This paper discusses and compares several methods, which aim to remove spurious peaks, i.e. ghost... more This paper discusses and compares several methods, which aim to remove spurious peaks, i.e. ghost particles, from the volume intensity reconstruction in tomographic-PIV. The assessment is based on numerical simulations of time-resolved tomographic-PIV experiments in linear shear flows. Within the reconstructed volumes, intensity peaks are detected and tracked over time. These peaks are associated with particles (either ghosts or actual particles) and are characterized by their peak intensity, size and track length. Peak intensity and track length are found to be effective in discriminating between most ghosts and the actual particles, although not all ghosts can be detected using only a single threshold. The size of the reconstructed particles does not reveal an important difference between ghosts and actual particles. The joint distribution of peak intensity and track length however does, under certain conditions, allow a complete separation of ghosts and actual particles. The ghosts can have either a high intensity or a long track length, but not both combined, like all the actual particles. Removing the detected ghosts from the reconstructed volume and performing additional MART iterations can decrease the particle position error at low to moderate seeding densities, but increases the position error, velocity error and tracking errors at higher densities. The observed trends in the joint distribution of peak intensity and track length are confirmed by results from a real experiment in laminar Taylor–Couette flow. This diagnostic plot allows an estimate of the number of ghosts that are indistinguishable from the actual particles.
Influence of Rotation Number to Coherent Structures and Torque Scaling in Turbulent Taylor-Couett... more Influence of Rotation Number to Coherent Structures and Torque Scaling in Turbulent Taylor-Couette Flow 1 SEDAT TOKGOZ, LaVision GmbH/TU Delft, GERRIT ELSINGA, RENE DELFOS, JERRY WESTERWEEL, TU Delft -Flow between two coaxial cylinders is called Taylor-Couette flow and has been studied extensively over the years. Due to the closed and well controlled character of the system, experimental studies with different measurement techniques mostly focused on the turbulent flow. Torque measurements performed at the turbulent range of Reynolds numbers showed change of the torque scaling with relative rotation speeds of the cylinders (i.e. rotation number). In this study, we use tomographic PIV to capture instantaneous three-dimensional flow structures in turbulent Taylor-Couette flow in order to study the mechanism that is responsible for the change of the torque. Time-averaging and auto-correlation of the data confirm the change of coherent structures with the rotation number in the mean flow. Spatial filtering of the instantaneous vector fields enables separating the contributions of small and large scale motions to the Reynolds stress. We show that combination of large scale azimuthal-small scale radial and large scale azimuthal-large scale radial motions are the dominant ones that are effecting the torque. Additionally, we observe change of the organisation of the coherent large scale structures with the rotation number, in relation to the change of the torque scaling.
The Netherlands -In this study, we use high-speed tomographic PIV to investigate the evolution of... more The Netherlands -In this study, we use high-speed tomographic PIV to investigate the evolution of turbulent flow structures in Taylor-Couette flow. High-speed tomographic PIV enables fully volumetric time-resolved measurements and is well-suited for this purpose. Presently, the turbulent flow is created by exact counter-rotation of the cylinders (Re i = -Re o , R Ω = 0.0), where the wall velocities are the same with opposite sign. Under these circumstances, the mean flow is zero in the bulk. Results indicate that the structures still advance in one direction despite the zero mean. However, the observation time for the flow structures is still at least one order magnitude longer than in the boundary layer flows, which were considered before. Results also revealed the presence of azimuthal velocity streaks. It is found that the intense vortical structures are mostly located in the shear layer between these streaks. Different events regarding the evolution of the vortical structures, such as stretching and break-up of vortices, are observed.
We report on the experimental investigation of the large-scale instantaneous flow structures in t... more We report on the experimental investigation of the large-scale instantaneous flow structures in turbulent Taylor-Couette flow using tomographic particle image velocimetry. The results indicate three distinct regimes for counter-rotating flow within a shear Reynolds number range of 11 000 < Re S < 47 000. Close to only inner cylinder rotation, large-scale structures are aligned in the azimuthal direction, similar to Taylor vortices. Near the point of only outer cylinder rotation, we observe columnar vortical structures in the axial direction, which are associated with small Rossby numbers. This is the first time such columnar structures are reported in a fully turbulent Taylor-Couette flow. A transition between these two regimes is observed around the point of exact counter-rotation, where the instantaneous azimuthal structures are inclined with respect to the walls. Furthermore, it is shown that the reported transitions in the turbulent flow structure modify the angular momentum transport, thereby affecting the torque scaling.
