Shear Layer Dynamics Research Articles

Numerical study of hole formation in a thin flapping liquid sheet sheared by a fast gas stream

This study reports aerodynamics effects of a fully developed double-shear layer gas flow in the perforation and disintegration of a thin liquid sheet using three-dimensional numerical simulations. A double-shear layer gas flow, where a thin quiescent gas layer is sandwiched between the two fast moving gas layers, is first simulated up to a statistically steady state. The downstream dynamics of the gas shear layers shows vortex shedding and three-dimensional rib-like vortical structures, similar to the wake of a bluff body. This gas flow is then used as the initial condition for a liquid-gas simulation, where the liquid phase is sandwiched by the top and bottom fast gas streams. A thin liquid sheet with a thickness of 25 μm is considered using air/water conditions. The liquid sheet oscillates and small holes, preceded by craters surrounded by ripples, are formed. The thinning of the liquid sheet, spanwise corrugations, and vortex separations within the liquid-gas boundary layer are shown to govern the dynamics of the liquid sheet. A localized pressure jump is also observed inside the liquid sheet and precedes the rupture of the liquid sheet by pushing the liquid away in the span direction. The holes formed subsequently grow in size, collide and merge with each other, and break the liquid sheet into multiple droplets.

The flow around a surface mounted cube: a characterization by time-resolved PIV, 3D Shake-The-Box and LBM simulation

The flow around a surface mounted cube with incoming turbulent or laminar boundary layer has been topic of many experimental and numerical investigations in the past decades. Despite its simple geometry the flow generates a set of complex vortical structures in front and around the cube, includes flow separation at the three front plane edges with corresponding subsequent shear layer dynamics enveloping recirculation zones. Downstream of the cube a large unsteady flow separation region is present which is associated with typical quasi-periodic bluff-body wake dynamics. Therefore the flow configuration is well suited to enhance the understanding of similar unsteady and separated flow phenomena in many aerodynamic and engineering applications. In the present experimental investigation we aim at resolving a large spectrum of spatial and temporal scales in the flow around a cube with incoming laminar and turbulent boundary layers by using the most recent developments of dense 3D Lagrangian particle tracking (LPT) and high resolution TR-PIV for Reynolds numbers based on cube size in the range text {Re}_H = U_infty ,H,nu ^{-1} = 2000 - 8000. The results documented in the present paper consist of snapshots and the analysis of long time-series of highly resolved 3D and 2D velocity fields suited to enhance the understanding of coherent structure dynamics and of corresponding statistical Lagrangian and Eulerian flow properties. Premultiplied velocity spectra and 3D pressure distributions are calculated and discussed as well. Finally, the measurement data is compared to results obtained with a simulation based on the lattice Boltzmann method (LBM).Graphic abstract

Spatio-temporal dynamics of flow separation induced by a forward-facing step submerged in a thick turbulent boundary layer

The effects of large-scale motion (LSM) on the spatio-temporal dynamics of separated shear layers induced by a forward-facing step submerged in a thick turbulent boundary layer (TBL) are investigated using a time-resolved particle image velocimetry. The Reynolds number based on the free-stream velocity and step height was 13 200. The oncoming TBL was developed over a cube-roughened surface and the thickness was 6.5 times the step height. The step height was chosen to coincide with the elevation where the dominant frequency of streamwise fluctuating velocity in the TBL occurred. At this elevation, the local turbulence intensity was 14.5 %. Distinct regions of elevated Reynolds stresses were observed upstream and downstream of the leading edge of the step. The unsteady dynamics of the separation bubbles upstream and downstream of the step was investigated using the reverse flow area. Both separation bubbles exhibit low-frequency flapping motion, and the dominant frequency of the downstream separation bubble is identical to the dominant frequency of the streamwise fluctuating velocity in the oncoming TBL at the step height. As the low-velocity region of LSM passes over the step, the downstream separation bubble is enlarged and subsequently undergoes a high-frequency oscillation. Turbulence motions were partitioned into low-, medium- and high-frequency regimes based on spectral analysis of the Reynolds stresses. The contributions from these partitioned turbulence motions are used to elucidate the effects of LSM on the elevated Reynolds stresses in the shear layers upstream and downstream of the step.

