scholarly journals Smooth Open Channel with Increasing Aspect Ratio: Influence on Secondary Flow

Water ◽  
2019 ◽  
Vol 11 (9) ◽  
pp. 1872
Author(s):  
Siyu Jing ◽  
Wenjun Yang ◽  
Yue Chen
Keyword(s):  
Aspect Ratio ◽  
Velocity Distribution ◽  
Secondary Flow ◽  
Open Channel ◽  
Maximum Velocity ◽  
Secondary Flows ◽  
Measuring Instruments ◽  
Contour Maps ◽  
Aspect Ratios ◽  
Logarithmic Distribution

A high-resolution particle image velocitmetry system is used to investigate the relationship between secondary flow and aspect ratio in a straight channel. Considering the symmetry of open channel flow, the flow parameters in half of the flume are measured. Since the variation of the aspect ratio has a direct impact on the intensity and structure of secondary flows, this study was conducted in a smooth open channel to study the influence of aspect ratio on the structure and strength of secondary flows with aspect ratio change from 3 to 7.5 under supercritical flow condition. Profiles and contour-maps of time-averaged stream-wise and vertical velocities were acquired using precise measuring instruments. The results show that there are several secondary flow cells in the cross section, and their structure affects the velocity distribution and energy distribution, which makes the velocity distribution deviate from the traditional logarithmic distribution, and the maximum velocity occur below the surface. The flow intensity of secondary flows is different under different aspect ratios. Results show great agreement with classical theory.

Secondary flows and developing, turbulent boundary layers in a rotating duct

1998 ◽  
Vol 373 ◽  
pp. 1-32 ◽  
Author(s):  
I. MACFARLANE ◽  
P. N. JOUBERT ◽  
T. B. NICKELS
Keyword(s):  
Aspect Ratio ◽  
Secondary Flow ◽  
Boundary Layers ◽  
Suction Side ◽  
Turbulent Boundary Layers ◽  
Secondary Flows ◽  
Turbulent Shear ◽  
Mean Flow ◽  
Flow Strength ◽  
Aspect Ratios

The work presented in this paper represents an experimental investigation into secondary flows, turbulent boundary layers and the interaction of the two as they develop in a zero-pressure-gradient rotating flow field. A duct of intermediate aspect ratio was used to examine secondary flows and determine when they begin to govern the boundary layer development. The aspect ratio (A) was defined as duct height/width at the upstream end of the working section. Measurements were taken at three aspect ratios: A=1, 2 and 4.A qualitative indication of secondary flow strength was established with mean-cross-stream-plane velocity measurements. A first-order analysis of the secondary flow is presented which provides a reasonable estimation of their strength. Mid-span mean-flow, turbulence and spectra profiles were measured on the duct walls parallel to the axis of rotation. Results are generally presented for A=2 and 1. For A=4 and 2 there were minor effects of secondary flows observed on the mid-span mean flow parameters. The turbulent shear measurements showed some secondary flow effect for A=2. All turbulence and mean-flow quantities were strongly affected by secondary flows for A=1. Spectra results presented for A=2 showed most variation at the low-to-mid wavenumber end. Spectra results for A=1 showed a bodily shift of the whole spectrum towards low wavenumber on the pressure side and high wavenumber on the suction side.

Numerical Investigation on Mist/Air Cooling in Rectangular Ribbed Channels With Various Aspect Ratios

2017 ◽  
Author(s):  
Junxiong Zeng ◽  
Tieyu Gao ◽  
Jun Li ◽  
Jianying Gong
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Secondary Flow ◽  
Gas Turbine ◽  
Gas Turbines ◽  
Secondary Flows ◽  
Air Cooling ◽  
Mass Ratio ◽  
Aspect Ratios

