Wavy Region Research Articles

Experimental investigation of hypersonic boundary layer instability with wavy wall located downstream of synchronization point

The wavy wall surface has been reported to be effective in suppressing the growth of second mode instability waves in hypersonic boundary layer, while the allocation of wavy wall relative to the synchronization point requires further study. Upon this work experimental investigation of hypersonic boundary layer instability with wavy wall located downstream of synchronization point based on 7° half-angle sharp cone at zero angle of attack was carried out in a hypersonic noisy wind tunnel. The evolution of instability waves with wavy wall surface is measured using both surface-mounted pressure fluctuation sensors and focused laser differential interferometer (FLDI). The results reveal that the growth of instability waves has been enhanced significantly when the wavy wall region is allocated downstream of the synchronization point when the wavy wall height is slightly smaller than the local boundary layer thickness. Additionally, second mode instability waves are observed at both crests and troughs within the wavy wall region based on the FLDI measurement and moreover, the high-frequency instability waves measured in the separation bubbles of troughs are speculated to be the transmission of second mode instability waves from the shear layer, rather than the local generation of second mode instability.

Double-diffusive magneto-natural convection of nanofluid in an enclosure equipped with a wavy porous cylinder in the local thermal non-equilibrium situation

The emphasis of this study is the numerical analysis via finite volume method of magneto-natural convection induced by double-diffusion of a nanofluid within an enclosure equipped with a wavy porous cylinder. The porous cylinder which is under the local thermal non-equilibrium (LTNE) situation is modeled using Darcy-Brinkman-Forchheimer design as momentum equation. Results are interpreted and evaluated in terms of the dimensionless controlling variables such as Rayleigh number (Ra), Lewis number (Le), Hartman number (Ha), buoyancy ratio parameter (N), Darcy number, (Da), and medium's porosity (ε) at a given heat transfer coefficient for the solid/fluid inter-phase (H) and modified conductivity ratio (γ) of the porous wavy cylinder region. Corcione's correlations with 4% of Al2O3 nanoparticles is used to estimate nanofluid's thermal conductivity and viscosity. Findings show that the heat and mass transfers are improved by Ra, Da, and ε augmentation and Ha and Le diminution. In general, as Le increases, the mass transfer rate rises while the corresponding heat transfer rate drops. Also, we identified a value of N where Le-dependent heat and mass transfer rates are minimal. Moreover, the heat transfer rate is found to be more sensitive to the changes in the porous medium's properties of the wavy porous cylinder than the mass transfer rate. Furthermore, the LTNE situation everywhere exists in the porous wavy cylinder except for the center where the LTE is realized.

Impact of Wavy Wall Surface on Hypersonic Boundary-Layer Instability of Sharp Cone Model

A comprehensive investigation of the impact of wavy wall on hypersonic boundary layers is carried out in a Mach 6 Ludwieg tube based on a 7 deg half-angle sharp cone. Linear stability theory is applied on the cone base flow with both smooth and wavy surfaces first. The evaluation of amplification rate as well as the frequency of instability waves reveals that the second mode instability is suppressed downstream of the wavy wall region. The development of instabilities is then characterized using surface flush-mounted pressure sensors and focused laser differential interferometer (FLDI). The FLDI measurement is conducted within the wavy wall zone both at troughs and at crests. Good agreement is found between the surface pressure measurement and computation, except that instability waves are invisible shortly downstream of the wavy wall. Additionally, bispectral analysis indicates that the nonlinear interaction of instability waves downstream of the wavy wall region is suppressed. FLDI measurement along the streamwise direction shows that the second mode instability waves disappear in the wavy wall region, and the boundary-layer transition process is delayed. Spatial measurement with FLDI of boundary layer identifies various types of disturbance at the troughs and crests; moreover, the co-existence of low-frequency and high-frequency disturbances is observed off the wall surface downstream of the wavy wall.

