FlowTracker Bias Caused by Flow

Disturbance

FlowTracker Bias Caused by Flow

DisturbanceDavid S. Mueller

Office of Surface Water

U.S. Geological Survey

The use of trade or brand names does not imply endorsement by the USGS

OverviewOverview FlowTracker sample volume Potential problem USGS tow tank tests USGS numerical model simulations Comparison of simulation to lab results Effect of flow angles Effect of hydrographer Proposed solution Conclusions Best practices

FlowTracker – How it WorksFlowTracker – How it Works

2-D most common the field Transmit transducer in center Receive transducers on arms Provides a point (small sample

volume) velocity Sample volume 0.7 cm long

and 0.7 cm in diameter centered at 11.3 cm from center transducer

Attached to wading rod.

Flow here mustbe undisturbed.

11.3 cm

Potential ProblemPotential Problem Tow-tank tests in Switzerland (BAFU) and at

SonTek’s facility in San Diego showed a bias caused by flow disturbance

The tow tank mounting bracket can have a significant effect on the flow in the sample volume

Using the probe only with a calibration or streamlined mount, verified SonTek’s calibration

Using a wading rod with Offset Bracket produced a negative bias

Office of Surface Water Action

Office of Surface Water Action

Completed tests using tow tank and jet tank at HIF Evaluated effect of flow angle Evaluate bias in flowing water

Initiated simulations using Flow-3D Compared numerical

simulations with tow-tank Evaluate difference between

tow-tank results and flowing water

HIF Tow Tank – Aligned Flow

HIF Tow Tank – Aligned Flow

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500-10.000

-8.000

-6.000

-4.000

-2.000

0.000

2.000

HIF Mount

J-Bracket

Flat S Bracket

Round S-Bracket

Cart Speed ft/s

Per

cen

t E

rro

r

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500-10.000

-9.000

-8.000

-7.000

-6.000

-5.000

-4.000

-3.000

-2.000

-1.000

0.000

J-Bracket

Flat S Bracket

Round S-Bracket

Cart Speed ft/s

Ad

just

ed P

erce

nt

Err

or

Comparison to Cart Speed Difference from HIF Mount

J-Bracket Flat S Round S

Mean > 0.5 fps -1.560 -1.596 -1.662HIF mount is used in the tow tank testing only and should minimize flow disturbance.

*

*

Numerical ModelNumerical Model Assume:

Uniform velocity distribution Infinite domain

Sensitivity simulations: Domain size to remove effect of boundaries Cell size Number of blocks Turbulence length parameter (TLEN)

Evaluation criteria Percent difference from ambient velocity along

centerline of center transducer at 11.3 cm

Flowing-Water ModelFlowing-Water Model Two nested blocks Inner block

X (cross stream): -20 x 15 cm Y (streamwise): -15 x 20 cm Z (vertical): -6 x 12 cm Cell size: 0.25 cm

Outer block X (cross stream): -100 x 100 cm Y (streamwise): -30 x 40 cm Z (vertical): -20 x 30 cm Cell size: 1 cm

FlowTracker STL from manufacturer Center of center transducer (0, 0, 0)

Water surface Initial simulations with fixed surface Later simulations with free surface

Turbulence TLEN=0.2

Data extracted Y: +/- 0.375 cm Z: +/- 0.375 cm X: 0 to 15 cm

Results Didn’t MatchResults Didn’t Match

Tow tank tests showed a 1-1.5% negative bias

Flowing water simulations showed a positive bias of about 0.5%

If the model does match verification data ALWAYS make sure you are modeling the same conditions!!!

Tow Tank ModelTow Tank Model Two nested blocks Inner block

X (cross stream): -20 x 15 cm Y (streamwise): -180 x 20 cm Z (vertical): -6 x 12 cm Cell size: 0.25 cm

Outer block X (cross stream): -100 x 100 cm Y (streamwise): -200 x 40 cm Z (vertical): -20 x 30 cm Cell size: 1 cm

FlowTracker STL from manufacturer Center of center transducer (0, 0, 0) GMO model used to move at fixed velocity

Water surface Initial simulations with fixed surface Later simulations with free surface

Data extracted Y: f(t) Z: +/- 0.375 cm X: 0 to 15 cm

FlowTracker CalibrationFlowTracker Calibration Factory calibrated in small tow tank at a

speed of 18 cm/s with a custom mount Calibration for flow disturbance evaluation:

In USGS tow tank at desired speed Simple mount directly to round rod Used to compare with wading rod and offset

