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Abstract
The aim of this contribution is to present the results of laboratory experiments on the dynamics of basic self-propagating vortices generated in a large volume of fluid when a linear (P) and an angular (M) momentum are applied locally to a fluid. Using the method proposed, it is possible to generate a whole family of isolated (net vorticity is equal to zero) vortices with different values of the nondimensional parameter ε, which is proportional to the ratio of linear to angular momentum (ε ∝ RP/M, R is the eddy size). Typical examples include monopole (ε = 0), quasi monopole (ε = 0.1–0.3), quasi dipole (ε ≈ 1), and dipole (ε = ∞).
One of the possible applications is the dynamics of oceanic eddies. Recently, Stern and Radko considered theoretically and numerically a symmetric barotropic eddy, which is subject to a relatively small amplitude disturbance of unit azimuthal wavenumber on an f plane. This case corresponds to a self-propagating quasi monopole. They analyzed the structure of the eddy and predicted that such an eddy remains stable and could propagate a significant distance away from its origin. This effect may be of importance for oceanographic applications and such an eddy was reproduced in laboratory experiments with the purpose of verifying these theoretical predictions.
Another possible application may include large eddies behind maneuvering bodies. Recent experiments by Voropayev et al. show that, when a submerged self-propelled body accelerates, significant linear momentum is transported to the fluid and unusually large dipoles are formed in a late stratified wake. When such a body changes its direction of motion, an angular momentum is also transported to the fluid and the resulting structure will depend on the value of ε.
Abstract
The aim of this contribution is to present the results of laboratory experiments on the dynamics of basic self-propagating vortices generated in a large volume of fluid when a linear (P) and an angular (M) momentum are applied locally to a fluid. Using the method proposed, it is possible to generate a whole family of isolated (net vorticity is equal to zero) vortices with different values of the nondimensional parameter ε, which is proportional to the ratio of linear to angular momentum (ε ∝ RP/M, R is the eddy size). Typical examples include monopole (ε = 0), quasi monopole (ε = 0.1–0.3), quasi dipole (ε ≈ 1), and dipole (ε = ∞).
One of the possible applications is the dynamics of oceanic eddies. Recently, Stern and Radko considered theoretically and numerically a symmetric barotropic eddy, which is subject to a relatively small amplitude disturbance of unit azimuthal wavenumber on an f plane. This case corresponds to a self-propagating quasi monopole. They analyzed the structure of the eddy and predicted that such an eddy remains stable and could propagate a significant distance away from its origin. This effect may be of importance for oceanographic applications and such an eddy was reproduced in laboratory experiments with the purpose of verifying these theoretical predictions.
Another possible application may include large eddies behind maneuvering bodies. Recent experiments by Voropayev et al. show that, when a submerged self-propelled body accelerates, significant linear momentum is transported to the fluid and unusually large dipoles are formed in a late stratified wake. When such a body changes its direction of motion, an angular momentum is also transported to the fluid and the resulting structure will depend on the value of ε.
Abstract
Intercomparisons have been made of results from laboratory experiments and a numerical model for the flow in the vicinity of an idealized submarine canyon located along an otherwise continuous shelf. Motion in the rotating and continuously stratified fluid was impulsively generated by suddenly changing the period of rotation, so that the resulting flow occurred with the coastline either on the left (upwelling favorable) or right (downwelling favorable) when facing downstream. A principal purpose of the study was to further develop the notion that laboratory experiments can be effectively utilized to provide datasets to benchmark the development of numerical models. Laboratory data are of two types: velocity fields on three horizontal planes at numerous times, and time series of isopycnal movement in the canyon area. Comparison of numerical and laboratory results shows that values for bottom friction and interior mixing in the numerical model are crucial. Once those friction/mixing parameters are set, “skill” statistics using observed and predicted horizontal velocity components indicate that the high quality of the numerical model description is maintained over the full measurement period. Two principal features of the circulation are early (<one rotation period) along-canyon flow followed by generation of a canyon vortex. In up- (down-) welling cases, the cyclone (anticyclone) develops along the upstream edge of the canyon and then advects into the canyon interior without significant local vortex stretching within the canyon itself. Numerical results for the case of an extra slow rotation rate change show that vortex creation is not an artifact of the fast rate of rotation change. The canyon vortices extend from just slightly above shelf depth to the deepest part of the canyon; the intensities of the up- and downwelling vortices are asymmetric with respect to the direction of forcing at shelf level, but basically symmetric deeper in the canyon. Upper column vorticity generation by stretching over the canyon rim and flow separation around the canyon headlands could explain this upper water column asymmetric response. The symmetric response in the lower water column is shown to be related to the flow separation only.
