What is Flow Control?
For both experiments and simulations with sufficiently high Reynolds number the flow in a channel, or near a boundary, can become turbulent. This means that streamwise vortices arise in the near-wall region, forcing high velocity flow towards the wall and low velocity flow away from the wall. It is the presence of these high and low speed streaks which cause drag and hence, in practical applications, more energy is required to keep flow at a certain rate. The idea of flow control is to reduce this effect via some specified mechanism and diminish the drag.
We can firstly divide flow control techniques into two major categories:- open-loop and closed-loop mechanisms. Open-loop systems do not use any kind of feedback to adjust the amount of flow control. This is currently our main area of study as they don't require any complex sensors or adaptation of the mechanism throughout the control process. Conversely, closed-loop control uses a feedback loop to modify the control method dependent on some measurement at a sensing device.
Flow control strategies can also be divided into passive and active control schemes. For passive mechanisms no energy input is required to reduce the drag. This means that for these strategies any drag reduction is an increase in net power due to the fact that no energy has to be put into the system. The following is an example of a passive control scheme:
- Riblets - These are streamwise grooves in the surface of a wall or boundary. The existence of these riblets affect the position at which the vortices can be created on the surface. The amount of drag reduction or increase depends on the spacing of the grooves.
Active flow control methods require some kind of energy input to achieve a reduction in drag. An important point to note is that not all drag reduction results in a net power increase due to the possible input of more energy than is saved. Some active control schemes are listed below:
- Blowing and Suction - This method uses jets of air which can either be applied at the wall or in the near-wall region. These jets oppose the force of the streaks and hence reduce there effect, or sometimes remove them altogether. The drag reduction is affected by the layout and power of these jets.
- Spanwise Wall Oscillations - We can oscillate the walls in the spanwise direction to achieve drag reduction (or possibly even a drag increase). The reasons for the drag reduction of this method are not entirely understood. The efficiency of this method depends on the amplitude and frequency of the oscillation.
- Streamwise Travelling Waves - For this approach a sinusoidal wave, which moves in a steamwise direction, is applied at the boundary. The spanwise oscillating wall technique is an example of a streamwise travelling wave which moves with infinite speed. The amount of drag reduction relies on the amplitude, wavelength and speed of the wave.
Challenges & Benefits
The main area of study in this field is to fully understand the physics of the system. The method in which drag reduction can be achieved is not well known and hence many of the current flow control schemes are poorly explained. To improve our knowledge of this subject it is necessary to find out how the flow control methods interact with the streamwise vortices and low/high speed streaks.
Another useful research topic is to look into the parameters of specific flow control techniques that cause the maximum drag reduction or power increase i.e. the optimal parameters. It is also very important to find mathematical relationships between the input variables and the amount of drag reduction/power increase.
Flow control has many applications in solving real world problems. One of the most obvious applications is the relation to CO2 emissions in air travel. If an efficient flow control method can be adapted to work on the exterior of an aircraft, then less energy will be required to get up to flight speeds. This means that less fuel is needed during flight and hence the carbon emissions of the aerospace industry would be greatly reduced. One current application of the riblets method is in swimming costumes of professional athletes, which has seen a significant increase in race speeds in the past few years.