On the Effects of Incidence Angle on the Mean Wake of a Surface-Mounted Finite-Height Square Prism

2015 ◽  
Author(s):  
Ayodele Ogunremi ◽  
David Sumner
Keyword(s):  
Incidence Angle ◽  
Flow Direction ◽  
Ground Plane ◽  
Vortex Pair ◽  
Tip Vortex ◽  
Pressure Probe ◽  
Mean Velocity ◽  
Square Prism ◽  
Finite Height ◽  
The Mean

The wake of a surface-mounted finite-height square prism of sub-critical aspect ratio AR = 3 was studied experimentally in a low-speed wind tunnel at a Reynolds number of Re = 3.7×104. The ratio of the boundary layer thickness on the ground plane, to the width of the prism, was δ/D = 1.5. The incidence angle of the prism was varied from α = 0° to 45°. Wake mean velocity measurements were made in vertical planes normal to and parallel to the main flow direction using a seven-hole pressure probe. As the prism is rotated from α = 0° to 45°, the mean wake progressively widens and the maximum streamwise extent of the mean recirculation zone increases. The mean streamwise tip vortex pair is symmetric at 0° and 45°, but becomes strongly asymmetric at intermediate α, where the tip vortex is found higher above the ground plane on the wider side of the wake. The wake and tip vortex asymmetry is most pronounced near the critical incidence angle.

Wake Interference Effects for Two Surface-Mounted Finite Cylinders in a Tandem Configuration

2014 ◽  
Author(s):  
David Sumner ◽  
He Li
Keyword(s):  
Circular Cylinder ◽  
Ground Plane ◽  
Flow Regimes ◽  
Vortex Structures ◽  
Finite Cylinder ◽  
Pressure Probe ◽  
Extended Body ◽  
Tandem Configuration ◽  
Finite Height ◽  
The Mean

The mean wake of two identical surface-mounted finite-height circular cylinders arranged in a tandem configuration was investigated in a low-speed wind tunnel using a seven-hole pressure probe. The Reynolds number was Re = 2.4×104, the cylinder aspect ratio was AR = 9, and the boundary layer thickness on the ground plane relative to the cylinder height was δ/H ≈ 0.4. Three centre-to-centre longitudinal pitch ratios of L/D = 1.125, 2, and 5 were examined, corresponding to the extended-body, reattachment, and co-shedding flow regimes, respectively. Reference measurements were also made in the wake of a single finite circular cylinder of AR = 9. For the tandem configurations, velocity measurements were made behind the downstream cylinder in two orthogonal vertical planes. Compared to the wake of the single surface-mounted finite-height circular cylinder, the mean downwash and upwash flows for the tandem cylinders, behind the downstream cylinder, were weaker, the mean recirculation zone behind the downstream cylinder was shorter, and the mean wake extended higher above the ground plane, for all three pitch ratios. Marked changes were also observed in the mean streamwise wake vortex structures, compared to the case of the single finite cylinder. For the extended-body and reattachment flow regimes, the tip vortex structures became elongated in the wall-normal direction. In the co-shedding regime, two sets of tip vortices were observed, with the second set possibly originating from the upstream cylinder.

An Experimental Investigation of Aspect Ratio and Incidence Angle Effects for the Flow Around Surface-Mounted Finite-Height Square Prisms

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
J. F. McClean ◽  
D. Sumner
Keyword(s):  
Aspect Ratio ◽  
Drag Coefficient ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Incidence Angle ◽  
Lift Coefficient ◽  
Ground Plane ◽  
Aspect Ratios ◽  
Square Prism ◽  
The Mean

