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"Local flow"
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Dead end : suburban sprawl and the rebirth of American urbanism
'Dead End' traces how the ideal of a safe, green, orderly retreat where hardworking members of the middle class could raise their children away from the city mutated into the McMansion and strip mall-ridden suburbs of today.
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A numerical study to investigate the flow pattern around parabolic dish receiver system
The efficiency of the parabolic dish system is significantly impacted by the thermal energy lost from the receiver. Well-established correlations can be used to determine the energy’s conduction and radiation modes. However, due to the complex flow behavior near the cavity receiver, estimating the loss of convective heat is difficult. Surprisingly, the majority of studies has solely examined forced convection from the cavity and has not considered the impact of the dish structure. The presence of the dish structure may alter the local wind patterns near the cavity, which may also affect heat loss. Given the importance of an improved thermal model, the behavior of the wind flow must thus be investigated by adding the dish structure. The numerical results obtained in this work confirm that the dish has a significant impact on the flow in the cavity region. Under normal operating conditions, the cavity receiver is protected from the free stream flow by the dish architecture. Because the local flow velocity is relatively low near the receiver, there is a noticeable reduction in heat loss from the device.
Journal Article
Experimental investigation on nonlinear flow and solute transport behavior in interlayer expansion fractures
After the mines closed, the groundwater level quickly rises, and pollutants in the underground space will migrate outward along the flow channel, causing pollution to the surrounding environment. The different morphologies of interlayer fractures in the goaf fracture zone are one of the main flow channels, including interlayer expansion fracture. Fracture expansion can impact a local flow field, which, in turn can affect solute transport characteristics. The microscale flow field and solute transport behavior in fracture with different expansion characteristics were investigated by a microscale visualization experiment. The experimental results indicate that there is a significant difference in the pressure gradient between different expansion regions. When nonlinear flow characteristics (reflux or eddy) begin to occur within the expansion region, the pressure gradient value also begins to deviate from Darcy's law under the same
Re
. The maximum velocity position changes from the centerline to the upper and lower surfaces of the expansion region, a low velocity region is generated in the main flow channel, which moves to the outlet as
Re
increase, and the cross-section velocity curve also transforms into a bullet shape as
Re
increase. By developing a set of programs to establish a relationship between the average image luminance and the tracer particles concentration, the solute concentration variation curve in the expansion region was obtained. The process of solute invasion into fractures is divided into four stages. The change in the local flow field caused by the different expansion characteristics directly affects the duration of these four stages. As the area of the region increases, the proportion of the main channel area decreases relatively, the difference between the solute concentration curve of the whole fracture and the expansion region gradually decreases.
Journal Article
Numerical Study on Concrete Pumping Behavior via Local Flow Simulation with Discrete Element Method
The use of self-consolidating concrete and advanced pumping system enables efficient construction of super high-rise buildings; however, risks such as clogging or even bursting of pipeline still exist. To better understand the fresh concrete pumping mechanisms in detail, the discrete element method is employed in this paper for the numerical simulation of local pumping problems. By modeling the coarse aggregates as rigid clumps and appropriately defining the contact models, the concrete flow in representative pipeline units is well revealed. Important factors related to the pipe geometry, aggregate geometry and pumping condition were considered during a series of parametric studies. Based on the simulation results, their impact on the local pumping performance is summarized. The present work demonstrates that the discrete element simulation offers a useful way to evaluate the influence of various parameters on the pumpability of fresh concrete.
