![Prerequisites
What is Boundary Layer ?
Even if the fluid is inviscid the viscosity effects cannot be neglected very close to the
surface.[No-slip condition]
A thin region near the surface where viscosity effects exists hence a velocity gradient develops, is
called (velocity) boundary layer.
Boundary layer separation:
Firstly, accelerated flow with pressure drop
Followed by decelerated flow with pressure increment
Here the difference occurs, fluid outside BL attains initial velocity while the fluid inside the BL
due to strong losses due to friction fails to attain.
It comes to a sort of standstill & is pushed backward into motion by pressure distribution of the
Outer flow.
This reverse flow is the basis of the drag increase.](https://image.slidesharecdn.com/flowseparationcontrolusing-150329214613-conversion-gate01/85/Flow-separation-control-using-plasma-vortex-generators-3-320.jpg)
![Purpose of Boundary Layer
Separation Prevention
i) The reverse flow causes appreciable drag increase [Drag
reduction/flow control.]
ii) Less Fuel consumption means more economical.
iii) Prevent the distortion of body means maintain the shape and
size.
iv) Apply less force to move the body means reduce the effort of
human.](https://image.slidesharecdn.com/flowseparationcontrolusing-150329214613-conversion-gate01/85/Flow-separation-control-using-plasma-vortex-generators-4-320.jpg)
![References
[1] G. Schubauer, W. Spangenberg, Forced mixing in boundary layers, J. Fluid Mech. (1960) 10-32.
[2] J. C. Lin, Control of turbulent boundary layer separation using micro vortex generator, 30th AIAA Fluid Dynamics Conference, Norfolk, VA
(1999).
[3] J. C. Lin and F. G. Howard, Small submerged vortex generator for turbulent flow separation control, J. spacecraft, 27 (1990).
[4] J.C. Lin, S. K. Robinson , R. J. McGhee and W. Valarezo, Separation control on high-lift airfoils via micro-vortex generator, J. Aircraft, 31
(1994).
[5] T. Melin, S. Crippa, M. Holly and M. Smidy, Investigating active vortex generators as a novel high lift device, 25th international congress
of the aeronautical sciences (ICAS 2006), Hamburg, Germany
[6] A. D. Culter, P. Bradshaw, Strong vortex/boundary layer interactions Part II. Vortices flow, Experiments in fluids, (1993) 393-401.
[7] F. Satta, D. Simoni, M. Ubaldi, P. Zunino, Velocity and turbulence measurements in a separating boundary layer with laser Doppler
velocimetry, Proceeding of ImechE, Part A: J. Power and Energy
[8] D. Lengani, D. Simoni, M. Ubaldi., P. Zunino and F. Bertini, Turbulent boundary layer separation control and loss evaluation of low
profile vortex generators, Experimental Thermal and Fluid Science
[9] C.M.Velte, M. Hansen and K. Jonck, Experimental and numerical investigation of the performance of vortex generators on separation
control, Journal of Physics: Conference Series 75 (2007).
[10] T. Duriez., J. Aider and J.E.Wesfreid, Base flow modification by streamwise vortices. Application to the control of separated flows, ASME
Joint U.S. - European Fluids Engineering Summer Meeting (2006).
[11] J.C. Lin, Review of research on low-profile vortex generators to control boundary-layer separation, Prog. Aerosp. Sci. (2002), 389–420.
[12] T. N. Jukes, K.-S Choi., Dielectric-barrier-discharge vortex generators: characterisation and optimisation for flow separation control,
Experiments in Fluids, (2012),329-345.
[13] A. N. M. Mominul Islam Mukut, H. Mizunuma, T. Segawa, H. Obara, PIV measurements of flow characteristics induced by mini platewing
plasma actuator, 63th Annual DFD Meeting (2010).
[14] H. Schlichting, “Boundary Layer Theory”, Chapter XXI, 7th ed. 1979.](https://image.slidesharecdn.com/flowseparationcontrolusing-150329214613-conversion-gate01/85/Flow-separation-control-using-plasma-vortex-generators-16-320.jpg)
Flow separation control using plasma vortex generators
- 1. Flow Separation Control Using Plasma Vortex Generator Journal- Procedia Engineering Publisher-Elsevier A.N.M. Mominul Islam Mukuta, Hiroshi Mizunumab, Obara Hiromichib aDhaka University of Engineering & Technology (DUET), Gazipur-1700, Bangladesh bTokyo Metropolitan University, Tokyo 192-0397, Japan
- 2. Abstract This paper represents a series of experiments to figure out plasma vortex generator as a means of separation flow control. Typical conventional vortex generator is a vane normal to wall and has a yaw angle against a main flow. This yaw angle gives a penalty of drag increase at the sacrifice of the separation suppression. A plasma vortex generator developed here has a yaw angle of 0°and thus the penalty could be minimized. An exposed electrode was installed on the Kapton vane tip, and another electrode was embedded in the vane. Stream-wise counter-rotating vortices are produced by the vane type plasma vortex generator at downstream that helps to control flow separation. The effect of the plasma vortex generator has been investigated on a turbulent boundary layer on a 20°-inclined slope. The separation flow was visualized by using a smoke wire, and the velocity profiles were measured using a hot-wire anemometer. The plasma vortex generators suppressed the separation similarly as the conventional vortex generator, and its performance was validated.