Sakarya University Journal of Science, Mar 16, 2022
In this study, we aim to investigate the low noise flights of owls in terms of aerodynamics. The ... more In this study, we aim to investigate the low noise flights of owls in terms of aerodynamics. The flow around cross-section of an owl wing, which is known for its nearly silent flight, is numerically analyzed using Computational Fluid Dynamics (CFD). The analysis are based on the parameters of angle of attack and the flight speed. The aerodynamic effects on the acoustic is compared in terms of vorticity and sound pressure level, where the frequency interval for the acoustic data is set to 0-7500Hz. It was seen that the vortical organisations around the airfoils are closely related to the acoustic results. The results show that the increase in both velocity and angle of attack affect the vorticity, thus lead to a rise in sound pressure level. It can be stated that the owl airfoil shape ensures a relatively silent flight.
Friction measurements were performed in a Taylor-Couette setup. Drag reduction was obtained with ... more Friction measurements were performed in a Taylor-Couette setup. Drag reduction was obtained with a riblet surface and indicated a drag reduction for a wide range of shear Reynolds numbers, with a maximum of 5.3% at Res 4.7 ¢ 10 4 (s 14). Tomographic PIV verified that the friction coefficients are strongly related to the flow regimes and structures. The bulk fluid rotation was changed by the application of the riblets, as the wall-bounded flow conditions at the inner cylinder wall were changed due to the surface modification and is called the rotation effect. A simple model was used to indicate the averaged bulk velocity shift (1.4%), after which the drag changes due to the rotation effect (-1.9%) and the riblet effect (-3.4%) were determined. The bulk velocity shift of 1.4% was verified by PIV measurements. Compliant surfaces will be further investigated to check their required conditions for drag reduction of wall-bounded flows.
In this paper, we study the spatial resolution and the accuracy of tomographic particle image vel... more In this paper, we study the spatial resolution and the accuracy of tomographic particle image velocimetry (PIV). Since the spatial resolution of tomographic PIV is limited by the maximum density of the recorded particle images, sometimes the number of vectors exceeds the number of reconstructed particles. Then the actual resolution of tomographic PIV comes into question. We use fully turbulent Taylor-Couette flow to investigate the question, where the Reynolds number is changed from 3800 to 47000. Taylor-Couette geometry consists of two coaxial transparent cylinders, which allows implementing non-intrusive measurement techniques in addition to torque measurements. Independent torque measurements on the cylinders provide information about Kolmogorov length scale and the energy dissipation rate. We compute the dissipation rate from tomographic PIV data and compare the values against the dissipation rate estimated by the torque measurements. We change the interrogation window size and overlap values to see the effect of different data processing parameters on the measurement results. The results show that for all cases the flow is under-resolved. However, the results improve for decreasing Reynolds numbers. It is shown that the actual spatial resolution is a function of both vector spacing and the interrogation window size. It is significant that an improvement is achieved by increasing window overlap ratio to 75%, compared against 0 and 50% overlap cases at a constant interrogation window size. We also propose a range of vector spacing values for fully resolving the turbulent flow scales.
Time-resolved tomographic PIV was used to investigate the time evolution of turbulent flow struct... more Time-resolved tomographic PIV was used to investigate the time evolution of turbulent flow structures in Taylor-Couette flow. Turbulence is created by the shear due to exact counter rotation of the cylinders, where the mean velocity is zero in the bulk flow. This enables to observe the structures longer than many other turbulent flow types, sometimes during their whole lifetime. Results showed that the structures are produced around streaks of positive and negative velocities. Larger structures appear in different shapes. Most dominant ones have tube-like shape and aligned in the azimuthal direction of the cylinders. Since the measurements are time-resolved in a 3D volume, it is possible to track individual structures over time and observe evolution of their shapes. The large scale structures are found to be advancing despite the zero mean velocity.
In this study, we used a combination of quantitative background oriented schlieren (BOS) and part... more In this study, we used a combination of quantitative background oriented schlieren (BOS) and particle image velocimetry (PIV) is used to simultaneously measure 2D temperature and velocity distribution inside a thin fluid layer, where optical access is limited to one side. Application of optical filters allowed us to make simultaneous BOS and PIV measurements.
