Bhupatindra Malla

Proper Orthogonal Decomposition on a Subsonic Jet Exhausted from an Axisymmetric Nozzle and Chevron Nozzles with Varying Penetration

This paper utilizes Proper Orthogonal Decomposition (POD) to study a jet engine exhaust to identify differences between a baseline axisymmetric jet and one with chevrons. The analysis is done using flow velocity data obtained from Particle Image Velocimetry and performed for the flow exhausting through a baseline conical nozzle, low penetration chevron nozzle, and high penetration chevron nozzle. The mean velocity and TKE results show the chevron configurations reduce the potential core length, increase the jet spread, and reduce TKE levels near the end of the potential core. POD was then used to further quantify these results. The POD modes for the chevron flows and baseline flows are very similar but the energy within the POD modes is lower for the chevron cases which are consistent with the TKE results. Projection of the PIV images from the chevron configurations onto the baseline POD modes is used to directly quantify changes to the energy distribution of the largescale structures. It is seen that both near the nozzle and downstream, the highest energy containing modes are attenuated for the chevron cases. The reduction in growth of the large-scale structures near the jet exit is due to the increased spread of the jet and ultimately the stabilization of the jet column mode instabilities.

Using Wavelet Stochastic Estimation to correlate near-field pressure to velocity POD modes in a supersonic jet

Streamwise PIV and near-field pressure measurements were made simultaneously to explore the physics of an over-expanded, supersonic, axisymmetric jet. The near-field pressure consisted of two linear arrays diametrically opposed and with 16 microphones on each array. Wavelet Stochastic Estimation was employed to extract frequency information from the pressure field and then correlate the selected frequencies to the velocity measurements. It was found that the shock associated component contained symmetric flow features in the mixing layer and a strong oscillation along the jet centerline. At the same time the pressure field was also highly correlated with low frequency instability waves, which was supported by the high correlation to the first few axial and radial POD modes. The ability to simultaneously measure velocity and near-field pressure and then correlate the two measurements has proven valuable to understanding the flow physics responsible for the near-field pressure signature.

Acoustic Signature of a Supersonic Jet Emanating from a Rectangular C-D Nozzle

We live in a world with ever increasing air traffic and the demand for fuel efficient low noise emitting aircraft is high. The use of blended wing bodies (BWB) has gained interests within the aerospace industry due to its potential for reduced fuel consumption. These type of aircraft are generally equipped with rectangular nozzles. The drawback of such nozzles is increased instability of the emanating jet which increases the risk of higher noise radiation. Understanding the instability patterns and the underlying flow physics is therefore the key to improved stability and reduced noise. In the presented paper, an LES/CAA approach is utilized to predict the flow dynamics and the radiated noise from a rectangular nozzle. The nozzle is operated at underexpanded conditions. The simulations are compared with experiments and are used as a complement to the experimental data for improved understanding of the flow physics. The supersonic jet is found to exhibit an intense flapping motion followed by a large jet spreading in the minor-axis plane. In general, the prediction of the most amplified frequency and higher harmonics observed in the near-field and far-field spectra is in agreement with the experiment. Two types of flow events associated with the generation of high amplitude acoustic waves are detected. These events are identified as vortex-collision and shock-leakage through the shear layer.

Optimization of Stochastic Estimation Techniques in a High-Speed, Axisymmetric Jet

Stochastic Estimation (SE) is utilized to investigate the large-scale structure of a jet exhausted through a baseline axisymmetric nozzle and two chevron nozzles with different penetrations. Particle image velocimetry (PIV) was used to measure the streamwise and radial velocity components on a 2D streamwise plane over the first 15 jet equivalent diameters. Two 16 microphone, linear, near-field arrays separated by 180° in the azimuthal plane were simultaneously acquired with the PIV measurements. The analysis included investigation of the near-field pressure spectra and flow field mean velocity and Turbulent Kinetic Energy. SE was used to reconstruct the velocity field from the acquired pressure data. The optimal number and optimal placement of sensors was investigated to obtain the best reconstructed velocity. Near the nozzle, the optimal sensor location was near the jet while further downstream, SE was better when the microphones were slightly farther away. This is directly related to the wavelength of the large-scale structures which increased downstream. SE was successful at reconstructing the TKE profiles in shape but significantly under predicted the amplitudes. It was also shown that SE was not as successful reconstructing flow exiting the chevron nozzles due to a drop in correlation between the flow velocity and near-field pressure. Even with the lower correlations, the shape of the TKE profiles were successfully reconstructed using SE for the chevron flow fields.

Proper Orthogonal Decomposition on LES and PIV data sets from a Mach 0.9 jet

This paper contains the complementary analysis of the results obtained through computational and experimental methods using the technique of Proper Orthogonal Decomposition (POD). A detailed study of the experimentally obtained flow field data using the method of POD has already been accomplished. The experimental data consists of the 2D visualization of the velocity field obtained using Particle Image Velocimetry (PIV). The computational process is initialized with the simulation of the 3D flow field using a steady state RANS flow solver. RANS is used as a platform to proceed towards capturing the unsteadiness of the flow field which is accomplished using the Large Eddy Simulation (LES). A significant match in the characteristics of the mode shapes has been observed, in both the axial and radial components of the velocity field, however, there is difference in the distribution of the flow turbulent kinetic energy over the modes. The LES predicts the energy values to be higher at the lowest modes in comparison to the PIV results and the energy drop off rate is higher. On the contrary, PIV predicts lower energy content at the first few modes; however the energy drop off rate is smaller.

Characterization of supersonic jet noise and its control

As supersonic aircraft and their turbojet engines become more powerful they emit more noise. The principal physical difference between the jets emanating from supersonic jets and those from subsonic jets is the presence of shocks in the supersonic one. This paper summarizes a study of noise reduction technologies applied to supersonic jets. The measurements are performed with a simulated exhaust of a supersonic nozzle representative of supersonic aircraft. The nozzle has a design Mach number of 1.56 and is examined at design and off-design conditions. Several components of noise are present including mixing noise, screech, broadband shock associated noise, and crackle. Chevrons and fluidic injection by microjets and a combination of them are shown to reduce the noise generated by the main jet. These techniques provide significant reduction in jet noise. PIV provides detailed information of the flow and brings out the physics of the noise production and reduction process.