Increase In Turbulent Kinetic Energy Research Articles

The influence of turbulence on sediment phosphorus sorption

The effect of mean flow velocity on phosphorus (P) partitioning between water and sediment has received much attention in recent decades. However, the impact of turbulence on the efficiency and capability of sediment adsorbing and desorbing dissolved inorganic phosphorus (DIP) is still unclear. A series of contrasting experiments on the sediment sorption and desorption of DIP with the flow turbulence kinetic energy (TKE) ranging from 1.95 to 2.93 pa have been conducted. It was found that the adsorbed P onto unit mass of sediment increases with the increase in TKE. It is because an increase in TKE results in a rise in the effective adsorption capacity of sediment (bm) by 20–30% during the adsorption process. The bm shows the maximum rise from 0.18 to 0.25 mg/g when TKE increases from 1.95 to 2.93 pa with a fixed sediment concentration of 0.5 g/L. To account for the direct effect of TKE on P adsorption, the Langmuir model is modified by introducing a newly defined coefficient (fA−TKE). The fA−TKE shows a good linear relationship with TKE. Comparison between the modified model and the classic model shows that the amount of adsorbed P could be overestimated by over 50% if the direct effect of turbulence intensity is ignored. The experimental data show that the increase in TKE also enhances the desorption process, with the degree of P desorption (Ddes) increased by 44%. The relation between Ddes and TKE can be well represented using a logarithmic function to quantify the direct effect of turbulence intensity on desorption of P.

Experiments into the interaction of side dilution jets with bluff-body stabilised annular jets

Fundamental insights into the effects of side dilution jets on the flow behaviour and turbulence field in gas turbine combustors further advances the ability to use these jets to control (upstream) flame stability and (downstream) emissions. Despite this, the interaction of side dilution jets with typical flame anchoring mechanisms, such as bluff-bodies, and annular flows, both swirling and non-swirling, remains elusive due to the challenges of undertaking highly resolved laser diagnostics in such complex geometries. In this work, Particle Image Velocimetry (PIV) is used to investigate the interaction of turbulent side dilution jets (Red = 18,000 and 11,300) with bluff-body stabilised annular jets (Res = 35500 and 17800), under both swirling (S = 0.3) and non-swirling (S = 0) conditions. Boundary conditions for the cases studied are well-defined through Constant Temperature Anemometry, which in conjunction with the PIV resolved flow field means the results presented additionally serve as good benchmark cases for modelling (not undertaken here). The experiments are conducted in an optically accessible 700 mm long square-like (310 mm × 310 mm) enclosure equipped with four peripheral side dilution jets on the rounded corner supports.Results show that irrespective of the flow configuration used (swirling or non-swirling), side dilution jets considerably modify the overall flow field and lead to the development of characteristic recirculation zones on the periphery of the annular jet, named as Peripheral Recirculation Zones (PRZ). In the upstream region, PRZ significantly promote turbulence in the outer shear layer of the bluff-body stabilised annular jet, leading to a massive 176% (at x/D = 1.4) and 218% (x/D = 1.8) increase in peak turbulent kinetic energy. These high levels of turbulence further magnify closer to the location of the side jets. In the downstream (beyond x/D > 2) of the bluff-body stabilised annular jet, noticeable increases in centreline velocity decay (13%) and jet spreading (32%) are observed in the presence of a PRZ, which indicates better mixing and flow entrainment. The overall effects of side dilution jets vary directly and inversely with the Reynolds number of side dilution jets and annular jets, respectively. To resolve the flow dynamic behaviour independent of heat release, all tests are undertaken isothermally (non-reacting).

