14th International Symposium on Particle Image Velocimetry – ISPIV 2021
August 1–5, 2021

The stereo-PIV investigation of the unsteady flow in the
draft tube of a model hydro turbine

I. Litvinov1,2∗, D. Sharaborin1,2, S. Shtork1,2, V. Dulin1,2,
S. Alekseenko1,2, and Kilian Oberleithner 3

1 Kutateladze Institute of Thermophysics, Novosibirsk, Russia
2 Novosibirsk State University, Novosibirsk, Russia

3 Technische Universität Berlin, Berlin, Germany
∗ litvinov@itp.nsc.ru

Abstract
Varying the generator load of a hydro turbine results in short-term changes in the rotation frequency of the
runner, leading inevitably to flow instability and strong flow swirling behind the turbine. This may lead to
the formation of unsteady flow regimes featured by vortex instability of the swirling flow behind the runner,
known as the precessing vortex core (PVC) Dörfler et al. (2012). This effect causes dangerous periodic
pressure pulsations that propagate throughout the water column in the draft tube. The present study reports
on stereo PIV measurements of the air flow field inside a transparent draft tube of a model hydro turbine for
a wide range of operation conditions. The research is focused on the time-averaged flow properties (mean
velocity field and the second-order moments of velocity fluctuations), pressure pulsations and coherent flow
structures in the velocity field.

A detailed description of the aerodynamic test rig, used in these experiments, is presented in Litvinov
et al. (2018). The test section consists of a scaled-down geometric model of the Francis-99 draft tube
Cervantes et al. (2015) with an inlet diameter D = 100 mm. The air flow inside the model is investigated.
To generate the required flow distribution at the inlet to the cone, a pair of swirlers is used: a stationary
swirler, acting as guide vanes, and a rotary swirler, representing an analogue of the turbine runner Litvinov
et al. (2018). The pair of swirlers is designed for the optimal operating conditions (the best efficiency point
– BEP) corresponding to a volumetric flow rate Qc = 48.5 l/s and a runner rotation speed nc = 40.5 Hz. The
experimental setup includes a computer for controlling the airflow and rotation frequency of the rotor with
an uncertainty of 1.5% and 0.5%, respectively. The flow is studied for the operating parameters of the flow
rate in the range from 0.5Qc to Qc to replicate the off-design and the BEP operation conditions of the hydro
turbine.

Figure 1 shows the used PIV equipment and a sketch of the test section. A Laskin nozzle aerosol
generator was used to seed the jet flow with seed oil droplets. A double-head Nd:YAG laser (Quantel,
EverGreen) illuminated the tracer particles. The laser beam was converted into a laser sheet with a thickness
of less than 1 mm by using a system of cylindrical and spherical lenses. On average, the pulses energy
was 70 mJ before the laser sheet optics (measured by a power meter Coherent LabMax). Particles images
were captured by a pair of CCD cameras (Bobcat ImperX). The PIV images (4904x3280 pixels is size) were
processed by using an in-house software ActualFlow. Time separation between two PIV laser pulses was
30µs. The images were preprocessed to remove background (minimal intensity for each pixel). During four
iterations of an adaptive cross-correlation algorithm the interrogation area size was reduced from 64×64 to
16×16 pixels. The spatial overlap rate between the interrogation areas was 50%, and the resulting spatial
resolution was 0.5 mm

Four Behringer ECM8000 microphones were used to record the pressure pulsations on the draft tube
walls in the cross-section A-A as shown in Figure 1. Microphone signals were digitized by an ADC and
amplified using Behringer preamplifiers. The PVC frequency was identified via time-resolved pressure
signals at a sampling rate of 20 kHz. The microphone signals were decomposed into azimuthal spatial and
time Fourier domains. Figure 2 shows an example of the power spectral density of the asynchronous part of
the pressure pulsations pas,i(t) = pi(t)−(1/4)∑

4
i=1 pi(t) for the cases of flow rates 0.5Qc (off-design) and Qc

(BEP regime). As shown, the most energetic peak in the spectra occurs for the case of 0.5Qc. This dominant



#4

#3

#1

#2

microphones

cone

PIV

domain
A-A

swirler

system

z

cross-section A-A

connection with

the draft

tube elbow

x

1 10 100 1000

0.0

5.0x10 -4

1.0x10 -3

1.5x10 -3

2.0x10 -3

3 fPVC

2 fPVC

PS
D

 [a
rb

. u
n.

]

f  [Hz]

 0.5Qc

 Qc

fPVC

Figure 1: 2D sketch of the test section (left) and photo of the PIV
configuration (right)

Figure 2: Spectra of the asynchronous
part of the pressure pulsation of four
acoustic sensors (the vertical axis
in relative units) for a set of flow
rates 0.5Qc (off-design) and Qc (BEP
regime)

peak is considered to be associated with the PVC. Figure 3 shows the mean velocity field for the flow rates
0.5Qc (off-design) and Qc (BEP regime). For the regime with 0.5Qc, when the flow dynamics is related to
strong pulsations due to the PVC, the time-averaged velocity field is characterized by a compact recirculation
zone with the length of 0.2D. For the BEP regime with Qc is characterized by a smaller recirculation region
(0.1D), which is associated with the flow past the centrebody. It is noteworthy that this flow is featured
by the flow core rotation in the opposite direction to the sense of the main flow swirl. Further analysis is
focused on the phase-averaged velocity fields and on the results of proper orthogonal decomposition (POD).

Figure 3: Mean velocity fields for the 0.5Qc case, the off-design condition with PVC (left) and the BEP
regime, Qc (right), (U0 is a bulk velocity, a solid line indicates recirculation region border)

Acknowledgements
The research is supported by Russian Foundation for Basic Research (Grant no. 20-58-12012). I. Litvinov
acknowledges support of the grant of the President of the Russian Federation (project no. MK-1504.2021.4,
in the part of pressure signal analysis). The funding of the German Research Foundation (grant no. 429772199)
is acknowledged.

References
Cervantes M, Trivedi C, Dahlhaug OG, and Nielsen T (2015) Francis-99 workshop 1: steady operation of

francis turbines. in J. Phys.: Conf. Ser. volume 579. page 011001

Dörfler P, Sick M, and Coutu A (2012) Flow-induced pulsation and vibration in hydroelectric machinery:
engineer’s guidebook for planning, design and troubleshooting. Springer Science & Business Media

Litvinov I, Shtork S, Gorelikov E, Mitryakov A, and Hanjalic K (2018) Unsteady regimes and pressure pul-
sations in draft tube of a model hydro turbine in a range of off-design conditions. Experimental Thermal
and Fluid Science 91:410–422