PIV measurements of airfoil wake-flow turbulence statistics and turbulent structures

1994 ◽  
Author(s):  
Chungsheng Yao ◽  
Keith Paschal
Keyword(s):  
Wake Flow ◽  
Turbulence Statistics ◽  
Turbulent Structures ◽  
Piv Measurements ◽  
Flow Turbulence

scholarly journals Flow Turbulence Characteristics and Mass Transport in the Near-Wake Region of an Aquaculture Cage Net Panel

Water ◽  
2021 ◽  
Vol 13 (3) ◽  
pp. 294
Author(s):  
Dongdong Shao ◽  
Li Huang ◽  
Ruo-Qian Wang ◽  
Carlo Gualtieri ◽  
Alan Cuthbertson
Keyword(s):  
Mass Transport ◽  
Near Field ◽  
Lateral Spreading ◽  
Wake Flow ◽  
Wake Region ◽  
Near Wake ◽  
Turbulence Characteristics ◽  
Macro Scale ◽  
Flow Turbulence ◽  
Fish Cages

Cage-based aquaculture has been growing rapidly in recent years. In some locations, cage-based aquaculture has resulted in the clustering of large quantities of cages in fish farms located in inland lakes or reservoirs and coastal embayments or fjords, significantly affecting flow and mass transport in the surrounding waters. Existing studies have focused primarily on the macro-scale flow blockage effects of fish cages, and the complex wake flow and associated near-field mass transport in the presence of the cages remain largely unclear. As a first step toward resolving this knowledge gap, this study employed the combined Particle Image Velocimetry and Planar Laser Induced Fluorescence (PIV-PLIF) flow imaging technique to measure turbulence characteristics and associated mass transport in the near wake of a steady current through an aquaculture cage net panel in parametric flume experiments. In the near-wake region, defined as ~3M (mesh size) downstream of the net, the flow turbulence was observed to be highly inhomogeneous and anisotropic in nature. Further downstream, the turbulent intensity followed a power-law decay after the turbulence production region, albeit with a decay exponent much smaller than reported values for analogous grid-generated turbulence. Overall, the presence of the net panel slightly enhanced the lateral spreading of the scalar plume, but the lateral distribution of the scalar concentration, concentration fluctuation and transverse turbulent scalar flux exhibited self-similarity from the near-wake region where the flow was still strongly inhomogeneous. The apparent turbulent diffusivity estimated from the gross plume parameters was found to be in reasonable agreement with the Taylor diffusivity calculated as the product of the transverse velocity fluctuation and integral length scale, even when the plume development was still transitioning from a turbulent-convective to turbulent-diffusive regime. The findings of this study provide references to the near-field scalar transport of fish cages, which has important implications in the assessment of the environmental impacts and environmental carrying capacity of cage-based aquaculture.

scholarly journals Stereo-PIV measurements of spatio-temporal turbulence correlations in an axisymmetric jet

2015 ◽  
Vol 778 ◽  
pp. 216-252 ◽  
Author(s):  
C. D. Pokora ◽  
J. J. McGuirk
Keyword(s):  
Fourth Order ◽  
Single Point ◽  
Aerodynamic Noise ◽  
Filter Method ◽  
Frequency Noise ◽  
Turbulent Structures ◽  
Validation Data ◽  
Piv Measurements ◽  
Temporal Correlations ◽  
Spatio Temporal

