F. Scarano
Delft University of Technology, Faculty of Aerospace Engineering, Faculty Member
Particle image velocimetry (PIV) measurements of the flow around a wing section are employed as a basis for non-intrusive aerodynamic mean loads characterisation, providing sectional lift, drag and pitching moment. The technique relies... more
Particle image velocimetry (PIV) measurements of the flow around a wing section are employed as a basis for non-intrusive aerodynamic mean loads characterisation, providing sectional lift, drag and pitching moment. The technique relies upon the application of control-volume approaches in combination with the deduction of the pressure from the PIV experimental data through application of the momentum equation. The treatment can also be applied when the flow is unsteady; in that case time-mean loads are obtained from velocity statistics, through the use of Reynolds-averaged formulation of the governing equations. The procedure was applied in the experimental investigation of a NACA 642A015 aerofoil, in which the PIV approach is validated against standard pressure-based methods (surface pressure distribution and wake rake). The chord Reynolds number considered in the investigation ranges between 1 – 7 × 105. In addition, the consistency and potential performance of the method was asses...
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A method to improve the reliability of the drag coefficient computation by means of particle image velocimetry measurements is made using experimental data acquired on a NACA0012 airfoil tested in the transonic regime, using the... more
A method to improve the reliability of the drag coefficient computation by means of particle image velocimetry measurements is made using experimental data acquired on a NACA0012 airfoil tested in the transonic regime, using the combination of a variable pulse separation with a new high-order Poisson spectral pressure reconstruction algorithm.
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This paper focuses on evaluating the feasibility of estimating loads on vertical axis wind turbine blades in dynamic stall with velocity data acquired with Particle Image Velocimetry. The study uses numerical simulation data of a 2D... more
This paper focuses on evaluating the feasibility of estimating loads on vertical axis wind turbine blades in dynamic stall with velocity data acquired with Particle Image Velocimetry. The study uses numerical simulation data of a 2D Vertical Axis Wind Turbine in dynamic stall to verify sources of error and uncertainty and estimate the accuracy of the method. The integration of the forces from the velocity field overcomes the difficulties and limitations presented by pressure sensors for estimating the local section loads, but adds the difficulty in determining the correct velocity field and its time and spatial derivatives. The analysis also evaluates the use of phase-locked average data as an estimator of average loads.
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With the development of high-speed PIV and tomographic PIV, a number of novel measurement approaches are emerging. One of those is “pressure-by-PIV” and enables the spatio-temporal pressure fluctuations generated by airflow to be measured... more
With the development of high-speed PIV and tomographic PIV, a number of novel measurement approaches are emerging. One of those is “pressure-by-PIV” and enables the spatio-temporal pressure fluctuations generated by airflow to be measured in the fluid region as well over the surface of solid objects. This experimental method is based on the application of three-dimensional time-resolved velocity data and invokes the Navier-Stokes equations to evaluate the instantaneous pressure gradient field from the numerical evaluation of fluid acceleration, in the assumption of incompressible flow. A number of experiments conducted first by thin-volume tomographic PIV and later by fully volumetric tomographic PIV at measurement rates up to 10 kHz demonstrate the applicability of the PIV-based pressure evaluation to determine broadband pressure fluctuations in a fully developed turbulent boundary layer at outer velocity of 10m/s.
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ABSTRACT A tomographic PIV system is introduced for the instantaneous measurement of the material acceleration (material derivative of velocity). The system is conceived to operate with short temporal separation (microseconds) and is... more
ABSTRACT A tomographic PIV system is introduced for the instantaneous measurement of the material acceleration (material derivative of velocity). The system is conceived to operate with short temporal separation (microseconds) and is therefore suitable for applications up to the high-speed flow regimes. The method of operation consists of tomographic imaging of a measurement volume using three arrays of four CCD cameras and two double-pulse laser systems. Advantages and shortcomings of the approach with respect to the most commonly used method based on light polarization are discussed. Various approaches are compared to determine the optimal utilization of four-pulse data to measure the material acceleration: Eulerian and Lagrangian schemes are compared to the recently introduced fluid trajectory correlation (FTC) technique from the authors. A synthetic image test case of a translating vortex is used to compare the schemes with and without the presence of noise. The truncation errors and sensitivity to random noise for each scheme are highlighted. A discussion is also given on the dynamic range of the schemes. The four-pulse tomographic system is used to measure the separated wake of an axisymmetric truncated base with afterbody at a Reynolds number of 68 000. The system calibration accuracy and the baseline measurement uncertainty of the velocity are evaluated with a zero-time delay test. A novel criterion is introduced to establish the relative accuracy of the material derivative measurement, based on the curl of the material acceleration field. The results indicate that the four-pulse tomo-PIV approach suits the measurement of the material acceleration using a variety of estimation schemes. In particular, the FTC technique gives the lowest error levels and is well-suited to perform accurate material acceleration measurements.
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Turbulent boundary layer (TBL) flow consists of manifold temporal and spatial scales and is governed by the organization and decay of self-sustaining coherent flow structures driven by entrained high momentum fluid. Generic flow... more
Turbulent boundary layer (TBL) flow consists of manifold temporal and spatial scales and is governed by the organization and decay of self-sustaining coherent flow structures driven by entrained high momentum fluid. Generic flow structures such as hairpin-like vortices and spanwise alternating wall bounded low- and high-speed streaks have been observed and extensively analyzed with both experimental and numerical methods. The role of these structures for the wall normal and spanwise fluid exchange has been highlighted mostly within an Eulerian reference frame. But for an understanding of the momentum exchange in turbulent wall flows a step towards a spatially resolved Lagrangian frame of reference would be advantageous. The data achieved from the present application of time-resolved tomographic PIV to a flat plate turbulent boundary layer flow enables for the first time the investigation of the flow structures and related particle motions within a temporally and spatially resolved Lagrangian and Eulerian frame of reference.
