Performance Assessment of Transition Models for Three-Dimensional Flow Over NACA4412 Wings at Low Reynolds Numbers

2018 ◽  
Vol 140 (12) ◽  
Author(s):  
İlyas Karasu ◽  
Mustafa Özden ◽  
Mustafa Serdar Genç
Keyword(s):  
Reynolds Number ◽  
Secondary Flow ◽  
Flow Separation ◽  
Three Dimensional ◽  
Axial Flow ◽  
Reynolds Numbers ◽  
3D Flow ◽  
Tip Vortices ◽  
Transition Models ◽  
Wing Tip

The performance of the transition models on three-dimensional (3D) flow of wings with aspect ratios (AR) of 1 and 3 at low Reynolds number was assessed in this study. For experimental work; force measurements, surface oil and smoke-wire flow visualizations were performed over the wings with NACA4412 section at Reynolds numbers of 2.5 × 104, 5 × 104, and 7.5 × 104 and the angles of attack of 8 deg, 12 deg, and 20 deg. Results showed that the AR had significant effects on the 3D flow structure over the wing. According to the experimental and numerical results, the flow over the wing having lower ARs can be defined with wingtip vortices, axial flow, and secondary flow including spiral vortex inside the separated flow. When the angle of attack and Reynolds number was increased, wing-tip vortices were enlarged and interacted with the axial flow. At higher AR, flow separation was dominant, whereas wing-tip vortices suppressed the flow separation over the wing with lower AR. In the numerical results, while there were some inconsistencies in the prediction of lift coefficients, the predictions of drag coefficients for two transition models were noticeably better. The performance of the transition models judged from surface patterns was good, but the k–kL–ω was preferable. Secondary flow including spiral vortices near the surface was predicted accurately by the k–kL–ω. Consequently, in comparison with experiments, the predictions of the k–kL–ω were better than those of the shear stress transport (SST) transition.

scholarly journals Steady flow separation patterns in a 45 degree junction

2000 ◽  
Vol 411 ◽  
pp. 1-38 ◽  
Author(s):  
C. ROSS ETHIER ◽  
SUJATA PRAKASH ◽  
DAVID A. STEINMAN ◽  
RICHARD L. LEASK ◽  
GREGORY G. COUCH ◽  
...  
Keyword(s):  
Flow Separation ◽  
Stokes Equations ◽  
Bypass Graft ◽  
Three Dimensional ◽  
Symmetry Plane ◽  
Axial Flow ◽  
Reynolds Numbers ◽  
Secondary Flows ◽  
Measured Velocity ◽  
Curved Tube

Numerical and experimental techniques were used to study the physics of flow separation for steady internal flow in a 45° junction geometry, such as that observed between two pipes or between the downstream end of a bypass graft and an artery. The three-dimensional Navier–Stokes equations were solved using a validated finite element code, and complementary experiments were performed using the photochromic dye tracer technique. Inlet Reynolds numbers in the range 250 to 1650 were considered. An adaptive mesh refinement approach was adopted to ensure grid-independent solutions. Good agreement was observed between the numerical results and the experimentally measured velocity fields; however, the wall shear stress agreement was less satisfactory. Just distal to the ‘toe’ of the junction, axial flow separation was observed for all Reynolds numbers greater than 250. Further downstream (approximately 1.3 diameters from the toe), the axial flow again separated for Re [ges ] 450. The location and structure of axial flow separation in this geometry is controlled by secondary flows, which at sufficiently high Re create free stagnation points on the model symmetry plane. In fact, separation in this flow is best explained by a secondary flow boundary layer collision model, analogous to that proposed for flow in the entry region of a curved tube. Novel features of this flow include axial flow separation at modest Re (as compared to flow in a curved tube, where separation occurs only at much higher Re), and the existence and interaction of two distinct three-dimensional separation zones.

scholarly journals Three-dimensional flows around low-aspect-ratio flat-plate wings at low Reynolds numbers

2009 ◽  
Vol 623 ◽  
pp. 187-207 ◽  
Author(s):  
KUNIHIKO TAIRA ◽  
TIM COLONIUS
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Vortex Dynamics ◽  
Large Time ◽  
Angle Of Attack ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Tip Vortices ◽  
Wake Vortices ◽  
Low Aspect Ratio

