Browsing by Subject "Plasma flow control"
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Item Investigation of a pulsed-plasma jet for separation shock/boundary layer interaction control(2010-05) Narayanaswamy, Venkateswa; Raja, Laxminarayan L.; Clemens, Noel T.; Varghese, Philip; Raman, Venkatramanan; Bengtson, RogerA pulsed-plasma jet (called a "spark-jet" by other researchers), is a high-speed synthetic jet that is generated by striking an electrical discharge in a small cavity. The gas in the cavity pressurizes owing to the heating and is allowed to escape through a small orifice. A series of experiments were conducted to determine the characteristics of the pulsed-plasma jet issuing into stagnant air at a pressure of 45 Torr. These results show that typical jet exit velocities of about 250 m/s can be induced with discharge energies of about 30 mJ per jet. Furthermore, the maximum pulsing frequency was found to be about 5 kHz, because above this frequency the jet begins to misfire. The misfiring appears to be due to the finite time it takes for the cavity to be recharged with ambient air between discharge pulses. The velocity at the exit of the jet is found to be primarily dependent on the discharge current and independent of other discharge parameters such as cavity volume and orifice diameter. Temperature measurements are made using optical emission spectroscopy and reveal the presence of considerable non-equilibrium between rotational and vibrational modes. The gas heating efficiency was found to be 10% and this parameter is shown to have a direct effect on the plasma jet velocity. These results indicate that the pulsed-plasma jet creates a sufficiently strong flow perturbation that is holds great promise as a supersonic flow actuator. An experimental study is conducted to characterize the performance of a pulsed-plasma jet for potential use in supersonic flow control applications. To obtain an estimate of the relative strength of the pulsed-plasma jet, the jet is injected normally into a Mach 3 cross-flow and the penetration distance is measured by using schlieren imaging. These measurements show that the jet penetrates 1.5 [delta], where [delta] is the boundary layer thickness, into the cross-flow and the jet-to-crossflow momentum flux ratio is estimated to be 0.6. An array of pulsed-plasma jets was issued from different locations upstream of a 30-degree compression ramp in a Mach 3 flow. Furthermore, two different jet configurations were used: normal injection and pitched and skewed injection. The pitched and skewed configuration was used to see if the jets could act as high-bandwidth pulsed vortex generators. The interaction between the jets and the separation shock was studied using phase-locked schlieren imaging. Results show that the plasma jets cause a significant disturbance to the separation shock and clearly influence its unsteadiness. While all plasma jet configurations tested caused an upstream motion of the separation shock, pitched and skewed plasma jets caused an initial downstream shock motion before the upstream motion, demonstrating the potential use of these plasma jets as vortex generator jets. The effect of the plasma jet array on the separation shock unsteadiness is studied in a time-resolved manner by using 10 kHz schlieren imaging and fast-response wall pressure measurements. An array of three pulsed-plasma jets, in a pitched and skewed configuration, is used to force the unsteady motion of the interaction formed by a 24° compression ramp in a Mach 3 flow. The Reynolds number of the incoming boundary layer is Re[theta]=3300. Results show that when the pulsed jet array is placed upstream of the interaction, the jets cause the separation shock to move in a quasi-periodic manner, i.e., nearly in sync with the pulsing cycle. As the jet fluid convects across the separation shock, the shock responds by moving upstream, which is primarily due to the presence of hot gas and hence the lower effective Mach number of the incoming flow. Once the hot gases pass through the interaction, the separation shock recovers by moving downstream, and this recovery velocity is approximately 1% to 3% of the free stream velocity. With forcing, the low-frequency energy content of the pressure fluctuations at a given location under the intermittent region decreases significantly. This is believed to be a result of an increase in the mean scale of the interaction under forced conditions. Pulsed-jet injection are also employed within the separation bubble, but negligible changes to the separation shock motion were observed. These results indicate that influencing the dynamics of this compression ramp interaction is much more effective by placing the actuator in the upstream boundary layer.Item Simulation of direct-current surface plasma discharges in air for supersonic flow control(2010-05) Mahadevan, Shankar, 1982-; Raja, Laxminarayan L.; Varghese, Philip L.; Carey, Graham F.; Hallock, Gary A.; Ezekoye, Ofodike A.Computational simulations of air glow discharge plasma in the presence of supersonic flow are presented. The glow discharge model is based on a self-consistent, multi-species, continuum description of the plasma with finite-rate chemistry effects. The glow discharge model is coupled to a compressible Navier-Stokes solver to study the effect of the plasma on the flow and the counter-effect of the flow on the plasma. A finite-rate air chemistry model is presented and validated against experiments from the literature at a pressure of 600 mTorr. Computational results are compared with experimentally measured V-I characteristics, axial positive ion densities and electron temperature, and reasonably good qualitative and quantitative agreement is observed. The validated air plasma model is then used to study the effect of the surface plasma discharge on M=3 supersonic flow at freestream pressure 18 Torr and the corresponding effects of the flow on the discharge structure in two dimensions. The species concentrations and the gas temperature are examined in the absence and presence of bulk supersonic flow. The peak gas temperature from the computations is found to be 1180 K with the surface plasma alone in the absence of flow, and 830 K in the presence of supersonic flow. Results indicate that O- ions can have comparable densities to electrons in the pressure range 1-20 Torr, and that O2- ion densities are at least two orders of magnitude smaller over the pressure range considered. Different ion species are found to be dominant in the absence and presence of supersonic flow, highlighting the importance of including finite-rate chemistry effects in discharge models for understanding plasma actuator physical phenomena. Electrode polarity effects are investigated, and the cathode upstream actuation is found to be stronger than the actuation strength with the cathode downstream, which is consistent with experimental findings of several groups. A parallel computing implementation of the plasma and flow simulation tools has been developed and is used to study the three-dimensional plasma actuator configuration with circular pin electrodes.Item Simulation of supersonic flow phenomena in the presence of direct-current surface plasma actuators(2006-12) Mahadevan, Shankar, 1982-; Raja, Laxminarayan L.A computational study of the effects of surface direct-current glow discharge actuation on supersonic flows is presented. A Navier-Stokes solver is used to compute the supersonic flow and the plasma actuators are modeled using a phenomenological approach. The actuator of interest consists of two electrodes flush-mounted on a flat plate. In the absence of external magnetic fields, the principal actuation mechanisms are electrostatic forcing and volumetric gas heating. The Navier-Stokes solver uses finite-volume spatial discretization and is based on an unstructured grid framework. A dual-time stepping algorithm is used to generate time-dependent solutions. The plasma actuator model comprises a separate electrostatic forcing model and a volumetric heating model to represent the effect of the plasma on the flow. The electrostatic forcing model is based on Child's law for a collisional cathode sheath and the volumetric heating effect is modeled using an asymmetric Gaussian distribution. Two modes of the glow discharge are studied using the actuator model in the presence of supersonic flow: a low-power (diffuse) mode and a high-power (constricted0) mode. The effects of electrostatic forcing and volumetric gas heating are examined individually and in combination using surface and field plots of pressure and temperature. Results from computations indicate that the diffuse mode discharge results in a stronger perturbation to the flow compared to the constricted mode discharge, which is mainly attributed to the volumetric heating of the bulk flow. The effects of the position of the electrodes relative to the bulk supersonic flow (cathode upstream and cathode downstream) are studied using sensitivity parameters in the electrostatic forcing model. The results show that electrostatic forcing, despite being small in magnitude, plays a significant role in the flow actuation. The effect of inter-actuator distances and coupled effects between two actuators placed in series are also studied. The magnitude of the surface pressure rise increases as the inter-actuator distance is made smaller.