Lower Turbulent Kinetic Energy Research Articles

Swimming behaviour of downstream migrating carp in accelerating flows

The downstream migration of fish is impacted by accelerating flow. This study coupled computational fluid dynamics (CFD) modelling and laboratory observations to investigate the effects of two different accelerating flow hydrodynamics on the swimming behaviours of two fish species. The silver carp (Hypophthalmichthys molitrix) and bighead carp (Hypophthalmichthys nobilis) were chosen as target fish species and four hydrodynamic parameters were selected as main indicators to affect fish swimming behaviours. Four specific swimming behaviours were classified in the contraction flume based on fish downstream trajectories. The results indicated that high accelerating flow, low turbulent kinetic energy (Tke ) and shear stress generated the symmetric distribution of fish along two boundaries of the contraction flume. It also indicated that discharge increase would increase the fish entrainment risk from intakes to diversion pipe, while two fish species exhibited different swimming behaviour patterns. Velocity and velocity gradient had more significant effects than other hydrodynamic parameters on fish swimming strategies. There was mutual compensation among fish tail-beat frequency, angle and amplitude. The results illustrated the relationship between the swimming behaviours and hydrodynamics, which could contribute to understanding the fish downstream migration behaviours in accelerating flow.

Wake Formation Flow Physics and Boundary Layer Analysis on the Sides of the Isosceles Triangular Cylinder with Apex Pointing Downstream

Boundary layer interaction with downstream flow structures was numerically studied to find the region of inactivity behind an 75° isosceles triangular cylinder with apex pointing downstream at intermediate Reynolds numbers (Re = 520, 640, 840 and 1040). The Standard k-ε model in OpenFOAM was used in the study. Numerical results were validated against Particle Image Velocimetry data. Results revealed the stable region of inactivity characterized by low turbulent kinetic energy and vorticity. The onset of secondary vortex and separation point, independent of Reynolds number, was identified. The onset of the secondary vortex was located at (x = 2 mm) from the base and (y = 1.5 mm) from the apex on either side of the cylinder. The ratio of modulus of absolute primary z-component of vorticity, |ω_z^1 |, to the modulus of secondary z-component of vorticity, |ω_z^2 |, was found to be approximately equal to 1.2. This ratio is invariant of the Reynolds number of the study. These findings have practical implications. The unique properties of the inactivity region forms an ideal location that can be used for injecting fluid, placing measurement probe, active flow control and drag reduction. The research problem is formulated in the introduction. Literature is reviewed next providing the background. Details about the range of parameters, governing equations, numerical study details and software used are given in the methodology section. The results section gives the numerical results, verified by mesh refinement test and validated against experimental results. The results are finally discussed in the next section.

An Investigation on Hydraulic Aspects of Rectangular Labyrinth Pool and Weir Fishway Using FLOW-3D

Two different arrangements of the weir (i.e., straight weir and rectangular labyrinth weir) were used to evaluate the effects of geometric parameters such as weir shape, weir spacing, presence of an orifice at the weir, and bed slope on the flow regime and the relationship between discharge and depth, variation and distribution of depth-averaged velocity, turbulence characteristics, and energy dissipation at the fishway. Computational fluid dynamics simulations were performed using FLOW-3D® software to examine the effects on flow conditions. The numerical model was validated by comparing the calculated surface profiles and velocities with experimentally measured values from the literature. The results of the numerical model and experimental data showed that the root-mean-square error and mean absolute percentage error for the surface profiles and normalized velocity profiles of plunging flows were 0.014 m and 3.11%, respectively, confirming the ability of the numerical model to predict the flow characteristics of the pool and weir. A plunging flow can occur at values of L/B = 1.83 (L: distance of the weir, B: width of the channel) and streaming flow at L/B = 0.61 for each model. The rectangular labyrinth weir model has larger dimensionless discharge values (Q+) than the conventional model. For the conventional weir and the rectangular labyrinth weir at submerged flow, Q is proportional to 1.56 and 1.47h, respectively (h: the water depth above the weir). The average depth velocity in the pool of a conventional weir is higher than that of a rectangular labyrinth weir. However, for a given discharge, bed slope, and weir spacing, the turbulent kinetic energy (TKE) and turbulence intensity (TI) values are higher for a rectangular labyrinth weir compared to conventional weir. The conventional weir has lower energy dissipation than the rectangular labyrinth weir. Lower TKE and TI values were observed at the top of the labyrinth weir, at the corner of the wall downstream of the weir, and between the side walls of the weir and the channel wall. As the distance between the weirs and the bottom slope increased, the average depth velocity, the average value of turbulent kinetic energy and the turbulence intensity increased, and the volumetric energy dissipation in the pool decreased. The presence of an opening in the weir increased the average depth velocity and TI values and decreased the range of highest TKE within the pool, resulted in larger resting areas for fish (lower TKE), and decreased the energy dissipation rates in both models.

