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This study deals with base pressure management in a duct for various values of the Mach number (M), namely, Mach number corresponding to sonic and four supersonic conditions. In addition to the Mach number, the nozzle pressure ratio (NPR), the area ratio, the rib dimension, and the duct length are influential parameters. The following specific values are examined at M = 1, 1.36, 1.64, and 2, and NPRs between 1.5 and 10. The base pressure is determined by positioning ribs of varying heights at predetermined intervals throughout the length of the square duct. When the level of expansion is varied, it is seen that the base pressure initially drops for overexpanded flows and increases for under-expanded flows. When ribs are present, the flow field in the duct and pressure inside the duct fluctuate as the base pressure rises. Under-expanded flows can achieve a base pressure value that is suitably high without experiencing excessive changes in the duct flow in terms of static pressure if a rib height around 10% of the duct height close to the nozzle exit is considered. Rectangular rib passive control does not negatively affect the duct's flow field. [ABSTRACT FROM AUTHOR]

This study presents an evaluation of semi-empirical single-point wall pressure spectrum models by comparing model predictions with wind tunnel and flight test data. The mean squared error was used to compare the power spectral density of the wall pressure fluctuations predicted by semi-empirical models with a large amount of experimental data. Results show that the models proposed by Goody and Smol'yakov have the lowest mean squared error when predicting the power spectral density for wind tunnel experiments and the Rackl and Weston model has the lowest mean squared error when predicting the power spectral density for flight test data. In addition, although current studies of the power spectra obtained in the wind tunnel are similar, they are not generally an accurate representation of flight test experiments. [ABSTRACT FROM AUTHOR]

Unsteady side loads observed in supersonic nozzles operating in over-expanded regimes are most often associated with intrinsic unsteadiness of the shock system and separation line, featuring random motions with mainly broadband low-frequency contributions. A tonal flow behaviour, rather associated with energy peaks of fluctuating wall pressure in the middle frequency range, is also found to emerge for particular operating conditions in a truncated ideally contoured nozzle. The corresponding flow field is here investigated to understand its origin and show how it modifies side-load properties. The temporal and spatial organization of wall pressure and jet velocity field are first experimentally characterized based on synchronized acquisition of both wall pressure along rings of pressure probes located within the nozzle, and high-rate time-resolved particle image velocimetry velocity fields measured in a plane section crossing the jet downstream of the nozzle exit. The external jet aerodynamics and internal wall pressure field are first shown to be clearly linked, but only at this frequency peak for which a significant coherence emerges between first azimuthal mode of fluctuating wall pressure and first azimuthal mode of fluctuating external velocity field. A delayed detached eddy simulation is carried out and validated against experimental results in order to reproduce this tonal flow dynamics. The analysis of simulation data shows that the tonal flow behaviour of first azimuthal mode is indeed more globally felt within the whole flow structure where both upstream and downstream propagating waves are shown to coexist, even far downstream of the nozzle exit. The analysis shows that both waves possess support in the jet core and have a non-negligible pressure signature in the separated region. The spectral proper orthogonal decomposition of fluctuating pressure field at this tonal frequency reveals that the nature and intensity of lateral pressure forces is directed by the resonance related to the upstream- and downstream-propagating coherent structures, which imposes the shock waves network to respond and modulate the pressure levels on the nozzle internal surface. [ABSTRACT FROM AUTHOR]

Tests were carried in a shock tunnel to determine the heating rate and the wall pressure on a test model flying at hypersonic velocity. The experiments were performed at Mach M = 6.5 and a total enthalpy of 1.2 MJ/kg. Helium was used as the driver gas and air as the driven gas. The effective test time during the tunnel testing was 3.5 ms. The vacuum sputtered gages were used to evaluate the heating rate on the test model. The evaluated heating rate agrees well with numerical simulation. The experimentally measured pressure also agrees with computational fluid dynamics [ABSTRACT FROM AUTHOR]

In pressurized water reactors (PWR), fluid–structure interaction provokes vibrations of the fuel rods leading to grid-to-rod fretting. To explore the origins of the local excitations of the rods by the flow, analytical experiments are performed within a 5 × 5 rod bundle maintained by spacer grids. New velocity (laser Doppler velocimetry) and pressure fluctuations measurements, taken for various Reynolds numbers, show the presence of frequency peaks in the spectra over a broad range of Reynolds numbers, i.e. 13,000 to 108,000. The Strouhal numbers obtained vary weakly with the Reynolds number and are in the range of 0.21–0.26. The phenomena being more pronounced downstream the dimples, this manuscript explores the flow at the exit of spacer grids downstream the dimples. To see through aligned rods, refractive index matching is implemented on parts of selected rods. The dilated scale of the experiments permits one to perform particle image velocimetry (PIV; grid of 51 × 89 points with a correlation window size of 0.032 Dh) to identify eddies smaller than the rods in the wake of the dimples. PIV measurements reveal the presence of multi-scale eddies with a periodic street of main eddies. The size of these eddies and the periodic length scale of the street are found to be in agreement with the integral length scale and the length scale built on the frequency peaks of pressure fluctuations. To the authors' knowledge, this is the first experimental evidence of the existence and the role of coherent eddies streets downstream from dimples of spacer grids with geometries relevant to PWR fuel assemblies. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of rotorcraft design and operating parameters on trailing-edge noise. A rotor trailing-edge noise prediction method is first developed where the aerodynamics and the turbulence wall pressure spectrum near the trailing edge on airfoils are predicted by a combination of the standard blade element momentum theory, a viscous boundarylayer panel method, and a recently developed empirical wall pressure spectrum model. The coordinate transformations are combined with the Amiet model to predict far-field noise. Compared to experimental data, the validation of this method demonstrates its advantages and validity for airfoil and rotorcraft broadband noise predictions. Then, this method is used to study the effects of rotorcraft design and operating parameters on rotor trailing-edge noise. It is found that helicopter broadband noise scales with the 4.5th to 5.0th power of the tip Mach number in which the range is determined by the typical helicopter collective pitch angle in operation. Detailed trend analyses of noise levels as a function of frequency are presented in terms of the collective pitch angle, twist angle, rotor solidity, rotor radius, disk loading, and number of blades. It is found that the collective pitch angle, twist angle, and chord length make noticeable impacts on low- and midfrequency noise. Finally, a semianalytic model is presented to predict the directivity and geometric attenuation of rotor trailing-edge noise. [ABSTRACT FROM AUTHOR]

