Correlation Of Pressure Fluctuations Research Articles

Updating of wavenumber-frequency spectrum models by a phased array measurement

This study mainly presents the updating of wavenumber-frequency spectrum models (W-F models) based on spatial correlation measurement by a phased array technique, which is critical for the evaluation of the structural vibration caused by a turbulent boundary layer. Measurements are carried out in a wind tunnel, and the effect of background noise on identifying the spatial correlations of pressure fluctuations is discussed. To improve the resolution of the beam map used to estimate the spatial correlation by the beamforming algorithm, focusing functions are introduced to accelerate beam focusing. Then, broadband fluctuations and flow-speed effects on spatial correlation identification are discussed to estimate the adaption scope of the phased array technique. To update W-F models of wall pressure fluctuations, the subsection approaching method (SAM) is employed to extract the power spectrum of wall pressure fluctuations based on the identified spatial correlation. The investigated results reveal that the spatial correlation of wall pressure fluctuations caused by the turbulent boundary layer obeys a statistical law but does not constitute an exact math expression, which further validates the necessity of the phased array technique in spatial correlation identification.

Uniqueness of an inverse source problem in experimental aeroacoustics

This paper is concerned with the mathematical analysis of experimental methods for the estimation of the power of an uncorrelated, extended aeroacoustic source from measurements of correlations of pressure fluctuations. We formulate a continuous, infinite dimensional model describing these experimental techniques based on the convected Helmholtz equation in or . As a main result we prove that an unknown, compactly supported source power function is uniquely determined by idealized, noise-free correlation measurements. Our framework further allows for a precise characterization of state-of-the-art source reconstruction methods and their interrelations.

Correlation of pressure fluctuations in turbulent wall layers

Profiles of $\ensuremath{\langle}p\phantom{\rule{0}{0ex}}p{\ensuremath{\rangle}}^{+}$ from direct numerical simulation calculations are correlated for various Reynolds numbers. The correlating functions are unusual; a single function in the outer region and a defect function in the inner region. This is exactly opposite of the correlating functions for the mean velocity.

Flow structure and flow-induced noise in an axisymmetric cavity with lids

Direct numerical simulations of incompressible turbulent flow through an axisymmetric cavity with or without lids were performed at Ret,in = 186 to examine the hydrodynamic effects of the lids on the flow-induced noise. The strength of the recirculation in the downstream region was weakened by the installation of the lids. Comparison of the acoustic sources of the Lighthill equation indicated that the lid in the downstream region attenuated the flow-induced noise substantially. Frequency spectra and spatio-temporal correlations of pressure fluctuations revealed the most energetic mode and the convective nature of the flow over the cavity. It was evident from a detailed investigation of the instantaneous flow fields that the introduction of lids into the cavity significantly weakened the interaction between the separated shear layer and the trailing edge of the cavity. The present results clearly showed that the installation of lids is an effective means of reducing flow-induced noise.

Correlation of pressure fluctuations with tangential stresses in a turbulent boundary layer

Correlation of pressure fluctuations with tangential stresses in a turbulent boundary layer

Effects of free-stream turbulence on surface pressure fluctuations in a separation bubble

Wind-tunnel experiments were conducted to investigate the cause of large pressure fluctuations near leading edges of sharp-edged bluff bodies. Measurements obtained with a blunt flat plate showed that very low pressures occur in a narrow region located approximately 0.25XR from the leading edge, where XR defines the distance from the leading edge to the mean reattachment location. This phenomenon occurs in the undisturbed flow as well as turbulent flow, although the magnitude of peak pressure fluctuations increases with both turbulence intensity, σu/u, and turbulence scale, LX.Flow visualization experiments conducted with a high-speed cine-camera reveal the process that causes large pressure fluctuations in separation bubbles. This process is initiated when a perturbation in the approaching flow causes a roll-up of the separated shear layer, producing a strong vortex near the surface. Conditional sampling of pressure data was used to determine the spanwise length of the vortex. A significant increase in the spanwise correlation of pressure fluctuations occurs when the shear layer rolls up. Coherence measurements indicate that the spanwise length of vortices in the separation bubble is not directly related to longitudinal velocity fluctuations in the free-stream.

