Identification of the Cold Flow Perturbation Sources in a Dump Combustor With a Tapered Exit

2013 ◽  
Vol 135 (1) ◽  
Author(s):  
N. P. Yadav ◽  
Abhijit Kushari
Keyword(s):  
Shear Layer ◽  
Separated Flow ◽  
Reynolds Numbers ◽  
Wall Pressure ◽  
Mean Velocity ◽  
Potential Core ◽  
Flow Perturbation ◽  
Dump Combustor ◽  
Power Spectral ◽  
Pressure And Velocity Measurements

This paper reports on the experimental investigation of the flow inside a low aspect ratio (length less than the reattachment length of separated flow) dump combustor with a tapered exit. The flow field in the combustor is evaluated using wall pressure and velocity measurements at varying flow Reynolds numbers. The mean velocity and turbulence variation closer to the wall of the combustor was found to be different from that at other radial locations due to the presence of recirculation and possible thickening of the shear layer caused by a decrease in the strength of the potential core. The power spectral study of the wall pressure and velocity fluctuations suggested the dominant presence of acoustic perturbations with amplitude modulation of such perturbations due to viscous dissipation in the shear layer.

Recognize the Cold Flow Perturbation Sources in a Dump Combustor With Taper Exit

2010 ◽  
Author(s):  
N. P. Yadav ◽  
Abhijit Kushari
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Velocity Measurement ◽  
Pressure Sensors ◽  
Separated Flow ◽  
Velocity Variation ◽  
Spectral Studies ◽  
Potential Core ◽  
Flow Perturbation ◽  
Dump Combustor

This paper reports on experimental investigation of the flow field inside a low aspect ratio dump combustor with tapered exit. The length of the combustor studied was less than the reattachment length for the separated flow. The flow field behavior in the combustor is evaluated from pressure and velocity measurement at varying flow Reynolds number. The pressure measurement inside the combustor at different locations was performed by the piezo-resistive pressure sensors. The velocity measurement inside the combustor was carried out by a hot wire probe. The rms (root mean square) velocity and turbulence intensity distributions are found to be axisymmetric due to the geometry of the combustor. The variation in pressure and velocity with locations and Reynolds number was studied. The velocity variation near the surface of the combustor (r/R = 0.66) was showing the opposite behaviour upto x/h = 4.837 than the radial locations (r/R = 0.0, 0.33) due to presence of recirculation and high separation of shear layer by deceased in the strength of the potential core. The pressure and velocity measurement studies support that the presence of the strong recirculation and high turbulence inside the combustor. The power spectral studies of the pressure and velocity fluctuations also suggested the presence of strong recirculation, acoustic modes and turbulence behaviour inside the combustor. The velocity distributions were corroborated by comparing the frequency spectrum with the wall pressure distributions and the results were found to be in very good qualitative agreement with each other.

Large eddy simulation of a circular jet: effect of inflow conditions on the near field

2009 ◽  
Vol 620 ◽  
pp. 383-411 ◽  
Author(s):  
JUNGWOO KIM ◽  
HAECHEON CHOI
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Reynolds Numbers ◽  
Momentum Thickness ◽  
Potential Core ◽  
Initial Momentum ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Mode 2 ◽  
Inflow Conditions

In the present study, the effects of the jet inflow conditions such as the initial momentum thickness (θ) and background disturbances on the downstream evolution of a circular jet are investigated using large eddy simulation (LES). We consider four different initial momentum thicknesses,D/θ = 50, 80, 120 and 180, and three different Reynolds numbers,ReD=UJD/ν = 3600, 104and 105, whereUJis the jet inflow velocity andDis the jet diameter. The present study shows that the jet characteristics significantly depend on both the initial momentum thickness and the Reynolds number. For all the Reynolds numbers considered in this study, vortex rings are generated at an earlier position with decreasing initial momentum thickness. In case of a relatively low Reynolds number ofReD= 3600, at smaller initial momentum thickness, early growth of the shear layer due to the early generation of vortex ring leads to the occurrence of large-scale coherent structures in earlier downstream locations, which results in larger mixing enhancement and more rapid increase in turbulence intensity. However, at a high Reynolds number such asReD= 105, with decreasing initial momentum thickness, rapid growth of the shear layer leads to the saturation of the shear layer and the generation of fine-scale turbulence structures, which reduces mixing and turbulence intensity. With increasingReθ(=UJθ/ν), the characteristic frequency corresponding to the shear layer mode (Stθ=fθ/UJ) gradually increases and reaches near 0.017 predicted from the inviscid instability theory. On the other hand, the frequency corresponding to the jet-preferred mode (StD=f D/UJ) varies depending onReDandD/θ. From a mode analysis, we show that, in view of the energy of the axial velocity fluctuations integrated over the radial direction, the double-helix mode (mode 2) becomes dominant past the potential core, but the axisymmetric mode (mode 0) is dominant near the jet exit. In view of the local energy, the disturbances grow along the shear layer near the jet exit: for thick shear layer, mode 0 grows much faster than other modes, but modes 0–3 grow almost simultaneously for thin shear layer. However, past the potential core, the dominant mode changes from mode 0 near the centreline to mode 1 and then to mode 2 with increasing radial direction regardless of the initial shear layer thickness.

