A Velocity Interferometric Study of the Performance of a

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1 A Velocity Interferometric Study of the Performance of a Gas Gun T. Matsumura*, H. Ohuchit, N. N arayanswami*, A. Sasoh* and K. Takayama* *Shock Wave Research Center, Institute of Fluid Science, Tohoku University Katahira, Aoba, Sendai 980, Japan t SONY Corporation, Kitashinagawa, Shinagawa, Tokyo 141, Japan Abstract. As a successful application of the Velocity Interferometry System for Any Reflector (VISAR) to the performance study of a gas gun, in-bore projectile velocity measurements of a 15 mm bore single-stage gas gun with VISAR are presented. For understanding the gas gun operation analytically, a one-dimensional numerical simulation code was developed and the results were compared to that of the experiments. In addition, in order to verify the accuracy of the VISAR measurement, an optical flow visualization of the flow around the launched projectile with holographic interferometry was also carried out. Key words: Single-stage Gas gun, Velocity measurement, Flow visualization 1. Introduction Gas guns, such as the two-stage light-gas gun and the single-stage gas gun, are capable of accelerating a projectile up to hypervelocity and are applicable to not only aerospace studies but also to the dynamics of shock in condensed matter generated by hypervelocity impacts. Performance studies of gas guns have been conducted (Charters et al. 1957, Canning et al. 1970, Groth and Gottlieb 1988), however, most of them were semi-empirical. It is still an important research topic to determine the optimal performance of gas guns by using advanced measuring techniques of high accuracy. A VISAR (Velocity Interferometer System for Any Reflector, Barker and Hollenbach 1972) has been installed at the Shock Wave Research Center (SWRC) of the Institute of Fluid Science, Tohoku University. This paper reports the results of an application of the VISAR system to in-bore projectile velocity measurement with a 15 mm bore single-stage gas gun. A series of experiments were conducted for a better understanding of the gas gun performance experimentally; i.e., effect of propellant amount and diaphragm rupturing pressure on the projectile velocity. In order to verify the accuracy ofthe VISAR measurement, the supersonic flow around the projectile launched into the test section was visualized by double exposure holographic interferometry. The muzzle velocity of the projectile was predicted from the hologram by considering the drag effect, and was compared to that of the measured velocity with VISAR. 2. Experiments 2.1. Single-stage gas gun The SWRC single-stage gas gun used in the present study consists of a propellant chamber, a launch tube (15 mm bore, 1.1 m long), a flight tube and a test section. Smokeless powder for sports shooting (Nihon Yushi, Co., SS-type) of weight 2 to 10 g is used as the propellant. A high density polyethylene cylinder, 4 g in weight, is used as a projectile. The flight tube forms the initial part of the test section and serves to absorb the muzzle blast and the propellant gases. In order to control the projectile launching velocity, a metal diaphragm of aluminum or steel is set between the launch tube and the propellant chamber. The launch tube and the test section are evacuated to 1 to 10 kpa. For measuring the muzzle velocity of the projectile, pick-up coils are set at the launch tube exit and a small magnet is imbedded in the projectile. The output signal of the pick-up coil is used as a trigger signal for the VISAR operation or for a holographic ruby laser, the light source of the optical flow visualization. Shock Marseille I Editors: R. Brun, L. Z. Dumitrescu Springer-Verlag Berlin Heidelberg 1995