Winter storms drive offshore transport and modulate phytoplankton blooms in Northern Taiwan, China

The East China Shelf Sea (ECSS) is subject to high-frequency storms during winter and spring, with these storm processes serving as a significant driving factor for initiating the outward diffusion of materials from the inner shelf. Inner shelf waters tend to be rich in nutrients, thus their horizontal diffusion is crucial to the marine primary productivity of the outer shelf. However, our understanding of the processes that govern the offshore water mass transport under winter storm oscillations and the mechanisms driving their impact on phytoplankton is currently limited. In this study, we used satellite and reanalysis data from 2003 to 2020 to investigate the modulation mechanism of winter offshore currents (WOC) on phytoplankton bloom (PB) stimulated by storms in Northern Taiwan (NTW). The results revealed that at synoptic scales, winter storms result in higher sea surface height (SSH) and potential energy on the inner side of the front. However, during periods of weakened storms (for example, during north wind relaxation or a shift to southerly winds), a strong WOC is produced in the top 30–50 m of the ocean due to continuous SSH adjustments caused by changed pressure gradients and induced by topography. PB occurs in the offshore waters of the front, driven by a continuous nutrient supply and an increase in photosynthetically available radiation (PAR). On an interannual scale, offshore transport is controlled by the storm's duration and the extent of its weakening. As the climate warms and winter winds in NTW have consistently weakened over the past 20 years, lower turbulent kinetic energy (TKE) and a shallower mixed layer depth (MLD) will promote phytoplankton growth. The results suggest that water level fluctuations caused by high-frequency changes in winter storms are strongly linked to the offshore sediment transport from near-shore waters, impacting the material transport and the health of the ecosystem on the continental shelf.

Numerical Simulation Study on Flow Characteristics of Multistage Centrifugal Pumps under Different Inlet Gas Void Fractions

The flow characteristics of multistage centrifugal pumps in transporting gas–liquid mixed media are highly complex; particularly, the unstable flow of the passage at high gas contents exacerbates energy losses, leading to a decrease in pump efficiency and stability. The excess energy loss in the pump is not conducive to the concept of sustainable development. In this study, a numerical simulation of multistage centrifugal pumps was conducted based on the Euler–Euler heterogeneous two-fluid model. This research revealed that gas primarily accumulates on the suction side of each impeller stage, and the gas distribution decreases progressively with each stage. As the interstage gas volume fraction (IGVF) increases, the gas accumulation within the pump becomes more pronounced. The gas volume fraction of each component is negatively correlated with the flow rate, and the change is more obvious in the impeller. The turbulent kinetic energy distribution inside the impeller is positively correlated with the gas distribution. There is a large fluctuation at 0.6Q and 0.8Q, and the distribution of the impeller vortex is asymmetric. However, the impeller has a lower turbulent kinetic energy near 1.2Q, which indicates that the emergence of gas in the medium increases the high-efficiency area of the multistage pump to a certain extent. This paper reveals how the gas phase is distributed, how energy is lost in the multistage pump, and the gas–liquid two-phase flow characteristics in the pump, which have certain guiding significance for the design and working-condition adjustments of the gas–liquid mixed pump.

