Pump

1. A fluid pump comprising:

a chamber which, in use, contains a fluid to be pumped, the chamber including a main cavity having a substantially cylindrical shape bounded by first and second end walls and a side wall and a secondary cavity extending radially outwards of the main cavity;

one or more actuators which, in use, cause oscillatory motion of the first end wall in a direction substantially perpendicular to the plane of the first end wall, the actuator including an active element which is either a piezoelectric ring or a magnetostrictive ring, the active element being excited in a radial mode to induce axial deflection of one or both of the end walls, the distance between the inner and outer circumferences of the ring being approximately one quarter of a wavelength of the actuator mode-shape; and

whereby, in use, the axial oscillations of the first end wall drives radial oscillations of the fluid pressure in the main cavity; and

wherein the secondary cavity spaces the side wall from the first end wall such that the first end wall can move relative to the side wall when the actuator is activated.

2. A fluid pump according to claim 1 , wherein a gap is provided between the top of the side wall and the first end wall.

3. A pump according to claim 2 , wherein a layer of compliant material is provided between the top of the side wall and the first end wall.

4. A pump according to claim 1 , wherein the secondary cavity includes a thinner portion between the side wall and the first end wall and a deeper portion radially outward of the side wall.

5. A pump according to claim 4 , wherein the side wall tapers towards the first end wall.

6. A pump according to claim 1 , wherein the first end wall is mounted on the radially outermost portion of the secondary cavity.

7. A pump according to claim 1 , further comprising at least two apertures through the chamber walls, at least one of which is a valved aperture.

8. A pump according to claim 7 , wherein any valved apertures in the chamber walls are located near the centre of the main cavity.

9. A pump according to claim 7 , wherein any unvalved apertures in the chamber walls are located at a distance of 0.63a plus or minus 0.2a from the centre of the main cavity, where a is the main cavity radius.

10. A pump according to claim 1 , further comprising a second actuator, wherein, in use, the second actuator causes oscillatory motion of the second end wall in a direction substantially perpendicular to the second end wall.

11. A pump according to claim 1 , wherein the outer circumference of the ring is substantially adjacent the radially outermost portion of the secondary cavity.

12. A pump according to claim 1 , wherein the thickness of the first end wall is shaped to optimise the actuator displacement profile for mode-shape matching.

13. A pump according to claim 1 , wherein the main cavity radius, a, and height h, satisfy the following inequalities:

a/h is greater than 1.2; and

h 2 /a is greater than 4×10 −10 m

and wherein the main cavity radius, a, also satisfies the following inequality:

k

0

·

c_min

2

⁢

π

⁢

⁢

f

<

a

<

k

0

·

c_max

2

⁢

π

⁢

⁢

f

,

where c_min is 115 m/s, c_max is 1970 m/s, f is the operating frequency and k 0 is a constant (k 0 =3.83).

14. A pump according to claim 13 , wherein the ratio

a

h

is greater than 20.

15. A pump according to claim 13 , wherein the volume of the main cavity is less than 10 ml.

16. A pump according to claim 13 , wherein the ratio

h

2

a

is greater than 10 −7 meters and the working fluid is a gas.

17. A pump according to claim 13 , wherein, in use, the motion of the driven end wall(s) and the pressure oscillations in the main cavity are mode-shape matched and the frequency of the oscillatory motion is within 20% of the lowest resonant frequency of radial pressure oscillations in the main cavity.

18. A pump according to claim 17 , wherein the amplitude of end wall motion approximates the form of a Bessel function.

19. A pump according to claim 17 , wherein, in use, the frequency of the oscillatory motion is equal to the lowest resonant frequency of radial pressure oscillations in the main cavity and this frequency is greater than 500 Hz.

20. A pump according to claim 1 , wherein one or both of the end walls have a frusto-conical shape such that the end was are separated by a minimum distance at the centre and by a maximum distance at the edge.

21. A fluid pump comprising:

a chamber which, in use, contains a fluid to be pumped, the chamber including a main cavity having a substantially cylindrical shape bounded by first and second end walls and a side wall and a secondary cavity extending radially outwards of the main cavity;

one or more actuators which, in use, cause oscillatory motion of the first end wall in a direction substantially perpendicular to the plane of the first end wall, the actuator including an active element which is either a piezoelectric ring or a magnetostrictive ring, the active element being excited in a radial mode to induce axial deflection of one or both of the end walls, the radial distance between the inner and outer circumferences of the active element ring being approximately one half of a wavelength of the actuator mode-shape; and

whereby, in use, the axial oscillations of the first end wall drives radial oscillations of the fluid pressure in the main cavity; and

wherein the secondary cavity spaces the side wall from the first end wall such that the first end wall can move relative to the side wall when the actuator is activated.

22. A pump according to claim 21 , wherein the inner and outer circumferences of the active element ring are located substantially at nodes of the actuator vibrational mode-shape.

23. A pump according to claim 21 , wherein the actuator is constructed such that the piezoelectric or magnetostrictive material is pre-compressed in the actuator rest position.