Ships – Building – Antifriction surfaces
Reexamination Certificate
2002-10-03
2004-02-03
Morano, S. Joseph (Department: 3617)
Ships
Building
Antifriction surfaces
Reexamination Certificate
active
06684801
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
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.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention generally relates to a supercavitation ventilation control system.
More particularly, the invention relates to a supercavitation ventilation control system in which a terminal end of a cavity boundary is controlled in accordance with vehicle travel at varying speed and depth.
(2) Description of the Prior Art
Supercavitation is a means of drag reduction. Cavitation in a liquid results in gas formation. The presence of gas in the place of liquid that normally contacts an underwater body greatly reduces skin friction and thus permits higher speed travel using the same levels of propulsion thrust.
FIG. 1
shows the general features of an underwater vehicle
10
having a forward end
12
and an aft end
14
, the underwater vehicle
10
using supercavitation for drag reduction. The direction of travel for the vehicle
10
is shown with arrow
16
. A cavitator
18
is positioned at the forward end
12
of the vehicle. The cavitator
18
is the portion of the vehicle body
10
that is in contact with the liquid
20
in which the vehicle is submersed. The motion of the cavitator
18
in the liquid
20
causes a low-pressure wake (not shown) to form aft of the cavitator
18
. The pressure in the wake falls as the speed of the vehicle
10
is increased. Eventually the pressure in the wake falls sufficiently such that a vapor pressure is reached and fluid changes state from liquid to gas, forming a cavity
22
surrounding the body
10
. The cavitator
18
is normally designed with a blunt forward section
18
a and sharp detachment points
18
b
. The cavity
22
forms at the detachment points
18
b
. The shape of the cavitator
18
and the speed and depth of the body
10
determines the size and shape of the cavity
22
. The body
10
is generally sized to utilize the cavity volume leaving space for a small clearance gap between the body
10
and the liquid
20
outside the cavity
22
designated as the cavity boundary
24
. While a fore end of the cavity
22
is nearly filled with the vehicle body
10
, an aft portion of the cavity
22
is nearly empty. The empty portion of the cavity
22
exhibits periodic sloshing of liquid called a re-entrant jet or a pair of vortex tubes
26
as shown.
In general, cavities formed by speed of the body alone are too small at any depth to be of practical use in drag reduction. Ventilation of the cavity is normally used to make larger cavities at a given speed or depth. In ventilated cavities, a source of high-pressure gas is introduced into the cavity. The gas causes a rapid expansion of the vaporous cavity, and the cavity continues to grow as ventilation gas enters the cavity, and the pressure in the cavity approaches the ambient depth pressure. A steady state cavity pressure is reached, as the rate of gas leakage from the cavity equals the rate of ventilation gas introduction into the cavity.
FIG. 2
shows the ability to grow a cavity by the introduction of ventilation gas. The cavitation number is the non-dimensional parameter that describes the pressure difference between the gas cavity and the ambient fluid. As the cavitation number decreases, the cavity grows in size. The Froude number is a measure of body speed and the five curves are for five constant Froude numbers increasing from curve
1
to curve
5
. The ventilation coefficient is the non-dimensional parameter that describes the volumetric flow of gas into the cavity. The data shows that as ventilation gas increases, the cavitation number lowers and hence the cavity grows. At some point, gas leakage increases dramatically and ventilation flow rate increases cannot be used to expand the size of a cavity. This behavior results from the basic cavity closure in the aft of the cavity and its interaction with the liquid flow.
The body
10
must provide the volume of gas required for ventilation and cavity envelopment of the body. Thus, high gas losses caused by normal cavity closure as outlined above causes increased volumetric requirements of the body
10
. This use of the body volume limits travel at certain depths and also limits the use and practicality of supercavitating bodies.
The forces on a supercavitating body are due primarily to contact of the body with wetted flow. Normally this contact is at the cavitator, control fins and the aft section of the body, which planes on the cavity interface. The control of the supercavitating body is not optimal as a result of the fluctuating cavity behavior and the structure of the normal cavity closure.
The following patents, for example, disclose cavitating structures, but do not disclose an apparatus to modify and thereby control the cavity boundary generated by a cavitator as does the present invention.
U.S. Pat. No. 3,016,865 to Eichenberger;
U.S. Pat. No. 3,875,885 to Balquet et al.;
U.S. Pat. No. 3,205,846 to Lang;
U.S. Pat. No. 5,955,698 to Harkins et al.; and
U.S. Pat. No. 6,167,829 to Lang.
Specifically, Eichenberger discloses a method and apparatus for reducing the drag of bodies or vehicles such as a torpedo or a submarine or the like submerged in a liquid such as water. More particularly, the invention relates to a method and apparatus for providing a reduction of such drag by stabilization of a laminar water boundary layer by a gas film introduced between the body and the surrounding liquid whereby the stabilization of the laminar water boundary layer also results in the stabilization of the water-gas interface.
The patent to Lang '846 discloses a torpedo body form and gas layer control. The underwater craft includes an elongated hull having generally rounded transverse sections there along. An annular gas cavity is generated adjacent to the hull and means are provided for communicating the cavity rearward from a predetermined circumferential cavity generation locus of the hull disposed near the nose of the craft to a predetermined circumferential cavity closure and rewet locus of the hull disposed near the tail of the craft. A gas is selectively and varyingly introduced into the cavity for maintaining a predetermined communication between the loci. Means are provided for measuring the thickness of the annular cavity, the means adapted to introduce a variable quantity of gas into to the cavity. In response to the determined thickness, the quantity of gas introduced into the cavity is controlled in an inverse relationship to the cavity thickness.
Balquet et al. discloses an air injection propulsion system for marine vessels including a primary gas injector for creating an axial gas flow beneath the vessel's hull, a primary aerator located beneath the vessel's hull for generating an aerated flow of water, and a secondary aerator, for further refining the aerated flow, includes a deflecting surface to provide the main propulsive effect. The primary aerator comprises a contoured surface positioned transversely to the gas flow, which, in one embodiment, has located therein a series of slots with their axes parallel to the gas flow. Axial and transverse aeration of the water flow adjacent the gas flow are generated simultaneously by the primary aerator from the same axial gas flow. The primary aerator further comprises a deflecting foil spaced from and positioned opposite to the contoured surface which complements both types of aeration generated by the contoured surface. The secondary aerator comprises one or more gas injectors spaced transversely across the inclined rear surface of the vessel's hull and one or more contoured surface diluting foils located rearward of the primary aerator and positioned transversely across the aerated flow from the primary aerator.
Harkins et al. discloses a supercavitating water-entry projectile having empennage on the aft end providing both aerodynamic and hydrodynamic stability and a supe
Kasischke James M.
Morano S. Joseph
Nasser Jean-Paul A.
Oglo Michael F.
The United States of America as represented by the Secretary of
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