Helical slipstream (a.k.a. prop wash) on a USMC MV-22 Osprey. Photo Credit: YoyoWall.com
A slipstream is essentially a region in the boundary layer along side of and a wake region behind an object moving through a fluid, in which the local velocity is very near that of the moving object. In simpler terms, the slipstream is fluid being pulled alongside of and behind an object at close to the same speed as the object is moving. These slipstreams can be found around/behind virtually any object moving through a fluid, can be a high pressure or low pressure region (depending on the Reynold’s number of the flow), can be created in both liquids and gases, and can either be a hindrance (by creating parasitic drag) or can prove advantageous (by the creation of additional thrust, lift, or by positively affecting other key parameters). We will look at slipstreams in three key applications and in each one will look at how slipstreams can be bad as well as how they can prove useful!
The first application we will look at is that of an object flying through the air. The most familiar example of these slipstreams is that which aircraft encounter; these are helical slipstreams that are produced by propellers as they rotate through the air, as seen trailing the rotors of the V-22 Osprey pictured above. This type of helical slipstream is commonly referred to as “propwash” and can commonly be made visible on a humid day as moisture may condense out of the air if the pressure and temperature within the slipstream core drop below the dew point. This propwash is usually seen as a major detriment with regards to how it may affect the ability of pilots to control smaller aircraft. As shown below, the slipstream can wrap around the plane and ultimately interfere with the vertical stabilizer (the big vertical fin at the back of the plane), causing the plane to yaw/rotate to the left, requiring the pilot to correct back to the right.
The effect of propwash on aircraft stability. Photo Credit: SimHQ.com
As one can imagine, this can serve as a major inconvenience and can be a bit unsettling for new pilots; in fact, in the early days of powered flight, this phenomena led to quite a few crashes, some of them fatal. So if this propwash is so bad, why did I say earlier that we would look at the usefulness of slipstreams in each of the 3 applications? While for aircraft, slipstreams are usually seen as bad, we must remember that aircraft are not the only things that fly!
Geese flying in a V-formation; Geese vortex surfing behind an ultralight aircraft.
Photo Credit: Stevetabone.Files.WordPress.com; Picture-Newsletter.com
Many species of birds tend to fly in a v-formation to make use of the slipstream present in the wingtip vortices of the birds in front of them. The slipstream within the wingtip vortex coming off of the lead bird’s wing creates upwash, which the next bird uses as a source of lift, and so on and so on, down the line.
The next application we will look at is that of objects moving air, while on the earth’s surface. If you have ever been too close to a train as it has passed, then you have felt the effect of this slipstream as it threatened to rip you from your feet and drag you alongside and into the train. This is not a comfortable feeling and is incredibly dangerous, so PLEASE DO NOT try this; rather, you can observe the trees and plants around the train tracks as they appear to get “sucked into” the path of the train. This slipstream can cause a high degree of drag, lead to more noise (through the turbulent vortices within the slipstream), and can be dangerous for passerby. So how is it that we may take advantage of these slipstreams on the earth’s surface?
CFD analysis of the turbulent vortices in the slipstream of a moving freight train. Photo Credit: Birmingham.ac.uk
Competition bicycle riders often take advantage of each others’ slipstreams in order to save on energy during races. Bicyclists refer to this as “drafting” and often will intentionally remain behind a competitor to save on energy and then breakout in front of their opponent at strategic locations (i.e. near the finish line). This technique is also used by speed skaters, runners, cross-country skiers, stock car racers, and many more!
Bicycle drafting CFD analysis. Photo Credit: SingleTrackWorld.com
The last application we will look at is that of objects moving through the water. Cavitation is a word that virtually every boat owner knows and has learned to loathe (not to worry, you are not alone, we here at Team UV have vowed to make cavitation our enemy and defeat it!). Cavitation is the rapid formation and subsequent collapse of air bubbles that accompanies a large pressure drop in a flow. When propellers move very fast, are improperly designed, or contain surface defects, the pressure of the flow along the surface of the propeller may drop below the vapor pressure (effectively boiling the water), forming air bubbles, which then collapse and in doing so effectively create implosions which cause damage to propellers through pitting, as seen below. In addition to this, these bubbles (when they form) are sucked along into the slipstream, where they create separation between the propeller blade and the working fluid (water), thus significantly reducing the efficiency of the propeller with respect to the creation of thrust. These slipstreams also threaten to expose the location of stealthy underwater craft, as the bubbles may be visible from under water (or from surface ships or anti-submarine aircraft) and may create noise as they collapse, thus diminishing acoustic stealth. With all of these bad things, how on earth could slipstreams prove useful for marine applications?!
Cavitation within the slipstream of a marine propeller; Pitting cavitation damage.
Photo Credit: Wikipedia.org (2)
Enter the Grim Vane Wheel (GVW), one of many devices specifically designed to take advantage of the slipstreams of marine propellers on surface vessels. The GVW is basically a freely rotating, large diameter propeller which is situated behind (aft of) the main (powered, smaller diameter) propeller. As the main propeller spins and creates its helical slipstream (hopefully without any cavitation), the slipstream continues to rotate and move on down the line. When the slipstream encounters the GVW it does two things: for the portion of the GVW that lies within the diameter of main propeller, the GVW acts as a free spinning turbine, which can be allowed to rotate freely or even be used to generate electricity for auxiliary power; for the portion of the GVW that lies outside of the diameter of the main propeller, the GVW acts as an additional propeller, creating more thrust! These devices are used in situations where rotational energy losses are high and thus the advantages of the GVW with regards to the recoverable energy in the slipstream may be justified as they compare to the added weight, complexity, and cost associated with the GVW.
Twin Grim Vane Wheels on a large ship. Photo Credit: BoatDesign.net
So there you have it, from airplanes and geese, to trains and bicyclists, to stealthy underwater vehicles and large cargo ships, we have explored just some of the many scenarios in which slipstreams may be found as well as a few of the ways in which they may either prove harmful or, alternatively, may be taken advantage of. Hopefully those of you who have read through this entire article (yes, I know it was a little long, haha) can walk away from your computer with a little bit more of an understanding with regards to the beauty present in fluid mechanics and just one of the virtually infinite ways that this vast subject of mechanical engineering manifests itself in our world!