Overall, the results demonstrate that laboratory and numerical experiments work hand in hand to decipher the complexities of circulation and hydrography undergoing rapid change in a model coastal canyon.
Abstract
Intercomparisons have been made of results from laboratory experiments and a numerical model for the flow in the vicinity of an idealized submarine canyon located along an otherwise continuous shelf. Motion in the rotating and continuously stratified fluid was impulsively generated by suddenly changing the period of rotation, so that the resulting flow occurred with the coastline either on the left (upwelling favorable) or right (downwelling favorable) when facing downstream. A principal purpose of the study was to further develop the notion that laboratory experiments can be effectively utilized to provide datasets to benchmark the development of numerical models. Laboratory data are of two types: velocity fields on three horizontal planes at numerous times, and time series of isopycnal movement in the canyon area. Comparison of numerical and laboratory results shows that values for bottom friction and interior mixing in the numerical model are crucial. Once those friction/mixing parameters are set, “skill” statistics using observed and predicted horizontal velocity components indicate that the high quality of the numerical model description is maintained over the full measurement period. Two principal features of the circulation are early (<one rotation period) along-canyon flow followed by generation of a canyon vortex. In up- (down-) welling cases, the cyclone (anticyclone) develops along the upstream edge of the canyon and then advects into the canyon interior without significant local vortex stretching within the canyon itself. Numerical results for the case of an extra slow rotation rate change show that vortex creation is not an artifact of the fast rate of rotation change. The canyon vortices extend from just slightly above shelf depth to the deepest part of the canyon; the intensities of the up- and downwelling vortices are asymmetric with respect to the direction of forcing at shelf level, but basically symmetric deeper in the canyon. Upper column vorticity generation by stretching over the canyon rim and flow separation around the canyon headlands could explain this upper water column asymmetric response. The symmetric response in the lower water column is shown to be related to the flow separation only.
Overall, the results demonstrate that laboratory and numerical experiments work hand in hand to decipher the complexities of circulation and hydrography undergoing rapid change in a model coastal canyon.
Abstract
Laboratory experiments were conducted to simulate the diurnal heating-cooling cycle in the vicinity of a ridge of constant cross section. In the model the fluid is a water solution stratified with salt to simulate the background stratification of the atmosphere. The flow is driven by recirculating water of a controlled temperature beneath the model; the model surface temperature is thus varied in a specified way to simulate the surface heating by solar insolation during the daytime hours and surface cooling by radiation during the nighttime.
The pertinent similarity parameters are shown to be G c , for daytime convective flow and G d for nocturnal flow; here G c = H b /H c , G d = H b /H d , where H b , is the mountain height, H c the neutral buoyancy height of free convection. and H d the characteristic thickness of the nighttime drainage flow. The model demonstrates some of the principal features of thermally driven mountain circulations, including daytime upslope winds and nocturnal downslope drainage flows. The spatial and temporal structures of these motion fields are delineated, with the following being among the most important observations: (i) during the daytime, the upslope convective flow in the vicinity of the mountain tends to suppress convective turbulence over the horizontal plains; (ii) during the early evening, horizontal jets, with the principal one directed toward the mountain, develop above the mountain surface, and vortices in the vertical cross section develop both above and below the jets, following the collapse of the convective motion over the mountain; and (iii) in the evening, a downslope drainage flow is initiated following the establishment of a vertical vortex on the mountain slope and under the jet.
Quantitative experimental observations are made, which demonstrate the variation of various flow observables with the pertinent similarity parameters. These results are applied to the atmosphere following similarity relations between the physical model and the atmosphere. The predicted characteristic speeds and length scales of the daytime upslope flow and the nocturnal drainage flow for typical atmospheric parameters are in reasonable agreement with limited field observations.
