Experimental and Numerical Study of the Three Dimensional Instabilities in the Wake of a Blunt Trailing Edge Profiled Body

2010 ◽  
Author(s):  
Arash Naghib Lahouti ◽  
Lakshmana Sampat Doddipatla ◽  
Horia Hangan ◽  
Kamran Siddiqui
Keyword(s):  
Reynolds Number ◽  
Numerical Study ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
The Body ◽  
Bluff Bodies ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Image Velocimetry

The wake of nominally two dimensional bluff bodies is dominated by von Ka´rma´n vortices, which are accompanied by three dimensional instabilities beyond a threshold Reynolds number. These three dimensional instabilities initiate as dislocations in the von Ka´rma´n vortices near the trailing edge, which evolve into pairs of counter-rotating vortices further downstream. The wavelength of the three dimensional instabilities depends on profile geometry and Reynolds number. In the present study, the three dimensional wake instabilities for a blunt trailing edge profiled body, composed of an elliptical leading edge and a rectangular trailing edge, have been studied in Reynolds numbers ranging from 500 to 1200, based on the thickness of the body. Numerical simulations, Laser Induced Fluorescence (LIF) flow visualization, and Particle Image Velocimetry (PIV) methods have been used to identify the instabilities. Proper Orthogonal Decomposition (POD) has been used to analyze the velocity field data measured using PIV. The results confirm the existence of three dimensional instabilities with an average wavelength of 2.0 to 2.5 times thickness of the body, in the near wake. The findings are in agreements with the values reported previously for different Reynolds numbers, and extend the range of Reynolds numbers in which the three dimensional instabilities are characterized.

scholarly journals Fish larvae exploit edge vortices along their dorsal and ventral fin folds to propel themselves

2016 ◽  
Vol 13 (116) ◽  
pp. 20160068 ◽  
Author(s):  
Gen Li ◽  
Ulrike K. Müller ◽  
Johan L. van Leeuwen ◽  
Hao Liu
Keyword(s):  
Larval Fish ◽  
Fish Larvae ◽  
Fluid Dynamic ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Bony Fish ◽  
The Body ◽  
Functional Explanation ◽  
Image Velocimetry ◽  
Median Fins

Larvae of bony fish swim in the intermediate Reynolds number ( Re ) regime, using body- and caudal-fin undulation to propel themselves. They share a median fin fold that transforms into separate median fins as they grow into juveniles. The fin fold was suggested to be an adaption for locomotion in the intermediate Reynolds regime, but its fluid-dynamic role is still enigmatic. Using three-dimensional fluid-dynamic computations, we quantified the swimming trajectory from body-shape changes during cyclic swimming of larval fish. We predicted unsteady vortices around the upper and lower edges of the fin fold, and identified similar vortices around real larvae with particle image velocimetry. We show that thrust contributions on the body peak adjacent to the upper and lower edges of the fin fold where large left–right pressure differences occur in concert with the periodical generation and shedding of edge vortices. The fin fold enhances effective flow separation and drag-based thrust. Along the body, net thrust is generated in multiple zones posterior to the centre of mass. Counterfactual simulations exploring the effect of having a fin fold across a range of Reynolds numbers show that the fin fold helps larvae achieve high swimming speeds, yet requires high power. We conclude that propulsion in larval fish partly relies on unsteady high-intensity vortices along the upper and lower edges of the fin fold, providing a functional explanation for the omnipresence of the fin fold in bony-fish larvae.

