Freestream Flow Speed Research Articles

Effect of overlapping and space between stages of a three-bladed double-stage Savonius hydrokinetic turbine for low flow speed perennial river application

ABSTRACT Savonius-like hydrokinetic turbine (SAHT) in low-speed flow streams of perennial rivers is not much explored. In this paper, a triple-bladed double-stage SAHT is designed, and its performance is investigated for low flow speeds within 0.45–0.65 m/s under the effect of blade overlapping (within 10–20% in each stage), turbine diameter (within 208–234 mm), and without and with space between the stages (5.0 mm) at different operating speed ratios. Different combinations of design and operating input parameters are studied to obtain a reliable assessment of SAHT performance. After analyzing the results, the best conditions of input parameters and output performance parameters are reported in their nondimensional forms for future use of the obtained results. The torque produced by the turbine increased with the applied brake load, leading to its maximum value at the highest brake load 0.83 times the maximum load. The highest coefficient of performance is obtained as 0.086 at a space 0.25 times the maximum space (i.e. 20 mm), overlapping 15%, brake load 0.83 times the maximum load, tip speed ratio 0.276. And for this, the corresponding power 0.85 times the maximum power (i.e. 0.201 W) at free-stream flow speed of 0.85 times the maximum flow speed (i.e. 0.65 m/s) is obtained. The present turbine with space and overlapping exhibits better performance than some available SAHT designs of single/double stage, two-blade/multi-blade with/without overlap configurations. The novelty of the work is that an improved SAHT has been developed for low-flow speed perennial river applications in the band of low tip speed ratios by combining design conditions like double-stage, space between stages, and overlapping.

Design and performance investigation of a triple blade dual stage Savonius-alike hydrokinetic turbine from low flow stream reserves

ABSTRACT A trending technology that is being employed to generate hydro energy from low-flow stream reserves is the Savonius-alike hydrokinetic turbine (SAHT). Clearance between the stages of a dual-stage two-bladed SAHT was found to improve its performance at low flow speeds; however, the impact of clearance on the triple-bladed configuration of SAHT was not studied earlier. In this paper, a triple blade dual stage configuration of SAHT is designed, and its performance is investigated in a water flume under various stage clearances (0,5,10,15 and 20 mm), low flow speeds (0.45,0.55 and 0.65 m/s) and different brake loads. Detailed turbine performance under different design and off-design conditions are investigated to obtain meaningful performance insights. The findings show that torque production by the turbine increases with the increase of brake load, with maximum hydrodynamic torque generated at the highest brake load. The highest coefficient of performance and torque of 0.071 and 0.261 are obtained at a stage clearance of 5 mm, tip speed ratio of 0.273, and free-stream flow speed of 0.55 m/s. The present SAHT under design condition has improved performance compared to a dual blade dual stage SAHT exhibiting a wider tip speed ratio range for its application in low flow stream reserves. Further, this turbine may also be recommended for torque generation to work as a motor in a flow speed condition less than 0.5 m/s. The novelty of this work is the application of an additional flow control measure in the form of flow-through clearance to negotiate vertical water thrust through the clearance and exert additional pressure on the advancing blades of the SAHT in the upper stage.

Corroboration of the Toms effect from a frictional drag reducing self-polishing copolymer

A novel frictional drag reducing self-polishing copolymer (FDR-SPC) was first developed by the authors. The FDR-SPC is a special derivative of an SPC that was designed to achieve skin frictional drag reduction in turbulent water flow by releasing polyethylene glycol (PEG) into water through a hydrolysis reaction. Thus, the FDR-SPC coating acts as a continuous medium accommodating countless, molecular-level polymer injectors. However, direct evidence of such PEG release has not yet been demonstrated. Here, we report the results of in situ PEG concentration measurement based on the planar laser-induced fluorescence (PLIF) method. Polyethylene glycol methacrylate (PEGMA) was probed by the fluorescent functional material dansyl, and the fluorescence intensity from dansyl-PEG was then measured to quantify the concentration in the flow. The near-wall concentration of dansyl-PEG is observed to range from 1 to 2 ppm depending on the flow speed, which corroborates the existence of a drag reducing function for the FDR-SPC. In the concurrent measurement of skin friction, the present FDR-SPC specimen exhibited a skin friction reduction ratio of 9.49% at the freestream flow speed U=5m/s. In the comparative experiment of dansyl-PEGMA solution injection, the skin friction was found to decrease by 11.9%, which is in reasonable accordance with that for the FDR-SPC.

