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An experimental flow facility for sustaining a column of nearly isotropic turbulence in air has been developed. Four arrays of synthetic jet actuators are arranged along the walls of a vertical tower, producing a homogeneous region spanning 13 integral length scales in the vertical direction. The isotropy, homogeneity, and various turbulence properties are quantified using optical measurement techniques. At the highest jet power setting, a root-mean-square velocity of 0.73 m/s and a Reynolds number R e λ of 240 are attained. Although the mean flows are not negligible, their contribution to the turbulence kinetic energy production is insignificant, given the small magnitude of the measured Reynolds stress. The flow exhibits a high degree of isotropy when comparing the longitudinal autocorrelation functions, although some anisotropy is observed in the large scales according to the rms velocity ratio, which ranges from 0.90 to 1.33 over the measured regions. Despite these deviations from ideal isotropic turbulence, the experimental design is well-suited for the study of inertial particle agglomeration during gravitational settling. [ABSTRACT FROM AUTHOR]

A series of particle image velocimetry measurements was conducted in a towing tank to analyze the propeller wake dynamics of a fully appended very large crude oil carrier model. The Froude number and the Reynolds number of the test condition based on the model length and towing speed were 0.142 and 2.32 × 106, respectively. The propeller wake dynamics was investigated in terms of vortical structure and wake acceleration. It was found that the propeller loading on the starboard side was larger than that on the port side, as upward component of the propeller inflow resulted in a higher inflow speed on that side. The different propeller loading deformed the wake structure and bent it towards the port side. Between neighboring tip vortices, the local maxima of the turbulence kinetic energy and corresponding local turbulence structure were found. The rudder effects on the propeller wake were also investigated, proving that the rudder blocked the progress of the hub vortex and straightened the propeller wake. [ABSTRACT FROM AUTHOR]

By direct numerical simulations of the plane Couette flow (PCF) in a large computational domain, it is shown that an isolated turbulent band decays monotonically at low Reynolds numbers but experiences transient growth before the eventual relaminarization at moderate Reynolds numbers. The lower bound Reynolds number of the transient-growth regime is determined as 286. The width, length, and tilt angle of the isolated band structure are defined based on the disturbance kinetic energy in the mid-plane, and the geometric characteristics of the band can be described with a tilted rectangle. It is illustrated that before its eventual fragmentation, the isolated turbulent band decays in a style of longitudinal contraction, where the center, width, and tilt angle of the band keep almost constant but the band length contracts with a statistically constant velocity. [ABSTRACT FROM AUTHOR]

In this paper, semi 3D models for segmental Bridge decks are created in ABAQUS CFD program with the support of MATLAB codes to simulate and analyze vortex shedding generated due to wind excitation through considering the stationary position of the deck. Three parameters (wind speed, deck streamlined length and dynamic viscosity of the air) are dedicated to study their effects on the kinetic energy of the system in addition to the shapes and patterns of the vortices. Two benchmarks from the literature Von Karman and Dyrbye and Hansen are considered to validate the vortex shedding aspects for the CFD models. Good agreement between the results of the benchmarks and the semi 3D models has been detected. Latin hypercube experimental method is dedicated to generate the surrogate models for the kinetic energy of the system and the lift forces. Variance based sensitivity analysis is utilized to calculate the main sensitivity indices and the interaction orders for all the three parameters. The kinetic energy approach performed very well in revealing the rational effects and the roles of each parameter in the generation of vortex shedding and predicting vortex induced vibration of the deck. [ABSTRACT FROM AUTHOR]

The turbulent properties of the fiber suspension in a turbulent round jet are numerically simulated and visualized, and some of the results are compared with the experimental data. The effects of the Reynolds number, fiber volume fraction, and aspect ratio are analyzed. The results show that the fiber injection in the flow has a delay effect on the streamwise velocity decay along the jet axis, and such an effect becomes more obvious with the increases in the fiber volume fraction and aspect ratio and the decrease in the Reynolds number. The flow with fibers shows an increase in the streamwise velocity along the radial direction, and the increase magnitude is directly proportional to the fiber volume fraction and aspect ratio and inversely proportional to the Reynolds number. The presence of fibers makes the turbulent kinetic energy and Reynolds stress increase, and the extent increases with the fiber volume fraction, Reynolds number, and fiber aspect ratio. [ABSTRACT FROM AUTHOR]

We performed a series of experiments with water drops which impacted on the surface of a water pool with velocities up to 3.4 m/s and produced an ejecta cone. The use of a high-speed camera allowed us to clarify some features of the ejecta process that we observed in our previous high-velocity impact experiments with aluminum projectiles and targets (Shumikhin et al. in Int J Impact Eng 50:90-98, 2012). The experiments with water were motivated not only by the high cost of high-velocity impact experiments and the difficulty of registering in-situ in such experiments, but also by the similarity of the ejecta processes in water and aluminum. The modeling was based on the number of Weber (We) and Reynolds (Re). The critical number We for the splashing threshold means that there is a criterion for ejecta that connects the kinetic energy of the projectile with the fracture energy of the projectile and target. Experiments with water allowed us to describe the mechanism of decrease of the number of ejecta spikes and thus to explain the presence of V-shaped chains of small craters that we observed in the high-velocity impact experiments with aluminum projectiles and targets. [ABSTRACT FROM AUTHOR]

