Final Report Summary - AFRODITE (Advanced Fluid Research On Drag reduction In Turbulence Experiments)
Benefits in accomplishing transition to turbulence delay are associated with fluid-to-surface heat transfer reduction as well as skin-friction drag reduction. This means that a well developed laminar flow control method can lead to an unforeseen impact considering the broad spectrum of industrial applications where reducing drag and/or heat transfer is a daily challenge.
Unexpected results, where circular roughness elements were used in order to delay boundary layer transition on a flat surface, was published by the PI in 2006 (Fransson et al. Phys. Rev. Lett., 96, 064501). The results caught worldwide media attention and were referred to as revolutionary. The reason was that, ever since the mid 50-ties the common opinion about surface roughness had been that any roughness would promote transition to turbulence in a boundary layer. In the project proposal AFRODITE (Advanced Fluid Research On Drag reduction In Turbulence Experiments), pro and cons about using circular roughness elements as control devices to delay transition to turbulence were discussed. New ideas were introduced on how these devices could be improved as successful control devices in real flow applications. One idea of using miniature vortex generators (MVGs) was coined by the PI. These devices are miniature with respect to classical vortex generators that have evolved and been optimized over decades for separation control. The MVGs were hypothesized to be far more suitable for real flow applications than circular roughness elements since the flow would pass straight through a pair of MVGs and significantly reduce the absolute instability region generated by the wake of any device. AFRODITE received funding as an ERC-Starting grant in 2010.
The physical mechanism behind the attenuation of velocity disturbances, that would otherwise lead to transition to turbulence, is based on the presence of the additional spanwise disturbance kinetic energy production term when spanwise periodic regions of high and low speed flow, often denoted streamwise streaks, are generated. Today, this control method is denoted the SVG method, which is an abbreviation of Spanwise mean Velocity Gradient and has evolved through AFRODITE. The attenuation efficiency of velocity disturbances by the SVG method is correlated to the amplitude of the streamwise streaks for a given spanwise wavelength and the robustness of the streaks play a key role in the control success, since unstable streaks at high amplitude often lead to breakdown to turbulence.
One way of producing streamwise streaks involve mounting physical control devices on the surface. These devices have evolved with respect to their performance and robustness of the generated streaks within the AFRODITE project, from rectangular and cylindrical roughness elements to MVGs and streamwise elongated humps. The drawback of fixed physical control devices is that they are always present, but there are applications where protrusive devices are not suitable or not even allowed. Hence, a new direction was also taken within the AFRODITE project, i.e. to seek solutions in setting up streamwise streaks with a minimal energy input to the control system with the feature of instantaneously being able to turn on and off the control. AFRODITE experiments of transition to turbulence delay using localized suction as well as by taking advantage of a well-known non-linear effect, namely the interaction of two oblique waves at high amplitude, have addressed this issue of turning on and off the control system. Both techniques work very well in delaying transition to turbulence and the results are under consideration for journal publication. Other exciting AFRODITE results are the usage of free-stream vortices as well as simple wavy surfaces that have also been proven to efficiently stabilize boundary layer disturbances and delay transition to turbulence.
In summary, the AFRODITE project has allowed fundamental wind tunnel experiments on a novel laminar flow control method, denoted SVG, to be performed. Today the research community is well informed about this new AFRODITE method and is now ready for real flow application tests.
Unexpected results, where circular roughness elements were used in order to delay boundary layer transition on a flat surface, was published by the PI in 2006 (Fransson et al. Phys. Rev. Lett., 96, 064501). The results caught worldwide media attention and were referred to as revolutionary. The reason was that, ever since the mid 50-ties the common opinion about surface roughness had been that any roughness would promote transition to turbulence in a boundary layer. In the project proposal AFRODITE (Advanced Fluid Research On Drag reduction In Turbulence Experiments), pro and cons about using circular roughness elements as control devices to delay transition to turbulence were discussed. New ideas were introduced on how these devices could be improved as successful control devices in real flow applications. One idea of using miniature vortex generators (MVGs) was coined by the PI. These devices are miniature with respect to classical vortex generators that have evolved and been optimized over decades for separation control. The MVGs were hypothesized to be far more suitable for real flow applications than circular roughness elements since the flow would pass straight through a pair of MVGs and significantly reduce the absolute instability region generated by the wake of any device. AFRODITE received funding as an ERC-Starting grant in 2010.
The physical mechanism behind the attenuation of velocity disturbances, that would otherwise lead to transition to turbulence, is based on the presence of the additional spanwise disturbance kinetic energy production term when spanwise periodic regions of high and low speed flow, often denoted streamwise streaks, are generated. Today, this control method is denoted the SVG method, which is an abbreviation of Spanwise mean Velocity Gradient and has evolved through AFRODITE. The attenuation efficiency of velocity disturbances by the SVG method is correlated to the amplitude of the streamwise streaks for a given spanwise wavelength and the robustness of the streaks play a key role in the control success, since unstable streaks at high amplitude often lead to breakdown to turbulence.
One way of producing streamwise streaks involve mounting physical control devices on the surface. These devices have evolved with respect to their performance and robustness of the generated streaks within the AFRODITE project, from rectangular and cylindrical roughness elements to MVGs and streamwise elongated humps. The drawback of fixed physical control devices is that they are always present, but there are applications where protrusive devices are not suitable or not even allowed. Hence, a new direction was also taken within the AFRODITE project, i.e. to seek solutions in setting up streamwise streaks with a minimal energy input to the control system with the feature of instantaneously being able to turn on and off the control. AFRODITE experiments of transition to turbulence delay using localized suction as well as by taking advantage of a well-known non-linear effect, namely the interaction of two oblique waves at high amplitude, have addressed this issue of turning on and off the control system. Both techniques work very well in delaying transition to turbulence and the results are under consideration for journal publication. Other exciting AFRODITE results are the usage of free-stream vortices as well as simple wavy surfaces that have also been proven to efficiently stabilize boundary layer disturbances and delay transition to turbulence.
In summary, the AFRODITE project has allowed fundamental wind tunnel experiments on a novel laminar flow control method, denoted SVG, to be performed. Today the research community is well informed about this new AFRODITE method and is now ready for real flow application tests.