Flow Over Model Buildings With Sloped Roofs

2003 ◽  
Author(s):  
Ge´rard J. Poitras ◽  
Laurent-E. Brizzi ◽  
Yves Gagnon
Keyword(s):  
Vertical Wall ◽  
Three Dimensional ◽  
Median Plane ◽  
Reynolds Numbers ◽  
Horseshoe Vortex ◽  
Flow Topology ◽  
Image Velocimetry ◽  
The One ◽  
Flat Roof ◽  
Flat Roofs

The flow field around model buildings with different sloped roofs was investigated using Particle Image Velocimetry (PIV). The flow around model buildings having flat roofs was studied by many authors. Although buildings with sloped roofs are the most common type of low rise buildings, the flow around these buildings are not well known. Most of the studies for these types of buildings were made for the determination of surface pressures. The aim of this study is to highlight the fundamental differences between flat roofs and sloped roofs for three-dimensional obstacle flows. The experiments were performed in a wind tunnel having a cross section of 300 mm × 400 mm. All the models were 30 mm high (vertical wall) and were placed in a thin turbulent boundary layer. Three Reynolds numbers, based on the height of the obstacle, were used (12000, 22 000, 32 000). Furthermore, the quantitative data is analyzed and statistical results describing the mean and fluctuating velocity fields are presented. Finally, the surface pressures on the median plane were studied in order to correlate these pressures with the flow topology of different sloped roofs. It was found that upstream of the obstacle, the flow topology for the model having sloped roofs was similar to that of a flat roof apart from an increase in size of the well-known horseshoe vortex. However, the flow topology is not the same over different roofs, on the sides of the models and immediately downstream of the models. For the Reynolds number studied, there are no coherent flow structures over the upstream sloped roofs while an arch vortex is created on the sides of the models. This arch vortex is similar to the arch vortex that is created over a flat roof. An arch vortex is also present downstream of the models. The lower part of this vortex is similar to the one created for a flat roof. However, the upper part of the arch vortex starts from the tip of the roof and continues downstream and has an ellipse shape. This vortex also increases in size with the slope of the roof.

scholarly journals Fish larvae exploit edge vortices along their dorsal and ventral fin folds to propel themselves

2016 ◽  
Vol 13 (116) ◽  
pp. 20160068 ◽  
Author(s):  
Gen Li ◽  
Ulrike K. Müller ◽  
Johan L. van Leeuwen ◽  
Hao Liu
Keyword(s):  
Larval Fish ◽  
Fish Larvae ◽  
Fluid Dynamic ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Bony Fish ◽  
The Body ◽  
Functional Explanation ◽  
Image Velocimetry ◽  
Median Fins

Larvae of bony fish swim in the intermediate Reynolds number ( Re ) regime, using body- and caudal-fin undulation to propel themselves. They share a median fin fold that transforms into separate median fins as they grow into juveniles. The fin fold was suggested to be an adaption for locomotion in the intermediate Reynolds regime, but its fluid-dynamic role is still enigmatic. Using three-dimensional fluid-dynamic computations, we quantified the swimming trajectory from body-shape changes during cyclic swimming of larval fish. We predicted unsteady vortices around the upper and lower edges of the fin fold, and identified similar vortices around real larvae with particle image velocimetry. We show that thrust contributions on the body peak adjacent to the upper and lower edges of the fin fold where large left–right pressure differences occur in concert with the periodical generation and shedding of edge vortices. The fin fold enhances effective flow separation and drag-based thrust. Along the body, net thrust is generated in multiple zones posterior to the centre of mass. Counterfactual simulations exploring the effect of having a fin fold across a range of Reynolds numbers show that the fin fold helps larvae achieve high swimming speeds, yet requires high power. We conclude that propulsion in larval fish partly relies on unsteady high-intensity vortices along the upper and lower edges of the fin fold, providing a functional explanation for the omnipresence of the fin fold in bony-fish larvae.

scholarly journals Evolution of Turbulent Horseshoe Vortex System in Front of a Vertical Circular Cylinder in Open Channel

Water ◽  
2019 ◽  
Vol 11 (10) ◽  
pp. 2079 ◽  
Author(s):  
Chen ◽  
Yang ◽  
Wu
Keyword(s):  
Open Channel ◽  
Evolutionary Process ◽  
Reynolds Numbers ◽  
Horseshoe Vortex ◽  
Local Scour ◽  
Time Resolved ◽  
Vortex Interactions ◽  
Initial Stage ◽  
Image Velocimetry ◽  
Underlying Mechanisms

