Unsteady vortex shedding dynamics behind a circular cylinder in very shallow free-surface flows

The turbulent wake generated by a horizontal circular cylinder in free-surface flows of increasing shallowness with submergence-to-diameter ratios between 0.5 and 2.1 are investigated using large-eddy simulation. At Froude number ( ) = 0.26, the free-surface deformation is small with little influence on the wake, whereas at = 0.53 there is a drop in the free-surface downstream of the cylinder that impacts the coherence of the vortex shedding. Irrespective to the relative submergence, the close location of the cylinder to the bottom wall generates an asymmetric von-Kármán vortex street. Proper Orthogonal Decomposition (POD) is used to analyse the spatio-temporal coherence of the turbulent structures shed in the cylinder wake. The spatial patterns of the first two POD modes, those containing the most energy, depict the von-Kármán vortices. As increases, the energy content of the first pair of POD modes decreases from 56% at = 0.26 to 26.8% at = 0.53, as large-scale vortices lose coherence more rapidly with shallower conditions. This energy redistribution leads to the smaller flow structures to contain a relatively higher energy when is larger. The frequency of the dominating vortex shedding determined from the spectra of the POD temporal coefficients unveils that the first two coefficients feature a dominant peak at the von-Kármán vortex shedding frequency. At 0.45, the reconstructed flow field using the first 20 POD modes agrees well with the instantaneous velocities from LES, whereas free-surface effects on the wake dynamics at increasing requires more POD modes to reconstruct the flow field with reduced error

Statistical Fluid Dynamics

Modeling micrometric and nanometric suspensions remains a major issue. They help to model the mechanical, thermal, and electrical properties, among others, of the suspensions, and then of the resulting product, in a controlled way, when considered in material formation. In some cases, they can help to improve the energy transport performance. The optimal use of these products is based on an accurate prediction of the flow-induced properties of the suspensions and, consequently, of the resulting products and parts. The final properties of the resulting micro-structured fluid or solid are radically different from the simple mixing rule. In this book, we found numerous works addressing the description of these specific fluid behaviors

Wake Characteristics of Wall-Mounted Finite Solid and Foam-Covered Cylinders

The wake characteristics of a wall-mounted finite foam-covered cylinder are explored by comparing the flow properties with the wake of a solid cylinder. The velocity data are acquired experimentally using planar particle image velocimetry (PIV) and numerically by large eddy simulation at Reynolds number of 35100 based on the flow depth and 13100 based on the cylinder diameter. The wake characteristics may vary significantly if the approach flow properties change and also depend on the relative height of the boundary layer thickness with respect to the cylinder height. Therefore, it is a necessity to achieve a fully developed approach flow to maintain the consistency and universality of the dataset. The fully developed flow is achieved numerically by recycling of flow variables from outlet to inlet, but it is not straightforward to achieve the fully developed flow experimentally. Therefore, experimental studies on the effects of the tripping intensity and the free surface perturbations are carried out for proper conditioning of the approach flow and to achieve a fully developed state in the open channel flume. Two flow depths at a similar Reynolds number are used for the study of tripping and the intensity of tripping is gradually increased until the flow reaches the fully developed state. The boundary layer thickness is determined based on the wall-normal distribution of Reynolds stresses and higher-order moments and this is found to be more consistent than the classical definition which suggests a wall-normal position of 99% of maximum velocity. The fully developed flow is ensured by the self-similarity and comparing the experimental data with the literature. The flow properties of the fully developed state are characterized by using uniform momentum zone analysis. Compared to the momentum zones of developing flow, the fully developed flow shows a vertical variability in the quadrant events and higher shear contribution from the sweep events in the outer boundary layer which is caused by the existence of the free surface. In the study of free surface perturbation, the effects of three different floaters are observed at a far downstream measuring station and compared with the fully developed flow. A dip in the mean velocity is noticed adjacent to the free surface and it gets larger with an increase in perturbation. It is also seen that the Reynolds shear stress becomes negative near the free surface and the dominant quadrant events shift from ejections and sweeps to inward and outward interactions due to the inverted shear layer developed from the floaters. The extent of turbulence penetration towards the bed is deeper with the increment in the level of perturbation. These floater boards are commonly used in open channel flow experiments to minimize the free surface fluctuations. However, they are found to have an unintended influence on the flow characteristics and therefore are not used in the study of the wake. The wakes of the solid and foam-covered cylinders are developed computationally and experimentally under a fully developed approach flow. The PIV data are used to validate the computational model which is further used to reveal three-dimensionality in the flow characteristics. The iso-surface of λ2 is used to depict the instantaneous vortical structures such as horseshoe vortex, arch vortex, tip vortex etc. These vortical structures are prominent for the solid cylinder but broken. In case of foam-covered cylinders, the formation of the flow structure is highly influenced by the inner solid cylinder, top plate and the foam covering. Especially, the foam structure interrupts the formation of any large flow structures and suppresses the oscillating behaviour of the wake. Consequently, Reynolds stress generation in the near-wake region of the foam-covered cylinder is found to be much less and no dominant frequency is identified in the FFT analysis. Finally, spectral proper orthogonal decomposition (SPOD) modes are used to visualize the coherent structures around the body. In case of a solid cylinder, the tip vortices move downward and reattaches with the bottom wall within a short distance. However, this downstream reattachment length for the foam-covered cylinder is significantly higher since the flow through the porous foam structures lifts up the tip vortices. A difference in the reattachment pattern can also be seen on the top face of the cylinder, which occurs in a straight line for the solid cylinder but in a curved line for the foam-covered cylinder. The SPOD modes on the mid-horizontal plane depicts an asymmetric generation of side vortices due to the asymmetry in the foam structure

