Engineering, 2013, 5, 967-974
Published Online December 2013 (http://www.scirp.org/journal/eng)
Open Access ENG
Experimental and Numerical Evaluation and Optimization
of a Non Standard Pitot/Sampling Probe
Andrea Shmueli1, Tor Erling Unander2, Ole Jørgen Nydal1
1Norwegian University of Science and Technology, Department of Energy and Process Engineering, Trondheim, Norway
2Department of Petroleum Research, SINTEF, Trondheim, Norway
Received September 4, 2013; revised October 4, 2013; accepted October 11, 2013
Copyright © 2013 Andrea Shmueli et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
An isokinetic sampling probe is designed and constructed to measure entrained liquid droplet fluxes in separated gas-
liquid pipe flows. This probe also has the capability of working as a non-standard Pitot tube when the sampling is
stopped. CFD simulations using the commercial software Ansys CFX were carried out for single phase gas flow to
analyze the non-standard design. Pitot tube velocity calculations and isokinetic sampling conditions were studied. The
predicted results were compared against theoretical velocity profiles from the literature and with gas single phase ex-
perimental data acquired in a horizontal 49 m long steel pipeline with an internal diameter of 69 mm. The experiments
were done by using a dense gas (SF6) at 7 bara. An asymmetry of the experimental velocity profiles reproduced with
the numerical simulations. The CFD simulations made it possible to verify the design and predict and correct an instal-
Keywords: Isokinetic Probe; Pitot Tube; CFD; Flow; Installation Effects
Pitot probes are devices which are commonly used in the
industry to measure the local velocity in gases flowing in
pipes or ducts. The dynamic pressure
the fluid stream is measured by the Pitot probe and the
velocity can be calculated from it. When the gas com-
pressibility effect can be neglected, the local velocity can
be calculated from:
where C is a calibration constant.
The Pitot tubes have been used for measuring the gas
dynamic pressure in gas-liquid flows [1-4]. However, the
calculation of the velocity from the pressure values is not
straightforward and will depend upon the flow regime
. In gas flow with liquid droplet entrainment, it is a
common practice to use isokinetic sampling probes to
extract flow from the main stream in order to get the lo-
cal dispersed droplet flux. These probes are designed to
have a Pitot-like geometry and pressure tapings and can
be designed to measure the local velocity when the sam-
pling is stopped. The probes are generally not standard so
their design must be tested and validated. Two aspects
that should be taken into account when designing a new
Pitot probe are the velocity measured by the Pitot probe
in an ideal flow and the effect of the probe presence on
the upstream flow . Due to the complexity of multi-
phase flows, the probe design and installation are evalu-
ated by using single phase gas conditions.
In this paper, a design and installation assessment of
the non standard Pitot/sampling probe is done by the
analysis of single phase gas experimental data and simu-
lation of the probe current design using CFD tools.
2. Probe Design
The sampling/Pitot probe is designed to be able to sam-
ple a liquid flux. Some authors have demonstrated that
the effect of the sampling tube diameter and length of the
probe on the measured droplet flux is negligible [7,8].
However in this study the criteria used to select the probe
diameter was associated with the maximum possible
droplet size. Two correlations were used: one for gas/
liquid systems and another one for liquid/liquid systems.
Kocamustafaogullari et al. (1994) [9 ] presented a corre-
lation for the maximum droplet diameter in annular flows.
A. SHMUELI ET AL.
The correlati on is a function of the fluid properties and
of the local energy dissipated by the turbulence.
The viscosity number is defined by,
Kubie and Gardner (1997)  developed a correla-
tion for maximum droplet size in liquid/liquid systems.
