Schlieren Visualization Research Articles

Affordable schlieren visualization methods for understanding three-dimensional supersonic flows

This article discusses affordable schlieren methods which can visualize hidden details in three-dimensional high-speed flow. Mainly, two schlieren methods are brought to light, namely inclined schlieren and focusing schlieren. These two methods have been applied to visualize three-dimensional flow fields like normal shock boundary layer interaction in ducts and flows with shock–shock interactions. The paper explains the possibility of measuring normal shock oscillation in spanwise/transverse direction. Shock height variation across the duct span is also illustrated, using inclined schlieren. Scanning of the focusing schlieren setup can give planar views of three-dimensional shock–shock interactions. It is also shown that the two methods can be merged to give the inclined focusing schlieren method which can also be used to understand three-dimensional flow fields. Further, the possibilities and limitations of these flow visualization methods have been discussed.

Flow interactions on supersonic projectiles in transitional ballistic regimes

A projectile moving in the transitional/intermediate ballistic regime encounters many complex flow phenomena as the flow field contains two blast waves and several flow interfaces. The aerodynamic characteristics of the projectile are highly influenced by the interaction of the projectile with the surrounding flow field. The present study aims to experimentally visualize the flow structures associated with a moving projectile in the immediate vicinity of the launch tube (transitional/intermediate ballistic regime). In this work, the main focus is to investigate three significant phenomena which normally occur in the transitional ballistic regime and have significant effects on the aerodynamic characteristics of the projectile. These are the moving projectile-standing shock interaction termed as unsteady shock diffraction, shock generation due to the transition in the relative projectile Mach number and the moving projectile-moving shock interaction known as the projectile overtaking phenomenon. Experiments are carried out with projectiles of various configurations for various projectile Mach numbers. The flow field is visualized using time-resolved schlieren and shadowgraph flow visualization techniques. The experiments could capture several interesting features of projectile-flow interactions such as the unsteady shock diffraction, shock generation and overtaking phenomenon in various flow regimes through visualization and quantification of the images using image processing techniques.

Visualization and Echo Sounding of Stratified Fluid Disturbances in Front of and Behind a Vertical Plate

With the use of the schlieren visualization and echo sounding method, the spatial structure of disturbances in front of a plate and behind it is investigated for the first time in the flow regime of intense internal-wave generation. In the echograms, fine structures are detected both in the upstream disturbances and in the wake within the solid-motion layer. In the schlieren graph, the fields of upstream disturbances are smooth, and thin interfaces are expressed in the confluent wake. Local peaks in the spatial disturbance spectra are expressed on scales that slightly exceed the length of internal waves, and, to a lesser extent, for lower scales. The thinnest interfaces are observed in the schlieren image of the wake immediately behind the plate.

Schlieren visualization of micro gas turbine exhaust plume with different shapes of nozzle

Schlieren visualization of the plume ejected from the microgas turbine nozzle was conducted to understand infrared signal characteristics of various shapes of the exhaust nozzle. At the same time, the precise temperature distribution and infrared signal measurement were performed and compared. The maximum thrust of the microgas turbine used in the experiment is 230 N, the maximum speed of revolution is 108,500 rpm, and the maximum exhaust gas temperature is 750 °C. Seven nozzles were used for this experiment which included a cone nozzle, five square nozzles with aspect ratios (AR) ranging from 1 to 5 and an S-shaped nozzle with aspect ratio 6. The infrared signal emitted from the exhaust plume decreased as the aspect ratio increased. Schlieren flow visualization images show that cone nozzle had a uniform flow pattern, while square nozzle had a bright triangle pattern in the dispersed plume. As the aspect ratio of square nozzles increased, a bright triangle pattern reduced in size. On comparing Schlieren visualization with the temperature distribution, it can be understood that the triangular shape of core plume plays a major role in the temperature diffusion with the surrounding air. Based on the temperature distribution and the results of the Schlieren visualization, three types of exhaust plume models are suggested. These three models are homogenous plume, intermediate plume and two-dimensional plume with hot core, which correspond to the cone nozzle, the AR2 square nozzle and AR5 square nozzle, respectively.

