ISSN 10283358, Doklady Physics, 2015, Vol. 60, No. 4, pp. 160–163. © Pleiades Publishing, Ltd., 2015. Original Russian Text © V.M. Fomin, K.A. Lomanovich, B.V. Postnikov, 2015, published in Doklady Akademii Nauk, 2015, Vol. 461, No. 6, pp. 653–656.
PHYSICS
Effect of ElectricDischarge Plasma on GasDynamic Flow Modes of a Supersonic Jet Impinging on a Barrier Academician V. M. Fomin*, K. A. Lomanovich, and B. V. Postnikov Received September 24, 2014
DOI: 10.1134/S1028335815040102
The supersonic gas jet emits quite intensely acous tic vibrations, which, however, results in no develop ment of modes with intense selfoscillations [1]. At the interaction of the supersonic gas jet with a flat axisym metrical barrier, various types of unstable flow can be implemented in dependence on gasdynamic flow parameters and the barrier configuration. There are two basic mechanisms describing the unsteady flow around a flat impenetrable barrier by a supersonic jet [2–9]. The first mechanism of generation of selfoscil lations is caused by the presence of an external acous tic feedback [2–4]. This mechanism is implemented due to the flowstability loss with respect to small per turbations, when the sound wave generated by a fanjet boundary propagates towards the nozzle interacts with the jet and generates a perturbation in it.
an external acoustic feedback operates in the periph ery of the region and the mechanism with periodic flow lockout is in the central part. Near interface 2, both mechanisms are involved. Due to the small inertia, it is considered that one of the promising ways of controlling the highvelocity flows is the action of the electricdischarge plasma on the flow. The region of investigations of methods of magnetoplasma control of highvelocity flows has been actively developed [10–12]. In this study, we investigated the action of the dc electricdischarge plasma initiated near the super sonicjet boundary to the change of gasdynamic modes of the shockwave flow around a barrier. The
The pressure distribution over the barrier surfaces is bellshaped with a maximum in the barrier center and decreasing to the periphery. The second mechanism is caused by the occurrence of internal vortex flows in the compressed jet layer [5, 6]. In this case, the flow directed from the periphery to the barrier center is observed near the barrier surface. Favorable condi tions arise for the tangentialdiscontinuity sticking to the surface. The pressure distribution over the barrier surfaces has a maximum at the point of the tangential discontinuity sticking with a pressure drop when dis placing to the periphery and the barrier center. On the basis of experimental data, the authors of [7] revealed the regularities and constructed the region that determines the intense selfoscillations of the jet shockwave structure in the generalized coordinates N–X, where N is the generalized offdesign, X is the generalized distance from the barrier to the nozzle (Fig. 1). In [9], it was shown that the mechanism with
Khristianovich Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *email:
[email protected] 160
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Fig. 1. Generalized region (N, X) of existence of selfoscil lations with the experimental data presented: (1) exist enceregion boundary for selfoscillations; (2) boundary h d of the effect of mechanisms; (3) M = 3.25, = 1.2, = D D h d h 1.2; (4) M = 3.0, = 1.2, = 1.2; (5) M = 1.0, = 3.0, D D D d = 2.0. D
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Fig. 2. Schematic representation of the experiment: (1) nozzle, (2) electrodes, (3) head shockwave, (4) triple point, (5) fanjet boundary, (6) tangential discontinuity, (7) barrier with drainage apertures, (8) jet compressed region, (9) pressure sensors and ADC, (10) power supply.
experimental investigations were carried out on jet installations of periodic operation with an established flow time of about one minute. We had Reynolds number Re = 5 × 105 along the nozzleedge diameter, the Mach numbers M = 3.0 and 3.25 at the nozzle exit for overexpanded jets, and M = 1.0 for underexpanded jets. The offdesign parameter n varied from 0.9 to 40. A schematic representation of the experiment is shown in Fig. 2. The supersonic profile or sound conic nozzle forming the axisymmetric jet decelerated on a flat impenetrable cylindrical barrier. At the jet impact to the barrier, there was a set of pressure shocks or shockwaves in the unsteady flowaround mode. In the case of overexpanded jets, the outflow occurred from a volume with atmospheric pressure into an evacuated volume. The underexpanded jets flowed out from the volume with a pressure up to 40 × 105 Pa into an envi ronment with atmospheric pressure. Near the nozzle edge outside the jet, the arc discharge was initiated on two discharge gaps located symmetrically with respect to the jet axis. The measured current on one discharge gap is 50–60 A, and the voltage is 25–30 V. The dis chargeburning time amounted to 0.5 s. The jet shock wave structure was visualized by means of the shadow device with adaptive visualizing underlines. The obtained image was recorded by a highspeed digital videocamera with imagerecording highest frequency DOKLADY PHYSICS
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up to 24 × 103 frame/s. The velocity field in the jet was monitored by means of a digital tracer visualization (Particle Image Velocimetry—PIV). Glycerin vapors served as markers. For measuring the pressure distribution, we fabri cated an aluminum barrier with drainage apertures, which were connected by means of pneumatic tracers to tensometric pressure sensors. The signal from pres sure sensors and the I–V characteristics of discharge were recorded by means of oscillographs and an ana logtodigital converter (ADC). An arbitrary selfoscillation process can be stopped by breaking the feedback. For example, in [3], the jet flow impinging to a barrier was stabilized by establish ing a diaphragm screen on the path to the acoustic feedback. In this study, we established an electrode unit instead of the diaphragm screen. The flow mode with selfoscillations of shockwave fronts for the Mach numbers M = 3.0 and 3.25 and the offdesign param eters n = 0.9 and 0.95 was implemented. We investi gated the ranges of relative barrier diameters d from D h 1.0 to 2.4 and the nozzle–barrier distances from 0.6 D to 2.0, where D is the diameter of the internal output nozzle cross section. The visualization of the jet
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Fig. 3. Pressure distribution over the barrier surfaces with d h the diameter = l.2 at the distance = 1.2. (1) M = 3.25 D D without discharge; (2) M = 3.25 with discharge; (3) M = 3.0 without discharge; (4) M = 3.0 with discharge.