Secondary instability and subcritical transition of the leading-edge boundary layer

2016 ◽  
Vol 792 ◽  
pp. 682-711 ◽  
Author(s):  
Michael O. John ◽  
Dominik Obrist ◽  
Leonhard Kleiser
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
High Speed ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Secondary Instability ◽  
Bypass Transition ◽  
Wall Suction ◽  
Transition Mechanism

The leading-edge boundary layer (LEBL) in the front part of swept airplane wings is prone to three-dimensional subcritical instability, which may lead to bypass transition. The resulting increase of airplane drag and fuel consumption implies a negative environmental impact. In the present paper, we present a temporal biglobal secondary stability analysis (SSA) and direct numerical simulations (DNS) of this flow to investigate a subcritical transition mechanism. The LEBL is modelled by the swept Hiemenz boundary layer (SHBL), with and without wall suction. We introduce a pair of steady, counter-rotating, streamwise vortices next to the attachment line as a generic primary disturbance. This generates a high-speed streak, which evolves slowly in the streamwise direction. The SSA predicts that this flow is unstable to secondary, time-dependent perturbations. We report the upper branch of the secondary neutral curve and describe numerous eigenmodes located inside the shear layers surrounding the primary high-speed streak and the vortices. We find secondary flow instability at Reynolds numbers as low as$Re\approx 175$, i.e. far below the linear critical Reynolds number$Re_{crit}\approx 583$of the SHBL. This secondary modal instability is confirmed by our three-dimensional DNS. Furthermore, these simulations show that the modes may grow until nonlinear processes lead to breakdown to turbulent flow for Reynolds numbers above$Re_{tr}\approx 250$. The three-dimensional mode shapes, growth rates, and the frequency dependence of the secondary eigenmodes found by SSA and the DNS results are in close agreement with each other. The transition Reynolds number$Re_{tr}\approx 250$at zero suction and its increase with wall suction closely coincide with experimental and numerical results from the literature. We conclude that the secondary instability and the transition scenario presented in this paper may serve as a possible explanation for the well-known subcritical transition observed in the leading-edge boundary layer.

Numerical Investigation on Turbulence Statistics and Heat Transfer of a Circular Jet Impinging on a Roughened Flat Plate

2021 ◽  
Vol 13 (4) ◽  
Author(s):  
Abdulrahman Alenezi ◽  
Abdulrahman Almutairi ◽  
Hamad Alhajeri ◽  
Abdulaziz Gamil ◽  
Faisal Alshammari
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Numerical Study ◽  
Roughness Element ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Target Plate ◽  
Cross Sectional ◽  
Flow Physics ◽  
Baseline Case

Abstract A detailed heat transfer numerical study of a three-dimensional impinging jet on a roughened isothermal surface is presented and is investigated from flow physics vantage point under the influence of different parameters. The effects of the Reynolds number, roughness location, and roughness dimension on the flow physics and heat transfer parameters are studied. Additionally, the relations between average heat transfer coefficient (AHTC) and flow physics including pressure, wall shear and flow vortices with thermodynamic nonequilibrium are offered. This paper studies the effect of varying both location and dimension of the roughness element which took the shape of square cross-sectional continuous ribs to deliver a favorable trade-off between total pressure loss and heat transfer rate. The roughness element was tested for three different radial locations (R/D) = 1, 1.5, and 2 and at each location its height (i.e., width) (e) was changed from 0.25 to 1 mm in incremental steps of 0.25. The study used a jet angle (α) of 90 deg, jet-to-target distance (H/D = 6), and Re ranges from 10,000 to 50,000, where H is the vertical distance between the target plate and jet exit. The results show that the AHTC can be significantly affected by changing the geometry and dimensions of the roughness element. This variation can be either an augmentation of, or decrease in, the (HTC) when compared with the baseline case. An enhancement of 12.9% in the AHTC was achieved by using optimal location and dimensions of the roughness element at specific Reynolds number. However, a diminution between 10% and 30% in (AHTC) was attained by the use of rib height e = 1 mm at Re = 50k. The variation of both rib location and height showed better contribution in increasing heat transfer for low-range Reynolds numbers.