Laser-Induced Plasma Ignition Experiments in a Direct-Connect Supersonic Combustor at Mach 3

A new direct-connect supersonic combustor has been designed and tested for the study of plasma-assisted combustion in the ACT-II supersonic combustion tunnel. Laser-induced plasma ignition was investigated in an ethylene-fueled cavity flame holder at Mach 3. Combustor design, geometry, and flow characteristics are discussed in the first part of the paper. The supersonic combustor includes a cavity flame holder, where ethylene fuel is injected, mixed with entrained air, and ignited. Two injector configurations are included, one horizontal and one vertical, which were designed in such a way as to enhance the cavity flow recirculation. Two different plasma ignition mechanisms were considered: 1) a laser-induced plasma located at the center of the cavity, and 2) a spark plug placed on the bottom surface of the cavity. The diagnostics tools tested to characterize the flow include time-resolved pressure measurements, high-speed schlieren visualization and chemiluminescence visualization, and instantaneous planar laser-induced fluorescence of the CH radical. The second part of the paper discusses the results of the comparison between horizontal and vertical fuel injection. The larger backpressurization of the isolator and a more uniform distribution within the cavity suggest a greater heat release and combustion efficiency when the horizontal jet is used.

Experimental investigation on compressible flow over a circular cylinder at Reynolds number of between 1000 and 5000

In the present study, a compressible low-Reynolds-number flow over a circular cylinder was investigated using a low-density wind tunnel with time-resolved schlieren visualizations and pressure and force measurements. The Reynolds number ( ) based on freestream quantities and the diameter of a circular cylinder was set to be between 1000 and 5000, and the freestream Mach number ( ) between 0.1 and 0.5. As a result, we have clarified the effect of on the aerodynamic characteristics of flow over a circular cylinder at . The results of the schlieren visualization showed that the trend of effect on the flow field, that are the release location of the Karman vortices, the Strouhal number of vortex shedding and the maximum width of the recirculation, is changed at approximately . In addition, the spanwise phase difference of the surface pressure fluctuation was captured by the measurement using pressure-sensitive paint at approximately of higher- cases. The observed spanwise phase difference is considered to relate to the spanwise phase difference of the vortex shedding due to the oblique instability wave on the separated shear layer caused by the compressibility effects. The Strouhal number of the vortex shedding is influenced by and , and those effects are nonlinear. However, the effects of and can approximately be characterized by the maximum width of the recirculation. In addition, the effect on the drag coefficient can be characterized by the maximum width of the recirculation region and the Prandtl–Glauert transformation.

Entrainment studies in inverse jet flame port burner

The present investigation aims to investigate the fundamental understanding of entrainment characteristics of a Circumferentially Arranged Fuel Port Inverse Jet Flame Burner (CAFP IJF). The high-speed flame Schlieren visualization is carried out experimentally for 8, 18 and 32 port CAFP IJF burner for varying air–fuel operating conditions to visualize the entrainment characteristics. The region of fuel entrainment is found to occur within the base region whereas intense mixing and combustion is observed in transition and main flame region respectively. The 32 port CAFP IJF burner exhibits enhanced entrainment characteristics and augmented ambient air–jet interactions for similar operating conditions as compared to a lesser number of circumferential fuel ports. The jet breakup length is employed to identify the interaction of ambient air with the combustion driven vorticial structures. The steady laminar flamelet model is used to investigate the entrainment characteristics of a 32 port CAFP IJF burner. The calculated entrainment coefficients at different operating conditions for reacting and non-reacting CAFP IJF indicate that the entrainment rate for reacting jet is lower due to the diminution of density across the flame cross-section at each axial plane. The strong ambient air entrainment is observed within the interstitial space between consecutive fuel ports which promote air–fuel mixing. Additionally, unlike in Normal Jet Flame (NJF), a clear distinction between air and fuel entrainment is found to be essential for the current CAFP IJF burner configuration.

