Spectra Of Pressure Fluctuations Research Articles

Investigation of the correlation between noise & vibration characteristics and unsteady flow in a circulator pump

Circulator pumps have wide engineering applications but the acoustics, vibration and unsteady flow structures of the circulator pump are still not fully understood. We investigated the noise and vibration characteristics and unsteady flow structures in a circulator pump at different flow rates. Three-dimensional, unsteady RANS equations were solved on high-quality structured meshes with SST k-ω turbulence model numerically. Measurements were made in a semi-anechoic chamber to get an overview of noise and vibration level of a pump at different flow rates. The 1/3 octave-band filter technique was applied to obtain the explicit frequency spectra of sound, pressure fluctuations and vibration signals and their principal frequencies were identified successfully. The air-borne noise level of the designed condition is lower than that of the off-design conditions, and the highest sound pressure level is found at part-load condition. The acoustic emission from the pump is mainly caused by unsteady flow structures and pressure fluctuations. In addition, both the link between airborne noise and pressure fluctuation, and the correlation between vibration and unsteady hydrodynamic forces, were quantitatively examined and verified. This work offers good data to understand noise and vibration characteristics of circulator pumps and the relationships among the noise, vibration and unsteady flow structures.

Modelling wall pressure fluctuations under a turbulent boundary layer

The derivation of the wave vector-frequency (w-f) spectrum of wall pressure fluctuations below a turbulent boundary layer developed over a rigid flat plate is re-considered. The Lighthill's equation for pressure fluctuations is derived in a frame of reference fix with respect to the plate, at low Mach numbers, and transformed into the convected frame moving with the flow. To model the source terms of the Lighthill equation, it is assumed that in the inertial range, the turbulence is locally isotropic in the convected frame. The w-f spectrum of isotropic turbulence is obtained from symmetry considerations by extending the isotropy to space time, based on the concept of sweeping velocity. The resulting solution for the pressure w-f spectrum contains a term (the mean shear-turbulence term) which does not fulfill the Kraichnan Philipps theorem, due to the form of the selected turbulent velocity spectrum. The viscous effects are accounted for by a cut-off depending on wall distance; this procedure allows extending the model beyond the inertial range contribution. The w-f pressure spectrum is derived and compared to the experimental low wavenumber data of Farabee and Geib (1991) [8] and Bonness et al. (2010) [5], for which a good agreement is obtained. The derived expression is also compared to Chase theoretical model Chase (1987) [6] and found to agree well in the vicinity of the convective ridge of the subsonic domain and to differ significantly both in supersonic and subsonic low wavenumber limits. The pressure spectrum derived from the model and its scaling are discussed and compared to experimental data and to the empirical model of Goody (2002) [23], which results from the compilation of a large set of experimental data. Very good agreement is obtained, except at vanishing frequencies where it is claimed that the experimental results lack of significance due to the limited size of the experimental facilities. This hypothesis supported by the results obtained from numerical simulations.

Influence of the Density of Microspherical Catalysts on the Operating Modes of Fluidized Beds

Two types of fine particles of the Geldart class A with different bulk density, ρ n = 1200 kg/m3 and ρ n = 1300 kg/m3, were used for experimental studies of the circulating fluidized bed. A bench (0.7 m in diameter and 5.75 m in height) was used for the studies at room temperature with air as the fluidizing gas. The fluidization speed ranged from 0.1 to 0.75 m/s. The bed was sectioned in height using a set of horizontal diffuser grids. Fluctuations in average pressure drops and pressure distribution among the height of the fluidized bed were measured to determine the influence of the particle density on the operating modes of the fluidized bed. The transition rate, U c , was determined from mean square deviations of pressure drop fluctuations to equal U c = 0.40 m/s for lighter particles and Uc = 0.35 m/s for heavier particles. The transition rate, Uc, determined from the power energy spectrum of pressure fluctuations was 0.45 and 0.40 m/s for the lighter and heavier particles, respectively. The results of pressure measurements along the bed height showed that the pressure decreased linearly upward the bed, the decrease being faster for the heavier particles than for the lighter particles.

