Aeroacoustics, aeroelasticity & wind energy

Unsteady blade interactions and flow phenomena in turbomachinery can cause considerable noise emissions. In addition, acoustic resonances can occur, which not only increase the noise level, but can also trigger vibrations in the machine structures. The analysis and evaluation of the generated and propagating noise are therefore crucial for the acoustic optimization and safety of turbomachinery. At the TFD, we are therefore working on the precise prediction of acoustic conditions. For this purpose, we use numerical methods in the time and frequency domain as well as analytical models.

Experimental acoustic investigations on conventional compressor and turbine test benches are often difficult due to the lack of controlled acoustic boundary conditions. At the TFD, however, the aeroacoustic wind tunnel (AWT) is a test rig that can specifically generate such conditions. By using a sound generator, artificial acoustic modes can be imposed on the flow field, the propagation behavior of which we can investigate in detail by means of experimental radial mode analysis through rows of blades. In addition to transient pressure transducers, such as Kulites and microphones, pressure-sensitive paint is used for the areal measurement of transient pressures.

In order to ensure the reproducibility of acoustic measurements and to examine blade rows under acoustically optimized boundary conditions, acoustic similarity relationships must be applied in addition to aerodynamic ones. This makes it possible to examine the blade series under representative acoustic boundary conditions in acoustically optimized test benches. At the TFD, corresponding similarity relationships for sound generation and propagation are developed and validated.

For the validation of the aeroelastic models, experimental validation data is generated on the TFD test benches. The investigations are carried out on the TFD's realistic turbine and compressor test benches. In addition, the advanced measurement techniques allow the experimental investigation of physical phenomena. Test rig configurations explicitly designed for aeroelastic investigations are available for this purpose.

The measurement techniques used are:

•     Tip timing
•     Strain gauges
•     Transient pressure transducers
•     Digital image correlation
•     Pressure-sensitive color

Due to the interactions between rotating and stationary blades in turbomachinery, unsteady excitation forces always occur, which can excite blade vibrations and lead to blade failure (high cycle fatigue). At the TFD, numerical time and frequency domain methods in structural and aerodynamics are used to improve the prediction of blade vibrations and to develop methods to reduce vibration amplitudes. Our research focuses on acoustic and multi-stage effects, thermal streaks, blade-rotor coupling, mistuning, fan-intake coupling, crosswind excitation in the fan and mode shape tailoring using fiber composites.

Non-synchronous vibrations occur at non-integer multiples of the rotational frequency and are therefore more difficult to predict. When designing turbomachinery, the aeroelastic stability of the blading, i.e. positive aerodynamic damping, must be guaranteed. If this is not the case, flutter will occur.  At the TFD, we therefore use an acoustic excitation system to determine blade damping during operation. Another relevant vibration phenomenon is convective non-synchronous vibrations, which are excited by aerodynamic disturbances rotating around the circumference (rotating instabilities). Both jeopardize safe operation. We at TFD are therefore working on improved methods for predicting and better understanding these phenomena. To avoid flutter and convective non-synchronous vibrations, we also use mode shape tailoring with fiber composites in addition to conventional measures.

Due to fluctuating wind fields and vibrations of the rotor blades of wind turbines, the profiles of wind turbines experience constantly varying inflow conditions. As a result, an airfoil can briefly generate a significantly higher lift than would be the case under static conditions. However, this dynamic stall also generates transient loads on the structure, which must be taken into account in the design. At the TFD, we are therefore developing reduced-order models that are suitable for the design process of wind turbines and can map these dynamic effects.

When a rotor blade runs past the tower of a wind turbine, transient interactions occur that can pose risks to the bearings, the drive train, the tower and the rotor blades and cause fluctuations in performance. In order to better understand these effects and take them into account in future designs, we are developing innovative measurement techniques at the TFD. These include, for example, pressure measuring belts that are attached to the tower. Based on these measurements, we develop reduced-order models, also known as tower influence models, which we develop, validate and continuously optimize for the design of wind turbines.

Significant deformations can occur during the operation of wind turbines, which affect both the performance and the safety of the turbines. However, conventional measurement systems on wind turbines do not provide sufficient data for a comprehensive analysis of such deformations. At the TFD, we therefore use digital image correlation (DIC) and are developing it further in order to carry out full-surface deformation measurements on the rotor blades. This allows both bending and torsional deformations and vibrations of the rotor blades to be precisely analyzed over longer periods of time.