Comparisons of Overall Performance Among Double-Jet Film Cooling Holes, Cylinder Holes, and Fan-Shaped Holes

Abstract In this paper, the film-cooling effectiveness (η) and heat transfer coefficient (h) of different film hole geometries are investigated, including double-jet film cooling (DJFC) holes, streamwise cylindrical holes, and fan-shaped holes, both experimentally and numerically. Results reveal that when the blowing ratio is less than 1.0, the DJFC holes have the highest η and the highest h, as well as the highest net heat flux reduction (NHFR). However, a higher blowing ratio (>1.0) leads to a quickly decreasing NHFR of DJFC holes. The asymmetric antikidney vortex and the high turbulent kinetic energy (TKE) are dominant in the performance of the DJFC holes. Owing to medium effectiveness and the lowest heat transfer coefficient, the fan-shaped holes possess the highest net heat flux reduction at M = 2.0 although the value is negative. The relatively weak kidney vortex and the low TKE can explain the phenomena. The cylindrical holes have the lowest η and the lowest NHFR due to the kidney vortex and relatively higher TKE.

Experimental study of the behavioral response of fish to changes in hydrodynamic indicators in a near-natural environment

The large-scale construction of hydropower projects has significantly altered the ecohydrological conditions of fish habitats, and habitat, migration and other behavior of fish have been affected to varying degrees. Although the behavioral characteristics of fish under different hydrodynamic conditions have been investigated in a number of indoor experiments, it is still difficult to apply the results of hydrodynamic indicators directly to the practice of ecological restoration. In order to investigate the response relationship between fish behavior and hydrodynamic indicators under near-natural conditions, and to clarify the structural linkage of multiple influences between the external ecological environment, external behavior and internal changes of fish, the study relies on a large ecological test site in which the flow rate of the rivers can be changed. A low-frequency radio identification device was used in several monitoring sections in the test river to monitor the behavioral trajectories of fish in real time. The study shows that the method of fish behavior tracking based on passive integrated transponder (PIT) tags is feasible in small near-natural rivers. The test fish were more active under a low flow rate and changed their upstream trajectory in response to flow changes. When the flow velocity reached 1.6–1.8 m/s, the fish upstream gradually shifted from along the mainstream direction to along the shoreline. There was no significant difference in the activity of the test fish between day and night, with fish preferring to inhabit areas of low flow velocity and low turbulent kinetic energy. There was a range of high flow velocities (1.6–1.8 m/s) and high turbulent kinetic energy (0.1–0.15 m2/s2) that stimulated the swimming behavior of the fish. In addition, the physical indicators and physiological and biochemical indicators of fish were also included in the research scope. The test fish with high activity were slender after long-term swimming training, and their hemoglobin concentration and red blood cell number were higher than those of inactive fish and cultured fish. The research results will be used to optimize river restoration projects, promote the recovery of fish resources and population size, and reduce the negative impact of hydropower projects on the ecological environment.

Exploring the effects of turbulent field on propagation behaviors in confined hydrogen-air explosion using OpenFOAM

Explosion always happens in turbulent field, which is hard to measure in reality. This paper explores the influence of turbulent kinetic energy on the propagation behaviors of flame and pressure of stoichiometric hydrogen-air explosion in confined vessel, using large eddy simulation. The momentum equation and progressive variable equation are adapted to consider interaction of turbulence and flame. The results reveal that with initial turbulent kinetic energy increasing, its promoting effects on flame and pressure propagation weaken. For flame at low initial turbulent kinetic energy and low initial pressure, it undergoes three propagating stages: acceleration, steady propagation and deceleration. In addition, flame deceleration is caused by rapid decrease of flow velocity which falls faster with initial turbulent kinetic energy increasing. Furthermore, explosion pressure propagates faster with initial turbulent kinetic energy increasing. Sharp fluctuation of flow velocity appears in the flame zone where big pressure gradient exists.

