Fluctuating Pressure Coefficients Research Articles

Effects of turbulence integral scales on characteristics of fluctuating wind pressures

To investigate the effects of turbulence integral scale, four rectangular models were tested in grid-generated turbulent flows, and the characteristics of fluctuating pressures on windward surfaces were obtained. The results show that all statistical parameters except the mean pressures depend on both the longitudinal integral scale (Lux) and depth (D) of the model. The root-mean-square (rms) values of the fluctuating pressure coefficients and their correlation as well as the spanwise coherence increase with the turbulence integral scale. As the integral scale decreases, the power spectra of the fluctuating pressures in the low-frequency region, which are controlled by the quasi-steady effect, remain almost unchanged, whereas those in the high-frequency region attenuate more rapidly. To seek a potential modification method for correcting the error margin caused by an imprecisely simulated turbulence integral scale, an empirical model of aerodynamic admittance function (AAF) of fluctuating pressure was proposed with consideration of Lux /D as a key variable. Finally, using the fluctuating pressure on the model with a side ratio of 1:3 as an example, the error margins of the fluctuating pressures were discussed. It was discovered that the maximum deviation of the rms value of the fluctuating pressure coefficient reached approximately 25% when the simulated turbulence integral scale was one-sixth the target value.

Time-resolved reconstruction of turbulent flows using linear stochastic estimation and sequential data assimilation

The present work concentrates on the reconstruction of the time-resolved turbulent flows from probe signals and low sampling rate flow fields using linear stochastic estimation (LSE) and sequential data assimilation (DA). The separated and reattached flow over a blunt plate is used as the benchmark configuration. Experimental data are acquired with a microphone array (34 probes) installed on the plate surface to capture the pressure fluctuation at a sampling rate 1000 Hz, and with planar particle image velocimetry (PIV) measuring the two-dimensional two-component (2D2C) velocity fields synchronized with the microphones at 1 Hz. LSE is conducted first to estimate the raw temporal sequence of the flow field from PIV and microphone data. This temporal sequence then serves as the observations for the DA process based on continuous adjoint formulation for the flow field correction and pressure determination. The LSE results show that an appropriate size of proper orthogonal decomposition (POD) database should be evaluated considering the combined error induced by the truncation of the mapping function M, the size of the POD database, and the scaling of the model coefficient for the compensation of M truncation. Subsequently, the LSE reconstructions using the POD database of size Nt = 100 are employed as the observations in the DA process. The mean flow field is recovered quite well, while the normal Reynolds stress also has a significant improvement compared to large-eddy simulation. The temporal variation of the LSE reconstruction is significantly improved, and the resultant fluctuating pressure coefficient distribution agrees reasonably well with the microphone measurement.

Experimental evidence dynamic pressures reduction on plunge pool floors downstream flip bucket for increasing downstream face slopes

Abstract In this research, the ejecting jet from a flip bucket downstream of a chute spillway was simulated using physical modeling. The effects of influencing parameters upon fluctuations and extreme values of dynamic pressure were investigated. The angles of 0°, 30°, 45°, and 60° were adopted for the mobile bottom wall. The discharges were set as 67, 86, 161, and 184 litre/s and the depths of water cushion on the mobile bottom wall were set as 0, 15, 30, and 45 cm. The method suggested by Castillo for computation of fluctuating coefficient of dynamic pressure (see Castillo (2007) Pressure characterization of undeveloped and developed jets in shallow and deep pool. Proceedings of the Congress-International Association for Hydraulic Research32 (2), 645) was validated via the laboratory data. The results showed that the increase in water cushion depth downstream has led to a decrease in mean pressure and in pressure fluctuations. The analyses showed that the fluctuating pressure coefficient was a function of water cushion depth, and its maximum value was taken when there was a water cushion on the mobile bottom wall. With an increase in discharge and mobile bottom wall angle, the maximum value of the fluctuating coefficient occurred in less water cushion depth. Moreover, with the growth of discharge, the maximum positive and negative fluctuations of the pressure increased first and then decreased.

