# Impact Performance of Helmholtz Self-Excited Oscillation Waterjets Used for Underground Mining
## 1 Introduction  
High-pressure water jets have been widely used in mechanical processing, petroleum, chemical industry, metallurgy, and other fields. At present, drilling assisted by water jets has become one of the conventional technical means in exploitation of fossil fuels [1]. Primary crushing was carried out by high pressure water jets at the front of the bit on the rock, which can improve the drilling efficiency of the drill bit. At the same time, water has the benefit of cleaning, lubricating, cooling, and avoiding spark production, which will greatly improve the safety performance in the process of drilling. Aiming at rock breaking assisted by high-pressure water jets, many scholars have carried out relevant research. As early as 1964, [PERSON] et al. [2] studied the downhole flow field of a mechanical drill bit with high-pressure jets nozzles through laboratory tests and obtained the velocity distribution of downhole overflow tank level and the distribution law of high-pressure jet impact pressure. [PERSON] et al. [3] experimentally studied the attenuation law of high pressure jets before reaching the bottomhole, and the index of half-attenuation distance of impact pressure was put forward, which drew the conclusion that high ambient pressure will reduce the impact performance of water jets. [PERSON] et al. [4] proposed a new predicting scheme to evaluate bottomhole circulating pressure in petroleum industry and this method was able to predict the complex relationship between inputs and outputs of a highly nonlinear system. [PERSON] et al. [5] comprehensively analyzed the results of an orthogonal experimental design and demonstrated the optimal structural parameters of a straight cone nozzle. [PERSON] et al. [6] proposed a novel method that used a self-propelled water-jet nozzle to dredge blocked boreholes in coal seams in order to improve the coal bed methane output and coal mine safety. The nozzle inlet pressure was optimized and a field experiment on the blockage removal of blocked coal bed methane drainage boreholes using the proposed method was conducted. [PERSON] et al. [7] designed a self-propelled water-jet drilling nozzle to drill three-type boreholes to improve gas extraction, and drilling experiments were conducted to optimize the nozzle configuration. [PERSON] et al. [8; 9] designed a new drilling tool to improve drilling rate in deep wells, based on the theory of jets pulsation. Oil field tests in five wells show good applicability of the tool to bit types, formation, drilling densities, flow rates, and dynamic hydraulic drilling motors, etc. As a result, penetration rates are improved ranging from 10.1 to 31.5%. To increase the damage of water jets on rock, a new water jet technology called'self-excited oscillation jets' has been developing gradually since the 1970s. Through a certain structure, the energy provided by the power source is stored and transmitted to the jets media intermittently, so that the jets media can obtain a huge amount of energy from the jets. Due to its outstanding characteristics of high energy accumulation, strong erosion performance, and high working efficiency, self-excited oscillating pulsed jets have great importance as a new technology with broad application prospects. The application of self-excited oscillating jets on water jets assisted drilling greatly, improved the efficiency of jets' rock breaking, and has become an important means of oil and gas drilling and production increase. [PERSON] [10] conducted an experimental study of a Helmholtz resonator driven by a round jet passing through it and drew the conclusion that jet instabilities coupled with the Helmholtz resonance to produce very powerful chamber-pressure oscillations at a frequency slightly higher than the chamber frequency. [PERSON] and [PERSON] [11; 12] studied the self-sustained oscillation phenomena produced by fluid flowing past different cavities and confirmed this conclusion through a lot of experiments.