Suppression of gas-dynamic self-oscillations in the outflow line of a gas control valve

This article is devoted to the investigation of the causes and peculiarities of high-frequency gas-dynamic self-oscillations arising in the outflow line of gas control valves. We have demonstrated that these self-oscillations result in a substantial increase in the level of dynamic loading of the control valve elements due to the occurrence of intense tonal components in the pressure fluctuation spectra in the pipeline, the vibrations of the output flange of the control valve, as well as the external noise of the whole assembly. We have provided the results of investigating the conditions for high-frequency gas-dynamic self-oscillations arising from the gas flow interaction with the control valve elements, as well as with supporting pylons of the central fairing. It is shown that an increase in the flow above a certain value of output speed (40 m/s) results in tonal components of the spectrum of pressure pulsations. We have described the measures to suppress gas-dynamic self-oscillations by means of a perforated sleeve installed in the outflow line of the control valve. Three modifications of the sleeve design have been modeled, numerical simulation of gas-dynamic processes in the control valve exhaust duct with a perforated sleeve installed has been carried out. It is shown that complete elimination of gas-dynamic self-oscillations can be made possible by installing the sleeve in the control valve exhast duct.

Spectral characteristics of mesoscale variability of the bottom currents in the Ocean

We performed spectral analysis of variations in the bottom currents on the basis of direct measurements in a number of regions of the entire Atlantic Ocean. We compared the spectra of the atmospheric pressure fluctuations at sea level and fluctuations of current velocities in the bottom layer. A significant energy of current fluctuations near the bottom with a synoptic period in the range 10–15 days that exceeds similar energies in the other regions of the ocean was found in the region of the Newfoundland energetically active zone, where increased cyclonic activity in the atmosphere is observed.

Unsteady laminar gas flows in an annular nozzle

On the basis of the Navier–Stokes equations, a parametric study of the influence of various factors on the development of initial perturbations caused by starting an air-blown annular nozzle is performed for the laminar model of flow. Flow regimes are found in which the start perturbations accompanying the initiation of the nozzle do not attenuate but pass to a quasi-periodic mode. The frequency Fourier spectrum of pressure fluctuations at the center of the thrust wall and the value of the thrust of the nozzle are determined. Typical pulsed pressure signals obtained in the calculation model and detected in the experiments are presented.

Shock wave–boundary layer interaction in supersonic flow over a forward-facing step

We study in the present work a Mach 2.5 flow over a forward-facing step. The focus of the work is the flow ahead of the step, in particular, the unsteady interactions between the shock, the boundary layer and the separation bubble. The primary geometrical parameter in the problem is the ratio of the step height to the incoming boundary layer thickness,$h/\unicode[STIX]{x1D6FF}$, which is kept fixed at 2. Results are presented from detailed particle image velocimetry (PIV) measurements in two orthogonal planes to obtain a reasonable picture of the whole flow field. The mean velocity field in the central cross-stream or wall-normal ($x$–$y$) plane shows that the incoming boundary layer separates upstream of the step forming a large separation bubble ahead of the step, which can be relatively well resolved in PIV measurements compared to the compression ramp cases. Wall pressure fluctuation spectra close to the separation location show a dominant frequency ($f$) that is two orders of magnitude smaller than the characteristic frequency of the incoming boundary layer ($U_{\infty }/\unicode[STIX]{x1D6FF}$), consistent with low-frequency motions of the shock that have received a lot of recent attention ($U_{\infty }$ $=$ free-stream velocity,$\unicode[STIX]{x1D6FF}$ $=$ boundary layer thickness). PIV measurements in the wall-normal plane show large variations in shock position with time. The shock position measured from velocity data outside the boundary layer is found to be well correlated with the reverse flow area ahead of the step, and weakly correlated to structures in the incoming boundary layer. In contrast, the shock foot, determined from velocity data within the boundary layer, is found to be well correlated to the low- and high-speed streaks in the incoming boundary layer, in addition to the reverse flow area ahead of the step. Instantaneous velocity fields in the spanwise ($x$–$z$) plane parallel to the lower wall show that the shock is broadly two-dimensional with small spanwise ripples, while the recirculation region has very large spanwise variations. The spanwise-averaged shock location is found to be well correlated to the most upstream location of the recirculation region over a spanwise length ($x_{r,min}^{sp}$). Instantaneous velocity fields show that when some part of the recirculation region is far upstream, the corresponding nearly two-dimensional shock is also far upstream. On the other hand, when$x_{r,min}^{sp}$is relatively downstream, the resulting shock is also found to be downstream. Hence, the present results suggest that for the forward-facing step configuration, the large-scale streamwise motions of the shock are mainly correlated to the most upstream point of the recirculation region, which has large spanwise variations.