Mean and Fluctuating Pressure Coefficient Distributions for Circular Closed and Open Dome Roofs

Mean and Fluctuating Pressure Coefficient Distributions for Circular Closed and Open Dome Roofs

Joint acoustic and wall-pressure measurements on a model A-pillar vortex

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.

Surface pressure on a cubic building exerted by conical vortices

Experiments were performed to study surface pressure on a cubic building underlying conical vortices, which are known to cause severe structural damage and failure. The focus is on the effects of turbulence in the incident flow. Three turbulent boundary layers were created in a boundary layer wind tunnel. A wall-mounted cube, i.e. a cube situated on the horizontal ground floor surface of the wind-tunnel test section, was used as an experimental model. The cube was subjected to the incidence flow at 40°. Steady and unsteady pressure measurements were performed on the cube surface. The analysis suggests that conical vortices developed above the top surface of the wall-mounted cube. A larger mean suction was observed on the top cube surface in the less turbulent boundary layer. With an increase in turbulence in the incoming flow, the strong suction zones decreased in size. The fluctuating pressure coefficient profiles retained their shape when the turbulence in the upstream flow of the cube increased. The fluctuating pressure coefficient was observed to be larger in more turbulent flows. The pressure fluctuations were larger on the cube surface underlying outer boundaries of the conical vortex. The fluctuating pressure coefficient under the conical vortex was three to four times larger than in the weak suction zone on the central area of the top cube surface. Close to the leading cube corner, the pressure spectra were dominated by a single low frequency peak. As the conical vortex developed, this primary peak weakened and a secondary peak emerged at a higher reduced frequency. There is a general trend of shifting the pressure spectra towards higher reduced frequencies when the turbulence in the undisturbed incident flow increases.

Numerical Investigation of Multi-SDBD Plasma Actuators for Controlling Fluctuating Wind Load on Building Roofs

The present research aims to explore, by large-eddy simulation (LES), the potentiality and mechanism of multiple surface dielectric barrier discharge (multi-SDBD) plasma actuators to manipulate mean and fluctuating wind loads on a low-rise building. Three actuator configurations are located on the roof to induce directional wall jets in different directions. The effects of these configurations on flow structure and wind loads are studied in absence and presence of approaching flow. Results show that all subgrid-scale models can obtain accurate roof pressure, and for the diffusion and convection terms, the bounded central differencing scheme can provide more accurate predictions for the roof pressure. The control impact of active actuators gradually weakens with the increase of the approaching flow velocity. The direction of the wall jet can determine the position of the limited roof region with the reduced mean pressure coefficient. The multi-SDBD actuators continue to absorb the upstream flow and blow this flow downstream, meaning the wall jet exerts strong pressure on the local roof area at the end of the jet, which results in a significant reduction of the mean pressure coefficient. Furthermore, the counter-rotating vortices caused by the wall jet restrain the size and strength of the vortex shedding, thereby achieving the purpose of reducing the fluctuating pressure coefficient. Further analysis of the instantaneous vorticity fields indicates that the intensity and size of streamwise shedding vortices can be restrained by small-scale spanwise vortices induced by the plasma actuators. Under the action of the wall jet blowing from the trailing edge to the leading edge, the fluctuating lift and drag coefficients can be reduced by over 15% and the fluctuating pressure coefficient can be reduced by about 20% from the no actuation situation.

