Movies

The following movies are either obtained through my research, as a part of my teaching in the course "Experimental Methods and Instrumentation in Physics", or from playing in the lab.

To save movies (unless from vimeo), right click and choose "save destination as". You can also send me an email and ask for a copy.

Vortex wakes of flapping foils

The following movies are obtained from numerical simulations of the two-dimensional flow past oscillating foils using a vortex particle method. Jens Honoré Walther from the Dept. of Mechanical Engineering, Technical University of Denmark made the simulations and I was in charge of the post-processing.

The video below shows a foil that pitches around the center of the rounded leading edge. The flapping leaves a periodic vortex street (wake) comprised by two vortex pairs shed each flapping period. 4 chord lengths downstream of the trailing edge, the instantaneous velocity profile is shown by black bars. The thick red line shows the velocity profile averaged over the last flapping period. The free-stream velocity is subracted from the velocity profiles.

The flapping parameters are expressed by the dimensionless flapping frequency, the Strouhal number,

StD = D f / U
and the dimensionless flapping amplitude
AD = 2A / D.
D is the foil with, f is the flapping frequency, U is the free-streaming flow speed to which the foil is presented, and 2A is the transverse peak-to-peak amplitude of the trailing edge. In the movie, StD=0.084 and AD=1.14.

A variety of flapping cases can be downloaded from the table below.

Filename Oscillation mode Wake type StD AD
case1001 Pitching 2P 0.084 1.14
case1002 Pitching drag von Karman 0.12 1.14
case1006 Pitching thrust (inverse) von K 0.22 1.81
case0165 Heaving drag von Karman 0.13 0.50
case0393 Heaving thrust von Karman 0.25 1.30
case0114 Heaving drag 2P 0.10 1.40
case0116 Heaving zero drag 2P 0.10 1.60
case0118 Heaving thrust 2P 0.10 1.80
To download: Right-click the filename and choose "Save As"

Soap bubble fun

Blowing soap bubbles

A slow motion footage of Martin (from soapbubble.dk) blowing a series of bubbles using a standard wand. The cine was captured at 4400 frames per second and it is played at 25 fps, which corresponds to time reduced by a factor 176.

Soap film catenoid

A soap film catenoid becomes unstable as the two rings are moved apart. This slow motion movie shows the transition from a catenoid to two planer and detached soap films while a little sattelite bubble is formed. As the catenoid breaks up, notice how the local shape of the soap film is reminiscent of a water droplet detaching from a faucet.

Vortex ballet

A flapping foil (top center) in a downwards flowing soap film sheds six vortices each oscillation period. The vortices comprise a wake such that two vortex pairs are formed, spaced by vortex singlets. Downstream of the foil, the vortices are advected under their mutual influence which appears as a graceful "vortex ballet"

Flapping flag in a soap film

This video shows a little flag (top) that flaps in a downwards flowing soap film. Vortices formed close to the flag are visualised with thin film inferometry.

Another beautiful example of a flapping flag. 15 frames are shown together in a nice visualisation of the flapping flag. And yes, it is a movie from an experiment :-)

At New York University's Applied Mathematics Laboratory I am working with Jun Zhang on the drag acting on this flag.

von Kármán wake

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An example of the periodic shedding of vortices comprising the classical von Kármán vortex street for Re ~ 1200. The visualization is made by penetrating a vertically flowing soap film perpendicuclarly by a cylindrical rod. Monochromatic light reflected at the front- and back side of the film show thickness variations that follow the flow as a passive tracer.

von Kármán wake types behind a "swimming" foil

These three videos show the wake from a biologically inspired foil that performs harmonic pitching oscillations in a vertically flowing soap film. As the flapping amplitude is increased (keeping the frequency constant), the wake changes from a von Kármán wake type (left) to an inverted von Kármán wake type (right). Between these two wake types we find a "aligned" wake, where the counter rotating vortices are aligned on the symmetry line (middle).

Viscous splash

After falling vertically 2.5 meters, a drop of glycerin impacts on a shallow layer of ethanol on a horizontal glass plate. The violent impact manifests itself initially as a "bowl" of glycerin that grows, until it breaks up forming a beautiful grid pattern due to the marangoni instability. Filmed at 2900 fps.

Filmed at the Department of Physics at the Technical University of Denmark (DTU).

For details see "Crown breakup by Marangoni instability" (2006), Thoroddsen et. al, J. Fluid Mech. 557.

Breakup of fluid jet

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A jet of water exiting a small nozzle breaks up due to the Rayleigh-Taylor instability. The bars to the left shows a mm scale and the recording framerate is 6200fps.