Heat Induced Separation in Upward Impinging Jet Flows

A numerical study of compressible jet flows is carried out using Reynolds averaged Navier–Stokes (RANS) turbulence models such as k-ɛ and k-ω-SST. An experimental investigation is performed concurrently using high-speed optical methods such as Schlieren photography and shadowgraphy. Numerical and experimental studies are carried out for the compressible impinging at various impinging angles and nozzle-to-wall distances. The results from both investigations converge remarkably well and agree with experimental data from the open literature. From the flow visualizations of the velocity fields, the RANS simulations accurately model the shock structures within the core jet region. The first shock cell is found to be constraint due to the interaction with the bow-shock structure for nozzle-to-wall distance less than 1.5 nozzle diameter. The results from the current study show that the RANS models utilized are suitable to simulate compressible free jets and impinging jet flows with varying impinging angles.

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Three-Dimensional Simulation of Turbulent Rectangular and Square Impinging Jet Flows

Volume 2, Parts A and B ◽

10.1115/ht-fed2004-56131 ◽

2004 ◽

Author(s):

Qingguang Chen ◽

Zhong Xu ◽

Yulin Wu ◽

Yongjian Zhang

Keyword(s):

Recirculation Zone ◽

Three Dimensional ◽

Flow Characteristics ◽

Adverse Pressure Gradient ◽

Impinging Jet ◽

Boundary Layer Separation ◽

Streamwise Velocity ◽

Impinging Jets ◽

Jet Flows ◽

Dimensional Simulation

Flow characteristics of turbulent impinging jets issuing, respectively, from a rectangular and a square nozzles have been investigated numerically through the solution of three-dimensional Navier-Strokes equations in steady state. Two geometries with two nozzle-to-plate spacings of four and eight times of hydraulic diameters of the jet pipes, and two Reynolds numbers of 20000 and 30000 have been considered with fully developed inlet boundary conditions. An RNG based k–ε turbulence model and a deferred correction QUICK scheme in conjunction with the wall function method have been applied to the prediction of the flow fields within semi-confined spaces. A common feature revealed by the computational results is the presence of a toroidal recirculation zone around the jet. An adverse pressure gradient is found at the impingement surface downstream the stagnation point. Boundary layer separation will occur if the gradient is strong enough, and the separation manifests itself as a secondary recirculation zone at the surface. In addition, three-dimensional simulations reveal the existence of two and four pronounced streamwise velocity off-center peaks at the cross-planes near to the impingement plate, respectively, in the rectangular and square impinging jet flows. These peaks are found forming at the horizontal planes where the wall jets start forming accompanied by two or four pairs of counter-rotating vortex rings. It is believed that the formation of the off-center velocity peaks is due to the vorticity diffusion along the wall jet as the jet impinges on the target plate.

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Experimental study of physical mechanisms in the control of supersonic impinging jets using microjets

Journal of Fluid Mechanics ◽

10.1017/s0022112008003091 ◽

2008 ◽

Vol 613 ◽

pp. 55-83 ◽

Cited By ~ 46

Author(s):

FARRUKH S. ALVI ◽

HUADONG LOU ◽

CHIANG SHIH ◽

RAJAN KUMAR

Keyword(s):

Shear Layer ◽

Nozzle Exit ◽

Feedback Loop ◽

Large Scale ◽

Impinging Jet ◽

Impinging Jets ◽

Operating Conditions ◽

Jet Flows ◽

Streamwise Vorticity ◽

Control Approach

Supersonic impinging jet(s) inherently produce a highly unsteady flow field. The occurrence of such flows leads to many adverse effects for short take-off and vertical landing (STOVL) aircraft such as: a significant increase in the noise level, very high unsteady loads on nearby structures and an appreciable loss in lift during hover. In prior studies, we have demonstrated that arrays of microjets, appropriately placed near the nozzle exit, effectively disrupt the feedback loop inherent in impinging jet flows. In these studies, the effectiveness of the control was found to be strongly dependent on a number of geometric and flow parameters, such as the impingement plane distance, microjet orientation and jet operating conditions. In this paper, the effects of some of these parameters that appear to determine control efficiency are examined and some of the fundamental mechanisms behind this control approach are explored. Through comprehensive two- and three-component velocity (and vorticity) field measurements it has been clearly demonstrated that the activation of microjets leads to a local thickening of the jet shear layer, near the nozzle exit, making it more stable and less receptive to disturbances. Furthermore, microjets generate strong streamwise vorticity in the form of well-organized, counter-rotating vortex pairs. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. Based on these results and a simplified analysis of vorticity transport, it is suggested that the generation of these streamwise vortices is mainly a result of the redirection of the azimuthal vorticity by vorticity tilting and stretching mechanisms. The emergence of these longitudinal structures weakens the large-scale axisymmetric structures in the jet shear layer while introducing substantial three-dimensionality into the flow. Together, these factors lead to the attenuation of the feedback loop and a significant reduction of flow unsteadiness.

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Numerical Simulation of Violent Impinging Jet Flows with Improved SPH Method

International Journal of Computational Methods ◽

10.1142/s0219876216410012 ◽

2016 ◽

Vol 13 (04) ◽

pp. 1641001 ◽

Cited By ~ 11

Author(s):

J. R. Shao ◽

S. M. Li ◽

M. B. Liu

Keyword(s):

Free Surface ◽

Pressure Field ◽

Surface Condition ◽

Time History ◽

Impinging Jet ◽

Jet Flows ◽

Solid Boundary ◽

Sph Method ◽

Pressure Time ◽

Kernel Gradient Correction

This paper presents an implementation of an improved smoothed particle hydrodynamics (SPH) method for simulating violent water impinging jet flow problems. The presented SPH method involves three major modifications on the traditional SPH method, (1) The kernel gradient correction (KGC) and density correction are used to improve the computational accuracy and obtain smoothed pressure field, (2) a coupled dynamic solid boundary treatment (SBT) is used to remove the numerical oscillation near the solid boundary and ensure no penetration condition, (3) a free surface condition, which is obtained from the summation of kernel function and volume, is used to describe the water jet accurately. Different cases about violent impinging jet flows are simulated. The influences of impact velocity and angles are investigated. It is demonstrated that the presented SPH method has very good performance with accurate impinging jet patterns and pressure field distribution. It is also found that the pressure time histories of observation points are greatly influenced by the rarefaction wave from surrounding air. Closer distance from free surface can lead to quicker decay of the pressure time history.

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