Experimental Investigation of Turbulent Flow With Separation Over a NACA 4412 Airfoil at Angle of Attack 20 Degree

2002 ◽  
Author(s):  
Omar O. Badran ◽  
Hans H. Bruun
Keyword(s):  
Separation Bubble ◽  
Angle Of Attack ◽  
Shear Stresses ◽  
Turbulence Structure ◽  
Separation Flow ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Trailing Edge ◽  
Mean Velocity ◽  
Airfoil Surface

This paper presents the measured mean flow and Reynolds stresses results, obtained on the center-line plane of the airfoil, covering the boundary layers over the upper surface, the potential flow region and the wake downstream of the trailing edge, at αa = 20°. The flying X-hot-wire probe was used to measure mean velocity and turbulence structure over the airfoil. An improved understanding of the physical characteristics of separation on the airfoil sections and in the region of the trailing edge is of direct value for the improvement of high lift wings for aircraft. From the study of the separation flow at angle of attack αa = 20°, the following can be concluded: A stable separation bubble has developed near the trailing edge of the airfoil, covering around 0.6c of the airfoil surface. Also it is found that values of the Reynolds normal and shear stresses move away from the surface with downstream distance, showing turbulence diffusion to be more evident in this flow. In the wake region, relatively large values of Reynolds stresses occurred, which were related to the vertical oscillations in the upper wake.

Turbulent Flow Over a NACA 4412 Airfoil at Angle of Attack 15 Degree

2003 ◽  
Author(s):  
O. O. Badran ◽  
H. H. Bruun
Keyword(s):  
Reverse Flow ◽  
Separation Bubble ◽  
Angle Of Attack ◽  
Flow Region ◽  
Shear Stresses ◽  
Separation Flow ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Trailing Edge ◽  
Short Period

This paper presents the measured mean flow and Reynolds stresses results, obtained on the center-line plane of the airfoil, covering the boundary layers over the upper surface, the potential flow region and the wake downstream of the trailing edge, at αa = 15°. The flying X-hot-wire probe was used to measure the U and V components of the flow field over the airfoil. An improved understanding of the physical characteristics of separation on the airfoil sections and in the region of the trailing edge is of direct value for the improvement of high lift wings for aircraft. From the study of the separation flow at angle of attack αa = 15°, the following can be concluded: (1) An intermittent reverse flow region occurred near the trailing edge of the airfoil. A separation bubble occurred for a short period of time and was then swept away with the stream wise flow. (2) The angle of attack αa = 15° corresponds to the position of maximum lift for a NACA 4412 airfoil section. (3) It is found that values of the Reynolds normal and shear stresses move away from the surface with downstream distance, and (4) In the wake region, relatively large values of Reynolds stresses occurred, which were related to the vertical oscillation in the lower wake.

scholarly journals Simulations of rib-roughened rough-to-smooth turbulent channel flows

2018 ◽  
Vol 843 ◽  
pp. 419-449 ◽  
Author(s):  
Umair Ismail ◽  
Tamer A. Zaki ◽  
Paul A. Durbin
Keyword(s):  
Large Scale ◽  
Wall Layer ◽  
Shear Stresses ◽  
Thin Wall ◽  
Computational Domain ◽  
Reynolds Stresses ◽  
Smooth Wall ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

High-fidelity simulations of turbulent flow through a channel with a rough wall, followed by a smooth wall, demonstrate a high degree of non-equilibrium within the recovery region. In fact, the recovery of all the flow statistics studied is incomplete by the streamwise exit of the computational domain. Above a thin wall layer, turbulence intensities significantly higher than fully developed, smooth-wall levels persist in the developing region. Within the thin wall layer, the profile shapes for turbulence stresses recover very quickly and wall-normal locations of characteristic peaks are established. However, even in this thin layer, complete recovery of magnitudes of turbulence stresses is exceptionally slow. A similar initially swift but eventually incomplete and slow relaxation behaviour is also shown by the skin friction. Between the turbulence shear and streamwise stresses, the turbulence shear stress shows a comparatively quick rate of recovery above a thin wall layer. Over the developing smooth wall, the balance is not merely between fluxes due to pressure and shear stresses. Strong momentum fluxes, which are directly influenced by the upstream roughness size, contribute significantly to this balance. Approximate curve fits estimate the streamwise distance required by the outer peaks of Reynolds stresses to attain near-fully-developed levels at approximately $20\unicode[STIX]{x1D6FF}{-}25\unicode[STIX]{x1D6FF}$, with $\unicode[STIX]{x1D6FF}$ being the channel half height. An even longer distance, of more than $50\unicode[STIX]{x1D6FF}$, might be needed by the mean velocity to approach near-fully-developed magnitudes. Visualizations and correlations show that large-scale eddies that are created above the roughness persist downstream, and sporadically perturb the elongated streaks. These streaks of alternating high and low momentum appear almost instantly after the roughness is removed. The mean flow does not re-establish an equilibrium log layer within the computational domain, and the velocity deficit created by the roughness continues throughout the domain. On the step change in roughness, near the wall, profiles for turbulence kinetic energy dissipation rate, $\unicode[STIX]{x1D716}$, and energy spectra indicate a sharp reduction in energy at small scales. Despite this, reversion towards equilibrium smooth-wall levels is slow, and ultimately incomplete, due to a rather slow adjustment of the turbulence cascade. The non-dimensional roughness height, $k^{+}$ ranges from 42 to 254 and the friction velocity Reynolds number at the smooth wall, $Re_{\unicode[STIX]{x1D70F}S}$, ranges from 284 to 1160 in the various simulations.

