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Diffraction is a fundamental phenomenon that occurs when blast or shock waves pass over sudden discontinuous surfaces. It generates a complex flow field consisting of diffracted waves, expansion waves, slipstream, contact surface, and an unstable shear layer, in addition to emitting acoustic waves. In this study, we investigated the diffraction of a blast wave passing over a series of grooved structures with different aspect ratios and geometrical shapes (rectangular, circular, and triangular) using high-speed shadowgraph images. The blast wave Mach number considered in our investigation is 1.34. The grooves feature leading-edge geometrical variations such as rectangular, circular arc, and wedge shapes positioned at various lateral locations from the exit of the shock tube. The aspect ratios of the rectangular grooves vary from 0.33, 0.5, and 0.67. The circular and triangular grooves have an aspect ratio of 0.33. The trajectories and velocities of the primary vortex are calculated by tracking the location of the vortex in the shadowgraph images. Our observations revealed that a large portion of the incident blast wave is abducted inside the groove as the aspect ratio increases in rectangular grooves, resulting in better attenuation of the blast wave. The grooves, which have circular shapes, produced weaker diffraction, which resulted in delayed and weak primary vortex. The triangular grooves produced the strongest primary vortex and have the highest attenuation characteristics among other grooves. The strength and trajectory of the primary vortex formed over the grooves strongly depend on the aspect ratio and the curvature of the leading edge for a given Mach number. Vortices generated from rectangular and triangular grooves exhibit considerable strength and longevity. [ABSTRACT FROM AUTHOR]

A quantifiable, reproducible, and repeatable definition of the three-dimensional spray width and depth for a canonical jet in an open-source supersonic crossflow is presented. An expanding Mach 2 dry-air crossflow is generated through a converging-diverging nozzle with a 25.4 mm by 230 mm wide throat area. A one-millimeter injector with ethanol seeding provides the liquid injection. Injector characteristics and losses are quantified through a calibrated cavitating venturi. Momentum flux ratios ranged from 0.1 to 20, and Reynolds number scaled by the injector diameter ranged from 5000 to 40,000. A shadowgraph setup with a telecentric lens provides uniform magnification for precise and repeatable measurements from injection to 150 mm downstream of the jet. A Phantom v2012 camera with a frame rate of 20 kHz and shutter time of 285 ns was employed. Light transmittance is defined and calculated for each image pixel with a ratio method paired with no-spray images collected immediately before injection. These values are then related to an attenuation coefficient by incorporating spray width profiles collected with cross-sectional Mie-scatter imaging at multiple axial locations with a burst mode laser. [ABSTRACT FROM AUTHOR]

This experimental study investigates the impact of two diametrically positioned sonic air tabs on the mixing characteristics of a Mach 2.1 circular jet. Positioned at an axial distance of 0.25D from the convergent-divergent nozzle exit, the air tabs' injection pressure ratio was systematically varied from 3 to 6, while maintaining nozzle pressure ratios of 3, 4, 5, and 6. Through Pitot pressure measurements and flow visualization, the study reveals that the sonic air tabs effectively reduce the core length of the Mach 2.1 jet across all nozzle pressure ratios. The accelerated mixing of the Mach 2.1 jet with the ambient fluid, facilitated by the air tabs, results in shorter core lengths. Importantly, the mixing enhancement by air tabs intensifies with increasing injection pressure ratio for all nozzle pressure ratios, with the maximum reduction in core length consistently occurring at an injection pressure ratio of 6. The observed maximum reductions in core length for nozzle pressure ratios 3, 4, 5, and 6 at an injection pressure ratio of 6 are 41.3%, 60.8%, 43.7%, and 43.5%, respectively. Visualization results confirm the air tabs' effectiveness in attenuating waves within the jet core, with the weakening of waves increasing with higher injection pressure ratios. These findings contribute valuable insights into optimizing supersonic jet performance through fluidic control techniques. [ABSTRACT FROM AUTHOR]

The use of fluid injection in supersonic jet control techniques provides several benefits, including thrust augmentation and mixing enhancement. This study aimed to investigate the effect of air tabs on a Mach 2.1 supersonic jet by varying the injection pressure ratio (IPR) from 3 to 7 for fixed nozzle pressure ratios (NPR) of 4 and 7. The core length of the jet with and without air tabs was quantified by analyzing shadowgraph images under controlled conditions. The results showed that increasing the IPR from the air tabs at the nozzle exit (0D) led to a reduction in core length for the fixed NPRs of 4 and 7. The rate of core length reduction increased with increasing IPRs from 3 to 7. For NPR 4, the maximum and minimum core length reduction rates were found to be 32% and 8% for IPRs 7 and 3, respectively. Similarly, for NPR 7, the core length reduction rate was highest for IPR 7 (38%) and lowest for IPR 3 (8%). The results demonstrate that the use of air tabs induces stream-wise vortices, leading to mixing enhancement and a reduction in core length. [ABSTRACT FROM AUTHOR]