Vortex shedding characteristics around a circular cylinder with flexible film

Abstract In this paper, an experimental study about the influence of flexible film on the vortex shedding characteristics behind a cylinder has been performed with increasing Reynolds number (Re) and the ratio of flexible film length to cylinder diameter (L/D). The smoke-wire technique is implemented for flow visualization. The detailed statistics and vortex shedding structures associated with pressure fluctuations owing to a flexible film behind a cylinder are presented. The focus of this paper is on efforts to reveal the influence on the distribution of fluctuation pressure and the frequency of vortex shedding resulting from flapping flexible film with different parameters of L/D. Our experimental results mainly reveal that when the flexible film is in asymmetric flapping mode behind the cylinder, the peak distribution of fluctuating pressure coefficients C p f will deviates from central area. The Strouhal number decreases with the increase of L/D. For L/D=3, the Strouhal number also decreases with increasing Re. It is found that the peak of fluctuating pressure spectrum is closely related with the non-dimensional parameter L/D, and the vortex shedding frequency obviously decreases with increasing of L/D. The fluctuating signal of flapping flexible film for L/D=5 effectively offsets the fluctuating signal behind a cylinder itself, and the peak of fluctuating pressure spectrum does not occur at Re=2500. Nevertheless, for L/D=3, the amplitude of fluctuating pressure spectrum obviously exceeds that of fluctuating pressure spectrum of L/D=0. Our results further reveal that flapping flexible film effectively controls the vortex shedding behind a cylinder with increasing of L/D. However, for L/D=3, the flapping flexible film results in harmonic excitation mode.

Numerical and Experimental Comparative Study on the Flow-Induced Vibration of a Plane Gate

A numerical method is applied here to simulate the unstable flow and the vibration of a plane gate. A combination of the large eddy simulation (LES) method and the volume of fluid (VOF) model is used to predict the three-dimensional flow field in the vicinity of a plane gate with submerged discharge. The water surface profile, the streamline diagrams, the distribution of turbulent kinetic energy, the power spectrum density curve of the fluctuating pressure coefficient at typical points underneath the gate, and the complete vortex distribution around the gate are obtained by LES-VOF numerical calculation. The vibration parameters of the gate are calculated by the fluid-structure coupling interface transferring the hydrodynamic load. A simultaneous sampling experiment is performed to verify the validity of the algorithm. The calculated results are then compared with experimental data. The difference between the two is acceptable and the conclusions are consistent. In addition, the influence of the vortex in the slot on the flow field and the vibration of the gate are investigated. It is feasible to replace the experiment with the fluid-structure coupling computational method, which is useful for studying the flow-induced vibration mechanism of plane gates.

Aerodynamic performance of twin-box decks: A parametric study on gap width effects based on validated 2D URANS simulations

2D URANS simulations are conducted aiming to study the aerodynamic performance under smooth flow of twin-box decks depending on the gap distance between girders. The Stonecutters Bridge is taken as the reference geometry. In this parametric study, 14 gap to depth ratios in the range 0≤G/D≤9.70 are investigated, and for each geometry, 11 angles of attack in the range −10∘≤α≤10∘ are considered. Specific goals of this research have been: identification of the fundamental flow features, study of mean and fluctuating pressure coefficients distributions, identification of the vortex shedding mechanisms and general aerodynamic characterisation based on force coefficients. The numerical results provided herein are validated with wind tunnel data previously reported in the literature, finding a good agreement. A critical gap to depth ratio at G/D=2.35, in terms of aerodynamic response, was identified, which is consistent with the value reported in the literature for a different bridge based on wind tunnel tests. The obtained set of data provide a general picture of the expected aerodynamic performance of a twin-box deck depending on the gap distance and could be of great value at the early design stage of long-span cable-supported bridges.

