Restart: Take Off

takeoffThis web site started in 2012 (total 165.000 views) now makes a restart representing the recent breakthrough of the New Theory of Flight revealing for the first time the Secret of Flight. The time line is the following:

  1. DFS = CFD based on first principle physics 1995 –.
  2. Book: Computational Turbulent Incompressible Flow 2007.
  3. Resolution of d’Alembert’s Paradox 2008.
  4. New Theory of Flight 2008 –.
  5. High Lift Workshop: DFS for full aircraft 2017.
  6. DFS as MOOC 2018.
  7. DFS as open source CFD launched by Icarus Digital Math 2018.
  8. Presentation of New Theory of Flight to Boeing 2019.
  9. New Flight Simulator based on DFS initiated 2019.

After a long incubation period the new theory and new computation of DFS thus enters into the real world of flight from design to pilot training, offering entirely new possibilities. This website will track the further development and offer updates of background material on both theory, computation and application.

Its is a travesty that all through the modern era of aviation a physically correct theory of flight has been lacking. NASA reports on its home page 3 incorrect theories, but no theory claimed to be correct. It is now time for the New Theory to replace the text book theory and form a new practice.

Model of Flow Separation

tornado

The holy grail of CFD as computational fluid mechanics is:

  • Turbulence modeling.
  • Flow separation.

DFS as Direct Finite Element Simulation offers answers to these problems:

  • Turbulence captured as best possible computational solution to the Euler equations.
  • Flow separation described as 3d rotational or parallel slip separation.  

To see details of this picture ponder the above picture of the dynamics of a tornado with air sucked horizontally along the land surface towards a low pressure center being redirected into a raising swirling air thus separating from the surface.  Then read this post and think!

Role of Shear Layer: No-Slip vs Slip

The book Computational Turbulent Incompressible Flow (Chap 36) describes in theory and computation the transition to turbulence in parallel shear flow such as Couette flow between two parallel plates and in a laminar boundary layer. The basic mechanism is the action of streamwise vorticity, generated from perturbations in incoming flow, which slowly redistributes the shear flow transversally into high and low speed streamwise flow streaks with increasing transversal velocity gradients, which trigger turbulence when big enough.

Continue here.

Bypass Transition from No-Slip Laminar Boundary Layer to Slip Boundary Condition

skinfriction10

The New Theory of Flight is supported by Direct Finite Element Simulation DFS as best possible computational satisfaction of Euler’s equations expressing first principle physics in the form (i) incompressibility, (ii) momentum balance and (iii) slip boundary condition on solid walls.

Observations and experiments (connecting to the so-called drag crisis) indicate that at a Reynolds number Re of about 1 million the boundary condition at a solid wall changes from no-slip at the wall accompanied with a thin laminar boundary layer, to effectively a slip condition as a thin film without layer.

Continue here.

 

Update of New Theory of Flight

Here is a short update of the New Theory of Flight as concerns the slip/small friction boundary condition which is instrumental, with reference to the last sequence of posts:

  1. The boundary layer of a wing initialised as laminar at stagnation point at leading edge, effectively turns into (acts like) slip with very small skin friction.
  2. This is because transition to a turbulent boundray layer on the leading edge is blocked by wall and damped by acceleration.
  3. The flow once turned into slip on leading edge stays with slip, because transition to turbulent boundary layer is not triggered by slip (no shear).
  4. The net is that the flow around a wing effectively acts as having slip, because transition to a turbulent boundray layer is not triggered by artificial device on leading edge.
  5. The large skin friction from flat plate experiments with artificial tripping should not be used for a wing. If used they give much too big skin friction drag.
  6. The new flight theory builds on slip. With no-slip (laminar or turbulent) the flow separates on crest destroying the functionality of the wing.
  7. We now can see slip as a limit form of a laminar boundray layer with very small skin friction (without the negative aspect of no-slip of 6.), not as a limit form of a turbulent boundary layer with large skin friction, because of “by-pass” as discussed in previous post.
  8. The correct way to add skin friction to DFS is by the friction coefficient of laminar flow, which is an order of magnitude smaller than that of a turbulent bounder layer (used in RANS et cet).
  9. Comparison between experiments for a wing with and without tripping (and other experiments) show skin friction coefficient of size 0.002-3, much bigger than laminar skin friction as shown in this plot:

skinfriction3

On the dream of a “laminar wing”

Without tripping the flow around a common wing under pre-stall conditions thus effectively satisfies a slip boundary condition with the very small friction of a laminar boundary layer, and then without the destructive crest separation from vanishing normal pressure in a laminar boundary layer.

This means that already a common wing realises the dream of very small skin friction drag associated with a “laminar wing” as a wing with a laminar boundary layer.  This explains why the search for further skin friction reduction by e g blowing or suction has not been successful.  To reduce something which is already very small can be very difficult.