Vortex formation on surging aerofoils with application to reverse flow modelling

2018 ◽  
Vol 859 ◽  
pp. 59-88 ◽  
Author(s):  
Philip B. Kirk ◽  
Anya R. Jones
Keyword(s):  
Surface Pressure ◽  
Reverse Flow ◽  
Vortex Formation ◽  
Three Dimensional ◽  
Tangential Velocity ◽  
Leading Edge ◽  
Field Measurements ◽  
Reduced Frequency ◽  
Advance Ratio ◽  
Convection Speed

The leading-edge vortex (LEV) is a powerful unsteady flow structure that can result in significant unsteady loads on lifting blades and wings. Using force, surface pressure and flow field measurements, this work represents an experimental campaign to characterize LEV behaviour in sinusoidally surging flows with widely varying amplitudes and frequencies. Additional tests were conducted in reverse flow surge, with kinematics similar to the tangential velocity profile seen by a blade element in recent high-advance-ratio rotor experiments. General results demonstrate the variability of LEV convection properties with reduced frequency, which greatly affected the average lift-to-drag ratio in a cycle. Analysis of surface pressure measurements suggests that LEV convection speed is a function only of the local instantaneous flow velocity. In the rotor-comparison tests, LEVs formed in reverse flow surge were found to convect more quickly than the corresponding reverse flow LEVs that form on a high-advance-ratio rotor, demonstrating that rotary motion has a stabilizing effect on LEVs. The reverse flow surging LEVs were also found to be of comparable strength to those observed on the high-advance-ratio rotor, leading to the conclusion that a surging-wing simplification might provide a suitable basis for low-order models of much more complex three-dimensional flows.

A scaling for vortex formation on swept and unswept pitching wings

2017 ◽  
Vol 832 ◽  
pp. 697-720 ◽  
Author(s):  
Kyohei Onoue ◽  
Kenneth S. Breuer
Keyword(s):  
Control Volume ◽  
Vortex Formation ◽  
Fluid Motion ◽  
Three Dimensional ◽  
Leading Edge ◽  
Sweep Angle ◽  
Reduced Frequency ◽  
Vortex Formation Time ◽  
The Stability ◽  
Vorticity Transport

We examine the dynamics of the leading-edge vortex (LEV) on a rapidly pitching plate with the aim of elucidating the underlying flow physics that dictates the stability and circulation of the LEV. A wide variety of flow conditions is considered in the present study by systematically varying the leading-edge sweep angle ($\unicode[STIX]{x1D6EC}=0^{\circ }$, $11.3^{\circ }$, $16.7^{\circ }$) and the reduced frequency ($f^{\ast }=0.064{-}0.151$), while keeping the pitching amplitude and the Reynolds number fixed. Tomographic particle image velocimetry is used to characterise the three-dimensional fluid motion inside the vortex core and its relation to the LEV stability and growth. A series of control volume analyses are performed to quantify the relative importance of the vorticity transport phenomena taking place inside the LEV to the overall vortex development. We show that, near the wing apex where tip effects can be neglected, the vortex develops in a nominally two-dimensional manner, despite the presence of inherently three-dimensional vortex dynamics such as vortex stretching and compression. Furthermore, we demonstrate that the vortex formation time and circulation growth are well-described by the principles of optimal vortex formation number, and that the occurrence of vortex shedding is dictated by the relative energetics of the feeding shear layer and the resulting vortex.

Determining the relative stability of leading-edge vortices on nominally two-dimensional flapping profiles

2015 ◽  
Vol 766 ◽  
pp. 611-625 ◽  
Author(s):  
Jaime G. Wong ◽  
David E. Rival
Keyword(s):  
Relative Stability ◽  
Particle Tracking Velocimetry ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reduced Frequency ◽  
Image Velocimetry ◽  
Vortex Size ◽  
Spanwise Vorticity ◽  
Vorticity Transport ◽  
Leading Edge Vortices

AbstractIt is hypothesized that the relative stability of leading-edge vortices (LEVs) on flapping profiles can be improved by moderating LEV growth through spanwise vorticity convection and vortex stretching. Moreover, it is hypothesized that the reduced frequency $k$ and profile sweep ${\it\Lambda}$ are critical in predicting relative LEV stability as determined by the aforementioned effects. These hypotheses are then confirmed experimentally with phase-averaged particle image velocimetry (PIV) and three-dimensional particle tracking velocimetry. In particular, more stable LEVs are observed at higher reduced frequencies, which is argued to represent the ratio between the limiting vortex size and the rate of vorticity feeding. The introduction of profile sweep increased both relative LEV stability and spanwise vorticity transport. It is thought that spanwise vorticity transport improved LEV stability by acting as a sink for vorticity generated in the leading-edge shear layer.