Iterative Maneuver Optimization in a Transverse Gust Encounter

This paper presents a framework based on either iterative simulation or iterative experimentation for constructing an optimal, open-loop maneuver to regulate the aerodynamic force on a wing in the presence of a known flow disturbance. The authors refer to the method as iterative maneuver optimization and apply it in this paper to regulate lift on a pitching wing during a transverse gust encounter. A candidate maneuver is created by performing an optimal control calculation on a surrogate model of the wing–gust interaction. Execution of the proposed maneuver in a high-fidelity simulation or experiment provides an error signal based on the difference between the force predicted by the surrogate model and the measured force. The error signal provides an update to the reference signal used by the surrogate model for tracking. A new candidate maneuver is calculated such that the surrogate model tracks the reference force signal, and the process repeats until the maneuver adequately regulates the force. The framework for iterative maneuver optimization is tested on a discrete vortex model as well as in experiments in a water towing tank. Experimental results show that the proposed framework generates a maneuver that reduces the magnitude of lift overshoot by 92% for a trapezoidal gust with peak velocity equal to approximately 0.7 times the freestream flow speed.

Hydrodynamics study on a traveling wave-based undulating surface of a hydrofoil in a free-stream flow

We numerically show that traveling wave-based surface-undulations on a hydrofoil generates thrust. Simulations demonstrate that the pressure suction mechanism generates thrust beyond a threshold ratio of the wave to free stream flow speed. This mechanism is reminiscent of thrust generation in whole body undulation, as seen in prior studies. Within the examined parameter space, two distinct vortex patterns are revealed: almost steady vortex-sheet and periodic vortex-street. Both forward and reverse types of vortex-sheet and vortex-street are found behind the hydrofoil.

Experimental Investigation of Tractor and Pusher Hexacopter Performance

Multicopters are increasingly becoming popular for hobby and commercial applications. A variety of quadcopter, hexacopter, and octocopter configurations have emerged, yet few publications carefully investigate their performance. This paper contributes results from a series of dynamometer, wind-tunnel, and autonomous flight tests over different propeller configurations, flight speeds, and angles of attack. A modular small hexacopter platform providing propulsion unit redundancy is tested in the standard tractor configuration and a less common pusher configuration. Static thrust as a function of propeller, motor revolutions per minute, and motor current was determined from dynamometer tests. The hexacopter was mounted on a load cell test stand, and data were collected in the University of Michigan’s wind tunnel over different freestream flow speeds, motor thrust percentages, and hexacopter angles of attack. To validate wind-tunnel performance findings, outdoor autonomous flight tests were conducted. Results show the pusher configuration generates approximately 15% more thrust (lift) than do tractor propellers; however, they exhibit a relatively poor lift-to-drag ratio. These results suggests that a pusher configuration hexacopter will have higher efficiency for local-area surveillance applications requiring hover and slow flight, whereas a tractor configuration is more efficient for payload transport applications that require forward flight at appreciable velocities.

Integration, Calibration, and Experimental Verification of a Speed Sensor for Swimming Animals

Animal-attached GPS loggers are used to monitor the movement of land animals, but obtaining the measurements of aquatic animals remains challenging. Current animal-attached speed sensors are placed close to the body and measure the speed of the disturbed fluid near the skin interface, potentially affecting the accuracy of the measurement. Furthermore, the absolute speed estimates derived from the sensor data are affected by the location on the animal, orientation with respect to flow, and device shape. Here, we evaluate the performance of a micro-turbine in both the steady and variable flows using particle image velocimetry to visualize and measure the flow field around the tag and sensor. A closed recirculating flume was used to generate a range of both steady and oscillating flows at the sensor. During steady flow, turbine rotation rate was linearly correlated with both the free stream and near-sensor flow speeds. Following the controlled measurements of fluid speed in the flume, a tag with the speed sensor was used to measure both the speed and the total distance traveled by the dolphins in a managed environment. The results compared well with the speed and distance made from the analysis of video collected by an overhead camera system. Finally, the measurement variability of four specially designed tags with speed sensors was tested in the towing tank in the Marine Hydrodynamics Laboratory at the University of Michigan. These results provide a foundation for interpreting in-situ speed measurements from swimming animals, and potentially autonomous underwater vehicles, and will guide the development of the improved algorithms for the localization and energetics estimation.