The characteristics of a sonic under-expanded coaxial jet with lip thickness 1.5Dp (where Dp is the exit diameter of primary jet equals 10 mm) with the primary jet operating at nozzle pressure ratio (NPR) of 3, 4 and 5. For NPR 3 operating primary jet, the secondary jet operates at NPR 2.5, 1.4 and 1.27. For primary jet NPR 4, the secondary jet operating NPR is 3.2, 1.6 and 1.4. For NPR 5 primary jet, the secondary jet NPR is 3.8, 1.89 and 1.52. The study is performed using a co-flow nozzle of bypass ratio (BR) 6.4, 1.4 and 0.7. The core length of the primary jet is used as a measure to quantify the mixing of the primary jet in the presence of coaxial jet. The shock structure present in the near field was viewed using shadowgraph technique. Centreline pitot pressure distribution, radial spread and waves present in the jet core were analyzed. The results show that the mixing associated with the high bypass coaxial jet is superior to the low bypass coaxial jet. This mixing superiority associated with high bypass coaxial jet prevails all levels of expansion. [ABSTRACT FROM AUTHOR]

Short takeoff and vertical landing aircraft configurations involve multiple jets that impinge onto the deck surface in tandem and lead to several adverse effects. This study reports on the experimental characterization of supersonic dual-impinging jets by systematically varying their relative jet temperature. A sonic converging and Mach 1.5 converging-diverging (CD) nozzles are employed. The expansion ratio of the converging nozzle is maintained at 0.96, 1.19, and 1.59, and the CD nozzle is operated at a fixed nozzle pressure ratio of 3. The temperature ratio of the jet from the CD nozzle is varied from 1.0, 1.3, and 1.7. For a fixed momentum of the jet pair, an increase in jet temperature intensified the nearfield noise and unsteadiness on the impingement surface. At short impingement heights, resonance in the heated jet was the primary source of unsteadiness. At a fixed impingement height, an increase in jet temperature led to a systematic increase in tonal frequency, while jet instability mode shapes were retained. Furthermore, the mean flowfield of sonic jet and fountain regions remain unaffected, and an increase in supersonic jet velocity is observed. This is also accompanied by an increase in unsteadiness in the fountain upwash. [ABSTRACT FROM AUTHOR]

Rotary compressors are widely used in split air conditioning units and have a significant global market share. However, the atomization of lubricant in the rotary compressor during discharge from the cylinder can cause negative effects on both heat transfer performance and reliability. Therefore, it is crucial to deepen the understanding of the behavior of atomized oil in the form of small droplets in order to improve the system's performance. In this study, lubricant droplets were analyzed at different locations, operating frequencies, and pressures using shadowgraph imaging. The droplets were extracted using the Canny operator and corrected using a Convolutional Neural Network, and their sizes were analyzed statistically. The results reveal that the droplet characteristic mean diameters increase with frequency and pressure, with larger sizes observed in the outer region above the rotor/stator and constant sizes in the radial direction at the discharge tube level. Additionally, the droplet's volume fraction increases with frequency, with higher values observed in the outer region above the rotor/stator and above the rotor/stator clearance at the discharge tube level. These findings provide valuable experimental data for setting up the boundary conditions used in computational fluid dynamics (CFD) simulations. • An algorithm to identify oil droplets from images recorded in the rotary compressor. • Oil droplet size versus frequency, pressure, and locations are measured in-situ. • The volume fraction of oil droplets is experimentally quantified. [ABSTRACT FROM AUTHOR]

Clean and efficient combustion of liquid fuels depends on spray fineness that aids fast fuel vaporization and better fuel–air mixing. Swirl-burst (SB) atomizers generate fine droplets at the injector exit rather than typical jet cores as seen in the conventional atomizers. It integrates the primary breakup by bubble bursting of the Flow Blurring (FB) atomization, and secondary atomization by Rayleigh–Taylor instabilities between the swirling atomizing air and liquid phase. Thus, SB atomization has achieved clean lean-premixed flames of fuels with distinct properties involving diesel and straight oils around fifteen times more viscous. This study gains insights into the effect of the varying internal geometry, H/D ratio, on the atomization process and quantitatively investigates these effects on the near-field spray characteristics of SB injectors using high-spatial-resolution Shadowgraph Imaging Technique (SIT) and particle image velocimetry (PIV) for water sprays. Results acquired by SIT show that the Sauter Mean Diameter (SMD) of the droplets decrease with the reducing H/D ratio. The PIV measurements quantitatively reveal that atomization completion length decreases as the H/D ratio is lowered. Weber number analysis signifies that mostly vibrational and occasionally bag breakup dominates the secondary atomization for all the three H/D ratios. Results also reveal the high scalability of SB concept and its doubled atomization efficiency compared to FB injection. [ABSTRACT FROM AUTHOR]