Stereoscopic three-component particle image velocimetry (3C-PIV) measurements have been made in a turbulent round jet to investigate the spatio-temporal correlations that are the origin of aerodynamic noise. Restricting attention to subsonic, isothermal jets, measurements were taken in a water flow experiment where, for the same Reynolds number and nozzle size, the shortest time scale of the dynamically important turbulent structures is more than an order of magnitude greater that in equivalent airflow experiments, greatly facilitating time-resolved PIV measurements. Results obtained (for a jet nozzle diameter and velocity of 40 mm and $1~\text{m}~\text{s}^{-1}$, giving $\mathit{Re}=4\times 10^{4}$) show that, on the basis of both single-point statistics and two-point quantities (correlation functions, integral length scales) the present incompressible flow data are in excellent agreement with published compressible, subsonic airflow measurements. The 3C-PIV data are first compared to higher-spatial-resolution 2C-PIV data and observed to be in good agreement, although some deterioration in quality for higher-order correlations caused by high-frequency noise in the 3C-PIV data is noted. A filter method to correct for this is proposed, based on proper orthogonal decomposition (POD) of the 3C-PIV data. The corrected data are then used to construct correlation maps at the second- and fourth-order level for all velocity components. The present data are in accordance with existing hot-wire measurements, but provide significantly more detailed information on correlation components than has previously been available. The measured relative magnitudes of various components of the two-point fourth-order turbulence correlation coefficient ($R_{ij,kl}$) – the fundamental building block for free shear flow aerodynamic noise sources – are presented and represent a valuable source of validation data for acoustic source modelling. The relationship between fourth-order and second-order velocity correlations is also examined, based on an assumption of a quasi-Gaussian nearly normal p.d.f. for the velocity fluctuations. The present results indicate that this approximation shows reasonable agreement for the measured relative magnitudes of several correlation components; however, areas of discrepancy are identified, indicating the need for work on alternative models such as the shell turbulence concept of Afsar (Eur. J. Mech. (B/Fluids), vol. 31, 2012, pp. 129–139).

Two-Dimensional Experimental Investigation on the Effects of a Fowler Flap Gap and Overlap Size on the Wake Flow Field

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
David Demel ◽  
Mohsen Ferchichi ◽  
William D. E. Allan ◽  
Marouen Dghim
Keyword(s):  
Experimental Investigation ◽  
Flow Field ◽  
Angle Of Attack ◽  
Wake Flow ◽  
Two Dimensional ◽  
Edge Region ◽  
Suction Surface ◽  
Trailing Edge ◽  
Piv Measurements ◽  
Image Velocimetry

This work details an experimental investigation on the effects of the variation of flap gap and overlap sizes on the flow field in the wake of a wing-section equipped with a trailing edge Fowler flap. The airfoil was based on the NACA 0014-1.10 40/1.051 profile, and the flap was deployed with 40 deg deflection angle. Two-dimensional (2D) particle image velocimetry (PIV) measurements of the flow field in the vicinity of the main wing trailing edge and the flap region were performed for the optimal flap gap and overlap, as well as for flap gap and overlap increases of 2% and 4% chord beyond optimal, at angles of attack of 0 deg, 10 deg, and 12 deg. For all the configurations investigated, the flow over the flap was found to be fully stalled. At zero angle of attack, increasing the flap gap size was found to have minor effects on the flow field but increased flap overlap resulted in misalignment between the main wing boundary layer (BL) flow and the slot flow that forced the flow in the trailing edge region of the main wing to separate. When the angle of attack was increased to near stall conditions (at angle of attack of 12 deg), increasing the flap gap was found to energize and improve the flow in the trailing edge region of the main wing, whereas increased flap overlap further promoted flow separation on the main wing suction surface possibly steering the wing into stall.

scholarly journals Fish swimming kinematics in a turbulent wake: To spill or not to spill?

2018 ◽  
Vol 40 ◽  
pp. 03024 ◽  
Author(s):  
Valentine Muhawenimana ◽  
Catherine A. M. E. Wilson ◽  
Jo Cable
Keyword(s):  
Three Dimensional ◽  
Turbulent Wake ◽  
Wake Flow ◽  
Complex Nature ◽  
Fish Length ◽  
Reynolds Stresses ◽  
Turbulent Structures ◽  
Lateral Velocity ◽  
Engineering Structures ◽  
Velocity Statistics

Due to the complex nature of fish-environment interactions, the effects of altered flow conditions on fish habitats and behaviour, particularly near obstructions, are not yet fully understood. Many hydro-engineering structures generate three-dimensional turbulent structures that differ in properties from naturally occurring ones, and are challenging to fish movement. Here, swimming stability and habitat usage of Nile tilapia (Oreochromis niloticus) were examined in the turbulent wake of a horizontally oriented cylinder (50 mm diameter, D) for a series of increasing cylinder Reynolds numbers (Red). Velocity statistics showed that the near wake downstream of the cylinder within a 2D distance had higher magnitudes of longitudinal, vertical and lateral velocity components as well as Reynolds stresses, turbulence intensity and turbulent kinetic energy than the remaining wake flow field. Flow accelerating over the cylinder top and underside generated two shear horizontal layers, where two coherent structures were formed due to the shear layer breakdown off the cylinder’s walls and alternating von Karman-type vortex shedding occurred in the wake of the cylinder. The eddy symmetry and vorticity off the cylinder’s edges was dependent on Red. The recorded total number of spills, defined as loss of balance, was inversely proportional to fish length and weight, and depended on proximity to the cylinder and the flume bed. Furthermore, the frequency of spills was closely linked to the orientation of vortices and magnitude of Reynolds stresses, suggesting that these parameters may govern the swimming stability of fishes. This information may be used to inform the design of fish-friendly obstacles including hydraulic structures and hydro turbines in riverine and estuarine systems.

