Kalyan Goparaju

Large-Eddy Simulation of Plasma-Based Active Control on Imperfectly Expanded Jets

Jet flow control is important for mixing enhancement and noise mitigation. In previous efforts, we have used validated simulations to examine the effect of localized arc filament plasma actuators (LAFPA) on perfectly expanded Mach 1.3 jets. Here, we extend the analysis to an underexpanded jet at the same Mach number to examine the effect of shocks and expansions on control authority. After validation of the baseline flow, it is shown that the downstream evolution is relatively independent of Reynolds number. Simulations performed at different values of upstream pressure indicate that the higher stagnation pressure yields shock cells that are quantitatively stronger but qualitatively similar to those observed for the lower upstream stagnation pressure condition. For control simulations, axisymmetric mode pulsing is considered at two different Strouhal numbers of St = 0.3 and St = 0.9. These simulations show that the response of the jet to flow control is a strong function of the actuation frequency. Relative to the no-control case, actuating at the column-mode instability frequency (St = 0.3) results in an increase in the rate of spreading of the shear layer. Phase-averaged results indicate the formation of large toroidal vortices formed as a result of amplification of the column-mode instabilities that are excited at this frequency. On the other hand, the higher frequency actuation affects the initial shear-layer instability and interferes with the formation of the large-scale structures. Detailed integral azimuthal length scale analyses reveal that despite the absence of the axisymmetric toroids, the St = 0.9 case shows the dominance of the axisymmetric mode even at large distances from the nozzle exit. This indicates that flow control methods need not always have a visual signature of their influence on the system.

Effect of Active Control on Under-Expanded Jets

In an effort to reduce the aircraft jet noise, control of jets has become one of the highly explored areas. In this work, we examine an underexpanded jet subjected to control with Localized Arc Filament Plasma Actuators (LAFPA) to complement prior results on perfectly expanded flow. High fidelity, Large Eddy Simulations (LES) are employed with a simple model for the actuators, eight of which are placed along the periphery of a Mach 1.2 converging nozzle exit. The axisymmetric mode (m=0) is excited at two different Strouhal numbers of 0.3 (corresponding to the most amplified jet-column mode) and 0.9, based on the exit diameter of the nozzle. Baseline (no control) simulations at two different Reynolds numbers (100,000 and 1.2 million) are also performed. Results indicate a good correlation between the numerical and the experimental results. Undulations are observed in the mean flow, which correspond to the increase and decrease of the flow velocity as the jet traverses the complex shock cell structures generated as a result of the under-expansion of the jet. Baseline simulations at the two chosen Reynolds numbers reveal no significant difference between the two cases indicating that the effect of Reynolds number is negligible. Phase-averaged results, for St=0.3, indicate the presence of large vortical structures generated as a result of amplification of the natural structures due to actuation. Two different kinds of structures are generated corresponding to the switching on and switching off of the plasma actuators. These structures are absent when the flow is actuated at St=0.9. Quantitative near field acoustic analysis is conducted using two-point correlation technique. The qualitative effect of forcing on far-field noise propagation is also investigated.Copyright

Investigation of a Twinjet Configuration with and without Flow Control

Jet noise has been an active area of research owing to public health concerns and aviation regulations. This problem is further amplified with the use of multiple engines placed in closed proximity to each other. In addition to significantly altered farfield noise as compared to a single-jet, previous experimental results show high levels of dynamic pressure fluctuations in the inter-nozzle region which can cause structural damage. In this work, Large Eddy Simulations are performed to study the dynamics of a supersonic twinjet configuration, with and without flow control. Results indicate that relative to single-jet configuration, twinjets show decrease in column lengths due to significant modifications in shear-layer properties. The simulations also corroborate previous experimental observations of very high levels of dynamic pressure fluctuations in the inter-nozzle region relative to a single-jet configuration for identical flow parameters. The phenomenon of noise shielding along the plane containing the two jets has been reported for all the twinjet cases explored in this work and trends of sound pressure levels obtained at various polar angles indicate an increased efficiency of shielding at lower polar angles. The sensitivity of the dynamic pressure fluctuations on the nozzle-exit boundary layer thickness is also characterized. Simulations are also performed to mimic the effects of Localized Arc Filament Plasma Actuators at different excitation frequencies (St=0.3 and 0.9) actuated in an axisymmetric fashion (m=0). Close to the nozzle exit, toroidal structures formed as a result of control result in significant increase in jet spreading leading to the formation of a recirculation region along the symmetry plane, relative to the uncontrolled case. This excitation scheme increased the levels of dynamic pressure fluctuations closer to the nozzle exit in the inter-nozzle region. In contrast, the near field shows significant reduction in pressure fluctuations, especially for the higher Strouhal number case.