A coherent flame model of premixed turbulent combustion in a counterflow geometry

Premixed combustion in turbulent counterflowing streams is studied theoretically by adopting a coherent flame model. With this model, a flame surface density Σ -described by a transport equation—is used to calculate the mean chemical reaction rate relative to the distribution of a Reynolds mean progress variable c . The value of a model constant β, which is connected to flame surface stretch and annihilation, is selected to match numerical solutions with experiments at different nozzle exit conditions giving various values of the mean progress variables at the stagnation point c ̄ (0) . The k ̃ − ϵ ̃ equations are used to predict the turbulence strain rate for the Σ balance equation and the turbulent viscosity μ t = C μ p ̄ k ̃ 2 ϵ ̃ for second-moment terms. In this turbulence model, the term representing correlation of pressure fluctuations and dilatation is found to be important to turbulence production in the reaction zone. It is found that two different regions exist in the counterflow field: an outer region, which is far from the stagnation point and without chemical reaction, and an inner sublayer, which is close to the stagnation point, which experiences a large bulk strain rate and is located in the combustion zone. In order to provide an appropriate theoretical description of the experimental conditions, the inner region is allowed to have a finite thickness, thus imposing a finite displacement on the outer solution, due to thermal expansion in the sublayer. A composite similarity solution is obtained by matching the boundary conditions of the inner and outer solutions at the cold flame edge. The effective mean bulk strain rate is increased slightly by the displacement effect. Theoretical predictions are found to be in good agreement with experimental data. The constant β is found to be approximately proportional to the mean bulk strain rate.

Fluctuating Uplift and Lining Design in Spillway Stilling Basins

The paper deals with the characterization and the control of transient uplift generated by turbulent pressure fluctuations in spillway stilling basins. Theoretical and experimental analyses are presented, defining the maximum instantaneous uplift acting on the linings, which lead to a design criterion for suitable floor slabs. The thickness of concrete linings (not accounting for the possible contribution of anchors) required for stability is defined in this paper as a function of (1) The incoming kinetic head and/or Froude number; (2) a dimensionless function of the longitudinal and transversal dimensions of the lining slabs, the areal extent of extreme turbulent pressure fluctuations, and the anisotropic integral scales of the instantaneous spatial correlation of pressure fluctuations; (3) the ratio of specific weights of water and immersed concrete; and (4) a dimensionless pressure coefficient qualifying the maximum difference of positive and negative pressure fluctuations about the (stationary) mean value. It is concluded that safe design of protection works is significantly affected by the structure of instantaneous spatial distribution of pressures at the bottom of hydraulic jumps. Consideration of this factor leads to recommendations for the design of slabs in stilling basins.

Instantaneous pressure fields at a corner associated with vortex impingement

The impingement of concentrations of vorticity upon a corner is studied using simultaneous flow visualization and correlation of pressure fluctuations, revealing the relations between the instantaneous pressure fields along the top and front faces of the corner and the instantaneous distortion of the incident vortex.By varying the transverse offset of the corner relative to the incident vortex, several distinct and consistent patterns of incident vortex–corner interaction are evident. The corresponding form and phasing of the instantaneous pressure fields are strongly dependent upon the nature of the vortex–corner interaction, though the maximum amplitude on the top and front faces is always of the same order. For certain interactions, there is separation of flow from the edge of the corner, as well as from the front face of the corner, leading to secondary-vortex formation and corresponding peaks in the local pressure fields.By integrating the instantaneous pressure fields along the top and front faces of the corner, the phasing between the respective forces is shown to exhibit varying degrees of dipole-like behaviour, depending upon the character of the incident vortex–corner interaction. Amplitudes of the corresponding moments of the pressure fields are remarkably insensitive to this interaction.

Field and wind-tunnel measurements of wind pressures acting on a tower

Turbulence of the natural wind causes buffeting of slender structures and their dynamic reaction. Commonly used theoretical models and design procedures linearly transfer the properties of the oncoming turbulence into pressures acting on the building. This implies that the dynamic energy of the fluctuating pressures as well as their horizontal and vertical correlations are distributed as in the velocity field, neglecting effects of separation and the shape of the building. Full-scale measurements on a lighthouse tower show that this is only true for wave-numbers greater than κ=0.01. Smaller eddies are distorted by the presence of the structure. Further, the correlation of pressure fluctuations primarily depends on the line of separation and the wake. The full-scale results are verified by experiments with a dynamic model of the tower placed in a boundary-layer wind tunnel.

Fluctuation-dissipation theorems on the basis of hydrodynamic propagators

Hydrodynamic propagators of forces are formulated and studied in the following cases: for uniform temperature, neglecting convection or using Oseen's linearization, with or without compressibility, and also for nonuniform temperature. Discussions and comparisons are given wherever possible. Such a propagator formalism is useful in cases of hydrodynamic interactions among solid bodies, as well as thermodynamic fluctuation forces. It is found that the velocity correlation functions describe a source with time-dependent or -independent Oseen propagators. The correlation of pressure fluctuations, which exists even in the absence of dissipation, agrees with the result derived from the relaxation function of the mechanical fluctuation-dissipation theorem, as well as with the thermodynamic equilibrium correlation. The correlation of temperature fluctuations describes a diffusion process of the variance κ/ρ0cp when cp = cν. In accordance with Curie's principle of nonequilibrium thermodynamics, it is found that the transverse vector field is not coupled to the scalar fields. Finally, a possible comparison with a wind tunnel experiment on temperature fluctuations is indicated.