Flow Computations with a Twist

Coastal research with a drone (from left to right): Michael Streßer (doctoral candidate), Dr Jochen Horstmann (department head) and Ruben Carrasco Alvarez (engineer). Photo: Hereon/Jan-Rasmus Lippels

The sky is slightly cloudy, the Elbe flows leisurely by the city of Lauenburg and a few ducks waddle about. Members of the Institute of Coastal Research’s Radar Hydrography Department aren’t looking at the water though ‒ they’re looking into the sky. A drone is flying above them with a camera. It’s controlled by Michael Streßer, doctoral student at the Institute of Coastal Research in the Radar Hydrography Department (KOR). His colleagues, Dr Jochen Horstmann and Ruben Carrasco Alvarez, are observing the situation. The three scientists wish to determine how high the flow velocity is at this location of the Elbe. Specifically, they want to find out if their new method using drones functions as well as they had imagined.

Linear wave theory – what’s that?

Linear wave theory states that waves move at different rates depending on their length. The researchers theoretically know exactly how fast a wave moves as soon as they have determined its length ‒ provided there would be no current. A current, however, almost always exists; it doesn‘t matter whether it‘s a creek, river or the sea ‒ the water is always in motion. This is why the researchers must initially measure how fast the wave is actually moving, then they can deduct the theoretical value. The resulting difference is the actual flow velocity. It’s similar to an escalator, explains Michael Streßer: “If we know how quickly a person ascends normal stairs and then measure how long the person needs on an escalator ‒ from the bottom to the top ‒ we can subtract this value from the person’s velocity and therefore can determine the speed at which the escalator is moving.”

What do drones have to do with it?

Foto: Hereon/Jan-Rasmus Lippels

The Hereon scientists have been using radar measurements for current calculations in the past. These measurements, however, are laborious and result in coarser resolution. This is still a good approach for measurements in the open sea. For small waves under a metre long ‒ for example, at times in the Elbe ‒ finer and higher resolution images are beneficial. Now, drones take video recordings of the wave movements that can later be studied. They capture the average flow of the upper ten centimetres over the entire area. The scientists are also much more flexible with the drone and can move from site to site more quickly. They can record larger areas more rapidly and more effectively using this method rather than with conventional measurements. In addition, the drone is altogether more costefficient than typical instruments used for current measurements. The scientists can come up with a current profile of the Elbe within five minutes using a drone.

Jochen Horstmann and his colleagues have only utilised the drone a few times. At the moment, they’re still testing how strong the results are, what impact other parameters, such as light conditions, have on the recordings and therefore on the results. For comparison, they also record profiles using a current metre, known as an Acoustic Doppler Current Profiler (ADCP). In this case, the relative water velocity is measured against the ship, from which the absolute velocity of the water can be determined. The Hereon’s research vessel Zwergseeschwalbe crosses the Elbe, zig-zagging through the area, something that takes a great deal of time. While it remains above the river at a particular position, the drone measures the same area within only a few minutes.

What happens with the video recordings?

Dr Jochen Horstmann heads the Radar Hydrography Department at the Institute of Coastal Research. He and his team started using drones for research in 2016. Photo: Hereon/Jan-Rasmus Lippels

The drone’s video recordings will be evaluated on the computer back at the research centre. Using imaging methods, a type of grid is placed over the recordings. This way, the researchers obtain fi ve-byfi ve metre areas, which they can analyse in more detail. The scientists then use a program they developed themselves, with which the current velocities can ultimately be calculated. The researchers need about an hour to analyse one minute of video material. They must view the material, make calculations and then think about whether the results are plausible. A number of weeks and several tests flights later, Dr Jochen Horstmann reports: “The drone results are a pretty close match to the those from the research vessel. This means that our method works.” They nevertheless need to optimise their analysis methods to minimise errors.

In the service of science

What Jochen Horstmann and his colleagues are testing on the Elbe on a small scale can also conceivably be utilised for larger measurement projects. How do the waves mutually affect each other? What are the prevailing current velocities in small eddies? These questions and many more are what the scientists at the Institute of Coastal Research are now trying to answer.

Possible applications: Why are current calculations useful?

Knowledge of currents, waves and eddies in the water enable scientists to identify vulnerable areas and efficiently plan and implement coastal protection measures. The flow rate can also help determine how much water, for example, flows down the Elbe during a certain time interval. This data can be of importance in other research: when, for example, scientists study coastal regions where a river flows into the sea, determining the volume of water flowing there can be crucial. Currents greatly influence the shifting of sand and silt deposits. They are responsible for the transport of nutrients as well as pollutants. Fast and efficient current measurements from the air can therefore also assist in fighting oil spills or similar accidents. Precise knowledge of the local current conditions is also necessary, for example, to ensure safe ship navigation. Using drones for current velocity measurements would also allow researchers to come to more accurate conclusions about where bathers can safely swim and which areas produce strong currents that could pose a threat to humans.