Project Name

InfrastructureCNRS_Coriolis
Project (long title)Internal mixing and near-bed dynamics induced by restricted stratified exchange flows
Campaign Title (name data folder)14CARR
Lead AuthorMagda Carr
ContributorJarle Berntsen, Alan Cuthbertson, Jonathan Kean, Janek Laanearu, Madis-Jak Lilover, Monika Sergejeva, Joel Sommeria, Oyvind Thiem, Samuel Viboud
Date Campaign Start26/05/2014
Date Campaign End18/07/2014

0 - Publications, reports from the project:

IAHR congress, Delft 2015: Abstract_IAHR.pdf Download

1 - Objectives

An experimental study of internal flow and near-bed dynamics induced by restricted stratified exchange flows is proposed. Two configurations are considered, the first one models the mouth of an estuary and the second that of a fjord. The experiments are run with a fixed obstruction under both rotating and non-rotating conditions. The main objectives of the experiments are (i) to obtain high-resolution velocity and density profiles in the vicinity of and close to the obstruction and (ii) to observe and quantify the internal mixing and near-bed dynamics generated by such exchange flows. Particular aims are (i) to determine the mixing efficiency of turbulence generated by the shear between two counter-flowing layers in hydraulically controlled buoyancy-driven exchange flows (ii) investigate the internal wave field generated in the fjoric configuration and (iii) to investigate the effect of boundary flow dynamics on sediment transport and the evolution of obstruction morphology. The overall objective of the study is an improved understanding of parametric controls such as density difference, basin stratification, relative fresh and saline volumetric fluxes, and rotation rate, on interfacial mixing and entrainment/detrainment processes for restricted exchange flows in silled fjordic basins, as well as boundary layer processes associated with sediment transport and bed morphological changes encountered in semi-enclosed estuaries.

2 - Experimental setup

2.1 General description

The above figure shows a schematic diagram of the main parameters and idealised experimental set-up. Two configurations are considered namely (a) the estuarine case and (b) the fjordic case. The main differences between the two are that in the estuarine case the upstream fluid in the tank is homogeneous fresh water of density $\rho_0$ and there is an inflow of freshwater $Q_0$ while in the fjordic case the upstream section of the tank is stratified and there is no freshwater infow $Q_0$.In both cases, saline water is introduced with a flow rate $Q_1$ from the filling system of the platform and injected via a manifold arrangement to ensure uniform distribution of the inflow into the lower layer of the sea basin (see figure).

In the estuarine case, the freshwater input is then circulated with a flow rate $Q_0$ using axial pumps sited on each side of the channel outlet of the sea basin. An inlet manifold in the estuarine basin is also designed to ensure uniform inflow of the freshwater with thin boundary layers.

In the fjordic runs a barrier plate is placed in the upstream end of the tank on the outside of the honey comb manifold to effectively seal off that end of the flume.

The net saline water inflow $Q_1$ is balanced by sinks at the bottom on each side of the channel such that the total water depth remains constant (see figure below). This is done manually by adjusting the valve.

Experimental measurements focus on obtaining high temporal and spatial resolution density and velocity fields. In the estuarine case measurements are taken both across the obstruction and within the estuarine and sea basins adjacent to the obstruction. In the fjordic case focus is on the fjord side of the obstruction (where gravity curreent of salty water occurs) and in particular in obtaining high resolution data near the bottom boundary of the obstruction.

Ultrasonic probes are placed near the upstream end wall to monitor the spectrum of the internal wave field generated. Two-dimensional Particle Image Velocimetry (PIV) is used to measure velocity fields in the horizontal and vertical planes. In this regard, a global field is provided by the top view, typically 4 m long, with illumination by a horizontal sheet, which can be scanned in height [see figure above].

Illumination is provided from the fjord side (left on the sketch above).

Flow visualization achieved via a side-mounted camera (two cameras are used where appropriate to obtain a wider field of view). Onde camera (named Dalsa2) is used to study the fjord side slope, while another camera (Dalsa1) is used to map the upper (flat) part of the obstruction. The fields of view of these two cameras have been adjusted for different experiments.A third camera (Nikon) has been used in a few experiments to get a wider view of the fjord side, with aim of studyintg the emission of internal waves.

Rapid-deploying, micro-conductivity probe profilers (4 to 6 probes), positioned both within the basins and across the sill obstruction are used to measure instantaneous density profiles at specific measurement locations. Acoustic Doppler Velocimeters (ADVs) collect velocity profiles at similar locations to the micro-conductivity probes in order to predict “instantaneous” gradient Richardson number profiles at these selected locations. These ADV probes and an available Acoustic Doppler Current Profiler (ADCP) are utilized in the erodible bed experiments to measure detailed development of velocity profiles across the erodible obstruction, while a 3D laser bed scanner is utilized to investigate morphological changes induced by the restricted exchange flows.

