## 2 Flow Field State Detection Technology for Multiphase Sink Vortex  
### Vortex State Detection Technology  
As a fluid drains from a vessel with a drainage hole at the bottom, the accumulated turbulent energy causes the free surface to become unstable. It reaches a specific height and produces a free sink vortex [14; 15]. If the sink vortex extends to a gaseous vortex core, the flow field forms a gas-liquid two-phase suction flow. When suspended solids or other liquids are involved in the vortex core along the free surface, the flow field becomes a complex gas-liquid-solid or gas-liquid-liquid three-phase flow system. Many industrial operations, including metallurgy pouring, chemical extraction, and hydropower energy conversion, would be adversely affected by this process [16; 17; 18; 19]. Meanwhile, the related flow control equipment frequently uses closed and invisible limited physical space to meet the stable and dependable operating requirements. As a result, the real-time detection of flow field states is challenging. A number of flow state detection technologies, including electromagnetic detection [20], infrared detection [20; 21; 22], ultrasonic detection [23], and vibration detection [24; 25; 26; 27], have been developed to overcome the aforementioned issues. Figure 1 shows the essential functioning concepts: (1) The electromagnetic detection method works on the idea of installing a transmitting and receiving coil on the container's wall and using the different permeabilities of the fluid and solid to detect the flow status [28]. This method, which has nothing to do with the nozzle valve opening, can see the amount of solid content in the fluid in real time. It has a high success rate and a quick response time, but it has various drawbacks in the detecting process, including a high price, a short service life, a complicated installation, and a high maintenance cost. (2) The infrared detection approach is based on pointing an infrared camera at a fluid and detecting it by comparing the thermal radiation properties of the fluid and the solid in the infrared frequency range [29]. The technology collects and processes a real-time image of the draining process using a contactless infrared camera. Visualization, vividness, and excellent reliability are all advantages. Due to the high temperature and severe environment, the infrared detection system can only collect a limited number of visible signals at the equipment entrance and is only utilized as a backup to other detection methods. The detection is always delayed, and it can only be noticed when a particular quantity of solid is incorporated into the fluid, limiting the precision and dependability. (3) The ultrasonic detection technique works based on the difference in ultrasonic reflected signals between pure fluid and mixed fluid to determine the solid content in the drainage process [30]. Like the infrared and electromagnetic methods, this approach is a posterior detection. It has various flaws, such as a low detecting accuracy and being damaged easily. (4) The concept of the vibration detection method is that when the nozzle pipe and the linked operational arm are subjected to fluid impact during the drainage process they will produce a specified range of vibrations. The flow is greater and the vibration is stronger when the nozzle opening is larger. Hence, the difference in fluid-induced vibration is employed [31; 32; 33]. It belongs to the prior detection category and has the following benefits: long service life, high operating stability, cheap production cost, and simple installation and maintenance. Scene experiments and related investigations revealed that the above detection technique might enable the online detection and active control of the vortex flow field to some extent.