Structure of a streamwise-oriented vortex incident upon a wing

2017 ◽  
Vol 816 ◽  
pp. 306-330 ◽  
Author(s):  
C. McKenna ◽  
M. Bross ◽  
D. Rockwell
Keyword(s):  
Root Mean Square ◽  
Cross Flow ◽  
Leading Edge ◽  
Streamwise Velocity ◽  
Mean Square ◽  
Swirl Ratio ◽  
Streamwise Vorticity ◽  
Image Velocimetry ◽  
Unsteady Loading ◽  
Drag Ratio

Impingement of a streamwise-oriented vortex upon a fin, tail, blade or wing represents a fundamental class of flow–structure interaction that extends across a range of applications. It can give rise to unsteady loading known as buffeting and to changes of the lift to drag ratio. These consequences are sensitive to parameters of the incident vortex as well as the location of vortex impingement on the downstream aerodynamic surface, generically designated as a wing. Particle image velocimetry is employed to determine patterns of velocity and vorticity on successive cross-flow planes along the vortex, which lead to volume representations and thereby characterization of the streamwise evolution of the vortex structure as it approaches the downstream wing. This evolution of the incident vortex is affected by the upstream influence of the downstream wing, and is highly dependent on the spanwise location of vortex impingement. Even at spanwise locations of impingement well outboard of the wing tip, a substantial influence on the structure of the incident vortex at locations significantly upstream of the leading edge of the wing was observed. For spanwise locations close to or intersecting the vortex core, the effects of upstream influence of the wing on the vortex are to: decrease the swirl ratio; increase the streamwise velocity deficit; decrease the streamwise vorticity; increase the azimuthal vorticity; increase the upwash; decrease the downwash; and increase the root-mean-square fluctuations of both streamwise velocity and vorticity. The interrelationship between these effects is addressed, including the rapid attenuation of axial vorticity in presence of an enhanced defect of axial velocity in the central region of the vortex. When the incident vortex is aligned with, or inboard of, the tip of the wing, the swirl ratio decreases to values associated with instability of the vortex, thereby giving rise to enhanced values of azimuthal vorticity relative to the streamwise (axial) vorticity, as well as relatively large root-mean-square values of streamwise velocity and vorticity.

Three-dimensional boundary-layer instability and separation induced by small-amplitude streamwise vorticity in the upstream flow

1993 ◽  
Vol 246 ◽  
pp. 21-41 ◽  
Author(s):  
M. E. Goldstein ◽  
S. J. Leib
Keyword(s):  
Boundary Layer ◽  
Small Amplitude ◽  
Three Dimensional ◽  
Cross Flow ◽  
Leading Edge ◽  
Streamwise Velocity ◽  
Linear Perturbation ◽  
Streamwise Vorticity ◽  
Dimensional Boundary ◽  
Layer Flow

We consider the effects of a small-amplitude, steady, streamwise vorticity field on the flow over an infinitely thin flat plate in an otherwise uniform stream. We show how the initially linear perturbation, ultimately leads to a small-amplitude but nonlinear cross-flow far downstream from the leading edge. This motion is imposed on the boundary-layer flow and eventually causes the boundary layer to separate. The streamwise velocity profiles within the boundary layer become inflexional in localized spanwise regions just upstream of the separation point. The flow in these regions is therefore susceptible to rapidly growing inviscid instabilities.

Direct numerical simulation of turbulence in injection-driven three-dimensional cylindrical flows

2011 ◽  
Vol 670 ◽  
pp. 176-203 ◽  
Author(s):  
JU ZHANG ◽  
THOMAS L. JACKSON
Keyword(s):  
Reynolds Number ◽  
Root Mean Square ◽  
Streamwise Velocity ◽  
Wall Turbulence ◽  
Wall Friction ◽  
Mean Square ◽  
The Mean ◽  
Wall Shear ◽  
Strong Injection ◽  
Near Wall

