Boundary Layer Pressure Fluctuations Research Articles

Passive Flow Velocity and Direction Measurement System Under Interference Environment Based on the Turbulent Boundary Layer Pressure Fluctuation Field

Passive Flow Velocity and Direction Measurement System Under Interference Environment Based on the Turbulent Boundary Layer Pressure Fluctuation Field

Similitude method for self-noise under boundary layer pressure fluctuation excitation.

In this paper, the authors propose a similitude method based on the Π-theorem to predict the noise generated by pressure fluctuations along the boundary layer of a full-scale model by using a small-scale model. We begin by investigating the theory of the similitude method and introducing an approach to control the flow regime. We also provide an account of active and passive methods of controlling the flow regime. We analyze a plate model and a rotary model by using the computational fluid dynamics method to validate the effectiveness of the proposed similitude method and subject it to error analysis. The results demonstrate that it can accurately predict flow-induced noise in the full-scale model based on the noise observed in the small-scale model. We also provide methods to identify transitions in flow in the Appendix.

Wall pressure fluctuations in supersonic boundary layers over compression ramps with different turning angles

In the present study, we investigate influences of shock intensity on wall pressure fluctuations by performing direct numerical simulations of supersonic turbulence boundary layers over compression ramps with different turning angles. We found that as the turning angle increases, low-frequency motions of the separation shock are enhanced, accompanied by enlarged energetic pressure structures with lower convection velocities. By inspecting wavenumber-frequency spectra under the assumption of streamwise homogeneity, we further identified two energetic modes convected at different velocities. The one with the lower convection velocity, namely, the “slow mode,” inherited from the upstream pressure fluctuations of the turbulent boundary layer, is decelerated when passing through the oblique shock, during which the “rapid mode” with pressure fluctuations convected at higher speeds are generated. The increasing turning angle decelerates the slow mode and intensifies the fast mode. The reconstruction of the flow field suggests that the rapid mode is associated with the shear layer generated adjacent to the interaction zone, while the slow mode is associated with the Görtler vortices on the ramp.

Acoustic optimization design of window of high-speed train under flow-induced vibration

In this study, the sound optimization design of a high-speed train window structure under a turbulent boundary layer (TBL) pressure fluctuation is investigated. First, the Corcos model is used to characterize the cross-spectrum of the dynamic surface pressure, and analytical solutions of the window speed response, radiated sound power, and transmission loss (TL) are obtained. The effects of the cavity thickness, cavity damping, panel thicknesses, and panel damping on the TL were investigated by numerical examples. In the structural sound optimization process, the quality reduction and the sound response are essentially in conflict with each other; therefore, non-dominated sorting genetic algorithm (NSGA)-II is adopted for a dual-objective design. The numerical results show that the train speed has a significant influence on the TL of the window structure, which decreases by 4 dB when the train speed increases by 100 km/h. With the increase in the train speed, the effect of improving the TL of the window structure by increasing the thickness and damping of the window cavity gradually decreases. The TL of the window structure is more sensitive to the thickness of the window panels than that of other panels parameters; however, the thickness sensitivity decreases with the increase in the speed, whereas the improvement effect of panel damping on the TL increases with the increasing train speed. The Pareto optimal design of the window structure at 350 km/h was obtained using the NSGA-II optimization design method. The optimal design reduced the mass by 41.6 % and increased the average transmission loss (TLavg) increased by 6.2 dB compared to those of the original window structure.

Experimental study of the influence of structural parameters on pressure fluctuations of cone–cylinder–hemisphere models

When a viscous fluid flows over the surface of an object, different regions of the wall form different boundary layers. Fluctuating pressure in this boundary layer acts on the surface of the structure, causing it to vibrate and radiate noise; simultaneously, structural deformations will also have an impact on the flow field, and boundary layer pressure fluctuation is the most important component of flow noise. The characteristic parameters of a model, such as its wall thickness and the length of its parallel body section, will affect the pressure fluctuations it experiences. However, most studies treat the structure of the model as a rigid body. Therefore, this paper conducted experiments to examine the influences of the wall thickness and the parallel body length of a model on the pressure fluctuations it experiences. It was found that the fluctuating pressure at a given measuring position increases with decreasing wall thickness, and it decreases with increasing parallel body length. Then, this study demonstrated through comparative experiments that elastic and scale effects are important factors that cannot be ignored in calculations and experiments relating to pressure fluctuations. In addition, according to the characteristics of pressure fluctuation test values in different regions, the pressure-fluctuation prediction empirical formulas for different regions of the boundary layer were established or improved on the basis of previous research on pressure fluctuation in different regions of the boundary layer. Finally, by pasting a flow exciter at the transition position in the boundary layer of the model can keep its flow noise down, the experimental results show that rough particles can split the large vortex into smaller vortices and reduce flow noise by more than 5 dB. These results and empirical formulas provide references for numerical and experimental research examining pressure fluctuations.