Large-Eddy Simulation of Wind Turbine Wakes in Forest Terrain

In this study, large-eddy simulation was employed to investigate the influence of the forest canopy on wind turbine wakes. Nine forest case studies were carried out with different vertical distributions of leaf area density (LAD) and values of leaf area index (LAI). It was found that the wake in forest canopies recovers at a faster rate when compared with the flat terrain. An interesting observation was the significant reduction in turbulence kinetic energy (TKE) in the lower part of the wake above the forest in comparison with the inflow TKE, which occurred for a wide range of turbine downstream positions. The increase of TKE, on the other hand, was mainly located in the region around the top tip. Analyses of the power spectral density showed that the increase in TKE happened at a certain range of frequencies for the forest canopy cases and at all the examined frequencies for the flat case. Wake meandering was also examined and was found to be of a higher amplitude and a lower dominant frequency for the forest cases compared with the flat case. In terms of the influence of forest canopy parameters, the LAI was found to have an impact greater than the vertical distribution of LAD. Specifically, the wake-added TKE and wake-added Reynolds shear stress were found to be approximately the same for cases with the same LAI, regardless of the vertical distribution of LAD.

Development and Validation of a CFD Combustion Model for Natural Gas Engines Operating with Different Piston Bowls

Nowadays, an accurate and precise description of the combustion phase is essential in spark-ignition (SI) engines to drastically reduce pollutant and greenhouse gas (GHG) emissions and increase thermal efficiency. To this end, computational fluid dynamics (CFD) can be used to study the different phenomena involved, such as the ignition of the charge, combustion development, and pollutant formation. In this work, a validation of a CFD methodology based on the flame area model (FAM) was carried out to model the combustion process in light-duty SI engines fueled with natural gas. A simplified spherical kernel approach was used to model the ignition phase, whereas turbulent flame propagation was described through two variables. A zero-dimensional evolution of the flame kernel radius was used in combination with the Herweg and Maly formulation to take the laminar-to-turbulent flame transition into account. To estimate the chemical composition of burnt gas, two different approaches were considered—one was based on tabulated kinetics, and the other was based on chemical equilibrium. Assessment of the combustion model was first performed by using different operating points of a light-duty SI engine fueled with natural gas and by using the original piston. The results were validated by using experimental data on the in-cylinder pressure, apparent heat release rate, and pollutant emissions. Afterward, two other different piston bowl geometries were investigated to study the main differences between one solution and the others. The results showed that no important improvements in terms of combustion efficiency were obtained by using the new piston bowl shapes, which was mainly due to the very low (+4%) or null increase in turbulent kinetic energy during the compression stroke and due to the higher heat losses (+20%) associated with the increased surface area of the new piston geometries.

Effect of coolant pulsation on film cooling performance on turbine vane

Combined with infrared thermography measurements, large eddy simulation was used to study the square-wave pulsating film cooling on turbine vane at the time-averaged blowing ratios of 0.5 and 1.5. On suction surface, the introduction of coolant pulsation results in the decrease of cooling effectiveness. On pressure surface, coolant pulsation at high blowing ratio can improve coolant coverage. Starting vortex is the typical structure in pulsating film cooling. However, on suction surface, due to fully-turbulent mainstream, plenty of small-scale vortices play a dominating role in flow fields, and the effect of starting vortex is unobvious. The statistic characteristics and power spectrum density (PSD) of velocity fluctuation were also analyzed in detail. The results show that the disturbance of coolant jet on suction-side mainstream boundary layer is very weak, However, on pressure side, coolant pulsation causes notable increase of PSD and turbulent kinetic energy.

Numerical Investigation of Flow Past Bio-Inspired Wavy Leading-Edge Cylinders

A numerical investigation is proposed to explore the flow past a novel wavy circular cylinder as a passive flow control, whose shape is determined by a sinusoidal function applied to its leading edge line, similar to studies with wavy leading-edge airfoils. The latter are motivated by the wavy-shaped tubercles found in the flippers of humpback whales, which are believed to improve their maneuverability. Our attempt is, therefore, to assess the effects of leading-edge waviness now on a simpler and canonical geometry: circular cylinders. The present work relies on iLES simulations conducted with Nektar++ at a Reynolds number of 3900. Besides the straight cylinder, two wavy geometries are assessed, which are determined by a single wavelength of 37.5% for two amplitudes, 3% and 11%, based on the mean diameter of the wavy cylinder. Our results showed that, contrary to what is usually the case with traditional wavy cylinders at similar Reynolds numbers, waviness caused a reduction in the near-wake recirculation length and an increase in the mean near-wake turbulent kinetic energy compared to the straight cylinder. This was followed by a reduction in base pressure (up to about 36%) leading to a rise in lift oscillations and also to a significant increase in the mean drag coefficient of up to about 28%. An attempt to detail the flow phenomena is provided, evidencing the emergence of counter-rotating pairs of streamwise vortices between peaks. It is argued that the differences observed in recirculation length, turbulent kinetic energy, and force coefficients start even prior to the formation of these coherent structures and end up with interactions with the near wake.

