General Stmilartty Hypothests for Jet and Wake Flows 
Since with the classical similarity assumptions, these approximations 
lead to different equations, predictions for general free-jet problems 
can be obtained in the asymptotic ranges at best. To overcome these 
deficiencies, different suggestions have been made. For example, 
Wygnanski and Fiedler [5] succeeded in solving the equation of motion 
with the help of the conventional similarity assumptions for the special 
case of a general laminar free jet with tailored pressure gradient in 
the outer stream. Unfortunately, the solutions are mainly of academic 
interest, because such special cases are seldom realized in practice. 
To obtain solutions for the development of any free-turbulence 
shear flow in an incompressible fluid, the senior author has derived a 
new, more general similarity analysis [6] . Through the formulation 
of similarity functions based on three dynamic scales, mean flow as 
well as turbulence characteristics were shown to have a strong ten- 
dency toward self-preserving profiles even for such flows as the tur- 
bulent wake of a self-propelled body and turbulent jets in moving 
coaxial and cross streams. All solutions were satisfactorily verified 
by comparison with experimental data Ee 8 | : 
The key part of the new similarity analysis is the replacement 
of the conventional velocity-type representation of the mean flow field 
by a momentum-type description. It is the main purpose of this paper 
to provide further evidence of the validity of this assumption. In order 
to achieve this, the new approach is applied to flows, for which the 
similarity assumption suffices to produce solutions without the intro- 
duction of unreliable hypotheses concerning the structure of turbulence. 
As flows to be described we have chosen laminar jets in uniform un- 
confined streams. In this way, we are not only able to perform a cru- 
cial test of the key assumption of the new theory ; at the same time, 
we can obtain more reliable predictions for the case of unconfined la- 
minar free-jet flow, and this not only in the asymptotic ranges but in 
the transition region as well, which is of most practical interest. 
THEORETICAL ANALYSIS 
From the observation of a jet developing in a uniform coflowing 
stream, one is led to accept that the flow can be separated in an irro- 
tational outer flow (potential flow) and a shear zone extending only 
slowly in the lateral direction. Since this narrow zone, which is domi- 
nated by large lateral velocity gradients, is characterized by a boundary- 
layer type of flow, the governing Navier-Stokes equations can be appro- 
ximated for large Reynolds numbers by the boundary-layer equations 
given by Prandtl. The same simplifications hold also for all other per- 
tinent transport equations. This procedure has been repeatedly verified 
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