332 research outputs found
A HIGH-ORDER DISCONTINUOUS GALERKIN SOLVER FOR 3D AERODYNAMIC TURBULENT FLOWS
No full textThis paper deals with the evaluation and validation of a recently developed parallel discontinuous Galerkin code for the numerical solution of the RANS and k-omega turbulence model equations. The main features of the code can be summarized as follows: a) high-order spatial accuracy on hybrid grids, b) fully coupled, implicit time discretization, c) non-standard, realizable k-omega model implementation, d) efficient parallel execution using METIS package for grid partitioning and PETSc library for the linear algebra. The paper reports the numerical results of second- and third-order accurate computations of the subsonic turbulent flow in the Stanitz elbow and of the transonic turbulent flow around the ONERA M6 wing
Algebraic modifications of the k-ω̃ and Spalart–Allmaras turbulence models to predict bypass and separation-induced transition
No full textMany reliable and robust turbulence models are nowadays available for the Reynolds-Averaged Navier-Stokes (RANS) equations to accurately simulate a wide range of engineering flows. However, turbulence models are not suited to correctly described flows with low to moderate Reynolds numbers, which are characterized by strong transitional phenomena. Therefore, numerical models able to accurately predict transitional flows are mandatory to overcome the limits of turbulence models for the efficient design of many industrial applications. The only ways to describe transition are Direct Numerical Simulation (DNS), Large Eddy Simulation (LES), and transition models, where the computational cost of DNS and LES is still too high for their routine use in industry. A modified version of the k-(omega) over tilde and Spalart-Allmaras turbulence models is here proposed to predict transition due to the bypass and separation-induced modes. The modifications are based on the gamma k-(omega) over tilde and the SA-BCM models and avoid complex formulations of transport equations ad-hoc defined for transition. Both the transition models are correlation-based algebraic models that rely only on local flow information and an intermittency function, which damps the turbulent production according to some transition onset requirements. The proposed transition models are implemented in a high-order discontinuous Galerkin (dG) solver and validated on benchmark cases from the ERCOFTAC suite to the Eppler 387 airfoil, with different transition mode, freestream Reynolds number and turbulent intensity, and pressure gradient
Entropy conserving implicit time integration in a Discontinuous Galerkin solver in entropy variables
No full textThis article presents a fully discrete entropy conserving/stable method based on a Discontinuous Galerkin (DG) discretization in entropy variables coupled with a modified Crank-Nicolson scheme. The entropy conserving time integration is inspired by the work of LeFloch [1], originally developed in the context of a Finite Volume method in conservative variables. This entropy conserving time integrator is here adapted to a DG discretization in entropy variables also demonstrating the fulfilment of entropy conservation regardless of the time step size and the type of elements used (quadrangular or triangular elements, possibly with curved edges). The performance of the implicit method will be demonstrated by computing several inviscid flow problems, i.e., the convection of an isentropic vortex, the double shear layer, the Kelvin-Helmholtz instability, the shedding flow past a triangular wedge, the Sod shock tube, the receding flow and the Taylor-Green vortex.(c) 2022 Elsevier Inc. All rights reserved
A PARALLEL HIGH-ORDER DISCONTINUOUS GALERKIN SOLVER APPLIED TO COMPLEX THREE-DIMENSIONAL TURBULENT FLOWS
No full textA high-order discontinuous Galerkin method for natural convection problems
No full textDiscontinuous Galerkin (DG) methods have proved to be very well suited for the construction of robust high-order numerical schemes on unstructured and possibly non conforming grids for a wide variety of problems. In this paper we consider natural convection flow problems and present a high-order DG method for their numerical solution. The governing equations are the incompressible Navier-Stokes (INS) with the Boussinesq approximation to represent buoyancy effects and the energy equation to describe the temperature field. The method here presented is an extension to natural convection flows of a novel high-order DG method for the numerical solution of the INS quations, recently proposed in [1]. The distinguishing feature of this method is the formulation of the inviscid interface flux which is based on the solution of local Riemann problems associated with the artificial compressibility perturbation of the incompressible Euler equations. The discretization of the viscous term follows the well established DG scheme named BR2 [2, 3]. The method is fully implicit and the solution is advanced in time using either a first order backward Euler or a second order Runge-Kutta scheme. To assess the capabilities of the DG method developed in this paper we computed second-, third-and fourth-order space-accurate solutions of several benchmark problems on natural convection in two-dimensional cavities
FULLY-DISCRETE ENTROPY CONSERVING/STABLE DISCONTINUOUS GALERKIN SOLVER FOR UNSTEADY COMPRESSIBLE VISCOUS FLOWS
No full textThe aim of this work is to contribute to the development of a high-order accurate discretization that is entropy conserving and entropy stable both in space and in time. To do this, the general framework is based on a high-order accurate discontinuous Galerkin (dG) method in space with entropy working variables, several entropy conservative and stable numerical fluxes and an entropy conserving modified Crank-Nicolson method. We present the first results, obtained with the discretizations here proposed, for two bi-dimensional unsteady viscous test-case: The Taylor-Green vortex and the double shear layer
On the kinematics and dynamics parameters governing the flow in oscillating foils
