Background

Current challenges in turbulence research are closely related with the grand challenges of our time, in particular with regard to Climate and Sustainability. An example is the need for deep understanding of growth of raindrops in turbulent clouds. Another is related with introduction of innovative engineering approaches to design and modify the small-scale properties of turbulence. To explore these themes we are looking for three enthusiastic PhD-candidates who want to take up these challenges. The positions are available within the Fluids and Flows group at Eindhoven University of Technology (Eindhoven, the Netherlands).

A promising but challenging idea is the development of innovative tools to control turbulence at the smallest scales. Although turbulence is everywhere, our fundamental understanding and capabilities to control it remain very limited. It is a remarkable property of turbulence that, rather irrespectively of the large-scale forcing mechanism, it rapidly tends to restore a universal (homogeneous and isotropic) state at smaller scales. This tendency to universality severely hinders our capability to modulate turbulence by acting on few (large-scale) degrees of freedom. A promising candidate for small-scale control is the possibility to shape turbulence with small (chiral or magnetic) particles, see Figures 1 and 2. Active forcing at the heart of turbulence, the smallest scales, may let us alter the structure of turbulence from bottom-up. This opens up a wealth of options, on a fundamental level but certainly also for industrial applications.Figure 1: Magnetic particles can
efficiently force turbulence at the
small scales. Taken from Falcon et al.
PRF2, 102601 (2017).Figure 2: Formation of chains of
(super-)paramagnetic particles
activated via external magnetic fields
(top) and how the interaction between
particles affects the flow (bottom).
Taken from Van Reenen et al. Lab
Chip 14, 1966 (2014).



























A different but intimately related puzzle, one of the open problems in cloud physics is the growth of droplets to raindrop size. In situ size measurements display the presence of a 'size-gap' between 10 and 100 μm in the droplet-size distribution, with a significantly lower density of droplets in this size range. This seems to indicate that in this size range an efficient growth process exists, letting droplets rapidly grow across the gap. The main candidate stimulating droplet growth in this size range is turbulence: it enhances the rate of droplet collision and coalescence by 'centrifuging' droplets out of turbulent swirls, with greatly increased local concentrations as a result, and thus rapid turbulence-induced droplet growth. A process most likely further promoted by hydrodynamic interactions between the droplets.

This turbulence program with three PhD positions will enter a so far unexplored terrain by experimentally studying turbulence-initiated droplet growth and by designing non-universal turbulence using 'smart' particles (either chiral, magnetic or with both properties) capable of applying appropriate small-scale forcing. Within this program the available experimental and computational capabilities will be combined in our search for understanding and solutions.





Project #1: Shaping turbulence with smart particles: experiments with magnetic beadsFigure 3: Sketch of the experimental setup
known as the "French washing machine".
Taken from Berg et al., PRE 74, 016304
(2006).In this project, the PhD candidate will investigate experimentally the role of magnetic particles in turbulence at low and high densities with the possibility to steer the turbulence via external (time-dependent) magnetic fields. An experimental setup for the generation of homogeneous isotropic turbulence (French washing machine) will be designed (see Fig. 3). It should satisfy certain constraints such as optical access for Particle Image Velocimetry (PIV) and 3D Particle Tracking Velocimetry (3D PTV) applications and facilities for the production of magnetic fields to control the dynamics of the particles. Measurement of magnetic particle aggregates as well as the statistics of turbulence will be made possible as function of turbulence and magnetic field strength. Typical size of the magnetic beads will be similar to the dissipative scale of turbulence; aggregates can form strings as long as the integral scale. Strong interaction and exchange of data with parallel PhD projects (including project #2) on chiral particles in turbulence and numerical studies of such systems is anticipated.