Separated Flow Response to Rapid Flap Deflection

The transient response of a massively separated flow over an airfoil to rapid flap actuation is presented. A NACA 0006 airfoil is oriented at a fixed incidence of 20 deg for a Reynolds number of . The experiments are performed in a water tunnel with a wing spanning the width of the test section to produce a nominally two-dimensional flowfield. The airfoil is bisected about the midchord position, resulting in a 50%-chord trailing-edge flap. The flap is rapidly deflected in a smoothed-ramp profile over a range of deflection speeds and amplitudes. The flap maneuver is completed in a fraction of a single convective time. Focus is given to a deflection amplitude of 2 deg to minimize geometric deviation from the nondeflected configuration. The desired response to such a flap motion is the evocation of vortical transients conducive to lift enhancement. Through this study, two distinct transient responses are observed that are directionally dependent on flap actuation. In motions resulting in an increase in airfoil camber, the lift is increased instantaneously to modest values before relaxation to a separated steady state. In effect, this mode expedites convergence to the steady-state value of the final airfoil configuration and is devoid of the “antilift” spike associated with the discrete actuation of conventional fluidic actuators. In motions resulting in a decrease in airfoil camber, the lift profile is characterized by an initial reduction before a surge in lift, culminating in a global peak and followed by relaxation. Both deflection modes prove disruptive to the leading-edge shear-layer dynamics through trailing-edge actuation and are cause for rollup of a leading-edge vortex. Ridges of the finite-time Lyapunov exponent field are used to determine that the net decrease in camber motion induces significant entrainment near the trailing edge, leading to a smaller recirculation region and reattachment of the flow above the suction surface trailing-edge region. The net increase in camber motion does not generate this entrainment, and therefore yields a significantly larger recirculation region.

Turbulence at water-vegetation interface in open channel flow: Experiments with natural-like plants

Riparian shrubs and trees present a complex, seasonally variable morphology, with flexible stems and leaves efficiently adapting to the flow forcing (reconfiguration). The aim of this paper is to investigate how foliage and reconfiguration affect the flow and mixing in a partly vegetated channel. Specific attention was placed on the velocity statistics, onset and coherence of turbulent structures, and lateral momentum transport at the horizontal interface between vegetation and open water. The experimental flume arrangement was novel in that it allowed investigating the lateral shear layer induced by flexible riparian plants. The natural-like vegetation consisted of emergent woody plants and a grassy understory, with density, morphology and reconfiguration behavior comparable to those found in real riparian areas. Investigations were conducted under foliated and leafless conditions to determine the seasonality effects. The mean and turbulent flow structure was determined with acoustic Doppler velocimetry, and dynamic plant motions were investigated from video footage. The presence of foliage enhanced the drag discontinuity at the interface, resulting in more pronounced velocity gradients between the vegetated and open areas compared to the leafless conditions. Foliation induced stronger shear layer-scale mixing, whereas, under leafless conditions, the local mixing induced by stems was more important. The reconfiguration decreased the coherence of the two-dimensional large-scale vortices at the interface while their characteristic frequency was consistent with the canonical mixing layer theory. Our results indicated that shear layer dynamics in partly vegetated channels was influenced strongly by morphology and reconfiguration of complex plants, with more efficient lateral momentum transport at the interface in the foliated conditions than previously reported for shear layers induced by simpler vegetation.

Statistical behavior of shear layers of reactive oxygen/kerosene spray

Reactive gaseous oxygen/kerosene spray was visualized by a shadowgraph imaging technique in a lab-scale rocket engine combustor. Dynamic behaviors of inner and outer shear layer were observed morphologically and analyzed quantitatively by image processing. Correlation methods were used to statistically analyze dynamic behaviors of each shear layer and interaction between two shear layers. Relationship between shear layer dynamics and combustion flow-field was investigated by analyzing periodicity of jet-core length, lump-detachment, and luminous flame intensity. The results of correlation and frequency analysis show that dominant frequency of behaviors and interaction of shear layers is 270 Hz and the degree of correlation becomes stronger along the flow direction. The dominant frequency of core length variation and luminous flame formation is around 270 Hz and, which is the same as that of behaviors and interaction of shear layers. This proposes that the surface behaviors and interaction of the inner and outer jets in the upstream affect the downstream combustion flow-field.

