Large eddy simulation of compressible round jets with coflow

Large eddy simulations of subsonic round jets are carried out using high order compact finite difference scheme and an explicit filtering based approximate deconvolution method. The jets have a Mach number of 0.9 and Reynolds number of $4.5\times {10}^5$ based on jet diameter and centerline velocity at inflow. Results obtained for the mean flow and turbulence intensities agree well with those in existing literature. We also study the effects of co-flow velocity ratio on the flow physics. Increase in potential core length and decrease in spreading rate of jet is observed in the presence of co-flow. The effects of co-flow velocity ratio on the axial Reynolds stress and turbulent kinetic energy budgets are also presented. It is observed that increasing co-flow velocity ratio leads to reduction in the turbulence intensities and near-field sound levels.     

Flow through an elbow: A direct numerical simulation investigating turbulent flow quantities

Turbulent flow in a sharp $90°$ elbow in a square duct was numerically investigated by performing a direct numerical simulation (DNS) and the results were compared to experimental and Reynolds Averaged Navier–Stokes (RANS) data. This is the first part of an effort to expand the understanding of particle transport in complex geometries. The paper is divided into two parts: a validation of the flow, and then a discussion of additional flow quantities that are important for modeling particles but were not measured experimentally. In the validation section the DNS results were compared to experimental and RANS data at a Reynolds number of $11,500$. Profiles of the mean and root-mean-square (RMS) fluctuating velocities were compared at various points along the elbow’s midplane. Upstream of the bend, the predicted mean and RMS velocities from the RANS and DNS simulations compared well with the experiment, differing only slightly near the walls. Downstream of the bend the DNS and the experimental results were virtually identical, varying by no more than $2\%$. However, the RANS results deviated, showing a more extended region of flow re-circulation, causing the mean and RMS velocities to differ by as much as $40\%$. After the validation, one of the additional quantities was the secondary flow structures in the plane perpendicular to the mean flow direction. The RANS and DNS showed similar results upstream of the bend, exhibiting in-plane vortices of the second-kind. Downstream, the vortical flows of the first-kind were observed with a magnitude of about $40\%$ of the mean flow and differed by about $5\%$ between the DNS and the RANS. Eulerian time scales at different locations upstream and downstream of the elbow were also evaluated. The upstream Eulerian time scales showed trends similar to channel flow data, with maximum time scales near the wall. The downstream time scales were qualitatively different showing non monotonic behavior across the channel and values that were significantly different than a channel flow.     

Prediction of drag reduction effect by streamwise traveling wave-like wall deformation in turbulent channel flow at practically high Reynolds numbers

A fully developed turbulent channel flow controlled by traveling wave-like wall deformation under a constant pressure gradient condition is studied numerically and theoretically. First, direct numerical simulation (DNS) at three different friction Reynolds numbers, ${\text{Re}}_{\tau }=90,$ 180, and 360, are performed to investigate the modification in turbulence statistics and their scaling. Unlike the previous study assuming a constant flow rate condition, suppression of the quasi-streamwise vortices is not observed in either drag decrease cases or drag increase cases. It is found in the drag reduction case, however, that the periodic component of the Reynolds shear stress (periodic RSS) is largely negative in the viscous sublayer and the buffer layer. For the maximum drag reduction case, the set of control parameters is found to be identical in wall units regardless of the Reynolds number, and the resulting mean velocity profiles are also observed to be approximately similar even with an additional case of ${\text{Re}}_{\tau }=720$. Based on this scaling, we propose a semi-empirical formula for the mean velocity profile modified by the present control. With this formula, about $20\%-25\%$ drag reduction effect is predicted even at practically high Reynolds numbers, ${\text{Re}}_{\tau }\sim {10}^5-{10}^6$.