Flow Visualization of Axisymmetric Impinging Jet on a Concave Surface

2018 ◽  
Vol 140 (8) ◽  
Author(s):  
Dong Hwan Shin ◽  
Yeonghwan Kim ◽  
Jin Sub Kim ◽  
Do Won Kang ◽  
Jeong Lak Sohn ◽  
...  
Keyword(s):  
Flow Visualization ◽  
Large Scale ◽  
Impinging Jet ◽  
Ambient Air ◽  
Concave Surface ◽  
Small Scale ◽  
Wall Jet ◽  
Entrainment Rate ◽  
Toroidal Vortex ◽  
Axisymmetric Jet

Flow visualization was performed to give a physical insight with vortical structures of an axisymmetric impinging jet on a concave surface. High-speed imaging was employed to get clear images with a laser light sheet illumination. An axisymmetric jet is issued into quasi-ambient air through a straight pipe nozzle with fully-developed velocity profile. A regular vertical pattern of an axisymmetric jet was observed with different flow entrainment rate. While an impinged jet turns to convert a wall jet along a concave surface, the flow interaction between the large-scale toroidal vortex and the concave surface was observed in the transition between the stagnation and wall jet zone. The ring-shaped wall eddies induced from a pair of toroidal vortices were also appeared to diverge into the radial direction along the concave surface. As the jet Reynolds number increases, small-scale vortices can be developed to a large-scale toroidal vortex. The location in which a large-scale toroidal vortex strikes is generally identical to the location where the secondary peak in heat transfer occurs. The frequency of large scale toroidal vortex on concave surface is found to be nearly similar as that of wall jet on flat surface. As the nozzle-to-target spacing (L/D) increases, it becomes shorter due to the loss of jet momentum. The flow behavior of axisymmetric impinging jet on a concave surface can be helpful to design the internal passage cooling for gas turbine blade.

Modeling the Impact of Convective Entrainment on the Tropical Tropopause

2006 ◽  
Vol 63 (3) ◽  
pp. 1013-1027 ◽  
Author(s):  
F. J. Robinson ◽  
S. C. Sherwood
Keyword(s):  
Heat Flux ◽  
Large Scale ◽  
Small Scale ◽  
Entrainment Rate ◽  
Subgrid Scale ◽  
Turbulence Parameterization ◽  
Tropical Tropopause ◽  
Scale Turbulence ◽  
Cold Point ◽  
The Impact

Abstract Simulations with the Weather Research and Forecasting (WRF) cloud-resolving model of deep moist convective events reveal net cooling near the tropopause (∼15–18 km above ground), caused by a combination of large-scale ascent and small-scale cooling by the irreversible mixing of turbulent eddies overshooting their level of neutral buoyancy. The turbulent cooling occurred at all CAPE values investigated (local peak values ranging from 1900 to 3500 J kg−1) and was robust to grid resolution, subgrid-scale turbulence parameterization, horizontal domain size, model dimension, and treatment of ice microphysics. The ratio of the maximum downward heat flux in the tropopause to the maximum tropospheric upward heat flux was close to 0.1. This value was independent of CAPE but was affected by changes in microphysics or subgrid-scale turbulence parameterization. The convective cooling peaked roughly 1 km above the cold point in the background input sounding and the mean cloud- and (turbulent kinetic energy) TKE-top heights, which were all near 16.5 km above ground. It was associated with turbulent entrainment of stratospheric air from as high as 18.25 km into the troposphere. Typical cooling in the experiments was of order 1 K during convective events that produced order 10 mm of precipitation, which implied a significant contribution to the tropopause energy budget. Given the sharp concentration gradients and long residence times near the cold point, even such a small entrainment rate is likely consequential for the transport and ambient distribution of trace gases such as water vapor and ozone, and probably helps to explain the gradual increase of ozone typically observed below the tropical tropopause.

scholarly journals Numerical Simulation of Powder-Snow Avalanches

1990 ◽  
Vol 36 (123) ◽  
pp. 229-237 ◽  
Author(s):  
Yusuke Fukushima ◽  
Gary Parker
Keyword(s):  
Large Scale ◽  
Ambient Air ◽  
Dense Core ◽  
Snow Avalanche ◽  
Entrainment Rate ◽  
Density Currents ◽  
Snow Avalanches ◽  
Snow Layer ◽  
Fresh Snow ◽  
Snow Particle

