Whirl and Stall Flutter Simulation Using CFD

This paper presents recent research on numerical methods for whirl and stall flutter using computational fluid dynamics. The method involves coupling of the HMB3 CFD solver of the University of Glasgow and a NASTRAN derived structural model. Based upon a literature survey, a significant amount of research has been conducted on the numerical investigation of tiltrotors, with a focus on the XV-15 and V-22 aircraft. Within this paper, the coupling procedure is presented along with a steady CFD computation to highlight the accuracy of the high-fidelity method. In addition to this, a simple method is used to investigate the whirl flutter boundary of a standard propeller and the XV-15 blade

Understanding Transonic Weapon Bay Flows

No abstract available

Towards High-order Methods for Rotorcraft Applications

This work presents CFD results obtained with an efficient, high-order, finite-volume scheme. The formulation is based on the variable extrapolation MUSCL-scheme, and high-order spatial accuracy is achieved using correction terms obtained through successive differentiation. The scheme is modified to cope with physical and multiblock mesh interfaces, so stability, conservativeness, and high-order accuracy are guaranteed. Results with the proposed scheme for steady flows, showed better wake and higher resolution of vortical structures compared with the standard MUSCL, even when coarser meshes were employed. The method was also demonstrated for unsteady flows using overset and moving grids for the UH-60A rotor in forward flight and the ERICA tiltrotor in aeroplane mode. The present method adds CPU and memory overheads of 47% and 23%, respectively, in performing multi-dimensional problems for routine computations

Simulation of Unsteady Aerdynamic Load for Rigid Coaxial Rotor in Forward Flight with Vortex Particle Method

Co-axial rotor systems are frequently used for high-speed helicopters. Nevertheless, issues related to rotor-head drag, aerodynamic performance and vibration should also be considered. Simulating the unsteady aerodynamic loads for a rigid coaxial rotor, including the aerodynamic interactions between rotors and rotor blades, is an essential part of analyzing their vibration characteristics. In this paper, an unsteady aerodynamic analysis based on the vortex-lattice method is presented. In this method, a reversed flow model on the retreating side of the coaxial rotor is proposed based on the unsteady panel method. To account for reversed flow, shedding a vortex from the leading-edge is used rather than from the trailing-edge. Moreover, vortex-blade aerodynamic interactions are modelled. The model considers the unsteady pressure term induced on a blade by tip vortices of other blades, and thus accounts for the aerodynamic interaction between the rotors and its contribution to the unsteady airloads. Coupling the reversed flow model and the vortex-blade aerodynamic interaction model with a viscous vortex particle method is used to simulate the complex wake of the coaxial rotor, closing the loop in modelling aerodynamic interactions of coaxial rotors. Following this, the unsteady aerodynamic loads on the X2 coaxial rotor are simulated in forward flight, and compared with the results of PRASADUM (Parallelized Rotorcraft Analysis for Simulation And Design, developed at the University of Maryland) and CFD/CSD computations with the OVERFLOW and the CREATE-AV Helios tools. The results of the present method agree with the results of the CFD/CSD method, and compare better than the PRASADUM solutions. Furthermore, the influence of the aerodynamic interaction between the coaxial rotors on the unsteady airloads, frequency, wake structure, induced flow and force distributions are analyzed. Additionally, the results are also compared against computation for a single rotor case, simulated at similar conditions as the coaxial rotor. It is shown that the effect of tip vortex interaction plays a significant role in unsteady airloads of coaxial rotors at low-speeds, while the rotor blade passing effect is obvious strengthened at high-speed

Acoustic Assessment of Twin-engined Turboprop Layout

No abstract available

Performance Improvement of Variable Speed Rotors by Gurney Flaps

Gurney flaps are used for improving the performance of variable speed rotors. An analytical model able to predict helicopter rotor power is first presented, and the flight data of the UH-60A helicopter is used for validation. The predictions of the rotor power are in good agreement with the flight test data, justifying the use of this tool in analyzing helicopter performance. A fixed Gurney flap can enhance the performance of variable speed rotors and expand the corresponding flight envelop, especially near stall and high speed flight. A retractable Gurney flap at 1/rev yields more power savings than a fixed Gurney flap or a retractable one with higher a harmonic prescribed motion. At a speed of 200km/h, the retractable Gurney flap at 1/rev can obtain 3.22% more power reduction at a rotor speed of 85% nominal rotor speed, and this value is 8.37% at a speed of 220km/h. The height corresponding to the minimum power increases slowly in low to medium speed flight, and increases dramatically in high speed flight. With increasing take-off weight (i.e. rotor thrust), the retractable Gurney flap at 1/rev can obtain more rotor power savings

Numerical aeroacoustic analysis of propeller designs

As propeller-driven aircraft are the best choice for short/middle-haul flights but their acoustic emissions may require improvements to comply with future noise certification standards, this work aims to numerically evaluate the acoustics of different modern propeller designs. Overall sound pressure level and noise spectra of various blade geometries and hub configurations are compared on a surface representing the exterior fuselage of a typical large turboprop aircraft. Interior cabin noise is also evaluated using the transfer function of a Fokker 50 aircraft. A blade design operating at lower RPM and with the span-wise loading moved inboard is shown to be significantly quieter without severe performance penalties. The employed Computational Fluid Dynamics (CFD) method is able to reproduce the tonal content of all blades and its dependence on hub and blade design features

Tiltrotor CFD part I: validation

This paper presents performance analyses of the model-scale ERICA and TILTAERO tiltrotors and of the full-scale XV-15 rotor with high-fidelity computational fluids dynamics. For the ERICA tiltrotor, the overall effect of the blades on the fuselage was well captured, as demonstrated by analysing surface pressure measurements. However, there was no available experimental data for the blade aerodynamic loads. A comparison of computed rotor loads with experiments was instead possible for the XV-15 rotor, where CFD results predicted the FoM within 1.05%. The method was also able to capture the differences in performance between hover and propeller modes. Good agreement was also found for the TILTAERO loads. The overall agreement with the experimental data and theory for the considered cases demonstrates the capability of the present CFD method to accurately predict tiltrotor flows. In a second part of this work, the validated method is used for blade shape optimisation

Accurate Predictions of Hovering Rotor Flows Using CFD

With work on the S-76 rotor providing encouraging results regarding the prediction of integral loads with CFD in hover, the XV-15 rotor is now analysed. Fully turbulent and transitional results are obtained showing the capability of modern CFD methods. The transition onset and distribution of skin friction are well predicted and were found to have a mild effect on the overall figure of merit. This work also shows the potential of transport-based models for transition prediction in complex 3D flows. Finally, hover simulations for the PSP blade are also shown in terms of surface pressure coefficient and wake visualisation

Real Time Wake Computations using Lattice Boltzmann Method on Many Integrated Core Processors

This paper puts forward an efficient Lattice Boltzmann method for use as a wake simulator suitable for real-time environments. The method is limited to low speed incompressible flow but is very efficient and can be used to compute flows “on the fly”. In particular, many-core machines allow for the method to be used with the need of very expensive parallel clusters. Results are shown here for flows around cylinders and simple ship shapes