In this study we experimentally investigate the change of torque at constant shear Re, and its re... more In this study we experimentally investigate the change of torque at constant shear Re, and its relation to the coherent flow structures in turbulent Taylor-Couette (TC) flow. Torque measurements at counterrotating turbulent regimes show a change depending on the rotation number. In order to understand the mechanism behind this change we used tomographic PIV and measured the instantaneous 3D flow structures in turbulent TC flow. The instantaneous flow fields are decomposed into large (ILS) and smaller-scale (ISS) motions to study their contributions separately. Three distinctive flow states were found at counterrotating turbulent flow, associated with clear changes in the ILS and ISS structure. Close to only inner cylinder rotation, where well-organised Taylor-vortex-like flow structures are observed, the mean flow is responsible for the torque values. Close to exact-counter rotation, inclined ILS vortices induce velocities in the azimuthal and radial directions, contributing significantly to the torque. Close to only outer cylinder rotation the ILS vortices start to align themselves in the axial direction, resembling co-rotating Taylor column-like structures, which reduces the measured torque. The change of the orientation of the ILS vortices is also confirmed quantitatively.
Quantitative background oriented schlieren (BOS) method is used to measure the 2D temperature dis... more Quantitative background oriented schlieren (BOS) method is used to measure the 2D temperature distribution inside a thin fluid layer, where optical access is limited to one side. Instantaneous temperature gradient fields are obtained by implementing a highly reflective surface below the fluid layer. The temperature gradients are integrated to compute the temperature distribution. Temperatures can be estimated with an accuracy of the order of 10 mK. In this study the BOS method is applied to parallel jet flows with temperature offset.
ABSTRACT The effect of flow structures to the torque values of fully turbulent Taylor-Couette flo... more ABSTRACT The effect of flow structures to the torque values of fully turbulent Taylor-Couette flow was experimentally studied using tomographic PIV. The measurements were performed for various relative cylinder rotation speeds and Reynolds numbers, based on a study of Ravelet et al. (2010). We confirmed that the flow structures are strongly influenced by the rotation number. Our analyses using time-averaged mean flow showed the presence of Taylor vortices for the two smallest rotation numbers that were studied. Increasing the rotation number initially resulted in the shape deformation of the Taylor vortices. Further increment towards only outer cylinder rotation, showed transition to the dominance of the small scale vortices and absence of Taylor vortex-like structures. We compared the transition of the flow structures with the curves of dimensionless torque. Sudden changes of the flow structures confirmed the presence of transition points on the torque curve, where the dominance of small and large scale vortical structures on the mean flow interchanges.
This paper assesses the spatial resolution and accuracy of tomographic particle image velocimetry... more This paper assesses the spatial resolution and accuracy of tomographic particle image velocimetry (PIV). In tomographic PIV the number of velocity vectors are of the order of the number of reconstructed particle images, and sometimes even exceeds this number when a high overlap fraction between adjacent interrogations is used. This raises the question of the actual spatial resolution of tomographic PIV in relation to the various flow scales. We use a Taylor-Couette flow of a fluid between two independently rotating cylinders and consider three flow regimes: laminar flow, Taylor vortex flow and fully turbulent flow. The laminar flow has no flow structures, and the measurement results are used to assess the measurement uncertainty and to validate the accuracy of the technique for measurements through the curved wall. In the Taylor vortex flow regime, the flow contains large-scale flow structures that are much larger than the size of the interrogation volumes and are fully resolved. The turbulent flow regime contains a range of flow scales. Measurements in the turbulent flow regime are carried out for a Reynolds number Re between 3,800 and 47,000. We use the measured torque on the cylinders to obtain an independent estimate of the energy dissipation rate and estimate of the Kolmogorov length scale. The data obtained by tomographic PIV are assessed by estimating the dissipation rate and comparing the result against the dissipation rate obtained from the measured torque. The turbulent flow data are evaluated for different sizes of the interrogation volumes and for different overlap ratios between adjacent interrogation locations. The results indicate that the turbulent flow measurements for the lowest Re could be (nearly) fully resolved. At the highest Re only a small fraction of the dissipation rate is resolved, still a reasonable estimate of the total dissipation rate could be obtained by means of using a sub-grid turbulence model. The resolution of tomographic PIV in these measurements is determined by the size of the interrogation volume. We propose a range of vector spacing for fully resolving the turbulent flow scales. It is noted that the use of a high overlap ratio, that is, 75 %, yields a substantial improvement for the estimation of the dissipation rate in comparison with data for 0 and 50 % overlap. This indicates that additional information on small-scale velocity gradients can be obtained by reducing the data spacing.