Instability evolution in the hypersonic boundary layer over a wavy wall

The effects of a wavy wall on the stability of a hypersonic boundary layer on a flared cone are investigated by detailed experimental measurements and direct numerical simulations. The non-contact optical measurement method of focused laser differential interferometry is used to measure the disturbance development within the wavy region. The measurement results show that the second mode for the wavy wall is suppressed significantly compared with the smooth wall, and that multiple disturbances at low frequencies appear within the wavy region. Numerical corroboration against experimental measurements reveals good quantitative agreement. It is found that the disturbances at $f=360$ kHz on the wavy wall are suppressed appreciably, which are very significant on the smooth wall. And the disturbances at $f=140$ kHz and $f=260$ kHz develop within the wavy region, and increase considerably. Also, the disturbances achieve a significant increase over the first half of a wavy trough and become more stable over the second half of a wavy trough. The physical mechanism is found to be due to the change in wall geometry and is attributed to the spatially modulated mean flow. The disturbance growth rate is closely related to the level of the mean-flow distortion.

Defect identification in thick porous and wavy composites with hybrid use of ultrasound non-reciprocity and scattering

Diverse defects in thick composites were characterized using through transmission inspection based on ultrasound non-reciprocity and scattering. Simulations were conducted using OnScale® to reveal wave interactions with fiber waviness and voids of varying sizes at different positions. To validate the numerical predictions, two phased array probes were used to capture two sets of transmitted signals by reversing the excitation and reception sequence on three different samples: porous non-wavy, wavy, and porous wavy composites. Both voids and fiber waviness were found to induce reductions in the transmitted signal amplitudes due to wave scattering. At an optimized frequency, voids do not introduce time-of-flight differences, while fiber waviness causes measurable differences between the two transmitted signals due to direction-dependent frequency filters in the wavy region. Therefore, with the hybrid use of wave scattering and non-reciprocity, a promising solution for ultrasonic defect identification in thick porous and wavy composites was proposed.

The relation between in-plane fiber waviness severity and first ply failure in thermoplastic composite laminates

The influence of in-plane fiber waviness on the first ply failure of quasi-isotropic thermoplastic composite laminates was investigated. The failure mode and stress at first failure were studied by using a four-point bending test and an end-loaded bending (ELB) test. The experimental results confirm that no undesirable stress concentration due to load introduction occurred in the ELB test, thereby supporting the use of this test as an alternative method to measure stress at failure. The experiments demonstrated that waviness severity affects stress at first failure and compressive damage development. The stress at first failure initially decreases with increasing maximum waviness angle and levels off as the angle starts exceeding 20°. These results reinforce the hypothesis from previous research which suggests that wavy ply compressive strength is less sensitive to changes in severity at larger angles. Furthermore, it was found that kinking failure was the dominant failure mode for maximum waviness angle up to about 45°. No kink band was observed in wavy regions with maximum angles above 45° when there was another wavy region with a lower maximum angle in the same specimen. This means that failure is not necessarily initiated at the location where waviness is most severe.

Through transmission ultrasonic inspection of fiber waviness for thickness-tapered composites using ultrasound non-reciprocity: Simulation and experiment

This study proposed the use of ultrasound non-reciprocity in periodic structures to inspect fiber waviness in thickness-tapered composites. Ultrasound propagation in plain and thickness-tapered composites with complex microstructure were precisely modelled using TexGen® and OnScale® simulation software. Ultrasound non-reciprocity and attenuation was comparatively calculated to inspect fiber waviness through both simulation and experiment. After comparison, energy of transmitted waves was found to be sensitive to the presence of fiber waviness in plain composites, however, thickness-dependent ultrasound attenuation introduces difficulties in determining the diagnosis baseline for thickness-tapered composites. On the other hand, fiber waviness introduces direction-dependent nonlinearity in the wavy region, which introduces a disparity between the two transmitted signals when the propagation direction is reversed. Ultrasound non-reciprocity, defined by the time-of-flight difference between the two transmitted signals, demonstrated its efficiency for fiber waviness inspection in both plain and thickness-tapered composites regardless of variations in the thickness.

Flow characteristics in partially vegetated channel with homogeneous and heterogeneous layouts.

