scholarly journals Entrainment in a Dense Current Flowing Down a Rough Sloping Bottom in a Rotating Fluid

2017 ◽  
Vol 47 (3) ◽  
pp. 485-498 ◽  
Author(s):  
Luisa Ottolenghi ◽  
Claudia Cenedese ◽  
Claudia Adduce
Keyword(s):  
Laboratory Experiments ◽  
Strong Dependence ◽  
Flow Dynamics ◽  
Shear Instability ◽  
Rotating Fluid ◽  
Ambient Fluid ◽  
Homogeneous Fluid ◽  
Roughness Elements ◽  
Consequent Reduction ◽  
Sloping Bottom

AbstractDense oceanic overflows descend over the rough topography of the continental slope entraining and mixing with surrounding waters. The associated dilution dictates the fate of these currents and thus is of fundamental importance to the formation of deep water masses. The entrainment in a dense current flowing down a sloping bottom in a rotating homogeneous fluid is investigated using laboratory experiments, focusing on the influence of the bottom roughness on the flow dynamics. The roughness is idealized by an array of vertical rigid cylinders and both their spacing and height are varied as well as the inclination of the sloping bottom. The presence of the roughness is generally observed to decelerate the dense current, with a consequent reduction of the Froude number, when compared to the smooth bottom configuration. However, the dilution of the dense current due to mixing with the ambient fluid is enhanced by the roughness elements, especially for low Froude numbers. When the entrainment due to shear instability at the interface between the dense current and the ambient fluid is low, the additional turbulence and mixing arising at the bottom of the dense current due to the roughness elements strongly affects the dilution of the current. Finally, a strong dependence of the entrainment parameter on the Reynolds number is observed.

Three-Dimensional Direct Numerical Simulation of Unsteady Transitional Round Fountains in a Homogeneous Fluid

2014 ◽  
Vol 553 ◽  
pp. 150-155
Author(s):  
Wen Xian Lin ◽  
Tao Liu ◽  
Wen Feng Gao ◽  
Steven W. Armfield
Keyword(s):  
Three Dimensional ◽  
Flow Dynamics ◽  
Jet Flows ◽  
Ambient Fluid ◽  
Homogeneous Fluid ◽  
Interfacial Surface ◽  
Negative Buoyancy ◽  
Turbulence Generation ◽  
Direction Opposite ◽  
Shed Light

Fountains are common both in nature and in industrial and environmental settings. These are jet flows with a negative buoyancy force acting in the direction opposite to the jet direction. The onset of asymmetry and unsteadiness, which occurs in transitional fountains with intermediate Reynolds (Re) and Froude (Fr) numbers, is the key to shed light on the turbulence generation mechanism in fountains. In this study, a series of three-dimensional direct numerical simulations are carried out for transitional round fountains with Re and Fr in the ranges of 1 ≤ Fr ≤ 8, 50 ≤ Re ≤ 500 to reveal their unsteady flow dynamics, in particular the onset of asymmetry, three-dimensionality, and unsteadiness. The numerical results show that the onset of asymmetry and unsteadiness can be detected and quantified by the tangent velocity on the interfacial surface between the fountain fluid and the ambient fluid. The results also demonstrate that a critical Re is at about 165 for Fr =2 fountains and is reduced to about 65 for Fr =3 fountains. Similarly, a critical Fr exists between 2 and 3 for the Re = 100 fountains, whereas for Re = 200 fountains it reduces to be between 1.8 and 2.

scholarly journals Surface Expression of a Wall Fountain: Application to Subglacial Discharge Plumes

2020 ◽  
Vol 50 (5) ◽  
pp. 1245-1263 ◽  
Author(s):  
Craig D. McConnochie ◽  
Claudia Cenedese ◽  
Jim N. McElwaine
Keyword(s):  
Free Surface ◽  
Laboratory Experiments ◽  
Surface Expression ◽  
Ambient Fluid ◽  
Buoyant Jet ◽  
Homogeneous Fluid ◽  
Length Width ◽  
Negatively Buoyant Jet ◽  
Negative Buoyancy ◽  
Negatively Buoyant

