# Performance Study of a Fluidic Hammer Controlled by an Output-Fed Bistable Fluidic Oscillator
## 1 Introduction  
Fluidic oscillators, based on the well-known Coanda effect [1] of jet attachment to a wall, are considered to be able to generate a self-sustained periodic oscillating jet without the necessity of any movable or deformable parts [2; 3]. Like other fluidic devices, they are compact, reliable, and robust, while exhibiting resistance to high temperatures, corrosive environments, and other extreme ambient conditions, which offers attractive application possibilities in furnaces, nuclear radiation, and other unusual conditions [4]. Various designs of fluidic oscillators have been proposed and investigated by previous researchers [5; 6; 7; 8]. The most common fluidic oscillators are those with a jet-deflection amplifier and destabilizing feedback loops based on the fundamental operating principles associated with the two classic designs [7; 9]: relaxation oscillator with a single loop (shown in Figure 1a) and the two-loops version, namely, negative feedback oscillator (shown in Figure 1b). Behavior of both is characterized by oscillating frequency increasing linearly in proportion to the inlet flow rate, leading to the wide application of them to the measurement of flow rates [10]. Besides, because of the oscillating features, they are also widely applied for mixers [11], sensors [12], and memory and control devices [13]. Another less common fluidic oscillator, as displayed in Figure 1c, is the resonance-channel fluidic oscillator, recently proposed by Tesar [14]. Instead of the flow signal propagating in loops,the flow in the resonance-channel fluidic oscillator is here switched by the action of pressure waves traveling in a resonance channel connected to one of the control nozzles. The oscillation frequency of the resonance-channel fluidic oscillator is determined by the length of the resonance channel and the propagation velocity, but does not depend on the flow rate passing through the oscillator [15]. This novel fluidic oscillator is widely used for flow-separation control [16]. In a fluidic oscillator, the available pressure energy of the fluid feed is converted, in the supply nozzle, into the kinetic energy of jet, creating a very low-pressure region in the space between the two attachment walls, and then, the kinetic energy is reconverted back into the pressure energy form at the entrance of the output channel. Due to this energy reconversion mechanism, the fluidic oscillators have been recognized as particularly desirable to serve as actuators, assisting in output force, moment, or torque, as the supplied flow alternatively flows into the output channels. One of the successful designs is the output-fed bistable fluidic oscillator is shown in Figure 1d. A striking feature of the output-fed bistable fluidic oscillator is the introduction of vents and the non-coplanar setting of feedback loops, making the fluidic oscillator become jet-switching insensitive. The supply jet would not detach from the wall and switch to the other attachment wall unless the hydraulic resistance downstream of the output channel met at a certain level. This working principle focuses more on fluid flow control, which is similar to the function of a fluidic amplifier. Given this, the performance of the output-fed bistable fluidic oscillator can be evaluated both as a fluidic oscillator and a fluidic amplifier. This distinctive feature makes the output-fed bistable fluidic oscillator very unique compared with the two classic types mentioned above, and makes a wide application for engineering practice since it was first suggested and patented in the 1970s. For instance, with the combination of an output-fed bistable fluidic oscillator and the friction reduction technology, [PERSON] et al. [17] proposed a novel drilling tool to reduce the drag and torque in horizontal directional drilling process. [PERSON] et al. [18] investigated a booster compressor worked in an ultrahigh-pressure-jet system, in which an output-fed bistable fluidic oscillator represents the core part for compressor-reversing control.