Effect of gap flow on the shallow wake of a sharp-edged bluff body – turbulence parameters

This experimental study was carried out to investigate the turbulent wake generated by a vertical sharp-edged flat plate suspended in a shallow channel flow with a gap near the bed. The objective of this study is to understand the effect of the gap flow on the turbulent wake by studying two different gap heights between the channel bed and the bottom edge of the bluff body. These two cases were compared to the no-gap case which is considered as a reference case. The maximum flow velocity was 0.45 m/s and the Reynolds number based on the water depthwas 45,000. Extensivemeasurements of the flow field in the vertical mid-plane and in the horizontal near-bed, mid-depth, and near-surface planes weremade using particle-image velocimetry (PIV). This paper is the second part of an extensive study to characterise the gap-flow effects and is primarily focused on the mean and instantaneous turbulence quantities as well as coherent structures. The results revealed that the gap flow increased the transfer of the turbulent kinetic energy (TKE) from the streamwise to the vertical component along the vertical mid-plane. In addition, there is a corresponding increase and spread of the transverse component in the transverse direction as the flow evolves in the downstream direction. The momentum exchange by the Reynolds stress is significantly weak in the vertical mid-plane particularly in the lower half of the water depth, but the gap flow enhanced the momentum exchange in the upper half of the water depth by up to 1% of the freestream velocity squared. Furthermore, the intensity and bursting direction of the turbulence fluctuations in the far field are also affected by the gap flow when it is large. Furthermore, the proper orthogonal decomposition results revealed that the flow contains a large number of structures, and their interactions are responsible for deforming and/or tearing apart the structures, and transferring fluid throughout the velocity field

Vortex Shedding Dynamics in Long Aspect-Ratio Aerodynamics Bodies

The focus of the current dissertation is to study qualitatively the underlying physics of vortex-shedding and wake dynamics in long aspect-ratio aerodynamics in incompressible viscous flow through the use of the KLE method. We carried out a long series of numerical experiments in the cases of flow around the cylinder at low Reynolds numbers. The study of flow at low Reynolds numbers provides an insight in the fluid physics and also plays a critical role when applying to stalled turbine rotors. Many of the conclusions about the qualitative nature of the physical mechanisms characterizing vortex formation, shedding and further interaction analyzed here at low Re could be extended to other Re regimes and help to understand the separation of the boundary layers in airfoils and other aerodynamic surfaces. In the long run, it aims to provide a better understanding of the complex multi-physics problems involving fluid-structure-control interaction through improved mathematical computational models of the multi-physics process. Besides the scientific conclusions produced, the research work on streamlined and bluff-body condition will also serve as a valuable guide for the future design of blade aerodynamics and the placement of wind turbines and hydrakinetic turbines, increasing the efficiency in the use of expensive workforce, supplies, and infrastructure. After the introductory section describing the main fields of application of wind power and hydrokinetic turbines, we describe the main features and theoretical background of the numerical method used here. Then, we present the analysis of the numerical experimentation results for the oscillatory regime right before the onset of vortex shedding for circular cylinders. We verified the wake length of the closed near-wake behind the cylinder and analysed the decay of the wake at the wake formation region, and then studied the St-Re relationship at the Reynolds numbers before the wake sheds compared to the experimental data. We found a theoretical model that describes the time evolution of the amplitude of fluctuations in the vorticity field on the twin vortex wake, which accurately matches the numerical results in terms of the frequency of the oscillation and rate of decay. We also proposed a model based on an analog circuit that is able to interpret the concerning flow by reducing the number of degrees of freedom. It follows the idea of the non-linear oscillator and resembles the dynamics mechanism of the closed near-wake with a common configured sine wave oscillator. This low-dimensional circuital model may also help to understand the underlying physical mechanisms, related to vorticity transport, that give origin to those oscillations