No full textBased on a high-order implicit discontinuous Galerkin method, numerical simulations of a two-dimensional oscillating foil are performed to explore the origin of basic aspects of the flow such as the generation of interesting flow structures in the wake and the associated aerodynamic forces. Dimensional arguments suggest that the flow is characterized by non dimensional aerodynamic coefficients depending on the kinematics of the oscillation, such its frequency and amplitude, and on the dynamics of the flow, such as the Reynolds number. Most of the studies have concentrated their attention on the role played by the kinematic of the oscillation with less or no attention to the effect of the Reynolds number. Here, we show that this effect cannot be neglected in the study of the phenomena at the basis of the generation of lift and thrust. We found that the Reynolds number plays a fundamental role for the development of thrust by defining critical values Rec for the switch from drag to thrust conditions. It is also shown that for Re>Rec, the Reynolds number defines additional subcritical values which are at the basis of flow instabilities leading to smooth and sharp transitions of the structure of the wake and of the related aerodynamic forces. For the analysis of the behaviour of the flow, the space of phases composed by the instantaneous lift and thrust (cL,cT) is introduced. It is shown how the orbits in the (cL,cT)-space allow us for a clear understanding of the physical evolution of the flow system and of the cyclical phenomena composing it
Numerical experiments in separating and reattaching flows
No full textWe report high-order implicit large Eddy simulations of flows around flat plates with massive flow separation and reattachment. The aim is to provide evidence of the influence of relevant flow parameters such as the geometry of the leading-edge corner, the presence of a trailing-edge flow separation, and of a flow coupling between the two sides of the plate. The results reveal that flows with right-angled corners develop taller flow recirculations, which promote very-slow instability of the bubble itself. This large-scale unsteadiness is then found to be the basis of negative turbulence production mechanisms that in turn enhance the height of the bubble itself, thus closing a self-sustained cycle. The absence of these phenomena in flows with smooth leading-edge corners is also found to explain their high sensitivity to free-stream turbulence. The observed behaviors may have strong repercussions for theories and closures of separating and reattaching flows and should be carefully taken into account in control strategies used in the applications
Investigation of flow phenomena in air-water safety relief valves by means of a discontinuous Galerkin solver
No full textSafety valves are mechanical devices designed to protect a system (typically a pressure vessel) against
excessive pressure; the operational behavior of spring loaded safety valves is governed by the difference
between the reclosing force operated by the spring and the opening force operated by the pressure field
acting on the opening device surface. The resulting flow path inside the valve body is very complicated
due to sharp edges and sudden curvature characterizing the device geometry, and to complex flow phenomena
like shock waves and supersonic expansions occurring when operating with compressible flows.
As a consequence, the design of safety relief valves is an hard task.
In some cases safety relief valves are required to operate with both compressible and incompressible
flows, e.g. in shell-type water–gas heat exchangers, but indeed the device behavior may significantly vary
when operating with different fluids. Since the valve performance is strongly dependent on the fluid
properties, a particular design of the valve trim (i.e. of the flow path) is thus required in order to guarantee
proper functional characteristics when the valve is working either with gases or liquids.
In this work an accurate Discontinuous Galerkin (DG) solver is applied to compute higher-order
approximations of the flow field within a 200 J 300 safety valve (according to API 526) designed for
steam–water double protection. The investigation aims at clarifying the role played by viscous losses
and compressibility effects in the discharge capability of the device when working with air or water.
The main features of the code used for incompressible flows relying on a fully implicit high-order DG discretization
of the coupled RANS and k–x turbulence model equations are also described
Visible Learning and synchronous online lesson in higher education: a study in engineering education
No full textThe Visible Learning (VL) approach to learning processes stems from Hattie's work based on synthesising meta-analyses regarding achievement in education. Although the model is used at many levels of instruction, its performance has been less studied in higher education, engineering education, and in the context of synchronous online learning in distance education. This study implements VL features and analyzes their ability to improve learning outcomes and teaching quality. To this end, a synchronous online lesson in a Fluid Dynamics course was implemented with 39 mechanical engineering students. The research method is a one-group pretest-posttest design and data were collected through a test and a 5-point Likert scale questionnaire. The learning achievement is measured using Cohen's d. The relevant effect size value obtained (d =2.32) stands out from those in the literature where meta-meta-analyses report an impact on learning close to that of a traditional lecture (d=0.08). Regarding the learning experience, students' ratings of both the lesson and the teacher's teaching quality are clearly positive. It can be concluded that the VL approach can produce significant learning gains and positive perceptions of instructional quality among students in the context of synchronous online instruction in engineering education
- …