Effects of fuel jet penetration height on supersonic combustion behaviors were investigated experimentally in a supersonic combustion ramjet model combustor at a Mach speed of 2 and at a stagnation temperature of 1900 K. The jet-to-crossflow momentum flux ratio was varied to control the fuel-jet penetration height, using several injectors with different orifice diameters: 2, 3, and 4 mm. First, transverse nitrogen jets were observed to identify a relationship between the fuel jet penetration height and the momentum flux ratio by focusing Schlieren photography. Then, supersonic combustion behaviors of ethylene were investigated through combustion pressure measurements. Simultaneously, time-resolved images of CH* chemiluminescence and shadowgraphs were recorded with high-speed video cameras. Furthermore, a morphology of supersonic combustion modes was investigated for various equivalence ratios and fuel penetration heights in a two-dimensional latent space trained by the shared Gaussian process latent variable models (SGPLVM), considering CH* chemiluminescence images and the shock parameters. The results indicated that the penetration height of nitrogen jets was a function of the jet momentum flux ratio; this function was expressed by a fitting curve. Five typical combustion modes were identified based on time-resolved CH* chemiluminescence images, shadowgraphs, and pressure profiles. Even for a given equivalence ratio, different combustion modes were observed depending on the fuel penetration height. For an injection diameter of 3 and 4 mm, cavity shear-layer and jet-wake stabilized combustions were observed as the scram modes. On the other hand, although the cavity shear-layer and lifted-shear-layer stabilized combustions were observed, no jet-wake stabilized combustion was observed for an orifice diameter of 2 mm. Fuel penetration heights above the cavity aft wall were expected to affect the combustion behavior. Finally, a morphology of the supersonic combustion modes was clearly shown in the two-dimensional latent space of the SGPVLM. [ABSTRACT FROM AUTHOR]

Hypersonic flows have high heat transfer rates, and their management is essential to avoid detrimental effects. Since accurate prediction and measurement of heat flux in hypersonic test facilities are complicated, heat flux at the stagnation point is mostly estimated using Fay and Riddell formulation with Newtonian tangential velocity gradient approximation. Although it is relatively accurate and reliable, some errors creep in due to incompetent modelling of the tangential velocity gradient. This article studies the applicability of Olivier's tangential velocity gradient formulation for a sphere in the estimation of stagnation heat flux for spherically blunt axisymmetric hypersonic models. Oliver's estimation accurately models the tangential velocity gradient of spherically blunt axisymmetric hypersonic models as the heat flux estimates deviated only by approx. 2%–4% from the measured heat flux. A simplified model for tangential velocity gradient using Shock Standoff Distance and density ratio is also derived and tested for accuracy. [ABSTRACT FROM AUTHOR]

Engine efficiency and emissions depend on the fuel atomization and dispersion. The fuel atomization and dispersion depend heavily on the ambient pressure and temperature. In this work, to study Diesel sprays in engine conditions, an electrically heated, constant-volume, pressurized vessel was designed and manufactured. The controlling electronics and software were developed and tested to ensure safe and precise operation. A commercial Bosch six-hole automotive Diesel injector was used. The spray spatial and temporal development were studied. In the literature, spray liquid length and cone angle are extensively used to quantify fuel dispersion. In this work, these parameters were quantified using a high-speed shadowgraph technique. Models were derived to describe the temporal evolution of the liquid core. Such models can be used to predict the Diesel spray behaviour and the engine performance. [ABSTRACT FROM AUTHOR]

Polyacrylamide gel electrophoresis (PAGE) with sodium dodecyl sulphate (SDS) and Coomassie brilliant blue (CBB) staining is widely used in protein research and requires time for electrophoresis, staining and destaining. Because the protein bands electrophoresed in the gel are invisible in most cases, the results cannot be observed until the whole process is complete. In this study, shadowgraph was applied to detect biomolecules such as proteins during electrophoresis. A simple optical system and camera-enabled real-time monitoring of migration and separation of individual protein bands in polyacrylamide gels without staining. The visibility was high enough that it was possible to visualize substances other than proteins, such as DNA. This method provides protein profiles instantly in the early stage of electrophoresis. The elimination of the staining and destaining steps will help save researchers' time. The method is also environmentally friendly and will help reduce the generation of waste solutions containing synthetic dyes. [ABSTRACT FROM AUTHOR]