Abstract
Laboratory experiments were conducted to simulate the diurnal heating-cooling cycle in the vicinity of a ridge of constant cross section. In the model the fluid is a water solution stratified with salt to simulate the background stratification of the atmosphere. The flow is driven by recirculating water of a controlled temperature beneath the model; the model surface temperature is thus varied in a specified way to simulate the surface heating by solar insolation during the daytime hours and surface cooling by radiation during the nighttime.
The pertinent similarity parameters are shown to be G c , for daytime convective flow and G d for nocturnal flow; here G c = H b /H c , G d = H b /H d , where H b , is the mountain height, H c the neutral buoyancy height of free convection. and H d the characteristic thickness of the nighttime drainage flow. The model demonstrates some of the principal features of thermally driven mountain circulations, including daytime upslope winds and nocturnal downslope drainage flows. The spatial and temporal structures of these motion fields are delineated, with the following being among the most important observations: (i) during the daytime, the upslope convective flow in the vicinity of the mountain tends to suppress convective turbulence over the horizontal plains; (ii) during the early evening, horizontal jets, with the principal one directed toward the mountain, develop above the mountain surface, and vortices in the vertical cross section develop both above and below the jets, following the collapse of the convective motion over the mountain; and (iii) in the evening, a downslope drainage flow is initiated following the establishment of a vertical vortex on the mountain slope and under the jet.
Quantitative experimental observations are made, which demonstrate the variation of various flow observables with the pertinent similarity parameters. These results are applied to the atmosphere following similarity relations between the physical model and the atmosphere. The predicted characteristic speeds and length scales of the daytime upslope flow and the nocturnal drainage flow for typical atmospheric parameters are in reasonable agreement with limited field observations.
Abstract
Alongshore oscillatory flows over an elongated topographic feature next to a vertical wall for a homogeneous, rotating fluid were investigated by means of numerical and laboratory experiments. The physical experiments were conducted in the Grenoble 13-m diameter rotating tank, in which an elongated obstacle of limited longitudinal extent was placed along the vertical sidewall. The background oscillating motion was obtained by periodically varying the platform angular velocity. Fluid motions were visualized and quantified by direct velocity measurements and particle tracking. The numerical model employed was a tridimensional model developed by Haidvogel et al. It consists of the traditional primitive equations, that is, the Navier-Stokes equations for a rotating fluid with the addition of the hydrostatic, Boussinesq, and incompressibility approximations. (The experiments described here employ the homogeneous version.) The numerical formulation uses finite differences in the horizontal and spectral representation in the vertical dimensions.
Both the laboratory and numerical experiments show that in the range of dimensionless parameters considered, two distinct flow regimes, based on general properties of the rectified flow patterns observed, can be defined. It is further shown that the flow regime designation depends principally on the magnitude of the temporal Rossby number, Ro t , defined as the ratio of the flow oscillation to the background rotation frequency. Good qualitative and quantitative agreement is found between the laboratory experiments and the numerical model for such observables as the spatial distribution of rectified flow patterns. Several other flow observables are defined and their relation with the system parameters delineated.
Abstract
Alongshore oscillatory flows over an elongated topographic feature next to a vertical wall for a homogeneous, rotating fluid were investigated by means of numerical and laboratory experiments. The physical experiments were conducted in the Grenoble 13-m diameter rotating tank, in which an elongated obstacle of limited longitudinal extent was placed along the vertical sidewall. The background oscillating motion was obtained by periodically varying the platform angular velocity. Fluid motions were visualized and quantified by direct velocity measurements and particle tracking. The numerical model employed was a tridimensional model developed by Haidvogel et al. It consists of the traditional primitive equations, that is, the Navier-Stokes equations for a rotating fluid with the addition of the hydrostatic, Boussinesq, and incompressibility approximations. (The experiments described here employ the homogeneous version.) The numerical formulation uses finite differences in the horizontal and spectral representation in the vertical dimensions.