The probe diameter should allow for measurements in
oil-water-gas flow systems. The probe has a 4 mm inner
diameter, a 0.2 mm wall thickness. It is possible to
measure within 4.2 mm of the pipe wall. To avoid dis-
turbing the flow, the opening of the sampling probe ex-
tends 50 mm upstream (≈11.4dp). The dynamic pressure
is read at 3.75dp and the static pressure sensed at the pipe
wall on the probe stem plane. Two hoses were connected
to transmit the dynamic pressure from the holes to the
differential pressure transducer. The probe can traverse
the vertical pipe diameter using a linear actuator and has
the ability to work as a sampling and Pitot probe when a
manual ball valve connected to the sampling line is
closed. The non standard Pitot probe used in this study is
schematically shown in Figure 1.
3.1. Experimental Setup
The experiments were carried out in the medium-scale
flow loop at SINTEF Multiphase Flow laboratory in
Trondheim, Norway. The facility consists of a horizontal
69 mm inner diameter, 50 m long pipeline. The probe
test section is located about 580D downstream the mix-
3.2. Experimental Measurements
Pitot measurements on the vertical diameter were carried
out using single gas phase SF6 (Sulphur hexafluoride) at
7 bara and 20˚C. Three experimental gas velocities were
tested 4, 6 and 8 m/s. The measured gas density and dy-
namic viscosity from the experiments are ρg = 41.91
g = 1.5e ‒ 05 Pas respectively.
The measurements are compared against two theoreti-
cal velocity profiles. Following  the velocity distri-
bution in the main body of flow can be written as shown
in Equation (2), where α is a power law constant that in
this case is 0.111 as is proposed by .
= y/R is the normalized distance from the wall to
the pipe center. The second profile is obtained by fol-
lowing the modified log-wake model  for turbulent
Figure 1. Pitot probe geometry.
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A. SHMUELI ET AL. 969
The local gas velocity measurements and the theoreti-
cal profiles are normalized by their maximum value and
shown in Figure 2. The experimental velocity profiles
for all the tests are not symmetric and a systematic error
is shown for all of them on the upper half section of the
pipe. However, there is a good fitting between the ex-
perimental velocity profile and the theoretical one on the
bottom part on the pipe.
4. CFD Model Details
3D CFD simulations of the flow around a non standard
Pitot probe were developed using the commercial soft-
ware ANSYS-CFX© (V-13) which employs the finite
volume method for solving the conservation equations.
4.1. Cases under Study
The simulations were carried out using single gas phase
SF6 (Sulphur hexafluoride) at 7 bara, 20˚C and 4 m/s.
The calculated Mach number for the tested conditions is
lower than 0.3 and thereby the fluid is considered as in-
compressible on the simulations . The goal of the
model is to simulate the flow around the designed non
standard Pitot probe in order to find the origin of the ex-
perimental velocity profile asymmetry, predicting a pos-
sible installation effect on the gas velocity calculation
and afterwards improving the current design. Two loca-
tions of the Pitot probe above the pipe center were nu-
merically studied (See Table 1).
4.2. CFD Model General Settings
The turbulence model was a homogeneous K-ε model.
All the simulations were considered as steady state con-
Figure 2. Experimental and theoretical normalized gas ve-
locity profiles for 4, 6 and 8 m/s.
Table 1. Simulated cases.
Case Location of the probe opening from the bottom of the pipe [mm]
ditions. For all the simulations, the convergence was
reached when the maximum and RMS residual error for
any parameter was reduced to less than 4e−04 and
4.3. Boundary Conditions and Simulation
The simulated domain consists of the probe and a section
of the pipe (3.8 m upstream of the probe and 314 mm
downstream of it). The hoses used for the total pressure
sensing were not included on the model. The imposed
boundary conditions were total pressure at the inlet (Up-
stream Boundary) and uniform mass flow at the outlet
boundary condition (Downstream Boundary). All the
walls were treated as no-slip walls.
The fluid domain was meshed using the commercial
Workbench CFX© Mesh Module. The created meshes
were non-structured formed by tetrahedral, wedge and py-
ramid elements. A grid dependence procedure was car-
ried out in order to select the right mesh (see Table 2).