The flow around a surface-mounted finite-height square prism was investigated using a low-speed wind tunnel. The experiments were conducted at a Reynolds number of Re = 7.3 × 104 for prism aspect ratios of AR = 3, 5, 7, 9, and 11 and incidence angles from α = 0 deg to 45 deg. The thickness of the boundary layer on the ground plane relative to the side length was δ/D = 1.5. Measurements of the vortex shedding frequency were made with a single-component hot-wire probe, and measurements of the mean drag and lift forces were obtained with a force balance. For all aspect ratios and incidence angles, the mean drag coefficient and Strouhal number were lower than those of an infinite prism, while the mean lift coefficient was of nearly similar magnitude. As the aspect ratio was increased from AR = 3 to 11, the force coefficients and Strouhal number slowly approached the infinite-square-prism data. The mean drag coefficient and Strouhal number for the finite prism were less sensitive to changes in incidence angle compared to the infinite square prism. The critical incidence angle, corresponding to minimum mean drag coefficient, minimum (most negative) mean lift coefficient, and maximum Strouhal number, shifted to a higher incidence angle compared to the infinite square prism, with values ranging from αcritical = 15 deg to 18 deg; this shift was greatest for the prisms of higher aspect ratio. The behavior of the force coefficients and Strouhal number for the prism of AR = 3 was distinct from the other prisms (with lower values of mean drag coefficient and mean lift coefficient magnitude, and a different Strouhal number trend), suggesting the critical aspect ratio was between AR = 5 and AR = 3 in these experiments. In the wall-normal direction, the power spectra for AR = 11 and 9 tended to have weaker and/or more broad-banded vortex shedding peaks near the ground plane and near the free end at α = 0 deg and 15 deg. For AR = 7 to 3, well-defined vortex shedding peaks were detected along the entire height of the prisms. For AR = 11 and 9, at α = 30 deg and 45 deg, vortex shedding peaks were absent in the power spectra in the upper part of the wake.

Aerodynamic Forces and Vortex Shedding for Surface-Mounted Finite Square Prisms and the Effects of Aspect Ratio and Incidence Angle

2012 ◽  
Author(s):  
John F. McClean ◽  
David Sumner
Keyword(s):  
Aspect Ratio ◽  
Drag Coefficient ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Incidence Angle ◽  
Lift Coefficient ◽  
Ground Plane ◽  
Aspect Ratios ◽  
Square Prism ◽  
The Mean

The flow around a surface-mounted square prism of finite height was investigated experimentally using a low-speed wind tunnel. Of interest were the effects of aspect ratio and incidence angle on the mean aerodynamic forces and vortex shedding. Compared to the case of the “infinite” (or two-dimensional) square prism, the flow around the finite square prism has not been extensively studied. The experiments were conducted at a Reynolds number of Re = 7.2 × 104 for aspect ratios of AR = 3, 5, 7, 9, and 11 and incidence angles of α = 0°, 15°, 30° and 45°. The thickness of the boundary layer on the ground plane relative to the side length was δ/D = 1.5. Measurements of the vortex shedding frequency were made with a single-component hot-wire probe in the wake, and measurements of the mean drag and lift forces were obtained with a force balance. For all aspect ratios and incidence angles, the Strouhal number and the mean drag coefficient were lower than those of an infinite prism, while the mean lift coefficient was of nearly similar magnitude. As the aspect ratio was increased from AR = 3 to 11, the force coefficients and Strouhal number slowly approached the infinite-square-prism data. The behaviours of the mean drag coefficient and Strouhal number with incidence angle were less sensitive compared to the case of the infinite square prism, although a minimum mean drag coefficient, minimum (most negative) mean lift coefficient, and maximum Strouhal number were found at α = 15°. The reduced sensitivity to incidence angle is attributed to the complex three-dimensional flow over the free end of the prism and the downwash flow that enters the near wake. The behaviour of the force coefficients and Strouhal number for the prism of AR = 3 was distinct from the other prisms (with lower values of drag coefficient and lift coefficient magnitude, and a different Strouhal number trend), suggesting the critical aspect ratio was between AR = 5 and AR = 3 in these experiments. In the wall-normal direction, the power spectra for AR = 11 and 9 tended to have weaker and/or more broad-banded vortex shedding peaks near the ground plane and near the free end at α = 0° and 15°. For AR = 7 to 3, well-defined vortex shedding peaks were detected along the entire height of the prisms. For AR = 11 and 9, at α = 30° and 45°, vortex shedding peaks were absent in the power spectra in the upper part of the wake.

