Evolution of unstable disturbances at a mean‐flow stagnation point

1988 ◽  
Vol 83 (S1) ◽  
pp. S57-S57
Author(s):  
Charles Thompson ◽  
Martin Manley
Keyword(s):  
Stagnation Point ◽  
Mean Flow ◽  
Flow Stagnation

URANS Prediction of Flow Instabilities of a Novel Atomizer Combustor Configuration

2005 ◽  
Author(s):  
P. Jochmann ◽  
A. Sinigersky ◽  
R. Koch ◽  
H.-J. Bauer
Keyword(s):  
Stagnation Point ◽  
Recirculation Zone ◽  
Vortex Breakdown ◽  
Experimental Studies ◽  
Expansion Ratio ◽  
Reacting Flow ◽  
Mean Flow ◽  
Design Guideline ◽  
The Impact ◽  
Spiral Type

In this paper, the flow in a gas-turbine combustor with a novel air blast atomizer configuration is studied by URANS (unsteady Reynolds-averaged Navier-Stokes) calculations and compared to experimental data. The flow is characterized by a Reynolds number of 52000, a swirl number of 0.52 and an expansion ratio of 5. It is well known that at this high swirl level, flows exhibit a vortex breakdown which is characterized by a sudden axial deceleration in combination with a radial expansion and the formation of a stagnation point followed by a recirculation zone. At high Reynolds numbers, like in the present case, the vortex breakdown is either of a quasi-axisymmetric bubble type or of a precessing spiral type. Previous experimental studies confirmed the presence of a spiral type vortex breakdown for the configuration under concern. For the non-reacting flow, the structure as well as the frequency of the precessing vortex core is captured almost perfectly by the URANS predictions which is demonstrated by a direct comparison to LDV (Laser-Doppler Velocimetry) measurements. However, it was found that a suitable discretization as well as full three-dimensional computations are crucial in order to successfully predict the precessing spiral structure. In this context also the impact of two-equation and full transport Reynolds-stress turbulence models is discussed. After validation of the URANS method, it has been applied for developing improved designs which aim to suppress the unsteady flow pattern. The investigation of different design variants revealed that, if the mean axial velocity distribution of the flow upstream of the stagnation point is jet-like, the flow is more stable and less susceptible to the spiral type vortex breakdown. This fact which is known from laminar vortex breakdown investigations seems to be also valid for the turbulent mean flow and can be used as a design guideline for achieving a stable nearly symmetric bubble-like recirculation zone.

Simulation of Submicron Particle Deposition in Laminar and Turbulent Stagnation Point Flows

1991 ◽  
Author(s):  
Salem Abuzeid ◽  
Ahmed A. Busnaina
Keyword(s):  
Brownian Motion ◽  
Stagnation Point ◽  
Deposition Rate ◽  
Particle Deposition ◽  
Aerosol Particles ◽  
Laminar Flows ◽  
Wafer Surface ◽  
Particle Sizes ◽  
Mean Flow ◽  
Gaussian Random Process

Abstract The two dimensional laminar and turbulence stagnation-point flow over a wafer surface within a cleanroom environment are numerically simulated. This study shows the relationship between particle capture area on the wafer and the particle size and flow conditions. The mean flow field is simulated using a two equation k-ϵ turbulence model. Trajectories of aerosol particles are evaluated by solving the corresponding Lagrangian equation of motion that includes effects of drag, gravity, lift force, Brownian motion and turbulence fluctuations. The Brownian motion is modeled as a white noise process and turbulence fluctuation is assumed to behave as Gaussian random process. Simulations are carried out for aerosol particles (of various sizes) released at different locations over the surface. Depositions of particles on the wall are evaluated and a capture area which varies with particle sizes is produced. The results show that Brownian motion becomes very significant when turbulence fluctuations start to disappear near the wall for particles smaller than 1 μm in diameter. The results also show that, deposition of particles in turbulent flows are usually higher than that in laminar flows for all particle sizes considered. The effect of fluid on particle deposition rate is predicted for fluid of air and water. The results show that, particles deposition rate in air is higher than that in water.