Wall pressure measurements are made in a convergent-divergent nozzle with strut injection to estimate the internal forces and moments. The strut facing the flow was of rectangular cross section and is placed at two thirds of the diverging length of the nozzle from the throat. The design exit Mach number of the nozzle is 1.84. The strut height varied at constant internal total pressure of 690 kPa. Experiments were conducted to explore the possibility of using a solid strut as an alternative to secondary fluid injection for thrust vector control. The wall pressure distribution of the nozzle is studied to interpret the flow interaction with the strut. The calculations based on the experimental data show that the presence of the strut and a variation in its height produce significant variations in side force and pitching moment which would be useful for thrust vector control. [ABSTRACT FROM AUTHOR]

This study investigates the noise generation and radiation of a three-dimensional A-pillar vortex, which is a three-dimensional unsteady structure found on most automotive models. A small model is used to generate the vortex and is tested in an anechoic wind tunnel. The flow is validated against the literature using wall-pressure measurements: the maxima in the fluctuating pressure coefficient are found under the vortical structure. A 31-channel acoustic beamforming array is deployed to identify the noise sources on this model in 2 perpendicular planes. The A-pillar is identified to be the main noise mechanism over a wide range of frequency (between 1 and 8 kHz), with a monopolar radiation in a plane perpendicular to the flow. Some coherence between the fluctuating wall-pressure and the far-field radiation is found around 1 kHz, but only for wall-pressure sensors located in the area outside of the vortical structure, where the fluctuating pressure coefficient is low. An additional experiment with an external noise source is conducted and confirms that the wall-pressure fluctuations convected by the vortex structure mask the acoustic pressure fluctuations which result in a low coherence level in the area where the fluctuating pressure coefficient is the highest. This experiment shows that wall-pressure measurements need to be associated to microphone array measurements to carry out a thorough experimental aeroacoustic study of a complex 3D system such as the A-pillar vortex. [ABSTRACT FROM AUTHOR]

The minimum wall pressure coefficient of orifice plate and its wall pressure distribution relating to tunnel’s and dissipater’s safety are important indices for this energy dissipater design. In the present paper, the minimum wall pressure coefficient of sharp-edged orifice plate was analyzed; meanwhile, the characteristics of sharp-edged orifice plate wall pressure distribution were also analyzed. The research result has shown that the minimum wall pressure coefficient was mainly dominated by the contraction ratio of the orifice plate. The less is the contraction ratio of the orifice plate, the larger is the minimum wall pressure coefficient. The effect of orifice plate’ thickness on the minimum wall pressure coefficient was not obvious and could be neglected. When Reynolds number is more than 105, it has little impact on the minimum wall pressure coefficient. Wall pressure begins to drop down dramatically before 0.5D orifice plate, reaches minimum at the end of orifice plate, and then recovers normally when flows arrival at 3D away after orifice plate. An empirical expression was presented to calculate the minimum wall pressure coefficient. Experimental data illuminate that calculation results by using empirical expression coincided with experiment results. [ABSTRACT FROM AUTHOR]

We report on the measurement of the pressure associated with a shock wave within a very thin layer (100 nm) in proximity of a boundary surface. In the experiments, the shock wave was emitted by a cavitation bubble generated by a pulsed pump laser in water. We developed a pump-probe setup based on the detection of the light scattered at the surface of a one-dimensional photonic crystal, which was purposely designed to sustain a surface electromagnetic wave in the visible range and to enhance the optical response. In order to better understand the phenomenon, we implemented numerical simulations to describe the light scattering intensity distributions through a modified Rayleigh's method. We report, with a LoD of ∼ 0.1 MPa , the measurements of the pressure at a surface in the presence of a laser-induced cavitation bubble generated at different distances from the surface and for different pulse energies. [ABSTRACT FROM AUTHOR]

Highlights • Pressure and thermal damage are competitors in laser irradiation of vessel lesions. • In 595 nm laser, vessel lesions constantly undergo pressure damage and thus rupture. • Regions of high vessel wall pressure determines rupture locations and extent. • In 1064 nm laser, vessel lesions often undergo thermal damage and thus constrict. • Optimized laser parameters aiming at different damage types are recommended. Abstract Port wine stains (PWSs) are congenital dermal vessel proliferations mainly treated with laser therapy. The complete removal of the vessel lesions is rarely achieved because of a lack of discriminatory analysis of the two competitive laser damages to blood vessels, namely, pressure damage and thermal damage. Unlike complete vessel constriction, which is caused by thermal damage that can be measured by temperature-related integral Ω, vessel rupture results from pressure damage, which has been seldom studied. In this study, the rupture potential index based on wall pressure (RPIP) was calculated as the ratio of locally acting pressure to the pressure threshold. RPIP > 1 and Ω > 103 were adopted as benchmarks to judge pressure damage (vessel rupture) and thermal damage (complete vessel constriction), respectively. A computational fluid dynamics simulation was carried out to provide the temperature and pressure field in the PWS vessel model during irradiation by 595 nm pulsed dye laser (PDL) or 1064 nm Nd:YAG laser. Numerical results showed that for the 595 nm laser, vessels constantly underwent rupture. The area of high RPIP determined the degree of rupture by predicting the large and multiple rupture locations of the vessel. By contrast, for the 1064 nm laser, complete constriction was the main damage type. To a single vessel of 100 μm diameter, the optimized laser parameters were E = 10 J/cm2 with t p = 6 ms for 595 nm PDL and E = 180 J/cm2 with t p = 6 ms 1064 nm for Nd:YAG laser. [ABSTRACT FROM AUTHOR]

