Optimal streaks in 2D wakes and their stabilizing effect on local and global instabilities
Event details
| Date | 26.05.2014 |
| Hour | 15:00 › 17:00 |
| Speaker | Prof. Carlo Cossu, Directeur de recherche CNRS, Institut de mécanique des fluides (IMFT), Toulouse |
| Location | |
| Category | Conferences - Seminars |
This talk will summarize recent results obtained on the forcing of optimal perturbations in wakes and on their stabilizing action on local and global wake instabilities.
In the first part of the talk, parallel wakes are considered. Optimal temporal transient energy growths of streamwise uniform perturbations are computed. Optimal initial conditions consist in streamwise vortices and the optimally amplified perturbations are streamwise streaks. It is shown that, when forced at finite amplitude in absolutely unstable wakes, optimal streaks can completely suppress the absolute instability converting it into a convective instability.
Next, a synthetic non-parallel and the circular cylinder wakes are considered.
Optimal spatial energy amplifications of steady disturbances are computed, finding again that streamwise streaks are the most amplified structures. It is shown that global instabilities can be stabilized by optimal streaks of sufficiently large amplitude.
We also show that the amplitude of optimal spanwise periodic (3D) perturbations of the basic flow required to stabilize the global instability is smaller than the one required by spanwise uniform (2D) perturbations despite the fact that the first order sensitivity of the global eigenvalue to basic flow modifications is zero for 3D spanwise periodic modifications and non-zero for 2D modifications.
We therefore conclude that first-order sensitivity analyses can be misleading if used far from the instability threshold, where higher order terms are the most relevant.
Bio: I'm interested in fluid mechanics and applied mathematics and, more specifically, in the understanding of transition to turbulence and the design of innovative flow control strategies. These topics are relevant to a number of applications ranging from the `vaccination' of boundary layers to delay transition and reduce drag to the understanding of the dynamics of accretion disks. You can learn more browsing the pages listed on the on the left. If you are a highly motivated student and you feel interested in these topics do not hesitate to contact me.
In the first part of the talk, parallel wakes are considered. Optimal temporal transient energy growths of streamwise uniform perturbations are computed. Optimal initial conditions consist in streamwise vortices and the optimally amplified perturbations are streamwise streaks. It is shown that, when forced at finite amplitude in absolutely unstable wakes, optimal streaks can completely suppress the absolute instability converting it into a convective instability.
Next, a synthetic non-parallel and the circular cylinder wakes are considered.
Optimal spatial energy amplifications of steady disturbances are computed, finding again that streamwise streaks are the most amplified structures. It is shown that global instabilities can be stabilized by optimal streaks of sufficiently large amplitude.
We also show that the amplitude of optimal spanwise periodic (3D) perturbations of the basic flow required to stabilize the global instability is smaller than the one required by spanwise uniform (2D) perturbations despite the fact that the first order sensitivity of the global eigenvalue to basic flow modifications is zero for 3D spanwise periodic modifications and non-zero for 2D modifications.
We therefore conclude that first-order sensitivity analyses can be misleading if used far from the instability threshold, where higher order terms are the most relevant.
Bio: I'm interested in fluid mechanics and applied mathematics and, more specifically, in the understanding of transition to turbulence and the design of innovative flow control strategies. These topics are relevant to a number of applications ranging from the `vaccination' of boundary layers to delay transition and reduce drag to the understanding of the dynamics of accretion disks. You can learn more browsing the pages listed on the on the left. If you are a highly motivated student and you feel interested in these topics do not hesitate to contact me.
Links
Practical information
- General public
- Free
Organizer
- Laboratory of Fluid Mechanics and Instabilities LFMI
Contact
- Prof. François Gallaire