Porous Windscreens Research Articles

Acoustic characterization of porous windscreens for sound pressure and particle velocity sensors

Acoustic measurements of ground vehicles, wind turbines or aircrafts are often performed outdoors in presence of wind. Air movement around an acoustic sensor may significantly disturb the measured signals due to flow-induced vibrations and turbulence. This is specially relevant for pressure-gradient and particle-velocity based acoustic transducers which are more sensitive to these perturbations. To attenuate this effect and improve measurement quality, the sensors are typically covered with windscreens made of porous materials. However, designing an appropriate windscreen poses a challenging task, demanding a deep understanding of sound transmission characteristics. This study works towards quantifying acoustic properties of rigid-frame porous materials through in-situ measurements and fitting a parametric fluid-equivalent model to find optimal model parameters. The parameters are subsequently investigated by comparing windscreen insertion loss through finite-element numerical simulations and measurements on a sound intensity PU probe.

A note on wind velocity and pressure spectra inside compact spherical porous microphone windscreens.

Simultaneous measurements of wind velocity and pressure fluctuations were conducted in a wind tunnel to investigate the wind noise source inside compact spherical open celled porous windscreens. The existing outdoor wind noise models are found to be inadequate to predict the wind noise inside a wind tunnel. This paper proposes a model to predict the interior stagnation pressure, which agrees with the wind noise measured inside the windscreen within a bandwidth, where the exterior turbulence-turbulence interaction pressure overestimates the wind noise level. The limitations of the proposed model and other potential sources for wind noise inside porous windscreens are discussed.

Experimental investigation of the effect of perimeter windscreens on air-cooled condenser fan performance

Perimeter windscreens are finding application in large air-cooled condensers (ACCs) as a mechanism to mitigate the negative effects of wind on ACC fan performance and dynamic blade loading. The limited literature specific to perimeter windscreens is inconsistent and the mechanisms by which these screens impact fan behaviour are not well understood. This study investigates the influence of perimeter windscreens on ACC fan volumetric effectiveness under windy conditions using a laboratory based experimental apparatus consisting of a scaled fan row consisting of three fans in the wind direction. Three different porous windscreen materials (differing solidity) were investigated along with the effect of varying windscreen height. A controllable cross-flow (wind) was achieved by connecting the apparatus to a low speed wind tunnel. Fan volumetric flow rate and shaft power were measured and flow visualization was achieved by means of particle imagery velocimetry to provide insight into the reasons for the observed fan behaviour. The apparatus corresponds to a full-scale ACC for which data relating to perimeter windscreen effects was available in the literature. A good correlation to these full-scale measurements was achieved giving confidence in the validity of the results of this study.Perimeter windscreens were observed to degrade edge fan volumetric effectiveness under windy conditions relative to the no-screen case in almost all cases considered in this study although evidence of a recovery of volumetric effectiveness was observed at high cross-flow (β = 0.36) for the case where a low solidity screen (α = 0.5) was installed across half or more of the platform inlet (L ≥ 0.5). An improvement in cooling effectiveness of up to 30% was observed relative to the no-screen case for the aforementioned screen configurations indicating that such perimeter windscreens may benefit the effective cost of cooling in ACCs under high wind conditions. The PIV flow visualization identifies deflection of the flow past the edge fan as the reason for the reduction in fan volumetric effectiveness when screens are installed. The negative impact of this deflection is counteracted by permeation of flow through the screen and into the edge fan when a low solidity screen covers a large portion of the platform inlet.It can therefore be concluded that, for the low platform ACC simulated in this study, perimeter windscreens are detrimental to ACC performance except at very high wind speeds. Use of low solidity retractable screens that can be deployed under high wind conditions (taking into account structural limitations) may have merit. This works extends the understanding of perimeter windscreen effects on ACC fans by investigating the effect of screen solidity and height in a controllable and validated experimental facility. It provides insight into the mechanisms through which perimeter screens influence ACC fans that can be used to further develop and improve the potential benefits of these screens.

Spatial decorrelation of wind noise with porous microphone windscreens.

This paper explores the wind noise reduction mechanism of porous microphone windscreens by investigating the spatial correlation of wind noise. First, the spatial structure of the wind noise signal is studied by simulating the magnitude squared coherence of the pressure measured with two microphones at various separation distances, and it is found that the coherence of the two signals decreases with the separation distance and the wind noise is spatially correlated only within a certain distance less than the turbulence wavelength. Then, the wind noise reduction of the porous microphone windscreen is investigated, and the porous windscreen is found to be the most effective in attenuating wind noise in a certain frequency range, where the windscreen diameter is approximately 2 to 4 times the turbulence wavelengths (2 < D0/ξ < 4), regardless of the wind speed and windscreen diameter. The spatial coherence between the wind noise outside and inside a porous microphone windscreen is compared with that without the windscreen, and the coherence is found to decrease significantly when the windscreen diameter is approximately 2 to 4 times the turbulence wavelengths, corresponding to the most effective wind noise reduction frequency range of the windscreen. Experimental results with a fan are presented to support the simulations. It is concluded that the wind noise reduction mechanism of porous microphone windscreens is related to the spatial decorrelation effect on the wind noise signals provided by the porous material and structure.

On the wind noise reduction mechanism of porous microphone windscreens.

