347 research outputs found
Proceedings of the EUROPIV 2 Workshop held in Zaragoza, Spain, March 31 - April 1, 2003
No full textThe Particle Image Velocimetry (PIV) measurement technique has undergone a strong development in the last 10 years. This book presents the proceedings of an international workshop held in Zaragoza, Spain, on March 31st and April 1st, 2003, containing contributions from worldwide leading teams in the development of the PIV method. Most of these papers have been funded by the EC via the European EUROPIV 2 consortium to improve the performances of this measurement technique toward applications in the European Aeronautical industry, including results which are of strong interest for the worldwide community in Fluid Dynamics
Repeller or Attractor? Selecting the Dynamical Model for the Onset of Turbulence in Pipe Flow
No full textThe collapse of turbulence, observable in shear flows at low Reynolds numbers, raises the question if turbulence is generically of a transient nature or becomes sustained at some critical point. Recent data have led to conflicting views with the majority of studies supporting the model of turbulence turning into an attracting state. Here we present lifetime measurements of turbulence in pipe flow spanning 8 orders of magnitude in time, drastically extending all previous investigations. We show that no critical point exists in this regime and that in contrast to the prevailing view the turbulent state remains transient. To our knowledge this is the first observation of superexponential transients in turbulence, confirming a conjecture derived from low-dimensional systems.Process and EnergyMechanical, Maritime and Materials Engineerin
Single-pixel PIV measurements of high-re turbulent pipe flow
No full textIn this paper we present PIV measurements of turbulent pipe flow at Reynolds numbers between 3.4×105 and 6.9×105. We apply a so-called 'single-pixel correlation' that yields a superior spatial resolution (Westerweel et al., 2004). We use the location and shape of the averaged correlation peak to obtain the mean velocity and normal and Reynolds stresses (Scharnowski et al., 2012). A novel aspect of the single-pixel correlation approach is the extension to determine the spatial correlation of the velocity fluctuations. In this paper we present the results for Re = 4.98×105, corresponding to a shear Reynolds number Reτ = 10.3×103, with a spatial resolution of ∆y+ = 18.Fluid MechanicsMulti Phase System
Particle Image Velocimetry; Progress towards Industrial Application
No full textThis book is a detailed presentation of the cooperative research activity undertaken by 20 European teams to contribute to the development of Particle Image Velocimetry. This research was performed over a period of two years, in the frame of a European Brite-Euram contract and was greatly enhanced by its cooperative aspect. The book presents extensive comparisons of both recording and analysis of PIV images. A database has been built, which is presented in detail and which is accessible via Internet. Industrial application in a large wind tunnel, validation of up-to-date numerical predictions and new developments including extension to 3C are also addressed in the different chapters of this book. In addition, 25 individual contributions are provided in the form of scientific papers which cover various aspects of the development of the method (PTV, video recording, stereo-PIV, holo-PIV). This book will be of interest to scientists and engineers active in the field
Direct numerical simulations of drag reduction in turbulent channel flow over bio-inspired herringbone riblet-texture
No full textThe use of drag reducing surface textures is a promising passive method to reduce fuel consumption. Probably most wellknown is the utilisation of shark-skin inspired ridges or riblets parallel to the mean flow. They can reduce drag up to 10%. Recently another bio-inspired texture based on bird flight feather riblets has been proposed. It differs from the standard riblets in two ways. First, the riblets are arranged in a converging/diverging or herringbone pattern. Second, the riblet height or groove depth changes gradually. Drag reductions as high as 20% have been claimed [2]. The objective of the present work is to study the drag reducing properties and mechanisms of this texture. To that purpose Direct Numerical Simulations (DNSs) of turbulent plane channel flow have been performed. Structured roughness has been applied to both walls and several geometric parameters have been varied. Marginal drag reductions on the order of 2.5% and significant drag increases well beyond 100% were found. The latter is attributed to a strong secondary flow that mixes momentum through the whole channel. In future optimization studies we might look for conditions at which secondary motions affect the near-wall cycle of turbulence only
Mechanics of inhomogeneous turbulence and interfacial layers
No full textThe mechanics of inhomogeneous turbulence in and adjacent to interfacial layers bounding turbulent and non-turbulent regions are analysed. Different mechanisms are identified according to the straining by the turbulent eddies in relation to the strength of the mean shear adjacent to, or across, the interfacial layer. How the turbulence is initiated and the topology of the region of turbulence are also significant factors. Specifically the cases of a layer of turbulence bounded on one, or two, sides by a uniform and/or shearing flow, and a circular region of a rotating turbulent vortex are considered and discussed.
The entrainment processes at fluctuating interfaces occur both at the outer edges of turbulent shear layers, with and without free-stream turbulence (e.g. jets, wakes and boundary layers), at internal boundaries such as those at the outside of the non-turbulent core of swirling flows (e.g. the ‘eye-wall’ of a hurricane) or at the top of the viscous sublayer and roughness elements in turbulent boundary layers. Conditionally sampled data enables these concepts to be tested. These concepts lead to physically based estimates for critical modelling parameters such as eddy viscosity near interfaces, entrainment rates, maximum velocity and displacement heights
Predicting growth rates of interfaces and internal layers in a turbulent boundary layer using a first order jump model
No full textExperimental research is presented on the characteristics of interfaces and internal layers that are present in a turbulent boundary layer (TBL). Both the turbulent non-turbulent interface (T/NT) and internal shear layers are detected in snapshots of the stereo-PIV data. It turns out that the internal layers exhibit similar characteristics compared to the T/NT interface. A theoretical approximation of the large scale boundary layer growth indicates that the correct boundary layer growth can be obtained by employing a modified first order jump model on the conditional statistics. Employing the same framework to the internal shear layers indicates that shear layers tend to move slower in close proximity to the wall, whereas they accelerate when moving away from the wall. Based on previous research it is believed that these internal layers separate large regions of approximately uniform momentum. Hence, boundary entrainment velocities may be interpreted as growth rates of large scale motions in a TBL
Experimental study of surface modification in a fully turbulent Taylor-Couette flow
No full textFriction measurements were performed in a Taylor-Couette setup. Drag reduction was obtained with a riblet surface and indicated a drag reduction for a wide range of shear Reynolds numbers, with a maximum of 5.3% at Re_s=47000 (s+=14). Tomographic PIV verified that the friction coefficients are strongly related to the flow regimes and structures. The bulk fluid rotation was changed by the application of the riblets, as the wall-bounded flow conditions at the inner cylinder wall were changed due to the surface modification and is called the rotation effect. A simple model was used to indicate the averaged bulk velocity shift (1.4%), after which the drag changes due to the rotation effect (-1.9%) and the riblet effect (-3.4%) were determined. The bulk velocity shift of 1.4% was verified by PIV measurements. Compliant surfaces will be further investigated to check their required conditions for drag reduction of wall-bounded flow
Flow over partially liquid filled cavity
No full textExperiments have been carried out to investigate the effect of liquid cavity filling on the behavior of the gas flow over a flat plate cavity. PIV measurements in the gas phase reveal that cavity filling can affect vortex shedding in the cavity mouth. Shear layer vortices can break-up into smaller vortices, thereby losing their periodic interaction with the aft wall and, hence, their sound producing potential. Expected is that this is one of the mechanisms causing sound mitigation in corrugated pipes with liquid addition, observed in literature
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