Unsteady aerodynamic characteristics of long-span roofs under forced excitation

Wind-induced vibration is significant for light and flexible structures, and aerodynamic effect has to be considered in the estimation of wind load and dynamic response of these structures. The unsteady aerodynamic characteristics of long-span roofs are investigated numerically in this paper using Large Eddy Simulation. Roofs under forced excitation are simulated, and the distributions of aerodynamic coefficients on rigid and vibrating roofs are estimated. The computed coefficients show good agreement with the experimental results. The influences of inflow turbulence, shape of roof and other parameters on the aerodynamic coefficients are also analyzed. It is found that the mean wind pressure coefficients mainly depend on the vortex shedding and reattachment positions. These coefficients in the cases studied remain almost the same with increasing excitation frequency, while the fluctuating pressure coefficients and unsteady aerodynamic coefficients change with frequency regularly. The probability density functions of wind pressure on the vibrating roofs show obvious non-Gaussian characteristics, and the wind pressure spectra exhibit peaks at the excitation frequencies. The dynamic response of long-span roof is then predicted using the mechanical admittance function. Results show that the displacement response of the structure will be under-estimated without considering the unsteady aerodynamic effect especially for flexible structures.

Separation Control on a Bridge Box Girder Using a Bypass Passive Jet Flow

In the present study, a bypass passive jet flow control method was proposed to mitigate unsteady wind loads and to manipulate the flow field around a single box girder of a bridge. With a geometric ratio of 1:125, the single box girder model was determined using the cross-section of the Great Belt East Bridge. During the experiments, one test model without control was adopted, while five different test models with different suction/jet configurations were employed to analyze the effects of the control method and to reveal the underlying mechanism of different control schemes. The incoming wind speed was fixed to 12 m/s and the wind attack angles were changed from −20° to 20°, resulting in a corresponding Reynolds number of Re = 0.28 × 105–0.74 × 105 based on the different attack angles. A six-component force balance, a set of digital sensor array (DSA) pressure transducers, and a particle image velocimetry (PIV) system was used to measure the aerodynamic forces, pressure distributions, and flow fields around the test models to evaluate the control effectiveness of different control cases. Detailed flow structures are presented and discussed for two test cases when the angles of attack are +15° and −20°. The effects of control on the aerodynamic forces were first investigated to determine and select the best one out of five control cases. The pressure distributions on the surface of the test model without control and the best control case were then compared to evaluate the control effectiveness of the pressure gradient and the fluctuating pressure coefficients. The flow fields around the test models demonstrate that the bypass passive jet flow control could decrease vortex strength, delay flow separation, and change recirculation region and size. The results of the aerodynamic forces, pressure distributions, and flow fields indicate that the bypass passive jet flow control method results in effective control.

Reduction of fluid forces acting on a square prism using a planar jet

This experimental study focused on the concurrent minimization of the mean and fluctuating forces acting on a square prism in crossflow by creating a continuous jet through a narrow spanwise slot in it. For this purpose, three different predetermined injection surfaces (i.e., front, top, and rear) were individually studied for the injection ratios (IR) of 0, 1, 1.5, 2, 2.5, and 3 at a Reynolds number (Re) of 8000. The results showed that the rear jet is the optimum injection surface. For the rear jet configuration, the optimum IR is 1.5 in terms of mean total drag coefficient reduction, 〈CDT〉. In this case, the reductions in the 〈CDT〉 and the level of root mean square (RMS) of fluctuating pressure coefficient (CP RMS) on the side surfaces are about 29.7% and 68%, respectively. However, the maximum reduction in the CP RMS level on the side surfaces occurred at IR=2. In this case, the reductions in the 〈CDT〉 and the level of CP RMS on the side surfaces are about 27.5% and 88%, respectively. The underlying mechanism of force reduction was also demonstrated. Various flow patterns were identified with respect to IR at incidence angle (α) of 0°. It was also proved that, at relatively small angles of incidence, from α=0° to 20°, the active flow control method is still effective in reducing 〈CDT〉 and in suppressing the fluctuating side forces, which is indicative of periodic vortex shedding from either side of the square prism.