This study presents the experimental results of the flow characteristics, such as the flow adjustment, velocity profiles, mixing layer, and the momentum exchange, in the partially vegetated channel with homogeneous and heterogeneous layouts. Three cases are considered, including two homogeneous canopies with uniform sparse and dense vegetation patches respectively, and a heterogeneous canopy consisting of alternating patches of both densities. Results show that heterogeneous canopy requires a longer adjustment distance to reach the quasi-equilibrium region, compared with the homogenous canopy of the same density. In heterogeneous canopy flow, the mixing layer width and the momentum thickness fluctuates with the alternation of vegetation density. The increased values for these two parameters compared to those values for the homogeneous canopies indicate that the greater resistance and momentum loss occur for the heterogeneous layout. A wavy region of the enhanced in-plane turbulence kinetic energy (TKE) is observed in the heterogeneous canopy, suggesting a comparatively more chaotic flow condition, whereas the contours of in-plane TKE are smooth in homogeneous canopies. The presence of the coherent structures in heterogeneous canopy is identified by spectral analysis and the quasi-periodic fluctuations of velocities. The Reynolds stress associated with the coherent structures is found to be the dominator of the contribution to the total Reynolds stress. The comparison between the homogenous canopies of different density is also conducted. These results will be of practical importance for the design of vegetation layouts in water ecological restoration projects and for river management.

Study on interfacial friction in the inverted annular film boiling regime

The interfacial friction factor is an important closure model in the two-fluid model for the inverted annular film boiling (IAFB) regime. Almost all the interfacial friction factor correlations used in IAFB modeling were obtained based on experimental data from annular two-phase flows. This paper first reviews the interfacial friction factor models/correlations used in IAFB modeling. A laminar vapor film model is then derived for the smooth IAFB region based on a laminar flow assumption. Experimental data obtained from subcooled and low-quality film boiling experiments in the literature are reduced to calculate the interfacial friction factor by solving one-dimensional two-fluid momentum equations for the liquid and vapor phases. The trend of the interfacial friction factor indicates that the transition from the smooth to wavy IAFB regions occurs at a void fraction range of 0.2 to 0.4. Comparisons between the laminar vapor film model and corresponding experimental data and nine correlations in the literature indicate that the laminar vapor film model appears to provide the lower bound of the interfacial friction factor in the smooth IAFB region. Two interfacial friction factor correlations from the literature are recommended for the smooth and wavy IAFB regions based on the analysis. It can be concluded that the interfacial friction factor in the smooth IAFB region is dependent on the gas Reynolds number and vapor film thickness, while in the wavy IAFB region, it is primarily dependent on the amplitude of the interfacial waves.

Process characteristics and interfacial microstructure in spot impact welding of titanium to stainless steel

Vaporizing foil actuator (VFA) spot welding was used to weld a titanium alloy (Ti-1.2ASN) to a stainless steel (436 SS) for the first time. The relation of process to microstructure was explored. Interfacial microstructure was studied through optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Chemical compositions of molten regions and jet zone were characterized by energy-dispersive X-ray spectroscopy (EDS). The microhardness across the interface was measured through Vickers and nanoindentation tests. The results show that an impact velocity of 800 m/s can be achieved at very low input energy. VFA spot welds present heterogeneity in microstructure at large and small length scales. At long length scales, several regions of weld can be seen. At the center there is no bonding; moving radially outward the collision angle increases from zero and this causes a transition from a flat area with considerable melting to a wavy region, and finally an area of accumulated jetted material in the corner. EDS analyses show that discontinuous intermetallics including TiCr2, Fe2Ti and FeTi, and Ti fragments coexisted in a mixed zone that probably has components of mechanical and liquid-state mixing. TEM results show that equiaxed nanograins exhibited in the recrystallization zone and mixed zone. There is an ‘impact affected zone’ near the interface where the hardness rises significantly. At the edges of the weld there is a mixed, and oxidized zone that supports the idea that solid-state impact jets the nascent surfaces away to allow interatomic bonding.