This paper investigates the wind noise reduction mechanism of porous microphone windscreens. The pressure fluctuations inside the porous windscreens with various viscous and inertial coefficients are studied with numerical simulations. The viscous and inertial coefficients represent the viscous forces resulting from the fluid-solid interaction along the surface of the pores and the inertial forces imposed on the fluid flow by the solid structure of the porous medium, respectively. Simulation results indicate that the wind noise reduction first increases and then decreases with both viscous and inertial coefficients after reaching a maximum. Experimental results conducted on five porous microphone windscreens with porosity from 20 to 60 pores per inch (PPI) show that the 40 PPI windscreen has the highest wind noise reduction performance, and this supports the simulation results. The existence of the optimal values for the viscous and inertial coefficients is explained qualitatively and it is shown that the design of the porous microphone windscreens should take into account both the turbulence suppression inside and the wake generation behind the windscreen to achieve optimal performance.

SIMULATION METHOD OF POROUS WIND SCREEN SCALE MODEL ON BRIDGE BY WIND TUNNEL TESTS

In order to investigate the simulation method of a scale model for a porous wind screen, the aerodynamic coefficients of vehicles under the action of a wind screen installed on the railway bridges are measured by wind tunnel tests. The wind screen is 2.05m high, with the porosity of 30%, and with different hole shapes. The effects of hole size and hole shape are analyzed, and the similarity between the wind screen with circular holes and the wind screen with longitudinal bars are discussed. The result shows that the circular hole size of a wind screen at the scale of 1∶20 should be in the range of 8mm to 12mm; the hole shape of a wind screen with the same porosity has some certain effect on the aerodynamic coefficients of the windward vehicle; the longitudinal bar wind screen with more than 10 gaps is equivalent to a wind screen with the circular hole size of 8mm. The results could provide some references for the wind tunnel test and numerical simulation of a wind screen.

The effect of screens on air-cooled steam condenser performance under windy conditions

Abstract The effect of porous wind screens, installed in a cross-type arrangement below the fan platform, on the performance of El Dorado Power Plant’s 6 × 5 fan air-cooled steam condenser (ACC) under windy conditions is investigated using computational fluid dynamics. The existing wind screen configuration is found to increase ACC performance under windy conditions relative to the no screens case. The ACC performance enhancement can be attributed to an increase in the performance of the fans upstream of the wind screens brought about by the stagnation effect of the screens on the flow below the fan platform. The potential for further ACC performance enhancement at El Dorado is identified since the existing screen configuration is found to reduce the performance of the fans downstream of its location. An improved screen configuration consisting of screens with a loss coefficient of Ksc = 10 installed to half the fan platform height, with the remaining space below the fan platform left open, is identified for El Dorado. This configuration provides the favorable stagnation effect upstream of its location while having less of a detrimental effect on the downstream fans. It is further found that additional performance benefits could be gained under the prevailing wind conditions at El Dorado by relocating the N–S orientated screens by one fan row, thus increasing the number of fans operating in the beneficial upstream stagnation region.

Simulation of fluid flow in and around a porous windscreen

Fluid dynamic simulation of wind noise (turbulent pressure fluctuations) around porous microphone windscreens in an atmospheric flow is made difficult by the broad range of spatial scales, from large atmospheric eddies down to the pore size of the windscreen, that is involved. Here, we describe a simplified simulation approach that is based on two new insights: (1) the ability of linear fluid flow approximations to describe wind noise [Z.C. Zheng and B. K. Tan, J. Acoust. Soc. Am. 113, 161–166 (2003)] and (2) a new set of macroscopic, time-domain relaxational equations for the linear interaction of a fluid flow with a porous material [D. K. Wilson, V. E. Ostashev, and S. L. Collier, J. Acoust. Soc. Am. 116, 1889–1892 (2004)]. An upstream flow condition consisting of quasiwavelets (analogous to turbulent eddies) is also used to create well-developed, realistic turbulence. This simulation approach is shown to provide detailed simulations of the production of pressure fluctuations in and around a porous windscreen.

Measurement of turbulence characteristics for flow past porous windscreen

Abstract In this study, measurements of the turbulence characteristics for a turbulent boundary layer flow past porous windscreen are reported. Measurement analysis includes maximum mean velocity reduction, maximum Reynolds stress reduction, turbulence intensity profile, turbulent velocity power spectrum, and Reynolds stress structure in the wake region of the windscreen. Results show that the maximum Reynolds stress reduction along the downstream distance from windscreen decays faster than the maximum mean wind speed reduction. The turbulence intensity profiles exhibit self-preserving in the wake region of windscreen for the cases of 15%, 30% and 50% porosity. Power spectrum analysis exhibits that most of the energy containing eddies in the wake region of the windscreen shift to the lower frequencies as the windscreen porosity increases. Quadrant analysis results show that the sweep and ejection events dominate the Reynolds stress contributions in the wake region of windscreen. The Reynolds stress contribution difference between sweep and ejection events becomes significant as the distance from the wall and downstream distance behind the windscreen decrease.

Acoustic mode coupling in a circular-section duct containing a concentric porous windscreen

A coupled waveguide model is developed that represents sound transmission in a flowing fluid which contains a windscreened microphone probe. The flow is contained in an outer annular region of a circular section duct. In the central region the fluid is quiescent, and these two regimes are separated by a porous windscreen concentric with the duct. This windscreen is characterized by a specific flow resistance R. Acoustic coupling occurs across the windscreen and the central region represents the probe where the acoustic quantities are to be measured. The use of modal theory permits this situation to be formulated as an eigenvalue problem which results in an appropriate dispersion relation. Solving this equation analytically is quite difficult since the eigenvalues are complex. An alternative approach is to solve the governing equation for the acoustic pressure numerically. With these pressure profiles an iteration process is devised, in which an optimization technique is used, to determine the wavenumbers and attenuation rates.