Send article to a friendPrint

Unsteady Aerodynamic Modelling

The knowledge of unsteady aerodynamic loads acting on an aircraft structure, which is moving either uniformly (steady) or accelerated (manoeuvring

Shock induced flow separation on the harmonically oscillating engine nacelle model WIONA, TAU-RANS simulation, rolling oscillation of pylon and nacelle, shown are pressure distribution and skin fiction lines.

.

Instantaneous vorticity distribution around two interfering NACA0012 airfoils, for heave oscillation, Ma=0.75, Re=10 million, red. frequency=1.257, TAU-RANS Simulation.

.

CFD grid deformation (upper) and Mach number distribution (lower) around an airfoil with oscillating flap, at shock induced flow separation, TAU-RANS Simulation.

.

Simplified CFD grid for an unsteady panel method (upper) and unsteady harmonic surface pressure distribution (lower left: magnitude, lower right: phase), computed from a reduced TAU-RANS data basis, for HALO research aircraft.

or oscillating), or within an unsteady environment (atmospheric turbulence, vortices, gusts), is important for the aeroelastic behaviour of the aircraft. Corresponding numerical prediction methods have to be further developed and improved, specifically for flight simulation in transonic and separated flow as well as for geometrically complex structures. Different applications need both, sophisticated methods solving the Reynolds-averaged Navier-Stokes (RANS) equations, and fast computing methods for rapid and robust estimation of aeroelastic characteristics within an aircraft’s multidisciplinary design and optimisation process.

Research and Modelling

Main topic is the reliable computation of unsteady aerodynamics, especially of phase shifts between harmonic structural oscillations and corresponding pressure fluctuations, for flutter prediction. Further topics are investigations of unsteady flow separation (buffet), of influences from gusts and vortices on lifting surfaces, impact of laminar-turbulent transition on aeroelasticity, as well as vortex separation on bluff bodies. These topics are mainly treated by RANS and Detached Eddy Simulations (DES).

Validation

Code validation regarding accuracy and regions of applicability of different unsteady aerodynamic CFD and fast methods is performed both by code to code comparison and by   comparison with windtunnel test results. A special focus is put on unsteady flow separation, shock-boundary layer interaction, laminar flow as well as on control surfaces, wing-tail and wing-nacelle interferences. These topics are an essential part of the DLR projects  HighPerFlex and iGREEN.

CFD development

Different unsteady aerodynamic simulation tools are applied, according to the different problems of aircraft research and development to be solved, first of all the TAU code of DLR, but in addition also faster tools, which are developed especially for unsteady applications both in time and frequency domain, at DLR Institute of Aeroelasticity. Examples are Euler boundary-layer coupling in time domain, and in the frequency domain TDLM and iSKEM, both extensions of the Doublet-Lattice Method (DLM),  as well as the linearised TAU code (in cooperation with DLR institute AS). The frequency domain methods need one or two steady flow field results as input for treating the superimposed unsteady flow. In addition Reduced Order Methods (ROM) are developed, which are for example based on a small number of  TAU-RANS simulations, thus providing a data set which can be used to reconstruct a manifold of possible solutions for a wide range of frequency, kind of motion and Mach number. All fast and reduced methods are designed with compatibility to commercial software systems like NASTRAN and ZAERO.

Generation of aerodynamic data sets

Unsteady airloads induced by oscillatory motions based on TDLM, Euler boundary-layer coupling, as well as by the reduced order RANS method CFD4Flutter have been generated and have supported NASTRAN and ZAERO flutter computations, flight mechanic modelling of complete aircraft, as well as design of windtunnel models. Among others this work has been carried out within the EC projects MOB and AWIATOR, within national research project MODYAS and for the research aircraft HALO.