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5486 results in Thermal-fluids engineering

Page 60 of 275

  • 4 - Roughness

  • Jinjun Wang, Lihao Feng
  • Book:
    Flow Control Techniques and Applications
    Published online:
    14 December 2018
    Print publication:
    13 December 2018, pp 65-93

  • Summary

    Roughness is generally used for boundary layer flow control. Some traditional knowledge from the literature is first presented and then recent progress on roughness control is explained. Traditionally, roughness was used as an approach to advance flow transition, however some recent work has found an interesting result that roughness could delay flow transition and thus reduce the friction drag. Roughness also influences the roadmap of bypass transition and the coherent structures in the turbulent boundary layer. Since roughness can be made to form very simple configurations and can be easily attached to the object surface, such findings suggest significant potential applications of roughness in engineering.

  • 2 - Gurney Flap

  • Jinjun Wang, Lihao Feng
  • Book:
    Flow Control Techniques and Applications
    Published online:
    14 December 2018
    Print publication:
    13 December 2018, pp 23-47

  • Summary

    The Gurney flap is a simple device that can be easily attached to the pressure surface of an airfoil. The control effects of Gurney flaps on airfoils, wings, and aircraft are introduced, indicating that the Gurney flap can effectively increase the lift coefficient. Thus, it shows significant ability to shorten the takeoff/landing distance of aircraft. In addition, the influence of different parameters is compared and the control mechanism is revealed. Finally, some suggestions for engineering applications are given, as it is expected that Gurney flaps could be actually used in the near future.

  • 11 - Closed-Loop Control

  • Jinjun Wang, Lihao Feng
  • Book:
    Flow Control Techniques and Applications
    Published online:
    14 December 2018
    Print publication:
    13 December 2018, pp 266-277

  • Summary

    A closed-loop control system includes fluid system, actuator, sensor, and control algorithm. The control parameters depend on information from the control system which in turn depend on the control. Thus, it could adapt to the flow condition and contribute an intelligent and robust system. The control algorithm could be based on the reduced-order model or the measured signal from the flow field. The reduced-order model is usually achieved by advanced data processing methods, such as POD. On the other hand, measured variables, such as pressure and force coefficients, could also be used for the design of the control algorithm. Some examples of closed-loop control are introduced in detail to show its effect. It is suggested that the realization of closed-loop control is important for engineering applications of active flow control techniques.

  • 10 - Lorentz Force

  • Jinjun Wang, Lihao Feng
  • Book:
    Flow Control Techniques and Applications
    Published online:
    14 December 2018
    Print publication:
    13 December 2018, pp 246-265

  • Summary

    Lorentz force is a novel active flow control device originating from electromagnetism. It can be generated in electric fluids with a specific arrangement of the electrodes and magnets, with forcing in either the streamwise or the spanwise direction. The Lorentz force can change its magnitude and direction periodically, and the boundary layer can therefore be changed with the generation of the near wall jet. Though streamwise Lorentz force may increase the friction drag of the boundary layer, spanwise Lorentz force could result in a considerable drag reduction up to about 40% for the turbulent boundary layer. It is found that periodic Lorentz force could induce spanwise vortices that might weaken the turbulence activities. In addition, Lorentz force could reduce the flow separation, thus it can increase the lift coefficient over airfoils and also reduce the lift fluctuation of a circular cylinder. Considering that electric fluids are required for the application of the Lorentz force, it is suitable for the flow control of sea vehicles, such as drag reduction in submarines.

