Keywords: Structured surfaces two-dimension structures grooves longitudinal drag reduction riblets
Benefits of conventional two_dimension structures
The benefits of structured surfaces have been known for long. However, since the origin of their discovery, experimental and theoretical studies focused on two dimension structures mostly for simplicity reasons. The drag reduction of these structures may reach 10 – 12 % in the most favourable situation (optimised shape with both optimised height and spacing). Economics carried out on 30 year operation pipelines have shown the same order of magnitude of cost reduction by applying structured surfaces on the surface of pipe internal coatings despite it may vary slightly with the operating case.
According to some publications and several patents, the flow drag could be reduced further by using three dimensional structures with increased saving in capital and operating costs. Considering the large energy dissipation attributed to turbulence at large Reynolds number (99% due to turbulence for only 1% due to viscosity), significant drag reduction could be obtained by using more optimised / sophisticated structures. Experimental studies would be extremely difficult to carry out considering the large number of unknown parameters. Theoretical studies are nowadays more accessible due to considerable progress in computers and software’s. Three types of Computational Fluid Dynamics codes (DNS, RANS, LES) are now available for studying the turbulence development near a wall or in the flow core and therefore for predicting, relatively accurately, energy dissipation. An evaluation of CFD codes is presented in this section together with flow simulations (RANS in this section and LES in the next section) based on two techniques of drag reduction.
Reynolds Average Navier Stockes codes, based on turbulence modelling (k – Epsilon model), can be used to determine the degree of drag reduction of a specific two dimension riblet versus its relative dimensions (or relative flow).
These codes may be used for flow drag comparison between different riblet types and for determining the optimum shape and optimum dimensions of 2D riblets.
The maximum flow drag reduction is obtained with knife blade riblets and dimensionless interval S+ of 18. This maximum value is 12 per cent. It goes down to 6 per cent in the case of structures with triangle shape and with 60 degree angles. These data are confirmed by the results quoted in previous research works.
Only the RSM turbulence model has been identified for providing suitable information on transverse secondary flows near structured surfaces.