurement technologies to replicate and measure the dynamic response of an aeronautical thin-walled panel under conditions close to its real operating state. Through ODS analysis, not only were the structural resonance frequencies obtained, but more importantly, its vibration patterns were visualized. This provides direct and critical data support for deeply understanding the mechanisms of acoustic fatigue and for conducting anti-fatigue design. This methodology represents a cutting-edge and effective approach in the fields of structural dynamics and fatigue research within aeronautics and astronautics.
