139 research outputs found
Amplitude and frequency modulation of the small scales in a jet
No full textThe present study is an experimental investigation of the relationship between the large- and small-scale motions in the far field of an air jet at high Reynolds number. In the first part of our investigation, the analysis is based on time series of hot-wire anemometry (HWA), which are converted into space series after applying the Taylor hypothesis. By using a spectral filter, two signals are constructed, one representative of the large-scale motions (2?T?L, where ?T is the Taylor length scale, and L is the integral length scale) and the other representative of the small-scale motions (1.5?5?, where ? is the Kolmogorov length scale). The small-scale signal is found to be modulated both in amplitude and in frequency by the energy-containing scales in an analogous way, both at the centreline and around the centreline. In particular, for positive fluctuations of the large-scale signal, the small-scale signal is locally stronger in amplitude (amplitude modulation), and it locally exhibits a higher number of local maxima and minima (frequency modulation). The extent of this modulation is quantified based on the strength of the large-scale fluctuations. The response of the small-scale motions to amplitude modulation can be considered instantaneous, being on the order of one Kolmogorov time scale. In the second part of our investigation we use long-range ?PIV to study the behaviour of the small-scale motions in relation to their position in either high-speed or low-speed regions of the flow. The spatially resolved velocity vector fields allow us to quantify amplitude modulation directly in physical space. From this direct estimation in physical space, amplitude modulation is only 25 % of the value measured from HWA. The remaining 75 % comes from the fixed spectral band filter used to obtain the large- and small-scale signals, which does not consider the local convection velocity. A very similar overestimation of amplitude modulation when quantified in the time-frame is also obtained analytically. Furthermore, the size of the structures of intense vorticity does not change significantly in relation to the large-scale velocity fluctuation, meaning that there is no significant spatial frequency modulation. The remaining amplitude modulation in space can be explained as a statistical coupling between the strength of the structures of vorticity and their preferential location inside large-scale high-velocity regions. Finally, the implications that the present findings have on amplitude and frequency modulation in turbulent boundary layers (TBLs) are discusse
Resolving vorticity and dissipation in a turbulent boundary layer by tomographic PTV and VIC+
No full textThe existing time-resolved tomographic particle image velocimetry (PIV) measurements by Jodai and Elsinga (J Fluid Mech 795:611–633; Jodai, Elsinga, J Fluid Mech 795:611–633, 2016) in a turbulent boundary layer (Reθ = 2038) are reprocessed using tomographic particle tracking velocimetry (PTV) and vortex-in-cell-plus (VIC+). The resulting small-scale flow properties, i.e. vorticity and turbulence dissipation, are compared. The VIC+ technique was recently proposed and uses the concept of pouring time into space to increase reconstruction quality of instantaneous velocity. The tomographic PTV particle track measurements are interpolated using VIC+ to a dense grid, making use of both particle velocity and Lagrangian acceleration. Comparison of the vortical structures by visualization of isosurfaces of vorticity magnitude shows that the two methods return similar coherent vortical structures, but their strength in terms of vorticity magnitude is increased when using VIC+, which suggests an improvement in spatial resolution. Further statistical evaluation shows that the root mean square (rms) of vorticity fluctuations from tomographic PIV is approximately 40% lower in comparison to a reference profile available from a DNS simulation, while the VIC+ technique returns rms vorticity fluctuations to within 10% of the reference. The dissipation rate is heavily underestimated by tomographic PIV with approximately 50% damping, whereas the VIC+ analysis yields a dissipation rate to within approximately 5% for y+ > 25. The fact that dissipation can be directly measured by a volumetric experiment is novel. It differs from existing approaches that involve 2d measurements combined with isotropic turbulence assumptions or apply corrections based on sub-grid scale turbulence modelling. Finally, the study quantifies the spatial response of VIC+ with a sine-wave lattice analysis. The results indicate a twofold increase of spatial resolution with respect to cross-correlation interrogation.</p
Air pollutant sinks on noise barriers: Where do they perform the best?
