초록 ▼

The boundary layer of a fluid travelling in a mean-flow direction relative to a surface of a wall of a body is controlled by generating in a near-wall region of the flow a magnetic field B having flux lines parallel to the surface of the wall and an electric current density J traversing the magnetic

The boundary layer of a fluid travelling in a mean-flow direction relative to a surface of a wall of a body is controlled by generating in a near-wall region of the flow a magnetic field B having flux lines parallel to the surface of the wall and an electric current density J traversing the magnetic flux lines in the fluid. An electrolyte or other conductivity-enhancing material is introduced into the flow to provide an electrical conductivity gradient in the near-wall region. The magnetic field B and the electric current density J create in the fluid a force J×B having a component normal to the surface of the wall that because of the increased conductivity gradient near the surface can stabilize or destabilize flow in the boundary layer. Numerous aspects of the fluid flow and its interaction with the body can thus be controlled. As examples, shear stress in the fluid at the wall can be decreased, with a corresponding reduction in viscous drag, the characteristics of the acoustic and pressure fields in the fluid surrounding the body can be controlled to reduce noise and fatigue, and boundary layer separation can be inhibited or induced.

대표청구항 ▼

1. An apparatus for controlling a boundary layer in a flow of a fluid having a predetermined electrical conductivity and moving relative to a surface, said apparatus comprising: conductivity altering means for providing an electrical conductivity gradient in a near-wall region of the flow proxima

1. An apparatus for controlling a boundary layer in a flow of a fluid having a predetermined electrical conductivity and moving relative to a surface, said apparatus comprising: conductivity altering means for providing an electrical conductivity gradient in a near-wall region of the flow proximate to the surface; magnetic field generating means for generating in the near-wall region of the flow a magnetic field B(x,y,z,t) having flux lines with a predetermined orientation with respect to the direction of relative movement of the fluid and the surface; and electric current generating means for generating in the near-wall region of the flow an electric current density J(x,y,z,t) traversing the magnetic flux lines, wherein said magnetic field generating means and said electric current generating means are disposed relative to each other such that the magnetic field B and the electric current density J create in the near-wall region of the flow a force L(x,y,z,t)=J×B having a component normal to the surface for controlling the flow. 2. An apparatus according to claim 1, wherein the electric current density J is spatially constant and parallel to the surface in a mean-flow direction of the fluid. 3. An apparatus according to claim 2, wherein the magnetic field B is spatially constant and parallel to the surface transverse to the mean-flow direction of the fluid. 4. An apparatus according to claim 1, wherein the component of the force J×B normal to the surface is in a direction toward the surface for stabilizing the boundary layer. 5. An apparatus according to claim 4, wherein the force J×B reduces the shear stress in the fluid at the surface. 6. An apparatus according to claim 4 , wherein the force J×B maintains attached flow over the surface. 7. An apparatus according to claim 1, wherein the component of the force J×B normal to the surface of the wall is in a direction away from the surface for destabilizing the boundary layer. 8. An apparatus according to claim 7, wherein the force J×B induces turbulent fluid flow in the boundary layer. 9. An apparatus according to claim 7, wherein the force J×B induces the boundary layer to separate from the surface. 10. An apparatus according to claim 1, wherein said magnetic field generating means and electric current generating means generate a magnetic field having flux lines perpendicular to the electric current. 11. An apparatus according to claim 1, wherein the surface comprises a lifting surface. 12. An apparatus according to claim 11, wherein said lifting surface is a control surface. 13. An apparatus according to claim 1, wherein the fluid is liquid and said conductivity altering means comprises means for bleeding an electrolyte into the boundary layer. 14. An apparatus according to claim 1, wherein the fluid is a gas and said conductivity altering means includes means for increasing the concentration of ions in the gas. 15. An apparatus according to claim 1, wherein said electric current generating means comprises a pair of electrodes disposed with the flux lines of the magnetic field between said electrodes. 16. An apparatus according to claim 1, wherein said magnetic field generating means comprises at least one permanent magnet. 17. An apparatus according to claim 1, wherein said magnetic field generating means comprises at least one electromagnet. 18. An apparatus according to claim 1, further comprising control means for varying at least one of the density and orientation of at least one of the magnetic field B and electric current density J. 19. An apparatus for controlling a boundary layer in a flow of an electrically conductive fluid moving relative to a surface, said apparatus comprising: magnetic field generating means for generating in the fluid a magnetic field B(x,y,z,t) having flux lines entering and leaving the surface; and electric current generating means for generating in the fluid an electric current density J(x,y,z,t) traversing the magnetic flux lines in the fluid, wherein said magnetic field generating means and said electric current generating means are disposed relative to each other such that the magnetic field B and the electric current density J create in the fluid a force L(x,y,z,t)=J×B having a component normal to the surface for controlling the flow. 20. A device for travelling through a fluid medium having a predetermined conductivity, the device comprising: a surface contacting the fluid medium so that a boundary layer is formed on said surface; and conductivity altering means for providing an electrical conductivity gradient in a near-wall region of the flow proximate to said surface; flow control means including magnetic field generating means for generating in the near-wall region of the flow a magnetic field B(x,y,z,t) having flux lines with a predetermined orientation with respect to the flow over said surface and electric current generating means for generating in the near-wall region of the flow an electric current density J(x,y,z,t) traversing the magnetic flux lines, wherein said magnetic field generating means and said electric current generating means are disposed relative to each other such that the magnetic field B and the electric current density J create in the near-wall region a force L(x,y,z,t)=J×B having a component normal to said surface for controlling the flow. 21. A device according to claim 20, wherein the surface is a lift-generating control surface and said device further comprises means for varying at least one of the density and orientation of at least one of the magnetic field B and the electric current density J to vary the lift generated by said control surface. 22. A device according to claim 21, wherein said lift-generating control surface is a surface on the wing of an airplane. 23. A method for controlling a boundary layer of a fluid moving relative to a surface and having an electrical conductivity gradient in a near-wall region of the flow proximate to the surface, said method comprising the steps of: determining a magnetic field B(x,y,z,t) to be provided in the near-wall region of the flow by a magnetic field generator such that flux lines of the magnetic field have a predetermined orientation with respect to the direction of relative movement of the fluid and the surface; determining an electric current density J(x,y,z,t) to be provided in the near-wall region of the flow by an electric current generator such that the electric current density traverses the magnetic flux lines in the fluid; and determining the relative positions at which the magnetic field B and the electric current density J are to be provided with respect to the surface and the meanflow direction of the fluid such that the magnetic field and the electric current to create in the fluid a force L(x,y,z,t)=J×B having a component normal to the surface for controlling the flow. 24. A method according to claim 23, wherein the magnetic field B and the electric current density J are constant spatially. 25. A method for controlling a boundary layer of an electrically conductive fluid travelling along a surface, said method comprising the steps of: determining a magnetic field B(x,y,z,t) to be provided in the fluid by a magnetic field generator such that flux lines of the magnetic field enter and leave the surface; determining an electric current density J(x,y,z,t) to be provided in the fluid by an electric current generator such that the electric current density traverses the magnetic flux lines in the fluid; and determining the relative positions at which the magnetic field B and the electric current density J are to be provided with respect to the surface and the meanflow direction of the fluid such that the magnetic field and the electric current to create in the fluid a force L(x,y,z,t)=J×B having a component normal to the surface for controlling the flow.