scholarly journals Numerical analysis of hydrodynamics influenced by deformed bed due to near-bank vegetation patch

2021 ◽  
Author(s):  
Zijing Yi ◽  
Yi Sun ◽  
Xiekang Wang ◽  
Daoxudong Liu ◽  
Xufeng Yan
Keyword(s):  
Longitudinal Velocity ◽  
Leading Edge ◽  
Horizontal Shear ◽  
Vegetation Patch ◽  
Bed Topography ◽  
Surface Gradient ◽  
Flow Adjustment ◽  
Flow Mixing ◽  
Flow Motion ◽  
Open Region

Abstract This study with a 2D hydro-morphological model analyzes hydrodynamics over flat and deformed beds with a near-bank vegetation patch. By varying the patch density, the generalized results show that the hydrodynamics over deformed beds differs a lot from those over flat beds. It is found that the deformed bed topography leads to an apparent decrease in longitudinal velocity and bed shear stress in the open region and longitudinal surface gradient for the entire vegetated reach. However, the transverse flow motion and transverse surface gradient in the region of the leading edge and trailing edge is enhanced or maintained, suggesting the strengthening of secondary flow motion. Interestingly, the deformed bed topography tends to alleviate the horizontal shear caused by the junction-interface horizontal coherent vortices, indicating that the turbulence-induced flow mixing is highly inhibited as the bed is deformed. The interior flow adjustment through the patch for the deformed bed requires a shorter distance, La, which is related to the vegetative drag length, (Cda)−1, with a logarithmic formula (La = 0.4ln[(Cda)−1] + b, with b = 3.83 and 4.03 for the deformed and flat beds). The tilting bed topographic effect in the open region accelerating the flow may account for the quick flow adjustment.

scholarly journals Impacts of Deformed Bed Topography on Hydrodynamics in a Near-Bank Vegetated Channel: A Numerical Study

2020 ◽  
Author(s):  
Daoxudong Liu ◽  
Wenjun Li
Keyword(s):  
Numerical Study ◽  
Water Discharge ◽  
Longitudinal Velocity ◽  
Leading Edge ◽  
Secondary Flows ◽  
Bed Topography ◽  
Surface Gradient ◽  
Flow Adjustment ◽  
Flow Mixing ◽  
Open Region

Understanding how the deformed bed topography induced by near-bank vegetation impacts the hydrodynamics is significant for understanding the maintenance condition of bed morphology and further fluvial evolution. This issue has rarely been addressed by current studies. This study with a 2D hydro-morphological model investigates the hydrodynamics over flat and deformed beds with a near-bank vegetation patch. By varying the patch density, the generalized results show that the hydrodynamics for the deformed bed differs a lot from those for the flat bed. It is found that deformed bed topography leads to an apparent decrease in longitudinal velocity and bed shear stress in the open region and longitudinal surface gradient for the entire vegetated reach. However, the transverse flow motion and transverse surface gradient in the region of the leading edge and trailing edge is enhanced or maintained, suggesting the strengthening of secondary flows. Interestingly, the deformed bed topography tends to alleviate the turbulent effect caused by the junction-interface horizontal coherent vortices, indicating that the turbulence-induced flow mixing is highly inhibited by the deformed bed. Alternatively, the enhanced secondary flows might provide compensation for the flow mixing for the deformed bed, confirmed by a faster recovery of the redistributed water discharge for the vegetated and open regions to the normal value (50%). The interior flow adjustment through the patch for the deformed bed requires a shorter distance, which links the vegetative drag length with a logarithmic relation. The tilting bed topographic effect in the open region to accelerate the flow may account for the faster flow adjustment.

