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.

scholarly journals Transitional Flow Dynamics Behind a Micro-Ramp

2019 ◽  
Vol 104 (2-3) ◽  
pp. 533-552
Author(s):  
J. Casacuberta ◽  
K. J. Groot ◽  
Q. Ye ◽  
S. Hickel
Keyword(s):  
Boundary Layer ◽  
Stability Analysis ◽  
Large Scale ◽  
Vortex Pair ◽  
Base Flow ◽  
Transitional Flow ◽  
Mean Flow ◽  
Primary Vortex ◽  
The Mean ◽  
Near Wall

AbstractMicro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.

Unsteady Effects on Trailing Edge Cooling

2005 ◽  
Vol 127 (4) ◽  
pp. 388-392 ◽  
Author(s):  
G. Medic ◽  
P. A. Durbin
Keyword(s):  
Turbine Blade ◽  
Pressure Side ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Trailing Edge ◽  
Cooling Flows ◽  
The Mean ◽  
Adiabatic Effectiveness ◽  
Surface Jet ◽  
Unsteady Effects

It is shown how natural and forced unsteadiness play a major role in turbine blade trailing edge cooling flows. Reynolds averaged simulations are presented for a surface jet in coflow, resembling the geometry of the pressure side breakout on a turbine blade. Steady computations show very effective cooling; however, when natural—or even moreso, forced—unsteadiness is allowed, the adiabatic effectiveness decreases substantially. Streamwise vortices in the mean flow are found to be the cause of the increased heat transfer.

The Simulation of Airframe Noise Applying Euler-Perturbation and Acoustic Analogy Approaches

2005 ◽  
Vol 4 (1-2) ◽  
pp. 69-91 ◽  
Author(s):  
R. Ewert ◽  
J.W. Delfs ◽  
M. Lummer
Keyword(s):  
Mach Number ◽  
Scaling Laws ◽  
Perturbation Approach ◽  
Far Field ◽  
Acoustic Analogy ◽  
Mean Flow ◽  
Trailing Edge ◽  
Airframe Noise ◽  
Trailing Edge Noise ◽  
The Mean

The capability of three different perturbation approaches to tackle airframe noise problems is studied. The three approaches represent different levels of complexity and are applied to trailing edge noise problems. In the Euler-perturbation approach the linearized Euler equations without sources are used as governing acoustic equations. The sound generation and propagation is studied for several trailing edge shapes (blunt, sharp, and round trailing edges) by injecting upstream of the trailing edge test vortices into the mean-flow field. The efficiency to generate noise is determined for the trailing edge shapes by comparing the different generated sound intensities due to an initial standard vortex. Mach number scaling laws are determined varying the mean-flow Mach number. In the second simulation approach an extended acoustic analogy based on acoustic perturbation equations (APEs) is applied to simulate trailing edge noise of a flat plate. The acoustic source terms are computed from a synthetic turbulent velocity model. Furthermore, the far field is computed via additional Kirchhoff extrapolation. In the third approach the sources of the extended acoustic analogy are computed from a Large Eddy Simulation (LES) of the compressible flow problem. The directivities due to a modeled and a LES based source, respectively, compare qualitatively well in the near field. In the far field the asymptotic directivities from the Kirchhoff extrapolation agree very well with the analytical solution of Howe. Furthermore, the sound pressure spectra can be shown to have similar shape and magnitude for the last two approaches.

Passive pitching of splitters in the trailing edge of elliptic cylinders

2017 ◽  
Vol 826 ◽  
pp. 363-375 ◽  
Author(s):  
Y. Jin ◽  
L. P. Chamorro
Keyword(s):  
Vortex Shedding ◽  
Semimajor Axis ◽  
Wake Flow ◽  
Recirculation Region ◽  
Mean Flow ◽  
Trailing Edge ◽  
Flow Measurements ◽  
High Background ◽  
Aspect Ratios ◽  
The Mean

The distinctive pitching of hinged splitters in the trailing edge of elliptic cylinders was experimentally studied at various angles of attack ($AoA$) of the cylinder, Reynolds numbers, splitter lengths, aspect ratios ($AR$) of the cylinder and freestream turbulence levels. High-resolution telemetry and hotwire anemometry were used to characterize and gain insight on the dynamics of splitters and wake flow. Results show that the motions of the splitters contain various dominating modes, e.g. $f_{p}$ and $f_{v}$, which are induced by the mean flow and wake dynamics. High background turbulence dampens the coherence of the regular vortex shedding leading to negligible $f_{v}$. For a sufficiently long splitter, namely twice the semimajor axis of the cylinder, dual vortex shedding mode exists close to the leading and trailing edges of the splitter. In general, the splitters oscillate around an equilibrium position nearly parallel to the mean direction of the flow; however, a skewed equilibrium is also possible with a strong recirculation region. This is the case with cylinders of low $AR$ and high $AoA$, where higher lift and drag occurs. Flow measurements at various transverse locations within the wake of the cylinder–splitter system indicate that the signature of the low-frequency splitter pitching is shifted in the wake in the cases with non-zero $AoA$ of the cylinder. Although the splitter pitching exhibits two dominant vortex shedding modes in various configurations, only the higher frequency is transmitted to the wake.

