Characterization Of Turbulent Flow Research Articles

Advection-based temporal reconstruction technique for turbulent velocity fields

The characterization of turbulent flows is challenging due to the interaction of widespread spatiotemporal scales. Experimental techniques such as particle image velocimetry can be used to obtain spatially resolved flow measurements; however, these systems often suffer from limited acquisition rates. The present work investigates the ability of an advection-based flow reconstruction technique to increase the temporal resolution of turbulent flow data without any prior knowledge of the flow physics. A semi-Lagrangian technique is suggested to obtain fluid trajectories through a forward and backward integration of the available spatiotemporal data. The estimates are then fused using a temporal weighting scheme to yield velocity fields at intermediate times. The performance of the method is verified against three-dimensional direct numerical simulation (DNS) data of a plane jet at Re = 10 000. Extracting time series data from the spatially and temporally resolved DNS results, five test cases with artificially lowered sampling frequencies were generated. Spectral analysis revealed that the characteristic frequency found in shear layer-dominated flows can be obtained even for the most extreme case. Additionally, spectral information up to two orders of magnitude beyond the Nyquist criteria is successfully recovered throughout the spatial domain—surpassing the performance of previously introduced methods. The maximum spectral reconstruction error of the suggested method, defined as net energy loss or gain, fell within the bounds of [−5, 3]%, with a corresponding global energy difference in the range of [−2, 1]%. Furthermore, the spatially averaged reconstruction error for the velocity fluctuations was bound by [6±6]%.

Influence of Turbulent Flows in the Nozzle on Melt Flow Within a Slab Mold and Stability of the Metal–Flux Interface

The design of the ports of a casting nozzle has profound effects on the fluid flow patterns in slab molds. The influence of these outlets have also considerable effects on the turbulent flow and turbulence variables inside the nozzle itself. To understand the effects of nozzle design, three approaches were employed: a theoretical analysis based on the turbulent viscosity hypothesis, dimensional analysis (both analyses aided by computer fluid dynamics), and experiments using particle image velocimetry. The first approach yields a linear relation between calculated magnitudes of scalar fields of e (dissipation rate of kinetic energy) and k 2 (square of the turbulent kinetic energy), which is derived from the wall and the logarithmic-wall laws in the boundary layers. The smaller the slope of this linear relation is, the better the performance of a given nozzle is for maintaining the stability of the melt–flux interface. The second approach yields also a linear relation between flow rate of liquid metal and the cubic root of the dissipation rate of kinetic energy. In this case, the larger the slope of the linear relation is, the better the performance of a given nozzle is for maintaining the stability of the melt–flux interface. Finally, PIV measurements in a mold water model, together with equations for estimation of critical melt velocities for slag entrainment, were used to quantify the effects of nozzle design on the dynamics of the metal–slag interface. The three approaches agree in the characterization of turbulent flows in continuous casting molds using different nozzles.

Characterization of Turbulent Flow in a Flume with Surfactant

Summary The drag reducing effect of a surfactant additive from the alkylpolyglycosides family was investigated experimentally. In thepresent study, PIV measurements were done to investigate theturbulent flow in the streamwise-spanwise plane of a flume aty 1 580, in the buffer zone, where about 80% of the energy pro- Fig. 4 PDF’s of the u rms8 O U q for water —circles– and surfactant—squares–. u rms8 —x,z– ˜ −—u—x,z– A −u—x,z–‰– 2 ‰and the PDF is rep-resenting the spatial distribution of the rms values, normalizedby the flow-rate velocity U q .Fig. 5 Two-point—auto–correlation function R uu of the stream-wise velocity along the streamwise x and spanwise z coordi-nates, for water —circles and plus symbols– and for surfactant—square and triangular markers–, respectively.Table 1 Statistical properties of the Reynolds stress A −u 8 w 8 ‰ O U q2 .Statistics Maximum Minimum Average Std. dev. SkewnessWater 0.05 20.035 0.35310 23 0.0045 0.93Surfactant 0.03 20.035 0.35310 24 0.0032 0.003Table 2 Statistical properties of the turbulent energy production term

Miniature fiber optic pressure sensor for turbomachinery applications

Development of pressure sensors for the instrumentation of experimental aerodynamic facilities has traditionally concentrated on electrical techniques. An improvement in the currently attainable temporal and spatial resolution in pressure measurement would be beneficial in the characterization of turbulent flows behind turbine rotor stages, for example. We present results obtained in a turbine test rig from a fiber optic pressure sensor based upon the interferometric response of an extrinsic cavity formed between the interrogation fiber and a reflective diaphragm. We discuss the design trade offs, optical interrogation and temperature sensitivity of such a configuration, and demonstrate the success of the design in small-scale shock tube experiments. We then describe the application of the sensor in a full scale turbine test facility in which pressure signals with frequency components exceeding 200 kHz were obtained.

Characterization of Turbulent Flows by NMR Measurements with Pulsed Gradients

Abstract A theoretical description is given of the dependence of the signal from a turbulent flowing liquid in a pulsed-gradient nuclear magnetic resonance measurement. The relationships between the characteristic time scale of velocity fluctuations of the fluid and the timing parameters of the gradients and NMR pulse sequence are made explicit. An experimental approach has been developed by which the time scale of random changes in the velocity can be determined, and this has been used to show that pipe flow at Reynolds numbers of 0-3000, and grid-generated turbulence, are characterized by correlation times that are relatively long on the scale of the NMR measurement. The turbulent intensity has been measured at different spatial locations distal to meshes of different spatial frequency as well as in a pipe with Reynolds numbers of 0-6500. The onset of turbulence at Re ≍ 2300 in plain pipe flow and the spatial variation of turbulent intensity distal to each mesh are in good agreement,vith measurements made by other techniques. NMR may have unique advantages for spatial mapping and characterization of some turbulent flows.