1,720,981 research outputs found
Influence of the thermic boundary conditions on the high speed impulsive flow around a thick flat plate
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Model for Enhancing Turbulent Production in Laminar Separation Bubbles
No full textLaminar separation bubbles are one of the main critical aspects of flows at low Reynolds numbers in the range of 104 –105 . The flow separates in the laminar regime, the turbulence developing inside the recirculation region enhances the momentum transport, and the flow can reattach. Models based on the Reynolds-averaged Navier–Stokes equations suffer two of main issues: the determination of the transition onset and the level of the pressure recovery downstream of the reattachment of the flow. A model addressing both issues is presented in this paper. It is based on the γ transition model for the transition detection. The production of the turbulent kinetic energy κ has been properly enhanced thanks to a correlation found between the necessary boosting of κ and the intermittency function behavior within the bubble. The low-Mach-number and Reynolds-number flows around the Selig–Donovan 7003, Eppler 387, and NACA 0015 airfoils are analyzed. The results are compared to experimental data and large-eddy simulations available in the literature. The model can be applied to the analysis of an arbitrary airfoil without need of preliminary calculation of the transition point within the bubble
Definition of an invariant lamb-vector-based aerodynamic force breakdown using far-field flow symmetries
No full textSome recent developments in the Lamb-vector-based aerodynamic force breakdown used the concept of vortex force to define the lift and to decompose the drag into lift-induced drag and profile drag. However, the Lamb vector formulation involves moment transformations, and the associated force breakdown may depend on the reference point adopted for their computation. Yet, the force acting on an airplane cannot be dependent on this point. Thus, a systematic method based on the far-field flow symmetries is here proposed to eliminate this dependence. Those symmetries also allow for a better understanding of the progressive invariance of the drag decomposition with respect to the size of the integration domain. Finally, a reference-point-invariant formulation is defined and tested for a 2-D transonic flow around an OAT15A airfoil and a 3-D flow around the NASA Common Research Model under cruise flight conditions
Riblets and form drag
No full textThe effectiveness of riblets in reducing friction drag has been well assessed by many experiments in flat plate flow in the end of the last century. Both theory and experiments showed that the effect of riblets only depends on the local Reynolds number, however some experiments in pressure gradient flow showed increased effectiveness of riblets. This effects are still debated because a physical motivation on the effect of riblets on form drag was not provided. In the present paper a numerical method for riblets simulation at high Reynolds numbers is presented and the effect of riblets on form drag is discussed providing a contribution to clarify the origin of this second-order but significant effect
INFLUENCE OF THE THERMIC BOUNDARY CONDITIONS ON THE HIGH SPEED IMPULSIVE FLOW AROUND A THICK FLAT PLATE
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The start-up vortex issuing from a semi-infinite flat plate
No full textThe subject of the present work is the start-up vortex issuing from a sharp trailing edge accelerated from rest in still air. A numerical simulation of the flow has been performed in the case of a semi-infinite flat plate by solving the Navier-Stokes equations in the psi-omega formulation. The numerical algorithm is based on a fast multigrid implicit integration of the difference equations in an unstructured mesh that is dynamically built to minimize the computational costs. A local refinement of the mesh near the edge of the plate increases the accuracy of the simulation. The results show that the asymptotic stage of the vortex evolution is self-similar in the mean, but the appearance of instabilities produces a time-dependent flow which is not instantaneously self-similar
Evaluation of Seismic Vulnerability of Santa Maria del Mar in Barcelona by an Integrated Approach Based on Terrestrial Laser Scanner and Finite Element Modelling
No full textStructural assessment of historical constructions is a particular issue as these structures were built with technical knowledge and construction methods based on in-field experience and on the observation of previous failures and successes of similar buildings. Their structural behavior is influenced by numerous aspects, sometimes not completely known (geometry, structural schemes and details, material mechanical properties), which often results in historical structures inadequate to achieve the safety level required for new constructions. In the case of earthquakes, these structures can suffer heavy local or global damages that sometimes lead to collapse. Thus static and seismic vulnerability assessment of historical buildings should therefore be performed by proper methodologies that differ from those used for new constructions. In Italy, an official document (Direttiva del Presidente del Consiglio dei Ministri [DPCM] 2007, revised in 2011) was released, defining a step-by-step procedure for data acquisition and vulnerability assessment of Cultural Heritage buildings. Such procedures can be coupled with new survey technologies such as terrestrial laser scanning (TLS), which, for performing a high-detailed three-dimensional geometrical survey, can provide detailed structural models refining the assessment results. In this study, the DPCM procedure is applied to Santa Maria del Mar Church in Barcelona (Spain) for seismic vulnerability assessment, using TLS three-dimensional survey and finite element analysis
Gradient based empirical cumulative distribution function approximation for robust aerodynamic design
No full textA robust optimization approach based on the use of the conditional value at risk function is presented, together with an application to a robust transonic aerodynamic design problem of the central section of a Blended Wing-Body configuration. The conditional value at risk is estimated using an approach based on the empirical cumulative probability distribution function. The quantities of interest of the risk function are the aerodynamic characteristics of the airfoil, namely lift, drag, and pitching moment coefficients, computed solving the Reynolds-averaged Navier-Stokes equations with the open-source fluid-dynamic solver SU2. Conditional value at risk computation is costly, so techniques and methods for the reduction of the computational cost are introduced. In particular, the empirical cumulative distribution function is approximated with a first-order series expansion using efficiently calculated gradients from SU2 discrete adjoint solver
Effect of body shape on riblets performance
No full textThe effect of partial slip flow on airfoil performance at high Reynolds numbers is analyzed in this paper. The link between the physical mechanism of drag reduction attained by many devices and the slip length concept has been well assessed in the literature. A slip length model is therefore here adopted in large eddy simulations to quantify the effect of slip flow on airfoil performance. The possibility to adopt a slip flow boundary condition to simulate riblets on airfoil is verified. Their effectiveness in reducing friction drag in turbulent flow has been well assessed since the end of the last century. Both theory and experiments proved that the effect of riblets only depends on the local Reynolds number. However, some experiments showed an increased effectiveness of riblets in the presence of pressure gradient. This secondary effect is still being debated and a physical explanation has not been found. This paper has the aim to provide a contribution to the understanding of this phenomenon. Large eddy simulations of flows around airfoils are proposed with an extensive analysis of riblet performance, obtained by a proper slip flow boundary condition. It is shown that riblets reduce the boundary layer displacement thickness inducing small but significant modifications to the pressure distribution, in particular in the adverse pressure gradient region. The reduced thickening of the equivalent body is the reason for the reduced form drag
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