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Helicopter Drag Prediction using Detached-Eddy Simulation

Final Report Summary - HELIDES (Helicopter Drag Prediction using Detached-Eddy Simulation)

Executive Summary:

Reduction of aerodynamic drag (and hence fuel consumption) of rotorcraft is a key goal to improving the sustainability of this means of transport. Helicopter drag in fast forward flight is dominated by the phenomenon of flow separation from the rear fuselage and rotor hub, which also gives rise to unsteady aerodynamics loads on the helicopter tail. Drag reduction may be achieved by various means, such as geometry optimisation or flow control devices. The cost-effective design of such measures requires predictive tools to enable their assessment in the preliminary design stage.

Advances in computer capacity have led to an increasingly important role being played by Computational Fluid Dynamics (CFD) in aerodynamic design across the ground and air transport sectors. The principle limiting factor in CFD arises from the phenomenon of turbulence, which introduces a fundamental cost-accuracy trade-off. The methodology applied in HELIDES, namely Detached-Eddy Simulation (DES), is a prominent member of a new family of turbulence modelling strategies aimed at offering a step forward in accuracy by exploiting near-future computational resources.

Project Context and Objectives:

The global objectives of HELIDES are:

• To contribute to the analysis of an existing fuselage and rotor-head geometry in terms of aerodynamic loads and interactional effects using state-of-the-art hybrid RANS/LES methods
- A milestone in DES for helicopter aerodynamics
• To assess the industrial feasibility of the adopted methods with respect to:
- Numerical performance, efficiency, parallel scalability
- Expected “readiness date” for routine industrial application
• The demonstration and assessment of the feasibility of the DES software process for this complex application.

Project Results:

State-of-the art Detached-Eddy Simulation (DES) models were implemented in the open-source, unstructured CFD software OpenFOAM®. Additionally, related solver features were integrated including a locally-adaptive hybrid convection scheme, balancing low numerical dissipation and robustness. Simulations using RANS and unsteady RANS (URANS) methods were also conducted for comparison. URANS represents the current state-of-the-art in industrial practice.

For configurations with direct comparability between experiment and CFD, excellent agreement was achieved by DES for the aerodynamic loads. The drag on the cabin exhibited very close agreement between DES and experiment. For RANS and URANS however, even the trend with angle of attack was incorrectly predicted. The superiority of DES over (U)RANS was confirmed across all flow quantities and for all simulated configurations. This included improved prediction of the wake flow topology, which in addition to affecting drag prediction of the cabin also affects the predicted empennage loads due to upwash. Unsteady phenomena are important to predict effects such as tail-shake, caused by the impact of the turbulent wake on the empennage. Also in this regard DES shows significant improvement over URANS.

The computational efficiency and parallel scalability of the simulations was recorded, allowing an assessment of the industrial feasibility of the methods from the point of view of computational expense. An arbitrary criterion for “industrial readiness” was defined as the ability to perform such simulations within a 24-hour (computational) turnaround time. The efficiency and scalability information was combined with various projections for the future increase in computational capacity (ranging from optimistic to pessimistic). The computing hardware used in HELIDES was first available in 2009, which was used as the datum for scaling. Finally, statistical convergence criteria were used to determine the number of time steps that must be computed to achieve acceptable accuracy in mean values and fluctuating quantities, respectively. The resulting readiness dates, ranging between 2014 and 2019 indicate that such methods will indeed become affordable for routine industrial application in the near future.

Potential Impact:

HELIDES has contributed to the industrial applicability of advanced predictive tools for aerodynamics, which will play a key role in the design of future rotorcraft with reduced drag and fuel consumption. A more direct outcome of the project is the rich aerodynamic data and analysis that can be exploited to understand the drag producing mechanisms of the flow.

DES and related approaches have been the subject of intensive method development research over the past decade. A significant contribution to this research was and continues to be made within European-funded projects such as FLOMANIA, DESider, ATAAC and Go4Hybrid. By demonstrating the concrete advantages of high-fidelity DES over the currently used URANS approaches, as well its applicability to highly complex geometries, this work has done much to improve the readiness level of DES.

In addition to the transfer of the relevant expertise and best practice to industry, the high computational resources demanded by DES present a principal hurdle to routine industrial adoption. The forecast for industrial readiness of DES for such helicopter applications provides important information to the decision makers in industry. Furthermore, it is clear from the proximity of the predicted readiness dates that work should already be undertaken to prepare industry for adoption of DES-like methods in order to maintain competitiveness.

List of Websites:

Dr.-Ing. Charles Mockett
Senior Research Consultant

CFD Software Entwicklungs- und Forschungsgesesllchaft mbH
Wolzogenstr. 4, 14163 Berlin

Tel.: +49 (0)30 59 00 83 320
Fax: +49 (0)30 59 00 83 220