Computational Fluid Dynamics Based Mixing Prediction for Tilt Pad Journal Bearing TEHD Modeling—Part II: Implementation With Machine Learning

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Jongin Yang ◽  
Alan Palazzolo
Keyword(s):  
Machine Learning ◽  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Three Dimensional ◽  
Leading Edge ◽  
Cfd Modeling ◽  
Inlet Temperature ◽  
Training Set ◽  
Simplified Approach ◽  
Flow Mixing

Abstract Reynolds based thermo-elasto-hydrodynamic (TEHD) simulations of tilting pad journal bearings (TPJBs) generally provide accurate results; however, the uncertainty of the pad’s leading edge thermal boundary conditions causes uncertainty of the results. The highly complex thermal-flow mixing action between pads (BPs) results from the oil supply nozzle jets and geometric features. The conventional Reynolds approach employs mixing coefficients (MCs), estimated from experience, to approximate a uniform inlet temperature for each pad. Part I utilized complex computational fluid dynamics (CFD) flow modeling to illustrate that temperature distributions at the pad inlets may deviate strongly from being uniform. The present work retains the uniform MC model but obtains the MC from detailed three-dimensional CFD modeling and machine learning, which could be extended to the radially and axially varying MC case. The steps for implementing an artificial neural network (ANN) approach for MC regression are provided as follows: (1) utilize a design of experiment step for obtaining an adaptable training set, (2) conduct CFD simulations on the BP to obtain the outputs of the training set, (3) apply an ANN learning process by Levenverg–Mardquart backpropagation with the Bayesian regularization, and (4) couple the ANN MC results with conventional TEHD Reynolds models. An approximate log fitting method provides a simplified approach for MC regression. The effectiveness of the Reynolds TEHD TPJB model with ANN regression-based MC distributions is confirmed by comparison with CFD based TEHD TPJB model results. The method obtains an accuracy nearly the same as the complete CFD model, but with the computational economy of a Reynolds approach.

An Experimental Investigation of the Flow in a 1½ Stage Axial Turbine With Regard to a High Level of Cooling-Air Injection

1999 ◽  
Author(s):  
U. Reinmöller ◽  
H. E. Gallus
Keyword(s):  
Film Cooling ◽  
Turbine Blades ◽  
Gas Injection ◽  
Leading Edge ◽  
Experimental Investigations ◽  
Flow Angle ◽  
Turbine Cooling ◽  
Flow Mixing ◽  
Reference Measurements ◽  
High Level

Experimental investigations of flow mixing due to film cooling of turbine blades have been performed. In a 1½-stage axial air turbine cooling gas (cool nitrogen down to −130 °C) was blown directly onto the leading edge of the first stator by special gas injector devices. In order to provide a database for the verification of numerical codes and to give an impression of the mixing process the gas has been injected at different radial positions. Furthermore the cooling massflow and cooling temperature were varied. The measuring data were obtained using pneumatic 5-hole probes with temperature sensors. The presented experimental data were simultaneous acquired in the planes behind both stators and the rotor. The results are compared and, discussed with reference measurements without cooling gas injection. It is shown that the effect of cooling gas injection is apparent in the wake of the first stator where it causes a small decrease in the pressure distribution as a result of increased flow mixing. Behind the first stator differences in the circumferentially averaged pitchwise flow angle due to the injected gas were not measured. Furthermore, temperature measurements clearly show the effect of the cooling gas injection in all planes. Even behind the second stator the different magnitudes of the temperature distribution are caused by the various injection of cooling gas.

scholarly journals A New Well-Balanced Reconstruction Technique for the Numerical Simulation of Shallow Water Flows with Wet/Dry Fronts and Complex Topography

Water ◽  
2018 ◽  
Vol 10 (11) ◽  
pp. 1661 ◽  
Author(s):  
Zhengtao Zhu ◽  
Zhonghua Yang ◽  
Fengpeng Bai ◽  
Ruidong An
Keyword(s):  
Shallow Water ◽  
Water System ◽  
Reconstruction Technique ◽  
Test Cases ◽  
Bed Topography ◽  
Surface Gradient ◽  
Shallow Water Flows ◽  
Shallow Water System ◽  
The One ◽  
Flow Variables

