Phase-locked eduction of vortex shedding in flow behind past and inclined flat plate

Flow past an inclined flat plate at an angle of attack of 30° and a Reynolds number of 30 000 is investigated experimentally. The velocity field in the wake is measured with a laser doppler anemometer (LDA) in the region from one plate breadth downstream to three and a half-plate breadths downstream. Controlled forcing is applied to the wake by vibrating the plate in the across-wind direction at a frequency in the middle of the lock-in range. The forcing serves to enhance the regularity and two-dimensionality of vortex shedding from the plate. It also facilitates phase-locked averaging of the LDA data. The LDA bursts are sorted according to their arrival instants relative to a particular phase of the vortex shedding cycles. The phase-averaged velocity results reveal large-scale vortical structures in the wake. Dynamical properties of these structures such as coherent vorticity and Reynolds stress production are discussed. The wake is found to be strongly asymmetric. The flow dynamics in the wake are dominated by a train of counterclockwise vortices shed from the trailing edge of the plate. The development, shedding and subsequent convection of these vortices are studied by following the consecutive phases of the shedding cycle. ©1996 American Institute of Physics.published_or_final_versio

Two-phase velocity measurement in a particle-laden jet

postprin

Characteristics of wind pressures on large cantilevered roofs: Effect of roof inclination

Wind pressure distributions on both surfaces of a cantilevered roof are measured on a wind tunnel model. The investigation covers a down-sloping roof configuration at -5° roof angle and two up-sloping roof configurations of roof angles 5° and 10°. The results supplement an earlier study made on a horizontal roof. The wind pressure pattern on the upper surface of an up-sloping roof suggests that there exists a preferred mode of flow separation on that surface. The pattern shows two localized regions of high suction, which are separately located towards the two ends of the roof span. On the up-sloping roofs, this pressure pattern is observed in the time-averaged mean distribution as well as during the occurrence instant of a peak total uplifting force on the roof. On the horizontal roof, this particular pattern of flow separation is not obviously observed in the time-averaged mean wind pressure pattern but is revealed by the conditional sampling technique which captures the wind pressure distribution during the generation of a peak uplifting force on the roof. The wind pressure signals are analysed to study the characteristics of the total wind force on the roof and also the wind forces on individual roof surfaces. © 2002 Elsevier Science Ltd. All rights reserved.postprin

Unsteady features of bluff body wake with application to building wake length

ABSTRACT: This study is targeted at obtaining fluctuating characteristics of building wakes through measurement of unsteady velocity fields with time-resolved particle image velocimetry (PIV). Proper orthogonal decomposition is applied to the PIV data to identify low-dimensional substructures of the building wake. This leads to a more reliable estimation of the fluctuating ground-level length of the building wake from an instantaneous PIV snapshot. The statistics of unsteady wake lengths is important in the probability-based risk assessment of critical events of wind environment around buildings such as dispersion of hazardous pollutants, pedestrian danger and risks to aircraft landings on airport runways.published_or_final_versio

Centerline velocity decay of a circular jet in a counterflowing stream

We use an advection hypothesis to analyze the decay of centerline velocity of a circular jet issuing into a counterflowing stream. Working in the Lagrangian frame, we follow the locations and velocity gradients of jet fluid particles along the jet central axis while the particles are being advected backwards by the counterflow. The spatial velocity gradient along the jet centerline is thus obtained and subsequently integrated to describe the spatial decay of axial velocities. Laser-doppler velocity measurements are performed in the laboratory and the data are well predicted by our analytical expression of centerline velocity decay. Looking from another view, our treatment supports that the effect of an external axial flow stream on the jet flow field can be represented by a certain degree of stretching or contracting of the jet in the axial direction. © 1998 American Institute of Physics.published_or_final_versio

Occurence of peak lifting actions on a large horizontal cantilevered roof

Wind tunnel tests are performed on a rigid model of a horizontal grandstand roof. The objective is to investigate the generation mechanism of wind pressure and peak lifting actions on a large cantilevered roof. The roof model is 780 mm wide, 150 mm deep and is equipped with 78 pairs of pressure taps covering both roof surfaces. With an electronic pressure scanning system, wind pressures at these 156 taps are measured in a near-simultaneous manner. Time histories of the wind forces on both roof surfaces and of the net wind force on the entire roof are obtained from the simultaneous pressure signals. These force signals are analyzed statistically to investigate the mechanism of peak lift forces. The conditional sampling technique is used to extract the wind pressure pattern on the lower and upper roof surfaces when a large total uplift is occurring on the roof. It is found that the occurences of peak lift and peak moments on the roof are connected with large lifting actions on both roof surfaces. Differences between the conditionally sampled pressure pattern and the time-averaged mean pressure pattern are observed and discussed. A study is made on the overestimation of net roof pressure if it is assumed to be the worst combination of peak pressures simultaneously occuring on both roof surfaces. © 2002 Elsevier Science Ltd. All rights reserved.postprin