Hydrodynamics and Sediment Transport of Downslope Turbidity Current Through Rigid Vegetation

AbstractSystematical downslope‐turbidity‐current experiments were performed to clarify the relationship between sediment‐transport modes and current propagation patterns caused by rigid vegetation, as well as adjustments in turbulence characteristics of current. The equations for predicting the front velocities of downslope turbidity currents with emergent vegetation were proposed and validated via experimental data. Experimental results reveal that sediment deposition causes the lower turbulent kinetic energy (TKE) peaks of turbidity currents to decrease or even disappear without affecting their upper TKE peaks, while the rigid vegetation has the opposite effect. Vegetation stems destroy the longitudinal low‐high speed streaks associated with the quasi‐streamwise eddies in the near‐bed region and increase the proportions of the outward and inward interactions. In addition, sediment deposition severely suppresses the turbulent bursting events within turbidity currents but does not influence the relative proportions of the four bursting types. Rigid vegetation and sediment deposition both degrade the absolute values of the third‐order moments of velocity fluctuations without changing their signs to reduce the generation frequency of sweep events. Emergent rigid vegetation accelerates the formation of the reflected bore to provide energy for the deposited sediment to move downstream continuously. On the other hand, the dense submerged vegetation makes turbidity currents easily form the two‐head propagating mode, which allows part of the sediments carried by the upper current head to be transported downstream rather than deposited within or upstream of the vegetation region. Furthermore, the sloping boundary provides favorable conditions to initiate these specific modes for sediment transport adjusted by rigid vegetation.

An Experimental Study of Fish Movement with Turbulent Kinetic Energy along the Pools of A Vertical Slot Fish Pass

Bangladesh is a land of river which is situated in the delta of the Ganges-Brahmaputra-Meghna (GBM) Rivers which are three of the largest rivers of the world. These rivers provide an arterial transportation network for fish movement. Fish migration depends on flow characteristic and surrounding flood plain connected to rivers. In recent time for flood control in monsoon and water storage in lean period a lot of structure has been built along and across the river [77]. As a result the natural sequence of flooding in floodplain of Bangladesh, food chain and life cycle of natural fish and other aquatic species has become under sever threat [64]. In this research an experimental approach has been carried out to understand the turbulence structures of the flow inside a laboratory flume with different local fish species. The structural set up consists of an adverse slope of 6% followed by a mild slope of 3.4%. The adverse slope is 2.54 m in length and the mild slope is 4.45 m in length. Total nine sets of experiments have been conducted. For each set of experiments, data points were distributed on a 10 cm (in the longitudinal direction) x 10 cm (in the transverse direction) grid at 0.6 hydraulic depth and 10 cm (in the longitudinal direction) x 20 cm (in the transverse direction) grid at 0.4 and 0.8 hydraulic depth. In this experimental setup turbulent kinetic energy and fish movement in all four pools will be observed. This experimental study consists of three types of fish species; Rui (Labeo rohita), Catla (Giberlion catla) and Mrigel (Cirrhinus mrigala) of fry size (1-2 cm body length), fingerline size (7-10 cm) and juvenile size (10-20 cm) have been selected for studying. The flow from slot travels through the center of the pool to the next slot with two recirculation region on either side of the jet. At the pool opening the flow appear almost in the form of a shooting jet. There are similarities with turbulence characteristics in each case. Fishes have avoided high turbulent kinetic energy area in the contour map. The comparison shoes the area with low turbulent kinetic energy was most suitable for fishes to pass the pools or taking rest inside the pools. The results may be useful insight on the turbulence characteristics of flow in the vertical slot fish pass and can be used to study fishes characteristic and also for guidance in the design of fish pass in the future.

Supercritical Heat Transfer and Pyrolysis Characteristics of n-Decane in Circular and Rectangular Channels

In this research, the effects of different channel cross-section shapes on the flow, heat transfer and pyrolysis characteristics of n-decane were analyzed and compared based on CFD simulations. The interactions between cracking, heat transfer and flow field in a circular tube and a rectangular tube were studied. The results showed that the mean pressure drop in the rectangular channel is 1.18 times as high as that in the circular channel with pyrolysis due to its smaller equivalent diameter. The maximum value of the chemical heat sink in the rectangular channel is 1.6 times as high as that in the circular channel. The high temperature zone of any cross section in the rectangular channel is much larger than that in the circular channel due to the superposition of the boundary layer and lower turbulent kinetic energy in the corners of the rectangular channel. The maximum value of the Nu in the circular channel is 1.3 times as high as that in the rectangular channel with pyrolysis due to larger heat capacity, lower viscosity and higher wall shear stress.