2 270 Performance of a gas gun 2.2. VISAR interferometer system The VISAR interferometer installed in the SWRC is a dual beam type system (ATA Associates, Model 305S). The optical setup for in-bore projectile motion measurement is shown in Fig.I. An argon-ion laser (Spectra Physics, Model 2030) is used as a light source for the VISAR measurement. The interface optical system consists of fiber optic cables and a collimator. In the test section of the gas gun, an expendable mirror of 80x80 mm is set. As shown in Fig.l, a small piece of reflecting tape (Scotchlite tape) is stuck on the projectile front face. test chamber muzzle location projectile design mirror coil projectile 20.0 ~@magnet t scotch lite tape Fig. 1. Optical setup and experimental apparatus Two sets of 2-channel 250 MHz digitizing oscilloscopes (Hewlett Packard, HP5451OA) are used for the data acquisition for the VISAR system, such as output signals of photo multipliers corresponding to beam intensity and sin - cos -components of reflected beam from the Scotchlite tape. A 32 bit CPU personal computer (Hewlett Packard, Vectra Q5/20) is used for the data analysis Optical flow visualization The flowfield around the supersonic projectile shows a bow shock, an expansion fan, a trailing shock and a wake. In order to visualize the flow, various optical flow visualization techniques, such as Schlieren and shadowgraph, for instance, are very useful. In the SWRC, the double exposure holographic interferometry, which has already been successfully used for various shock tube experiments, was applied to the aeroballistic range study with the single-stage gas gun. A holographic ruby laser (Apollo Lasers Inc., Model 22HD, 10 J/pulse) was used as the light source, and the signal from the pick-up coil triggered the laser. By using this image holographic interferometry, a two-dimensional image of the phenomenon is recorded onto the hologram. In the reconstructed hologram, the density change of the flowfield between the two exposure processes is represented by the fringe pattern. This fringe pattern also helps to verify the result of numerical simulation of the supersonic flow around the projectile. From the information of the reconstructed hologram, the muzzle speed of the projectile, Vm, is predicted by the following formula: where Vi is the flying speed of the projectile at the test section, CD is the drag force coefficient of the cylindrical projectile, and L is the distance between the test section and the muzzle. p, A and m are the flow density, cross section area and mass of the projectile, respectively. (1)

3 Performance of a gas gun Numerical simulation In order to understand the physics of the gas gun operation, the various processes within the gun were modeled theoretically. The processes include; (a) propellant combustion, (b) projectile movement and (c) wave dynamics in the gas ahead of the projectile. The wave dynamics (process c) was modeled using a one-dimensional Random Choice (RCM) scheme. The gun barrel (launch tube) geometry, and the geometry of the propellant chamber were accurately specified. For the gas in the launch tube, the governing equations are the one-dimensional nonlinear Euler equations in conservation form, which are written as follows: Ut+Fx=-A+H P 1 [ gu 1-1 da [ pu2 1 U = pu, F = pu + P, A = A d pu [ p(e+~) u(pe+p~+p) x u(pe+p~+p),h= m (2) (3) where p, u, p, 'Y and e denote the density, velocity, pressure, specific heat ratio and the internal energy per unit mass, respectively; A, F/ and q denote the cross-sectional area, rictional and heat loss factors, respectively. The combustion process of the powder explosives used here is modeled by the lumped parameter method of Groth and Gottlieb (1988), Matsumura et al. (1990), Narayanswami et al. (1993). The burning rate of the powder, V, is assumed to be governed by a pressure exponent law of the form: V = (3pO: where p is the average pressure inside the propellant chamber, Q and (3 are the burning rate constants of the powder. In general, these two constants are determined from closed bomb tests employing large amounts of the smokeless powder (even up to hundreds of kilograms in certain cases). However, there is little accurate data available for small amounts of powder «10 g). In this study therefore, the two coefficients (Q and (3) were chosen such that they gave a close fit to a reference set of experimental data. 4. Results and discussion 4.1. VISAR measurements Projectile motion inside the launch tube of the single-stage gas gun was continuously measured with a VISAR interferometer. In the previous study of Matsumura et al. (1990, 1991), the repeatability of this facility was discussed only in terms of the projectile muzzle velocity. By using the VISAR, it is possible to measure the projectile velocity profile along the