Table Of ContentINVESTIGATION OF A VARIABLE GEOMETRY DIFFUSER
FOR A MACH NUMBER 4.0 WIND TUNNEL
A Thesis
Presented to
the Faculty of the Department of Mechanical Engineering
The University of Southern California
In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Aeronautics and Guided Missiles
by
Lee B. James
Arpad A. Kopcsak
Albert F. Rollins
William Teir
June 1950
UMI Number: EP54573
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This thesis, written by
Lee B. James, Arpad A. Kopcsak,
. A lb e r t _ _ F. _ _ Roll ins *.. W.L1 l i m . _T_e_i r...
under the guidance ofiho.lT.. Faculty Committee,
and approved by all its members, has been
presented to and accepted by the Council on
Graduate Study and Research in partial fulfill
ment of the requirements for the degree of
Master of Science in Aeronautics.._and
..Quids d...M.is.aI.l.e.s.
Date.... June.. 1950.
Faculty Committee
TABLE OF CONTENTS
CHAPTER PAGE
I. THE PROBLEM AND DEFINITIONS OF TERMS USED . . . 1
The problem .................................. 1
Statement of the problem ................... I
Importance of the s t u d y ................... 2
Definitions of terms and symbols ............. 2
T e r m s ...................................... 2
Symbols .................................... 2
Subscripts .................................. 3
II. DIFFUSER HISTORY.............................. . 4
III. THEORY AND COMPUTATIONS.......................... 7
Diffuser design and operation ............... 7
Diffuser efficiency .......................... 8
Determination of second throat area ........ 8
Derivation of equation for running time . . . 10
Stagnation pressure losses in the tunnel . . . 12
Computation of running times................ 14
IV. EQUIPMENT, PROCEDURE, AND RESULTS .............. 16
Equipment.................................... 16
General installation . . . . ............... 16
Nozzle blocks .............................. 16
Diffusers.................................. 17
Measuring devices .......................... 17
Quick opening valves....................... 17
ili
CHAPTER PAGE
Procedure and results....................... 17
Construction of a Mach number 4*0 wind
tunnel system ............... • • • • • • 18
Test of wooden wedge diffuser.............. 19
Tests with the flat board replacing the
diffuser.................................. 20
Tests with the rod replacing the diffuser • 22
Tests with increased upstream pressure • • • 23
Test of Mach number distribution • • • • • • 24
V. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS . . . 26
Summary...................................... 26
Conclusions.................................. 27
Recommendations............................. 28
BIBLIOGRAPHY .......................................... 45
LIST OF FIGURES
FIGURE PAGE
1. Side View of the Variable Geometry Diffuser • . • 30
2. Schematic Diagram of the Wind Tunnel System . . . 31
3. Vacuum Tanks and Downstream P i p i n g .......... 32
4. Air Drier..................................... 33
5. Wooden Model of Diffuser in Starting Position . • 34
6* Flat Board Replacing Diffuser .................... 35
7, One Inch Rod Replacing Diffuser.............. 36
8. Mach Number Distribution Probe Rake.......... 37
9* Shadowgraph S y s t e m ........................... 38
10. Schematic Diagram of Downstream Quick Opening
Valve....................................... 39
11. Schematic Diagram of Upstream Quick Opening Valve 40
12. Receding Normal Shock Wave, With Upstream Valve • 41
13. Two Photographs of Oblique Shock Waves, Using
Downstream V a l v e .......................... 42
14. Shock Wave on Measuring Rod During Pressure-
Vacuum R u n ................................. 43
15. Flow Breakdown During Pressure-Vacuum Run . . . . 44
CHAPTER I
THE PROBLEM AND DEFINITIONS OF TERMS USED
The purpose of any wind tunnel diffuser is to maintain
the maximum stagnation pressure by converting dynamic pressure
to static pressure# A larger second throat is necessary for
starting a supersonic wind tunnel than is required during
operation# A diffuser which would have a sufficient second
throat area for starting and a reduced second throat area
after starting would allow the tunnel to start with an initial
high stagnation pressure ratio, and would permit operation at
a reduced stagnation pressure ratio. Since high Mach number
wind tunnels require especially high starting pressure ratios,
such a diffuser would be desirable.
I. THE PROBLEM
Statement of the problem. The purpose of this study
was to design, construct, and test a variable geometry
diffuser to be used in a Mach number 4.0 wind tunnel which
would be operated automatically by the changing static
pressure in the diffuser section of the tunnel.
The problem was divided into four phases.
1. To construct a Mach number 4.0 wind tunnel and to
install the vacuum and piping system.
2. To determine the minimum second throat starting
2
area by testing wooden models of the diffuser in
the starting position.
3. To determine the final design of the variable
geometry diffuser.
4. To construct and test the variable geometry
diffuser.
Importance of the study. The successful completion of
this investigation would increase the efficiency of operation
of high Mach number "blow-down” wind tunnels. It would make
available a diffuser that would permit the tunnel to start
with a high starting pressure ratio, and allow continued
operation at a lower stagnation pressure ratio, or higher
efficiency.
II. DEFINITIONS OF TERMS AND SYMBOLS
Terms. "Blow-down." A "blow-down" wind tunnel Is one
that operates on the pressure differential between atmospheric
pressure and a partial vacuum.
"Core stream." A core stream is a stream of super
sonic flow in a subsonic field.
Symbols.
A Area.
V Volume of vacuum tanks.
T Temperature.
Density.
Velocity,
Viscosity.
Reynolds number.
Coefficient of friction based on wetted area.
Coefficient of friction based on frontal area.
Ratio of specific heats * 1.4.
Pressure•
Speed of sound.
Time.
Mach number.
Dynamic pressure.
Gas constant.
Function of Mach number.
cripts.
Stagnation conditions.
Throat conditions.
Final conditions.
Initial conditions.
Conditions before a change.
Conditions after a change.
Atmospheric conditions.
CHAPTER II
DIFFUSER HISTORY
Efficient diffuser operation is a major problem in
supersonic wind tunnel design. Some of the types of diffusers
which have been proposed are:
1. Normal shock.
2. Multiple oblique shock.
3. Continuous compression.
4. Variable geometry.
The normal shock diffuser consists of a constant area
duct followed by an expanding cross-sectional area. With
supersonic flow and the proper exit pressure a normal shock
wave occurs near the entrance to the diffuser. This shock
wave reduces the flow from supersonic to subsonic, and the
expanding diffuser area further decelerates the flow. This
type of diffuser has poor efficiency because of the single
strong shock wave used to decelerate the flow. At Mach
number 4.0 its stagnation pressure recovery is about 14 per
cent.
The multiple oblique shock diffuser consists of one or
more wedges which produce an oblique shock pattern in the
flow field. These wedges can be arranged in a number of ways.
Three of the more common proposed configurations are the
convergent-divergent duct, the single wedge, and the multiple
wedge.