Experimental study of the wake of multi-rotor turbine

This paper describes a comparative experimental study of the wake of a multi-rotor turbine and a single-rotor turbine. The wake merge process in the multi-rotor wake is analyzed from various perspectives. A jet-like gap flow region and a momentum redistribution region are found in the region before and after the full merge of the individual rotor wakes, respectively. Compared to the single-rotor wake, the fully merged multi-rotor wake is found to have a slower recovery rate and a lower turbulent kinetic energy. Furthermore, two sources are found for the peak frequency in the multi-rotor wake which results in different peak frequencies at different locations. One of the two sources is the rotor wake meandering with the meandering frequency maintain the same throughout the wake, and another one corresponds to the dynamic of the shear layer in the gap region with its peak frequency decrease in the streamwise direction. The yawed multi-rotor wake is found to be similar to the non-yawed case except for the wake deflection of the yawed case. And the deflection is more pronounced in the yawed multi-rotor wake than in the yawed single-rotor wake which indicates a higher yaw efficiency for the multi-rotor turbine.

Large-scale convective flow sustained by thermally active Lagrangian tracers

Non-isothermal particles suspended in a fluid lead to complex interactions – the particles respond to changes in the fluid flow, which in turn is modified by their temperature anomaly. Here, we perform a novel proof-of-concept numerical study based on tracer particles that are thermally coupled to the fluid. We imagine that particles can adjust their internal temperature reacting to some local fluid properties, and follow simple, hard-wired active control protocols. We study the case where instabilities are induced by switching the particle temperature from hot to cold depending on whether it is ascending or descending in the flow. A macroscopic transition from stable to unstable convective flow is achieved, depending on the number of active particles and their excess negative/positive temperature. The stable state is characterized by a flow with low turbulent kinetic energy, strongly stable temperature gradient, and no large-scale features. The convective state is characterized by higher turbulent kinetic energy, self-sustaining large-scale convection, and weakly stable temperature gradients. Individually, the particles promote the formation of stable temperature gradients, while their aggregated effect induces large-scale convection. When the Lagrangian temperature scale is small, a weakly convective laminar system forms. The Lagrangian approach is also compared to a uniform Eulerian bulk heating with the same mean injection profile, and no such transition is observed. Our empirical approach shows that thermal convection can be controlled by pure Lagrangian forcing, and opens the way for other data-driven particle-based protocols to enhance or deplete large-scale motion in thermal flows.

Influence of Channel‐Spanning Engineered Logjam Structures on Channel Hydrodynamics

AbstractNature‐based solutions to flood risk management, such as engineered logjams (ELJs), contribute to the reintroduction of wood in rivers. As part of stream restoration, and utilized in tributaries, ELJs increase upstream water levels, causing the flow to spill onto surrounding floodplains, resulting in the desynchronization of peak flows in a river network. To understand the effect of ELJs on local river hydrodynamics, we experimentally investigate the flow field upstream and downstream of six ELJs, using acoustic Doppler velocimetry and flow visualization. We consider channel‐spanning structures designed with a gap (b0) underneath, allowing unhindered baseflow. Our results revealed that upstream of the logjams, flow diverted toward the lower gap, creating a primary jet exiting underneath the structures, whose strength depends on the physical logjam design. Maximum jet velocities remained constant until a downstream distance of 4b0 for all logjams. The upper wake was structure‐dependent, with logjam structures allowing distinct internal flow paths generating secondary jets, which influenced near wake decay (x < 4b0) and turbulent mixing. The highest turbulence in the near wake was found for the non‐porous and short, porous logjam designs, while the upper wake of all long, porous logjams was characterized by low turbulent kinetic energy levels. Far wake decay (x > 4b0) was self‐similar for all logjams and resulted in near flow recovery at downstream streamwise distances greater than 35b0. ELJs are likely to enhance bed shear stress, increasing the risk of local scour and sediment mobilization. Our study expands the current knowledge of ELJ hydrodynamics and highlights potential implications for the riverine ecosystem.

