RESEARCH

Focal points

Four “focal points” have been selected from the three research themes, which receive special attention. A brief description of these “focal points” is given below.

Bio fluid mechanics

Most biological organisms live in a flowing medium (air or water). Nature has found solutions for fluid mechanical problems which enable fish to swim fast or cellular organisms to propel. These solutions are intriguing to understand and may lead to new solutions for technical problems. Similarly, fluid flow is essential inside the human body, where the blood is pumped around and the air is inhaled. Deposition of aerosols in the lung, sound production by speaking, atherosclerotic plaque formation at well determined positions, gene activation at cellular levels are all more or less determined by fluid mechanic processes. Finally, diagnostic and therapeutic techniques make use of fluid mechanic and heat transfer insights. The development of heart valves and the monitoring of temperatures inside the body during operations are examples for that.

Although most of the above mentioned problems can be solved with known physical principles, the complicated geometrical structures and the combination of phenomena (for example the transitional flow of non-Newtonian media in elastic bifurcating channels like blood vessels and airways) form an exciting new area for the development of advanced numerical and experimental techniques. Due to the geometry, three-dimensional unstructured meshes are to be used and the most efficient solvers are required to solve the flow at the relevant dimensionless parameters. Micro-PIV systems are needed to analyse the flow field in micro-vessels and fast optical techniques will enlighten the perfusion in permeable tissues.
New physical insights are needed for several areas, especially in multidisciplinary science. Some examples are given: The combination of fluid mechanics and solid mechanics is apparent in the phenomena at the focal folds where the unsteady flow separation is strongly influenced by the complex movement of the structure. Many modern uses of micro-bubble ultrasound contrast agents rely on the highly nonlinear response of the bubbles to a driving ultrasonic field and a quantitative model is lacking. The heat transfer processes in the anaestesized body are strongly determined by control mechanisms that are only globally known. Drag reduction occurs at the skin of several fish and reverse transition from turbulent to laminar flow is present in the nasal cavity and stenotic blood vessels; the relation with the wall structure is unclear. Fluid mechanical parameters stimulate the activation of genes in cells, with striking downstream effects - unexplained. The interaction between the non-Newtonian mucus layer in the airways and the oscillating airflow during cough is undescribed. The settlement and growth of settlements in aquatic ecosystems require the combination of advanced flow and mass transport models. As we have noted, research on this topic is extremely diverse and complex, because it involves a large number of different areas of expertise and advanced techniques. Therefore, this theme is an excellent area for collaboration between research groups inside and outside the fluid-mechanics community.

Granular matter

Granular matter exhibits many fascinating phenomena and is attractive both from a fundamental and an applied point of view. Its economic potential is enormous: it has been estimated that no less than 40 percent of the capacity of the industries that process granular matter is wasted due to problems connected to the handling of these materials.

Depending on the situation, granular matter can behave similar to a solid, a liquid, or a gas. E.g., when dry sand is poured, it acts as a fluid. The pile on which it is poured is solid-like, stabilised by forces in between the sand beeds. These forces organise themselves in tree-like networks. Finally, when dry sand is strongly shaken or fluidized through a gas stream, it behaves gas-like. The transition from one to the other regime can be very sudden and the dynamics of such a transition is very rich. When in a gas-like or fluid-like state, the granular particles can all the locally sudden cluster. In many applications this can lead to serious problems, as whole production lines get stuck or the free available surface of some heterogenous catalysator all the sudden gets to small. So it is crucial to better understand the transition to the clustered state in order to avoid it.

The origin of the potential to cluster lies in the inelasticity of the particle-particle collision: If two particles collide, they loose kinetic energy and will thus stay closer to each other, trapping even further particles in the developing cluster. Even without the phase transitions granular dynamics is difficult to understand. For the fluidised phase the brute-force approach is molecular dynamical simulations, based on some interaction potential between the particles. If this potential is chosen realistically (i.e., rather hard), the time step of advancing the numerical simulation can only be extremely slow, making this approach inpracticable. Better results have been obtained with either (unrealistically) soft potentials or with event driven codes. The ultimate goal must be to achieve at some continuum description, similar to the Navier-Stokes equation for fluid dynamics. Though considerable success in this direction has meanwhile been achieved, the problem is far from being solved. One of the main questions is how to pick the boundary conditions for such a continuum field.

One of the current physical questions one wants to answer is: How do average velocity profiles and velocity fluctuations look like in granular flow? On the experimental side, tomographical methods have turned out to be very successful to reveal these questions. Another intriguing problem of granular dynamics is size segregation. The most famous example presumably is the so called "Brazil nut" effect: In vibro-fluidised granular material big particles tend to "swim" to the top. Two explanations compete. The original interpretation was that the smaller particles can easier fall into gaps which the big ones are leaving when jumping up. In this way the big particles would be pushed towards the top. The second explanation is based on convection roles and channels which would form, which are too small for the big particles to dive down again, so that they must stay on the top. Both of these interpretations are challenged by the recent discovery of an inverse Brazil nut effect which pushes big particles to the bottom.
Finally, we would like to mention the interaction of granular matter ("sand") with water, which often leads to pattern formation, e.g., the famous sand ripples on the beach. On a larger scale, this interaction is crucial (in particular for the Netherlands) for the protection of the coastline.

