Project (long title)Mixing and length scales in Stratified Turbulence
Campaign title (name data folder)18MILESTONE
Lead authorErik LINDBORG
Date campaign start17-05-2016
Date campaign end09-07-2016

0 - Publications, reports from the project:

1 - Objectives

During the last decade a new theory of stratified turbulence has emerged. The project aims at testing the predictions of this theory in controlled laboratory experiments for the first time. A series of experiments of high Reynolds number and low Froude number stratified turbulence will be carried out at the Coriolis platform. Turbulence generated by an array of translating vertical cylinders in the presence of background stratification, with and without system rotation, will be studied by means of PIV. Three-dimensional velocity fields from the wake will be retrieved, from which spectra, structure functions as well as different length scales will be calculated. Mixing efficiency for different buoyancy Reynolds numbers will be estimated. The results will be compared with theoretical predictions as well as results from Direct Numerical Simulations.

2 - Experimental setup

The experimental setup is presented on this sketch (view from above). A channel is delimited by three walls (transparent where needed).

A more technical representation of the experiments with hardware and software can be found on this page: setup. See also the page todolist.

2.1 - Forcing

The flow is forced by an array of cylinders attached to a moving carriage. The mesh is 0.75 m and the diameter of the cylinders is equal to 0.25 m (solidity 0.33). See the dedicated page: forcing.

2.2 - Stratification

Experiments will be carried out in pure water (homogeneous) and with density stratifications with salt and with salt and ethanol in order to get an homogeneous refractive index. See the dedicated page: stratification.

2.3 - Measurements

The velocity is measured by PIV (by correlation of image and maybe particle tracking). There are 3 fields of view:

- vertical field of view (~ 1 m x 0.6 m), measurements/verticalpiv

- large horizontal field of view (~ 2.4 m x 3 m), measurements/horizontalpiv

- small horizontal field of view (~ 1 m x 1.25 m). measurements/horizontalpiv

Density is measured by 5 conductivity probes in different configurations (see measurements/probes):

- 3 probes are attached to linear traverses and are used to measure density profiles,

- 1 probe is attached to the carriage,

- 2 fixed probes measure the density at the top and the bottom.

Positions of the probes.

/!\ Probes on profilers (C0, C2 and C5) have been titlted at a certain time in order to prevent hydrodynamic disturbance on the signal

Probe x (cm) y (cm) z (cm) x tilted (cm)y tilted (cm)
C0 -411 194 moving -390 194
C2 127 -226.5 moving 127 239
C5 -215 -230 moving -175 -235
C4 -297 -230 ?
C3 -297 -230 ?
C1 x_carriage + 53 121 ?

2.4 - Software systems

See dedicated page for quick reference to different software-systems:

2.5 - Time origin and coordinates

The time origin is taken at the start of the grid motion.

The coordinate x is along the carriage motion, y spanwise horizontal, with origin at the channel centre. z is vertical upward with origin at the tank bottom.

Camera calibration is done in 4 steps:

* 3D calibration with the calibration grid tilted in different orientations giving te intrinsic parameters of the camera (focal length, quadratic deformation parameter) * image with the calibration grid in a reference plane giving the plane (x,y), giving the extrinsic parameters: translation and rotation between the phys and camera coordinates. * image with a ruler giving the absolute coordinates, while the calibration grid is generally not positioned at a known position. * replication for the actual images with the additional information of the laser plane position(s) and tilt angle(s), which are defined for each j image index for 3D scanning.

The cameras Dalsa with vertical fields of view are calibrated directly in water, while the PCO are calibrated in air for simplicity and a transform is applied to take into account refraction effect.

All the images used for calibration are stored in 16MILESTONE/Calibrations. Images used for Dalsa are in the subfolder /Dalsa:

  • /calib_3D_eau for 3D calibration
  • /calib_ref_plane_laser for the calibration grid in the reference vertical plane of the laser at 45 cm of the channel edge (y=206 cm).
  • /calib_ruler_2016-06-16 for a vertical ruler in the laser sheet, giving the x and z coordinates.

The calibration images for the 3 PCO cameras are in PCO_cam.

  • 3D_air/ for 3D calibration: h1, h2, h3, 3 different plane orientations, and 3cm_air_PCO for the reference horizontal plane. 72cm-air_PCO is used for checking the scale of the horizontal grid at the new height z=72 cm.
  • ruler_air is used to adjust to the phys coordinate axis using a ruler.
  • water_ provides additional test of the scaling of the calibration grid image in water.

Note that for PCO bottom, the reverse height is used: z'=H-z where H=81 cm is the water height. This is to account for the opposite (upward) line of view.

Calibration is introduced in all the xml files of the same camera by the uvmat tool geometry_calib/REPLICATE (this has to be done after the timing has been introduced in the same xml files).

Then the tool set_slices is used to introduce the position(s) and tilt angle(s) of the laser sheet, also stored in the xml files for each image sequence.

3 - Procedure to make an experiment

See the dedicated pages howtoexp and prepare_data_after_exp.

4 - Data and list of experiments

See the dedicated page listexp.

5 - Post-processing

A lot of processing of the raw data will be done on LEGI clusters. To connect to the frontal machine at LEGI, follow the instruction on this page ssh_x2go_legi

For loading data, post-processing and plotting, we use the Python packages fluiddyn, fluidimage and fluidcoriolis. See this page for a presentation of how to install and use them: postproc/fluiddyn.

From the images :

Calcul of displacement fields from the images postproc/calculpiv

Post-processing of the PIV fields postproc/frompiv

From the probes :

- postproc/probes