Schlieren image velocimetry (SIV) is based on light deflection through flow heterogeneities and image cross-correlations. This is a low-cost and relatively low complexity technique that allows measurement of the droplet velocity field in a large region of a spray. A Z-type Toepler schlieren system with a high-speed camera was used to determine mean vertical and horizontal droplet velocities, as well as the cone angles of sprays produced by a pressure swirl injector with characteristic geometric constant K = 2. Different LEDs and digital filters were evaluated for edge detection and improvement of image contrast. Open software was adopted for digital image processing and velocimetry. Interrogation windows and overlaps of different sizes were tested to obtain an appropriate correlation for determination of the velocity field. The digital images were obtained with 5 × 103 fps and a resolution of 2.77 pixels/mm. Since the swirl sprays analysed presented instabilities, a number of 100 cross-correlations of images was required to reduce mean velocity fluctuations. Injection pressures varied from 0.05 to 7 bar and mass flow rates varied from 1.389 to 13.89 g/s, using water as test fluid. The wideband warm white LED with Laplacian or high-pass filters provided velocity data for a larger range of injection pressures. Mean axial velocities varied from 3.3 to 11.3 m/s, approximately, with mean horizontal velocities varying from around 0.17 to 3.3 m/s for pressures from 0.05 to 3.22 bar. The velocity data were compared to microscopic shadowgraphy results, showing a good agreement. Spray cone angles ranged from about 32.5o to 69.5°, for injection pressures from 0.05 to 7 bar, and results of triangulation with a blue LED were closer to semi-empirical data. [ABSTRACT FROM AUTHOR]

The Wiener index of a graph G , denoted by W (G) , is defined as W (G) = ∑ u ≠ v d (u , v) , where the sum is taken through all unordered pairs of vertices of G and d (u , v) is distance between two vertices u and v of G. Let G = (V 1 , E 1) and H = (V 2 , E 2) be two graphs. For a graph G , let v ′ be a copy of v and V ′ (G) = { v ′ : v ∈ V (G) }. The F -sum G + F H is a graph with the set of vertices V (G + F H) = (V (G) ∪ V ′ (G)) × V (H) and two vertices u = (u 1 , u 2) and v = (v 1 , v 2) of G + F H are adjacent if and only if [ u 1 = v 1 ∈ V 1  and  (u 2 , v 2) ∈ E (H) ] or [ u 2 = v 2  and  (u 1 , v 1) ∈ E (F (G)) ] , where F (G) be one of the shadowgraph D 2 (G) or closed shadowgraph D 2 [ G ]. In this paper, we reported the Wiener index of these graphs. [ABSTRACT FROM AUTHOR]

Visualizing two variables in the fluid flow requires a complex optical system to fulfill such a purpose. Furthermore, in these applications, measuring the temperature fields of the fluid is sometimes necessary. In this work, we use a color digital camera to record in one snapshot shadowgraph and two direction sensitivity Schlieren images. Furthermore, the Schlieren images obtained are used to measure temperature fields. The optical system, as main components, employs an rgb Light Emitting Diode (LED), two knife edges represented by shortpass and longpass ultra-thin filters, and a color digital camera. The results of this study show that shadowgraph images are contaminated with the crosstalk effect, and correction against it was applied. On the other hand, the reached resolution of the optical system to measure temperature values is approximately one degree. [ABSTRACT FROM AUTHOR]

In the future, ammonia is expected to become a carbon-free fuel for internal combustion engines. However, the flammability of ammonia is poorer compared to conventional fuels such as gasoline and diesel fuel. Pre-chamber jet ignition may be an effective way to ensure stable ignition and enhance the combustion of ammonia. In this paper, the effects of pre-chamber geometric parameters, including volume and orifice diameter, on the jet ignition and combustion processes were studied using visualization methods, combined with pressure acquisition. The results showed that ignition energy increased and the jet duration was prolonged with the increase in pre-chamber volume, resulting in a higher maximum pressure and pressure rise rate in the main chamber. The jet characteristics of a larger volume pre-chamber exhibited higher stability when the ambient parameters were changed. The smaller volume pre-chamber showed the superiority of a shorter flame propagation distance inside the pre-chamber, which advanced the timing of the jet appearance and shortened the ignition delay when the flammability of the pre-mixture was adequate. The larger pre-chamber orifice diameter caused an earlier jet ignition timing, shorter ignition delay, and higher ignition location. The jet duration for the pre-chamber with a smaller orifice was longer, which was beneficial for increasing the pressure rise rate in the main chamber. Too small a pre-chamber orifice led to ignition failure in the main chamber. [ABSTRACT FROM AUTHOR]

• Experiments are presented detailing particle migration in inertial Taylor–Couette flow of complex fluids. • Particles are observed to migrate oppositely within Taylor vortices with inelastic and elastic fluids. • A weaker elastic fluid shows a transition between an initial migration to the center of the Taylor vortices, switching to migration to the perimeter of the vortices as rotation rate is increased. • A preliminary numerical Euler-Euler calculation using a White–Metzner constitutive model captures the migration. A drilling process comprises a drill-pipe rotating within a borehole where fluid is pumped down the pipe and returns, with drilled cuttings, along the annulus. Predominantly the axis of the system is horizontal. Thus, in the absence of axial flow the process geometry is that of a Taylor–Couette flow. Formulated drilling fluids themselves are usually regarded as Bingham or Hershel-Bulkley in nature, but nevertheless encompass elastic behaviour. We have thus studied the distribution of dense (i.e. sedimenting) non-Brownian solid particles in Taylor–Couette flow of model drilling fluids as a function of center body rotation speed. In all cases Taylor vortices are formed above some critical, fluid dependent, Taylor number. However, depending on the fluid properties, particles decorate the vortices differently: particles in a polymeric fluid move to the centroids of the vortices, whereas in a colloidal fluid they move to the outer periphery of the vortices, as previously observed for Newtonian fluids. With a mixed fluid, a clear transition between the two regimes is found. We postulate that this behaviour is a result of a balance between elastically derived lift forces and inertially driven Saffman lift forces acting antagonistically on the particles. [ABSTRACT FROM AUTHOR]

