Turbulent Pressure Fluctuations Research Articles

Distinctive Features of Pressure Fluctuation Fields on the Surface of Steps

Parametric experimental investigations of the fields of turbulent wall pressure fluctuations on the surface of steps are performed. A considerable effect of the obstacle height on the spectral composition of the pressure fluctuations behind the leading edge is shown. The transformation of the spectral composition at different observation points is illustrated. The influence of the boundary layer thickness and the sweep angle is shown to be relatively weak. The spatial variation of the local correlation scales and the convective properties of the pressure fluctuation field on the step surface is illustrated.

Plastron Regeneration on Submerged Superhydrophobic Surfaces Using In Situ Gas Generation by Chemical Reaction

Superhydrophobic surfaces submerged under water appear shiny due to total internal reflection of light from a thin layer of air (plastron) trapped in their surface texture. This entrapped air is advantageous for frictional drag reduction in various applications ranging from microfluidic channels to marine vessels. However, these aerophilic textures are prone to impregnation by water due to turbulent pressure fluctuations from external flows and dissolution of the trapped gas into the water. We demonstrate a novel chemical method to replenish the plastron in situ by using the decomposition reaction of hydrogen peroxide on superhydrophobic surfaces prepared with a catalytic coating. We also provide a thermodynamic framework for designing superhydrophobic surfaces with optimal texture and chemistry for underwater plastron regeneration. We finally demonstrate the practical utility of this method by fabricating periodic microtextures on aluminum surfaces that incorporate a cheap catalyst, manganese dioxide. We perform drag-reduction experiments under turbulent flow conditions in a Taylor-Couette cell (TC cell), which show that more than half of the drag increase ensuing from plastron collapse can be recovered spontaneously by injection of dilute H2O2 into the TC cell. Thus, we present a low-cost, scalable method to enable in situ plastron regeneration on large surfaces for marine applications.

Thermal transition and its evaluation of liquid hydrogen cavitating flow in a wide range of free-stream conditions

The objectives of this paper are to (1) validate the present numerical modeling framework for liquid hydrogen cavitating flow, (2) investigate the dynamic evolution of liquid hydrogen cavitating flows in a wide range of free-stream conditions and (3) propose a thermal parameter to evaluate and predict the transition process of two typical cavitation dynamics in thermo-fluids. The dynamic evolutions of liquid hydrogen cavitating flows in a wide range of free-stream conditions (T∞ = 14–33 K, U∞ = 63.9 m/s, 90 m/s, σ∞ = 0.38, 0.8, 1.2) are numerically investigated. General agreements are obtained between the numerical results and the experimental measurements, including the pressure distribution, temperature drop as well as the cavity structures. The results show that the cavitation behaviors near the triple point are similar to cavitating flows without thermodynamic effects. Two typical cavitation dynamics named the quasi-isothermal mode and the thermo-sensitive mode in liquid hydrogen are observed under the same cavitation number and flow velocity. When the free-stream T∞ is below 30 K (TN ≤ 0.85), as the temperature increases, the cavity area increases to the maximum around the thermal transition temperature and then decreases when the cavitation dynamics transits from the quasi-isothermal mode to the thermo-sensitive mode. Further analysis indicates that the thermal transition temperature for liquid hydrogen is approximately at T∞ = 17 K and the transition temperature slightly increases with the increasing free-stream velocity and cavitation number. When the free-stream T∞ is above 30 K (TN > 0.85), the thermodynamic effects significantly decrease due to the suddenly changed physical properties. And then, the cavitation dynamics turns from the thermo-sensitive mode to the quasi-isothermal mode with the increasing temperature. The modified C-factor including both the thermodynamic effects and the effects of turbulent pressure fluctuations proposed in this paper could quantitatively evaluate and predict dynamics transition from the quasi-isothermal mode to the thermo-sensitive mode in thermos-fluids cavitating flows. It could be utilized as a parameter to design the operating conditions for thermos-fluids apparatus or system to avoid the maximum cavitation aggressiveness around the thermal transition temperature.