Stagnation-point flow under free-stream turbulence

2007 ◽  
Vol 590 ◽  
pp. 1-33 ◽  
Author(s):  
ZHONGMIN XIONG ◽  
SANJIVA K. LELE
Keyword(s):  
Heat Transfer ◽  
Stagnation Point ◽  
Reynolds Stress ◽  
Free Stream ◽  
Leading Edge ◽  
Stagnation Point Flow ◽  
Mean Flow ◽  
Free Stream Turbulence ◽  
Local Boundary ◽  
The Mean

In this paper, the effects of free-stream turbulence on stagnation-point flow and heat transfer are investigated through large eddy simulation (LES) of homogeneous isotropic turbulence impinging upon an isothermal elliptical leading edge. Turbulent mean flow and Reynolds stress profiles along the stagnation streamline, where the mean flow is strain dominant, and at different downstream locations, where the mean flow gradually becomes shear-dominated, are used to characterize evolution of the free-stream turbulence. The Reynolds stress budgets are also obtained, and the turbulence anisotropy is analysed through the balance between the mean flow strain and the velocity pressure gradient correlation. In the presence of free-stream turbulence, intense quasi-streamwise vortices develop near the leading edge with a typical diameter of the order of the local boundary-layer thickness. These strong vortices cause the thermal fluxes to peak at a location much closer to the wall than that of the Reynolds stresses, resulting a greater sensitivity to free-stream turbulence for the heat transfer than the momentum transfer. The heat transfer enhancement obtained by the present LES agrees quantitatively with available experimental measurements. The present LES results are also used to examine the eddy viscosity and pressure-strain correlations in Reynolds stress turbulence models.

Thermal boundary-layer theory near the stagnation point in three-dimensional fluctuating flow

1970 ◽  
Vol 43 (3) ◽  
pp. 465-476 ◽  
Author(s):  
S. Ghoshal ◽  
A. Ghoshal
Keyword(s):  
Skin Friction ◽  
Stagnation Point ◽  
Critical Frequency ◽  
Equations Of Motion ◽  
Three Dimensional ◽  
The Body ◽  
Phase Advance ◽  
Mean Flow ◽  
The Mean ◽  
Oncoming Stream

The equations of motion and energy governing a three-dimensional fluctuating flow of an incompressible fluid in the vicinity of a stagnation point on a regular surface have been integrated analytically. The velocity of the oncoming flow relative to the body oscillates in magnitude but not in direction.It has also been shown that the analysis of Lighthill for the two-dimensional fluctuating flow may be extended to the three-dimensional flow (both chordwise and spanwise), namely for each point on the body there is a critical frequency ω0 such that for frequencies ω > ω0 the oscillations are to a close approximation ordinary ‘shear waves’, unaffected by the mean flow; the phase advance in the skin friction is then 45°. For frequencies ω < ω0 the oscillations may be closely approximated by the sum of two parts: one quasi-steady part and the other proportional to the acceleration of the oncoming stream. The phase advance in the skin friction is then tan−1 (ω/ω0).

Influence of incidence angle on sound generation by airfoils interacting with high-frequency gusts

1995 ◽  
Vol 292 ◽  
pp. 271-304 ◽  
Author(s):  
Matthew R. Myers ◽  
E. J. Kerschen
Keyword(s):  
High Frequency ◽  
Incidence Angle ◽  
Outer Region ◽  
Sound Field ◽  
Leading Edge ◽  
Edge Region ◽  
Sound Generation ◽  
Mean Flow ◽  
Flow Stagnation ◽  
Trailing Edges

A theoretical model is developed for the sound generated when a convected vortical or entropic gust encounters an airfoil at non-zero angle of attack. The theory is based on a linearization of the Euler equations about the steady subsonic flow past the airfoil. High-frequency gusts, whose wavelengths are short compared to the airfoil chord, but long compared to the displacement of the mean-flow stagnation point from the leading edge, are considered. The analysis utilizes singular-perturbation techniques and involves four asymptotic regions. Local regions, which scale on the gust wavelength, are present at the airfoil leading and trailing edges. Behind the airfoil a ‘transition’ region, which is similar to the transition zone between illuminated and shadow zones in optical problems, is present. In the outer region, far away from the airfoil edges and wake, the solution has a geometric-acoustics form. The primary sound generation is found to be concentrated in the local leading-edge region. The trailing edge plays a secondary role as a scatterer of the sound generated in the leading-edge region. Parametric calculations are presented which illustrate that moderate levels of airfoil steady loading can significantly affect the sound field produced by airfoil–gust interactions.

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