Flushing control strategies to improve the stability of a biogas slurry drip irrigation system: Behavioral characteristics and mechanisms

Biogas slurry drip irrigation can mitigate environmental pollution and reduce the use of chemical fertilizers to enable sustainable development. However, the stability of the biogas slurry drip irrigation system (BSDIS) is disrupted by emitter clogging; hence, it is essential to explore the flushing control strategy of BSDIS. By means of combining actual measurement and simulation, this study investigates the BSDIS stability based on the three technical parameters of the flushing control strategy. Appropriate flushing control strategies can improve system stability and cause spatial differences on the drip irrigation tape. Under various flushing control strategies, the system stability primarily undergoes delays, sensitivity, and ineffectiveness of flushing with time. Compared with the without flushing and emitter outlet downward-oriented treatment, the optimal flushing combination (the high frequency flushing + emitter outlet upward-oriented treatment) reduces the emitter clogging content by approximately 70.97% and increases system stability by 189.1%. In the internal hydrodynamics, the laying direction of emitters does not change the movement characteristics of water flow, although the clogging particles do not completely follow the water flow, with some particles settling owing to gravity, thereby clotting the emitters. When clogging occurs, the increase in flushing speed is conducive to the increase in turbulent kinetic energy on the inlet surface of the emitter, which facilitate the flushing of clogged substances. This study proposes optimal flushing strategy parameters along with a new management mode for the waste liquid represented by biogas slurry.

Enhancement of liquid-solid mixing and particle hydrophobization by an impeller-less flotation pulp conditioning device

An impeller-less device was proposed to employ opposite rotational jet flows conditioning pulp before flotation. The flow field characteristics and relevant effects on the liquid-solid mixing performance were investigated by the numerical simulation of computational fluid dynamics that was proved effective by the particle image velocimetry technique. The outcome of particle surface modification was clarified by variations in the contact angle and surface species of quartz particles. Results showed that the increase of turbulent kinetic energy induced by the opposite rotational flows remarkably improved the dispersion of mineral particles. Smaller particle size and larger circulation rate were conductive to a more uniform distribution of particles. In the case of a volume flow of 3.6 m3/h and a feed volume fraction of 7 % with an average particle size of −45 μm, nearly 4/5th of the whole device area exhibits a particle volume fraction within the range of 6 % and 7 %, and the coefficient of variation was as low as 0.151. Meanwhile, strong turbulence and high shear induced by opposite rotational flows provided favorable conditions for the collector adsorption and particle hydrophobization, which was responsible for the increase in contact angle under relatively higher volumetric flow rates. However, when the volume flow exceeded 3.0 m3/h, over-increasing shear stresses and excessive turbulence caused the desorption of adsorbed collectors, reflected as the decreased nitrogen content on quartz surfaces from 0.85 % to 0.51 %, thereby accounting for the declined contact angle of conditioned quartz particles. This study highlighted the dominant effect of turbulence and shear strain on particle surface modification, and the significance of proper parameter control.