Aeroacoustics research in Europe: The CEAS-ASC report on 2022 highlights, Contribution: Uncertainty quantification for aircraft noise emission simulation: methods and limitations

Review paper about the research highlights in the field of aeroacoustics. Assembled by the CEAS ASC, i.e., editors Christophe Schram and Gareth Bennett. The contribution of the authors presents research activities in the field of uncertainty quantification associated with aircraft system noise prediction

Aeronautical Engineering: a Continuing Bibliography with Indexes (Supplement 243)

This bibliography lists 423 reports, articles, and other documents introduced into the NASA scientific and technical information system in August 1989. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Large eddy simulation of flow over submerged cylinders and leaky barriers

Extreme weather events are increasing their frequency due to climate change, leading to more recurrent destructive flooding incidents over the recent years, which require the development of potential solutions. For this, leaky barriers are a natural-based flood mitigation solution that can reduce and delay peak flow events. Understanding the local hydrodynamics involved in the flow around these mostly-submerged hydraulic structures is essential to enhance their performance in retaining flood events but also to ensure their structural integrity. Numerical methods arise as a complementary tool to experimental approaches that enable a further understanding of the fluid dynamics around submerged cylinders used in these leaky barriers. This thesis adopts a large-eddy simulation (LES) computational approach, incorporating the level-set method (LSM) to capture free-surface deformation. The hydrodynamics around a single cylinder are investigated, finding a critical Froude number threshold when free-surface effects become pronounced and influence the hydrodynamic coefficients, vortex shedding patterns, and wake structures downstream of the cylinder. Proper-orthogonal decomposition (POD) is employed to quantify and analyse energetic coherent structures developed behind the cylinder, revealing redistribution in the energy contribution as flow conditions approach shallower conditions. Furthermore, POD is used to compare flow pattern predictions from two separate LESs of flow past a single horizontal cylinder in very shallow conditions, highlighting the limitations of traditional rigid-lid modelling and emphasising the importance of adopting LSM for accurate free surface and flow dynamics. The hydrodynamics of leaky barriers are simulated and analysed with LES to investigate the impact of barrier’s inclination and length on the flow. Results reveal configurations with flatter inclinations or shorter barrier lengths lead to reduced bed scour risk and improved performance. Two novel methodologies for estimating water depths and velocities around leaky barriers have been proposed and validated using experimental and simulation datasets, providing an easy-to-use design tool for eco-friendly wood structures in future flood management. This thesis seeks to enhance the current understanding of the complex hydrodynamic phenomena developed in the flow around fully-submerged horizontal circular cylinders and leaky barriers, providing essential insights for practical flood management strategies and environmental conservation efforts

Turbulent Flow Analysis and Coherent Structure Identification in Experimental Models with Complex Geometries

Turbulent flows and coherent structures emerging within turbulent flow fields have been extensively studied for the past few decades and a wide variety of experimental and numerical techniques have been developed for measurement and analysis of turbulent flows. The complex nature of turbulence requires methods that can accurately estimate its highly chaotic spatial and temporal behavior. Some of the classical cases of turbulent flows with simpler geometries have been well characterized by means of the existing experimental techniques and numerical models. Nevertheless, since most turbulent fields are of complex geometries; there is an increasing interest in the study of turbulent flows through models with more complicated geometries. In this dissertation, characteristics of turbulent flows through two different facilities with complex geometries are studied applying two different experimental methods. The first study involves the investigation of turbulent impinging jets through a staggered array of rods with or without crossflow. Such flows are crucial in various engineering disciplines. This experiment aimed at modeling the coolant flow behavior and mixing phenomena within the lower plenum of a Very High Temperature Reactor (VHTR). Dynamic Particle Image Velocimetry (PIV) and Matched Index of Refraction (MIR) techniques were applied to acquire the turbulent velocity fields within the model. Some key flow features that may significantly enhance the flow mixing within the test section or actively affect some of the structural components were identified in the velocity fields. The evolution of coherent structures within the flow field is further investigated using a Snapshot Proper Orthogonal Decomposition (POD) technique. Furthermore, a comparative POD method is proposed and successfully implemented for identification of the smaller but highly influential coherent structures which may not be captured in the full-field POD analysis. The second experimental study portrays the coolant flow through the core of an annular pebble bed VHTR. The complex geometry of the core and the highly turbulent nature of the coolant flow passing through the gaps of fuel pebbles make this case quite challenging. In this experiment, a high frequency Hot Wire Anemometry (HWA) system is applied for velocity measurements and investigation of the bypass flow phenomena within the near wall gaps of the core. The velocity profiles within the gaps verify the presence of an area of increased velocity close to the outer reflector wall; however, the characteristics of the coolant flow profile is highly dependent on the gap geometry and to a less extent on the Reynolds number of the flow. The time histories of the velocity are further analyzed using a Power Spectra Density (PSD) technique to acquire information about the energy content and energy transfer between eddies of different sizes at each point within the gaps

