Research interests

I am interested in unsteady fluid dynamics at moderate to high Reynolds numbers. My interest is motivated by the complexity of the physics and the derived challenges for both experimentalists and theorists, but also by the beauty of nature that is often met.

Post Doc research

Currently I am working on swirling flow confined to a cylinder with a dominant axial flow. This research is not only motivated by its many open questions of fundamental nature, but also by its vast industrial applications.

We study the swirling, scavenging flow of a down-scaled and simplified model of a two-stroke marine diesel engine. The experiment that we have build is by far the most technically complex that I have ever used: Laser doppler anemometry, stereo Particle image velocimetry, fast dynamic pressure gauges, linear motors, air-fans, high-quality glass are only a few of the components used for this research. I will update this page with results soon - until then please enjoy the photo below showing a low Re swirling flow just downstream the cylinder's inlet section.

Visualization of the in-cylinder swirling flow just downstream of the inlet.
Photo credit: Barrie Thomson and Brendan Faulke.

Ph.d. research

During my time as a ph.d. student, I worked with simple experiments on unsteady fluid dynamics of flapping foils. Below this section a few of our results and findings are introduced. I was very careful in constructing experimental set-ups in a spirit of simplicity, such that fundamental processes and questions could be studied in a "clean" setting. As the images show, we have been so fortunate to lure mother nature to unravel some of her beauty and complexity before our very eyes (and camera).

Tandem objects in a two dimensional wind

From everyday experience on the bicycle path we all know that riding behind a fellow cyclist reduces the muscle work required to ride against the wind. However, this idea of "conventional drafting" where the follower enjoys a reduced drag does not apply when the objects are passive "flags" that flap in the wind. The image above, to the left, shows a single flag that flap in the fast downwards flowing soap film. The image to the right shows how the flapping amplitude of the leading flag is limited by the downstream placed identical flag. This indicates a reduced drag on the leading flag. This recent discovery of inverted drafting was made by Jun Zhang and Leif Ristroph at New York University.

With Jun I am working on more general aspects of this observation: How a body can downstream neighbor manipulate a flapping flag?

A section of the wake shown to the right has made it to the front cover of Benny Lautrup's book Physics of Continuum Matter. I think that is pretty cool :-)

"Japanese Fan" Flow

The Japanese geishas are famous for using decorated flapping fans to create a cooling breeze on warm summers days. Here we show the complex flow caused by a two-dimensional "fan": A fast pitching foil in a stationary horizontal soap film. The colorful patterns stemming from thickness variations visualize the downward streaming clearly seen in the stripes on both sides of the foil (top center). Two vortical structures are formed at the tip in each period. These structures are advected downwards, while being stretched in the lateral direction and compressed in the streamwise direction. This forms a broadening layered pattern which merges into two beautiful eye-like rolls and gives the whole flow a butterfly shape. Take a look at the foil and follow the vortex pattern on one side downwards - the row of vortices can be discerned all the way into the eye, as footprints of the fan motion of the past. The rigid foil is 12 mm long, 2 mm wide, and it is driven with simple harmonic pitching oscillations around the center of the leading edge at a frequency of 100 Hz and a lateral tip excursion of 1.5 mm. We speculate that the remarkable stability of the layered pattern is partly due to non-Newtonian effects.

NOTE: The picture participated in the 2009 Gallery of Fluid Motion contest, held at the 62nd Annual Meeting of the APS Division of Fluid Dynamics (link). And by golly: We won! You can see the poster here.

The picture is also shown:

  • On the front cover of the April 2011 edition of danish magazine Kvant.
  • On National Geographic's homepage.
  • On Discovery's homepage.
  • In a gallery of spectacular pictures from science in Denmark hosted by one of the largest communicators of news related to engineering and science in Denmark. See the text here (in danish) and the gallery here.
  • In the danish newspaper Weekendavisen section "IDEER" on January 22, 2010.
  • On the danish site for popular science on June 23rd 2010.

    Bio inspired fluid dynamics

    I investigated the vortex wakes behind a symmetric foil performing simple harmonic pitching oscillations in a vertically flowing soap film. In a phase plane spanned by flapping amplitude and frequency, I mapped out the rich variety of periodic vortex wakes and provided understanding of some of the very subtle details of the vortex shedding. This research left many questions open, such as the transition from drag to thrust and the mechanisms that determines the topological structure of a wake. The image below shows three examples of the beauty and complexity of periodic vortex wakes.

    M.Sc. research: Sand ripples under oscillatory flow

    This work was conducted as a part of my master's thesis with Tomas Bohr.

    When exposed to a sufficiently strong, oscillating flow, a flat bed of sand is unstable and starts forming beautiful ripples perpendicular to the direction of flow (see picture below). The physics of sand ripples is very complicated since it involves the interplay between a separated flow and a granular medium. With Tomas Bohr (Technical University of Denmark and Center for Fluid Dynamics) and Keith Mertens (University of North Carolina) I have worked on developing an amplitude equation to catch important features of the sand ripples. Some of our results are compiled in our paper (2008).