aerospace
characteristics of turbulence,’ CD-adapco’s Mendonca added. ‘Instead, we try to solve these characteristics implicitly from equations rather than the models themselves. Tere is a lot of effort being made towards integrated solutions with high level physics and integrated processes.’ Te faster the industry can get answers with complex turbulence models based on large eddy simulations, the better, he said.
The hybrid approach Te intense computational demands of large eddy simulations have led to the development of hybrid models that combine both RANS and LES approaches. Academic institutions, research organisations, and commercial companies have been collaborating in recent years on the progression of these models. One example is the EU-funded project, Desider (Detached Eddy Simulation for Industrial Aerodynamics), which brought together 18 partners from the European Union and Russia, representing industry, research institutions and universities, to improve existing computational fluid dynamics (CFD) methods and demonstrate the capabilities of hybrid RANS- LES approaches. Te development group at Ansys actively
participated in this project. Brian Bell commented on the merit of the research: ‘Tese hybrid models make it possible to solve problems at realistic Reynolds numbers with, potentially, orders of magnitude reduction in the computation expense. Tey make it possible to predict the boundary layer turbulence without the stringent grid resolution requirements of large eddy simulations. ‘Tese simulations can now be used for
applications that are simply not possible to do purely with LES at the level of computing power we have today. Ultimately, engineers can obtain more precise information from simulation.’ Bell warned, however, that these are not mature models and therefore require a lot
Patran/Nastran aero-structure coupling for aeroelasticity analysis
of understanding regarding their performance and how they should be applied. A big driver behind projects such as
Desider is the demand for higher fidelity and predictive accuracy, without the corresponding computational cost. MSC Soſtware’s Doug Neill has observed that high-fidelity CFD is being used to produce time domain equations for the calculation of noise and fatigue. ‘But where we truly see CFD and turbulence and gust load alleviation coming to bear is in the flight mechanics, such as the control system design,’
THE INTENSE
COMPUTATIONAL DEMANDS OF LARGE EDDY SIMULATIONS HAVE LED TO THE DEVELOPMENT OF HYBRID MODELS
he said. ‘Te actual vehicle design criteria for gusts and loads are being developed with much more simplified aerodynamics, and that seems to work acceptably, so far. Te problem is that the moment you get high-fidelity fluid dynamics you also need a pretty high-fidelity structural model or you start to measure spurious elastic effects.’ Neill continued by saying that, whereas simplified aerodynamics would ignore these, CFD won’t. ‘Ten you get
unintended interactions that are wrong. It’s not about increasing the fidelity; it’s about getting the right fidelity,’ he added.
Finding the right solution All CFD soſtware must, to a certain extent, handle turbulent flow. Choosing the solution that best meets each user’s needs comes down to the breadth of capabilities and the level of technical support being offered. MSC Soſtware began in the aerospace industry and the company’s solutions cover the spectrum from external loads and aero-acoustics to the mechanical behaviour of advanced composite materials. Te simulation suite is designed to enable aerospace companies to apply advanced materials to help them solve their regulatory compliance issues. Doug Neill stated that MSC Soſtware is continuing to push its OEM partners in the industry to increase its use of simulation in the certification of aircraſt. CD-adapco’s solution, STAR-CCM+, is able to
solve Navier-Stokes equations with any level of turbulence assumptions. Te code is integrated in that this level of very complex physical modelling is built into the menu choice for modelling the physics. Te complex geometry of an aircraſt, such as the wings and landing gear, can be modelled and then automatically translated into a computational model. Te full geometry is taken and then discretised. Once the physics have been solved on this virtual model, the data needs to be processed in order to understand what the pressure loading on the system is or the effect of liſt and drag on individual components. Each step of the process is integrated so that, if engineers wish to go back and make changes to the geometry, the soſtware will automatically go through the remembered steps. Design optimisation is critical – especially
Flow over generic fighter airplane configuration. Left: flow structures; right: comparison of experiments and SAS axial flow component
www.scientific-computing.com l @scwmagazine
with industry pressures regarding fuel economy and regulatory compliance. Esteco’s platform, modeFrontier, automates the process of executing simulations and then runs optimisation algorithms on top of that. Te solution aggregates multiple simulations, changing parameters for the models or for the design configurations, in order to reach the optimal values. Turbulence models need to be calibrated in many situations, and calibration itself is a complex task that requires optimisation techniques. Because
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EADS Germany GmbH - Military Air Systems
MSC Software
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