AbstractWe use laboratory experiments and theoretical modeling to investigate the surface expression of a subglacial discharge plume, as occurs at many fjords around Greenland. The experiments consider a fountain that is released vertically into a homogeneous fluid, adjacent either to a vertical or a sloping wall, that then spreads horizontally at the free surface before sinking back to the bottom. We present a model that separates the fountain into two separate regions: a vertical fountain and a horizontal, negatively buoyant jet. The model is compared to laboratory experiments that are conducted over a range of volume fluxes, density differences, and ambient fluid depths. It is shown that the nondimensionalized length, width, and aspect ratio of the surface expression are dependent on the Froude number, calculated at the start of the negatively buoyant jet. The model is applied to observations of the surface expression from a Greenland subglacial discharge plume. In the case where the discharge plume reaches the surface with negative buoyancy the model can be used to estimate the discharge properties at the base of the glacier.

scholarly journals Mixing in a density-driven current flowing down a slope in a rotating fluid

2008 ◽  
Vol 604 ◽  
pp. 369-388 ◽  
Author(s):  
CLAUDIA CENEDESE ◽  
CLAUDIA ADDUCE
Keyword(s):  
Flow Rate ◽  
Systematic Study ◽  
Laboratory Experiments ◽  
Flow Regimes ◽  
Rotating Fluid ◽  
Bottom Slope ◽  
Turbulent Entrainment ◽  
Dense Fluid ◽  
Wide Range ◽  
Sloping Bottom

We discuss laboratory experiments investigating mixing in a density-driven current flowing down a sloping bottom, in a rotating homogenous fluid. A systematic study spanning a wide range of Froude, 0.8 < Fr < 10, and Reynolds, 10 < Re < 1400, numbers was conducted by varying three parameters: the bottom slope; the flow rate; and the density of the dense fluid. Different flow regimes were observed, i.e. waves (non-breaking and breaking) and turbulent regimes, while changing the above parameters. Mixing in the density-driven current has been quantified within the observed regimes, and at different locations on the slope. The dependence of mixing on the relevant non-dimensional numbers, i.e. slope, Fr and Re, is discussed. The entrainment parameter, E, was found to be dependent not only on Fr, as assumed in previous studies, but also on Re. In particular, mixing increased with increasing Fr and Re. For low Fr and Re, the magnitude of the mixing was comparable to mixing in the ocean. For large Fr and Re, mixing was comparable to that observed in previous laboratory experiments that exhibited the classic turbulent entrainment behaviour.

Critical layers in accelerating two-layer flows

1988 ◽  
Vol 197 ◽  
pp. 429-451 ◽  
Author(s):  
Donald B. Altman
Keyword(s):  
Laboratory Experiments ◽  
Single Mode ◽  
Shear Instability ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Interfacial Wave ◽  
Flow Acceleration ◽  
Critical Layers ◽  
The Mean ◽  
Processing Software

A series of laboratory experiments on accelerating two-layer shear flows over topography is described. The mean flow reverses at the interface of the layers, forcing a critical layer to occur there. It is found that for a sufficiently thin interface, a slowly growing recirculating region, the ‘acceleration rotor’, develops on the interfacial wave at mean-flow Richardson numbers of O(0.5). This, in turn, can induce a secondary dynamical shear instability on the trailing edge of the wave. A single-mode, linear, two-layer numerical model reproduces many features of the acceleration rotor if mean-flow acceleration and bottom forcing are included. Velocity measurements are obtained from photographs using image processing software developed for the automated reading of particle-streak photographs. Typical results are shown.

Experiments with density currents on a sloping bottom in a rotating fluid

2000 ◽  
Vol 31 (1-4) ◽  
pp. 139-164 ◽  
Author(s):  
D Etling ◽  
F Gelhardt ◽  
U Schrader ◽  
F Brennecke ◽  
G Kühn ◽  
...  
Keyword(s):  
Density Currents ◽  
Rotating Fluid ◽  
Sloping Bottom

scholarly journals On the dynamics of starting plumes

2017 ◽  
Vol 833 ◽  
Author(s):  
N. Bhamidipati ◽  
Andrew W. Woods
Keyword(s):  
Laboratory Experiments ◽  
Added Mass ◽  
Rate Of Change ◽  
Small Scale ◽  
Maximum Height ◽  
Ambient Fluid ◽  
A Value ◽  
Proposed Model ◽  
The Difference ◽  
Reasonable Description