Three-dimensional flows over impulsively translated low-aspect-ratio flat plates are investigated for Reynolds numbers of 300 and 500, with a focus on the unsteady vortex dynamics at post-stall angles of attack. Numerical simulations, validated by an oil tow-tank experiment, are performed to study the influence of aspect ratio, angle of attack and planform geometry on the wake vortices and the resulting forces on the plate. Immediately following the impulsive start, the separated flows create wake vortices that share the same topology for all aspect ratios. At large time, the tip vortices significantly influence the vortex dynamics and the corresponding forces on the wings. Depending on the aspect ratio, angle of attack and Reynolds number, the flow at large time reaches a stable steady state, a periodic cycle or aperiodic shedding. For cases of high angles of attack, an asymmetric wake develops in the spanwise direction at large time. The present results are compared to higher Reynolds number flows. Some non-rectangular planforms are also considered to examine the difference in the wakes and forces. After the impulsive start, the time at which maximum lift occurs is fairly constant for a wide range of flow conditions during the initial transient. Due to the influence of the tip vortices, the three-dimensional dynamics of the wake vortices are found to be quite different from the two-dimensional von Kármán vortex street in terms of stability and shedding frequency.

scholarly journals An Experimental Flow Field Study of a Bio-inspired Corrugated Wing at Low Reynolds Number

2019 ◽  
Vol 11 (3) ◽  
pp. 55-65 ◽  
Author(s):  
Y. D. DWIVEDI ◽  
Y. B. SUDHIR SASTRY
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Flat Plate ◽  
Three Dimensional ◽  
Longitudinal Axis ◽  
Reynolds Numbers ◽  
Subsonic Wind Tunnel ◽  
The Right ◽  
Moderate Range ◽  
Wing Tip

The present paper examined experimentally the glide flight flow visualization and boundary layers of a bio-inspired corrugated dragonfly wing performing a comparison with the results obtained with a flat plate, at low to moderate range of chord Reynolds numbers. The experimental work is performed in an open-end low speed subsonic wind tunnel at different angles of attack ranging from 0 to 120 and Reynolds number 2.25×105. The boundary layer measurements were done at a fixed chord location (0.7 x/c) and three different semi span locations such as 30%, 60% and 90% of the wing’s semi span from the right side of the longitudinal axis of the wing. The flow patterns were visualized by using colored tufts, placed at different span locations. The flow reversal was observed at selected Reynolds numbers and angles of attack only. The boundary layer measurements demonstrated that there exists a clear distinction on the pressure and velocity parameters in all the three tested locations on both types of the wings. The corrugated wing showed significant delay in stall and flow separation compared with the flat plate. The visualization of flow in both wings showed that there subsists a spanwise flow moving from wing tip to root, indicating three dimensional natures of airflows.

Three-dimensional features in low-Reynolds-number confined corner flows

2010 ◽  
Vol 668 ◽  
pp. 33-57 ◽  
Author(s):  
LAURA GUGLIELMINI ◽  
R. RUSCONI ◽  
S. LECUYER ◽  
H. A. STONE
Keyword(s):  
Reynolds Number ◽  
Secondary Flow ◽  
Cross Section ◽  
Low Reynolds Number ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Channel Cross Section ◽  
Radius Of Curvature ◽  
Matched Asymptotic Expansion ◽  
Low Reynolds

In recent microfluidic experiments with solutions of bacteria we observed the formation of biofilms in the form of thread-like structures, called ‘streamers’, which float in the middle plane of the channel and are connected to the side walls at the inner corners. Motivated by this observation, we discuss here the pressure-driven low-Reynolds-number flow around a corner bounded by the walls of a channel with rectangular cross-section. We numerically solve the flow field in a channel of constant cross-section, which exhibits 90° sharp corners, or turns with constant curvature, or portions with slowly changing curvature along the flow direction, for finite, but small, values of the Reynolds numbers and including the limit of vanishingly small Reynolds numbers. In addition, we develop a matched asymptotic expansion solution for the flow around two boundaries intersecting at an angle α and spanning the small gap h between two horizontal plates. We illustrate the basic features of the flow in these channel geometries by describing the three-dimensional velocity field and the distribution of streamwise vorticity and helicity, and comparing the numerical solutions with predictions based on the asymptotic approach. We demonstrate that near a corner or a change in the curvature of the side wall the flow is three-dimensional and pairs of counter-rotating vortical structures are present, as identified by Balsa (J. Fluid Mech., vol. 372, 1998, p. 25). Finally, we discuss how this secondary flow depends on the significant geometric parameters, the aspect ratio of the channel cross-section, the radius of curvature of the turn and, more generally, the variation of the curvature of the channel side boundary. We believe that these three-dimensional secondary flow structures are relevant to transport problems where accumulation of material at the boundary is possible.