The Atmospheric Boundary Layer Over Urban-Like Terrain: Influence of the Plan Density on Roughness Sublayer Dynamics

We investigate the effect of the packing density of cubical roughness elements on the characteristics of both the roughness sublayer and the overlying turbulent boundary layer, in the context of atmospheric flow over urban areas. This is based on detailed wind-tunnel hot-wire measurements of the streamwise velocity component with three wall-roughness configurations and two freestream flow speeds. The packing densities are chosen so as to obtain the three near-wall flow regimes observed in urban canopy flows, namely isolated-wake, wake-interference and skimming-flow regimes. Investigation of the wall-normal profiles of the one-point statistics up to third order demonstrates the impossibility of finding a unique set of parameters enabling the collapse of all configurations, except for the mean streamwise velocity component. However, spectral analysis of the streamwise velocity component provides insightful information. Using the temporal frequency corresponding to the peak in the pre-multiplied energy spectrum as an indicator of the most energetic flow structures at each wall-normal location, it is shown that three main regions exist, in which different scaling applies. Finally, scale decomposition reveals that the flow in the roughness sublayer results from a large-scale intrinsic component of the boundary layer combined with canopy-induced dynamics. Their relative importance plays a key role in the energy distribution and influences the near-canopy flow regime and its dynamics, therefore suggesting complex interactions between the near-wall scales and those from the overlying boundary layer.

Measurement of unsteady flow structures in a low-speed wind tunnel using continuous wave laser-based TR-PIV: near wake behind a circular cylinder

The unsteady measurement of spatiotemporally varying flow structures in a low-speed wind tunnel using a continuous wave (CW) laser-based time-resolved particle image velocimetry (TR-PIV) setup was extensively evaluated in the near wake behind a circular cylinder. A CW laser with a maximum power of 25 W in combination with a high-speed camera operating at 7 kHz was used to determine the wake flows at two different free-stream flow speeds: U 0 = 5 and 10 m/s. Three different camera exposure times were selected for comparison: τ = 20, 50, and 80 μs. In the experiments, the low-repetition conventional PIV setup using the high-power pulsed laser (τ = 8 ns, 135 mJ/pulse) was used to determine the time-mean and statistical flow quantities, which served as the reference for determining the deviation in the TR-PIV measurements. At the lower flow speed of U 0 = 5 m/s, the time-mean-separated flow patterns and the streamwise velocity profiles in all of the TR-PIV systems showed satisfactory agreement with the conventional PIV measurements, along with accurate capture of the large-scale Karman vortex and its harmonic behaviors. At the higher flow speed of U 0 = 10 m/s, the measurement at τ = 50 μs gave a relatively accurate representation of the statistical flow quantities. At the longest exposure time of τ = 80 μs, considerable deviations in the time-mean streamwise fluctuation intensity and the TKE (turbulence kinetic energy) were observed due to the streaky particle image. The strong swirling motion of the large-scale vortical structures increased the deviation in the TR-PIV measurements, which increased with the increasing camera exposure time. Further POD analysis demonstrated that the leading energetic modes in the system with τ = 50 μs accurately determined the spatial features of the Karman-like vortex and its harmonic events. However, inaccurate vector representation of the second harmonic events was observed in the system with τ = 80 μs. Finally, for both flow speeds, the lower-order reconstructed phase-dependent representations of the Karman-like vortex and its harmonic behaviors were composed of the time-series velocity vector fields determined using the system with τ = 50 μs, thus providing a straightforward quantitative view of the coupled unsteady events.

Isothermal velocity measurements in two HyperVapotron geometries using Particle Image Velocimetry (PIV)

HyperVapotron beam stopping elements are high heat flux devices able to transfer large amounts of heat (of the order of 10–20MW/m2) efficiently and reliably making them strong candidates as plasma facing components for future nuclear fusion reactors or other applications where high heat flux transfer is required. They employ the Vapotron effect, a two phase complex heat transfer mechanism. The physics of operation of the device are not well understood and are believed to be strongly linked to the evolution of the flow fields of coolant flowing inside the grooves that form part of the design. An experimental study of the spatial and temporal behaviour of the flow field under isothermal conditions has been carried out on two replicas of HyperVapotron geometries taken from the Mega Amp Spherical Tokamak (MAST) and the Joint European Torus (JET) experiments. The models were tested under three isothermal operating conditions to collect coolant flow data and assess how the design and operational conditions might affect the thermal performance of the devices for single phase heat transfer. It was discovered that the in-groove speeds of MAST are lower and the flow structures less stable but less sensitive to free stream speed perturbations compared to the JET geometry. The MAST geometry was found to suffer from hydrodynamic end effects. A wake formation was discovered at the top of the groove entrance for the JET geometry, while this is absent from the MAST geometry. The wake does not affect significantly the mean operation of the device but it may affect the coolant pumping load of the device. For the JET variant, there is evidence that the typical operation with free stream flow speed of 6m/s is advantageous.