CFD study of dynamic wind actions on faces of a tall building

Section B: Numerical simulation and theoretical analysisThe article can be viewed at: http://wgy091.ust.hk:8080/document/HKWE1papers.pdfThis paper computes and investigates the fluctuating wind forces on different building faces of a 6:1:1 square-section tall building with LES. Time histories of aerodynamic forces on building faces are computed from which power spectra and cross correlation analysis are made. The computation results are compared to past wind tunnel data from base balance and wind pressure measurements. Most results show qualitative agreement.published_or_final_versio

Asymmetric vortex shedding flow past an inclined flat plate at high incidence

This paper reports an experimental investigation of the vortex shedding wake behind a long flat plate inclined at a small angle of attack to a main flow stream. Detailed velocity fields are obtained with particle-image velocimetry (PIV) at successive phases in a vortex shedding cycle at three angles of attack, α=20°, 25° and 30°, at a Reynolds number Re≈5,300. Coherent patterns and dynamics of the vortices in the wake are revealed by the phase-averaged PIV vectors and derived turbulent properties. A vortex street pattern comprising a train of leading edge vortices alternating with a train of trailing edge vortices is found in the wake. The trailing edge vortex is shed directly from the sharp trailing edge while there are evidences that the formation and shedding of the leading edge vortex involve a more complicated mechanism. The leading edge vortex seems to be shed into the wake from an axial location near the trailing edge. After shedding, the vortices are convected downstream in the wake with a convection speed roughly equal to 0.8 the free-stream velocity. On reaching the same axial location, the trailing edge vortex, as compared to the leading edge vortex, is found to possess a higher peak vorticity level at its centre and induce more intense fluid circulation and Reynolds stresses production around it. It is found that the results at the three angles of attack can be collapsed into similar trends by using the projected plate width as the characteristic length of the flow. © 2004 Elsevier SAS. All rights reserved.postprin

Velocity and concentration measurements in initial region of submerged round jets in stagnant environment and in coflow

Velocity and concentration fields are measured in submerged round jets in a stagnant environment and in coflow using laser Doppler anemometry and laser-induced fluorescence. Measurements are made in the initial region within distances of 40 jet exit diameter at jet Reynolds number between 1000 and 5000 and coflow-to-jet velocity ratio from 0 to 0.43. Different behaviors of jet spreading and dilution are found in jets at three different ranges of Reynolds number in which the jets are classified as initially laminar, transitional or turbulent. In the zone of established flow, the jet centerline velocity and concentration decay with downstream distance at different rates in the three groups of jets. For jets in coflow, axial development of normalized forms of centerline mean excess velocity and mean concentration at different velocity ratios can be reasonably well collapsed onto universal trends through the use of momentum length scale. Turbulence properties inside a jet are increased by the presence of a strong coflow. Inside the zone of flow establishment, some strange features are observed on jet turbulence properties. The length of zone of flow establishment increases from the turbulent jets, to the transition jets and to the laminar jets. The zone lengths for concentration are shorter than those for velocity by one to two jet exit diameters. Both lengths are shortened further in the presence of a coflow. For jets a stagnant environment and in the strong jet flow region of jets in coflow, jet widths increase linearly with downstream distance in transitional and turbulent jets. Self-similarity of radial profiles of mean velocity or excess velocity, mean concentration, turbulence intensities and concentration fluctuation level is explored in the zone of established flow. © 2009 International Association for Hydraulic Engineering and Research, Asia Pacific Division.postprin

Mechanistic basis of ligand efficacy in the calcium-activated chloride channel TMEM16A

Agonist binding in ligand-gated ion channels is coupled to structural rearrangements around the binding site, followed by the opening of the channel pore. In this process, agonist efficacy describes the equilibrium between open and closed conformations in a fully ligand-bound state. Calcium-activated chloride channels in the TMEM16 family are important sensors of intracellular calcium signals and are targets for pharmacological modulators, yet a mechanistic understanding of agonist efficacy has remained elusive. Using a combination of cryo-electron microscopy, electrophysiology, and autocorrelation analysis, we now show that agonist efficacy in the ligand-gated channel TMEM16A is dictated by the conformation of the pore-lining helix α6 around the Ca$^{2+}$ -binding site. The closure of the binding site, which involves the formation of a π-helix below a hinge region in α6, appears to be coupled to the opening of the inner pore gate, thereby governing the channel's open probability and conductance. Our results provide a mechanism for agonist binding and efficacy and a structural basis for the design of potentiators and partial agonists in the TMEM16 family