Aeronautics and astronautics – Miscellaneous
Reexamination Certificate
2001-05-31
2003-03-04
Swiatek, Robert P. (Department: 3644)
Aeronautics and astronautics
Miscellaneous
C244S130000
Reexamination Certificate
active
06527221
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to facilitating the movement of objects through a fluid and, more particularly, to modifying shock waves within the fluid.
BACKGROUND OF THE INVENTION
When a fluid is driven to flow at a relative speed, with respect to the fluid it encounters, that exceeds the speed of sound within the encountered fluid, one or more shock waves can develop. The driving of the fluid can occur when the fluid is pressed forward by an object or body propagating through the fluid. Alternatively, the fluid can be accelerated by a pressure gradient generated by any other means, such as in wind tunnels, propulsive units, jets, and rapid heating/expansion. When a shock wave is formed in a supersonic stream of a fluid, several undesirable effects can occur.
If, for example, the supersonic stream of fluid results from a propulsive effluent stream, such as the discharge of a jet aircraft, then pressure jump(s) due to the difference in pressure across a shock wave can reduce the efficiency of the desired momentum transfer from the vehicle to the effluent stream. Additionally, a series of shock waves within the supersonic stream can augment the acoustic signature of the supersonic stream in certain frequency ranges. This augmentation of the acoustic signature is undesirable for both environmental and detection avoidance reasons. As a further example, if solid (or liquid) particles in multi-phase supersonic flow are directed to propagate across a shock wave, such as during supersonic spray deposition, a potential problem is that particles of different sizes and/or densities are affected differently when they cross the shock wave. This can result in an undesired segregation of particles, or particle size redistribution at the shock wave depending on the shock parameters and the size and/or densities of the particles. Furthermore, when a body or vehicle is driving a fluid forward, the driving body will typically feel the strong increase in pressure across the shock wave as a drag force that impedes the forward motion of the body. Another problem associated with the increase in pressure across a shock wave is an increase in temperature. Again, if the shock is being driven by a body or vehicle, high temperatures behind the shock wave can result in undesirable heating of the vehicle materials and/or components behind the shock wave. The deleterious effect of interacting shock waves and their high temperatures and pressures can be yet stronger.
The control of shock waves by reducing the strength of the shock wave or completely eliminating the shock wave is sometimes referred to as flow control. This term is used because the fluid flow is being controlled by manipulating or affecting the shock wave(s) within the fluid. When considering vehicles/bodies, flow control also encompasses processes which reduce drag. This drag can be the overall or total drag, the reduction of which is intended to optimize the performance and efficiency of the vehicle. Alternatively, the drag reduction can be preferentially applied to generate moments or torque, which is useful in maneuvering the vehicle or maintaining certain angles of attack. Flow control can also be used to reduce heating and modify acoustic signatures such as a sonic boom, which result directly from the shock waves.
As a fluid element crosses from one side of the shock wave to the other, the fluid element experiences a sharp and theoretically discontinuous increase in pressure. The magnitude of this increase or “pressure jump” is typically larger for stronger shock waves, which is characterized by a greater difference between the pressures on either side of the shock wave along a perpendicular line across the shock wave. As used herein, the term “reducing the strength” of a shock wave involves reducing the pressure difference across the shock wave along the original direction of flow by reducing or eliminating the pressure discontinuity within the fluid flow and/or diffusing or broadening the pressure jump to create a shallower pressure gradient across the shock wave in this original direction of flow. When a shock wave has been removed or eliminated, the formerly shocked flow becomes subsonic in the original direction of fluid flow although, however, the flow may be supersonic or shocked in directions transverse (not limited to orthogonal) to the original direction of the fluid flow in the specific spatial region in question.
Reducing the strength of the shock wave, or eliminating it completely, can advantageously reduce or remove a sometimes significant portion of the drag force acting on the body due to the shock wave. This can be beneficial to such bodies because a reduction in drag force increases the range and/or speed of the body. Therefore, the reduction in drag requires less energy/fuel to propel the vehicle and/or allows for a greater payload of the vehicle or body for the same amount of fuel/propellant required without invoking any drag reduction.
Another benefit of being able to reduce the strength of or eliminate the shock wave is the ability to steer the body or vehicle. If only certain portions of the shock wave are reduced in strength at a given time, such as to one side of the body, then drag on the body can be preferentially and selectively controlled. Being able to control the drag on certain parts of the body allows the body to be steered by preferentially controlling the strength of the associated shock wave(s) as well as the resulting pressure distribution along the body.
Since the first supersonic vehicle, there have been many developments to reduce the strength of shock waves; increase shock standoff distance from the vehicle; and reduce the stagnation pressure and temperature. One of the first developments was that of the aerospike
10
, as illustrated in FIG.
1
. This is typically a pointed protrusion extending ahead of the nose of the vehicle
12
or other critical shock-generating surfaces. The aerospike
10
effectively increases the “sharpness” of the vehicle
12
, and is based on the idea of using a mechanical structure to physically push air to seed transverse motion in the fluid, thus allowing the fluid to start moving laterally out of the way before the fluid actually encounters a larger part of the vehicle
12
. Because the aerospike
10
pushes air, a shock wave
14
actually begins to develop when the ambient air encounters the tip of the aerospike
10
.
Other developments, as illustrated in
FIG. 2
, have been the injection of fluids
16
, such as streams of water, gas, and heated and/or ionized fluid, toward the shock wave
14
from the vehicle
12
. These fluid extensions behave similarly to the aerospike and obtain similar effects and benefits, because the counter-flowing fluid also pushes the ambient air forward and laterally before the air reaches a larger part of the vehicle
12
. More recently, there have been attempts to ionize the air ahead of a vehicle and its shock wave using by radio frequency (RF) or microwave radiation. Electromagnetic methods have the benefit that they can pass through the gas without “pushing,” or imparting any momentum, to the gas. The electromagnetic radiation can therefore pass through a shock wave without significantly affecting it.
The microwave methods involve creating a spot ahead of the shock wave using a microwave intensity high enough to heat and/or ionize the gas. One proposed method, as illustrated in
FIG. 3A
, is to focus a microwave beam
26
emanating from the front of a supersonic vehicle
24
to a point
28
ahead of the shock wave. Another proposed method using microwaves, as illustrated in
FIG. 3B
, has been to mount microwave horns
20
on the wings
22
on both sides of the vehicle fuselage
24
. Each microwave horn
20
emits a microwave beam
26
that is alone too weak to ionize the gas. However, when the two beams
26
are crossed in front of the vehicle
24
, the combined electric field
28
is strong enough to ionize the gas. Both of the aforementioned methods using microwaves disadvantageously must be operated conti
McDermott & Will & Emery
Swiatek Robert P.
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