Exchange Processes Induced by Large Horizontal Coherent Structures in Floodplain Vegetated Channels

AbstractMangrove forests along the Mekong delta estuaries are usually observed to degrade together with the increasing extension of fish farms. In this limited‐width condition, the formation of coherent structures, their interactions with mangroves, the role of the width of the mangrove forest, and their effects on the exchange processes between the open channel and the adjacent floodplain are still not well examined. In order to obtain more insight, a unique laboratory experiment of a vegetated compound channel mimicking estuarine mangroves has been conducted. The results show that in a compound vegetated channel with a very gentle transverse slope, the vegetated shear layer dynamics resembled that associated with vegetation rather than that associated with a depth differential. Furthermore, the flow field under the effect of large horizontal coherent structures (LHCSs) shows a spatially and temporally cycloid motion with associated flow events, namely, sweep, ejection, stagnant, and reverse flows. It is also suggested that the coherent structures can have an influence on a broader area than the vegetation area into which the eddy structures can penetrate. In terms of the exchange processes, the momentum transfer and the intensity of transverse fluctuations induced by the LHCSs can be related to this phenomenon. Consequently, decreasing the mangrove width can significantly affect the pattern of the LHCSs, disturb the transverse exchange processes induced by these structures, and thereby changing the shear layer, creating unfavorable conditions for sedimentation inside forests and for river bank stability.

Effect of syngas composition on high frequency combustion instability in a non-premixed turbulent combustor

An experimental investigation on the effect of syngas composition (H2/CO) on thermodynamic instability of a bluff-body combustor is studied. Three syngas compositions, namely, 25%H2–75%CO, 50%H2–50%CO and 75%H2–25%CO, and pure hydrogen and 75%H2–25%CH4 are acoustically characterized by varying the inlet Reynolds number, Re, in the range of 2200–8100. The combustor is observed to undergo two modes of thermo-acoustic instability across the above Re range for all syngas compositions and pure hydrogen, whereas for the H2CH4 composition, the instability manifests at a single frequency ∼130 Hz. For all the syngas compositions, the pressure oscillations exhibit low frequency (130 Hz) at low Re, and bifurcate to higher frequency (∼500 Hz) at mid-range Re values, while pure hydrogen exhibits bifurcation from the low frequency to the first harmonic (∼260 Hz). The high frequency observed with the syngas compositions is at the third harmonic natural acoustic mode of the combustor and matches thrice the Strouhal frequency of the hydrodynamic oscillations of the shear layer separating from the bluff body. Further investigation into the nature of the dynamic behaviour across the listed fuels is performed using simultaneous time resolved imaging of OH* and CO2* chemiluminescence and two-component particle image velocimetry. The peak in the CO2* chemiluminescent intensity is found to be axially staggered downstream of that of OH*. This is due to sequential oxidation of H2 and CO, the former producing OH as an intermediate consumed by the latter. Spatio-temporal analysis of the time-resolved data reveals that the presence of the flame in the shear layer of the bluff-body plays a significant role in modulating the high frequency acoustic excitation. Further, a spatial Rayleigh map is derived based on both the OH* and CO2* chemiluminescence, which reveals the bluff-body shear layer dynamics to yield the key driving and damping regions in the present study.

Modification of turbulent wake characteristics by two small control cylinders at a subcritical Reynolds number

In this paper, the flow around a main circular cylinder with two small control cylinders is numerically investigated at a subcritical Reynolds number of 3900. Two small control cylinders with diameter ratio of d/D = 0.04 (d and D are the diameters of the control cylinders and the main cylinder, respectively) are symmetrically placed inside the separated shear layers emanating from the main cylinder. The computation is performed with highly resolved direct numerical simulations employing a high-order spectral/hp element method. For validation purposes, the results of the flow over a single cylinder are presented at first, and the current simulation is in good agreement with the existing experiments and large eddy simulations. For the main cylinder with the two control cylinders, the dynamics of shear layers is found to be modified strongly owing to the disturbance from the small control cylinders. As a consequence, the recirculation length is decreased by 89% as compared to that of a single cylinder. The presence of the small control cylinders further leads to a significant increase in the fluctuating lift and the mean drag exerting on the main cylinder. Furthermore, the statistical analysis and turbulent wake visualization imply that the turbulent intensity is amplified in the near wake region and becomes weaker further downstream. This study then suggests that the small control cylinders located in the near wake region of the main cylinder may work differently in the turbulent wake against that in the laminar wake.