scholarly journals Turbulence structure of non-uniform rough open channel flow

2018 ◽  
Vol 40 ◽  
pp. 05039
Author(s):  
Priscilla Williams ◽  
Vesselina Roussinova ◽  
Ram Balachandar
Keyword(s):  
Pressure Gradient ◽  
Channel Flow ◽  
Friction Velocity ◽  
Open Channel ◽  
Open Channel Flow ◽  
Shear Stresses ◽  
Turbulence Structure ◽  
Mean Flow ◽  
Rough Bed ◽  
The Mean

This paper focuses on the turbulence structure in a non-uniform, gradually varied, sub-critical open channel flow (OCF) on a rough bed. The flow field is analysed under accelerating, near-uniform and decelerating conditions. Information for the flow and turbulence parameters was obtained at multiple sections and planes using two different techniques: two-component laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Different outer region velocity scaling methods were explored for evaluation of the local friction velocity. Analysis of the mean velocity profiles showed that the overlap layer exists for all flow cases. The outer layer of the decelerated velocity profile was strongly affected by the pressure gradient, where a large wake was noted. Due to the prevailing nature of the experimental setup it was found that the time-averaged flow quantities do not attained equilibrium conditions and the flow is spatially heterogeneous. The roughness generally increases the friction velocity and its effect was stronger than the effect of the pressure gradient. It was found that for the decelerated flow section over a rough bed, the mean flow and turbulence intensities were affected throughout the flow depth. The flow features presented in this study can be used to develop a model for simulating flow over a block ramp. The effect of the non-uniformity and roughness on turbulence intensities and Reynolds shear stresses was further investigated.

scholarly journals Numerical Analysis of an Instrumented Turbine Blade Cascade

2018 ◽  
Author(s):  
Bryn N. Ubald ◽  
Jiahuan Cui ◽  
Rob Watson ◽  
Paul G. Tucker ◽  
Shahrokh Shahpar
Keyword(s):  
Numerical Analysis ◽  
Measurement Accuracy ◽  
Free Stream ◽  
Separation Bubble ◽  
Leading Edge ◽  
Pressure Probe ◽  
Mean Flow ◽  
Trailing Edge ◽  
Free Stream Turbulence ◽  
Jet Trajectory

The measurement accuracy of the temperature/pressure probe mounted at the leading edge of a turbine/compressor blade is crucial for estimating the fuel consumption of a turbo-fan engine. Apart from the measurement error itself, the probe also introduces extra losses. This again would compromise the measurement accuracy of the overall engine efficiency. This paper utilizes high-fidelity numerical analysis to understand the complex flow around the probe and quantify the loss sources due to the interaction between the blade and its instrumentation. With the inclusion of leading edge probes, three dimensional flow phenomena develop, with some flow features acting in a similar manner to a jet in cross flow. The separated flow formed at the leading edge of the probe blocks a large area of the probe bleed-hole, which is one of the reasons why the probe accuracy can be sensitive to Mach and Reynolds numbers. The addition of 4% free stream turbulence is shown to have a marginal impact on the jet trajectory originated from the probe bleedhole. However, a slight reduction is observed in the size of the separation bubble formed at the leading edge of the probe, preceding the two bleedhole exits. The free stream turbulence also has a significant impact on the size of the separation bubble near the trailing edge of the blade. With the addition of the free stream turbulence, the loss observed within the trailing edge wake is reduced. More than 50% of the losses at the cascade exit are generated by the leading edge probe. A breakdown of the dissipation terms from the mean flow kinetic energy equation demonstrates that the Reynolds stresses are the key terms in dissipating the counter rotating vortex pairs with the viscous stresses responsible for the boundary layer.