Incompressible turbulent flow in a periodic circular pipe with strong injection is studied as a simplified model for the core flow in a solid-propellant rocket motor and other injection-driven internal flows. The model is based on a multi-scale asymptotic approach. The intended application of the current study is erosive burning of solid propellants. Relevant analysis for easily accessible parameters for this application, such as the magnitudes, main frequencies and wavelengths associated with the near-wall shear, and the assessment of near-wall turbulence viscosity is focused on. It is found that, unlike flows with weak or no injection, the near-wall shear is dominated by the root mean square of the streamwise velocity which is a function of the Reynolds number, while the mean streamwise velocity is only weakly dependent on the Reynolds number. As a result, a new wall-friction velocity $\(u_\tau{\,=\,}\sqrt{\tau_w/\rho}\)$, based on the shear stress derived from the sum of the mean and the root mean square, i.e. $\(\tau_{w,inj} {\,=\,} \mu |{\partial (\bar{u}+u_{rms})}/{\partial r}|_w\)$, is proposed for the scaling of turbulent viscosity for turbulent flows with strong injection. We also show that the mean streamwise velocity profile has an inflection point near the injecting surface.

scholarly journals Pitched and Yawed Circular Jets in Cross-Flow

2002 ◽  
Author(s):  
Ivana M. Milanovic ◽  
K. B. M. Q. Zaman
Keyword(s):  
Experimental Investigation ◽  
Flow Field ◽  
Turbulence Intensity ◽  
Momentum Flux ◽  
Cross Flow ◽  
Flux Ratio ◽  
Streamwise Velocity ◽  
Streamwise Vorticity ◽  
Downstream Distance ◽  
Circular Jets

Results from an experimental investigation of flow field generated by pitched and yawed jets discharging from a flat plate into a cross-flow are presented. The circular jet was pitched at α = 20° and 45° and yawed between β = 0° and 90° in increments of 15°. The measurements were performed with two X-wires providing all three components of velocity and turbulence intensity. These data were obtained at downstream locations of x = 3, 5, 10 and 20, where the distance x, normalized by the jet diameter, is measured from the center of the orifice. Data for all configurations were acquired at a momentum-flux ratio J = 8. Additionally, for selected angles and locations, surveys were conducted for J = 1.5, 4, and 20. As expected, the jet penetration is found to be higher at larger α. With increasing β the jet spreads more. The rate of reduction of peak streamwise vorticity, ωxmax, with the downstream distance is significantly lessened at higher β but is found to be practically independent of α. Thus, at the farthest measurement station x = 20, ωxmax is about five times larger for β = 75° compared to the levels at β = 0°. Streamwise velocity within the jet-vortex structure is found to depend on the parameter J. At J = 1.5 and 4, ‘wake-like’ velocity profiles are observed. In comparison, a ‘jet-like’ overshoot is present at higher J.

Turbulent Plane Couette Flow and Scalar Transport at Low Reynolds Number

2003 ◽  
Vol 125 (6) ◽  
pp. 988-998 ◽  
Author(s):  
Chun-Ho Liu
Keyword(s):  
Reynolds Number ◽  
Couette Flow ◽  
Root Mean Square ◽  
Correlation Coefficients ◽  
Streamwise Velocity ◽  
Conductive Heat ◽  
Mean Square ◽  
Plane Couette Flow ◽  
Maximum Root ◽  
Channel Center

The turbulence structure and passive scalar (heat) transport in plane Couette flow at Reynolds number equal to 3000 (based on the relative speed and distance between the walls) are studied using direct numerical simulation (DNS). The numerical model is a three-dimensional trilinear Galerkin finite element code. It is found that the structures of the mean velocity and temperature in plane Couette flow are similar to those in forced channel flow, but the empirical coefficients are different. The total (turbulent and viscous) shear stress and total (turbulent and conductive) heat flux are constant throughout the channel. The locations of maximum root-mean-square streamwise velocity and temperature fluctuations are close to the walls, while the location of maximum root-mean-square spanwise and vertical velocity fluctuations are at the channel center. The correlation coefficients between velocities and temperature are fairly constant in the center core of the channel. In particular, the streamwise velocity is highly correlated with temperature (correlation coefficient ≈−0.9). At the channel center, the turbulence production is unable to counterbalance the dissipation, in which the diffusion terms (both turbulent and viscous) bring turbulent kinetic energy from the near-wall regions toward the channel center. The snapshots of the DNS database help explain the nature of the correlation coefficients. The elongated wall streaks for both streamwise velocity and temperature in the viscous sublayer are well simulated. Moreover, the current DNS shows organized large-scale eddies (secondary rotations) perpendicular to the direction of mean flow at the channel center.

