=Paper=
{{Paper
|id=Vol-2744/paper14
|storemode=property
|title=Image-based System for 3D Visualization of Flow in Hydrodynamic Tunnel
|pdfUrl=https://ceur-ws.org/Vol-2744/paper14.pdf
|volume=Vol-2744
|authors=Vladimir Knyaz,Michail Novikov,Vladimir Kniaz,Vladimir Mizginov,Eugeny Ippolitov
}}
==Image-based System for 3D Visualization of Flow in Hydrodynamic Tunnel==
https://ceur-ws.org/Vol-2744/paper14.pdf
Image-based System for 3D Visualization of Flow in
Hydrodynamic Tunnel ?
Vladimir Knyaz1,2[0000−0002−4466−244X] , Mikhail Novikov3[0000−0003−0626−793X] ,
Vladimir Kniaz1,2[0000−0003−2912−9986] , Vladimir Mizginov2[0000−0003−1885−3346] ,
and Eugeny Ippolitov3[0000−0002−0622−6727]
1
Moscow Institute of Physics and Technology (MIPT), Dolgoprudny, Russia
2
State Research Institute of Aviation System (GosNIIAS), 125319 Moscow, Russia
{knyaz,vl.kniaz,vl.mizginov}@gosniias.ru
http://www.mipt.ru
3
ILIT RAS – Branch of the FSRC “Crystallography and Photonics” of Russian Academy of
Sciences, Shatura, Russia
novikov@rambler.ru,ippevg@ya.ru
Abstract. Aircraft safety depends on wing flow process, so the study of air flow
in different flight conditions is one of the most important parts of aircraft design
and exploiting. The effective method of aerodynamic processes modeling is ex-
periment in wind (aerodynamic) tunnel or water (hydrodynamic) tunnel. They
allow to perform experiments with a scaled model of an aircraft and to visu-
alize the wing flow process. A visualization and video registration of the wing
flow yields useful qualitative information about flow, but it is more important to
retrieve quantitative 3D data of flow for 3D visualization and analysis. The pre-
sented study addresses to creating an image-based system for accurate 3D flow
acquisition for further diverse 3D visualization and quantitate evaluation of 3D
flow parameters in a hydrodynamic tunnel for aircraft icing influence exploration.
Being an initial part of a long-term research project, this study is aimed at de-
veloping stereolithography (SLA) modeling technique for flow visualization in
hydrodynamic tunnel and a photogrammetric system for accurate flow 3D cap-
tion. The results of first experiments of the system calibration and application are
given along with preliminary results of flow jets 3D reconstruction.
Keywords: Image-based 3D Measurements, Hydrodynamic Tunnel, Aircraft Ic-
ing, Multimedia Imaging, 3D Visualization.
1 Introduction
A comprehensive study of flow behavior is crucially important for aircraft performance
and safety. Nowadays a scaled modeling of flow in aerodynamic or hydrodynamic tun-
nel confirmed to be an effective tool for flow analysis.
Copyright © 2020 for this paper by its authors. Use permitted under Creative Commons Li-
cense Attribution 4.0 International (CC BY 4.0).
?
The reported study was funded by Russian Foundation for Basic Research (RFBR) according
to the research project 19-29-13040.
2 V. Knyaz et al.
Physical flow modeling in aerodynamic or hydrodynamic tunnel plays important
role in the process of an aircraft design due to possibility of controlled study of flow
behavior for various flight conditions, including critical attack angles or wing icing [1].
The basis for such modeling is the similitude concept. The adequateness of modeling
is provided by keeping the similarity of a model geometry and dynamic characteristics
for real and model processes. Similitude is widely applied in aerodynamic and hydro-
dynamic to study flow behavior using scaled models.
The similitude concept allows to use liquid instead of air when studying flow be-
havior. Such possibility arises from the key condition of similarity real and model flow
process expressed in the equivalence of main dimensionless similarity criteria such as
Reynolds number, Mach number, Prandtl number. Flow modeling in a hydrodynamic
tunnel has the important advantage of relatively slow velocity of flow, comparing with
aerodynamic case. This feature makes hydrodynamic modeling to be an attractive tool
for flow visualization and studying.
While well-established techniques for 2D flow registration and visualization based
on colored smoke or liquid exist, the accurate 3D flow registration and measurement
are more challenging tasks. The main challenges for the accurate 3D measurement by
photogrammetric techniques are imaging through optical interfaces and image-based
3D reconstruction of flow shape.
The paper presents the developed photogrammetric system for accurate flow 3D
registration and processing aimed at further 3D visualization of the flow.
