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Industrial fluid dynamics insights


Fluid Dynamics Hobbies III: Whitewater Kayaking

You might never know when one seemingly minor decision could change your life.

One summer weekend, just before entering my third year in the Civil & Environmental Engineering program at Tufts, I found myself on a whitewater kayaking class for beginners run by volunteer instructors with the Appalachian Mountain Club. A friend recruited me to join at the last minute; they needed more new “boaters” to reach their minimum capacity.

Some combination of perfect weather, good company, and new challenges that weekend got me hooked on the sport. The more time I spent on the river, the more folks I met who had degrees and careers related to hydrology or engineering. That would eventually include me, too – my love for this hobby & fluid dynamics led me to work here at Alden.

When I returned to school in the fall, I took my first fluid dynamics course. The coursework and the new hobby complemented each other – spending time in a boat made it easier for me to understand certain fluid mechanics topics.

One of those topics is the concept of a stagnation point: an obstruction in a flow field, like a rock or bridge abutment in a river, will cause the fluid to slow down to a velocity of zero at the object’s surface, resulting in high static pressure.

For a boater, the stagnation point is a dangerous place to be. You and your boat can get pinned on the upstream side of an obstruction, and if you can’t get free quickly, you could be injured or drown.

What makes the stagnation point so dangerous? Consider conservation of energy and the Bernoulli equation, a key concept in fluid dynamics:

Bernoulli equation:   Bernoulli equation 
 where:  p = pressure
   rho = density
   V = velocity
   g = gravitational acceleration
   z = elevation

 

The Bernoulli equation only applies to steady, incompressible, frictionless flow along a streamline. Under these conditions, total mechanical energy per unit mass of water (which is the sum of pressure (p/rho), kinetic (V2/2), and potential (gz) energies) is constant along a streamline. This concept can clearly explain why a kayaker doesn’t want to get stuck at the stagnation point. 

River stagnation point

Figure 1: Bridge Abutment Stagnation Point

 

Follow the light blue dotted streamline through the middle of the river in Figure 1. If we estimate negligible friction and minimal change in elevation as the streamline approaches the bridge abutment, Bernoulli’s equation says the energy from a high velocity in the river upstream of the abutment will change to a velocity of zero, along with high static pressure at the stagnation point.

For a boater, this means that if you float head on into an obstruction and become pinned, you will have a velocity of zero, and high water pressure will hold you in place against the obstruction. Yikes! It is best to avoid this situation by navigating around the obstruction, but boaters should always be prepared for the worst by staying up to date on swift water rescue techniques and carrying appropriate safety equipment. Thanks for the safety tip, Bernoulli!

Whitewater kayaking was also helpful in understanding my favorite dimensionless parameter: the Froude number! This parameter compares inertial to gravitational forces. It is also a ratio of the fluid’s velocity to the speed that a surface wave travels across the fluid (AKA wave celerity).

 Froude number:   Froude number
 where:  L = characteristic length descriptive of the flow
field (e.g., water depth for open channel flow)

Froude numbers less than 1 indicate subcritical flow: the water is deep and moving relatively slowly. Froude numbers higher than 1 indicate supercritical flow: the water is shallow and moving quickly. At a Froude number of 1, we have critical flow: gravitational and inertial forces are equivalent. Critical flow can be achieved where the slope of the channel or river is zero, such as over the top of a weir. If you measure the depth (L) of water over the top of the weir as well as the width of the river along the weir, you can determine the flow area, flow velocity (using the Froude number), and ultimately the total flow rate of the river. The Froude number also helps us understand how waves form, and why kayakers are able to surf on them.

When conditions are just right in a river, standing waves and holes (shown in Figure 2) can form; they stay in the same place and don’t move with the flow of the water. Kayakers can have fun with these features by surfing on them, balancing on top of the wave or hole, and staying in the same location relative to the river bank in a fast-moving river.

Whitewater hole and wave

Figure 2: Whitewater Hole & Wave1

 

In fluid dynamics, this phenomenon is called a hydraulic jump.