A turbulent horseshoe vortex (HV) system around a wall-mounted cylinder in open channel is characterized by random variations in vortex features and an abundance of vortex interactions. The turbulent HV system is responsible for initiating the local scour process in front of the cylinder. The evolution of the turbulent HV system is investigated statistically and quantitatively with time-resolved particle image velocimetry. The cylinder Reynolds numbers of the flow are 8600, 10,200, and 13,600, respectively. A novel vortex tracking method was proposed to obtain the variations in position, size, and strength of the primary HV (PHV) which dominates the system most of the time. Relationships between the various features of the PHV during its evolutionary process were obtained through correlation analyses. Results show that the dimensionless mean lifespan of the PHV is about 5.0. Statistically, the downstream movement of the PHV toward the cylinder is accompanied with its bed-approaching movement and decreasing in size, and the opposite is true. The circulation strength of the PHV decreases and increases dramatically in the region downstream of its time-averaged position when the PHV approaches and departs from the cylinder, respectively. Meanwhile, mechanisms responsible for the generation, movement, variation, and disappearance of the PHV are re-investigated and enriched based on its interactions with vortices in the separation region and structures in the incoming flow. The obtained change trends of the features of the PHV and the underlying mechanisms for its evolution are valuable for predicting and controlling the initial stage of the local scour in front of cylinders.

Experimental Study of Three-Dimensional Laminar Wall Jets of Non-Newtonian Fluid

2009 ◽  
Author(s):  
Kofi K. Adane ◽  
Mark F. Tachie
Keyword(s):  
Newtonian Fluid ◽  
Open Channel ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Jet Flows ◽  
Wall Jet ◽  
Particle Image Velocimetry Technique ◽  
Piv Measurements ◽  
Image Velocimetry ◽  
Shear Thinning Fluid

A particle image velocimetry technique was employed to study three-dimensional laminar wall jet flows of a non-Newtonian shear-thinning fluid. The wall jet was created using a circular pipe of diameter 7 mm and flows into an open channel. The Reynolds numbers based on the pipe diameter and jet exit velocity were varied from 250 to 800. The PIV measurements were performed in various streamwise-transverse and streamwise-spanwise planes. From these measurements, the velocity profiles, jet growth rate and spread rates were obtained to study the characteristics of three-dimensional wall jet flows of a non-Newtonian fluid.

Effect of Sinusoidal Oscillatory Flow on a Vertical Wall-Mounted Cylinder

2020 ◽  
Author(s):  
HaKun Jang ◽  
Celalettin Emre Ozdemir ◽  
Mayank Tyagi ◽  
Jun-Hong Liang
Keyword(s):  
Numerical Solutions ◽  
Stokes Equations ◽  
Vertical Wall ◽  
Three Dimensional ◽  
Finite Depth ◽  
Horseshoe Vortex ◽  
Vortex Structures ◽  
Navier Stokes ◽  
Wide Range ◽  
Sinusoidal Flow

Abstract The purpose of this study is to numerically investigate the bed shear stress and near-bed mixing due to coherent vortex structures in the vicinity of a vertically wall-mounted circular cylinder subject to an imposed finite-depth oscillatory sinusoidal flow. Previous studies reveal that the Keulegan–Carpenter (KC) number influences the formation of lee-side wake vortex structures as well as the horseshoe vortex in front of a cylinder. Therefore, parametric studies in a moderately wide range of KC from 5 to 20 are numerically performed. In the present study, Direct Numerical Simulation (DNS) is conducted using the open-source software, OpenFOAM, that solves the three-dimensional unsteady incompressible Navier-Stokes equations using finite volume method. Nondimensional parameters used in the simulations are carefully chosen to represent the real physics. The numerical solutions are first validated using an analytical solution for the oscillating Stokes flow and the results are then systematically and quantitatively compared with the experimental measurements. The results show that the lee-side wake is significantly influenced by KC, and distinctive types of the lee-side wake are generated and classified based on KC. It is also found that both KC and the ratio of the thickness of the Stokes boundary layer to the water depth are heavily associated with the stability of the lee-side wake. In addition, the simulated size and lifespan of the horseshoe vortex agree well with the experimental data.