Experimental Investigation of the Effect of Camber on the Aerodynamic Characteristics of Airfoils in Ground Effect

2003 ◽  
Author(s):  
M. R. Ahmed ◽  
G. M. Imran ◽  
S. D. Sharma
Keyword(s):  
Experimental Investigation ◽  
Angle Of Attack ◽  
Ground Effect ◽  
Chord Length ◽  
Wake Region ◽  
Trailing Edge ◽  
Mean Velocity ◽  
Large Defect ◽  
Freestream Velocity ◽  
Ground Clearance

In the present paper, results from an experimental investigation of aerodynamic ground effect on two airfoils are presented. The flow characteristics over a symmetrical airfoil (NACA 0015) and a cambered airfoil (NACA 4415) were studied in a low speed wind tunnel. Experiments were carried out by varying the angle of attack from 0° to 10° and ground clearance from zero to one chord length. Pressure distribution on the surface of the airfoil was obtained with the help of pressure tappings. Mean velocity distributions were obtained over the surface of the airfoil. Profiles of mean velocity and turbulence intensity were obtained in the wake region at 0.5 and 1.0 chord length downstream of the trailing edge. It is found that pressure increases on the lower surface as the ground is approached. The flow accelerates over the airfoil, and a considerably higher mean velocity is observed near the suction peak location. For the symmetrical airfoil, the mean velocity over the surface was found to increase by nearly 30%, while for the cambered airfoil, an increase of nearly 60% was recorded for an angle of attack of 7.5°. The flow was found to separate almost near the trailing edge for angles of attack upto 10°, resulting in a thinner wake region and lower turbulence intensities for the symmetrical airfoil; while for the cambered airfoil, an early separation for an angle of attack of 10° was observed. Measurements in the wake region showed a defect in mean velocity profile at the corresponding values of ground clearance. For lower angles of attack, turbulence levels were higher in the wake region for the symmetrical airfoil, while for an angle of attack of 10°, very large defect in velocity was observed for the cambered airfoil model and the minimum velocity reduced to 20% of the freestream velocity.

Control of trailing-edge separation by tangential blowing inside the bubble

2004 ◽  
Vol 108 (1086) ◽  
pp. 419-425 ◽  
Author(s):  
P. R. Viswanath ◽  
K. T. Madhavan
Keyword(s):  
Separation Bubble ◽  
Effective Means ◽  
Separated Flow ◽  
Wake Flow ◽  
Turbulent Shear ◽  
Trailing Edge ◽  
Mean Velocity ◽  
Turbulent Shear Stress ◽  
Edge Separation ◽  
Tangential Blowing

Abstract Experiments have been performed investigating the effectiveness of steady tangential blowing, with the blowing slot located downstream of separation (but inside the separation bubble) to control a trailing-edge separated flow at low speeds. Trailing-edge separation was induced on a two-dimensional aerofoil-like body and the shear layer closure occurred in the near-wake. Measurements made consisted of model surface pressures and mean velocity, turbulent shear stress and kinetic energy profiles in the separated zone using a two-component LDV system. It is explicitly demonstrated that the novel concept of tangential blowing inside the bubble can be an effective means of control for trailing-edge separated flows as well. Blowing mass and momentum requirements for the suppression of wall and wake flow reversals have been estimated.

The turbulence structure of the annular non-swirling flow past an axisymmetric poppet valve

1998 ◽  
Vol 212 (6) ◽  
pp. 455-471 ◽  
Author(s):  
S Nadarajah ◽  
S Balabani ◽  
M J Tindal ◽  
M Yianneskis
Keyword(s):  
Kinetic Energy ◽  
Swirling Flow ◽  
Laser Doppler Anemometry ◽  
Turbulence Kinetic Energy ◽  
Shear Stresses ◽  
Turbulence Structure ◽  
Reynolds Stresses ◽  
Length Scales ◽  
Gaussian Form ◽  
Poppet Valve

This paper describes an experimental investigation of the non-swirling flow through an axisymmetric port and poppet valve assembly under steady flow conditions using laser Doppler anemometry. The three velocity components and the associated Reynolds stresses were measured by ensemble-averaged techniques and the turbulence kinetic energy and its production rate were determined. Time-resolved measurements were also taken in order to determine turbulence time and length scales and the dissipation rate of the turbulence kinetic energy. The Reynolds number, based on the minimum cross-sectional area of the port, was 25000. The flow is characterized by an annular jet which forms two vortices, one on either side of the jet. A jet flapping instability is also evident since the skewness and kurtosis of the velocity probability distribution function depart from the Gaussian form. This instability causes an intermittent mixing between eddies in the jet region and the vortices which introduces a non-turbulent contribution to the measured quantities. The production rates of the turbulence kinetic energy were found to be negative in some regions of the flow, indicating counter-gradient transport of momentum by turbulence; according to the coherent structures approach, the distribution of the Reynolds shear stresses and the length scales in these regions imply possible changes in the orientation of eddies.