2 Related work
Vision-based methods of registration, visualization, and analysis have proven to be an
effective tool for qualitative and quantitative flow characteristics study in aerodynamics
and hydrodynamics.
3D laser scanning is successfully applied for ice accretion geometry acquiring and
further applying the obtained results for disturbed flow investigation. A commercial
3D laser scanning system was used for recording ice accretion geometry in the NASA
Icing Research Tunnel [2] with the aim of the geometric assessment of the 3D laser
scanning system on a 2D (straight wing) and a 3D (swept wing) airfoil geometries. The
comparison of scanned ice accretion with castings of the same ice accretion has proved
the possibility of wing icing modeling.
The ice-accretion molds has been used to produce one set of artificial ice shapes
from polyurethane castings [3]. The laser-scan data has been utilized to fabricate an-
other set of artificial ice shapes, using rapid prototype manufacturing (stereolithogra-
phy). The iced-airfoil results with both sets of artificial ice shapes were compared to
estimate the aerodynamic simulation accuracy of the laser-scan data. For four of the
six ice-accretion cases, there was excellent agreement in the iced-airfoil aerodynamic
performance between the casting and laser-scan based simulations.
Mid-infrared (MIR) laser scanning demonstrated high measuring performance for
acquiring the 3D ice shape for various types of ice: clear ice, rime ice, mixed ice, or even
supercooled water droplets or films [4]. The advantage of these wave is the property
of mid-infrared radiation to penetrate ice and water only within a depth of less than 10
Image-based System for 3D Visualization of Flow in Hydrodynamic Tunnel 3
micrometers. Several sequential scans were applied to trace the 3D shape evolution of
the continuous ice accretion on an airfoil in an icing wind tunnel. The ice growth process
can be well observed in the results. The mid-infrared scan shows a good agreement with
the traditional visible laser scan on a plastic replication of the final ice shape made by
the mold and casting method.
To obtain accurate 3D geometric parameters of the flow in a hydrodynamic tun-
nel it is required to account for multimedia optical working space, including air, glass
and liquid (oil or water). Special models of imaging and calibration of an optical mea-
surement system provides reliable and accurate 3D data. Various approaches to optical
system calibration for measurement through multimedia optical interfaces can be found
in [5,6,7,8], including development of distance dependent distortion or application of
special ports for camera [9,10] to reduce refraction effects. A special technique for pho-
togrammetric system calibration directly considering imaging through two media inter-
faces [11] allows to perform accurate 3D measurements in a hydrodynamic tube. This
calibration technique provides not only qualitative but also quantitative characteristics
of a flow that are needed for icing process study.
The standard approach to densely reconstruct the flow motion utilizes high-contrast
particles for flow visualization. Then a set of synchronized high-speed cameras ac-
quire video sequences of flow motion for further processing. The acquired registra-
tions are processed in two separate steps, utilizing either Eulerian or Lagrangian ap-
proach [12,13,14,15]. Eulerian methods perform a voxel-based reconstruction of parti-
cles per time step, followed by 3D motion estimation, with some form of dense match-
ing between the precomputed voxel grids from different time steps [16,12]. Alterna-
tively, Lagrangian methods reconstruct an explicit, sparse set of particles and track the
individual particles over time. Physical constraints can only be incorporated in a post-
processing step when interpolating the particle tracks to a dense motion field [13].
Recently methods for object shape 3D reconstruction appears that incorporate deep
learning for multi-view or even single-view 3D shape reconstruction [17,18,19,20].
These methods look promising for further research in 3D flow analysis.
3 Hardware and algorithms
3.1 Hydrodynamic tunnel
The possibility of studying flow motion at low flow speed makes hydrodynamic tun-
nel (Figure 1) an attractive solution for complicated cases, when high velocity (when
modeling in aerodynamic tunnel) prevents for detailed consideration of a process.
The adequateness of such modeling to real air flow is provided by correspondence
of the similitude criteria such as Reynolds number R, indicating the ratio of inertial
forces to viscous forces within a fluid. Having equal Reynolds number liquid flow speed
is significantly lower than the speed of air flow for the same flow characteristics. So
hydrodynamic tunnel allows to work with very low flow velocity (flow velocity V =
2 − 10 cm/s) for the considered conditions.
An aircraft icing process and its influence on aircraft flying characteristic is studied
in a very low-speed hydrodynamic tunnel HDT-400 of the Central Aerohydrodynamic
4 V. Knyaz et al.
Fig. 1. Flow visualization in hydrodynamic tunnel
Institute (TsAGI). HDT-400 has a working part size of 400 × 400 mm2 . This hydro-
dynamic tunnel has been successfully applied for the analysis of unsteady localization
phenomena of “explosion” of vortices.