Hydraulic jumps suitable for surfing sometimes occur naturally in rivers, and can also be designed and installed where river flow and gradient (along with local stakeholders and regulatory agencies) permit. To create a hydraulic jump, a weir-like structure in a river (shown as a “play feature” in Figure 2) can be used to make the flow to transition from critical to supercritical (i.e., the water is shallow and moving quickly). An abrupt drop leading into the downstream pool creates a discontinuity along the river bed. The flow immediately becomes subcritical, getting deeper and slowing down. This abrupt change is mirrored at the river surface, shown in these pictures as a “seam” in a hole or as a “trough” in a wave, where the green water of the supercritical upstream flow meets the hydraulic jump. 

Throwing a loop in a kayak

Figure 3: Me, trying to throw a loop, in Tariffville CT. Photo credit goes to Andrew Nitchske.

 

The velocity of the subcritical flow is very low, and can in some cases reverse so the water is flowing upstream, which makes it possible to surf and do tricks in a wave or hole without floating downstream.

The flow is critical at the seam or trough, which means that the Froude number is equal to 1.  The wave/hole will not move around too much since the flow velocity is equal to the wave celerity.

Kayak surfing (made possible by the Froude number) is my favorite thing to do!! Check it out:

Figure 4: Katelyn Green (yellow kayak) and me (blue kayak), surfing in Tariffville CT. 

 

So… what’s the difference between a wave and a hole? Tune in next time for a post from my colleague Ben Mater!

 

Remember to use the comment field to share your favorite fluid dynamics hobbies with us.

 

[1] McLaughlin Whitewater Design Group, Ben Nielsen, April 15, 2014 Presentation “Recreational Whitewater: Keys to Successful Management”, available online at https://www.slideshare.net/rshimoda2014/nielsen-ben-rms-recwwworkshop2014submittededit (slide 38 of 66). McLaughlin is one of a small number of companies that create whitewater parks for surfing, and Alden has been lucky enough to collaborate with McLaughlin on occasion!


 




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Avatar  David Schowalter last year

I was also thinking this might have to do with weight distribution in the boat. I have noticed an instability like this when sitting up sledding in a one person toboggan. Because most of the weight is in the back, the drag on the front of the sled will create a higher moment than the drag in back, and the sled will want to go backwards, where the drag moments are in a stable configuration. I would think the drag on the bottom of the boat from the water surface could cause a similar phenomenon.

Avatar  Orli Gottlieb last year

Richard, I'm not sure about the oscillation you mentioned (I'll have to pay attention for that this weekend!!), but the sudden spinning around when you're trying to go straight on flat water is just like catching an edge or carving around a turn on moving water. This is something folks might be familiar with from other sport like skiing or snowboarding. Even on flat water (or flat, snow-covered ground if you've ever tried cross country skiing), many of the factors that contribute to carving or catching an edge are still present: the boat will probably not be level laterally as the paddler leans to the side to put in a stroke; the paddle stroke (even a good forward stroke) will generate a small amount of rotation and a slight difference in water velocity relative to the boat from one side to the other (causing water to flow under the hull at angle!); the shape of the boat/ski/vehicle as well as the center of mass will also impact the dynamics of the turn. Whitewater kayaks are generally not designed to travel in a straight line - they are shorter (as compared to a sea kayak), and often have sharp edges to allow the boater to make quick maneuvers around obstacles. However, with good technique (e.g., maintaining the angle between the hull and water surface while putting in good forward strokes), more experienced whitewater enthusiasts are able to keep moving in a relatively straight line without spinning around on flat water.

Avatar  Richard Bloom last year

New kayakers often complain that they can't make their boats go straight. I have noticed that ww kayaks, especially in flat water, tend to suddenly start to spin and once they begin that turn, they are very hard to stop. If you let them go, they often seem to spin almost 180 deg, then oscillate like a pendulum. Could this be the result of water flowing under the hull at an angle?