Comparative Tests of Adjusted Inlet in Blow-Down and Hot-Shot Wind Tunnels

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1 Proceedings of the 8 th International Symposium on Experimental and Computational Aerothermodynamics of Internal Flows Lyon, July 27 Paper reference : ISAIF8-49 Comparative Tests of Adjusted Inlet in Blow-Down and Hot-Shot Wind Tunnels Francois Falempin, Eric Wendling MBDA-France, 2 rue Beranger BP CHATILLION Cedex - France Marat Goldfeld, Aleksey Starov, Konstantin Timofeev Institute of Theoretical and Applied Mechanics, Institutskaya str., 4/1, 639 Novosibirsk, Russia The subject of this paper is experimental investigations of the hypersonic inlets with different methods of control. The conditions of boundary layer separation and the possibility of separation prevention by diffuser configuration changing and boundary layer bleed were research. The inlet model tests were fulfilled in blow-down and hot-shot wind tunnel at Mach numbers from 2 to 6 and from 5 to 8, correspondingly. Following measurements were performed: static and total pressure distribution on the walls and in the characteristics, airflow ratio, and total pressure recovery. It was obtained that the porous bleed is an effective procedure to achieve inlet start. The porous bleed allows achieving more than twice the growth of the airflow ratio on the model with sidewalls. It was also established that the downstream porous bleed was the most efficient. The possibility of shock start was checked in the blow-down wind tunnel. It was shown that the shock start could be realized at the tests in blow-down wind tunnel. The characteristics obtained in the blow-down wind tunnel coincide with the data obtained in the hot-shot wind tunnel with time operation 1-15ms. In the last case, the shock start happens naturally and simultaneously with the shock start of the wind tunnel. Keywords: inlet, boundary layer, wind tunnel, shock start, porous bleed Introduction For the design of future hypersonic space transportation planes, air-breathing propulsion is one of the key technologies. The inlet particularly is of great importance for the efficiency of the propulsion system and, therefore, for the maximal attainable payload of the whole transportation plane [1-3]. In airframe-integrated scramjet, the vehicle forebody performs the initial compression in bow-shock. In these configurations, hypersonic inlet must swallow the thick boundary layers that develop on the airframe of space planes. In hypersonic inlets, viscous effects have an important influence on the inlet flow field and, by this, on the efficiency of the inlet system. One of the dominating flow phenomena in a hypersonic inlet is the interaction of oblique shocks induced by the inlet ramps with the boundary layer developing on the inlet walls [4]. Francois FALEMPIN: Deputy Director of Department

2 2 Proceedings of the 8 th International Symposium on Experimental and Computational Aerothermodynamics of Internal Flows Nomenclature M Mach number PB porous bleed P static pressure SW inlet side walls T temperature f mass flow coefficient Re unit Reynolds number (1/m) Greek letters x longitudinal coordinate ν total pressure recovery EG exit gate of bleed system Subscripts SD starting device free stream parameters The flow on compression surfaces of the inlet is characterized by shock-shock interaction, shockboundary-layer interaction, expansion-fan-boundary -layer-interaction, and development of thick turbulent boundary layers over the forebody, which can lead to adverse effects on the inlet performance [5, 6]. The separation of the ingested boundary layer promotes engine stall as a result of inlet unstart. To prevent the engine unstart and to extend the engine operating range, a boundary layer bleeding systems were researched for different inlets [7] One of the main problems of ground testing is providing relevant flow parameters and tests conditions in different kind of wind tunnels for obtaining representative data for future flight. Hypersonic inlet must be operating on a wide range of flow parameters, including different level of turbulence. Test conditions of wind tunnel vary to influence on the properties of a boundary layer. At these experiments, it is necessary take into account also conditions of laminar-turbulent transition, which can significantly differ from transition at the flight. These factors strongly influence on boundary layer state. As a result, the thickness of the boundary layer and its possible separation differ from real state. In this case, start of the inlet and its performances will be contrary to reality. Then, the understanding and the control of the boundary layer development along the forebody becomes a key problem for the air inlet design. The ordinary way is to apply the boundary layer bleed in the inlet throat or through a slot located ahead of the first compression ramp. As is shown in the previous tests [8], the described methods cannot always guarantee