G. Chabert d'Hières

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

Abstract The behaviour of a density current on a sloping bottom in a rotating system is investigated by laboratory experiments. The main result is that the dense bottom outflow induces cyclonic vortices in the upper fluid layer, which are formed periodically and move to the west parallel to coast. Two regimes of vortex formation have been identified. For strong density currents and weak rotation, vortices are formed by stretching of the upper layer near the source as found also in the experiments by Lane-Serff and Baines (1998) [Lane-Serff, G.F., Baines, P.G., 1998. Eddy formation by dense flows on slopes in a rotating fluid. J. Fluid Mech. 363, 229–253]. For weak density currents and strong rotation vortices are due to instability of the bottom plume itself as found in the numerical simulations of Jiang and Garwood (1996) [Jiang, L., Garwood, W. Jr., 1996. Three-dimensional simulations of overflows on continental slopes. J. Phys. Oceanogr. 26, 1224–1233].

A laboratory study of the lift forces on a moving solid obstacle in a rotating fluid

Abstract A series of experiments is described in which a novel technique is used to measure directly the lift force on a right cylynder circular moving through a homogeneous rotating fluid. These experiments have been conducted on the large rotating table facility at the Institut de Mecanique de Grenoble, in order to investigate simultaneously flows at high Reynolds number ( Re ), low Rossby ( Ro ), and low Ekman ( Ek ) number. The measurements have shown that within the range of parameters 0.25 Ro −6 Ek −6 ; 1.50 × 10 4 Re 5 , the cylinder experiences a mean lift due to (i) the structural asymmetry between cyclonic and anticyclonic eddies in its wake, and (ii) the Coriolis force on its excess mass. The total mean lift coefficient C ∗ shows no significant dependence upon Ek , but exhibits a linear relationship with Ro −1 . Similarly, the contribution of the mean lift coefficient C p due to the wake asymmetry alone is shown to vary approximately linearly with Ro −1 . In addition to the net mean lift, the cylinder experiences a fluctuating lift force generated by periodic vortex shedding. The amplitude of the fluctuating lift coefficient also increases linearly with increasing values of Ro −1 , though the Strouhal frequency shows no variation with either Ro or Ek . Measured values of the Strouhal number do not differ significantly from values typical of non-rotating flow past circular cylinders.

Experimental studies of lift and drag forces upon cylindrical obstacles in homogeneous, rapidly rotating fluids

Abstract A series of laboratory measurements is described, in which simultaneous drag and lift forces have been measured directly on full and truncated cylinders moving uniformly through homogeneous rotating fluids. For the full cylinder experiments, it has been demonstrated that within the Reynolds (Re) and Rossby (Ro) number ranges 9.8 × 103 ≤ Re ≤5.6 × 104 and 0.1 ≤ Ro ≤0.9, the mean total lift coefficient and the mean lift coefficient because of wake asymmetry both increase with decreasing Rossby number Ro. For cases in which the fluid is non-rotating, the total lift coefficient is shown to be zero, and the values of the drag coefficient for the cylinder are shown to be in accord with existing drag for this geometry. When the value of the Reynolds number is sufficiently small, the presence of background rotation is shown to result in a two- to three-fold increase in the drag coefficient. Experiments with truncated cylinders show similar behaviour to the full cylinder cases with regard to the dependence of the drag coefficient upon Ro and Re. However, values of the lift coefficient are shown to be increased considerably over corresponding full cylinder cases throughout the Reynolds number range, particularly at low and intermediate values of the inverse Rossby number Ro−1. The increase in lift is attributed to (1) the accentuation of wake asymmetry caused by vortex tube stretching as fluid passes below the truncated cylinder, and (2) the generation of an unbalanced Coriolis force on the volume of fluid displaced by the cylinder.

A laboratory simulation of mesoscale flow interaction with the Alps

Abstract A series of laboratory experiments, aimed at the simulation of some aspects of Alpine lee cyclogenesis has been carried out in the rotating tank of the Coriolis Laboratory of LEGI-IMG in Grenoble. Dynamic and thermodynamic processes, typical of baroclinic development triggered by the orography, were simulated. The background flow simulating the basic state of the atmosphere consisted of a stream of intermediate density fluid introduced at the interface between two fluid layers. The structure of the intermediate current was established by mixing fluid obtained from the upper layer of fresh water with fluid removed from the heavier salty layer below. The dynamical similarity parameters are the Rossby ( Ro ), Burger ( Bu ) and Ekman ( Ek ) numbers, although this last, owing to its small values, need not be matched between model and prototype, since viscous effects are not important for small time scales. The flow in both the prototype and laboratory simulation is characterized by hydrostatics; this requires ( Ro 2 δ 2 / Bu )⪡1 (where δ = H / L is the aspect ratio of the obstacle) which is clearly satisfied, in the atmosphere and oceans, and for the laboratory experiment. A range of experiments for various Rossby and Burger numbers were conducted which delimited the region of parameter space for which background flows akin to that found to the northwest of the Alps prior to baroclinic cyclogenesis events, were observed. One such experiment was carried out by placing a model of the Alps at the appropriate place in the flow field. The subsequent motion in the laboratory was observed and dye tracer motions were used to obtain the approximate particle trajectories. The density field was also analyzed to provide the geopotential field of the simulated atmosphere. Using standard transformations from the similarity analysis, the laboratory observations were related to the prototype atmosphere. The flow and the geopotential fields gave results compatible with the particular atmospheric event presented.

A laboratory experiment on the development of cyclogenesis in the lee of a mountain

SummaryOne of the most intriguing problems concerning the interaction of subsynoptic and synoptic atmospheric flows with topographic features is orographic cyclogenesis. A fully satisfactory prediction theory is not yet available, but a lot of efforts have been made by theoreticians to implement reliable numerical models simulating the different phases of this complex phenomenon. An attempt to perform a laboratory experiment to simulate physically this kind of interaction has been made by us, through the generation of a baroclinic frontal system in the rotating hydraulic platform of the «Coriolis Laboratory—LEGI-IMG—Grenoble». The adopted technique consists in a device which produces, at the interface separating two water layers of different density (ϱ1 and ϱ2), a stream of stratified fluid whose density has an intermediate value ϱ1 < ϱm < ϱ2. This stream is generated at the height of the interface between the two layers; due to the rotation of the platform, the attainment of geostrophic equilibrium brings about an intermediate-water flow running along a wall, giving rise to a three-layer baroclinic structure which can represent some of the main outstanding features of an atmospheric frontal system impinging on a mountain. In a well-defined range of the Rossby and Burger numbers, the instability of this current gives rise to a couple of persistent cyclonic and anticyclonic vortices, whose horizontal dimensions and vertical extents reproduce quite faithfully the synoptic situation supporting the onset of the orographic cyclogenesis, with its characteristic cold front stretching between the two vortex structures. It is enough to place an obstacle of a suitable size in the proper geographic position, to make the cyclogenesis start. The first results of our simulations have been encouraging, showing the occurrence of lee cyclogenesis when the stream conditions in our model correspond to the synoptic features which have been recognized as the precursors of orographic cyclogenesis in the lee of the Alps.