Engine Inlet Air Research Articles

Experimental Study of a PowerCore Filter Bed Operating in a Two-Stage System for Cleaning the Inlet Air of Internal Combustion Engines

Small dust grains cause a higher intensity of increase in the flow resistance of the fibrous filter bed, which, due to the established value of the permissible resistance, results in a shorter period of operation of the air filter and the vehicle. At the same time, the mass of dust per unit of filtration area takes on smaller values. Such a phenomenon occurs in the two-stage “multicyclone-baffle filter” engine inlet air filtration system. The main objective of this study was to experimentally determine the mass of dust retained per unit of filtration area (dust absorption coefficient km) of the PowerCore filter bed operating under two-stage filtration conditions, which cannot be found in the available literature. The original methodology and conditions for determining the dust absorption coefficient km of a PowerCore filter bed operating under two-stage filtration conditions are presented. Tests were carried out on the characteristics of filtration efficiency and accuracy, as well as on the flow resistance of a filtration unit consisting of a single cyclone and a PowerCore test filter with an appropriately selected surface area of filter material. During the tests, conditions corresponding to the actual conditions of vehicle use and air filter operation were maintained, including filtration speed and the dust concentration in the air. The experimentally determined dust absorption coefficient of the PowerCore research filters operating in a two-stage filtration system took on values in the range of km = 199–219 g/m2. The dust absorption coefficient km of the PowerCore research filter operating under single-stage filtration conditions reached a value of km = 434 g/m2, which is twice as high. Prediction of the mileage of a car equipped with a single-stage and two-stage “multi-cyclone-partition” filtration system was carried out, showing the usefulness of the experimentally determined dust absorption coefficients km.

A Study on the Effect of Inlet Air Pollution on the Engine Component Wear and Operation

This paper systematically reviews the research progress in the field of the influence of air pollutants in the engine inlet on the accelerated wear of the elements of the association: piston, piston rings, cylinder liner (P-PR-CL), and plain bearing (journal–panel). It was shown at the outset that the primary component of air pollution is road dust. Its main components are dust grains of hard minerals (SiO2, Al2O3), which penetrate the oil film area between two frictionally mating surfaces causing their abrasive wear. Therefore, the effect of three dust parameters (grain size and hardness, and dust concentration in air) on the accelerated wear of the friction pair: piston, piston rings, cylinder liner(P-PR-CL), and plain bearing (journal–pan) is presented extensively. It was noted that the wear values of the same component were obtained by different researchers using different testing techniques and evaluated by different indices. It has been shown that the greatest wear of two frictionally cooperating surfaces is caused by dust grains with sizes equal to the thickness of the oil film at a given moment, which in typical combustion engine associations assumes varied and variable values in the range of 0–50 µm. The oil film thickness between the upper ring and the cylinder liner varies and depends on the crankshaft rotation angle, engine speed and load, and oil viscosity, and takes values less than 10 µm. It was shown that the maximum wear of the cylinder liner, resulting from the cooperation with the piston rings, occurs in the top dead centre (TDC) area and results from unfavorable (high temperature, low piston speed) operating conditions of these elements. From the extensive literature data cited, it follows that abrasive wear is caused by dust grains of specific dimensions, most often 5–20 µm, the greater the wear the greater the hardness of the grains and the sulfur content of the fuel. At the same time, it was shown that the main bearing, crankshaft bearing, and oil ring experienced maximum wear by a different range of particle size, respectively: 20–40, 5–10, and 20–80 μm. It was shown that the mass of dust that enters the engine cylinders and thus the wear of the components is determined by the concentration of dust, the value of which is definitely reduced by the air filter. However, it was pointed out that the low initial filtration efficiency and the presence of large dust grains in the purified air in the initial period of the filter operation (after replacement of the filter element with a new one) may have an impact on the accelerated wear of mainly (P-PR-CL) association. The next stage of the paper presents the effects of excessive wear of the cylinder liner and piston rings of the engine, resulting from actual vehicle operation and bench tests on the decrease in compression pressure and engine power, increase in the intensity of exhaust gas blow-by into the oil sump and increase in oil consumption and exhaust gas toxicity. This paper addresses the current problem of the effect of engine inlet air contaminants on the performance of the air flow meter, which is an essential sensor of the modern internal combustion engine. The phenomenon of deposition of contaminants (mineral dust, salt, carbon deposit, and moisture) on the measuring element (wire or layer anemometer) of the air flow meter has been analyzed. The empirical results presented show that the mineral dust layer on the measuring element of the air flow meter causes a 17.9% reduction in output voltage, and the dust and oil layer causes a 46.7% reduction in output voltage. This affects the decrease in engine power and exhaust toxicity.

