Research at the department of Engine Acoustics is dedicated to the sound produced by the flow within aero-engines of civil aircraft. Sound may also be induced by the vibrations of the blades, however this mechanism is not the dominant one in aero-engines.
The Acoustic Analogy proposed by Lighthill in 1952 has strongly influenced the field of Aeroacoustics. It postulates that flow phenomena (i.e. turbulent eddies impinging onto a blade) generate sound sources, which then emit sound waves that propapate through the flow field without interacting with it. This model applies the simple principle of cause and effect applied to flow and sound and it is appropriate as long as the flow and the sound fields are decoupled. Over the past 60 years, the application of the Lighthill analogy has enabled significant improvements in the acoustic design of turbomachines
However, for a further reduction of sound emissions in modern engine designs, we face the increasing need for an extended understanding of flow-induced sound generation beyond the simple concept of cause and effect. This problem is approached with experimental, analytical and numerical methods in the department of Engine Acoustics. Special attention is given to the relation between sound and unsteady features of the flow and a focus is put on the noise generated by rotating blades (e.g. fan, compressor, or turbine blades) and how they interact with struts or bifurcations in the bypass
Rotating instability (RI) occurs at off-design conditions in turbomachinery components, predominantly in axial compressor configurations with large tip clearances. RI is an indicator for critical operating conditions and is the source of the clearance noise. Characteristic spectral signatures with side-by-side peaks typically arise in the clearance region next to the leading edge (LE) at critical operating conditions. Each peak can be assigned to a dominant circumferential mode (see Figure).
So far, there is no theoretical model of the RI to predict the observed frequencies and modal structures. Such a model is a prerequisite for the development of safer, more efficient, and noise reduced compressors. The stochastic and unsteady characteristics of the RI phenomenon were proven by using time-resolved circumferential mode decomposition techniques in experiments both on an annular cascade (stator row) as well as on a laboratory axial fan stage (rotor-stator).
Further Reading & Publications:
B. Pardowitz, U. Tapken und L. Enghardt (2012): “Time resolved rotating instability waves in an annular cascade” in: 18th AIAA/CEAS Aeroacoustics Conference, 4.-6. Juni, Colorado Springs, USA, AIAA 2012-2132.
B. Pardowitz, U. Tapken, R. Sorge, P. U. Thamsen und Lars Enghardt (2013): “Rotating instabilitiy in an annular cascade: Detailed analysis of the instationary flow phenomena” in: 58th ASME Turbo Expo Conference, 3.-7. Juni, Texas, USA, GT 2013-95820.
Numerical methods for the prediction of broadband sound sources in fans
The sound emission of a fan can be split into two components: A deterministic and a stochastic part.
To overcome these problems, the department Technical Acoustics of the DLR-Institute of Aerodynamic and Flow Technology develops the Random Particle Mesh (RPM) method. It is used to reconstruct the stochastic fluctuations from the time-averaged quantities of a RANS. If these fluctuations are coupled to sound propagation tools such as PIANO, we can do broadband-noise prediction computations. I order to apply this method to the prediction of broadband noise in fans, we are working to expand it to cyclo-stationary processes and develop new models for interaction noise.
Experimental methods to reduce broadband noise in turbomachinery
Within the EU-funded projects PROBAND and FLOCON, experiments were conducted at the DLR Berlin lab-scale fan rig in order to identify the dominant sources of broadband noise. Three experimental methods for the reduction of fan broadband noise were investigated
Gurney flaps were located at the trailing edge of the stator vanes in order to modify the chordwise distribution of loading, especially to reduce the velocity peak on the suction side of vanes that are inclined relative to the mean flow direction (see figures 1 and 2).
Applying suction to the tip vortex produced by the rotor blades (see figure 3).
The blowing of pressurized air from the trailing edges of the rotor blades. This should fill the wakes and reduce the velocity gradients that are the source of strong turbulence production. As a result, the rotor-stator interaction noise may be diminished by decreasing the turbulent intensity impingin onto the stator vanes. This concept is shown in Fig. 4, the rotor blades are marked in red, the stator vane in black. A hot-wire probe is located between the rotor and the stator in order to measure the effect of the blowing from the trailing-edge on the shape of the wake and the turbulence intensity (see figure 4).
Fig. 2: Stator vanes equipped with Gurney Flaps
Fan stage interaction with installations in the bypass duct
Aero-engine development today strongly involves the reduction of fuel consumption by constructing shorter and thus lighter engines. But the shorter distances between individual components inside the engines increase the risk of generating noise by the interaction of components. An important noise generation mechanism is the interaction of a blade row with the potential field of a second blade row that rotates relative to the first. In order to prevent or at least to reduce this noise generation mechanism in an aero-engine fan stage, the rotor and the stator are usually separated by a reasonably large axial distance.
Less is known about the interaction of the fan stage with components further downstream in the bypass duct. In the project OPAL, it was shown that the potential field of bifurcations (large support struts used to mount the engine to the airframe) can have a strong impact on the generation of tonal noise in the fan stage. The typical tonal noise of a fan stage operating at undisturbed flow conditions consists of only a few well-defined modal structures. But the bifurcations can excite of a large number of additional modes and the total tonal acoustic energy can increase. The most important mechanism in the interaction of the fan stage with the bifurcation was identified to be the modification of the trajectories of the rotor-wakes. The potential field of the bifurcations deflects the rotor wakes into one azimuthal direction or the other. The phase of the wake impingement onto the stator varies with the azimuthal position of the stator vanes. This changes the phases of the acoustic sources on the stator blades and increases the number of excited acoustic modes. Furthermore, the interaction between the fan stage and the bifurcation can compromise the Cut-Off-Design of the fan which aims to selectively excite only those interaction modes that are not able to generate noise.