Abstract The present work discusses the influence of diamond shaped strut profile on combustion characteristics of a supersonic combustor. The 2D DLR scramjet model was used to investigate the effects of parametric variables which include Mach number, wall pressure ratio and inlet temperatures respectively. Further, interaction between shock-shock, shock-shear layer, and shock-near wall boundary layer is also discussed. It has been observed that the fluid flow behind the strut is oscillating while the primary and secondary recirculation region is present along the centre line of the strut. The study reveals that the optimal size of recirculation region is found at Mach number 2.5 which results into stable combustion. It is observed that the combustion efficiency starts to increase at the trailing edge of the strut due to the strong interaction of shock and the performance of a scramjet combustor is found to deteriorate at very high temperatures and very low Mach number. An optimal combustion efficiency equal to 80% has been obtained at Mach = 2.5, T = 696 K and P j / P ∞ = 0.8 corresponding to minimum pressure loss. Highlights • Effect of diamond shaped strut on performance of scramjet combustor is examined. • Interplay of shockwaves and recirculation zone on combustion is discussed. • Optimal size of recirculation zone for stable combustion is found at Mach no. 2.5. [ABSTRACT FROM AUTHOR]

Abstract The paper deals with buckling of metal cylindrical silos composed of corrugated sheets and thin-walled open-sectional columns. A simplified silo wall segment model with a few columns was proposed for calculations of the global buckling strength of silos instead of comprehensive 3D computations of the entire silo structure. Calculations were carried out for three real silos with a different geometry (two slender silos and one intermediate slender silo). Comprehensive linear bifurcation/buckling analyses and geometrically non-linear stability analyses for both imperfect and perfect silos were applied. Silos and silo wall segments were mainly modelled both with an equivalent orthotropic shell for walls and 1D beam elements for thin-walled columns and with shell elements for walls and columns. The silo walls were loaded by tangential wall pressure, horizontal wall pressure and patch load according to Eurocode 1. The simplified silo wall segment models fully proved their efficiency in FE calculations of the buckling strength of silos. Graphical abstract Linear bifurcation/buckling analyses for silo loaded by vertical tangential wall pressure - buckling load factor λ versus column number n for different models: a) 3D equivalent orthotropic model with reduced column cross-section positioned eccentrically with respect to wall, b) wall segment model (three columns), c) wall segment model (four columns), d) Eurocode (small column separation), e) Eurocode (large column separation) and f) Eurocode amendment (large column separation). fx1 Highlights • Buckling of cylindrical metal silos with corrugated sheets was analysed. • Three-dimensional linear/non-linear buckling analyses were performed. • An equivalent orthotropic shell was mainly used for walls. • A simplified silo wall segment model with few columns was proposed. [ABSTRACT FROM AUTHOR]

The unsteady dynamics of shock interactions on the crotch of two typical V-shaped blunt leading edges have been investigated numerically and experimentally with a freestream Mach number 6 for different ratios (R), defined as the rounding radius at the crotch to the blunt radius at the leading edge. The primary flow features observed in the shock tunnel experiments are reproduced by the large-eddy simulations. The time-averaged flow structures in the crotch are clearly shown as counterrotating vortices originating from the collision of jets near the stagnation point. These jets and vortices undergo unsteady motions coupled with the dynamics of shock interactions. Of great interest, two typical global oscillations, i.e., swing oscillation and arch-recover oscillation corresponding to the two values of R , are identified. The coherent structures of the oscillations are analyzed using the proper orthogonal decomposition technique. It is demonstrated that the swing oscillation and arch-recover oscillation are characterized by an antisymmetric pattern and a symmetrical pattern, respectively. These two oscillations are also characterized by the energetic middle-frequency components of the broadband wall pressure spectra. Two feedback models are proposed for the prediction of such middle-frequency components. The results show that the swing oscillation causes much more severe pressure load, and the local impingement of the transmitted shock on the crotch is responsible for the peak value of the pressure fluctuation. This study illustrates that the geometry of R has a key impact on the unsteady shock interactions and, therefore, should be considered critically in practical applications, such as the cowl lip of a hypersonic inward-turning inlet. [ABSTRACT FROM AUTHOR]

Abstract In this work, the cross-statistics of acceleration and wall pressure fluctuations generated by an incompressible jet interacting with a tangential flat-plate are presented. The results are derived from an experimental test campaign on a laboratory-scale model involving simultaneous velocity and wall pressure measurements. The pressure footprint of the jet on the surface was measured through a cavity-mounted microphone array, whereas pointwise velocity measurements were carried out by a hot wire anemometer. The time derivative of the velocity signal has been taken as an estimation of the local acceleration of the jet. The multivariate statistics between acceleration and wall pressure are achieved through cross-correlations and cross-spectra, highlighting that the causality relation is more significant in the potential core where the Kelvin–Helmholtz instability is dominant. The application of a conditional sampling procedure based on wavelet transform allowed us to educe the acceleration flow structures related to the energetic wall-pressure events. The analysis revealed that, unlike the velocity, the acceleration signatures were detected only for positions where the jet had not yet impinged on the plate, their shape being related to a convected wavepacket structure. [ABSTRACT FROM AUTHOR]