Analysis of Water-Cooled Intercooler Thermal Characteristics

With the incremental power of construction machinery diesel engines, the power performance of diesel engines and the pollutant emissions from the exhaust gas have imposed increasingly stringent requirements on the intake cooling system of diesel engines. This paper compared the j/f evaluation factors for fin unit bodies of water-cooled intercooler (including straight fins and rectangular misaligned fins) by means of CFD simulation, and found that the rectangular misaligned fins had an 8% advantage in comprehensive performance. With the rectangular staggered fin intercooler, it was found that under the same conditions, the cooling efficiency of the dual-pass water-cooled intercooler is higher than that of the single-pass water-cooled intercooler, and the uniformity factor of the temperature difference field of the dual-pass water-cooled intercooler is 1.5% higher than that of the latter. The accuracy of the overall simulation of the intercooler is verified by the field test. The dual-pass and single-pass water-cooled intercooler both can maintain heat balance under working conditions, and its average air inlet temperature is 10 °C lower than that of the original air-cooled intercooler, which provides support for further reducing the engine air inlet temperature. The results provide a theoretical basis for the performance improvement of water-cooled intercoolers.

Numerical Simulation and Experimental Research on Engine Room Heat Dissipation Performance of a Hybrid Car

Compared with the fuel cars, the hybrid cars has higher thermal load of engine room and has higher requirements for heat dissipation performance of the cooling module. Under idling condition, a hybrid car has an actual problem that higher temperature of engine room and inlet air temperature of cooling module. In this paper, a mathematic model of car thermal management is established, numerical simulations on flow and temperature fields of car were performed by using STAR-CCM+ software, the feasibility of the simulation results was verified by the thermal balance test of the cars. The results show that the reflow of hot air and the inlet air temperature of the cooling module are reduced by sealing the cooling module, the front end module and optimizing the structure of deflector, thus, the heat dissipation efficiency of the cooling module is improved.

Material Properties Analysis with Addition of Nanofibres for Air Intake Filtration in Internal Combustion Engines

The aim of this study is to provide an experimental properties evaluation of a standard filter material (cellulose) and materials with fiber layer addition with small diameters (nanofibers). Filter media, including cellulose, used in the internal combustion engine inlet air filtration are made of high diameter fibres, approx. 15 µm. Significantly higher separation and filtration efficiency performance are obtained for materials with lower fibre diameters (nanofibres), however, at the expense of a significantly higher pressure drop, affecting the engine performance. Filter media manufacturers mainly specify the structure parameters (pore size, air permeability and thickness), without giving any information on the dust filtration performance and rate. The literature includes test results for models of different filter media structures. Filtration process modelling using polydisperse dust with particles of different shape and density and irregular filter media structure is possible using advanced computer techniques, however, the process is complex and requires many simplifications. Test results can be applied directly in the automotive industry. The data can be obtained by experimental tests on filter medium specimens, complete filter elements or air filters which are costly and time-consuming tests, however, those test methods are the most reliable. Conditions and testing methodology for intake air filter materials used in internal combustion engines were developed. Filtration and flow resistance efficiency and accuracy were done depending on test dust mass stopped per unit area. Tested materials filtration efficiency was assessed by a filtration quality factor, which includes experimentally determined efficiency and accuracy as well as flow resistance values. Much higher efficiency and filtration accuracy of dust grains below 5 µm in filtration materials with nanofibers addition compared to standard filtration material (filter paper) were demonstrated. For the same flow resistance values, filter materials with nanofibers addition accumulate smaller dust mass than standard filter paper. Usage of materials with nanofibers addition used in motor vehicles intake air filtration ensures their high efficiency and accuracy. It minimises its components wear, but at the expense of faster flow resistance increase, which shortens filter life and increases filter replacement frequency. Results obtained during the experimental research partly fill the gap when it comes to the basic material properties used in internal combustion engines intake air filter partitions production.

