The German engineer Ludwig Prandtl pioneered the science of flow control at the beginning of the twentieth century. Today, concerns about the environment and future supplies of fossil fuel motivate the search for better flow control strategies for transport vehicles and numerous other man-made devices. Methods of control to effect transition delay, separation postponement, lift enhancement, drag reduction, turbulence augmentation and noise suppression are sought. The potential benefits of realizing efficient flow-control systems range from saving billions of dollars in annual fuel costs for land, air and sea vehicles to achieving economically and environmentally more competitive industrial processes involving fluid flows. Laser-induced fluorescence visualization of a wing undergoing a pitching maneuver.
Flow is from left to right, and the wing's shadow partially obstructs the lower portion of the flow.
Ranging from simple to complex, from inexpensive to expensive, from passive to active to reactive, and from market ready to futuristic, the fluids engineer has a great variety of control strategies to choose from. Tough design compromises are, however, always in the forefront since flow control goals are often adversely interrelated. When designing a flow control device to achieve a particular goal such as skin-friction reduction, the engineer's foremost task is to secure a minimum and most benign tradeoff. For example, energy expenditure by active control actuators must be rationed carefully to insure a net gain. In contrast, classical control strategies are often ineffective for fully turbulent flows.