Modelling and Simulation of helicopters and tilt-rotors in Vortex Ring State

Lead Research Organisation: University of Glasgow
Department Name: School of Engineering

Abstract

Progress with computational fluid dynamics based on the Navier-Stokes equations allows for the computation of flows around propellers within realistic time-scales. The most established method for such simulations is to employ the Unsteady Reynolds-Averaged Navier-Stokes method that is a good compromise between accuracy and efficiency. This is because URANS doesn't need to resolve all flow scales which leads to CFD meshes of the size of millions of cells and time steps that are of 1/100 of the chord travel-time of particles. The results can be used for the computation of the mean pressure and velocity fields as well as the slowest frequencies present in the flow. When translated to acoustics, the method can only give tone-noise while the broadband spectrum remains under-resolved.

In terms of geometric complexity, the use of structured or unstructured grids is possible, and by employing overset or sliding meshes, the predictions can account for the interaction between the wing and nacelle of an installed propeller system.

The Helicopter Multi-Block method of Glasgow has all the ingredients for computing the flow around installed propellers based on the URANS approach and it is therefore a good starting point for research work in this area.

On the other hand, the need to resolve more and more harmonics as well as the broad-band part of the acoustic spectrum present in propeller flows, requires more sophisticated techniques that are based on simulation rather than modelling of turbulence. Glasgow has substantial experience with Detached Eddy Simulation for the computation of flows inside weapon bays and helicopter rotors. DES could, in principle, be used for propellers and should allow for the resolution of a large part of the flow spectrum at the expense of more computational resources. For a flow around a propeller blade, grids of the order of 10 million cells should be used with this method and time steps of the order of 1/10000 of the chord travel-time. This of course leads to an increase of the required CPU time by a factor of 100 in comparison to URANS (due to the use of efficient time-integration schemes in the HMB solver). To avoid the penalty associated with this method. The recently-developed Structure-Adaptive-Simulation or SAS should be tested for propeller flows. This method should give results close to the DES at almost twice the cost of the URANS method.

It is therefore advisable to adopt a triple strategy that begins with the evaluation of the URANS and DES for propeller flows and then compare the potential gains of SAS with the promised reduction in CPU time in comparison to the DES method.

The CFD results of any of the above method can be combined with a number of tools for the further exploitation of the pressure field in conjunction with far-field aeroacoustics methods.

Glasgow has experience with the FW-H method that is popular in the field of helicopter rotors. The method uses the CFD-generated unsteady pressure and based on the linearized acoustics equations, produces the acoustic signature of the propeller at distances far apart from the source of noise.

Thickness, loading and broadband noise sources could be resolved in the near-field and propagated further of the CFD domain with the FWH method. In addition, trailing edge noise should be resolved by a fine-mesh DES solution.

A second method that could be combined with the CFD results could lead to the prediction of the noise-level inside the cabin of an aircraft equipped with propellers. This would lead to an integrated simulation environment where the propeller performance and its acoustics could be studied and used for the far-field and cabin noise predictions at the same time.

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509668/1 30/09/2016 29/09/2021
1804616 Studentship EP/N509668/1 02/10/2016 24/08/2020 Ross John Higgins
 
Description Following an initial investigation into vortex ring state, it became clear that the study of propeller blades have greater importance due to the change in modern design and hence, the aeroelastic phenomena of flutter was investigated. Under certain conditions, the response of a propeller blade can become unstable and lead to potential unsafe flying conditions. Due to the danger associated with such conditions, accurate simulation methods are required to ensure such a phenomena can be captured. Such a method has been developed and applied through this award. In collaboration with DOWTY Propellers, the UK specialist is propeller design and manufacturing, the response of one of DOWTY's blades was validated and subsequently analysed under different conditions, with the insight and findings to be published within the PhD thesis. Although the method was validated via a known experiment, one of the key findings from this research was that a distinct lack of fully comprehensive experimental results were available. This has since lead on to further projects focusing on experimental study of propeller blades.
Exploitation Route Following this investigation, it became clear that a modern experimental test was required. Such findings from this award can be validated through the experimental campaign with a greater amount of detail to validate from. This award has shown that high-fidelity modelling techniques are required for such a study. This is due to the complex flow physics associated with stalled propeller flow which only resolved computational fluid dynamics can capture accurately.
Sectors Aerospace, Defence and Marine

 
Description Aid in the development of higher-fidelity methods in propeller modelling and design.
First Year Of Impact 2019
Sector Aerospace, Defence and Marine
 
Description Support provided by DOWTY Propellers for the investigation of propeller blades 
Organisation Messier-Dowty Ltd
Country United Kingdom 
Sector Private 
PI Contribution The support allowed for the validation of the method. Based upon this validated model, the blade was subsequently investigated under different conditions to provide a greater understanding for Dowty.
Collaborator Contribution Support was provided by Dowty Propellers to study propeller blades during flutter. Both geometry and experimental results were provided by Dowty.
Impact Several journal papers and conference proceedings were published as a result of this collaboration. 1) http://dx.doi.org/10.2514/1.J058463 2) https://doi.org/10.1017/aer.2019.135 3) http://dx.doi.org/10.2514/6.2019-1102
Start Year 2017
 
Title HMB3 - Helicopter Multi-Block 3 
Description The HMB3 CFD solver has been extended and validated for stall flutter flows. This is a new application area for out in-house CFD solver. Apart from the validation and extension of the tool to cover this particular type of fluid-structure interaction, additional documentation has been produced for HMB3 and additional test cases were added to its validation database. 
Type Of Technology Software 
Year Produced 2019 
Impact The advent of small, personal flying machines, requires the use of safe propeller blades, free from flutter. The HMB3 method as extended in this project can cover this gap and deliver computations of high fidelity that can be used to de-risk propeller designs for stall flutter. 
URL http://www.gla.ac.uk/cfd