This study develops a new well-balanced scheme for the one-dimensional shallow water system over irregular bed topographies with wet/dry fronts, in a Godunov-type finite volume framework. A new reconstruction technique that includes flooded cells and partially flooded cells and preserves the non-negative values of water depth is proposed. For the wet cell, a modified revised surface gradient method is presented assuming that the bed topography is irregular in the cell. For the case that the cell is partially flooded, this paper proposes a special reconstruction of flow variables that assumes that the bottom function is linear in the cell. The Harten–Lax–van Leer approximate Riemann solver is applied to evaluate the flux at cell faces. The numerical results show good agreement with analytical solutions to a set of test cases and experimental results.

scholarly journals Numerical Study of Horizontal Shear Instability Waves along Narrow Cold Frontal Rainbands

2011 ◽  
Vol 68 (4) ◽  
pp. 878-903 ◽  
Author(s):  
Masayuki Kawashima
Keyword(s):  
Numerical Study ◽  
Leading Edge ◽  
Shear Instability ◽  
Vertical Shear ◽  
Horizontal Shear ◽  
Edge Waves ◽  
Instability Waves ◽  
Low Level ◽  
Wide Range ◽  
The Mean

Abstract The effects of variations in low-level ambient vertical shear and horizontal shear on the alongfront variability of narrow cold frontal rainbands (NCFRs) that propagate into neutral and slightly unstable environments are investigated through a series of idealized cloud-resolving simulations. In cases initialized with slightly unstable sounding and weak ambient cross-frontal vertical shears, core-gap structures of precipitation along NCFRs occur that are associated with wavelike disturbances that derive their kinetic energy mainly from the mean local vertical shear and buoyancy. However, over a wide range of environmental conditions, core-gap structures of precipitation occur because of the development of a horizontal shear instability (HSI) wave along the NCFRs. The growth rate and amplitude of the HSI wave decrease significantly as the vertical shear of the ambient cross-front wind is reduced. These decreases are a consequence of the enhancement of the low-level local vertical shear immediately behind the leading edge. The strong local vertical shear acts to damp the vorticity edge wave on the cold air side of the shear zone, thereby suppressing the growth of the HSI wave through the interaction of the two vorticity edge waves. It is also noted that the initial wavelength of the HSI wave increases markedly with increasing horizontal shear. The local vertical shear around the leading edge is shown to damp long HSI waves more strongly than short waves, and the horizontal shear dependency of the wavelength is explained by the decrease in the magnitude of the vertical shear relative to that of the horizontal shear.

scholarly journals Flow adjustment and interior flow associated with a rectangular porous obstruction

2011 ◽  
Vol 680 ◽  
pp. 636-659 ◽  
Author(s):  
JEFFREY T. ROMINGER ◽  
HEIDI M. NEPF
Keyword(s):  
Aquatic Vegetation ◽  
Leading Edge ◽  
Low Flow ◽  
Rectangular Array ◽  
Canopy Flow ◽  
Flow Adjustment ◽  
Interior Flow ◽  
Flow Blockage ◽  
Adjustment Length ◽  
Canopy Drag

The flow at the leading edge and in the interior of a rectangular porous obstruction is described through experiments and scaling. The porous obstruction consists of an emergent, rectangular array of cylinders in shallow flow, a configuration that mimics aquatic vegetation. The main features of the flow depend upon the non-dimensional canopy flow-blockage, which is a function of the obstruction width and porosity. For the ranges of canopy flow-blockage tested in this paper, the fluid decelerates upstream of the obstruction over a length scale proportional to the array width. For high flow-blockage, the interior adjustment length within the porous obstruction is set by the array width. For low flow-blockage, the array's frontal area per unit volume sets the interior adjustment length. Downstream of the adjustment regions, the interior velocity is governed by a balance between the lateral divergence of the turbulent stress and canopy drag, or by a balance between the pressure gradient and canopy drag, depending on the lateral penetration into the array of Kelvin–Helmholtz (KH) vortices, which is set by the non-dimensional canopy flow-blockage. For a porous obstruction with two stream-parallel edges, the KH vortex streets along the two edges are in communication across the width of the array: a phenomenon that results in cross-array vortex organization, which significantly enhances the vortex strength and creates significant lateral transport within the porous obstruction.