Journal Article
Comparative Analysis of Local Flow Fields of Typical Inner Jet Holes-Type Reverse Circulation Drill Bit for Pneumatic Hollow-Through DTH Hammer Based on CFD Simulation
The reverse circulation drill bit is the key component for the efficient and smooth implementation of the pneumatic hollow-through down-the-hole (DTH) hammer reverse circulation continuous coring (sampling) technology. To obtain the structural form of a reverse circulation drill bit with better reverse circulation performance, revealing its local flow fields by computational fluid dynamics (CFD) simulation is an effective approach. Taking the inner jet holes-type reverse circulation drill bit as the research object, three kinds of symmetrical and asymmetrical structures of inner jet holes were proposed. The CFD simulation results show that increasing the air volume supply and the number of inner jet holes leads to an increase in the velocity of air flow jet within the inner jet holes, an increase in the negative pressure formed in the central through channel below the inner jet holes, an enhancement of the reverse circulation performance and suction capacity formed by the reverse circulation drill bit, and an acceleration of the upward flow velocity of the rock cores (samples) located at the bottom of the borehole. Additionally, the reverse circulation performance formed by the reverse circulation drill bit with staggered arranged inner jet holes is superior to that of the reverse circulation drill bit with uniformly distributed inner jet holes. Under the same simulation conditions, the static pressure (i.e., negative pressure) and the upward flow velocity formed by the JB6 model are 2.34 kPa and 30.778 m/s higher than those formed by the JB3-3 model, while these two values formed by the JC6 model are 0.197 kPa and 3.689 m/s higher than those formed by the JB6 model, respectively. In conclusion, an asymmetric structural design would be more reasonable for the design of the inner jet holes-type reverse circulation drill bit.
Journal Article
A Model Based on the Random Forest Algorithm That Predicts the Total Oil–Water Two-Phase Flow Rate in Horizontal Shale Oil Wells
Due to variables like wellbore deviation variation and flow rate, the local flow velocity in the output wellbore of horizontal shale oil wells varied significantly at various points in the wellbore cross-section, making it challenging to calculate the total single-layer production with accuracy. The oil–water two-phase flow rate calculation techniques for horizontal wells developed based on particular flow patterns and array spinners had excellent applicability in their respective niches but suffered from poor generalizability and demanding experience levels for logging interpreters. In this study, we employed five spinners in a triangular walled array instrument to create the multi-decision tree after figuring out how many leaf nodes there were and examining the defining characteristics of the observed values gathered under various experimental setups. The construction of the entire oil–water two-phase flow prediction model was made possible when the random forest regression approach was used with it. The total oil–water flow rate at each perforated layer was predicted using the model in sample wells, and the mean square error with the third party’s interpretation conclusion was 1.42, indicating that the model had an excellent application effect. The approach, which offered a new interpretation method for calculating the oil–water two-phase flow rate of horizontal wells based on multi-location local flow rate, required less interpretation knowledge from the interpreter and had a stronger generalization capacity.
Journal Article
Effect of Volute-Tongue Clearance on the Aerodynamic Performance and Noise of Multi-Wing Centrifugal Fan for Air Conditioning
Multi-wing centrifugal fans are wildly used in the central air-conditioning. The influence of dimensionless clearance of the volute-tongue on aerodynamic performance and noise is studied by numerical simulation and experimental tests in this paper. The complicated internal flow related to unsteady flow in a centrifugal fan with multiple wings is investigated by numerical simulation. Besides, the influence of circumstance on the noise is analyzed. It is testified that the internal flow of centrifugal fans is ameliorated using appropriate volute tongue clearance. Reduced eddy current decreased the local-flow loss near the volute tongue and exit. The experimental results show that the static pressure of model △t/R2=0.12 rose to 7.5 Pa and the aerodynamics noise value reduced to 4 dB compared with that of a reference model. Meanwhile, an obvious reduction of aerodynamics noise by 3.74 dB is obtained for model △t/R2=0.12 installed in the air conditioning unit. The static pressure of centrifugal fan is significantly improved for the model with a cochlear tongue clearance ratio of △t/R2=0.12. It is further demonstrated that the proper dimensionless distance effectively suppresses the aerodynamic noise of forward multi-wing fans.
