Wake Zone Research Articles

A hybrid performance prediction method for centrifugal compressors based on single-zone and two-zone models

High-accuracy models are essential for rapid performance estimations and reliable parameter refinements during the preliminary design of centrifugal compressors. In this paper, a hybrid performance prediction method is proposed by combining the loss models of the single-zone method and the division idea of the two-zone method. Physically-based loss models are adopted for the jet zone analysis in substitute of the empirical parameters in the two-zone method. Two additional parameters are introduced for the wake zone analysis to implicitly evaluate the loss not fully understood in the single-zone method. For these two-zone parameters, prediction models are proposed by first analyzing the dominant factors from one-dimensional level and then extracting the coefficients from diverse datasets with statistical techniques. The hybrid method is validated on four centrifugal compressors with flow coefficients ranging from 0.09 to 0.58 and pressure ratios up to 8.5. The results show that, compared with the single-zone and two-zone methods, the hybrid method provides more reliable performance predictions with higher accuracy in the whole operating range.

CFD analysis of thermal-hydraluic behaviors in a LBE cooled 19-pin wire wrapped bundle under porous lumped blockage conditions

Flow blockage in fuel assemblies is a severe accident that must be considered in reactor design and safety analysis. Porous type internal blockages are more likely to occur within fuel assemblies by the accumulation of suspended impurities following coolant flow. For lead-based reactor, the deposition of oxide impurities with low thermal conductivity are expected to represent a safety concern. In this paper, three-dimensional flow and heat transfer behaviors of LBE were obtained using commercial CFD software. A parametric study including blockage geometry, its position and blockage particle properties were performed systematically. It was found that the local hotspot may occur in the wake zone downstream of blockage for plate type blockage, while for column type blockage, the hotspot occurred within the blockage zone. The peak cladding temperature could exceed the design limit (usually 500 °C for LBE system) even for a small length column blockage. But flow resistance is not strongly influenced by the axial growth of blockage. That means the column type porous blockage, which is more likely to occur in wire spacer bundle, is difficult to be detected in advance. Blockage particle properties have great impacts on the peak cladding temperature. Moreover, cladding that are partly exposed to blockage are subjected to large circumferential temperature variation (>140 °C for case 4) and may result in huge thermal stress.

A Comparison of Edge Detectors in the Framework of Wake Pattern Modeling for Wind Turbines

To monitor wind turbine health, wind farm operators can take advantage of the historical SCADA (supervisory control and data acquisition) data to generate the wake pattern beforehandfor each wind turbine, and then decide in real time whether observed reduction in power generation is due to wake or true faults. In our earlier efforts, we proposed an effective wakepattern modeling approach based on edge detector using Linear Prediction (LP) with entropy-thresholding, and smoothing using Empirical Mode Decomposition (EMD) on the windspeed difference plots. In this paper, we compare the LP based edge detector with two other predominant edge detectors, Sobel and Canny edge detectors, to quantitatively justifythe appropriateness and effectiveness of the LP based edge detector in wind turbine wake pattern analysis. We generate a fused wake model for the turbine of interest with multiple neighboring turbines, and then analyze the wake effect on turbine power generation. With a fused wake pattern, we do not need to identify the individual source of wake any more. Weexpect that wakes cause reduced wind speed and hence reduced power generation, but we have also observed from the SCADA data that the wind turbines in wake zones tend to overreact when the wind speed is not yet close to the highwind- shut-down threshold, which causes further power generation loss.