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Tomographic-PIV was used to quantitatively visualize the 3D coherent structures in a Mach 2 supersonic turbulent boundary layer between y/δ = 0.15 and 0.47. The Reynolds number based on momentum thickness Reθ = 34,000. The instantaneous... more
Tomographic-PIV was used to quantitatively visualize the 3D coherent structures in a Mach 2 supersonic turbulent boundary layer between y/δ = 0.15 and 0.47. The Reynolds number based on momentum thickness Reθ = 34,000. The instantaneous velocity fields show hairpin vortices aligned in streamwise direction forming very long zones of low-speed fluid. The observed hairpin structure is also a statistically
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ABSTRACT Particle image velocimetry (PIV) has been used to measure the flow around a 2D square-section cylinder at incidence. A detailed statistical flow field characterization was obtained in terms of the spatial distributions of the... more
ABSTRACT Particle image velocimetry (PIV) has been used to measure the flow around a 2D square-section cylinder at incidence. A detailed statistical flow field characterization was obtained in terms of the spatial distributions of the mean velocity and turbulence components. The observed change of the mean flow pattern with incidence agrees with earlier studies and is consistent with previously reported behavior of the normal force component. At small angles of attack a downward force results, as a consequence of the increased curvature of the lower shear layer. At higher incidence, this trend is reversed due to reattachment of the separated shear layer on the afterbody. Furthermore, experimental force data are provided that were obtained from PIV velocity field data through application of a control-volume approach.
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The purpose of this study is to investigate the blade-to-blade flow in an axial compressor, providing a detailed description of the entire flow-field on multiple layers through the use of Particle Image Velocimetry (PIV). Typical problems... more
The purpose of this study is to investigate the blade-to-blade flow in an axial compressor, providing a detailed description of the entire flow-field on multiple layers through the use of Particle Image Velocimetry (PIV). Typical problems that relate to flow seeding, light-sheet generation, and camera-laser timing control are successfully dealt with, bringing additional knowledge in the domain. An extensive data set is analyzed to obtain statistical flow information (mean and fluctuating quantities). Spatial data reconstruction procedures allow the complete planar flow distributions to be determined. The multiplanar mean velocity field constitutes the final result of a three-dimensional reconstruction of the domain geometry and the proper introduction of the available planar measurements. [S0889-504X(00)00304-4]
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The present study investigates the mechanisms associated with tonal noise emission from a NACA 0012 aerofoil at moderate incidence ($0^{\circ },1^{\circ },2^{\circ }$ and $4^{\circ }$ angle of attack) and with Reynolds numbers ranging... more
The present study investigates the mechanisms associated with tonal noise emission from a NACA 0012 aerofoil at moderate incidence ($0^{\circ },1^{\circ },2^{\circ }$ and $4^{\circ }$ angle of attack) and with Reynolds numbers ranging from 100 000 to 270 000. Simultaneous time-resolved particle image velocimetry (PIV) of the aeroacoustic source region near the trailing edge and acoustic measurements in the far field are performed in order to establish the correspondence between the flow structure and acoustic emissions. Results of these experiments are presented and analysed in view of past research for a number of selected cases. Characteristics of the acoustic emission and principal features of the average flow field agree with data presented in previous studies on the topic. Time-resolved analysis shows that downstream convecting vortical structures, resulting from growing shear layer instabilities, coherently pass the trailing edge at a frequency equal to that of the dominant to...
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The positive and negative high-amplitude pressure peaks (HAPP) are investigated in a turbulent boundary layer at $R{e}_{\theta } = $ 1900 in order to identify their turbulent structure. The three-dimensional velocity field is measured... more
The positive and negative high-amplitude pressure peaks (HAPP) are investigated in a turbulent boundary layer at $R{e}_{\theta } = $ 1900 in order to identify their turbulent structure. The three-dimensional velocity field is measured within the inner layer of the turbulent boundary layer using tomographic particle image velocimetry (tomo-PIV). The measurements are performed at an acquisition frequency of 10 000 Hz and over a volume of $418\times 149\times 621$ wall units in the streamwise, wall-normal and spanwise directions, respectively. The time-resolved velocity fields are applied to obtain the material derivative using the Lagrangian method followed by integration of the Poisson pressure equation to obtain the three-dimensional unsteady pressure field. The simultaneous volumetric velocity, acceleration, and pressure data are conditionally sampled based on local maxima and minima of wall pressure to analyse the three-dimensional turbulent structure of the HAPPs. Analysis has as...
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An experimental study is carried out to investigate the three-dimensional instantaneous structure of an incident shock wave/turbulent boundary layer interaction at Mach 2.1 using tomographic particle image velocimetry. Large-scale... more
An experimental study is carried out to investigate the three-dimensional instantaneous structure of an incident shock wave/turbulent boundary layer interaction at Mach 2.1 using tomographic particle image velocimetry. Large-scale coherent motions within the incoming boundary layer are observed, in the form of three-dimensional streamwise-elongated regions of relatively low- and high-speed fluid, similar to what has been reported in other supersonic boundary layers. Three-dimensional vortical structures are found to be associated with the low-speed regions, in a way that can be explained by the hairpin packet model. The instantaneous reflected shock wave pattern is observed to conform to the low- and high-speed regions as they enter the interaction, and its organization may be qualitatively decomposed into streamwise translation and spanwise rippling patterns, in agreement with what has been observed in direct numerical simulations. The results are used to construct a conceptual mod...