scholarly journals Numerical Study of Precipitation Core-Gap Structure along Cold Fronts

2007 ◽  
Vol 64 (7) ◽  
pp. 2355-2377 ◽  
Author(s):  
Masayuki Kawashima
Keyword(s):  
Thermal Stratification ◽  
Cold Front ◽  
Numerical Study ◽  
Leading Edge ◽  
Shear Instability ◽  
Vertical Shear ◽  
Horizontal Shear ◽  
Cold Fronts ◽  
Simulated Precipitation ◽  
Gap Structure

Abstract The mechanism responsible for the core-gap structure of precipitation along narrow cold-frontal rainbands (NCFRs) is investigated through analyses of idealized cloud-resolving simulations of cold fronts. The control simulation, in which the prefrontal thermal stratification is characterized by a weak convective instability at low levels with convective available potential energy (CAPE) of ∼60 J kg−1, reproduces the typical alongfront variability of observed NCFRs. The simulated NCFR is broken up into regularly spaced, ellipsoidal cores oriented at a clockwise angle to the cold front. While horizontal-shear instability (HSI) has frequently been proposed as a mechanism leading to the alongfront variability of NCFRs, no characteristic features of HSI are recognized in the simulated vertical vorticity field at the leading edge of the cold front. The alongfront variability in precipitation is attributed to the formation of a wavelike disturbance just above the leading edge of the cold front. The wave phase lines are oriented nearly perpendicular to the direction of mean vertical shear, with enhanced (suppressed) precipitation occurring at the wave updrafts (downdrafts). An analysis of the eddy kinetic energy budget indicates that the wavelike disturbance derives most of its energy from the mean vertical shear and the buoyancy. Sensitivity experiments reveal a systematic relationship between the alongfront variability of NCFRs and the stability of the prefrontal thermal stratification. Simulated precipitation cores remain essentially parallel to the cold front when the prefrontal environment is absolutely stable or almost neutral to surface parcel ascent. The typical alongfront variability of NCFRs is reproduced for weakly unstable environments with small amounts of CAPE (≤140 J kg−1). On the other hand, simulations with sufficiently unstable environments produce precipitation cores oriented counterclockwise to cold fronts.

Effect of inlet distortion on the performance of axial transonic contra-rotating compressor

2016 ◽  
Vol 232 (1) ◽  
pp. 42-54 ◽  
Author(s):  
Hanru Liu ◽  
Yangang Wang ◽  
Songchuan Xian ◽  
Wenbin Hu
Keyword(s):  
Total Pressure ◽  
Pressure Ratio ◽  
Leading Edge ◽  
Tip Leakage Flow ◽  
Stall Margin ◽  
Tip Leakage ◽  
Inlet Distortion ◽  
Flow Mixing ◽  
Inflow Conditions ◽  
Total Pressure Ratio

The present paper numerically conducted full-annulus investigation on the effects of circumferential total pressure inlet distortion on the performance and flow field of the axial transonic counter-rotating compressor. Results reveal that the inlet distortion both deteriorates the performance of the upstream and downstream rotors resulting in reduction of total pressure ratio, efficiency and stall margin of the transonic contra-rotating compressor. Regarding the development of distortion inside compressor, the downstream rotor reinforces the air-flow mixing effects and, thus, attenuates the distortion intensity significantly. Under the distorted inflow conditions, the detached shockwave at the leading edge of downstream rotor interacts with the tip leakage flow and causes the blockage of the blades passage, which is one important reason for the transonic contra-rotating compressor stall.

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