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A vortex-controlled variable flow resistance device ideal for use in a backpressure tool for advancing drill string in extended reach downhole operations. The characteristics of the pressure waves generated by the device are controlled by the growth and decay of vortices in the vortex chamber(s) of

A vortex-controlled variable flow resistance device ideal for use in a backpressure tool for advancing drill string in extended reach downhole operations. The characteristics of the pressure waves generated by the device are controlled by the growth and decay of vortices in the vortex chamber(s) of a flow path. The flow path includes a switch, such as a bi-stable fluidic switch, for reversing the direction of the flow in the vortex chamber. The flow path may include multiple vortex chambers, and the device may include multiple flow paths. A hardened insert in the outlet of the vortex chamber resists erosion. This device generates backpressures of short duration and slower frequencies approaching the resonant frequency of the drill string, which maximizes axial motion in the drill sting and weight on the bit. Additionally, fluid pulses produced by the tool enhance debris removal ahead of the bit.

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1. A variable flow resistance device defining at least one flow path, the flow path comprising: an inlet and an outlet;a vortex chamber;wherein the vortex chamber has first and second inlet openings each fluidly connected to the inlet of the flow path and first and second feedback outlets, wherein t

1. A variable flow resistance device defining at least one flow path, the flow path comprising: an inlet and an outlet;a vortex chamber;wherein the vortex chamber has first and second inlet openings each fluidly connected to the inlet of the flow path and first and second feedback outlets, wherein the first and second inlet openings of the vortex chamber are positioned to direct fluid in opposite, tangential paths into the vortex chamber so that fluid entering the first input inlet opening produces a clockwise vortex and fluid entering the second inlet opening produces a counterclockwise vortex, and wherein the first and second feedback outlets of the vortex chamber are positioned to direct fluid in opposite, tangential paths out of the vortex chamber, whereby fluid in a clockwise vortex will tend to exit through the second feedback outlet and fluid in a counterclockwise vortex will tend to exit through the first feedback outlet, wherein the first inlet opening is adjacent the first feedback outlet and the second inlet opening is adjacent the second feedback outlet, and wherein the first and second inlet openings and the first and second feedback outlets are all within about a 180 degree segment of the periphery of the vortex chamber;a switch for changing the direction of the vortical flow in the vortex chamber;a feedback control circuit for operating the switch, wherein the feedback control circuit is controlled by fluid output from the vortex chamber through the first and second feedback outlets;wherein the vortex chamber has a vortex outlet fluidly connected to the outlet of the flow path; andwherein the device comprises a structure split into two halves with opposing internal faces, and wherein the flow path is defined in at least one of the opposing internal faces. 2. The device of claim 1 wherein the switch is a fluidic switch. 3. The device of claim 2 wherein the fluid switch is a Y-shaped bi-stable fluidic switch. 4. The device of claim 3 wherein the switch includes first and second control ports. 5. The device of claim 4 wherein the feedback control circuit includes first and second feedback channels, wherein the first feedback channel connects the first feedback outlet to the first control port, and the second feedback channel connects the second feedback outlet with the second control port. 6. The device of claim 5 wherein wherein the first and second inlet openings in the vortex chamber are about between about 60 degrees and about 90 degrees apart. 7. The device of claim 6 wherein the first and second feedback outlets are positioned on opposite sides of the vortex chamber. 8. The device of claim 5 wherein the switch comprises a nozzle, a jet chamber that directs fluid from the nozzle to the legs of the Y-portion of the switch, and wherein the first and second control ports are on opposite side of the jet chamber to move the fluid to the opposite leg of the Y. 9. The device of claim 5 wherein the first and second feedback channels are formed in a single plane. 10. The device of claim 1 wherein the vortex chamber comprises vanes partially surrounding the vortex outlet. 11. A downhole tool comprising the device of claim 1. 12. A bottom hole assembly comprising the tool of claim 11. 13. A drill string comprising the bottom hole assembly of claim 12. 14. A drilling rig comprising the drill string of claim 13. 15. The downhole tool of claim 11 wherein the tool comprises a tool housing and the device comprises a insert captured inside the tool housing. 16. The downhole tool of claim 15 wherein the insert in generally cylindrical in shape. 17. The downhole tool of claim 16 the flow path is partially defined by each of the two opposing internal faces. 18. The downhole tool of claim 15 wherein the insert is retrievable from the tool housing. 19. The downhole tool of claim 15 wherein the insert is nonretrievable from the tool housing. 20. The device of claim 1 further comprising an erosion-resistant liner in the vortex chamber surrounding the outlet. 21. The device of claim 1 wherein the device is elongate. 22. The device of claim 21 wherein the device is cylindrical. 23. The device of claim 1 wherein the flow path is partially defined by each of the two opposing internal faces.