Effect of the Reynolds number on the aeroacoustic fields of a transitional supersonic jet

The Reynolds number effect on the aeroacoustic fields of a supersonic jet was experimentally investigated via particle image velocimetry (PIV), schlieren visualization, and near-field acoustic measurements. Cold supersonic jets under ideally expanded conditions at a Mach number of 2.0 were produced with the Reynolds number, based on the diameter of the nozzle exit, ranging from 105 to 106. This study focuses on the laminar-to-turbulent transition of a supersonic jet, including a transitional condition (Re ≈ 105). The PIV results of the high-Reynolds-number jet (Re = 106), which exhibited a fully turbulent shear layer at the nozzle exit, show a linear growth of the shear layer width. Conversely, a significant increase in the turbulent fluctuations and a drastic change in the shear-layer-growth rate were observed near the transition region in the case of the low-Reynolds-number jet (Re = 105). The schlieren visualization and acoustic measurements imply that the turbulent fluctuations of the transition generated strong Mach waves. The effect of disturbing the nozzle flow was also investigated because the laminar-to-turbulent transition is sensitive to the initial disturbance. A physical disturbance added in the inlet of the low-Reynolds-number jet (Re = 105) can promote an earlier transition and suppress a significant increase in the turbulence fluctuations outside the nozzle. The changes in the flow fields due to the disturbance result in a decrease in the sound pressure level. Therefore, the aeroacoustic fields of the low-Reynolds-number jet with the disturbance show similar trends to those of the high-Reynolds-number jet (Re = 106), which already exhibited a fully turbulent shear layer at the nozzle exit.

Experimental investigation of end-gas autoignition-to-detonation transition for an n-decane/O2/Ar mixture

The transition between different combustion regimes is investigated experimentally for a stoichiometric argon-diluted n-decane/O2 mixture. The focus is put on the influence of initial temperature (T = 420–470 K) and pressure (P = 1.5–3 bar) on the regime transitions. Fast schlieren visualization (≥ 120 kHz) and high-speed pressure and temperature measurements are used to monitor the evolution of the reactive processes inside a combustion chamber of square cross-section (40 mm × 40 mm × 172 mm). Results for P ≥ 2.5 bar and the entire temperature range and for P = 2 bar and T ≥ 440 K show three distinct stages following the adiabatic compression of fresh gases induced by the propagation of a flame into the chamber/test section, namely a cool flame, a main heat release stage, and detonation onset. For P = 1.5 bar, however, only the first two stages of the process are observed in the temperature range studied. A two-stage autoignition phenomenon, typical of large hydrocarbons, occurs systematically in the end gas and generates consecutive reactive fronts. The transition to detonation appears to result from the acceleration of the aforementioned fronts toward the speed of sound in fresh gases. Notably, the compression history plays a key role in setting conditions for detonation onset. Our results are in agreement with classical transition maps available in the literature.

Vortices Induced by a Dielectric Barrier Discharge Plasma Actuator Under Burst-Mode Actuation

The formation and motion of vortices generated by a dielectric barrier discharge (DBD) plasma actuator under periodic burst-mode actuation are investigated. The flowfield is studied using particle image velocimetry and schlieren visualization. The induced flowfield is very different from the single starting vortex generated by turning on a continuous steady-state actuation of the DBD actuator. A train of periodic vortices is formed by the periodic burst-mode actuation, with each burst cycle generating one vortex that travels along and away from the wall. The effects of the burst frequency and duty-cycle ratio on the vortex motion and structure are studied, and similarity laws are discovered. The velocity at which the vortex moves is shown to be independent of the duty-cycle ratio of the actuation. The velocity component along the wall, however, is found to increase with the burst frequency. A time-averaged wall jet velocity based on the actuation time during each burst cycle is identified and found to be independent of the burst frequency but increase with the duty-cycle ratio of the actuation. Self-similar patterns of the vortex flowfield are demonstrated for a short time duration after the vortex is formed within each burst cycle.

Unsteady Flow Structures Induced by Single Microsecond-Pulsed Plasma Actuator

The unsteady flowfield around a single microsecond-pulsed dialectic barrier discharge plasma actuator in quiescent air is investigated using particle image velocimetry, Schlieren visualization, and a pressure-field microphone. A single microsecond pulse with a rising time of and a decaying time of is employed. It is the first time that a series of planar-type pressure waves which is induced by the microsecond-pulsed dialectic barrier discharge plasma actuator and propagates outward in the rising period is observed. It is likely that the streamer discharge which occurs in the rising stage and conduces to numerous current pulses with energy release could play an important role in generating these pressure waves. During the plateau of the constant voltage of 10 kV, a wall jet that has a deflection angle of 60 deg is formed, and the maximum velocity is approximately , while a new type of arched jet is observed during the plateau of the constant voltage of 0 kV. These wall jets provide favorable conditions for creating some perturbations and promoting mixing between the mainstream flow and the boundary flow.