Differently from conventional electrospray studies on the 'cone-jet mode' to produce homogenous droplets, a wet electrostatic precipitator requires high flowrate which shows significantly different spray patterns and droplet sizes. Since the droplet are distributed much sparser than cone-jet electrospray, this study focused on the high flowrate water electrospray characteristics. The size measurement methods were compared between shadowgraph and phase Doppler anemometry (PDA). In shadowgraph method, the droplet sizes were measured from the electrospray photos through multi-stage image processing algorithm. PDA failed to determine most of the large droplets, whereas shadowgraph measured all droplets larger than 20µm. In terms of the corresponding volume measured using the diameter distribution, small sized droplets had a very small effect on the total atomization. This study proffers the appropriate method to measure the distribution of large size droplets in a disperse high flowrate electrospray, which may provide essential parameters for wet electrospray precipitator design. [ABSTRACT FROM AUTHOR]

To generate a radial force an alternating electric tension, HT ht , is applied at the inner cylinder while the outer is grounded. The transition from the base unicellular flow to the thermoelectric instability occurs for all HT ht between 6 kV and 8 kV. Figure 3 shows the post-treated shadowgraphs obtained for three different HT ht and three different HT ht . The resulting shadowgraph exhibits an axisymmetric light intensity profile which is still visible for HT ht , and is not distinguishable for higher temperature differences between the two cylinders. [Extracted from the article]

The behavior of a dual-fuel compression engine on a marine scale was investigated using a Rapid Expansion-Compression Machine (RCEM) fueled by gas oil emulsion and methane-inert gases dilution. The methane-inert gas blend was utilized as the primary injected fuel, with a pilot dose of diesel fuel acting as an ignitor. The findings demonstrate enhancements in the case of gas dilution besides the gas oil emulsion. Integrating inert gases with methane fuel improves flame penetration and increases the combustion duration. Blending inert gases with methane boosts the heat release rate of the combustion, leading to an increase in in-cylinder pressure peak by 3% for all tested fuels. In the case of gas oil (GO) emulsion, 95% reduction in NOx emissions compared to GO was reached. Using a methane-nitrogen mixture with 17.5% nitrogen gas reduces NOx emissions to 62% of the gas oil fuel emissions. Generally, diluting the methane with inert gas has the same effect as water-Gas Oil emulsion on combustion enhancement and emission reduction. This experimental study suggests using the Methane–Nitrogen combination as a direct-injected fuel with 17.5 percentage nitrogen gas. [ABSTRACT FROM AUTHOR]

Early stage of underwater electrical wire explosion was observed by shadowgraph. By decreasing initial energy, the early stage of wire explosion was slowed down and possible to capture shadow photos during phase transitions. The vaporization process of wire is usually axially non-uniform due to random local micro explosions along the wire. The degree of non-uniform will be significantly decreased when increasing the initial energy. During explosion of the wire, more than one shock waves were generated not only from phase transition, but also from different pulses of deposition power. A modified piston model was introduced to explain the phenomenon.

Engine efficiency and emissions depend on the fuel atomization and dispersion. The fuel atomization and dispersion depend heavily on the ambient pressure and temperature. In this work, to study Diesel sprays in engine conditions, an electrically heated, constant-volume, pressurized vessel was designed and manufactured. The controlling electronics and software were developed and tested to ensure safe and precise operation. A commercial Bosch six-hole automotive Diesel injector was used. The spray spatial and temporal development were studied. In the literature, spray liquid length and cone angle are extensively used to quantify fuel dispersion. In this work, these parameters were quantified using a high-speed shadowgraph technique. Models were derived to describe the temporal evolution of the liquid core. Such models can be used to predict the Diesel spray behaviour and the engine performance.