shockwave structure showed that, if we initiate the electric discharge, the shockwave oscillations are stopped with subsequent renewal when the discharge is switched off. The selfoscillation frequency varied from 0.8 × 103 to 1.6 × 103 Hz, and the oscillation dou ble amplitude reached 0.8D. The time of the flow reaction to the presence of discharge and the flow mode change (the selfoscillation suppression) is smaller than 50 ms and, in certain cases, 1 ms. When switching off the discharge, the selfoscillations are reconstructed with a substantial time delay to 200 ms, which is typical for establishing the gasdynamic pro cesses. In Fig. 3, we show the measured pressure distribu tions over the barrier surface when switching on the discharge. Dependences 1 and 2 correspond to point 3 on the diagram in Fig. 1. It is the region of the predom inant effect of the acoustic feedback. From the graphs in Fig. 3, it can be seen that the discharge insignifi cantly modifies the bellshaped pressure distribution. At the same time, at the switchedon discharge under the conditions of the experiment characterized by point 4 in the diagram, we observed a substantial change in the pressure distribution (curves 3, 4). The pressure minimum in the central region transforms into the maximum at the burning discharge, which testifies to the absence of a toroidal vortex. The problem of the plasma effect on an unsteady shockwave flow pattern in the jet, the shockwave oscil lations in which are caused by internal vortex flows, is strongly complicated due to the absence of the possi
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Fig. 4. Pressure distribution over the barrier surfaces with d h = 2.0 at the distance = 3.0 for M = 1.0 D D and n = 6.0: (1) without discharge; (2) with discharge. the diameter
bility of a direct effect on the feedback. In this case, the pressure distribution over the barrier surface (Fig. 4) is similar to that in [5] and has a maximum at the point of sticking of the tangential discontinuity to the barrier surface. The pressure distribution (curves 1 and 2) is typical for the central regions of the diagram of selfoscilla tion existence (point 5 in Fig. 1). In the experiments with underexpanded jets, steel cylindrical barriers with the diameters d from 2.0 to 6.0 established from the D nozzle edge at the distance h from 1.0 to 6.0 were D used. When switching on the discharge, we observed an increase in pressure in the central barrier region, whereas the changes are insignificant in the periphery at relative radii 2r > 0.8. It shows a decrease in the vor D texflow intensity in a compressed shock layer of the jet. In a number of experiments with underexpanded jets, the selfoscillations were also suppressed at the initiation of the electric discharge. Here the mecha nism of the discharge effect is most likely thermal. The thermal trace from the discharge in the jetmixture layer interacts with the triple point and affects the development of internal flows in the compressed shock layer near the barrier surface. Thus, the new method of plasma control with the help of the electric discharge for supersonic jet flows impinging to a barrier was investigated in the study. The possibility of stabilization of unsteady flow is DOKLADY PHYSICS
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shown by the initiation of the electric discharge near the nozzle edge outside the jet. The measurements of the pressure distribution over the barrier showed that the electric discharge affects the development of the selfoscillatory process by the mechanism with both external and internal acoustic feedback, due to the interaction of the recircular flow with the thermal trace of the discharge. The applicability of this method of control of overexpanded and underexpanded super sonic jets was shown. REFERENCES 1. G. Sinibaldi, G. Lacagnina, L. Marino, and G. P. Ro mano, Phys. Fluids 25, 1 (2013). 2. V. N. Glaznev, V. I. Zapryagaev, V. N. Uskov, et al., Jet and Unsteady Flows in Gas Dynamics (Izdvo SO RAN, Novosibirsk, 2000) [in Russian]. 3. V. N. Glaznev, V. S. Demin, and N. A. Zheltukhin, Izv. SO AN SSSR, Ser. Tekh. Nauk, No. 13 (3), 138 (1973).
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4. V. N. Glaznev, Prikl. Mekh. Tekh. Fiz., No. 4, 59 (1991). 5. I. P. Ginzburg, B. G. Semiletenko, V. S. Terpigor’ev, and V. N. Uskov, Inzh.Fiz. Zh. 19 (3), 412 (1970). 6. B. G. Semiletenko, B. N. Sobkolov, and V. N. Uskov, Izv. SO AN SSSR, Ser. Tekh. Nauk, No. 13 (3), 39 (1972). 7. V. A. Ostapenko and A. V. Solotchin, Izv. SO AN SSSR, Ser. Tekh. Nauk, No. 8 (2), 66 (1974). 8. A. Powel, J. Acoust. Soc. Am. 83 (2), 515 (1988). 9. S. G. Mironov, Prikl. Mekh. Tekh. Fiz., No. 1, 94 (1993). 10. P. Bletzinger and B. N. Ganguly, J. Phys. D: Appl. Phys. 38, R33 (2005). 11. Eric Moreau, J. Phys. D: Appl. Phys. 40, 605 (2007). 12. Proc. Workshop on MagnetoPlasma Aerodynamics in Aerospace Applications 2008–2014, Moscow.
Translated by V. Bukhanov