Unsteady shear layer flow under excited local body-force for flow-separation control in downstream of a two-dimensional hump

We present a detailed numerical investigation of unsteady shear layer dynamics in downstream under an excited local body force, based on the assumption that a plasma actuator is positioned near the top of a two-dimensional hump for flow-separation control. A local body force works in temporal burst mode, which is homogeneous in the spanwise direction. In our previous report (Yakeno et al., 2015), the most effective frequency to cause early reattachment is fh=0.2, which corresponds to what Hasan (1992) and many other past studies referred to as the step mode. A periodic excitation generates two-dimensional roll vortices and other three-dimensional turbulence between downstream rolls, such as rib structures. These vortex characteristics significantly depend on the excitation frequency. In the study, we discuss these multi-scale turbulence motion separately by considering decomposition of temporal phase-locked periodic statistics of the excitation frequency and non-periodic turbulence fluctuation. At first, we found that non-periodic turbulence kinetic energy due to three-dimensional rib structure increases the most at the optimal frequency fh=0.2, although that frequency corresponds to the time scale that a hump-height vortex grows. It seems that non-periodic turbulence energy growth near separation point correlates with the control performance more than two-dimensional roll vortex increase. We operated linear hydrodynamic stability analysis on a free shear layer and confirmed that periodic phase fluctuation at high frequency grew on the Kelvin-Helmholtz instability. At low-frequency, periodic turbulence fluctuation is not reproduced with the exponential assumption, while its magnitude is large. From those results, we consider that the time and spanwise-averaged non-periodic turbulence energy becomes strong near the separation point the most at fh=0.2 because a hump-height vortex occurs the most times at this frequency, which is associated with a generation of the rib structure around it. Temporal-periodic momentum balance based on the decomposition is also investigated. A difference of terms contribution at high and low frequencies to the term of a pressure gradient in the wall-normal direction is discussed. Finally, we investigated how excitation position affects a total drag around a hump and found that, in some cases, two recirculation regions separately emerge in the downstream of the hump, and thus the control performance is degraded. At fh=0.2, one recirculation occurs regardless of the excitation position, while the most effective position is near the inflection point of the mean velocity of the uncontrolled flow near the wall.

Shear Layer Dynamics in a Supersonic Rectangular Multistream Nozzle with an Aft-Deck

Large-eddy simulations are employed to analyze the flowfield generated by a multistream propulsion system, comprised of a core Mach 1.6 single-sided expansion ramp stream separated by a splitter plate from a sonic stream evolving on an aft-deck. The emphasis is on deriving a fundamental understanding of three-dimensional unsteadiness due to the different interacting free and wall-bounded shear layers. Simulation results on several grids compare well to data from a companion experiment, including particle image velocimetry measurements, time-resolved Schlieren images, and deck pressure spectra, thereby confirming simulation fidelity for the primary physics of interest. Even at design conditions, the asymmetry induced by the single-sided expansion and the aft-deck results in a complex three-dimensional shock train, whose interactions with the bounding shear layers yield many of the principal observations, including local separation of boundary layers developing on the expansion ramp and aft-deck, as well as plume unsteadiness. The mean transverse deflection of the plume, whether upward or downward, is shown to be related to the nature of the shock train components at the aft-deck trailing edge. As the plume develops downstream, three-dimensional characteristics become prominent, including corner effects and asymmetric entrainment. Spectral analyses and space–time correlations demonstrate that the signature of a shedding instability initiated at the splitter plate trailing edge permeates the entire flowfield, and influences the development of the shock train, deck pressure, and shear layers.

Shear Layer Dynamics in a Reacting Jet in Crossflow

This study explores the effect of heat release on the growth of the shear layer vortical structures in a reacting jet in crossflow. Jets composed of mixtures of hydrogen, helium and nitrogen were used to independently vary the momentum flux ratio (J), jet to crossflow density ratio (S) and heat release. Velocity fields were obtained from 10 kHz high-speed stereoscopic particle image velocimetry (SPIV) and regions of elevated temperature/combustion products from simultaneous OH planar laser induced fluorescence (OH-PLIF). The shear layer vortices (SLV) originating from instabilities in the windward and leeward shear layers were identified using vortex identification indicator functions in order to track their spatial location and strength. The results show that the asymmetries in shear layer strength between the windward and leeward shear layers are dependent primarily on J, for both reacting and non-reacting flow-fields. The SLV growth rate dependencies on J and S is found to match trends noted by previous studies for non-reacting jets, where SLV growth rates increase with degree of global instability of the JICF. Heat release is also shown to suppress the SLV growth rates relative to non-reacting cases with the same jet parameters. Related to this point, the degree of lifting of the flame also has a significant impact on SLV growth. As flame lifting is directly related to autoignition times, this point shows strong coupling between kinetic rates and jet hydrodynamic stability.