HIF Calibration Mount

Wading Rod with J-Bracket

2D Probe Tow Tank Simulations

Cart Speed: 30 cm/s

2D Probe Tow Tank Simulations

Cart Speed: 30 cm/sFree Surface Fixed Surface

Mount HIF * Free Surf. Fixed Surf

HIF Mount 0.08 1.09 0.76

J-Bracket -1.30 -0.41 -0.48

Difference -1.38 -1.50 -1.24

7 8 9 10 11 12 13 14-0.7

-0.5

-0.3

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

X: 11.38Y: 1.093

Range from transmitting transducer (cm)

Per

cen

t d

evia

tio

n f

rom

am

bie

nt

velo

city

X: 11.38Y: -0.4115

HIF-Mount

J-Bracket

7 8 9 10 11 12 13 14-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Range from transmitting transducer (cm)P

erce

nt

dev

iati

on

fro

m a

mb

ien

t ve

loci

ty

X: 11.38Y: -0.4818

X: 11.38Y: 0.7572

HIF-Mount

J-Bracket

*FlowTracker calibrated by manufacturer

Simulation ComparisonsSimulation ComparisonsTow Tank

HIF Calibration MountTow Tank

Wading Rod and J-BracketFlowing Water with

Wading Rod and J-Bracket

1.0

0.67

0.33

0.0

-1.0

-0.67

-0.33

% Deviation

1.0

0.67

0.33

0.0

-1.0

-0.67

-0.33

% Deviation

1.0

0.67

0.33

0.0

-1.0

-0.67

-0.33

% Deviation

Jet Tank (Aligned)Jet Tank (Aligned)

0 2 4 6 8-4

-3

-2

-1

0

1

2

3

Actual Water Speed (ft/s)

Per

cen

t D

evia

tio

nRotation: 0

Error bars represent 2*StdDev

0.8 1.3

Simulated Error for 2D Probe

with Flowing Water (30 cm/s)

Simulated Error for 2D Probe

with Flowing Water (30 cm/s)Free Surface Fixed Surface

Mount HIF Jet~ 1 ft/s

Free Surf. Fixed Surf

HIF -- 1.09 0.76

J-Bracket -0.75* (-0.69) 0.47 0.43

Difference -0.75 (-0.69) -0.62 -0.33

7 8 9 10 11 12 13 14-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Range from transmitting transducer (cm)P

erce

nt

dev

iati

on

fro

m a

mb

ien

t v

elo

city

X: 11.38Y: 0.7572

X: 11.38Y: 0.4312

X: 11.38Y: -0.4818

HIF Mount TTJ-Bracket TTJ-Bracket Flowing

7 8 9 10 11 12 13 14-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Per

cen

t d

evia

tio

n f

rom

am

bie

nt

velo

city

Range from transmitting transducer (cm)

X: 11.38Y: 1.093

X: 11.38Y: 0.4673

X: 11.38Y: -0.4115

HIF Mount TTJ-Bracket FlowingJ-Bracket TT

*Standard deviation about mean: 0.23

Tow Tank EffectTow Tank Effect

Still Water Moving Instrument

Moving Water Still Instrument

Mount HIF * Free Surf. Fixed Surf

HIF Mount 0.08 1.09 0.76

J-Bracket -1.30 -0.41 -0.48

Difference -1.38 -1.50 -1.24

*FlowTracker calibrated by manufacturer

Mount HIF Jet~ 1 ft/s

Free Surf. Fixed Surf

HIF -- 1.09 0.76

J-Bracket -0.75 (-0.69) 0.47 0.43

Difference -0.75 (-0.69) -0.62 -0.33

Comparison of 2D and 2D/3D Probes

Comparison of 2D and 2D/3D Probes

7 8 9 10 11 12 13 140.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

Range from transmitting transducer (cm)

Per

cent

dev

iatio

n fr

om a

mbi

ent

velo

city

Flowing water tests of 2D and 2D/3D probes with free surface(TLEN=0.2)

30 cm/s 2D

50 cm/s 2D30 cm/s 2D/3D

50 cm/s 2D/3D

0 10 20 30 40 50 60 70-0.7

-0.69

-0.68

-0.67

-0.66

-0.65

-0.64

-0.63

-0.62

-0.61

-0.6

Actual Water Speed (ft/s)

Per

cen

t D

evia

tio

n

Rotation: 0

SimulationsAligned, Varying Velocity

SimulationsAligned, Varying Velocity

Actual Water Speed (cm/s)