scholarly journals On the challenge of five-hole-probe measurements at high subsonic Mach numbers in the wake of transonic turbine cascades

2018 ◽  
Vol 2 ◽  
pp. JPRQQM
Author(s):  
Marcel Boerner ◽  
Martin Bitter ◽  
Reinhard Niehuis
Keyword(s):  
Flow Field ◽  
Flow Regimes ◽  
Reynolds Numbers ◽  
Sensitivity Study ◽  
Wake Flow ◽  
Piv Measurements ◽  
Image Velocimetry ◽  
Measurement Results ◽  
Transonic Turbine ◽  
Probe Measurements

Five-hole-probes are common use in turbomachinery flow investigations, even though, inserting a probe into a flow field inevitably induces perturbations to the flow which can falsify the measurement results, especially when exposed to transonic flows. The objective of the investigations presented here is to evaluate the Mach number measurements of a five-hole-probe (5HP) in the wake flow of a transonic turbine cascade at engine relevant Reynolds numbers by comparing them to the results of particle image velocimetry (PIV). Furthermore, PIV measurements were performed with inserted probe to investigate the influence of the probe on the wake flow field. Together with a sensitivity study of 5HP measurements in flow regimes close to Ma = 1, the results demonstrate how the measurement uncertainty can be improved in high subsonic flow regimes.

A Comparative Study of the Dynamic Wind Loads and Wake Flow Characteristics of a Turbine Sited in Onshore and Offshore Wind Farms

2014 ◽  
Author(s):  
Wei Tian ◽  
Ahmet Ozbay ◽  
Hui Hu
Keyword(s):  
Flow Field ◽  
Wind Turbine ◽  
Wind Farms ◽  
Field Measurements ◽  
Wake Flow ◽  
Offshore Wind ◽  
Wind Loads ◽  
Offshore Wind Farms ◽  
Piv Measurements ◽  
Turbine Model

An experimental study was conducted to compare the characteristics of the dynamic wind loads and evolution of the unsteady vortex and turbulent flow structures in the wake of a wind turbine sited in onshore and offshore wind farms. A scaled three-blade Horizontal Axial Wind Turbine (HAWT) model was placed in Atmospheric Boundary Layer (ABL) winds with different mean and turbulence characteristics to simulate the wind conditions in onshore and offshore wind farms. In addition to measuring dynamic wind loads acting on the wind turbine model by using a high-sensitive force-moment sensor unit, a high-resolution digital Particle Image Velocimetry (PIV) system was used to achieve flow field measurements to quantify the characteristics of the turbulent flow in the wake of the wind turbine model. Besides conducting “free-run” PIV measurements to determine the ensemble-averaged statistics of the flow quantities such as mean velocity, Reynolds stress, and Turbulence Kinetic Energy (TKE) distributions in the wake, “phase-locked” PIV measurements were also performed to elucidate further details about evolution of the unsteady vortex structures in the wake flow in relation to the position of the rotating turbine blades. The detailed flow field measurements are correlated with the dynamic wind loads measurements to elucidate underlying physics in order to gain further insight into changes of the dynamic wind loads and wake characteristics behind the wind turbine operating in either onshore or offshore wind farms.

PIV Measurements of the Near-Wake Flow of an Airfoil Above a Free Surface

2007 ◽  
Vol 19 (4) ◽  
pp. 482-487 ◽  
Author(s):  
Daichin ◽  
Wen Kang ◽  
Li-li Zhao
Keyword(s):  
Free Surface ◽  
Wake Flow ◽  
Near Wake ◽  
Piv Measurements
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