Detonation transition criteria from the interaction of supersonic shock-flame complexes with different shaped obstacles

The present study is an experimental investigation of the last stages of the deflagration-to-detonation transition. A fast flame following a lead shock was generated by passing a detonation wave through a perforated plate. The shock flame complex then interacts with an obstacle of different shape. We study the influence of the obstacle shape on the transition mechanism to a detonation. The obstacles studied are a single round or square obstacle, a flat plate, a C-shaped and an H-shaped obstacle. The experiments were performed in a thin transparent channel permitting high speed schlieren visualization. Stoichiometric propane-oxygen was investigated at sub-atmospheric conditions. For each obstacle configuration, the initial pressure was changed to modify the flame burning velocity and the Mach number of the leading shock. The burning velocity prior to the interaction was measured experimentally from the displacement velocity of the flame in the videos. This required estimating the speed of the gas ahead of the flame. A linear correction to the speed immediately behind the lead shock was applied using the shock change equations and the measured pressure gradient behind the lead shock in order to account for the non-steadiness of the lead shock and viscous losses to the walls. Three main findings were that the obstacle shape had a minimal influence on the critical flame strength required for transition, although obstacles with a forward facing cavity were able to suppress the transition by isolating the re-initiation event inside the cavity. The main transition mechanism for all geometries was the enhancement of the flame burning velocity through the flame interaction with the shock reflected on the obstacle leading to Richtmyer-Meshkov instability. Finally, it was found that the flame burning velocity of the initial flame required for transition was closely approximated by the Chapman-Jouguet burning velocity. Consistent with the visual observations, this supports the view that transition is favored when the flame is in phase with the acoustic waves, and strong internal pressure waves can be amplified.

Effect of electrical conductivity of water on plasma-driven gas flow by needle-water discharge at atmospheric pressure

In this study, we investigated the process of gas flow formation, which is driven by atmospheric-pressure plasma. The plasma discharge was produced by a high voltage of 5–7.5 kV0p and was generated between a needle electrode and water surface. Gas flow formation caused by the discharge was observed using the Schlieren visualization technique. Mie-scattered light was detected after the introduction of micrometer-sized particles, which were illuminated by a laser light source. It was determined that the generated gas flow velocity had a strong dependence on the magnitude of the water conductivity. The gas flow velocities for water conductivity values of 0.8 μS/cm and 5 μS/cm were approximately 28 m/s and 8 m/s, respectively. The time evolution of the electric field between the needle electrode and water surface caused different flow formations, and the evolution was affected by the process of charge accumulations in the water and its glass cell container. This charging effect determined the strength of the electric field. The formation of the gas flow required a high electric field, and the process occurred in approximately 10 μs As the pulse width of the applied voltage decreased, the velocity of the resulting gas flow also reduced. It was therefore concluded that the gas flow formation was dependent on the strength and time duration of the present electric field.

Effect of CO2 Dilution on the Laminar Burning Velocities of Premixed Methane/Air Flames at Elevated Temperature

