570
P. Liu
If the distributed roughness elements are added, the development of crossflow vortex is determined by the roughness elements. Then, we can find
roughness elements which can delay the transition by adjusting the parameters. The roughness here is different from that of the wall itself, which is
close to a very flat small cylinder. With the deepening of the research, Saric’s
team carried out laminar flow control test under actual flight conditions on a
Cessna O-2A Skymaster aircraft. The test results show that the distributed
roughness elements can keep more than 80% chord length covered by
laminar flow under certain conditions.
Later, Fransson et al. proposed to use distributed roughness elements to
control the development of TS wave. Although distributed roughness is used,
the principle used here is not the same as that used to control cross-flow
transition. Here, the periodic change along spanwise direction is generated by distributed roughness elements, and the gradient along spanwise is
introduced into the two-dimensional boundary layer flow, then the negative production term of Reynolds stress is formed during the evolution of
disturbance, which restrains the further growth of disturbance in the flow.
Based on the same principle, the experiment of using micro-vortex generators to produce strips in the laminar boundary layer to restrain the growth
of TS wave and thus delay the transition has also been successful, and if
multiple groups of vortex generators are used at the same time, the laminar
flow coverage can be continuously extended. After careful design, the plasma
actuator has been successfully used to delay the transition caused by TS wave.
These new control methods are still in the stage of experimental research and
test, and have not been tested in the field.
Regarding delaying the transition of the boundary layer, there are also some
ideas based on observations of nature. For example, Kramer proposed the use
of flexible walls to delay the transition based on the study of dolphins (as
shown in Fig. 7.172). The research in this area is relatively tortuous. For a
long time, no one could repeat the effect of the delayed transition mentioned
by Kramer until Gaster carefully designed the test of unstable disturbance
growth on the flexible wall and carried out the corresponding theoretical
analysis, which is consistent with the experimental results, thus bringing new
vitality to the research on transition control of flexible walls. Although the
laminar flow control method of the flexible wall surface is currently under
research, it is considered to be very promising for laminar drag reduction
control of aircraft.
P. Liu
If the distributed roughness elements are added, the development of crossflow vortex is determined by the roughness elements. Then, we can find
roughness elements which can delay the transition by adjusting the parameters. The roughness here is different from that of the wall itself, which is
close to a very flat small cylinder. With the deepening of the research, Saric’s
team carried out laminar flow control test under actual flight conditions on a
Cessna O-2A Skymaster aircraft. The test results show that the distributed
roughness elements can keep more than 80% chord length covered by
laminar flow under certain conditions.
Later, Fransson et al. proposed to use distributed roughness elements to
control the development of TS wave. Although distributed roughness is used,
the principle used here is not the same as that used to control cross-flow
transition. Here, the periodic change along spanwise direction is generated by distributed roughness elements, and the gradient along spanwise is
introduced into the two-dimensional boundary layer flow, then the negative production term of Reynolds stress is formed during the evolution of
disturbance, which restrains the further growth of disturbance in the flow.
Based on the same principle, the experiment of using micro-vortex generators to produce strips in the laminar boundary layer to restrain the growth
of TS wave and thus delay the transition has also been successful, and if
multiple groups of vortex generators are used at the same time, the laminar
flow coverage can be continuously extended. After careful design, the plasma
actuator has been successfully used to delay the transition caused by TS wave.
These new control methods are still in the stage of experimental research and
test, and have not been tested in the field.
Regarding delaying the transition of the boundary layer, there are also some
ideas based on observations of nature. For example, Kramer proposed the use
of flexible walls to delay the transition based on the study of dolphins (as
shown in Fig. 7.172). The research in this area is relatively tortuous. For a
long time, no one could repeat the effect of the delayed transition mentioned
by Kramer until Gaster carefully designed the test of unstable disturbance
growth on the flexible wall and carried out the corresponding theoretical
analysis, which is consistent with the experimental results, thus bringing new
vitality to the research on transition control of flexible walls. Although the
laminar flow control method of the flexible wall surface is currently under
research, it is considered to be very promising for laminar drag reduction
control of aircraft.