Highlights • Direct numerical simulation was performed for the pipe flow with a 90 bend. • Force oscillation caused by unsteady wall pressure was found. • Various measures of the swirl-switching frequency were tested. • The swirl-switching frequency was greatly affected by the local flow properties. • Correlation between the mass flow oscillation and swirl-switching frequency was established. Abstract Direct numerical simulation (DNS) has been performed for a spatially developing 90° bend pipe flow to investigate the unsteady flow motions downstream of the bend. A recycling method is implemented to generate a fully-developed turbulent inflow condition. The Reynolds number of the pipe flow is R e D = 5300 and the bend curvature is γ = 0.4. A long straight pipe section (40 D) is attached in the downstream of the bend to allow the flow to develop. Flow oscillations downstream of the bend are measured using several methods, and the corresponding oscillation frequencies are estimated. It is found that different characteristic frequencies are obtained from various flow measurements. The stagnation point movement and single-point velocity measurements may not be good measures to determine the swirl-switching frequency. The oscillations of the lateral pressure force on the pipe wall and half-sided mass flow rate are proposed to be a more unambiguous measure of the unsteady flow motions downstream of the bend. [ABSTRACT FROM AUTHOR]

In this paper we study the wall pressure and vorticity fields of the Stokes boundary layer in the intermittently turbulent regime through direct numerical simulation (DNS). The DNS results are compared to experimental measurements and a good agreement is found for the mean and fluctuating velocity fields. We observe maxima of the turbulent kinetic energy and wall shear stress in the early deceleration stage and minima in the late acceleration stage. The wall pressure field is characterized by large fluctuations with respect to the root mean square level, while the skewness and kurtosis of the wall pressure show significant deviations from their Gaussian values. The wall vorticity components show different behaviours during the cycle: for the streamwise component, positive and negative fluctuations have the same probability of occurrence throughout the cycle while the spanwise fluctuations favour negative extrema in the acceleration stage and positive extrema in the deceleration stage. The wall vorticity flux is a function of the wall pressure gradients. Vorticity creation at the wall reaches a maximum at the beginning of the deceleration stage due to the increase of uncorrelated wall pressure signals. The spanwise vorticity component is the most affected by the oscillations of the outer flow. These findings have consequences for the design of wave energy converters. In extreme seas, wave induced fluid velocities can be very high and extreme wall pressure fluctuations may occur. Moreover, the spanwise vortical fields oscillate violently in a wave cycle, inducing strong interactions between vortices and the device that can enhance the device motion. [ABSTRACT FROM AUTHOR]

The reasonable determination of wall pressure is critical for the design of silo structures. In this study, the primary objective is to present four novel wall pressure coefficients based on four true triaxial strength criteria in the quasiplane strain state. These four strength criteria are the Drucker-Prager (D-P) criterion, the Matsuoka-Nakai (M-N) criterion, the Lade-Duncan (L-D) criterion, and the unified strength theory (UST), and they all consider the effect of the intermediate stress yet to different extent. These coefficients have a wide application range and are readily used to predict the distribution of wall pressure for deep and squat silos. Comprehensive comparisons are made between the predictions from the wall pressure coefficients described herein and experimental data reported in the literature as well as the results from the European, American, and Chinese silo standards or the Rankine and the modified Coulomb theories. It is found that the effect of the intermediate stress on the wall pressure is very significant for both deep and squat silos; the wall pressure of the D-P criterion is underestimated, whereas that of the Mohr-Coulomb (M-C) criterion is overestimated; the L-D criterion is recommended to be adopted to calculate the soil wall pressure. [ABSTRACT FROM AUTHOR]

A large-eddy simulation of laminar transonic buffet on an airfoil at a Mach number M = 0.735, an angle of attack α = D 4°, a Reynolds number Rec = 3 × 106 has been carried out. The boundary layer is laminar up to the shock foot and laminar/turbulent transition occurs in the separation bubble at the shock foot. Contrary to the turbulent case for which wall pressure spectra are characterised by well-marked peaks at low frequencies (St = f ּּ· c/U∞ ≃ 0.06–0.07, where St is the Strouhal number, f the shock oscillation frequency, c the chord length and U∞ the free-stream velocity), in the laminar case, there are also well-marked peaks but at a much higher frequency (St = 1.2). The shock oscillation amplitude is also lower: 6% of chord and limited to the shock foot area in the laminar case instead of 20% with a whole shock oscillation and intermittent boundary layer separation and reattachment in the turbulent case. The analysis of the phase-averaged fields allowed linking of the frequency of the laminar transonic buffet to a separation bubble breathing phenomenon associated with a vortex shedding mechanism. These vortices are convected at Uc/U∞ ≃ 0.4 (where Uc is the convection velocity). The main finding of the present paper is that the higher frequency of the shock oscillation in the laminar regime is due to a different mechanism than in the turbulent one: laminar transonic buffet is due to a separation bubble breathing phenomenon occurring at the shock foot. [ABSTRACT FROM AUTHOR]