This paper discusses and compares several methods, which aim to remove spurious peaks, i.e. ghost... more This paper discusses and compares several methods, which aim to remove spurious peaks, i.e. ghost particles, from the volume intensity reconstruction in tomographic-PIV. The assessment is based on numerical simulations of time-resolved tomographic-PIV experiments in linear shear flows. Within the reconstructed volumes, intensity peaks are detected and tracked over time. These peaks are associated with particles (either ghosts or actual particles) and are characterized by their peak intensity, size and track length. Peak intensity and track length are found to be effective in discriminating between most ghosts and the actual particles, although not all ghosts can be detected using only a single threshold. The size of the reconstructed particles does not reveal an important difference between ghosts and actual particles. The joint distribution of peak intensity and track length however does, under certain conditions, allow a complete separation of ghosts and actual particles. The ghosts can have either a high intensity or a long track length, but not both combined, like all the actual particles. Removing the detected ghosts from the reconstructed volume and performing additional MART iterations can decrease the particle position error at low to moderate seeding densities, but increases the position error, velocity error and tracking errors at higher densities. The observed trends in the joint distribution of peak intensity and track length are confirmed by results from a real experiment in laminar Taylor–Couette flow. This diagnostic plot allows an estimate of the number of ghosts that are indistinguishable from the actual particles.
Influence of Rotation Number to Coherent Structures and Torque Scaling in Turbulent Taylor-Couett... more Influence of Rotation Number to Coherent Structures and Torque Scaling in Turbulent Taylor-Couette Flow 1 SEDAT TOKGOZ, LaVision GmbH/TU Delft, GERRIT ELSINGA, RENE DELFOS, JERRY WESTERWEEL, TU Delft -Flow between two coaxial cylinders is called Taylor-Couette flow and has been studied extensively over the years. Due to the closed and well controlled character of the system, experimental studies with different measurement techniques mostly focused on the turbulent flow. Torque measurements performed at the turbulent range of Reynolds numbers showed change of the torque scaling with relative rotation speeds of the cylinders (i.e. rotation number). In this study, we use tomographic PIV to capture instantaneous three-dimensional flow structures in turbulent Taylor-Couette flow in order to study the mechanism that is responsible for the change of the torque. Time-averaging and auto-correlation of the data confirm the change of coherent structures with the rotation number in the mean flow. Spatial filtering of the instantaneous vector fields enables separating the contributions of small and large scale motions to the Reynolds stress. We show that combination of large scale azimuthal-small scale radial and large scale azimuthal-large scale radial motions are the dominant ones that are effecting the torque. Additionally, we observe change of the organisation of the coherent large scale structures with the rotation number, in relation to the change of the torque scaling.
The Netherlands -In this study, we use high-speed tomographic PIV to investigate the evolution of... more The Netherlands -In this study, we use high-speed tomographic PIV to investigate the evolution of turbulent flow structures in Taylor-Couette flow. High-speed tomographic PIV enables fully volumetric time-resolved measurements and is well-suited for this purpose. Presently, the turbulent flow is created by exact counter-rotation of the cylinders (Re i = -Re o , R Ω = 0.0), where the wall velocities are the same with opposite sign. Under these circumstances, the mean flow is zero in the bulk. Results indicate that the structures still advance in one direction despite the zero mean. However, the observation time for the flow structures is still at least one order magnitude longer than in the boundary layer flows, which were considered before. Results also revealed the presence of azimuthal velocity streaks. It is found that the intense vortical structures are mostly located in the shear layer between these streaks. Different events regarding the evolution of the vortical structures, such as stretching and break-up of vortices, are observed.
We report on the experimental investigation of the large-scale instantaneous flow structures in t... more We report on the experimental investigation of the large-scale instantaneous flow structures in turbulent Taylor-Couette flow using tomographic particle image velocimetry. The results indicate three distinct regimes for counter-rotating flow within a shear Reynolds number range of 11 000 < Re S < 47 000. Close to only inner cylinder rotation, large-scale structures are aligned in the azimuthal direction, similar to Taylor vortices. Near the point of only outer cylinder rotation, we observe columnar vortical structures in the axial direction, which are associated with small Rossby numbers. This is the first time such columnar structures are reported in a fully turbulent Taylor-Couette flow. A transition between these two regimes is observed around the point of exact counter-rotation, where the instantaneous azimuthal structures are inclined with respect to the walls. Furthermore, it is shown that the reported transitions in the turbulent flow structure modify the angular momentum transport, thereby affecting the torque scaling.
