K.M. Lam

Physical Modeling of Flow Field inside Urban Street Canyons

Abstract The flow characteristics inside urban street canyons were studied in a laboratory water channel. The approaching flow direction was horizontal and perpendicular to the street axis. The street width was adjusted to form street canyons of aspect ratios 0.5, 1.0, and 2.0. The velocity field and turbulent intensity were measured with a laser Doppler anemometer at various locations within the street canyons, which were used to elucidate the flow pattern inside the street canyons. It was found that the previous numerical modeling results are in good agreement with the current experimental results at most locations. For the street canyon of aspect ratio 0.5, which belongs to the wake interference flow regime, the mean and fluctuating velocity components were more difficult to measure as compared with the other two cases because of its more complicated flow pattern. Some guidelines for numerical modeling were developed based on the measurement results. The data presented in this paper can also be used as...

Phase‐locked eduction of vortex shedding in flow past an 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.

Evaluation of pedestrian-level wind environment around a row of tall buildings using a quartile-level wind speed descripter

Wind environment around the base of a row of identical tall buildings was investigated through wind-tunnel experiments. Statistical records of wind speeds and wind angles at the pedestrian level were obtained with a split-fibre probe for the side-by-side arrangement of buildings. This enabled us to evaluate the wind environment with two different pedestrian-level wind descripters, namely a quartile-level wind speed descripter and an “effective wind speed”. The presented results showed how pedestrian-level winds were affected by locating a tall building in a row of identical buildings.

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.

Numerical simulation of vortex shedding from an inclined flat plate

Abstract: Vortex shedding flow from a flat plate inclined to a uniform flow at an angle of attack between 20o and 45o is simulated with a finite volume CFD code with RNG k-ω turbulence model. The unsteady flow simulation at Re=2X104 with RANS shows two trains of vortices shed from the two different edges of the plate forming a vortex street in the wake of the plate. The computed results provide support to previous experimental observations that in this asymmetric flow geometry, the two trains of vortices in the vortex street possess different vortex strengths. There is further evidence that the vortex from the plate leading edge is actually shed from a location near the trailing end of the plate. The computed flow at successive phases of a vortex shedding cycle show different development and shedding mechanisms for the two trains of vortices. The study also explores the generation mechanism of the fluctuating lift and drag on the plate and its relationship with the vortex shedding processes.

Interference effect of an upstream larger cylinder on the lock-in vibration of a flexibly mounted circular cylinder

Experiments have been carried out to investigate the flow-induced vibration response of a flexibly mounted circular cylinder located in the vicinity of a larger cylinder and subjected to cross-flow. The interfering larger cylinder was placed upstream and had a diameter twice that of the vibrating cylinder. Complex interaction was observed between the flow over the two cylinders. The vibration responses of the flexible cylinder were classified into different regimes according to the relative positions of the two cylinders. In the-side-by-side arrangement and the tandem or near-tandem arrangement, flow-induced vibrations of the flexible cylinder were greatly suppressed. In the staggered arrangement which covered a large portion of the relative cylinder positions being investigated, vibrations of the smaller cylinder were greatly amplified. The vibration response curves were also largely modified with a broadening of the lock-in resonance range. A shift of the peak reduced velocity for maximum vibration response was also found. Flow visualizations and wake velocity measurements suggested that the modifications of the vibration responses were related to the presence or absence of constant or intermittent flow through the gap region between the two cylinders. The proposed mechanisms of flow interactions and the resulting vibration response characteristics could explain previous observations on flow-induced vibrations of two equal-sized circular cylinders reported in the literature.

Formation of vortex street and vortex pair from a circular cylinder oscillating in water

Abstract The flow around a circular cylinder undergoing sinusoidal oscillating movement in still water is investigated by phase-locked PIV measurements. The pattern and development of large-scale vortex structures in the flow are studied from the velocity vectors and vorticity contours obtained at eight successive phases of an oscillating cycle. Experiments are performed at three Keulegan–Carpenter numbers; KC =12, 6.28 and 4.25. Results at KC =12 reveal the mechanism of vortex formation and the development of the shed vortices into a vortex street at a lateral direction to the line of cylinder movement. The role of a biased flow stream and the length of the cylinder stroke in the formation of the vortex street are discussed. At the lower KC numbers, a symmetric pair of vortices is found attached to the leeward face of the cylinder. The vortex pair exhibits an increasing degree of asymmetry when KC increases from 4.25 to 6.28. An explanation in terms of the length of the cylinder strokes and the degree of flow asymmetry is offered for the transition of flow regimes from a vortex pair to a vortex street. The present results are compared with the observations made in previous experimental and numerical studies in the literature.

Analytical determination of equivalent modal damping ratios of a composite tower in wind-induced vibrations

Abstract We present two methods to determine the equivalent modal damping ratios of a composite tower under wind-induced vibrations. Different damping characteristics arise from the construction of the tower with two materials; for the lower main part, say of reinforced concrete and an upper part, say of steel. The first method employs a detailed numerical integration procedure in which the static condensed stiffness matrices are established by the transfer matrix method. The damping matrices of the upper and the lower substructures are modelled with the Rayleigh damping formulation. The second method makes use of a simplified approximation of two lumped masses. Computation using the two methods is demonstrated with a numerical example for a composite T.V. tower of a total height 459m. Results show that, while the detailed method gives good accuracy but demands intensive computation, the approximate method involves simple calculations and produces data of enough precision for engineering applications.

Wind-induced natural ventilation of re-entrant bays in a high-rise building

Abstract This paper reports a systematic computational study of wind-induced natural ventilation and pollutant transport of re-entrant bays on a total of 30 generic building models of different building heights and with bays of different dimensions. Mean wind flow around each building model and wind-induced flow inside re-entrant bays are computed. To determine the ventilation efficiency of the bay, the computed flow field is used to disperse a scalar pollutant initially occupying the entire bay at a uniform concentration. The subsequent time decay of pollutant concentration inside the bay is studied and the ventilation efficiency is quantified by the retention time. The results show that wind-induced flow inside the bay, especially on the building side face, is complex and highly three-dimensional. Air exchange rates through the roof opening and vertical side opening are analyzed for each bay and their relationship to the ventilation efficiency is discussed. The bays on the building side faces are much worse ventilated than those on the windward or leeward building face. The deeper the side bay, the worse is the air exchange and ventilation. The building height is found to have a governing effect on the ventilation of the windward and leeward re-entrant bays.

Vortex formation processes from an oscillating circular cylinder at high Keulegan-Carpenter numbers

Development of vortex patterns around a circular cylinder oscillating in quiescent water is investigated using time-resolved particle image velocimetry. Experiments are performed at Keulegan–Carpenter (KC) numbers between 8 and 36 with Reynolds number kept constant at 2400. Similar to previous studies, three modes of vortex patterns are identified and denoted as modes I, II, and III. The development of vortices in each mode at successive phases of cylinder oscillation is studied in details. The classification of modes is based on the development mechanism of shear layers around the cylinder, the number of vortices shed in each half cycle, and the characteristics of the vortex street. Modes I, II, and III are characterized by one, two, and three (or more) vortices generated, respectively, in each half cycle. The appropriate vortex formation length is applied to explain the dependence of number of vortices formed in each cylinder cycle on KC. Vortex shedding in mode I occurs only on one side of the line of ...