Computational Investigation of the Three-Dimensional Flow Structure and Losses in a Low Reynolds Number Microturbine

2011 ◽  
Author(s):  
M. Omri ◽  
L. G. Fre´chette
Keyword(s):  
Reynolds Number ◽  
Heat Engine ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Flow Patterns ◽  
Tip Clearance ◽  
3D Flow ◽  
Low Reynolds Numbers ◽  
Blade Passage ◽  
Low Reynolds

In this work, three dimensional numerical studies of the aerodynamics in laminar subsonic cascades at relatively low Reynolds numbers (Re < 2500) are presented. The stator and rotor blade designs are those for a MEMS-based Rankine microturbine power-plant-on-a-chip with 100 micron chord blades. Blade passage calculations in 2D and 3D were done for different Reynolds numbers, four different tip clearances (0%, 5%, 10% and 20%) and four incidences (0°, 5°, 10° and 15°) to determine the flow patterns and compute losses. These conditions are applied to a blade passage without rotation (stator) and with rotation (rotor), both for a stationary and moving outer casing. The 3D blade passage (without tip clearance) indicates the presence of two large symmetric vortices due to the interaction between flow curvature and hub/casing boundary layers. With tip clearance, a secondary vortex appears due to tip flow. This so-called tip vortex becomes dominant in the case of tip clearance above 10%. Relative wall motion also impacts the 3D flow patterns due to the important tangential drag at these low Reynolds numbers. Two dimensional calculations characterize well the flow at the mid-height plane, but are not sufficient for loss predictions due to the omission of the 3D flow structures. The 3D total losses increase dramatically for Re<500, which is similar to 2D studies. This suggests an operating Reynolds number greater than this to obtain efficiency levels necessary to operate a heat engine. The losses also increased monotonically with increasing tip clearance and incidence.

A Leading-Edge Alula-Inspired Device (LEAD) for Stall Mitigation and Lift Enhancement for Low Reynolds Number Finite Wings

2018 ◽  
Author(s):  
Mihary R. Ito ◽  
Chengfang Duan ◽  
Leonardo P. Chamorro ◽  
Aimy A. Wissa
Keyword(s):  
Aspect Ratio ◽  
Flow Separation ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Tip Vortices ◽  
Low Reynolds Numbers ◽  
Lift Enhancement ◽  
Low Reynolds ◽  
Torque Transducer

Even though Unmanned Aerial Vehicles (UAVs) operating at low Reynolds numbers are becoming common, their performance and maneuverability are still greatly limited due to aerodynamic phenomena such as stall and flow separation. Birds mitigate these limitations by adapting their wings and feather shapes during flight. Equipped with a set of small feathers, known as the alula, located near the leading edge and covering 5% to 20% of the span, bird wings can sustain the lift necessary to fly at low velocities and high angles of attack. This paper presents the effect on lift generation of different placements of a Leading-Edge Alula-inpsired Device (LEAD) along the span of a moderate aspect-ratio wing. The device is modeled after the alula on a bird, and it increases the capability of a wing to maintain higher pressure gradients by modifying the near-wall flow close to the leading-edge. It also generates tip vortices that modify the turbulence on the upper-surface of the wing, delaying flow separation. The effect of the LEAD can be compared to traditional slats or vortex generators on two-dimensional wings. For finite wings, on the other hand, the effect depends on the interaction between the LEADs tip vortices and those from the main structure. Wind tunnel experiments were conducted on a cambered wing at post-stall and deep-stall angles of attack at low Reynolds numbers of 100,000 and 135,000. To quantify the aerodynamic effect of the device, the lift generated by the wing with and without the LEAD were measured using a 6-axis force and torque transducer, and the resulting lift coefficients were compared. Results show that the location of the LEAD yielding the highest lift enhancement was 50% semi-span away from the wing root. Lift improvements of up to 32% for post stall and 37% for deep stall were obtained at this location, demonstrating that the three-dimensional effects of the LEAD are important. The lift enhancement was also more prominent on a finite moderate aspect-ratio wing (3D) than on an airfoil (2D), confirming that the LEAD is a three-dimensional device. Identifying the configurations and deployment parameters that improve lift generation the most is needed to design an adaptive LEAD that can be implemented on a UAV wing for increased mission-adaptability.

Experiments on the Velocity Field Around a Low Aspect Ratio Wing at Low Reynolds Numbers

2009 ◽  
Author(s):  
Daniel R. Morse ◽  
James A. Liburdy
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Shear Stresses ◽  
Tip Vortices ◽  
Low Reynolds Numbers ◽  
Low Aspect Ratio ◽  
Low Reynolds ◽  
Wing Tip

The flow structure around a low aspect ratio wing at low Reynolds numbers and a fixed angle of attack of 20° is discussed using flow visualization as well as Three-Component Time-Resolved Particle Image Velocimetry (3C TR PIV). Mean quantities and statistical measurements of velocity were obtained and used to describe the average and transient characteristics of the flow field. Effects of spanwise variation from centerline to wingtip and Reynolds number variation from 1.3×104 to 6.6×104 are discussed. The role of the wing tip vortices is observed to be large in a low aspect ratio wing. The transfer of momentum via Reynolds shear stresses is shown in the leading edge region. Normalized spanwise shear stresses associated with the wing tip vortices are observed to increase with increasing Reynolds number.