Numerical investigations of turbulent heat transfer enhancement in circular tubes via modified internal profiles

Convective heat transfer enhancement in turbulent pipe flows via patterned surface textures is studied numerically via large eddy simulation (LES). The Reynolds and Prandtl numbers of the flow are 90,000 and 0.836, respectively. The patterned surface textures consist of ellipsoidal inward-facing elements, a wire coil, and spiral corrugations. The governing equations are solved on three-dimensional grids with the finite volume method using second-order temporal and spatial schemes. The mean Nusselt number and friction factor are calculated in each textured pipe and compared to an untextured baseline configuration. The inward-facing ellipsoidal elements showed the best performance yielding 22 − 42 % heat transfer enhancement with 41 − 97 % pressure loss penalty. The mechanisms driving convective heat transfer enhancement are examined via order-of-magnitude analysis of the first and second laws of thermodynamics. An effective convective mixing parameter is defined to reflect the interplay between the radial turbulence and local thermal gradients. The best enhancement technique yields 46% larger effective convective mixing with 26% lower turbulent kinetic energy compared to the least efficient case. The analysis shows that high heat transfer enhancement is promoted by inducing strong radial turbulent fluctuations in high-temperature-gradient regions while keeping other regions undisturbed. • Heat transfer enhancement via surface textures is studied via large eddy simulation. • Inward ellipsoids yield 22% higher heat transfer rate and 41% larger pressure drop. • The process driving heat transfer enhancement is explained via total energy budgets. • Near-wall radial fluctuations contribute the most towards enhancing heat transfer. • Axial vortices yield the best heat transfer enhancement and low entropy generation.

Spatial heterogeneity in sediment and carbon accretion rates within a seagrass meadow correlated with the hydrodynamic intensity

The majority of the carbon stored in seagrass sediments originates outside the meadow, such that the carbon storage capacity within a meadow is strongly dependent on hydrodynamic conditions that favor deposition and retention of fine organic matter within the meadow. By extension, if hydrodynamic conditions vary across a meadow, they may give rise to spatial gradients in carbon. This study considered whether the spatial gradients in sediment and carbon accretion rates correlated with the spatial variation in hydrodynamic intensity within a single meadow. Field measurements were conducted in three depth zones across a Zostera marina L. (eelgrass) meadow in Nahant Harbor, Massachusetts. Four sediment cores were collected in each zone, including one outside the meadow (control) and three within the meadow at increasing distances from the nearest meadow edge. Sedimentation and carbon accretion rates were estimated by combining the measurements of dry bulk density, organic carbon fraction (%OC), 210Pb, and 226Ra. Tilt current meters measured wave velocities within each zone, which were used to estimate turbulent kinetic energy (TKE). Both sediment and carbon accretion rates exhibited spatial heterogeneity across the meadow, which were correlated with the spatial variation in near-bed TKE. Specifically, both accretion rates increased with decreasing TKE, which was consistent with diminished resuspension associated with lower TKE. A method is proposed for using spatial gradients in hydrodynamic intensity to improve the estimation of total meadow accretion rates.

Analysis of Movements and Behavior of Bighead Carps (Hypophthalmichthys nobilis) Considering Fish Passage Energetics in an Experimental Vertical Slot Fishway.

Simple SummaryHydraulic structures have modified the natural waterways, and they have inevitably blocked fish migration routes and reduced fish habitat. To mitigate the impact on fish, fish passages have been developed as an effective way to restore riverine connectivity. The design of fish passage facilities refers to both hydrodynamics and fish behavior, and the information on the energetic expenditure of fish can be used to identify movement zones that are suitable for fish migration. Thus, this paper intended to identify fish movement behavior in response to water flow field information by means of estimating the energetic expenditure using an IBM approach. The results demonstrated that the fish required more energy in high TKE zones, and they were therefore likely to utilize the low TKE zones. This study provides a reference for optimizing the design of fish passages, and the method can be applied to assess the efficiency of fish passages and other fish bypass structures.An understanding of fish movement behavior in response to flow field variables is important for exploring the hydrodynamic strategies of fish in fish passages. In this paper, bighead carps were taken as an example. The fish movement behavior response to water flow field information by means of estimating the energetic expenditure using an IBM approach in an experimental fishway was investigated. Fish swimming velocity, drag force, and energy expenditure were analyzed in varied flow conditions related to hydraulic variables, including velocity (V), turbulent kinetic energy (TKE), and strain rate (SR). The result indicated that the fish will require more energy in high TKE zones. This study provides a reference for optimizing the design of fish passages and fisheries management. This method can be applied to assess the efficiency of fish bypass structures and conduct fish survival studies.