To visualize flow jets a specially designed model of a plane and colored liquid was
used. The aircraft model during the test in HDT-400 is shown in Figure 1. Special
channels inside the aircraft model provide coloring of flow jets for observation.
3.2 SLA-technique for flow visualization
To visualize flow jets in a hydrodynamic tunnel, it is necessary that the test model has
internal channels for the passage of paint, which will color the studied flow. One of
the most accurate and fast ways to obtain such a model is additive manufacturing using
laser stereolithography (SLA).
Fig. 2. SLA model for flow study in HDT-400
Image-based System for 3D Visualization of Flow in Hydrodynamic Tunnel 5
This technology allows to make thin sealed channels of any shape, without using
the support structure, and the remains of the photo-polymerized composition can be
removed by purging and washing, provided that the polymer has sufficient fluidity.
Another advantage of modeling in hydrodynamic tunnel is relatively low dynamic
loads on an aircraft model, so models produced by laser stereolithography technology
can be directly used for study. Stereolithography allows to produce very complicated
forms with high accuracy, including producing complex inner topology of a model (Fig-
ure 2).
The complicated shape of the inner channels requires special means for aircraft
model producing. The powerful technique for creating such complicated models is ad-
ditive technology. The model of the aircraft shown in Figure 2 is produced with the aid
of SLA-150 equipment developed by Institute of Laser Information Technology (ILIT)
RAS.
3.3 Optical 3D measurement system
Image-based system for 3D flow motion registration leverages photogrammetric tech-
niques. Its core is 3D motion capture system ”Mosca” [21] extended for 3D measure-
ments through several optical media interfaces and 3D reconstruction of flow jets (Fig-
ure 3).
Fig. 3. The laboratory 3D measurements setup.
The scalable 3D motion capture system ”Mosca” provides high speed image ac-
quisition and high accuracy of photogrammetric 3D measurements. For flow study in
HDT-400 it has been equipped with DMK 37AUX273 USB 3.1 cameras and adjusted
for working space of 300 × 300 × 300 mm. Main characteristics of DMK 37AUX273
camera is shown in Table 1.
The developed original software supports image acquisition from 2 . . . 4 cameras in
synchronization mode at frame rate up to 200 frames per second and accurate 3D motion
6 V. Knyaz et al.
Table 1. Main characteristics of DMK 37AUX273 camera.
Parameter Value
Vision Standard USB3 Vision
Dynamic Range 10 bit
Resolution 1440x1080
Frame Rate at Full Resolution 238
Pixel Formats
8-Bit Monochrome
12-Bit Packed Monochrome
16-Bit Monochrome
Optical Interface
IR-Cut filter No
Sensor Type Sony IMX273LLR-C
Shutter Type Global
Sensor Format 1/2.9 inch
Pixel Size 3.45 µm
Lens Mount C/CS
Electrical Interface
Interface USB 3.1gen1
Supply voltage 4.75 VDC to 5.25 VDC
Current consumption approx 380 mA @ 5 VDC
Mechanical Data
Dimensions H: 36 mm, W: 36 mm, L: 25 mm
Mass 70 g
Adjustments
Shutter 1 µs to 30 s
Gain 0 dB to 48 dB
reconstruction based on original calibration and point tracking procedures [22]. The
”Mosca” system has been configured for image acquisition in hydrodynamic tunnel.
The laboratory setup for system calibration and 3D measurements is shown in Figure 3.
For applying ”Mosca” system for 3D measurements in hydrodynamic tunnel it is
necessary to account for a phenomenon of light refraction at interfaces of different op-
tical media. The original techniques for calibration and 3D measurements, based on the
detailed analysis of light passing through the different optical media has been devel-
oped and evaluated [11]. The developed techniques provide accurate measurements of
3D coordinates in working space 300 × 300 × 300 mm with the accuracy of 0.03 mm.
4 Visualization of the captured data
Test video sequences of flow modeling have demonstrated reasonable quality of flow
visualization for qualitative 2D analysis of flow behavior for given modeling conditions.
Figure 4 presents samples of images acquired in HDT-400 during SLA-model testing.
At current stage of the study the imaging configuration of 3D flow motion registra-
tion is determined. The laboratory experiments proved high accuracy of 3D measure-
Image-based System for 3D Visualization of Flow in Hydrodynamic Tunnel 7
Fig. 4. Registration of an image sequence of a flow
ment of the developed system. The system demonstrates necessary technical character-
istics for its applying for flow registration in HDT-400 hydrodynamic tunnel.