Large-eddy simulation is used to investigate very-large-scale motions (VLSMs) in the neutrally stratified atmospheric boundary layer at a very high friction Reynolds number, $$Re{_{\tau }} \sim \mathcal {O}(10^{8})$$ . The vertical height of the computational domain is $$L_{z}=1000$$ m, which corresponds to the thickness of the boundary layer. In order to make sure that the largest flow structures are properly resolved, the horizontal domain size is chosen to be $$L_{x}=32\pi L_{z}$$ and $$L_{y}=4\pi L_{z}$$ , which is much larger than the standard domain size, especially in the streamwise direction (i.e., the direction of elongation of the flow structures). It is shown that the contributions to the resolved turbulent kinetic energy and the resolved shear stress from streamwise wavelengths larger than $$10 L_{z}$$ are up to 27 and 31 % respectively. Therefore, the large computational domain adopted here is essential for the purpose of investigating VLSMs. The spatially coherent structures associated with VLSMs are characterized through flow visualization and statistical analysis. The instantaneous velocity fields in horizontal planes give evidence of streamwise-elongated flow structures of low-speed fluid with negative fluctuation of the streamwise velocity component, and which are flanked on either side by similarly elongated high-speed structures. The pre-multiplied power spectra and two-point correlations indicate that the scales of these streak-like structures are very large, up to $$20L_{z}$$ in the streamwise direction and $$0.6 L_{z}$$ in the spanwise direction. These features are similar to those found in the logarithmic and outer regions of laboratory-scale boundary layers by direct numerical simulation and experiments conducted at low to moderate Reynolds numbers. The three-dimensional correlation map and conditional average of the three components of velocity further indicate that the low-speed and high-speed regions possess the same elongated ellipsoid-like structure, which is inclined upward along the streamwise direction, and they are accompanied by counter-rotating roll modes in the cross-section perpendicular to the streamwise direction. These results are in agreement with recent observations in the atmospheric surface layer. [ABSTRACT FROM AUTHOR]

This study focuses on the internal jet interactions and the oscillation mechanism of the feedback-free fluidic oscillator at low flow rate, corresponding to a Reynolds number of 1,350 (based on exit nozzle width and average exit velocity). Particle image velocimetry (PIV) was used in this study with a refractive index-matched fluid to minimize reflections that would otherwise occur at the fluid-acrylic interface in the test setup. A simple microphone-tube sensor configuration generated a reference signal, with a phase-averaging method based on each quarter period for velocity time history reconstruction. PIV results revealed the existence of a vortex of fluctuating size, shape, and strength on each side of the oscillator; and two transient vortices that are formed in the dome region of the oscillator by each of the jets once per period. The dome vortices periodically bifurcate each of the jets and transfer some of the kinetic energy of that jet to the opposing jet. This kinetic energy transfer mechanism dictates the dominance of either jet at the exit, and this mechanism repeats itself to sustain the oscillations created by the fluidic oscillator. At this flow rate, the two jets form a continuous mutual collision, and the jets are never completely cut off from the exit. The oscillatory behavior at this flow rate is due to a complex combination of jet interactions and bifurcations, vortex-shear layer interactions, vortex-wall interactions, and saddle point formations. [ABSTRACT FROM AUTHOR]

The properties of low-vorticity pockets within the turbulent region (named as bubbles) and high-vorticity pockets within the potential flow region (named as drops) in the vicinity of turbulent/non-turbulent (T/NT) interface in turbulent boundary layers are first experimentally investigated using two-dimensional time-resolved particle image velocimetry. The T/NT interface is detected with a turbulent kinetic energy (TKE) criterion. The bubbles and drops which enlarge the T/NT interface area will promote the entrainment process. A detailed study revealed that the T/NT interface above bubbles is generally higher than the mean interface position, whereas the T/NT interface below drops is lower. Due to different sizes of bubbles and drops, though the temporal probability of bubbles is much larger than drops, the spatial probability is similar. Moreover, the conditional mean velocity distribution and the convection speed of the bubbles and drops are first obtained, especially in experiments. The streamwise velocity of fluids in bubbles at different Reynolds number is larger than surrounding turbulent flow, while in drops, the streamwise velocity is smaller than surrounding free-stream. The conditional mean streamwise velocity difference can be well scaled with respective friction velocity. Besides, bubbles and drops will convect downstream with the velocity of surrounding fluids. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to examine the formation of secondary circulation in a grid-mixing box and to determine its effect on turbulence. This apparatus has been used extensively to study turbulence and mixing in a variety of geophysical contexts, and it is commonly assumed that turbulence is nearly isotropic and horizontally homogenous and that it is a zero-mean shear flow. Exceptions to these assumptions, however, have been reported, where a secondary flow pattern has been observed consisting of two roughly symmetric large-scale circulations with upward flow in the center of the box and downward return flow along the sides. These secondary flows appear to be associated with different grid oscillation conditions and box and grid geometries, and criteria have been proposed to describe conditions when secondary flow may be expected. Experiments were conducted for three different combinations of the grid oscillation stroke and frequency, while maintaining a near constant grid Reynolds number, to examine the formation and strength of the secondary flow and its effect on the magnitude and distribution of turbulence within the box. Velocity characteristics were obtained by particle image velocimetry (PIV). Results show that (1) secondary circulations were present for all combinations of the grid oscillation conditions; (2) as stroke length increased, the intensity of the secondary circulation and the contribution of these motions to total kinetic energy increased; and (3) the presence of secondary circulation results in greater overall mixing and turbulent transport in the region close to the grid. These insights are expected to be relevant to a wide range of mixing box applications. [ABSTRACT FROM AUTHOR]