Investigation of thermo-physical and chemical effects of in-cylinder steam injection on gasoline engine performance

In this study, for a gasoline engine, the effects of thermo-physical properties and chemistry of in-cylinder steam injection were discussed. The properties of steam directly influence the combustion and emissions characteristics of engines. A computational fluid dynamic model was established to investigate the combustion characteristics of engines with steam injection. A premixed laminar flame model was developed to examine the influence of steam injection on laminar flame speed. The findings of this study show that with increase in steam quantity, flow velocity and turbulent kinetic energy increased and tended to the core of combustor, and laminar flame speed decreased. When the steam dilution ratio changed from 0 to 0.5, the maximum laminar flame speed decreased from 37.3 cm/s to 26.9 cm/s. The effect of thermo-physical properties of steam injection was 80 % more potent than its chemical effect on the laminar flame speed. The chemical effect was gradually strengthened in high initial temperature and poor oxygen. The maximum consumption rate of the reaction, C8H18+H⇌C8H17+H2, decreased by 27.7 % and 3.2 % due to the thermal-physical and chemical effects of steam, respectively. The mole fractions of O, H and OH decreased with increase in steam quantity. The findings of this study demonstrate that by manipulating the thermo-physical properties of steam injection, engine performance can be improved.

Influence of Feed Rate on the Performance of Hydrocyclone Flow Field

In order to clarify the influence of feed rate on a hydrocyclone flow field, numerical simulation was employed to model the influence of feed rate on the pressure field, velocity field, air column, turbulent kinetic energy, and split ratio. The results revealed that static pressure, tangential velocity, and radial velocity increased with an increase in the feed rate. When the feed rate at the inlet increases from 1 m/s to 5 m/s, the static pressure increases from 5.49 kPa to 182.78 kPa, tangential velocity increases from 1.97 m/s to 11.16 m/s, and radial velocity increases from 0.20 m/s to 1.16 m/s demonstrating that a high feed rate facilitated the strengthening separation of the flow field. Meanwhile, with the increase in the feed rate, the split ratio of the hydrocyclone decreased, indicating that the concentration effect of the hydrocyclone improved. Additionally, the formation time of the air column was reduced, and the flow field became more stable. Nevertheless, the axial velocity and the turbulent kinetic energy also increased with the increase in the feed rate, and the increase in the axial velocity reduced the residence time of the material in the hydrocyclone, which was not conducive to the improvement of separation accuracy. In addition, the increase in turbulent kinetic energy led to an increase in energy consumption, which was not conducive to the improvement of the comprehensive performance of the hydrocyclone. Therefore, choosing an appropriate feed rate is of great significance to the regulation of the flow field and the improvement of hydrocyclone separation performance.

An experimental and numerical study of turbulent heat transfer enhancement for graphene nanofluids produced by pulsed discharge

• Time and cost-effective production of graphene nanofluids by pulsed discharge is tried and tested. • A new two-step method for producing graphene nanofluids has been developed. • Graphene nanofluids generated by pulsed discharge and ultrasound, like other graphene nanofluids, have high turbulent heat transfer performance (35% higher than water). • The numerical results show considerable increase in turbulent kinetic energy (8%) and velocity gradients near the wall due to physical interaction between continuous phase(water) and dispersed phase (graphene nano-particles). • The exchange of momentum and energy due to the movement of particles is considered as a cause-and-effect relationship between the aggregation of particles and the change in physical quantities in the continuous phase. Developing production methods for graphene nanofluids that can produce them in large quantities at once in a short time and at a low cost will contribute to the development of a wide range of industries that use nanofluids, and understanding their properties through mathematical modeling and experiments will lead to the development of new nanofluids. In this study, the new production method of graphene nanofluids is proposed and experimental and numerical investigations of the turbulent heat transfer performance of graphene nanofluids in a horizontal circular tube subjected to constant heat flux are conducted. The experimental investigation is conducted to evaluate the turbulent heat transfer performance of graphene nanofluids which are made by a new method using pulsed discharge and to compare them with numerical results. In the numerical investigation, the finite volume method with a Realizable k-ε model and Two-layer model is employed to solve the continuity, momentum, and energy conservation in two-dimensional domains. The Lagrangian two-phase model is applied to consider the physical interaction between the dispersed phase and continuous phase. In this model, the particles are tracked in a Lagrangian manner and coupled with the Eulerian flow description. Since the computational burden would be tremendous if individual nanoparticles were tracked in a Lagrangian manner, we defined a cluster of particles, called a parcel and treated it statistically. This concept of parcels allowed the average behavior of nanoparticles to be established as an analytical parameter. As a result, graphene nanofluids are not produced by pulsed discharge alone but are successfully produced by applying ultrasonic. Experimental investigations show a 33% increase in the Nu number of graphene nanofluid produced by pulsed discharge compared to water as the flow velocity increased. Numerical investigations confirm changes in the properties (Turbulent kinetic energy, Velocity distribution, Temperature distribution) of the continuous phase due to physical interactions between the dispersed and continuous phases. It is also shown that these changes contribute to the improved turbulent heat transfer performance of the nanofluid and that the aggregation of particles into the center of the tube is responsible for these changes.

