Lagrangian measurements of inertial particle accelerations in a turbulent boundary layer

2008 ◽  
Vol 617 ◽  
pp. 255-281 ◽  
Author(s):  
S. GERASHCHENKO ◽  
N. S. SHARP ◽  
S. NEUSCAMMAN ◽  
Z. WARHAFT
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Probability Density ◽  
Isotropic Turbulence ◽  
Free Stream ◽  
Probability Density Functions ◽  
Density Functions ◽  
Inertial Particles ◽  
Free Stream Turbulence ◽  
The Mean

Two-dimensional Lagrangian acceleration statistics of inertial particles in a turbulent boundary layer with free-stream turbulence are determined by means of a particle tracking technique using a high-speed camera moving along the side of the wind tunnel at the mean flow speed. The boundary layer is formed above a flat plate placed horizontally in the tunnel, and water droplets are fed into the flow using two different methods: sprays placed downstream from an active grid, and tubes fed into the boundary layer from humidifiers. For the flow conditions studied, the sprays produce Stokes numbers varying from 0.47 to 1.2, and the humidifiers produce Stokes numbers varying from 0.035 to 0.25, where the low and high values refer to the outer boundary layer edge and the near-wall region, respectively. The Froude number is approximately 1.0 for the sprays and 0.25 for the humidifiers, with a small variation within the boundary layer. The free-stream turbulence is varied by operating the grid in the active mode as well as a passive mode (the latter behaves as a conventional grid). The boundary layer momentum-thickness Reynolds numbers are 840 and 725 for the active and passive grid respectively. At the outer edge of the boundary layer, where the shear is weak, the acceleration probability density functions are similar to those previously observed in isotropic turbulence for inertial particles. As the boundary layer plate is approached, the tails of the probability density functions narrow, become negatively skewed, and their peak occurs at negative accelerations (decelerations in the streamwise direction). The mean deceleration and its root mean square (r.m.s.) increase to large values close to the plate. These effects are more pronounced at higher Stokes number. In the vertical direction, there is a slight downward mean deceleration and its r.m.s., which is lower in magnitude than that of the streamwise component, peaks in the buffer region. Although there are free-stream turbulence effects, and the complex boundary layer structure plays an important role, a simple model suggests that the acceleration behaviour is dominated by shear, gravity and inertia. The results are contrasted with inertial particles in isotropic turbulence and with fluid particle acceleration statistics in a boundary layer. The background velocity field is documented by means of hot-wire anemometry and laser Doppler velocimetry measurements. These appear to be the first Lagrangian acceleration measurements of inertial particles in a shear flow.

scholarly journals Analysis of the forward and backward in time pair-separation probability density functions for inertial particles in isotropic turbulence

2017 ◽  
Vol 830 ◽  
pp. 63-92 ◽  
Author(s):  
Andrew D. Bragg
Keyword(s):  
Probability Density ◽  
Isotropic Turbulence ◽  
Probability Density Functions ◽  
Density Functions ◽  
Inertial Particles ◽  
Mean Square ◽  
Inertial Particle ◽  
Particle Pair ◽  
Pair Separation ◽  
Dissipation Range

In this paper we investigate, using theory and direct numerical simulations (DNS), the forward in time (FIT) and backward in time (BIT) probability density functions (PDFs) of the separation of inertial particle pairs in isotropic turbulence. In agreement with our earlier study (Bragg et al., Phys. Fluids, vol. 28, 2016, 013305), where we compared the FIT and BIT mean-square separations, we find that inertial particles separate much faster BIT than FIT, with the strength of the irreversibility depending upon the final/initial separation of the particle pair and their Stokes number $St$. However, we also find that the irreversibility shows up in subtle ways in the behaviour of the full PDF that it does not in the mean-square separation. In the theory, we derive new predictions, including a prediction for the BIT/FIT PDF for $St\geqslant O(1)$, and for final/initial separations in the dissipation regime. The prediction shows how caustics in the particle relative velocities in the dissipation range affect the scaling of the pair-separation PDF, leading to a PDF with an algebraically decaying tail. The predicted functional behaviour of the PDFs is universal, in that it does not depend upon the level of intermittency in the underlying turbulence. We also analyse the pair-separation PDFs for fluid particles at short times, and construct theoretical predictions using the multifractal formalism to describe the fluid relative velocity distributions. The theoretical and numerical results both suggest that the extreme events in the inertial particle-pair dispersion at the small scales are dominated by their non-local interaction with the turbulent velocity field, rather than due to the strong dissipation range intermittency of the turbulence itself. In fact, our theoretical results predict that for final/initial separations in the dissipation range, when $St\gtrsim 1$, the tails of the pair-separation PDFs decay faster as the Taylor Reynolds number $Re_{\unicode[STIX]{x1D706}}$ is increased, the opposite of what would be expected for fluid particles.

scholarly journals Relaxation process modeling in a turbulent boundary layer with nonzero free stream turbulence

1997 ◽  
Vol 3 (3) ◽  
pp. 255-265
Author(s):  
Eugen Dyban ◽  
Ella Fridman
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Process Modeling ◽  
Free Stream ◽  
Equations Of Motion ◽  
Turbulent Shear ◽  
Shear Stresses ◽  
Free Stream Turbulence ◽  
Relaxation Effects ◽  
Averaged Equations