Continuously increasing gas turbine inlet temperature to further improve thermal efficiency and power output of gas turbines leads to acquiring a higher cooling effectiveness of gas turbine blades and vanes to protect them from high temperature corrosion and creeping damage. One of the new and promising technologies to greatly increase heat transfer is mist cooling by injecting a small amount of tiny droplets into coolant flow. This paper aims to numerically study the flow and heat transfer behaviors of mist/air coolant in rectangular ribbed channels with various aspect ratios of 1/4, 1/2, 1/1, 2/1 and rib angle of 60°. In addition, the distribution of secondary flows in the four ribbed channels and its effect on heat transfer are analyzed in detail. The effects of Reynolds number ranging from 10,000 to 60,000, mist mass ratios ranging from 1% to 4%, and droplet sizes ranging from 5 μm to 20 μm on heat transfer characteristics of mist/air cooling are investigated. As a comparison, the air-only coolant is also considered in the present study. The Eulerian-Lagrangian particle tracking method is adopted in this study to simulate the two-phase flow mist/air cooling. Turbulence model validation has been conducted for air-only, indicating that the numerical results with SST k-ω model are fairly consistent with experimental data. The results show that the aspect ratio has insignificant influence on longitudinal secondary flow distribution in the four ribbed channels, but greatly affects the size of main secondary flows. The channel with a smaller aspect ratio obtains a larger size of main secondary flow, which may result in decreasing the heat transfer coefficient. The average Nu on ribbed surfaces presents an increasing trend with Reynolds number and mist mass ratio for mist/air cooling. The heat transfer enhancement of mist/air as compared to air-only increases from 12.3% to 91.86% when Reynolds number ranges from 10,000 to 60,000 with injecting 2% mist into air coolant, while that increases from 7.96% to 113.15% when mist mass ratio increases from 1% to 4%. The average Nu initially increases with droplet size and then decreases. A peak value of average Nu is obtained in the case of 15μm mist among all the sizes of droplets. The case of AR = 2/1 obtains the highest average Nu, followed by the cases of AR = 1/2, 1/1 and 1/4 for both air-only and mist/air. The channel with aspect ratio of 1/2 obtains the best thermal performance in mist/air cooling channel.

scholarly journals Secondary Flow Control in Low Aspect Ratio Vanes Using Splitters

2016 ◽  
Author(s):  
Christopher Clark ◽  
Graham Pullan ◽  
Eric Curtis ◽  
Frederic Goenaga
Keyword(s):  
Kinetic Energy ◽  
Aspect Ratio ◽  
Secondary Flow ◽  
Current Practice ◽  
Leading Edge ◽  
Horseshoe Vortex ◽  
Secondary Flows ◽  
Optimization Approach ◽  
Flow Strength ◽  
Low Aspect Ratio

Low aspect ratio vanes, often the result of overall engine architecture constraints, create strong secondary flows and high endwall loss. In this paper, a splitter concept is demonstrated that reduces secondary flow strength and improves stage performance. An analytic conceptual study, corroborated by inviscid computations, shows that the total secondary kinetic energy of the secondary flow vortices is reduced when the number of passages is increased and, for a given number of vanes, when the inlet endwall boundary layer is evenly distributed between the passages. Viscous computations show that, for this to be achieved in a splitter configuration, the pressure-side leg of the low aspect ratio vane horseshoe vortex, must enter the adjacent passage (and not “jump” in front of the splitter leading edge). For a target turbine application, four vane designs were produced using a multi-objective optimization approach. These designs represent: current practice for a low aspect ratio vane; a design exempt from thickness constraints; and two designs incorporating splitter vanes. Each geometry is tested experimentally, as a sector, within a low-speed turbine stage. The vane designs with splitters geometries were found to reduce the measured secondary kinetic energy, by up to 85%, to a value similar to the design exempt from thickness constraints. The resulting flowfield was also more uniform in both the circumferential and radial directions. One splitter design was selected for a full annulus test where a mixed-out loss reduction, compared to the current practice design, of 15.3% was measured and the stage efficiency increased by 0.88%.

Performance Prediction for High Turning Low Aspect Ratio Stator Cascades in the Transonic Regime

1970 ◽  
Vol 92 (4) ◽  
pp. 390-398
Author(s):  
H. F. L. Griepentrog
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Flow Field ◽  
Performance Prediction ◽  
Transonic Flow ◽  
Secondary Flows ◽  
Compressor Stage ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Boundary Layer Interaction

This paper describes a method for the prediction of the transonic flow field in a high solidity, high turning cascade, suitable for use as stator of a shock-in-rotor supersonic compressor stage. Effects of shock boundary layer interaction is taken into account by empirical correlation, valid for blade aspect ratios below unity. Use of partial slots for reduction of the secondary flows is briefly discussed and a correlation on slot efficiency is presented.

