Experimental Study of Free Stream Turbulent Effects on Dynamic Stall of Pitching Airfoil by Using Particle Image Velocimetry

2012 ◽  
Vol 225 ◽  
pp. 103-108 ◽  
Author(s):  
Tzong Shyng Leu ◽  
J.M. Yu ◽  
C.C. Hu ◽  
J.J. Miau ◽  
S.Y. Liang ◽  
...  
Keyword(s):  
Particle Image Velocimetry ◽  
Turbulence Intensity ◽  
Particle Image ◽  
Free Stream ◽  
Vertical Axis ◽  
Dynamic Stall ◽  
Vertical Axis Wind Turbine ◽  
Free Stream Turbulence ◽  
Pitching Airfoil ◽  
Image Velocimetry

The unsteady flow fields above NACA 0015 airfoil pitching with/without upstream turbulence generator are investigated in a water tunnel by mean of particle image velocimetry (PIV). The turbulence was generated by a square bar mesh situated at the inlet of the test section. The airfoil pitching waveform is performed under the condition calculated from the angle of attack histogram of a vertical axis wind turbine (VAWT). By using PIV, the instantaneous vortex structures above the pitching airfoil can be revealed. It allows us to study the free stream turbulence effects on dynamic stall over an airfoil at pitching waveform the same as VAWT. It is found that the free stream turbulence intensity has significant impacts on the dynamic stall process. The dynamic stall process is delayed to higher incidence angles on increasing the turbulence intensity.

Particle image velocimetry study of fractal-generated turbulence

2012 ◽  
Vol 711 ◽  
pp. 306-336 ◽  
Author(s):  
R. Gomes-Fernandes ◽  
B. Ganapathisubramani ◽  
J. C. Vassilicos
Keyword(s):  
Particle Image Velocimetry ◽  
Turbulence Intensity ◽  
Particle Image ◽  
Free Stream ◽  
Length Scale ◽  
Hot Wire ◽  
Interaction Length ◽  
Free Stream Turbulence ◽  
Image Velocimetry ◽  
Wake Interaction

AbstractAn experimental investigation involving space-filling fractal square grids is presented. The flow is documented using particle image velocimetry (PIV) in a water tunnel as opposed to previous experiments which mostly used hot-wire anemometry in wind tunnels. The experimental facility has non-negligible incoming free-stream turbulence (with 2.8 % and 4.4 % in the streamwise (${u}^{\ensuremath{\prime} } / U$) and spanwise (${v}^{\ensuremath{\prime} } / U$) directions, respectively) which presents a challenge in terms of comparison with previous wind tunnel results. An attempt to characterize the effects of the incoming free stream turbulence on the grid-generated turbulent flow is made and an improved wake-interaction length scale is proposed which enables the comparison of the present results with previous ones for both fractal square and regular grids. This length scale also proves to be a good estimator of the turbulence intensity peak location. Furthermore, a new turbulence intensity normalization capable of collapsing${u}^{\ensuremath{\prime} } / U$for various grids in various facilities is proposed. Comparison with previous experiments indicates good agreement in turbulence intensities, Taylor microscale, as well as various other quantities, if the improved wake-interaction length scale is used. Global and local isotropy of fractal-generated turbulence is assessed using the velocity gradients of the two-component (2C) two-dimensional (2D) PIV and compared with regular grid results. Finally, the PIV data appear to confirm the new dissipation behaviour previously observed in hot-wire measurements.

Particle image velocimetry measurements of a transitional boundary layer under free stream turbulence

2012 ◽  
Vol 702 ◽  
pp. 215-238 ◽  
Author(s):  
K. P. Nolan ◽  
E. J. Walsh
Keyword(s):  
Boundary Layer ◽  
Particle Image Velocimetry ◽  
High Speed ◽  
Particle Image ◽  
Free Stream ◽  
Bypass Transition ◽  
Turbulent Structures ◽  
Free Stream Turbulence ◽  
Image Velocimetry ◽  
Hot Film

