Flow in turbo-machines is characterized by a large number of transient effects. This is particularly true for the flow around blades. Downstream wakes of each blade in a row affect the following rows. This gives poor aerodynamic quality and leads to blade vibrations as well as negative acoustic effects throughout the engine. By time-resolved measurements of the velocity field and wall pressure unsteady interaction effects can be visualized and thus help to understand flow physics. Hot-wire, total pressure and wall-mounted pressure probes as well as visualization techniques such as laser and PIV can be used to acquire those data sets. Turbulence parameters can be determined, which can be used for the validation of fluid mechanics and aero-acoustic computational models.
Unsteady aerodynamic in turbo-machines
Hot-wire anemometry and unsteady CFDsimulations
One way to visualize unsteady effects such as wakes behind blade rows is hot-wire anemometry. With this technique very thin wires are electrically heated and cooled by the fluid around it. This cooling effect can be measured by monitoring the change in the resistors and converting it into a velocity. Due to the very high sampling rates that are possible, the hot-wire anemometry has a very high temporal resolution. This allows the measuring and visualization of unsteady flow effects over a wide range. The acquired data can be used as boundary conditions for numerical simulations. By applying Fast-Fourier-Transformations (FFT) to the time-resolved velocity data sets one is able to calculate the level of turbulence, which is an important input parameter for turbulence modelling and accurate CFD simulations.
For the simulation of the temporal fluctuations in a turbo-machinery stage the CFD solver TRACE is used. TRACE solves the compressible Navier-Stokes equations. From these CFD simulations the spatial and temporal change of the deterministic component for each flow parameter can be examined. At selected locations, these quantities can be compared with data from experiments in order to validate the results of the simulations. A detailed analysis of the spatial and temporal gradients allows a better understanding of the physics behind the flow.
Unsteady aerodynamic and noise prediction
Unsteady flow phenomena are responsible for the formation of noise. In a turbo-machine they appear in form of convective wakes and rotating pressure fields. To predict sound with analytical tools (e.g. PropNoise) it is important to map the occurring flow variations properly. For resource and time reasons, it is often not possible to perform unsteady flow simulations. In this case one has to resort to a less expensive stationary RANS calculation. The unsteady effects have to be reconstructed accordingly in order to obtain the necessary input variables for the acoustic models. This is part of the current research in the Department of engine acoustics.
Example: Rotating Instabilities
The so called Rotating Instability (RI) is a phenomenon attributive to the field of unsteady aerodynamics in turbomachinery. 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. Existing models assume that the unsteady characteristics of the clearance vortex system induces RI. So far, these models are not able to predict the observed frequencies and modes. A new model is a prerequisite for the development of safer, more efficient, and noise reduced compressors. Rotating Instability was observed in axial rotor and stator configurations but also in centrifugal- and radial compressors and low pressure turbine stages. 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 (more details are given here).
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