Fluidic Oscillators, Feedback Channel Effect under Compressible Flow Conditions

Abstract

Fluidic oscillators are often used to modify the forces fluid generates on any given bluff body; they can also be used as flow, pressure or acoustic sensors, with each application requiring a particular oscillator configuration. Regarding the fluidic oscillators’ main performance, a problem which is not yet clarified is the understanding of the feedback channel effect on the oscillator outlet mass flow frequency and amplitude, especially under compressible flow conditions. In order to bring light to this point, a set of three-dimensional Direct Numerical Simulations under compressible flow conditions are introduced in the present paper; four different feedback channel lengths and two inlet Reynolds numbers Re = 12,410 and Re = 18,617 are considered. From the results obtained, it is observed that as the inlet velocity increases, the fluidic oscillator outlet mass flow frequency and amplitude increase. An increase of the feedback channel length decreases the outlet mass flow oscillating frequency. At large feedback channel lengths, the former main oscillation tends to disappear, the jet inside the mixing chamber simply fluctuates at high frequencies. Once the Feedback Channel (FC) length exceeds a certain threshold, the oscillation stops. Under all conditions studied, pressure waves are observed to be traveling along the feedback channels, their origin and interaction with the jet entering the mixing chamber are thoroughly evaluated. The paper proves that jet oscillations are pressure-driven.

Analysis of a Novel Fluidic Oscillator under Several Dimensional Modifications

“…Regardless of the case studied, the onset of the self-sustained oscillations is proven to be mostly due to the pressure forces acting onto the jet as it enters the mixing chamber; the forces due to the mass flow, although smaller, have the same order of magnitude as the pressure forces in the present configuration. This is at odds with what was previously observed in [25][26][27], where the self-sustained oscillations were pressure-driven and the mass flow forces had a minor influence, but the FO studied in these references was a completely different one, with much longer and narrower feedback channels than the ones studied here. Another characteristic which differentiates the present configuration from the previously studied ones is the large reverse flow existing in the feedback channels; we have proven that this reverse flow interacts with the main jet inside the mixing chamber, generating random pressure fluctuations in the mixing chamber.…”

“…For all cases studied, a large reverse flow at the FC has been observed (see Figures 7b, 12b and 16b). In fact, this large reverse flow was not previously observed in the FO configurations studied by [14,[25][26][27] or even by [28], where the same FO configuration was studied but at a very low Reynolds number. In the present subsection, an oscillation cycle is carefully analyzed, linking the velocity and pressure fields at different instants with the forces acting over the jet; the concept here is to unveil the forces acting on the jet as it enters the mixing chamber and understand why the jet keeps oscillating, even when this large reverse flow exists.…”

“…All these graphs clarify that for the FO configuration presented in this study, the forces acting onto the jet as it enters the mixing chamber are mostly due to the pressure operating at the FC outlets, yet the forces due to the FC mass flow term are of the same order of magnitude as the pressure ones. At this point, it is important to recall the work performed by [25][26][27], where it was stated that the jet was pressure-driven and mass flow forces were playing a secondary role. The much more relevant role played by the FC mass flow forces in the present FO configuration is believed to be due to the particularly short and wide feedback channels employed in the present fluidic oscillator.…”

“…In 2020, they used the same model and studied the effect of several design modifications [26]. Bergada et al [27] studied the effect of feedback channel (FC) length on FO performance for compressible flow conditions; they found that at large feedback channel lengths, the former main oscillation tends to disappear, the jet inside the mixing chamber simply fluctuates at high frequencies, and as the feedback channel (FC) length exceeds a certain threshold the FO stops oscillating. In Sarvar et al [28], a novel shape of a fluidic oscillator (FO) in the laminar regime at a very low Reynolds number was studied; they observed that the jet sweeping angle amplitude is more pronounced for a two-dimensional FO as compared to a three-dimensional one at a fixed given Reynolds number, and the instability of the output jet becomes slightly chaotic at very low Reynolds numbers.…”

Aerodynamic Efficiency Improvement on a NACA-8412 Airfoil via Active Flow Control Implementation

“…(3) Fluidic actuators (FA), create pulsating flow and in some configurations no moving parts are required. In some particular FA designs, the origin of the self-sustained oscillations was recently unveiled in [15][16][17].…”