AbstractHigh-speed particle image velocimetry (PIV) measurements of bypass transition reveal the breakdown of the ubiquitous streaks into turbulent spots. Individual streak velocity profiles are examined and contrasted with the root mean square profiles typically reported. An estimation of streak amplitude based on the modulation of the instantaneous boundary layer thickness is proposed. Examination of the PIV velocity fields shows how turbulent spot precursors, identified with concurrent hot-film recordings, consist of streamwise arrangements of positive and negative streaks. As secondary instability progresses, the interface between these streaks is observed to result in turbulent structures. In an attempt to further elucidate the role of the free stream turbulence, correlation maps are generated to determine the extent of the wall-normal fluctuations. Significant damping of the free stream is found within the boundary layer for all Reynolds numbers prior to the onset of spot precursors.

scholarly journals Particle image velocimetry measurements of induced separation at the leading edge of a plate

2016 ◽  
Vol 804 ◽  
pp. 278-297 ◽  
Author(s):  
J. P. J. Stevenson ◽  
K. P. Nolan ◽  
E. J. Walsh
Keyword(s):  
Particle Image Velocimetry ◽  
Shear Layer ◽  
Particle Image ◽  
Reverse Flow ◽  
Leading Edge ◽  
Quadrant Analysis ◽  
Bypass Transition ◽  
Reynolds Stresses ◽  
Free Stream Turbulence ◽  
Image Velocimetry

The free shear layer that separates from the leading edge of a round-nosed plate has been studied under conditions of low (background) and elevated (grid-generated) free stream turbulence (FST) using high-fidelity particle image velocimetry. Transition occurs after separation in each case, followed by reattachment to the flat surface of the plate downstream. A bubble of reverse flow is thereby formed. First, we find that, under elevated (7 %) FST, the time-mean bubble is almost threefold shorter due to an accelerated transition of the shear layer. Quadrant analysis of the Reynolds stresses reveals the presence of slender, highly coherent fluctuations amid the laminar part of the shear layer that are reminiscent of the boundary-layer streaks seen in bypass transition. Instability and the roll-up of vortices then follow near the crest of the shear layer. These vortices are also present under low FST and in both cases are found to make significant contributions to the production of Reynolds stress over the rear of the bubble. But their role in reattachment, whilst important, is not yet fully clear. Instantaneous flow fields from the low-FST case reveal that the bubble of reverse flow often breaks up into two or more parts, thereby complicating the overall reattachment process. We therefore suggest that the downstream end of the ‘separation isoline’ (the locus of zero absolute streamwise velocity that extends unbroken from the leading edge) be used to define the instantaneous reattachment point. A histogram of this point is found to be bimodal: the upstream peak coincides with the location of roll-up, whereas the downstream mode may suggest a ‘flapping’ motion.

scholarly journals Effect of turbulence on the wake of a wall-mounted cube

2016 ◽  
Vol 804 ◽  
pp. 513-530 ◽  
Author(s):  
R. Jason Hearst ◽  
Guillaume Gomit ◽  
Bharathram Ganapathisubramani
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Reattachment Length ◽  
Free Stream Turbulence ◽  
Boundary Layer Development ◽  
Image Velocimetry ◽  
Shedding Frequency ◽  
Layer Development

The influence of turbulence on the flow around a wall-mounted cube immersed in a turbulent boundary layer is investigated experimentally with particle image velocimetry and hot-wire anemometry. Free-stream turbulence is used to generate turbulent boundary layer profiles where the normalised shear at the cube height is fixed, but the turbulence intensity at the cube height is adjustable. The free-stream turbulence is generated with an active grid and the turbulent boundary layer is formed on an artificial floor in a wind tunnel. The boundary layer development Reynolds number ($Re_{x}$) and the ratio of the cube height ($h$) to the boundary layer thickness ($\unicode[STIX]{x1D6FF}$) are held constant at $Re_{x}=1.8\times 10^{6}$ and $h/\unicode[STIX]{x1D6FF}=0.47$. It is demonstrated that the stagnation point on the upstream side of the cube and the reattachment length in the wake of the cube are independent of the incoming profile for the conditions investigated here. In contrast, the wake length monotonically decreases for increasing turbulence intensity but fixed normalised shear – both quantities measured at the cube height. The wake shortening is a result of heightened turbulence levels promoting wake recovery from high local velocities and the reduction in strength of a dominant shedding frequency.

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