Effect on a laminar boundary layer of small-amplitude streamwise vorticity in the upstream flow

2001 ◽  
Vol 426 ◽  
pp. 229-262 ◽  
Author(s):  
DAVID W. WUNDROW ◽  
M. E. GOLDSTEIN
Keyword(s):  
Reynolds Number ◽  
Small Amplitude ◽  
Experimental Studies ◽  
Leading Edge ◽  
Transition Process ◽  
Streamwise Velocity ◽  
Shear Layers ◽  
Linear Perturbation ◽  
Streamwise Vorticity ◽  
Upstream Flow

This paper is a generalization of a previous analysis of the effects of a small-amplitude, steady, streamwise vorticity field on the flow over an infinitely thin flat plate in an otherwise uniform stream. That analysis, which is given in Goldstein & Leib (1993), required that the disturbance Reynolds number (i.e. the Reynolds number based on the disturbance velocity and length scale) be infinite while the present paper considers the more general case where this quantity can be finite. The results show how an initially linear perturbation of the upstream flow ultimately leads to a small-amplitude but nonlinear cross-flow far downstream from the leading edge. This flow can, under certain conditions, cause the streamwise velocity profiles to develop distinct shear layers in certain localized spanwise regions. These shear layers, which are remarkably similar to the ones that develop in Tollmien–Schlichting-wave transition (Kovasznay, Komoda & Vasudeva 1962), are highly inflectional and can therefore support the rapidly growing inviscid instabilities that are believed to break down into turbulent spots (Greenspan & Benney 1963, and, subsequently, many others). Numerical computations are carried out for input parameters which approximate the flow conditions of some recent experimental studies of the so-called Klebanoff-mode phenomenon. The results are used to explain some of the experimental observations, and, more importantly, to explain why the averaged quantities usually reported in these experiments do not correlate well with the turbulent-spot formation and therefore with the overall transition process.

The influence of a drag-reducing surfactant on a turbulent velocity field

1999 ◽  
Vol 388 ◽  
pp. 1-20 ◽  
Author(s):  
MICHAEL D. WARHOLIC ◽  
GAVIN M. SCHMIDT ◽  
THOMAS J. HANRATTY
Keyword(s):  
Shear Stress ◽  
Velocity Field ◽  
Root Mean Square ◽  
Reynolds Shear Stress ◽  
Rectangular Channel ◽  
Streamwise Velocity ◽  
Laser Doppler Velocimeter ◽  
Shear Stresses ◽  
Mean Square ◽  
Velocity Fluctuations

A two-component laser-Doppler velocimeter, with high spatial and temporal resolution, was used to study how the introduction of a drag-reducing surfactant to water changes the fully-developed velocity field in an enclosed rectangular channel. Measurements were made for four different Reynolds numbers, Re = 13300; 19100; 32000, and 49100 (based on the bulk viscosity, the half-height of the channel, and the viscosity of water). For a fixed volumetric flow the pressure drop was reduced by 62 to 76% when compared to a Newtonian flow with an equal wall viscosity. Measurements were made of the mean streamwise velocity, the root mean square of two components of the fluctuating velocity, the Reynolds shear stress and the spectral density function of the fluctuating velocity in the streamwise direction. The Reynolds shear stress is found to be zero over the whole channel and the spectra of the streamwise velocity fluctuations show a sharp cutoff at a critical frequency, fc. The ratio of the cutoff frequency to the root mean square of the streamwise velocity fluctuations is found to be approximately equal to 1 mm−1. The observation of a zero Reynolds shear stress indicates the existence of additional mean shear stresses (or mean transfers of momentum) that are not seen with a Newtonian fluid. Furthermore, the presence of a random fluctuating velocity field suggests a production of turbulence by a mechanism other than that usually found for a fully developed flow. Possible explanations for this behaviour are presented.

The Relationship of Root Mean Square Value of Electromyography and Isometric Torque of Quadriceps in Normal Subjects

2016 ◽  
Vol 26 (1) ◽  
pp. 58
Author(s):  
Qiurong XIE ◽  
Zheng JIANG ◽  
Qinglu LUO ◽  
Jie LIANG ◽  
Xiaoling WANG ◽  
...  
Keyword(s):  
Root Mean Square ◽  
Normal Subjects ◽  
Mean Square ◽  
Isometric Torque ◽  
Relationship Of ◽  
The Relationship
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