torsdag 12 december 2019

CFD State-of-the-Art/NASA 2030 Vision vs DFS

DFS Direct Finite Element Simulation offers revolutionary new possibilities in CFD Computational Fluid Dynamics.

Let us give perspective to DFS starting with the survey of state-of-the-art and future prospects of CFD given by P. Spalart and V. Venkatakrishnan at Boeing as prime user of CFD, presented in 2016 before the 737 Max disaster in 2019:
  1. Boeing and its competitors are very conservative companies, first of all because of their passion for safety, but also because of the extreme industrial consequences of any design mistakes.
  2. Flaws uncovered during assembly or flight test of a new model cause considerable disruptions for the entry into service. The corresponding financial impacts are very large, and the possibility that the new aircraft model would be impossible to certify short of, say, a complete redesign of the wing would be a nightmare. 
  3. As a result, the penetration of CFD is gradual, often involving agreement amongst large communities, from engineers to top managers to company pilots, and acceptance by government agencies such as the Federal Aviation Administration (FAA).
  4. We believe automatic grid adaptation, or ‘self-gridding,’ is a very powerful ingredient of CFD; however, it has proven very difficult, and even the talent in government, industry, and academia and the competition amongst CFD code suppliers have had only modest levels of success.
  5. At present, CFD and wind tunnel are used in a complementary fashion.
  6. Potential new areas for CFD to contribute are in the certification of various phases of an aircraft development
  7. Concerted efforts are needed if much of the database in the flight simulator is to be populated using CFD. 
  8. The level of confidence in CFD when dealing with flow past complex configurations such as high lift (with leading-edge and trailing-edge devices deployed) is considerably less compared to the high-speed clean-wing area. 
  9. To set the stage in our industry, we may consider the problem of calculating the flare and landing maneuver of an airliner, therefore a configuration with high-lift devices, landing gear, spoilers, moving control surfaces, ground effect, thrust reversers, and unsteadiness lasting many seconds.  More specifically, as of today a solution for this landing maneuver that is accurate to the degree needed in our industry is out of reach even with the least costly type of turbulence modelling, namely, RANS.
  10. It is conceivable that computing power will someday make DNS in aeronautics possible, so that modelling proper would disappear, and the turbulence considerations would be reduced to ensuring that the grid and time resolution are adequate. 
  11. In 2000, one of us boldly anticipated this to happen around 2080, but by now we are not confident of this for the 21st century, or even that it will ever happen. 
  12. Our prediction in 2000 that LES would prevail in the 2045 time frame assumed wall modelling, and a few other generous assumptions. 
  13. The widely expected substitution of CFD for the vast majority of ground and flight testing in the aerospace and similar industries, although announced in the 1970s, will take decades from today to complete, gradually expanding from the center to the edges of the operational envelope, from isolation to complete collaboration with other disciplines, and from innocuous to safety-critical decisions. 
In this perspective DFS offers the stunning new possibility of simulating the full flight characteristics of an airplane including critical dynamic moments of start, landing, climb, turn and stall. DFS can be be viewed as DNS in sense of capturing the critical elements of (i) turbulence and (ii) flow separation without turbulence and wall model. This is way beyond the above Boeing perspective.

And even more stunning: DFS is realised with todays super computer power in readily available open source form by Icarus Digital Math.

How is this possible? It seems way beyond the bleak perspective of a conservative Boeing. The breakthrough of DFS has been possible by circumventing the main road block to progress built by Prandtl as the Father of Modern Fluid Mechanics, namely the idea that fluid flow critically depends on the presence of thin boundary layers where a fluid meets a solid wall with a no-slip velocity boundray condition, so thin that computational resolution is impossible with any foreseeable computer power. 

Prandtl, thus claimed that both lift and drag of a wing comes from a thin boundary layer around the wing surface. This connects to the "butterfly effect" as a large effect (tornado in Texas) resulting from a vanishingly small detail/cause (butterfly in Brazil). 

Such an effect can be virtually impossible to actually verify because a vanishingly fine resolution can be needed to computationally resolve the little detail. But to disprove the reality of the effect is possible: Take away the little detail/cause/butterfly and observe that the effect/tornado is still there. 

This is what DFS does (in addition to capturing turbulence): DFS uses a slip/small friction force boundary condition without boundary layer, instead of no-slip with layer, and yet computes lift and drag of a wing in full accordance with observations, and more as full flight characteristics. 

DFS thus breaks the Spell of Prandtl which has paralysed CFD during the 20th century and in the above Boeing perspective will continue to do so during the 21st.  

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