Loss Mechanisms and Flow Control for Improved Efficiency of a Centrifugal Compressor at High Inlet Prewhirl

2015 ◽  
Author(s):  
Zheng Xinqian ◽  
Huang Qiangqiang ◽  
Liu Anxiong
Keyword(s):  
Flow Control ◽  
Mass Flow ◽  
Centrifugal Compressor ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Leading Edge ◽  
Flow Region ◽  
Percentage Points ◽  
Stable Conditions ◽  
Loss Mechanisms

Variable inlet prewhirl is an effective way to suppress compressor instability. Compressors usually employ a high degree of positive inlet prewhirl to shift the surge line in the performance map to a lower mass flow region. However, the efficiency of a compressor at high inlet prewhirl is far lower than that at zero or low prewhirl. This paper investigates the performances of a centrifugal compressor with different prewhirl, discusses the mechanisms thought to be responsible for the production of extra loss induced by high inlet prewhirl and develops flow control methods to improve efficiency at high inlet prewhirl. The approach combines steady three-dimensional Reynolds average Navier-Stockes (RANS) simulations with theoretical analysis and modeling. In order to make the study universal to various applications with inlet prewhirl, the inlet prewhirl was modeled by modifying the velocity condition at the inlet boundary. Simulation results show that the peak efficiency at high inlet prewhirl is reduced compared to that at zero prewhirl by over 7.6 percentage points. The extra loss is produced upstream and downstream of the impeller. Severe flow separation was found near the inlet hub which reduces efficiency by 2.3 percentage points. High inlet prewhirl works like a centrifuge gathering low-kinetic-energy fluid to hub, inducing the separation. A dimensionless parameter C is defined to measure the centrifugal component of flow. As for the extra loss produced downstream of the impeller, the flow mismatch of impeller and diffuser at high prewhirl causes a violent backflow near the diffuser vanes’ leading edges. An analytical model is built to predict diffuser choking mass flow which proves that the diffuser flow operates outside of stable conditions. Based on the two loss mechanisms, hub curve and diffuser stager angle were modified and adjusted for seeking higher efficiency at high prewhirl. The efficiency improvement of a modification of the hub is 1.1 percentage points and that of the combined optimization is 2.4 percentage points. During optimizing, constant distribution of inlet prewhirl was found to induce reverse flow at the leading edge of the blade root, which turned out being uncorrelated with blade angle. By revealing loss mechanisms and proposing flow control ideas, this paper lays a theoretical basis for overcoming the efficiency drop induced by high inlet prewhirl and for developing compressors with high inlet prewhirl.

3D Effects in the Wake Vortex Formation of an Elliptic Flat Plate in Hover

2017 ◽  
Author(s):  
Vivek Nair ◽  
Siddarth Chintamani ◽  
B. H. Dennis
Keyword(s):  
Flat Plate ◽  
Vortex Formation ◽  
Three Dimensional ◽  
Leading Edge ◽  
Leading Edge Vortex ◽  
Elliptic Geometry ◽  
3D Effects ◽  
Edge Vortex ◽  
Formation Number ◽  
Vorticity Transport

A Numerical Analysis is conducted to investigate the Leading Edge Vortex (LEV) dynamics of an elliptic flat plate undergoing 2 dimensional symmetric flapping motion in hover. The plate is modeled with an aspect ratio of 3 and a flapping trajectory resulting in Reynolds number 225 is studied. The leading edge vortex stability is analyzed as a function of the non dimensional formation number and a vorticity transport analysis is carried to understand the flux budgets present. The LEV formation number is found to be 2.6. The results of vorticity analysis show the highly three dimensional nature of the LEV growth for an elliptic geometry.