On the mechanism of high-incidence lift generation for steadily translating low-aspect-ratio wings

2017 ◽  
Vol 813 ◽  
pp. 110-126 ◽  
Author(s):  
Adam C. DeVoria ◽  
Kamran Mohseni
Keyword(s):  
Vortex Pair ◽  
Tip Vortex ◽  
Mean Flow ◽  
Flow Topology ◽  
Trailing Edge ◽  
Negative Contribution ◽  
High Incidence ◽  
The Mean ◽  
Stall Angle ◽  
Strong Vorticity

High-incidence lift generation via flow reattachment is studied. Different reattachment mechanisms are distinguished, with dynamic manoeuvres and tip vortex downwash being separate mechanisms. We focus on the latter mechanism, which is strictly available to finite wings, and isolate it by considering steadily translating wings. The tip vortex downwash provides a smoother merging of the flow at the trailing edge, thus assisting in establishing a Kutta condition there. This decreases the strength/amount of vorticity shed from the trailing edge, and in turn maintains an effective bound circulation resulting in continued lift generation at high angles of attack. Just below the static lift-stall angle of attack, strong vorticity is shed at the trailing edge indicating an increasingly intermittent reattachment/detachment of the instantaneous flow at mid-span. Above this incidence, the trailing-edge shear layer increases in strength/size representing a negative contribution to the lift and leads to stall. Lastly, we show that the mean-flow topology is equivalent to a vortex pair regardless of the particular physical flow configuration.

Measurements with a pulsed-wire and a hot-wire anemometer in the highly turbulent wake of a normal flat plate

1976 ◽  
Vol 77 (3) ◽  
pp. 473-497 ◽  
Author(s):  
L. J. S. Bradbury
Keyword(s):  
Turbulent Flow ◽  
Flat Plate ◽  
Flow Direction ◽  
Operating Conditions ◽  
Turbulent Intensity ◽  
Hot Wire ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean ◽  
Better Than

This paper describes an investigation into the response of both the pulsed-wire anemometer and the hot-wire anemometer in a highly turbulent flow. The first part of the paper is concerned with a theoretical study of some aspects of the response of these instruments in a highly turbulent flow. It is shown that, under normal operating conditions, the pulsed-wire anemometer should give mean velocity and longitudinal turbulent intensity estimates to an accuracy of better than 10% without any restriction on turbulence level. However, to attain this accuracy in measurements of turbulent intensities normal to the mean flow direction, there is a lower limit on the turbulent intensity of about 50%. An analysis is then carried out of the behaviour of the hot-wire anemometer in a highly turbulent flow. It is found that the large errors that are known to develop are very sensitive to the precise structure of the turbulence, so that even qualitative use of hot-wire data in such flows is not feasible. Some brief comments on the possibility of improving the accuracy of the hot-wire anemometer are then given.The second half of the paper describes some comparative measurements in the highly turbulent flow immediately downstream of a normal flat plate. It is shown that, although it is not possible to interpret the hot-wire results on their own, it is possible to calculate the hot-wire response with a surprising degree of accuracy using the results from the pulsed-wire anemometer. This provides a rather indirect but none the less welcome check on the accuracy of the pulsed-wire results, which, in this very highly turbulent flow, have a certain interest in their own right.