the inlet start and satisfactory flow rate characteristics of it. That is why other configurations of the boundary layer bleed have to be explored. Among the possible configurations is a porous bleed on the external compression surface. The other effect related to the presence or absence of the boundary layer in the inlet channel appears in the tests in hot-shot wind tunnels a shock start. This effect is also achievable in blow wind tunnels, if one can provide a fast movement from close to open state for the air inlet door (shock start device). Purposes of the presented work are: a) comparison of results obtained in hot-shot and blow-down wind tunnels to check up the adequacy of tests result in the different types of wind tunnels; b) experimental study of the boundary layer bleed control effect on the inlet characteristics and its start; c) the test of the shock start device in the blow-down wind tunnel. Experimental Model and Methods In order to assess the achievable performances with this concept, an experimental evaluation of the air inlet has been performed. Using past experience and numerical simulation, a reference configuration has been designed for this inlet taking into account its integration into a given forebody shape Fig. 1. Model scheme. 1 forebody, 2 inlet compression surfaces, 3 adjusted cowl, 4 central body, 5 flowmeter entrance.

3 Francois FALEMPIN et al. Comparative Tests of Adjusted Inlet in Blow-Down and Hot-Shot Wind Tunnels 3 Starting Device Constant Inlet Throat Section Cowl Translation Changeable Length of Cowl Throat Bleed Fig. 2. Scheme of starting device. The model (Fig. 1) consisted of three parts: 1) A forebody, which presented a half-cone with an elliptic cross section and highly blunted side edges. This part simulated a nose part of an aircraft with the preliminary compression. 2) Two-dimensional two-shock inlet with the adjusted cowl (due to its lengthwise motion). 3) A flowmeter with throttling device. The modular construction of model allowed implementing additional modification of internal compres- EG sion due to change of cowl position by means of the longitudinal translation (Fig. 2). Position of a leading edge of cowl have changed depending on Mach number and was implemented in case of need of increase of internal compression under condition of growth of Mach number. The model has been tested with six variants of the sidewalls installed on surfaces of external compression, including a variant without sidewalls. Fig. 3. Scheme of porous bleed. Compression ramp as insertion units could be changed as well as sidewalls In the same time, it is possible to use or not two kinds of boundary layer bleeding systems: An internal boundary layer bleed with open and closed cavity configuration in throat, for which it has to be underlined that the large modularity of the inlet model enlarges the possibility to optimize the configuration. A porous bleed located on compression ramp, for which an exit gate allows to test open and closed bleed configuration (Fig. 3). Porous bleed has been located on last (second) surface of compression ahead the inlet throat. The porous bleed consists of two panels with holes oriented of 3 toward the free stream. Both panels of bleed could be closed separately (Fig. 3). Photo of second ramp of inlet with downstream porous insertion are shown in Fig. 4. The relative area of the bleed holes was 3%. The common channel of porous bleed could be closed smoothly and repeatedly open again during experiment. Such constructive decision has allowed de- Fig. 4. Inlet compression surfaces with porous bleed holes. fining efficiency of the boundary layer bleed, located in various sections of the second surface of compression. The scheme of the shock start device is presented in Fig. 2. At the initial time moment, the leading edge of the cowl was set on the inlet compression ramp (closed entrance), and a special device registered such a position. The leading edge of the cowl was designed in such a way that the inlet channel opening was governed by a spring and aerodynamic force of the air stream. After the flow onset in the blow down wind tunnel, the catch

4 4 Proceedings of the 8 th International Symposium on Experimental and Computational Aerothermodynamics of Internal Flows released a spring, and the cowl came back into the working position. The axis of leading edge rotation lied in the throat plane. Measurements and Test Conditions Following measurements were performed in all runs. The measurements included the definition of distribution of pressure along the central body of model and cowl in longitudinal and cross-section directions. Static and Pitot pressure distributions in model throat for different fixed position of moving cowl were measured. These data allowed obtaining main characteristics of the inlet: total pressure recovery, compression ratio and throat Mach number. Measurement of Pitot and static pressure at the exit of flowmeter nozzles allowed obtaining mass