Vehicle comprising at least one apparatus for reducing drag on the vehicle

1. A vehicle comprising at least one pressure-drag-reducing apparatus for reducing the pressure drag on the vehicle, the apparatus comprising:

a housing at least partly defining a cavity and an aperture in the housing, the cavity in fluid communication via the aperture with the environment surrounding the vehicle; and

oscillating means for creating an oscillation in pressure across the aperture between the cavity and the environment;

the cavity volume, the frequency of oscillation, the aperture size and the amplitude of oscillation arranged to cause fluid to be alternately drawn into, and ejected from, the cavity through the aperture, and the apparatus positioned relative to the remainder of the vehicle such that the ejected fluid entrains fluid from the environment to reduce pressure drag on the vehicle;

wherein the apparatus is arranged so that the fluid ejected from its aperture is ejected in a direction substantially parallel to the direction of travel of the vehicle to create a highly sheared region in which viscous stresses dominate; and

wherein the vehicle is a large goods vehicle.

2. A vehicle according to claim 1 , comprising a driving means arranged to drive the oscillating means.

3. A vehicle according to claim 2 , wherein the oscillating means comprises a piezoelectric material bonded to a diaphragm.

4. A vehicle according to claim 3 , wherein the driving means is arranged to drive the oscillating means by applying an alternating voltage to the piezoelectric material.

5. A vehicle according to claim 2 , wherein the driving means is arranged to drive the oscillating means using amplitude modulation.

6. A vehicle according to claim 1 , wherein the apparatus is positioned at the back of the vehicle with respect to the direction of travel of the vehicle.

7. A vehicle according to claim 1 , wherein the apparatus is arranged substantially around the perimeter of the back of the vehicle with respect to the direction of travel of the vehicle.

8. A vehicle according to claim 1 , wherein the apparatus is arranged so that its aperture lies in a plane substantially perpendicular to the direction in which fluid would usually flow over the vehicle.

9. A vehicle according to claim 1 , wherein the apparatus is additionally arranged selectively to increase pressure drag.

10. A method of operating at least one pressure-drag-reducing apparatus for reducing the pressure drag on a vehicle, the apparatus comprising:

a housing at least partly defining a cavity and an aperture in the housing, the cavity in fluid communication via the aperture with the environment surrounding the vehicle; and

oscillating means for creating an oscillation in pressure across the aperture between the cavity and the environment;

the cavity volume, the frequency of oscillation, the aperture size and the amplitude of oscillation arranged to cause fluid to be alternately drawn into, and ejected from, the cavity through the aperture, and the apparatus positioned relative to the remainder of the vehicle such that the ejected fluid entrains fluid from the environment to reduce pressure drag on the vehicle;

wherein the apparatus is arranged so that the fluid ejected from its aperture is ejected in a direction substantially parallel to the direction of travel of the vehicle to create a highly sheared region in which viscous stresses dominate; and

wherein the vehicle is a large goods vehicle;

the method comprising the step of:

(a) operating the pressure-drag-reducing apparatus to decrease the pressure drag on the vehicle.

11. A method according to claim 10 , wherein the oscillating means is driven at an effective frequency determined in dependence on the speed of the vehicle.

12. A method according to claim 11 , wherein in response to determining a change in speed of the vehicle, the effective frequency of oscillation of the oscillating means is increased or decreased such that it remains within the range of frequencies which produce a decrease in pressure drag on the vehicle.

13. A method according to claim 11 , wherein the effective frequency of oscillation is the frequency of amplitude modulation of the oscillations of the oscillating means and increasing or decreasing the effective frequency in step (a) comprises respectively increasing or decreasing the frequency of the envelope of the oscillations.

14. A method according to claim 10 , wherein step (a) comprises driving the oscillating means to oscillate at the resonance frequency of the apparatus.

15. A method according to claim 13 , wherein step (a) comprises increasing the frequency of the envelope of the oscillations of the oscillating means when the speed of the vehicle increases.

16. A method according to claim 13 , wherein step (a) comprises decreasing the frequency of the envelope of the oscillations of the oscillating means when the speed of the vehicle decreases.

17. A method according to claim 10 , wherein the method comprises the step (b) of driving the oscillating means at an effective amplitude determined in dependence on the speed on the vehicle.

18. A method according to claim 17 , wherein in response to determining a change in speed of the vehicle, the effective amplitude of oscillation of the oscillating means is increased or decreased such that it remains within the range of amplitudes which produce a decrease in pressure drag on the vehicle.

19. A method according to claim 10 , wherein the method comprises the step of modelling the wake region of the vehicle, including determining the pressure at a position at or adjacent the aperture of the pressure-drag-reducing apparatus; and the step of driving the oscillating means at an effective amplitude and or effective frequency determined in dependence on the model of the wake region of the vehicle.

20. A method according to claim 19 , wherein there is more than one pressure-drag-reducing apparatus, and the method comprises increasing or decreasing the effective amplitude and or frequency of the oscillating means individually, in dependence on the model of the wake region.

21. A method according to claim 10 , wherein the method is additionally for selectively increasing pressure drag.

22. A method according to claim 21 , wherein the method comprises the step of increasing or decreasing the effective frequency and or amplitude of the oscillations of the oscillating means to maintain the effective frequency and or amplitude at a frequency and or amplitude at which the ejected fluid increases pressure drag on the vehicle.

23. A method according to claim 10 , wherein the method comprises the step of driving the oscillating means at an effective frequency that is greater than or equal to six times the frequency of the shear layer mode.

24. A method according to claim 10 , wherein the method comprises the step of driving the oscillating means at an effective amplitude at which the oscillating means generates a primary vortex of maximum strength.

25. A method according to claim 24 , wherein the effective amplitude is selected according to the relationship:

C

μ

=

u

j

2

⁢

A

j

U

2

·

A

wherein where u j is the jet amplitude measured at the centre of the jet aperture, A j is the aperture area, U is the vehicle speed, and A is the area of the vehicle base.