Instability analysis of a pump-turbine in vaneless space by turbulence kinetic energy production term

Unsteady flow in a high head pump-turbine was simulated using the Partially-Averaged Navier-Stokes (PANS) method. To study the improvement of “S” characteristics of a pump-turbine with misaligned guide vanes (MGV), turbulence kinetic energy production term and pressure fluctuations are numerical studied, comparing with results of pump-turbine with synchronized guide vane (SyV). Instability in the vaneless space of the model pump-turbine is analyzed based on turbulence kinetic energy equation. The maximum value of turbulence kinetic energy production(Pk) with MGV is doubled compared with the maximum value with SyV, which are in accordance with the increase of the amplitude of pressure fluctuation compared with SyV. The change of Pk is determined by the change of the normal stress term. The change of normal stress term will influence the amplitude of pressure fluctuation. The maximum value of 3D term with MGV increases compared with the result with SyV. It may be caused by the nouniform flow with MGV.

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.

Mean Velocity, Reynolds Shear Stress, and Fluctuations of Velocity and Pressure Due to Log Laws in a Turbulent Boundary Layer and Origin Offset by Prandtl Transposition Theorem

The maxima of Reynolds shear stress and turbulent burst mean period time are crucial points in the intermediate region (termed as mesolayer) for large Reynolds numbers. The three layers (inner, meso, and outer) in a turbulent boundary layer have been analyzed from open equations of turbulent motion, independent of any closure model like eddy viscosity or mixing length, etc. Little above (or below not considered here) the critical point, the matching of mesolayer predicts the log law velocity, peak of Reynolds shear stress domain, and turbulent burst time period. The instantaneous velocity vector after subtraction of mean velocity vector yields the velocity fluctuation vector, also governed by log law. The static pressure fluctuation p′ also predicts log laws in the inner, outer, and mesolayer. The relationship between u′/Ue with u/Ue from structure of turbulent boundary layer is presented in inner, meso, and outer layers. The turbulent bursting time period has been shown to scale with the mesolayer time scale; and Taylor micro time scale; both have been shown to be equivalent in the mesolayer. The shape factor in a turbulent boundary layer shows linear behavior with nondimensional mesolayer length scale. It is shown that the Prandtl transposition (PT) theorem connects the velocity of normal coordinate y with s offset to y + a, then the turbulent velocity profile vector and pressure fluctuation log laws are altered; but skin friction log law, based on outer velocity Ue, remains independent of a the offset of origin. But if skin friction log law is based on bulk average velocity Ub, then skin friction log law depends on a, the offset of origin. These predictions are supported by experimental and direct numerical simulation (DNS) data.

Flow noise in planar sonar applications

In this paper an investigation of flow noise in sonar applications is presented. Based on a careful identification of the dominant coupling effects, the acoustic noise at the sensor position resulting from the turbulent wall pressure fluctuations is modelled with a system of hydrodynamic, bending and acoustic waves. We describe for the first time an analytical solution of the problem which is based on a biorthogonal system which can be solved in the spectral domain without the usual simplifying assumptions. Finally, it is demonstrated that the analytical solution describes the flow noise generation and propagation mechanisms of the considered sea trials.

Sound Radiation Analysis of a Front Side Window Glass of DrivAer Model under Wind Excitation

To study the car radiated noise caused by turbulent pressure fluctuation on the side glass under wind excitation, a clay model of DrivAer was constructed except that the front left side window was built with real glass. Firstly, the constraint boundary condition of the side glass was set equivalent to a series of springs for the modelling with Finite Element Method (FEM). Then, a platform based on Matlab‐Abaqus cosimulation was originally developed, and the genetic algorithm was applied to find out the best fitted spring stiffness, which was used to build the equivalent model. Then working as excitation, the Corcos model was applied to calculate the power spectrum density of the turbulence pressure fluctuations acting on the side glass. Subsequently, the finite element modal superposition method was used to solve the vibroacoustic coupling equation to get the noise level of the driver’s ear position in the vehicle interior. Finally, the surface vibration velocity distribution of side glass under wind excitation was measured with Laser Vibrometer and then with Boundary Element Method (BEM), the radiated noise into interior was calculated as well (semisimulation). Through comparison of these two results, it shows good agreement up to 1000 Hz. It demonstrates that the above method is applicable to calculate the sound radiation caused by the side glass’ vibration at the low and middle frequency range. Therefore, an approach of calculating sound radiation of a vibrating glass caused by the air convective pressure fluctuation was explored.