Propagating helical waves as a building block of round turbulent jets

Turbulent jets are known to support large-scale vortical wave packets traveling downstream. We show that a propagating helical wave represents a common form of the optimal eigenfunction tracking these structures from the near to the far field of a round jet issuing from a pipe. Two first mirror-symmetric modes containing around 5% of the total turbulent kinetic energy capture all significant large-scale events and accurately replicate the full shear-layer dynamics of the azimuthal wave number m=1. A family of the most energy-containing traveling waves represents low wave numbers and is described in terms of empirical dispersion laws.

Energy fluxes in turbulent separated flows

Turbulent separation in channel flow containing a curved wall is studied using a generalised form of Kolmogorov equation. The equation successfully accounts for inhomogeneous effects in both the physical and separation spaces. We investigate the scale-by-scale energy dynamics in turbulent separated flow induced by a curved wall. The scale and spatial fluxes are highly dependent on the shear layer dynamics and the recirculation bubble forming behind the lower curved wall. The intense energy produced in the shear layer is transferred to the recirculation region, sustaining the turbulent velocity fluctuations. The energy dynamics radically changes depending on the physical position inside the domain, resembling planar turbulent channel dynamics downstream.

Flame-acoustic coupling of combustion instability in a non-premixed backward-facing step combustor: the role of acoustic-Reynolds stress

Combustion instability in a laboratory scale backward-facing step combustor is numerically investigated by carrying out an acoustically coupled incompressible large eddy simulation of turbulent reacting flow for various Reynolds numbers with fuel injection at the step. The problem is mathematically formulated as a decomposition of the full compressible Navier–Stokes equations using multi-scale analysis by recognising the small length scale and large time scale of the flow field relative to a longitudinal mode acoustic field for low mean Mach numbers. The equations are decomposed into those for an incompressible flow with temperature-dependent density to zeroth order and linearised Euler equations for acoustics as a first order compressibility correction. Explicit coupling terms between the two equation sets are identified to be the flow dilatation as a source of acoustic energy and the acoustic Reynolds stress (ARS) as a source of flow momentum. The numerical simulations are able to capture the experimentally observed flow–acoustic lock-on that signifies the onset of combustion instability, marked by a shift in the dominant frequency from an acoustic to a hydrodynamic mode and accompanied by a nonlinear variation of pressure amplitude. Attention is devoted to flow conditions at two Reynolds numbers before and after lock-on to show that, after lock-on, the ARS causes large-scale vortical rollup resulting in the evolution of a compact flame. As compared to acoustically uncoupled simulations at these Reynolds numbers that show an elongated flame with no significant roll up and disturbance in the upstream flow field, the ARS is seen to alter the shear layer dynamics by affecting the flow field upstream of the step as well, when acoustically coupled.

Visualization of Shear Layer Dynamics in a Transversely Forced Flow and Flame

This study addresses the response of a swirling annular jet flow and flame to transverse acoustic excitation in order to better describe key velocity-coupled processes during transverse combustion instabilities in lean, premixed flames. In particular, visualization and velocimetry techniques provide information about the effects of acoustic excitation on unsteady vortex development in the shear layers. Without acoustic forcing, the shear layers roll up into small vortices, driven by the Kelvin–Helmholtz instability, that convect downstream and grow. In the presence of high-amplitude acoustic forcing, as would be present during a combustion instability, the acoustic oscillations drive shear layer vortices to undergo a strong rollup event. Smoke visualization provides visual evidence of the rollup, while particle image velocimetry measurements show the development and trajectory of this large structure. Finally, planar laser-induced fluorescence of OH shows how the large coherent structure causes flame wrinkling at the base of the flame. The vortex rollup, responding at the frequency of acoustic forcing, creates large-scale wrinkles on the flame and can be the main driver of flame response during combustion instability.

