**5.4 Hydrodynamics in self-purification of water bodies**

*Vortex Dynamics Theories and Applications*

countries due to its simple design [39].

fluid-particle separation.

**5.3 Hydrodynamics in food and paper processing**

refinement conifer rich pulp samples [47].

**5.2 Hydrodynamics in solid-liquid separation**

flows indicating shear forces imposed by the flow conditions were above the critical levels for biofilm attachment. Experimental data from these studies were used to characterize the hydrodynamic behavior for numerical simulation and validation [37, 38]. Low-cost pipe-based pathogen reduction system was also demonstrated by Thomas et al. Their approach has a huge potential for application in developing

In the design of process reactors, it is often necessary to tailor the separation technique to the dynamics and characteristics of the waste slurry that is being treated. Hence, several studies have been conducted to determine the influence of hydrodynamics on sludge processing. The optimum dosage values, which were obtained when flocculation performance was assessed based on surrogate indicators such as sludge volume index and supernatant turbidity, confirmed polymer bridging as the primary flocculation mechanism. Specific apparatus construction and reactor geometry were found to help maintain sludge suspension in a metastable state that is crucial for the formation of pellet-like compact agglomerates with better dewaterability properties [2, 40]. Similarly, sludge disintegration, using rotor-stator type hydrodynamic cavitation reactor (HCR), has been experimentally investigated [41]. To determine the effects of cavitation (including thermal energy) and shear stress on sludge disintegration, the performance of the HCR with and without the dimples and temperature control was analyzed. The results indicated that when dimples were present and there was no temperature control, the reduction of sludge particles increased by 50–80%. Further, the disintegration performance increased with the rotational speed and was minimized at the highest inlet pressure. Several other studies leveraging on the hydrodynamics of the process reactor for fluid-particle separation are available in literature [42, 43]. Many of these lab-scale studies have demonstrated the feasibility of turbulence-induced

Hydrodynamic cavitation (HC) is a process in which high energy is released in a flowing liquid upon bubble implosion due to decrease and subsequent increase in local pressure. In food and beverage industries, hydrodynamic cavitators can be utilized for the purpose of extraction, emulsification, sterilization, disinfection, and homogenization [44]. HC, which can effectively induce sonochemistry by mechanical means, creates extraordinarily high of pressures of ~1000 bar, local hotspots with ~5000 K, and high oxidation (hydroxyl radicals) in room environment, without introducing new chemicals. For possible industrial application, the efficiency of HC has been studied by comparing the chemical oxygen demand (COD) removal efficiency of a Venturi device to that of an orifice plate. A sucrose solution and an effluent from a sucrose-based soft drink industry were treated. Results showed that the Venturi device recorded 90% COD removal efficiency after treatment period of three minutes. On the other hand, the orifice plate recorded 90% COD removal efficiency after 9 minutes [44–46]. Developing high-performance HCRs and revealing the corresponding disinfection mechanisms constitute the most crucial issues today [45]. Refining of cellulose pulp is a critical step in obtaining high quality paper characteristics, however, this process is slow and costly especially for refining longer conifer fibers which are the preferred source for high quality paper production and give the paper its strength. Recently, hydrodynamic cavitation was applied to the

**12**

The self-purification ability of water bodies is related to the prevailing hydrodynamic conditions. Coupled hydrodynamic and water quality models have been used to investigate the spatial and temporal water quality variations of the water bodies. Using an Acoustic Doppler Velocimeter, the efficiency of aeration plug-flow device (APFD) in terms of water flow and dissolved oxygen (DO) have been determined experimentally [48–50]. Recent findings have shown that discharges from several rivers flowing into the New York/New Jersey (NY/NJ) harbor interact and interfere with one another. Such interactions can improve or inhibit water and contaminant flushing from the harbor. In Poyang Lake, three-dimensional velocity at various locations as well as the velocity distribution and turbulence characteristics were assessed, and plug-flow characteristics were analyzed. The two patterns of velocity and turbulence in horizontal sections observed are (1) near the aeration plug-flow device (APFD), the water flow was intensively pushed downstream and simultaneously recirculated; (2) farther away, the reflux area gradually decreased, and the velocity and turbulence distribution were more or less uniform. At the interfaces between two immiscible fluids – water and alkane of small carbon number, the amphiphilic PEO chain diffuses laterally, experiencing hydrodynamic drags from both phases. The absolute values of interfacial diffusion coefficients demonstrate a bigger contribution from the hydrodynamics from the water phase, which may be attributed to a stronger attraction between water and the PEO molecules [51].

## **5.5 Vortex patterns in fluid-particle reactors**

The flow vortex patterns in many fluid-particle reactors are quite diverse and their analysis and characterization can provide an insight into the fluid and particle dynamics within the reactor. One important feature in many of these flow types is the presence of rotation or swirling or a combination of both resulting in anisotropic turbulence. A few of the dominant vortex patterns will be discussed in this section. It is also worth mentioning that the flow pattern is a function of the reactor geometry, stirrer or agitator, baffles, and the operating conditions. The focus will be on the common reactor geometries and flow regimes that are typically encountered in many practical applications.

A qualitative and quantitative analysis of the flow pattern in a processing reactor may be performed experimentally using an optical diagnostic imaging technique such as PIV or PDA or by numerical modeling. The selected approach in a given scenario will depend on the required detail of the flow field as well as the available resources. While the later provide a flexible option for the investigation of fluid flow problems, the former often complements or used in the validation of numerical model. A detailed description of specific techniques is beyond the scope of this submission. A few of the vortex patterns in some of these reactors are discussed below.

## *5.5.1 Rotatory flow vortex pattern (e.g., tubular reactor)*

This type of flow pattern is commonly found in rotating tubular reactors with an enclosed flow induced by a stirrer. Previous studies have shown that this flow pattern has a profound influence on the performance of fluid-particle reactors, and this type of flow pattern can be found in many technical applications [29, 39, 52]. For instance, in terms of the stirrer-vessel configuration, the flocculation performance is significantly influenced by the impeller type and its speed. The axial impeller has been found to promote floc formation over a range of impeller speeds as it produces

## *Vortex Dynamics Theories and Applications*

a more homogeneous distribution of local velocity gradients in comparison to the radial impeller. Also, high velocity gradients occur in the region around the impeller, which might significantly hamper the high rate activated sludge (HRAS) flocculation process [53, 54].
