Jens Nitzsche

Prediction of aeroelastic limit-cycle oscillations based on harmonic forced motion oscillations

Aeroelastic limit-cycle oscillations due to aerodynamic nonlinearities are usually investigated using coupled fluid–structure interaction simulations in the time domain. These simulations are compu...

Accurate load prediction by BEM with airfoil data from 3D RANS simulations

In this paper, two methods for the extraction of airfoil coefficients from 3D CFD simulations of a wind turbine rotor are investigated, and these coefficients are used to improve the load prediction of a BEM code. The coefficients are extracted from a number of steady RANS simulations, using either averaging of velocities in annular sections, or an inverse BEM approach for determination of the induction factors in the rotor plane. It is shown that these 3D rotor polars are able to capture the rotational augmentation at the inner part of the blade as well as the load reduction by 3D effects close to the blade tip. They are used as input to a simple BEM code and the results of this BEM with 3D rotor polars are compared to the predictions of BEM with 2D airfoil coefficients plus common empirical corrections for stall delay and tip loss. While BEM with 2D airfoil coefficients produces a very different radial distribution of loads than the RANS simulation, the BEM with 3D rotor polars manages to reproduce the loads from RANS very accurately for a variety of load cases, as long as the blade pitch angle is not too different from the cases from which the polars were extracted.

Numerical Modeling of Wind Tunnel Walls for the Investigation of Oscillating Airfoils

The numerical modeling of airfoil oscillations in wind tunnels is investigated on the basis of frequency response functions of the flow field due to a forced motion input employing unsteady RANS simulations. The analyzed models reasonably predict the unsteady wind-tunnel wall effects, including the acoustic wind tunnel resonance. For high frequencies, however, the model shows significant spurious fluctuations related to non-physical reflections at the outflow boundary. Therefore, both increasing the distance to the computational boundaries and a nominally less-reflective boundary condition according to Hedstrom (1979, J. Comput. Phys. 30:222–237) are examined leading to slightly improved results.

Numerical Experiments on Aerodynamic Resonance in Transonic Airfoil Flow

We present the results of 2-d URANS simulations of unsteady shock/-boundary layer interaction on a supercritical airfoil in transonic flow. At constant Mach and Reynolds number the angle of attack is gradually increased until self-sustained periodic shock buffet oscillations set in. Subsequently, we focus on the subcritical flow field dynamics below the identified shock buffet onset, where already damped flow oscillations can be observed. Therefore, various fixed-point stable flows are perturbed with small time-periodic deflections of the airfoil geometry or random impulses, after which the particular flow response is analyzed in the frequency domain to identify the dominant aerodynamic eigenvalue. Furthermore, we demonstrate an effective stabilization of sub- and supercritical shock buffet flows by means of a closed-loop controller.

Bifurcations of limit-cycle oscillations of a two degree-of-freedom airfoil caused by aerodynamic non-linearities

Flutter is usually predicted using linearised theory. In reality, flutter is always non-linear and might already occur below the linearly predicted flutter boundary. Whether this is the case for limit-cycle oscillations (LCOs) caused by aerodynamic non-linearities is not known, since these LCOs can only be predicted using expensive wind-tunnel tests or coupled Computational Fluid Dynamics (CFD)-Computational Structural Mechanics (CSM) simulations. However, it is important to know whether a sufficiently large disturbance can already cause LCOs below the flutter boundary predicted from linearised theory. Furthermore, since structural properties and the flow conditions will vary, it is necessary to study the resulting variations of the Hopf bifurcation behaviour of the LCO solutions near the flutter point. In this work viscous and inviscid transonic flows are considered. The LCO bifurcation behaviour was found to vary significantly when the uncoupled structural natural frequency ratio and the location of the elastic axis are changed. When the non-linearity is relatively weak, a change in the Hopf bifurcation type might result. A Mach number variation in inviscid flow showed that the effective flutter boundary might significantly deviate from that predicted using linearised theory. For both the structural parameter variations and the Mach number variation, LCOs were observed below the linearly predicted flutter boundary. At the nominal structural parameters, the amplitude-dependent behaviour of the phase of the lift was found to be responsible for the type of bifurcation of the LCO solution that occurs. Inspection of the local force distributions at various pitch amplitudes showed that the motion of the shock wave on the lower surface is responsible for the behaviour of the phase of the lift and hence for the bifurcation behaviour of the LCOs observed in this work.