Four parameters were compared in order to select the
right mesh for the simulations: The pressure loss on the
pipe segment, maximum Y+ value, the gas velocity on
the probe opening calculated from Equation (1) using the
static pressure value at the current location Ps-1 (See
Figure 3 and 4).
Figure 3. Location of the specific calculated parameters in
Figure 4. Mesh dependency study. Variable: velocity at
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A. SHMUELI ET AL.
The velocity profile on a line is located on the probe
opening Ps-2, (See Figure 5). A percent error lower than
3% between each mesh and the finest one of every
physical variable compared was used as selection criteria.
There are some qualitative differences between all the
meshes especially close to the probe tip. The biggest dif-
ference between Mesh 1 and Mesh 4 is 7%. The selected
mesh for the simulations was Mesh 3.
5.1. Pitot Geometry Effect
One possible cause of the asymmetry in velocity profiles
around the pipe centerline is a blockage effect of the
stem in the flow upstream the probe tip. For this reason,
the vertical velocity profiles in different locations down-
stream and upstream the probe tip are plotted in Figures
6 and 7. The profiles from the pipeline inlet to the probe
location are compared (Figure 6) showing symmetry
with respect to the pipe axis. The flow in the pipe is de-
veloped before 20D. There are no upstream distur-
bances on the profiles due to the presence of the Pitot.
However, the probe stem has a blockage effect on the
profiles downstream the tip of the probe, mainly due to
the reduction of the flow area (Figure 7). This behavior
Figure 5. Mesh dependency study variable: velocity profile
on a line located at the probe opening.
Table 2. Generated meshe s for case 1.
Mesh 1 2 3 4
Max Y+ 13.27 13.52 13.56 13.55
number 3 - 46 3 - 46 2 - 44 2 - 50
Element vol ratio 1 - 52 1 - 48 1 - 93 1 - 82
Min face angle 18 - 84 17 - 86 16 - 86 12 - 87
Max face angle 65 - 130 66 - 136 66 - 131 64 - 134
Edge length ratio 1 - 43 1 - 43 1 - 16 1 - 16
Elements 270,328 334,852 1,105,596 1,756,149
Nodes 92,570 111,118 383,564 600,838
Figure 6. Velocity profiles from the pipe inlet. Case 1.
Figure 7. Velocity profiles downstream the probe tip. Case
was expected as the ratio of the Pitot tube diameter to the
pipe diameter does exceed 0.02 .
5.2. Pitot Vertical Location
Two cases were studied as presented in Table 1. The
vertical location of the probe has a qualitative and quan-
titative effect on the static pressure distribution around
the Pitot. The closer the Pitot is to the upper side of the
pipe the lower is the pressure on the probe plane (See
Figure 8). Near the top of the pipe the probe disturbs the
flow and accelerates it creating a low pressure area on
the top of the probe caused by the area reduction between
the stem of the probe and the pipe.
The velocity at the probe opening (Table 3) is calcu-
lated using Equation (1) with the total pressure value at
the probe tip and the static pressure values at Ps-1, Ps.2
and Ps-3 (See Figure 3). The percentage errors in the
velocity calculation using Ps-1 or Ps-2 are 10% and 17
for Cases 1 and 2 respectively. The percentage errors
when sensing the static pressure in front the Pitot tip
(Ps-3) are 0.5% and 0.7% for Cases 1 and 2 respectively.
A comparison of the calculated velocity values and the
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A. SHMUELI ET AL. 971
Figure 8. Static pressure contour case 2.
Table 3. Velocity at the probe opening calculated with total
pressure and static pressur e at point 1 and 2.
Velocity at the
Ps-1 699,344 - 4.92
Ps-2 699,428 - 4.49
Ps-3 699,434 4.46
Ps-1 699,003 - 4.89
Ps-2 699,151 - 4.10
Ps-3 699,156 4.08
Pt - 699,504 -
experimental one is made in Figure 9. Using the static
pressure at the probe tip plane to calculate the local ve-
locity shows a better fit with the theoretical profiles.
