Strength Of Streamwise Vortices Research Articles

Influence of rounding corners on the wake of a finite-length cylinder: An experimental study

This study aims to investigate experimentally the influence of rounding corners (r) as well as aspect ratio (AR) on the flow structures of a surface-mounted finite cylinder. The cylinders with sharp (r* = r/D = 0) and rounded corners (r*=0.167, 0.25 and 0.5) and aspect ratio or height-to-width/diameter ratio (AR = H/D) between 2 and 7 are utilized. The experiments are based on the five-hole probe and hot-wire measurements as well as the oil flow visualization. Wake measurements are made in an open return wind tunnel at the Reynolds number, Re = 1.6 × 104, where Re is defined based on the side width/diameter (D) of the cylinder cross-section and the freestream velocity. It is found that r* and AR have significant effects on the flow structure from the perspective of wake topology, strength of streamwise vortices, and vortex shedding frequency. For all r* considered, the wake is characterized by a quadrupole type (both the tip and base vortices are present) at AR = 7, while a dipole type occurs for AR = 2 and 4 (the base vortices are absent). The strength (circulation) of the streamwise vortex structures is affected by r*. For all AR examined in the present study, the strengths of tip and base vortex structures decrease with increasing r*. The oil flow visualization demonstrates that the features of the horseshoe vortex are sensitive to r* and AR. With increasing r*, the location of the separation line moves downstream and the distance between horseshoe vortex legs decreases. Velocity measurements reveal that the downwash flow enhances with increasing r*. It is also found that the Strouhal number increases progressively by 60% as r* increases from 0 to 0.5, regardless of AR.

Real-time processing methods to characterize streamwise vortices

One of the bottlenecks of an active control of turbulent boundary layers based on particle image velocimetry (PIV) measurements relates to the ability to process PIV images and extract useful for control information in real-time, on a time scale relevant for the selected control strategy. We propose two methodologies to extract useful information from PIV measurements in the y−z spanwise - wall normal cross-section, based on characterisation of properties such as size and strength of streamwise vortices. The vortices are created by active vortex generators embedded in a fully turbulent boundary layer. The proposed methodologies combine vortex identification and characterisation methods that use instantaneous PIV realisations to extract centers and strengths of streamwise vortices. The main purpose is to compare a standard vortex identification method that provides a robust and accurate estimate with another, ad-hoc method that is less robust or accurate but has a large computational speed gain potential. For demonstration purposes we use PIV measurements obtained at the wind tunnel facility downstream active vortex generators (Carlier and Stanislas, 2005; Foucaut et al., 2014). The first algorithm uses the Q-criteria and the integration of vorticity of each extracted vortex. This robust and accurate method requires the full spatially resolved PIV field followed by the computationally expensive spatial derivatives calculations and an integration. The second algorithm utilises the prior knowledge about the presence and shape of streamwise vortices expected in the measurement, therefore uses only a single line velocity profile estimate and can be parallelized to obtain few horizontal velocity profiles at different wall-normal distances. We compare the two methods and discuss their potential in term of computation times and robustness. Results show that in a turbulent boundary layer, a moving window average of a number of instantaneous fields is needed for both methods.

Management of Stronger Wall Jet by a Streamwise Vortex with Periodic Perturbation (Analysis by Phase Averaging)

In a stronger wall jet managed by a streamwise vortex, effect of periodic perturbation on the interaction and evolution processes has been investigated in the equations derived from phase averaging and triple decomposition. Transverse and spanwise locations of vortex center have time variations associated with variation in strength of streamwise vortex. Normal Reynolds stress difference (V2-W2) and shear stress VW generated by periodic perturbation reduce decay rate of maximum vorticity and accelerate growth rate of spanwise vortex radius in terms of production terms in transport equation for streamwise vorticity. Prediction of Reynolds shear stress component VW by taking account of time variation of both vortex center and strength successfully explains experimental data of conventional time average stress of vw.

Dual stream axisymmetric mixing in the presence of axial vorticity

An experimental investigation was undertaken to study mixing in a coaxial jet where the inner nozzle is a lobed mixer. Four inner jet nozzle geometries were explored: a baseline circular jet and three different sixlobed nozzles. Also, three velocity ratios of 3:1, 1:1, and 1:3 (inner:outer) were examined. Flow visualizations using a passive scalar were performed using a laser sheet from a Nd:YAG laser. The laser has a 9-ns pulse duration that effectively freezes the flow. Instantaneous cross-sectional images were taken as well as stream wise views. The effects of various parameters such as the interfacial area increase due to the lobed nozzle geometry, the strength of stream wise vortices, and the large-scale structures on the mixing process are evaluated and discussed. Results showed highly enhanced mixing as the strength of the streamwise vortices increased. In addition, the presence of large-scale structures due to the Kelvin-Helmholtz instabilities, and their interaction with streamwise vortices generated by the lobes were found to be crucial for enhanced mixing. The fraction of mixing enhancement due to streamwise vorticity (relative to mixing enhancement due to increased interfacial contact area) was found to increase as velocity ratio increased. This fraction also increased with downstream distance.