Unsteady Pressure Spectra Research Articles

Identification of Compressor Vibration Aerodynamic Forcing Mechanisms by Spectral Characteristics

Abstract Nonsynchronous vibration (NSV) in axial compressors can be caused either by (1) unsteady aerodynamic forces that are not related to motion of the blades or (2) motion-dependent aerodynamic forcing (e.g., flutter). Aerodynamic forcing mechanisms can be challenging to identify in experimental observations of NSV because the temporal vibration characteristics for both forcing mechanisms can appear similar. This work proposes a method for distinguishing between the two mechanisms using spectral characteristics. The method provides an interpretation of experimental data explicitly consistent with the analytical models used to differentiate between forced response and flutter. Two cases of NSV were observed in a 1.5-stage axial compressor at near-stall conditions. The circumferential wavenumber-dependent unsteady pressure spectra and nonintrusive stress measurement system (NSMS) spectra were observed to have distinct characteristics for the two NSV cases. Based on these distinct spectral characteristics, the first case was identified as blade-row aerodynamic forcing, while the second was identified as motion-dependent (flutter). Numerical simulations confirmed low aerodynamic damping at the conditions where flutter was observed.

On the unsteady dynamics of partially shrouded compressible jets

We experimentally investigate a partially shrouded sonic jet (a sonic free-jet shielded by a solid wall-extension on one side) exiting from a planar nozzle at two different nozzle pressure ratio ($\zeta=4$ and $5$). We experimentally show that the inherent jet unsteadiness from the shock-induced flow separation on the wall and the emitted noise in the far-field is strongly coupled through a series of experiments like high-speed schlieren, wall-static pressure, unsteady pressure spectra, and microphone measurements. The partially shrouded jet's lateral free expansion is also identified to be complicated, three-dimensional, and the produced noise is directional. The emitted acoustic pulses from the flapping-jet, the radiated noise from the shock-induced separation on the wall, and the shock-shear layer interaction on the other side of the wall are responsible for the generated acoustic disturbances. The non-uniform aeroacoustic forcing on the top and bottom portion of the partially wall-bounded jet shear layer leads to a self-sustained jet oscillation and a discrete sound emission. The vital features are identified through the proper orthogonal decomposition of high-speed schlieren images and supplemented by other measurements.

Using High-Frequency Pulsed Supersonic Microjets to Control Resonant High-Speed Cavity Flows

A novel actuator concept is evaluated in a series of active flow control experiments on a resonant high-speed cavity flow. The actuator generates pulsed supersonic microjets by using the resonance of an impinging microjet source, and with smart materials incorporated into its design, the actuator’s resonant frequency can be actively controlled. The actuator was designed such that its resonant frequency would lie within the range of predicted cavity resonance. Results from Mach 1.5 flow over a cavity of length are presented, and the actuator’s performance is evaluated across three modes of operation: pulsed, active pulsed, and steady. In the active pulsed mode, the smart materials are used to actively vary the actuator’s resonant frequency, enabling frequency modulation of the actuator’s output. When the actuator operates in both pulsed modes, the amplitude of the dominant peak in the cavity’s unsteady pressure spectra is reduced by as much as 7 dB. However, steady microjet injection yields more effective control. The dominant peak is substantially attenuated (by 25 dB) in this mode of operation. The effectiveness of the steady mode is attributed to higher momentum flux that can be attained in contrast to the pulsed modes of operation.

Dynamical aspects of a backward-facing step flow at large Reynolds numbers

A contribution to the study of separating and reattaching flows over a backward-facing step configuration is reported. Simultaneous unsteady wall-pressure and velocity field measurements are made to investigate the dynamical aspects of the separated flow field at high Reynolds numbers, up to 182,600. An array of 25 unsteady pressure sensors in the streamwise direction coupled with PIV velocity fields is used to analyze the statistical properties as well as the streamwise time–space characteristics of the separated flow. A comparison with some previous studies is done to highlight the expansion ratio influence and dependency on the main separation flow. Emphasis is placed on the dynamical aspects of turbulent flows at a wide range of Reynolds numbers, specifically in the convective motion of the vortical flow structures. Unsteady pressure spectra show, even for high Reynolds numbers, the relative dominance of low-frequency flapping motion over high-frequency modes of the large-scale vortical structure. The latter shows a ‘waked mode’ behavior, where these large-scale coherent structures grow in place and further accelerate in the downstream direction. The ability to understand the flow field unsteadiness will lead future investigations to develop active and/or passive flow control techniques in separating and reattaching flows.

Design of an experiment to measure unsteady shear stress and wall pressure transmitted through an elastomer in a turbulent boundary layer

A flat plate that is exposed to a turbulent boundary layer (TBL) experiences unsteady velocity fluctuations which result in fluctuating wall pressures and shear stresses on the surface of the plate. There is an interest in understanding how fluctuating shear stresses and normal pressures generated on the surface of an elastomer layer exposed to a TBL in water are transmitted through the layer onto a rigid backing plate. Analytic models exist which predict these shear stress and normal pressure spectra on the surface of the elastomer as well as those transmitted through the elastomer. The design of a novel experiment is proposed which will utilize Surface Stress Sensitive Films (S3F) to measure the fluctuating shear stress and hydrophones to measure fluctuating normal pressure at the elastomer-plate interface. These experimental measurements would then be compared to models of unsteady shear and unsteady pressure spectra within a TBL for purposes of model validation. This work will present the design of an experiment to measure the unsteady pressure and unsteady shear at the elastomer-plate interface and the methodology for comparing the measured results to the analytic model predictions.