Experimental schedule

Configuration Bed conditionRotationOrderRemarks
FjordFixedNo1
EstuaryFixedNo2
EstuaryFixedYes3

2.2 Definition of the coordinate system

$x$ along the channel, origin at the start of the fjord side slope, oriented along the salt water current toward fjord).

$y$ horizontal across the channel, origin a the middle of the channel

$z$ vertical upward, origin at the tank bottom.

2.3 Relevant fixed parameters:

Notation DefintionValueremarks
$W$Channel width$W$=150 cm.....
$\alpha$Slope angle$\alpha=\textrm{20}^\circ$.....

2.4 Definition of the variable control parameters

Notation DefinitionUnitInitial estimated valuesRemarks
$\Delta\rho$Density difference$\textrm{kg/m}^3$5, 10, 20
$H_s$Obstruction submergence depthm0.25, 0.5
$L_s$Obstruction lengthm0.5, 2Start with the 2m, only vary if sufficient time
$\Omega$Angular Velocity1/s0, $\pi/15$, $\pi/30$
$Q_0$Freshwater flow ratel/s30 (start then one interation)
$Q_1$Salt water flow ratel/s20 (start then 4 iterations)
$h_1$Initial height of pycnocline in fjord casem0.1, 0.2, 0.3, 0.4

2.5 Definition of the relevant non-dimensional numbers

Horizontal Reynolds number across the obstruction, $Re = u_1h_1/\nu$.

Internal composite Froude number across the obstruction, $Fr = q/(g'h^3)^{1/2}$, $q=Q/W$, $g' = g(\Delta\rho)/\rho_0$.

Rossby number, $Ro = u/(lf)$.

Internal Rossby radius of deformation, $r = (g'h_1/f^2)^{1/2}$.

Shields Parameter, $\tau_*=\rho u^2_*/(\rho_s-\rho)gD$, where $\rho$ is the density of the fluid, $\rho_s$ is the density of the sediment, $u_*$ is the boundary layer velocity, typically one twentieth of the free stream velocity and $D$ is the diameter of the sediment particle.

Rouse Number, $P=w_s/\kappa u_*$, where $w_s$ is the fall velocity of the sediment.

Equating Stokes force $F_d=6\pi\rho \nu a w_s$ where $a$ is the radius of the particle with the effective gravity $4(\rho_s-\rho)g\pi a^3/3$ implies that the fall velocity $w_s=2(\rho_s-\rho)ga^2/9\nu$. So for a particle of radius $a=100 \mu m$ and density $\rho_s=1050 kg/m^3$ the fall velocity is $w_s = 10^{-3} m/s$. Similarly, a radius of $a=400 \mu m$ with density $\rho_s=1050 kg/m^3$ gives a fall velocity of $w_s = 16\times10^{-3} m/s$. Assuming a free stream velocity of the order $0.1 m/s$ implies we can get a Rouse number ranging from $0.5$ (for the $100 \mu m$ particles) to 8 (for the $400 \mu m$ particles). This covers all possible scenarios (wash load, suspended load, bed load). The corresponding Shields parameter is $1/4$ for the $100 \mu m$ particle $1/16$ for the $400 \mu m$ particle.

3 Instrumentation and data acquisition

3.1 Instruments

In the estuarine case stick to camera positions detailed above and/or add a zoomed in field of view at the bed of the obstruction ? Use an array of vertically traversing probes to monitor the exhange flow or is LIF better ? Note if we use LIF we will not then be able to use the water for PIV in the fjord case.

In the fjord case we would like to capture velocity data close to the bed of the sill on the stratified (fjord) side. We would also like a bigger view of the internal wave field generated. Suggest one/two camera(s) zoomed in on the bed of the sill and one camera giving a larger field of view say up to 2m in horizontal extent. We will need to know what the initial undisturbed stratification is and so require a density probe in the fjord side. Ultrasonic probes may be useful at the upstream end of the tank to monitor the internal wave field. PIV can be used for velocity data and seeding will tend to congregate at interface so it can also be used for visualising the interface. LIF should not be necessary unless we see interesting dynamics which we'd like to quantify better.

3.2 Definition of time origin and instrument synchronisation

The initial experimental configuration is one in which the channel is filled with fresh water of density $\rho_0$. Dense saline water of density $\rho_1$ is then added on the sea side ($x<0$) very slowly until the saline water extends from the input source to the obstruction. Filling continues at a slow rate until the interface between the saline water and fresh water reaches a height of half the obstruction. At this point the fresh water inflow begins with the opening of the valve for flow $Q_0$ and $Q_1$ is turned up to the experimental reference value. For a given experiement, the flow rate of the fresh water $Q_0$ will be held fixed and the flow rate of the saline water $Q_1$ will be increased incrementaly over a range of values such that (i) the salt water is erroded and unable to propagate upstream (ii) the salt water propagates onto the sill and is then arrested (iii) the salt water intrudes into the fresh water basin (iv) the intrusion rate rate is increased (v) a stratification is established in the upstream basin. $Q_1$ is then reduced over the same incremental levels and the flow features observed once more. Flow rates $Q_0$ and $Q_1$ are then switched off and the estuarine experiment is concluded.

