Observations of turbulent flow close to a wall reveal turbulent eddies which are elongated in the flow direction. This has motivated the use of a slender body assumption to simplify the Navier Stokes equations. Derivatives in the flow-direction are neglected so that three velocity components are calculated in a plane. The application of this 2 1/2D model to the viscous wall region (y+ < 40) shows that the turbulent velocity field can be represented by interaction of two eddies with spanwise wavelengths of 100 and 400 wall units. This model has been used to investigate the effect of favorable pressure gradients on a turbulent boundary-layer and to explore what determines the size of the stress producing eddies close to the wall. The accuracy of the basic physical assumptions are explored by examining resulte from a computer simulation of the three-dimensional time dependent turbulent flow in a channel. Some possible improvements are discussed, which make use of the observation that spatial derivatives in the flow direction can be related to time derivatives by using a convection velocity.

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