Time-resolved and time-averaged stereo-PIV measurements of a unit-ratio cavity

Immer, Marc Christian; Allegrini, Jonas; Carmeliet, Jan

doi:10.1007/s00348-016-2186-9

Cited by 23 publications

(7 citation statements)

References 28 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Figure 6 shows a wider range of turbulent intensity relative to the friction velocity u i /u τ for flows over street canyons of AR = 1:2 (d-type flows) compared with AR = 1:12 (k-type flows). Flows over both street canyons of AR = 1:2 and 1:12 exhibit more frequent events of sweep Q4 and ejection Q2, in line with the laboratory measurements over a single street canyon available in the literature [70]. For d-type flows over street canyons of AR = 1:2, P (u , w ) illustrates a similar shape over building roofs and street canyons such that stronger sweeps Q4 and ejection Q2 are more frequent than outward interaction Q1 and inward interaction Q3.…”

Section: Quadrant Analysissupporting

confidence: 86%

Street-Level Ventilation in Hypothetical Urban Areas

2017

Atmosphere

View full text Add to dashboard Cite

Street-level ventilation is often weakened by the surrounding high-rise buildings. A thorough understanding of the flows and turbulence over urban areas assists in improving urban air quality as well as effectuating environmental management. In this paper, reduced-scale physical modeling in a wind tunnel is employed to examine the dynamics in hypothetical urban areas in the form of identical surface-mounted ribs in crossflows (two-dimensional scenarios) to enrich our fundamental understanding of the street-level ventilation mechanism. We critically compare the flow behaviors over rough surfaces with different aerodynamic resistance. It is found that the friction velocity u τ is appropriate for scaling the dynamics in the near-wall region but not the outer layer. The different freestream wind speeds (U ∞ ) over rough surfaces suggest that the drag coefficient C d (= 2u 2 τ /U 2 ∞ ) is able to characterize the turbulent transport processes over hypothetical urban areas. Linear regression shows that street-level ventilation, which is dominated by the turbulent component of the air change rate (ACH), is proportional to the square root of drag coefficient ACH ∝ C 1/2 d . This conceptual framework is then extended to formulate a new indicator, the vertical fluctuating velocity scale in the roughness sublayer (RSL) w RSL , for breathability assessment over urban areas with diversified building height. Quadrant analyses and frequency spectra demonstrate that the turbulence is more inhomogeneous and the scales of vertical turbulence intensity w w 1/2 are larger over rougher surfaces, resulting in more efficient street-level ventilation.

show abstract

Section: Quadrant Analysissupporting

confidence: 86%

Street-Level Ventilation in Hypothetical Urban Areas

2017

Atmosphere

View full text Add to dashboard Cite

show abstract

“…In order to show the influence of the choice of different scales and filter functions (parameters of the method) on the generated flow, simulations were performed on a turbulent boundary layer (see Section 3). The inflow statistics were derived from time resolved stereo-Particle Image Velocimetry (PIV) measurements [18].…”

Section: Turbulent Inflow Generationmentioning

confidence: 99%

“…The boundary layer measurements are reported in [18] and [16], where the boundary layer was measured upstream of a cavity. The boundary layer is not fully developed and therefore it is expected that the boundary layer will still further develop in flow direction in the LES simulations.…”

Section: Boundary Layer Simulationmentioning

confidence: 99%

RETRACTED ARTICLE: Evaluation of the Filtered Noise Turbulent Inflow Generation Method

Allegrini

Carmeliet

2016

Flow Turbulence Combust

Self Cite

View full text Add to dashboard Cite

Flows around buildings and in urban areas have the ability to exchange mass and momentum through mixing layers. The complex dynamical phenomena arising in mixing layers can be studied using Large Eddy Simulation (LES). As mixing layers depend on the turbulence conditions upstream of the buildings or urban areas, appropriate turbulent inlet conditions have to be provided to a simulation. Due to the high efficiency and level of control, the filtered noise inflow method was selected. The control over the Reynolds stresses as well as nine length scales make this method suitable to replicate conditions measured in experiments. In this paper, a formal method to obtain the filter coefficients is presented. This is achieved by relating the spatial filtering to a Finite Impulse Response (FIR) filter and the temporal filtering to an Autoregressive (AR) model. Three closed-form solutions for the spatial filter coefficients are presented having a Gaussian, doubleexponential and exponential correlation function. By means of an LES simulation of a turbulent wall-bounded flow, the input-output behaviour is investigated. It was found that a combination of a Gaussian filter with length scales that increase with increasing wall distance result in the fastest downstream development of the artificial turbulence and the smallest loss of turbulent kinetic energy. Keywords Inflow Generator • LESWe want to thank for the essential contribution by Marc Immer, who obtained the doctoral degree at ETH Zurich (DISS. ETH NO. 23208). We also thank ETH Zurich for their support by means of the Hartmann Müller-Fonds on grant ETH-32 11-2.

show abstract

“…Much effort has been done in recent years to analyze urban canopy flows by means of CFD, often using Reynolds-averaged NavierStokes (RANS) simulations of two-dimensional (2D) arrays of buildings. e interest of the scientific community for such a simplified building arrangement is justified by the fact that the 2D array can be considered as an archetype for more complex geometries [10][11][12][13]. Huang et al [14] carried out 2D simulations to investigate the effect of wedge-shaped roofs on the flow in an urban street canyon and found that they have significant influence on the vortex structure and pollutant distribution pattern.…”

Section: Introductionmentioning

confidence: 99%

CFD Analysis of Urban Canopy Flows Employing the V2F Model: Impact of Different Aspect Ratios and Relative Heights

Nardecchia

Bernardino

Pagliaro

et al. 2018

Advances in Meteorology

View full text Add to dashboard Cite

Computational fluid dynamics (CFD) is currently used in the environmental field to simulate flow and dispersion of pollutants around buildings. However, the closure assumptions of the turbulence usually employed in CFD codes are not always physically based and adequate for all the flow regimes relating to practical applications. e starting point of this work is the performance assessment of the V2F (i.e., v 2 − f ) model implemented in Ansys Fluent for simulating the flow field in an idealized array of twodimensional canyons. e V2F model has been used in the past to predict low-speed and wall-bounded flows, but it has never been used to simulate airflows in urban street canyons. e numerical results are validated against experimental data collected in the water channel and compared with other turbulence models incorporated in Ansys Fluent (i.e., variations of both k-ε and k-ω models and the Reynolds stress model). e results show that the V2F model provides the best prediction of the flow field for two flow regimes commonly found in urban canopies. e V2F model is also employed to quantify the air-exchange rate (ACH) for a series of two-dimensional building arrangements, such as step-up and step-down configurations, having different aspect ratios and relative heights of the buildings. e results show a clear dependence of the ACH on the latter two parameters and highlight the role played by the turbulence in the exchange of air mass, particularly important for the step-down configurations, when the ventilation associated with the mean flow is generally poor.

show abstract

Time-resolved and time-averaged stereo-PIV measurements of a unit-ratio cavity

Abstract: resulting in a significantly different cavity flow. Large turbulent sweep and ejection events in the wake flow suppress the typical shear layer and sporadic near wall sweep events generate coherent vortices at the upstream edge.

Cited by 23 publications

References 28 publications

Street-Level Ventilation in Hypothetical Urban Areas

Street-Level Ventilation in Hypothetical Urban Areas

RETRACTED ARTICLE: Evaluation of the Filtered Noise Turbulent Inflow Generation Method

CFD Analysis of Urban Canopy Flows Employing the V2F Model: Impact of Different Aspect Ratios and Relative Heights

Contact Info

Product

Resources

About