The fjordic experiments are initiated in one of two ways.

(1) After an estuarine experiment the tank is left for a period of time until there is no residual motion. The stratification in both basins are measured and if deemed suitable used as initial conditions for a given fjordic run.

(2) The channel is filled with fresh water then $Q_1$ is switched on, slowly at first and gradually increasing until the salt water reaches the top of the sill, it is then allowed to spill over, slowly, forming a stratification in the upstream basin.

In the fjord case, the experiment will run until the dense water level rises by typically 10 cm on the fjord side. With a half channel length 450 cm and width 150 cm, it corresponds to a volume 700 l, so the experiment duration ranges from 27 s ($Q_1$=25l/s) to 135 s (for ($Q_1$=5l/s). The measurements can be made longer, typically 10 mn, to record the internal waves. See Exp_proc_Fjordic_case.pdf Download for details.

In the estuarine case, $Q_0$ may start at the same time as $Q_1$ when the sea side is already filled with dense water. The total time of the experiment must be chosen when a steady regime is obtained.

3.3 Requested final output and statistics

Overall flow velocity in the central vertical plane (y=0) and interface shape during the time of the experiment.

Overall flow velocity in a few horizontal planes to check the uniformity.

4 - Methods of calibration and data processing:

see DataProcessing.

5 - Organization of data files:

All data related to the project are in Coriolis /.fsnet/project/coriolis/2014/'project name'

  • 0_DOC: miscellaneous documentation and reports
  • 0_MATLAB_FCT: specific matlab functions
  • 0_PHOTOS: photos of set-up
  • 0_REF_FILES: files of general use (calibration data, grids ...)
  • 0_RESULTS: processed data (time series, statistics...) and figures.
  • EXP1, EXP2, folder for each experiment with names given in the table below. The names refer to the FJORD, ESTUARY or GULF(effect of rotation) cases.
    • Within each experiments, images from successive runs are put in subdirectories labelled by a, b, c... Probes data are put in a folder 'PROBES'. Data from the acoustic anemometer are in a folder ADV.
      • Each folder of Images contains subfolders DeviceFolder named after the camera name, 'DALSA1', 'DALSA2', 'NIKON' (used only for a few experiments).
      • Results from data processing are stored in a folder DeviceFolder.ext where ext is set by the processing program.
      • An xml file DeviceFolder.xml specifies information from the devices, for instance timing and calibration parameters for an image series.
    • Probe data are put in the folder PROBES, in files with names labelled with a, b , c corresponding to the run. The original text format .lvm is translated into netcdf (extension .nc) for faster reading and standardisation.

6 - Table of experiments

List of parameter, Param1... , denoted by names defined in section 2.4. ESTUARY 2-7 correspond to EX2-7 in the publication to appear in environmental fluid mechanics

NameDate$\rho_0$$\rho_1$$\Delta\rho$ (kg/m3)$H_s$(cm)$L_s$(cm)$\Omega$(s$^{-1}$)$Q_0$$Q_1$(m3/h)$h_1$Remarks
#FJORD127/06100010055502000060?PIV, many problems
#FJORD201/071000.41015.114.743.32000010?PIV, problems - see below
#FJORD302/071000.41013.613.241.62000025?PIV, problems - see below
#FJORD403/071000.11007.67.543.02000025?PIV, problems - see below
#FJORD504/071000.11007.47.345.12000025?LIF, good images (a)&(b)
#FJORD609/071000.01003.33.343.42000025?LIF/PIV? (Fluorescene dye added)
#FJORD709/071001.01006.65.6?2000025?LIF/PIV? ( " " " )
#FJORD810/071001.11005.34.240.52000025?LIF (rhodamine dye added)
#ESTUARY110/071001.11005.34.240.52000Varies14.5, 3.0?Test experiment for exchange flows
#ESTUARY2=EX215/071000.01005.15.143.02000Varies25?PIV
#ESTUARY3 =EX316/071000.01009.69.645.02000Varies25?PIV
#ESTUARY4 =EX416/071000.01009.69.635.02000Varies25?PIV
#ESTUARY5=EX516/071000.01009.69.635.02000Varies9.5?PIV/LIF (rhodamine added)
#ESTUARY6=EX617/071000.01004.74.735.42000Varies10.5?
#ESTUARY7=EX717/071000.01004.74.734.92000Varies25?
#GULF118/071000.01004.74.735.02000.0167Varies10.0?Rotating exchange flow run
#GULF218/071000.01004.74.735.02000.0167Varies10.0?Rotating exchange flow run

7 - Diary:

FJORD1 - 27/06/2014

first test experiments. Initial condition poorly controled: bubbles in salt water flux + leaks in topography? leak in the salt water filling system.

probes to be checked ...

camera dalsa1 (sill top) did not record ?

camera Dalsa2 gives poor images ( lines ), not enough particles. Yet some PIV can be done .

