Umberto Iemma

Boundary integral equations and conservative dissipation schemes for full potential transonic flows

A boundary integral formulation for the nonlinear aerodynamic analysis of three-dimensional full-potential transonic flows is presented. The emphasis here is on the analysis of the effects on the solution of artificial dissipation schemes, which are necessary in order to capture properly the physics of the phenomenon. The main novelty is the use of conservative schemes, never previously used in boundary integral formulations where all the existing approaches are based on non-conservative ones. The conservative scheme presented here is an adaptation of concepts used in the CFD community. Specifically, a linear dissipation term is added directly to the continuity equation: hence the name artificial mass-generation scheme. Both conservative and non-conservative full-potential expressions for the nonlinear terms are discussed. The corresponding TSP (transonic small perturbation) formulation are also analyzed. Numerical results, for two-dimensional steady flows are presented in order to assess the different schemes. Good agreement is obtained with existing finite-difference and finite-volume results.

Aeroacoustoelasticity in state-space format using CHIEF regularization

This paper deals with aeroacoustoelastic modeling for analysis of the acoustic field inside an aircraft cabin. The aim is the identification of a state-space format for aeroacoustoelasticity equations applicable, for instance, for synthesis of an active control law devoted to cabin noise abatement. Specifically, attention is focused on the development of the aeroelastic operator, starting from a boundary integral equation method for the exterior compressible-aerodynamics solution. As is well known, in such a type of application of the boundary integral equation method, singularities occur in the algebraic equations resulting from discretization of the integral operator. Here, the discretized aerodynamic operator is regularized by using the CHIEF technique, that consists of augmenting the algebraic problem with homogeneous conditions at points in the interior domain (the cabin space, in our problem). Then, in order to obtain the state-space format model of the aeroacoustoelastic operator, the resulting trascendental aerodynamic transfer functions between structural Lagrangean variables and generalized aerodynamic forces are approximated through rational polynomials, and the additional aerodynamic states induced by their poles are included in the set of state-space variables.

Community Noise Considerations in Multidisciplinary Optimization for Preliminary Design of Innovative Configurations

The paper proposes an approach for MDO/PD (Multi‐Disciplinary Optimization for Preliminary Design) of innovative aircraft configurations in the presence of aeroacoustics considerations. The specific innovative configuration of interest here is a box‐wing aircraft denoted as the Prandtl-Plane, which has, as a distinguishing feature, a low induced drag. Thus, one of the advantages is less noise at take-o. Hence here, as a very first step towards our long range goal, we apply the algorithm to the optimization of the Prandtl-Plane with the objective function modified by adding an empirical term representative of the take-o noise. The fact that the configurations are innovative requires that the formulation be first‐principle based, since in this case the designer cannot rely upon past experience. The mathematical model used is reviewed. The formulation is based upon an integrated modeling of structures, aerodynamics, aeroelasticity and flight mechanics developed primarily by the authors. The methodology is geared specifically towards MDO/PD for civil aviation. The emphasis here is on wing design ‐ the fuselage is assumed as given. The stress analysis is based on finite elements for beams, whereas the structural dynamics is based upon natural modes, which are evaluated by the same finite‐ element algorithm. For the aerodynamic analysis, a boundary‐element quasi‐potential‐flow method is used for both steady and unsteady aerodynamics. An elementary boundary layer model is used to include the steady viscous eects and estimate the drag. A reduced order model (ROM) for the unsteady‐aerodynamics forces is used in dynamic aeroelasticity. Numerical results on the optimized Prandtl-Plane configuration are included. It is shown how the addition of the community-noise term in the objective function aects the configuration and produces noise reduction.

On the vorticity generated sound: a transpiration-velocity/power-spectral-density approach

to be addressed. The objective of the paper is to present a formulation for the evaluation of the power spectral density of the acoustic pressure at any given point in the field in terms of the power spectral density of the transpiration velocity: this is a quantity defined in terms of the vorticity and is closely related to the equivalent source concept introduced by Lighthill 7 (the relationship between the two is addressed in Appendix B). Specifically, the formulation used allows one to obtain, in the frequency domain (Fourier transform), a matrix relationship between the transpiration velocity at a number of points on the surface of the object (those arising from the boundary‐element discretization) and the pressure at given points in the irrotational region. 1 From this, the relationship between the corresponding power spectral densities is easily obtained using the Wiener-Khintchine theorem. The approach used here is based upon a formulation introduced for aerodynamics in Ref. 11, and refined in Refs. 13 and 14. The commonality between aerodynamics and aeroacoustics is addressed in Ref. 12 (which provides a synthesis of all the preceding work), and is exploited here. Although applications to aeroacoustics implicitly imply compressibility, for the sake of clarity in the main body of the paper the formulation is presented for an incompressible flow (the formulation for compressible flows is presented in Appendix C).

Wall pressure fluctuations in hypersonic boundary layer: a strategy to design the passive noise control systems

The state of the boundary layer is of high importance since skin friction drag and heat transfer rates in a turbulent boundary layer can be several times higher than those of a laminar one. A lot of different strategies are used to delay or prevent the transition process. One possibility to manipulate the transition is using porous surfaces to influence the growth of the second mode in a passive way. The 2nd or so called Mack mode is dominant for the transition process at hypersonic Mach numbers. Some studies showed that an ultrasonically absorptive coating (UAC) can suppress the 2nd instability and then delay the transition of hypersonic boundary layer. The acoustic scattering problem affects the design of ultrasonic absorptive coatings for hypersonic laminar flow control. To investigate this phenomenon a finite element method (FEM) was employed to formulate 2-D simple model of UAC. The results provided are in strong agreement with existing results achieved by direct N-S solution (DNS) and theoretical models. DNS requires important computational costs while theoretical modelling is not adaptable to any type of geometry. Conversely, the FEM is extremely flexible and computationally affordable, resulting as the best candidate to perform design optimizations of more efficient UAC. Pressure time histories for coating of different porosities, various forcing frequencies and single or double cavity configurations are presented. Overall, the amplitude reflection by the UAC decreases with higher porosity and, in most cases, by reducing the dimensionless wavelength of the forcing. Hence, on the basis of our parametric study of the geometrical factors, we identified the dimensionless wavelength of the forcing λ∗ and the porosity ϕ as the most important parameters for UAC design. Moreover a comparison between UAC based on single-depth and double-depth rectangular cavity has been performed suggesting an interesting strategy to design a novel generation of UAC.

BEM for aerodynamics and aeroacoustics of rotors in subsonic/transonic forward flight

This paper deals with some recent developments of a boundary element methodology for the unified aerodynamic/aeroacoustic analysis of subsonic and transonic potential flows past helicopter rotors in hover and forward flight. In this methodology, first the potential solution is derived on the rotor-blade surfaces. Then, the boundary integral representation gives the potential everywhere in the field, and the Bernoulli theorem yields the acoustic pressure. Numerical results are presented for helicopter rotors in hover and forward flight at both subsonic and transonic rotor speeds. Comparisons with existing numerical results and experimental data are included.