Real-Time Simulation of Rotor Blade Aeroelasticity for the Multidisciplinary Design of Rotorcraft

This paper elaborates on the theoretical development of a mathematical approach, targeting the real-time simulation of aeroelastic rotor blade dynamics for the multidisciplinary design of rotorcraft. A Lagrangian approach is formulated for the rapid estimation of natural vibration characteristics of rotor blades with nonuniform structural properties. Modal characteristics obtained from classical vibration analysis methods, are utilized as assumed deformation functions. Closed form integral expressions are incorporated, describing the generalized centrifugal forces and moments acting on the blade. The treatment of three-dimensional elastic blade kinematics in the time-domain is thoroughly discussed. In order to ensure robustness and establish applicability in real-time, a novel, second-order accurate, finite-difference scheme is utilized for the temporal discretization of elastic blade motion. The developed mathematical approach is coupled with a finite-state induced flow model, an unsteady blade element aerodynamics model, and a dynamic wake distortion model. The combined aeroelastic rotor formulation is implemented in a helicopter flight mechanics code. The aeroelastic behavior of a full-scale hingeless helicopter rotor has been investigated. Results are presented in terms of rotor blade resonant frequencies, airframe–rotor trim performance, oscillatory structural blade loads, and transient rotor response to control inputs. Extensive comparisons are carried out with wind tunnel and flight test measurements found in the open literature, as well as with non-real-time comprehensive analysis methods. It is shown that, the proposed approach exhibits good agreement with flight test data regarding trim performance and transient rotor response characteristics. Accurate estimation of structural blade loads is demonstrated, in terms of both amplitude and phase, up to the third harmonic component of oscillatory loading. It is shown that, the developed model can be utilized for real-time simulation on a modern personal computer. The proposed methodology essentially constitutes an enabling technology for the multidisciplinary design of rotorcraft, when a compromise between simulation fidelity and computational efficiency has to be sought for in the model development process.

Numerical Investigations on the Conceptual Design of a Ducted Contra-Rotating Fan

The Advisory Council for Aeronautics Research in Europe (ACARE) has set an ambitious array of objectives to be accomplished by 2050. It is often claimed that complying with those targets will not require evolution but, rather, revolution. If the growth in aviation has to be sustained in the future then we must come up with radical aircraft and engine configurations which can meet the demands of future aviation. The contra-rotating fan is one such system which can play an important role in the future engine configurations, such as the hybrid engine configuration that is being investigated in the EU cofounded AHEAD project. In order to design a CRF system, a 1-D code has been developed based on the inverse Blade Element Method (BEM) to design a contra rotating fan. The CRF design obtained from this methodology is then analyzed with a full 3D RANS simulation. The numerical analysis revealed that the performance of the first rotor satisfies with the given design requirements in terms of both pressure ratio and isentropic efficiency, thus proving the efficacy of using the 1-D code for designing the CRF. However, the performance of the rear rotor does not reach the design demands. It was observed that there is a strong flow separation around the root and a strong normal shock in the blade passage near the tip. It was found that there is a great difference between the blade metal inlet angles and the relative flow inlet angles near the root of the rear rotor. One of the main reasons for this is the calculation of the axial velocity depending on the vortex design and the resolution of the radial equilibrium. Based on the CFD simulations, the design code could be further modified to improve the design of CRF.

Accessory Gear Box Architecture Evaluation Tool

An architectural modeling tool has been developed to support Accessory Gearbox (AGB) design. This is a structured approach which allows the Preliminary Design team to design an AGB with high fidelity in a short time and earlier in the design process. The program can accommodate a large variety of accessories with different attributes to generate a gearbox of minimum size. The final gearbox shape includes curvature which enables a close fit to the engine carcass. By setting the size, shape and location of the gearbox and accessories earlier, the designer can begin the process of locating external configurations around the gearbox more efficiently. The tool systematically explores the design space to optimize geometry based on multiple design criteria. It successfully constructs the intermediate gear train needed within the gearbox. It provides graphical output to the user with primitive models of the gears, accessories and housing. Once located in engine coordinates, these primitives provide the reference for the next level for detailed design.