flow rate coefficient of inlet for tested range of parameters. These measurements have allowed to supervise start of an air inlet and realization of flow in the throat and in divergent channel of inlet. Also, small-sized throttling device for obtaining throttling performances during one run in blow-down wind tunnel. Discrete position of throttling grid was used in hot-shot wind tunnel. The model of hypersonic adjusted inlet was tested in the blow-down wind tunnel T-313 at Mach numbers from 2 to 7 and unit Reynolds numbers from 8 to 54 million per meter. Wind tunnel has 2D profiled nozzles with test section size mm and test duration from 3 to 6 minutes. In the hot-shot wind tunnel IT-32M, the experiments were conducted at Mach numbers M=5, 6, 7 and 8. Wind tunnel has profiled nozzles with exit diameter of 3 mm and test duration up to 15ms. Flow parameters in hot-shot wind tunnel are presented in the Table 1 including variation range during test operation. Table 1. Wind tunnel T-313 M P, kpa T, K Re 1-6, 1/m Wind tunnel IT-32M (time interval 5-6ms) M P, kpa T, K Re 1-6, 1/m Results and Discussion The researches of an inlet without control of a boundary layer demonstrated that start of an inlet with side walls was not realized. In this case, an essential reduction of the mass flow rate owing to strong lateral spilling. The similar effect of spilling took place also at bleed of the boundary layer in a throat of inlet. Porous bleed Investigation of the inlet start with porous bleed within the Mach number 2-6 have revealed that this method of porous bleed control is more effective at Mach number 4. In this case, the mass flow rate coefficient increased approximately by 3% and the total pressure recovery coefficient grew almost in two times in comparison with variants without porous bleed (Fig. 5). At the Mach number 6 (Fig. 6), increase of the flow.3 ν without PB with PB Fig. 5. Porous bleed effect on throttling performance М=4. rate coefficient was not so significant and did not exceed 2%. It is associating with the effect of internal compression and the inlet start conditions improving at Mach number increase. At Mach number of 2, the inlet start did not occur and the flow rate coefficient, as well as total pressure recovery coefficient, has remained approximately constant..1 ν EG closed EG open Fig. 6. Porosity effect on throttling performance М=6. f f

5 Francois FALEMPIN et al. Comparative Tests of Adjusted Inlet in Blow-Down and Hot-Shot Wind Tunnels 5 Simultaneously with measurement of the mass flow rate and total pressure recovery coefficient, measurement of pressure distribution on walls of the model channel was carried out. It is obvious, that at change of geometrical configuration of the model channel or at change of a method of the boundary layer bleed control, an essential changings of pressure distribution in the channel of model occur. At inlet investigation without boundary layer control, inlet start with sidewalls was not realized. On model without sidewalls there was an essential decrease of the flow rate coefficient owing to strong lateral air spilling. The similar spilling effect took place also at boundary layer bleed in the inlet throat. At the same time, use porous bleed on model with sidewalls not only led to inlet start (Fig. 7а), that is expressed in pressure decrease on the central body (х < 58 mm) as a consequence of inlet start, but also leads significant pressure increase (approximately threefold) in the channel of model. Besides, it is possible to see, that pressure distribution in the channel has characteristic sawtooth structure that testifies to a supersonic stream mode. P/P 35 a) P/P 35 3 without PB and SW with PB and SW x, mm b) without PB and SW with PB and SW The similar positive result has been obtained at Mach number 4 (Fig. 7b). However, only approximately twofold pressure increase in the channel was obtained and distribution of pressure was smooth, that testifies to realization sonic, or even subsonic, a flow in the channel. It has been ascertained that the major effect of porous bleed depended on the second panel of the bleed, which is situated closer to the inlet throat. The closed first panel made almost no effect on the mass flow rate. If the bleed channel will be closed after the inlet start, but the porosity of surface is preserved it did not result in the break-down of the inflow into the inlet and was not accompanied by deterioration of its characteristics. At the same time it was found, that at closing boundary layer bleed, but at preservation of porosity of a surface, the flow rate increases (Fig. 8, sample 21), f open EG closed EG Samples Fig. 8. Throttling performance and flow rate coefficient with porous bleed, М=6. owing to turbulizing influence of a rough porous surface for a boundary layer. Schlieren visualization of the flow structure shown on Fig. 9.] x, mm Fig. 7. Static pressure distribution along model a) M=6; b) M=4. Fig. 9. Flow structure visualization, M=4.

