The structure of a jet in cross flow at low velocity ratios

Abstract

This paper examines in detail the flow structure and associated wall pressure fluctuations caused by the injection of a round, turbulent jet into a turbulent boundary layer. The velocity ratio, r, ratio of mean jet velocity to the mean cross flow, varies from 0.5 to 2.5 and the Reynolds number based on the cross flow speed and jet diameter is 1.9ϫ10 4 . Particle image velocimetry is used to measure the flow and flush mounted pressure sensors installed at several locations used to determine the wall pressure. The results consist of sample instantaneous flow structures, distributions of mean velocity, vorticity and turbulence intensity, as well as wall pressure spectra. The flow structure depends strongly on the velocity ratio and there are two distinctly different regions. At low velocity ratios, namely rϽ2, a semicylindrical vortical layer ͑''shell''͒ forms behind the jet, enclosing a domain with slow moving reverse flow. The vorticity in this semicylindrical shell originates from the jet shear layer. Conversely, at high velocity ratios, namely rϾ2, the near-wall flow behind the jet resembles a Karman vortex street and the wall-normal vortical structures contain cross flow boundary layer vorticity. Autospectra of the pressure signals show that the effect of the jet is mainly in the 15-100 Hz range. At rϽ2, the wall pressure fluctuation levels increase with r. At rϾ2, the wall pressure levels reach a plateau demonstrating the diminishing effect of the jet on the near-wall flow. Consistent with the flow structure, the highest wall pressure fluctuations occur off the jet centerline for rϽ2 and along the jet centerline for rϾ2. Also, the advection speed of near-wall vortical structures increase with r at rϽ2, while at rϾ2 it is a constant.

A Theoretical Approach to Facilitating Transition Phase Motion in a Positively Buoyant Autonomous Underwater Vehicle

“…Furthermore, detailed experimental studies of jets emitting from flat plates into ambient flows have shown that for low speed ratios the jet dominates the ambient flow. Hence the low pressure region is created solely by the induced separation of the ambient flow boundary layer [18,19]. That is, for low speed ratios the reduction in effective force is solely a function of the ambient flow speed and is independent of the jet exit speed.…”

Modelling Tunnel Thrusters for Autonomous Underwater Vehicles

“…Figure 1 shows data presented in terms of the speed ratio, however some authors have presented data to show that the decrease in effective force is solely a function of the ambient flow speed and therefore independent of the jet speed (and hence thrust) (Brix & Bussemaker, 1973;Karlikov & Sholomovich, 1998). In addition to this, detailed experimental studies of jets emitting from flat plates into ambient flows have shown that for small speed ratios the jet dominates the ambient flow and hence the low pressure region is created solely by the separation of the ambient flow boundary layer (Gopalan et al, 2004;Fric & Roshko, 1994). That is, for low speed ratios the reduction in effective force is solely a function of the ambient flow speed and is independent of the jet exit speed.…”

Unsteady wake vortices in jets in cross-flow

“…Wake vortices are variously referred to in the literature as 'fire devils ' (McMahon et al 1971), 'spin-off vortices' (Wu et al 1988), 'fingers' (Lozano et al 1994), 'upright' (Kelso et al 1996), 'zipper' (Morton & Ibbetson, 1996), 'tornado-like' or Fric's vortices (Rivero et al 2001), and the 'wall-normal' vortices (Gopalan et al 2004). Although the wake vortices of the JICF have been investigated since the 1970s (e.g., McMahon et al, 1971), the question of their origin, statistics, and potential technological applications still persists.…”

“…The pattern recognition analysis of the velocity field revealed strong fluctuations of CVP driven by these structures. Gopalan et al (2004) used particle image velocimetry (PIV), flow visualization, and wall pressure measurement to study the near-field of a water jet issuing from a fully developed turbulent pipe flow into a turbulent boundary layer, at velocity ratios 0.5 B VR B 2.5 and a Reynolds number of 19,000. For VR \ 2, a semicylindrical 'vortical shell' extending from the underside of the jet and connecting to the wall enclosed the reverse flow region.…”