Study on the interaction between coolant pulsation and recirculation vortexes by large-eddy simulation

In the recirculation zone behind the bluff-body, LES method was used to study the interaction mechanism of between the square-wave pulsating film cooling jet and recirculation vortexes. Combined with infrared thermography test results, having a detailed analysis at the time-averaged blowing ratios of 0.5 and 1.5. In model A, at low blowing ratio, the introduction of pulsating jet results in the decrease of cooling effectiveness, and can obviously reduce the turbulent kinetic energy (TKE) at low frequency. At high blowing ratio, pulsating jet at low amplitude can improve film cooling effectiveness, while has adverse effect at high amplitude, the region with high TKE level is obviously elevated and the introduction of pulsation results in the increase of TKE. In model B, at two blowing ratios, film cooling effectiveness can be improved by pulsating jet at low amplitude, but reduced at high amplitude. The effect of pulsating jet on TKE is not obvious at low blowing ratio, while at high blowing ratio, the region with high TKE level is obviously elevated and the introduction of pulsation results in the increase of TKE. In the time-averaged flow field, the entrainment of recirculation vortexes plays a dominant role in the near-wall region. In two models, the introduction of pulsating jet has almost no effect on the recirculation vortexes and velocity profiles. The results show that the varying regularity of film cooling can be explained by the synergistic relationship between velocity field and temperature field, and the pulsating amplitude and frequency play different roles under different conditions respectively.

Experimental study of turbulence intensity on the wake characteristics of a horizontal-axis wind turbine

ABSTRACT A time-resolved particle image velocity (TR-PIV) system was used to test the wake flow of a small horizontal-axis wind turbine at tip speed ratios (TSRs) in the range of 4–6 under uniform inflow and turbulent inflow in order to study the impact of turbulence intensity on wind turbine wake characteristics. The impact of incoming turbulence on the wake structure of a wind turbine and its mechanism was examined by comparing and analyzing the turbulence properties of the wake flow, including velocity field, vortex structure, additional turbulence intensity, and energy distribution coefficient. It was found that tip vortex structure remained intact in the range of 9 R under uniform inflow, and the longitudinal distribution of turbulent intensity had double peaks, indicating that it was in the near wake zone. Under turbulent inflow, the three vortex systems could not be distinguished at the position of 5 R, and the longitudinal distribution of turbulent intensity did not have peaks, the wake vortex system was broken and the wake transitioned to the far wake zone. With the increase in the turbulence intensity and tip speed ratio, the contribution of the longitudinal component of turbulence intensity to the turbulent kinetic energy increases, which is also the reason for the rapid wake recovery. Additionally, the recovery of the wake velocity was sped up in the range of 3 R and gradually decreased after 3 R.

Numerical study of bubble induced mixing in stratified fluids

Direct numerical simulations of up to 146 bubbles rising in unbounded stratified fluids using finite-volume/front-tracking method are performed. Both the bubble dynamics and destratification effects caused by the bubble motion are analyzed. The importance of bubble deformability and bubble Reynolds numbers on the induced background mixing are studied by varying the Eo¨tvo¨s number in the range 1.55 to 4.95 and Reynolds number in the range 25 to 200. Highly deformable, high Reynolds number bubbles undergo path instabilities and give rise to higher levels of mixing. Liquid and bubble velocity fluctuations and pseudo-turbulence caused by the bubble motion are examined and are seen to play an important role in mixing statistics. An increase in turbulent kinetic energy (TKE) levels with void fraction is noted. TKE levels are seen to decrease slightly as the stratification strength is increased, indicating increasing stability and resistance to destratification. The dependence of mixing parameters on the void-fraction of bubbles and stratification strength of the liquid is also presented. An increase in buoyancy flux across pycnoclines is observed as void fraction increases. A similar increase in vertical mass flux is observed for a decrease in stratification strength. The bubble dispersion and wake flow patterns are seen to strongly influence mixing properties.