Hot-Wire Measurements Inside a Centrifugal Fan Impeller

1989 ◽  
Vol 111 (4) ◽  
pp. 363-368 ◽  
Author(s):  
A. Kjo¨rk ◽  
L. Lo¨fdahl
Keyword(s):  
Suction Side ◽  
Design Flow ◽  
Shear Stresses ◽  
Centrifugal Fan ◽  
Reynolds Stresses ◽  
Low Velocity ◽  
Mean Velocity ◽  
Attached Flow ◽  
The Mean ◽  
Velocity Region

Measurements of the three mean velocity components and five of the Reynolds stresses have been carried out in the blade passage of a centrifugal fan impeller. The impeller was of ordinary design, with nine backward curved blades, and all measurements were carried out at the design flow rate. The mean velocity measurements show that the flow can be characterized as an attached flow with almost linearly distributed velocity profiles. However, in a region near the suction side close to the shroud a low velocity region is created. From the turbulence measurements it can be concluded that relatively low values of the turbulent stresses are predominating in the center region of the channel. Closer to the walls higher values of the normal as well as shear stresses are noted.

Added stresses because of the presence of FENE-P bead–spring chains in a random velocity field

1997 ◽  
Vol 337 ◽  
pp. 67-101 ◽  
Author(s):  
HESHMAT MASSAH ◽  
THOMAS J. HANRATTY
Keyword(s):  
Shear Stress ◽  
Velocity Field ◽  
Turbulent Flow ◽  
Drag Reduction ◽  
Velocity Gradient ◽  
Gradient Field ◽  
Shear Stresses ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Random Velocity

FENE-P bead–spring chains unravel in the presence of large enough velocity gradients. In a turbulent flow, this can result in intermittent added stresses and exchanges of energy between the chains and the fluid, whose magnitudes depend on the degree of unravelling and on the orientations of the bead–spring chains. These effects are studied by calculating the average behaviour at different times of an ensemble of chains, contained in a fluid particle that is moving around in a random velocity field obtained from direct numerical simulation of turbulent flow of a Newtonian fluid in a channel. The results are used to evaluate theoretical explanations of drag reduction observed in very dilute solutions of polymers.In regions of the flow in which the energy exchange with the fluid is positive, the possibility arises that turbulence can be produced by mechanisms other than the interaction of Reynolds stresses and the mean velocity gradient field. Of particular interest, from the viewpoint of understanding polymer drag reduction, is the finding that the exchange is negative in velocity fields representative of the wall vortices that are large producers of turbulence. One can, therefore, postulate that polymers cause drag reduction by selectively changing the structures of eddies that produce Reynolds stresses. The intermittent appearance of large added shear stresses is consistent with the experimental finding of a stress deficit, whereby the total local shear stress is greater than the sum of the Reynolds stress and the time-averaged shear stress calculated from the time-averaged velocity gradient and the viscosity of the solvent.

Full-coverage film cooling. Part 1. Three-dimensional measurements of turbulence structure

1980 ◽  
Vol 101 (1) ◽  
pp. 129-158 ◽  
Author(s):  
S. Yavuzkurt ◽  
R. J. Moffat ◽  
W. M. Kays
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Film Cooling ◽  
Outer Boundary ◽  
Turbulence Structure ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Test Plate ◽  
Apparent Paradox ◽  
Full Coverage

Hydrodynamic measurements were made with a triaxial hot wire in the full-coverage region and the recovery region following an array of injection holes inclined downstream, at 30° to the surface. The data were taken under isothermal conditions at ambient temperature and pressure for two blowing ratios: M = 0·9 and M = 0·4. (The ratio M = ρjetUjet/ρ∞U∞, where U is the mean velocity and ρ is the density. Subscripts jet and ∞ stand for injectant and free stream, respectively.) Profiles of the three mean-velocity components and the six Reynolds stresses were obtained at several spanwise positions at each of five locations down the test plate.In the full-coverage region, high levels of turbulence kinetic energy (TKE) were found for low blowing and low TKE levels for high blowing. This observation is especially significant when coupled with the fact that the heat transfer coefficient is high for high blowing, and low for low blowing. This apparent paradox can be resolved by the hypothesis that entrainment of the mainstream fluid must be more important than turbulent mixing in determining the heat transfer behaviour at high blowing ratios (close to unity).In the recovery region, the flow can be described in terms of a two-layer model: an outer boundary layer and a two-dimensional inner boundary layer. The inner layer governs the heat transfer.

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