Direct measurement of the wall pressure loading subjected to the near-field underwater explosion is of great difficulty. In this article, an improved methodology and a lab-scale experimental system are proposed and manufactured to assess the wall pressure loading. In the methodology, a Hopkinson bar (HPB), used as the sensing element, is inserted through the hole drilled on the target plate and the bar’s end face lies flush with the loaded face of the target plate to detect and record the pressure loading. Furthermore, two improvements have been made on this methodology to measure the wall pressure loading from a near-field underwater explosion. The first one is some waterproof units added to make it suitable for the underwater environment. The second one is a hard rubber cylinder placed at the distal end, and a pair of ropes taped on the HPB is used to pull the HPB against the cylinder hard to ensure the HPB’s end face flushes with loaded face of the target plate during the bubble collapse. To validate the pressure measurement technique based on the HPB, an underwater explosion between two parallelly mounted circular target plates is used as the validating system. Based on the assumption that the shock wave pressure profiles at the two points on the two plates which are symmetrical to each other about the middle plane of symmetry are the same, it was found that the pressure obtained by the HPB was in excellent agreement with pressure transducer measurements, thus validating the proposed technique. To verify the capability of this improved methodology and experimental system, a series of minicharge underwater explosion experiments are conducted. From the recorded pressure-time profiles coupled with the underwater explosion evolution images captured by the HSV camera, the shock wave pressure loading and bubble-jet pressure loadings are captured in detail at 5  mm, 10  mm, …, 30  mm stand-off distances. Part of the pressure loading of the experiment at 35  mm stand-off distance is recorded, which is still of great help and significance for engineers. Especially, the peak pressure of the shock wave is captured. [ABSTRACT FROM AUTHOR]

In the present study, swirling coaxial confined impinging turbulent air jets issuing from a novel designed nozzle is studied experimentally. Heat transfer characteristics and pressure distribution on the impingement plate are analyzed. Experiments have been conducted at different dimensionless nozzle-to-plate distances ( H / D  = 0.5, 1.0, 1.5, 2.0 and 2.5) and dimensionless flow rates ( Q ∗  = 0.25, 0.50 and 0.75) for a constant total flowrate of 1.33 × 10 −3  m 3  s −1 (80 L/min). The results show that the flowrate ratio improves the uniformity of the heat transfer through the impingement surface and increases the average Nusselt number. Also, the intensity of convective heat transfer is shown to enhance significantly with decreasing nozzle-to-plate distance. With regards to the pressure distribution, subatmospheric regions occur on the impingement plate. Contribution of swirl is also compared against the pure circular impingement jet condition ( Q ∗  = 0.0). [ABSTRACT FROM AUTHOR]

The bubble dynamics behaviors and the pressure in the wall center are investigated through experimental method and numerical study. In the experiment, the dynamics of an underwater explosion (UNDEX) bubble beneath a rigid wall are captured by high-speed camera and the wall pressure in the wall center is measured by pressure transducer. To reveal the process and mechanism of the pressure on a rigid wall during the first bubble collapse, numerical studies based on boundary element method (BIM) are applied. Numerical results with two different stand-off parameters (γ=0.38 and γ=0.90) show excellent agreement with experiment measurements and observations. According to the experimental and the numerical results, we can conclude that the first peak is caused by the reentrant jet impact and the following splashing effect enlarged the duration of the first jet impact. When γ=0.38, the splashing jet has a strong impact on the minimum volume bubble, a number of tiny bubbles, formed like bubble ring, are created and collapse more rapidly owing to the surrounding high pressure and emit multi shock waves. When γ=0.90, the pressure field around the bubble is low enough only a weak rebounding bubble peak occurs. [ABSTRACT FROM AUTHOR]

Velocity fluctuations are widely used to identify the behavior of developing turbulent flows. The pressure on the other hand, which is strongly coupled with the gradient of the mean velocity and fluctuations, is less explored. In this study, we report the results of wall pressure measurements for the development of pipe flow at high Reynolds numbers along the axial direction. It is found that the pressure fluctuations increase exponentially along the pipe with a self-similarity scaling. The exponential growth of the pressure fluctuations along the pipe saturates after reaching a critical position around 50 diameters from the inlet. It qualitatively agrees with the critical position usually adopted for fully developed turbulence, which was obtained from earlier velocity fluctuations at various locations along the pipe centerline. Results also show that the exponential growth of the pressure fluctuations is weakly affected by the presence of ring obstacles placed close to the pipe inlet. Finally, it is found that the pressure fluctuations decrease as a function of Reynolds number, contrary to the boundary layer flow. [ABSTRACT FROM AUTHOR]

The effects of thermal throat on flame stabilization in a kerosene fueled supersonic combustor were numerically and experimentally studied in the present paper. The results were obtained under the inflow condition of stagnation temperature 1100 K, stagnation pressure 1.0 MPa and Mach number 2.0. Wall pressure measurements and flame emission images were made during the experiments in an attempt to better understand the flame development, results showed the combustion organization scheme had a great effect on flame stabilization in the combustor. The flame development process in each case was different according to the flame luminosity pictures. When the pilot hydrogen was removed, the flame of case 1 and case 2 was still stable, and that of case 3 was blown off by the high-speed air flow, but it was surprised to find that the flame of case 4 was blowout even the pilot hydrogen existed. A shock train had generated in the isolator due to the thermal throat existing, which decreased the high-speed main stream, and increased the temperature and pressure, also enhanced the fuel mixing efficiency in the combustor, this was why the flame stabilization was achieved. But if the thermal throat disappeared, the shock train would be pushed out of the combustor by the high-speed air flow. The residence time for mixing, ignition and combustion was getting shorter, the flame was blown off finally. [ABSTRACT FROM AUTHOR]

