Tham khảo tài liệu 'the mems handbook mems applications (2nd ed) - m. gad el hak episode 2 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Flow Control 13-5 FIGURE Classification of flow control strategies. effects on the flowfield influencing particularly the shape of the velocity profile near the wall and thus the boundary layer susceptibility to transition and separation. Different additives such as polymers surfactants micro-bubbles droplets particles dust or fibers can also be injected through the surface in water or air wall-bounded flows. Control devices located away from the surface can also be beneficial. Large-eddy breakup devices also called outer-layer devices or OLDs acoustic waves bombarding a shear layer from outside additives introduced in the middle of a shear layer manipulation of freestream turbulence levels and spectra gust and magneto- and electro-hydrodynamic body forces are examples of flow control strategies applied away from the wall. A second scheme for classifying flow control methods considers energy expenditure and the control loop involved. As shown in the schematic in Figure a control device can be passive requiring no auxiliary power and no control loop or active requiring energy expenditure. For the action of passive devices some prefer to use the term flow management rather than flow control Fiedler and Fernholz 1990 reserving the latter terminology for dynamic processes. Active control requires a control loop and is further divided into predetermined or reactive. Predetermined control includes the application of steady or unsteady energy input without regard to the particular state of the flow. The control loop in this case is open as shown in Figure and no sensors are required. Because no sensed information is being fed forward this open control loop is not a feedforward one. This subtle point is often confused in the literature blurring predetermined control with reactive feedforward control. Reactive control is a special class of active control where the control input is continuously adjusted based on measurements of some kind. The control loop in