Investigation of the influence of the surface material of a streamlined round cylinder on the parameters of the near wake using PIV and POD methods

In this paper, the influence of the surface material of a streamlined cylinder on the characteristics of large-scale velocity pulsations is experimentally investigated using the methods of studying the flow structure, namely PIV (Particle Image Velocimetry) and POD (Proper Orthogonal Decomposition). A comparison of the parameters of the near wake in the range of Reynolds numbers of 70 000 – 180 000 is presented for cylinders made of vacuum rubber (hydrophilic material) and fluoroplast (hydrophobic material) of the same diameter. Using the PIV method, instantaneous and averaged velocity fields are obtained. The size of the separation zone is determined based on the average velocity fields. The spatial shape and kinetic energy of the observed large-scale ripples in the wake region are analyzed using the POD statistical method. In the studied range, the transition to the critical flow mode for the fluoroplast cylinder is recorded. For the case of flow around a rubber cylinder, the character of pulsations in the wake zone remains unchanged throughout the studied range.

Experimental Study on Aerodynamic Characteristics of a Gurney Flap on a Wind Turbine Airfoil under High Turbulent Flow Condition

The objective of the current work is to experimentally investigate the effect of turbulent flow on an airfoil with a Gurney flap. The wind tunnel experiments were performed for the DTU-LN221 airfoil under different turbulence level (T.I. of 0.2%, 10.5% and 19.0%) and various flap configurations. The height of the Gurney flaps varies from 1% to 2% of the chord length; the thickness of the Gurney flaps varies from 0.25% to 0.75% of the chord length. The Gurney flap was vertical fixed on the pressure side of the airfoil at nearly 100% measured from the leading edge. By replacing the turbulence grille in the wind tunnel, measured data indicated a stall delay phenomenon while increasing the inflow turbulence level. By further changing the height and the thickness of the Gurney flap, it was found that the height of the Gurney flap is a very important parameter whereas the thickness parameter has little influence. Besides, velocity in the near wake zone was measured by hot-wire anemometry, showing the mechanisms of lift enhancement. The results demonstrate that under low turbulent inflow condition, the maximum lift coefficient of the airfoil with flaps increased by 8.47% to 13.50% (i.e., thickness of 0.75%), and the Gurney flap became less effective after stall angle. The Gurney flap with different heights increased the lift-to-drag ratio from 2.74% to 14.35% under 10.5% of turbulence intensity (i.e., thickness of 0.75%). However, under much a larger turbulence environment (19.0%), the benefit to the aerodynamic performance was negligible.

Hydrodynamic effects on the overall adsorption rate of phenol on activated carbon cloth through the advection-diffusion model application

In this work, a mathematical formulation based on fundamental principles of momentum and mass transport with a kinetic adsorption model at the liquid-solid interface is proposed. This formulation was numerically solved and used for the interpretation of adsorption rate data of phenol onto an activated carbon cloth in a stirred tank adsorber under different agitation rates. The mathematical model solution was compared with the obtained by mathematical models that neglect the local velocity and concentration profiles in the solution; external mass transfer model (EMTM) and the first-order kinetic model (FOM). The results showed that under all studied stirring speeds (30−200rpm) the proposed model was able to capture the dynamics of the concentration decay curves, while EMTM and FOM models were only able to interpret the data at high stirring speeds, indicating that velocity profiles play an important role during phenol adsorption. Particular “wake zones” were identified behind the mobile adsorbent, which importantly promotes the transport of solute toward the activated carbon cloth. As a result, it was found that convective transport is much more important than diffusive transport in the solution.

Experimental and numerical studies on the vortex-induced vibration of two-box edge girder for cable-stayed bridges

Due to the blunt aerodynamic configuration, the edge-girder bridge deck may sustain severe vortex-induced vibrations (VIVs) that adversely affect bridge usability and security. In the present study, the VIV performance and the related aerodynamic mechanism of the two-box edge girder for a cable-stayed bridge are systematically investigated. Based on the flow pattern and vortex structure around the section simulated by computational fluid dynamics, it is inferred that the aerodynamic interactions of the large-scale vortices under the deck and the shedding vortices in the wake zone periodically change the fluctuating pressures on the section and induce the vibrations of the deck. Moreover, several aerodynamic mitigation countermeasures are designed. Their effects on the vertical VIV are experimentally investigated in detail. It is found that the vertical VIVs are considerably suppressed at different attack angles (−3°, 0°, and +3°) by installing the mini-triangular wind fairings, which can satisfy the bridge wind-resistant design requirements. Further numerical simulations show that after the installation of the effective countermeasures, vortex shedding in the wake zone is mitigated, and the strengths and distortions of large-scale vortices around the upper and lower surfaces of the section also decrease, thereby the vertical VIV is suppressed effectively.

