Counterflow Jet Research Articles

Experimental Analysis on the Use of Counterflow Jets as a System for the Stabilization of the Spatial Hydraulic Jump

This study presents an investigation on the use of submerged counterflow jets as a means for stabilizing the spatial hydraulic jump occurring in abruptly expanding channels. The characteristics of the flow downstream from the stilling basin and the main parameters influencing the effectiveness of the device in improving flow uniformity and reducing scouring potential are examined in laboratory tests, under several geometric configurations and hydraulic boundary conditions. The position within the stilling basin and the jet density (i.e., the number of orifices issuing the counterflow jets) were found to be important parameters influencing the performance of the device. Overall, the results indicate that this dissipation system has promising capabilities in forcing the transition from supercritical to subcritical flow, by significantly shortening the protection length needed to limit the phenomena of instability associated with spatial hydraulic jumps.

Observation of unsteady motion induced by counterflow jet on blunt body in hypersonic flow

Observation of unsteady motion induced by counterflow jet on blunt body in hypersonic flow

Modeling and Analysis of Shock Reduction through Counterflow Plasma Jets

The study presents a numerical investigation of aerodynamic drag reduction by implementing a counterflow plasma jet, emanating from the stagnation point of an aerodynamic surface in a supersonic regime with a constant pressure ratio PR = 3 , and compares findings with a conventional opposing jet. The computational study is carried out by solving three-dimensional and axisymmetric Navier–Stokes equations for counterflow plasma-jet interaction. The calculations are performed at free-stream Mach ( M ∞ = 1.4) with sea level stagnation conditions. The weakly ionized argon plasma jet generated by a plasma torch has constant stagnation pressure and temperature of 303,975 Pa and 3000 K . The effect of the Mach number and the angle of attack variation on plasma-jet effectiveness is also analyzed. The results indicate that the counterflow plasma jet reduces more drag (in twice) compared to the conventional jet (nonplasma). The gravitational, magnetic field effect and chemical processes in the plasma formation are considered negligible. It is inferred that the effectiveness of the counterflow plasma jet strongly depends upon the jet stagnation temperature.

The effects of O2 concentrations on the ignition and MILD combustion of intensely diluted C2H6 in a counterflow jets

A detailed study on the ignition characteristics of C2H6 diluted intensively in non-premixed systems at the four O2 volume ratio(10%, 21%, 30% and 40%) has be conducted in this paper. IDIM 2.0 for predicting the ignition temperature of C1–C2 has been developed, and been verified by the ignition temperatures of CH4 and C2H6 from our and Fotache et al.‘s experiments. Through the S-curves analysis, the appearance of the “slow” radical concentration growth is not the important indicator of the effect of dilution on the ignition of C2H6. The reason for the emergence of slow growth zone in the case of 100% C2H6 is that many reactions making important positive contribution to the ignition are endothermic. The characteristics of dilution effects are mainly reflected in two aspects: 1)the oxidation pathway is extended further to CO2 under intense dilution conditions; 2)the enhancement of the pathway, CH3→CO→CO2. The effect of O2 concentration on the ignition temperature will change as the fuel is diluted. For 7% C2H6, the order of ignition temperatures of various O2 concentrations from low to high is 21% < 10%<40%. Within the scope of this study, the preheating temperature to maintain MILD combustion under 21% O2 concentration is the lowest for intensely diluted C2H6.

Numerical Modeling of Flow Pattern and Heat Transfer at Injection of Counter-Flowing Wall Jet

Numerical Modeling of Flow Pattern and Heat Transfer at Injection of Counter-Flowing Wall Jet

Effects of counterflow jet on the performance of a generic rocket

Effects of a jet emanating from the nose of a generic rocket in the opposite direction of the incoming freestream have been studied to determine the effects of the jet in reducing the drag and the heat loads on the rocket. The effects are comparatively evaluated for different jet exit Mach numbers from ideally expanded nozzles, designed using the method of characteristics, operating at a total temperature of 500 K and at total pressures with predetermined ratios of the freestream total pressure. Axisymmetric computations were conducted with the generic rocket flying at a Mach number of 6.0 and 40,000 ft altitude with zero-degree angle of attack. Results indicate that a jet generated by the Mach 1.5 nozzle, operating at a total pressure corresponding to 1/4th of the freestream total pressure, resulted in the best combined overall drag and temperature reduction. The area-averaged surface temperature and drag were reduced by 52%, and 22%, respectively, in comparison to the body without the flow control.