In side channel blowers, the pressure field is the result of complex, inner flow mechanisms. While there are already experimental investigations on the wall pressure distributions, little is known about the pressure in the rotating system, i.e., on the blade surface. In this work, we present an experimental setup for measuring the unsteady blade surface pressure in several positions. The acquired data will be complemented by and compared to the additionally measured wall pressure on the side channel housing. Miniature pressure sensors are integrated into the impeller. It is modified to ensure flush mounted membranes of the sensors and to avoid impacting on the flow field. A telemetric system is used for a wireless transfer of the data from the rotating system to the data recorder. As a result, we show the time-resolved pressure distribution as well as its phase-locked ensemble average. The variations of the pressure field are related to the integral pressure difference across the turbomachine and to its rotational speed. Due to the high temporal resolution of the measurement data, an exact spatial localization of crucial flow phenomena is achieved. Low integral pressure differences show a nearly linear increase of the pressure in circumferential direction, while greater integral pressure differences evolve exponentially over the azimuth. The results confirm the circulatory flow theory. Different rotational speeds elicit a comparable behavior. The stripper is a dominant source for pressure fluctuations. Its individual geometric discontinuities are correlated to the flow field. Our results provide a deeper understanding of the flow phenomena in side channel blowers and the theory of pressure generation. Although the measurements were performed for only one type of side channel blower with a double-flow configuration and open blades, the energy transfer mechanism is the same for other modifications like single flow or closed blade versions. [ABSTRACT FROM AUTHOR]

This study aims at validating an experimental method for characterizing the vibration behavior of panels excited by a turbulent boundary layer (TBL) excitation as a possible alternative to standard means like wind tunnels or in situ tests. The approach takes advantage of an explicit separation of the excitation contribution from the dynamic behavior of the panel. Based on the measurement of deterministic transfer functions on the panel, called “sensitivity functions,” which are then combined with either measurements or a model of the wall-pressure fluctuations induced by the TBL excitation, the vibration response under such an excitation can be retrieved. For validation purposes, the wall-pressure fluctuations of the turbulent flow generated in an anechoic wind tunnel are measured with a flush-mounted microphone array. The decay rates and the convection velocity, which mainly characterize the excitation, are extracted from these measurements. The plate velocity response to this excitation is estimated following the proposed method using the measured sensitivity functions and the model of Mellen fed with experimentally estimated decay rates and convection velocity. A comparison between a directly measured vibration auto-spectrum under the actual flow and the one predicted following the suggested method shows satisfactory agreement. [ABSTRACT FROM AUTHOR]

The present study addresses the computation of the wall pressure spectrum for a turbulent boundary layer flow without pressure gradient, at high Reynolds numbers, using a new model, the Kriging-based elliptic extended anisotropic model (KEEAM). A space–time solution to the Poisson equation for the wall pressure fluctuations is used. Both the turbulence–turbulence and turbulence–mean shear interactions are taken into account. It involves the mean velocity field and space–time velocity correlations which are modelled using Reynolds stresses and velocity correlation coefficients. We propose a new model, referred to as the extended anisotropic model, to evaluate the latter in all regions of the boundary layer. This model is an extension of the simplified anisotropic model of Gavin (PhD thesis, 2002, The Pennsylvania State University, University Park, PA) which was developed for the outer part of the boundary layer. It relies on a new expression for the spatial velocity correlation function and new parameters calibrated using the direct numerical simulation results of Sillero et al. (Phys. Fluids, vol. 26, 2014, 105109). Spatial correlation coefficients are related to space–time coefficients with the elliptic model of He & Zhang (Phys. Rev. E, vol. 73, 2006, 055303). The turbulent quantities necessary for the pressure computation are obtained by Reynolds-averaged Navier–Stokes solutions with a Reynolds stress turbulence model. Then, the pressure correlations are evaluated with a self-adaptive sampling strategy based on Kriging in order to reduce the computation time. The frequency and wavenumber–frequency wall pressure spectra obtained with the KEEAM agree well with empirical models developed for turbulent boundary layer flows without pressure gradient. [ABSTRACT FROM AUTHOR]

This paper compares several approaches for the prediction of the noise emitted by a NACA0012 airfoil at 0 ∘ and 4 ∘ of angle of attack and a Reynolds number R e = 1.5 × 10 6 . Amiet's semi-analytical model for trailing-edge noise is combined with two-dimensional Reynolds-Averaged Navier-Stokes (RANS) computations. The wall-pressure spectrum, which constitutes the cornerstone of Amiet's model, is obtained by processing the boundary layer data extracted from the simulations. The specific contribution of the paper is a comparison of two families of prediction methods: semi-empirical wall-pressure models that are fitted to experimental databases, and statistical approaches based on the integration of the Poisson equation across the boundary layer profile. The semi-empirical models that were calibrated on airfoil databases provide better predictions with experiments, while the models based on flat-plate boundary layer data fail to reproduce the measured spectra. Considering the statistical approach, it was shown to predict the general spectral features, but with an overall under-prediction of about 3 dB. It can be concluded from this study that the statistical approach proves indeed more robust than semi-empirical models when the latter were not precisely calibrated for the flow under consideration. Further improvements of the statistical approach are suggested for future work. [ABSTRACT FROM AUTHOR]

An idea was carried out on an open cavity having a free stream Mach number of 0.8. The main objective of the present study is to design a passive venting that connects the high pressure region on the rear wall to a low pressure region downstream of the cavity. The baseline condition shows that the cavity flow oscillate with the total amplitude of 155 m. The vortex clipping which is responsible for the self-sustained nature of the cavity flow and the corresponding time history of the pressure on the cavity rear wall is discussed. A cross correlation analysis was carried out between the pressure history at the points on the rear wall of the cavity and the ones on the wall downstream of the cavity. Two points that defines the passive venting were identified based on the rear wall Overall Sound Pressure Level (OASPL) and the minimum time lag computed by cross correlation. The venting is shown to reduce the cavity oscillations and the rear wall pressure is shown to reach a steady state value which is desired in the perception of low cavity noise that can be used to reduce the structural loads and the acoustic loads on electronic components kept near the cavity. [ABSTRACT FROM AUTHOR]

