None.
The present invention is directed to a device for mitigating physical shocks that utilizes quiescent cavitation to temporarily store impact energy and is further directed to a concussion mitigation helmet employing this device.
A primary use of helmets is to shield the head from injuries caused by impact or sudden accelerations. It is well known in the art of helmet design to provide various types of impact absorbing material between an outer surface of a helmet and a wearer's head. When the helmet is subjected to an impact this material collapses to absorb energy from the impact and to distribute the energy over a wider surface area for a longer period of time.
Injuries can also occur from sudden acceleration or deceleration. A common traumatic brain injury is caused by what is called a “coup, contrecoup” impact. This injury occurs when the skull is suddenly decelerated bringing the head forward. The brain impacts the frontal area of the skull in what is referred to as the “coup” injury. The skull then rebounds backwards until the neck stops the head and the brain impacts the back of the skull resulting in the “contrecoup” injury. Existing helmets do not guard against accelerations that can cause injury.
Cavitation is the phase change of a liquid to its vapor state by lowering the pressure in the liquid below its vapor pressure. This commonly occurs on the low pressure side of propellers, pumps, and venturi nozzles and is predicted by the cavitation number as follows:
where Plocal is the local pressure, Pv is the vapor pressure of the liquid which is needed to cause cavitation, ρ is the density of the liquid, and V is the velocity. Cavitation becomes likely when the cavitation number C is below 1. When the cavitation number is above 1 cavitation is unlikely and the fluid remains as a liquid.
In the classic experiment of subjecting a liquid filled bottle to sudden acceleration, it is observed that this causes cavitation bubbles at the bottom of the bottle. The bubbles collapse with sufficient force to shatter glass bottles.
It is thus desirable to have a means for delaying and dissipating physical shocks that is passive and compact.
It is a first object of the present invention to mitigate physical shock applied to a joined object.
Another object is to provide shock mitigation with a passive system.
Yet another object is to provide shock mitigation to a variety of different objects.
Accordingly, there is provided a shock mitigator for mitigating physical shock to a joined object. The shock mitigator includes a hollow body capable of being affixed to the object and having two ends defining a volume therein. A cavitating liquid is disposed in the hollow body volume. At least one end cap is slidingly disposed within the hollow body to seal at least one end thereof. When exposed to a physical shock the cavitating liquid changes phase from a liquid to a vapor, absorbing energy from the shock.
Reference is made to the accompanying drawings in which are shown an illustrative embodiment of the invention, wherein corresponding reference characters indicate corresponding parts, and wherein:
In order to use the energy storage capacity of potential energy stored in cavitation bubbles, it is desirable to characterize the onset of cavitation caused by acceleration. This can be done by utilizing the incompressible Navier-Stokes in the y-direction and continuity equations to describe the system of
In this equation u is a velocity vector, μ is viscosity, and g7 is gravity. Assuming that the fluid is inviscid and that all the fluid flow is in the vertical direction gives the following equation:
This allows simplification of equation (2) to give:
Defining
ignoring gravity and using finite difference to simplify the remaining terms, gives a cavitation number for accelerations Ca described as:
This number can be used with the knowledge that values below 1 indicate accelerations that are likely to cause cavitation bubbles.
With knowledge of this acceleration cavitation number, Ca, a shock mitigator 30 is provided as shown in
End caps 34 are sealed against the interior of hollow body 32 and are slidable within hollow body 32. End caps 34 each have a sliding body 38 and a head 40. Sliding body 38 is positioned within hollow body 32. Head 40 is sized to interfere with hollow body 32 in order to prevent end caps 34 from sliding fully within hollow body 32. End caps 34 can be made from polycarbonate or another material. Density of the end caps 34 can be selected to influence cavitation.
A cavitation liquid 42 is positioned within the interior of hollow body 32 and retained by end caps 34. Cavitation liquid 42 can be any liquid having an appropriate vapor pressure, Pv, and density ρ for cavitating when subjected to an expected acceleration. At ordinary temperatures and pressures, it is believed that degassed water is suitable; however, other liquids such as corn syrup, ethanol or mineral oil can be utilized.
Retaining housings 44 and 46 can be provided at the ends of hollow body 32 around end caps 34. Retaining housings 44 and 46 are optional but can serve several purposes. Primarily, these are provided in order to restrain end caps 34 from excessive travel within hollow body 32. It is expected that end caps 34 will generally be retained within hollow body 32 by hydrodynamic forces created by the seal between end caps 34 and hollow body 32. Retaining housings 44 and 46 also serve to prevent mechanical interference with end caps 34 by other objects. Additionally, housings 44 and 46 can act to seal the ends of hollow body 32. This will allow maintenance of a pressurized environment at these ends to further tailor cavitation characteristics. Sealed ends also protect against cavitation liquid leakage or from environmental fluids entry. While retaining housings 44 and 46 can seal the region around end caps and hollow body, these can also be open structures such as cages or other structures.
Cavitation in region 50 stores the impulsive kinetic energy given by acceleration 48 as phase change potential energy. This acts to provide an inertial force resisting acceleration impulse 48. Upon termination of impulse 48, cavitation bubbles collapse back into liquid and equilibrium is restored. This acts to increase the duration of the acceleration impulse and mitigate it.
Shock mitigators 30 can be curved. This allows the mitigator to be contoured to the applied object. A 1/radius dependency is provided in the Navier-Stokes equation in cylindrical coordinates:
An assumption can be made that there will be no velocity in the radial direction (ur=0), and that the radial velocity does not change as a function of radial angle
giving:
Setting the derivative of radial velocity with respect to time equal to the radial acceleration, and simplifying the derivative of pressure with respect to change in angle utilizing the finite difference method gives:
It can be seen that this result is similar to the linear quiescent cavitation number with the exception of 1/r. The further the curved mitigator is from the center of rotation, the more movement the tube will undergo, increasing cavitation potential.
Quiescent cavitation is dependent on the pressure inside the tube, the length of the tube, and the vapor pressure of the liquid being used. These three variables can be altered to make the mitigators cavitate at lower or higher accelerations. This is important because the tubes can be set to cavitate at lower accelerations for helmets having different purposes and users.
Shock mitigators such as those described herein could be installed in a variety of different applications. One such application includes automotive applications to absorb impact energy during a collision. One or more mitigators could be installed along the longitudinal access of the vehicle near the under carriage. During impact, the tube would cavitate temporarily, converting energy into cavitation bubbles before converting the energy back into kinetic energy.
Another likely application is for shock hardening equipment. Shock mitigators such as those described herein could be retrofit on existing equipment to absorb impact energy and make grade B equipment shock hardened to grade A.
It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. For example, hollow body can have a geometric configuration that causes an enhanced pressure drop at a chosen area resulting in cavitation at a lower acceleration.
The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive, nor to limit the invention to the precise form disclosed. Obviously, many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
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Number | Date | Country | |
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62462572 | Feb 2017 | US |