Organization Development in Precipitating Shallow Cumulus Convection: Evolution of Turbulence Characteristics

Abstract Horizontal organization or mesoscale variability is an important mechanism in the interaction of the boundary layer with the large-scale conditions. The development of organization in a precipitating cumulus trade wind boundary layer is studied using large-eddy simulations with extensive horizontal domains, up to 160 × 160 km2 and fine grid resolution (40 m). The cloud fields vary between different computational domain sizes. Mean profiles and vertical velocity statistics do not vary significantly, both with respect to the domain size and when large-scale organization develops. Turbulent kinetic energy (TKE) rapidly increases when organization develops. The increase of TKE is attributed to the horizontal component, whereas the vertical velocity variance does not change significantly. The large computational domains blend the boundary between local convective circulations and mesoscale horizontal motions leading to the dependence of horizontal TKE on the LES domain size. Energy-containing horizontal length scales are defined based on the premultiplied spectra. When large-scale organization develops, the premultiplied spectra develop multiple peaks corresponding to the characteristic horizontal scales in the boundary layer. All flow variables have a small length scale of 1–2 km, which corresponds to local convective motions, e.g., updrafts and cumulus clouds. Organization development creates additional larger length scales. The growth rate of the large length scale is linear and it is about 3–4 km h−1, which agrees well with the growth rate of the cold pool radii. A single energy containing length scale is observed for vertical velocity for the entire run (even after organized convection develops) that is fairly constant with height.

Effects of bed permeability on the hydrodynamic characteristics in a channel with a vegetation patch: A modeling study

• The impact of sediment layer characteristics on flow-vegetation interactions was investigated. • The bed permeability has a threshold effect on the hydrodynamic characteristics of channels with vegetation patches. • The permeability Reynolds number is suitable for characterizing the bed permeability and its threshold effect. Vegetation patches grow on permeable beds, but the effects of bed permeability on the hydrodynamic characteristics of channels with vegetation patches are rarely discussed. In this study, emergent rigid cylinders are used to represent a vegetation patch, and porous media are utilized to represent the sediment. Multiple groups of conditions with various bed permeabilities are simulated. The results show that when the bed permeability is high, a jet flow forms downstream of the patch. With increasing bed permeability, the velocity distribution of the Karman vortex block is closer to the level in front of the patch, and the peak value of the turbulent kinetic energy increases and extends downstream. However, when the bed permeability is low, the hydrodynamic characteristics in a channel with a vegetation patch (HCCVPs) change negligibly with the bed permeability. The bed permeability has a threshold effect on the HCCVPs. A further discussion suggests that the permeability Reynolds number Re k is suitable for characterizing the bed permeability and its threshold effect. The magnitude of the Re k threshold that corresponds to the change in hydrodynamic characteristics is 1. These new results also explain the process differences in sediment transport and patch expansion under different bed permeability conditions.

Abrasive flow finishing of micro-channel produced by selective laser melting

ABSTRACT Abrasive flow machining (AFM) process and ultrasonically assisted AFM technique (UAAFM) were carried out to polish micro-channel internal faces produced by selective laser melting (SLM). Both numerical simulation and experimental studies were investigated. Simulation results confirmed that the distribution of the flow velocity, absolute pressure, and turbulence intensity inside the S-shaped micro-channel exhibit significantly different. After loading ultrasonic-vibration, cavitation can indeed enhance the turbulence intensity. As the ultrasonic frequency increases, the cavitation area and turbulent kinetic energy increase accordingly. A set of AFM and UAAFM experiments were conducted. Compared with traditional AFM process, UAAFM owns better surface roughness and more uniform material removal at the same experimental condition.