Secondary Flow and Hydraulic Losses Within Sinuous Conduits of Rectangular Cross Section

1992 ◽  
Vol 114 (4) ◽  
pp. 593-600 ◽  
Author(s):  
Yukimaru Shimizu ◽  
Yoshiki Futaki ◽  
C. Samuel Martin
Keyword(s):  
Aspect Ratio ◽  
Secondary Flow ◽  
Cross Section ◽  
Rectangular Cross Section ◽  
Flow Patterns ◽  
Geometric Configuration ◽  
Hydraulic Losses ◽  
Aspect Ratios ◽  
Wide Range ◽  
The Relationship

This paper describes the relationship between hydraulic losses and secondary flow within sinuous conduits with complicated bends. It has been found that the nature of secondary flow present in the bends is quite sensitive to the geometric configuration of the bend and the actual aspect ratio of the conduit section. Indeed, many different secondary flow patterns have been found to exist as the bend geometry is altered. A wide range of experiments has been conducted for various aspect ratios of a rectangular conduit with different curvatures.

Wake Flow and Vortex Shedding Patterns Behind Rotating Finite Length Cylinders

2019 ◽  
Author(s):  
Amber Donaldson ◽  
John C. Vaccaro ◽  
David M. Rooney
Keyword(s):  
Aspect Ratio ◽  
Vortex Shedding ◽  
Rotational Speed ◽  
Maximum Velocity ◽  
Velocity Profiles ◽  
Wake Flow ◽  
Circular Cylinders ◽  
Mean Velocity ◽  
Constant Length ◽  
Aspect Ratios

Abstract An experimental wind tunnel study was performed to assess the effect of aspect ratio and rotational speed of circular cylinders of varying diameter on the flow patterns behind the cylinders in the presence of a uniform upstream crossflow. Six circular cylinders of constant length but different diameters, producing aspect ratios 6 ≤ AR ≤ 32 were examined at a single upstream velocity such that the Reynolds number varied between 1920 ≤ Re ≤ 10240. Rotational speeds from stationary up to 3600 rpm were applied to the cylinders, so that the maximum relative velocity α = πfD/U∞ = 0.80. Mean velocity profiles were measured three diameters downstream of the cylinder axis at 6 equidistant locations, and PSD power spectral density were generated for 26 equidistant locations along the cylinder, to create a comprehensive record of spanwise variations under all rotational conditions. For the highest aspect ratio tested, the wake velocity profiles were independent of rotational speed at all spanwise locations, whereas at lower aspect ratios, the maximum velocity defect diminished with increasing rotational speed along most of the span and became asymmetric near the free end. Two distinct shedding cells were found only for a cylinder with an aspect ratio of twelve at three relative spin rates of 0.067, 0.27, and 0.4. In cases where only a single cell existed, increased rotational speed produced a higher vortex shedding frequency on a given aspect ratio cylinder.

Effects of Aspect Ratio on Higher-Order Moments, Conditional Statistics, TKE Budget and Anisotropy in Narrow Open Channel Flow

2020 ◽  
Author(s):  
Minakshee Mahananda ◽  
Prashanth Reddy Hanmaiahgari ◽  
Ram Balachandar ◽  
Vesselina Roussinova
Keyword(s):  
Aspect Ratio ◽  
Open Channel ◽  
Outer Region ◽  
Higher Order ◽  
Third Order ◽  
Velocity Fluctuations ◽  
The Third ◽  
Aspect Ratios ◽  
Ejections And Sweeps ◽  
Conditional Statistics

The paper investigates the influence of aspect ratio on the higher-order statistics of velocity fluctuations in hydraulically rough narrow OCF. In the experiments, the aspect ratios were varied between 2.5 and 4. Velocities were measured with ADV. The third-order moments were found to be sensitive to the aspect ratio in the outer region. The contributions of all quadrant events are approximately equal in lower aspect ratio flows, whereas ejections and sweeps are the dominant as the aspect ratio increases. The upward transfer of TKE flux increases in the outer layer with increase in aspect ratio. The TKE production and dissipation are found to be dependent on the aspect ratio. The analysis of Reynolds stress AIM reveals that for low aspect ratio flows turbulence tends to attain rod like axisymmetric turbulence only in the intermediate layer whereas for higher aspect ratio, turbulence attains rod like axisymmetric turbulence throughout the depth.

scholarly journals Secondary Flow Control in Low Aspect Ratio Vanes Using Splitters

2017 ◽  
Vol 139 (9) ◽  
Author(s):  
Christopher J. Clark ◽  
Graham Pullan ◽  
Eric Curtis ◽  
Frederic Goenaga
Keyword(s):  
Kinetic Energy ◽  
Aspect Ratio ◽  
Secondary Flow ◽  
Current Practice ◽  
Leading Edge ◽  
Secondary Flows ◽  
Optimization Approach ◽  
Flow Strength ◽  
Low Aspect Ratio ◽  
End Wall