Acoustical characteristics of two-phase horizontal intermittent flow through an orifice

This study addresses acoustical characteristics, in particular, sound generation of intermittent flow regime in a horizontal pipe through an orifice, based on recorded pressure fluctuations and videos of upstream and downstream flow of the orifice. The flow regimes of interest are plug flow and slug flow, respectively, where sharp-edged and rounded entrance orifice are tested. In principle, the noise generated in the intermittent flow regime through an orifice increases with a higher gas flow rate. The pressure fluctuation spectra show a haystack configuration with a peak at a low frequency associated with a sequence of elongated gas bubbles. It demonstrates that the sound generation is predominately governed by dynamic of elongated gas bubbles in intermittent flow through the orifice. Particularly, in plug flow, an additional small hump is detected close to St = 0.2, i.e., vortex-shedding at the orifice exit contributes the sound generation in addition to the bubble dynamic. Furthermore, non-dimensional power of the downstream intermittent flow decays with St-a. The value of a increases with a higher liquid rate, and the haystack-like shape at the peak frequency widens as the gas rate increases. In plug flow, the rounded entrance orifice generates less noise by resulting in a smaller pressure drop compared to the sharp-edged orifice in general; however, it generates noise in high frequency.

Acoustic responses of the composite sandwich plates with lattice truss core to the subsonic turbulent boundary layer

Acoustic responses of composite sandwich plates with lattice truss core to the subsonic turbulent boundary layer are investigated. The lattice truss core is composed of four sets of orthogonal stiffeners, two sets of oblique rods and one set of vertical columns, which establishes a significant transfer path of the reactive loadings. Bending and axial motion of the vertical columns makes a great impact on the power spectral density (PSD) of the transverse displacements for the sandwich plates with lattice truss core. The convective region of the high subsonic turbulent boundary layer pressure fluctuation spectra is the predominant vibrational source in the wide frequency bands, which makes the skin plate effectively radiate sound power into the mean air flow and the trim plate transmit sound waves into the static air by wavenumber conversion. Radiated acoustic power of skin plate is much larger than transmitted sound power of the trim plate and the sandwich plates can attenuate wave propagation of the airflow turbulent wall-pressure. The principal energy of the turbulent boundary layer wall-pressure for the water flow with a low speed can be transformed into the vibrational and acoustic energy of the sandwich plates in the low frequency band, to great extent.

Pressure Loadings in a Rectangular Cavity with and Without a Captive Store

Simulations of the flow past a rectangular cavity containing a model captive store are performed using a hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation model. Calculated pressure fluctuation spectra are validated using measurements made on the same configuration in a trisonic wind tunnel at Mach numbers of 0.60, 0.80, and 1.47. The simulation results are used to calculate unsteady integrated forces and moments acting on the store. Spectra of the forces and moments, along with correlations calculated for force/moment pairs, reveal that a complex relationship exists between the unsteady integrated forces and the measured resonant cavity modes, as indicated in the cavity wall pressure measurements. The structure of identified cavity resonant tones is examined by visualization of filtered surface pressure fields.