launch tube continuously and with higher resolution. Fig.2 shows a comparison of the two VISAR measurements of the projectile velocity variations under identical initial conditions. In these shots, an aluminum diaphragm of 1 mm thickness and 6 g and 10 g smokeless powder were used. Very good agreement was obtained for both 6 g and 10 g cases. The maximum difference of velocity in the two cases is less than 4%. The repeatability of the gas gun was thus found to be very good. Figs.3 and 4 show the projectile velocity profiles for various powder weights. In these two series of experiments, aluminum diaphragms of 1 mm thickness and no-diaphragm, respectively, were used. In the no-diaphragm case, it is found that the initial projectile acceleration is lower and its duration of acceleration is longer than when an aluminum diaphragm was used. From comparison of these two figures, it is seen that the muzzle velocity for 2 g powder weight in the no-diaphragm case attained only 80% of that in the diaphragm-on case. For the case of the powder weight exceeding 6 g, there is no difference between the case without diaphragm and that of the 1 mm aluminum diaphragm. Fig.5 shows the projectile velocity profiles for various diaphragms. This figure shows the effect of the diaphragm selection, i.e., the rupturing pressure. Three types of diaphragms, (1) aluminum of 1 mm thickness, (2) mild steel of 0.5 mm and (3) mild steel of 1 mm thickness were used: results were compared with the powder weight fixed to 6 g. By increasing the diaphragm rupturing pressure, the initial acceleration of the projectile is increased and the muzzle velocity

4 272 Performance of a gas gun 2.5 with 1.0 mm aluminum diaphragm!,he time when a projcc1ile passed lhemuzzle 2.5 With 1.0 mm aluminum diaphragm.!the time When a Pfoiectlle passed Ihe muzzle I _ 1.5 ~ 1.0 propeltantweight 6, TIME (ms) Fig. 2. Comparison of projectile velocities (powder weight: 6 g and 10 g) TIME(ms) Fig. 3. Projectile velocities for various powder weights (diaphragm: All mm) veloclly hisiory lor various diaphragm Ithe lime when apfotecllle passed lhe muzzle 2.5 WllMoldoaphragm. l'he~"'()whenap<ojocbklpljssedlh<1muzli() I 1.5 >- ~ 1.0 ~ sleel1 Omm aluminum 1.001m no diaphragm 0.5 propelltmlweight6g TIME{ms) Fig. 4. Projectile velocities for various powder weights (no diaphragm) TIME (ms) Fig. 5. Comparison of projectile velocities for various diaphragms (powder weight: 6 g) is also increased. According to the VISAR measurement, the order of the magnitude of the initial acceleration does not change significantly even if the diaphragm rupturing pressure is changed. This implies that there is an optimal selection of diaphragm strength, which, while keeping the initial acceleration low, can increase the muzzle velocity. In particular, in the case of 1 mm steel diaphragm, the muzzle velocity increased up to 2.16 km/s which represents an increase of 144% from the no diaphragm case (1.5 km/s) Numerical simulation The projectile velocity profile along the launch tube obtained by the numerical simulation was compared with the experimental result obtained with the VISAR. Fig.6 shows the no-diaphragm case. The powder weight in this case is 6 g. Except between the time of 1 to 1.5 ms, and the final part, i.e., for time> 1.6 ms, the average shape of the profile is almost the same. In particular, the initial acceleration process is predicted very well. This implies that the burning rate constants values were selected properly. The burning rate constants in this case are Q = and f3 = X 10-5, respectively. The 1 mm aluminum diaphragm case is shown in Fig.7. Smokeless powder amount in this case is also 6 g. As in the previous case, the average profile of the present simulation is very similar to the experiment. Burning rate constants in this case are Q = 0.79 and f3 = X 10-5 The value of Q is slightly different from that of the no-diaphragm case. This shows that the combustion process of the smokeless powder may be different under the condition with or without using a diaphragm, or under different diaphragm rupturing pressures, even if the powder amount is fixed. As a future work, it must be tested as to whether or not similar results are obtained for other (different) diaphragm cases for a fixed amount of smokeless powder.