Both the laboratory and numerical experiments show that in the range of dimensionless parameters considered, two distinct flow regimes, based on general properties of the rectified flow patterns observed, can be defined. It is further shown that the flow regime designation depends principally on the magnitude of the temporal Rossby number, Ro t , defined as the ratio of the flow oscillation to the background rotation frequency. Good qualitative and quantitative agreement is found between the laboratory experiments and the numerical model for such observables as the spatial distribution of rectified flow patterns. Several other flow observables are defined and their relation with the system parameters delineated.
Abstract
Previous laboratory experiments and associated numerical models of laminar flows forced by oscillatory, along-shelf background currents are extended to include some of the effects of boundary-generated turbulence. The experiments are conducted in the 13-m-diameter rotating-flow facility in Grenoble, France. Two pairs of case studies, one at a large forcing velocity (designated as FAST) for which the boundary layers are fully turbulent during part of the flow cycle and one at relatively smaller forcing (SLOW) for which transitional boundary layers are operative at the higher speeds of the background flow, are conducted. Smooth and artificially roughened boundaries are considered, respectively, for each of these pairs. Phase-averaged and time-mean velocity, vertical vorticity, and horizontal divergence fields are found to be qualitatively similar to those of previous laminar experiments. The similarities in the time-mean fields are that (i) within the canyon they are dominated by cyclonic vorticity with maxima centered near the shelf break; (ii) within and in the vicinity of the canyon the general circulation pattern includes a net transport into the canyon through its mouth, a net upwelling in the canyon interior, a transport away from the canyon over the shelf break along both sides of the canyon, and, by inference, a return flow to the deep ocean; and (iii) the interior time-mean flow is characterized by a well-defined coastal current whose axis follows the shelf in the vicinity of the shelf break, with the coast on the right. It is found that the measurements of the characteristic speed of the residual or time-mean flow within the canyon for the transitional and fully turbulent experiments do not follow the scaling law derived earlier for laminar experiments. An alternative scaling analysis for large-Reynolds-number flows is thus derived. Although sufficient numbers of experiments are not available to test the hypothesis fully, the measurements available for the fully turbulent flows are consistent with the theory advanced.
Abstract
Previous laboratory experiments and associated numerical models of laminar flows forced by oscillatory, along-shelf background currents are extended to include some of the effects of boundary-generated turbulence. The experiments are conducted in the 13-m-diameter rotating-flow facility in Grenoble, France. Two pairs of case studies, one at a large forcing velocity (designated as FAST) for which the boundary layers are fully turbulent during part of the flow cycle and one at relatively smaller forcing (SLOW) for which transitional boundary layers are operative at the higher speeds of the background flow, are conducted. Smooth and artificially roughened boundaries are considered, respectively, for each of these pairs. Phase-averaged and time-mean velocity, vertical vorticity, and horizontal divergence fields are found to be qualitatively similar to those of previous laminar experiments. The similarities in the time-mean fields are that (i) within the canyon they are dominated by cyclonic vorticity with maxima centered near the shelf break; (ii) within and in the vicinity of the canyon the general circulation pattern includes a net transport into the canyon through its mouth, a net upwelling in the canyon interior, a transport away from the canyon over the shelf break along both sides of the canyon, and, by inference, a return flow to the deep ocean; and (iii) the interior time-mean flow is characterized by a well-defined coastal current whose axis follows the shelf in the vicinity of the shelf break, with the coast on the right. It is found that the measurements of the characteristic speed of the residual or time-mean flow within the canyon for the transitional and fully turbulent experiments do not follow the scaling law derived earlier for laminar experiments. An alternative scaling analysis for large-Reynolds-number flows is thus derived. Although sufficient numbers of experiments are not available to test the hypothesis fully, the measurements available for the fully turbulent flows are consistent with the theory advanced.
Abstract
Synoptic classification is used to identify meteorological conditions characteristic of high-pollution periods at Nogales, Arizona. Low surface winds determined by local surface cooling at night with little vertical mixing were found to be most important. This condition was simulated in a 0.79-m-square box filled with water with the lower surface made to model a 12-km-square region of the surface topography of the United States-Mexico border at Nogales. The aluminum base was cooled to induce the downslope flows. Photographs of dye initially placed on the surface at many locations were used to obtain a set of surface velocities that formed the input to the Diagnostic Wind Model (DWM). The DWM provided hourly velocity data with grids of 500- and 250-m spacings.
