Andreas Spohn

Experiments on vortex breakdown in a confined flow generated by a rotating disc

The steady-state flow generated by a rotating bottom in a closed cylindrical container and the resulting vortex breakdown bubbles have been studied experimentally. By comparing the flow inside two different container geometries, one with a rigid cover and the other with a free surface, we examined the way in which the formation and structure of the breakdown bubbles depend on the surrounding flow. Details of the flow were visualized by means of the electrolytic precipitation technique, whereas a particle tracking technique was used to characterize the whole flow field. We found that the breakdown bubbles inside the container flow are in many ways similar to those in vortex tubes. First, the bubbles are open with in- and outflow and second, their structure is, like in the case of vortex breakdown in pipe flows, highly axisymmetric on the upstream side of the bubble and asymmetric on their rear side. However, and surprisingly, we observed bubbles which are open and stationary at the same time. This shows that open breakdown bubbles are not necessarily the result of periodic oscillations of the recirculation zone. The asymmetry of the flow structure is found to be related to the existence of asymmetric flow separations on the container wall. If the angular velocity of the rotating bottom is increased the evolution of the breakdown bubbles is different in both configurations: in the rigid cover case the breakdown bubbles disappear but persist in the free surface case.

Cluster-based reduced-order modelling of a mixing layer

We propose a novel cluster-based reduced-order modelling (CROM) strategy of unsteady flows. CROM combines the cluster analysis pioneered in Gunzburgers group (Burkardt et al. 2006) and and transition matrix models introduced in fluid dynamics in Eckhardts group (Schneider et al. 2007). CROM constitutes a potential alternative to POD models and generalises the Ulam-Galerkin method classically used in dynamical systems to determine a finite-rank approximation of the Perron-Frobenius operator. The proposed strategy processes a time-resolved sequence of flow snapshots in two steps. First, the snapshot data are clustered into a small number of representative states, called centroids, in the state space. These centroids partition the state space in complementary non-overlapping regions (centroidal Voronoi cells). Departing from the standard algorithm, the probabilities of the clusters are determined, and the states are sorted by analysis of the transition matrix. Secondly, the transitions between the states are dynamically modelled using a Markov process. Physical mechanisms are then distilled by a refined analysis of the Markov process, e.g. using finite-time Lyapunov exponent and entropic methods. This CROM framework is applied to the Lorenz attractor (as illustrative example), to velocity fields of the spatially evolving incompressible mixing layer and the three-dimensional turbulent wake of a bluff body. For these examples, CROM is shown to identify non-trivial quasi-attractors and transition processes in an unsupervised manner. CROM has numerous potential applications for the systematic identification of physical mechanisms of complex dynamics, for comparison of flow evolution models, for the identification of precursors to desirable and undesirable events, and for flow control applications exploiting nonlinear actuation dynamics.

Bluff body drag manipulation using pulsed jets and Coanda effect

The impact of fluidic actuation on the wake and drag of a three-dimensional blunt body is investigated experimentally. Jets blowing tangentially to the main flow allow to force the wake with variable frequency and amplitude. Depending on the forcing conditions, two flow regimes can be distinguished. First, in case of broadband actuation with frequencies comprising the natural wake time scale, the convection of the jet structures enhances wake entrainment, shortens the length of the recirculating flow and increases drag. Secondly, at higher actuation frequencies, shear-layer deviation leads to fluidic boat-tailing of the wake. It additionally lowers its turbulent kinetic energy thus reducing the entrainment of momentum towards the recirculating flow. The combination of both mechanisms produces a raise of the base pressure and reduces the drag of the model. Both actuation regimes are characterized by complementary velocity, pressure and drag measurements at several upstream conditions and control parameters. By adding curved surfaces to deviate the jets by the Coanda effect, periodic actuation is reinforced and drag reductions of about 20% are achieved. The unsteady Coanda blowing not only intensifies the flow deviation and the base pressure recovery but also preserves the unsteady high-frequency forcing effect on the turbulent field. The present results encourage further development of fluidic control to improve the aerodynamics of road vehicles and provide a complementary insight into the relation between wake dynamics and drag.

Resonances in the forced turbulent wake past a 3D blunt body

We study the resonances of a forced turbulent wake past a flat-based bluff body using symmetric and antisymmetric actuation modes. The natural, unforced wake flow exhibits broadband dynamics superimposed on oscillatory motions linked to the reminiscent laminar Benard-von Karman instability in the turbulent flow. Harmonic and subharmonic resonances can be controlled by the phase relationship of periodic forcing and are linked to the symmetry properties of vortex shedding. Symmetric forcing leads to a strong subharmonic amplification of vortex shedding in the wake, but no harmonic excitation. The robustness of the subharmonic resonance is confirmed at different Reynolds numbers. Antisymmetric actuation, however, promotes a harmonic resonance with very similar wake and drag features.