A sparse recovery technique for the turbulent boundary layer wavenumber-frequency spectrum with non-uniformly spaced array of pressure transducers

Historically, determination of wavenumber-frequency spectra of turbulent wall pressure fluctuations has employed the use of evenly spaced linear arrays of sensors. The reason for this is that even spacing of pressure samples in both the temporal and spatial domains facilitates analysis by direct application of 2-D Fourier transforms. The spacing of pressure measurements directly impacts the resolution of the resulting wavenumber-frequency spectra. This rigid spacing requirements of traditional methods may preclude analysis altogether with even a single sensor failure within the array. Rather than determining sensor locations based on the simplest analysis method, the sparseness of the wavenumber-frequency spectra can be leveraged to allow non-uniform sampling in both temporal and spatial domains. Sparse recovery techniques as an analysis tool for wavenumber-frequency spectra will be demonstrated both theoretically and experimentally for uniformly spaced arrays, uniformly spaced arrays with missing measure...

Computation of vortical flow and flow induced noise by large eddy simulation with FW-H acoustic analogy and Powell vortex sound theory

The sound generated by a NACA0012 airfoil in the wake of a rod is numerically simulated by two approaches, one is the large eddy simulation (LES) with the FW-H acoustic analogy and the other is the LES with the Powell vortex sound theory, in order to compare the accuracies of their predictions. The vortical structures around the rod-airfoil are computed by the LES and captured by the vortex identification (Q). The acoustic predictions are verified by the measurements. It is shown that the computed results by the two hybrid approa- ches (LES and FW-H, LES and Powell) are very similar. Both are shown to be satisfactory in the prediction of the noise generated by an unsteady flow. Subsequently, the numerical simulations of the wall pressure fluctuations and the flow-induced noise of a NACA0015 airfoil are made by the two hybrid approaches. At two angles of attack (0° and 8°), the wall pressure fluctuations of the NACA0015 airfoil are computed. The obtained power spectra of the wall pressure fluctuations are analyzed and compared with the measured data. And the vortical structures around the airfoil at two angles of attack are simulated and analyzed. After that, the flow induced noises of the NACA0015 airfoil at two angles of attack are predicted by the two hybrid approaches (LES and FW-H, LES and Powell). The radiated sound spectra are analyzed and compared with the experimental data. Comparisons show that both are robust, credible and satisfactory in the numerical prediction of the flow induced noise. All numerical simulations are carried out by parallel processing in the Wuxi supercomputing center.

Fast PSP measurements of wall-pressure fluctuation in low-speed flows: improvements using proper orthogonal decomposition

Fast pressure-sensitive paint (PSP) is very useful in flow diagnostics due to its fast response and high spatial resolution, but its applications in low-speed flows are usually challenging due to limitations of paint’s pressure sensitivity and the capability of high-speed imagers. The poor signal-to-noise ratio in low-speed cases makes it very difficult to extract useful information from the PSP data. In this study, unsteady PSP measurements were made on a flat plate behind a cylinder in a low-speed wind tunnel (flow speed from 10 to 17 m/s). Pressure fluctuations (ΔP) on the plate caused by vortex–plate interaction were recorded continuously by fast PSP (using a high-speed camera) and a microphone array. Power spectrum of pressure fluctuations and phase-averaged ΔP obtained from PSP and microphone were compared, showing good agreement in general. Proper orthogonal decomposition (POD) was used to reduce noise in PSP data and extract the dominant pressure features. The PSP results reconstructed from selected POD modes were then compared to the pressure data obtained simultaneously with microphone sensors. Based on the comparison of both instantaneous ΔP and root-mean-square of ΔP, it was confirmed that POD analysis could effectively remove noise while preserving the instantaneous pressure information with good fidelity, especially for flows with strong periodicity. This technique extends the application range of fast PSP and can be a powerful tool for fundamental fluid mechanics research at low speed.