Abstract With increased interest in reducing emissions, the staged combustion concept for gas turbine combustors is gaining in popularity. For this work, the effect of CO2 dilution on laminar burning velocities of premixed methane/air flames was investigated at elevated temperature through both experiments and numerical simulations. Validation of the experimental setup and methodology was completed through experimental testing of methane/air mixtures at 1 bar and 298 K. Following validation, high temperature experiments were conducted in an optically accessible constant volume combustion chamber at 1 bar and 473 K. Laminar burning velocities of premixed methane/air flames with 0%, 5%, 10%, and 15% CO2 dilution were determined using the constant pressure method enabled via schlieren visualization of the spherically propagating flame front. Results show that laminar burning velocities of methane/air mixtures at 1 bar increase by 106–145% with initial temperature increases from 298 K to 473 K. Additions of 5%, 10%, and 15% CO2 dilution at 1 bar and 473 K cause a 30–35%, 51–54%, and 66–68% decrease in the laminar burning velocity, respectively. Numerical results were obtained with CHEMKIN (Kee et al., 1985, “PREMIX: A Fortran Program for Modeling Steady Laminar One-Dimensional Premixed Flames,”) using the GRI-Mech 3.0 (Smith et al., 2019) and the San Diego (“Chemical-Kinetic Mechanisms for Combustion Applications,” San Diego Mechanism Web Page, Mechanical and Aerospace Engineering (Combustion Research), University of California at San Diego, San Diego, CA) mechanisms. It is concluded that the GRI-Mech 3.0 (Smith et al.., 2019) better captures the general overall trend of the experimental laminar flame speeds of methane/air/CO2 mixtures at 1 bar and 473 K. Additionally, the dilution, thermal-diffusion, and chemical effects of CO2 on the laminar burning velocities of methane/air mixtures were investigated numerically by diluting the mixtures with both chemically active and inactive CO2 following the determination of the most important elementary reactions on the burning rate through sensitivity analysis. Finally, it was shown that CO2 dilution suppresses the flame instabilities during combustion, which is attributable to the increase in the burned gas Markstein length (Lb) with the addition of diluent.

Experimental investigation of unstart dynamics driven by subsonic spillage in a hypersonic scramjet intake at Mach 6

Understanding start–unstart behavior of intakes in hypersonic Mach numbers is essential for seamless operation of scramjet engines. We consider a high compression ratio intake (CR = 40) at a Mach number of M = 6 in this work. Start–unstart characteristics are studied in a hypersonic wind tunnel at a flight realistic Reynolds number (Re = 8.7 × 106/m, M = 6). A flap provided at the rear end of the isolator simulates the effect of backpressure for throttling ratios in the range of 0–0.69. Experiments are conducted in two modes: (a) with the flap fixed at a particular throttling ratio and (b) the flap moved to a particular throttling ratio after the started flow has been established. Unsteady pressure measurements and time-resolved Schlieren visualization are undertaken. Modal analysis of pressure (using fast Fourier transform) and Schlieren images (using dynamic mode decomposition) are carried out. The intake shows started behavior for throttling ratios up to 0.31 and a dual behavior, where it remains started in dynamic flap runs but unstarted in fixed flap runs for throttling ratios of 0.35 and 0.42. The intake exhibits a staged evolution to a large amplitude oscillatory unstart for throttling ratios of 0.55 and 0.69, with frequencies of 950 Hz and 1100 Hz, respectively. For the first time, a staged evolution (5 stages) to a subsonic spillage oscillatory unstart of a hypersonic intake is detailed using corroborative evidence from both time-resolved Schlieren and pressure measurements. A precursor to the final large amplitude oscillatory unstart is identified, and the flow mechanism for sustained oscillations is explained.

Modeling and analysis of the visualized gas-assisted laser cutting flow from both conical and supersonic nozzles

A coaxial high-pressure gas jet is normally used to assist and enhance laser cutting quality and capability. However, the gas nozzles are mostly of conical type causing the deterioration of the dynamic characteristics of the exit jet. In this research, the effect of both nozzle type and nozzle dimensions on the uniformity and behavior of the exit jet has been investigated. A total of three different supersonic nozzles have been designed in accordance to gas dynamics theory and manufactured using wire electrical discharge machining (WEDM). The exit jet patterns from these supersonic nozzles and a reference conical nozzle have been numerically modeled and compared with the experimental observations made through Schlieren visualization. The experimental results are found to match and hence validate the simulations. It is concluded that the exit jet from the supersonic nozzle is marked by uniformity, better dynamic characteristics, and longer effective exit jet length compared with conical nozzle.