Dissimilar joining of Al/SS sheets with interlayers by ultrasonic spot Welding: Microstructure and mechanical properties

Ultrasonic spot welding (USW) has received a great deal of attention by the automobile manufacturer to build lightweight structural components. However, the versatility of using this process is still limited due to the substantial instabilities in weld quality. These fluctuations are mainly due to the lack of scientific understanding of this process and poor control of intermetallic compound formation. Therefore, this study is focused on the improving of the joint strength and its quality by ultrasonic welding of Al3003 and SS304 sheets with molybdenum (Mo) and zinc (Zn) powder as interlayers. The results revealed that the optimum thickness for Mo and Zn interlayers are found to be 0.11 mm and 0.15 mm, respectively. In comparison, Al3003/SS304 joints with Zn interlayer exhibits the maximum tensile shear failure load about 626.44 N at 1.5 s of weld time, which is better than those of the joints produced with Mo interlayer. The microstructural morphologies at the weld interface reveal the different characteristics like mechanical interlocking at the outside zone of the sonotrode knurl indentation, wavy type region under the sonotrode knurl, and swirling like diffusion zone inside the sonotrode knurl.

Stability of space-periodic flow with separation of a laminar boundary layer

An unstable laminar flow over a wavy region of a plate placed parallel to the oncoming air flow is studied. The results have been obtained by the hot-wire method in a low-turbulent wind tunnel at low subsonic velocities. Flow near a wall with transverse undulation, whose amplitude is sufficiently large and can contribute to the formation of local regions of boundary layer separation, is considered. The external harmonic (acoustic) forcing of the periodic flow leads to the suppression of perturbations that dominate in the spectrum of velocity disturbances near the model surface. The results of this work may be useful in developing methods for controlling spatially inhomogeneous flows.

Effect of pipe rotation on flow pattern and pressure drop of horizontal two-phase flow

In this paper, the effect of pipe rotation on two-phase flow patterns and pressure drop of a horizontal pipe is experimentally investigated. To obtain two-phase flow patterns and related transition boundaries, a unique experimental set-up was constructed to measure flow patterns under different pipe rotational speeds. A Plexiglas pipe, with 4000 mm length and 25.4 mm ID was used in experiments to allow direct observation of flow pattern. For rotation of the pipe, an electromotor coupled with gearbox was used to allow different rotational speeds. The set-up also supports up to ±25° inclination. Air and water were used as the gas and liquid phase, respectively. Over 3800 experiments were conducted to draw flow pattern maps at six different rotational speeds of 0, 50, 100, 200, 300 and 400 rpm in both horizontal and 10° inclined pipe. To validate the results, experimental findings were compared with previous research for a horizontal fixed pipe case. Results show that pipe rotation has a significant effect on the flow pattern map and the transition boundaries. It was found that in horizontal pipe case, the stratified smooth flow regime decreases as the pipe rotation increases and disappears at high revolution speeds. Moreover, the annular regime enlarges with increasing pipe rotational speed. For a 10° inclined pipe, the stratified wavy region appears as the pipe rotation speeds up. It was observed that for both horizontal and inclined pipe, pressure drop considerably increases as the pipe rotational speed increases. Furthermore, the effect of pipe inclination on the pressure drop is reduced as the rotational speed increases.

Manipulating the structure and mechanical properties of thermoplastic polyurethane/polycaprolactone hybrid small diameter vascular scaffolds fabricated via electrospinning using an assembled rotating collector.

The success of blood vessel transplants with vascular scaffolds (VSs) highly depends on their structure and mechanical properties. The fabrication of small diameter vascular scaffolds (SDVSs) mimicking the properties of native blood vessels has been a challenge. Herein, we propose a facile method to fabricate thermoplastic polyurethane (TPU)/polycaprolactone (PCL) hybrid SDVSs via electrospinning using a modified rotating collector. By varying the ratio between the TPU and the PCL, and changing the electrospinning volume, SDVSs with a wavy configuration and different properties could be obtained. Detailed investigation revealed that certain TPU/PCL hybrid SDVSs closely resembled the mechanical behaviors of blood vessels due to the presence of a wavy region and the combination of flexible TPU and rigid PCL, which mimicked the properties of elastin and collagen in blood vessels. The fabricated TPU/PCL SDVSs achieved lumen diameters of 1-3mm, wall thicknesses of 100-570µm, circumferential moduli of 1-6MPa, ultimate strengths of 2-8MPa, over 250% elongation-at-break values, toe regions of 5.3-9.4%, high recoverability, and compliances close to those of human veins. Moreover, these TPU/PCL SDVSs possessed sufficient suture retention strength and burst pressure to fulfill transplantation requirements and maintain normal blood flow. Human endothelial cell culture revealed good biocompatibility of the scaffolds, and cells were able to grow on the inner surface of the tubular scaffolds, indicating promising prospects for use as tissue-engineered vascular grafts.