Pulsed laser ablation in liquid (PLAL) is gaining an important role as a methodology for producing nanostructures without the use of chemicals and stabilizers. Several nanomaterials have been produced and the engineering of PLAL is becoming an important task for the dissemination of this approach for nanostructure production. Monitoring the processes involved in the PLAL during nanostructure production can be extremely useful for improving the experimental methods and for pushing PLAL to new material formation. In this paper, we discuss the use of optical techniques for investigating the specific stages involved in the production of nanomaterials with PLAL. In particular, the recent advancements of these optical techniques for each specific stage of the PLAL process will be discussed: optical emission spectroscopy and imaging for the investigation of the plasma phase, shadowgraph imaging for the investigation of the cavitation bubble dynamics and different scattering techniques for the visualization of the produced nanostructure. [ABSTRACT FROM AUTHOR]

Visualizing two variables in the fluid flow requires a complex optical system to fulfill such a purpose. Furthermore, in these applications, measuring the temperature fields of the fluid is sometimes necessary. In this work, we use a color digital camera to record in one snapshot shadowgraph and two direction sensitivity Schlieren images. Furthermore, the Schlieren images obtained are used to measure temperature fields. The optical system, as main components, employs an rgb Light Emitting Diode (LED), two knife edges represented by shortpass and longpass ultra-thin filters, and a color digital camera. The results of this study show that shadowgraph images are contaminated with the crosstalk effect, and correction against it was applied. On the other hand, the reached resolution of the optical system to measure temperature values is approximately one degree.

In this paper, we define new graph operations F-composition F (G)[H], where F (G) be one of the symbols S(G),M(G),Q(G),T(G),Λ(G),Λ[G],D2(G),D2[G]. Further, we give some results for the Wiener indices of the these graph operations.

Mixing efficiency helps the thrust gain and jet noise reduction. Aerospace and aviation research communities around the world are constantly demanding cleaner, quieter and more efficient commercial aircrafts. Though there are many sources of noise during flight, one such is deficient mixing of air at the rear of the nozzle, uneven expansion of nozzle and corresponding engine noise is quite dominant. The mixing proficiency is achieved by mixing of cold air at the surroundings with the hot air that exits the nozzle. To reduce the engine noise, elliptical tabs are employed at the exit of convergent nozzle. These are passive noise controllers which create vortices to the outlet flow and entrain the cold mass from the surroundings in order to reduce the noise level. This paper is concerned with the various shapes and sizes of tabs to test the mixing efficiency at different Mach numbers. Further, mixing efficiency is evaluated by both qualitative and quantitative analysis of shadowgraph system and by employing a propitious pressure scanner. [ABSTRACT FROM AUTHOR]

Resumo A utilização de experimentos é uma das formas mais eficientes para o ensino de física, uma vez que os alunos tornam-se mais motivados quando conceitos teóricos fundamentais são associados a aplicações tecnológicas. O desenvolvimento de carros, aeronaves e foguetes, por exemplo, requer o conhecimento detalhado dos escoamentos de fluidos ao redor e no interior desses veículos. O presente artigo descreve os fundamentos da óptica geométrica em meios heterogêneos e apresenta as técnicas ópticas shadowgraph e schlieren que são amplamente utilizadas em centros de pesquisa, universidades e empresas no estudo da aerodinâmica e dos escoamentos em geral. Esses métodos ópticos são baseados na deflexão dos raios de luz causada por variações na massa específica do escoamento. O método schlieren-Toepler é apresentado e discutido em detalhes, derivando-se equações para o contraste, sensibilidade e o alinhamento óptico. Uma implementação do método shadowgraph direto para uso em sala de aula é demonstrada e, ao longo do artigo, são apresentadas diversas imagens obtidas pelos métodos ópticos descritos.

Mixing efficiency helps the thrust gain and jet noise reduction. Aerospace and aviation research communities around the world are constantly demanding cleaner, quieter and more efficient commercial aircrafts. Though there are many sources of noise during flight, one such is deficient mixing of air at the rear of the nozzle, uneven expansion of nozzle and corresponding engine noise is quite dominant. The mixing proficiency is achieved by mixing of cold air at the surroundings with the hot air that exits the nozzle. To reduce the engine noise, elliptical tabs are employed at the exit of convergent nozzle. These are passive noise controllers which create vortices to the outlet flow and entrain the cold mass from the surroundings in order to reduce the noise level. This paper is concerned with the various shapes and sizes of tabs to test the mixing efficiency at different Mach numbers. Further, mixing efficiency is evaluated by both qualitative and quantitative analysis of shadowgraph system and by employing a propitious pressure scanner. [ABSTRACT FROM AUTHOR]

The GRADFLEX experiment of ESA has shown that under microgravity conditions a stationary thermodiffusion process is accompanied by giant non-equilibrium fluctuations with size as large as the size of the sample. In the presence of small concentration gradients, the features of the non-equilibrium fluctuations can be described by means of linearized hydrodynamics. However, the linear models are not suitable to describe most cases of applicative interest, such as fluctuations induced by large gradients and under non-stationary conditions. Moreover, presently the investigation of non-equilibrium fluctuations has mainly involved single component fluids and binary mixtures, but recently transport processes in ternary mixtures have attracted increasing interest due to the experiments performed on the International Space Station in the framework of the DCMIX project of ESA. The Giant Fluctuations (NEUF-DIX) project of ESA will investigate non-equilibrium fluctuations during diffusive processes occurring in complex multi-component mixtures, where one of the components is a macromolecule, such as a polymer, a colloid or a protein. Important objectives will be the exploration of the features of the fluctuations under non-ideal conditions, such as large gradients, transient processes, and concentrated samples, and the understanding of how the fluctuations affect the interactions between macromolecules. The project involves the development of a dedicated facility, consisting of an array of shadowgraph optical instruments working in parallel, each one equipped with a thermal gradient cell. Here we outline the design concept of the facility and the results of performance tests performed on a breadboard to evaluate the suitability of the designed instrument to carry out scientific measurements of non-equilibrium concentration fluctuations in space. [ABSTRACT FROM AUTHOR]