scholarly journals Numerical and experimental investigation of the mean and turbulent characteristics of a wing-tip vortex in the near field

2014 ◽  
Vol 228 (13) ◽  
pp. 2516-2529 ◽  
Author(s):  
Micheál S O’Regan ◽  
Philip C Griffin ◽  
Trevor M Young
Keyword(s):  
Reynolds Stress ◽  
Turbulence Model ◽  
Near Field ◽  
Tip Vortex ◽  
Mean Flow ◽  
Turbulent Characteristics ◽  
The Mean ◽  
Wing Tip Vortex ◽  
Wing Tip ◽  
Good Agreement

The near-field (up to three chord lengths) development of a wing-tip vortex is investigated both numerically and experimentally. The research was conducted in a medium speed wind tunnel on a NACA 0012 square tip half-wing at a Reynolds number of 3.2 × 105. A full Reynolds stress turbulence model with a hybrid unstructured grid was used to compute the wing-tip vortex in the near field while an x-wire anemometer and five-hole probe recorded the experimental results. The mean flow of the computed vortex was in good agreement with experiment as the circulation parameter was within 6% of the experimental value at x/ c = 0 for α = 10° and the crossflow velocity magnitude was within 1% of the experimental value at x/ c = 1 for α = 5°. The trajectory of the computed vortex was also in good agreement as it had moved inboard by the same amount (10% chord) as the experimental vortex at the last measurement location. The axial velocity excess is under predicted for α = 10°, whereas the velocity deficit is in relatively good agreement for α = 5°. The computed Reynolds shear stress component 〈 u′v′〉 is in good agreement with experiment at x/ c = 0 for α = 5°, but is greatly under predicted further downstream and at all locations for α = 10°. It is thought that a lack of local grid refinement in the vortex core and deficiencies in the Reynolds stress turbulence model may have led to errors in the mean flow and turbulence results respectively.

scholarly journals Effect of Fluid Viscoelasticity on Turbulence and Large-Scale Vortices behind Wall-Mounted Plates

2014 ◽  
Vol 6 ◽  
pp. 823138
Author(s):  
Takahiro Tsukahara ◽  
Masaaki Tanabe ◽  
Yasuo Kawaguchi
Keyword(s):  
Viscoelastic Fluid ◽  
Large Scale ◽  
Vortex Pair ◽  
Mean Flow ◽  
Turbulent Structures ◽  
Frictional Drag ◽  
Local Skin ◽  
Smooth Channel ◽  
The Mean ◽  
Free Area

Direct numerical simulations of turbulent viscoelastic fluid flows in a channel with wall-mounted plates were performed to investigate the influence of viscoelasticity on turbulent structures and the mean flow around the plate. The constitutive equation follows the Giesekus model, valid for polymer or surfactant solutions, which are generally capable of reducing the turbulent frictional drag in a smooth channel. We found that turbulent eddies just behind the plates in viscoelastic fluid decreased in number and in magnitude, but their size increased. Three pairs of organized longitudinal vortices were observed downstream of the plates in both Newtonian and viscoelastic fluids: two vortex pairs were behind the plates and the other one with the longest length was in a plate-free area. In the viscoelastic fluid, the latter vortex pair in the plate-free area was maintained and reached the downstream rib, but its swirling strength was weakened and the local skin-friction drag near the vortex was much weaker than those in the Newtonian flow. The mean flow and small spanwise eddies were influenced by the additional fluid force due to the viscoelasticity and, moreover, the spanwise component of the fluid elastic force may also play a role in the suppression of fluid vortical motions behind the plates.

On the periodically excited plane turbulent mixing layer, emanating from a jagged partition

2007 ◽  
Vol 589 ◽  
pp. 479-507 ◽  
Author(s):  
E. KIT ◽  
I. WYGNANSKI ◽  
D. FRIEDMAN ◽  
O. KRIVONOSOVA ◽  
D. ZHILENKO
Keyword(s):  
Turbulent Mixing ◽  
Mixing Layer ◽  
Two Dimensional ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Trailing Edge ◽  
Mean Velocity ◽  
Turbulent Mixing Layer ◽  
Arithmetic Average ◽  
The Mean