Lattice Boltzmann very large eddy simulations of a turbulent flow over covered and uncovered cavities

Microphone measurements in a closed test section wind tunnel are affected by turbulent boundary layer (TBL) pressure fluctuations. These fluctuations are mitigated by placing the microphones at the bottom of cavities, usually covered with a thin, acoustically transparent material. Prior experiments showed that the cavity geometry affects the propagation of TBL pressure fluctuations toward the bottom. However, the relationship between the cavity geometry and the flowfield within the cavity is not well understood. Therefore, a very large-eddy simulation was performed using the lattice Boltzmann method. A cylindrical, a countersunk and a conical cavity are simulated with and without a fine wire-cloth cover, which is modeled as a porous medium governed by Darcy's law. Adding a countersink to an uncovered cylindrical cavity is found to mitigate the transport of turbulent structures across the bottom by shifting the recirculation pattern away from the cavity bottom. Covering the cavities nearly eliminates this source of hydrodynamic pressure fluctuations. The eddies within the boundary layer, which convect over the cover, generate a primarily acoustic pressure field inside the cavities and thus suggesting that the pressure fluctuations within covered cavities can be modeled acoustically. As the cavity diameter increases compared to the eddies' integral length scale, the amount of energy in the cut-off modes increases with respect to the cut-on modes. Since cut-off modes decay as they propagate into the cavity, more attenuation is seen. The results are in agreement with experimental evidence.

Turbulence and pressure fluctuations in rough wall boundary layers in pressure gradients

Turbulence and pressure fluctuations in rough wall boundary layers in pressure gradients

Wall-cooling effects on pressure fluctuations in compressible turbulent boundary layers from subsonic to hypersonic regimes

Pressure fluctuations play an essential role in the transport of turbulent kinetic energy and vibrational loading. This study focuses on examining the effect of wall cooling on pressure fluctuations in compressible turbulent boundary layers by high-fidelity direct numerical simulations. Pressure fluctuations result from the vorticity mode and the acoustic mode that are both closely dependent on compressibility. To demonstrate the effects of wall cooling at various compressibility intensities, three free-stream Mach numbers are investigated, i.e. $M_\infty =0.5$ , 2.0 and 8.0, with real gas effects being absent for $M_\infty =8.0$ due to a low enthalpy inflow. Overall, opposite effects of wall cooling on pressure fluctuations are found between the subsonic/supersonic cases and the hypersonic case. Specifically, the pressure fluctuations normalized by wall shear stress $p^\prime _{rms}/\tau _w$ are suppressed in the subsonic and supersonic cases, while enhanced in the hypersonic case near the wall. Importantly, travelling-wave-like alternating positive and negative structures (APNS), which greatly contribute to pressure fluctuations, are identified within the viscous sublayer and buffer layer in the hypersonic cases. Furthermore, generating mechanisms of pressure fluctuations are explored by extending the decomposition based on the fluctuating pressure equation to compressible turbulent boundary layers. Pressure fluctuations are decomposed into five components, in which rapid pressure, slow pressure and compressible pressure are dominant. The suppression of pressure fluctuations in the subsonic and supersonic cases is due to both rapid pressure and slow pressure being suppressed by wall cooling. In contrast, wall cooling strengthens compressible pressure for all Mach numbers, especially in the hypersonic case, resulting in increased wall pressure fluctuations. Compressible pressure plays a leading role in the hypersonic case, mainly due to the APNS. Essentially, the main effects of wall cooling can be interpreted by the suppression of the vorticity mode and the enhancement of the acoustic mode.

Spectral analysis of turbulent boundary layer pressure fluctuations collected by a MEMS microphone array

Spectral analysis of turbulent boundary layer pressure fluctuations collected by a MEMS microphone array

Experimental design and stochastic modeling of hydrodynamic wave propagation within cavities for wind tunnel acoustic measurements

