Florian Ostermann

Phase-Averaging Methods for the Natural Flowfield of a Fluidic Oscillator

The presented study examines various methods for phase averaging the naturally oscillating flowfield of a scaled-up fluidic oscillator. No external trigger is employed to control the oscillation of the flow. Mathematical and signal conditioning approaches for phase averaging the data are categorized and described. The results of these methods are evaluated for their accuracy in capturing the natural flowfield. The respective criteria are based on the minimum fluctuation in oscillation period length, the conservation of velocity amplitudes, and the number of snapshots per phase-averaging window. Although all methods produce reasonable qualitative results, only two methods are identified to provide the desired quantitative accuracy and suitability for the investigated flowfield. The first method is based on conditioning a time-resolved pressure signal from the feedback channels in the oscillator. An autocorrelation applied to the reference signal improves the period identification. The second method employs...

Phase-Averaging Methods for a Naturally Oscillating Flow Field

The presented study examines various methods for phase-averaging the naturally oscillating flow field of an enlarged fluidic oscillator acquired by a high-speed PIV system. Because of the absence of an external trigger, phase-averaging the acquired data is challenging. Mathematical and physical methods are categorized and described. The results of these methods are evaluated for their accuracy in capturing the natural flow field. It is found that the mathematical methods, especially the method of proper orthogonal decomposition, produce reasonable qualitative results. However, compared to the physical methods, shortcomings in quantitative accuracy are revealed. The physical methods require a time-resolved reference signal. Two possibilities to identify the oscillation periods in the reference signal are described and compared. It is found that applying an autocorrelation on the reference signal improves the period identification due to consideration of a locally fluctuating mean value. This period identification method and according phase-averaging yields the best results regarding the minimum fluctuation of the oscillation period lengths. The according procedure is described in detail and applied to the internal and external flow field of the fluidic oscillator.

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

Experimental Three-Dimensional Velocity Data of a Sweeping Jet from a Fluidic Oscillator

DFG, 289230680, Die Interaktion zwischen einem raumlich oszillierenden Strahl und einer Querstromung