Unstable waves on an axisymmetric jet column

1974 ◽  
Vol 65 (3) ◽  
pp. 541-560 ◽  
Author(s):  
G. E. Mattingly ◽  
C. C. Chang
Keyword(s):  
Flow Field ◽  
Shear Layer ◽  
Stability Theory ◽  
Jet Flow ◽  
Jet Flows ◽  
Water Tank ◽  
Mean Velocity ◽  
Core Diameter ◽  
Potential Core ◽  
Axisymmetric Jet

The growth of infinitesimal disturbances on an axisymmetric jet column is investigated theoretically and experimentally. The theoretical analysis is based upon inviscid stability theory, wherein axisymmetric, helical and double helical disturbances are considered from the spatial reference frame. In the jet flow field near the source, the mean velocity profile is observed to have a potential core and a thin, but finite, shear layer between the potential core and the quiescent ambient fluid. With downstream distance, the potential core diameter decreases and the shear-layer thickness increases. To incorporate these variations into the theory, a quasi-uniform assumption is adopted, whereby successive velocity profiles are analysed individually throughout the region in the jet flow where disturbances are observed to be small. The results of the theory indicate that initially, in the jet flow where the shear layer is thin and the potential core is larger, all disturbances considered are unstable. The dominant disturbance in the jet is an axisymmetric one. However, further downstream in the jet, where the half-breadth thickness of the shear layer is 55% of the potential core radius, a helical disturbance is found to dominate the axisymmetric and double helical modes. Nowhere in the jet flow field examined was the double helical disturbance found to be dominant. The cross-stream distributions of velocity and vorticity for the dominant disturbance modes are presented according to the spatial stability theory.The downstream development of the jet column and the characteristics of the disturbances amplifying on it were also studied in a water tank. No artificial stimulation of any particular disturbance was used. The experimental results show good agreement with the results of the theory in the region where the disturbances are small. However, conclusive confirmation of the switch in the hierarchy of dominant disturbances was not found. Half of the time the disturbance observed experimentally exhibits an axisymmetric character and the other half a helical one. This apparently is due to the similar spatial amplification rates experienced by both of these disturbance modes. It is concluded that this switching of dominant modes is, in large part, responsible for (i) the well-known natural drifting of disturbance characteristics in jet flows, and (ii) the wide variety of observations made in previous jet experiments.

scholarly journals An analysis of the correlations between the turbulent flow and the sound pressure fields of subsonic jets

2007 ◽  
Vol 583 ◽  
pp. 71-97 ◽  
Author(s):  
C. BOGEY ◽  
C. BAILLY
Keyword(s):  
Shear Layer ◽  
Sound Pressure ◽  
Flow Region ◽  
Reynolds Numbers ◽  
Axial Distance ◽  
Noise Component ◽  
Potential Core ◽  
Vortical Structures ◽  
Round Jets ◽  
The Times

Noise generation is investigated in subsonic isothermal round jets at Mach numbers M = 0.6 and M = 0.9, with Reynolds numbers ReD = 1700 and ReD ≥ 105, using causality methods on data provided by large-eddy simulations. The correlations between broadband sound pressure signals and broadband turbulence signals along the jet axis and the shear layer are calculated. The normalized correlations are found to be significant between the pressure emitted in the downstream direction and centreline flow quantities. They are much smaller in the cases involving flow quantities along the shear layer, and fall for large emission angles. The maximum correlations obtained between centreline turbulence and downstream sound pressure are observed just at the end of the potential core for time delays corresponding to the times of propagation evaluated along ray paths. They also appear to be lower as the Mach number is reduced, and to be enhanced as the Reynolds number is decreased. These correlation levels can reasonably be attributed to the noise source which is predominant at small emission angles. This source is therefore located on the jet centreline at the end of the potential core, in a flow region which is shown to be characterized by a dominant Strouhal number over a large axial distance, by a strong level of intermittency, and by a high convection velocity. This supports the contention that the downstream jet-noise component is connected to the periodic and intermittent intrusion of vortical structures into the jet core.

Effects of Reynolds Number on Turbulent Characteristics of Surface Jet

2016 ◽  
Author(s):  
M. S. Rahman ◽  
M. F. Tachie
Keyword(s):  
Reynolds Number ◽  
Reynolds Shear Stress ◽  
Maximum Velocity ◽  
Reynolds Numbers ◽  
Mean Velocity ◽  
Potential Core ◽  
Turbulent Characteristics ◽  
Reynolds Number Effects ◽  
Surface Jet ◽  
Jet Characteristics

Experimental study was carried out to investigate the Reynolds number effects on surface jet characteristics. The surface jet was produced using orifice nozzle with offset height ratio of 2. Six different Reynolds numbers ranging from 2300 to 11900 were investigated. Potential core region of the jet decreased with Reynolds number up to the Reynolds number of 5500. Reattachment point was sensitive to Reynolds number within the range of the present study. The maximum velocity decay and jet spread were nearly independent of Reynolds number. The streamwise mean velocity, streamwise turbulence intensity and Reynolds shear stress distribution along surface-normal direction were affected by the free surface and showed Reynolds number independency at the Reynolds numbers beyond 5500.

Wake Flow Patterns Behind Rotating Smooth Spheres and Baseballs

2018 ◽  
Author(s):  
Patrick Mortimer ◽  
John C. Vaccaro ◽  
David M. Rooney
Keyword(s):  
Rotational Speed ◽  
Reynolds Numbers ◽  
Boundary Layer Separation ◽  
Wake Flow ◽  
Hot Wire ◽  
Mean Velocity ◽  
Magnus Effect ◽  
Power Spectral ◽  
The Mean ◽  
Smooth Sphere

An experimental investigation into the flow field behind baseballs at two different seam orientations as well as a smooth sphere of the same diameter was undertaken at Reynolds numbers of 5 × 104 and 1 × 105. The rotational speed of the three spheres varied from 0 to 2400 rpm, with data collected in increments of 400 rpm which correspond to relative spin rates between 0 and 0.94. Mean velocity profiles, turbulence in intensity profiles, and power spectral density of the signals were taken using hot-wire anemometry. The smooth sphere wake was seen to change in orientation over a range of relative rotational speeds. The Strouhal number remained constant around 0.24 for relatively low spin rates. The seams on the baseball suppressed any measurable vortex shedding once rotation began, also eliminating any significant change in wake orientation as evidenced by the mean velocity deficit and turbulence intensity profiles. It was concluded that the so-called inverse Magnus effect recorded by previous investigators at a specific Reynolds number / relative rotational speed of a sphere exists only for a smooth sphere and not for a sphere where the boundary layer separation is governed by raised seams.

Dynamics of Swirling Flow in a Dump Combustor at Elevated Pressures

2009 ◽  
Author(s):  
N. P. Yadav ◽  
Abhijit Kushari
Keyword(s):  
Flow Field ◽  
Swirling Flow ◽  
Separated Flow ◽  
Wall Pressure ◽  
Operating Conditions ◽  
Needle Valve ◽  
Chamber Pressure ◽  
Dump Combustor ◽  
Pressure Distributions ◽  
Inlet Swirl

This paper reports an experimental investigation of the flow field inside a low aspect ratio dump combustor with inlet swirl and choked exit. The length of the combustor studied was less than the reattachment length for the separated flow. The tapered exit of the combustor was choked by a needle valve to investigate the effect of elevated chamber pressure on the flow field of the combustor. The variation in wall pressure and velocity at different locations and Reynolds number was studied. It was observed that the turbulent intensity increased with the swirling flow and decreased with an increase in the chamber pressure. The exit choking reinforced the recirculation. The velocity distributions were corroborated by comparing the frequency spectrum with the wall pressure distributions and the results were found to be in good qualitative agreement with each other. This study will be helpful to design the combustor for different operating conditions.

A numerical and experimental study of transition processes in an obstructed channel flow

1994 ◽  
Vol 260 ◽  
pp. 185-209 ◽  
Author(s):  
E. P. L. Roberts
Keyword(s):  
Numerical Simulation ◽  
Shear Layer ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Stability Study ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Cross Sectional ◽  
Low Reynolds Numbers ◽  
Steady And Unsteady Flows

Incompressible Newtonian flow in a two-dimensional channel with periodically placed sharp edged baffles has been studied both by numerical simulation and by experimental flow visualization. The flow was observed to be steady and symmetric at low Reynolds numbers, with recirculating eddies downstream of each baffle. At a critical Reynolds number (based on channel width and cross-sectional mean velocity) of approximately 100 the flow became asymmetric and unsteady. This transition to unsteadiness led to an eddy shedding regime, with eddies formed and shed successively from each baffle. A stability study suggested that the mechanism for transition to unsteady flow is a Kelvin–Helmholtz instability associated with the shear layer formed downstream of the sharp edged baffles. The frequency of the unsteadiness is, however, dependent on the full flow field, and not only the shear layer characteristics. Experimental observations show that the instability is followed by a secondary transition to three-dimensional disordered flow. Experimentally observed flows in the two-dimensional regime were found to be in close agreement with the numerical simulation for both the steady and unsteady flows.

Free Shear Layer Behavior in Rotating Systems

1979 ◽  
Vol 101 (1) ◽  
pp. 117-120 ◽  
Author(s):  
P. H. Rothe ◽  
J. P. Johnston
Keyword(s):  
Shear Layer ◽  
Separated Flow ◽  
Reynolds Numbers ◽  
Two Dimensional ◽  
Free Shear Layer ◽  
Rotation Direction ◽  
Rotating Systems ◽  
System Rotation ◽  
Steady Rate ◽  
Extensive Flow

Experiments are reported concerning turbulent separated flow downstream of a backward-facing step in a two-dimensional channel that was rotated at a steady rate about a spanwise axis. Reattachment distance is reported as a function of Reynolds number, rotation direction and number and passage aspect ratio. Extensive flow visualization films have been produced. It is demonstrated that turbulent motions in a free shear layer may be suppressed or enhanced by system rotation according to the sense of the rotation. Two-dimensional, spanwise vortices which have been observed in the free shear layer are found to be relatively insensitive to system rotation in the stabilizing direction. These vortices are believed to be important contributors to the high rates of free shear layer entrainment, even in stationary systems at moderate Reynolds numbers.

Mean and Fluctuating Velocity Characteristics of a Separated Shear Layer Past a Surface Mounted Block

2006 ◽  
Vol 129 (2) ◽  
pp. 200-208 ◽  
Author(s):  
Ü. Özkol ◽  
C. Wark ◽  
D. Fabris
Keyword(s):  
Shear Layer ◽  
Reynolds Stress ◽  
Reynolds Shear Stress ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Separated Shear Layer ◽  
Block Height ◽  
High Reynolds

The mean velocity, Reynolds stress, and mean vorticity regions of a separated shear layer over a surface mounted block are investigated by 2D Digital Particle Image Velocimetry (DPIV) for three Reynolds numbers (Rea=500, 1000, and 2500) and two channel-to-block height ratios (H∕a=1.825 and 4.6). The recirculation region’s height and length are determined for the separated shear layer by means of U¯=0 contours. It is observed that the high Reynolds stress regions lay just outside of the U¯=0 contours. The flow visualization and DPIV measurement of vorticity indicate that the differing normalized Reynolds stresses between Rea=500 and 1000 are most probably due to the initiation of the vortex shedding between these two Reynolds numbers while, differences are minimal between Rea=1000 and 2500. A sign change in the Reynolds shear stress distribution of the separated shear layer near the leading edge of the block was recognized for every Reynolds number and channel width.

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