Anatomy of a Burble
by Jan Meyer
A burble is what skydivers call the dead air space over a face to Earth skydiver's back. Engineers use the term wake. Wakes or burbles are common to all objects moving through a fluid or fluids moving past objects.
A wake region will exist whenever there is relative motion between the fluid and object. Fluid dynamics does not depend on whether an object moves through a fluid or whether the fluid flows past an object.
Everyday examples of wake dominated flows are: cars and trucks moving through the air, wind blowing by buildings, a submarine through the ocean, a pearl dropped in a bottle of shampoo and skydivers falling through the air.
Aerodynamics refers to moving air, as opposed to hydrodynamics that studies moving water and fluid dynamics that studies any kind of moving fluid.
The size and shape of wakes depend on the fluid's viscosity, the relative speed between the fluid and object and the object's shape and size. Viscosity is a measure of a fluid's thickness or resistance to move. Oils are more viscous than water. Water is more viscous than air. The air's viscosity changes with altitude. The very low viscosity of the air means that wakes (burbles) will become random very quickly and at relatively low speeds.
Burble Size and Structure
The relative speed between a fluid and an object influences the wake structure. Convected air flow past a burning cigarette is slow speed flow. Air gets heated by the cigarette and rises in a series of random, but distinguishable, curves. When the air flow increases, by moving the cigarette through the air or by blowing across the tip, the flow becomes more and more random. Single traces of smoke are no longer visible. The air on the downwind side becomes a hazy, smokey area. There doesn't appear to be any kind of air movement in any particular direction. This is the type of wake (burble) structure that skydivers have when falling through the air.
The width of wakes or burbles that exist behind skydivers is a little bit smaller than the width of the skydiver. Skydivers' burbles are almost as wide as their shoulders and almost as long as their torso and thighs, as illustrated in Fig. 1. Limbs (arms and legs) also make burbles that are almost as wide and long as the limb or jumpsuit, as shown in Fig. 2.
Wakes (burbles) are 3-dimensional. The wake structure varies in the direction along the wake. Very close to a skydiver, air recirculates, as shown in Fig. 3. Air actually moves toward a skydiver's back in the center region of the near wake (burble). The length of this region is about 1 to 2 times the size of the object. Skydivers' near wake region is about 2 to 4 feet long, depending on body position and rig thickness.
The near wake (burble) is were hesitating pilot chutes hang out. Hand deploy pilot chutes should be cast out to the side of a skydiver to reduce the chance of pilot chute hesitations. A spring loaded pilot chute should have at least a 6 ft long bridle line. This will allow the pilot chute to emerge from the near wake recirculation zone before the bridle line becomes taut to pull the bag from the container.
Airflow Outside of Burble
The air right next to a skydiver moves faster than air further away from him. The airspeed gradually decreases as you move away from a skydiver. This change in speeds is illustrated in Fig. 4. The length of the arrows represent airspeed. The direction of flow is the same as the direction of the arrow. The flow on the upwind side is slower and gets turned nearly 90 degrees. As this air passes around a skydiver it speeds up and gets turned back to almost the same direction as before. Far away from a skydiver's side, the airflow remains undisturbed.
A burble from one skydiver can interfere with the burble from another skydiver. The airflow becomes more distorted when two skydivers get close together. If jumpers are about 2 to 4 feet away, the airflow between them goes a little bit faster than if the skydivers were far apart. The burbles don't interfere much when skydivers are this far apart. As skydivers move closer together, the airflow between them speeds up. Their burbles remain unchanged. (Fig. 5)
If skydivers get very close together, say less than a foot apart, then the airflow between them becomes restricted and starts to become blocked. (Fig. 6) In essense, the burble that two skydivers, less than a foot apart, make is larger than the sum of two individual burbles of skydivers far apart.
Skydivers can minimize these interference effects by keeping their torsos far apart and reach each other with extended arms. The interference between one skydiver's torso and another skydiver's arm is very small. An extended arm gripping another skydiver's leg strap will not significantly block the airflow past either skydiver. Contrastingly, the interference between one skydiver in a very close side body dock is substantial. The airflow is effectively blocked and slowed down as it passes between the skydivers torso and the other skydiver's head and shoulders.
The airflow past jumpers is fastest right next to a jumper's torso, decays as you move laterally away from a jumper, and recirculates over a jumper's back. The burble or wake has air that moves toward a jumper's back. This air sometimes traps pilot chutes. A jumper merely needs to roll about 30 degrees to disturb the burble enough to get faster air to carry the pilot chute away from his back.
Interference between skydiver's burbles is greatest when skydivers are very close together, say less than a foot. Very little interference occurs when skydivers take grips with fully extened arms. Most docks are compromises between these two extreme positions. Most docks do produce interference between burbles.
Originally published in Sport Parachutist's Safety Journal V1, #3 Sept/Oct 1988.
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