Three-Dimensional Vortex Structure in a Leading-Edge Separation Bubble at Moderate Reynolds Numbers

1991 ◽  
Vol 113 (3) ◽  
pp. 405-410 ◽  
Author(s):  
Kyuro Sasaki ◽  
Masaru Kiya
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Plate Thickness ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Vortex Filaments ◽  
Separated Shear Layer ◽  
Hydrogen Bubbles ◽  
Edge Separation

This paper describes the results of a flow visualization study which concerns three-dimensional vortex structures in a leading-edge separation bubble formed along the sides of a blunt flat plate. Dye and hydrogen bubbles were used as tracers. Reynolds number (Re), based on the plate thickness, was varied from 80 to 800. For 80 < Re < 320, the separated shear layer remains laminar up to the reattachment line without significant spanwise distortion of vortex filaments. For 320 < Re < 380, a Λ-shaped deformation of vortex filaments appears shortly downstream of the reattachment and is arranged in-phase in the downstream direction. For Re > 380, hairpin-like structures are formed and arranged in a staggered manner. The longitudinal and spanwise distances of the vortex arrangement are presented as functions of the Reynolds number.

Low Reynolds numbers flow past an ellipsoid of revolution of large aspect ratio

1965 ◽  
Vol 23 (4) ◽  
pp. 657-671 ◽  
Author(s):  
Yun-Yuan Shi
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Three Dimensional ◽  
Major Axis ◽  
Reynolds Numbers ◽  
The Body ◽  
Large Aspect Ratio ◽  
Low Reynolds Numbers ◽  
Ellipsoid Of Revolution ◽  
Limiting Case

The results of Proudman & Pearson (1957) and Kaplun & Lagerstrom (1957) for a sphere and a cylinder are generalized to study an ellipsoid of revolution of large aspect ratio with its axis of revolution perpendicular to the uniform flow at infinity. The limiting case, where the Reynolds number based on the minor axis of the ellipsoid is small while the other Reynolds number based on the major axis is fixed, is studied. The following points are deduced: (1) although the body is three-dimensional the expansion is in inverse power of the logarithm of the Reynolds number as the case of a two-dimensional circular cylinder; (2) the existence of the ends and the variation of the diameter along the axis of revolution have no effect on the drag to the first order; (3) a formula for drag is obtained to higher order.

CFD Study of the Pressure Distribution on the Back of a 3-D Bluff Body (CUBE) of Air Flow in Different Reynolds Numbers

2009 ◽  
Author(s):  
Mohammad Javad Izadi ◽  
Pegah Asghari ◽  
Malihe Kamkar Delakeh
Keyword(s):  
Reynolds Number ◽  
Bluff Body ◽  
Free Stream ◽  
Fluid Flows ◽  
Reynolds Numbers ◽  
The Body ◽  
Stream Velocity ◽  
Bluff Bodies ◽  
Unsteady Forces ◽  
Flow Round

The study of flow around bluff bodies is important, and has many applications in industry. Up to now, a few numerical studies have been done in this field. In this research a turbulent unsteady flow round a cube is simulated numerically. The LES method is used to simulate the turbulent flow around the cube since this method is more accurate to model time-depended flows than other numerical methods. When the air as an ideal fluid flows over the cube, flow separate from the back of the body and unsteady vortices appears, causing a large wake behind the cube. The Near-Wake (wake close to the body) plays an important role in determining the steady and unsteady forces on the body. In this study, to see the effect of the free stream velocity on the surface pressure behind the body, the Reynolds number is varied from one to four million and the pressure on the back of the cube is calculated numerically. From the results of this study, it can be seen that as the velocity or the Reynolds number increased, the pressure on the surface behind the cube decreased, but the rate of this decrease, increased as the free stream flow velocity increased. For high free stream velocities the base pressure did not change as much and therefore the base drag coefficient stayed constant (around 1.0).

