Showing papers by "James W. Gregory published in 2018"

Oscillation characteristics of mutually impinging dual jets in a mixing chamber

Abstract: In this study, we consider the oscillatory behavior of mutually impinging jets in an enclosed, dome-shaped mixing chamber. The frequency of the impinging jet oscillations is dictated by the flow rate, with the oscillatory behavior being grouped into three regimes: a low flow rate regime (Re 2000). The detailed characteristics of the oscillations in the high flow rate regime (Re = 6800 in the present study) are investigated through simultaneous frequency and refractive-index-matched particle image velocimetry measurements. The oscillation mechanism in the high flow rate regime was found to be similar to that of the other two regimes, where jets collide and interact in an oscillatory manner. However, in the high flow rate regime, there is a distinct and phase-evolving process of saddle point formation and jet bifurcation that is not present at the lower flow rates. The jet bifurcation process is also distinctly related to the balance of vortical structures inside the mixing chamber, and saddle point formation plays a key role in the internal and external flow field of this configuration. The external sweep angle of the exiting jet increases with the flow rate throughout the low and transition flow rate regimes, but a constant sweep angle was found to persist in the high flow rate regime. Thus, formation and location of the internal saddle point is directly linked to the external sweep angle of the jet.

Sweeping Jet Film Cooling on A Turbine Vane

Abstract: The cooling performance of sweeping jet film cooling was studied on a turbine vane suction surface in a low-speed linear cascade wind tunnel. The sweeping jet holes consist of fluidic oscillators with an aspect ratio (AR) of unity and a hole spacing of Pd/D = 6. Infrared (IR) thermography was used to estimate the adiabatic film effectiveness at several blowing ratios and two different freestream turbulence levels (Tu = 0.3% and 6.1%). Convective heat transfer coefficient was measured by a transient IR technique, and the net heat flux benefit was calculated. The total pressure loss due to sweeping jet film cooling was characterized by traversing a total pressure probe at the exit plane of the cascade. Tests were performed with a baseline shaped hole (SH) (777-shaped hole) for comparison. The sweeping jet hole showed higher adiabatic film effectiveness than the 777-shaped hole in the near hole region. Although the unsteady sweeping action of the jet augments heat transfer, the net positive cooling benefit is higher for sweeping jet holes compared to 777 hole at particular flow conditions. The total pressure loss measurement showed a 12% increase in total pressure loss at a blowing ratio of M = 1.5 for sweeping jet hole, while 777-shaped hole showed a 8% total pressure loss increase at the corresponding blowing ratio.

Optimum-wavelength forcing of a bluff body wake

Abstract: Three-dimensional forcing of the wake of a circular cylinder was studied experimentally to determine the optimal spatial-forcing-wavelength for drag reduction. Dielectric-barrier-discharge plasma actuators were mounted on a cylinder in a square-wave pattern to create the three-dimensional forcing. Six spatial wavelengths (1d–6d) and two blowing ratios (Uj/U∞ = 0.2 and 0.6) were tested at a Reynolds number of 4700. For most spatial wavelengths and blowing ratios, the segmented plasma actuators produced streamwise vorticity that altered the wake development, formation length, and drag. A spatial wavelength of 4d emerged as the optimum wavelength for the high-blowing-ratio case. Forcing with this optimum wavelength significantly attenuated vortex shedding, leading to maximum drag reduction in the high-blowing-ratio case. This optimum wavelength of 4d exists because longer wavelengths reduce the spatial extent of induced velocity and shorter wavelengths inhibit the development of streamwise vorticity.

Turbine Vane Leading Edge Impingement Cooling With a Sweeping Jet

Abstract: A low speed linear cascade was used to investigate sweeping jet impingement cooling in a nozzle guide vane leading edge at an engine-relevant Biot number. Sweeping and steady jets were studied at varying mass flow rates and freestream turbulence intensities. Infrared thermography and a thermal inertia technique were used to determine the overall cooling effectiveness and internal heat transfer coefficients of the impingement cooling configurations. The circular jet array provided higher overall effectiveness values at both freestream turbulence intensities. The sweeping jet array provided a broader heat transfer profile due to the spreading of the jet. Pressure drop was measured for each jet geometry, and the circular jet was found to have less pressure drop than the sweeping jet at a given mass flow rate.

