Particle Image Velocimetry Study of Rough-Wall Turbulent Flows in Favorable Pressure Gradient

2009 ◽  
Vol 131 (6) ◽  
Author(s):  
G. F. K. Tay ◽  
D. C. S. Kuhn ◽  
M. F. Tachie
Keyword(s):  
Reynolds Number ◽  
Particle Image Velocimetry ◽  
Pressure Gradient ◽  
Particle Image ◽  
Low Reynolds Number ◽  
Plane Channel ◽  
Mean Velocity ◽  
Image Velocimetry ◽  
Favorable Pressure Gradient ◽  
Low Reynolds

This paper reports an experimental investigation of the effects of wall roughness and favorable pressure gradient on low Reynolds number turbulent flow in a two-dimensional asymmetric converging channel. Flow convergence was produced by means of ramps (of angles 2 deg and 3 deg) installed on the bottom wall of a plane channel. The experiments were conducted over a smooth surface and over transitionally rough and fully rough surfaces produced from sand grains and gravel of nominal mean diameters 1.55 mm and 4.22 mm, respectively. The dimensionless acceleration parameter was varied from 0.38×10−6 to 3.93×10−6 while the Reynolds number based on the boundary layer momentum thickness was varied from 290 to 2250. The velocity measurements were made using a particle image velocimetry technique. From these measurements, the distributions of the mean velocity and Reynolds stresses were obtained to document the salient features of transitionally and fully rough low Reynolds number turbulent boundary layers subjected to favorable pressure gradient.

Separation-Bubble-Transition Measurements on a Low-Re Airfoil Using Particle Image Velocimetry

2005 ◽  
Author(s):  
B. R. McAuliffe ◽  
M. I. Yaras
Keyword(s):  
Reynolds Number ◽  
Particle Image Velocimetry ◽  
Large Scale ◽  
Particle Image ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Piv Measurements ◽  
Image Velocimetry ◽  
Low Reynolds

This paper presents experimental results on separation-bubble transition at low Reynolds number and low freestream turbulence, measured on an airfoil using particle image velocimetry (PIV). The two-dimensional PIV measurements have been performed over the suction surface of a low-Reynolds-number airfoil in a water tow-tank facility. Reynolds numbers, based on airfoil chord length and towing speed, of 40,000 and 65,000 have been examined at various angles of incidence, providing a range of streamwise pressure distributions and transitional separation-bubble geometries. The types of bubbles observed range from a short and thick bubble with separation near the leading edge of the airfoil, to a long and thin bubble with separation far downstream of the suction peak. The PIV measurements facilitate visualization of the vortex dynamics associated with separation-bubble transition. The growth of instability waves within the separated shear layer and eventual breakdown into turbulence is documented through the instantaneous vector fields. For all cases examined, large-scale vortex shedding and multiple reverse-flow zones are observed in the reattachment region. A technique for estimating the location of transition onset based on statistical turbulence quantities is presented, and comparisons are made to existing transition models.

scholarly journals Experimental insights into flow impingement in cerebral aneurysm by stereoscopic particle image velocimetry: transition from a laminar regime

2013 ◽  
Vol 10 (82) ◽  
pp. 20121031 ◽  
Author(s):  
Takanobu Yagi ◽  
Ayaka Sato ◽  
Manabu Shinke ◽  
Sara Takahashi ◽  
Yasutaka Tobe ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Particle Image Velocimetry ◽  
Cerebral Aneurysm ◽  
Particle Image ◽  
Low Reynolds Number ◽  
Stereoscopic Particle Image Velocimetry ◽  
Patient Specific ◽  
Laminar Regime ◽  
Image Velocimetry ◽  
Low Reynolds

This study experimentally investigated the instability of flow impingement in a cerebral aneurysm, which was speculated to promote the degradation of aneurysmal wall. A patient-specific, full-scale and elastic-wall replica of cerebral artery was fabricated from transparent silicone rubber. The geometry of the aneurysm corresponded to that found at 9 days before rupture. The flow in a replica was analysed by quantitative flow visualization (stereoscopic particle image velocimetry) in a three-dimensional, high-resolution and time-resolved manner. The mid-systolic and late-diastolic flows with a Reynolds number of 450 and 230 were compared. The temporal and spatial variations of near-wall velocity at flow impingement delineated its inherent instability at a low Reynolds number. Wall shear stress (WSS) at that site exhibited a combination of temporal fluctuation and spatial divergence. The frequency range of fluctuation was found to exceed significantly that of the heart rate. The high-frequency-fluctuating WSS appeared only during mid-systole and disappeared during late diastole. These results suggested that the flow impingement induced a transition from a laminar regime. This study demonstrated that the hydrodynamic instability of shear layer could not be neglected even at a low Reynolds number. No assumption was found to justify treating the aneurysmal haemodynamics as a fully viscous laminar flow.