Processing camera: first image at 16h51:43, gravity current along slope clearly visible with detachment at the level of the interface. Projection done on a grid along the slope.

Processing probes:

To check meaning of the time in the file (start of data acquisition?. Check time correspondance with the clock of the image computer.problem of ground for the electrical signal?

exp0.lvm:

time record 1600 s starting at 16h39:39 (end at about 17h06). camera signal (5 Hz) visible in t=153-212 s (300 images), t=258-337 s, (395 images), 452-1565s (5565 images). Profiles (motor) done at t=20-105 s, and for t=1146-1212 s, pos z=78 cm in between (not good).

  1. C5: strange signal, may-be displacement wrong.
  2. T5: (temperature), noisy around 1.9 Volt
  3. C6: no signal (v=-5 with noise)
  4. T6: very noisy, around V=7 volt
  5. C2: like C5 but noisy
  6. T2: signal -5 to -4 Volt
  7. ADVP: noisy around 0
  8. I1: slow increase in time beyond t=400, may correspond to the fillling of the basin.
  9. I2: bad signal, V=+7.5 V.
  10. I3: interesting oscillations around V=-7.5 V
  11. I4: noisy, around V=-3.2 V

exp0_1.lvm: 17h12:38, duration 240s, single profile down-up, no camera signal (but noise 1.5 Volt peak to peak due to motor)

  1. C5: seems a good profile (although some noise).
  2. T5: (temperature), profile visible, but noise and shift due to motor (around 1.9 Volt).
  3. C6: no signal (v=-5 with noise)
  4. T6: very noisy, around V=7 volt
  5. C2: like C5 but noisy
  6. T2: signal -5 to -4 Volt, related to the profile.
  7. ADVP: noisy around 0
  8. I1: about -8 Volt, noisy.
  9. I2: bad signal, V=+7.5 V.
  10. I3: signal around V=-7.3 V, noise 1.5 V peak to peak due to motor.
  11. I4: noisy, around V=-3.2 V

FJORD2 - 01/07/2014

Second test experiment. Again, problems with inlet salt water flow - difficult to know what initial stratification in the seawater basin is as no conductivity probes sited there - also unsure how to interpret acoustic probe signals. Appears to be leaks in saltwater basin to outer tank - when dense water level is set to sill cret level and left, interface elevation drops off over time.

Only probe C5 gives out sensible profiles of conductivity - C2 and C6 do not work - replace.

Processing camera: Dalsa 1 camera (horizontal sill) images seem to be missing. Check files on G:\. Images acquired from Dalsa 2 (fjordic slope) camera only. FJORD2a and b combined into one PIV acquisition folder (FJORD2a_b).

FJORD2a:

Dalsa 2 (fjordic slope) camera - nothing observed, no overspill recorded.

FJORD2b:

Dalsa 2 (fjordic slope) camera - images of overspill have refraction problems - also not so many particles for PIV. Gravity current arrives in field of view at image 9900 (of 14380 total images). Spill duration = 60 sec, measured from this point forward (i.e. until image 10200). Salt water flow Q1 then switched off and PIV images continued until overflow completely diminished. Dense water gravity current shown to propagate down slope with little or no mixing with ambient fluid. No indication of detachment observed in EXP1 (FJORD1) run.

FJORD2c:

Dalsa 2 (fjordic slope) camera - images look reasonable with decent amount of seeding particles at start. Gravity current arrives in field of view at image 3874 (of 8261 total images). Spill duration = 60 sec, measured from this point forward (i.e. until image 4174). Very few particles in dense water overspill, also problems again with refraction. Dense water overspill layer seems very thin (Q1 = 10 m3/hr only) with little or no mixing on down slope. No indication of current detachment from slope. Density interface in fjordic side appear in image later on - increased levels of refraction observed. Not probably possible to analyse images from this run.

Processing probes:

General observation and notes on FJORD2 run:

  1. Gate in the fjordic basin was open during FJORD2a and 2b - this meant that no stratification was able to build up in the fjordic basin.
  2. PIV image files for FJOTD2a and b are combined into one acquisition folder (see processing camera above).
  3. Nothing happened during FJORD2a - partially because salt water flow rate was so low (Q1 = 10 m3/hr), meaning that it took a long time to spill over the crest.
  4. FJORD2b run showed a very thin gravity current to be generated across the sill due to low flow rate Q1. Limited mixing observed in overspill with ambient fluid. Possibly because of high density difference $\Delta\rho$ = 14.7 $kg/m3$ limiting mixing of overspill??
  5. Low salt water flow rate (Q1 = 10m3/hr) was due to supply valve from gravity feed controlling salt water inflow being partially closed. When opened for subsequent experiments (see below), the maximum salt water flow rate Q1 increased to 25 m3/hr.
  6. Density of water at outflow drain from fjord side on 02/07/14 am was recorded as 1008.7kg/m3. Water temperature was 22 degrees.