Circadian VIPergic Neurons of the Suprachiasmatic Nuclei Sculpt the Sleep-Wake Cycle

SummaryAlthough the mammalian rest-activity cycle is controlled by a “master clock” in the suprachiasmatic nucleus (SCN) of the hypothalamus, it is unclear how firing of individual SCN neurons gates individual features of daily activity. Here, we demonstrate that a specific transcriptomically identified population of mouse VIP+ SCN neurons is active at the “wrong” time of day—nighttime—when most SCN neurons are silent. Using chemogenetic and optogenetic strategies, we show that these neurons and their cellular clocks are necessary and sufficient to gate and time nighttime sleep but have no effect upon daytime sleep. We propose that mouse nighttime sleep, analogous to the human siesta, is a “hard-wired” property gated by specific neurons of the master clock to favor subsequent alertness prior to dawn (a circadian “wake maintenance zone”). Thus, the SCN is not simply a 24-h metronome: specific populations sculpt critical features of the sleep-wake cycle.

Passive flow control on Ahmed body by rear linking tunnels

This paper concentrates on the interaction of a car body with the surrounding airflow, in order to provide small changes for reducing vehicle fuel consumption by reducing drag. There are many passive and active methods for this purpose, the main goal of this project is the development of passive drag reduction devices for the simplified reference model (Ahmed body with 25°, 30°, and 35° slant angle) used to represent bluff body vehicles. Passive flow control is performed by adding linking tunnels at the rear of the bluff body, blowing the flow from the sidewalls (high-pressure zone) to the wake region (low-pressure zone). The movement of a high-pressure flow to the wake zone reduces the wake size, which reduces the pressure drag. A numerical and experimental analysis of this passive method is carried out. The optimal configuration was reached by numerical analysis (showing a drag reduction of up to 5%) and after that, experimental tests were performed in a wind tunnel to check the effect of this passive flow control on an Ahmed body 25°.

Airflow Characteristics Downwind a Naturally Ventilated Pig Building with a Roofed Outdoor Exercise Yard and Implications on Pollutant Distribution

The application of naturally ventilated pig buildings (NVPBs) with outdoor exercise yards is on the rise mainly due to animal welfare considerations, while the issue of emissions from the buildings to the surrounding environment is important. Since air pollutants are mainly transported by airflow, the knowledge on the airflow characteristics downwind the building is required. The objective of this research was to investigate airflow properties downwind of a NVPB with a roofed outdoor exercise yard for roof slopes of 5°, 15°, and 25°. Air velocities downwind a 1:50 scaled NVPB model were measured using a Laser Doppler Anemometer in a large boundary layer wind tunnel. A region with reduced mean air velocities was found along the downwind side of the building with a distance up to 0.5 m (i.e., 3.8 times building height), in which the emission concentration might be high. Additional air pollutant treatment technologies applied in this region might contribute to emission mitigation effectively. Furthermore, a wake zone with air recirculation was observed in this area. A smaller roof slope (i.e., 5° slope) resulted in a higher and shorter wake zone and thus a shorter air pollutant dispersion distance.