6 6 Proceedings of the 8 th International Symposium on Experimental and Computational Aerothermodynamics of Internal Flows During tests in hot-shot wind tunnel IТ-32М, porous bleed did not have significant influence on inlet typical. However, for higher Mach numbers, the porosity on the compression ramp surface influenced the structure of the flow and generated non-uniformity of pressure behind the inlet throat with, as consequence, some losses on total pressure recovery. At all Mach numbers, the increase of angle of attack on flow structure and inlet characteristics was essential and led to growth of mass flow rate coefficient and total pressure recovery while influence of the angle of slip was practically not apparent. Shock inlet start The performed investigations of the shock inlet start in the blow wind tunnel have allowed to compare test specifications of two various types of wind tunnels and adequacy of obtained results. Measurements in Т-313 have shown that before channel opening, which was closed by the cowl, pressure in the channel was close to zero. After cowl opening, it quickly reached the value, which close to calculation value for the started inlet at Mach number 6 with pressure distribution, typical for supersonic flow in the channel (Fig. 1). P/P SD closed SD open x, mm Fig.1. Static pressure distribution along model, М=6. At Mach numbers less than 6, starting device was inefficient and the flow rate corresponded to the flow rate of not started inlet. The characteristic example of flow rate change at joint action of shock start and porous bleed at Mach number 5 is shown in Fig. 11. It can be seen, that starting device has not provided inlet start and the flow rate corresponds to the flow rate of not started inlet (f=.17, sample 2). Addition porous bleed has been used for realization of inlet start. As a result, the inlet has been started and the flow rate has increased more than in 2 times and was close to designed value (sample 3). After closing boundary layer bleed, the inlet remained started (sample.8 f SD closed, EG closed EG open EG closed SD open, EG closed EG closed EG open Samples Fig. 11. Throttling performance and flow rate coefficient with porous bleed, М=5. 4) and the throttle characteristic has been obtained at closed porous bleed. After full opening throttle grids, the inlet was not started (sample 21) and repeated opening porous bleed for its restart was necessary (sample 22). It can be seen (sample 22), that the flow rate has reached the maximal value. The same result has been obtained at Mach number 4 (Fig. 12). If the inlet.5 f EG closed SD closed EG open EG closed EG open SD open EG closed, SD open Samples Fig. 12. Throttling performance and flow rate coefficient with porous bleed, М=4. has not been started, the flow rate did not exceed.14. However, its value increased almost in 3 times if porous bleed was used. Restart of inlet was also realized at repeated opening of porous bleed (sample 21). The performed measurements of the inlet flow rate in two various types of wind tunnels have allowed to compare results of tests and to check up their adequacy, including comparison of efficiency of shock start. Comparison of flow rates for Mach number 6 is shown in Fig.13 where data obtained in blow-down Т-313 and hot shot IT-32M wind tunnels are presented. Shock start of inlet in natural conditions of hot-shot wind tunnel occurs when the inlet is started simultaneously with start of a wind tunnel. Obviously good agree-

