Abstract
This research work presents an experimental study's outcomes to reveal the impact of an O-ring on the flow control over a sphere placed in a turbulent boundary layer. The investigation is performed quantitatively and qualitatively using particle image velocimetry (PIV) and dye visualization. The sphere model having a diamater of 42.5 mm is located in a turbulent boundary layer flow over a smooth plate for gap ratios of 0≤G/D≤1.5 at Reynolds number of 5x103 . Flow characteristics, including patterns of instantaneous vorticity, streaklines, time-averaged streamlines, velocity vectors, velocity fluctuations, Reynolds stress correlations, and turbulence kinetic energy (<TKE>), are compared and discussed for a naked sphere and spheres having O-rings. The boundary layer velocity gradient and proximity of the sphere to the flat plate profoundly influence the flow dynamics. At proximity ratios of G/D=0.1 and 0.25, a wall jet is formed between lower side of the sphere and flat plate, and velocity fluctuations increase in regions close to the wall. At G/D=0.25, the jet flow also induces local flow separations on the flat plate. At higher proximity ratios, the velocity gradient of the boundary layer causes asymmetries in the mean flow characteristics and turbulence values in the wake region. It is observed that the O-ring with various placement angles (Θ) on the sphere has a considerable alteration in the flow structure and turbulence statistics on the wake. At lower placement angles, where the O-ring is closer to the forward stagnation point of the sphere, the flow control performance of the O-ring is limited; however, its impact on the flow separation becomes pronounced as it is moved away from the forward stagnation point. At G/D=1.50 for O-ring diameters of 4.7 (2 mm) and 7 (3 mm) percent of the sphere diameter, the -ring exhibits remarkable flow control at Θ=50° and Θ=55° before laminar flow separation occurrence on the sphere surface, respectively. This conclusion is yielded from narrowed wakes and reductions in turbulence statistics compared to the naked sphere model. The O-ring with a diameter of 3 mm and placement angle of 50° exhibits the most effective flow control. It decreases, in sequence, streamwise velocity fluctuations and length of wake recovery region by 45% and 40%, respectively, which can be evaluated as source of decrement in drag force.
Key Words
flow control; O-ring; PIV; sphere; turbulent boundary layer
Abstract
Gust factor is an important parameter for the conversion between peak gust wind and mean wind speed used for the
structural design and wind-related hazard mitigation. The gust factor of typhoon wind is observed to show a significant
dispersion and some differences with large-scale weather systems, e.g., monsoons and extratropical cyclones. In this study, insitu measurement data captured by 13 meteorological towers during a strong typhoon Morakot are collected to investigate the
statistical characteristics, height and wind speed dependency of the gust factor. Onshore off-sea and off-land winds are
comparatively studied, respectively to characterize the underlying terrain effects on the gust factor. The theoretical method of
peak factor based on Gaussian assumption is then introduced to compare the gust factor profiles observed in this study and given
in some building codes and standards. The results show that the probability distributions of gust factor for both off-sea winds and
off-land winds can be well described using the generalized extreme value (GEV) distribution model. Compared with the off-land
winds, the off-sea gust factors are relatively smaller, and the probability distribution is more leptokurtic with longer tails. With
the increase of height, especially for off-sea winds, the probability distributions of gust factor are more peaked and right-tailed.
The scatters of gust factor decrease with the mean wind speed and height. AS/NZ's suggestions are nearly parallel with the
measured gust factor profiles below 80 m, while the fitting curve of off-sea data below 120 m is more similar to AIJ, ASCE and
EU.
