Numerical investigation of internal flow characteristics and oscillation stability of a fluidic oscillator under restricted-inlet conditions

As a periodic excitation device capable of generating oscillating jets without moving parts, fluidic oscillators (FOs) have been widely used in flow control, heat transfer enhancement, and other fields, and have shown great application potential in engineering fields such as food drying and industrial cleaning. However, due to the limitations of practical piping connections and installation space, a circular-to-rectangular inlet transition structure is often required at the inlet of the FO, which increases the risk of internal flow instability. In this study, we employed the unsteady Reynolds-averaged Navier-Stokes (URANS) method to investigate the effects of restricted-inlet conditions on the internal flow characteristics of the FO and its oscillation stability under thicknesses of H = 5−25 mm and inlet-outlet pressure differences of ΔP = 10−700 kPa. The influence of the inlet-restricted structure was quantitatively evaluated by comparison with an unrestricted-inlet fluidic oscillator (UIFO). The results showed that the separation vortices and low-pressure regions in the mixing chamber of the inlet-restricted fluidic oscillator (IRFO) were key to maintaining the oscillating jet. As the thickness H increased, the oscillation performance of the IRFO decreased significantly, and the influence of the inlet-restricted structure on the oscillation performance became more pronounced. Under ΔP = 10 kPa, when H = 15 mm, the oscillation frequency and outlet sweeping angle of the IRFO decreased to 30.5 Hz and 72°, respectively, which were approximately 63.3% and 25.0% lower than those of the UIFO. When the thickness increased to H ≥ 18 mm, oscillation failure occurred in the IRFO, and no regular dominant frequency could be identified. Increasing ΔP could restore regular oscillation within a certain range, but it became difficult to recover when H ≥ 20 mm. For the geometry investigated in this study, 1 ≤ H/D ≤ 3.4 can be used as a safe reference range to avoid oscillation failure. These findings deepen the understanding of the flow characteristics and oscillation stability of FOs under restricted-inlet conditions and provide a basis for structural design and reliable application under practical engineering constraints.