Analysis of Effect of Advanced Technique Configurations on Overall Performance of High Bypass Turbofan Engine

An analytical study was undertaken using the performance model of a two spool direct drive high BPR 300kN thrust turbofan engine, to investigate the effects of advanced configurations on overall engine performance. These include variable bypass nozzle, variable cooling air flow and more electric technique. For variable bypass nozzle, analysis on performance of outer fan at different conditions indicates that different operating points cannot meet optimal performance at the same time if the bypass nozzle area kept a constant. By changing bypass nozzle throat area at different states, outer fan operating point moves to the location where airflow and efficiency are more appropriate, and have enough margin away from surge line. As a result, the range of variable area of bypass nozzle throat is determined which ensures engine having a low SFC and adequate stability. For variable cooling airflow, configuration of turbine cooling air flow extraction and methodology for obtaining change of cooling airflow are investigated. Then, base on temperature analysis of turbine vane and blade and resistance of cooling airflow, reduction of cooling airflow is determined. Finally, using performance model which considering effect of cooling air flow on work and efficiency of turbine, variable cooling airflow effect on overall performance is analyzed. For more electric technique, the main characteristic is to use power off-take instead of overboard air extraction. Power off-take and air extraction effect on overall performance of high bypass turbofan engine is compared. Investigation demonstrates that power offtake will have less SFC.

Lip Separation and Inlet Flow Distortion Control in Ducted Fans Used in VTOL Systems

This paper describes a novel ducted fan inlet flow conditioning concept that will significantly improve the performance and controllability of ducted fan systems operating at high angle of attack. High angle of attack operation of ducted fans is very common in VTOL (vertical take off and landing) UAV systems. The new concept that will significantly reduce the inlet lip separation related performance penalties in the edgewise/forward flight zone is named DOUBLE DUCTED FAN (DDF). The current concept uses a secondary stationary duct system to control inlet lip separation related momentum deficit at the inlet of the fan rotor occurring at elevated edgewise flight velocities. The DDF is self-adjusting in a wide edgewise flight velocity range and its corrective aerodynamic effect becomes more pronounced with increasing flight velocity due to its inherent design properties. Most axial flow fans are designed for an axial inlet flow with zero or minimal inlet flow distortion. The DDF concept is proven to be an effective way of dealing with inlet flow distortions occurring near the lip section of any axial flow fan system, especially at high angle of attack. In this present paper, a conventional baseline duct without any lip separation control feature is compared to two different double ducted fans named DDF CASE-A and DDF CASE-B via 3D, viscous and turbulent flow computational analysis. Both hover and edgewise flight conditions are considered. Significant relative improvements from DDF CASE-A and DDF CASE-B are in the areas of vertical force (thrust) enhancement, nose-up pitching moment control and recovery of fan through-flow mass flow rate in a wide horizontal flight range.

Analysis of the Noise Level Generated by Axial Flow Fan Composed of Radial-Bladed Centrifugal Rotor

The objective of this work is to present, by means of experimental, analytical and numerical techniques that sound pressure level generated by radial-bladed centrifugal fans of electric motor cooling systems may be expressed by a logarithmical ratio of the peripheral velocity of rotor, volumetric flow and efficiency of the fan. The proposed methodology proved to be efficient and simple in the prediction of generated noise by radial-bladed centrifugal fans of TEFC motors with accuracy of ± 3 dB. In addition, the acoustic resonance mode of the fan cavity were determined by means of numerical simulations, which its results were validated through experiments using waterfall spectrum.

Tonal Noise Reduction in a Radial Fan With Forward-Curved Blades

The main focus of this work is on the geometrical modifications that can be applied to the fan wheel and the volute tongue of a radial fan to reduce the tonal noise. The experimental measurements are performed by using the in-duct method in accordance with ISO 5136. In addition to the experimental measurements, CFD (Computational Fluid Dynamics) and CAA (Computational Aeroacoustics) simulations are carried out to investigate the effects of different modifications on the noise and performance of the fan. It is shown that by modifying the blade outlet angle, the tonal noise of the fan can be reduced without affecting the performance of the fan. Moreover, it is indicated that increasing the number of blades leads to a significant reduction in the tonal noise and also an improvement in the performance. However, this trend is only valid up to a certain number of blades, and a further increment might reduce the aerodynamic performance of the fan. Besides modifying the impeller geometry, new volute tongues are designed and manufactured. It is demonstrated that the shape of the volute tongue plays an important role in the tonal noise generation of the fan. It is possible to reduce the tonal noise by using stepped tongues which produce phase-shift effects that lead to an effective local cancellation of the noise.