5.3. Pitot Probe Sampling under an Isokinetic
The main goal of this simulation was to obtain the pres-
sure loss inside the probe working under isokinetic con-
dition. It is important to establish the pressure loss be-
tween the probe tip and the dynamic pressure ports inside
it. For this simulation just Case 2 was analyzed.
The simulation and mesh selection was done following
the steps explained on section 4. The flow conditions and
fluid are the same as in the previous simulations. The
imposed boundary conditions were total pressure at the
inlet (Upstream Boundary), uniform mass flow at the
outlet of the probe (calculated from the isokinetic condi-
tion) and uniform mass flow at the outlet boundary con-
dition (Downstream Boundary). All the walls were treat-
ed as no-slip walls. The k-ε turbulence model was used
on the simulations.
The streamlines approaching the probe opening and
inside it are plotted in Figure 10. All the streamlines are
undisturbed as the probe is not present and the velocity
of the flow entering the probe is the same as in the main
body of the flow so the conditions for isokinetic sam-
pling are accomplished.
The total pressure is averaged on planes perpendicular
to the flow inside the probe (See Figure 11). The total
pressure holes are currently located on the position
marked by the dotted line. The pressure loss between the
probe opening and the total pressure holes is 33 Pa.
Figure 9. Comparison between experimental, theoretical
and CFD normalized gas velocity profile at 4 m/s.
Figure 10. Streamlines entering the probe.
Figure 11. Total pressure averaged on planes inside the
probe where location 1 is the probe opening.
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A. SHMUELI ET AL.
6. Experiments on the Upgraded Installation
New experiments at 4 m/s were developed using an up-
graded setup in which the static pressure tap was located
on the same plane of the probe tip. The velocity profiles
are symmetric in accordance to the pipe axial direction.
The experiments are compared against the CFD profile
showing an agreement and improvement in comparison
with the previous setup (See Figure 12).
7. Conclusions and Remarks
A non-standard Pitot/sampling probe was experimentally
tested and simulated using Ansys CFX (V13) to validate
its accuracy and to determine any design or installation
problem. The current experimental setup caused asym-
metries on the measured gas velocity profiles.
The experiments were conducted for 3 velocities while
the numerical simulations were carried out for one veloc-
ity. The obtained experimental asymmetries show a sys-
tematic behavior so the numerical results are extrapolated
to other flow conditions.
The CFD simulations were concentrated on two probe
vertical locations where the velocity profiles present the
asymmetry. The effect of the probe location on the verti-
cal pipe diameter, static pressure port location on the
pipe wall, probe stem location and dynamic pressure
ports location was studied. The location of the static
pressure tap at either the probe tip plane or upstream of it
will give more real and accurate velocity values.
As a result from the simulations, the static pressure tap
at the top of the pipe was relocated in order to have a real
and accurate reading of the static pressure. The probe
itself generates disturbances of the flow downstream of it
but not upstream of it.
A simulation of the isokinetic sampling probe was car-
ried out to obtain the pressure drop inside the probe. It
Figure 12. Comparison of the experiments carried out the
upgraded facility at 4 m/s with previous experiments and
was recommended for further probe designs to place the
dynamic pressure holes on the probe closer to the probe
tip in order to avoid the pressure losses and have a better
reading of the pressure.
Experiments with a new configuration with the static
pressure tap on the same plane as the probe tip were car-
ried out at 4 m/s showing an improvement on the meas-
urements with symmetric velocity profiles which agreed
with the theoretical predictions.
The financial and research support from Total E&P and
Sintef Petroleum Research are greatly appreciated.
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A. SHMUELI ET AL.
D Pipe diameter
C Pitot calibration constant ≈ 1
R Pipe radius
Umax Velocity at the pipe center
dmax Maximum droplet size
dh Hydraulic diameter
dp Pitot diameter
Cw Coefficient in Kocamustafaogullari correlation
y Distance from the wall
Normalized distance from the wall to the pipe center
Power law constant
Re Reynolds number
f Friction factor
Wem Modified Weber number
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