Rotation and Pulsation of a Blunt Body
The ability to manipulate a flowfield actively or passively to effect a desired change is of immense technological importance, and this undoubtedly accounts for the subject’s being more hotly pursued at present by more scientists and engineers than any other topic in fluid mechanics. The potential benefits of realizing efficient flow-control systems range from saving billions of dollars in annual fuel costs for transportation to achieving economically and environmentally more competitive industrial processes involving fluid flows.
Under certain conditions, wall-bounded flows separate. To improve the performance of natural or manmade flow systems, it may be beneficial to delay or advance this detachment process. Of all the various types of flow control now extant, control of flow separation is probably the oldest and most economically important. Separation control is of immense importance to the performance of air, land and sea vehicles, turbomachines, diffusers, and a variety of other technologically important systems involving fluid flow. Generally it is desired to postpone separation so that form drag is reduced, stall is delayed, lift is enhanced, and pressure recovery is improved. However, in some instances it may be beneficial to provoke separation. For example, to improve the subsonic high-lift performance of an airfoil optimized for supersonic flight, a flap may be used to initiate leading-edge separation followed by reattachment.
In the present research, a novel combination of body rotation and radial pulsation is used to delay or prevent separation around a prototypical blunt body, a circular cylinder whose axis is placed normal to a uniform flow. Flow visualization is used to provide global information and minute ‘hot-wires’ are used to measure the local, detailed velocity field. The idea is to understand such complex flowfield for eventual practical application, in which the control mechanism may ultimately morph into a much simpler form as compared to the present global rotation and pulsation. We have established the existence of an important energy transfer mechanism from the mean flow to the turbulence fluctuations.
Radial pulsations cause and enhance that energy transfer. Certain values of the amplitude and frequency of the pulsations lead to negative drag (i.e., thrust). The nonlinear interaction between the Magnus effect induced by the steady rotation of the cylinder and the near-wake region modulated by the bluff body’s pulsation leads to alteration of the omnipresent Kármán vortices and the possibility of optimizing the lift-to-drag ratio as well as the rates of heat and mass transfer. Other useful applications include the ability to enhance or suppress the turbulence intensity, and to avoid the potentially destructive lock-in phenomenon in the wake of bridges, electric cables and other structures.