The present projects aims at shedding light onto the structure of turbulence in ducts and channels with complex shape. A primary objective will be characterizing the nature and strength of the secondary motions which arise, and which are not present in canonical flows, e.g. plane channel and pipe flow. Direct numerical simulations (DNS) will be used as a high-fidelity tool to obtain full information on the statistical properties of the flows. Several geometries will be considered, including rectangular, trapezoidal and semi-circular ducts, with either closed or open upper boundary, hence also covering flows relevant for the hydraulic engineering. The numerical solver will rely on the immersed-boundary method, which will also be used to carry out numerical simulations of rectangular duct flow with embedded complex geometries, as those found for instance in the cooling channels of rocket engines. The DNS database herein developed will be used as a basis for the assessment and development of advanced turbulence models based on the Reynolds-averaged Navier-Stokes equations. Significant impact of the study is foreseen in terms of improved prediction of heat and momentum transfer in internal flows, and of mechanisms of sedimentation and erosion in closed and open channels.
As stated in the description of the state-of-the-art of the subject, it appears that high-fidelity numerical simulations of complex turbulent flows is ducts are all but lacking. This is mainly because of the reduced efficiency of incompressible fluid solvers for body-conformal meshes, coupled with the large number of nodes (cells) demanded by DNS. To the best of our knowledge, the use of the immersed-boundary (IB) method for DNS of internal turbulent flows has never been attempted. Although the use of the IB method prevents clever use of grid stretching in the wall-normal directions, the vastly larger computational efficiency and parallel scalability of numerical algorithms for Cartesian meshes still guarantees feasibility of the numerical simulations. We believe that the adoption of high-fidelity numerical solvers for the study of complex duct flows can yield substantial advancement with respect to the state-of-the-art in at least two respects: i) greater insight into the complex phenomena involved; ii) development of advanced turbulence models for simplified engineering prediction. The second item is especially important as flows of technological relevance invariably feature high Reynolds number which cannot be directly accessed through DNS. In hydraulics, models for turbulent transport of sediments and erosion are of utmost importance, and in this respect having a correct representation of the shape and intensity of secondary currents would clearly allow greatly improved predictive capabilities. In the aerospace industry, the correct prediction of friction and heat transfer associated with the use of metallic foams will certainly help bring those innovative components of cooling systems to routine use in rocket engines.
In summary, we believe that the activities to be carried out within th present project will improve the understanding of complex physical phenomena by now not fully understood, but at the same time they will have significant impact from the standpoint of the everyday engineering design.
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