Detached-eddy and large-eddy simulations of wind effects on a high-rise structure

Abstract In this study, detached-eddy simulations (DES) and large-eddy simulations (LES) of wind effect on a high-rise structure with elliptical shape are performed. The aim of this study is to examine the accuracy of the numerical simulations for wind flow around a complex high-rise structure and investigate the effects of surface roughness on the wind loading. In order to reproduce appropriate inflow turbulence, the improved recycling method combined with the weighted amplitude wave superposition (WAWS) method is used to generate the inflow turbulence in the streamwise direction. Also, to study the surface roughness effects on the wind loading, the rough wall boundary conditions are adopted in the DES models. Typical results including the mean and fluctuating pressure coefficients, force spectra, Reynolds stresses and wind-induced top-floor response are obtained and compared with the corresponding wind tunnel experimental data. Through the cross-comparisons, the numerical results by the DES and LES models are found to be comparable with the experimental results. Additionally, the surface roughness effects on the wind loading is well reproduced with the DES models, such as the slow pressure recovery in the separation regions and the increased mean drag force coefficients. It is demonstrated through the validations that the DES and LES models and the numerical treatments adopted in this study can provide reasonably good results.

Simulation of turbulent flows around a prism in suburban terrain inflow based on random flow generation method simulation

In the study, the modified discretizing and synthesizing random flow generation (MDSRFG) was adopted to generate an anisotropic boundary layer inlet for large-eddy simulation. The mean velocity, turbulence intensity and turbulence length scale distributions at inlet, were defined according to the measurements at TKU wind tunnel. The von Kármán model was used as the target spectrum. Wind tunnel pressure measurements on a square prism model with aspect ratio of 3 was used for validation of numerical simulation. Results show that turbulence energy is well maintained from the inlet to the downstream. The relative differences between the measurement and predicted results are 3.4% (mean drag coefficient), 11% (fluctuating drag coefficient), 25.6% (fluctuating side force coefficient) and 4.7% (Strouhal number). The simulated mean and fluctuating pressure distributions showed good agreements with the experiments. The averaged differences between measurement and predicted results are 14.49% (mean pressure coefficient) and 13.74% (fluctuating pressure coefficient). This indicates that the adoption of a reasonable process based on the MDSRFG method is an effective tool to generate a spatially correlated atmospheric boundary layer flow field.

Plunge pool dynamic pressures: a temporal analysis in the nappe flow case

ABSTRACTRectangular jet, or nappe flow, constitutes one of the energy dissipation methods used in the overtopping of dams. This paper reports new laboratory data on pressures, velocities and air entrainment and analyses the degree of break-up of a rectangular jet before entering the basin and the pressure fluctuations on the bottom of the pool. It is found that the maximum mean dynamic pressure coefficient of 0.86, which is valid for circular jets, is reduced to 0.83 for a rectangular jet. The study has also led to a relationship between the fluctuating pressure coefficient and the degree of the jet break-up. The peak pressure coefficient occurred for break-up length ratios (i.e. falling height/length) in the range of 1.00–1.20. The scaling effects are negligible for scales less than 1:10. The new set of characteristics given in the paper fills the current knowledge gap related to rectangular jets.

An experimental study on the unsteady vortices and turbulent flow structures around twin-box-girder bridge deck models with different gap ratios

In the present study, an experimental investigation was conducted to characterize the unsteady vortices and turbulent flow structures around twin-box-girder (TBG) bridge deck models with and without cross beams. While the oncoming wind speed was fixed at U∞=8.0m/s (i.e., the corresponding Reynolds number, Re=1.01×104, based on the height of the box girder) during the experiments, the gap width between the TBG was varied to have four different gap ratios (i.e., the ratio of the gap width between the TBG to the deck height). The corresponding test cases were classified into two categories: the cases with relatively small gap ratios (i.e., gap ratio=0.85 and 1.70) and the cases with relatively large gap ratios (i.e., gap ratio=2.55 and 3.40). In addition to measuring the surface pressure distributions around the TBG bridge deck models using an array of digital pressure transducers, a high-resolution particle image velocimetry (PIV) system was utilized to perform detailed flow field measurements to quantify the evolution of the unsteady vortex structures around the TBG bridge deck models. The measurements reveal that as the gap ratio increases, the vortex shedding moves from the trailing edge of the leeward box to the rear edge of the windward box, which simultaneously increases the turbulent kinetic energy in the gap region and the fluctuating pressures on the leeward box. The vortex dimensions and the core-to-core distances between two neighboring vortices are also affected by the gap ratio. Combining with the estimation of the pressure field and the measured fluctuating pressure coefficient distributions on the TBG models, it is found that the TBG bridges with larger gap ratios will dramatically strengthen the fluctuating pressure coefficients on the leeward box which are greatly higher than those for the small gap ratio cases. Moreover, for large gap ratio cases, the test model with cross beams also has higher fluctuating pressure coefficients on the leeward box which just decrease a little comparing with the test model without cross beams.

