## Use of suction or blowing to prevent separation of a turbulent boundary layer

##### Authors

Dodds, James Ian

##### Date

1961-02-21##### Awarding Institution

University of Cambridge

##### Author Affiliation

Faculty of Engineering.

##### Qualification

Doctor of Philosophy (PhD)

##### Type

Thesis

##### Metadata

Show full item record##### Citation

Dodds, J. I. (1961). Use of suction or blowing to prevent separation of a turbulent boundary layer (Doctoral thesis). https://doi.org/10.17863/CAM.14127

##### Description

The work described divides itself into three parts. The first of
these describes an experimental investigation into the influence of a line
sink on a turbulent boundary layer, the object of which was to ascertain
the overall effect on the values of boundary layer thickness and. mean
velocity profile shape factor of removing a given amount of fluid. To
this end, an axisymmetrical boundary layer duct was constructed. W1 thin
the limitations of the experimental investigation, which was restricted
to the case of only one initial value of shape-factor, it was found
possible to represent the effect of a suction strip on a boundary layer
in a semi-empirical manner. It was also apparent that the transient
effects as represented by the lack of universality of the mean velocity
distribution only persisted for a limited extent downstream of the suction
strip.
The second part of this work considers the problem of the optimum
. distribution of suction in order to suppress the separation of a boundary
layer. A fairly comprehensive theoretical treatment of this problem is
presented which can be used to define the distribution of suction for
any surface over which the boundary layer flow is essentially two dimensional.
The basis of this approach is that the suction distribution
can be defined by specifying .an upper limit on the value of either one
of the two parameters which are normally taken as defining the state at
a boundary layer, i.e., momentum thickness and shape factor. The precise
value lot this upper limit is defined. by the condition that the suction
power required should be a minimum. A series of calculations have been Wldertaken which illustrate the general validity of this approach and. which
:f'urther result in a prediction of the minimum suction quantity necessary
in order to obtain a given lift coefficient. These results may be used as
the basis tor a project study of an aircraft which utilises this type o£'
boundary layer control, and also as a starting point for an experimental
investigation which would introduce the influence of the various methods
of attaining an idealised porous surface in practise.
The third part of the work considers the alternative of boundary
layer control by tangential blowing. Experimental measurements on a plane
wall jet are compared with Glauert' s theoretical predictions and it is
noted that, whereas the basic idea behind Gl.auert's approach is confirmed,
some of the detailed predictions show significant discrepancies. The
existence of a region of universal mean velocity distribution near the
surface is confirmed.
A method of caloulating the streamwise variation of the maximum
velocity of a wall jet is proposed which is based on the principles of
similarity of the mean velocity distribution, continuity and variation
of momentum due to the action of the surface shearing force. consideration
is made of the effects of . surface curvature and the superposition of a
tree stream on the development of a wall jet and it is noted that the
latter effect is small. In the case of :flap blowing, it is shown that
the non-dimensional blowing momentum coefficient can be interpreted
directly in terms of the value of the ratio of maximum jet velocity to
local stream velocity where both are measured at the trailing edge.