Low aspect ratio vanes, often the result of overall engine architecture constraints, create strong secondary flows and high end-wall loss. In this paper, a splitter concept is demonstrated that reduces secondary flow strength and improves stage performance. An analytic conceptual study, corroborated by inviscid computations, shows that the total secondary kinetic energy (SKE) of the secondary flow vortices is reduced when the number of passages is increased and, for a given number of vanes, when the inlet end-wall boundary layer is evenly distributed between the passages. Viscous computations show that, for this to be achieved in a splitter configuration, the pressure-side leg of the low aspect ratio vane horseshoe vortex, must enter the adjacent passage (and not “jump” in front of the splitter leading edge). For a target turbine application, four vane designs were produced using a multi-objective optimization approach. These designs represent current practice for a low aspect ratio vane, a design exempt from thickness constraints, and two designs incorporating splitter vanes. Each geometry is tested experimentally, as a sector, within a low-speed turbine stage. The vane designs with splitter geometries were found to reduce the measured secondary kinetic energy, by up to 85%, to a value similar to the design exempt from thickness constraints. The resulting flow field was also more uniform in both the circumferential and radial directions. One splitter design was selected for a full annulus test where a mixed-out loss reduction, compared to the current practice design, of 15.3% was measured and the stage efficiency increased by 0.88%.

scholarly journals The impact of volute aspect ratio and tilt on the performance of a mixed flow turbine

2021 ◽  
pp. 095765092199822
Author(s):  
Samuel P Lee ◽  
Simon M Barrans ◽  
Ambrose K Nickson
Keyword(s):  
Aspect Ratio ◽  
Secondary Flow ◽  
Engine Performance ◽  
Vital Role ◽  
Secondary Flows ◽  
Low Velocity ◽  
Mixed Flow ◽  
Flow Development ◽  
Inlet Conditions ◽  
The Impact

Current trends in the automotive industry towards engine downsizing means turbocharging now plays a vital role in engine performance. The purpose of turbocharging is to increase the engine inlet air density by utilising, the otherwise wasted energy in the exhaust gas. This energy extraction is commonly accomplished through the use of a radial turbine. Although less commonly used, mixed flow turbines can offer aerodynamic advantages due to the manipulation of blade leading (LE) angles, improving performance at low velocity ratios. The current paper investigates the performance of a mixed flow turbine with four volute designs, two radial and two tilted volutes each with one variant with an aspect ratio (AR)=0.5 and one with AR = 2. To ensure constant mass flow parameter (MFP) for aerodynamic similarity, volute area to radius ratio (A/r) was manipulated between the design variants. The maximum variation of cycle averaged normalized efficiency measured between the designs was 2.87%. Purely in the rotor region, the variation in normalized cycle averaged efficiency was 3%. The smallest volute AR designs showed substantial secondary flow development. The introduction of volute tilt further complicated the secondary flow development with the introduction of asymmetry to the flows. It was established that both AR and tilt have a notable effect on secondary flows, rotor inlet conditions and over all mixed flow turbine performance.

scholarly journals PIV-Measurements of Centrifugal Instabilities in a Rectangular Curved Duct with a Small Aspect Ratio

Fluids ◽  
2021 ◽  
Vol 6 (5) ◽  
pp. 184
Author(s):  
Afshin Goharzadeh ◽  
Peter Rodgers
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Velocity Distribution ◽  
Channel Wall ◽  
Secondary Flows ◽  
Working Fluid ◽  
Measured Velocity ◽  
Outer Channel ◽  
Dean Vortices ◽  
Curved Duct

In this study, experimental measurements were undertaken using non-intrusive particle image velocimetry (PIV) to investigate fluid flow within a 180° rectangular, curved duct geometry of a height-to-width aspect ratio of 0.167 and a curvature of 0.54. The duct was constructed from Plexiglas to permit optical access to flow pattern observations and flow velocity field measurements. Silicone oil was used as working fluid because it has a similar refractive index to Plexiglas. The measured velocity fields within the Reynolds number ranged from 116 to 203 and were presented at the curved channel section inlet and outlet, as well as at the mid-channel height over the complete duct length. It was observed from spanwise measurements that the transition to unsteady secondary flows generated the creation of wavy structures linked with the formation of Dean vortices close to the outer channel wall. This flow structure became unsteady with increasing Reynolds number. Simultaneously, the presence of Dean vortices in the spanwise direction influenced the velocity distribution in the streamwise direction. Two distinct regions defined by a higher velocity distribution were observed. Fluid particles were accelerated near the inner wall of the channel bend and subsequently downstream near the outer channel wall.

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