In order to analyze the relaxation effects in a turbulent boundary layer with zero and nonzero free stream turbulence, the Reynolds-averaged equations of motion and energy are solved. As the closure of the Reynolds-averaged equations, the transport equation for turbulent shear stresses is used. The proposed approach leads to calculation of the relaxation scales in the turbulent boundary layer with zero and nonzero free stream turbulence. Results for friction coefficients, velocity profiles, shear stresses, thickness of the boundary layer and so called “superlayer” in a flat-plate turbulent boundary layer are presented. The results obtained are in agreement with those available from the experimental data.

scholarly journals Effect of turbulence on the wake of a wall-mounted cube

2016 ◽  
Vol 804 ◽  
pp. 513-530 ◽  
Author(s):  
R. Jason Hearst ◽  
Guillaume Gomit ◽  
Bharathram Ganapathisubramani
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Reattachment Length ◽  
Free Stream Turbulence ◽  
Boundary Layer Development ◽  
Image Velocimetry ◽  
Shedding Frequency ◽  
Layer Development

The influence of turbulence on the flow around a wall-mounted cube immersed in a turbulent boundary layer is investigated experimentally with particle image velocimetry and hot-wire anemometry. Free-stream turbulence is used to generate turbulent boundary layer profiles where the normalised shear at the cube height is fixed, but the turbulence intensity at the cube height is adjustable. The free-stream turbulence is generated with an active grid and the turbulent boundary layer is formed on an artificial floor in a wind tunnel. The boundary layer development Reynolds number ($Re_{x}$) and the ratio of the cube height ($h$) to the boundary layer thickness ($\unicode[STIX]{x1D6FF}$) are held constant at $Re_{x}=1.8\times 10^{6}$ and $h/\unicode[STIX]{x1D6FF}=0.47$. It is demonstrated that the stagnation point on the upstream side of the cube and the reattachment length in the wake of the cube are independent of the incoming profile for the conditions investigated here. In contrast, the wake length monotonically decreases for increasing turbulence intensity but fixed normalised shear – both quantities measured at the cube height. The wake shortening is a result of heightened turbulence levels promoting wake recovery from high local velocities and the reduction in strength of a dominant shedding frequency.

Influence of Free-Stream Turbulence on Turbulent Boundary Layer Heat Transfer and Mean Profile Development, Part I—Experimental Data

1983 ◽  
Vol 105 (1) ◽  
pp. 33-40 ◽  
Author(s):  
M. F. Blair
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Boundary Layer Flow ◽  
Free Stream ◽  
Full Range ◽  
Stream Velocity ◽  
Free Stream Turbulence ◽  
Turbulent Boundary Layer Flow ◽  
Layer Flow

An experimental research program was conducted to determine the influence of free-stream turbulence on zero pressure gradient, fully turbulent boundary layer flow. Connective heat transfer coefficients and boundary layer mean velocity and temperature profile data were obtained for a constant free-stream velocity of 30 m/s and free-stream turbulence intensities ranging from approximately 1/4 to 7 percent. Free-stream multicomponent turbulence intensity, longitudinal integral scale, and spectral distributions were obtained for the full range of turbulence levels. The test results with 1/4 percent free-stream turbulence indicate that these data were in excellent agreement with classic two-dimensional, low free-stream turbulence, turbulent boundary layer correlations. For fully turbulent boundary layer flow, both the skin friction and heat transfer were found to be substantially increased (up to ∼ 20 percent) for the higher levels of free-stream turbulence. Detailed results of the experimental study are presented in the present paper (Part I). A comprehensive analysis is provided in a companion paper (Part II).

The reduction and elimination of a closed separation region by free-stream turbulence

2001 ◽  
Vol 446 ◽  
pp. 271-308 ◽  
Author(s):  
M. KALTER ◽  
H. H. FERNHOLZ
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Free Stream ◽  
Reverse Flow ◽  
Flow Region ◽  
Adverse Pressure Gradient ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Free Stream Turbulence ◽  
The Mean

This paper is an extension of an experimental investigation by Alving & Fernholz (1996). In the present experiments the effects of free-stream turbulence were investigated on a boundary layer with an adverse pressure gradient and a closed reverse-flow region. By adding free-stream turbulence the mean reverse-flow region was shortened or completely eliminated and this was used to control the size of the separation bubble. The turbulence intensity was varied between 0.2% and 6% using upstream grids while the turbulence length scale was on the order of the boundary layer thickness. Mean and fluctuating velocities as well as spectra were measured by means of hot-wire and laser-Doppler anemometry and wall shear stress by wall pulsed-wire and wall hot-wire probes.Free-stream turbulence had a small effect on the boundary layer in the mild adverse-pressure-gradient region but in the vicinity of separation and along the reverse-flow region mean velocity profiles, skin friction and turbulence structure were strongly affected. Downstream of the mean or instantaneous reverse-flow regions highly disturbed boundary layers developed in a nominally zero pressure gradient and converged to a similar turbulence structure in all three cases at the end of the test section. This state was, however, still very different from that in a canonical boundary layer.

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