Supersonic Flow over a Shallow Open Rectangular Cavity

An experimental investigation was conducted on a supersonic flow at a freestream Mach number of 2 over a shallow open cavity, including the effects of adding streamwise serrated edges. These flows have relevance to weapons bays and airframe gaps on high-speed aircraft. The measurements consisted of single-shot and time-resolved schlieren visualization, as well as unsteady pressure spectra. The length-to-depth ratio of the cavity was 8. The tests conducted at different Reynolds numbers with the baseline cavity (straight leading and trailing edges) showed that increasing the Reynolds number increases the root-mean-square pressure inside the cavity. The addition of serrations to the cavity leading or trailing edge did not show any significant effect on the separating shear layer nor in controlling the oscillations of the shear layer. There was also no noticeable effect on the overall sound pressure levels inside the cavity. A new expression for calculating shallow-cavity resonant frequencies applicable at supersonic Mach numbers is proposed. It is shown that it compares well with experiments and the well-known modified Rossiter relation.

Role of coherent structures in supersonic impinging jets

This paper describes the results of a study examining the flow field and acoustic characteristics of a Mach 1.5 ideally expanded supersonic jet impinging on a flat surface and its control using steady microjets. Emphasis is placed on two conditions of nozzle to plate distances (h/d), of which one corresponds to where the microjet based active flow control is very effective in reducing flow unsteadiness and near-field acoustics and the other has minimal effectiveness. Measurements include unsteady pressures, nearfield acoustics using microphone and particle image velocimetry. The nearfield noise and unsteady pressure spectra at both h/d show discrete high amplitude impinging tones, which in one case (h/d = 4) are significantly reduced with control but in the other case (h/d = 4.5) remain unaffected. The particle image velocimetry measurements, both time-averaged and phase-averaged, were used to better understand the basic characteristics of the impinging jet flow field especially the role of coherent vortical structures in the noise generation and control. The results show that the flow field corresponding to the case of least control effectiveness comprise well defined, coherent, and symmetrical vortical structures and may require higher levels of microjet pressure supply for noise suppression when compared to the flow field more responsive to control (h/d = 4) which shows less organized, competing (symmetrical and helical) instabilities.

Computational investigation of cavity flow control using a passive device

AbstractIn this paper, flow control effectiveness of a passive device in relation to open cavity flowfield is investigated computationally and compared with experimental work. Specifically the modification in the cavity flowfield due to the presence of a spoiler is studied in details to explain the physics behind the flow control effects. A combination of 2D and 3D flow visualisation tools are used to understand the flow behaviour inside the cavity and the quantitative analysis of the unsteady pressure fluctuations is also performed to assess the unsteady effects. Flow simulations with a turbulence model based on a hybrid RANS/LES (commonly known as Detached-Eddy Simulation (DES)) are used in this study. The time-mean flow visualisation clearly showed the presence of three dimensional effects inside the empty cavity whereas the 3D effects were found to diminish in the presence of a spoiler. In the unsteady flow analysis, near-field acoustic spectra were computed for empty cavity as well as cavity-with-spoiler cases. Study of unsteady pressure spectra for the cavity-with-spoiler case was found to record the complete suppression of the dominant tones in the presence of the spoiler. The analysis has indicated that the main reason behind this suppression is due to the inability of faintly energised vortical structures (faintly energised as a result of the extraction of turbulent kinetic energy by the spoiler) to maintain the unsteady flapping of the separated shear layer.

Computational study of flowfield characteristics in cavities with stores

AbstractIn this paper, the results of computational studies on the unsteady flow features in three-dimensional empty cavities and cavities with a representative store are presented. Flow simulations with a turbulence model based on a hybrid method, which behaves as a standard Reynolds-averaged Navier-Stokes (RANS) model within the attached boundary layer and as a Large-Eddy Simulation LES sub-grid scale model in the rest of the flow (commonly known as Detached-Eddy Simulation (DES)) are used in this study. The time-mean flow study showed the presence of three-dimensional effects inside the cavities. The mean flowfield visualisation also clearly showed the presence of a pair of ‘tornado-like’ vortices in the upstream half of the cavity which merge to a single, large recirculation further downstream. Visualisation for the cavity-with-store case revealed that the mean flowfield was effectively divided into two halves with significant reduction of the spanwise flow across the cavity width. In the unsteady flow study, near-field acoustic spectra were computed for the empty cavity and cavity-with-store cases. Study of unsteady pressure spectra for the cavity-with-store case found the presence of many peaks and the corresponding mode frequencies were found to agree well with the Rossiter modes. The blockage effect of store and strut on the spanwise flow is thought to have reduced the interaction, and subsequent non-linear coupling, between the Rossiter modes. This may be the reason for the co-existence of multiple modes without the coupling among them.

Thermal Cavitation Experiments on a NACA 0015 Hydrofoil

The present paper illustrates the main results of an experimental campaign conducted in the Thermal Cavitation Tunnel of the Cavitating Pump Rotordynamic Test Facility (CPRTF) at Centrospazio/Alta S.p.A. Experiments were carried out on a NACA 0015 hydrofoil at various incidence angles, cavitation numbers, and freestream temperatures, in order to investigate the characteristics of cavitation instabilities and the impact of thermal cavitation effects. Measured cavity length, surface pressure coefficients, and unsteady pressure spectra are in good agreement with the data available in the open literature and suggest the existence of a strong correlation between the onset of the various forms of cavitation and instabilities, the thermal cavitation effects, and the effects induced by the presence of the walls of the tunnel. Further analytical investigations are planned in order to provide a better interpretation of the above results.