Acoustic Transducer of Turbulent Pressure Fluctuations in a Temperature-Stratified Medium

The operation of an acoustic transducer in a temperature-stratified medium is investigated. The formation of a response of piezoceramic transducers of pressure fluctuations under the action of temperature fluctuations in a working medium on the sensor element is considered. The attenuation of the temperature signal of a pressure transducer in a turbulent boundary layer is calculated numerically. The effect of distortions of the spectral levels of pressure fluctuations detected by a sound transducer in the field of temperature inhomogeneities is investigated for the example of measurements of turbulent pressure fluctuations in a boundary layer during vertical ascent of the device to the surface from a specified depth in a deep sea.

Three-dimensional pressure fluctuation fields in the vicinities of cantilevered cylindrical obstacles

The fields of turbulent near-wall pressure fluctuations in the vicinities of cylindrical bodies mounted orthogonal to the surface in a flow are experimentally investigated. The considerable inhomogeneity and three-dimensionality of the pressure fluctuation field in the measurement area is shown. The dependence of the main characteristics of the inhomogeneous pressure fluctuation field on geometric parameters is presented. A weak influence of the boundary layer thickness and the obstacle height, when greater than the cylinder diameter, is demonstrated.

Turbulent velocity and pressure fluctuations in gas turbine combustor exit flows

This paper presents the results of two test programmes using novel instrumentation to characterise the pressure and turbulent velocity fields in gas-turbine combustor exit flows. The probes are uncooled, therefore a fast-insertion traverse system is employed to prevent thermal degradation of the instrumentation in these severely hostile high-temperature environments. High-bandwidth ultra-miniature pressure transducers are used to measure unsteady total pressure, whilst a Pitot tube is employed to measure time-averaged total pressure. The probes are 4 mm in diameter with a measurement bandwidth of the order of 100 kHz. In the first test programme, the probes are used to characterise the streamwise turbulent velocity field approximately two axial chords downstream of an uncooled single-stage turbine in a turbojet engine. Established data reduction methods and calibration against a hot-wire are used to obtain turbulent velocity fluctuations from unsteady total pressure measurements. Comprehensive turbulence results are presented including time-histories, power spectra, intensities, and lengthscales obtained at four-engine conditions and at two radial and two circumferential measurement locations. In the second test programme the probes are demonstrated in an industrial combustor rig, featuring a can combustor with swirler nozzle and no dilution holes, at temperatures up to 1500 K. Static pressure fluctuations are obtained up to 100 kHz, and some typical combustor spectral features are identified.

Simulation of turbulent boundary layer wall pressure fluctuations via Poisson equation and synthetic turbulence

Flat plate turbulent boundary layers under zero pressure gradient are simulated using synthetic turbulence generated by the fast random particle–mesh method. The stochastic realisation is based on time-averaged turbulence statistics derived from Reynolds-averaged Navier–Stokes simulation of flat plate turbulent boundary layers at Reynolds numbers $\mathit{Re}_{\unicode[STIX]{x1D70F}}=2513$ and $\mathit{Re}_{\unicode[STIX]{x1D70F}}=4357$. To determine fluctuating pressure, a Poisson equation is solved with an unsteady right-hand side source term derived from the synthetic turbulence realisation. The Poisson equation is solved via fast Fourier transform using Hockney’s method. Due to its efficiency, the applied procedure enables us to study, for high Reynolds number flow, the effect of variations of the modelled turbulence characteristics on the resulting wall pressure spectrum. The contributions to wall pressure fluctuations from the mean-shear turbulence interaction term and the turbulence–turbulence interaction term are studied separately. The results show that both contributions have the same order of magnitude. Simulated one-point spectra and two-point cross-correlations of wall pressure fluctuations are analysed in detail. Convective features of the fluctuating pressure field are well determined. Good agreement for the characteristics of the wall pressure fluctuations is found between the present results and databases from other investigators.

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.

