John M. Seiner

Model and full scale study of twin supersonic plume resonance

This paper examines the effect of both nozzle geometry and scale on the twin supersonic plume resonance phenomenon associated with aircraft having engine nozzle center-to-center spacings less than two diameters. Exit plane near field dynamic pressures were measured for both single and dual nozzle operation in 4.7 percent model and full scale under static conditions. The frequencies associated with this phenomenon were predicted to within 5 percent for a full scale F-15 aircraft. Amplitude levels associated with this phenomenon were found to dominate the dynamic pressure fluctuations in the inter-nozzle region, and reach a level near the structural design limit for this aircraft. The model scale studies, which involved both axisymmetric and rectangular geometry, indicated that amplitude levels could be expected to be much higher in flight. High amplitude levels would likely occur in the overexpanded region for axisymmetric geometry, and in the underexpanded region for rectangular geometry.

Bicoherence analysis of model-scale jet noise

Bicoherence analysis has been used to characterize nonlinear effects in the propagation of noise from a model-scale, Mach-2.0, unheated jet. Nonlinear propagation effects are predominantly limited to regions near the peak directivity angle for this jet source and propagation range. The analysis also examines the practice of identifying nonlinear propagation by comparing spectra measured at two different distances and assuming far-field, linear propagation between them. This spectral comparison method can lead to erroneous conclusions regarding the role of nonlinearity when the observations are made in the geometric near field of an extended, directional radiator, such as a jet.

Measurements of Store Separation Dynamics

The separation of munitions from payload bays at supersonic speeds was studied experimentally in the tri-sonic wind tunnel at the National Center for Physical Acoustics. A new mechanism for model scale store release was designed and its repeatability evaluated. A 1/15 scale GBU-38 was designed using a “light scaling” approach and the corresponding ejection force was determined. Test drops were performed into a Mach 1.5 free-stream using both the scaled ejection force and a lower ejection force for comparison. An optical tracking method based on PIV was used to measure the trajectory and attitude of the stores. Additionally, the cavity pressure oscillations were measured synchronously with the store release trigger. The results show that the phase relationship between the store release and the cavity pressure oscillations is a key initial condition in determining the resulting flight of the store.

The Effects of Gappy POD on Higher-Order Turbulence Quantities

Velocity data obtained using particle image velocimetry is often marred by missing data in various spatial locations due to inconsistent seeding, variations in illumination, and other factors. In order to examine instantaneous or time-varying quantities, it is often necessary to provide as estimate for the missing data. The Gappy POD method provides a means to acquire this estimate using the proper orthogonal decomposition which takes into account the statistics of the data set along with all the valid data in a given snapshot to estimate the missing data in that snapshot. This provides a higher level of fidelity compared to interpolating values based only on neighboring points. The Gappy POD method was evaluated by applying it to 2-D PIV data of a subsonic cold jet with a jet Mach number of 0.85. The estimated velocity was used to compute instantaneous Reynold’s stresses and vorticity which were compared to that obtained from the available data. The results demonstrate that the Gappy POD can provide an estimate that is acurate to within the experimental uncertainty of the measured data. One drawback is observed in that the estimate is optimized using the measured values; therefore, at best it will inherit the experimental uncertainty from the available data.

External nozzle flap dynamic load measurements on F-15 S/MTD model

Dynamic pressure loads were obtained on 1/12 scale models of the F-15B production aircraft and the F-15 S/MTD experimental aircraft with rectangular nozzles and canards. Flight Mach numbers from 0.51 to 1.20 were studied for aircraft angles of attack from 0 to 10 deg and nozzle pressure ratios from 1.00 to 5.09. The results show that dynamic levels are lower in the internozzle region of twin rectangular nozzles than are levels found with twin axisymmetric nozzles. At other locations, the levels associated with both geometries are of the same order of magnitude when normalized by aircraft dynamic Q. At Mach number of 0.51, the loads spectrum is dominated by plume shock noise processes for both geometries. Above Mach 0.51, this mechanism is associated with either vortex bursting from a forward location or turbulent boundary layer separation over the nozzle external flaps. At supersonic speeds both geometries show significantly decreased load levels.

Application of an Image Tracking Algorithm in Fire Ant Motion Experiment

An image tracking algorithm, which was originally used with the particle image velocimetry (PIV) to determine velocities of buoyant solid particles in water, is modified and applied in the presented work to detect motion of fire ant on a planar surface. A group of fire ant workers are put to the bottom of a tub and excited with vibration of selected frequency and intensity. The moving fire ants are captured with an image system that successively acquires image frames of high digital resolution. The background noise in the imaging recordings is extracted by averaging hundreds of frames and removed from each frame. The individual fire ant images are identified with a recursive digital filter, and then they are tracked between frames according to the size, brightness, shape, and orientation angle of the ant image. The speed of an individual ant is determined with the displacement of its images and the time interval between frames. The trail of the individual fire ant is determined with the image tracking results, and a statistical analysis is conducted for all the fire ants in the group. The purpose of the experiment is to investigate the response of fire ants to the substrate vibration. Test results indicate that the fire ants move faster after being excited, but the number of active ones are not increased even after a strong excitation.