• Interaction of coal particles with lean premixed methane-air flames is studied. • Discrete phase model is used for the transport of micron-sized coal particles. • Short kinetic mechanism, soot and radiation sub-models are included. • Variation of laminar flame speed with coal dust concentration is presented. • Secondary flame zone and devolatilization zones are discussed in detail. Interaction of combustible particles with flames occurs in fire scenarios. In this study, numerical investigation of interaction of coal dust or micron-sized particles with lean premixed methane-air flames is presented. A two-dimensional axisymmetric domain is employed to simulate conical premixed flames from lab-scale Bunsen burner. A chemical kinetic mechanism having 25 species and 121 elementary reactions, temperature dependent thermo-physical properties, multi-component diffusion with Soret effect and radiation model accounting for gas and soot radiation are used. Discrete Phase Model (DPM) is used to simulate the transport of coal particles. Coal particles in varies size ranges and concentrations are injected into the premixed reactant mixture at equivalence ratio between 0.75 and 0.85. Multiple species from devolatilization of coal particles are considered to enter the gas-phase. Laminar flame speeds are predicted using numerical shadowgraphs and validated against the experimental data from literature. Injection of coal particles affects the laminar burning velocity and flame structure. The numerical model is able to predict the variation trends in the laminar flame speed data quite reasonably. A detailed analysis of injection of coal particles on the resultant flame dynamics are presented using the fields of temperature, flow, species, net reaction rate, heat release rate and DPM. [ABSTRACT FROM AUTHOR]

This paper is a comparative study on the characteristics of high-speed liquid jets injected in surrounding water and air using shadowgraph technique. One of the main objectives is to investigate the effects of liquid's physical properties, used to generate the high-speed liquid jets, on jet generation's characteristics. Moreover, comparative investigations on effects of those liquid jets after injected in water and air are reported. The high-speed liquid jets were generated by the impact of a projectile launched by a horizontal single-stage power gun. The impact-driven high-speed liquid jets were visualized by shadowgraph technique and images were recorded by a high-speed digital video camera. The process of impact-driven high-speed liquid jet injection in air and water, oblique shock waves, jet-induced shock waves, shock waves propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud regeneration were clearly observed. It was found that different properties of liquid (surface tension and kinematic viscosity) affect the jet maximum velocity and shape of the jet. Bubble behaviors were only found for the jet injected in water. From the shadowgraph images, it is found that the maximum average jet velocity, expansion and contraction velocities of bubble in axial direction increase when the value of the multiplied result of surface tension by kinematic viscosity increases. Therefore, surface tension and kinematic viscosity are the significant physical properties that affect characteristics of high-speed liquid jets. [ABSTRACT FROM AUTHOR]

The Hydrogen Unconfined Combustion Test Apparatus (HUCTA) was designed and built to study the blast waves produced from unconfined hydrogen/oxygen deflagrations. The HUCTA uses evacuated balloons of up to 2 m in diameter which are filled with a combustible combination of gaseous hydrogen–oxygen mixtures. The well-mixed gases are ignited with an electric spark at the center of the sphere, resulting in a gaseous deflagration propagating through the mixture and a shock wave produced in the air. The combinations of balloon size and fuel/oxidizer ratios allow for a wide range of blast waves to be produced. Overpressures are measured with standard blast gauges at a variety of locations, demonstrating a high degree of radial symmetry and repeatability in the shock wave pressures, as well as the ability to produce non-ideal shock wave pressure profiles under some conditions. The range of peak pressures and explosive impulses obtainable is described as a function of mixture ratio. High-speed retroreflective shadowgraphy is used to visualize shock wave propagation and coalescence in individual frames and digital streak images. Since HUCTA is elevated approximately 2 m off the ground, there is a significant area around the apparatus where non-noisy, un-reflected, symmetric blast waves propagate; this area is ideal for testing items whose response to blast waves is desired for safety considerations. [ABSTRACT FROM AUTHOR]

