Effect Of Microjets Research Articles

The resistance of S235JR steel to cavitation erosion

The erosion resistance of S235JR steel to flow cavitation was investigated. In order to understand an influence of steel properties and cavitation intensities on the degradation mechanisms, low-carbon S235JR steel was tested in the as-received state and after thermal treatment (annealing) under four different flow velocities. Annealing decreased steel hardness, which made S235JR steel less resistant to cavitation erosion. The performed investigations showed that the state of this steel affects the degradation process, e.g. erosion rate and development of surface roughness. An increase in flow velocity caused a linear increase in erosion rate of S235JR steel in as-received state, and an exponential increase in erosion rate of S235JR steel after thermal treatment. Regardless of the state of S235JR steel, an increase in the flow velocity caused a logarithmic increase in the area with increased roughness for Ra > 0.5 μm, where Ra parameter is the arithmetic average of all profile deviations from the mean line over the evaluation length. The increase in surface roughness and the Ra parameter depended on the flow velocity. However, for high flow rates, the Rp parameter, which is the mean of the five maximum profile peak heights from the mean line over the evaluation length, represented an effect of micro-jets impacts on the surface development better than the Ra parameter. A new parameter P, which combines elongation, impact energy and hardness, has been proposed. With increased value of this parameter, the resistance to cavitation erosion decreases. The performed investigations showed that the volume loss can be described by equation in the form of ΔV = C(t)∙(v-v0)n(t), where the function C(t) describes the development of cracks in the surface layer and the function n(t) describes the development of the hardened surface layer.

Influence of individual roughnesses nozzle edge on the length of the supersonic section of an underexpanded microjet

The paper presents the results of the experimental investigation of the influence of a small group of roughnesses uniformly located on the edge of the convergent nozzle on the length of the supersonic core of an underexpanded air microjet. The tests are carried out on the low-pressure test bench with the aid of the Pitot tube. The set of nozzles with the diameter of 2 mm, with the smooth output edge and the edge of the similar shape roughnesses (from 2 to 4) is used. The underexpanded microjets flowing out from the nozzles of the diameter 10.6; 16.1; 21.4, and 34.8 μm are simulated by the outflow Reynolds number. It is shown that the presence of roughnesses can reduce essentially the supersonic section length and eliminate the previously found effect of microjet “relaminarization”, however the reduction value dubiously depends on the amount of the roughnesses on the nozzle edge.

Numerical Simulations of a CD Nozzle and the Influence of the Duct Length

A numerical method is used to observe the effect of microjets control on wall pressure spreading in sudden expansion two-dimensional planar duct. In order to find the microjet effectiveness 2-jets of 1 mm diameter orifice located precisely at 900 of intervals along a pitch-circle-distance (PCD) of 1.3 times the exit diameter of the nozzle in the base were employed to control actively. At the present study, the Mach number was used to calibrate the entry to duct was 2.2, and the area ratio of 2.56. The focus in this study and investigate the influence of length-to-diameter ratio (L/D) of a suddenly expanded duct and its effect on the development of the flow field. Hence, to achieve this, the duct length has been varied from 2 to 10. Nozzles are producing such Mach numbers the experiments were performed operating at nozzle pressure ratio (NPR) 3, 5, 7, 9, and 11. The convergent-divergent nozzle geometry has been studied using the K-ε standard wall function turbulence model and independently check with the ANSYS software.