Application of oxygen enrichment and adiabatic humidification to suction air for reducing exhaust emissions in a gasoline engine

ABSTRACT This paper aims to reduce the exhaust emissions of a gasoline engine by conditioning the engine intake air. The oxygen enrichment of inlet air to reduce the HC and CO emissions and humidifying the inlet air to control NOX emissions was applied. The intake air oxygen concentration was increased from 21% to 25% by 2% interval. The oxygen enrichment of the engine inlet air increased the combustion rate, and thus combustion improved, and thermal efficiency increased. This situation led to a reduction in HC emissions and CO emissions. However, the combustion improvement caused a striking rise in NOX emissions because of the high cylinder temperature and high oxygen concentration. This undesirable effect was tried to be alleviated by adiabatic humidification of the engine inlet air. For this purpose, the relative humidity of the intake air was increased from 35% to 65% before and then to 85%. For this, the intake air passed through the conditioning chamber, the dry thermometer temperature lowered by 4 °C for a relative humidity of 85% thanks to the evaporative cooling of water. The results showed that oxygen enrichment caused a 13.5% and 9.3% increase in thermal efficiency in order of half and full load. The humidification method resulted in a 14.3% and 18.3% decrease in thermal efficiency for half and full load. The results also showed that the humidification method could only control NOX increment generated at half load and 23% oxygen condition.

The impact of volute aspect ratio and tilt on the performance of a mixed flow turbine

Current trends in the automotive industry towards engine downsizing means turbocharging now plays a vital role in engine performance. The purpose of turbocharging is to increase the engine inlet air density by utilising, the otherwise wasted energy in the exhaust gas. This energy extraction is commonly accomplished through the use of a radial turbine. Although less commonly used, mixed flow turbines can offer aerodynamic advantages due to the manipulation of blade leading (LE) angles, improving performance at low velocity ratios. The current paper investigates the performance of a mixed flow turbine with four volute designs, two radial and two tilted volutes each with one variant with an aspect ratio (AR)=0.5 and one with AR = 2. To ensure constant mass flow parameter (MFP) for aerodynamic similarity, volute area to radius ratio (A/r) was manipulated between the design variants. The maximum variation of cycle averaged normalized efficiency measured between the designs was 2.87%. Purely in the rotor region, the variation in normalized cycle averaged efficiency was 3%. The smallest volute AR designs showed substantial secondary flow development. The introduction of volute tilt further complicated the secondary flow development with the introduction of asymmetry to the flows. It was established that both AR and tilt have a notable effect on secondary flows, rotor inlet conditions and over all mixed flow turbine performance.

Case Study: The Cause Analyses for Helicopter (AS-565MB) Flying Accident using Star-CCM Program

헬리콥터 사고가 빈번하게 발생되고 있으나, 사고원인을 조사하기 위한 정보의 한계와 실험의 한계 등으로 정확한 사고원인을 추정하기 어려운 경우가 많다. 이에 국립과학수사연구원으로 최근 의뢰된 사고 중 헬리콥터 엔진부의 이물질 유입으로 인한 공기압축기 파손사고는 자칫 큰 인명피해로 이어질 수 있는 사고였으나, 사전에 발견되어 대형사고를 미연에 방지할 수 있는 경우였다. 본 연구에서는 전산유체해석에 있어서 범용적으로 사용되고 있는 Star-CCM 프로그램으로 유동해석하여 헬리콥터 외부에서의 이물질 유입에 대한 가능성을 검토하고자, 이·착륙 비행과 제자리 비행 시 주로터 회전과 압축기 공기 유량으로 생성되는 전기체 주변의 유동장, 압력장, 속도벡터 등을 분석하여 엔진 공기흡입구로 이물질이 유입될 수 있는 가능성을 살펴보았다.Helicopter accidents have been constantly occurring by performing the mission of anti-disaster, prevention of the breeding and extermination, putting out a forest fire, saving a life, and so on. these accidents do not on the decrease in spite of the social big issue that once these happens is likely to lead to the heavy casualties and property damages. The reason is thought that it is not active research because of limitation of experimental analysis, limitation of information access, etc. in investigation and lacks the replacement of deteriorated Helicopter, the safety management effort and the research of operation, maintenance, training. In this paper, we introduce the inflow possibility in engine air inlet of AS-565MB helicopter by foreign objects in take-off and landing, in hovering using Star-CCM program (Computer Fluid Dynamics program).