Heat Transfer and Entropy Generation Analysis of Slit Pillar Array in Microchannels

Abstract The slit pillar allows a small fraction of the mainstream flow through pillar to disturb the pillar wake zone fluid and eventually enhance the local and global heat transfer performances in microchannels. In this study, three-dimensional full-domain numerical simulations on the hydrodynamic and thermal characteristics of slit pillar array in microchannels are performed. Effects of slit angle and height over diameter (H/D) ratio on the fluid flow and heat transfer are studied. Comparisons with the nonslit pillar array are conducted on pressure drop, surface temperature, Nusselt number, and thermal performance index (TPI). Furthermore, the results are analyzed by using the entropy generation. As a result of secondary flows and enhanced convective heat transfer area, all cases at H/D ratio of 0.3 demonstrate enhanced heat transfer performance at an increase of 18.0–34.7% on Nusselt number, while a reduction of 3.4–12.9% on pressure drop in comparison to the criterion case at the same conditions. Among them, slit 15–15 deg shows the best comprehensive heat transfer performance. Due to the improved uniformities of velocity and temperature distributions, all slit pillar array microchannels show decreased entropy generation. The maximum entropy generation reduction can reach up to 15.8%, as compared with the criterion case at the same conditions. The above results fully demonstrate that the novel slit pillar array microchannel heat sink can be used as an effective approach for heat transfer enhancement.

Performance improvement of a Savonius vertical axis wind turbine using a porous deflector

The present study proposed a novel porous deflector in front of the Savonius wind turbine to improve the performance. This new deflector was specifically proposed to minimize the severe effects of the complex wake zone created behind the conventional solid deflector that had not been investigated yet. This caused a deeper investigation into the effect of severe negative effects on the turbine performance to achieve an appropriate design configuration. Therefore, different configuration parameters were taken into account during the numerical process. The unsteady Reynolds-averaged Navier-Stokes equation in conjunction with the SST k-ω turbulence model was numerically solved. Comprehensive comparisons of torque, power, and flow structures between deflector configurations and the standard Savonius turbine were performed to illustrate the mechanism of how the porous deflector improved the turbine performance. Using a porous deflector caused the wind flow passed through the porous zone resulted in a breakdown of the wake zone behind the deflector. Thereby, the flow structures became more regular with fewer fluctuations. The results demonstrated that there is an appropriate porosity value to obtain the highest torque and power coefficients. The maximum value of the power coefficient increased by approximately 10% using the favorable configuration of porous deflector at a tip speed ratio of 1. The notable observation was that the appropriate configuration increased the static torque coefficient approximately 2 times in the rotation angles between 0° and 30° which proved the self-starting ability besides power coefficient enhancement.

Study on Crack Closure Considering Welding Residual Stress Under Constant Amplitude Loading and Overloading

Welding residual stress in the engineering structure has a non-negligible influence on crack propagation, and crack closure is a significant factor affecting the crack propagation. Based on the elastoplastic finite element method and crack closure theory, we studied crack closure and residual compressive stress field of butt-welded plates under constant amplitude loading and overloading regarding the stress ratio, maximum load, overload ratio, and number of overloads. The results show that the welding residual tensile stress can decrease the crack closure because of a decrease in the residual compressive stress in the wake zone, but the effect is gradually reduced with increased stress ratio or maximum load. And the combined effect of welding residual tensile stress and overload can produce a stronger retardation effect on crack propagation.

Flow behaviour around spur dyke founded in cohesive sediment mixtures

This paper presents the results of flow and turbulence characteristics measured by a down-looking 16 MHz micro acoustic Doppler velocimeter around partially submerged spur dyke founded in a clay–gravel sediment mixture. Velocity and turbulence data were collected at various cross-sections at the wake and upstream sides of the spur dyke within and outside of the scour hole. The longitudinal velocity, turbulence intensities and Reynolds stresses were measured and quantitative analysis was carried out. Very small values of the longitudinal velocity component (negative) were obtained within the scour hole whereas larger values were obtained beyond the scour hole region. Turbulence intensities and Reynolds stresses were larger within the scour hole in the wake zone of the spur dyke. Bursting events, including outward interaction, ejection, inward interaction and sweep, were also analysed around the spur dyke. Quadrant analysis of the velocimeter data showed that the values of outward and inward interaction were higher within the scour hole around the spur dyke compared with the values of ejection and sweep events, while outside of the scour hole, the values of ejection and sweep events were higher than the values of outward and inward interaction events.