Introduction of an Universal Scale-Up Method for the Efficiency of Axial and Centrifugal Fans

Acceptance tests on large fans to prove the performance (efficiency and total pressure rise) to the customer are expensive and sometimes even impossible to perform. Hence there is a need for the manufacturer to reliably predict the performance of fans from measurements on down-scaled test fans. The commonly used scale-up formulas give satisfactorily results only near the design point, where inertia losses are small in comparison to frictional losses. At part- and overload the inertia losses are dominant and the scale-up formulas used so far fail. In 2013 Pelz and Stonjek introduced a new scaling method which fullfills the demands ( [1], [2]). This method considers the influence of surface roughness and geometric variations on the performance. It consists basically of two steps: Initially, the efficiency is scaled. Efficiency scaling is derived analytically from the definition of the total efficiency. With the total derivative it can be shown that the change of friction coefficient is inversely proportional to the change of efficiency of a fan. The second step is shifting the performance characteristic to a higher value of flow coefficient. It is the task of this work to improve the scaling method which was previously introduced by Pelz and Stonjek by treating the rotor/impeller and volute/stator separately. The validation of the improved scale-up method is performed with test data from two axial fans with a diameter of 1000 mm/250mm and three centrifugal fans with 2240mm/896mm/224mm diameter. The predicted performance characteristics show a good agreement to test data.

Development of a New Design Method for High Efficiency Swept Low Pressure Axial Fans With Small Hub/Tip Ratio

For a competitive low pressure axial fan design low noise emission is as important as high efficiency. In this paper a new design method for low pressure fans with a small hub to tip ratio including blade sweep is introduced and discussed based on experimental investigations. Basis is an empirical axial and tangential velocity distribution at the rotor outlet combined with a distinctive sweep angle distribution along the stacking line. Several fans were designed, built and tested in order to analyze the aerodynamic as well as the aeroacoustic behavior. For the aerodynamic performance particular attention was paid to compensate the influence of reduced pressure rise and efficiency due to increasing blade sweep. This was achieved by a method of increasing the blade chord depending on the local sweep angle which is based on single airfoil data. The tested fans without this compensation revealed a significant noise reduction effect of up to approx. 6 dB(A) for a tip sweep angle of 64° which was accompanied by an unsatisfactory effect of reduced overall aerodynamic performance. The second group of fans did not only confirm the method of the aerodynamic compensation by a nearly unchanged pressure rise and efficiency characteristic but also revealed an increased aeroacoustic benefit of in average 9.5 dB(A) compared to the unswept version. Beside the overall characteristics the individual differences between the designs are also discussed using results of wall pressure measurements which show some significant changes of the blade tip flow structure.

Hybrid RANS/ILES for Aero Engine Intake

Hybrid, Implicit Large Eddy Simulations (ILES) for an idealized aero engine intake in a crosswind is performed. The ILES zone is smoothly blended to a near wall Reynolds Averaged Navier-Stokes (RANS) zone. The flow has a region of high favourable pressure gradient (FPG) where the streamwise acceleration parameter (KS) is found to be greater than 3×10−6. This is sufficient to laminarize the boundary layer (BL). As a consequence, the turbulence in the boundary is severely suppressed and this interacts with a shock causing separation and distortion at the engine fan face. This is known to be undesirable for aero engines. The separated shear layer reenergizes turbulence and this promotes reattachment. The calculation in the RANS zone has been enhanced with a novel three-component RANS model and this is used in the hybrid RANS/ILES framework. Simulations also consider the modelling of roughness. The turbulent statistics and the engineering relevance of these are also discussed in this work. Broadly, encouraging agreement is found with measurements. Substantial accuracy improvements are found relative to standard RANS model simulations. The accuracy of the hybrid simulations is also contrasted with pure ILES and the critical need for the RANS layer shown for modest grids.