Drag and Heat Flux Reduction using Counterflow Jet and Spike - Analysis of their Equivalence for a Blunt Cone Geometry at Mach 8

This study aims to explore equivalence between active and passive flow control techniques in reducing the wave drag and surface heat flux over a blunt cone model kept in Mach 8 stream. Computational investigations were carried out by using finite volume-based compressible flow solver. Throughout the study, the solution of governing equations is sought by assuming two dimensional-axisymmetric nature of the flowfield. Both counter flow-stagnation point injection and forward facing-physical spike are considered to mitigate the excess drag and heat flux experienced by a blunt body representing the nose cone section of a hypersonic vehicle. Eventually, based on identified drag reductions, the present study proposes equivalence cases between these two methods. It is shown that a pointed spike of L/D=1 provides almost the same drag reduction as the counterflow injection jet with a pressure ratio of 8.25. Similarly, other equivalence cases are identified and the physics behind them is explored. The identified equivalence is expected to help the designers in effectively replacing one technique with another according to the requirement. Equivalence matrix is presented for different spike cases in terms of injection ratios of counterflow injection.

Studies of autoignition-assisted nonpremixed cool flames

Autoignition-assisted nonpremixed cool flames of diethyl ether (DEE) are investigated in both laminar counterflow and turbulent jet flame configurations. First, the ignition and extinction limits of laminar nonpremixed cool flames of diluted DEE are measured and simulated using detailed kinetic models. The laminar flame measurements are used to validate the kinetic models and guide the turbulent flame measurements. The results show that, below a critical mixture condition, for elevated temperature and dilute mixtures, the cool flame extinction limit and the low-temperature ignition limit merge, leading to autoignition-assisted cool flame stabilization without hysteresis. Based on the findings from the laminar flame experiments, autoignition-assisted turbulent lifted cool flames are established using a Co-flow Axisymmetric Reactor-Assisted Turbulent (CARAT) burner. The lift-off heights of the turbulent cool flames are quantified using formaldehyde planar laser-induced fluorescence. Based on an analogy with autoignition-assisted lifted hot flames, a correlation is proposed such that the autoignition-assisted cool flame lift-off height scales with the product of the flow velocity and the square of the first-stage ignition delay time. Using this scaling, we demonstrate that the kinetic mechanism that most accurately predicts the laminar flame ignition and extinction limits also best predicts the turbulent cool flame lift-off height.

Drag reduction analysis of counterflow jets in a short penetration mode

The characteristics around counterflow jets are difficult to predict, because of the many variables involved. To analyze counterflow jets with different freestreams, a numerical simulation was conducted, by solving a two-dimensional axisymmetric RANS equation. The trends in drag reduction were reviewed in short penetration mode, with freestream conditions (altitude: 10 to 19 km, M∞: 2.0 to 4.0). According to the results of the numerical analysis, it was determined that the differences in overall flow structure arise from the freestream Mach number. The recirculation zone is the key feature of the flow. In the flow mechanism, the recirculation zone is affected by the velocity and pressure at the boundary, and the position and pressure of the reattachment point are the main parameters influencing the recirculation zone. Observed differences in the flow structure depending on the freestream Mach number can be explained by these parameters. As a result, different drag reduction curves can be drawn for each freestream Mach number, and a maximum value on drag reduction exists.

Effect of Counterflowing Jet on Supersonic Slender-Body Configurations: A Numerical Study

Previous studies have demonstrated that the use of counterflowing supersonic jets operating in the long penetration mode (LPM), directed from the nose of blunt bodies against a supersonic/hypersonic freestream, can greatly reduce the drag and heat loads on the vehicle. However, the benefits of LPM for slender bodies of revolution operating at low supersonic speeds, applicable to supersonic transport applications, remain largely unknown. This Paper conducts a systematic parametric study for the effects of a counterflowing jet on two slender body models (cone-cylinder and quartic body of revolution, respectively) operating at a freestream Mach number of 1.6, using time-accurate computations performed with an unstructured Navier–Stokes solver. Different ranges of jet flow rates, jet exit Mach numbers, nozzle exit diameters, and jet-to-base diameter ratios are examined systematically. The jet-to-base diameter ratio is determined to be the most important parameter to facilitate the establishment of the LPM regime. Depending on the geometry of the slender body, longer jet penetration (thus larger shock-standoff distance) associated with the LPM does not always result in larger drag reduction. Furthermore, current results indicate that inclusion of an LPM jet is likely to cause large oscillations of surface pressure and drag on these slender geometries.

ILES of an array of three subsonic counter-flow jets issuing from a wing leading edge exposed to hypersonic aerodynamic heating

The flow of an active thermal protection system exploiting subsonic counter-flow jets for wing leading edges of hypersonic vehicles is numerically studied on the basis of the three dimensional Navier-Stokes equations. The coolant air issuing from around the stagnation point as an array of three jets spreads over both the upper and the lower sides of the cylinder surface and about 40 ~ 60% cooling effectiveness is achieved in the range up to 5 degrees angle of attack despite the occurrence of various three-dimensional fluid-dynamic instabilities. The numerical scheme is second order accurate but simple inclusion of high order polynomial approximation in the reconstruction enables the capturing of finer structure of the flow field.

