Theory and Experiments for Distributed Roughness Effects in Laminar Boundary Layers

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Identifying and modeling how distributed surface roughness affects laminar boundary layers is an important challenge in fundamental fluid mechanics. Its technological relevance relates to the role surface roughness plays in predicting aircraft skin friction, wind-turbine power production, and the performance of similar aerodynamic systems. Although aerodynamic roughness has been studied for nearly a century, a good understanding of its role has not yet been achieved. One reason may be that most studies have started from isolated roughness elements and attempted to build understanding from single elements to multiple elements. This research will pursue an alternative approach that addresses an entire field of distributed roughness using an analysis technique known as triple-deck asymptotics. The analytical results will be compared to wind-tunnel measurements of roughness fields to be manufactured using 3D printing. Additionally, the project will provide unique educational experiences for South Texas high-school students with STEM interests. These experiences will focus on the critical skills of dimensional analysis and scaling, key engineering concepts that underly the usefulness of both wind-tunnel testing, and triple-deck asymptotic analysis.To develop a whole-field approach, this project will pursue three research objectives. First, a theoretical model based on triple-deck asymptotic analysis will be implemented. This model will accommodate distributed roughness in both linear and nonlinear amplitude ranges. This technique is not widely used in an era of easy access to supercomputer-class numerical solutions. However, asymptotic analysis is ideally suited to studying distributed roughness because parameter studies are needed to characterize many roughness fields. Second, wind-tunnel measurements will be made over manufactured roughness specimens designed to validate key aspects of the theoretical predictions. Third, the validated model will be used to perform a parameter study that explores a wide range of roughness configurations and advances a new understanding of distributed roughness. Combining asymptotic analysis with validation experiments and then using the validated tool to perform wide-ranging parameter studies will significantly advance understanding of the roughness problem.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.