A Numerical Investigation of Localized and Steady Energy Addition to High Speed Airflows

ABSTRACT This work presents a numerical analysis of the gas dynamic problem of localized and steady energy addition to uniform high-speed airflows. Firstly, the general effects caused by a localized energy deposition on the flow were investigated, then an extensive parametric analysis concerning the effects of energy deposition rate and dispersion for different free stream flow speeds was performed. As a general result, localized and steady energy deposition generates compression waves and constant property flow stream tube downstream to the source. The parametric analysis results have shown that either increasing the rate or decreasing dispersion in the orthogonal direction to the flow of the deposited energy has the effect of enhancing property flow changes, which is even more pronounced for lower flow speeds. In contrast, energy dispersion in the flow direction has presented very little effect on the flow changes. The results of this numerical analysis should be very helpful in studies of energy addition applications in hypersonic.

The simulation of unsteady cavitating flows with external perturbations

In this study, the unsteady cavitating flows arising from the abrupt or gradual changes in the freestream flow speed and emergence of cavitator are simulated. The flows are governed by the one-fluid compressible Euler equations with cavitation phenomenon modelled by the cut-off cavitation formulation. The two-dimensional (2D) axisymmetric cavitating flows around a cylinder with a blunt or sharp-edged head are resolved to investigate the cavity evolution associated with the freestream flow speed variation and emergence of cavitator, respectively. The numerical study indicates that the cavity over the cylinder may collapse with an abrupt and severe change of the freestream flow speed. A very strong pressure wave can be generated following the cavity collapse. However, if the freestream flow speed is gradually increased, the original cavity may be sustained without complete collapse. For a given amplitude of speed increase, the longer response time allowed for the change corresponds to a slower and smoother deformation of the cavity. On the other hand, the larger amplitude of speed variation leads to a more significant deformation during the cavity evolution for a given response time. Separately, an emerging cavitator is introduced to the cylinder head to control the cavity development for a given flow speed. It is found that the introduced cavitator can amplify the original cavity. The emerging cavitator may change a partial cavity to a supercavity and therefore potentially reduces the friction drag on the object.

Flow-induced vibrations of a side-by-side arrangement of two flexible circular cylinders

Laboratory experiments with a side-by-side arrangement of two vertical, high aspect ratio (length over diameter) and low mass ratio (mass over mass of displaced fluid) cylinders, pin-jointed at the ends and vibrating at low mode number, were carried out in a free-surface water channel. The dynamic response of the models under two different wake interference situations is presented here. Initially, one of the cylinders was fixed and the other was completely free to move. In a second battery of experiments both cylinders were free to vibrate. A very large parameter space was covered by varying the free-stream flow speeds, the natural frequencies of the system and the separation between the models, allowing the identification of vortex-induced vibrations (VIV) and wake-coupled VIV (WCVIV). Amplitudes, frequencies and phase synchronisation between the models are presented.

Positive flow dependence in the initial colonization of a fouling community: results from in situ water current manipulations

During late winter/early spring (1992–1993), a series of 12 tapered channels were placed subtidally in a Long Island, New York (U.S.A) estuary to examine total fouling community development. Rather than relying upon flow-dependent correlations among spatially segregated sites, these channels directly accelerated or decelerated the existing tidal currents ± 50% (ambient flow speeds ranged from < 2 to > 50 cm/s). In response to experimentally manipulated water flows over five months, the arborescent hydroid Obelia longissima Pallas (1766) accumulated onto Tygon tubing in direct proportion to mean free-stream flow speed, whereas barnacle ( Balanus eburneus Gould) initial settlement and cumulative recruitment along plywood ceilings increased three-fold and five-fold, respectively, with a long-term doubling of water speed. These positive flow-dependent results directly contrast those from previous experiments in the same channel apparatus which failed to document any flow enhancement on the growth of suspension-feeding bivalves.

Chemical Induction of Larval Settlement Behavior in Flow.