Wake behind a rough (metal foam-covered) cylinder in cross-flow

The flow structures behind a circular cylinder are associated with various instabilities. These instabilities are characterized by the Reynolds number and they include the wake, separated shear layer and boundary layer. Depending on the physical application of the cylinder, increasing the level of turbulence by wrapping the cylinder with metal foam would be a target for drag reduction or heat transfer enhancement. In contrast to the extensive consideration that has been devoted to the flow around bare cylinders, the flow structures around the foam-covered cylinders and the characteristics of the wake behind such surfaces has received relatively little attention. Since the present study explores the possibility of using metal foams as replacements for fins on heat exchanger tubes, investigation on the flow-field structures downstream of a foam-covered cylinder and compare it to the bare cylinder specifies the feasibility of using the foam- covered cylinders in heat exchangers. Moreover, the outcome of this study can be used to develop an appropriate boundary condition for the porous-air interface modelling also increases the knowledge of turbulence in porous media. As it will be discussed more in detail in the following chapters, pressure drop and drag coefficient are directly linked to the wake and recirculation region, and to study these two phenomena, it is necessary to have in depth study on the flow-field down-stream of the cylinder. Hence, the purpose of this study is to investigate the wake region behind a foam- covered cylinder by means of Particle Image Velocimetry (PIV) and Hot-Wire Anemometer. PIV is providing instantaneous whole-flow-field velocity vector measurements in a cross- section of a flow, which makes it possible to study the instabilities of the flow-field and also the flow structures downstream of the model. By applying Proper Orthogonal Decomposition (POD) and Linear Stochastic Estimation (LSE) to the PIV results and comparing the results with what exists in literature for bare cylinder, we can conclude if the foam-covered cylinder increases the turbulence level. Moreover, using hot-wire anemometry, to investigate the energy of the flow inside and outside of the foam-covered cylinder’s wake, let us know if a foam-covered cylinder can be treated as an obstacle to the incident flow with a rough surface or the whole foam can be considered as the combination of local jets that are coming out of the pores and disperse in random directions

Structural Response and Wake Vortex Dynamics of Cylindrical Structures Undergoing VIV with Elliptic Trajectories

Vortex Induced Vibrations (VIV) of a pivoted circular cylinder were examined through experiments conducted in a water tunnel facility at the University of Calgary. The structural response and wake dynamics of the system were evaluated with the goal of quantitatively describing the attendant fluid-structure interaction. Experiments were performed at a fixed Reynolds number of 3100, a mass ratio of 10.8, and a range of reduced velocities, 4.42<U*<9.05. Laser based displacement sensors and time-resolved, two-component particle image velocimetry (PIV) measurements were used to quantify the structural response and flow development, respectively. The results provide new insight into the fluid-structure interaction for pivoted cylinders, which is of practical engineering interest as this phenomenon is encountered in various civil structures and mechanical devices. For the investigated conditions, the cylinder traces elliptic trajectories, with the experimental conditions producing three out of four possible combinations of orbiting direction and primary axis alignment relative to the incoming flow. The study focuses on the quantitative analysis of wake topology and its relation to this type of structural response. The planar phase-averaged wake topology generally agrees with the Morse and Williamson shedding map for one-degree-of-freedom vortex induced vibrations, with 2S, 2P0, and 2P shedding patterns observed within the range of reduced velocities studied here. A new Eulerian vortex identification, tracking, and strength quantification methodology is developed and applied to analyze the vortex shedding process. In the case of 2S vortex shedding, vortices are shed when the cylinder is approaching the maximum transverse displacement and reaches the streamwise equilibrium. 2P vortices are shed approximately half a period earlier in the cylinder's elliptic trajectory. Leading vortices shed immediately after the peak in transverse oscillation and trailing vortices shed near the equilibrium of transverse oscillation. The orientation and direction of the cylinder's elliptic trajectory are shown to influence the timing of vortex shedding, inducing changes in the 2P wake topology. Three dimensional reconstructions of the phase averaged wake velocity measurements reveal 2S shedding along the span of a stationary cylinder and hybrid shedding for the VIV cases at U*=5.48 and 7.08, with planar wake topology transitioning from 2S to P+S to 2S, and 2S to P+S, respectively. The observed wake topologies show significant deviation from predictions based on the Morse and Williamson shedding map. Examination of the time averaged wake characteristics shows the formation length, wake half-width, and maximum velocity deficit exhibit spanwise trends that support the observed region of wake transition between different shedding regimes. Spectral analysis of the wake velocity indicates cycle-to-cycle variations of the spanwise location of wake topology transition as well as transience in the frequency of vortex shedding

Reduced order modeling of some fluid flows of industrial interest

Some basic ideas are presented for the construction of robust, computationally efficient reduced order models amenable to be used in industrial environments, combined with somewhat rough computational fluid dynamics solvers. These ideas result from a critical review of the basic principles of proper orthogonal decomposition-based reduced order modeling of both steady and unsteady fluid flows. In particular, the extent to which some artifacts of the computational fluid dynamics solvers can be ignored is addressed, which opens up the possibility of obtaining quite flexible reduced order models. The methods are illustrated with the steady aerodynamic flow around a horizontal tail plane of a commercial aircraft in transonic conditions, and the unsteady lid-driven cavity problem. In both cases, the approximations are fairly good, thus reducing the computational cost by a significant factor

Dynamics of gap flow interference in a vibrating side-by-side arrangement of two circular cylinders at moderate Reynolds number