Experimental Research of Wall Pressure Distribution and Effect of Micro Jet at Mach 1.5

In this paper, a study on the effect of the control on the wall pressure as well as the quality of the flow when tiny jets were employed. The small jet aimed to regulate the base pressure at the base region of the suddenly expanded duct and wall pressure distribution is carried out experimentally. The convergent-divergent (CD) nozzle with a suddenly expanded duct was designed to observe the wall pressure distribution with and without control using small jets. In order to obtain the results with the effect of controlled four tiny jets of 1 mm diameter located at a ninety-degree interval along a pitch circle diameter (PCD) of 1.3 times the CD nozzle exit diameter in the base, region was employed as active controls. The Mach numbers of the rapidly expanded are 1.5. The jets were expanded quickly into an axis-symmetry duct with an area ratio of 4.84. The length-todiameter (L/D) ratio of the rapid expansion duct was diverse from 10 to 1. There is no adverse effect due to the presence of the tiny jets on the flow field as well as the quality of the flow in the duct

CFD Analysis of CD Nozzle and Effect of Nozzle Pressure Ratio on Pressure and Velocity for Suddenly Expanded Flows

A numerical work was carried out to study the effectiveness of micro-jets to control base pressure in suddenly expanded two-dimensional planar duct. Two micro-jets of 1 mm orifice diameter located at 90° intervals along a pitch circle distance of 1.5 times the nozzle exit diameter in the base region were employed as active controls. The calibrated Mach numbers at the entry to suddenly expanded duct was 1.87. The length-to-diameter ratio (L/D) of suddenly expanded duct was 10. Nozzles generating the calibrated Mach numbers were operated with nozzle pressure ratio (NPR) 3, 5, 7, 9 and 11. From the present investigation it is evident that for a given Mach number and effect of NPR will result in maximum increase/decrease of pressure and velocity. The convergent-divergent nozzle geometry has been modeled and simulated employing turbulence models: K-e standard wall function turbulence model from the code was independently checked with the commercial computational fluid dynamics.

Jet arrays in supersonic crossflow — An experimental study

Jet injection into a supersonic crossflow is a classical fluid dynamics problem with many engineering applications. Several experimental and numerical studies have been taken up to analyze the interaction of a single jet with the incoming crossflow. However, there is a dearth of the literature on the interaction of multiple jets with one another and with the crossflow. Jets in a supersonic crossflow are known to produce a three-dimensional bow-shock structure due to the blockage of the flow. Multiple jets in a streamwise linear array interact with both one another and the incoming supersonic flow. In this paper, a parametric study is carried out to analyze the effect of microjet (sub-mm diameter) injection in a Mach 1.5 supersonic crossflow using flow visualization and velocity field measurements. The variation of the microjet orifice diameter and spacing within an array is used to study the three-dimensional nature of the flow field around the jets. The strength of the microjet-generated shock, scaling of the shock wave angle with the momentum coefficient, averaged streamwise, spanwise, and cross-stream velocity fields, and microjet array trajectories are detailed in the paper. It was found that shock angles of the microjet-generated shocks scale with the momentum coefficient for the three actuator configurations tested. As the microjets issue in the crossflow, a pair of longitudinal counter-rotating vortices (CVPs) are formed. The vortex pairs remain coherent for arrays with larger spanwise spacing between the micro-orifices and exhibit significant three-dimensionality similar to that of a single jet in crossflow. As the spacing between the jets is reduced, the CVPs merge resulting in a more two-dimensional flow field. The bow shock resulting from microjet injection also becomes nearly two-dimensional as the spacing between the micro-orifices is reduced. Trajectory estimations yield that microjets in an array have similar penetration as single jets. A notional schematic depicting the interaction of the microjet arrays with the supersonic crossflow is presented summarizing the principal features of the interaction.

