TU/e MTP / WDY Fluid Dynamics Seminar

Announcements

Our next guests will visit us on December 12th.

General information

The WDY (Turbulence and Vortex Dynamics) and MTP (Mesoscopic Transport Phenomena) groups of the Department of Applied Physics would like to invite you to our seminar series. The seminar takes place every first week of the month, preferably on Mondays in Cascade 2.21 (building 23 on the campus map, grid square D5) at 12:45.

If you would like to receive email announcements of these seminars, feel free to contact one of the organizers. You are also invited to subscribe to our calendar.

Schedule

WeekDateSpeakerTitle / abstract
502016-12-12 dr. Roger Jaspers
Science and Technology of Nuclear Fusion (FUSION), TU Eindhoven

452016-11-07 Ir. Sabine Rijnsburger
Environmental Fluid Mechanics, Civil Engineering, TU Delft
Frontal dynamics in the mid-field region of the Rhine river plume

Freshwater fronts in coastal areas can impact the transport of fine sediment. They can transport fine sediment offshore or exert a high stress on the bottom what causes resuspension. During the presentation I will show which processes change the frontal dynamics off the Dutch coast and under which conditions the fronts are strong enough to impact the fine sediment transport. The study uses X-band radar images and field data, which is collected 10 km north of the Rhine river outflow at 1.5 and 6 km offshore.

402016-10-06 dr. Ad Stoffelen
KNMI
Earth's winds from space

Knowledge and measurement of winds is essential for, among others, atmospheric dynamics, mesoscale weather analysis, atmospheric dispersion, oceanography, marine forecasting of winds, waves and storm surges, off-shore wind energy applications, and from cloud- to climate-scale processes. But how can we measure winds in the Earth's atmosphere?
In-situ observations are rather sparse in relation to the grid dimensions of today's Earth system models, particularly in the vertical. In this seminar, given by Dr. Ir. Ad Stoffelen (KNMI), several space-borne wind measurement techniques will be presented, based on measurements of ocean roughness, on Doppler shifts of atmospheric particles, and on the radiometric or geometric tracking of atmospheric features.

222016-06-02 ir. Johan Bosschers
Research & Development MARIN
Vortex cavitation on ship propellers and its radiated noise

High propulsive efficiency of marine propellers goes with increasing cavitation extents, cavitation being the process of water evaporation and condensation. The dynamics of cavitation, especially the collapse, is an efficient noise source and it may lead to erosion on the propeller blade. Ships with high comfort class requirements, e.g. cruise vessels, or underwater signature requirements (e.g. naval vessels) optimize their propellers to achieve minimum cavitation such that often only a cavitating tip vortex remains. There is however remarkably little known on vortex cavitation as most cavitation research has and still is focused on either bubble cavitation or sheet cavitation as their nuisance is typically higher than of vortex cavitation. The presentation will give a short introduction on various types of vortex cavitation observed on marine propellers and will focus on some fundamental aspects of vortex cavitation describing both the kinematics and the dynamics. A practical method in development to predict hull pressure fluctuations and radiated noise for industrial applications will be presented and future challenges will be addressed.

142016-04-04 dr. ir. Chiel C. van Heerwaarden
Wageningen University
Growth and decay of a convective boundary layer over a surface with a constant temperature

The growth and decay of a convective boundary layer (CBL) over a surface with a constant surface temper- ature that develops into a linear stratification is studied, and a mathematical model for this system is derived. The study is based on direct numerical simulations with four different Reynolds numbers; the two simulations with the largest Reynolds numbers display Reynolds number similarity, suggesting that the results can be extrapolated to the atmosphere. Due to the interplay of the growing CBL and the gradually decreasing surface buoyancy flux, the system has a complex time evolution in which integrated kinetic energy, buoyancy flux and dissipation peak and subsequently decay. The derived model provides characteristic scales for bulk properties of the CBL. Even though the system is unsteady, self-similar vertical profiles of buoyancy, buoyancy flux and the velocity variances are recovered. There are two important implications for atmospheric modeling. First, the magnitude of the surface buoyancy flux sets the time scale of the system, thus over a rough surface the roughness length is a key variable. Therefore, the performance of the surface model is crucial in large-eddy simulations of convection over water surfaces. Second, during the phase in which kinetic energy decays, the integrated kinetic energy never follows a power law, because the buoyancy flux and dissipation balance until the kinetic energy has almost vanished. Therefore, the applicability of power law decay models to the after- noon transition in the atmospheric boundary layer is questionable; the presented model provides a physically sound alternative.

