Notes on numerical fluid mechanics

A zonal hybrid Reynolds-averaged Navier-Stokes large-eddy simulation (RANS/LES) approach, called zonal-DES, used to handle a two-dimensional high-lift configuration with deployed slat and flap is presented. This method allows to reduce significantly the cost of an accurate numerical prediction of the unsteady flow around wings compared to a complete LES. Some issues concerning grid generation as well as the use of zonal-detached-eddy simulation for a multi-element airfoil are discussed. The basic planar grid has 250,000 points and the finest spanwise grid has 31 points with Az/c = 0.002. The effort is geared toward detailed comparison of the numerical results with the Europiv2 experimental particle image velocimetry data including both mean and fluctuating properties of the velocity field (Arnott and al)

Zonal-detached-eddy simulation of the flow around a high-lift configuration

Buffeting flow on transonic aerofoils serves as a model problem for the more complex three-dimensional flows responsible for aeroplane buffet. The origins of transonic aerofoil buffet are linked to a global instability, which leads to shock oscillations and dramatic lift fluctuations. The problem is analysed using the Reynolds-averaged Navier–Stokes equations, which for the foreseeable future are a necessary approximation to cover the high Reynolds numbers at which transonic buffet occurs. These equations have been shown to reproduce the key physics of transonic aerofoil flows. Results from global-stability analysis are shown to be in good agreement with experiments and numerical simulations. The stability boundary, as a function of the Mach number and angle of attack, consists of an upper and a lower branch – the lower branch shows features consistent with a supercritical bifurcation. The unstable modes provide insight into the basic character of buffeting flow at near-critical conditions and are consistent with fully nonlinear simulations. The results provide further evidence linking the transonic buffet onset to a global instability.

Origin of transonic buffet on aerofoils

Mesh generation: Art or science?

Buffeting flow on transonic aerofoils serves as a model problem for the more complex three-dimensional flows responsible for aeroplane buffet. The origins of transonic aerofoil buffet are linked to a global instability, which leads to shock oscillations and dramatic lift fluctuations. The problem is analysed using the Reynoldsaveraged Navier–Stokes equations, which for the foreseeable future are a necessary approximation to cover the high Reynolds numbers at which transonic buffet occurs. These equations have been shown to reproduce the key physics of transonic aerofoil flows. Results from global-stability analysis are shown to be in good agreement with experiments and numerical simulations. The stability boundary, as a function of the Mach number and angle of attack, consists of an upper and a lower branch – the lower branch shows features consistent with a supercritical bifurcation. The unstable modes provide insight into the basic character of buffeting flow at nearcritical conditions and are consistent with fully nonlinear simulations. The results provide further evidence linking the transonic buffet onset to a global instability.

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Origin of transonic buffet on airfoils.

An evaluation of RANS turbulence modelling for aerodynamic applications

The purpose of this paper is to assess the capabilities of the newly developed tool U-ZEN, a code solving the Unsteady Reynolds Averaged Navier-Stokes equations. The activity is being performed within a CIRA project focusing on Flow Control; the specific goal for the CFD task is to simulate the unsteady flow phenomena due to particular fluid dynamic conditions (Mach, Reynolds, !) or to time dependent boundary conditions (e.g. flow control devices) around complex 3D configurations. The CIRA steady state flow solver ZEN (Zonal Euler Navier-Stokes) has been extended to treat unsteady problems by introducing a time discretization scheme, namely the Dual Time Stepping (DTS) technique. The validation is performed simulating unsteady flows around bluff obstacles or airfoils/wings at high angles of attack which are available experimental test cases.

