L Christoffer Johansson

L Christoffer Johansson is a zoologist working as a lecturer at Lund University in Sweden (Personal webpage). His research focuses on fluid dynamics of animal locomotion and to understand morphological adaptations, with a current focus on animal flight. He is member of the Animal flight lab at Lund University (AFL page), working mainly with quantitative flow measurements (using PIV) of animals flying freely in a wind tunnel.

Christoffer earned his PhD in 2002 at Göteborg University, Sweden, with a thesis focused on functional morphology and hydrodynamics of swimming birds. In 2003, he did a postdoc at Harvard University, USA. As part of the Lauder lab, he studied the hydrodynamics of swimming in frogs. Since returning to Sweden in 2004, Christoffer has been working with animal flight at Lund University, becoming a Lecturer/Associate professor in 2010.

Research focus

Flight

Birds

Christoffer’s work on bird flight covers both studies of the aerodynamic mechanism involved in generating the forces and how the cost of flight varies across speed. He has been part of showing that birds can use leading-edge vortices (LEVs) to boost their force production when flying slowly and that the split feathers at the wing tip of many species work as individual airfoils. He has also been part of showing that birds tend to be more efficient fliers than bats and that the cost of flight varies with speed in a manner different from predicted by traditional models. The latter probably due to how drag changes across speed due to the birds tilting their bodies when flying slowly. The work has also involved studies where cost of flight has been modelled, creating an updated and more accurate model of flight costs in flapping flight than the one that has been dominating for decades.

Hybrid robotic wing in Lund University wind tunnel.

Notably, Christoffer has also studied bird flight using an advanced flapping robot, where he and the coauthors where able to show the benefits of folding the wing during the upstroke and that the most efficient use of the upstroke was to generate some lift, just like many bird species do (but opposite to what bats do).

Bats

Bat wake

Animal flight lab in Lund was the first to measure and describe the wake left behind bats flying freely in their wind tunnel. In addition, they performed the first on wing measurements, showing that the bats used leading-edge vortices (LEVs) to boost their force production when flying slowly. These papers have been highly influential on our understanding of bat flight.

In addition, Christoffer’s work has also involved studying how flying close to a surface influences the cost of flight. Flight close to a surface generates what is known as ground effect and by comparing the aerodynamic cost for Daubentons’ bats flying in or out of ground effect they could show that the bats saved more energy than predicted by models. Additional work has concerned comparing the cost of flight in bats with different sized ears and the efficiency of converting physiological to mechanical power.

Insects

Christoffer has worked on several species and aspects of insect flight, including to understand how the elytra, the hard cover wings of beetles, affects the flight performance. Despite being evolved as a protection for the beetles, the elytra flap and add to the lift production. However, at the same time the reduce the efficiency of the flight.

Butterfly take off

Insects use unsteady aerodynamic mechanisms to generate enough force to stay aloft. Christoffer’s work has focused on two of them, the leading edge vortex (LEV) and the clap and fling. He has performed the first quantitative measurements of the strength of the LEV in a freely flying insect, showing that it is stronger and more unstable than previously thought. His work on wing clap in butterflies has showed that the butterflies use the upstroke to clap their wings together and due to the flexibility of the wings the clap is more effective and more efficient than if the wings had been stiff.

Swimming

Christoffer’s work on swimming has focused on secondary swimmers, animals that have returned to a life in water after their ancestors lived on land. Through studies of the motion pattern of swimming and diving birds and physical models of bird feet he has discovered and described novel ways of swimming. Foot propulsion has traditionally been considered to be based on the drag the feet generate as they are pushed in the opposite direction to swimming, but Christoffer have described two separate mechanisms that allows foot propelled birds to instead use lift, which makes their swimming more efficient and allows for faster swimming. One example is found in grebes, which have lobed toes resembling feathers, where the feet are swept sideways from underneath the body to a position above and behind the body. Almost like a propeller turning a third of a full turn. The other example applies to birds with triangular feet and where he has shown that the feet can work as delta wings. Unlike the bird examples, Christoffer has also worked with swimming in frogs showing that they use the expected drag-based paddling.

For a complete list of publications see Google scholar profile

References

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