102016-03-08 Dave Weij
Delft University of Technology
Unstable breaching

Breaching is a gradual retreat of a sub-aqueous slope, which is steeper than the angle of repose. While mainly investigated due to the application of this process in the breaching process, the breaching process can also be a cause for unwanted slope instability and failure. These failures are usually caused by unstable breaching, where the size of the retreating slope increases during the process. The aim of this project is to investigate the unstable breaching process using numerical methods.

102016-03-08Frans van Grunsven
Delft University of Technology
Modeling offshore turbidity sources

Accurate predictions of the environmental impact remain a major challenge in the licensing phase of offshore mining projects. A significant part of the environmental impact of offshore mining is caused by an increase in turbidity of the oceanic waters. Turbidity levels are increased by the suspension of sediment during the excavation process and due to the discharge of return water and possibly tailings into the ocean. Current consensus prescribes releasing discharges in close proximity of the seafloor to decrease the impact area of the turbidity plume. Main engineering choices remain a challenge. Available engineering models and software are unable to include the interaction between the turbulent slurry mixture and the (deep) seafloor. This introduces an uncertainty into the predictions of the turbidity source. Uncertainties increase as the source is used as input for long-term and far-field environmental impact calculations.
The presented research aims to develop a three-dimensional numerical model capable of simulating turbulent turbidity sources and their interaction with a sedimentary bed. With the use of a drift-flux and large-eddy turbulence model, significant improvements are expected in the prediction of the dominant transport processes.

92016-03-01 dr. ir. Werner M.J. Lazeroms
IMAU / Utrecht University
Describing subgrid-scale processes in geophysical models with focus on atmospheric turbulence

Numerical models of weather and climate are important application areas for fluid dynamics. These models aim at describing a vast amount of physical processes on Earth and are very computationally expensive. Therefore, it is impossible to completely resolve all the processes and subgrid-scale models are needed in many cases. The prime example of a process that needs to be modelled is turbulence, both in the atmosphere and in the ocean, but also at engineering scales. This talk will focus on the new explicit algebraic turbulence model for stratified flows that has recently been developed at KTH in Stockholm. I briefly explain the differences between this model and the more classical eddy-viscosity approach. A significant improvement is obtained when the model is tested in various test cases, e.g. turbulent channel flow and a stably stratified atmospheric boundary layer. If time allows it, I will briefly discuss ongoing modelling work in Utrecht for a different geophysical process, namely the melting of the Antarctic ice sheet by ocean circulations.

52016-02-03 dr. Giordano Lipari
Watermotion | Waterbeweging
From academic knowledge to tradable knowledge -- Clues from experience. Cues from fluid dynamics

Moving to a consultancy job after an extended academic experience is a change of habitat that has many dimensions. At a personal level, it requires willingness to reformulate one's own experience according to different expectations, demands and needs. At a social level, it demands the ability to articulate and embody this willingness while maintaining and accruing your expertise, lest performance outplays learning. Without pretence on general applicability, I will try to suggest some mindset adaptations and bring forth some heuristic tools that might give orientation when consultancy is the new setting where knowledge is being produced.
I hope to promote the relevance of three vaguely-defined attitudes: serendipity, sagacity and integrity. I will even endeavour to instil motivation as to why scholars in fluid dynamics can use their acquaintance (wrestling?) with it to their advantage, prime a disposition to versatility and have a better grip on chances for professional development. My considerations will hint at a personal repertoire of experiences in research and consultancy. As underpinnings for the conversation, I will recall fragments of topics in environmental fluid dynamics that I happened to investigate:

  • About 35 years worth of experimental data for turbulent incompressible round jets
  • The occurrence of short-lived gyres in the Western Scheldt estuary that pose great danger to 
the navigation on the fairway towards the harbour of Antwerp
  • Time permitting, the generation of storm surges in the Wadden Sea and why the cinematic 
expectation of perfect storms can be awed and misleading.
This talk's ambition is to verbalize and blend three key dimensions of knowledge – expert, lay, tacit – and highlight a saucerful of resources that everyone can leverage upon one's own judgement. I will also tap from more or less in-depth readings in diverse topical areas, such as organizational and social psychology and personnel recruitment. 
 For the sake of casting the net wide, the argumentation is unlikely to completely avoid a little stereotyping, un-scholarly intuitions and anecdotal support. Nonetheless, it is hoped that, on the whole, the talk will help raise the eyesight over perceived, possible and probable obstacles especially in two key moments of this change. Job applications and project execution.

452015-11-02 Prof. Murray Rudman
Monash University Melbourne
Turbulent Pipe Flow of non-Newtonian Fluids - Direct Numerical SImulation et al.
All of us are familiar with Newtonian fluids such as air and water - they are part of our daily life. What we may not realise is that we are also quite familiar with non-Newtonian fluids, i.e. fluids in which the ratio of fluid stress to fluid strain rate is not a constant (this ratio is normally called "viscosity"). Examples from everyday life include toothpaste, mayonnaise, shampoo, hair gel - the list is large. While non-Newtonian fluids may often display phenomena such as visco-elasticity (i.e. "memory" effects), there is an important class of non-Newtonian fluids which are simpler. In so-called Generalised Newtonian Fluids (or GNFs) there are no elastic effects and the the fluids stress is still written as a viscosity multiplied by a fluid strain rate, however the viscosity is no longer constant. Typically it is a function of strain rate, and numerous models have been proposed to capture this physics including power law, Bingham, Herschel-Bulkley, Carreau, and so on. This study is motivated by an important sub-set of GNFs that are important to the Australian economy - mining tailings slurries. In this talk I will describe the basics of how these slurries are created, what kind of properties they have and why their flow (usually in a pipe) is important. I describe how important rheological characterision is and how it has usually been treated quite crudely. Using Direct Numerical SImulation we investigate the turbulent flow of GNFs in an attempt to understand how and why rheological parameters such as yield stress, shear thinning/thickening modify the turbulence and consequently basic engineering quantities such as friction factor. The open questions and next steps will also be briefly mentioned. .

302015-07-20Riccardo Scatamacchia
Univ. Tor Vergata, Rome
Dispersion of particles from localized sources in turbulence

We present a detailed investigation of particles relative separation in homogeneous isotropic turbulence. We use data from a 3D direct numerical simulations with 1024^3 collocation points and Re = 300 following the evolution of a large number of passive tracers and heavy inertial particles, with Stokes numbers in the range St = [0:5; 5]. Many studies have focused on the subject, including extensions to the case of particles with inertia. In particular, our simulation aims to investigate extreme events characterizing the distribution of relative dispersion in turbulent flows. To do that, we seed the flow with hundred millions of particles emitted from localized sources in time and in space. Thanks to such huge statistics, we are able to assess in a quantitative way deviations from Richardson’s prediction for tracers. Furthermore, we present the same kind of measures for heavy particles to understand how the inertia affects the pair separation statistics.

