7th International Symposium on Particle Image Velocimetry PIV2007 Paper nnnn
Roma, Italy, September 11-14, 2007
Scanning PIV study on the laminar separation bubble on the SD
W. Zhang, R. Hain, C.J. Kähler
The laminar separation bubble (LSB) is a classic topic in fluid mechanics due to its importance in laminar-
turbulence transition. Increasing interest in LSB is aroused by the development of the micro air vehicles (MAVs),
which normally cruise in the Reynolds number range of 50,000-200,000. In this low Re flow regime, LSBs bring
about significant adverse aerodynamic effects, especially the augment of the pressure drag. The mean flow
characteristics and unsteadiness of the LSB have been studied with theoretical, numerical and experimental means
(Ol et al., 2005; Windte et al., 2006), however, the description of the LSB, especially the physical mechanism of the
transition is far from complete due to its elusive nature. A time-resolved 3D measurement technique is desirable
because of the unsteady 3D vortex structure of the LSB. The scanning PIV technique employed in this study is a
time-resolved quasi-3D measurement technique, extended from the standard PIV methods.
This paper aims to study the LSB over the SD 7003 airfoil at the angle of attack α=4° and at Re=60,000 using the
scanning PIV technique. The SD 7003 airfoil is designed for model planes operating in the Reynolds number of
50,000-200,000, hence the flow phenomenon in this typical case is expected to be different to that occurs at
Re=20,000 reported by Burgmann et al., 2006.
Experimental Apparatus and Methods
Experiments were performed in a Goettingen type water channel with a test section of 1.25 mL×0.25 mW×0.33 mH in
the Institute of Fluid Mechanics, Technical University at Braunschweig. A transparent SD 7003 airfoil was mounted
upside down between the side walls and parallel to the freestream direction, exposing the suction side to the optical
access form bottom. Measurements were carried out from two orthogonal views: in the streamwise-wall-normal
planes from side and the airfoil surface-parallel planes from bottom. A 20 W diode-pumped dual-head Nd:Yag laser
was employed as the light source and operated at 1000 Hz. For generating scanning light sheets, an oscillating
mirror mounted on an optical scanner (VM1000, GSI Lumonics) and a long focal length cylindrical lens (f=+300
mm) were applied in addition to the optics serving to expand the laser beam. The light sheet thickness δ in the
measured area was estimated to be 0.2 mm. The entire scanning volume was Z=5.2 mm in the spanwise wall-normal
planes and 4.0 mm in the surface parallel planes, respectively. A high speed CMOS camera (Redlake) served as the
master to synchronize the scanning PIV system. A pulse generator was applied to trigger the optical scanner to
starting a scanning cycle once it received a signal from the camera. The laser head was also triggered by the camera
to give a pulse at each scanned plane. In this study, the oscillating mirror swept five steps at 100 Hz with the double
pulse mode instead of the single pulse mode. Hollow glass beads (d~10 µm) were employed as the seeding particles.
The particle images were recorded by the camera with the full resolution of 1504 × 1128 pixels at 1000 fps. The
particle image evaluation was performed using Davis 7.1 software, with a second order multi-grid, multi-pass
method and image deformation.
Results and Discussions
Figure 1 represents a selective instantaneous iso-surfaces of the spanwise vorticity (ωz) of the scanning volume in
the streamwise wall-normal planes. For clear visibility only iso-surfaces of |ωz|=0.1, 0.2 and 0.3 (1/s) are shown.
Large well organized vortices, formed in the separated shear layer owing to linear instability, roll up and shed
downstream, split into a number of small structures and break down to turbulence. Because the instantaneous flow
field is highly unsteady in transition, especially near the reattachment point, not only the vertical thickness but the
length of the LSB are variable with time. Observation of the time sequence clearly shows an overall flapping motion
W. Zhang, Aerospace Engineering Department, Iowa State University, Ames, IA 50011, US
R. Hain, C. J. Kähler, Institute of Fluid Mechanics, Technical University of Braunschweig,
Bienroder Weg 3, 38106 Braunschweig, Germany
Dr. C. J. Kähler, Institute of Fluid Mechanics, Technical University of Braunschweig,
Bienroder Weg 3, 38106 Braunschweig, Germany, E-mail: firstname.lastname@example.org
of the separated shear layer. The fluctuation in the height of LSB also indicates that outer fluid is continuously
entrained to ensure the vortex shedding process.
The scanning volume composed of surface parallel planes enables one to observe the vortex evolution in the
orthogonal view. The overlapping of the wall-normal vorticity iso-surfaces (|ωy|=0.08 1/s) at a selective time
sequence at Re=60,000 is shown in Fig. 2. Two observations are made based on behavior of the vorticity packets in
this time span: Firstly, vorticity packets are generated from the wall and transported downstream. In the
incompressible flow the wall is the only source of the vorticity generation due to the non-slip wall condition.
Secondly, the frequently appeared structures are the paired vorticity packets (either symmetric or not), with positive
and negative vorticity values though vortices propagate in the reattached turbulent boundary layer showing
miscellaneous size and forms,. It could be conjectured that, these paired vorticity packets are formed due to the
development of three-dimensional motion during the laminar-turbulence transition, after the vortex breakdown in
the vicinity of the reattachment position. In this way they could transport fluid away from the near wall region to the
outer layer of the boundary layer. It is noticeable that the well-organized structures can be identified from the
random turbulent surroundings, though mutual interaction among them occurs all the time.
Fig. 1 Spanwise vorticity iso-surfaces (|ωz|=0.1, 0.2 Fig. 2 Wall-normal vorticity iso-surfaces (|ωy|=0.08 1/s)
and 0.3 1/s) of a scanning volume in the streamwise of a scanning volume in the surface parallel planes
By using the scanning PIV technique, the LSB on the suction side of the SD 7003 airfoil was studied at the angle of
attack α=4° and at Re=60,000. Vortex shedding in transition near the reattachment region of the LSB was clearly
identified in the spanwise wall-normal direction. Further vortex evolution in the reattached turbulent boundary layer
was characterized by downstream convection of paired positive and negative vorticity packets.
This research has been supported by the German Research Foundation (DFG) in the priority program 1147.
Ol MV; Hanff E; McAuliffe B; Scholz U; Kähler CJ (2005) Comparison of laminar separation bubble
measurements on a low Reynolds number airfoil in three facilities. 35th AIAA fluid dynamics conference and
exhibit, Toronto, Ontario, June 6–9 2005, AIAA Paper 2005–5149
Windte J; Radespiel R; Scholz U; Eisfeld B (2004) RANS simulation of the transitional flow around airfoils at
low Reynolds Numbers for steady and unsteady onset conditions. Specialists meeting on enhancement of NATO
military flight vehicle performance by management of interacting boundary layer transition and separation, Prague,
Czech Republic, RTO-MP-AVT-111-P-03
Burgmann S; Brücker Ch; Schröder W (2006) Scanning PIV measurements of a laminar separation bubble.
Experiments in Fluids 41, 319-326