Effect of Axial Sweep and Tip Extension on Performance of an Axial Fan

2016 ◽  
Author(s):  
Ankit Bhai Patel ◽  
K. Viswanath ◽  
Dhyanjyoti Deb Nath
Keyword(s):  
Performance Improvement ◽  
Flow Direction ◽  
Pressure Rise ◽  
Aerodynamic Performance ◽  
Secondary Flows ◽  
Tip Vortex ◽  
Low Flow ◽  
Pressure Probe ◽  
Axial Fan ◽  
Stall Margin

Performance enhancement in terms of stall margin increment, increased pressure rise coefficient and increased efficiency is of great need for low speed axial fans. Stacking line modifications in terms of sweep, skew, dihedral or combination of these, as well as blade tip geometry modifications are assumed to be one of the ways to achieve finite performance improvement. Non radial stacking of blade profiles modifies secondary flows, tip vortex effects, hub passage vortex and thus affects aerodynamic performance parameters such as stall margin, efficiency, pressure rise, blade loading. In literature many studies have confined to comparison of few cases which led to conflicting results as modification of stacking line may have different effects in different cases. In the present work, comparison of performance of axial fan rotor with three different blade configurations BSL (baseline), SWP (swept blade) and EXTN (swept blade with extended tip) are considered. The BSL configuration is designed on basis of non-free vortex design. The SWP configuration is obtained by shifting radial stacking line of the BSL in axial flow direction by 10° (Forward sweep). The EXTN configuration is obtained by extending tip profile on pressure surface as well as suction surface by 3% locally. Experiments have been conducted on these three configurations to study effects of these modifications on aerodynamic performance. The flow field has been surveyed using Kiel probe, Three hole pressure probe at many flow rates starting from fully open to fully closed. Unsteady flow analysis at exit of rotors of all configurations is carried out using fast response pressure probe. Experimental results show slight performance improvement in terms of increased stall margin, efficiency, as well as total pressure rise for SWP rotor as well as EXTN rotor compared to BSL rotor at low flow coefficients.

Anisotropic pressure correlation spectra in turbulent shear flow

2012 ◽  
Vol 694 ◽  
pp. 50-77 ◽  
Author(s):  
Yoshiyuki Tsuji ◽  
Yukio Kaneda
Keyword(s):  
Mixing Layer ◽  
Inertial Subrange ◽  
Turbulent Shear ◽  
Pressure Probe ◽  
Anisotropic Pressure ◽  
Turbulent Shear Flow ◽  
Velocity Correlation ◽  
Mean Velocity ◽  
Correlation Spectrum ◽  
The Mean

AbstractWe measured the correlation spectrum ${\hat {Q} }_{p} (\mathbi{k})$ of pressure fluctuations in a driving mixing layer with a Taylor-scale Reynolds number ${R}_{\lambda } $ up to ${\simeq }700$ by a newly developed pressure probe with spatial and temporal resolutions that are sufficient to analyse inertial-subrange statistics. The influence of the mean velocity gradient tensor ${S}_{ij} $ in the mixing layer, which is almost constant near its centreline, is studied using an idea similar to that underlying the linear response theory developed in statistical mechanics for systems at or near thermal equilibrium. If we write the spectrum ${\hat {Q} }_{p} (\mathbi{k})$ as ${\hat {Q} }_{p} (\mathbi{k})= { \hat {Q} }_{p}^{(0)} (\mathbi{k})+ \mrm{\Delta} {\hat {Q} }_{p} (\mathbi{k})$, where ${ \hat {Q} }_{p}^{(0)} (\mathbi{k})$ is the isotropic Kolmogorov spectrum in the absence of mean shear, then for small ${S}_{ij} $ the deviation $ \mrm{\Delta} {\hat {Q} }_{p} (\mathbi{k})$ due to the shear is approximately linear and is determined by a few non-dimensional universal constants in addition to ${S}_{ij} $, $k$ and the mean energy dissipation rate. We also measured the pressure–velocity and velocity–velocity correlation spectra. Deviations from isotropy due to shear are shown to be approximately proportional to ${S}_{ij} $ at large ${R}_{\lambda } $.