202015-05-11Dr. Carles Panadés
IMAU
THE 1:2 SPATIAL RESONANCE IN CYLINDRICAL FLOWS DRIVEN BY SIDEWALL OSCILLATIONS

Besides the per se appealing interest of the instability phenomena occurring in a periodically driven cylindrical cavity, the flow that is engendered in this system is analogous to the von Kármán vortex sheet from the point of view of the symmetries. The streamwise flow at any time is reflected respect to the mid plane after advancing half of the forcing period (spatio-temporal symmetry). Additionally, the flow is also invariant to translations and reflections in the spanwise direction (spatial symmetry). Thus, the basic flow is axisymmetri c and periodic with the sidewall oscillation. Depending on the Reynolds number and the Stokes number, which are proportional to the amplitude and frequency of the forcing respectively, the spatial symmetry is broken via different Fourier modes, resulting in various three-dimensional states. Furthermore, several c odimension-2 points, where two different modes are unstable at the same time, have been found. The dynamics around one of these points (C1) have been studied, yielding a wide variety of possible states. In C1, two Fourier modes with m = 1 and 2 that preserve the spatio-temporal symmetry bifurcate at the same time. Actually, this is equivalent to a 1:2 spatial resonance for a discrete dynamical system because of the forcing of the si dewall. The normal form around this codimension-2 point has shed more light on the different transitions. For instance, this analysis has corroborated the existence of the states found in the numerical simulations and has unraveled other states, such as a modulated standing wave, that has gone unnoticed because t his state exists in a narrow region of the parameter space.

142015-03-31Henk Schuttelaars
TU Delft
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92015-02-23Andrea Cimatoribus
NIOZ
Statistics of temperature from high resolution ocean measurements.

Thermistors with high precision and high sampling rate have been developed at NIOZ over the last decade. These autonomous instruments can be deployed for several months on moorings in the deep, open ocean. The high resolution datasets produced by these instruments provide a unique view on turbulence and internal waves in the open and deep ocean. I will discuss in particular the results from a mooring approximately 100m tall, with 144 thermistors, deployed above the slopes of a seamount in the North Eastern Atlantic Ocean. Turbulence at this location is strongly affected by the semidiurnal tidal wave. The statistics of temperature fluctuations and temperature increments obtained from observations are compared with those measured in the laboratory and available in the literature. In particular, we try to asses the passive/active nature of the scalar, characterising intermittency in this strongly stratified environment.

72015-02-11Martijn van Rijsbergen
MARIN
Micro-scale challenges in cavitation research

Although cavitation inception (the start of evaporation of a liquid by a decrease in pressure) has been studied for several decades, the exact working mechanism of sheet cavitation inception is not yet understood. High-speed micro-scale observations on a foil have shown that the surface characteristics of the foil play a major role in the inception process as well as the free stream nuclei. RANS simulations have shown a minimum pressure coefficient at an isolated roughness element which is 1.5 times lower than on a smooth surface. To investigate the effect of the nuclei content on cavitation on propeller models a nuclei measurement system will be developed.

412014-10-06Marie-Jean Thoraval
UT
Breakup of submicron air films in a liquid

We use ultra-high-speed video imaging, at frame-rates up to 1 million fps, to study the dynamics and breakup of submicron films of air in a liquid pool. When a drop impacts at very low impact velocities, the air under it can cushion the impact and prevent direct contact between the drop and the pool. The thin air layer under the drop is then stretched into a hemisphere and only ruptures when it becomes of the order of 100 nm thick. The breakup of this air is extremely rapid, but the high-speed imaging allows for well controlled studies of the air film puncturing and resulting entrapment of micro-bubbles. We discuss the possible influence of van der Waals forces in the breakup mechanism, and the similarities with dewetting patterns of a liquid film on a solid plate.

362014-09-01Bernard J. Geurts and Rudie P.J. Kunnen
UT and TU/e
Rotating turbulent Rayleigh–Bénard convection subject to harmonically forced flow reversals

The characteristics of turbulent flow in a cylindrical Rayleigh–Bénard convection cell can be modified considerably in case rotation is included in the dynamics. By incorporating the additional effects of an Euler force, i.e., effects induced by non-constant rotation rates, a remarkably strong intensification of the heat transfer efficiency can be achieved. We consider turbulent convection at Rayleigh number Ra = 10^9 and Prandtl number = 6.4 under a harmonically varying rotation, allowing complete reversals of the direction of the externally imposed rotation in the course of time. Both slow and fast flow-structuring and heat transfer intensification are induced due to the forced flow reversals. Depending on the magnitude of the Euler force, increases in the Nusselt number of up to 400 % were observed, compared to the case of constant or no rotation. It is shown that a large thermal flow structure accumulates all along the centreline of the cylinder, which is responsible for the strongly increased heat transfer. This dynamic thermal flow structure develops quite gradually, requiring many periods of modulated flow reversals. In the course of time the Nusselt number increases in an oscillatory fashion up to a point of global instability, after which a very rapid and striking collapse of the thermal columnar structure is seen. Following such a collapse is another, quite similar episode of gradual accumulation of a next thermal column. We perform direct numerical simulation of the incompressible Navier-Stokes equations to study this system. Both the flow structures and the corresponding heat transfer characteristics are discussed at a range of modulation frequencies. We give an overview of typical time scales of the system response.