Effect of Mach number on Wall Pressure Flow Field for Area Ratio 2.56

This paper presents the experimental results on the flow characteristics of a suddenly expanded flow from the convergent nozzle for subsonic flow. In the present study micro jets were used to investigate the effect of micro jets on wall pressure flow field in the enlarged duct. Accordingly an active control in the form of four micro jets of 1 mm orifice diameter located at 90 0 intervals along a pitch circle diameter of 1.3 times the nozzle exit diameter in the base region was employed. The Mach numbers of the present studies were M = 0.9, 0.8, and 0.6 and the area ratio (ratio of area of suddenly expanded duct to nozzle exit area) studied was 2.56. The length-to-diameter (i.e. L/D) ratio of the sudden expansion duct was varied from 10 to 1. From the results, it is seen that the flow in the base region is dominated by the waves, the general perception what we have that correctly expanded flow will be from waves is proved to be wrong; also, it is found that for L/D in the range L/D = 10 and 8 the flow remains oscillatory mostly for all the Mach numbers. However, these oscillations are suppressed gradually either with the increase in the L/D ratio in the range 3 to 6 or with decrease in the level of inertia level. The present study explicitly reveals that, the wall pressure in a suddenly expanded axi-symmetric duct can be controlled by employing micro jets. Since the flow field in the enlarged duct remained undisturbed hence, we can say that the control in the form of micro jets don't disturb the field.

Supersonic Jet Noise Reduction by Microjet Injection

The effect of microjet (μjet) injection on the noise from supersonic jets is investigated. One convergent and three convergent-divergent (C–D) nozzles, covering design Mach numbers ( MD) 1.0 to 2.2, are used in the study; all nozzles have the same exit diameter ( D). The μjets are injected perpendicular to the primary jet at the nozzle lip from six equally-spaced ports; each port has a diameter of 0.0054 D. Effects in the over-expanded and under-expanded regimes as well as one case of fully expanded condition are explored. Relative to the effect on subsonic jets, larger reduction in the overall sound pressure level (OASPL) is achieved in most supersonic conditions. While the injection readily eliminates screech tones with the convergent nozzle it becomes increasingly ineffective in screech reduction with nozzles of higher MD. With all nozzles, the injection is often found to amplify the broadband shock-associated noise; the overall noise reduction occurs due to a suppression of broadband levels especially at lower frequencies. With the shock still within the divergent section of the C–D nozzles, there is ‘excess broadband noise’ and sometimes there are ‘transonic tones’; the injection is found to affect the tones very little but reduce the broadband noise component significantly. For shock-free, fully expanded condition, the OASPL reduction in the peak noise radiation direction is comparable to that in a subsonic case; the same correlation, found earlier for subsonic cases, applies.

Fluidic control with microjets in dump combustors

This work explores the use of microjets as an effective means to improve the performance of dump combustors. Design studies are conducted for implementation of microjets using Reynolds averaged Navier–Stokes (RANS) equations in the framework of standard k- ε turbulence models. The effect of microjets on the primary flow and combustion is studied in two dump combustor geometries, namely round (axisymmetric) and planar dump combustors. Microjets are placed on the step face at a 45° angle. Compared to the baseline case without microjets, simulations show that adding microjets results in an increase in heat release rate of ∼22% and ∼140% in round and planar dump combustors, respectively. A parametric study is conducted by varying the microjet velocity. The finalized design for implementation of microjets, based on RANS simulations, has been used for experiments. The data collected from the experiments qualitatively confirm the trends predicted by the simulations.

Numerical study of the influence of temperature and microjets on the radiated noise for a subsonic jet

Jet noise remains the principal nuisance source for an aircraft at take‐off conditions. A previous work [1] showed that using micro‐jets could be effective to modify the turbulence development and thus decrease the radiated pressure. This micro‐jets effect is numerically studied for two jets. LES simulations of a cold (M = 0.9) and a hot (M = 0.636) jets, with and without micro‐jets, are computed and the acoustic post‐processing is performed using the Ffowcs‐Williams and Hawkings surface formulation. The temperature effect on the noise is analyzed for the reference computations and, with micro‐jets, the noise reduction is compared to measurements for a similar configuration [1]. Aerodynamic comparisons with experiments give a good comparison, excepted a too short potential core and overestimated turbulent kinetic energy. The noise decrease with the higher temperature and the modifications of the spectral densities are well captured by the simulations. The micro‐jets provide a reduction up to more than 2 dB for the best angles. Turbulence modification is still under investigation and results will be presented. [1] Castelain, T., Béra, J., Sunyach, M., and Juvé, D., “Effect of microjets on a high‐subsonic jet. Parametric study of far‐field noise reduction,” 12th AIAA/CEAS Aeroacoustics Conference (2006).