Sinusoidal Excitation of a Free Turbulent Jet With Constant Amplitude Transverse Pressure Gradients Near the Nozzle Exit: Part I: Measurement of Mean Shear Velocities

1973 ◽  
Vol 95 (2) ◽  
pp. 167-173
Author(s):  
A. K. Stiffler ◽  
J. L. Shearer
Keyword(s):  
Nozzle Exit ◽  
Flow Direction ◽  
Excitation Frequency ◽  
Velocity Profiles ◽  
Turbulent Jet ◽  
Pressure Gradients ◽  
Mean Velocity ◽  
Effective Velocity ◽  
Gaussian Distribution Function ◽  
The Mean

A free turbulent jet is perturbed transverse to the flow direction by a sinusoidal pressure gradient near the nozzle exit. Velocities in the jet are determined by hot wire anemometer measurements. Moving effective mean velocity profiles are defined and reconstructed from the point-by-point stationary measurements of the mean velocity and of the harmonic content of the time varying signal. The effective velocity profiles are described by the Gaussian distribution function where the spread parameter decays as the cube of the product of the excitation frequency and the downstream location from the nozzle. These profile measurements and analysis of their characteristics lead to a better understanding of the factors determining the gain of a fluidic amplifier under conditions of high frequency operation.

Synchronous Vortex Shedding (Vortex Pumping) Downstream of a Flat Plate Array

1999 ◽  
Vol 122 (1) ◽  
pp. 183-185 ◽  
Author(s):  
D. W. Guillaume ◽  
J. C. LaRue
Keyword(s):  
Flow Visualization ◽  
Vortex Pair ◽  
Flat Plates ◽  
Mean Velocity ◽  
Trailing Edges ◽  
The Mean ◽  
Plate Array ◽  
Adjacent Channels ◽  
Bounding Surfaces ◽  
Zero Time

Flow visualization and statistics, obtained downstream of an array of flat plates, are presented. Flow visualization shows that the mean separation between streaklines for the flow downstream of adjacent channels is negatively correlated. Consistent with this observation, the zero-time cross correlation of the fluctuating velocity between adjacent plates is negative. Vortices shed at the trailing edges of the bounding surfaces that form a channel in the plate array are in-phase, but they are nearly 180 deg out-of-phase with the vortices shed at the trailing edges of the bounding surfaces of adjacent channels. Relative to the mean velocity, the axial velocity on the midplane of the channel increased in the region between each pair of vortices. At downstream positions not near the vortex pair, the velocity on the midplane is relatively decreased. Hence, vortices shed at the trailing edges of the channel surfaces lead to “vortex pumping,” which is consistent with the negative correlation of the fluctuating velocity on the midplanes between adjacent channels. [S0098-2202(00)01701-6]

Effect of Freestream Turbulence and Flow Recovery in the Wake of a Flat Plate

2002 ◽  
Author(s):  
F. N. Krampa-Morlu ◽  
R. Balachandar
Keyword(s):  
Flat Plate ◽  
Turbulence Intensity ◽  
Flow Direction ◽  
Wake Region ◽  
Freestream Turbulence ◽  
Near Wake ◽  
Mean Velocity ◽  
Channel Boundary ◽  
The Mean ◽  
Velocity Turbulence

The study of the recovery of an open channel boundary flow in the presence of increased freestream turbulence (FST) generated in the wake region of a surface mounted flat plate is presented. Detailed LDA velocity measurements were obtained upstream and downstream of the flat plate, which is 3 mm in thickness and has a thickness-to-chord ratio of 0.12. The chord is placed parallel to the flow direction. The characteristics of the mean velocity, turbulence intensity, and the velocity skewness and flatness factors were investigated. The skin friction was increased while the strength of the boundary layer wake parameter decreased in the wake region. The turbulence intensity profiles in the wake region increasingly deviated significantly from the upstream profile. Generally, the increased FST noticed in the near-wake region was observed to decay with downstream distance. As a result, the mean velocity and turbulence intensity profiles showed a general sense of recovery towards the state of the approaching flow.

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