This study investigates how embedding microphones in different cavity geometries along the wall of a wind tunnel reduces the measured turbulent boundary layer pressure fluctuations. The effect of these cavities on the measured signal-to-noise ratio of an acoustic source with flow present was also quantified. Twelve cavity geometries defined by their depths, diameters, chamfer, opening percentage, and mesh covering were tested. The cavity geometries were selected using a design of experiments methodology. The application of design of experiments enabled a statistically sound and efficient test campaign. This was done by applying a D-optimal selection criterion to all potential cavity geometries in order to select 12 cavities to allow for the individual effect of the geometric parameters such as depth and diameter to be quantified with statistical confidence. The resulting wind tunnel test data were fit to a generalized additive model. This approach quantified the relative effect of these parameters on the turbulent boundary layer pressure spectral energy and signal-to-noise ratio while accounting for non-linear frequency dependence. This experimental investigation quantified how much increasing depth reduces the turbulent boundary layer spectral energy and increases signal-to-noise ratio. It also showed that a mesh covering reduces the boundary layer noise by 8 dB. It was also quantified how much reducing the cavity area from the opening of the cavity to the base of the microphone reduces the measured boundary layer spectral energy. Additionally, the model quantified the interactions between the mesh and cavity area as well as the change in area.

Wall-pressure spectra models for supersonic and hypersonic turbulent boundary layers

This paper presents an investigation of pressure fluctuations spectra beneath supersonic and hypersonic turbulent boundary layers at zero pressure gradient. High-order implicit Large Eddy Simulations have been used to provide numerical data for assessing existing models and further improving their qualitative accuracy in predicting wall-pressure spectra. Several different models have been investigated and it is shown that existing models fail to capture the correct behaviour of pressure fluctuations in supersonic and hypersonic boundary layers across a broad range of frequencies. The models have been modified by introducing compressibility corrections. The modified models are validated against implicit Large Eddy Simulations, Direct Numerical Simulations, and experimental data. The qualitative accuracy of the models is discussed and the most promising model is identified.

The control of aerodynamic sound due to boundary layer pressure gust scattering by trailing edge serrations

A theoretical model is proposed in the current work to shed light on the noise-reduction mechanisms of trailing-edge serrations, which have been shown to suppress noise generated at the trailing edge due to the scattering of boundary layer pressure fluctuations. The current analytical model, which is developed by incorporating Fourier series expansions and the Wiener-Hopf method, can quickly predict noise reductions due to various types of trailing-edge serrations at low Mach numbers. The present model is validated by comparing its predictions with relevant published theoretical and experimental results. Moreover, from the Winer-Hopf analysis, an effective method based on Fourier series expansion is proposed to approximately evaluate the noise-reduction performance of different serrated geometries under various working conditions. The current work shows that trailing-edge serrations reduce noise by producing scattered waves with spanwise modes which could be evanescent, and explains why some serrated geometries are more effective than others in suppressing noise. The efficient noise prediction capability of the proposed model makes it very suitable to be used in the trailing-edge designs and evaluations of low-noise aircraft components such as wings and aero-engine fan, compressor and turbine cascades.

Modal energy analysis for mechanical systems excited by spatially correlated loads

Abstract MODal ENergy Analysis (MODENA) is an energy-based method, which is proposed to deal with vibroacoustic problems. The performance of MODENA on the energy analysis of a mechanical system under spatially correlated excitation is investigated. A plate/cavity coupling system excited by a pressure field is studied in a numerical example, in which four kinds of pressure fields are involved, which include the purely random pressure field, the perfectly correlated pressure field, the incident diffuse field, and the turbulent boundary layer pressure fluctuation. The total energies of subsystems differ to reference solution only in the case of purely random pressure field and only for the non-excited subsystem (the cavity). A deeper analysis on the scale of modal energy is further conducted via another numerical example, in which two structural modes excited by correlated forces are coupled with one acoustic mode. A dimensionless correlation strength factor is proposed to determine the correlation strength between modal forces. Results show that the error on modal energy increases with the increment of the correlation strength factor. A criterion is proposed to establish a link between the error and the correlation strength factor. According to the criterion, the error is negligible when the correlation strength is weak, in this situation the correlation strength factor is less than a critical value.

Analytical Modeling of the Vibro-Acoustic Response of a Double-Walled Cylindrical Shell With Microperforation Excited by Turbulent Boundary Layer Pressure Fluctuations

An analytical model is developed to investigate the vibro-acoustic response of a double-walled cylindrical shell with the inner wall perforated when excited by the external turbulent boundary layer (TBL) pressure fluctuations. The shell motion is governed by the Donnell’s thin shell theory, and the mean particle velocity model is employed to describe the boundary condition between the microperforated shell and fluid media. Numerical results indicate that the transmission loss (TL) for the configuration of microperforating the inner wall could be larger than that for the conventional solid double-walled cylindrical shell with and without the core of porous material over a wide frequency range. Comparison between TL results with excitations from the TBL and the acoustic diffuse field (ADF) shows that with the thought of microperforating the inner shell, to reduce the acoustical excitation will be of more importance than the flow excitation over the ring frequency for a quiet interior space. Parametric studies illustrate that the perforation ratio is the main factor affecting the sound insulation performance through the total reactance.

