A holistic approach to propeller performance
Propeller performance can be defined by a number of metrics, from hydrodynamic efficiency and cavitation susceptibility to mechanical strength and noise and vibration. These performance criteria often pull in opposing directions, such that the right balance needs to be driven by the specific, weighted requirements of each customer and the propeller geometry defined to meet that specification.
Until recently, propeller design was driven by empirical data and long duration trial and error improvement and was viewed as a dark art by those on the outside looking in. Teignbridge utilises new powerful simulation tools to understand the interaction between the propeller structure and the high-velocity fluid flowing around its complex surfaces.
This enables our designers to complement 45 years of experience with performance evaluation before the design leaves the engineering office, let alone gets wet. Initially, the preserve of big ship propellers, these tools are now available to the sub 2.5m diameter propeller market. With a clear customer specification and powerful tools to predict performance, our designs can intentionally target areas of specific areas of performance.
Noise and vibration performance for example…
In addition to reviewing basic design principles such as blade passing rates and propeller to engine matching, propeller designers can now explore more complex sources of propeller noise and vibration performance. This work requires the simulation and evaluation of a variety of complex phenomena relating to cavitation, pressure distribution in the fluid surrounding the propeller and transmission of vibration and noise through the fluid and vessel structures.
One-off boat designs and varying vessel mission profiles mean that almost every application requires a bespoke approach. The following are just some of the tools and methods available to designers to explore these phenomena in ever-increasing detail:
Most highly loaded propellers will cavitate, and whilst erosive cavitation (causing damage to blade surfaces) is often the principal area of concern, the presence of cavitation (erosive or not) can also represent a problematic source of broadband noise. Transient computational fluid dynamic (CFD) simulations can be used to simulate the location, extent and type of cavitation.
In order to manage computational time and effort, this will typically mean inferring regions of cavitating flow from the cavitation number (ratio of local pressure drop to fluid kinetic energy in a region of flow), although where problems get very complex, a two-phase simulation can be used to simulate the vapour/gas-filled voids that form in the fluid during cavitation.
Trailing edge vortex shedding:
Von Kármán vortex street shedding from the trailing edge (TE) of a Propeller blade can cause excitation of the propeller’s natural frequencies (see note below), resulting in strong tonal noise emission from the propeller. Hand calculations, 2D and 3D CFD simulation can be used to identify problematic trailing edge geometry and make targeted changes to avoid problematic vortex patterns.
Identification of propeller natural frequencies:
Structural finite element analysis (FEA) can be used to carry out a modal analysis on the propeller, identifying mode shapes and frequencies (not forgetting the effect of submergence in water on frequency). Identifying correlations between sources and locations of excitation (including TE vortex shedding) and propeller natural frequencies can be key to identifying and avoiding problems.
Underwater radiated noise:
It is now possible to place digital ‘hydrophones’ in the CFD simulation domain, enabling conversion of time-varying pressure distribution in the fluid to an estimate of the frequency and amplitude of underwater radiated noise.
The above is a snapshot of what is possible with advanced simulation tools and methods and each propeller application will have different risk factors that need exploring to maximise performance in relation to propeller noise and vibration.
Why go to the effort?
Propeller geometry is necessarily complex (don’t buy a propeller with straight edges!) with inter-related parametric definitions of pitch, skew, rake, blade section and more. Navigating the available design space to drive performance in a particular direction whilst operating in the complex wake of a boat is not a simple or a quick task, but Teignbridge is dedicated to getting it right. The approach pays dividends in fuel reduction, noise and vibration reduction and overall customer satisfaction.
Propellers are the last and vital connection between your power plant and the water. Demand more performance from your propeller designer and be prepared to pay a little more for it – it will be money well spent.