Vapor Layer Reduces Drag On Objects Moving Through Liquid
Magnetically held steel spheres illustrate the two drag-reduction methods compared by Vakarelski and his colleagues: bubble injection (right) and gas layer (left). A fully encompassing gas layer around an object greatly reduces the drag forces on it as it moves through a liquid. Bubbles around an object reduce drag forces to a lesser extent.
Efficient drag-reduction design is needed by supercars, speedboats, and fighter-jets to travel at high-speeds. It maximizes the vehicle's speed by minimizing the effects of drag force deceleration. Now, scientists at King Abdullah University of Science and Technology (KAUST) are looking to add the unconventional method of vapor layers to the pool of drag-reduction options currently available.
Improving hydrodynamic drag reduction methods can help increase efficient use of energy in, for example, nautical applications, pipeline transport, and microfluidic devices.
Reducing drag cuts down the amount of energy needed to move an object through water, or increases its speed or distance it's able to go with the same amount of energy. Some current nautical drag-reduction methods employ bubble formation to decrease drag on an object in water.
However KAUST researchers Dr. Ivan Vakarelski, Dr. Jeremy Marston, and Professor Sigurdur Thoroddsen, along with colleague Derek Chan, a professor at the University of Melbourne, both in Australia, show in their recent Physical Review Letters paper "Drag Reduction by Leidenfrost Vapor Layers" that an undisrupted vapor layer offers much greater drag-reduction potential. The study was highlighted in Nature Physics, Nature Middle East, and Wired.
A New Focus For An Old Observation
The researchers utilized a concept first described more than 250 years ago—the Leidenfrost effect. The effect is easily noticed when a drop of water skates around on a hot pan. The water moves around because liquid that's placed on a surface heated past a critical point generates an insulative gas layer, protecting the liquid from evaporating and allowing it to move around.
The researchers inverted this centuries-old observation to generate a drag-reducing gas layer around an object moving through liquid—instead of dropping a liquid onto a heated surface, they dropped a heated surface into a liquid.
Usually, when a solid object moves through a liquid, the surrounding fluid molecules directly grip it, reducing the object's movement. For example, a submarine moving through the ocean needs energy to counteract these drag forces. But with a protective gas layer, the liquid doesn't attach to the object and it can move more freely, Vakarelski says.
A Model Approach
To create a continuous gas layer around steel spheres and compare it's effect on the sphere's movement through liquid, Vakarelski and his colleagues heated steel spheres to different temperatures. They then dropped the spheres into an perfluorinated liquid composed primarily of perfluorohexane, a fluid that vaporizes at 56 °C.
Because this liquid has a lower boiling point than water, the researchers could gather data on the gas-layer drag-reduction concept without heating the steel balls as much as they would have if they used water.
The team also used this experiment to compare the research with existing gas layer generation methods such as bubble injection or superhydrophobic surfaces.
Spheres superheated past the Leidenfrost temperature point generated gas layers while spheres heated past the liquid's boiling point, but below the Leidenfrost temperature, generated bubbles instead. The gas layer acts like an insulative lubricant around the object, and the researchers found it could reduce hydrodynamic drag by up to 85 per cent.
Calculating The Benefits
Vakarelski and colleagues used high-speed video to record what happened when they dropped the spheres into the perfluorinated liquid. The videos enabled them to determine the spheres' terminal velocity, which they used to calculate the spheres' drag coefficient.
Drag coefficient is related to both an object's velocity and the energy needed to move it. The researchers can use the value to predict the velocity a gas-layer-enclosed object can achieve as well as the force needed to reach that velocity. With this predictive ability, researchers could determine whether the energy input for the gas-layer drag-reduction model would be appropriate for certain applications.
The team is now focusing on sustainable formation of gas layers in water, which will bring the concept closer to application, such as to reduce energy use of submarines or ships and to optimize liquid flow through microfluidic devices.