- 3. Prerequisites What is Boundary Layer ? Even if the fluid is inviscid the viscosity effects cannot be neglected very close to the surface.[No-slip condition] A thin region near the surface where viscosity effects exists hence a velocity gradient develops, is called (velocity) boundary layer. Boundary layer separation: Firstly, accelerated flow with pressure drop Followed by decelerated flow with pressure increment Here the difference occurs, fluid outside BL attains initial velocity while the fluid inside the BL due to strong losses due to friction fails to attain. It comes to a sort of standstill & is pushed backward into motion by pressure distribution of the Outer flow. This reverse flow is the basis of the drag increase.
- 4. Purpose of Boundary Layer Separation Prevention i) The reverse flow causes appreciable drag increase [Drag reduction/flow control.] ii) Less Fuel consumption means more economical. iii) Prevent the distortion of body means maintain the shape and size. iv) Apply less force to move the body means reduce the effort of human.
- 5. Boundary Layer Separation Prevention Well ordered motion promotes BL separation For preventing separation, momentum transfer should be enhanced Hence turbulence is introduced before hand(prior to point of separation) From the source of turbulence, the eddies diffuse into free stream & lead to equilateral momentum distribution Eddies basically help to rotate the parallel vectors of free stream. We just wish to speed up the slow layer
- 6. Types of flow control- i) Active flow control – External energy needs to be provided. For e.g.. Suction, Heating of surface, Active Vortex Generators ii) Passive flow control- Doesn’t requires external energy. For e.g. Modifying surface profile, Slat and slot( secondary winglets) etc.
- 7. Vortex Generators (VG) as an active flow control device Vortex generators create stream-wise vortices close to the surface Mixing between boundary layer and free stream is increased due to the formation of vortices which bring momentum to the near wall region. Hence near wall flow become energies to withstand more adverse pressure gradient and flow separation become delayed The conventional vane-type VG has a height of the order of the boundary layer thickness, and has a penalty of drag increase (WAKE DRAG)
- 8. Plasma Vortex Generator (PVG) Plasma actuator is the new alternative without the drag penalty to control the separation.( zero yaw angle ) The authors developed a winglet type plasma actuator and characterized the jet flow induced from the winglet Winglet-type plasma actuator is acted like vane type VG The plasma induced flow from the vane tip to the hub produces a pair of stream-wise counter-rotating vortices downstream Authors studied the flow control effects of both VG and PVG contemporarily.
- 9. Experimental setup and measurement methodology The experiment was carried out in an open type wind tunnel composed of a straight entrance section and a diffuser section. Free stream v= 4ms-1 Bottom wall of the diffuser section was inclined by 20 degrees.
- 10. A tripping wire of 2 mm diameter was fixed at x = 50 mm in the straight entrance section to develop a turbulent BL. The BL profile followed 1/7th law. Flow separation was- measured by hot wire anemometer visualised by smoke sheet synchronised with 3 video cameras.
- 11. Configurations of VG and PVG VG VG used here was a vane-type and its height was designed to submerge in the BL The yaw angle between the flow and the VG was fixed to 25° The same dimensions were applied to the PVG, but the yaw angle was fixed to zero. Three VGs were installed on the wall of the entrance straight section. PVG The PVG has an exposed electrode of 2 mm width on the each side and a covered electrode of 5 mm width. Kapton film was used as the dielectric material. AC pulses of 4 kV and 5 kHz was applied to the exposed electrode using an electric power supply
- 12. Result and Discussions (Flow separation under no actuation by vortex generators) From smoke visualisation point of separation was at 10mm downstream from the start of the diffuser section velocity profile plotted at x = 450 mm, 550 mm, 630 mm, 660 mm, 690 mm, and 730 mm. The pictures in (a) were taken at 10ms after a high voltage pulse was loaded into the smoke wire. (a) Visualization by a smoke wire. (b) Time averaged velocity profiles obtained from a hot wire anemometer.
- 13. (Flow control under VG ) (a) Visualization by a smoke wire. (b) Time averaged velocity profiles obtained from a hot wire anemometer.
- 14. (Flow control under PVG ) (a) Visualization by a smoke wire. (b) Time averaged velocity profiles obtained from a hot wire anemometer.