Dynamics of shear layers at the interface of a highly porous medium and a pure fluid

In this paper, we report on shear flows in domains that contain a macroscopic interface between a highly porous medium and a pure fluid. Our study is based on the single-domain approach, according to which, the same set of governing equations is employed for both inside the porous medium and in the pure-fluid domain. In particular, we introduce a mathematical model for the flows of interest that is derived directly from a continuum theory for fluid-saturated granular materials. The resulting set of equations is a variation of the well-known unsteady Darcy-Brinkman model. First, we employ this model to perform a linear stability analysis of inviscid shear layers over a highly porous medium. Our analysis shows that such layers are unconditionally unstable. Next, we present results from numerical simulations of temporally evolving shear layers in both two and three dimensions. The simulations are performed via a recently designed algorithm that employs a predictor-corrector time-marching scheme and a projection method for the computation of the pressure field on a collocated grid. According to our numerical predictions, the onset of the Kelvin-Helmholtz instability leads to the formation of vortices that extend to both sides of the material interface, thus producing substantial recirculation inside the porous medium. These vortices eventually merge, leading to significant growth of the shear layer and, in three dimensional flows, transition to turbulence. The dynamics of the shear layers, including growth rate and self-similarity, is presented and analysed. Finally, the structure of these layers is described in detail and compared to the one of plain mixing layers.

Trailing-edge dynamics and morphing of a deformable flat plate at high Reynolds number by time-resolved PIV

The present paper investigates the turbulent wake structure in the near-region past the trailing edge of a deformable inclined plate. The plate is actuated by shape memory alloys. Using these actuators a significant deformation (bending) can be achieved (≈10% of the chord) under the aerodynamic loads corresponding to a Reynolds number of 200000. The shear-layer dynamics as well as the mean velocity and turbulent stresses have been quantified for a reference case (flat plate inclined at 10°). The present study investigates the modification of the shear-layer and near-wake dynamics achieved by means of the dynamic deformation of the plate compared with static cases that include three intermediate positions of the deformed plate. The comparison of the static cases with the dynamic regime discusses the validity of the quasi-static hypothesis for the present low frequency actuation. It is found that the present actuation enhances the shearing mechanisms past the trailing-edge and modifies the von-Kármán mode as well as the structure of the shear-layer, Kelvin–Helmholtz eddies. Moreover, the increase of the bending enhances the appearance of the pairing mechanism between successive shear-layer eddies and the interaction between the von-Kármán and shear-layer instability modes. Furthermore, it has been found that the increase of the plate׳s curvature leads to an attenuation of the shear-layer amplitude and of the overall spectral energy, concerning the most deformed position.

Time-resolved particle image velocimetry measurements of vortex and shear layer dynamics in the near wake of a tethered sphere

The coupling between shear layer, near wake dynamics, and structural oscillations downstream of a tethered spherical pendulum undergoing vortex induced vibrations (VIV) has been experimentally investigated using time-resolved particle image velocimetry in a wind tunnel. One quarter of the sphere was imaged in the field of view (spatial resolution 0.043D) that extended to 1.17D from the sphere's center (D is the sphere diameter). Reynolds numbers based on D, ranged between 493 ≤ Re ≤ 2218 and reduced velocities between 3.18 ≤ U* ≤ 14.1, covering a non-oscillating sphere, periodic oscillations, and the onset of non-stationary sphere oscillations. After the first Hopf bifurcation, the sphere exhibited large amplitude periodic oscillations and the near-wake vortices periodically interacted with the sphere and flapping shear layer. At U* = 5.97, a “secondary” counterclockwise rotating vortex seemed to facilitate shear layer pinch-off. In agreement with the onset of shear layer instabilities for a stationary sphere, only at Re = 2218 power spectra of velocity fluctuations inside the shear layer indicated a weak, broad frequency peak centered at 15 Hz similar as those measured for stationary cylinders and spheres. This peak was consistent with the results of linear instability theory indicating that despite the inherent three-dimensionality of the shear layer, its instability characteristics (at least for the Re investigated here) can be considered to be quasi-two-dimensional. Small-scale, near-wake structures were observed in the instantaneous swirling strength maps at all U* and it is conjectured here that their interaction with the sphere and separating shear layer is the feedback mechanism through which VIV occurs and is sustained.

Modelling considerations for examining the mean and unsteady flow in a simple urban-type street canyon

Different approaches to wind tunnel modelling of the flow in a single urban street canyon are discussed, with the emphasis on achieving approach flow conditions that most closely represent the full-scale case whilst maintaining a simple geometry for studying the mean and turbulent dynamics that ventilate pollutants from the street. Wind tunnel measurements using particle image velocimetry within a square section canyon of lateral length/height from 1 to 20, immersed in an array of cubic roughness elements of the same height, are compared with those from studies with different upstream roughness. It is shown that the canyon mean flow is dependant upon the nature of the flow separation from the roof of the upstream canyon building which, in turn, is related to the approaching flow conditions. Unless care is taken when modelling these conditions it is unlikely that the resulting canyon shear layer dynamics will be represented correctly.