Flow Characterization of a Three-Dimensional Separated Annular Diffuser

2009 ◽  
Author(s):  
Jason J. Dunn ◽  
Mark Ricklick ◽  
J. S. Kapat
Keyword(s):  
Vertical Wall ◽  
Three Dimensional ◽  
Separated Flow ◽  
Outer Wall ◽  
Reynolds Numbers ◽  
Pressure Gradients ◽  
Cfd Models ◽  
Flow Characterization ◽  
Annular Diffuser

Experiments were performed on two annular diffusers to characterize the flow separation along the outer wall. Both diffusers had the same fully developed inlet flow condition, however, the expansion of the two diffusers differed such that one diffuser replicated a typical compressor discharge diffuser found in a real machine while the other would create a natural separated flow along the outer wall. Both diffusers were tested at two Reynolds numbers, 5×104 and 1×105, with and without a vertical wall downstream of the exit to replicate the dump diffuser that re-directs the flow from the compressor outlet to the combustor. It was shown that the separation happens quite quickly within the diffuser after which the pressure recovery remains fairly constant. The results also further validate claims that CFD models can not accurately predict flows with high adverse pressure gradients.

Experimental Investigation of the Wake of a Discontinuous Cylinder

2010 ◽  
Author(s):  
V. S. R. Mandava ◽  
Gregory A. Kopp ◽  
Joan Herrero ◽  
Francesc Giralt
Keyword(s):  
Structural Characteristics ◽  
Dominant Role ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Wake Flow ◽  
Near Wake ◽  
Vortical Structures ◽  
Image Velocimetry ◽  
Entrainment Process ◽  
Thin Steel Plate

The effects of a discontinuous cylinder geometry on the near wake structures was investigated experimentally. This ‘discontinuous’ circular cylinder has gaps so that solid segments 5D long are followed by gaps 2.5D long, in a repeating pattern, where D is the diameter of the cylinder. A thin steel plate was used to hold all of the cylinder pieces together. Thus, a three-dimensional (3D) wake was created at the origin with the intent to force the near wake flow to have similar structural characteristics as the far wake behind an ‘infinite/continuous’ cylinder, i.e., a near wake flow with horseshoes or double rollers formed by rapid kinking of Ka´rma´n-like vortices. Since the kinetic energy associated with the fluctuations of these near-wake 3D vortical structures is high, the flow system is considered suitable to clarify the role of these velocity patterns in the entrainment process of wake flows, which is still the subject of controversy. Particle Image Velocimetry (PIV) and Hot-Wire Anemometry (HWA) techniques were used to analyze the flow at two Reynolds numbers, Re = 10000 and 4000, in the wake of the discontinuous cylinder up to x/D = 190 downstream. The development of double rollers resulting from the interaction between the high momentum flow through the gaps and the Ka´rma´n-like vortices formed behind the solid cylindrical segments was confirmed. The Strouhal number of the double rollers in the wake is 0.14. These vortices have a dominant role in the initial wake growth. Thus, the overall flow dynamics are similar to the momentum transfer that takes place at the scale of the intermittent turbulent bulges that protrude from the wake in the far region and that were reported to be associated with double rollers.

scholarly journals Mixing in a vortex breakdown flow

2013 ◽  
Vol 731 ◽  
pp. 195-222 ◽  
Author(s):  
P. Meunier ◽  
K. Hourigan
Keyword(s):  
Scaling Laws ◽  
Vortex Breakdown ◽  
Three Dimensional ◽  
Stationary Flow ◽  
Reynolds Numbers ◽  
Exchange Flow ◽  
Convective Mixing ◽  
Homogenization Time ◽  
Image Velocimetry ◽  
Stationary Disc

AbstractIn this paper we present experimental and theoretical results on the mixing inside a cylinder with a rotating lid. The helical flow that is created by the rotation of the disc is well known to exhibit a vortex breakdown bubble over a finite range of Reynolds numbers. The mixing properties of the flow are analysed quantitatively by measuring the exponential decay of the variance as a function of time. This homogenization time is extremely sensitive to the asymmetries of the flow, which are introduced by tilting the rotating or the stationary disc and accurately measured using particle image velocimetry (PIV). In the absence of vortex breakdown, the homogenization time is strongly decreased (by a factor of 10) with only a moderate tilt angle of the rotating lid (of the order of $1{5}^{\circ } $). This phenomenon can be explained by the presence of small radial jets at the periphery which create a strong convective mixing. A simple model of exchange flow between the periphery and the bulk correctly predicts the scaling laws for the homogenization time. In the presence of vortex breakdown, the scalar is trapped inside the vortex breakdown bubble, and thus increases substantially the time needed for homogenization. Curiously, the tilt of the rotating lid has a weak effect on the mixing, but a small tilt of the stationary disc (of the order of ${2}^{\circ } $) strongly decreases (by a factor of 10) the homogenization time. Even more surprising is that the homogenization time diverges when the size of the bubble vanishes. All of these features are recovered by applying the Melnikov theory to calculate the volume of the lobes that exit the bubble. It is the first time that this technique has been applied to a three-dimensional stationary flow with a non-axisymmetric perturbation and compared with experimental results, although it has been applied often to two-dimensional flows with a periodic perturbation.

