Brunel University School of Engineering and Design PhD Theses
Abstract
This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel UniversityDrag reduction in wall-bounded flows can be achieved by the passive flow control
technique using riblets and surface grooves aligned in the mean direction of an
overlying turbulent flow. They were inspired by the skin of fast sharks covered
with small longitudinal ribs on their skin surfaces. Although it was found that the
drag reduction depends on the riblets’ geometrical characteristics, their physical
mechanisms have not yet been fully understood in the scientific terms.
Regarding riblets sizing, it has been critically explained in the literature how riblets with vanishing size interact with the turbulent flow and produce a change in the drag proportional to their size. Their shapes are focused upon because these are
most significant from a technological perspective, and also less well understood.
Different riblet shapes have been designed, some with complicated geometries, but
except for the simple ones, such as U and V grooves, there has not been enough
study regarding shape features. Therefore, special effort is undertaken to the design
of an innovative type of ribleted surface, e.g. the Serrate-Semi-Circular shape, and
its effect on the skin friction and drag reduction. In this work, the possible physical mechanisms of riblets for turbulent drag reduction have been explored. The modelling and experiments concerning the
relationship between the riblets features and the turbulent boundary layer structure
have also been reviewed. Moreover, numerical simulations on riblets with different shapes and sizes are presented and studied in detail. An accurate treatment based on k-ε turbulence model was adopted to investigate the flow alteration and the consequent drag
reduction on ribleted surfaces. The interaction of the overlying turbulent flow with riblets and its impact on their drag reduction properties are further investigated. In addition, the experimental facilities, instrumentation (e.g. hotwires) and measurement techniques (e.g. time-averaged turbulence structure) have been
employed to experimentally investigate the boundary layer velocity profiles and
skin friction for smooth and micro-structured surfaces (the proposed riblet shape, respectively and the presented new design of riblets with serration inside provides 7% drag reduction. The results do not show significant reduction in momentum
transfer near the surface by riblets, in particular, around the outer region of the
turbulent boundary layer. Conclusions with respect to the holistic investigation on the drag reduction with Serrate-Semi-Circular riblets have been drawn based on the research objectives as achieved. Recommendations for future work have been put forward particularly for further future research in the research area.Brunel University and KIMM (Korea Institute of Machinery and Materials
