Why drag loads are underwhelming at damping floating system motion
Originally published on dsaocean.com
Snow is rare in my Canadian city. This might surprise you, given Canada’s reputation as a northern and wintry country, but I am lucky to live near the coast. The marine climate helps keep the weather surprisingly mild. Despite this, I do own a snow shovel. It’s a big one, too. It has a wide scoop, all the better to clear a large swath along the path on our driveway. I expected it to do well and help quickly clean up any snow that came along.
But that expectation threw me off on our last rare snowfall. The problem with snow in a marine climate is that it often comes down super wet and thick. Because wet snow is so dense, a massive scoopful in my large shovel was too heavy to lift. Piling up a giant heavy glob of dripping snow almost broke my shovel and my back at the same time! I had big expectations for my large shovel, but in wet, heavy snow, it was underwhelming.
An underwhelming response can throw anyone off. It can cause nasty surprises, especially if there are big expectations. In the world of hydrodynamics, there are often big expectations with drag loads. Any analyst should be wary of drag forces – after all, they are very sensitive to current speed and can become a dominating effect. But there are some circumstances when they fall flat and have little impact on ships and buoys. But first, it helps to understand why drag loads can be so powerful in the first place.
Why can drag loads be overwhelming?
Viscous drag forces appear when there is relative flow past a structure. Whether the structure is moving, the flow is moving, or a combination of both, you’ll see drag loads. The key is that there needs to be some relative motion involved. Generally, the greater this relative motion, the greater the magnitude of forces involved.
One portion of drag on a hull is a skin friction effect, and while important in some ways, it tends to be a much smaller effect in general. The other critical portion of drag is a pressure differential caused by eddies and wake structures that form in the lee of the relative flow behind the hull. Eddies and wakes are regions of low pressure in the flow field, and the resulting imbalance in pressure on the hull causes the drag force. But when are these effects overwhelming?
Drag forces from current can be overwhelming
The drag on the side of a ship hull or buoy in a current can be so significant as to dominate all other effects, overpowering a ship or destroying a mooring system. When a hull and flow have a large relative velocity, the pressure differential from these wake structures grows larger.
These forces increase rapidly with the square of relative flow speed. But it doesn’t mean they are always significant just because they are so sensitive. Viscous forces can also be weak in certain circumstances, and in particular, this can happen in oscillation.
Drag forces are underwhelming at damping oscillation
Oscillation, or any cyclical back-and-forth motion, is common for any hull in the marine environment. For example, a ship or a buoy will bob up and down vertically in heave. You see this in the tilt degrees of freedom of pitch and roll, too. So why does oscillation mean underwhelming drag forces?
A fully developed wake structure doesn’t appear instantly: it takes time for the low-pressure region to form behind the hull. Only once it is fully formed is the pressure differential greatest. This means there will be a time lag for the drag forces to come up to full strength.
But when a hull oscillates back and forth, it’s also constantly changing the flow structure back and forth. There’s almost no chance for a wake to form on either side of the system as the flow regularly reverses one way and the other. In the short term, fluid flows over the contours of the hull, causing some skin friction drag but without forming these larger low-pressure regions, preventing significant drag forces from forming.
The key is that in any kind of back-and-forth motion, the relative flow speed hovers around zero, as you would expect when you see a buoy or ship roll back and forth. The flow speeds can be high at a moment in time if the motion is violent, but the resulting drag loads are underwhelming because the flow structures haven’t had time to form and create large pressure differentials.
But aren’t there other more significant hull damping forces?
Drag and viscous effects are not the only sources of damping in motion oscillations. Wave radiation can be substantial in certain conditions. Ship motion in pitch is usually controlled with damping by a totally different force: wave radiation. Smaller structures like buoys can some wave radiation effects, but it’s not as significant. Regardless, buoys often use a mooring that helps to control motion to some degree.
Sometimes drag is the only source of motion damping
This is a particular issue in specific buoy systems in pitch and roll and spar hulls in heave. It’s also a common problem in ship roll. These are only a few particular cases. Generally, it’s best if you are careful to understand both how your real system will work and how that reflects in your analysis of the floating hull dynamics.
Let’s look at an example
Here are two motion decay tests of the Generic Frigate computed by the ProteusDS ShipMo3D toolset. These are time-domain simulations in calm conditions with an initial deflection. The first is a decay test with the Generic Frigate starting at 10 degrees pitch. In this case, wave radiation dominates the damped system response, and after about 20 seconds, the motion is entirely damped.
The second scenario is a decay test with the Generic Frigate starting at 10 degrees roll. The big difference from pitch motion is that there’s no significant wave radiation in roll because of the slender hull, and this means the damping effects are essentially completely from viscous drag loads. The decay response is far different than in pitch: after 100 seconds, it’s still going!
Drag loads can be devastating when there are high relative flow speeds. Low-pressure wake structures cause a large differential pressure on the hull that we think of as the drag force. But when a floating hull oscillates, these wake structures don’t often have time to develop fully. Even if the oscillation velocity can be high, it’s only for a moment in time before the system reverses backward. It can mean drag loads become vanishingly small and ineffective at damping oscillatory motion. The result can be underwhelming – much like my disappointing giant snow shovel. By the way, my son’s toy snow shovel ended up working much better at clearing my driveway of heavy snow. Because it had a small scoop, it was far easier to quickly scoop and heave loads of snow out of the way, and it took far less time to get the job done!
In this article, we saw how viscous drag forces fizzle in oscillation motion – particularly in rotation. In the next article, we focus on roll motion and see why it’s such an important topic in ship design. Read more on why the sensitivity of roll damping makes naval architects sweat here.