Investigation of a variable Mach number supersonic tunnel with nonintersecting characteristics Page: 6 of 29
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NACA RM No. E8J13 a 5
At the Mach number for which the straightening wall is designed,
the test section is bounded by the final-expansion Mach line, the
tangent plate to the expansion cylinder, and a Mach line originating
at the end of curvature of the straightening wall (fig. 2). (The
straightening wall is assumed to be terminated at this point.) The
resultant test section is triangular. For operation at Mach numbers
below the design value (fig. 3(a)), the same restrictions govern
the test section except that the final-expansion Mach line inter-
oepts the straightening wall at some point on the curved section.
The flow therefore undergoes a compressive turning downstream of
this point, the compression region being bounded by a Mach line.
The lower limit of operation of the tunnel occurs when compression
downstream of the test section causes choking in the passage between
the straightening wall and the tangential plate.
For tunnel operation at Mach numbers greater than the design
value (fig. 3(b)), a flow expansion occurs in the region between the
expansion Mach line corresponding to the design Maoh number and the
final-expansion Mach line. The expansion region is bounded by a
Mach line so that the effective test section is again triangular.
The theoretical maximum test-section lengths over a range of
Mach numbers of a variable Mach number tunnel of the type shown in
figures 2 and 3 are compared in figure 4 with the test-section
lengths of conventional syrmetrical-expansion tunnels having either
constant test-section heights or constant mass flows. In order to
make the results nondimensional, the calculated test-section lengths
were divided by the test-section heights at M = 2.0. The variable
Mach number tunnel was assumed to be designed for M = 2.0 with oper-
ation between M = 1.5 and M = 2.5 and to have a cylinder-radius
to throat-height ratio R/rO of 1.0. The maximum test-section
length for all three tunnels was assumed to be the distance required
for a Mach line originating on the center line of the test section
to be reflected back to the center line. The center line of the
variable Mach number tunnel with nonintersecting characteristics
was taken at one-half the maximum test-section height at each con-
dition.
In conventional symmetrical-expansion tunnels having fixed
test-section dimensions, the test-section length-height ratio varies
only as the cotangent of the Mach angle. The test-section height
of conventional tunnels with constant mass flow, on the other hand,
so varies with Maoh number that the length-height ratio of figure 4
is less than for the fixed-test-section tunnel at Mach numbers
below 2.0 and greater for Mach numbers above 2.0.
t_
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Evvard, John C. & Wyatt, DeMarquis D. Investigation of a variable Mach number supersonic tunnel with nonintersecting characteristics, report, November 15, 1948; (https://digital.library.unt.edu/ark:/67531/metadc64572/m1/6/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.