Applied Hydraulic Transients

Recent advances in the shock wave/boundary layer interaction and its control in internal and external flows

This article reviews techniques applicable to enhancing the mixing of jets, with particular emphasis on infrared (IR) signature reduction of high-speed jets. Following a brief introduction to the IR signature of jet plumes and the fundamentals of jet mixing, this paper discusses rapid mixing technologies under the categories of: geometric modifications (to the nozzle); high shear stress mixing; normal stress mixing; self-acoustic excitation; external acoustic excitation; mechanically oscillated; self-oscillated. It is shown that mixing enhancements of the order of 100 per cent are possible with some techniques and that by combining techniques this can be increased by at least as much again. Simple geometric calculations are presented which demonstrate that with rectangular nozzles such high levels of mixing enhancement may be necessary in order to reduce IR signature. Some apparent rapid mixing technologies, however, have been shown to increase jet spreading without increasing entrainment, whereas...

A review of jet mixing enhancement for aircraft propulsion applications

Experiments were carried out to study the effect of the tabs with limiting length, termed Rathakrishnan limit, on controlling the mixing and acoustic characteristics of free jets. The limiting tab is found to be efficient in promoting the jet mixing even in the presence of an adverse pressure gradient. Furthermore, the limiting tab is found to bemost efficient in promoting the mixing near the correct expansion rather than at an underexpanded condition, with a favorable pressure gradient at the nozzle exit. The reason behind theworking of the limiting tab in the formof a cross wire is discussed forMach 1.6, 1.79, and 2.0 jets. As high as 50%reduction in the core length is achieved using a cross wire forMach 1.79 jet operatedwith nozzle pressure ratio 5.66. The combination of thewaves in the jet core and their interaction with the detached shock envelop generated by the wire has a strong influence on the overall jet noise and its spectral content. Even though thewire reduced the overall noise of the jet and the reduction is significant formany cases of the present study, it caused an increase of screech amplitude for some cases. Jet noise reduction was found to be sensitive to the wire orientation.

Experimental Studies on the Limiting Tab

A gap between eddy-viscosity-type and second-order models is bridged using the results of a two-scale direct-interaction approximation developed for the study of turbulent shear flows. This work provides a method for incorporating the findings from models of eddy-viscosity-type into second-order models, and vice versa. Specifically, the effect of helicity controlling energy-cascade processes is incorporated into a second-order model. Then, a higher-order eddy-viscosity-type expression for the Reynolds stress is derived through the application of an iterative approximation to the second-order model

Bridging between eddy-viscosity-type and second-order turbulence models through a two-scale turbulence theory

Studies on the Effect of Notches on Circular Sonic Jet Mixing

The present work reviews the CFD studies carried out in the last more than fifty years to understand the uncontrolled and controlled jet characteristics. The chronological evolution of investigations on jets are reviewed, however they are presented according to their relevance in the discussion. The study also highlights the complexities involved in adaptive turbulence models in a realistic simulation of free jets. These complexities are largely due to entrainment of jets, large perturbations present at low Reynolds numbers in subsonic jets and strong wave interactions in jets exiting at supersonic Mach numbers from convergent-divergent nozzles. The accurate capturing of these perturbations, shock-cell structures in jets poses a real challenge in the CFD analysis. The complexities of capturing shock and expansion waves accurately in supersonic jets, is the critical step in the simulation.

Review of Computational Fluid Dynamics Studies on Jets

To have an efficient combustion, the aircraft engine intakes operating at supersonic and hypersonic speeds necessarily decelerate the flow to subsonic level before entering the combustor, which is achieved by a combination of oblique and normal shock waves in the intake-isolator, and thus the high-speed intakes are called the mixed-compression intakes. The advantages of shock-enabled compression, however, does not come standalone rather associated with colossal losses due to shock and boundary layer interactions (SBLIs). The repercussions in the flow due to these interactions may include; intake unstart, abrupt thickening, separation of the boundary layer, unsteady shock oscillations, etc. Therefore, the SBLIs must be controlled to minimize the losses. Control of these interactions by manipulating the boundary layer using micro-vortex generators (MVGs) has gained prominence. In this study, a new ramped-vane MVG configuration, deployed near the shock impact point in the Mach 2.2 mixed-compression intake at varied contraction ratios, has been experimentally investigated. Plain intake and the intake controlled with conventional MVGs are also investigated for comparison. The heights of all the MVGs were varied as; 600 μm, 400 μm and 200 μm. The ramped-vane MVGs of height 200 μm are found to be the most efficient in causing a favorable pressure drop at the locations; near-upstream (x = 0.48 L) and near-downstream (x = 0.7 L) of the MVGs. The maximum reductions in static pressures about 11% at the intake contraction ratio of 1.20 at x = 0.48 L, and about 24% at the contraction ratio of 1.23 at x = 0.7 L, are achieved. The Schlieren pictures clearly demonstrate the effectiveness of all the tested ramped-vane MVGs (particularly 200 μm MVGs) in weakening the waves and reducing the separation length.

Experimental Studies on Micro-Vortex Generator Controlled Shock/Boundary-Layer Interactions in Mach 2.2 Intake

PART I: THEORETICAL AERODYNAMICS -- Chapter 1: The Standard Atmosphere -- Chapter 2: Basic Concepts -- Chapter 3: Governing Equations of Fluid Flows -- Chapter 4: Potential Flow Theory -- Chapter 5: Thin Airfoil Theory -- Chapter 6: Finite Wing Theory -- Chapter 7: Panel Methods -- Chapter 8:Thermodynamics of Fluids in Motion -- Chapter 9: Compressible Flows -- Chapter 10: Hypersonic Flows -- Chapter 11: Boundary Layers -- Chapter 12: Wind Tunnels -- PART II: APPLIED AERODYNAMICS -- Chapter 13: Supersonic Jets -- Chapter 14: Shock Wave and Boundary Layer Interactions -- Appendix.

Theoretical and Experimental Aerodynamics

Abstract The efficacy of introducing mixing promoting small-scale vortices by two rectangular tabs, of aspect ratio 1.0, 1.5 and 2.0, placed at diametrically opposite locations at the exit of a Mach 1.73 convergent–divergent circular nozzle has been experimentally investigated, for NPRs from 4 to 8, covering overexpanded, correctly expanded and underexpanded states of the jet. The area blockage due to the each tab was 2.5% of the nozzle exit area. Keeping the blockage constant, the aspect ratio (defined as the ratio of length to width of the tab) was varied. A maximum core length reduction of 84.6% was caused by the tabs of aspect ratio 1.0, at underexpanded conditions corresponding to NPR (nozzle pressure ratio) 6. At this NPR, tabs of aspect ratio 1.5 and 2.0 caused core length reduction of 76.9% and 61.5%, respectively. The mixing promoting efficiency of aspect ratio 1.0 is found to be better than 1.5 and 2.0, at all NPRs of the present study, except NPR 5. The shadowgraph pictures of the uncontrolled and controlled jets clearly demonstrate the effectiveness of the tabs in weakening the waves in the jet core.

Mrinal Kaushik

Papers

Studies on the Effect of Notches on Circular Sonic Jet Mixing

Review of Computational Fluid Dynamics Studies on Jets

Experimental Studies on Micro-Vortex Generator Controlled Shock/Boundary-Layer Interactions in Mach 2.2 Intake

Theoretical and Experimental Aerodynamics

Tab Aspect Ratio Effect on Supersonic Jet Mixing