AbstractAppropriate expressions describing the motion of powder-snow avalanches are derived. The model consists of four equations, i.e. the conservation equations of fluid mass, snow-particle mass, momentum of the cloud, and kinetic energy of the turbulence. Insofar as the density difference between the avalanche and the ambient air becomes rather large compared with the density of the ambient air, the Boussinesq approximation, which is typically used to analyze density currents, cannot be adopted in the present case. As opposed to previous models, the total buoyancy of a powder-snow avalanche is allowed to change freely via erosion from and deposition on to a static snow layer on a slope. In the model, the snow-particle entrainment rate from the slope is directly linked to the level of turbulence.A discontinuous, large-scale powder-snow avalanche occurred on 26 January 1986 near Maseguchi, Niigata Prefecture, Japan. The avalanche appears to have had a dense core at its base. The present model is employed to simulate that part of the avalanche above any dense core. The depth of the layer of fresh snow is considered to be an important parameter in the model. The larger the depth of fresh snow, the larger is the concentration of snow attained in the avalanche, and the faster its speed. It is seen that the model provides a reasonable description of the powder-snow avalanche generated near Maseguchi. In particular, the model prediction that a powder-snow avalanche strong enough to reach Maseguchi requires a depth of fresh snow of at least 2 m is in agreement with the observed depth just before the event.

scholarly journals Heat and mass transfer enhancement strategies by impinging jets: A literature review

2020 ◽  
pp. 227-227
Author(s):  
Florin Bode ◽  
Claudiu Patrascu ◽  
Ilinca Nastase
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat And Mass Transfer ◽  
Large Scale ◽  
Impinging Jet ◽  
Impinging Jets ◽  
Nozzle Geometry ◽  
Small Scale ◽  
Mixing Enhancement ◽  
Transfer Enhancement

Heat and mass transfer can be greatly increased when using impinging jets, regardless the application. The reason behind this is the complex behavior of the impinging jet flow which is leading to the generation of a multitude of flow phenomena, like: large-scale structures, small scale turbulent mixing, large curvature involving strong normal stresses and strong shear, stagnation, separation and re-attachment of the wall boundary layers, increased heat transfer at the impinged plate. All these phenomena listed above have highly unsteady nature and even though a lot of scientific studies have approached this subject, the impinging jet is not fully understood due to the difficulties of carrying out detailed experimental and numerically investigations. Nevertheless, for heat transfer enhancement in impinging jet applications, both passive and active strategies are employed. The effect of nozzle geometry and the impinging surface macrostructure modification are some of the most prominent passive strategies. On the other side, the most used active strategies utilize acoustical and mechanical oscillations in the exit plane of the flow, which in certain situations favors mixing enhancement. This is favored by the intensification of some instabilities and by the onset of large scale vortices with important levels of energy.

Stability of a Compressible Laminar Wall-Jet With Heat Transfer

1996 ◽  
Vol 118 (4) ◽  
pp. 824-828 ◽  
Author(s):  
O. Likhachev ◽  
A. Tumin
Keyword(s):  
Heat Transfer ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Small Scale ◽  
Wall Jet ◽  
Boundary Layer Equations ◽  
Incompressible Boundary ◽  
Flow Approximation ◽  
Ambient Gas ◽  
The Stability

The flow of a plane, laminar, subsonic perfect gas wall jet with heat transfer through the wall was investigated theoretically. For the case under consideration the entire surface was maintained at a constant temperature which differed from the temperature of the ambient gas. The velocity and temperature distribution across the flow were calculated for a variety of temperature differences between the ambient gas and the surface. The boundary layer equations representing these flows were solved by using the Illingworth-Stewartson transformation, thus extending the classical Glauert’s solution to a thermally non-uniform flow. The effects of heat transfer on the linear stability characteristics of the wall jet were assessed by making the local parallel flow approximation. Two kinds of unstable eigenmodes coexisting at moderate Reynolds numbers are significantly affected by the heat transfer. The influence of cooling or heating on the stability of the flow was expected in view of the experience accumulated in incompressible boundary layers, i.e. heating destabilizes and cooling stabilizes the flows. Cooling of the wall affects the small scale disturbances more profoundly, contrary to the results obtained for the large scale disturbances.

scholarly journals Behaviour of small-scale turbulence in the turbulent/non-turbulent interface region of developing turbulent jets

2019 ◽  
Vol 879 ◽  
pp. 187-216 ◽  
Author(s):  
M. Breda ◽  
O. R. H. Buxton
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Direct Action ◽  
Subsequent Development ◽  
Turbulent Jets ◽  
Small Scale ◽  
Entrainment Rate ◽  
Potential Core ◽  
Velocity Gradients ◽  
Canonical State