5 /1 ::1 o 8 ~ I 04U~ ~ E,,."meot 00 CFD - Performance of a gas gun 273 Experiment CFD j-,~"-tt--'-;---'--"tt,,--,-;---.--rl D.O time(ms) D lime(ms) 12 Fig. 6. Comparison of projectile velocity: CFD vs. experiment (powder weight: 6 g; no diaphragm) Fig. 7. Comparison of projectile velocity: CFD vs. experiment (powder weight: 6 g; 1 mm-al diaphragm) Fig. 8. Reconstructed image hologram (M=4.7) 4.3. Image holographic interferogram A reconstructed image hologram is shown in Fig.S. Smokeless powder of 6 g, and 1 mm aluminum diaphragms were used. Initial pressure in the test section was kpa. A trend of density change is observed in almost all regions behind the bow shock. Symmetric fringes are also observed in the wake region. This suggests that recompression of the flow is occurring in the wake region, and a wake shock is formed. The projectile posture in flight is slightly inclined, however, the flight Mach number of the projectile can be obtained from the stand-off distance of the bow shock. By using Eq.1, the muzzle velocity of the projectile is predicted to be 1.61 km/s. The difference between this predicted value and the other two values of projectile muzzle velocity obtained from (a) the VISAR measurements, and (b) from the numerical simulation, is less than 5% and 10%, respectively. Therefore, the optical flow visualization is very useful not only for quantitative measurements of the flowfield around the flying projectile but also as a verification tool of the projectile muzzle velocity measurement with VISAR interferometer. 5. Summary and future work In order to analyse the performance of a gas gun, the VISAR interferometer technique was applied to in-bore projectile velocity measurement. In order to understand the gas gun operation, one-dimensional numerical simulations were conducted and the results compared with that of the experiments. In addition, the flow around the flying projectile was visualized with double exposure holographic interferometry, and verifed the accuracy of the VISAR measurements. In the next phase, the following aspects will be given further attention:

6 274 Perlonnance of a gas gun (1) Application of the VISAR system to much higher (typically 5 km/s) or lower speed (typically 200 m/s) phenomena. (2) Improvement of the numerical simulation code, for more accurate predictions of the projectile movement. (3) Design of an advanced (more efficient) single-stage gas gun facility. Acknowledgements The authors express their gratitude to Messrs. O. Onodera, H. Ojima and T. Ogawa of the Shock Wave Research Center of the Institute of Fluid Science, Tohoku University for their assistance in conducting the present experiments. Thanks also to our colleagues Mr. S. Kitashima of Chugoku Kayaku Co., Ltd. for their help on firing the powder explosives and to Mr. N. Okoshi, a Graduate Student of the Faculty of Engineering, Tohoku University, for his helpful cooperation. In conducting the VISAR measurements the kind support of Dr. W. Isbell, president of ATA Associates is acknowledged with thanks. References Barker LM, Hollenbach RE (1972) Shock-wave studies of PMMA, Fused silica and Sapphire. J. App!. Phys. 43, 11: Canning TN, Seiff A, James CS (1970) AGARDograph No.138 on Ballistic-Range Technology. pp Groth CPT, Gottlieb JJ (1988) Numerical study of two-stage light-gas hypervelocity projectile aunchers. UTIAS Rep. No.327 Isbell WM (1991) A simplified compact VISAR: concept and construction. Proc. 42nd Aeroballistic Range Association Meeting, Adelaide, South Australia, No.11 Matsumura T, Funabashi S, Saito T, Takayama K (1993) A holographic interferometric study of the axisymmetric supersonic flow around a flying projectile. Rep. Inst. Fluid Sci., Tohoku Univ., Vo!.5, pp Narayanswami NT, Matsumura T, Sasoh A, Saito T, Takayama K (1993) Theoretical prediction of hypervelocity launching device performance. (Abstract submitted to 5th Int. Symp. on Compo Fluid Dyn). Takayama K, Matsumura T, Ohuchi H (1992) Measurements of a projectile motion inside a powder gun with VISAR system. Proc. 43rd Aeroballistic Range Association Meeting, Columbus, OH, Vo!'2, No.29