Numerical simulation of dynamic flow characteristics in a centrifugal water pump with three-vaned diffuser

The complex three-dimensional turbulent flow field in a centrifugal water pump with three asymmetrical diffusers was numerically simulated. The characteristics of pressure and force fluctuations inside the model pump were investigated. Fast Fourier transformation was performed to obtain the spectra of pressure and force fluctuations. It indicates that the dominant frequency of pressure fluctuations is the blade passing frequency in all the sub-domains inside the pump and the first blade passing frequency energy (first order of blade passing frequency) is the most significant. The dominant frequency of pressure fluctuations at the location of diffuser outlet is featured by low frequency (less than 1 Hz), which may be due to the locally generated eddy structures. Besides, the dominant frequency force fluctuations on the impeller blades are also the blade passing frequency. The existence of the three asymmetrical diffusers has damping effect on the pressure fluctuation amplitude and energy amplitude of pressure fluctuations in the diffuser domain dramatically, which indicates that the diffusers can effectively control the hydraulically excited vibration in the pump. Besides, the prediction of the dominant frequency of pressure fluctuations inside the pump can help to utilize the pump effectively and to extend the pump life. The main findings of this work can provide prediction of the pump performance and information for further optimal design of centrifugal pumps as well.

Influence of stator vane number on performance of the axial-flow pump

The interplay of impeller and stator is investigated for an axial-flow pump. Three stator vane numbers of 5, 7 and 9 are devised to match the same impeller. The renormalization group k-e turbulence model is used to simulate three-dimensional flows in three pumps with different vane numbers. Axial velocity distributions at impeller outlet and stator outlet are comparatively analyzed. Experiments assess operation performance of the three pumps. Vibration parameters and static pressure fluctuations are measured as well. It is indicated that the influence of vane number on both pump head and pump efficiency is insignificant. Large stator vane number contributes to the improvement of the uniformity of axial velocity distribution at impeller outlet. At stator outlet, similar tendency is revealed. Severe vibration occurs near the outlet bend of the pump, as is particularly remarkable at vane number of 9. For the three cases, blade passing frequency and its harmonics are predominant in the frequency spectra of pressure fluctuations. As flow rate increases from 0.8Q to 1.0Q, high-frequency pressure fluctuations are suppressed considerably at vane number of 9, while the other two cases also manifest a decline in overall pressure fluctuation amplitude. The 7-vane case is the most preferable one among the three cases in terms of both pump performance and pressure fluctuation between the impeller and the stator.

Trailing edge noise estimation by tomographic Particle Image Velocimetry

The feasibility of estimating broadband trailing edge noise with high-speed tomographic Particle Image Velocimetry (PIV) measurements is studied. A thin plate terminating in a sharp trailing edge provides a generic test case for turbulent boundary layer trailing edge interaction noise.Far-field noise is linked to the wavenumber–frequency spectrum of the surface pressure fluctuations in proximity of the trailing edge through diffraction theory. High-speed tomographic PIV measurements return volumetric and time-resolved information about all velocity components for the resolved spatio-temporal scales and can therefore provide the required statistical quantities. For the turbulent boundary layer interacting with the trailing edge, these statistics include the auto-spectral density, spanwise correlation length, and convection velocity of the unsteady surface pressure, which are thus estimated.Acoustic phased array measurements in an anechoic environment provide a reference for comparison. Over the resolved frequency band, PIV based noise estimation results compare favorably with the reference measurements. Especially at lower frequencies, where existing, empirical models for the unsteady surface pressure spectrum are not accurate, tomographic PIV can offer an alternative approach to complex and intrusive model instrumentation for assessing the relevant statistical quantities.