Obtaining phase velocity of turbulent boundary layer pressure fluctuations at high subsonic Mach number from wind tunnel data affected by strong background noise

Boundary layer measurements at high subsonic Mach number are evaluated in order to obtain the dominant phase velocities of boundary layer pressure fluctuations. The measurements were performed in a transonic wind tunnel which had a very strong background noise. The phase velocity was taken from phase inclination and from the convective peak in one- and two-dimensional wavenumber spectra. An approach was introduced to remove the acoustic noise from the data by applying a method based on CLEAN-SC on the two-dimensional spectra, thereby increasing the frequency range where information about the boundary layer was retrievable. A comparison with prediction models showed some discrepancies in the low-frequency range. Therefore, pressure data from a DNS calculation was used to substantiate the results of the analysis in this frequency range. Using the measured data, the DNS results and a review of the models used for comparison it was found that the phase velocity decreases at low frequencies.

Implicit large eddy simulation of acoustic loading in supersonic turbulent boundary layers

This paper investigates the accuracy of implicit large eddy simulation in the prediction of acoustic phenomena associated with pressure fluctuations within a supersonic turbulent boundary layer. We assess the accuracy of implicit large eddy simulation against direct numerical simulation and experiments for attached turbulent supersonic flow with zero-pressure gradient, and further analyze and discuss the effects of turbulent boundary layer pressure fluctuations on acoustic loading both at the high and low frequency regimes. The results of high-order variants of the simulations show good agreement with theoretical models, experiments, as well as previously published direct numerical simulations.

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.

Using flow-induced vibrations for structural health monitoring of high-speed naval ships

It has been demonstrated theoretically and experimentally that an estimate of the impulse (or structural) response between two receivers can be obtained from the long-time average of the cross-correlation of diffuse vibrations (or ambient noise) recorded at these two receivers in various environments and frequency ranges of interest: ultrasonics, underwater acoustics, seismology, and structural health monitoring. Indeed, those estimated impulse responses result from the cumulated contributions over time of random vibrations (e.g., as created by flow-induced vibrations) traveling along the test structure and being recorded by both. Hence, this technique provides a means for structural health monitoring using only the ambient structure-borne noise (e.g., generated by flow-induce vibrations) only, without the use of active sources. We will review work conducted using (1) high-frequency random vibration (100 Hz–5 kHz) data induced by turbulent boundary layer pressure fluctuations and measured on a hydrofoil and a plate at the Navy’s William B. Morgan Large Cavitation Channel. (2) Low frequency random vibration data (1 Hz–50 Hz) collected on high-speed naval ships during at-sea operations were strong wave impact loading took place. [Work sponsored by ONR.]

Flow-induced noise calculations for vector hydrophones in towed arrays

The purpose of this study is to develop an integrated analytical method of flow-induced noise for vector hydrophones in towed arrays and discuss the parameters that influence flow-induced noise character. Based on Carpenter turbulent boundary layer pressure fluctuation model for slender cylinder, the power spectra and cross power spectra of acoustic pressure and particle velocity of flow-induced noise, are deduced by utilizing the wavenumber-frequency spectral analysis. It is shown that the flow-induced noise is determined by the towed speed, the size of both vector hydrophones and elastomer tube, the material parameters of elastomer tube and so on. In addition, the condition that cylindrical vector hydrophones are distributed non-axially in elastomer tube is also taken into account. Considering the influence of the axis-off distance on acoustic pressure, axial particle velocity and radial particle velocity, a set of numerical results show that the influence of the axis-off distance on the high-frequency component of the acoustic pressure and axial particle velocity is greater than that on the low-frequency component. The radial particle velocity is influenced by the axis-off distance within the full frequency range. The impact of the axis-off distance on the particle velocity is far greater than that on acoustic pressure.

Reynolds number effects in DNS of pipe flow and comparison with channels and boundary layers

Direct numerical simulations of turbulent pipe flow were performed at four Reynolds numbers: Reτ=180,500,1002and2003. Beyond Reτ=1000 viscous scaling holds near the wall for the mean velocity, Reynolds shear stress and wall-normal velocity variance. Streamwise and spanwise velocity variances do not exhibit inner (viscous) scaling due to increasing large-scale energy contributions. A comparison with channel and boundary layer DNS data shows negligible statistical differences between pipes and channels, whereas the transverse velocities for pipes/channels are significantly different when compared with boundary layers. A further comparison displays that the boundary layer pressure fluctuations is greater than pipes/channels. In addition, is it shown that the higher pressure fluctuations in the boundary layer is not the sole mechanism responsible for a stronger wake region in the flow.