Time-resolved sphere and fluid motions in turbulent boundary layers

This paper extends the study by Tee et al. (2020) to investigate the effect of large coherent structures on motion of spheres with specific gravities of 1.006 (P1) and 1.152 (P3) at Reτ = 670 and 1300 (d+ = 56 and 116). The sphere and fluid motions are tracked simultaneously via 3D particle tracking and stereoscopic particle image velocimetry over the streamwise-spanwise plane, respectively. With sufficient mean shear, sphere P1 lifts off of the wall upon release before descending back towards the wall at both Reτ. It typically accelerates strongly over a streamwise distance of less than one boundary layer thickness before approaching an approximate terminal velocity. By contrast, the denser sphere P3 does not lift off upon release but mainly slides along the wall. At lower Reτ where wall friction is stronger, this sphere translates with unsteady velocity, significantly lagging the local fluid. The streamwise velocities of both spheres correlate strongly with the fast- and slow-moving zones that approach and move over them. In most runs, both spheres lag the local coherent structures and travel with either fast- or slow-moving zones throughout the observed trajectories. Vortex shedding, which is most prevalent for sphere P3 at Reτ = 670, is also important. The sphere spanwise motion is prompted by wall friction, spanwise fluid motion, and/or meandering of the coherent structures, and spheres do not appear to migrate preferentially into slow-moving zones.

Effect of Gust Wind on Flow over a Wall-Mounted Fence

In this work, the characteristics of incoming and wake flows downstream of wall-mounted fences under wind gust were explored with wind tunnel experiments. A time-resolved particle image velocimetry was used to capture the flow dynamics across two different fence heights. The results show that during the gust period, the wake presents distinct meandering and strong flow mixing. The Probability Density Function distribution of flow velocities indicates that the mixing effect increases with the streamwise distances. Specifically, for locations above the fence top tip, the growth of streamwise distance decreases the footprint of wind gust. However, for locations lower than the fence top tip, the local wind flows exhibit stronger variations before and after wind gust with the growth of downstream distance. Overall, at the same relative streamwise and spanwise locations downstream of fences within the wake region, the higher fence better suppresses the influence of gust wind.

Wind turbine wake models' evaluation for different downstream locations

The land use limitations, especially for onshore applications, have led modern Wind Turbines (WTs) to be aggregated in wind parks under the scope of minimizing the necessary area required. Within this framework, the trustworthy prediction of the wind speed deficiency downstream the WTs' hub (known also as the “wake effect”) and the meticulous wind park micrositing are of uppermost importance for the optimized WTs siting across the available land area. In this context, substantial effort has been made by the academic and research community, contributing to the deployment of several analytical, numerical and semi-empirical wake models, attempting to estimate the wind speed values at different locations downstream a WT. The accuracy of several semi-empirical and analytical wake models, serving also as the basis for pertinent commercial software development, is investigated in the present work, by comparing their outcome with experimental data from a past research work that concerns the wake flow. The dimensionless streamwise distance (known also with the term “downstream distance”) from the WT's hub is used as benchmark in order to categorize and evaluate the calculation results. A dedicated comparison between the wind speed cases investigated is conducted, striving to properly assess the wake models' prediction accuracy. The notable findings obtained for the wake models examined designate the requirement for subsequent research to enlighten the wake effect dynamic behavior.

Experimental investigation of energy harvesting eel in the wake of bluff body under ocean waves

Investigation of the energy harvesting from deep water waves by using flexible piezoelectric eel in a controlled environment is studied. Energy harvesting potential is examined as a function of streamwise distance from the fixed cylinder and spanwise gap along with the cylinder at different wave conditions. Output voltage and eel flapping behavior are dependent on cylinder vortices caused by local wavelength and wave amplitude. Maximum energy is harvested when the eel is placed near to the surface caused by high flapping amplitude and frequency. Similarly, at greater depth low flapping amplitude is observed resulting in small output voltage. Maximum output voltages are found at the shorter wavelength and at a streamwise distance of gx = 1.25 (where gx is the ratio of spacing “S” between cylinder and eel to the diameter of cylinder “D”) for all spanwise gaps along with the cylinder and minimum voltages are calculated at a longer wavelength and streamwise distance gx = 0.75. An increase of 65% in energy harvesting is observed by switching longer wavelengths (λ) to a shorter one and changing the piezo-eel spanwise gap from deep to the shallow depth. Whereas, an increase of 31.5% was found by keeping wavelength constant and changing the spanwise gap of eel. Furthermore, it is observed that energy harvesting from the wake of a bluff body in the wavy motion of water is sensitive to the wavelength and wave height.

CFD based analysis of flow-driven vertical axis hydrokinetic turbines: blade hydrofoil and interactions effects

In last years, interest in hydrokinetic energy conversion (HEC) technology has grown. However, HEC technology requires advancements to become successful for practical in-situ conditions. In this study we investigated the Darrieus type vertical axis hydrokinetic turbines (HKT). Simulations are carried out using commercial CFD package to study the hydrodynamic behavior of a three bladed turbine. Two HKT with same solidity equal to 0.17 were considered, a 1st with symmetric-NACA0018 blade hydrofoils and a 2nd with cambered-NACA4415 blades. Our simulations show a shift in instantaneous torque coefficients for two different tip-speed ratio (TSR) values. For TSR = 1, the 2nd HKT has a delay compared to the 1st HKT, while for TSR = 2 it has a lead. NACA0018 provides highest troque coefficients. For a hydrofarm use, it is important to consider the interactions between hydrokinetic turbines. We investigated the effect of streamwise distance on performance of a Darrieus turbine. A distance of about 11 times the diameter of HKT along the streamwise direction seems essential.