Bifurcation Characteristics of Flows in Rectangular Sudden Expansion Channels

2005 ◽  
Author(s):  
Francine Battaglia ◽  
George Papadopoulos
Keyword(s):  
Reynolds Number ◽  
Laminar Flow ◽  
Critical Reynolds Number ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Expansion Ratio ◽  
Sudden Expansion ◽  
Two Dimensional ◽  
Effective Expansion ◽  
Three Dimensional Simulations

The effect of three-dimensionality on low Reynolds number flows past a symmetric sudden expansion in a channel was investigated. The geometric expansion ratio of in the current study was 2:1 and the aspect ratio was 6:1. Both experimental velocity measurements and two- and three-dimensional simulations for the flow along the centerplane of the rectangular duct are presented for Reynolds numbers in the range of 150 to 600. Comparison of the two-dimensional simulations with the experiments revealed that the simulations fail to capture completely the total expansion effect on the flow, which couples both geometric and hydrodynamic effects. To properly do so requires the definition of an effective expansion ratio, which is the ratio of the downstream and upstream hydraulic diameters and is therefore a function of both the expansion and aspect ratios. When the two-dimensional geometry was consistent with the effective expansion ratio, the new results agreed well with the three-dimensional simulations and the experiments. Furthermore, in the range of Reynolds numbers investigated, the laminar flow through the expansion underwent a symmetry-breaking bifurcation. The critical Reynolds number evaluated from the experiments and the simulations was compared to other values reported in the literature. Overall, side-wall proximity was found to enhance flow stability, helping to sustain laminar flow symmetry to higher Reynolds numbers in comparison to nominally two-dimensional double-expansion geometries. Lastly, and most importantly, when the logarithm of the critical Reynolds number from all these studies was plotted against the reciprocal of the effective expansion ratio, a linear trend emerged that uniquely captured the bifurcation dynamics of all symmetric double-sided planar expansions.

Using Turbulence Intensity and Reynolds Number to Predict Flow Separation on a Highly Loaded, Low-Pressure Gas Turbine Blade at Low Reynolds Numbers

2018 ◽  
Author(s):  
Kenneth Van Treuren ◽  
Tyler Pharris ◽  
Olivia Hirst
Keyword(s):  
Reynolds Number ◽  
Gas Turbine ◽  
Turbine Blade ◽  
Turbulence Intensity ◽  
Flow Separation ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
High Altitudes ◽  
Low Reynolds Numbers ◽  
Low Pressure Turbine

The low-pressure turbine has become more important in the last few decades because of the increased emphasis on higher overall pressure and bypass ratios. The desire is to increase blade loading to reduce blade counts and stages in the low-pressure turbine of a gas turbine engine. Increased turbine inlet temperatures for newer cycles results in higher temperatures in the low-pressure turbine, especially the latter stages, where cooling technologies are not used. These higher temperatures lead to higher work from the turbine and this, combined with the high loadings, can lead to flow separation. Separation is more likely in engines operating at high altitudes and reduced throttle setting. At the high Reynolds numbers found at takeoff, the flow over a low-pressure turbine blade tends to stay attached. At lower blade Reynolds numbers (25,000 to 200,000), found during cruise at high altitudes, the flow on the suction surface of the low-pressure turbine blades is inclined to separate. This paper is a study on the flow characteristics of the L1A turbine blade at three low Reynolds numbers (60,000, 108,000, and 165,000) and 15 turbulence intensities (1.89% to 19.87%) in a steady flow cascade wind tunnel. With this data, it is possible to examine the impact of Reynolds number and turbulence intensity on the location of the initiation of flow separation, the flow separation zone, and the reattachment location. Quantifying the change in separated flow as a result of varying Reynolds numbers and turbulence intensities will help to characterize the low momentum flow environments in which the low-pressure turbine must operate and how this might impact the operation of the engine. Based on the data presented, it is possible to predict the location and size of the separation as a function of both the Reynolds number and upstream freestream turbulence intensity (FSTI). Being able to predict this flow behavior can lead to more effective blade designs using either passive or active flow control to reduce or eliminate flow separation.

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