No full textWhile laboratory experiments, numerical simulations as well as field tests have underlined the influence of noise barriers in dispersing vehicular emissions and reducing downwind peak concentrations, these pollutants still remain in the atmosphere. Artificial pollutant sinks (for example, particle capturing or toxic gas treating devices) installed on top of noise barriers can further alleviate this problem by eliminating the pollutants passing through it. However, it is not known how the installation of a semi-permeable pollutant sink affects the aerodynamics of the pollutants’ flow. By finding an optimal position and orientation for these sinks, the mass of the pollutants reaching the sink inlet can be maximized. Scaled down water tunnel experiments have been used to investigate the effectiveness of installing such a pollutant sink, of fixed dimensions, on top of a noise barrier adjacent to a highway. It is found that installing a sink is more beneficial on top of shorter barriers and that vertically elevating the sink, only slightly, can enhance its pollutant capturing performance. Using a sink in a ‘highway canyon’ (two noise barriers placed symmetrically with respect to the highway) must be done cautiously as there are several flow regimes observed, which are sensitive not only to the canyon aspect ratio (ratio between canyon width and height), but also to the presence/absence of the sink. The results here not only demonstrate the effectiveness of installing pollutant sinks on noise barriers, but also provide ballpark estimates on the optimal placement, orientation and performance of these devices, prior to field tests or even large-scale installation.Accepted Author ManuscriptMulti Phase SystemsFluid Mechanic
Tin layer behaviour and microstructural development during high–speed resistance mash seam welding of tinplate–packaging steel
No full textMechanical, Maritime and Materials EngineeringMaterials Science and Engineerin
Tomographic-PIV measurement of the flow around a zigzag boundary layer trip
No full textTomographic-PIV was used to measure the boundary layer transition forced by a zigzag trip. The resulting instantaneous three-dimensional velocity distributions are used to quantitatively visualize the flow structures. They reveal undulating spanwise vortices directly behind the trip, which break up into individual arches and then develop into the hairpin-like structures typical of wallbounded turbulence. Compared to the instantaneous flow structure, the structure of the average velocity field is very different showing streamwise vortices. Such streamwise vortices are often associated with the low-speed streaks occurring in bypass transition flows, but in this case clearly are an artifact of the averaging. Rather, the present streaks in the separated flow region directly behind the trip are resulting from the waviness in the spanwise vortices as introduced by the zigzag trip. Furthermore, these streaks and the separated flow region are observed to be related to a large-scale, spanwise uniform unsteadiness in the flow that contributes significantly to the velocity fluctuations over large downstream distances (up to at least the edge of the present measurement domain).Aerospace Engineerin
social housing on the move
No full textBoth the public and the European Commission have criticized the past actions of Dutch housing associations. A new housing act has become applicable, which would force housing associations to change their conduct on the rental housing market. Important developments include the allocation of households with a low income to social dwellings, a distinction between the social and commercial rental market, and lastly, a landlord levy. A system dynamics model was developed to identify whether the rental housing market would become a more dual rental market in the future. The model was made for one specific housing association: Havensteder, a large housing association in the region of Rotterdam. The position of Havensteder and some objectives are taken into consideration for the system dynamics model. The outcomes of the system dynamics model show that the structure of the social housing market tends towards a more dual rental market, although not all indicators point to this result. Therefore, it cannot be concluded that the social housing market is in fact transitioning to a dual rental housing market.System Engineering, Policy Analysis and ManagementOTBTechnology, Policy and Managemen
De waarde van Koopgarant. Koopgarant en de waardeontwikkeling van wijken
No full textParallelsessie II. E. De waarde van Koopgaran
Lessons from 150 years of inter-municipal residential planning: An ex-post evaluation of factors that determine the dynamics of inter-municipal decision-making on residential planning
No full textSince the financial crisis of 2008-2009, Dutch municipalities develop policy solutions to address financial losses on land development. This report explores one possible strategy: inter-municipal cooperation on residential planning. Through cooperation, municipalities should be able to control the oversupply of new housing. By means of an institutional analysis and case studies, the following question has been answered: what factors have previously determined inter-municipal decision-making concerning residential planning, and how can inter-municipal cooperation be used as a mean to manage regional supply of new housing? The institutional analysis introduces four types of institutional regimes that vary in the capacity to facilitate inter-municipal cooperation. The regime of inter-governmental covenants, which was in force during the 1990s is considered most facilitative. Policy network theory (Van Bueren, Klijn and Koppenjan 2003) argues that institutional, process and substantive factors explain the course and outcomes of inter-municipal decision-making. Case studies of regions Haaglanden and Rijnmond during the VINEX-era confirm the usefulness of these factors. This study further specifies the explanatory factors with respect to inter-municipal residential planning. The report concludes with recommendations for provinces and municipalities to improve inter-municipal decision-making.SEPAMPolicy, Organisation, Law & GamingTechnology, Policy and Managemen
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