Tomographic particle image velocimetry experiments were conducted in the near and intermediate fields of two different types of jet, one fitted with a circular orifice and another fitted with a repeating-fractal-pattern orifice. Breda & Buxton (J. Vis., vol. 21 (4), 2018, pp. 525–532; Phys. Fluids, vol. 30, 2018, 035109) showed that this fractal geometry suppressed the large-scale coherent structures present in the near field and affected the rate of entrainment of background fluid into, and subsequent development of, the fractal jet, relative to the round jet. In light of these findings we now examine the modification of the turbulent/non-turbulent interface (TNTI) and spatial evolution of the small-scale behaviour of these different jets, which are both important factors behind determining the entrainment rate. This evolution is examined in both the streamwise direction and within the TNTI itself where the fluid adapts from a non-turbulent state, initially through the direct action of viscosity and then through nonlinear inertial processes, to the state of the turbulence within the bulk of the flow over a short distance. We show that the suppression of the coherent structures in the fractal jet leads to a less contorted interface, with large-scale excursions of the inner TNTI (that between the jet’s azimuthal shear layer and the potential core) being suppressed. Further downstream, the behaviour of the TNTI is shown to be comparable for both jets. The velocity gradients develop into a canonical state with streamwise distance, manifested as the development of the classical tear-drop shaped contours of the statistical distribution of the velocity-gradient-tensor invariants $\mathit{Q}$ and $\mathit{R}$. The velocity gradients also develop spatially through the TNTI from the irrotational boundary to the bulk flow; in particular, there is a strong small-scale anisotropy in this region. This strong inhomogeneity of the velocity gradients in the TNTI region has strong consequences for the scaling of the thickness of the TNTI in these spatially developing flows since both the Taylor and Kolmogorov length scales are directly computed from the velocity gradients.

On the scaling of air entrainment from a ventilated partial cavity

2013 ◽  
Vol 732 ◽  
pp. 47-76 ◽  
Author(s):  
Simo A. Mäkiharju ◽  
Brian R. Elbing ◽  
Andrew Wiggins ◽  
Sarah Schinasi ◽  
Jean-Marc Vanden-Broeck ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Weber Number ◽  
Large Scale ◽  
Air Entrainment ◽  
Small Scale ◽  
Length Scale ◽  
Entrainment Rate ◽  
Two Dimensional ◽  
Wide Range ◽  
Stable Cavity

AbstractThe behaviour of a nominally two-dimensional ventilated partial cavity was examined over a wide range of size scales and flow speeds to determine the influence of Froude, Reynolds, and Weber number on the cavity shape, dynamics, and gas entrainment rate. Two geometrically similar experiments were conducted with a 14:1 length scale ratio. The results were compared to a two-dimensional semi-analytical model of the cavity flow, and Froude scaling was found to be sufficient to match basic cavity shapes. However, the air flux required to maintain a stable cavity did not scale with Froude number alone, as the dynamics of the cavity closure changed with increasing Reynolds number. The required air flux differed over one order of magnitude between the lowest and highest Reynolds number flows. But, for sufficiently high Reynolds numbers, the rate of scaled entrainment appeared to approach Reynolds number independence. Modest changes in surface tension of the small-scale experiment suggested that the Weber number was important only at the lowest speeds and smaller length scale. Otherwise, the Weber numbers of the flows were sufficiently high to make the effects of interfacial tension negligible. We also observed that modest unsteadiness in the inflow to the large-scale cavity led to a significant increase in the required air flux needed to maintain a stable cavity, with the required excess gas flux nominally proportional to the flow’s perturbation amplitude. Finally, discussion is provided on how these results relate to model testing of partial cavity drag reduction (PCDR) systems for surface ships.

Two-layer thermal convection in miscible viscous fluids

1999 ◽  
Vol 379 ◽  
pp. 223-253 ◽  
Author(s):  
ANNE DAVAILLE
Keyword(s):  
Thermal Convection ◽  
Large Scale ◽  
Scaling Laws ◽  
Three Dimensional ◽  
Interfacial Instability ◽  
Small Scale ◽  
Entrainment Rate ◽  
Viscous Layer ◽  
Stable Chemical ◽  
Large Scale Flow

The influence of a viscosity stratification on the interaction between thermal convection and a stable density discontinuity is studied, using laboratory experiments. Initially, two superposed isothermal layers of high-Prandtl-number miscible fluids are suddenly cooled from above and heated from below. By adjusting the concentrations of salt and cellulose, Rayleigh numbers between 300 and 3×107 were achieved for density contrasts between 0.45 % and 5 % and viscosity ratios between 1 and 6.4×104. Heat and mass transfer through the interface were monitored.Two-layer convection is observed but a steady state is never obtained since penetrative convection occurs. A new interfacial instability is reported, owing to the nonlinear interaction of the unstable thermal and stable chemical density gradients. As a result, the temperature condition at the interface is highly inhomogeneous, driving, on top of the classical small-scale thermal convection, a large-scale flow in each layer which produces cusps at the interface. Entrainment, driven by viscous coupling between the two layers, proceeds through those cusps. The pattern of entrainment is asymmetric: two-dimensional sheets are dragged into the more viscous layer, while three-dimensional conduits are produced in the less viscous layer. A simple entrainment model is proposed and scaling laws for the entrainment rate are derived; they explain the experimental data well.