7 Francois FALEMPIN et al. Comparative Tests of Adjusted Inlet in Blow-Down and Hot-Shot Wind Tunnels 7 f IT-32М Т-313, model without starting device Т-313, model with starting device Nevertheless, shown in Fig. 14 Schlieren visualization of flow in channel confirms good accordance in flow structure obtained in different aerodynamic wind tunnels. Performed experiments have confirmed reproducibility of experimental data at various methods of boundary layer control Samples Fig. 13. Inlet throttling performance. ment of flow rate coefficients in two wind tunnels. It can be seen also, that the flow rate remains to constants during an operating mode of a hot-shot wind tunnel, despite of pressure and Reynolds number reduction during experiment. The similar result has been obtained at Mach number 5, however mass flow rate concurrence was a little bit worse, because inlet start conditions at Mach number reduction worsen, and influence of internal compression and a boundary layer intensifies. T-313 Conclusions The researches of the two-dimensional adjusted inlet model conducted at the Mach numbers from 2 to 6 in the blow down wind tunnel and at the Mach numbers from 5 to 8 in the hot-shot wind tunnel allow to draw following conclusions. Comparison of the inlet start conditions has shown that the inlet start in the hot-shot wind tunnel occurs at a smaller throat area than in the blow down wind tunnel as a result of shock start of inlet. The porous bleed is an effective method of inlet start control and improvement of its integral characteristics. Performed investigations have shown efficiency of the applied boundary layer control systems for maintenance of inlet start and achievement of its characteristics closed to a designed level. The possibility of shock start was checked in the blow wind tunnel. It showed that the shock start could be realized only at Mach of 5 and 6. The characteristics obtained in the blow wind tunnel coincide with the data obtained in the hot shot wind tunnel IT-32M. Obtained data allow concluding that investigations of hypersonic inlets in two wind tunnel of different types is adequate and results of measurement of inlet characteristics agree closely despite time operation difference and drop of flow parameter during the test time. Methods of inlet control can be used in both types of wind tunnels but effect of control can be different and depend on Mach number. Acknowledgement The work has been done within the ISTC Contract No. 887, and has been supported by MBDA-France. References IT-32 M Fig. 14. Schlieren flow visualization in T-313 and IT-32M wind-tunnels, M=5. [1] Billig, F. S.: Supersonic Combustion Ramjet Missile, J. of Propulsion and Power, vol. 11, No.6, pp , (1995). [2] VanWie, D. T-313 M., Kwok, F. T., Walsh, R. F.: Starting Characteristics of M=5 Supersonic Inlets, AIAA Paper No , (1996).

8 8 Proceedings of the 8 th International Symposium on Experimental and Computational Aerothermodynamics of Internal Flows [3] Falempin, F., Serre, L.: LEA Flight Test Program a Mandatory Step in The European R&T Effort Hypersonic Airbreathing Propulsion, ISABE No , (25). [4] Koschel, W., Rick, W., Ruggeberg, T.: Study of Flow Phenomena in High Speed Intakes, AIAA Paper No , (1992). [5] Dolling, D.S.: Fifty Years of Shock-Wave / Boundary-Layer Interaction Research: What Next?, AIAA J. vol. 39, No.8, pp.3 15, (21). [6] Goldfeld, M.A., Starov, A.V., Semenova, Yu.V.: Compressible Boundary Layer Investigation for Ramjet / Scramjet Inlets and Nozzles, Proc. of Fifth European Symposium on Aerothermodynamics for Space Vehicles, Cologne, Germany, p.p , (24). [7] Schulte, D., Henckels, A.,. Neubacher, R.: Manipulation of Shock / Boundary-Layer Interactions in Hypersonic Inlets, J. of Propulsion and Power, vol. 17, No.3, p.p , (21). [8] Falempin, F., Goldfeld, M.A., Nestoulia, R.V., Starov, A.V.: Experimental Investigation of Variable Geometry Inlet at Mach Number from 2 to 8, AIAA/IS , (23).