The use of experiments is one of the most efficient ways to teach physics, since students become more motivated when fundamental theoretical concepts are associated with technological applications. The development of cars, aircraft and rockets, for example, requires detailed knowledge of the flow of fluids around and inside these vehicles. This article describes the fundamentals of geometric optics in heterogeneous media and presents the shadowgraph and schlieren optical techniques that are widely used in research centers, universities and companies in the study of aerodynamics and flows in general. These optical methods are based on the deflection of the light rays caused by variations in the flow density. The schlieren-Toepler method is discussed in detail, deriving equations for contrast, sensitivity and optical alignment. An implementation of the direct shadowgraph method for classroom use is demonstrated and, throughout the article, several images obtained by the described optical methods are presented. [ABSTRACT FROM AUTHOR]

• Analysis of shock boundary layer and shock-separation interactions in a wavy-shaped geometry. • 10 kHz shadowgraph for quantification of unsteady shock, shear layer, and separation regions. • Pressure Sensitive Paint (PSP) and Kulite measurements to quantify pressure unsteadiness on the walls and frequency spectra associated with the different flow features. • Higher-order dynamic mode decomposition and spectral proper orthogonal decomposition to identify the features. A canonical wavy surface exposed to a Mach 2 flow is investigated through high-frequency Pressure Sensitive Paint, Kulite measurements, and shadowgraph imaging. The wavy surface features a compression and expansion region, two shock-boundary layer interactions, and two shock-separation regions. The unsteady characteristics of the wall pressure and shock angles are presented, demonstrating an increase in the amplitude of the instabilities when traveling through the shock systems. The pressure sensitive paint measurements confirm a two-dimensional flow pattern with small transversal unsteadiness. Higher-Order Dynamic Mode Decomposition and Spectral Proper Orthogonal Decomposition are implemented to dissect the different flow features, revealing several dominant low-frequency and medium-frequency phenomena. The separation region appears at frequencies with Strouhal numbers between 0.01 and 0.2, confirmed by the frequency content in the local pressure measurement using Kulites and the pressure sensitive paint. [ABSTRACT FROM AUTHOR]

Pulsed laser ablation in liquid (PLAL) is gaining an important role as a methodology for producing nanostructures without the use of chemicals and stabilizers. Several nanomaterials have been produced and the engineering of PLAL is becoming an important task for the dissemination of this approach for nanostructure production. Monitoring the processes involved in the PLAL during nanostructure production can be extremely useful for improving the experimental methods and for pushing PLAL to new material formation. In this paper, we discuss the use of optical techniques for investigating the specific stages involved in the production of nanomaterials with PLAL. In particular, the recent advancements of these optical techniques for each specific stage of the PLAL process will be discussed: optical emission spectroscopy and imaging for the investigation of the plasma phase, shadowgraph imaging for the investigation of the cavitation bubble dynamics and different scattering techniques for the visualization of the produced nanostructure.

Reactive gaseous oxygen/kerosene spray was visualized by a shadowgraph imaging technique in a lab-scale rocket engine combustor. Dynamic behaviors of inner and outer shear layer were observed morphologically and analyzed quantitatively by image processing. Correlation methods were used to statistically analyze dynamic behaviors of each shear layer and interaction between two shear layers. Relationship between shear layer dynamics and combustion flow-field was investigated by analyzing periodicity of jet-core length, lump-detachment, and luminous flame intensity. The results of correlation and frequency analysis show that dominant frequency of behaviors and interaction of shear layers is 270 Hz and the degree of correlation becomes stronger along the flow direction. The dominant frequency of core length variation and luminous flame formation is around 270 Hz and, which is the same as that of behaviors and interaction of shear layers. This proposes that the surface behaviors and interaction of the inner and outer jets in the upstream affect the downstream combustion flow-field. • Behavior of two shear layers in reactive spray was detected from shadowgraph images. • Correlation methods were used to characterize behavior of shear layers and interaction between them. • Behavior and interaction of shear layers correlate to inner jet breakup and formation of luminous flame. [ABSTRACT FROM AUTHOR]

• Transition experimentally found from natural convection to thermal electro-hydrodynamic convection. • Validation of linear stability analysis. • Vertically aligned stationary columns found after first transition. • Enhancement of heat transfer due to dielectrophoretic force-driven convection. A radial temperature difference together with an inhomogeneous radial electric field gradient is applied to a dielectric fluid confined in a vertical cylindrical annulus inducing thermal electro-hydrodynamic convection. Identification of the stability of the flow and hence of the line of marginal stability separating stable laminar free (natural) convection from thermal electro-hydrodynamic convection, its flow structures, pattern formation and critical parameters. Combination of different measurement techniques, namely the shadowgraph method and particle image velocimetry, as well as numerical simulation are used to qualify/quantify the flow. We identify the transition from stable laminar free convection to thermal electro-hydrodynamic convective flow in a wide range of Rayleigh number and electric potential. The line of marginal stability found confirms results from linear stability analysis. The flow after first transition forms a structure of axially aligned stationary columnar modes. We experimentally confirm critical parameters resulting from linear stability analysis and we show numerically an enhancement of heat transfer. [ABSTRACT FROM AUTHOR]