Combination of counterflow jet and cavity for heat flux and drag reduction

In the present work, a computational study is carried out to explore the potential of a combination of a forward-facing cavity and a counterflow jet for drag and heat flux mitigation for an axisymmetric body in a supersonic free stream of Mach 2. A comparison between the axisymmetric body with a forward-facing cavity and counterflow jet and the baseline geometry without the cavity and jet is made. The first part of this study focuses on capturing the transition point of the two counterflow jet modes, Long Penetration Mode (LPM) and Short Penetration Mode (SPM), by changing the pressure ratio of the nozzle. It gave an appropriate nozzle operating pressure selection based on the design requirement. The LPM is found to be suitable when only drag reduction is required, while the SPM provides both drag and heat flux reduction, but at a higher operating jet pressure. Next, a study is conducted to characterize the effect of the angle of attack on system performance. The LPM shows improvement over the baseline geometry for only a narrow range of angles of attack, while the SPM provides improved performance for a wider range of angles of attack. The effect of cavity and cavity dimensions on performance is then studied.

Review and numerical investigation of the mean flow features of a round turbulent jet in counterflow

A turbulent jet in counterflow is a lesser-studied jet configuration which exhibits great potential for mixing applications in sustainable energy production. In this paper, a comprehensive literature review of the research in counter-flowing jets is performed. Experimental and numerical results for mean and turbulent quantities are reviewed, and similarities and differences between datasets are discussed. Additionally, large eddy simulations (LESs) are carried out in order to study a turbulent jet in counterflow at several jet-to-counterflow velocity ratios (α = 2.2, 3.4, 5.1). The effect of two different jet inflow conditions is investigated. The LES results are directly compared to the available literature, and the subsequent analysis sheds light on the differences seen in the review. Finally, a set of recommendations and best practices are provided in order to aid future studies of jets in counterflow.

Shock Reduction through Opposing Jets—Aerodynamic Performance and Flight Stability Perspectives

In this research paper, investigations of counter flow (opposing) jet on the aerodynamic performance, and flight stability characteristics of an airfoil with blunt leading-edge in supersonic regime are performed. Unsteady Reynolds-Averaged Navier-Stokes ( U R A N S ) based solver is used to model the flow field. The effect of angle of attack ( α ), free-stream Mach number ( M ∞ ), and pressure ratio ( P R ) on aerodynamic performance of airfoil with and without jet are compared. The results indicate that the opposing jet reduces drag from 30 % to 70 % , improves the maximum lift-to-drag ratio from 2.5 to 4.0, and increases shock stand-off distance from 15 % to 35 % depending on flow conditions. The effect of opposing jet on longitudinal flight stability characteristics, studied for the first time, indicate improvement in dynamic stability coefficients ( C m q + C m α ˙ ) at low angles of attack. It is concluded that the opposing jet can help mitigate flight disturbances in supersonic regime.

Characteristics of two-dimensional counter-flowing wall jets

Wastewater plant effluents discharged into streams are expected to undergo dilution in a short reach. Jets provide rapid mixing and enhance dilution of the effluent discharged into the stream. The counter-flowing wall jet expansion angle, the geometry of the dividing streamline and the jet penetration length were found using the measured mean axial jet velocity data. These jet characteristics were expressed in terms of the jet velocity ratio (jet exit velocity against main stream velocity). A salt solution of known concentration was injected into the jet flow. The distribution of the salt concentration was determined at several cross-sections of the jet wake using a concentration meter. The standard deviation of the salt concentration provided a measure of jet mixing. When standard deviation reached zero at a cross-section, mixing was considered to be complete.

Study on the combined effect of various injection conditions on the drag reduction by a counter-flow jet in supersonic flow

Parametric investigations of drag reduction by counter-flow jet in supersonic flow have been conducted through numerical simulations. An axisymmetric RANS equation was solved to analyze the drag reduction effect of each parameter. This study revealed a general tendency of the drag reduction effect on various shapes and counter-flow jet conditions and confirmed the conditions for obtaining the maximum drag reduction. In addition, the counter-flow jet structure and flow transition were also analyzed. Five parameters, jet mass flow rate, nozzle size, nozzle exit Mach number, gas total temperature, and gas types, were applied to analyze the drag reduction and flow structure. The results showed that the jet mass flow rate is the dominant factor of drag reduction and flow transition by counter-flow jet and the nozzle size has little effect on drag reduction. Regarding the individual effects of each parameter, a high nozzle exit Mach number, high total temperature, and low molecular weight of the gas are desirable for a higher drag reduction ratio with a low mass flow rate. On the other hand, because a thrust reversal phenomenon tends to occur easily at high temperatures, proper trade-off between each parameter is essential for obtaining the maximum drag reduction ratio. In addition, the nozzle flow conditions, particularly the static pressure ratio at the nozzle exit, have a large effect on the flow structure of the counter-flow jet.