Wake Characteristics of a Tidal Stream Turbine under Combined Wave and Current

Lin, X.; Zhang, J.; Guan, D.; Zhang, J.; Zhang C., and Gan, M., 2020. Wake characteristics of a tidal stream turbine under combined wave and current. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1558-1562. Coconut Creek (Florida), ISSN 0749-0208.Tidal stream energy has been considered as one of the most promising renewable and clean resources to ease energy shortage. This study experimentally investigates wake characteristics of a tidal stream turbine subject to combined wave and current, using an advanced acoustic Doppler velocimeter to measure its wake properties. The phase-space filter is used to remove spikes in raw data, and the phase-averaged method is adopted for further data analyses. The experimental results indicate that the mean wake characteristics under combination of wave and current share a similar trend with that under pure current. However, it's found that the wave-current interaction fastens the wake recovery and increases the turbulence intensity level. Besides, the supporting structure is found to play an important role in the near wake. A region with low velocity and high turbulence intensity is observed in the lee side of the supporting structure. Furthermore, the wake zone is found to shift upward to the free surface due to wake merging of the turbine rotor and the supporting structure.

Flow field impact assessment of a tidal farm in the Putuo-Hulu Channel

Waters surrounding Zhoushan Archipelago have the most abundant tidal current energy in China, which makes it a promising site for tidal farm construction in the future. Before deploying tidal current turbines, the potential environmental impacts on surrounding waters must be evaluated. In this work, we focus on the flow field impact assessment of an assumed tidal farm installed in Zhoushan Putuo-Hulu Channel. First, in order to evaluate the tidal characteristics there, a 2-month ADCP campaign is carried out, and a 3-D hydrodynamic model is established and validated with the in-situ measurements. Additionally, tidal current velocity and water depth as well as flow heading asymmetry information are used to evaluate the tidal stream exploitability within this channel. Second, a tidal farm consisting of 115 turbines is planned and assumed to be deployed throughout this channel. Blade element momentum method is used to numerically characterize the flow-turbine interactions, and then the spatial-temporal flow field variations caused by the assumed tidal farm are evaluated. It can be seen from the results that the peak flow speed reduction produced by the tidal farm could be over 0.5 m/s within the wake zones, while the flow velocity increment along two islands could reach 0.6 m/s. In particular, a flow speed phase shift phenomenon can be observed. Besides, the water level experiences a small overall rise, and the difference compared with the reference is within ±3.0 cm. Moreover, the seabed shear stress within the wake zones could drop by 65%, while the areas along the two islands might see over 100% increase, indicating the risk of sediment deposition and coastal erosion.

Characteristics of a co-flowing jet with varying lip thickness and constant velocity ratio

PurposeThis study aims to analyze co-flowing jets (CFJs) with constant velocity ratio (VR) and varying primary nozzle lip thickness (LT) to find a critical LT in CFJs below which mixing enhances and beyond which mixing inhibits.Design/methodology/approachCFJs were characterized with a constant VR and varying LTs. A single free jet with a diameter equal to that of a primary nozzle of the CFJ was used for characteristic comparison. Numerical simulation is carried out and is validated with the experimental results.FindingsThe results show that within a critical limit, the mixing enhanced with an increase in LT. This was signified by a reduction in potential core length (PCL). Beyond this limit, mixing inhibited leading to the elongation of PCL. This limit was controlled by parameters such as LT and constant VR. A new region termed as influential wake zone is identified.Practical implicationsIn this study, the VR is maintained constant and bypass ratio (BR) was varied from low value to very high values. Presently, subsonic commercial turbo fan operates under low to ultra-high BR. Hence the present study becomes vital to the current scenario.Originality/valueTo the best of the authors’ knowledge, this is the first effort to find the critical value of LT for a constant VR for compressible co-flow jets. The CFJs with constant VR and varying LT have not been studied in the past. The present study focuses on finding a critical LT below which mixing enhances and above which mixing inhibits.