Fluctuating wind pressure distribution around full-scale cooling towers

At present, external wind pressure description around cooling towers in various loading Codes is based on full-scale measurement data. The data has been collected during 1960s–1990s for hyperbolic cooling towers with heights of about 90m~120m, and focus has been usually on the average wind pressure distribution. In fact, modern cooling towers taller than 165m show sensitivity to wind-induced effects under strong wind excitation. The performance of these flexible structures is closely related to the fluctuating wind action in the atmospheric boundary layer. Modeling of these features is underscored by the difficulty in matching supercritical Reynolds number in boundary layer wind tunnels. Accordingly, it has been difficult to accurately establish a relationship between fluctuating pressures on the surface of a cooling tower to inflow conditions. This has caused a bottleneck for enhancements in the state of wind resistant structural design of larger cooling towers. In view of this difficulty, high Reynolds number flows were simulated in the wind tunnel using scaled model with a combination of surface roughness elements to establish a relationship between the inflow turbulence intensity and pressure fluctuations. This was supplemented by a validation using data from long-term measurements of wind pressure around a 166.68m cooling tower. Wind tunnel experiments were in a general agreement with full-scale observations which offered a relationship between the fluctuating wind pressure distribution and the incoming turbulence intensity at supercritical Reynolds number conditions (Re≥4×10E7).

Investigation and modelling of the turbulent wall pressure fluctuations on the bulbous bow of a ship

For the effective operation of sonar systems mounted inside the bulb of fast ships, it is important to reduce all the possible noise and vibration sources that radiate noise and interfere with sonar sensor response. In particular, pressure fluctuations induced by turbulent boundary layers on the sonar dome surface represent the major source of self-noise for on-board sensors. Reliable calculations of structural vibrations and noise radiated inside the dome require valid statistical descriptions of wall pressure fluctuations beneath the turbulent boundary layer. Previous research about wall pressure fluctuations deals with equilibrium turbulent boundary layers on flat plates in zero pressure gradient flow, for which scaling laws for power spectral densities and empirical models for the cross spectral densities are well established. On the contrary, turbulent boundary layers on bulbous bow exhibit the combined effects of three-dimensionality, streamline and spanwise curvatures and pressure gradients. In order to collect information about realistic configurations, wall pressure fluctuations were measured in an experimental campaign performed in a towing tank; data were collected at two different locations along a large scale model of a ship bulb and their spectral characteristics were investigated in terms of auto and cross spectral densities. Mean flow parameters of the boundary layer, required in the analysis, were obtained by a finite volume code that solves the Reynolds Averaged Navier Stokes Equations. The applicability of classical scaling laws for pressure spectra on zero pressure gradient flat plate was investigated, together with the spatial characterization of the wall pressure fluctuations in the space-frequency domain; parameters of some semi-empirical models available in the scientific literature were tuned to fit the measured pressure field.

Special features of the pressure fluctuation field structures in the vicinity of bluff bodies (cylinders)

The fields of turbulent near-wall pressure fluctuations on a surface in a flow are experimentally investigated in the presence of cylinders located in the proximity of the surface. It is shown that the main characteristics of the pressure fluctuation fields can be essentially transformed in the case of nonzero distance (gap) between the cylinder and the surface in a flow, namely, the spectral levels increase over the entire frequency range, a narrow-band frequency maximum is formed, and the spatial distribution of the pressure fluctuation intensity changes. The dependence of the main parameters of the pressure fluctuation fields is presented and a weak influence of the boundary layer thickness is shown.

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.

Observations of the Transfer of Energy and Momentum to the Oceanic Surface Boundary Layer beneath Breaking Waves

AbstractMeasurements just beneath the ocean surface demonstrate that the primary mechanism by which energy from breaking waves is transmitted into the water column is through the work done by the covariance of turbulent pressure and velocity fluctuations. The convergence in the vertical transport of turbulent kinetic energy (TKE) balances the dissipation rate of TKE at first order and is nearly an order of magnitude greater than the sum of the integrated Eulerian and Stokes shear production. The measured TKE transport is consistent with a simple conceptual model that assumes roughly half of the surface flux of TKE by wave breaking is transmitted to depths greater than the significant wave height. During conditions when breaking waves are inferred, the direction of momentum flux is more aligned with the direction of wave propagation than with the wind direction. Both the energy and momentum fluxes occur at frequencies much lower than the wave band, consistent with the time scales associated with wave breaking. The largest instantaneous values of momentum flux are associated with strong downward vertical velocity perturbations, in contrast to the pressure work, which is associated with strong drops in pressure and upward vertical velocity perturbations.

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...