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