Analysis of high speed jet flow physics with time-resolved PIV

This work focuses on a Mach 0.6 turbulent, compressible jet flow field with simultaneously sampled near and far-field pressure, as well as 10 kHz time-resolved PIV. Experiments have been conducted in the fully anechoic chamber and jet facility at Syracuse University. The PIV measurements were taken in the streamwise plane of the jet along the center plane at various downstream locations. In addition, measurements were taken off of the center plane to obtain a three-dimensional view of the jet flow. Active flow control (both open and closed-loop) was performed in order to see the effects on the potential core length and overall sound pressure levels. Various reduced-order models have been used to analyze previous experimental data sets at Syracuse University. This paper will focus on the analysis of the flow physics, using the time-resolved velocity field coupled with the simultaneously sampled pressure. Novel modeling approaches such as observable inferred decomposition and cluster-based reduced-order modeling have been implemented in an effort to link the near-field velocity with the far-field acoustics.

Cluster-based reduced-order modelling of shear flows

Cluster-based reduced-order modelling (CROM) builds on the pioneering works of Gunzburgers group in cluster analysis [1] and Eckhardts group in transition matrix models [2] and constitutes a potential alternative to reduced-order models based on a proper-orthogonal decomposition (POD). This strategy frames a time-resolved sequence of flow snapshots into a Markov model for the probabilities of cluster transitions. The information content of the Markov model is assessed with a Kullback-Leibler entropy. This entropy clearly discriminates between prediction times in which the initial conditions can be inferred by backward integration and the predictability horizon after which all information about the initial condition is lost. This approach is exemplified for a class of fluid dynamical benchmark problems like the periodic cylinder wake, the spatially evolving incompressible mixing layer, the bi-modal bluff body wake, and turbulent jet noise. For these examples, CROM is shown to distil nontrivial quasi-attr...

Data-Driven Methods in Fluid Dynamics: Sparse Classification from Experimental Data

This work explores the use of data-driven methods, including machine learning and sparse sampling, for systems in fluid dynamics. In particular, camera images of a transitional separation bubble are used with dimensionality reduction and supervised classification techniques to discriminate between an actuated and an unactuated flow. After classification is demonstrated on full-resolution image data, similar classification performance is obtained using heavily subsampled pixels from the images. Finally, a sparse sensor optimization based on compressed sensing is used to determine optimal pixel locations for accurate classification. With 5–10 specially selected sensors, the median cross-validated classification accuracy is ≥ 97 %, as opposed to a random set of 5–10 pixels, which results in classification accuracy of 70–80 %. The methods developed here apply broadly to high-dimensional data from fluid dynamics experiments. Relevant connections between sparse sampling and the representation of high-dimensional data in a low-rank feature space are discussed.

Aero-Acoustic Performance of Fractal Spoilers

One of the major environmental problems facing the aviation industry is that of aircraft noise. The work presented in this paper, done as part of the EU’s OPENAIR Project, looks at reducing spoiler noise whilst maintaining aerodynamic performance, through means of large-scale fractal porosity. It is hypothesised that the highly turbulent flow generated by fractal grids from the way the multiple-length-scales are organised in space, would reduce the impact of the re-circulation region and with it, the low frequency noise it generates. In its place, a higher frequency noise is introduced which is more susceptible to atmospheric attenuation and is less offensive to the human ear. A total of nine laboratory scaled spoilers were looked at, seven of which had a fractal design, one with a regular grid design and one solid for reference. The spoilers were inclined at an angle of 30◦. Force, acoustic and flow visualisation experiments on a flat plate were carried out and it was found that the present fractal spoilers reduce the low frequency noise by 2.5dB. Results show that it is possible to improve the acoustic performance by modifying a number of parameters defining the fractal spoiler, some of them very sensitively. From these experiments, two fractal spoilers were chosen for a detailed aero-acoustic study on a three-element wing system, where it was found that the fractal spoilers had a reduction of up to 4dB in the sound pressure level while maintaining similar aerodynamic performances as conventional solid spoilers on the measured wing system.

Effects of Unsteady Coanda Blowing on the Wake and Drag of a Simplified Blunt Vehicle

The impact of periodic forcing on the wake past a square back bluff body is experimentally studied. By the use of pulsed jets in combination with a Coanda effect, shear layer forcing allows to recover over 30 % of the model’s base pressure. The actuation frequency is an order of magnitude higher than the natural flow instabilities. Velocity measurements indicate that this direct wake control modifies the vorticity development along the shear layers and shrinks the width of the recirculating flow region down. At the same time, the velocity fluctuations in the near wake decrease, without measurable impact on the oscillatory vortex shedding. With this control strategy, both the flow deviation and the base pressure recovery are dependent on the upstream Reynolds number. Particle image velocimetry data and pressure measurements are used to discuss the origin of these observations.