Mantra of Bluff Body Flow

New Theory of Flight is a spin-off of the following Basic Mantra:

  • bluff body flow = potential flow + 3d rotational slip separation into turbulent wake.

It is a fundamental break from the view of Prandtl as the Father of Modern Fluid Mechanics based on the concept of boundary layer connecting free flow velocity to zero relative velocity on a solid wall.

The Basic Mantra is understandable since potential flow is understandable and so is 3d rotational slip separation. This is a major step forward in fluid mechanics, where  d’Alembert’s paradox from 1755 has prevented understanding lasting into our time.

The large lift and small drag of the flow around a wing is explained by the Basic Mantra in terms of the following two basic elements with slip boundary condition being instrumental:

  1. Wing flow as potential flow stays attached on the (upper) suction side of the wing and thereby generates large lift.
  2. The flow separates at the trailing edge with small drag without the pressure rise of potential flow destroying lift.

The key is thus that wing flow before stall stays close to potential flow until the trailing edge, but there separates differently as quasi-stable 3d rotational slip separation without pressure rise, because potential flow separation is unstable.  Here is the generic picture revealing the secret of flight showing potential flow with zero drag and lift being modified at the trailing edge by streamwise vorticity into flow with large lift and small drag, with L low pressure and H high pressure:

secretflight10

In computational form the Basic Mantra is expressed as DFS Direct Finite Element Simulation as best possible solution of Euler’s equations with a slip/small friction boundary condition. DFS computes lift and drag of a wing in close agreement with observation and also captures stall with flow separation on the suction side  before the trailing edge.

DFS makes fluid mechanics computable, while it is not with Prandtl’s boundary layers beyond computational resolution.

The website unfolds the message of the Basic Mantra. Compare with The Secret.

The Basic Mantra can be summarised as:

  • Bluff body flow as potential flow generates large lift before trailing edge.
  • Bluff body flow with 3d rotational separation keeps large lift while adding small drag.