Numerical research on vortex tube separator for special vehicle engine inlet air filter

A two-stage inlet air filtration system used in the special vehicle combustion engine is presented. A comparative analysis of the filtration properties of return and vortex tube separators (VTS) was carried out. Factors determining the efficiency of vortex tube separators are presented. The aerosol filtration process in the cyclone was discussed. The decisive influence of the inertia force, and the drag force of the medium on cyclone efficiency was demonstrated. The impact of design factors and flowing aerosol on the separation efficiency, and pressure drop of cyclone flow was assessed using commercial Ansys Fluent engineering software. A Discrete Phase Model was used, which characterizes in that the particle trajectory is determined using Lagrange’a formulations. Real vortex tube separator geometry was used to build the cyclone model (VTS), which was acquired as a result of reverse-digital 3D scanning engineering. The developed numerical model verification was carried out on the results basis of numerical, and experimental tests of the vortex tube separator. Numerical and experimental separation efficiency tests and vortex tube separator pressure drop were carried out for different vortex tube separator inlet speeds in the range of 2.5 ÷ 15 m/s. A domestic substitute for AC fine polydisperse dust (PTC-D test dust) was used. In the numerical calculations the particle size distribution of PTC-D was taken into account using the Rosin-Rammler model. As a result of numerical tests, the characteristics of vortex tube separator separation efficiency are similar to those resulting from experimental tests.

An experimental examination of the effects of n-hexane and n-heptane fuel blends on combustion, performance and emissions characteristics in a HCCI engine

Homogenous charge compression ignition (HCCI) engines are low-temperature combustion engines with high thermal efficiency. The operation range of HCCI engines, which are not directly controlled on combustion, is limited by the problems of knocking and misfiring. At this point, it is aimed to eliminate the problem of knocking at the high engine loads by the charge mixture composition and various operating parameters. In this study, the experiments were performed in a single cylinder, four stroke, port injection HCCI gasoline engine to investigate the effects of n-hexane, n-heptane and n-hexane/n-heptane blends. Combustion characteristics, engine performance and exhaust emissions were determined at different engine speeds (800–1800 rpm), lambda (λ = 1.5–3.0) and inlet air temperatures (60 and 80 °C). Operating range of the HCCI engine was determined by using both pure n-hexane and pure n-heptane fuels. The experiments showed that n-hexane, which has higher octane number than that of n-heptane, has more resistance to knocking. In-cylinder pressure decreased with the increase of lambda for all test fuels. Thermal efficiency increased about 28% by using n-hexane compared to n-heptane at constant lambda (λ = 2.35). The results showed that H75N25 provided larger operating range compared with other test fuels.

Diesel Engine Acoustic Emission Airflow Clogging Diagnostics With Machine Learning

A diesel engine electrical generator set (“gen-set”) was instrumented with in-cylinder indicating sensors as well as acoustic emission microphones near the engine. Air filter clogging was emulated by progressive restriction of the engine's inlet air flow path during which comprehensive engine and acoustic data were collected. Fast Fourier transforms (FFTs) were analyzed on the acoustic data. Dominant FFT peaks were then applied to supervised machine learning neural network analysis with matlab-based tools. The progressive detection of the air path clogging was audibly determined with correlation coefficients greater than 95% on test data sets for various FFT minimum intensity thresholds. Further, unsupervised machine learning self-organizing maps (SOMs) were produced during normal-baseline operation of the engine. The degrading air flow engine sound data were then applied to the normal-baseline operation SOM. The quantization error (QE) of the degraded engine data showed clear statistical differentiation from the normal operation data map. This unsupervised SOM-based approach does not know the engine degradation behavior in advance, yet shows clear promise as a method to monitor and detect changing engine operation. Companion in-cylinder combustion data additionally shows the degrading nature of the engine's combustion with progressive airflow restriction (richer and lower density combustion).

Assessment of Turbocharge System for an Existing Gas Engine

S Turbocharger is a turbine driven air compressor and forced induction device powered by exhaust gases from the internal combustion engine. A turbocharger consists of two chambers connected by center housing. The two chambers contain a turbine wheel and a compressor wheel connected by a shaft which passes through the center housing. The response of the turbine to the engine exhaust, dictates the response of the compressor and thus the engine air inlet. Turbocharger response time (also known as turbo lag, is directly related to the size of the turbine and compressor wheels. Small wheels accelerate rapidly; large wheels accelerate slowly. While small wheels would seem to have an advantage over larger ones, they may not have enough airflow capacity for an engine. Therefore, to minimize turbo lag, the intake and exhaust breathing capacities of an engine must be matched to the exhaust and intake airflow capabilities of the turbocharger. The results of the current study showed that the horsepower of 347 hp and torque 380 ft-lb before turbocharge reached to 483.3 hp, torque 552 ft-lb after turbocharge at a speed of 4600 rpm.

