Final Report Summary - PTURB (Particles in Turbulence: from tracers to rain formation)
The research of M Gibert was focused on two complementary themes:
- 1-Fully developed turbulence
The Goettingen wind tunnel http://www.euhit.org/i-gtf.html has the advantage to allow us to generate extremely high level of turbulence (high Reynolds number), almost comparable to geo/astrophysical flows, in well-controlled laboratory conditions. The entire turbulence group (2 permanent researchers, 5 postdocs and 3 students) is working on this project together. The ultimate goal of this project is to achieve 3D-LPT measurements in the wind tunnel. It involves a lot of technical issues due to this particular atmosphere (15 bar SF6). M Gibert's role in this team was to solve the optical problems due to the change of index of refraction when changing the pressure of the SF6. This regroups the optics of the fast cameras that have to be adjustable and the laser path in the wind tunnel that has to move with the mean wind speed (5m/s). He is also working with E.W. Saw (postdoc in the group) on the seeding of the flow. Additionally he is involved in an international collaboration with the LEGI (Grenoble - France), in the context of the ICTR, to setup a vorticity measurement acoustic probe in the wind tunnel.
The research of M Gibert on fully developed turbulence also took place in smaller devices. At his arrival in the MPI-DS, he designed and built a Von Kármán flow (two coaxial counter rotating disks) in water, able to generate high turbulence level with no mean velocity (at the center) in a small volume experiment. Numerous of this type of turbulence generator, with no mean velocity, were developed in the last 10 years since it is particularly well suited for Lagrangian measurements (like 3D-LPT). A group of international researchers (including M Gibert) decided to compare quantitatively a variety of these flows (including numerical simulations) to uncover the influence that the large scales of a turbulent flow have on the small ones.
2-Inertial particles dynamics in fully developed turbulence
The dynamical coupling between a turbulent flow and inertial particles motion is of crucial interest in a wide range of applications. By inertial particles, we refer to particles that are heavier than the fluid in which they evolve and/or bigger than the Kolmogorov length scale of the flow. The application of this research spans from industrial mixing process to climate change or pollution spreading. It is also very relevant for all the modern visualisation based fluid mechanics measurement techniques (such as PIV, LDA or LPT). Indeed, all these techniques rely on the fact that the particles used to seed the flow can be considered as tracer particles that have the same velocity as the underlying fluid. The two important parameters to consider are the density and size of the particles. In order to decouple the effect due to the density mismatch and the size, M. Gibert studied two limit cases.
The first one is focused on small but heavy particles. In this particular case lots of studies have been done on single particle statistics (meaning velocity and/or acceleration probability density function). M Gibert extended those studies to two-particles statistics using 3D-LPT. This allowed him to study quantitatively how heavy particle separates one from another and the effect of preferential concentration on these statistics.
The second limit that he studied is the case of iso-dense but big particles. This case is even more challenging because it cannot be addressed by numerical simulations in the next decade, and no model is available today. To make a significant advance in this field, there was a need to measure the full motion of the particle (translation and rotation) together with the fluid velocity around it. This is the experimental challenge that M Gibert has been able to overcome by developing a very advance 3D particle-tracking algorithm.
- 1-Fully developed turbulence
The Goettingen wind tunnel http://www.euhit.org/i-gtf.html has the advantage to allow us to generate extremely high level of turbulence (high Reynolds number), almost comparable to geo/astrophysical flows, in well-controlled laboratory conditions. The entire turbulence group (2 permanent researchers, 5 postdocs and 3 students) is working on this project together. The ultimate goal of this project is to achieve 3D-LPT measurements in the wind tunnel. It involves a lot of technical issues due to this particular atmosphere (15 bar SF6). M Gibert's role in this team was to solve the optical problems due to the change of index of refraction when changing the pressure of the SF6. This regroups the optics of the fast cameras that have to be adjustable and the laser path in the wind tunnel that has to move with the mean wind speed (5m/s). He is also working with E.W. Saw (postdoc in the group) on the seeding of the flow. Additionally he is involved in an international collaboration with the LEGI (Grenoble - France), in the context of the ICTR, to setup a vorticity measurement acoustic probe in the wind tunnel.
The research of M Gibert on fully developed turbulence also took place in smaller devices. At his arrival in the MPI-DS, he designed and built a Von Kármán flow (two coaxial counter rotating disks) in water, able to generate high turbulence level with no mean velocity (at the center) in a small volume experiment. Numerous of this type of turbulence generator, with no mean velocity, were developed in the last 10 years since it is particularly well suited for Lagrangian measurements (like 3D-LPT). A group of international researchers (including M Gibert) decided to compare quantitatively a variety of these flows (including numerical simulations) to uncover the influence that the large scales of a turbulent flow have on the small ones.
2-Inertial particles dynamics in fully developed turbulence
The dynamical coupling between a turbulent flow and inertial particles motion is of crucial interest in a wide range of applications. By inertial particles, we refer to particles that are heavier than the fluid in which they evolve and/or bigger than the Kolmogorov length scale of the flow. The application of this research spans from industrial mixing process to climate change or pollution spreading. It is also very relevant for all the modern visualisation based fluid mechanics measurement techniques (such as PIV, LDA or LPT). Indeed, all these techniques rely on the fact that the particles used to seed the flow can be considered as tracer particles that have the same velocity as the underlying fluid. The two important parameters to consider are the density and size of the particles. In order to decouple the effect due to the density mismatch and the size, M. Gibert studied two limit cases.
The first one is focused on small but heavy particles. In this particular case lots of studies have been done on single particle statistics (meaning velocity and/or acceleration probability density function). M Gibert extended those studies to two-particles statistics using 3D-LPT. This allowed him to study quantitatively how heavy particle separates one from another and the effect of preferential concentration on these statistics.
The second limit that he studied is the case of iso-dense but big particles. This case is even more challenging because it cannot be addressed by numerical simulations in the next decade, and no model is available today. To make a significant advance in this field, there was a need to measure the full motion of the particle (translation and rotation) together with the fluid velocity around it. This is the experimental challenge that M Gibert has been able to overcome by developing a very advance 3D particle-tracking algorithm.