The bubble motion in a compressible liquid under a pressure pulsing condition was studied numerically by solving the conservation equations of mass and momentum. The bubble deformation was computed by a level-set method, which was modified to include the compressibility effects of bubble and liquid. The numerical results demonstrate that the presence of a bubble can significantly amplify the wall pressure near the bubble, and the wall also considerably increases the bubble pressure. The effects of surface tension, viscosity and the magnitude of pressure pulse on the bubble motion were quantified. [ABSTRACT FROM AUTHOR]

This paper proposes a modified TNO model for the prediction of aerofoil trailing-edge noise for wind turbine applications. The capabilities of the current modified model and four variants of the TNO model are analysed through a comprehensive study which includes 10 aerofoils and involves two different wind tunnels. The Reynolds numbers considered are between 1.13 and 3.41 million, and the effective angles of attack are between −2.20° and 13.58°. The merit of a model is assessed by comparing two aspects of the numerically predicted and the experimentally measured sound pressure level spectra: the sound pressure level difference between two different aerofoils at similar lift coefficients within a certain frequency range (referred to as the delta noise); and the closeness in terms of spectral magnitude and shape of the predicted and measured sound pressure level spectra. The current modified model is developed by deriving new formulations for the computation of the wall pressure fluctuation spectrum. This is achieved by using the approximate ratio of the normal Reynolds stress components for an anisotropic flow over a flat plate to estimate the vertical Reynolds stress component, and by introducing new stretching factors to take the effects of turbulent flow anisotropy into account. Compared with the four TNO model variants tested, the current modified model has strong delta noise prediction ability, and is able to predict sound pressure level spectra that are more consistent and closer to measurements for the vast majority of aerofoils and flow conditions tested in the two wind tunnels. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Flow-induced vibrations in a pressurized water reactor (PWR) core can cause fretting wear in fuel rods. These vibrations can compromise safety of a nuclear reactor. So, it is necessary to know the random fluctuating forces acting on the rods which cause these vibrations. In this paper, simplified 3D models like square spacer grid, circular spacer grid and symmetric mixing vanes have been used inside an annular pipe. Hydrodynamic and wall pressure characteristics are evaluated using large eddy simulations (LES). Structured meshes are generated as far as possible. Simulations are compared with an experiment. Results show that the grid and vanes have a combined effect: grid accelerates the flow whereas the vanes contribute to the swirl structures. Spectral analysis of the simulations illustrate vortex shedding phenomenon in the wake of spacer grids. This initial study opens up interesting perspectives towards improving the modeling strategy and understanding the complex phenomenon inside a PWR core. [ABSTRACT FROM AUTHOR]

This study presented an extensive analysis of the influence of chord-to-thickness ratios on unsteady behaviors of the separated flow and their coupling with wall-pressure fluctuations at the Reynolds number Re D = 1.58 × 10 4 . Three systems with different chord-to-thickness ratios were chosen for comparison: L / D = 3.0, 6.0, and 9.0. The separated flow fields and wall-pressure fluctuations were measured using planar Particle Image Velocimetry (PIV) operating at 1 Hz and a microphone array sampling at 1 kHz, respectively. The distributions of the turbulent kinetic energy and the reverse-flow intermittency indicated that the shortest plate ( L / D = 3.0) was subjected to full coverage of the separated flow. In the moderate system ( L / D = 6.0), the time-averaged reattachment station was determined to be x / D = 4.75, while in 7% of all realizations the fluid close to the trailing edge surface remained slightly reversed due to the unsteady reattachment of the separated shear layer to the trailing edge and to irregular trailing edge vortex shedding. However, the separated flow always reattached on the plate’s surface in the longest system L / D = 9.0. The measured wall-pressure fluctuations demonstrated combinative influences of hydrodynamic pressure of the large-scale vortical structures above the plate and acoustic excitation propagated by the energetic unsteady wake. This resulted in the salient phase changeover characteristics of the correlative patterns near x / D = 1.5 in the shortest system, L / D = 3.0. Close to the leading edge, the wall-pressure fluctuations in the L / D = 6.0 system were over-exposed to acoustic excitation propagated by the unsteady wake behind the trailing edge, while in the longest system, L / D = 9.0, such influences were relatively attenuated. Proper Orthogonal Decomposition (POD) analysis was performed on the down-sampled wall-pressure fluctuations paired with the flow fields, resulting in a series of energetic flow structures and their corresponding wall-pressure fluctuation profiles. Subsequently, the conditional averaging technique was applied to determine the relationship between the spatial characteristics of the large-scale vortical structures and profiles of the wall-pressure fluctuations. [ABSTRACT FROM AUTHOR]

This paper considers the inherent unsteady behavior of the three-dimensional (3D) separation in the corner region of a subsonic linear compressor cascade equipped of 13 NACA 65-009 profile blades. Detailed experimental measurements were carried out at different sections in spanwise direction achieving, simultaneously, unsteady wall pressure signals on the suiface of the blade and velocity fields by time-resolved particle image velocimetry (PTV) measurements. Two configurations of the cascade were investigated with an incidence of 4 deg and 7deg, both at Re = 3.8 X 105 and Ma = 0.12 at the inlet of the facility. The intermittent switch between two statistical preferred sizes of separation, large, and almost suppressed, is called bimodal behavior. The present PIV measurements provide, for the first time, time-resolved flow visualizations of the separation switch with an extended field of view covering the entire blade section. Random large structures of the incoming boundaiy layer are found to destabilize the separation boundary. The recirculation region, therefore, enlarges when these high vorticity perturbations blend with larger eddies situated in the aft part of the blade. Such a massive detached region persists until its main constituting vortex suddenly breaks down and the separation almost completely vanishes. The increase of the blockage during the separation growth phase appears to be responsible for this mechanism. Consequently, the proper orthogonal decomposition (POD) analysis is carried out to decompose the flow modes and to contribute to clarify the underlying cause-effect relations, which predominate the dynamics of the present flow scenario. [ABSTRACT FROM AUTHOR]