282014-07-07Dr. Willem van de Water
TU/e
Writing in turbulent air

When a Gaussian blob of passive scalar is released in turbulence, it will spread due to the combined action of turbulence and molecular diffusion. The question is if diffusion helps the spreading due to turbulence, or suppresses it. To answer this elementary question, we make a Gaussian blob in a turbulent flow of air by painting the molecules (which are normally blue) in a different color using strong lasers, and then see where the colored ones go. It is an example of the flow tagging technique, that needs no tracers as we use the molecules of air itself as tracers. We will see the tagging process in action, and answer the question about the joint action of dispersion and diffusion at short times.

232014-06-02George Schramkowski and Tomas Van Oyen
Flanders Hydraulics Research
Upcoming hyperturbidity reseach at FHR

This talk describes a four years research project that will start later this year. It is devoted to modeling and understanding the shift that has been observed in some estuaries from relatively low to rather high sediment concentrations (~ 1 g/l). This situation is undesired from both an economical and ecological point of view. The aim of the research is to develop a reduced mathematical model (aka idealized model) that still includes all relevant physical mechanisms but describes the above-mentioned shift.

Muddy problems in tidal harbors

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92014-02-26Dr. Ton Hoitink
Wageningen University
Monitoring flow, turbulence and suspended sediment concentration using coupled ADCPs

Turbulence strongly controls the exchange of momentum and suspended sediment in geophysical surface flows occurring in rivers, estuaries and in the coastal ocean. Acoustic Doppler current profilers offer a promising means of measuring turbulent quantities. We introduce a new technique to measure profiles of each term in the Reynolds stress tensor using two coupled ADCPs. The technique is based on the variance method which is extended to the case with eight acoustic beams. Using the acoustic backscatter of the ADCPs, vertical profiles of suspended sediment concentration can be obtained continuously. The covariances between radial velocities and calibrated acoustic backscatter allow the determination of the three Cartesian components of the turbulent flux of suspended sediment. The main advantage of this new approach is that flow velocity and sediment concentration measurements are exactly collocated, and allowing for profiling over long ranges. Results show that vertical profiles of the inverse turbulent Prandtl-Schmidt number are coherent with corresponding profiles of the sediment diffusivity, rather than with profiles of the eddy viscosity.

82014-02-19Prof. Dr. ir. Gert-Jan van Heijst
TU/e
When Waves collide, an experimental investigation of people waves in a football stadium

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62014-02-05Dr. Hanneke Gelderblom
Physics of Fluids, University of Twente
Drop deformation and fragmentation by laser impact

In Extreme Ultraviolet (EUV) lithography EUV light is generated by hitting liquid tin drops with a pulsed laser. The laser-drop interaction results in a strong pressure pulse exerted on the drop. As a consequence, the drop is propelled in horizontal direction and deforms into a thin sheet which expands, becomes unstable and eventually fragments into tiny droplets. To study this process, we perform experiments with both liquid tin and water drops that are hit by a 10-ns laser pulse. The drop deformation and displacement are captured by two CCD cameras with stroboscopic illumination. From these images, we extract the drop shape, velocity and fragmentation moment. In addition, we develop a theoretical model to characterize the drop deformation and fragmentation in terms of the laser energy, pulse duration, liquid properties and initial drop size.

Show previous talks.

Organization

Currently the seminar is being organized by Kim Alards (WDY) and Matias Duran Matute (WDY).