Address
Zihang Liu:State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China
Genshen Fang, Lin Zhao, Shuyang Cao and Yaojun Ge:1)State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China
2)Key Laboratory of Transport Industry of Wind Resistant Technology for Bridge Structures, Tongji University, Shanghai 200092, China
Abstract
A comparative study of aeroelastic vibrations of spring-mass cylinder and chimneys, with the help of a few wake
oscillator models available in the literature, is presented. The models include those proposed by Facchinetti, Farshidian and
Dolatabadi method-I, Farshidian and Dolatabadi method-II, de Langre, Skop and Griffin. Besides, the linear model proposed by
Simiu and Scanlan is also incorporated in the study. For chimneys, the first mode oscillation is considered, and the top
displacements of the chimneys are evaluated using the considered models. The results of the analytical model are compared with
those obtained from the numerical solution of the wake–oscillator coupled equations. The response behavior of the cylinder and
three chimneys of different heights are studied and compared with respect to critical parametric variations. The results of the
study indicate that the numerical analysis is essential to capture the effect of non-linear aeroelastic phenomena in the solutions,
especially for small damping. Further, except for the models proposed by Farshidian and Dolatabadi, other models predict nearly
the same responses. The non-linear model predicts a much higher response as compared to the linear model.
Key Words
numerical analysis; non-linear analysis; Van der Pol oscillator; wake-oscillator models
Address
Saba Rahman:Department of Civil Engineering, Indian Institute of Technology (IIT), Hauz Khas 110016, New Delhi, India
Arvind K. Jain:Department of Civil Engineering, Indian Institute of Technology (IIT), Hauz Khas 110016, New Delhi, India
S.D. Bharti:Department of Civil Engineering, Malaviya National Institute of Technology (MNIT), Malaviya Nagar 302017, Jaipur Rajasthan, India
T.K. Datta:Department of Civil Engineering, Malaviya National Institute of Technology (MNIT), Malaviya Nagar 302017, Jaipur Rajasthan, India
Abstract
Tall buildings are often subjected to steady and unsteady forces due to external wind flows. Measurement and
mitigation of these forces becomes critical to structural design in engineering applications. Over the last few decades, many
approaches such as modification of the external geometry of structures have been investigated to mitigate wind-induced load.
One such proven geometric modification involved the rounding of sharp corners. In this work, we systematically analyze the
impact of rounded corner radii on the reducing the flow-induced loading on a square cylinder. We perform 3-Dimensional (3D)
simulations for high Reynolds number flows (Re=1X=105) which are more likely to be encountered in practical applications. An
Improved Delayed Detached Eddy Simulation (IDDES) method capable of capturing flow accurately at large Reynolds numbers
is employed in this study. The IDDES formulation uses a k-ωShear Stress Transport (SST) model for near-wall modelling that
prevents mesh-induced separation of the boundary layer. The effects of these corner modifications are analyzed in terms of the
resulting variations in the mean and fluctuating components of the aerodynamic forces compared to a square cylinder with no
geometric changes. Plots of the angular distribution of the mean and fluctuating coefficient of pressure along the square
cylinder's surface illustrate the effects of corner modifications on the different parts of the cylinder. The windward corner
Key Words
long-span suspension footbridge; galloping instability; web opening; Den Hartog criterion; damping
Address
Nivedan Vishwanath, Aditya K. Saravanakumar, Kush Dwivedi, Kalluri R.C. Murthy, Pardha S. Gurugubelli, Sabareesh G. Rajasekharan:Department of Mechanical Engineering, BITS Pilani Hyderabad Campus, Hyderabad, India 500078
Abstract
While the aerodynamics of solid bluff bodies is reasonably well-understood and methodologies for their reliable
numerical simulation are available, the aerodynamics of porous bluff bodies formed by assembling perforated plates has
received less attention. The topic is nevertheless of great technical interest, due to their ubiquitous presence in applications
(fences, windbreaks and double skin facades to name a few). This work follows previous investigations by the authors, aimed at
verifying the consistency of numerical simulations based on the explicit modelling of the perforated plates geometry and their
representation by means of pressure-jumps. In this work we further expand such investigations and, contextually, we provide
insight into the flow arrangement and its sensitivity to important modelling and setup configurations. To this purpose, Unsteady
Reynolds-Averaged Navier-Stokes (URANS) and Large-Eddy Simulations (LES) are performed for a 5:1 rectangular cylinder at
null angle of attack. Then, using URANS, porosity and attack angle are simultaneously varied. To the authors' knowledge this is
the first time in which LES are used to model a porous bluff body and compare results obtained using the explicit modelling
approach to those obtained relying on pressure-jumps. Despite the flow organization often shows noticeable differences, good
agreement is found between the two modelling strategies in terms of drag force.
