Eric F. Spina

Mode-switching and nonlinear effects in compressible flow over a cavity

Multiple distinct peaks of comparable strength in unsteady pressure autospectra often characterize compressible flow-induced cavity oscillations. It is unclear whether these different large-amplitude tones (i.e., Rossiter modes) coexist or are the result of a mode-switching phenomenon. The cause of additional peaks in the spectrum, particularly at low frequency, is also unknown. This article describes the analyses of unsteady pressure data in a cavity using time-frequency methods, namely the short-time Fourier transform (STFT) and the continuous Morlet wavelet transform, and higher-order spectral techniques. The STFT and wavelet analyses clearly show that the dominant mode switches between the primary Rossiter modes. This is verified by instantaneous schlieren images acquired simultaneously with the unsteady pressures. Furthermore, the Rossiter modes experience some degree of low-frequency amplitude modulation. An estimate of the modulation frequency, obtained from the wavelet analysis, matches the low-fr...

Control of high-speed impinging-jet resonance

The intense pressure fluctuations generated by high-speed impinging jets dramatically increase in amplitude and exhibit peaks at discrete frequencies under certain flow conditions. It is generally accepted that a feedback loop consisting of downstream-convecting organized motions and upstream-propagating acoustic disturbances is responsible for the maintenance of this flow resonance. We describe an effort to control impinging-jet resonance through the addition of an annular stream. Fluctuating wall-pressure and near-field acoustic measurements were made in a high-speed (M j =0.8-1.6) impinging cold jet issuing from coaxial round converging nozzles. Coherent motions critical to the maintenance of the feedback loop were examined via real-time conditional acquisition of schlieren images

The structure of a supersonic turbulent boundary layer subjected to concave surface curvature

This paper reports an experimental investigation of the instantaneous structure of a supersonic turbulent boundary layer ( M = 2.86, Re θ = 82000) over a short region of longitudinal concave surface curvature. The radius of curvature was 12 initial boundary-layer thicknesses and the turning angle was 16°. Severe distortion of the boundary layer occurred, as evidenced by an alteration of the mean velocity profiles and an increase in wall shear stress of 125%. The large-scale organized motions in the boundary layer were significantly altered as illustrated by changes in the character of the mass flux ‘fronts’ (large gradients in the fluctuating streamwise mass flux).

Constant-temperature anemometry in hypersonic flow: critical issues and sample results

The critical issues are examined in the application of constant-temperature hot-wire anemometry to hypersonic boundary layers. While continuous turbulence measurements are more challenging to make in hypersonic flows, it is shown here that the difficulties can be overcome for a wide range of flow conditions. An extensive review of the literature reveals that many of the heat transfer complexities associated with hypersonic anemometry have already been resolved. Frequency-response tests, calibration results and boundary-layer measurements in hypersonic flow are also presented. A hot-wire frequency response of about 500 kHz was obtained in a Mach 11 flow, and the resulting boundary-layer spectra are smooth and repeatable.

Convection velocity in supersonic turbulent boundary layers

The convection velocity of large‐scale organized motions was measured in a Mach 3 boundary layer using streamwise‐separated hot wires. The broadband convection velocity, determined from space‐time correlations, is 0.9U∞ across the entire outer region of the boundary layer. The convection velocity of individual structures was deduced using a pattern recognition technique, and this revealed that the majority of the large‐scale structures convect at nearly the same velocity. A comparison of the present results to previous measurements indicates negligible Reynolds number and Mach number effects on convection velocity.

Progress in hot-film anemometry for hypersonic flow

A robust microsensor hot-film probe has been developed to obtain continuous turbulence measurements in moderately severe hypersonic environments. This probe represents a significant advancement of an existing concept. Optimization for hypersonic flow is achieved through sensor placement, the use of high-temperature materials, and state-of-the-art microphotolithographic fabrication techniques. Tests in Mach 6 air and Mach 11 helium flows have been very promising. The probe has exhibited excellent characteristics, including a frequency response that is a factor of 5 higher than that of previous hot-film sensors. The qualitative spectra of the anemometer fluctuations has been smooth and repeatable. Despite these improvements, heat conduction into the substrate still complicates the probe response. A review of the physics of this problem is presented along with a plan for the characterization of the response via several dynamic calibration techniques.

An improved analysis method for cross-wire signals obtained in supersonic flow

The use of crossed-wire probes to measure simultaneously the instantaneous stream-wise and normal velocities in supersonic turbulent flows has enabled researchers to investigate the characteristics of organized structures more fully. This paper examines both the practical aspects of using crossed-wire probes in supersonic flow and several methods of converting the resulting signals into useful quantities. Three small perturbation methods are compared in a Mach 2.9 boundary layer, and it is shown that the higher-order terms neglected in the traditional first-order perturbation analysis can alter the instantaneous velocity signals. This is particularly true for regions of intense streamwise mass flux fluctuations. A fourth method, which calculates the instantaneous flow angle directly from the inclined-wire formulation of Kings Law, is introduced and discussed. While this method is potentially more accurate than the small perturbation techniques, it is more sensitive to parameter drift during the period between the wire calibration and actual testing.

Determination of the frequency response of a constant-voltage hot-wire anemometer

The dynamic response of the CVA system was investigated both analytically and experimentally. The frequency response functions of the CVA system for a number of different circuit parameters and operating conditions (Re, r) were determined via laser-based radiative heating of the hot-wire sensor. A 2-order linear systems model of the CVA was developed to provide insight to the dynamic response and to help interpret the experimental results. With the use of properly selected circuit components, a bandwidth in excess of 100 kHz can be achieved (« 350 kHz in this study). The qualitative variations in the frequency response function with changes in circuit parameters are in agreement with the 2-order model. The frequency-response functions of the CVA systems used in this study were found to have little dependence on the operating conditions of the wire.

Research and educational initiatives at the Syracuse University Center for Hypersonics

The Department of Mechanical, Aerospace, and Manufacturing Engineering and the Northeast Parallel Architectures Center of Syracuse University have been funded by NASA to establish a program to educate young engineers in the hypersonic disciplines. This goal is being achieved through a comprehensive five-year program that includes elements of undergraduate instruction, advanced graduate coursework, undergraduate research, and leading-edge hypersonics research. The research foci of the Syracuse Center for Hypersonics are three-fold; high-temperature composite materials, measurements in turbulent hypersonic flows, and the application of high-performance computing to hypersonic fluid dynamics.

Elimination of impinging-jet resonance

The pressure fluctuations generated by highspccd impinging jets, normally intense, dramatically increase in amplitude and exhibit peaks at discrete frequencies under certain flow conditions. I t is generally accepted that a feedback loop consisting of downstream-convecting organized motions and upstream-propagating acoustic disturbances is responsible for the maintenance of this flow resonance. This paper describes an effort to control impinging jet resonance through the addition of an annular stream. Fluctuating wall-pressure and near-field acoustic measurements were made in a high-speed (Mj = 0.8 to 1.6) impinging je t issuing from coaxial round, converging w nozzles. Coherent motions critical to the maintenance of the feedback loop were examined via real-time conditional acquisition of schlieren images. The results of the passive control were dramatic. The discrete impingement tone was eliminated and the broadband noise was decreased by an order of magnitude when the Mach number of the annular jet was properly selected. It is hypothesized that the annular stream acoustically shields the nozzle lip from the full strength of the upstream-propagating disturbances. This interferes with the creation and amplification of thc downstreamconvecting organized motions, thereby interrupting the fluid-dynamic feedback.