МЕТОД ВИЗНАЧЕННЯ ХОЛОДОПРОДУКТИВНОСТІ ТЕРМОТРАНСФОРМАТОРА ЗА МАКСИМАЛЬНИМ ТЕМПОМ ПРИРОЩЕННЯ ТЕРМОЧАСОВОГО ПОТЕНЦІАЛУ ОХОЛОДЖЕННЯ ПОВІТРЯ

It is proved a possibility of using the thermohour cooling potential method, developed by the author, for defining the installed (design) refrigeration capacity of term transformer (refrigeration machine), providing a maximum rate of thermo-hour cooling potential increasing according to the current climatic conditions for a definite period of operation.It is proposed to define the effect, gained due to cooling air, in particular at the inlet of GTU, depends on duration and depth of cooling, by thermohour potential ÕS,°С·h, as air temperature decrease Δta multiplied by duration τ of GTU operation at decreased temperature: ÕS = ∑(Δta ∙°τ), which to some extent characterizes heat load on the cooling system.It is shown that taking into consideration a different rate of annual thermohour cooling potential arising with increasing the installed refrigeration capacity of term transformer, caused by changing the heat load according to current climatic conditions during a year, it is necessary to choose such design heat load on the air cooling system (refrigeration capacity of term transformer) that provides a maximum value of annual thermohour cooling potential or close it with relatively high rates of its increasing. To define the installed refrigeration capacity, providing a maximum rate of annual thermohour cooling potential increasing, it is analyzed the dependence of annual thermohour cooling potential related to the installed refrigeration capacity of term transformer, from the installed refrigeration capacity of term transformer. As a result of the investigation, it is proposed the method of defining the design heat load (installed refrigeration capacity) of term transformer with maximum rates of increasing thermohour cooling potential, as a further development of methodology of rational designing of them transformers for combustion engine inlet air cooling on the base of thermohour potential, developed by author

ЗАКОНОМІРНОСТІ ТЕЧІЇ ДВОФАЗНОГО ПОТОКУУ ВХІДНОМУ ПРИСТРОЇ ВЕРТОЛІТНОГО ГТД

The results of finite element modeling of the airflow contains dust particles of various sizes in the helicopter gas turbine engine air inlet duck with mounted and dismounted dust protective device are given. The main laws of the behavior of multiphase flow in the gas turbine engine inlet duct in response to the particle size are determined. It clarifies that the use of dust-protective devices does not eliminate dust particles in full manner. An estimation of the possibility of replacing the simulation of a two-phase flow with one-phase in the GTE compressor has been made

Experimental Research of Fuel Mass Flow Rate Distribution Effectiveness in Fuel Manifold on Engine Starting Performance

The component test results of fuel flow divider connecting fuel manifolds show that, when the starting fuel supply is given and the primary fuel manifold relative unfold pressure is at constant, the adjustment of the secondary fuel manifold turn-on pressure has effects on fuel flow through the primary fuel manifold and the time of fuel into the combustion chamber. The verification results of the secondary fuel manifold unfold pressure influence on engine starting performance shows that, when the secondary fuel manifold unfold pressure is low, the time of fuel into the combustion chamber is short, the engine starts fast; but because of rotation speed is low and engine inlet air flow is less, the gas temperature in staring phase is high. The test research results have important guidance and reference meaning for confirming the secondary fuel manifold unfolds pressure.

The Design of a Shell-Tube Heat Exchanger as Evaporator an Absorption Chiller Cycle to Reduce the Temperature of the Air Entering a Diesel Engine Operating at Full Load Engine Medium

Survey, design evaporator heat exchanger is an absorption chiller cycle. To cooling air to a four-cylinder diesel engine medium, at full load at different speeds to increase the useful power output. Heat can be recycled to cooling air to the engine, by the absorption chiller, cooling to be converted. By reducing the engine inlet air temperature by evaporating, at full load, increased air density. And consequently more engine volumetric efficiency. As a result, the effective power output increases. In this study, the design of the structure of the evaporator, Solid Works was done by software. And the relationship between formulas and equations of heat transfer in the MATLAB code and the output results is provided in this article.