FJORD3 - 02/07/2014

Three experimental runs (FJORD3a, 3b & 3c) conducted at a higher salt water inflow rate Q1 = 25 m3/hr. Density difference $\Delta\rho$ = 13.6 $kg/m3$.

Processing camera: Images from both Dalsa 1 (horizontal sill) and Dalsa 2 (fjordic slope) camera fields were obtained.

FJORD3a:

Dalsa 1 (horizontal sill) camera - Reasonable amount of seeding particles in ambient fluid. Difficult to distinguish start of gravity current, seems to be a thick (dilute?) initial intrusion into left side of Dansa 1 image field at 4900 (out of a total of 10202 images). This layer is well seeded and PIV should work well. Difficault to distinguish and interpret what it is though (possibly just an ambient flow patch with good seeding). No refaction problems observed at this time.

Dalsa 2 (fjordic slope) camera - Spill observed on slope at 5250 (out of 10202 images in total). Good number of particles (although appearing as streaks in gravity current). Not too bad in terms of refraction. Overflow layer seems quite thick (dilute?). Spill lasts for 60 sec - until image 5550 (out of 10202 total). Remainder of images show gradual decrease on overspill on fjordic slope. No evidence of stratification forming in fjordic basin.

FJORD3b: Run abandoned after 35 sec of spill (no particles observed in gravity current)'

Dalsa 1 (horizontal sill) camera - Thin gravity current intrusion appears into left of view field at image 1190 (of 1873 total images). This shows a distince head and body formation, but has no particles within the current itself. Very little mixing observed with ambient fluid on horizontal sill - very sharp interface with overlying ambient fluid. Thickness of interface seen to decrease after salt water inflow Q1 switched off (after 35 sec).

Dalsa 2 (fjordic slope) camera - Gravity current overspill observed at image 1350 (out of 1873 total images). Large amounts of initial mixing between gravity current head and ambient fluid results in significant refraction problems. Again, no particles in the spill layer, combined with the refraction ossues, means it is impossible to track this overspill layer. These refraction problems only occur at some distance down the slope - is this the location where the overflow becomes unstable and results in entrainment of ambient water?? This position seems to initially rise up the slope (as the dense spill rate increases), then lower (as the spill rate decreases). Also interesting to note that the image field position is changing with time (fjordic sloping bed appears to be rising with time) - this is due to the increasing water level depth within the basin. Salt water inflow Q1 switched off after 35 sec (at image 1525). The remainder of the images then observes the decrease in overspill -

FJORD3c:

Dalsa 1 (horizontal sill) camera - PIV camera acquisition started at 16:59:39 and ended at 17:19:52.

Dalsa 2 (fjordic slope) camera - As above.

FJORD4 - 03/07/2014

Three experimental runs (FJORD4a, 4b and 4c) were conducted at the higher flow rate Q1 = 25 m3/hr. Density difference $\Delta\rho$ = 7.5 $kg/m3$.

Processing Camera: Images from both Dalsa 1 (horizontal sill) and Dalsa 2 (fjordic slope) camera fields were obtained. Seeding particles were added directly to the ambient water in the channel and to the salt water input by dosing the upper supply tank. Both cameras were also refocused on the laser light sheet.

CALIB_FJORD4 folder set up for camera calibration images. This calibration applies to experiments FJORD2, FJORD3 and FJORD4.

FJORD5 - 04/07/2014

Three experimantal runs (FJORD5a, 5b and 5c) were conducted at the higher flow rate Q1 = 25 m3/hr. Density difference $\Delta\rho$ = 7.3 $kg/m3$. These conditions were largely similar to FJORD4. Both cameras (Dalsa 1 and 2) were repositioned (moved upwards). Dalsa 2 (fjordic basin slope) in particular was set to look down on the dense water overflow to attempt to minimise refrection issues. New claibration images were acquired for the DALSA 1 and 2 cameras following camera position adjustments for FJORD5. These are stored in 'CALIB_FJORD5' folder. This is a LIF run and so fluorescene (unspecified concentration) was added to the dense water at the upper feed tank via a peristaltic pump at a fixed flow rate of 0.8 ml/s.

Processing Camera: Images from both Dalsa 1 (horizontal sill) and Dalsa 2 (fjordic slope) camera fields were obtained. The PIV software crashed during FJORD5b run and PIV data between 11:50 and 11:52 should be ignored.

FJORD6 - 09/07/2014

Three experimantal runs (FJORD6a, 6b and 6c) were conducted at the higher flow rate Q1 = 25 m3/hr. Density difference $\Delta\rho$ = 3.3 $kg/m3$. The PIV cameras were again adjusted and refocused on the zones of interest. An additional video camera (PIKE) was positioned outside the tank, looking through a window in the side wall to record the development of internal waves in the fjordic basin.