U-ZEN: A Computational Tool Solving U-RANS Equations For Industrial Unsteady Applications

A numerical investigation of the three-dimensional turbulent flow in a solid rocket motor with a submerged, vectored nozzle is presented in this paper. The simulations have been performed employing an Eulerian-Lagrangian approach: the full Navier-Stokes equations are solved numerically for the gas phase and a Lagrangian deterministic model is adopted for the discrete phase. Computations have been carried out on a realistic segmented solid rocket motor. Both an axisymmetric configuration and a 3-D geometry with a canted nozzle have been studied: a nozzle deflection of 6 degrees has been assumed. Three grid levels have been used in all computations to check grid independence. Very strong 3-D effects are predicted: an asymmetric pressure gradient is established at motor aft-end, and a vortical and circumferential flow pattern in the aft-dome is obtained. Alumina droplets trajectories have been computed for two diameters and different injection points; in most cases, three-dimensional, swirling paths have been predicted. Introduction Many solid rocket motors employ aluminized propellants to increase performance. During the combustion process liquid droplets of aluminum oxide are produced at the propellant grain surface, and are dragged into the combustion chamber by the internal flow. Some of these droplets, under certain circumstances, collect in the aft-end of the booster and give rise to alumina slag deposition which results in motor performance loss', damage of thermal protection, and possible sloshing and ejection of liquid agglomerates through the nozzle;' 3 this, in turn, is believed to cause pressure pulses and thrust imbalance. The possibilities that large lumps of alumina are exhausted through the nozzle is even greater during the thrust vector controlled phases of the vehicle ascent, in which the gimbaled nozzle operates at a certain deflection angle. This gimbaled position of the nozzle may induce strong threedimensional effects in the booster internal flow, especially in proximity of the motor aft-dome. These effects may lead to significant changes in the flow field patterns and in the alumina droplet trajectories with respect to an axisymmetric configuration, which may influence the deposition process significantly. Experimental evidence of 3-D motions in the booster aft-end region exists today. Slag movement and preferential accumulation in one sector of the motor were observed during the testing of a strategic-size solid rocket motor. Also, circumferential flows were observed by Waesche et al. in water tunnel tests when the submerged nozzle was gimbaled. Booster flow field characteristics in the motor aft-dome are very difficult to predict accurately due to the two-phase turbulent nature of the flow, mass injection from the grain surface, regions of strong recirculation and complex combustion chamber geometry, especially in three-dimensional configurations. Several numerical analyses of the internal flow field in a solid rocket motor directed to the investigation of the slag deposition phenomenon have been performed." A variety of models have been adopted: inviscid rotational or viscous flow models, including laminar and turbulent models, for the gas * Aerospace Engineer, Propulsion Section, Head; Member AIAA Aerospace Engineer, Computational Methods Section; currently with the Center for Turbulence Research, Stanford University, Stanford, CA Aerospace Engineer, Computational Methods Section, Head Copyright © 1998 by CIRA. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission American Institute of Aeronautics and Astronautics

Numerical investigation of 3D two-phase turbulent flows in solid rocket motors

The objective of this paper is to investigate the accuracy of Reynolds-Averaged Navier- Stokes (RANS) techniques in predicting the effect of steady and unsteady flow control devices. This is part of a larger effort in applying numerical simulation tools to investigate of the performance of synthetic jets in high Reynolds number turbulent flows. RANS techniques have been successful in predicting isolated synthetic jets as reported by Kral et al. Nevertheless, due to the complex, and inherently unsteady nature of the interaction between the synthetic jet and the external boundary layer flow, it is not clear whether RANS models can represent the turbulence statistics correctly.

RANS Simulation of the Separated Flow over a Bump with Active Control

RANS simulations of boundary layer flows with stationary and pulsating active control devices have been carried out. Two computational codes and several turbulence models have been compared to available experimental data. Two configurations have been selected: a twodimensional bump with a slot and a threedimensional circular, oscillating jet. The results for the first test case indicate that the sensitivity of the results to the choice of the RANS closure is higher for the flow without control. None of the model used allows to predict this baseline flow with satisfactory agreement. The results for the threedimensional problem are in good agreement with the measurements away from the orifice but pronounced differences are observed in the vicinity of the jet.

Marcello Amato

Papers

An evaluation of RANS turbulence modelling for aerodynamic applications

U-ZEN: A Computational Tool Solving U-RANS Equations For Industrial Unsteady Applications

Numerical investigation of 3D two-phase turbulent flows in solid rocket motors

RANS Simulation of the Separated Flow over a Bump with Active Control

Rans modeling and simulations of synthetic jets