INVESTIGATION 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 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI EP54573 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 A& Or 'So JZl 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.