Processing Camera: Images from both Dalsa 1 (horizontal sill) and Dalsa 2 (fjordic slope) camera fields were obtained. As this was a LIF run, fluorescene (unspecified concentration) was again added to the upper supply tank for the salt water via a peristaltic pump at a fixed flow rate of 0.8 ml/s. However, as this was not done from the start of the dense water inflow into the channel, there may have been an initial saltwater spill across the obstruction before the dyed saltwater intrusion is observed.

FJORD7 - 09/07/2014 - 10/07/2014

Four experimantal runs (FJORD7a, 7b, 7c and 7d) were conducted at the higher flow rate Q1 = 25 m3/hr. Density difference $\Delta\rho$ = 5.6 $kg/m3$. Test was conducted over two days (FJORD7a and 7b on 09/07/2014; FJORD7c and 7d on 10/07/2014). Dense bottom water in fjordic basin partially drained between FJORD7b and 7c. No changes to camera positions during FJORD7.

FJORD8 - 10/07/2014

Only one experimantal run (FJORD8a) were conducted at the higher flow rate Q1 = 25 m3/hr. Density difference $\Delta\rho$ = 5.3 $kg/m3$. Dye rhodamine 6G introduced for LIF: solution 1 [g:l g/ll] prepared using alcool to improve solubility. Then dilution by a factor 100 (100 ml in 10 l) into a bacquet. Dye is taken by a peristaltic pump in the top filling tank with fixed flow rate 0.8 ml/s (100 ml in 2 min). After mixing with salt water at flow rate 25 [m3:h=7 m3/h=7] l/s, this adds a dilution factor 10000, leading to a final dye concentration 10^-6 [g:l g/l]. It turns out to be unvisible, so we increase by a factor of 10, to 10^-5 [g:l g/l], leading to good LIF luminosity.

Processing camera: Poor quality PIV/LIF images obtained in this run - gravity current was observed spilling over the obstruction for approximately 5 minutes, but dye trace only observed for part of this time.

Freshwater Flow Calibration - 10/07/2014

PIV and ADV measurements made for different pump flow rates (Dial position 2, 4, 6, 8, 10 - i.e. increasing freshwater flow). Flow depth in channel = 90.5 cm (LEGI control screen).

ESTUARY1 - 10/07/2014

Trial forced exchange flow runs conducted with counterflowing dense water intrusion and fresh water surface outflow. Density and ADV profiles taken with freshwater flow dial settings of 6 and 10 for saltwater inflow Q1 = 14 ms/hr. Similar measurements made at freshwater flow dial setting of 10 and Q1 = 3 m3/hr. Density difference $\Delta\rho$ = 5.3 $kg/m3$ in both cases. No PIV measurements taken? Flow depth in channel = 90.5 cm (at start of run).

ESTUARY2 - 15/07/2014

Four experimantal runs (ESTUARY2a, 2b, 2c and 2d) were conducted at the higher salt water flow rate Q1 = 25 m3/hr and variable freshwater flow conditions (dial settings 2, 4, 6, 8). Density difference $\Delta\rho$ = 5.1 $kg/m3$. Flow depth in channel = 93 cm (at start of run) and 94.9 cm (at end of run). Flow seeded for PIV measurements.

Processing camera: Dalsa 1 and 2 cameras were repositioned for estuarine experiments to capture more of the dynamics across the sill obstruction. Calibration of the camera fields of view were conducted - see 'ESTUARY2_CALIB'. Freshwater pump was then recalibrated for a flow depth of 95 cm.

ESTUARY3 - 16/07/2014

Six experimantal runs (ESTUARY3a - 3f) were conducted at the higher salt water flow rate Q1 = 25 m3/hr and variable freshwater flow conditions (dial settings 2, 3, 4, 6, 8 & 10). Density difference $\Delta\rho$ = 9.6 $kg/m3$. PIV and probe measurements taken over 100 s and 190 s, respectively, for each salt-fresh water flow conbination. ADV measurements were made continuously during the run. An additional set of PIV/probe data was obtained at the end of the experiment for freshwater flow dial setting 10 (ESTUARY3g). Flow depth in channel kept constant = 95 cm throughout experiment (selective removal of dense water from outer basin). Flow seeded for PIV measurements.

No sign of arrestment of salt water intrusion across obstacle during any runs.

ESTUARY4 - 16/07/2014

Seven experimantal runs (ESTUARY4a - 4g) were conducted at the higher salt water flow rate Q1 = 25 m3/hr and variable freshwater flow conditions (dial settings 2, 3, 4, 6, 8 & 10x2). Density difference $\Delta\rho$ = 9.6 $kg/m3$. PIV and probe measurements taken over 190 s and 100 s, respectively, for each salt-fresh water flow conbination. ADV measurements were made continuously during the run. Flow depth in channel kept constant = 85 cm throughout experiment (selective removal of dense water from outer basin).