An Image Pattern Tracking Algorithm for Time-resolved Measurement of Mini- and Micro-scale Motion of Complex Object

An image pattern tracking algorithm is described in this paper for time-resolved measurements of mini- and micro-scale movements of complex objects. This algorithm works with a high-speed digital imaging system, which records thousands of successive image frames in a short time period. The image pattern of the observed object is tracked among successively recorded image frames with a correlation-based algorithm, so that the time histories of the position and displacement of the investigated object in the camera focus plane are determined with high accuracy. The speed, acceleration and harmonic content of the investigated motion are obtained by post processing the position and displacement time histories. The described image pattern tracking algorithm is tested with synthetic image patterns and verified with tests on live insects.

Computational and Experimental Investigations of Jet Noise Reduction Concepts for Low-Bypass Ratio Military Gas Turbine Engines

Computational and experimental activities supporting the design and evaluation of jet noise reduction concepts for low bypass-ratio military gas turbine engines are presented. Work has been focused on evaluations of lobed nozzle concepts, utilizing typical Field Carrier Landing Practice (FCLP) engine cycle conditions. At FCLP, the engine operates slightly over-expanded, with the result that jet noise emissions also include shock noise contributions. Reduction of nozzle exit area via introduction of lobes permits the nozzle to operate closer to ideal conditions, while also introducing vortices into the plume that may alter turbulence structure and noise emissions. Structured grid, computational fluid dynamics (CFD) investigations of isolated noise nozzles have been conducted. Determination of the optimal number of lobes for a nozzle has been a key objective of our studies. It was found that a six-lobe configuration is optimal, and that two-, three- and four- and twelve-lobe designs fail to provide performance improvement relative to the six-lobe design. While all lobe designs provided the same amount of area reduction for the baseline nozzle, the twelve-lobe configuration restricted the growth of the vortices introduced into the plume by the lobes. With reduced number of lobes, the far-field mixing of the exhaust plume remained unaffected, although some near-field benefits were observed. However, the turbulence characteristics, and hence noise, were not altered and the six-lobe design provided best performance, as demonstrated during tests at NCPA. The noise attenuation benefits of nozzle beveling were also assessed. These studies demonstrate the strong effect of dual jet interactions for the closely spaced, inward canted exhaust nozzles of a twin-engined aircraft. These dual nozzle plume-plume interactions have been found to have a very large effect on the turbulence structure, and hence noise. The analysis of lobed nozzle concepts with engine-engine interactions and vehicle aerodynamic/plume interactions has required the usage of multi-element unstructured grid numerics. Evaluations of aspirated lobe concepts were also conducted, where mass flux is introduced into the plume at the trailing edge of the lobes. Sensitivities to mass flow rates were examined and laboratory measurements of noise emissions in NCPA’s anechoic chamber are presented. Finally, impact of the lobes on nozzle performance during altitude flight are presented, along with CFD modeling upgrades required for performing simulations of complete aircraft/plume interactions.Copyright

Tracking an aerodynamic model in a wind tunnel with a stereo high-speed imaging system

A method is presented to track the 3D motion of an aerodynamic model in wind tunnel tests with a stereo high-speed imaging system. The imaging system includes two high-speed cameras that work at 4,000 frames per second with a digital resolution of 1024×512 pixels. Stereo image recordings of tracking markers, which are precisely distributed on the model surface, are captured with a limited view angle difference between the two cameras. The stereo images are calibrated and 2D positions of tracking markers in the image frame are determined with a correlation-based algorithm. The camera positions and view angles are calibrated with a target shifting test. The determined 2D positions of both camera views in each stereo image recording are used to reconstruct instantaneous 3D coordinates of the tracking markers. A minimum quadratic difference approach is used to determine the center coordinates, roll angle, pitch angle and yaw angle of the aerodynamic model in the wind tunnel. Tests and simulations were conducted to verify the method.

Velocity and Pressure Measurements of a Mach 0.85 Axisymmetric Jet

The turbulent properties of a heated and unheated Mach 0.85 axisymmetric jet have been studied. The velocity field of the jet at static temperature ratios of 0.87 and 2.34, was measured in the streamwise radial plane using Particle Image Velocimetry. The velocity measurements were acquired between streamwise locations of 3D and 8D downstream from the nozzle exit. Proper Orthogonal Decomposition (POD) was applied to the velocity field using snapshot POD. The POD analysis showed that the eigenvalues of the heated jet had higher fraction of energy. The POD eigenfunctions or modes of the streamwise velocity of both jets were similar, while the POD modes of the radial velocity of both jets were very different. The POD modes of radial velocity of the unheated jet were symmetrical about the jet centerline, and the modes of the heated jet seemed to merge at the centerline. The near field pressure measurements were acquired just outside the shearlayer. A linear array of five pressure transducers was placed at 7° to the nozzle lipline, so that it would be parallel to the shear layer. The transducers in the array were spaced one diameter apart. Pressure measurements were acquired at streamwise locations between 4.25D and 10.25D from the nozzle exit. Based on the slope of the pressure spectra, the propagating events in the two jets were identified. The POD was also applied to the pressure data, and the POD modes of the two jets were compared. The peak in the amplitude of the POD mode of the heated jet was at a higher frequency. With increasing mode numbers, the peak in the POD mode of both jets shifted to a downstream location.Copyright