## 1 Introduction  
In the literature, the jet behavior for a variety of free and bounded geometries were investigated considerably. A multitude of studies is devoted to investigating the emitting jet in a 2D channel. This kind of jet in a cavity might be considered as a flow that can produce and generate self-sustaining oscillations. Based on the geometry, self-sustaining oscillations could coarsely be classified as jet edge, hole tone, organ type, sudden expansion, jet confined in a cavity, and fluidic oscillator (Figure 1). The confinement's effect on the entrainment of the confined self-sustaining oscillating jet is broadly appealing and has enticed scholars to investigate that. Self-sustaining phenomena are carried out in a wide range of Reynolds number from 100 [1; 2; 3; 4] up to 170,000 [5], which leads to the conclusion that in both laminar and turbulent jets, this mechanism can be attained. In addition, for different ranges of Reynolds numbers, several studies about a jet in the sudden expansion were reported. [PERSON] et al. [6] by using the laser-Doppler anemometry, experimentally analyzed the velocity characteristics of asymmetric flow for low Reynolds number (110-1000), which forms in symmetric sudden-expansion geometry. [PERSON] et al. [7] numerically and experimentally studied the flow at Reynolds numbers in the range of 25-380 and described that the flow becomes time-dependent at higher Reynolds numbers. [PERSON] and [PERSON] [8] investigated the turbulent flow through the sudden expansion at Re = 10,000 by performing 2D-PIV measurements. They observed an asymmetric flow and concluded that the flow structure is slightly sensitive to the variation of the Reynolds number in the considered range. So far, there are no studies to explain the unique characteristic of an oscillating jet from a double feedback fluidic oscillator, which is emitted in a 2D channel with sudden expansion. Thus, in the current work, the oscillating jet emitting in a fully confined external domain (Figure 2a), is compared with the free oscillating jet. Fluidic oscillators are widely used as actuators for active flow control, because these devices exert a high momentum and a wide range of frequency, they do not have any moving part, and have a simple design. Based on the number of feedback loops, fluidic oscillators are divided into three main groups--feedback-free, single-feedback, and double-feedback jets. The most popular type is double-feedback channels, as illustrated in Figure 2b. When the supply fluid enters the mixing chamber, it attaches to one side of the chamber because of the Coanda effect [9]. One part of the fluid goes out to the external domain, while the other part near the exit is diverted through the feedback channel and impinges on the main jet at the inlet. This part causes the jet to attach to another side of the mixing chamber, and the process is done continuously  
Figure 1: Classification of self-sustained oscillator. with a specific frequency. Therefore, an oscillating jet is emitted to the external domain, without using mechanical parts. In recent decades, this type of fluidic oscillator attracted much research interest and is used in various applications, like heat transfer enhancement [10; 11; 12; 13; 14; 15; 16], mixing enhancement [17; 18; 19; 20; 21], and separation control [22; 23; 24]. In the literature, it is alleged that the oscillation frequency of the jet is dependent on the internal region. For the first time, [PERSON] et al. [25] experimentally investigated the internal part of a fluidic oscillator. Then, some numerical and experimental studies were also conducted to identify the mechanisms that drive the jet to oscillate with a certain frequency [25; 26; 27; 28; 29]. They concluded that by increasing the mass supply for the fixed geometry, the frequency increases.