The flow in a turbulent mixing layer resulting from two parallel different velocity streams, that were brought together downstream of a jagged partition was investigated experimentally. The trailing edge of the partition had a short triangular ‘chevron’ shape that could also oscillate up and down at a prescribed frequency, because it was hinged to the stationary part of the partition to form a flap (fliperon). The results obtained from this excitation were compared to the traditional results obtained by oscillating a two-dimensional fliperon. Detailed measurements of the mean flow and the coherent structures, in the periodically excited and spatially developing mixing layer, and its random constituents were carried out using hot-wire anemometry and stereo particle image velocimetry.The prescribed spanwise wavelength of the chevron trailing edge generated coherent streamwise vortices while the periodic oscillation of this fliperon locked in-phase the large spanwise Kelvin–Helmholtz (K-H) rolls, therefore enabling the study of the inter- action between the two. The two-dimensional periodic excitation increases the strength of the spanwise rolls by increasing their size and their circulation, which depends on the input amplitude and frequency. The streamwise vortices generated by the jagged trailing edge distort and bend the primary K-H rolls. The present investigation endeavours to study the distortions of each mode as a consequence of their mutual interaction. Even the mean flow provides evidence for the local bulging of the large spanwise rolls because the integral width (the momentum thickness, θ), undulates along the span. The lateral location of the centre of the ensuing mixing layer (the location where the mean velocity is the arithmetic average of the two streams,y0), also suggests that these vortices are bent. Phase-locked and ensemble-averaged measurements provide more detailed information about the bending and bulging of the large eddies that ensue downstream of the oscillating chevron fliperon. The experiments were carried out at low speeds, but at sufficiently high Reynolds number to ensure naturally turbulent flow.

scholarly journals Mechanisms for the Generation of Mesoscale Vorticity Features in Tropical Cyclone Rainbands

2006 ◽  
Vol 134 (10) ◽  
pp. 2649-2669 ◽  
Author(s):  
Charmaine N. Franklin ◽  
Greg J. Holland ◽  
Peter T. May
Keyword(s):  
Tropical Cyclone ◽  
Rossby Waves ◽  
Primary Source ◽  
Vortex Pair ◽  
Cloud Microphysics ◽  
Vortex Center ◽  
Mean Flow ◽  
Fourier Decomposition ◽  
Freezing Level ◽  
The Mean

Abstract A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. Analysis of the vorticity interactions that occur within the simulated rainbands demonstrates that couplets of cyclonic–anticyclonic mesovortices can be produced in outer bands. The primary source of this vorticity is the upward tilting of system-generated horizontal vorticity by diabatic heating gradients. The vertical heating gradient in the stratiform cloud also creates a potential vorticity (PV) dipole that accelerates the tangential flow and develops a midlevel jet. The strength of the jet is enhanced by the vortex pair that is oriented radially across the rainband. The Fourier decomposition of the absolute vorticity field shows two counterpropagating vortex Rossby waves associated with the rainband. The wave located on the inner side of the band transports energy toward the vortex center. The outer wave is made up of high wavenumbers and uses the vorticity gradients generated by the rainband. The results support the hypothesis that the heating profile in the stratiform regions of rainbands generates cyclonic PV across the freezing level, which develops a midlevel jet. This mechanism creates a vorticity gradient that enables the propagation of vortex Rossby waves that could allow the rainbands to interact with the mean flow and potentially influence the evolution of the storm by contributing to the symmetric component of vorticity and the development of secondary eyewalls.

The Effect of Land Taper Angle on Trailing Edge Slot Film Cooling

2015 ◽  
Vol 137 (7) ◽  
Author(s):  
Julia Ling ◽  
Christopher J. Elkins ◽  
John K. Eaton
Keyword(s):  
Film Cooling ◽  
Concentration Distribution ◽  
Turbine Blades ◽  
Mean Flow ◽  
Trailing Edge ◽  
Taper Angle ◽  
Flow Features ◽  
The Mean ◽  
Slot Film Cooling ◽  
The Impact

Trailing edge slot film cooling is a widely used method of protecting the thin trailing edge of turbine blades from hot gas impingement. The structures that separate the slots, known as “lands,” come in a variety of configurations which can be broadly classified as either “tapered” or “straight.” This paper examines the effect of the land taper angle on the mixing of the coolant flow with the main flow by comparing three configurations: a case with straight lands, a previously reported case with slightly tapered lands, and a case with strongly tapered lands. In each case, the slot width and the land width at the plane of the slot exit are kept constant. For each configuration, the mean volumetric coolant concentration distribution and three-component velocity field were measured using magnetic resonance imaging (MRI) techniques. It is shown that the land taper angle has a strong effect on the mean flow features and coolant surface effectiveness. Furthermore, the impact of the lands configuration on the flow field and concentration distribution is seen not just in the cutback region, but also in the wake of the blade.

Turbulent flow of a fluid with anisotropic viscosity

2016 ◽  
Vol 792 ◽  
pp. 252-273 ◽  
Author(s):  
Tim Grünberg ◽  
Thomas Rösgen
Keyword(s):  
Turbulent Flow ◽  
Large Scale ◽  
Tracer Particle ◽  
Mean Flow ◽  
Flow Topology ◽  
Velocity Statistics ◽  
The Mean ◽  
High Reynolds ◽  
Ultrasound Velocimetry ◽  
Anisotropic Viscosity

We ask if and how the large-scale structure of a turbulent flow depends on anisotropies introduced at the smallest scales. We generate such anisotropy on the viscous scale in a paramagnetic colloid whose rheology is modified by an external, uniform magnetic field. We report measurements in a high Reynolds number turbulence experiment ($R_{{\it\lambda}}=120$). Ultrasound velocimetry provides records of tracer particle velocity. Distinct changes in the velocity statistics can be observed from the dissipative scales up to the mean flow topology.

Sign in / Sign up

Export Citation Format

Share Document