- 15. Conclusion The plasma vortex actuator does not have a yaw angle against a main flow. Thus the penalty of drag increase is expected to be low when the plasma is switched off. When the plasma is switched off, the flow returned to the separation flow. This result showed the advantage of PVG over convectional VG that PVG can be switched off to minimize the drag penalty when it is not necessary to control the flow. Plasma vortex generator would be a better means of flow control device in the field of flow control engineering considering above mentioned criteria and comparison with convectional vortex generator. As clearly visible the PVG is not as effective as the conventional VG. This is one of the reasons why it is an active area of research.
- 16. References [1] G. Schubauer, W. Spangenberg, Forced mixing in boundary layers, J. Fluid Mech. (1960) 10-32. [2] J. C. Lin, Control of turbulent boundary layer separation using micro vortex generator, 30th AIAA Fluid Dynamics Conference, Norfolk, VA (1999). [3] J. C. Lin and F. G. Howard, Small submerged vortex generator for turbulent flow separation control, J. spacecraft, 27 (1990). [4] J.C. Lin, S. K. Robinson , R. J. McGhee and W. Valarezo, Separation control on high-lift airfoils via micro-vortex generator, J. Aircraft, 31 (1994). [5] T. Melin, S. Crippa, M. Holly and M. Smidy, Investigating active vortex generators as a novel high lift device, 25th international congress of the aeronautical sciences (ICAS 2006), Hamburg, Germany [6] A. D. Culter, P. Bradshaw, Strong vortex/boundary layer interactions Part II. Vortices flow, Experiments in fluids, (1993) 393-401. [7] F. Satta, D. Simoni, M. Ubaldi, P. Zunino, Velocity and turbulence measurements in a separating boundary layer with laser Doppler velocimetry, Proceeding of ImechE, Part A: J. Power and Energy [8] D. Lengani, D. Simoni, M. Ubaldi., P. Zunino and F. Bertini, Turbulent boundary layer separation control and loss evaluation of low profile vortex generators, Experimental Thermal and Fluid Science [9] C.M.Velte, M. Hansen and K. Jonck, Experimental and numerical investigation of the performance of vortex generators on separation control, Journal of Physics: Conference Series 75 (2007). [10] T. Duriez., J. Aider and J.E.Wesfreid, Base flow modification by streamwise vortices. Application to the control of separated flows, ASME Joint U.S. - European Fluids Engineering Summer Meeting (2006). [11] J.C. Lin, Review of research on low-profile vortex generators to control boundary-layer separation, Prog. Aerosp. Sci. (2002), 389–420. [12] T. N. Jukes, K.-S Choi., Dielectric-barrier-discharge vortex generators: characterisation and optimisation for flow separation control, Experiments in Fluids, (2012),329-345. [13] A. N. M. Mominul Islam Mukut, H. Mizunuma, T. Segawa, H. Obara, PIV measurements of flow characteristics induced by mini platewing plasma actuator, 63th Annual DFD Meeting (2010). [14] H. Schlichting, “Boundary Layer Theory”, Chapter XXI, 7th ed. 1979.
Editor's Notes
- #4: Separation of the boundary layers occurs whenever the flow tries to decelerate quickly or the pressure gradient is positive, sometimes referred to as adverse pressure gradient. ii)At the diffused surface local external pressure decreases and the flow should accelerate as the potential energy of the pressure field is converted to kinetic energy. iii)However, because of viscous losses, not all kinetic energy is recovered and the flow reverses around the separation point.
- #9: A gas discharge is created when an electric field of sufficient amplitude is applied to a volume of gas to generate electron–ion pairs through electron impact ionization of the neutral gas A yaw angle of the vane is zero against a main flow, and thus the drag penalty is expected to be low if the plasma is switched off. The effect of the separation suppression was validated and compared with that of the conventional VGs.
- #10: The coordinate system of x, y, and z was defined as shown in Fig. 1. The x-wise lengths of these sections were 0.63m and 1.4m, respectively. The cross section of the entrance was 0.2 m by 0.2 m. Fan at leftmost STRAIGHT ENTRANCE & DIFFUSER SECTION
- #11: Turb BL is comparatively difficult to separate Smoke sheet & video cameras were controlled by a MICROCOMPUTER Smoke wire works in PULSES The trigger sent signals to the electric pulse generator and the video camera were controlled by a microcomputer
- #12: Plasma pheno starts from EXPOSED ELECTRODE and ends on the COVERD
- #13: The BL is 1st allowed to separate A hot wire anemometer cannot discriminate the flow direction, and thus the velocity plotted shows the absolute value including reverse flow
- #14: U can see that VG is suppressing the separation the flow separation As is visible from the flow visualisation
- #15: PVG is also supressing te downstream separation U can easily make out from the 3 velocity profile downstream But the thing is that pvg is not as effective as the VG , as is clear from the profiles hence is the area of research