Project #2: Smart particles in turbulence: study of the bulk propertiesFigure 4: Example of a hybrid magnetic-chiral particle.
Particles can be produced in different sizes and for
applications particles with sizes up to the range of
millimeters may be used. Taken from Zhang et al.
Nano Lett. 9, 3663-3667 (2009).In this project, the PhD-candidate will investigate experimentally and numerically the role of particle shape and properties, in combination with magnetic actuation protocols, on the turbulence statistics and the bulk properties. An example of a magnetic-chiral particle is shown in figure 4. When employing smart particles to influence turbulence statistics the aim is to achieve the largest possible effect with the smallest possible number of particles (i.e. having a strong non-linear dependency from the particle volume fraction). Moreover, we aim at exploring experimentally (based on the setup designed and built in project #1, see figure 3) a large parameter space clarifying, e.g. the influence of particle size, of  polydispersity, the influence of shapes of particles and of the external (magnetic) forcing protocols on the bulk properties of the turbulent flow. From a numerical point of view accurate and point-wise models, capable to embed the physics of chirality, will be developed and analytical models for chiral particles and aggregates of magnetic particles will be further validated via Immersed Boundary Methods and Stokesian Dynamics simulations with few particles and confronted with experimental datasets from projects #1 and #2.  

Project #3: Droplets in turbulenceFigure 5: The turbulence chamber with
optical diagnostics available at TU/e,
Fluids and Flows group.In this project, the PhD student will initiate experimental investigations to generate and track micron-sized droplets in a turbulent airflow. The setup used for this investigation is a football-shaped turbulence chamber with a diameter of about 1 m available within the Fluids and Flow group at TU/e, see Fig. 5. Loudspeakers agitate turbulence inside; a spinning-disc generator provides the droplets. Laser illumination of a small central volume of a few cm3 allows for 3D position tracking of droplets from four viewing angles with fast cameras. These cameras are adequate for the reconstruction of droplet trajectories in 3D (via 3D PTV algorithms) and quantification of clustering, but also suitable for sufficient statistical accuracy of droplet tracking and the resolution of droplet velocities. In previous studies it was observed that hydrodynamic interactions might promote droplet collisions. This experimental investigation will therefore focus on the effect of hydrodynamic interaction on the droplet velocity, and how this will affect droplet collisions and coalescence.

Location

These projects will be carried out within the Fluids and Flows group at the Department of Applied Physics (https://www.tue.nl/en/research/research-groups/fluids-and-flows/) of Eindhoven University of Technology. The PhD candidates will be supervised by a team consisting of prof. H.J.H. Clercx, prof. F. Toschi and dr. R.P.J. Kunnen. This program is part of a collaboration with University of Twente that encompasses also international partners.

We are looking for enthusiastic PhD-students with an excellent background in fluid dynamics and turbulence, and depending on the project a strong background in experimental measurement techniques or affinity with computational physics. You have an MSc in (applied) physics or mechanical engineering. Knowledge of statistical mechanics is an asset. As an ideal candidate you should be able to work in a team. You also have good written and oral communication skills in English.

• An exciting job in the active group Fluids and Flows, with strong national and international collaborations
• A full-time appointment for four years (start date on or before February 1st 2022) at Eindhoven University of Technology http://www.tue.nl/en/
• To develop your teaching skills, you will spend 10% of your employment on teaching tasks.
• To support you during your PhD and to prepare you for the rest of your career, you will make a Training and Supervision plan and you will have free access to a personal development program for PhD students (PROOF program).
• A gross monthly salary and benefits (such as a pension scheme, pregnancy and maternity leave, partially paid parental leave) in accordance with the Collective Labor Agreement for Dutch Universities.
• Additionally, an annual holiday allowance of 8% of the yearly salary, plus a year-end allowance of 8.3% of the annual salary.
• Should you come from abroad and comply with certain conditions, you can make use of the so-called '30% facility', which permits you not to pay tax on 30% of your salary.
• A broad package of fringe benefits, including an excellent technical infrastructure, moving expenses, and savings schemes.
• Family-friendly initiatives are in place, such as an international spouse program, and excellent on-campus children day care and sports facilities.