scholarly journals Numerical Simulation of Powder-Snow Avalanches

1990 ◽  
Vol 36 (123) ◽  
pp. 229-237 ◽  
Author(s):  
Yusuke Fukushima ◽  
Gary Parker
Keyword(s):  
Large Scale ◽  
Ambient Air ◽  
Dense Core ◽  
Snow Avalanche ◽  
Entrainment Rate ◽  
Density Currents ◽  
Snow Avalanches ◽  
Snow Layer ◽  
Fresh Snow ◽  
Snow Particle

Abstract Appropriate expressions describing the motion of powder-snow avalanches are derived. The model consists of four equations, i.e. the conservation equations of fluid mass, snow-particle mass, momentum of the cloud, and kinetic energy of the turbulence. Insofar as the density difference between the avalanche and the ambient air becomes rather large compared with the density of the ambient air, the Boussinesq approximation, which is typically used to analyze density currents, cannot be adopted in the present case. As opposed to previous models, the total buoyancy of a powder-snow avalanche is allowed to change freely via erosion from and deposition on to a static snow layer on a slope. In the model, the snow-particle entrainment rate from the slope is directly linked to the level of turbulence. A discontinuous, large-scale powder-snow avalanche occurred on 26 January 1986 near Maseguchi, Niigata Prefecture, Japan. The avalanche appears to have had a dense core at its base. The present model is employed to simulate that part of the avalanche above any dense core. The depth of the layer of fresh snow is considered to be an important parameter in the model. The larger the depth of fresh snow, the larger is the concentration of snow attained in the avalanche, and the faster its speed. It is seen that the model provides a reasonable description of the powder-snow avalanche generated near Maseguchi. In particular, the model prediction that a powder-snow avalanche strong enough to reach Maseguchi requires a depth of fresh snow of at least 2 m is in agreement with the observed depth just before the event.

Particle resuspension by a periodically forced impinging jet

2017 ◽  
Vol 820 ◽  
pp. 284-311 ◽  
Author(s):  
Wen Wu ◽  
Giovanni Soligo ◽  
Cristian Marchioli ◽  
Alfredo Soldati ◽  
Ugo Piomelli
Keyword(s):  
Large Scale ◽  
Flow Region ◽  
Impinging Jet ◽  
Small Scale ◽  
Outer Flow ◽  
Particle Resuspension ◽  
Lift Off ◽  
Secondary Vortices ◽  
Impingement Surface ◽  
Near Wall

When hovering over sandy terrain, the rotor of helicopters generates a downward jet that induces resuspension of dust and debris. We investigate the mechanisms that govern particle resuspension in such flow using an Eulerian–Lagrangian approach based on large-eddy simulation of turbulence. The wake generated by the helicopter is modelled as a vertical impinging jet, to which a sequence of periodically forced azimuthal vortices is superposed. The resulting flow field provides a unique range of flow scales with which the particles can interact. Downstream of the impingement region, layers of negative azimuthal vorticity (secondary vortices) form on the upwash side of the primary azimuthal (large-scale) vortices. These layers then detach from the surface together with the near-wall (small-scale) vortices populating the wall-jet region. We show how the dynamics of sediments is governed by its interaction with these structures. After initial lift off from the impingement surface, particles accumulate in regions where near-wall vortices roll around the impinging azimuthal vortex, forming rib-like structures that either propel particles away from the azimuthal vortex or entrap them in the shear layer between the azimuthal and secondary vortices. We demonstrate that these trapped particles are more likely to reach the outer flow region and generate a persistent cloud of airborne particles. We also show that, in a time-averaged sense, particle resuspension and deposition fluxes balance each other near the impingement surface.

An Experimental Investigation of the Flowfield and Dust Resuspension Due to Idealized Human Walking: Disk Model

2007 ◽  
Author(s):  
Yoshihiro Kubota ◽  
Joseph W. Hall ◽  
Hiroshi Higuchi
Keyword(s):  
Experimental Investigation ◽  
Flow Visualization ◽  
Large Scale ◽  
Human Movement ◽  
Human Walking ◽  
Wall Jet ◽  
Disk Model ◽  
Piv Measurements ◽  
Particle Resuspension ◽  
Dust Resuspension

To better understand how human movement causes particles to be resuspended from the ground, we performed flow visualization and PIV measurements on idealized human walking, a disk moved normal to the ground. The flow visualization indicates that particles are resuspended on both the down step and the up step of the walking process by a purely aerodynamic mechanism. The results suggest that a wall jet formed beneath the disk is responsible for particle resuspension, whereas large scale vortices created in the wake of the disk are responsible for the rapid redistribution of the resuspended particles.

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