Paul J Strykowski

The effect of counterflow on the development of compressible shear layers

A compressible countercurrent shear layer was investigated experimentally by establishing reverse flow around the perimeter of a supersonic jet. Measurements demonstrate that spatial growth rates of the countercurrent shear layer significantly exceed those of the classical coflowing layer at comparable density ratios and levels of compressibility. Experiments also reveal the presence of coherent three-dimensional structures in the countercurrent shear layer at convective Mach numbers where similar structures are not present in coflowing layers. It is argued that these kinematic differences are responsible for the enhanced diffusion of the shear layer with counterflow. The spatio-temporal theory is used to examine the connection between the experimental observations and the existence of a transition from convective to absolute instability in high-speed shear layers.

Turbulent structure and entrainment in heated jets : the effect of initial conditions

The turbulent structure in the near field of heated jets was investigated at a Reynolds number of 10 000 for density ratios between 1.0 and 0.5; the corresponding mixing and entrainment of these low density jets was examined. The exit conditions of the jet were carefully controlled using extension tubes to alter the nozzle‐exit boundary layer thickness, as well as using screens to generate turbulent conditions while keeping the boundary layer thickness approximately constant. Heated jets with initially laminar exit conditions were dominated by the formation and pairing of vortex structures. Increasing the boundary layer thickness produced longer wavelength structures which saturated and paired at farther downstream distances; under these conditions jet momentum mixing was reduced, giving rise to an increase in the jet potential core length. The shear layer structures also became more organized as the jet density was reduced relative to the density of the ambient fluid, resulting in increased momentum mixing and a dramatic visual spreading of the jet. Turbulent exit conditions disrupted the formation of these large‐scale vortex structures for all of the density ratios investigated, producing smaller spreading rates and significantly lower mixing and entrainment.

Mixing enhancement due to global oscillations in jets with annular counterflow

A novel control technique is described which leads to the enhancement of mixing in axisymmetric jets. The approach relies on the self-excitability of the jet and does not require elaborate feedback control schemes or actuators. Self-excited global oscillations are established in the jet by applying suction to an annular cavity placed around the jet periphery. These oscillations are responsible for enhanced mixing between the jet and the surrounding fluid, and can be maintained at relatively low levels of counterflow. Furthermore, the global oscillations and increased mixing appear to be insensitive to the jet initial conditions. The present study included jet Reynolds numbers between 3.4×10 4 and 1.1×10 5 for initially laminar and turbulent separated shear layers

Stabilization effects in flow through helically coiled pipes

When a flow through a straight pipe is passed through a coiled section, two stabilizing effects come into play. First, in a certain Reynolds number range, the flow that is turbulent in the straight pipe becomes completely laminar in the coiled section. Second, the stabilization effect of the coil persists to a certain degree even after the flow downstream of the coil has been allowed to develop in a long straight section. In this paper, we report briefly on aspects related to these two effects.

Heat transfer - A review of 2001 literature

2. Conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 2.1. Contact conduction and contact resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 2.2. Micro/nanoscale thermal effects, laser pulse heating, and hyperbolic heat transport . . 1892 2.3. Composites, heterogeneous media and complex geometries . . . . . . . . . . . . . . . . . . . 1893 2.4. Conduction with convection, phase change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893 2.5. Analytical, numerical and experimental studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893 2.6. Thermomechanical problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893 2.7. Miscellaneous and special applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893

Origin of streamwise vortices in supersonic jets

Our purpose is to clarify the initial conditions necessary for the development of streamwise vortices observed in underexpanded jets. In particular, we examine whether the Taylor-Gortler instability established in the curved shear layers of an underexpanded jet is a sufficient condition for the formation of streamwise vortices

The influence of velocity and density ratio on the dynamics of spatially developing mixing layers

The dynamics of countercurrent mixing is examined in the shear layer of an axisymmetric jet. Experiments were designed to establish conditions of absolute instability in a spatially developing shear layer and to document how the instability influences the jet development. By applying suction around the jet periphery, shear‐layer velocity ratios R greater than 1 could be studied. Here, R=(U1−U2)/(U1+U2), where U1 is the velocity of the forward jet stream and U2 is the velocity of the counterflowing stream created by suction. The density ratio S=ρ1/ρ2 of the mixing layer was also varied to determine the stability boundary in the S‐R plane. The density of the forward stream ρ1 was increased by adding sulfur hexafluoride to the air jet, which provided density ratios between 1 and 5.1. Hot‐wire anemometry and flow visualization revealed that a global transition occurs when conditions of absolute instability are established in the jet shear layers. One consequence of this transition is an abrupt decrease in the...

Counterflow Thrust Vector Control of Subsonic Jets: Continuous and Bistable Regimes

Fluidic thrust vectoring of a subsonic jet was examined by the asymmetric application of countere ow to the jet shear layers. When countere ow is applied to one side of a 4:1 aspect ratio rectangular jet in the proximity of a control surface, the shear-layer entrainment characteristics are altered asymmetrically giving rise to a cross-stream pressure gradient and e ow vectoring. Thrust vector control up to 20 deg was possible at Mach numbers up to 0.5 using countere ow. Two regimes of e uidic control were identie ed, a continuous regime and a bistable one. During continuous vectoring, proportional control could be achieved between jet response and the pressure conditions established in the countere owing stream. However, under certain circumstances, proportional control was lost leading to jet bistability. Since such bistability is undesirable for aircraft control applications, the phenomenon was studied in detail. A parametric study of the collar geometry was used to minimize jet attachment and expand the operating domain over which proportional control could be maintained.

Multiaxis Fluidic Thrust Vector Control of a Supersonic Jet Using Counterflow

A relatively new fluidic control technique, which employs secondary flow traveling in a direction opposite to the primary jet - namely, counterflow control - has been explored by Strykowski et al. at jet Mach numbers up to 2. These studies conclusively show that the primary jet can be continuously vectored up to angles approaching 20 deg. Futhermore, if implemented correctly, counterflow thrust vectoring (CFTV) does not suffer from the bistability problems encountered with earlier fluidic control schemes. We describe the results of a study in which a Mach 2 diamond-shaped jet was used to extend the single-axis vectoring concept of rectangular jets to multiaxis operation.

Vectoring Thrust in Multiaxes Using Confined Shear Layers

A fluidic scheme is described which exploits a confined countercurrent shear layer to achieve multiaxis thrust vector response of supersonic jets in the absence of moving parts. Proportional and continuous control of jet deflection is demonstrated at Mach numbers up to 2. for pitch vectoring in rectangular nozzles and multiaxis vectoring in axisymmetric nozzles. Secondary mass flow rates less than approximately 2% of the primary flow are used to achieve thrust vector angles exceeding 15 degrees. Jet slew rates up to 180 degrees per second are shown, and the fluidic scheme is examined in both static and wind-on configurations. Thrust performance is studied for external coflow velocities between Mach 0.3 and 0.7