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The paper describes experimental investigation results on wheat flow in a medium-size steel cylindrical flat bottom silo with horizontally corrugated walls and columns composed of open-sectional thin-walled profiles. Several experiments on silo filling and emptying with wheat were performed by inducing a different flow pattern. The following quantities were measured in the silo: horizontal wall pressures, wall frictional tractions and vertical reaction forces under columns. Horizontal wall pressures and wall frictional tractions were registered along the silo height and silo circumference in the lower silo region during three different flow patterns: mass flow, mixed mass-funnel flow and eccentric flow. Experimental results were compared with Eurocode 1. Some discrepancies between experiments and code formulae were observed. [ABSTRACT FROM AUTHOR]

In this paper, the effect of heat release on movement characteristics of shock train is numerically investigated in an isolator. It is found that the combustion heat release has a distinct effect on the shock train movement characteristics in the isolator. With increasing heat release, a shock train gradually forms and then propagates toward isolator entrance. In process of shock train formation, separation bubbles before injection ports entrain the high temperature burning gas into the boundary layer, which causes the shock train to shrink and stretch, and changes in configuration and number of shock waves. At the same time, the system force fluctuates. In addition, the shock train movement is divided into three stages, which have different wall pressure distribution. It is believed that these findings have a help the better understanding of the effect of heat release on the movement characteristics of shock train in an isolator. [ABSTRACT FROM AUTHOR]

The pressures exerted on the walls of rectangular planform steel flexible-walled silos by several different stored granular bulk solids are investigated using a validated finite element model that has been used in several previous studies. These pressures and the state of stress in the bulk solid are explored for a range of silo geometries and stored bulk solids. The results show that the horizontal pressure distribution across a silo wall is generally not uniform. This demonstrates that widely used theories may be adequate for stiff concrete silos, are far from suited to flexible-walled steel silos, and the differences can be used to produce much lighter structures. These findings match previously published experimental and analytical results for square planform silos where much larger pressures develop in the corners. The present analyses show that rectangular silos differ from those of square section, in that the mean pressure and degree of pressure variation is different on the two walls. The mechanisms causing these changes are investigated. The results further demonstrate that relatively small changes in the properties of a stored solid can produce significant changes in the pressure magnitudes and patterns, and hence greatly influence the silo structural design. The paper concludes that existing design guidance is seriously deficient and leads to metal silos that are considerably more expensive than is necessary. [ABSTRACT FROM AUTHOR]

A more complete understanding of the physical relationships, between wall-pressure and turbulence, is required for modeling flow-induced noise and developing noise reduction strategies. In this study, the wall-pressure fluctuations, induced by low Reynolds number turbulent boundary layers, are experimentally studied using a high-resolution microphone array. Statistical characteristics obtained using traditional cross-correlation and cross-spectra analyses are complimented with wall-pressure–velocity cross-spectra and wavelet cross-correlations. Wall-pressure–velocity correlations revealed that turbulent activity in the buffer layer contributes at least 40% of the energy to the wall-pressure spectrum at all measured frequencies. As Reynolds number increases, the low-frequency energy shifts from the buffer layer to the logarithmic layer, as expected for regions of uniform streamwise momentum formed by hairpin packets. Conditional cross-spectra suggests that the majority of broadband wall-pressure energy is concentrated within the packets, with the pressure signatures of individual hairpin vortices estimated to decay on average within traveling ten displacement thicknesses, and the packet signature is retained for up to seven boundary layer thicknesses on average. [ABSTRACT FROM AUTHOR]

The main objective of a dual bell nozzle is the enhancement of performances based on the principle of auto-adaptation in accordance with the altitude. Indeed, this system has as advantage the auto-adaptation of the flow for two operating modes (at low and high altitude) without mechanical activation. The principle is theoretically simple but structural forces involved can be significant. In this study, a numerical method for the design of this type of nozzle is developed. On the one hand, it is based on a transonic flow approaches to define the starting line on which the supersonic calculations will be initiated. On the other hand, the method of characteristics is used to draw the base nozzle profile. Knowing that the latter is assimilated as a polynomial of the second degree, its constants are calculated from initial conditions. In order to minimize the weight of this nozzle, its truncation proves necessary; this is performed at a point where the best compromise (weight / performances) was respected. The profile of the second curve is calculated to give a constant wall pressure. This is achieved by using the direct method of characteristics applied for a centered expansion wave that the intensity is P 2 /P 1 at the junction. Once the profile is generated, an analysis of the thermodynamic-parameters evolution (such as: pressure, Mach number) and aerodynamic performances is conducted. For more consistency, our results are compared with numerical databases of ONERA nozzle. Simulations of flow in the nozzle with Ansys 13.0 environment for different types of meshes are presented. Also, to offset the effects of the boundary layer, the simulations were performed by using the k-ω SST turbulence model. The obtained results by the method of characteristics and numerical simulation are compared to the computed results of the literature and it was found good agreement and similarity. [ABSTRACT FROM AUTHOR]