Operating properties of non-woven fabric panel filters for internal combustion engine inlet air in single and two-stage filtration systems

Operating properties of non-woven fabric panel filters for internal combustion engine inlet air in single and two-stage filtration systems

Study of the Variations in Cylinder-Exhaust-Gas Temperature Over Inlet Air and Operation-Input Parameters of Compression-Ignition Engines

Cylinder-exhaust-gas temperature (Texh) of a turbocharged compression-ignition engine indicates the levels of engine thermal loading on cylinder and exhaust components, thermal efficiency performance, and engine exhaust emissions. In consideration that Texh is affected by engine air inlet condition that primarily includes inlet air temperature (Ti) and pressure (pi), this paper studies the variation (ΔTexh) of Texh over varying the engine inlet air parameters of compression-ignition engines. The study is to understand ΔTexh with appropriate relations between the inlet parameters and Texh being identified and simply modeled. The regarded effects on Texh and ΔTexh for both naturally aspirated and turbocharged engines of this type are analyzed and predicted. The results indicate that Texh increases as Ti increases or pi decreases. The rate of variation in ΔTexh over varying Ti or pressure pi is smaller in a turbocharged engine than that in a naturally aspirated engine, as reflected from the model and results of the analysis. The results also indicate, for instance, Texh would increase approximately by ∼2 °C as Ti increases by 1 °C or increase by ∼35 °C as pi decreases by 10−2 MPa, as predicted for a typical high-power turbocharged diesel engine operating at a typical full-load condition. The design and operating parameters significant in influencing ΔTexh along with varying Ti or pi are studied in addition. These include the degree of engine cylinder compression, the level of intake manifold air temperature, the magnitude of intake air boost, and the quantity of cycle combustion thermal input. As those parameters change, the rate of variation in Texh varies. For instance, the results indicate that the rate of ΔTexh versus the inlet air parameters would increase as the quantity of cycle combustion thermal input becomes higher. With the understanding of ΔTexh, the engine output performances of thermal loading, efficiency, and exhaust emissions, concerning engine operation at variable ambient temperature or pressure, can be understood and evaluated for the purpose of engine analysis, design, and optimization.

Prediction of Peak Cylinder Pressure Variations Over Varying Inlet Air Condition of Compression-Ignition Engine

Peak cylinder pressure (pmax) of a compression-ignition engine can be affected by the engine inlet air condition, such as its inlet air temperature (Ti) and pressure (pi). The variation of peak cylinder pressure due to varying inlet air temperature or pressure is analytically studied. A model is developed and simplified, and thus the variations of pmax can be predicted along with varying inlet air temperature or pressure. The analysis and prediction indicate that cylinder active compression ratio (CR) and intake air boost ratio (pm0∕pi0) play relatively significant roles in affecting the variation of pmax over inlet air temperature and pressure, and the pressure variation is proportional to CRk and ratio pm0∕pi0. Comparison between the predicted results using the simplified model and those from engine experiments shows a close agreement in both the trend and magnitude. The investigation and prediction also include modeling the variation in pmax due to varying the cylinder TDC clearance volume (Vc). The simplified model is presented and shows that the change in pmax versus varying Vc also depends on the cylinder compression ratio. It is indicated that for a certain change in the clearance volume, a higher compression-ratio configuration would produce a greater change in pmax than a lower one does, especially as the rest of the engine design and operating parameters remain unchanged.

Tangentially directed acoustic jet controlling boundary layer

The nozzle of an acoustic jet directs high momentum flux gas particles essentially tangentially into the boundary layer of the flow in a diffuser, an engine air inlet, a jet engine gas flow path or on the suction surface of an airfoil, the gas particles in the chamber of the tangential acoustic jet being replenished with approaching low momentum flux particles drawn from the gas flow in a direction normal to the surface, thereby to provide a net time-averaged flow of increased momentum flux particles to defer the onset of boundary layer separation and/or reduce the thickness of the boundary layer. The acoustic jet is driven by a gas pressure oscillation generator which may be a loudspeaker, a resonant solenoid piston, a cranked piston, or the like.