A Flow Study of Pulsed Jet Cross-Flow Interaction by Micro Particle Image Velocimetry

2006 ◽  
Author(s):  
Kenichi Watanabe ◽  
Tomonori Nakatsuka ◽  
Daichi Suzuki ◽  
Takashi Nagumo ◽  
Masahiro Motosuke ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Particle Image Velocimetry ◽  
Flow Control ◽  
Particle Image ◽  
Low Reynolds Number ◽  
Cross Flow ◽  
Synthetic Jet ◽  
Pulsed Jet ◽  
Micro Particle Image Velocimetry ◽  
Image Velocimetry

Recent development of micro devices is remarkable as in the examples of Micro-TAS, Lab-on-a-chip or ultra micro gas turbine. In order to make the micro devices smaller and more effective, an appropriate use of a micro scale jet as an actuator can be a key technology. Aiming at the development of a measurement system of the micro flow control devices in the future micro aerodynamics, we have established a system to measure a continuous jet, a pulsed jet and a synthetic jet for the flow control in the low Reynolds number air flow with a micro length scale. The two-dimensional flow field around the micro jet using micro particle image velocimetry (PIV) was measured. The jet was injected through the device using an acoustic speaker. It was observed that a saddle point existed at the certain phase where the velocity is 0 at the boundary of the jet blowing and suction phase for the synthetic jet into a still air. It was found that the pulsed jet and the synthetic jet are more effective in the fluid mixing in the low Reynolds number flow than the continuous jet. The dead water region was observed downstream of the jet in case of the jet injection into cross flow. It was recognized that the synthetic jet at the certain oscillation frequency generated a vortex pair near the jet hole.

Near-surface particle image velocimetry measurements in a transitionally rough-wall atmospheric boundary layer

2007 ◽  
Vol 580 ◽  
pp. 319-338 ◽  
Author(s):  
SCOTT C. MORRIS ◽  
SCOTT R. STOLPA ◽  
PAUL E. SLABOCH ◽  
JOSEPH C. KLEWICKI
Keyword(s):  
Reynolds Number ◽  
Particle Image Velocimetry ◽  
Environmental Science ◽  
Particle Image ◽  
Shear Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
Near Surface ◽  
Image Velocimetry ◽  
The Mean

The Reynolds number dependence of the structure and statistics of wall-layer turbulence remains an open topic of research. This issue is considered in the present work using two-component planar particle image velocimetry (PIV) measurements acquired at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility in western Utah. The Reynolds number (δuτ/ν) was of the order 106. The surface was flat with an equivalent sand grain roughness k+ = 18. The domain of the measurements was 500 < yuτ/ν < 3000 in viscous units, 0.00081 < y/δ < 0.005 in outer units, with a streamwise extent of 6000ν/uτ. The mean velocity was fitted by a logarithmic equation with a von Kármán constant of 0.41. The profile of u′v′ indicated that the entire measurement domain was within a region of essentially constant stress, from which the wall shear velocity was estimated. The stochastic measurements discussed include mean and RMS profiles as well as two-point velocity correlations. Examination of the instantaneous vector maps indicated that approximately 60% of the realizations could be characterized as having a nearly uniform velocity. The remaining 40% of the images indicated two regions of nearly uniform momentum separated by a thin region of high shear. This shear layer was typically found to be inclined to the mean flow, with an average positive angle of 14.9°.

Experimental study of flow field on stepped airfoil at very low Reynolds number

2016 ◽  
Vol 231 (9) ◽  
pp. 1706-1717
Author(s):  
Reza Kamyab Matin ◽  
Hojat Ghassemi ◽  
Abbas Ebrahimi ◽  
Bahman Ghasemi
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Particle Image ◽  
Low Reynolds Number ◽  
Flow Structures ◽  
Wake Region ◽  
Particle Image Velocimetry Technique ◽  
Vortical Structures ◽  
Image Velocimetry ◽  
Low Reynolds

In this article, the flow field around NACA0024 airfoil with step at lower and upper surfaces is experimentally investigated. For this purpose, particle image velocimetry technique based on the instantaneous flow structures is used to investigate the flow field around the airfoil at different times. All the experimental measurements in current study are conducted at very low Reynolds number condition based on the chord of the airfoil (Re=2000) and at angles of attack at 0° and 5° where the flow around airfoils is separated. The differences between vortical structures, mean streamlines, sizes of the wake regions, and vortex shedding of the stepped airfoils compared to unmodified airfoil are observed. The results disclose that using step in airfoil leads to a decrease in the Strouhal number. In addition, the formation of vortices in wake region and their positions at different times are discussed.

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