Field Measurement of Wind Effects of Roof Accessory Structures on Gable-Roofed Low-Rise Building

The components and claddings of low-rise buildings are usually destroyed first during typhoon disasters in coastal areas. Roof accessory structures can change the flow pattern on the roof, thus effectively reducing the wind load on the roof surface and the damage to the low-rise buildings. Three types of aerodynamic mitigation plates, that is, (1) 0.3 m high full-length roof-edge plate, (2) 0.3 m high and 0.5 m + 0.5 m long roof-corner plate, and (3) discrete roof-edge plates with different spaces (which can be used as advertisement boards), are studied on the basis of the field measurement results under roof pitches of 10° and 18.4°. By comparing the results of the roof with and without constructed plates, it is implied that the three types of plates can affect the formation of conical vortexes and can significantly reduce the mean and fluctuating pressure coefficients in the windward corner. Compared with the constructed plate, the roof ridge has a larger influence on the wind loads on leeward roof.

Suppression of vortex-induced vibration of a circular cylinder using suction-based flow control

In the present study, a flow control method is employed to mitigate vortex-induced vibration (VIV) of a circular cylinder by using a suction flow method. The VIV of a circular cylinder was first reproduced in a wind tunnel by using a spring–mass system. The time evolution of the cylinder oscillation and the time histograms of the surface pressures of 119 taps in four sections of the circular cylinder model were measured during the wind tunnel experiments. Four steady suction flow rates were used to investigate the effectiveness of the suction control method to suppress VIV of the circular cylinder. The vibration responses, the mean and fluctuating pressure coefficients, and the resultant aerodynamic force coefficients of the circular cylinder under the suction flow control are analyzed. The measurement results indicate clearly that the steady suction flow control method exhibits excellent control effectiveness and can distinctly suppress the VIV by dramatically reducing the amplitudes of cylinder vibrations, fluctuating pressure coefficients and lift coefficients of the circular cylinder model. By comparing the test cases with different suction flow rates, it is found that there exists an optimal suction flow rate for the maximum VIV control. The cases with higher suction flow rates do not necessarily behave better than those with lower suction flow rates. With the experimental setting used in the present study, the suction flow control method is found to behave better for VIV suppression when the ratio of the suction flow velocity to the oncoming flow velocity is less than one.

Wind pressure characteristics for a double tower high-rise structure in a group of buildings

Wind pressure characteristics on a double tower high-rise structure, which is disturbed by surrounding buildings, were investigated using large eddy simulation (LES) and 1:300 scale wind tunnel experiments. The computational simulation technique and wind tunnel experimental technique were described in detail initially. Comparisons of computational results with the experimental data have subsequently been carried out to validate the reliability of LES. Comparisons have been performed in detail for the mean and fluctuating pressure coefficients. Detailed explanations of each comparison were given in the paper. To study further on the pressure coefficients on the building surfaces, parametric studies on shape coefficient and spatial correlation were performed and investigated. The numerical and experimental results presented in this paper advance understanding on wind field around buildings and the application of LES and wind tunnel tests.