The Interaction of the Precessing Vortex Core and Reverse Flow Zone in the Exhaust of a Swirl Burner

1994 ◽  
Vol 208 (1) ◽  
pp. 27-36 ◽  
Author(s):  
N Syred ◽  
T O'Doherty ◽  
D Froud
Keyword(s):  
Vortex Core ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Tangential Velocity ◽  
Basic Structure ◽  
Flow Zone ◽  
Three Dimensional Flow ◽  
Laser Anemometry ◽  
Precessing Vortex Core ◽  
First Time

This paper describes recent work at Cardiff to gain further understanding of the fundamental processes occurring in swirl burners. The phenomenon of the precessing vortex core has been characterized via the use of a two-component laser anemometry system and the signal from a hot-wire anemometry probe for triggering purposes. This has allowed the rotating three-dimensional flow associated with the precessing vortex core to be characterized for the first time at different downstream sections. Regions of reversed mean tangential velocity have been identified while new insights into the basic structure of the reverse flow zone have been provided.

Vortex development on pitching plates with lunate and truncate planforms

2013 ◽  
Vol 732 ◽  
pp. 332-344 ◽  
Author(s):  
Colin Hartloper ◽  
David E. Rival
Keyword(s):  
Vortex Formation ◽  
Three Dimensional ◽  
Leading Edge ◽  
Strongly Correlated ◽  
Near Wake ◽  
Relaxation Phase ◽  
Force Relaxation ◽  
Edge Vortex ◽  
Drag Ratio ◽  
Lift To Drag Ratio

AbstractThe three-dimensional flow field and instantaneous forces are measured on pitching rectangular, lunate and truncate planforms of aspect-ratio four. The leading-edge vortex on the rectangular planform is compressed as it grows, and subsequently forms an arch-shaped vortex. For the lunate and truncate planforms, which both have identical spanwise leading-edge curvature but differ in planform area, outboard-directed convection of vorticity, rather than vortex stretching, mitigates arch-vortex formation. The vortical near wake that is formed by the planforms with spanwise leading-edge curvature is found to be strongly correlated with a favourable lift-to-drag ratio during the force-relaxation phase.

Three-Dimensional Vortex Structure in the Wake of an Oscillating Discoid Airfoil

2011 ◽  
Author(s):  
K. Nakagawa ◽  
H. Hasegawa
Keyword(s):  
Near Field ◽  
Three Dimensional ◽  
Wind Tunnel Test ◽  
Leading Edge ◽  
Fluid Force ◽  
Fluid Forces ◽  
Reduced Frequency ◽  
Unsteady Fluid ◽  
Two Peaks ◽  
Oscillating Motion

Flow fields around an oscillating airfoil at the low Reynolds number region are extremely unsteady because the change direction of leading edge produces unsteady vortex motions. Studies of unsteady propulsion system of birds, insects, and fish are few and inconclusive. It has been noted that the unsteady fluid force plays an important role in biological flight. To evaluate the force correctly, it is necessary to know the unsteady properties determined from the vortex dynamics. The actual motion of a hand in swimming is obviously unsteady, and time-dependent fluid forces must be considered, because a quasi-steady-state approach towards predicting the fluid forces acting on a hand under unsteady conditions yielded errors. The three-dimensionality and the unsteady effect of a hand must be important to the estimation of the fluid forces acting on a swimmer’s hand. The purpose of this study is to investigate the relationship between unsteady fluid forces and vortex structures for a three-dimensional airfoil during the pitch-oscillating motion. Particle image velocimetry (PIV) was used to aid in understanding the flow field in the near field of the airfoil edge for the wind tunnel test. It is confirmed that the unsteady fluid forces were affected by the vortices shed from the airfoil edge during up-stroke in pitching oscillation. There are two peaks in the fluid force during one pitch-oscillating due to the vortex behavior. The vortex behavior was strongly affected by the reduced frequency, and the fluid force acting on the airfoil model increases with increasing the reduced frequency.

