Evanescent Field Optical Sensor for Shock Wave Detection
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Solution Overview
Problem
Controlling oscillatory behavior in inlet-isolator flows of supersonic air-breathing engines is challenging due to complex shock wave/boundary-layer interactions, which can lead to engine unstart and instability issues.
Innovation Solution
An evanescent field-based optical sensor system using directionally coupled optical fibers is employed to detect shock waves in fluid flows. This system transmits and receives monochromatic light, allowing for the measurement of changes in the index of refraction and energy transfer through the evanescent field, thereby detecting shock waves and their directionality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional electrical or electronic sensors are used in supersonic inlet-isolator flows, then they can provide straightforward measurement capability, but they are unsafe for use in hazardous areas and susceptible to electromagnetic interference
Solution Approach 1:
The patent replaces conventional electrical/electronic sensors with fiber optic sensors that use light transmission through optical fibers. This substitution eliminates susceptibility to electromagnetic interference and enables safe operation in hazardous areas while maintaining measurement capabilities through optical detection of shock wave-induced refractive index changes
Solution Approach 2:
The patent introduces an intermediary optical sensing system that uses laser light transmitted through fiber optic cables to detect shock waves. The fiber optic sensor acts as a mediator between the harsh supersonic flow environment and the protected measurement equipment, allowing remote sensing without exposing electronic components to hazardous conditions
2Object-affected harmful factors
If fiber optic sensors are used to detect shock waves, then immunity to electromagnetic interference and safety in hazardous areas are achieved, but the detection mechanism requires complex evanescent field coupling and directional coupling arrangements
Solution Approach 1:
The patent creates a localized evanescent field interaction region where the optical fiber is positioned in close proximity to the shock wave propagation path. This local configuration enables selective detection of shock wave effects at specific measurement points while maintaining the overall simplicity of the fiber optic sensing approach
Solution Approach 2:
The patent utilizes the evanescent field extending perpendicular to the optical fiber axis to detect shock waves. By sensing in this additional spatial dimension rather than requiring direct contact or complex angular arrangements, the system simplifies the coupling mechanism while maintaining sensitivity to shock wave-induced refractive index changes
3Reliability
If shock wave detection systems are implemented in supersonic inlets, then control of shock train location and mitigation of engine unstart can be achieved, but the complex shock wave/boundary-layer interactions make control challenging
Solution Approach 1:
The patent implements a feedback control system where fiber optic sensors continuously detect shock wave position and strength, and this information is used to adjust inlet geometry or actuator positions. The real-time monitoring of refractive index changes provides feedback signals that enable dynamic control of the shock train location to prevent engine unstart and maintain stable operation
Solution Approach 2:
The patent replaces complex conventional pressure tap arrays and electronic sensor networks with a simplified fiber optic sensing system. The single-mode or multi-mode fiber optic sensors directly detect shock wave characteristics through evanescent field interactions, reducing measurement complexity while improving reliability in the harsh supersonic environment
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The optical sensor system effectively detects shock waves and their direction, providing sensitive measurements of pressure gradients and index of refraction changes. This capability aids in controlling shock train location in scramjet isolators, mitigating engine unstart and instability.
Implementation Method 1
The laser light is positioned by the system in a flow field of a fluid via the optic fiber. The optical sensor includes an evanescent field coupling with the receiving fiber.
Implementation Method 2
allowing for the measurement of changes in the index of refraction and energy transfer through the evanescent field, thereby detecting shock waves and their directionality
Implementation Method 3
They can be used to transmit and receive monochromatic light
Implementation Method 4
A shock wave detection system includes: an optical sensor configured to generate a sensor signal based on laser light
Data Source
AI summary
A shock wave detection system includes an optical sensor configured to generate a sensor signal based on the received laser light, a processor, and a memory. The memory includes instructions stored thereon, which when executed by the processor cause the system to: generate a sensor signal based on the laser light; perform a digital fast Fourier transform on the sensor signal; determine a power spectral density of the sensor signal based on the digital fast Fourier transform; determine a difference in a frequency content before, during, and after a shock wave transition event based on the power spectral density; and determine a passing of the shock wave based on the difference in the frequency content.