Athanasios Dafnis

Dynamic aero-structural response of an elastic wing model

Abstract A highly elastic rectangular wing model with a supercritical airfoil was designed and manufactured to study aero-structural equilibrium configurations and aerodynamic damping at various speeds in a subsonic wind tunnel. The supporting structure is a cross-shaped spar. A foam with negligible stiffness is used to provide the aerodynamic surface of the wing. Experimental data are used to examine a coupled algorithm for simulating fluid–structure interaction which simultaneously solves the Euler or Navier–Stokes equations and the structural dynamics equations in the time domain. The elastic wing is modelled by a generalized Timoshenko-type beam with six degrees of freedom for a material cross-section. Correct reproduction of the aero-structural equilibrium shape of the wing model and the deflection under zero lift and nonzero lift conditions, time consistency of unsteady deformations and pressure fluctuations are examined by comparing numerical and experimental results.

Structural Concept and Design for Modular and Serviceable Spacecraft Systems

The performance of spacecraft systems can be improved significantly by a modular and serviceable design approach. Maintenance operations as well as system upgrades in context of On-Orbit Servicing (OOS) become feasible. To enable autonomous maintenance and reconfiguration operations in orbit, one major challenge is the development of a suitable structural concept. The general idea to achieve a modular design is based on the fragmentation of common satellite systems into standardized building blocks. This paper depicts the design process of the primary structural components and the required docking interface. Different concepts for both parts are presented, designed and finally assessed by multiple criteria.

Aero-structural Dynamics Experiments at High Reynolds Numbers

The elastic wing model, its excitation and comprehensive high frequency measuring equipment for the High Reynolds Number Aero-Structural Dynamics (HIRENASD) tests in the European Transonic Windtunnel (ETW) are shortly described. Some of the stationary polars are presented in terms of wing deformation, as well as aerodynamic coefficients and pressure distributions. Then unsteady processes observed in the measurements of static aerodynamic coefficients, are regarded with focus on small amplitude pressure waves travelling upstream from the trailing edge and triggering periodically break-down and redeployment of the local supersonic domains with transonic shock waves to run upstream and to disappear. Another focus is on stochastic vibrations excitation while moving forward during nominally static experiments. Emphasis is put on measured variations of pressure distribution on the wing surface caused by defined vibration excitation applying internal force couples at the wing root, whereby the exciter frequencies were chosen close to natural frequencies of the wing model. Phase and magnitude of measured local lift fluctuations as well as real and imaginary parts of pressure distributions are presented.

Design, Qualification and Experimental Investigation on Flexible Wind Tunnel Wing Models

The purpose of this work on highly flexible wing models is the validation of the numerical direct simulation, developed in the Collaborative Research Center SFB 401. The first step in the validation process is the investigation of a flexible straight wing model in the subsonic region. The design and construction of this wing model will be described under consideration of aerodynamic and structural constraints. A short introduction in the measurement techniques and the tests under wind-off and windon conditions will be given. Different data reduction methods to obtain the frequency and the damping factor during wind tunnel tests will be introduced. Some experiments under steady state and transient flow conditions will be presented for validation of the numerical calculations.

Numerical analysis of acoustic loads generated by supersonic jets

A hybrid computational fluid dynamics (CFD) and computational aeroacoustics (CAA) method is used to compute the flow and the acoustic field of supersonic jets at a Mach number of 3.6. The flow simulations are performed by highly resolved large-eddy simulations (LES) from which sound source terms are extracted to compute the acoustic field by solving the acoustic perturbations equations (APE). The acoustic loads are determined on the structural components to obtain the correct dynamic behavior of non-rigid surfaces at atmospheric flight conditions using fluid-structure interaction (FSI) methods.