Effect of splitter angles and orientations attached to pin fin on heat transfer and hydraulic characteristics in a jet impingement rectangular channel

Heat transfer improvement of heat sinks is very interesting in many applications including electronic devices cooling. In this study, the influence of rotational angle of splitters attached to pin fins for two cases of identical and opposite directions is investigated. Five angles including 0, 30, 45, 60 and 90 are considered in the analysis conducted for the rectangular channel. Air as the heat transfer fluid at 302 K and 35,000 Reynolds number enters the channel. The temperature of the bottom of the channel is assumed to be constant. 18 rows of circular pin fins on which splitters are attached for enhancement of heat transfer are examined. In the numerical analysis, Nusselt number, turbulent kinetic energy, friction factor and pressure drop are analyzed parameters. The simulation of the models presented in the current study is endeavored using ANSYS Fluent software, which employs finite volume method. Results revealed that, for the identical-direction case, as the angle rises, the average Nusselt number, turbulent kinetic energy, friction factor and pressure drop increase, in a way that for the case of 90 degree, Nusselt number is 9 % and turbulent kinetic energy is 96 % higher than the base case of zero angle. Moreover, friction factor and pressure drop of the vertical-splitter case are 4.8 and 4.6 times more than those of the base case. It is also concluded that increasing angle from 0 to 45° for the opposite-direction case, Nusselt number diminishes. However, extra increase in the angle from 45 to 90 degree results in Nusselt number enhancement. Similar trend of variation is also reported for turbulent kinetic energy. Results also indicated that for the opposite-direction case, friction factor and pressure drop rise; for the case of 90 degree, friction factor and pressure drop increase 4.4 and 4.32 times, respectively.

Investigation into Influence of Wall Roughness on the Hydraulic Characteristics of an Axial Flow Pump as Turbine

Pump as turbine (PAT) is a factual alternative for electricity generation in rural and remote areas where insufficient or inconsistent water flows pose a threat to local energy demand satisfaction. Recent studies on PAT hydrodynamics have shown that its continuous operations lead to a progressive deterioration of inner surface smoothness, serving the source of near-wall turbulence build-up, which itself depends on the level of roughness. The associated boundary layer flow incites significant friction losses that eventually deteriorate the performance. In order to study the influence of wall roughness on PAT hydraulic performance under different working conditions, CFD simulation of the water flow through an axial-flow PAT has been performed with a RNG k-ε turbulence model. Study results have shown that wall roughness gradually decreases PAT’s head, efficiency, and shaft power. Nevertheless, the least wall roughness effect on PAT hydraulic performance was experienced under best efficiency point conditions. Wall roughness increase resulted in the decrease of axial velocity distribution uniformity and the increase of velocity-weighted average swirl angle. This led to a disorderly distribution of streamlines and backflow zones formation at the conduit outlet. Furthermore, the wall roughness impact on energy losses is due to the static pressure drop on the blade pressure surface and the increase of turbulent kinetic energy near the blade. Further studies on the roughness influence over wider range of PAT operating conditions are recommended, as they will lead to quicker equipment refurbishment.

Observations of boundary layer wind and turbulence of a landfalling tropical cyclone

This study investigates the atmospheric boundary layer structure based on multiple-level tower observations with a height of 350 m during the landfall of Super Typhoon Mangkhut (2018). Results show a layer of log wind profile outside of the radius of maximum wind speed with a height of 100 m or larger. The log layer height increases with the wind speed. The height of the constant flux layer reaches ~ 300 m for 10-m wind speeds less than 13 m s−1 while this height decreases with the wind speed. Momentum fluxes and turbulent kinetic energy increase with the wind speed at all vertical levels. The drag coefficient and surface roughness length estimated at the tower location have values of 7.3 × 10–3 and 0.09 m, respectively, which are independent of wind speed. The estimated vertical eddy diffusivity and mixing length increase with height up to ~ 160 m and then slowly decrease with height. The vertical eddy diffusivity increases with the wind speed while the vertical mixing length has no dependence on the wind speed. Comparing our results with previous work indicates that the vertical eddy diffusivity is larger over land than over ocean at a given wind speed range.