Less drag for ships

The US Defense Advanced Research Projects Agency (DARPA) has announced some scientific breakthroughs that could lead to reduced drag for future Navy and commercial ships.

The US Defense Advanced Research Projects Agency (DARPA) has announced some scientific breakthroughs that could lead to reduced drag for future Navy and commercial ships, increasing their range and decreasing fuel consumption.

It has been known for decades that adding polymers or microbubbles to the flow of water around flat plates can reduce friction drag by as much as 80% at laboratory scales.

Despite such promising results, however, the use of additives has never been reduced to practice in shipboard systems.

DARPA is undertaking a radically new approach in the Friction Drag Reduction (FDR) program. The FDR program is developing a multi-scale modelling capability that will allow researchers to run full-scale experiments on a computer, thus more easily discovering the techniques that will bring optimal results.

Using computational techniques that leverage massively parallel computer architectures, researchers in the FDR program have developed the capability to predict from first principles how turbulence is modified by the presence of polymers and microbubbles.

Researchers have simulated, for the first time, realistic polymers in turbulent flow. Researchers have also learned that polymers that organise into sheets or filaments produce dramatically enhanced drag reduction.

Small-scale experiments with 30-micron bubbles have also clearly demonstrated for the first time what has long been suspected: like polymers, bubbles must be located within a very thin layer near the wall to be effective.

The FDR programme manager at DARPA, Dr Lisa Porter, said past attempts at implementing full-scale drag reduction were hit-or-miss due to a lack of understanding of the physical mechanisms involved. Computer modelling and small-scale experiments have allowed her to predict how and why water turbulence is affected by the presence of polymers or microbubbles.

Several US universities are taking part in the FDR programme. A Stanford University team has developed a theory of how turbulent eddies or vortices are weakened when polymers are added. This knowledge should allow the team to figure out the best way to inject the polymers into the water stream.

Meanwhile, work carried out by the University of Illinois showed that when an injected polymer has a concentration greater than about 500 parts per million, polymer threads form. Although not fully understood, it is believed that the threads are stretched by the vortices to their fullest extent. This alters the fluid viscosity of the water, which has the effect of reducing drag.

‘Drag reduction is enhanced if the polymer is in the form of these filaments,’ said Porter. The findings mean that this autumn, a University of Michigan team can examine polymer behaviour at near full-scale. Porter said another Michigan team will explore the use of micro-bubble drag reduction at the highest speeds and largest sizes possible short of sea trials.

‘This experiment has been devised to span the gap between smaller-scale experiments and full-size ships,’ said Porter. The test model is 3m wide and nearly 13m long. It will be tested in the US Navy’s William B Morgan Large Cavitation Channel, the world’s largest recirculating water tunnel.

‘Compressed air will be injected near the leading edge of the model, and the corresponding reduction in the friction drag will be measured at multiple downstream locations,’ said Porter. Electrodes, lasers and gamma rays will be used to gauge the behaviour of the bubbles.

Any additive that is developed will have to meet environmental concerns. Porter said that in the past the most commonly used polymer has been polyethylene oxide, which is biodegradable. She does not anticipate that the injection of air bubbles, however, would have any impact on marine life.

One of the as-yet unanswered questions is why polymers and microbubbles – additives that are fundamentally so different – appear in turbulent flow in remarkably similar ways. Porter hopes this could lead to a ‘grand unified theory’ of drag reduction and the discovery of ‘a truly optimal additive’.