Rio Baidya

Enhancing Tomo-PIV reconstruction quality by reducing ghost particles

A technique to enhance the reconstruction quality and consequently the accuracy of the velocity vector field obtained in Tomo-PIV experiments is presented here. The methodology involves detecting and eliminating spurious outliers in the reconstructed intensity field (ghost particles). A simulacrum matching-based reconstruction enhancement (SMRE) technique is proposed, which utilizes the characteristic shape and size of actual particles to remove ghost particles in the reconstructed intensity field. An assessment of SMRE is performed by a quantitative comparison of Tomo-PIV simulation results and DNS data, together with a comparison to Tomo-PIV experimental data measured in a turbulent channel flow at a matched Reynolds number (Reτ = 937) to the DNS study. For the simulation data, a comparative study is performed on the reconstruction quality based on an ideal reconstruction determined from known particle positions. The results suggest that a significant improvement in the reconstruction quality and flow statistics is achievable at typical seeding densities used in Tomo-PIV experiments. This improvement is further amplified at higher seeding densities, enabling the use of up to twice the typical seeding densities currently used in Tomo-PIV experiments. A reduction of spurious vectors present in the velocity field is also observed based on a median outlier detection criterion. The application of SMRE to Tomo-PIV experimental data shows an improvement in flow statistics, comparable to the improvement seen in simulations. Finally, due to the non-iterative nature of SMRE, the increase in processing time is marginal since only a single pass of the reconstruction algorithm is required.

Spatial averaging of streamwise and spanwise velocity measurements in wall-bounded turbulence using ∨- and ×-probes

The effect of finite dimensions of ?- and ?-probes is investigated for the measurement of mean and variances of streamwise and spanwise velocities in wall-turbulence. The probes are numerically simulated using a Direct Numerical Simulation database of channel flow at a friction Reynolds number (Re?) of 934 by varying the probe parameters, namely, the wire-lengths (l), the angle between the wires (?) and the spacing between the wires (?s). A single inclined wire is first studied to isolate the effect of l and ?. Analytical expressions for the variances of the streamwise and spanwise velocities are derived by applying a linear-box-type filter to the unfiltered velocity field for both ?- and ?-probes (at ? = 45?, and arbitrary l and ?s). A similar expression for the streamwise variance in the case of a single inclined wire (for arbitrary l and ?) is also derived. These analytical expressions, supplemented with a model for the correlation over the wire-length, compare favourably with the numerical simulation results, and more importantly explain various trends that are observed in the variances with varying parameters. Close to the wall (where the errors are generally higher) the errors in spanwise variances of the ?-probes are much lower than the ?-probes, owing to an ?error-cancelling? mechanism present in ?-probes due to the effect of l and ?s, as well as due to the procedure of recovering the velocities from two wires. The errors in the streamwise variances are comparable for both ?- and ?-probes. On the other hand, mean velocities are measured with almost no error by the ?-probe, whereas the ?-probe induces finite errors in mean velocities due to the fact that the two wires experience different mean velocities in ?-probes unlike ?-probes. These results are explained using the corresponding analytical results, which also show that under the effect of a linear filter, measured variances depend only on the fluctuating velocities (more precisely on the two-point correlations) and the measured means depend only on the unfiltered mean. The various results are found to be in accordance with the experimental measurements carried out in a turbulent boundary layer at Re? ? 5000. Finally, considering the physical positioning of wires in the ?- and ?-probes combined with the above results suggests that ?-probes might be more suitable for stream?spanwise velocity measurements than the ?-probes in conventional wind tunnels.

Distance-from-the-wall scaling of turbulent motions in wall-bounded flows

An assessment of self-similarity in the inertial sublayer is presented by considering the wall-normal velocity, in addition to the streamwise velocity component. The novelty of the current work lies in the inclusion of the second velocity component, made possible by carefully conducted subminiature ×-probe experiments to minimise the errors in measuring the wall-normal velocity. We show that not all turbulent stress quantities approach the self-similar asymptotic state at an equal rate as the Reynolds number is increased, with the Reynolds shear stress approaching faster than the streamwise normal stress. These trends are explained by the contributions from attached eddies. Furthermore, the Reynolds shear stress cospectra, through its scaling with the distance from the wall, are used to assess the wall-normal limits where self-similarity applies within the wall-bounded flow. The results are found to be consistent with the recent prediction from the work of Wei et al. [“Properties of the mean momentum bala...

Two-dimensional energy spectra in high-Reynolds-number turbulent boundary layers

Here we report the measurements of two-dimensional (2-D) spectra of the streamwise velocity (

Amplitude modulation of all three velocity components in turbulent boundary layers

u

Assessment of tomographic PIV in wall-bounded turbulence using direct numerical simulation data

) in a high Reynolds number turbulent boundary layer. A novel experiment employing multiple hot-wire probes was carried out at friction Reynolds numbers ranging from 2400 to 26000. Taylors frozen turbulence hypothesis is used to convert temporal-spanwise information into a 2-D spatial spectrum which shows the contribution of streamwise (

Temporally optimized spanwise vorticity sensor measurements in turbulent boundary layers

\lambda_x

Structure function tensor scaling in the logarithmic region derived from the attached eddy model of wall-bounded turbulent flows

) and spanwise (

Large coherence of spanwise velocity in turbulent boundary layers

\lambda_y

Trajectory of a synthetic jet issuing into a high Reynolds number turbulent boundary layer

) length scales to the streamwise variance at a given wall height (