Wintertime Easterly and Southeasterly Airflow in the ‘Alenuihāhā Channel, Hawaii

Abstract During the wintertime, easterly (E) to southeasterly (SE) flow in the Hawaiian coastal waters is frequent. These wind regimes alter the location and magnitude of channel and tip jet accelerations and the orientation and horizontal extent of the wake zones from east-northeast (ENE) trade wind conditions. The differences are the result of changes in orographic blocking by the Big Island and Maui, with respect to the prevailing wind. During an E wind event, the fastest winds over the ‘Alenuihāhā Channel (>9 m s−1) occur in the channel exit with sinking of the inversion, which rises again downstream. Although the upstream wind speed is similar to typical summer ENE trade winds (7–8 m s−1), the maximum channel wind speed is 3–4 m s−1 slower in the exit. The SE flow is characterized by maximum (~6 m s−1) northeasterly (NE) channel winds along Maui’s south shore and at the channel exit. These winds are the result of orographic blocking on the eastern end of Maui as the northwestern tail of a tip jet off the northeastern coast of the Big Island impinges on Mount Haleakalā. Channel wind speeds are modulated by the speed and direction of this tip jet, which itself varies diurnally and throughout the approach of a midlatitude cold front. Removal of the Big Island shows how the tip jet speed and orientation modulate the pressure gradients and winds in the ‘Alenuihāhā Channel. Removal of the Maui County terrain reveals the impact of orographic blocking on the occurrence of channel winds off Maui’s south shore.

Modeling Axial Mixing of Fuel Particles in the Dense Region of a Fluidized Bed

A semi-empirical model for the axial mixing of fuel particles in the dense region of a fluidised bed is presented and validated against experimental magnetic particle tracking in a fluid-dynamical, down-scaled fluidised bed that resembles hot, large-scale conditions. The model divides the bottom region into three mixing zones: a rising bubble wake solids zone; a zone with sinking emulsion solids; and the splash zone above the dense bed. In the emulsion zone, which is crucial for the mixing, the axial motion of the fuel particle is shown to be satisfactorily described by a force balance that applies experimental values from the literature and an apparent emulsion viscosity of Newtonian character. In contrast, the values derived from the literature for key model parameters related to the bubble wake zone (such as the upwards velocity of the tracer), which are derived from measurements carried out under cold laboratory-scale conditions, are known to underestimate systematically the measurements relevant to h...

The impact of building surface temperature rise on airflow and cross-contamination around high-rise building.

This paper numerically studies the characteristics of flow field around a high-rise building and the cross-contamination when the building surface is heated by the solar radiation. Firstly, the normalized concentration Kc is used to evaluate the dispersion characteristics under different source locations without surface temperature rise. Under iso-thermal condition, the near-wall pollutant dispersion features revealed by the predicted results are similar to our previous wind tunnel experiment. Then, the effect of wall surface temperature rise on the cross-contamination and the flow fields is evaluated based on the near-wall concentration distributions and the wake zone vortex core positions, respectively. When the building surface temperature rises, the location of vortex core obviously changes comparing with that under iso-thermal condition. The correction formula for the vortex core location with the leeward wall surface temperature rise below 15K is developed. The windward wall surface temperature rise brings more serious pollutant accumulation. The near-wall concentrations increase with the rise of temperature when the pollutant is released from the bottom and middle of leeward wall surface, while the top-release scenario exhibited a contrary tendency. For the three interval ranges of generally recognized Richardson number Ri (Ri < 0.1; 0.1 < Ri < 10; Ri > 10), these results indicate that when Ri is less than 0.1, the effect of wall surface temperature rise on near-wall flow and cross-contamination of small-scale model cannot be ignored.