  • 9 - Plasma Actuator

  • Jinjun Wang, Lihao Feng
  • Book:
    Flow Control Techniques and Applications
    Published online:
    14 December 2018
    Print publication:
    13 December 2018, pp 206-245

  • Summary

    The plasma actuator is an effective device that could be used for active flow control. There are different kinds of plasma actuators, such as DBD plasma actuator, surface corona discharge actuator, and plasma spark-jet actuator, and the DBD plasma actuators are paid particular attention. The two main features of the DBD plasma actuator are that it can induce a wall jet and a starting vortex. Firstly, the conventional applications of the plasma actuators are introduced. The plasma actuator can delay laminar to turbulence transition. It can also delay or eliminate flow separation, and thus improve aerodynamic performance of airfoils, straight wings, delta wings, aircraft, and bluff bodies. On the other hand, some novel flow control conceptions based on plasma actuators are also introduced, including plasma synthetic jet, plasma Gurney flap, plasma circulation control, plasma vortex generator. It has been indicated that those techniques could achieve similar control effects to traditional techniques. Meanwhile, plasma-based techniques are easier to conduct in real-time and unsteady active control, and more convenient to implement than conventional techniques. Finally, the disadvantages of DBD plasma actuators and their solutions are discussed.

  • 3 - Vortex Generator

  • Jinjun Wang, Lihao Feng
  • Book:
    Flow Control Techniques and Applications
    Published online:
    14 December 2018
    Print publication:
    13 December 2018, pp 48-64

  • Summary

    The vortex generator is a simple device to induce streamwise vortex for flow separation control, thus reducing the pressure-difference drag. The fundamental characteristics of vortex generators and the corresponding parameter influence are first introduced. Then the applications of vortex generators in various fields for flow separation control and heat transfer enhancement are discussed. Note that vortex generators have been actually applied in a few industrial fields, such as modern passenger aircraft. This shows the advantages of vortex generators, since most other control devices are still at the lab-investigation stage.

  • 6 - Biological Techniques

  • Jinjun Wang, Lihao Feng
  • Book:
    Flow Control Techniques and Applications
    Published online:
    14 December 2018
    Print publication:
    13 December 2018, pp 108-140

  • Summary

    Biological flow control techniques are derived from nature. It is expected that we can learn from animals and plants to copy or mimic these features to improve the performance of man-made systems. The techniques that are specifically introduced here include hairy coating, leading-edge tubercles, riblet, and cactus-shape modification. Hairy coating can adapt to flow, and thus reduce the drag coefficient of bluff bodies while increasing the lift coefficient of airfoils. Leading-edge tubercles can induce a streamwise vortex from each protuberance to enhance momentum mixing for the separated flow. Surface riblets can reduce the momentum exchange properties of the streamwise vortices, leading to a friction drag reduction of up to 10%. The cactus-shape modification may decrease the size of the wake vortices and the strength of their interaction, thus reducing the lift fluctuation and vortex-induced vibration. It is indicated that biomimetic techniques are easy to mplement with high control effectiveness, and thus show great potential in engineering applications.

  • 7 - Geophysical Flow

  • W. O. Criminale, University of Washington, T. L. Jackson, University of Florida, R. D. Joslin
  • Book:
    Theory and Computation in Hydrodynamic Stability
    Published online:
    22 November 2018
    Print publication:
    06 December 2018, pp 191-224

  • Summary

    The instability of geophysical flows are covered in Chapter 7. From the class of geophysical flows, there are three classes that are distinct and that illustrate the salient properties when viewed from the basis of perturbations. These cases include the effects of density variations and rotation. The cases considered in this chapter are stratified flow, rotation (Rossby waves) and the Ekman layer.

  • 8 - Transient Dynamics

  • W. O. Criminale, University of Washington, T. L. Jackson, University of Florida, R. D. Joslin
  • Book:
    Theory and Computation in Hydrodynamic Stability
    Published online:
    22 November 2018
    Print publication:
    06 December 2018, pp 225-258

  • Summary

    Chapter 8 addresses the intial value problem, x, where the effect of initial conditions are sought within the linear disturbance regime. Laplace transforms, moving coordinates and numerical approaches are all discussed. Examples of the latter include channel flows and the Blasius boundary layer. The chapter concludes with an in-depth discussion of optimizing the initial conditions for subcritical Reynolds numbers to obtain the maximum energy as a function of time. The concept of algebraically instability is discussed within this context, such that when the normalized energy density is greater than one, the flow is said to be algebraically unstable.

Page 60 of 275