Speedskating Santa Barbara -- Vortex Generators

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6. Vortex Generators
(for Speedskating Drag Reduction) P. J. Baum, 3-11-01

What They Do

An airplane wing at a modest angle of attack is shown here below. Without vortex generation (left) the streamlines separate from the wing before reaching the downstream edge. With vortex generation (right) the streamlines do not separate from the wing. If the flow stays attached to the wing the drag pressure force is low. For a more bluff object or for this wing at a greater angle the separation occurs farther upstream.(© 1992, Micro AeroDynamics, Inc.)

When the angle is increased (below) we see the early separation and the wide turbulent region that is generated when there is no vortex generator (left). On the right side the vortex generator keeps the streamlines attached to the wing. (© 1992, Micro AeroDynamics, Inc.)

The Upper Body Problem

The skater's case is shown below. The cyclist shown on the left here models the skater's back. The lower part of the photo is specific to cycling as the bike obstructs the flow there. A stream of smoke has been injected from the left here and it passes by the rider leaving a streamline pattern. There is a problem just behind the head (helmet) and another problem toward the rear of the back as the streamline separates from the back leaving a low pressure region which is evidenced as drag.


A head-mounted vortex generator should remove the low pressure region behind the head. It is not clear just where the separation point farther along the back will be moved by a head-mounted vortex generator. The photo suggests that another vortex generator strip just before the major separation point on the back might help. The Dutch are doing such tests on real skaters.	The skier's case (Nat. Res. Council Canada) is shown here. The drag behind the helmet has been 'fixed' and the rear pressure seems aided. But -- the smoke has been injected close to the helmet so the post-helmet region may be over-represented compared to the biker. Other differences: skier bends lower, strap across helmet, suit materials, ski poles may generate vortices.

How They Work

The series of vortex generators are attached to the top-front of the wing on an airplane. The picture on the left below shows one vortex generator attached to the right wing. Here the View is from the front along the top of the wing.

Above: computational result for air flow around a delta-shape wing (Tohoku University). Imagine the delta to be a solid \/ in the case of the Dutch 'Go-Faster Stripe'. The point of the \/ generates vortex flow to either side of it.

Left: This figure shows schematically how the 'Go-Faster Strip' might generate vortex flow around the leg. The vortices should stay attached to the leg rather than widening into a broad bounday layer due to separation from the leg.

Referring to the photo on the left above, consider the generator strip (about like on upside-down 'T') to be the / portion of the \/ or V in the Dutch 'Go-Faster Stripe'. When the air passes the generator it is spun off in a helical spring-like pattern. These twisted air tubes tend to remain close to the wing so the pressure drag is reduced.

For the case of a skater we can imagine that the v-strips generate a sheet of vortices along the inside and outside edge of both lower legs and generate another sheet of vortices from the head along the back. Apparently the drag is reduced more than in the trip wire effect -- at least at skating speeds. It is not clear what effect the vortices have on the surface tangential drag on the skater but it is claimed that laminar drag is less than turbulent drag so it may also be reduced. Since the air is spinning the tangential surface drag will not just be in the downstream direction but will have a strong component in the spin direction (up the skater's leg on one side, down the skater's leg on the other side; across the top of the skater's back perpendicular to the direction of motion). These components should not slow the skater in the direction of motion.

So far we have assumed that the vortices passively stay parallel to one another as they thread by the skater. The figure above shows that this is not necessarily true. It comes from Dr. Williamson's site at Cornell University and shows the interaction of two vortices in water. Laser Fluorescence produced the animation above where two vortices turn into four donuts. The vortex is like a twisting rubber band which twists outward and reconnects with the neighboring vortex generating a spinning donut. Presumably this occurs primarily downstream from the skater.

However it raises an interesting possibility-- normally you want to draft the skater in front of you to get in the low pressure pocket behind him. But, if he has an effective vortex generator you may get hit with high pressure vortex tubes or donuts of air which remove the advantage of drafting!

Why Place Vortex Strips on the Lower Legs?

Relative to the upper body the legs appear to have much smaller area. So why do the Dutch place four of their five 'go-faster strips' on the lower legs?

Reasons might include:

The lower legs are not too streamlined -- the curvature to the inside edge of the shins is 'backwards' and may lead to early separation of the streamlines and significant drag.
The drag is velocity dependent and the lower legs move faster than the torso during recovery so their drag force increases then. Suppose the skater moves with a speed V relative to the air. Depending on details of individual skater technique I estimate that the recovery leg will move forward at a speed in the range of V/4 to V/2. So relative to the air the lower legs will move at a speed in the range of 1.25V to 1.5V. And if the wind drag force varies as velocity squared, then the drag force on the lower leg will be 1.56 to 2.25 times its gliding value during recovery.
So the lower legs may appear to the air to be several times larger than their apparent size based on these effects. The second effect above suggests that a speed skater's boots should be made more streamlined than they currently are.

7. Analysis and Summary

The American Speedwyre® technology is claimed to trip turbulence in the downstream boundary layer which reduces the drag. The Dutch 'Go-Faster Stripe' may also use the trip wire effect. However, it also incorporates a zigzag V-shape similar to the riblet effect. The Dutch version may have some similarity with the riblet -- reducing the tangential drag although probably not down to the flat plate limit. The V-shapes must modify the downstream boundary layer in such a way that the pressure is more nearly equalized in the upstream-downstream directions. Apparently this drops the pressure drag force more than the Speedwyre® does.
The claimed decrease in elapsed time (up to nearly 7 seconds for a 5000 meter race) is close to what would be expected from the riblet effect but the dimensions do not appear to be close to those usually associated with riblets. It seems likely that the V-shape strips generate vortices which keep the air flow close to the skater rather than letting it separate into a broad boundary layer which would have more drag. It would be surprising if the Dutch have reached the optimum design. It is likely that there is more Olympic Gold to be mined here. Who's got a wind tunnel?

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