Experimental and Numerical Study of the Three Dimensional Instabilities in the Wake of a Blunt Trailing Edge Profiled Body

2010 ◽  
Author(s):  
Arash Naghib Lahouti ◽  
Lakshmana Sampat Doddipatla ◽  
Horia Hangan ◽  
Kamran Siddiqui
Keyword(s):  
Reynolds Number ◽  
Numerical Study ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
The Body ◽  
Bluff Bodies ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Image Velocimetry

The wake of nominally two dimensional bluff bodies is dominated by von Ka´rma´n vortices, which are accompanied by three dimensional instabilities beyond a threshold Reynolds number. These three dimensional instabilities initiate as dislocations in the von Ka´rma´n vortices near the trailing edge, which evolve into pairs of counter-rotating vortices further downstream. The wavelength of the three dimensional instabilities depends on profile geometry and Reynolds number. In the present study, the three dimensional wake instabilities for a blunt trailing edge profiled body, composed of an elliptical leading edge and a rectangular trailing edge, have been studied in Reynolds numbers ranging from 500 to 1200, based on the thickness of the body. Numerical simulations, Laser Induced Fluorescence (LIF) flow visualization, and Particle Image Velocimetry (PIV) methods have been used to identify the instabilities. Proper Orthogonal Decomposition (POD) has been used to analyze the velocity field data measured using PIV. The results confirm the existence of three dimensional instabilities with an average wavelength of 2.0 to 2.5 times thickness of the body, in the near wake. The findings are in agreements with the values reported previously for different Reynolds numbers, and extend the range of Reynolds numbers in which the three dimensional instabilities are characterized.

Wavelet Analysis of the 3-D Wake Topology for a Square Prism

2002 ◽  
Author(s):  
Adrian Dobre ◽  
Horia Hangan
Keyword(s):  
Wavelet Analysis ◽  
Three Dimensional ◽  
Wake Flow ◽  
Flow Topology ◽  
Three Dimensional Flow ◽  
Square Prism ◽  
Wavelet Techniques ◽  
High Reynolds ◽  
The One ◽  
Topological Models

The three-dimensional flow topology of square prism intermediate wake flow based on multi-point hot-wire measurements is investigated using wavelet analysis. A number of continuous wavelet techniques combining time and scale analysis are applied systematically to detect the primary and the secondary flow structures and to validate previously proposed topological models. Our results suggest that a preferred alternate rib spanwise arrangement similar to the one proposed by Meiburg and Lasheras [1] is plausible for the vortex topology of high Reynolds number square prism intermediate wakes. Moreover, it seems that the wake topology has a preferred spanwise wavelength of approximately 2b = 2.4d, which gives a relative spanwise/ streamwise wavelength ( 2b / λ ) of 2/5.

Full Simulation of Natural Waves in Falling Films

2001 ◽  
Author(s):  
Edwin de Korte ◽  
Jim B. W. Kok ◽  
Theo H. van der Meer
Keyword(s):  
Stokes Equations ◽  
Vertical Wall ◽  
Reynolds Numbers ◽  
Navier Stokes ◽  
Falling Film ◽  
Complete Understanding ◽  
Navier Stokes Equations ◽  
The One ◽  
The Waves ◽  
Full Simulation

Abstract The two dimensional Navier Stokes equations have been solved for a falling film flow, i.e. a laminar flow running down a vertical wall driven by gravity. The aim of this research is to describe the onset of natural waves by using full simulation of the Navier-Stokes equations. In order to find the natural wave the flow velocity at the the inlet boundary (top horizontal plane) is perturbed. With the boundary condition described above the fastest growing wave is triggered for a range of Reynolds numbers in the laminar wavy range. The wave numbers and velocities agree with results based on Orr-Sommerfeld theory reported by Pierson & Whitaker [7] for Re = 5.90 and Re = 11.8. For Re = 23.5 a different wavelength triggered than the one predicted by linear stability analysis. However the wavespeeds are in reasonable agreement with each other. A complete understanding for this phenomenon is not found yet. There is shown that the behavior of the waves is strongly non-linear and transient. In this work is to illustrate that the wave profiles can be calculated by simply solving the Navier-Stokes equations without any prior assumptions.

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