Sakarya University Journal of Science, Mar 16, 2022
In this study, we aim to investigate the low noise flights of owls in terms of aerodynamics. The ... more In this study, we aim to investigate the low noise flights of owls in terms of aerodynamics. The flow around cross-section of an owl wing, which is known for its nearly silent flight, is numerically analyzed using Computational Fluid Dynamics (CFD). The analysis are based on the parameters of angle of attack and the flight speed. The aerodynamic effects on the acoustic is compared in terms of vorticity and sound pressure level, where the frequency interval for the acoustic data is set to 0-7500Hz. It was seen that the vortical organisations around the airfoils are closely related to the acoustic results. The results show that the increase in both velocity and angle of attack affect the vorticity, thus lead to a rise in sound pressure level. It can be stated that the owl airfoil shape ensures a relatively silent flight.
Friction measurements were performed in a Taylor-Couette setup. Drag reduction was obtained with ... more Friction measurements were performed in a Taylor-Couette setup. Drag reduction was obtained with a riblet surface and indicated a drag reduction for a wide range of shear Reynolds numbers, with a maximum of 5.3% at Res 4.7 ¢ 10 4 (s 14). Tomographic PIV verified that the friction coefficients are strongly related to the flow regimes and structures. The bulk fluid rotation was changed by the application of the riblets, as the wall-bounded flow conditions at the inner cylinder wall were changed due to the surface modification and is called the rotation effect. A simple model was used to indicate the averaged bulk velocity shift (1.4%), after which the drag changes due to the rotation effect (-1.9%) and the riblet effect (-3.4%) were determined. The bulk velocity shift of 1.4% was verified by PIV measurements. Compliant surfaces will be further investigated to check their required conditions for drag reduction of wall-bounded flows.
In this paper, we study the spatial resolution and the accuracy of tomographic particle image vel... more In this paper, we study the spatial resolution and the accuracy of tomographic particle image velocimetry (PIV). Since the spatial resolution of tomographic PIV is limited by the maximum density of the recorded particle images, sometimes the number of vectors exceeds the number of reconstructed particles. Then the actual resolution of tomographic PIV comes into question. We use fully turbulent Taylor-Couette flow to investigate the question, where the Reynolds number is changed from 3800 to 47000. Taylor-Couette geometry consists of two coaxial transparent cylinders, which allows implementing non-intrusive measurement techniques in addition to torque measurements. Independent torque measurements on the cylinders provide information about Kolmogorov length scale and the energy dissipation rate. We compute the dissipation rate from tomographic PIV data and compare the values against the dissipation rate estimated by the torque measurements. We change the interrogation window size and overlap values to see the effect of different data processing parameters on the measurement results. The results show that for all cases the flow is under-resolved. However, the results improve for decreasing Reynolds numbers. It is shown that the actual spatial resolution is a function of both vector spacing and the interrogation window size. It is significant that an improvement is achieved by increasing window overlap ratio to 75%, compared against 0 and 50% overlap cases at a constant interrogation window size. We also propose a range of vector spacing values for fully resolving the turbulent flow scales.
Time-resolved tomographic PIV was used to investigate the time evolution of turbulent flow struct... more Time-resolved tomographic PIV was used to investigate the time evolution of turbulent flow structures in Taylor-Couette flow. Turbulence is created by the shear due to exact counter rotation of the cylinders, where the mean velocity is zero in the bulk flow. This enables to observe the structures longer than many other turbulent flow types, sometimes during their whole lifetime. Results showed that the structures are produced around streaks of positive and negative velocities. Larger structures appear in different shapes. Most dominant ones have tube-like shape and aligned in the azimuthal direction of the cylinders. Since the measurements are time-resolved in a 3D volume, it is possible to track individual structures over time and observe evolution of their shapes. The large scale structures are found to be advancing despite the zero mean velocity.
In this study, we used a combination of quantitative background oriented schlieren (BOS) and part... more In this study, we used a combination of quantitative background oriented schlieren (BOS) and particle image velocimetry (PIV) is used to simultaneously measure 2D temperature and velocity distribution inside a thin fluid layer, where optical access is limited to one side. Application of optical filters allowed us to make simultaneous BOS and PIV measurements.