We explore the dynamics of starting plumes by analysis of a series of new small-scale laboratory experiments combined with a theoretical model for mass, momentum, and buoyancy conservation. We find that the head of the plume ascends with a speed which is approximately 0.6 times the characteristic speed of the fluid in the following steady plume, in accord with Turner (J. Fluid Mech., vol. 13 (03), 1962, pp. 356–368), and so the fluid released from the source eventually catches the head of the flow. On reaching the top of the plume it recirculates and mixes in the plume head. We estimate that approximately $0.61\pm 0.04$ of the total buoyancy released from the source accumulates in the plume head, with the remainder in the following steady plume. Using measurements of the volume of the head, we estimate that a fraction $0.16\pm 0.08$ of the volume of the head is entrained directly from the ambient, with the remainder of the fluid in the head being supplied by the following steady plume. These results imply that the buoyancy force exerted on the plume head plus the momentum flux supplied by the following plume exceeds the rate of change of momentum of the plume head even including the added mass of the plume head. We propose that the difference is associated with a drag force resulting from the displacement of ambient fluid around the plume head. Using our experimental data, we estimate that the drag coefficient $C_{d}$ has a value $4.2\pm 1.4$, with the range in values associated with the uncertainty in our estimate of entrainment of fluid directly into the plume head. As a test, the proposed model is shown to provide a reasonable description of a starting plume rising through a stratified environment in the region below the maximum height of rise of the associated steady plume, although, above this point, the shape of the plume head changes and the model breaks down.

Distortions of inertia waves in a rotating fluid cylinder forced near its fundamental mode resonance

1994 ◽  
Vol 265 ◽  
pp. 345-370 ◽  
Author(s):  
Richard Manasseh
Keyword(s):  
Circular Cylinder ◽  
Fundamental Mode ◽  
Mathematical Problem ◽  
Flow Instabilities ◽  
Rotating Fluid ◽  
Inertial Oscillations ◽  
Homogeneous Fluid ◽  
Elliptical Flow ◽  
Ill Posed ◽  
Axis Of Symmetry

A series of experimental observations is presented of a flow in which inertial oscillations are excited. The homogeneous fluid is contained in a completely filled right circular cylinder. The cylinder is spun about its axis of symmetry and a small ‘forced precession’ (or coning motion) is impulsively started. The flow is visualized by an electrolytic dyeline method. The mathematical problem for linear inviscid inertial oscillations in this system, although ill-posed in general, admits a solution in terms of wave modes for the specific boundary conditions considered here. The experiments show that while this linear inviscid theory provides some facility for predicting the flow structure at early times, the flow rapidly and irreversibly distorts away from the predicted form. This behaviour is seen as a precursor to some of the more dramatic breakdowns described by previous authors, and it may be pertinent to an understanding of the breakdowns reported in experiments on elliptical flow instabilities.

Boundary-layer noise induced by arrays of roughness elements

2013 ◽  
Vol 727 ◽  
pp. 282-317 ◽  
Author(s):  
Qin Yang ◽  
Meng Wang
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Strong Dependence ◽  
Pressure Fluctuations ◽  
Turbulent Structures ◽  
Dipole Strength ◽  
Eddy Simulation ◽  
Unsteady Separation ◽  
Roughness Elements ◽  
Roughness Arrays

AbstractSound induced by arrays of $10\times 4$ roughness elements in low-Mach-number turbulent boundary layers at ${\mathit{Re}}_{\theta } = 3065$ is studied with Lighthill’s theory and large-eddy simulation. Three roughness fetches consisting of hemispheres, cuboids and short cylinders are considered. The roughness elements of different shapes have the same height of $0. 124\delta $, the same element-to-element spacing of $0. 727\delta $ and the same flow blockage area. The acoustically compact roughness elements and their images in the wall radiate sound primarily as acoustic dipoles in the plane of wall. The dipole strength, orientation and spatial distribution show strong dependence on the roughness shape. Correlations between dipole sources associated with neighbouring elements are found to be small for these sparsely distributed roughness arrays. Correlations and coherence between roughness dipoles and surface pressure fluctuations are analysed, which reveals the importance of the impingement of upstream turbulence and surrounding vortical structures to dipole sound radiation, especially in the streamwise direction. For roughness shapes with sharp frontal edges, the edge-induced unsteady separation and reattachment also play important roles in sound generation. Large-scale turbulent structures in the boundary layer have a relatively low influence on roughness dipoles, except for the first row of elements.

Sign in / Sign up

Export Citation Format

Share Document