A Formulation of the Thrust Coefficient for Representing Finite-Sized Farms of Tidal Energy Converters

Tidal energy converter (TEC) arrays in tidal channels generate complex flow phenomena due to interactions with the local environment and among devices. Models with different resolutions are thus employed to study flows past TEC farms, which consider multiple spatial and temporal scales. Simulations over tidal cycles use mesoscale ocean circulation models, incorporating a thrust coefficient to model the momentum sink that represents the effects of the array. In this work, we propose an expression for a thrust coefficient to represent finite-sized farms of TEC turbines at larger scales, C t F a r m , which depends on the spatial organization of the devices. We use a coherent-structure resolving turbulence model coupled with the actuator disk approach to simulate staggered turbine configurations in more detail, varying the separation among devices and the ratios between the channel depths and hub heights. Based on these simulations, we calculate the resultant force for various subsets of devices within the farm, and their corresponding effective thrust coefficient, C t F a r m . We conclude that the thrust coefficient depends solely on the lateral separation of the devices, S y , for farms with only two rows. For farms with more than two rows, the streamwise distance, S x , must be considered as well. With the proposed expression, it is possible to calculate efficiently the effects of finite-sized TEC farms and incorporate a momentum sink into ocean circulation models, without assuming a constant coefficient derived from an infinite farm approximation.

Numerical Analysis of Thermofluid Performance of Fin-and-Tube Heat Transfer Surface Using Rectangular Winglets

This paper numerically investigates the heat transfer enhancement using rectangular winglet pairs in a fin-and-tube type heat transfer surface having five inline rows of tubes. The influence of number of winglets, attack angles of the winglets, and their location has been analyzed under laminar flow conditions with Reynolds number ranging 400–1500. To account for the combined effect of heat transfer enhancement and pressure drop penalty, an enhancement factor is also discussed by changing the winglet pair's number and location. The numerical results show that pressure drop can be reduced significantly by placing the winglet more toward the exit of the flow channel. Streamwise distance and spanwise distance of the winglet pairs have been investigated for maximum enhancement factor. The numerically obtained results show that the winglets number and their placement at different locations have a major influence on enhancement factor. The results show that both the heat transfer and the pressure drop increase with an increase in attack angle of the winglets and best angle for the highest enhancement factor has been found out. Correlations have been developed for streamwise distance, spanwise distance, and angle of attack for different range of Reynolds numbers.

The turbulent Kármán vortex

This work focuses on the temperature (passive scalar) and velocity characteristics within a turbulent Kármán vortex using a phase-averaging technique. The vortices are generated by a circular cylinder, and the three components of the fluctuating velocity and vorticity vectors, $u_{i}$ and $\unicode[STIX]{x1D714}_{i}$ ($i=1,2,3$), are simultaneously measured, along with the fluctuating temperature $\unicode[STIX]{x1D703}$ and the temperature gradient vector, at nominally the same spatial point in the plane of mean shear at $x/d=10$, where $x$ is the streamwise distance from the cylinder axis and $d$ is the cylinder diameter. We believe this is the first time the properties of fluctuating velocity, temperature, vorticity and temperature gradient vectors have been explored simultaneously within the Kármán vortex in detail. The Reynolds number based on $d$ and the free-stream velocity is $2.5\times 10^{3}$. The phase-averaged distributions of $\unicode[STIX]{x1D703}$ and $u_{i}$ follow closely the Gaussian distribution for $r/d\leqslant 0.2$ ($r$ is the distance from the vortex centre), but not for $r/d>0.2$. The collapse of the distributions of the mean-square streamwise derivative of the velocity fluctuations within the Kármán vortex implies that the velocity field within the vortex tends to be more locally isotropic than the flow field outside the vortex. A possible physical explanation is that the large and small scales of velocity and temperature fields are statistically independent of each other near the Kármán vortex centre, but interact vigorously outside the vortex, especially in the saddle region, due to the action of coherent strain rate.

Effects of Streamwise Distance and Density Ratio on Film-Cooling Effectiveness for Double-Jet Film-Cooling on a Flat Plate

Several double-jet film-cooling (DJFC) hole geometries on a flat plate are investigated in this paper. Pressure sensitive paint (PSP) is used to measure the film-cooling effectiveness on the flat plate. The streamwise distance between the DJFC holes varies as s/d = 3.0, 4.0, 5.0, and 6.0, while three different spanwise distance conditions are considered (p/d = 0, 0.5, and 1.0). The diameter (d) of the DJFC holes is 7 mm, and the pitch (P) between each set of the DJFC holes is 8d. The holes have an inclination angle (θ) of 35°, and a compound angle (β) of ± 45°. The density ratio (DR) varies as 1.0, 1.5, and 2.5, and the blowing ratio (M) is 0.5, 1.0, 1.5, and 2.0. Both effects of the streamwise distance between DJFC holes, and the density ratio on film-cooling effectiveness are focused. Results show, though the effect of the streamwise distance is not monotonic all the time, a general trend exists when comparing with the baseline case of s/d = 3.0. For p/d = 0, increasing the streamwise distance leads to wider lateral coverage and higher laterally averaged effectiveness; however, for p/d = 0.5 and 1.0, an increased s/d may weaken the interaction between the two jets, thus the film coverage and effectiveness decrease. A higher density ratio reduces the jet momentum, therefore better attachment and higher effectiveness are obtained. Effects of the streamwise distance and the density ratio on double-jet film-cooling are more distinct when the blowing ratio rises. However, in most cases, both of them show minor influence on the range of lateral coverage, as it is mainly dominated by the spanwise distance. For each operation condition (DR and M), an optimal DJFC configuration (among the geometries in this study) is identified by comparing the area averaged effectiveness.