Experiments With Three-Dimensional Passive Flow Control Devices on Low-Pressure Turbine Airfoils

2004 ◽  
Vol 128 (2) ◽  
pp. 251-260 ◽  
Author(s):  
Douglas G. Bohl ◽  
Ralph J. Volino
Keyword(s):  
Reynolds Number ◽  
Flow Control ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Trailing Edge ◽  
Low Pressure Turbine ◽  
Wide Range ◽  
Turbine Airfoils

The effectiveness of three-dimensional passive devices for flow control on low pressure turbine airfoils was investigated experimentally. A row of small cylinders was placed at the pressure minimum on the suction side of a typical airfoil. Cases with Reynolds numbers ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) were considered under low freestream turbulence conditions. Streamwise pressure profiles and velocity profiles near the trailing edge were documented. Without flow control a separation bubble was present, and at the lower Reynolds numbers the bubble did not close. Cylinders with two different heights and a wide range of spanwise spacings were considered. Reattachment moved upstream as the cylinder height was increased or the spacing was decreased. If the spanwise spacing was sufficiently small, the flow at the trailing edge was essentially uniform across the span. The cylinder size and spacing could be optimized to minimize losses at a given Reynolds number, but cylinders optimized for low Reynolds number conditions caused increased losses at high Reynolds numbers. The effectiveness of two-dimensional bars had been studied previously under the same flow conditions. The cylinders were not as effective for maintaining low losses over a range of Reynolds numbers as the bars.

Wake dynamics of external flow past a curved circular cylinder with the free stream aligned with the plane of curvature

2007 ◽  
Vol 592 ◽  
pp. 89-115 ◽  
Author(s):  
A. MILIOU ◽  
A. DE VECCHI ◽  
S. J. SHERWIN ◽  
J. M. R. GRAHAM
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Free Stream ◽  
Three Dimensional ◽  
Axial Flow ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
The Body ◽  
Vertical Extension ◽  
Flow Past

Three-dimensional spectral/hp computations have been performed to study the fundamental mechanisms of vortex shedding in the wake of curved circular cylinders at Reynolds numbers of 100 and 500. The basic shape of the body is a circular cylinder whose centreline sweeps through a quarter section of a ring and the inflow direction lies on the plane of curvature of the quarter ring: the free stream is then parallel to the geometry considered and the part of the ring that is exposed to it will be referred to as the ‘leading edge’. Different configurations were investigated with respect to the leading-edge orientation. In the case of a convex-shaped geometry, the stagnation face is the outer surface of the ring: this case exhibited fully three-dimensional wake dynamics, with the vortex shedding in the upper part of the body driving the lower end at one dominant shedding frequency for the whole cylinder span. The vortex-shedding mechanism was therefore not governed by the variation of local normal Reynolds numbers dictated by the curved shape of the leading edge. A second set of simulations were conducted with the free stream directed towards the inside of the ring, in the so-called concave-shaped geometry. No vortex shedding was detected in this configuration: it is suggested that the strong axial flow due to the body's curvature and the subsequent production of streamwise vorticity plays a key role in suppressing the wake dynamics expected in the case of flow past a straight cylinder. The stabilizing mechanism stemming from the concave curved geometry was still found to govern the wake behaviour even when a vertical extension was added to the top of the concave ring, thereby displacing the numerical symmetry boundary condition at this point away from the top of the deformed cylinder. In this case, however, the axial flow from the deformed cylinder was drawn into the wake of vertical extension, weakening the shedding process expected from a straight cylinder at these Reynolds numbers. These considerations highlight the importance of investigating flow past curved cylinders using a full three-dimensional approach, which can properly take into account the role of axial velocity components without the limiting assumptions of a sectional analysis, as is commonly used in industrial practice. Finally, towing-tank flow visualizations were also conducted and found to be in qualitative agreement with the computational findings.

