Floating-Element Shear-Stress Sensor with Optical Detection
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Solution Overview
Problem
Current technologies are inadequate for accurately measuring mean and fluctuating wall shear-stress data, particularly due to limitations in thermal management, electromagnetic interference, and the inability to be flush-mountable without generating flow disturbances, which hinders precise measurement in high-temperature and high-Reynolds number environments.
Innovation Solution
A shear-stress sensor system utilizing a floating element connected to a structure, with optical fibers positioned non-orthogonally or orthogonally to detect deflections through intensity or phase modulation, enabling direct, high-temperature measurements using geometric and interferometric optical techniques, and employing high-temperature materials like sapphire and silicon carbide to minimize thermal stress and interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional macro-scale measurement technology is used, then the measurement system is simple and robust, but the measurement precision of wall shear stress is insufficient
Solution Approach 1:
The patent replaces conventional mechanical measurement systems with a micro-electromechanical system (MEMS) that uses optical detection methods. The floating element deflection is detected through optical means rather than mechanical strain gauges or pressure transducers, enabling higher precision measurements of wall shear stress while maintaining a compact form factor.
Solution Approach 2:
The patent changes the scale of the measurement device from macro-scale to micro-scale. By fabricating the sensor using MEMS techniques, the physical dimensions are reduced while the measurement capability is enhanced. The micro-fabricated floating element and optical detection system enable precise measurement of shear stress effects that were previously inaccessible to conventional macro-scale instruments.
2Ease of manufacture
If thermal sensors are used, then the fabrication is simpler and the structure is more robust, but the measurement precision for quantitative measurements is difficult to achieve
Solution Approach 1:
The patent replaces thermal sensing mechanisms with direct mechanical deflection measurement of a floating element. Instead of inferring shear stress through thermal conduction analogies, the system directly measures the mechanical response of a floating element to shear stress, providing more accurate quantitative measurements while maintaining ease of fabrication through standard MEMS processes.
3Measurement precision
If optical MEMS laser-Doppler anemometers are used, then the measurement capability is improved, but the ability to generate a sufficiently small measurement volume in high-Reynolds number sublayer is challenging
Solution Approach 1:
The patent replaces optical Doppler velocimetry with direct optical detection of floating element deflection. Instead of measuring velocity gradients through complex optical scattering and Doppler shift analysis, the system uses a floating element whose deflection directly reflects shear stress, enabling smaller measurement volumes while maintaining precision in high-Reynolds number flows.
4Measurement precision
If floating-element structures with capacitive or piezoresistive techniques are used, then direct quantitative measurements are possible, but thermal management issues and electromagnetic interference occur
Solution Approach 1:
The patent replaces capacitive and piezoresistive transduction techniques with optical detection methods. Instead of using electrical signals that are susceptible to thermal and electromagnetic interference, the system uses optical detection to measure floating element deflection, eliminating these harmful factors while maintaining direct quantitative measurement capability.
Solution Approach 2:
The patent introduces optical detection as an intermediary between the floating element and the measurement system. The optical detection system acts as a mediator that converts mechanical deflection into optical signals, which are then processed to determine shear stress. This intermediary approach isolates the measurement system from thermal and electromagnetic interference that would otherwise affect direct electrical sensing methods.
5Reliability
If floating-element structures with wire bonds are used, then the transduction is established, but flow disturbances are generated
Solution Approach 1:
The patent removes wire bonds and other protruding elements from the floating element structure. By extracting these flow-disturbing components, the system achieves flush mounting that minimizes flow disturbances while maintaining the transduction capability through optimized optical detection pathways.
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
Enables direct, high-temperature measurement of skin friction with improved accuracy and reduced interference, allowing for precise determination of shear stress in complex flow phenomena.
Implementation Method 1
an optical fiber in proximity to the floating element and being operably connected to the light source for delivery of the optical signal to the floating element
Implementation Method 2
a second optical fiber in proximity to the floating element and positioned non-orthogonal to the floating element. The second optical fiber can receive a reflected signal based on the optical signal
Implementation Method 3
an interferometer operably connected to the optical fiber. The optical fiber can receive a reflected signal based on the optical signal, where the reflected signal is phase modulated and delivered to the interferometer
Data Source
AI summary
A shear-stress sensing system can include a floating element whose displacement can be detected through use of optical measurements. The system can utilize high temperature materials to deliver the optical signal to the structure to be measured, which can also utilize high temperature materials. In one embodiment, an intensity modulation or phase modulation of a reflected signal can be measured to determine the shear stress. In another embodiment, a Moire fringe pattern can be used to determine the shear stress.