# How Do You Map a Von Karman Vortex Street?

Physicist Tim Divett spends his time using powerful computers to solve the mathematical equations (which he describes as "beautiful") that describe fluid dynamics. In his presentation to possible artists last November, he showed the solutions to his equations as computer-generated visuals. These visuals were of eddies coming off turbine blades, which are called von Karman vortex streets. He also commented that he loved the colours of these visuals and was hoping to find an artist who also liked them.

The concept of tidal current-generated power is one that I am very interested in, given the amount of ocean surrounding New Zealand. This, and the wonderful visuals produced by Tim in his introductory presentation, immediately drew me towards his work.

At our first meeting, Tim gave me some of his work to read. While most of it was in language only another physicist could understand, sprinkled throughout were diagrams showing a variety of computer-generated von Karman vortex streets. He also gave me a couple of web addresses to have a look at actual tidal turbines at work.

My overwhelming first thought was to try and produce a work with movement to mimic the von Karman vortex street. That said, how to end up with such a work seemed like a huge if not impossible task.

I first looked at what kind of fabric I could use that would be light enough to move, sheer enough to see through and sturdy enough to work with. I tested a variety of chiffons and organzas. None had the combination I was looking for. A bit of brainstorming and looking though my collection of fibre art books set me off to look at some more unusual fabrics--fine interfacing Vlisofix, Mistyfuse, Lutradur and Bondaweb. Mistyfuse, Vlisofix and Bondaweb are all types of double-sided iron-on fusible webbing--mostly used in textile work to bond two pieces of fabric together. Lutradur is a non-woven polyester cloth similar to dressmakers' interfacing. It comes in several weights, the finest of which is almost transparent.

Further tests brought the choices down to Mistyfuse and Lutradur. Both had the attributes I was looking for--very fine, almost transparent yet not totally fragile, able to be painted or dyed easily, and lightweight enough to produce movement.

I tested both types of fabrics with a variety of paints and dyes in various strengths to see what the resultant colour would be and how well the fabric would 'take' it. In addition to this process, I also used a fan to see how each type of fabric moved in a breeze.

The final choice was Lutradur, and with 6m purchased and sent to me, I started on the project of colouring it. After cutting the fabric into three two-metre lengths, I laid out each piece separately on the table on top of lengths of silicone baking paper. Using the colours I had chosen, I painted each piece separately, mostly spraying the paint onto the fabric. The first piece I painted was the darkest--using the dye/paint at almost 1-1 strength, with water added to make the painting process easier. The following two pieces were painted at 1-2 and 1-3 strength, making each one slightly lighter than the previous example.

The next challenge was working out how to represent the von Karman vortex street in such a way that it could be seen through a layer of Lutradur, but still be light enough to blow in the breeze. My first ideas revolved around using some type of multi-coloured wool or thread appliqued onto the Lutradur in the design I wanted. Some of the threads I used had potential, and some didn't, but being able to machine applique them onto the Lutradur using the bobbin applique technique had quite a few drawbacks, as well as making the fabric too heavy to float easily. I then tried needle-felting wool yarn and roving in the design I wanted. While I was surprised at how well the Lutradur stood up to this, the test pieces turned out much too unwieldy.

Another session of brainstorming brought the suggestion of using foil to create the design. The fine metallic foil is laid on a cellophane background. A design is placed on fabric using some form of glue medium--including Vlisofix, glue gun, PVA or specially formulated foil glue. Once the glue is in place and is almost dry, the foil is placed coloured side down onto the glue and rubbed until the foil comes off.

I chose a rainbow foil as the best colour to use, and I made several samples to find out how best to use the glue. Applying the glue with a fine brush meant that enough glue stuck to the design for the foil to come off easily, while not adding much weight to the final piece.

The last piece of the puzzle was how to use Tim's von Karman vortex street design. The picture I was using showed the design at about 2cm high and about 15cm long. How then was that to be translated into a design 50cm high and 200cm long? Further discussions with Tim led him to do an enlargement on his computer and send it to me. After a couple of tries, I finally managed to enlarge it to the correct proportions for the size of my fabric.

I pinned the enlarged design to a table, covered it with silicone baking paper and pinned the middle of the three coloured Lutradur sheets on top. As the fabric was so see-through, it was easy to still see the pattern. I applied glue evenly but thinly, following the pattern and left until just tacky. The foil was then laid over the top of the glue and rubbed off. Because of the many angles of the design, the foiled area does not show the rigid stripes that are on the original foil.

Once this process was completed, I found a suitable fan and tested this with all three layers attached to the fan by hooks to find which was the best angle and speed for the fan, and the best placement option for the fabrics.

Once installed, the completed piece was everything I had first envisioned, and I was very happy to find out that Tim was also pleased with it.

Working with Tim opened a whole new field for me, and I found his input and suggestions helped me in forming my ideas for the finished piece. Tim talks about experiencing both the peace and well-being as well as the terror of the maelstrom as described in Edgar Allan Poe's writing during his work on his PhD. I felt similarly while working on this piece--the terror as something went wrong, and the peace each time I worked out part of the puzzle.

I look forward in the not-so-distant future to seeing some of Tim's work being showcased in our waterways.

Megan Griffiths graduated from the Dunedin School of Art in 2016 with an Honours degree in visual arts (textiles). Now in 2018 she has returned to the Dunedin School of Art to start work on her Master of Fine Arts. Megan has been working with textiles for more than 30 years.

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Caption: Figure 1. Fabric Samples. Photograph: Megan Griffiths.

Caption: Figure 2. Testing paint on Lutradur and Mistyfuse. Photograph: Megan Griffiths

Caption: Figure 3. Trying to map a von Karman vortex street. Photograph: Megan Griffiths.

Caption: Figure 4a-d. (a) Enlarged pattern pinned out; (b) Covered with silicone paper; (c) Lutradur pinned on top; (d) Finished piece with complete foiling. Photographs: Megan Griffiths

Caption: Figure 5. Installation piece. Photograph: Pam McKinlay.