Sweeping jet impingement heat transfer on a simulated turbine vane leading edge

Abstract: With the development of additive manufacturing technology, it is now possible to design complex and integrated internal cooling architecture for a gas turbine engine. In search of a spatially uniform heat transfer at the leading edge of a turbine nozzle guide vane, a sweeping jet impingement cooling strategy was proposed. Experiments were conducted in a low-speed wind tunnel to investigate sweeping jet impingement cooling in a faired cylinder leading edge model at an engine-relevant Biot number (Bi). Sweeping jets were generated with additively manufactured fluidic oscillator and steady jets were produced by a cylindrical orifice (with length to diameter ratio of 1). Both sweeping and steady jets were studied at varying mass flow rates, jet-to-wall spacing (H/D), jet pitch (P/D), and freestream turbulence. The effect of varying aspect ratio (AR) of the sweeping jet geometries was also studied. The overall cooling effectiveness of each configuration was estimated using infrared thermography (IR) measurements of the external surface temperature of the leading edge model. The sweeping jet provided higher overall cooling effectiveness values compared to steady jet in specific configurations. The pressure drop across each jet was also measured for each geometry, and the sweeping jet shows comparable pressure drop to steady jet.

Iterative Blind Deconvolution Algorithm for Deblurring a Single PSP/TSP Image of Rotating Surfaces.

Abstract: Imaging of pressure-sensitive paint (PSP) for pressure measurement on moving surfaces is problematic due to the movement of the object within the finite exposure time of the imager, resulting in the blurring of the blade edges The blurring problem is particularly challenging when high-sensitivity PSP with a long lifetime is used, where the long luminescence time constant of exponential light decay following a burst of excitation light energy results in blurred images One method to ameliorate this effect is image deconvolution using a point spread function (PSF) based on an estimation of the luminescent time constant Prior implementations of image deconvolution for PSP deblurring have relied upon a spatially invariant time constant in order to reduce computational time However, the use of an assumed value of time constant leads to errors in the point spread function, particularly when strong pressure gradients (which cause strong spatial gradients in the decay time constant) are involved This work introduces an iterative method of image deconvolution, where a spatially variant PSF is used The point-by-point PSF values are found in an iterative manner, since the time constant depends on the local pressure value, which can only be found from the reduced PSP data The scheme estimates a super-resolved spatially varying blur kernel with sub-pixel resolution without filtering the blurred image, and then restores the image using classical iterative regularization tools A kernel-free forward model has been used to generate test images with known pressure surface maps and a varying amount of noise to evaluate the applicability of this scheme in different experimental conditions A spinning disk setup with a grazing nitrogen jet for producing strong pressure gradients has also been used to evaluate the scheme on a real-world problem Results including the convergence history and the effect of a regularization-iteration count are shown, along with a comparison with the previous PSP deblurring method

Variable characteristics fluidic oscillator and fluidic oscillator with three dimensional output jet and associated methods

Abstract: Various implementations include a fluidic oscillator having at least one control port. The at least one control port is for introducing a control fluid into the fluidic oscillator or suctioning the fluid stream from the fluidic oscillator. The introduction of a control fluid into the fluidic oscillator or suction of the fluid stream from the fluidic oscillator alters the frequency and sweeping angle of the oscillating fluid stream as it exits the fluidic oscillator. Various other implementations include a fluidic oscillator having a first control port defined by the first portion of the outlet nozzle and a second control port defined by the second portion of the outlet nozzle. The introduction of a control fluid into the fluidic oscillator or suction of the fluid stream from the fluidic oscillator through the control ports alters the exit angle of the oscillating fluid stream as it exits the fluidic oscillator.