Underexpanded Supersonic Jets from Elliptical Nozzle with Aft Deck

An experimental investigation of an underexpanded supersonic jet issuing from a circular nozzle, an elliptical nozzle, and an elliptical nozzle with an aft deck at different nozzle pressure ratios (NPRs) is reported in this paper. Static-pressure measurements over the nozzle walls were analyzed to understand the internal flow structure. Schlieren visualization and conical probe measurements were conducted to elucidate the development of the jets issuing from the nozzles. The sound-pressure level in the near field was measured, and it was found that the screech amplitudes were 26 dB lower in the elliptical nozzle with an aft deck. The screech-tone frequencies were observed to be above and near audible region (16 kHz) in the presence of the aft deck at NPRs 3 and 4. The jets emanating from the circular and truncated elliptical nozzles were observed to be flapping about the jet axis at NPRs 3 and 4. The unsteadiness of the jet was reduced by locking the jet onto the aft deck. The centerline Mach-number decay revealed that, due to the screeching effect, the truncated elliptical nozzle has a shorter potential core length and larger jet spreading. The flow separation and reattachment were observed over the aft-deck surface for higher NPR conditions.

Investigation on the Inert Gas-Assisted Laser Cutting Performances and Quality Using Supersonic Nozzles

In the present work, three different supersonic nozzles were designed and manufactured to operate at various stagnation pressures during laser cutting. Several cutting experiments were performed on stainless steel samples of various thicknesses (2, 4, 6 mm) using a fiber laser of 3 kW with a head adapted to fit with both the proposed supersonic nozzles and a commercial reference conical nozzle. The flow through these nozzles was numerically modeled and compared with the Schlieren visualization measurements. The mass flow rate, the Mach number, and the pressure distributions were selected in detail in order to analyze the dynamic characteristics of the exit jet and to comparatively assess the achieved cutting quality (roughness perpendicularity) and capability (maximum thickness, cutting speed). The numerical and the experimental results were found to be in high agreement in terms of the flow structure and mass flow rate. In addition, a significant reduction of the assistance gas consumption of up to 65% on average was achieved by using supersonic nozzles as opposed to conical ones, accompanied with the decrease of the operating pressure and the increase of the cutting speed.

Numerical modeling and Schlieren visualization of the gas-assisted laser cutting under various operating stagnation pressures

The uniformity of the exit jet pattern in high pressure gas-assisted laser cutting represents the main feature in order to achieve high cutting quality and capability. Therefore, the effect of both inlet stagnation pressure and nozzle geometry on the behavior of the exit jet has been investigated in this research. Quasi 1-D gas dynamics theory has been used to calculate the exact-design operating conditions for three different supersonic nozzles that were fabricated by means of Wire Electrical Discharge Machining. The jet flow through these nozzles has been numerically modeled and experimentally checked, using Schlieren visualization, under exact-design, over-expansion and under-expansion operating conditions coming to a good numerical-experimental agreement in terms of flow structure. As main result, the exit jet was found to preserve its uniform distribution with parallel boundaries and low divergence under the exact-design operating condition, differently to what observed for the others two conditions, especially for nozzle with small divergence angle.

Effects of Angle of Attack and Bluntness on Heating Rate Distribution of Blunt Models at Hypersonic Speeds

The effects of nose radius on stagnation and surface heat transfer rate along the surface are addressed in this research paper. Experiments are carried out in hypersonic shock tunnel, at hypersonic Mach number of 6.56 for 11.38° apex angle blunt cone with nose radius of 0.2R, base radius of R. Similarly, experiments are carried out at Mach 7.32 for 13.87° apex angle blunt cone models with nose radius of 0.18R′, base radius of R′. Test is performed at stagnation enthalpy of 1.4 and 2 MJ/kg with effective test time of 3.5 ms. Convective heat transfer measurements have been carried out on the test model at two different angles of attack, namely 0° and 5° with angle of rotation of 0°, 90°,180° with platinum thin film sensors. ANSYS-Fluent used to simulate the flow over the blunt models at different Mach numbers. The measured shock standoff distance from Schlieren visualization images compared with theory and computational fluid dynamic study for both configurations. The measured stagnation heating value is compared with theoretical value estimated using Fay-Riddell expression and numerical simulation. The measured heat transfer rate is higher for configuration 1 than configuration 2. The increases in heat transfer rate is due higher density ratio across the shock wave and the reduced shock layer thickness. The measured shock layer thickness is 2.06 mm for Mach 6.56 and 3.45 mm for Mach 7.32. The heat transfer rate is higher for Mach 6.56 as compared to Mach 7.32.