# Research Progress and Engineering Applications of Viscous Fluid Mechanics
## 4 Future Trends and Directions  
Viscous fluid mechanics, as an important branch of fluid mechanics, has made remarkable progress in the past decades and has been widely used in several engineering and scientific fields. The future research and development outlook can be viewed from the following aspects (Figure 19):  
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2. Future research in viscous fluid mechanics will pay more attention to the coupling of multiple physical fields. The physical fields and their effects that need attention include the following: (1) Thermal field: Temperature changes can significantly affect the viscosity of the fluid; usually, a temperature increase will lead to a decrease in the viscosity of the fluid, while a decrease in temperature will increase the viscosity. This coupling is particularly important in high-temperature lubrication and polymer fluid handling. (2) Chemical reactions: Chemical reactions can change the composition of a fluid and thus affect its viscosity. For example, cross-linking reactions in polymer fluids can lead to nonlinear changes in viscosity, which is critical for the processing and control of viscoelastic materials. (3) Electromagnetic fields: Magnetic fluids can produce morphological changes and controlled flow behavior under an applied magnetic field, such as electromagnetically driven fluids in liquid metals. This coupling can have applications in liquid metal metallurgy, aerospace technology, and biomedical sensors. (4) Solid mechanics: in fluids in contact with solid walls, the morphology and roughness of the solid surface can significantly affect the friction and adhesion behavior of the fluid. (5)Acoustic effects: Acoustic fields can affect the viscoelasticity and flow properties of fluids, for example, in the ultrasonic acoustic field for microfluidic manipulation and in biomedical imaging. This coupling helps to control the mixing and stirring process of microfluidics; not only limited to the above physical fields, or even using two or more kinds of physical fields coupled together, numerical simulation or complex experimental methods can be used in studies and analysis in order to add a comprehensive, in-depth understanding of complex engineering phenomena and natural phenomena. This multiphysical field coupling research method is expected to provide new ideas and solutions for innovations in the fields of energy conversion, environmental protection, and biomedicine. I believe that the optimization trends in this area are as follows: (1) for complex systems and phenomena, such as biofluids, multiphase flows, and non-Newtonian fluids, multiscale modeling and simulation methods should be improved to capture the details in different spatial and temporal scales; (2) for numerical simulation, new numerical simulation techniques and algorithms such as mixed-element methods, implicit-coupling methods, and adaptive mesh techniques should be improved to facilitate the numerical modeling and analysis of complex fluid systems;(3)experimental techniques, such as microfluidic experimental platforms and advanced imaging techniques, should be improved to further improve the precision of experimental device components and optimize real-time sensing techniques, which can more accurately validate theoretical models and numerical predictions. 3. One of the future research priorities will be the in-depth understanding and control of complex flow phenomena. Viscous fluid complex flow phenomena include the following: (1) non-Newtonian fluid flow: non-Newtonian fluid flow is not only affected by the velocity gradient but may also be affected by the non-uniformity and time-dependence of the flow field, resulting in complex flow behaviors, such as shear thinning and shear thickening phenomena; (2) multiphase flow: this refers to the simultaneous presence of two or more substances (e.g., gases, liquids, or solid particles) of the flow system, with a focus on gas-liquid, liquid-liquid, solid-liquid, and other combinations of flow, such as foam flow, bubble flow, the movement ofparticles in suspension, etc.