CLAIMS :

1.

A fluid dynamic wall surface provided with a series, of elongate projections extending in the direction of fluid flow relative to the surface for modification of a boundary layer of a fluid flow on said surface, said projections having heights which change with distance along the wall surface in the direction of fluid flow in dependence upon a plurality of factors from the group of factors consisting of: (a) counteraction for a selflimiting effect of the projections caused by progressive displacement of boundary layer turbulence away from the wall surface by the projections, (b) compensation for effects dependent upon Reynolds Number, (c) counteraction of the progressive displacement of boundary layer turbulence away from the wall surface by virtue of a streamwise increase of height of the projections, (d) compensation for variations of skin friction in flow direction.

2.	A wall surface according to claim 1 wherein said variation for counteracting the selflimiting effect of the projections caused by their progressive displacement of the boundary layer turbulence away from the wall surface is at a rate of increase of between 5% and 10% per meter.

3.	A wall surface according to claim 1 or claim 2 wherein said compensation for Reynolds Number dependent effects is at a rate of increase of between 10% and 20% per meter.

4.	A wall surface according to any one of claims 1 to 3 wherein the counteraction for the movement of the boundary layer turbulence further from the wall surface by the increasing projection height is at a rate of between 0 and 300% per meter.

5.	A wall surface according to any one of the preceding claims wherein the compensation for said variations of skin friction is at a rate of between 20% decrease per meter and 500% increase per meter.

6.

A fluid dynamic wall surface provided with a series of elongate projections extending substantially in the direction of fluid flow relative to the surface for modifying the interaction of a boundary layer of said fluid flow with said surface, the surface having an upstream region in which said boundary layer is laminar and the projections are scaled in proportion to the boundary layer thickness to increase in height in the direction of the flow, said surface further having a downstream region in which said boundary layer is turbulent and in which at least some of the projections have an initial height in the range of 2 to 45 wall units and increase in height progressively in the direction of the flow, said surface also having a transitional flow region located between said upstream and downstream regions and adjoining the downstream end of the upstream region and the upstream end of the downstream region, the projections extending at least substantially continuously from said upstream region through said transition region and into said downstream region.

7.	A wall surface according to claim 6 wherein the projections in said transition region have heights which change along their length to blend with the downstream heights of the projections in the upstream region and with the upstream heights of the projections in the downstream region.

8.	A wall surface according to claim 6 or claim 7 wherein said projections in said upstream region have a height in the range of 0.03 to 0.2 of the momentum boundary layer thickness.

9.	A wall surface according to any one• of claims 6 to 8 wherein said projections in said upstream region have a height not substantially less than 0.1 of the boundary layer thickness.

10.	A wall surface according to any one of claims 6 to 9 wherein the projections decrease in height in the direction of flow within the transitional region.

11.	A wall surface according to any one of claims 6 to 10 wherein the projections increase in height in the direction of fluid flow in said upstream region at a rate of between 20% and 100% per meter length.

12.

A wall surface according to any one of the preceding claims wherein the projections are spaced from each other at their upstream, ends and have base widths which vary with their height whereby a progressive increase in height of said projections is accompanied by a decrease in the spacing between said projections along their longitudinal extent.

13.	A wall surface according to claim 11 wherein said spacings between the projections comprise flatbottomed valleys substantially coplanar with the wall surface.

14.

A method of operating a fluid dynamic wall surface in a fluid flow in which said surface has a series of elongate projections in an upstream region extending substantially in the direction of fluid flow relative to the surface, the height of the projections varying along their length, in said upstream region the flow of said fluid having a laminar boundary layer close to said wall surface, the changing height projections interacting with said boundary layer in said upstream region so as to extend" the downstream limit of the laminar flow condition of said boundary layer, said projections continuing at least to the extended limit of said laminar region.

15.	A method according to claim 14 in which said projections are continued into a region of onset of transitional flow.

16.	A metho according to claim 15 wherein said projections extend downstream from the onset of transitional flow from said laminar flo to to limit the rise of skin friction at least in an intermediate region in which the fluid flow is transitional.

17.	A method according to claim 15 wherein projections have a height within the laminar flow region of between 0.03 and 0.2 of the boundary layer thickness and the height of the projections is reduced at the onset of the transition region.

18.

A fluid dynamic wall surface provided with a series of elongate projections extending in the direction of fluid flow relative to the surface for modifying the interaction of a boundary layer of said flow with said surface, said flow comprising a laminar flow region and a transition region immediately downstream of said laminar flow region, said projections having a height within the laminar flow region of between 0.03 and 0.2 of the boundary layer thickness and the height of the projections being reduced at the onset of the transition region.

19.	A fluid dynamic wall surface constructed and arranged for use substantially as described herein with reference to the accompanying drawings.