Elimination of gas dynamic resonance modes of vibration loads of gas distribution station pipeline fitting

The article contains a study of the modes of resonance interaction of a high-speed gas flow exiting from the metering sections of pressure regulators with their elements. It is shown that the interaction leads to a significant increase of the valve noise due to the tone component of the spectrum of vibration energy which occurs at the frequency characteristics of pressure fluctuations in the pipeline, the vibration of the controller output flange, as well as the external valve noise. The results of analyzing the prerequisites for resonant interaction of the gas flow with the regulator structural elements including the pylons of the central fairing are presented in the paper. It is shown that an increase in the output flow rate above a certain value (about 40 m / s) gives rise to discrete components of the spectrum of pressure fluctuations. The results of experimental analysis of the pressure regulator elements showing practical coincidence of the frequencies of discrete components of the flow pulsation spectrum and free oscillations of construction elements are also presented. Modal analysis of the natural oscillations of regulator elements is performed by tapping. Measures to remove resonant interaction of the output flow with the pylons that consist in installing special flow-splitting structures - perforated cylindrical sleeves are proposed. Other measures to eliminate the resonant interaction under consideration are also possible.

Numerical investigation of tandem-cylinder noise reduction using plasma-based flow control

AbstractThe noise of flow over tandem cylinders at $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{\mathit{Re}}_D= 22\, 000$ and its reduction using single dielectric barrier discharge (SDBD) plasma actuators are simulated numerically both to confirm and extend experimental results. The numerical approach is based on large-eddy simulation (LES) for the turbulent flow field, a semi-empirical plasma actuation model, and Lighthill’s theory for acoustic calculation. Excellent agreement between LES and experimental results is obtained for both the baseline flow and flow with plasma control in terms of wake velocity profiles, turbulence intensity, and frequency spectra of pressure fluctuations on the downstream cylinder. The validated flow-field results allow an accurate acoustic analysis based on Lighthill’s equation, which is solved using a boundary-element method. The effectiveness of plasma actuators for reducing noise is clearly demonstrated. In the baseline flow, the acoustic field is dominated by the interaction between the downstream cylinder and the upstream wake. Through suppression of vortex shedding from the upstream cylinder, the interaction noise is reduced drastically by the plasma flow control, and the vortex-shedding noise from the downstream cylinder becomes equally important. At a free-stream Mach number of 0.2, the peak sound pressure level is reduced by approximately 16 dB. This suggests the viability of plasma actuation for active aeroacoustic control of airframe noise.

Extension of Lysak’s Approach to Evaluate the Wall Pressure Spectrum for Boundary Layer Flows

An analytical model proposed by Lysak to evaluate the wall pressure fluctuation spectrum and variance for pipe flows is extended to use it as a post-processor of RANS computations, for attached boundary layers, whatever the turbulence model. Model extension also deals with three-dimensional flows. The model fairly reproduces key features of the wall pressure spectrum such as the f 2 behaviour in the low frequency range, the inertial f −1 region and the viscous cut-off. Wall pressure spectra and variance can be fairly well predicted. The model allows identifying the contribution of each part of the boundary layer to the wall pressure spectrum. The model was validated for zero pressure gradient boundary layer flows. Sources of differences in the predictions according to the turbulence model and ways of improvement were identified. For adverse pressure gradient boundary layers, departures from experiments are observed and several plausible causes are pointed out.

Numerical Study on the Characteristics of Pressure Fluctuations in an Axial-Flow Water Pump

Flow induced vibration due to the dynamics of rotor-stator interaction in an axial-flow pump is one of the most damaging vibration sources to the pump components, attached pipelines, and equipment. Three-dimensional unsteady numerical simulations were conducted on the complex turbulent flow field in an axial-flow water pump, in order to investigate the flow induced vibration problem. The shear stress transport (SST) k-ω model was employed in the numerical simulations. The fast Fourier transform technique was adopted to process the obtained fluctuating pressure signals. The characteristics of pressure fluctuations acting on the impeller were then investigated. The spectra of pressure fluctuations were predicted. The dominant frequencies at the locations of impeller inlet, impeller outlet, and impeller blade surface are all 198 Hz (4 times of the rotation frequency 49.5 Hz), which indicates that the dominant frequency is in good agreement with the blade passing frequency (BPF). The first BPF dominates the frequency spectrum for all monitoring locations inside the pump.