Print Email Facebook Twitter Turbulence in the wake of a roughness patch Title Turbulence in the wake of a roughness patch Author Voermans, J.J. Contributor Uijttewaal, W.S.J. (mentor) Van Prooijen, B.C. (mentor) Breugem, W.P. (mentor) Faculty Civil Engineering and Geosciences Department Hydraulic Engineering Programme Environmental Fluid Mechanics Date 2013-03-05 Abstract Little research was done in the past concerning the propagation of three dimensional effect in shallow wake flow caused by a roughness patch. Today’s research on related subjects is dominated by emerging obstructions in shallow water where the flow can be assumed as (quasi) two dimensional. However the relevance of a submerged obstruction with increased roughness can be found in wake control, oyster reefs, river- and estuary bottoms and heterogeneous land occupancy. To get a better understanding of the consequences of the three dimensionality of the flow structures, experiments are performed in a wide shallow flume to examine these structures. The main objective is to examine whether the wake structure of a roughness patch can be treated as (quasi)-two-dimensional. The objective has been answered by a combination of a literature study and an experiment performed at the faculty’s laboratory. The results show four dominant mechanisms in the wake of a roughness patch: transverse mass flux, bottom friction, mixing layer and the secondary circulation. Based on a momentum balance the transverse mass flux and the bottom friction are the largest contributions to this balance. Although the contribution of the mixing layer and the secondary circulation to the recovery of the wake are of the order of 10%, their influence on the flow structure is more pronounced. The mixing layer is shifted towards the wake centerline due to the presence of a transverse mass flux forming a misalignment between the maximum spanwise Reynolds stress and the position of the wake half width. Since this shift is of limited influence on the position of the secondary circulation, a misalignment if formed between the maximum momentum transport by the secondary circulation and the mixing layer causing a lower streamwise velocity at the edge of wake with respect to the wake of an emerging obstruction. The secondary circulation is responsible for the transport of low momentum fluid towards the edge of the wake near the bottom, and high momentum fluid towards the wake centerline near the surface. This behavior is responsible for the cross gradient in the streamwise velocity profiles as shown by the data obtained. For modeling purposes of well mixed quantities, a (quasi)-two-dimensional approach only holds if the weaker streamwise velocity near the edge of the wake is taken into account. In the case a prediction of depth varying quantities is desired, the cross gradient caused by the secondary circulation needs to be implemented as well which results in the need of a three-dimensional modeling approach. Subject secondary circulationmixing layerroughness patchshallow wake flowturbulence To reference this document use: http://resolver.tudelft.nl/uuid:bb68ada6-c2cf-4dcd-b863-c4b8f0d31617 Embargo date 2013-03-15 Related item https://doi.org/10.4121/uuid:042d0fa4-bdfe-4f4b-885c-9731e80c1ed2 Part of collection Student theses Document type master thesis Rights (c) 2013 Voermans, J.J. Files PDF Thesis_Joey_Voermans_1315 ... 032013.pdf 6.58 MB Close viewer /islandora/object/uuid:bb68ada6-c2cf-4dcd-b863-c4b8f0d31617/datastream/OBJ/view