In this study we experimentally investigate the change of torque at constant shear Re, and its re... more In this study we experimentally investigate the change of torque at constant shear Re, and its relation to the coherent flow structures in turbulent Taylor-Couette (TC) flow. Torque measurements at counterrotating turbulent regimes show a change depending on the rotation number. In order to understand the mechanism behind this change we used tomographic PIV and measured the instantaneous 3D flow structures in turbulent TC flow. The instantaneous flow fields are decomposed into large (ILS) and smaller-scale (ISS) motions to study their contributions separately. Three distinctive flow states were found at counterrotating turbulent flow, associated with clear changes in the ILS and ISS structure. Close to only inner cylinder rotation, where well-organised Taylor-vortex-like flow structures are observed, the mean flow is responsible for the torque values. Close to exact-counter rotation, inclined ILS vortices induce velocities in the azimuthal and radial directions, contributing significantly to the torque. Close to only outer cylinder rotation the ILS vortices start to align themselves in the axial direction, resembling co-rotating Taylor column-like structures, which reduces the measured torque. The change of the orientation of the ILS vortices is also confirmed quantitatively.
Quantitative background oriented schlieren (BOS) method is used to measure the 2D temperature dis... more Quantitative background oriented schlieren (BOS) method is used to measure the 2D temperature distribution inside a thin fluid layer, where optical access is limited to one side. Instantaneous temperature gradient fields are obtained by implementing a highly reflective surface below the fluid layer. The temperature gradients are integrated to compute the temperature distribution. Temperatures can be estimated with an accuracy of the order of 10 mK. In this study the BOS method is applied to parallel jet flows with temperature offset.
ABSTRACT The effect of flow structures to the torque values of fully turbulent Taylor-Couette flo... more ABSTRACT The effect of flow structures to the torque values of fully turbulent Taylor-Couette flow was experimentally studied using tomographic PIV. The measurements were performed for various relative cylinder rotation speeds and Reynolds numbers, based on a study of Ravelet et al. (2010). We confirmed that the flow structures are strongly influenced by the rotation number. Our analyses using time-averaged mean flow showed the presence of Taylor vortices for the two smallest rotation numbers that were studied. Increasing the rotation number initially resulted in the shape deformation of the Taylor vortices. Further increment towards only outer cylinder rotation, showed transition to the dominance of the small scale vortices and absence of Taylor vortex-like structures. We compared the transition of the flow structures with the curves of dimensionless torque. Sudden changes of the flow structures confirmed the presence of transition points on the torque curve, where the dominance of small and large scale vortical structures on the mean flow interchanges.
This paper assesses the spatial resolution and accuracy of tomographic particle image velocimetry... more This paper assesses the spatial resolution and accuracy of tomographic particle image velocimetry (PIV). In tomographic PIV the number of velocity vectors are of the order of the number of reconstructed particle images, and sometimes even exceeds this number when a high overlap fraction between adjacent interrogations is used. This raises the question of the actual spatial resolution of tomographic PIV in relation to the various flow scales. We use a Taylor-Couette flow of a fluid between two independently rotating cylinders and consider three flow regimes: laminar flow, Taylor vortex flow and fully turbulent flow. The laminar flow has no flow structures, and the measurement results are used to assess the measurement uncertainty and to validate the accuracy of the technique for measurements through the curved wall. In the Taylor vortex flow regime, the flow contains large-scale flow structures that are much larger than the size of the interrogation volumes and are fully resolved. The turbulent flow regime contains a range of flow scales. Measurements in the turbulent flow regime are carried out for a Reynolds number Re between 3,800 and 47,000. We use the measured torque on the cylinders to obtain an independent estimate of the energy dissipation rate and estimate of the Kolmogorov length scale. The data obtained by tomographic PIV are assessed by estimating the dissipation rate and comparing the result against the dissipation rate obtained from the measured torque. The turbulent flow data are evaluated for different sizes of the interrogation volumes and for different overlap ratios between adjacent interrogation locations. The results indicate that the turbulent flow measurements for the lowest Re could be (nearly) fully resolved. At the highest Re only a small fraction of the dissipation rate is resolved, still a reasonable estimate of the total dissipation rate could be obtained by means of using a sub-grid turbulence model. The resolution of tomographic PIV in these measurements is determined by the size of the interrogation volume. We propose a range of vector spacing for fully resolving the turbulent flow scales. It is noted that the use of a high overlap ratio, that is, 75 %, yields a substantial improvement for the estimation of the dissipation rate in comparison with data for 0 and 50 % overlap. This indicates that additional information on small-scale velocity gradients can be obtained by reducing the data spacing.
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