Spatio-temporal Signatures of Elasto-inertial Turbulence in Viscoelastic Planar Jets

The interplay between viscoelasticity and inertia in dilute polymer solutions at high deformation rates can result in inertio-elastic instabilities. The nonlinear evolution of these instabilities generates a state of turbulence with significantly different spatio-temporal features compared to Newtonian turbulence, termed elasto-inertial turbulence (EIT). We explore EIT by studying the dynamics of a submerged planar jet of a dilute aqueous polymer solution injected into a quiescent tank of water using a combination of schlieren imaging and laser Doppler velocimetry (LDV). We show how fluid elasticity has a nonmonotonic effect on the jet stability depending on its magnitude, creating two distinct regimes in which elastic effects can either destabilize or stabilize the jet. In agreement with linear stability analyses of viscoelastic jets, an inertio-elastic shear-layer instability emerges near the edge of the jet for small levels of elasticity, independent of bulk undulations in the fluid column. The growth of this disturbance mode destabilizes the flow, resulting in a turbulence transition at lower Reynolds numbers and closer to the nozzle compared to the conditions required for the transition to turbulence in a Newtonian jet. Increasing the fluid elasticity merges the shear-layer instability into a bulk instability of the jet column. In this regime, elastic tensile stresses generated in the shear layer act as an "elastic membrane'" that partially stabilizes the flow, retarding the transition to turbulence to higher levels of inertia and greater distances from the nozzle. In the fully turbulent state far from the nozzle, planar viscoelastic jets exhibit unique spatio-temporal features associated with EIT. The time-averaged angle of jet spreading, an Eulerian measure of the degree of entrainment, and the centerline velocity of the jets both evolve self-similarly with distance from the nozzle. LDV measurements of the velocity fluctuations at the jet centerline reveal a frequency spectrum characterized by a $-3$ power-law exponent, different from the well-known $-5/3$ power-law exponent characteristic of Newtonian turbulence. We show that the higher spectral energy of long wavelength modes in the EIT state results in coherent structures that are elongated in the streamwise direction, consistent with the suppression of streamwise vortices by elastic stresses

Building a Criterion-Referenced Test in Measurement and Evaluation and Determining Its Cut-Off Score by Several Methods

The current study aimed to build a criterion-referenced test in measurement and evaluation and determine its cut-off score by several methods. The primary test form had 45 items, which a group of professors and measurement and evaluation experts reviewed; their comments and feedback were taken into account, and the final test form had 40 items. The test has been presented to 174 university students to examine its psychometric characteristics. Multiple statistical techniques were later performed using the SPSS program, and the results show that the discrimination and difficulty coefficients ranged from 0.36 to 0.82. Additionally, the test reliability was calculated using the Kuder-Richardson -20 and Spilt half statistical methods, and the concurrent validity was 0.76. The results showed that the value of the Kuder-Richardson -20 method was 0.81, while the value of the Spilt-Half method was 0.79. Finally, the cut-off score has been calculated using four methods, and the results indicate that the Angoff method value was 65%, the Nedelsky method was 64%, the contrasting groups’ method was 68%, and the criterion groups’ method was 62%. Keywords: criterion-referenced test, measurement, and evaluation, university student, the cut-off score. DOI: 10.7176/JEP/14-1-06 Publication date: January 31st 202

Spectral Universality of Elastoinertial Turbulence

Dissolving small amounts of polymer into a Newtonian fluid can dramatically change the dynamics of transitional and turbulent flows. We investigate the spatiotemporal dynamics of a submerged jet of dilute polymer solution entering a quiescent bath of Newtonian fluid. High-speed digital Schlieren imaging is used to quantify the evolution of Lagrangian features in the jet revealing a rich sequence of transitional and turbulent states. At high levels of viscoelasticity, we identify a new distinct transitional pathway to elastoinertial turbulence (EIT) that does not feature the conventional turbulent bursts and instead proceeds via a shear-layer instability that produces elongated filaments of polymer due to the nonlinear effects of viscoelasticity. Even though the pathways to the EIT state can be different, and within EIT the spatial details of the turbulent structures vary systematically with polymer microstructure and concentration, there is a universality in the power-law spectral decay of EIT with frequency, f^{-3}, independent of fluid rheology and flow parameters