Role of viscosity in the preferential concentration of heated, bidispersed particles

We study the effect of temperature-dependent viscosity on the preferential concentration of bidispersed, externally-heated solid particles in decaying isotropic turbulence via direct numerical simulations (DNS). More specifically, we investigate the role of liquid- and gas-like viscosity–which respectively decrease and increase with temperature–on the preferential concentration of small and large particles due to turbulence. The bidispersed particles enhance the overall distribution compared to monodispersed flows as the voids created by the particles of one size are occupied by the other particles. Particle clusters emerge irrespective of the Stokes number although the clustering characteristics differ based on the functional form of the temperature-dependent viscosity. When particles are externally heated in a variable viscosity flow, the Kolmogorov-based Stokes number, Stη, is not sufficient to predict preferential concentration. Increased clustering is observed, especially for small-sized particles, as the fluid is heated and turbulence decays. This increased clustering is explained through a viscous capturing mechanism in which the initial clustering, prior to the onset of heating, is responsible for the creation of local hot-spots in the flow as the particles are heated. In a gas, these higher temperature regions have higher viscosity which cause other particles to be captured due to the increased drag. The increased drag of the gas results in a lower fluid-particle drift velocity as compared to the liquid, despite the significantly lower turbulent kinetic energy of the flow. The relative distribution of the particles as a function of vorticity and strain rate magnitude reveals a bimodal distribution in which a higher proportion of the particles aggregate in mid- and high strain/vorticity regions as the turbulence decays.

Influence of Geometric Parameters of Tiny Blades on the Shroud of a Centrifugal Pump on the Cavitation Suppression Effect

In this paper, the effect of adding non-connected forms of tiny blades with different parameters of radial position, width, and length to the shroud of the impeller on the cavitation performance of a centrifugal pump is researched. A modified SST k-ω turbulence model combined with the Zwart–Gerber–Belamri cavitation model is used for numerical simulation of the model pump. The results show that the numerical prediction of the original pump agrees well with the experimental results. Adding tiny blades with different radial positions, widths, and lengths has a small effect on the pump head and efficiency under each working condition. Adding tiny blades near the impeller inlet has better suppression of cavitation in the initial stage, and adding tiny blades in the middle and backward positions has a better suppression effect on all stages of cavitation. There is an optimal tiny blade width to make the cavitation suppression effect optimal, and the optimal width of the model in this study is 3/4 of that of the main blade. The effect of a tiny blade length on cavitation performance is small. The existence of tiny blades slightly increases the turbulent kinetic energy in the low-turbulent kinetic energy region near the impeller inlet, significantly reduces the turbulent kinetic energy in the high-turbulent kinetic energy region near the outlet, and reduces the overall pressure pulsation main frequency amplitude during pump operation, making its operation more stable.

Microbarometer measurements to probe turbulent kinetic energy in the atmospheric boundary layer

Microbarometer networks are typically in place for the detection of atmospheric infrasound waves. The monitoring of such waves can be of interest for source characterization, which include a wide array of geophysical and man-made sources, as well as atmospheric infrasound propagation studies. In such studies, the presence of turbulence and other wind-induced effects are considered a nuisance. For this reason, wind noise filters are typically in place for suppression. As Cuxart et al. [BLM (2013)] pointed out, microbarometer measurements indeed have a value for detection of local and remote turbulence and measurements can be used to estimate turbulent kinetic energy (TKE). In this study, we compare microbarometer observations at the Cabauw atmospheric research site to co-located in-situ TKE measurements. Moreover, we compare turbulence timeseries to predictions from the high-resolution HARMONIE weather model in which this quantity is parameterized. This approach has two foreseen applications: (1) modeled TKE fields could possibly help in identifying regions that are most appropriate for infrasound monitoring due to low TKE values and (2) microbarometer observations could possibly be of use in the further refining of the modeled TKE parameter.