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Why drag force is more than just resistance

Originally published on dsaocean.com

It’s tough to beat Nature. You don’t have to look in a top secret lab to find one of the most miraculously slippery fluids on the planet. You can just look at your knee! Lining many of your joints is an egg-white coloured substance called Synovial Fluid. Among its many purposes is to keep your joints moving. But not just for a few days – for your entire life. And through all this time, it manages to control and minimize joint resistance.

Certainly, when resistance gets out of control, everything can come to a grinding halt. When it comes to hydrodynamics, when you think of resistance, drag forces may be the first thing that comes to mind. But there’s more to drag than just resistance, and this is what we’re going to cover in this article.

What is the drag force?

Forces appear when there are changes in momentum. Momentum means mass in motion, so for the specific case of fluids, this could mean water or air flowing in a current. The drag force arises when there is a change in momentum in fluids. More specifically, the drag force transfers momentum between a structure and a fluid it is immersed in. This might sound like an abstract idea, but you already have some experience with it every day.

You feel this effect when your hand is in water

A big part of this is the drag force. But if you are in a pool or bathtub, the water isn’t moving around – it’s your hand – and so the drag force in this case will feel like resistance to you. In this case, it’s your hand that has momentum: it is the mass in motion. The drag force is then transferring momentum from your hand into the water.

What does momentum look like in water?

Well, if it’s your bathtub, there will be swirling and churning water – turbulence, and probably some waves. All this moving water is mass in motion, too. But the drag force does work in reverse, too – it can transfer momentum from water into structures.

What happens when there are ocean waves or currents in the water?

Ocean waves and water currents are also examples of mass in motion, too. If a structure, like a mooring buoy or ship gets in the way of moving water, there’s going to be some drag forces. These forces will transfer momentum from the waves and currents into the structure. So in this way, drag force is really about the exchange and transfer of momentum.

Sometimes this means it creates resistance and slows down a structure. But it can create motion in floating structures, too. The key is that drag is proportional to the relative velocity between a structure and the fluid.

The drag force is one of the essential forces in hydrodynamics

It acts like a resisting effect on many structures. If you are trying to understand what will happen to an ocean robot driving around in the water, it will have a big impact on energy consumption as well as how it can maneuver.

But the drag force is also a key element in mooring designs. In oceanographic mooring designs, the aggregate drag on all the components and the mooring line causes the system to deflect in an ocean current. In reality, the water current loses some momentum from this drag force from the mooring – the wake from the float and mooring components reduces the ocean current flow speed by some small amount.

Drag is also a key element in the excitation forces when ocean waves are around. You need to know these excitation forces to properly design a system to perform the way you want in the ocean environment. But knowing drag forces is one thing. Knowing how big they are is really the million-dollar question.

How do we know what the actual drag force is?

The drag force is measured from an experiment. These experiments might be a physical test in a lab, or in more modern times, they might be virtual tests using fluid dynamics software. These experiments resolve what the drag forces are in certain specific conditions. There have been hundreds of thousands of tests completed in laboratories for many decades measuring the drag force on a vast array of shapes in different flow speeds and fluid mediums. But how do we take all this information on drag force and then use it in a specific application?

The key is the drag coefficient

Similar shapes produce similar and predictable drag forces. The concept of a drag coefficient works well and covers a wide range of fluid types and conditions. Ultimately, if you’re looking at something like a sphere, you can use a drag coefficient associated with a sphere and predict reasonably well what the drag forces will be for a good range of wind or water current speeds. These drag coefficients are often available in look up tables to help the design process.

But what about the drag on mooring lines?

A mooring line is indeed a much more complicated structure than a sphere. Depending on its orientation to the flow, the local drag can change drastically. The drag forces on mooring lines require a calculation that considers the local flow speed and tilt of the line of the whole system. Suffice to say, it’s not a back-of-the-envelope type calculation you can do like that of the drag on a sphere! But this is what we have programs like ProteusDS Oceanographic that help address these complexities.

It’s summary time

The drag force is an effect that arises when momentum transfers between a fluid and a structure. Yes, a structure will feel resistance when it’s moving in a fluid, but the reverse is true, too: a moving fluid, like ocean currents and waves, can also cause a structure to move around, too – and the drag force plays a big role in that. The drag force is a very important effect that affects a range of systems from vehicle dynamics to oceanographic mooring design.

Even super low friction fluids like those in your knee joints have some drag effects, too. While you don’t need to do much over your life to keep your knee joints going, you do need to be mindful of drag when designing structures in the ocean.

Next step

Figuring out a drag coefficient for a particular structure isn’t always obvious. While ProtesuDS includes drag coefficients for a variety of shapes, you can learn more about different ways to resolve drag coefficients here.