The ability of dissolved chemical cues to induce larval settlement from the water column has long been debated. Through computer-assisted video motion analysis, we quantified the movements of individual oyster (Crassostrea virginica) larvae in a small racetrack flume at free-stream flow speeds of 2.8, 6.2, and 10.4 cm/s. In response to waterborne chemical cues, but not to seawater (control), oyster larvae moved downward in the water column and swam in slow curved paths before attaching to the flume bottom. Effective stimuli were adult-oyster-conditioned seawater (OCW) and a synthetic peptide analog (glycyl-glycyl-L-arginine) for the natural cue. The chemically mediated behavioral responses of oyster larvae in flow were essentially identical to those responses previously reported in still water. Our experimental results therefore demonstrate the capacity of waterborne cues to evoke settlement behavior in oyster pediveligers under varying hydrodynamic conditions.

Effect of flow velocity and body size on swimming trajectories of sea scallops, Placopecten magellanicus (Gmelin): a comparison of laboratory and field measurements

The effect of flow velocity and body size on swimming trajectories of Placopecten magellanicus (Gmelin) was studied in a 5 m, flow-through laboratory flume and in a shallow (7 m depth) tidal channel in Lunenburg Bay, Nova Scotia, Canada. In the flume, scallops of 4 size classes, 11–20, 21–30, 31–50 and 51–80 mm shell height, (SH) were induced to swim at mean free-stream flow speeds of 0, 5, 9, 14 and 21 cm s −1. Swimming (active) and post-swimming (passive sinking) phases of individual trajectories were quantified in terms of swimming height, net displacement and direction, swimming time and swimming speed. Smaller scallops (≤ 30 mm) were adverted by the flow during both phases while larger scallops (> 30 mm) were advected only during the passive phase. Net horizontal displacement (active and passive stages combined) was positively related to flow speed for all size classes but the proportion of the variance explained by flow speed decreased with increasing scallop size. Most scallops swimming under flow had a downstream net swimming direction (finished their swimming phase downstream of the start point). For small scallops (≤ 30 mm SH), the proportion swimming downstream was independent of flow speed at flows of 0–5 cm s ( p > 0.30), but not independent of the flow speed at higher flows ( p < 0.01). For large scallops (> 30 mm SH), the proportion swimming downstream was independent of flow speed. In the field study, scallops ranging from 17–72 mm were induced to swim at current speeds ranging from 7–55 cm s −1. Model II regression and circular-linear correlation analysis showed a positive relationship between net horizontal displacement and current speed for scallops landing downstream and a negative relationship for scallops landing upstream, indicating the effect of advection. For small scallops that swam at medium (~ 18 cm s −1) current speeds, the distribution of displacement directions was significantly non-random and the mean direction did not differ significantly from the mean current direction. For large scallops that swam at both medium and high (~ 38 cm s −1) current speeds, the distribution of displacement directions was not significantly different from random. However, at high current speeds the mean size of scallops that landed upstream was significantly greater and the mean swimming height was significantly lower than for those that landed downstream. Flume and field measurements of swimming trajectories demonstrated that predictability of dispersion patterns of P. magellanicus released in bottom culture based on near-bed current data will be poor, especially when scallops larger than 30 mm SH are used.

Colony fission increases particle capture rates of a soft coral: Advantages of being a small colony

The rate at which a passive suspension feeder captures food particles depends on the rate at which ambient currents bring particles into contact with its feeding apparatus. This flux of particles is controlled in part by the turbulence and velocity of water as it moves over a surface of a given topography, and is thus strongly influenced by the size and shape of an organism, and by the size and spatial distribution of neighboring individuals. The effects of (1) colony size and (2) the presence of conspecific neighbors on particle capture rates were quantified in laboratory experiments conducted with colonies of an undescribed species of alcyonacean soft coral. Soft coral colonies were observed feeding in a recirculating flow tank at freestream flow speeds ranging from 0.04-0.51 m/s. At all experimental flow speeds, per polyp particle capture rates decreased as colony size increased. At low flow speeds, colonies had lower particle capture rates when surrounded by neighboring colonies than when neighbors were absent; however, at high flow speeds, particle capture rates were enhanced slightly in the presence of neighboring colonies. These alcyonacean colonies, as well as other colonial organisms in diverse taxa, undergo continual asexual fission as they grow, thereby forming aggregations of small, physiologically discrete daughter colonies. The results obtained here suggest that this growth strategy may be an effective means of maximizing nutrient intake (hence growth rate and sexual reproductive potential) per unit biomass of the genotype. The experimental results can be used to predict the size at which a colony should undergo fission in a given flow environment in the absence of other selective forces acting on colony size.