Freshwater pumps were stopped and restarted prior to ESTUARY4a run due to excessive bubbles in flow.

No sign of arrestment of salt water intrusion across obstacle during any runs.

ESTUARY5 - 16/07/2014

Seven experimantal runs (ESTUARY5a - 5g) were conducted at a lower salt water flow rate Q1 = 9.5 m3/hr and variable freshwater flow conditions (dial settings 2, 3, 4, 6, 8 & 10x2). Density difference $\Delta\rho$ = 9.6 $kg/m3$. PIV and probe measurements taken over 190 s and 100 s, respectively, for each salt-fresh water flow conbination. ADV measurements were made continuously during the run. Flow depth in channel kept constant = 85 cm throughout experiment (selective removal of dense water from outer basin).

Runs ESTUARY5a - 5d are PIV measurements with seeded flow.

Runs ESTUARY5e & 5f are LIF (rhodamine used in dense water inflow) due to PIV particles running out.

No sign of arrestment of salt water intrusion across obstacle during any runs.

ESTUARY6 - 17/07/2014

PIV laser adjusted to take account of reduced water level.

Camera positions (Dalsa1 & 2) adjusted. New calibration images taken and stored in 'ESTUARY6_CALIB'

Seven experimantal runs (ESTUARY6a - 6g) were conducted at a lower salt water flow rate Q1 = 10.5 m3/hr and variable freshwater flow conditions (dial settings 2, 3, 4, 6, 8 & 10x2). Density difference $\Delta\rho$ = 4.7 $kg/m3$. PIV and probe measurements taken over 190 s and 100 s, respectively, for each salt-fresh water flow conbination. ADV measurements were made continuously during the run. Flow depth in channel kept constant = 85 cm throughout experiment (selective removal of dense water from outer basin).

Salt water intrusion across obstable was arrested duning run ESTUARY6f.

ESTUARY7 - 17/07/2014

Seven experimantal runs (ESTUARY7a - 7g) were conducted at a higher salt water flow rate Q1 = 25 m3/hr and variable freshwater flow conditions (dial settings 2, 3, 4, 6, 8 & 10x2). Density difference $\Delta\rho$ = 4.7 $kg/m3$. PIV and probe measurements taken over 190 s and 100 s, respectively, for each salt-fresh water flow conbination. ADV measurements were made continuously during the run. Flow depth in channel kept constant = 85 cm throughout experiment (selective removal of dense water from outer basin).

GULF1 - 18/07/2014

For this rotating exchange flow experiment, the Coriolis basin was spun up overnight for 16 hours with a rotation period of 60 s ($\Omega = 0.0167 s-1).

Six experimantal runs (GULF1a - 1f) were conducted at a lower salt water flow rate Q1 = 10 m3/hr and variable freshwater flow conditions (dial settings 0, 0, 0, 2, 3 & 0). Density difference $\Delta\rho$ = 4.7 $kg/m3$. PIV and probe measurements taken over 190 s and 100 s, respectively, for each salt-fresh water flow conbination. ADV measurements were made continuously during the run. Flow depth in channel kept constant = 85 cm throughout experiment (selective removal of dense water from outer basin).

Salt water intrusion across the obstruction sill arrested during run GULF1d.

GULF2 - 18/07/2014

Eight experimantal runs (GULF2a - 2g) were conducted at a lower salt water flow rate Q1 = 10 m3/hr and variable freshwater flow conditions (dial settings 0, 0, 2, 2, 4, 4, 4 & 4). Density difference $\Delta\rho$ = 4.7 $kg/m3$. PIV and probe measurements taken over 190 s and 100 s, respectively, for each salt-fresh water flow conbination. ADV measurements were made continuously during the run. Flow depth in channel kept constant = 85 cm throughout experiment (selective removal of dense water from outer basin).

Freshwater and saltwater pumps remained on from GULF1 run.

Acquisition frequency of Nikon camera set at 1 Hz.

Salt water intrusion across obstruction sill arrested during run GULF2g.



Appendix A: Preliminary Fjordic Calculations

The plan is to start with the fjordic configuration. Some preliminary calculations and commentary is given below to assess the range of parametric conditions possible under this configuration:

  • If we assume critical overflow conditions (i.e. internal hydraulic control, $F_1 = 1$) in the dense overflow at the sill crest:

$F_1$ = $u_1 / (g' h_1)^{1/2}^$ then $u_1 = (g' h_1)^{1/2}^$ (A)

  • where $g'$ is the reduced gravational acceleration; $h_1$ is the overflow layer thickness; $u_1$ is the average overflow velocity. With the volumetic flow rate $Q_1$ across the sill given by the expression:

$Q_1 = u_1 b h_1$ (B)

  • where $b$ is the channel width. Combining Eq. (A) and (B), we get:

$Q_1$ = $(g' h_1)^{1/2}^$ $b h_1$

$h_1^{3/2}$ = $(Q_1/b) g'^{-1/2}^$

$h_1 = (Q_1/b)^{2/3}^$ $g'^{-1/3}^$ (C)

  • The Reynolds number $Re$ for the overflow can also be estimated as follows:

$Re = U_1 h_1 /\nu$ (D)

  • where $\nu = 10^{-6}^$ $m^2^$$s^{-1}^$

Thus, for the range of experimental conditions we can potentially run:

$Q_1$ $(m^3^/s)$$\Delta\rho$ $(kg/m^3)$$g'$ $(m/s2)$$u_1$ $(m/s)$$h_1$ $(m)$$Re$
0.005200.1960.0870.0383333
0.005100.0980.0690.0483333
0.00550.0490.0550.0613333
0.01200.1960.1090.0616667
0.01100.0980.0870.0776667
0.0150.0490.0690.0976667
0.02200.1960.1380.09713333
0.02100.0980.1090.12213333
0.0250.0490.0870.15413333
0.04200.1960.1740.15426667
0.04100.0980.1380.19426667
0.0450.0490.1090.24426667

Another interesting calculation we did was to work out how quickly the upstream fjordic basin was likely to fill to obstruction crest level (i.e. z = 0.5 m) under a range of spill conditions. We have decided to make the fjordic basin slightly larger than the estuarine basin (i.e. offset the obstruction).

If we assume length of the fjordic bottom is $L_b = 3 m$, the total volume of the fjordic basin (up to the crest level) is given by:

$V$ = 3.5 x 0.5 x 1.5 = 2.625 $m^3^$ (i.e. accounting for upstream obstruction slope = 1/2)

Thus, the total fill time (assuming no mixing) for different $Q_1$ overflow spill rates can be estimated roughly from:

$t = V/Q_1$

Hence,

For $Q_1$ = 0.005 $m^3^$$s^{-1}^$: t = 2.625/0.005 = 525 s (8.75 min)

For $Q_1$ = 0.010 $m^3^$$s^{-1}^$: t = 2.625/0.010 = 263 s (4.4 min)

For $Q_1$ = 0.020 $m^3^$$s^{-1}^$: t = 2.625/0.020 = 131 s (2.2 min)

For $Q_1$ = 0.040 $m^3^$$s^{-1}^$: t = 2.625/0.040 = 66 s (1.1 min)


Thus, if we use a high $Q_1$ value, the basin fill time is so short that the spill condition set up for each run will not have sufficient time to reach a quasi-steady overflow condition before we stop it again for the next pycnocline set-up position within the fjordic basin. This means we would only ever consider transient overflow conditions rather than steady overflow conditions (is this a problem???). In addition, the required time for the overspill dynamics across the obstruction to adjust to this steady condition may be too long (relative to the basin filling time) for steady conditions ever be achieved.

The interfacial elevation within the fjordic basin may also adjust to quickly due to the dense water inflow to record the near-bed dynamics successfully. The field of view that we need to look at to resolve near-bed dynamics would need be about 20 cm x 20 cm. As it is difficult to adjust the camera position during an individual experimental run, we have to be certain that the initial experimental condition is adjusted appropriately to ensure we measure the near-bed dynamics we are looking for. In other words, there may be no real benefit in trying to look at a number of initial interface elevations within the fjordic basin during any particular experiment [especially as we cannot move the near-field camera view anyway].

COMMENT ON ABOVE FROM MAGDA:

I don't think just looking at transient dynamics is a big problem, I didn't expect to see anything too steady to be honest although I hadn't appreciated just how quickly we can fill and how quickly things will change. We may need to concentrate on the slower fill rates. If we start with ${Q_1}$ = 0.005 ${m^3^/s}$ we can then decide which way to iterate - we may in fact want to go down instead of up, is it possible to fill at ${Q_1}$ = 0.001 ${m^3^/s}$ for example?

AC RESPONSE - OK, lets start with ${Q_1}$ = 0.005 ${m^3^/s}$ and go from there.

I hear what you're saying about the changing interface position within a given experiment and the restricted field of view we are going to have. As a starting guess I'd suggest we start with an interface position of approximately 20cm (or even higher, having a low interface height can be questioned physically). I guess then the most sensible thing to do would be to centre the near bed field of view on the pycnocline and then stop the experiment once the interface goes out of our field of view. T. If the dynamics are interesting higher than 30 cm at the bed then may be we could do a repeat run with the camera repositioned higher ? Or stop $Q_1$, relocate the camera and start again for the higher interface position ?

AC RESPONSE - Agreed, lets start with an initial interface position within the fjordic basin of 30 cm (this seems to be a more realistic configuration). The near-bed camera image field (20 cm x 20 cm) can be centred at this point.

Appendix B - Presentation at Hydralab Joint Users Meeting, Lisbon 2 - 4 July 2014

Attachments