The similarity arguments used to analyze the relationship of the physical model to the atmosphere are discussed. Although the magnitude of the wind vectors in the physical model cannot be matched to the atmosphere, the directions can be used to assess the accuracy of the wind field obtained from a sparse set of field sites. A range of locations of these sites is analyzed to determine a strategy for obtaining sufficient wind data to depict satisfactory wind fields in this complex terrain.
Abstract
Synoptic classification is used to identify meteorological conditions characteristic of high-pollution periods at Nogales, Arizona. Low surface winds determined by local surface cooling at night with little vertical mixing were found to be most important. This condition was simulated in a 0.79-m-square box filled with water with the lower surface made to model a 12-km-square region of the surface topography of the United States-Mexico border at Nogales. The aluminum base was cooled to induce the downslope flows. Photographs of dye initially placed on the surface at many locations were used to obtain a set of surface velocities that formed the input to the Diagnostic Wind Model (DWM). The DWM provided hourly velocity data with grids of 500- and 250-m spacings.
The similarity arguments used to analyze the relationship of the physical model to the atmosphere are discussed. Although the magnitude of the wind vectors in the physical model cannot be matched to the atmosphere, the directions can be used to assess the accuracy of the wind field obtained from a sparse set of field sites. A range of locations of these sites is analyzed to determine a strategy for obtaining sufficient wind data to depict satisfactory wind fields in this complex terrain.
Abstract
The problem of the oscillatory motion of a homogeneous, rotating fluid in the vicinity of an isolated topographic feature is investigated in the laboratory and numerically. The laboratory experiments are conducted by fixing a cosine-squared body of revolution near the outer boundary of a circular tank rotating about a vertical axis with an angular velocity Ω(t)=Ω0+Ω1sinωt, where Ω0 is the mean background rotation and Ω0 and ω are the magnitude and frequency of an oscillatory component. Experiments with an oscillatory flow show clearly that a mean anticyclonic vortex is formed in the vicinity of the topographic feature. Surface floats are used to determine typical particle paths for various flow conditions and these are shown to vary markedly with the Rossby and temporal Rossby numbers of the background flow. Eulerian velocity profiles along and normal to the streamwise axis are used to quantify the anticyclonic vortex. A scaling analysis is advanced to show how the strength and distribution of the anticyclonic current varies with the various system parameters. The laboratory findings are in good agreement with the scaling analysis and with the theoretical model of Wright and Loder.
A nonlinear numerical model, using the quasi-geostrophic potential vorticity equation, is considered; the results correlate well with the scaling analysis and the laboratory experiments. The laboratory and numerical experiments are used to estimate the magnitude of the mean anticyclonic motion that might be expected in the vicinity of Fieberling Guyot. Future laboratory and numerical experiments will consider the additional feature of background stratification.
Abstract
The problem of the oscillatory motion of a homogeneous, rotating fluid in the vicinity of an isolated topographic feature is investigated in the laboratory and numerically. The laboratory experiments are conducted by fixing a cosine-squared body of revolution near the outer boundary of a circular tank rotating about a vertical axis with an angular velocity Ω(t)=Ω0+Ω1sinωt, where Ω0 is the mean background rotation and Ω0 and ω are the magnitude and frequency of an oscillatory component. Experiments with an oscillatory flow show clearly that a mean anticyclonic vortex is formed in the vicinity of the topographic feature. Surface floats are used to determine typical particle paths for various flow conditions and these are shown to vary markedly with the Rossby and temporal Rossby numbers of the background flow. Eulerian velocity profiles along and normal to the streamwise axis are used to quantify the anticyclonic vortex. A scaling analysis is advanced to show how the strength and distribution of the anticyclonic current varies with the various system parameters. The laboratory findings are in good agreement with the scaling analysis and with the theoretical model of Wright and Loder.
A nonlinear numerical model, using the quasi-geostrophic potential vorticity equation, is considered; the results correlate well with the scaling analysis and the laboratory experiments. The laboratory and numerical experiments are used to estimate the magnitude of the mean anticyclonic motion that might be expected in the vicinity of Fieberling Guyot. Future laboratory and numerical experiments will consider the additional feature of background stratification.