An investigation of the wind noise reduction mechanism of porous microphone windscreens

Wind energy is a green way to produce electricity without carbon emissions. However, the infrasound and low frequency audible sound radiated by wind turbines may adversely affect the nearby communities. To investigate the impact of wind farm noise and to understand its noise generation mechanism and propagation, the sound level of wind farm noise must be measured under windy conditions. However, it is often a challenge to measure wind turbine noise under windy conditions in quiet rural residential areas due to wind noise, especially for infrasound and low frequency audible sound. Wind noise is the pseudo sound pressure generated on microphones due to turbulent pressure fluctuations and is indistinguishable from the acoustic signals to be measured. Various microphone windscreens have been utilized to reduce wind noise. However, the physical mechanism of wind noise reduction by windscreens has been unclear to date. The aim of this PhD research is to investigate the mechanisms of wind noise generation and the wind noise reduction mechanism of porous microphone windscreens, and then develop a new compact acoustic measurement system that is insensitive to wind noise. To achieve this objective, a critical literature review is first presented to summarise the state-of-the-art research results in the field of wind noise and its reduction. Then, the research is focused on three aspects: the mechanisms of wind noise generation, the wind noise reduction mechanism of porous microphone windscreens, and wind noise reduction with a compact spherical microphone array. In the first aspect of this thesis, the generation mechanism of wind noise is explored and two theoretical models are proposed to predict wind noise spectra. One model is for outdoor atmospheric turbulence where the Reynolds number based on the Taylor microscale varies from 4250 to 19500, and the other is for indoor fan generated turbulent flows where the Reynolds number based on the Taylor microscale is estimated to be around 432. The proposed theoretical models are validated with existing simulations and experimental results from the literature, as well as measurement results conducted as part of this thesis in a car park for outdoor wind noise and in a laboratory for wind noise from an axial fan. In the second aspect of this thesis, the mechanism of wind noise reduction by porous microphone windscreens is investigated. It is shown that the wind noise reduction of porous microphone windscreens is caused by viscous and inertial forces introduced by the porous structure. Simulation results indicate that the design of porous microphone windscreens should take into account both turbulence suppression inside and wake generation behind the windscreens to achieve optimal performance. Besides, porous windscreens are found to be the most effective in attenuating wind noise in a certain frequency range, where the windscreen diameter is approximately 2 to 4 times the turbulence wavelengths. It is also found that the wind noise reduction is related to the spatial decorrelation of the wind noise signals provided by porous microphone windscreens. The simulation findings are validated with measurement results from an axial fan in a laboratory. In the last aspect of this thesis, a method for wind noise reduction with the spherical microphone array is proposed, and the effect of wind noise on the beamforming performance of a spherical microphone array is investigated. The characteristics of the wind noise is explored and compared with the sound signals in the spherical harmonics domain, based on which a spherical harmonics domain low pass filter method is proposed to reduce wind noise without degrading the desired sound signal. Experimental results demonstrate the feasibility of the proposed method. On the other hand, the effects of wind noise on the beamforming performance of the spherical Plane Wave Decomposition (PWD), Delay and Sum (DAS) and Maximum Variance Distortionless Response (MVDR) beamformers are studied. The experimental results demonstrate that the MVDR beamformer is insensitive to wind noise and able to localise the sound source direction under windy conditions. In summary, two theoretical models are proposed in this PhD research to predict the wind noise spectra in outdoor, large Reynolds number, atmospheric turbulence and indoor, small Reynolds number, turbulent flows, respectively; the physical mechanism of wind noise reduction by porous microphone windscreens is found to be related to the spatial decorrelation effect on the wind noise signal due to the porous structure, and it is demonstrated that the design of porous windscreens should take into account both turbulence suppression inside and wake generation behind the windscreen to achieve optimal performance; the effect of wind noise on the beamforming performance of a spherical microphone array is investigated and a spherical harmonic domain low pass filtering method is proposed to attenuate wind noise without degrading the desired sound signal

Aeronautical engineering. A continuing bibliography with indexes

This bibliography lists 326 reports, articles, and other documents introduced into the NASA scientific and technical information system in January 1982. Topics on aeronautical engineering and aerodynamics such as flight control systems, avionics, computer programs, computational fluid dynamics and composite structures are covered

Aeronautical engineering: A continuing bibliography with indexes (supplement 272)

This bibliography lists 719 reports, articles, and other documents introduced into the NASA scientific and technical information system in November, 1991. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics