Single Free Jet Research Articles

Jet Noise Reduction in Co-Flowing Jets with Finite Lip Thickness

Passive control for suppressing mixing noise from Co-Flowing Jets (CFJ) is presented in this study. The idea behind this is to reduce the convective Mach number of turbulent eddies that produce intense sound radiation. The present study analyses co-flowing jets with a bypass ratio of 6.3 and the primary nozzle lip thickness of 10 mm. The aim of the study is to find the jet noise level in finite lip thickness in co-flowing jets. CFJ with finite lip thickness forms a recirculation zone (in the near field). The secondary core and recirculation zone are shielding the primary core by reducing the jet noise. A single free jet with a diameter equal to that of a primary nozzle of co-flowing jet is also studied for comparison. The results show that co-flow jet with finite lip thickness of 10mm for various emission angles and the Overall Sound Pressure Level (OASPL) level gets reduced when compared with the single free jet.

Characteristics of a co-flowing jet with varying lip thickness and constant velocity ratio

PurposeThis study aims to analyze co-flowing jets (CFJs) with constant velocity ratio (VR) and varying primary nozzle lip thickness (LT) to find a critical LT in CFJs below which mixing enhances and beyond which mixing inhibits.Design/methodology/approachCFJs were characterized with a constant VR and varying LTs. A single free jet with a diameter equal to that of a primary nozzle of the CFJ was used for characteristic comparison. Numerical simulation is carried out and is validated with the experimental results.FindingsThe results show that within a critical limit, the mixing enhanced with an increase in LT. This was signified by a reduction in potential core length (PCL). Beyond this limit, mixing inhibited leading to the elongation of PCL. This limit was controlled by parameters such as LT and constant VR. A new region termed as influential wake zone is identified.Practical implicationsIn this study, the VR is maintained constant and bypass ratio (BR) was varied from low value to very high values. Presently, subsonic commercial turbo fan operates under low to ultra-high BR. Hence the present study becomes vital to the current scenario.Originality/valueTo the best of the authors’ knowledge, this is the first effort to find the critical value of LT for a constant VR for compressible co-flow jets. The CFJs with constant VR and varying LT have not been studied in the past. The present study focuses on finding a critical LT below which mixing enhances and above which mixing inhibits.

AERODYNAMICS OF JETS INTERACTING WITH A FLAT SURFACE

In this paper, the development features of a single free jet of hightemperature nitrogen interacting with a flat surface were studied. Calculation of the heat exchange process during heating by the attacking jets is very difficult to implement analytically due to complexity of the gas-dynamic processes occurring both in a single jet and in a system of jets interacting with the metal. The computational difficulties are aggravated by the fact that when interacting with the surface the jet as such disappears. The flat (fan) flow interacts with the surface: form, aerodynamic properties and thermal state of the flow strongly differ from those of the original jet. The studies were conducted on the basis of numerical simulation in the FloEFD software and computing complex for multiphysical simulation based on solution of the equations of gas dynamics and heat transfer. The solved system of equations consisted of Navier-Stokes equations, equations of energy and continuity and was supplemented by k – ε turbulence model. A three-dimensional model was developed for simulation, the necessary properties, initial and boundary conditions were specified. In the study of aerodynamics of a single high-temperature jet interacting with the surface, the main defining values were: nitrogen flow rate from the nozzle U0 , nitrogen temperature T, internal diameter of the nozzle d0 , distance from the nozzle section to the surface h, distance from the critical point (point of intersection of the jet axis with the surface) along the flow radius r. Data on the gas velocity decrease as the jet develops due to the loss of initial energy to engage the motionless surrounding gas in motion, is presented. The studies have shown that increase in the initial velocity of gas outflow brings the area of higher velocities closer to the surface both in the jet itself and in the fan jet. This factor contributes to heat transfer intensification. In addition, high speeds increase the total thickness of the fan flow and reduce the thickness of hydrodynamic boundary layer, which increases with distance from the critical point.

Characteristics of a co-flowing jet with varying lip thickness and constant bypass ratio

PurposeSubsonic commercial aircraft operate with turbo-fan engines that operate with moderate bypass ratio (BR) co-flowing jets (CFJ). This study aims to analyse CFJ with constant BR 6.3 and varying primary nozzle lip thickness (LT) to find a critical LT in CFJ below which mixing enhances and beyond which mixing inhibits.Design/methodology/approachCFJ were characterized with a constant BR of 6.3 and varying lip thicknesses. A single free jet with a diameter equal to that of a primary nozzle of the co-flowing jet was also studied for comparison.FindingsThe results show that within a critical limit, the mixing enhanced with an increase in LT. This was signified by a reduction in potential core length (PCL). Beyond this limit, mixing inhibited leading to the elongation of PCL. This limit was controlled by parameters such as LT and magnitude of BR.Practical implicationsThe BR value of CFJ in the present study was 6.3. This lies under the moderate BR value at which subsonic commercial turbofan operates. Hence, it becomes impervious to study its mixing behavior.Originality/valueThis is the first effort to find the critical value of LT for a constant BR for compressible co-flow jets. The CFJ with moderate BR and varying LT has not been studied in the past. The present study focuses on finding a critical LT below which mixing enhances and above which mixing inhibits.

Influence of bypass ratio on subsonic and correctly expanded sonic co-flowing jets with finite lip thickness

The effects of bypass ratio on co-flowing subsonic and correctly expanded sonic jet decay have been studied experimentally. Co-flowing jets with lip thickness 1.0 Dp (where Dp is the diameter of primary nozzle and is equal to 10 mm) with bypass ratios of around 0.7, 1.4, and 6.4 at primary jet exit Mach numbers 0.6, 0.8, and 1.0 have been analyzed. A single free jet equivalent to primary nozzle of the co-flowing nozzle was considered for comparison. Primary jet centerline total pressure decay, spread, and static pressure variation were investigated. The results show that the mixing of the high bypass ratio co-flowing jet with lip thickness 1.0 Dp is superior to low bypass ratio co-flowing jet. Both lip thickness and bypass ratio have a strong influence on the co-flowing jet mixing. Bypass ratio 6.3 experiences a significantly higher mixing than bypass ratio 0.7 and 1.4. Selected jets were also investigated computationally. The computations capture the salient flow physics and reproduce well with the experiments.

Prediction on Turbulent Flow of Two Parallel Plane Jets Using κ-ε Model

In the present study, computational details of two parallel plane jets with nozzle separation ratio = 4.25 and Re = 11,000 are presented. A study of the turbulence has also been done. The computational domain size, grid resolution, and different discretization schemes on the predictions are discussed. The existence of a recirculation flow region, a merging region and a combined region in the two parallel plane jets configuration has been predicted qualitatively by model. Power law scheme is used for discretizing the convective terms. Calculations were made using a Power law scheme. A code is used to solve a laminar, two dimensional viscous fluid flow and heat transfer. At the last it is shown that this code is ready to use for modification for mean flow solution of the turbulent two parallel plane jets. The numerical results are compared with the experimental results. Computational details of various mean flow and turbulent parameters are presented and described with separation ratio of 4.25 and Re = 11,000. The results are compared with Nasr and Lai and are found to be in good agreement with it. The two parallel plane jets develop like a single free jet in the combined region. The outer shear layer spreads faster than the inner shear layer in the near field.

Applicability of Reichardt's Hypothesis to the Prediction of Velocity Field of Multiple Parallel Plane Jets

AbstractIn this paper the velocity distribution for multiple parallel plane jets based on the superposition of Reichardt's solution for single free jet is derived. The extend of the applicability of Reichardt's hypothesis to prediction of multiple parallel plane jets is examined. Both ventilated and unventilated two parallel plane jets data published in the literature and obtained for this study were used for comparisons with the theoretical results obtained from Reichardt's hypothesis. LDA measurements of mean streamwise velocities and turbulence characteristics of an array of 5 parallel plane jets were also made in order to establish the conditions under which this heuristic approach is valid. Results show that provided the pressure gradient in the lateral direction is small and hence the deflection of the individual jet center line is small, there is a good agreement between experimental results and predictions based on Reichardt's hypothesis for multiple parallel plane jets.

Suppression of Jet Noise by Staged Water Injection during Launch Vehicle Lift-Off

Jet noise during the lift-off of a launch vehicle is complicated by the simultaneous flow of multiple jets, and their deflection by jet deflectors. Further, the presence of the launch pedestal, the service structure, and the moving vehicle itself, act as reflecting surfaces, which contribute to the noise environment. The present work involves the suppression of noise as measured at different parts of the launch vehicle in a small-scale replica of a full launch pad, for different locations of the vehicle along its vertical lift-off trajectory. The primary source of noise is the two jets emerging from the base of the launch vehicle at a Mach number of 3.38. Noise is suppressed by water injection at different locations in the launch pad such as the upstream and downstream edges of the jet deflector cover-plate, bottom and top of the launch pedestal, and at two different locations on the service structure. The effect of staged injection of water, i.e., without and with injection at different heights relative to the position of the jets, as the vehicle is at different locations in its trajectory, is examined. It is found that sustained suppression of noise is obtained only with injection of water in successive stages closer to the nozzle exit, as the jet position is moved up. The effect of angle of injection of water is investigated to verify previously reported results that an injection angle of 60°, i.e., along the flow, is superior to injection at 90° to the jet. The effect of injection pressure shows an optimum that supports the effective atomization of the injected water jets into droplets due to shear by the gas jet flow. Investigation with hot jets shows increased noise levels by at least ~2 dB relative to cold jets of nearly the same nozzle-exit Mach number. However, an increased reduction of noise by water injection is apparent with hot jets. The reduction is nearly independent of the jet temperature in the 600-900 K range. The present results collectively confirm the qualitative applicability of different previous studies on single free jets to the complicated launch vehicle noise scenario.

Comparison of flow characteristics in the near field of two parallel plane jets and an offset plane jet

Two parallel plane air jets and offset air jets have several common features such as the existence of a subatmospheric pressure region and the formation of a flow recirculation zone adjacent to the nozzle plate. In particular, the symmetry plane that exists between two parallel plane jets may appear to affect the flow field in much the same way as a solid wall does in a reattaching offset jet. It is obvious, however, that there are significant differences far downstream from the nozzles because the two parallel plane jets will combine to form a single free jet while the offset jet will develop into a wall jet. Differences in the near field between these two jet configurations with a separation ratio of 2.125 are examined here under identical initial flow exit conditions through laser Doppler anemometer measurements of the mean velocity components, turbulence intensities, and Reynolds shear stress. The results indicate that the wall exerts significant retarding and turbulence suppression effects on the offset jet in the flow development region. Through the use of comparisons, the interaction of two inner shear layers on both sides of the symmetry plane in two parallel plane jets results in a much more turbulent near field than that of the offset jet.

Further study of high speed single free jets

AbstractExperiments to study the behaviour of a compressible free jet issuing from an axisymmetric convergent nozzle are reported. Detailed measurements of the mean flow field for subsonic, sonic and underexpanded sonic jets was made within the pressure ratio range of 1.13 to 4. The results show that the spread and decay rates of the jet vary significantly with pressure ratio. The results further indicate the radial velocity/total pressure collapse within the two-dimensional shear layer data. The underexpanded jet propagates to a greater distance downstream than the low speed jets; this leads to enhancement of the far field mixing.