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Multidisciplinary Optimization of Aircraft Propeller Blades

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VKI PHDT 2002-03, Benoît Marinus, Multidisciplinary Optimization of
Aircraft Propeller Blades, ISBN 978-2-87516-024-9, 267 pgs

Multidisciplinary Optimization of Aircraft Propeller Blades

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Multidisciplinary Optimization of Aircraft Propeller Blades

By Benoît Marinus

PhD Thesis from the von Karman Institute/Royal Military Academy and “Doctor in Acoustics”, Ecole Centrale de Lyon, November 2011, 978-2-87516-024-9, 267 pgs

Abstract

Open rotors are known to have significant advantages in terms of propulsive efficiency. These advantages translate directly in reduced fuel burn so that they nowadays benefit from a surge of interest. At the same time, recent advances in numerical simulations make the application of multidisciplinary optimization  for the demanding design of transonic propeller blades, an affordable option.  Therefore, an optimization  method in which the performance objectives of aerodynamics, aeroacoustics and aeroelasticity compete against each other, is developed and applied for the design of high-speed single-rotation propellers.

The optimization  is based on Multi-Objective  Differential  Evolution  (MODE). This technique is a particular kind of evolutionary algorithm that mimics the natural evolution of populations by relying on the selection, recombination and eventually mutation  of blade designs, each of them being represented by a vector of design variables (e.g. chord width,  tip  sweep, etc).   MODE  has the advantage of dealing concurrently with  all the objectives in the selection of potentially  promising designs among a population.   In  order to  keep the computational  cost within  reasonable margins, the assessment of the performance of proposed designs is done in a two-level approach. A metamodel provides performance estimates for each proposed design at extremely low computational effort while high-fidelity analysis codes provide accurate performance values on some promising designs at much higher cost. To safeguard the accuracy of the estimates, the metamodel is initially trained on a population that is specifically assembled for that  purpose. The training  is repeated from time to time with the high-fidelity  performance values of promising designs. Different high-fidelity tools have been developed and used for the assessment of performance.

The CFD-tool performs steady RANS simulations of a single blade passage of the isolated propeller in free air under zero angle of attack.  These simulations provide the aerodynamic performance values.  The full  propeller is modelled  thanks to cy- clic boundary conditions.  The k ? ? turbulence model is used in combination with wall treatment.   Adiabatic  no-slip wall conditions are imposed on the spinner and blade surfaces whereas the test-section radial  boundary is reproducing the effects of a pressure far-field.  This approach has proven its robustness and, above all, its accuracy as satisfactory agreement with experimental results has been found for dif- ferent  operating conditions over a wide range of blade shapes, as well as sufficient grid independency.

In the post-processing of the aerodynamic results, the Sound Pressure Level (SPL) is computed for tonal noise at various observer locations by the aeroacoustic solver (CHA).  Formulation  1A from Farassat is used for this purpose. This formulation  is related to the inhomogeneous wave equation derived from Lighthill’s acoustic analogy by Ffowcs Williams  and Hawkings (FW-H).  It benefits from the partial  decoupling of the acoustic and aerodynamic aspects and is particularly suited to compute the noise from propellers. The thickness noise and loading noise are expressed by separate equations in the time-domain whereas the quadrupole source term is dropped from the original FW-H equation. The blade surface is chosen as integration surface and a newly developed truncation  technique is applied to circumvent the mathematical sin gularity  arising when parts of the blade reach sonic conditions in terms of kinematics with  respect to the observer. This approach delivers accurate values at acceptable computational cost.

Besides, CSM-computations make use of a finite elements solver to compute the total mass of the blade as well as the stresses resulting from the centrifugal and aero- dynamic forces. Considering the numerous possibilities to tailor  the blade structure so that it properly takes on the stresses, only a simplified blade model is implemented. The simplified blade is a monocoque design. The shell is composed of several layers of braided composite and the core is filled with  foam.  Additionally, the aeroelastic problem is decoupled from the aerodynamic one as the analysis is performed solely on the ’cold’ blade shape. Hence the CSM computations provide a convenient, yet rudi- mentary, sanity check from a structural  point of view. Nonetheless, the optimization tends to minimize the mass and stresses; to some respect, this should also benefit a tailored structure analyzed with full coupling.

A first optimization  is performed by integrating  the CFD and CHA tools in the optimizer.  It uses a broad set of design variables controlling the spanwise variations of chord, thickness, sweep and twist  of the  blade as well as the airfoil  shape. The propellers are assessed under different operating conditions corresponding to cruise (M?  = 0.75) and take-off/landing  (M?  = 0.2), each time at various rpm so that off- design performance is also taken into account. The objectives aim at increasing the efficiency while reducing the noise in and out of the propeller plane. This optimization delivered a small number of optimized blades (28). All of them satisfy an extensive set of operational constraints.  Moreover, it pointed out the weaknesses and deficiencies of the way the optimization  method and high-fidelity  tools have been implemented. A few specific blade designs are analyzed in more detail to better  understand the potential  of the set of design variables.   These designs have peculiar geometrical features and offer only small improvement in terms of efficiency but a substantial one in terms of noise.

A second optimization  is performed with complete integration of the CFD, CHA and CSM tools. The design variables allow again complete control of the blade shape. Propellers are assessed under both operating conditions, though this time cruise is at M? = 0.7. One objective is defined to improve the aerodynamic efficiency, another to minimize noise emissions and a last one to decrease blade mass and stress levels. The optimization  provided a set of 61 optimized designs that are fully compliant with operational as well as structural constraints. Specific designs are once more subject to a thorough analysis. These designs also have innovative geometrical features. They come with a substantial increase in efficiency but a moderate decrease of the emitted noise. Their geometrical features do not cause unacceptable  levels of stresses.

Both optimizations illustrate  the feasibility and the capabilities of the method. It is proficient in exploring the search space and delivering designs with  features that are worth  further  investigation.   One of them is the humps on the blade obtained by smoothly increasing the chord of a specific region of the blade.  Finally,  general conclusions are drawn and improvements to the method are proposed.

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