Control of suddenly expanded flow

PurposeThis paper presents an experimental investigation to study the effectiveness of micro jets under the influence of Over, Under, and Correct expansion to control the base pressure in suddenly expanded axi‐symmetric ducts.Design/methodology/approachFour micro jets of 1 mm orifice diameter located at 90° intervals along a pitch circle diameter of 1.3 times the nozzle exit diameter in the base region was employed as active controls. The tests were conducted for Mach numbers 1.5, 1.3, 1.6, 1.8, 2.0, 2.5, and 3.0. The jets were expanded suddenly into an axi‐symmetric tube with cross‐sectional area 2.56 times that of nozzle exit area. The L/D ratio of the sudden expansion tube was varied from 10 to 1.FindingsFrom the present studies, it was found that the maximum increase in base pressure is 152 percent for Mach number 2.58. It is found that the micro jets do not adversely influence the wall pressure distribution.Research limitations/implicationsAs a result of developments in space flights and missile technology, the base flows at high Reynolds numbers continues to be an important area of research. Our understanding of many features of base flows remains poor, due to inadequate knowledge of turbulence, particularly in the presence of strong pressure gradient.Originality/valueThe paper shows that micro jets can serve as an effective controller raising the base suction to almost zero level for some combination for parameters. The nozzle pressure ratio has a definite role to play in fixing the base pressure with and without control. There is no adverse effect of the micro jets on the flow field in the duct.

Control of Suddenly Expanded Flows from Correctly Expanded Nozzles

The effectiveness of micro jets to control base pressure in suddenly expanded axi-symmetric ducts has been presented in this paper. Four micro jets of 1 mm orifice diameter located at 90° intervals along a pitch circle diameter of 1.3 times the nozzle exit diameter in the base region were employed as active controls. The Mach numbers of the suddenly expanded flows were 1.25, 1.30, 1.48, 1.6, 1.8 and 2.0. The jets were expanded suddenly into an axi-symmetric tube with cross-sectional area 2.56, 3.24, 4.84, and 6.25 times that of nozzle exit area. The length-to-diameter ratio of the sudden expansion tube was varied from 10 to 1. The jets at all Mach numbers were correctly expanded. It is found that the micro jets can serve as active controllers for base pressure. Also, the wall pressure distribution is not adversely influenced by the micro jets.

On the use of microjets to suppress turbulence in a Mach 0.9 axisymmetric jet

We have experimentally studied the effect of microjets on the flow field of a Mach 0.9 round jet. Planar and three-dimensional velocity field measurements using particle image velocimetry show a significant reduction in the near-field turbulent intensities with the activation of microjets. The axial and normal turbulence intensities are reduced by about 15% and 20%, respectively, and an even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The required mass flow rate of the microjets was about 1% of the primary jet mass flux. It appears that the microjets influence the mean velocity profiles such that the peak normalized vorticity in the shear layer is significantly reduced, thus inducing an overall stabilizing effect. Therefore, we seem to have exploited the fact that an alteration in the instability characteristics of the initial shear-layer can influence the whole jet exhaust including its noise field. We have found a reduction of about 2 dB in the near-field overall sound pressure level in the lateral direction with the use of microjets. This observation is qualitatively consistent with the measured reduced turbulence intensities.

The effect of three irrigation practices on phytophthora crown and root rot of apple trees under field conditions

The effect of microjet, drip, and two durations of sprinkler irrigation systems on phytophthora crown and root rot of apple trees was examined under field conditions. This eight year study indicates that crown and root rot caused byPhytophthora cactorum was most severe where young MM. 106 rootstock trees were watered by microjet irrigation for 2.3 h each day. There was no difference in infection byP. cactorum when trees were irrigated either by drip or sprinkler irrigation systems. The MM.106 apple rootstock trees watered by drip irrigation for 2.6 h each day were least affected by phytophthora crown and root rot.