Although an accurate and fast-speed registration of a flow by the developed optical
3D measurement system in HDT-400 hydrodynamic tunnel is the next step of the study,
the preliminary experiments for flow 3D reconstruction and 3D visualization were car-
ried out, using available flow registration (Figure 4). As video sequence acquisition has
been made by cameras with unknown parameters of interior and exterior orientation,
only qualitative 3D reconstruction could be performed.
A 3D Shape from Silhouette [23,24,25] technique has been applied for qualitative
3D flow jets reconstruction. The silhouette, or occluding contour of a shape in an image
contains some information about the 3D shape of the object. Specifically, an object
silhouette in an image defines the solid angle based on the projection of this object to
the image plane. This solid angle can be found by back-projecting the area inside the
silhouette using the parameters of camera orientation.
An exterior orientation parameters include camera location X, Y, Z and angular
orientation α, ω, κ in some reference system of coordinates. For the cameras used for
acquiring test images (Figure 4) this information is not available. So the following tech-
nique for determining exterior orientation parameters has been applied.
The geometry of the aircraft stereolithography model applied for the experiment
corresponds (with high level of the accuracy) to the digital CAD-model, that serves for
the stereolithography 3D printing process. This CAD model contains full and accurate
information about the geometry of the aircraft model. Using CAD information about a
set of aircraft specific points as reference data, cameras orientation parameters in CAD-
model coordinate system has been estimated. These parameters then were used for 3D
shape estimation of the jets.
As the number of cameras was minimal (only two), the additional supposition has
been used for shape reconstruction, specifically, that flow jets have symmetry relatively
the central trace of a jet. Another word, it has been supposed that two more (virtual)
cameras has been located symmetrically to the real ones, and they has acquired two
additional (symmetrical) images.
8 V. Knyaz et al.
Figure 5 presents the results of the 3D reconstruction of colored flow jets by voxel-
based Shape from Silhouette techniques.
Fig. 5. 3D visualization of flow jets.
Standard voxel-based approach has been used for a flow jets shape reconstruction.
It includes the following main steps:
1. Split up the working space of HDT-400 into a 3D voxel grid of given voxel size;
2. Determine a flow jet borders (silhouette) in the acquired image;
3. For each camera calculate an intersection of silhouette solid angle from the projec-
tion center with the 3D voxel grid;
4. Find the resulting flow jet voxel 3D shape as an intersection of the all solid angles.
More detailed description of the pipeline for a flow jet 3D shape reconstruction is
presented as Algorithm 1.
The results of preliminary flow jets 3D reconstruction demonstrate the reasonable
quality. With increasing the number of registering cameras and system accurate cal-
ibrating, the quality of the developed 3D reconstruction technique allows to receive
quantitative data needed for flow motion analysis.
5 Conclusion
The developed image-based system for accurate 3D flow acquisition and 3D analy-
sis utilizes a set of calibrated high-speed cameras for synchronized flow motion reg-
istration. A specially developed original calibration technique allows to carry out 3D
measurement in hydrodynamic tunnel with relative accuracy at the level of 1/10000 of
working space.
Stereolithography technology is used for producing accurate an aircraft 3D model
with interior channels, that serve for colored jets injecting needed for the visualization
and further 3D reconstruction of flow jet shape. The results of preliminary flow jets 3D
reconstruction using the Shape from Silhouette technique demonstrate the quality, that
corresponds the requirements of the icing process study.
Image-based System for 3D Visualization of Flow in Hydrodynamic Tunnel 9
Algorithm 1: Flow jet 3D shape reconstruction
Input:
exterior orientations of acquisition cameras {Pkeo },
array of K current images of flow {Ick } came from acquisition cameras {Ck }
Output:
voxel shape of flow jet SF j for current image array {Ick }
1 Split working space into voxel grid V of Nx × Ny × Nz volume elements (voxels) vi ;
2 for each camera C k of acquisition system do
3 Detect borders of the flow jet F j in image Ick ;
4 Determine projecting ray rb for every point of the border b in the image of the flow
jet F j ;
5 Determine solid angle ΩF j of flow jet F j for camera Ck as array of rb ;
6 for each voxel vi of voxel grid V do
7 if vi ∈ ΩF j then
8 save vi in flow jet voxel array VFkj ;
9 else
10 skip;
11 end
12 end
13 store the flow jet voxel array VFkj
14 end
T k
15 Find shape of flow jet as intersection of flow jet voxel arrays: SF j = VF j ;
k
Acknowledgements
The reported study was funded by Russian Foundation for Basic Research (RFBR)
according to the research project 19-29-13040.
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