-20 -10 0 10 20-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

Rotation (degrees)

Per

cen

t D

evia

tio

n

10 cm/s30 cm/s60 cm/s

SimulationsVarying Angle, Varying

Velocity

SimulationsVarying Angle, Varying

Velocity

Results adjusted for 1.09% tow tank calibration

61.2

60.6

60.0

59.4

58.8

15.08.01.0-6.0-13.0-20.0

-20.0

-15.0

Y-V

EL

OC

ITY

(cm

/s)

61.2

60.6

60.0

59.4

58.8

15.08.01.0-6.0-13.0-20.0

-20.0

-15.0

Y-V

EL

OC

ITY

(cm

/s)

Approximate Sample Volume

Y-Velocity ContoursY-Velocity Contours

-20 degrees +20 degrees

Comparison to Tow TankComparison to Tow Tank

-20 -10 0 10 20-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Rotation (degrees)

Per

cen

t D

evia

tio

n

10 cm/s Simulation30 cm/s Simulation60 cm/s Simulation9 cm/s Tow Tank30 cm/s Tow Tank60 cm/s Tow Tank

Legs ModelLegs Model Three nested blocks Inner block

X (cross stream): -20 x 15 cm Y (streamwise): -15 x 20 cm Z (vertical): -6 x 12 cm Cell size: 0.25 cm

Second block X (cross stream): -60 x 80 cm Y (streamwise): -30 x 65 cm Z (vertical): -15 x 20 cm Cell size: 0.6 cm

Outer block X (cross stream): -100 x 170 cm Y (streamwise): -40 x 100 cm Z (vertical): -20 x 30 cm Cell size: 1 cm

FlowTracker STL from manufacturer Center of center transducer (0, 0, 0)

Legs Two 15 cm diameter cylinders Aligned with flow 46 cm to side of rod 10 cm downstream from front o rod

Water surface Later simulations with free surface

Data extracted Y: +/- 0.375 cm Z: +/- 0.375 cm X: 0 to 15 cm

Leg positions based on recommended hydrographer location from Rantz and others (1982) and Pierce (1941)

Extended Simulation TimeExtended Simulation Time

0 10 20 30 40 50 60 7030

30.1

30.2

30.3

30.4

30.5

30.6

30.7

30.8

Simulation Time (sec)

Y-V

elo

city

(cm

/s)

7 8 9 10 11 12 13 141.7

1.8

1.9

2

2.1

2.2

2.3

2.4

Range from transmitting transducer (cm)

Per

cen

t d

evi

atio

n f

rom

am

bie

nt

velo

city

14 sec21 sec28 sec35 sec42 sec49 sec56 sec

Effect of Hydrographer (30 cm/s)

Effect of Hydrographer (30 cm/s)

7 8 9 10 11 12 13 140

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

X: 11.38Y: 1.818

Range from transmitting transducer (cm)

Per

cen

t d

evia

tio

n f

rom

am

bie

nt

velo

city

X: 11.38Y: 1.093

X: 11.38Y: 0.4673

HIF-Mount TTJ-Bracket FlowingJ-Bracket + Legs Flowing

Mount % Dev %Dev

HIF 1.09 1.09

J-Bracket 0.47

Hydrographer 1.82

Difference -0.62 0.73

Conclusions from Model Results

Conclusions from Model Results

Flow-3D proved to be a valuable tool for investigating this problem

The FlowTracker disturbs the flow in its sample volume The orientation, type of probe, and type of mount all affect the

magnitude of the flow disturbance Because the FlowTracker disturbs its sample volume tow tank

calibrations/tests do not exactly represent turbulent flowing water conditions.

Within +/- 10 deg the results are within 1% (manufacturer spec). The position of the hydrographer in the stream has a

significant effect on the flow disturbance.

SonTek’s SolutionSonTek’s Solution Observed bias is due to flow disturbance

caused by mount and/or wading rod Average observed bias is 1.2% for probes

aligned with flow Firmware upgrade will provide option to

apply a user specified correction to measured velocities, if wading rod is selected as deployment method.

Correction can be applied or removed in both the handheld and post processing software.

Correction does not vary with flow angle.

Best PracticesBest Practices

Hydrographer should take care to position themselves to minimize flow disturbance while collecting data.

Select sections with aligned flow to minimize angles Rehmel 2007 showed average difference of 0.1% for

55 field comparisons. Currently no correction to the FlowTracker

measurements are recommended