Journal Article
Performance and wake characteristics of tidal turbines in an infinitely large array
The efficiency of tidal stream turbines in a large array depends on the balance between negative effects of turbine-wake interactions and positive effects of bypass-flow acceleration due to local blockage, both of which are functions of the layout of turbines. In this study we investigate the hydrodynamics of turbines in an infinitely large array with aligned or staggered layouts for a range of streamwise and lateral turbine spacing. First, we present a theoretical analysis based on an extension of the linear momentum actuator disc theory for perfectly aligned and staggered layouts, employing a hybrid inviscid-viscous approach to account for the local blockage effect within each row of turbines and the viscous (turbulent) wake mixing behind each row in a coupled manner. We then perform large-eddy simulation (LES) of open-channel flow for 28 layouts of tidal turbines using an actuator line method with doubly periodic boundary conditions. Both theoretical and LES results show that the efficiency of turbines (or the power of turbines for a given bulk velocity) in an aligned array decreases as we reduce the streamwise turbine spacing, whereas that in a staggered array remains high and may even increase due to the positive local blockage effect (causing the local flow velocity upstream of each turbine to exceed the bulk velocity) if the lateral turbine spacing is sufficiently small. The LES results further reveal that the amplitude of wake meandering tends to decrease as we reduce the lateral turbine spacing, which leads to a lower wake recovery rate in the near-wake region. These results will help to understand and improve the efficiency of tidal turbines in future large arrays, even though the performance of real tidal arrays may depend not only on turbine-to-turbine interactions within the array but also on macro-scale interactions between the array and natural tidal currents, the latter of which are outside the scope of this study.
Journal Article
Design and Simulation of a MEMS Flow Velocity Sensor Based on a Narrow Channel Structure
Inspired by the constricted channel structures observed in the lateral line systems of fish, this study proposes a flow velocity sensor design that integrates a narrow channel structure with a MEMS piezoresistive sensing element. A locally constricted channel is introduced near the sensing unit to enhance the local flow velocity, thereby improving the response strength of the MEMS piezoresistive element. The sensing element adopts a beamcilium configuration and outputs a voltage signal, facilitating system integration and signal processing. Multiphysics simulation results demonstrate that the proposed design achieves a significant voltage gain at a representative flow velocity of 1 m/s, with sensitivity improved by more than 50% compared to designs without the constricted structure. This research provides a novel approach for optimizing high-sensitivity MEMS flow velocity sensors and holds promising application potential in underwater intelligent equipment and microscale fluid detection.
Journal Article
Global drag reduction and local flow statistics in Taylor–Couette turbulence with dilute polymer additives
We present an experimental study on the drag reduction by polymers in Taylor–Couette turbulence at Reynolds numbers ($Re$) ranging from $4\\times 10^3$ to $2.5\\times 10^4$. In this $Re$ regime, the Taylor vortex is present and accounts for more than 50 % of the total angular velocity flux. Polyacrylamide polymers with two different average molecular weights are used. It is found that the drag reduction rate increases with polymer concentration and approaches the maximum drag reduction (MDR) limit. At MDR, the friction factor follows the $-0.58$ scaling, i.e. $C_f \\sim Re^{-0.58}$, similar to channel/pipe flows. However, the drag reduction rate is about $20\\,\\%$ at MDR, which is much lower than that in channel/pipe flows at comparable $Re$. We also find that the Reynolds shear stress does not vanish and the slope of the mean azimuthal velocity profile in the logarithmic layer remains unchanged at MDR. These behaviours are reminiscent of the low drag reduction regime reported in channel flow (Warholic et al., Exp. Fluids, vol. 27, no. 5, 1999, pp. 461–472). We reveal that the lower drag reduction rate originates from the fact that polymers strongly suppress the turbulent flow while only slightly weaken the mean Taylor vortex. We further show that polymers steady the velocity boundary layer and suppress the small-scale Görtler vortices in the near-wall region. The former effect reduces the emission rate of both intense fast and slow plumes detached from the boundary layer, resulting in less flux transport from the inner cylinder to the outer one and reduces energy input into the bulk turbulent flow. Our results suggest that in turbulent flows, where secondary flow structures are statistically persistent and dominate the global transport properties of the system, the drag reduction efficiency of polymer additives is significantly diminished.
Journal Article