Multi-Diameter Propeller Blade Asymmetry for Noise Reduction
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Solution Overview
Problem
Contra-rotating open rotor turbomachines face significant acoustic challenges due to noise generated by vortex structures from the upstream propeller interacting with the downstream propeller, which is difficult to mitigate without compromising thrust or complexity.
Innovation Solution
The upstream propeller is designed with blades of varying lengths, where at least one blade is shorter than others by 0.5% to 5%, causing vortices to strike the downstream propeller at different radial positions, de-phasing acoustic sources and reducing noise levels without significant thrust loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the downstream propeller is of smaller diameter than the upstream propeller (clipping), then interaction noise is reduced, but thrust is compromised and the solution is ineffective at large angles of attack
Solution Approach 1:
The patent applies asymmetry by making the blades of the upstream propeller have different lengths (multi-diameter configuration). This asymmetric blade length distribution causes vortex structures to be emitted at different radial positions, which then strike the downstream propeller at different locations, creating phase differences that reduce noise interference while maintaining sufficient thrust across all operating conditions including large angles of attack.
2Object-affected harmful factors
If the geometry of the blades of the upstream propeller is modified to reduce vortex intensity, then interaction noise is reduced, but the complexity increases and thrust is reduced
Solution Approach 1:
The patent segments the upstream propeller blades into multiple groups with different lengths. Instead of modifying the geometry of all blades uniformly, the solution divides the blade set into discrete segments (groups of blades with equal length), where each segment has a specific length. This segmentation approach simplifies the design compared to continuous geometry modification while effectively reducing vortex intensity and interaction noise.
3Device complexity
If uniform blade lengths are used in the upstream propeller, then the architecture is simple, but interaction noise remains high especially at large angles of attack
Solution Approach 1:
The patent applies local quality by assigning different lengths to different groups of blades in the upstream propeller. Rather than making all blades uniform, specific local variations in blade length are introduced. Each group of blades has a specific length tailored to contribute to noise reduction, while maintaining overall architectural simplicity. This local differentiation effectively reduces interaction noise without significantly complicating the overall propeller design.
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
This design effectively reduces radiated noise levels by de-phasing acoustic sources, maintaining thrust efficiency and simplifying the propeller architecture, while allowing operation at large angles of attack.
Implementation Method 1
the principal source of noise comes from vortex structures leaving the blades of the upstream propeller and striking the blades of the downstream propeller
Implementation Method 2
causing vortices to strike the downstream propeller at different radial positions, de-phasing acoustic sources and reducing noise levels
Data Source
AI summary
A turbomachine including at least two unducted propellers, one of which is an upstream propeller and one a downstream propeller, the upstream propeller including a plurality of blades, at least one first blade of which has a different length from that of a second blade.