C. Oliver Paschereit

Spectral proper orthogonal decomposition

The identification of coherent structures from experimental or numerical data is an essential task when conducting research in fluid dynamics. This typically involves the construction of an empirical mode base that appropriately captures the dominant flow structures. The most prominent candidates are the energy-ranked proper orthogonal decomposition (POD) and the frequency-ranked Fourier decomposition and dynamic mode decomposition (DMD). However, these methods are not suitable when the relevant coherent structures occur at low energies or at multiple frequencies, which is often the case. To overcome the deficit of these ‘rigid’ approaches, we propose a new method termed spectral proper orthogonal decomposition (SPOD). It is based on classical POD and it can be applied to spatially and temporally resolved data. The new method involves an additional temporal constraint that enables a clear separation of phenomena that occur at multiple frequencies and energies. SPOD allows for a continuous shifting from the energetically optimal POD to the spectrally pure Fourier decomposition by changing a single parameter. In this article, SPOD is motivated from phenomenological considerations of the POD autocorrelation matrix and justified from dynamical systems theory. The new method is further applied to three sets of PIV measurements of flows from very different engineering problems. We consider the flow of a swirl-stabilized combustor, the wake of an airfoil with a Gurney flap and the flow field of the sweeping jet behind a fluidic oscillator. For these examples, the commonly used methods fail to assign the relevant coherent structures to single modes. The SPOD, however, achieves a proper separation of spatially and temporally coherent structures, which are either hidden in stochastic turbulent fluctuations or spread over a wide frequency range. The SPOD requires only one additional parameter, which can be estimated from the basic time scales of the flow. In spite of all these benefits, the algorithmic complexity and computational cost of the SPOD are only marginally greater than those of the snapshot POD.

The impact of heating the breakdown bubble on the global mode of a swirling jet: Experiments and linear stability analysis

This study investigates the dynamics of non-isothermal swirling jets undergoing vortex breakdown, with an emphasis on helical coherent structures. It is proposed that the dominant helical coherent structure can be suppressed by heating the recirculation bubble. This proposition is assessed with Stereo Particle Image Velocimetry (PIV) measurements of the breakdown region of isothermal and heated swirling jets. The coherent kinetic energy of the dominant helical structure was derived from PIV snapshots via Proper Orthogonal Decomposition. For one set of experimental parameters, mild heating is found to increase the energy content of the dominant helical mode. Strong heating leads to a reduction by 30\% of the coherent structures energy. For a second set of experimental parameters, no alteration of the dominant coherent structure is detectable. Local linear stability analysis of the time-averaged velocity fields shows that the key difference between the two configurations is the density ratio at the respective wavemaker location. A density ratio of approximately 0.8 is found to correlate to a suppression of the global mode in the experiments. A parametric study with model density and velocity profiles indicates the most important parameters that govern the local absolute growth rate: The density ratio and the relative position of the density profiles and the inner shear layer.

A Kinetics Model For The Shockless Explosion Combustion

Constant-volume (pressure-gain) combustion cycles show much promise for further increasing the efficiency of modern gas turbines, which in the last decades have begun to reach the boundaries of modern technology in terms of pressure and temperature, as well as the ever more stringent demands on reducing exhaust gas emissions. The thermodynamic model of the gas turbine consists of a compressor with a polytropic efficiency of 90%, a combustor modeled as either a pulse detonation combustor (PDC) or as an isobaric homogeneous reactor, and a turbine, the efficiency of which is calculated using suitable turbine operational maps. A simulation is conducted using the one-dimensional reacting Euler equations to obtain the unsteady PDC outlet parameters for use as turbine inlet conditions. The efficiencies for the Fickett–Jacobs and Joule cycles are then compared. The Fickett–Jacobs cycle shows promise at relatively low compressor pressure ratios, whereas the importance of the harvesting of exhaust gas kinetic energy for the cycle performance is highlighted.

