By default the marine simulations performed in Orca3D Marine CFD treats the wall boundaries corresponding to the hull, deck, and other model surfaces as perfectly smooth, with none of the roughness due to paint, fouling and other factors that generally exist in physical vessels. Roughness will affect the flow inside of the boundary layer, generally resulting in an increase in drag, and in many cases this increase can be significant enough to include in the performance predictions.
SimericsMP does offer a built-in model to estimate the roughness effects based on the boundary layer theory of Schlichting (Boundary Layer Theory, 6th Edition, 1968 ISBN 07-055329-7 pg. 580). As shown below, this roughness model can be enabled after changing the Marine module to "Extended Mode" and then selecting the desired boundary(ies) and setting the roughness properties. Choosing an appropriate "Roughness Height" for this model presents its own set of challenges. The typical roughness height of 150 microns noted in several ITTC publications (such as ITTC - Recommended Procedures and Guidelines: 1978 ITTC Performance Prediction Method, 7.5-02-03-01.4) carries with it some assumptions on how roughness is measured. Surface roughness on ship and boat hulls is not typically uniform over the surface, so the conversion of a single roughness value that can be physically measured with a specified approach to one that is appropriate for use in a CFD roughness model is not trivial. There are numerous papers on this topic such as "An experimental investigation into the surface and hydrodynamic characteristics of marine coatings with mimicked hull roughness ranges" by Yeginbayeva and Atlar that help to study this. Anecdotal data from one of our Orca3D customers relies on an approximate ratio of 1/5 to convert from roughness height as noted by ITTC documents to those appropriate for the CFD roughness model. In this case, a user would enter a value of 30 microns (3e-5 m) to represent the 150 micron ITTC value. At Orca3D, we have not performed any validation on this value to date and leave it to the user to decide whether to use this approach or the empirical approach in the following discussion.
For ships the ITTC recommends an empirical approach to accounting for hull roughness (ITTC - Recommended Procedures and Guidelines: 1978 ITTC Performance Prediction Method, 7.5-02-03-01.4), and this approach is currently implemented in the Orca3D Marine CFD reporting tool. In the original version of this ITTC guidance (1999, Rev 00), an increment of frictional resistance, DCf, was used to represent the effects of roughness as shown in the formulation below, where ks is the roughness height and Lwl is the waterline length in consistent units. Although this was labeled as a roughness allowance in the ITTC document, it really served as an overall model-ship correlation factor including any allowances beyond just roughness. That said, there were no specifics given in the document regarding how much of this allowance was due just to roughness. When using this option, the user should realize that the resulting values account for roughness "and more."
Later on, the ITTC developed a new performance prediction method (2014 Rev 03) that modifies the prior (1999) formulation by explicitly defining a drag component due to roughness, delta Cf, and a component due to differences between model measurements and full-scale trial results, Ca. The new formulation is presented below. In this new formulation the guidelines state that Ca is defined "to give values of delta Cf + Ca that approximates the values of delta Cf of the original 1978 ITTC method."
Strictly speaking, when using this formulation through the Orca3D Marine CFD reporting tool, the user may want to zero out the column corresponding to Ca since this is specifically intended for expanding model scale measurements to full-scale, which is not the goal of the reporting tool.
To apply hull roughness corrections using the Orca3D CFD reporting tool, select the appropriate Report Options as shown in the image below. Selecting "None" applies no additional roughness corrections to the computed CFD results. Choosing "ITTC 1978 Ca" applies the older ITTC guidelines where a single Ca value is computed to represent roughness and any other allowances. The roughness height defaults to 0.00015 m but can be adjusted and applied according to the formulae above. Similarly, "ITTC Modified" applies the separate computation of Ca and DCf in the report, and as noted earlier you may choose to zero out the Ca column if desired. Finally the last option allows the user to enter a constant value of DCf to use for effects of roughness. This is useful when the user has better guidance on the effects of roughness for their specific design and bottom coatings.