In this work, the coupled dynamics of the gap flow and the vortex-induced vibration (VIV) on a side-by-side (SBS) arrangement of two circular cylinders is numerically investigated at moderate Reynolds number 100 < Re < 800. Of particular interest is to establish a relationship between the VIV, the gap flow and the near-wake instability behind bluff bodies. We find that the kinematics of the VIV regulates the streamwise vorticity concentration, which accompanies with a recovery of two-dimensional hydrodynamic responses at the peak lock-in stage. On the other hand, the near-wake instability may develop around an in-determinant two-dimensional streamline saddle point along the interfaces of a pair of imbalanced counter-signed vorticity clusters. The vorticity concentration difference of adjacent vorticity clusters and the fluid momentum are closely interlinked with the prominence of streamwise vortical structures. In both SSBS and VSBS arrangements, the flip-flopping frequency is significantly low for the three-dimensional flow, except at the VIV lock-in stage for the VSBS arrangement. A quasi-stable deflected gap flow regime with negligible spanwise hydrodynamic (i.e., two-dimensional) response is found at the peak lock-in stage of VSBS arrangements. Owing to the gap-flow proximity interference, a high streamwise vorticity concentration is observed in its narrow near-wake region. The increase of the gap-flow proximity interference tends to stabilize the VIV lock-in, which eventually amplifies the spanwise correlation length and weakens the streamwise vortical structures. We employ the dynamic mode decomposition procedure to characterize the space-time evolution of the primary vortex wake

A review of experiments on stationary bluff-body wakes

Experimental studies dealing with the wake of isolated stationary bluff-bodies are reviewed. After briefly recalling the pioneering works in this domain, the paper focuses on recent research conducted with the latest experimental methods and techniques. The review encompasses a range of topics, including, the effects of bluff-body geometry (non-circular cross sections and nonuniformity in spanwise direction), steady and unsteady (periodic and non-periodic) inflow conditions; surface proximity (rigid wall, confinement and water free surface) and non-Newtonian fluids. Focus is brought to the flow physics of the wakes, including especially the complex threedimensional and oscillatory behaviours induced by the periodic vortex shedding phenomenon. The paper aims to offer a critical and systematic review of new knowledge and findings on the subject area, as well as emerging? and the most frequently adopted experimental techniques. The review also helps identifying knowledge gaps in the literature that need to be addressed in future investigations

LARGE EDDY SIMULATION OF FLOW AROUND A FINITE SQUARE CYLINDER

The main objective of this research is to develop, document and study numerically the flow around finite-height square cylinders mounted on a ground plane, particularly in the near-wake region, under various geometrical conditions. Both the time-averaged and instantaneous flow fields are studied. This thesis consists of three main parts: a comprehensive study of flow over an aspect ratio AR = 5 square cylinder, the effect of sub-grid scale (SGS) models on the numerical simulation and the effect of aspect ratio on the flow structure. The first part of the thesis presents the time-averaged and instantaneous flow fields for flow over a wall-mounted finite-height square cylinder of aspect ratio of AR = 5 at a Reynolds number of Re = 500. The time-averaged flow field results are shown to be in good agreement with experiments. Comparison of the time-averaged results with the velocity field for a square cylinder immersed in a thicker boundary layer, suggests that the boundary layer thickness especially affects the upwash flow (Wang et al., 2009). The instantaneous velocity fields provide an in-depth view of the unsteady nature of the flow field. For the flow over a square cylinder of AR = 5, the instantaneous velocity fields are symmetric near the free end. However, antisymmetric patterns observed downstream may be an indication of the presence of periodic von-Karman type vortices. Since the wake regions are characterized by large-scale unsteady motions, turbulent flow over bluff bodies is well suited to large eddy simulation in which the large energy-containing scales of motion, which are responsible for most of the momentum transport, are resolved whereas the small-scale turbulent fluctuations are modeled. In the second part of the thesis, the performance of the three SGS models, the Smagorinsky model (SM), dynamic Smagorinsky model (DSM) and dynamic non-linear model (DNM) are studied for two grid sets of lower and higher resolution. The results indicated that in case of the DSM insufficient grid resolution leads to erroneous predictions, whereas the DNM is a major improvement as the predictions are similar on both the coarse and fine grids. In the third and final part of the thesis, the effect of aspect ratio on the flow over a wall-mounted finite-height square cylinder is numerically investigated. The wake of a finite square cylinder is studied for three aspect ratios of AR = 3, 5 and 7. The time-averaged vorticity was shown to vary with aspect ratio, e.g. as the aspect ratio increases, the vortex structures in a horizontal plane at mid-height became shorter and rounder in shape. The flow field of the finite cylinder is known to be strongly affected by the aspect ratio (Adaramola et al., 2006). For cylinders with relatively small aspect ratios, the two ends affect the flow patterns and significantly alter the flow structure

Aeronautical Engineering: A continuing bibliography, supplement 120

This bibliography contains abstracts for 297 reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1980