Video: Sweeping Jet from a Fluidic Oscillator in Crossflow

Jets in crossflow are a fundamental flow scenario relevant for various technical applications. Recent studies have indicated promising improvements for several applications by using spatially oscillating jets (i.e., sweeping jets) generated by fluidic oscillators. These are devices that are able to emit a sweeping jet without requiring any moving parts. This advantage makes them attractive for flow control applications. Several studies have proven their effectiveness for mixing enhancement, reduction of drag and noise, and separation control [1]. However, the reasons for their effectiveness remains widely unknown due to the lack of fundamental knowledge on the oscillators themselves and on the interaction between the sweeping jet and a crossflow. Ostermann et al. described the time-resolved internal mechanisms inside a fluidic oscillator [2,3] and investigated the interaction of a sweeping jet with a crossflow [4,5]. The video presented here provides a time-resolved, threedimensional visualization of the experimentally acquired flow field of a sweeping jet interacting with a crossflow. The experiments are conducted in a wind tunnel where the fluidic oscillator is installed inside a splitter plate. The oscillation plane spanned by the sweeping jet is perpendicular to the crossflow direction. The velocity ratio between jet and crossflow is three and the oscillation frequency is 67 Hz. A traversable stereoscopic particle image velocimetry (PIV) system is employed for acquiring the flow field plane-by-plane. A pressure signal provides a temporal correlation between the various PIV planes, which enables phase-averaging to yield the three-dimensional and time-resolved flow field for one oscillation period [6]. More information on the setup are provided in Ref. [4]. After describing the setup, the video displays the velocity magnitude, which shows the sweeping movement of the jet. It also indicates that the jet is being bent into the direction of the crossflow. The streak volume of the jet (Fig. 1) reveals more detailed flow features. The streak volume is obtained by tracing virtual particles in the flow field through time. Due to its visual similarity to ink-based visualizations, it enables a more intuitive interpretation of the flow field and exposes dominant flow features. Two counter-rotating vortices oriented in streamwise direction are very prominent. They form alternatingly downstream of the jet. An instantaneous cross-section through the flow field visualizes their sense of rotation (Fig. 2). In-plane velocity vectors infer that their sense of rotation is opposite to that of the counter-rotating vortex pair formed downstream of a steady jet in crossflow. Therefore, fluid motion toward the wall is induced in between these vortices. Based on inviscid vortex dynamics, this sense of rotation keeps the vortices close to the wall over a longer downstream distance, thereby affecting a greater area. The presence of such dominant streamwise vortices and their sense of rotation is suspected to be one aspect for the high efficacy of sweeping jets in flow

Poster: A graphical representation of the spectral proper orthogonal decomposition

We consider the spectral proper orthogonal decomposition (SPOD) for experimental data of a turbulent swirling jet. This newly introduced method combines the advantages of spectral methods, such as Fourier decomposition or dynamic mode decomposition, with the energy-ranked proper orthogonal decomposition (POD). This poster visualizes how the modal energy spectrum transitions from the spectral purity of Fourier space to the sparsity of POD space. The transition is achieved by changing a single parameter – the width of the SPOD filter. Each dot in the 3D space corresponds to an SPOD mode pair, where the size and color indicates its spectral coherence. What we notice is that neither the Fourier nor the POD spectrum achieves a clear separation of the dynamic phenomena. Scanning through the graph from the front plane (Fourier) to the back plane (POD), we observe how three highly coherent SPOD modes emerge from the dispersed Fourier spectrum and later branch out into numerous POD modes. The spatial properties of these three individual SPOD modes are displayed in the back of the graph using line integral convolution colored by vorticity. The first two modes correspond to single-helical global instabilities that are well known for these flows. Their coexistence, however, has not been observed until now. The third mode is of double- helical shape and has not been observed so far. For this considered data set and many others, the SPOD is superior in identification of coherent structures in turbulent flows. Hopefully, it gives access to new fluid dynamic phenomena and enriches the available methods.

Feasibility Study of a Recuperated Turboshaft-Engine Based on a Micro-Gasturbine

The present paper shows the current state of a feasibility study of the University of Applied Sciences Esslingen. The intention of this study is the conversion of a model-turbine into a turboshaft-engine for variable applications, with as few as possible modifications. The shaft power of the engine is estimated on 20 kW at least. It is intended to use a recuperator to augment its efficiency. After a general introduction possible applications are discussed and the previous design-process is explained: Subsequent to the concept-phase cycle parameters were calculated and the power turbine was designed and manufactured. At present turbine tests are running. The recuperator is of counter flow type. To shorten the flow path it is mounted directly around the combustor. Currently different variations are being designed which will be optimised. The pressure loss within the exhaust manifold between power turbine and recuperator has already been reduced by simulations and tests. This will be minimised through application of a genetic optimisation software.Copyright