Measurement techniques

Optical diagnostics become more important for the investigation of flows. The principal differences with conventional methods, such as hot-wire anemometry, is that these optical methods can be considered as non-intrusive and that they provide data on the instantaneous spatial structure of the flow field. These optical methods can be divided into two categories: one in which the flow information is extracted from tracer particles added to the fluid (seeded flows), and one in which the fluid information is extracted from the fluid itself (spectroscopic methods).

Seeded flows
The motion of the flow can be detected by adding to the fluid very small tracer particles that are small enough to consider the method as non-intrusive. Essentially the motion is recorded by measuring the displacement of the tracers between to recordings taken with a small time delay. These methods are collectively known as particle image velocimetry, or PIV. In its most basic implementation, the fluid motion is recorded in a planar cross section of the flow, yielding between 103 and 105 velocity vectors per image, with a precision better than 1%. By using stereoscopic recording, it is possible to measure all three velocity components in a plane. This can now be considered as a standard configuration that can be applied for a broad range of applications, ranging from creeping flows to transonic flows. The challenge in the near future is to further extend the capabilities of these methods:

Combination of PIV methods with other (optical) diagnostics makes it possible to determine more complex flow properties. For example, the combination of PIV with measurements of the concentration field or temperature field makes it possible to directly measure scalar flux and heat flux;
Currently under development is a PIV method that can be used for the investigation of two-phase flow, in which one fluid (viz., liquid) is seeded with tracer particles, and the second fluid (viz., bubbles, droplets, or solid particles) is observed simultaneously.
Here the challenge is to obtain measurements in a flow system with very strong optical aberrations due to the second phase; One major challenge is to be able to measure the full three-dimensional flow field. Within the JMBC a photogrammetric technique is developed and applied to various flow problems, and a 3D holographic recording method for PIV is under development.

Spectroscopic methods
The development of laser diagnostic techniques is essential for detailed non-intrusive studies of physical and chemical processes in reactive and non-reactive gas flows. At the University of Nijmegen various sensitive detection techniques have been developed and applied to different systems, such as laminar flames, optically accessible diesel engines and non-reactive turbulent flows. Most of these techniques are molecule specific, such as Laser Induced Fluorescence (LIF) detection, Cavity Ring Down Spectroscopy (CRDS) or Raman scattering, which allows for the determination of molecular concentrations. By the application of optical imaging techniques using CCD camera's two-dimensional density distributions can be determined with high spatial and temporal resolution. The obtained data are used to validate numerical model calculations, which are being performed by other collaborating JMBC groups.
For the study of non-reactive flows both Rayleigh and Raman scattering is applied to characterise the density distribution close to boundaries, whereas filtered Rayleigh scattering and Molecular Tagging Velocimetry (MTV) are used for non-seeding velocity measurements. Recently a new promising MTV technique has been developed at Nijmegen, Air Photolysis And Recombination Tracking (APART), which can be used also at high pressure (at least up to 40 bar) to measure velocities with very high spatial resolution. This latter technique is applied for the study of turbulence in collaboration with the groups of Nieuwstadt and Van de Water. In the near future these laser techniques will be further improved and applied to both combustion and non-reactive flow research. At the University of Groningen LIF, CARS, infrared Cavity Ringdown Spectroscopy and spontaneous Raman scattering are being used for quantitative characterisation of the physics and chemistry of combustion processes, specifically pollutant formation and ignition processes, at atmospheric and reduced pressure.

Advanced Numerical Techniques

An essential tool in studying flow problems is computational fluid dynamics (CFD). CFD is a collective term for a large number of numerical techniques, often each with its own area of application. The last decades have shown a growing knowledge of the fundamental concepts of CFD, and the efficiency of numerical algorithms has progressed at a considerable pace. It is foreseen that this growth will continue for some time. Although much emphasis is on turbulent flows at high Reynolds number and multi-phase or reacting flows (which are posing the more challenging problems from a physical point of view), insights in simpler problems may be equally useful and often even essential for constructing stable and efficient methods, to be used in more general contexts. Of the latter kind one should mention basic progress in iterative methods and in discretisation approaches. Iterative developments have shown widespread use of multigrid methods and special fast solvers for large linear systems. Combination with implicit time-integration can deal with the issue of stiffness in e.g. reacting flow. Discretisation methods are challenged by complex geometries and moving boundaries, and by large ranges of length- and time scales. Within the JMBC both Cartesian and unstructured (adaptive) grid approaches are being pursued to deal with the geometric and topological challenges. The (structured) Cartesian grid approach is combined with local grid refinement based on defect correction. The scale resolution problem is tackled e.g. by symmetry-preserving finite-volume methods (with a benign behaviour on underresolved flow features) and by space-time discontinuous finite-element methods (offering flexible spatial and temporal grid adaptation). Also unified algorithms for low-Mach number flow are under development. Further, following a different discretisation philosophy, Lattice-Boltzmann methods are being studied, which possess potential advantages in multi-phase flow simulation. Another important tool in enabling the computations to be performed in a 'reasonably limited' time is parallellisation. Besides a more straightforward use of multiprocessors where the parallelism is taken care of by the compiler, a variety of domain decomposition techniques is in development. In particular within a context where different flow modelling is used in the individual subdomains, research will open up interesting applications, e.g. in turbulent flow simulation where a mixture of RaNS, LES and/or DNS modelling can be envisaged.

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