A Study of Flow Downstream of a T-Junction and the Implications of its Effect (In Crude Oil Flow) on Water Droplet Size and Distribution

1993 ◽  
Vol 207 (2) ◽  
pp. 123-132
Author(s):  
C P Lenn ◽  
J Hemp ◽  
R C Baker ◽  
E R Hayes ◽  
A D Harper
Keyword(s):  
Crude Oil ◽  
Water Droplet ◽  
Three Dimensional ◽  
Tangential Velocity ◽  
Field Measurements ◽  
Maximum Size ◽  
Radial Component ◽  
Mean Velocity ◽  
Grab Sampling ◽  
Oil Flow

A laser Doppler anemometer (LDA) is used to measure velocity profiles and turbulence levels of water flow in the first few diameters downstream of a T-junction. The ‘vertical’ limb of the T-junction is half the diameter of the ‘horizontal’ limb, one end of which is blanked off. Flow passes from the smaller into the larger tube and LDA measurements of axial and tangential velocity components are conducted in the larger tube up to 3.75 diameters downstream of the T-junction at Reynolds numbers of 10.5 × 104 and 7.42 × 104. The pipe geometry is a commonly occurring configuration in crude oil pipelines and is of interest because of its possible ability to break up and mix water droplets to an extent sufficient for accurate grab sampling. LDA measurements of r.m.s. velocity fluctuations give information on the level of turbulent diffusivity and hence the maximum size of droplets that can be present in crude oil flow in the same geometry. A novel mathematical technique is used to interpolate between LDA measurements of mean velocity and to calculate the radial component of mean velocity. The three-dimensional velocity distributions thus formulated are used to predict water droplet concentration profiles downstream of the T-junction using the Segev approach—that is by solving numerically a differential equation for concentration of a contaminant under conditions of turbulent diffusion. Results are compared with field measurements in a similar geometry.

Experimental Study of 3D Unsteady Flow Around Oscillating Blade With Part-Span Separation

2000 ◽  
Author(s):  
O. J. R. Queune ◽  
L. He
Keyword(s):  
Three Dimensional ◽  
Leading Edge ◽  
Test Facility ◽  
Separation Flow ◽  
Pressure Measurements ◽  
Blade Surface ◽  
Unsteady Pressure ◽  
Reduced Frequency ◽  
Bending Mode ◽  
Linear Behaviour

This paper documents an investigation conducted on the aerodynamic response of a turbine blade oscillating in a three-dimensional bending mode under massive tip separation. Flow separation near the tip of the blade was realized by use of a step placed upstream of the blade’s leading edge. Extensive blade surface steady and unsteady pressure measurements were obtained from a test facility with clearly defined boundary conditions for a range of reduced frequency. The experiment is designed to produce detailed and reliable 3D test cases for aeroelastic applications. A complete set of steady and unsteady blade surface pressure measurements is provided for five spanwise sections at 10 %, 30 %, 50 %, 70 % and 90 % of span. In addition, the issue of linearity is addressed. Experimental results demonstrate a predominant linear behaviour of the unsteady pressure response.

Vortical Features of a Rotor Blade in Reversed Flow

2015 ◽  
Author(s):  
Dhwanil Shukla ◽  
Nandeesh Hiremath ◽  
Narayanan Komerath
Keyword(s):  
Radial Flow ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Leading Edge ◽  
Core Diameter ◽  
3 Dimensional ◽  
Rotating Blade ◽  
Flow Phenomena ◽  
Component Velocity ◽  
Blunt Edge

The nature of the flow around a rotating blade in reverse flow is described, integrating results from fixed and rotary wing experiments. The highly 3-dimensional flow phenomena do not conform to expectations based on 2-D airfoil aerodynamics. Fixed-wing results from load measurements and flow visualization showed that the sharp-edge vortex (SEV) is a primary feature when the blade is yawed either forward or backward. The loads are better modeled using the Polhamus leading edge suction analogy. Vortex-induced pressure gradient induces an inward radial flow overcoming centrifugal effects, but away from the vortex, outward radial flow is evident everywhere. A strongly three-dimensional and attached SEV is evident under the blade at 240 degrees azimuth. This detaches and convects with the flow, remaining close to the blade by 270 degrees. The vortex seen at 300 degrees is clearly detached, but growth of the core diameter corresponding to vortex bursting, causes strong suction under the blade. The flow around the blunt edge is again strongly 3-D. Some evidence of intermittent separation is seen, but the azimuth-resolved, ensemble-averaged flow is mostly attached around the blunt edge. Preliminary static pressure contours derived from the measured 3-component velocity field are presented.

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