Numerical Investigation of the Three-Dimensional Flow Structure About a Revolving Wing

2012 ◽  
Author(s):  
Daniel J. Garmann ◽  
Miguel R. Visbal ◽  
Paul D. Orkwis
Keyword(s):  
Turbulent Flows ◽  
Numerical Study ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Vortex System ◽  
Aerodynamic Loads ◽  
Eddy Simulation ◽  
Data Reduction Technique ◽  
The Stability

A numerical study is conducted to examine the vortex structure about a revolving wing in quiescent flow employing a high-fidelity, implicit large eddy simulation (ILES) technique found to be effective in simulating flows that exhibit interspersed regions of laminar, transitional, and turbulent flows. The revolving wing configuration consists of a single, aspect ratio one rectangular plate extended out a distance of 0.5 chords from the origin. Shortly after the onset of the motion, the rotating wing generates a stable and coherent vortex system across the leading edge and wing root that remains throughout the motion. The aerodynamic loads are also analyzed and found to remain mostly constant during the maneuver. Transitional effects on the vortex system are investigated over a range of Reynolds numbers (3,000 < Re < 15,000). It is found that higher Reynolds numbers promote more breakdown of the leading edge and root vortices, but do not alter the stability of the vortex system. The aerodynamic loads also show little sensitivity to Reynolds number with the higher Reynolds numbers producing only moderately higher forces. Comparisons with recent experimental PIV measurements using a PIV-like data reduction technique applied to the computational solution show very favorable agreement with the mid-span velocity and vorticity contours.

scholarly journals Experimental investigations on the sharp leading-edge separation over a flat plate at zero incidence using particle image velocimetry

2020 ◽  
Vol 61 (9) ◽  
Author(s):  
K. Fujiwara ◽  
R. Sriram ◽  
K. Kontis
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Particle Image ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Reattachment Length ◽  
Large Scale Structures ◽  
Image Velocimetry ◽  
Scale Structures ◽  
Sharp Leading Edge

Abstract Leading-edge separated flow field over a sharp flat plate is experimentally investigated in Reynolds numbers ranging from 6.2 × 103 to 4.1 × 104, using particle image velocimetry (PIV) and its statistics. It was observed that the average reattachment length is nearly independent of Reynolds number and the small secondary bubble observed near the leading edge was found to shrink with increasing Reynolds number. The wall-normal profiles of the statistical values of kinematic quantities such as the velocity components and their fluctuations scaled well with average reattachment length lR and freestream velocity U∞. Their magnitudes compare well with previous investigations even though the current triangular shaped sharp leading edge is different from previous flat-faced or semi-circular ones. The shear layer was observed to exhibit 2 different linear growth rates over 2 distinct regions. Instantaneous PIV realizations demonstrate unsteady nature of the separation bubble, whose origins in the upstream portion of the bubble are analysed. Bimodal nature of the probability density function (PDF) of fluctuating streamwise velocity at around x/lR = 0.08–0.15 indicates successive generation and passage of vortices in the region, which subsequently interact and evolve into multiscale turbulent field exhibiting nearly Gaussian PDF. Shedding of vortices with wide range of scales are apparent in most of the instantaneous realizations. Proper Orthogonal Decomposition (POD) of the velocity fluctuation magnitude field revealed that the flow structures of the dominant modes and their relative energies are independent of Reynolds number. In each of the dominant modes (first 3 modes), the length scales corresponding to the large scale structures and their spacing are the same for all Reynolds numbers, suggesting that their Strouhal number (observed to be ~ 0.09–0.2 at Reynolds number of 6.2 × 103) of unsteadiness should also be independent of Reynolds number. A single large structure- comparable in size to lR—was apparent well before reattachment in a few instantaneous realizations, as compared to multiple small-scale structures visible in most realizations; at Reynolds number of 6.2 × 103, realizations with such large-scale structures occurred approximately after every 20–30 realizations, corresponding to non-dimensional frequency of 0.4–0.6, which is identified to be the “regular shedding”. It was possible to reconstruct the large-scale structure during the instances from just the first 3 POD modes, indicating that the Strouhal number of regular shedding too is independent of Reynolds number. Graphic abstract

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