Heat Flux and Pressure Reduction Using Aerospike and Counterflowing Jet on Complex Hypersonic Flow

A numerical study on a reduction of aerodynamic heating and pressure by an active or passive control strategy has been conducted. Three-dimensional double ellipsoid models with aerospike or opposing jet are investigated. In numerical analysis, three-dimensional Navier–Stokes equations are solved by a finite-volume method, and hybrid LES/RANS turbulence model is used. The results show that the use of aerospike or counterflowing jet bring about an obvious decrease in pressure and heat flux values of the nose. However, both the pressure and heat flux appears an increase on body surface either for aerospike model or opposing jet model. For aerospike model, the aerospike will push the bow shock standing away from the first ellipsoid and creates an elliptical shaped recirculation zone, in which the pressure and heat flux keep at a low level. After that, the shear layer will reattaches to the shoulder and creates the peak value of the heat flux and pressure. For opposing jet model, the counterflowing jet flows out the nozzle and pushes the bow shock away from the nose. Meanwhile, the jet flow is pushed back by the freestream and reattached to the ellipsoid shoulder surface. Then, the heat flux and pressure peaks are created by reattachment. These separation and reattachment lead to the ups and downs of the surface pressure and heat flux.

Hypersonic nonequilibrium flow simulations of a hemispherical nose with a counterflowing jet

In this study, numerical analysis is performed to investigate a hemispherical nose with a counterflowing jet in hypersonic flow with a Mach 19 free stream at an altitude of 60 km. Three jet velocities (Mach = 0.91,1.82 and 3.63) corresponding to steady, unsteady and near steady jet states, respectively, are simulated to explore the mechanisms of these three jet states and the effects on aerodynamic drag and aerodynamic heat. Vibrational excitations, chemical reactions and nonequilibrium due to high-temperature effects are considered. To explore the influences of these factors on the two surface quantities (the drag coefficient (Cd) and heat flux), four simulation methods are used involving single temperature without reaction (SingleT-NR), single temperature with reaction (SingleT-R), two temperatures without reaction (TwoT-NR) and two temperatures with reaction (TwoT-R). The results show that different methods have little effect on the change trend of Cd with time and the values of average Cd, which indicates that high-temperature effects barely affect the jet state and the aerodynamic drag of hemispherical nose. For the heat flux, regardless of the unsteady state, the SingleT-R method which not considered the nonequilibrium effect significantly underestimate the results for the hemispherical nose with a high Mach number, while the TwoT-NR method which not include chemical reactions overestimates the results.

Numerical Investigation of Interaction of Counter Flow Jet and Hypersonic Capsule Flow Via Modified DsmcFoam

Abstract In this work, numerical investigation of interaction of counter flow jet and hypersonic re-entry capsule flow has been carried out through modification of DsmcFoam solver. The DsmcFoam modification includes implementation of variable soft sphere (VSS) collision model for more accurate collision model, adjustment of nonuniform initial condition for faster convergence and nonuniform boundary condition, calculation of local Knudsen number in postprocessing for adaptive grid generation, and implementation of different gas species for multigas flow interaction simulations. Therefore, the modified DsmcFoam can be used for investigation of the effect of counter flow jet on the vehicle aerodynamics and aerothermodynamics. New validation test cases from Von Karman gas dynamics facility (VKF) tunnel data of Apollo and blunt-cone re-entry geometries are studied via DsmcFoam in which a suitable agreement of results is observed compared to experimental data and also MONACO code computations. Also, the influence of counter flow jet has been presented, i.e., changing of bow shock configuration and its distance from the vehicle. Consequently, it is observed that by increasing counter flow jet velocity or density, reduction of the drag coefficient and heat flux on the vehicle will occur. Furthermore, variation of the velocity or density of counter flow jet leads to different jet-flow interaction patterns which are presented evidently.

Premixed Flame Response to a Counterflowing Non-thermal Plasma Jet

ABSTRACT The response of the laminar premixed flame to a counterflowing non-thermal plasma jet has been studied experimentally and numerically. The plasma is driven by alternating current and sustained in a co-axial dielectric-barrier-discharge (DBD) generator. Particle tracking methods and optical diagnostics of flame emission provide spatio-temporal resolutions of the flow field and subsequent flame response. The deflection of the streamline and flame sheet due to hydrodynamic disturbances results in oscillation of the total heat release rate. Numerical analysis is performed by specifying the flow fluctuation using inflow boundary conditions, as well as solving the flow field and flame structure in the small cylindrical domain sandwiched between the opposed nozzle outlets. The contour plots of temperature and velocity vectors in the simulation solution verify that the flow fluctuation causes deflection, displacement, creases and localized quenching of the flame front.