A multi-parameter model for stiffness prediction of composite laminates with out-of-plane ply waviness

In this paper, an analytical multi-parameter model based on the classical lamination theory (CLT) is developed and the study of the stiffness response for unidirectional composite laminates with out-of-plane ply waviness is followed. A range of parameters associated with the waviness defect, including the waviness geometry, the fiber off-axis angle, the extent of wavy region and the waviness pattern, are incorporated into the analytical model. The fiber volume fraction and the type of reinforcement are also taken into account in the parametric analysis. The three-dimensional stiffness parameters of thick wavy laminates are investigated systematically for different cases. An observation of the negative in-plane Poisson’s ratios for wavy laminated composites reinforced with carbon fibers is presented under the given assumptions of CLT. This phenomenon provides a possible, new technique to design the auxetic materials. The modeling approach provides an effective procedure to quantify the effects of ply waviness on the stiffness of composite laminates.

Gradient effect in the waved interfacial layer of 304L/533B bimetallic plates induced by explosive welding

Relationship between the nano-mechanical behavior and microstructure in the wavy interface zone of explosive welded 304L/533B steel plates was explored. Nanoindentation tests were carried out on the interfacial wavy region across the wave peak and wave valley. The microstructure and chemical components of the interfacial region were obtained. Both across the wave peak and wave valley of interfacial 533B steel, the hardness gradually decreased and then went back to a plateau value of ~ 3.5GPa. There was no distinct difference in hardness with the distance from interface between the wave peak and wave valley. The reduction of cementite content and the decarburization of interfacial 533B steel were responsible for the decreasing of hardness in the interfacial regions. Moreover, this variation in hardness with the distance from interface was basically inverse to that in ferritic grain size. On the side of 304L austenite steel, the nanohardness gradually increased from the outer region to the interface both across wave peak and wave valley. The relationship between the hardness and the grain size in the graded interfacial zone of 304Lsteel followed the classical Hall-Petch strengthening equation. The graded variation in hardness could be due to the combined strengthening mechanisms of gradual refined grain size, as well as martensitic transformation strengthening, carbon precipitation strengthening and dislocation strengthening. Also, the hardness in the wave peak zone was larger than that in the wave valley zone of 304L steel, which could be due to the severer plastic deformation occurring in the peak zone.

Material Performance in Laser Cladding of a Laminated Plate with Wavy Bond Interface

A laminated plate with excellent mechanical properties is commonly used for a bulkhead part in which the interface structure of diffusion bonded is detrimental to the bending quality. In this study, 304 stainless steel and Q235 carbon steel were cladded by laser cladding technique. Microstructure, yield strength, elements diffusion, hardness and tensile fracture morphology of the clad metals were evaluated. The bond interface of the cladded metals demonstrated a wavy morphology in this work. Tensile test results shows that yield strength reaches 400-410MPa and tensile strength reaches 405-419MPa. They are higher than the standard value 235MPa and 370MPa, respectively. In wavy interface region, transition layer of metallurgical bond was formed as a result of mutual diffusion of Fe, Cr and Ni. The hardness is increased in the substrate and cladding plate near the bond interface. The mechanical properties of the low-carbon steels is increased by laser cladding with austenitic stainless steel, which are far beyond the national standard and other bending methods.

Laminar–turbulent transition and wave–turbulence interaction in stratified horizontal two-phase pipe flow

Stratified cocurrent flow of air and water was studied experimentally in a 5 cm diameter horizontal pipe. The velocity in the liquid phase was measured using planar particle image velocimetry, and the instantaneous interfacial profile was recorded using a separate camera. The resulting velocity fields extended from the pipe wall to the wavy interface. The principal aims of the study were to investigate the laminar–turbulent transition of the liquid phase in stratified gas–liquid flow, and to explore the interaction between the transition process and the interfacial waves. The boundaries of transition were determined in both the smooth and the wavy region. The occurrence of waves had the effect of increasing the Reynolds numbers at the end of transition. On the other hand, the transition to turbulence caused a change from the ‘2D small-amplitude’ to the ‘3D small-amplitude’ wave pattern, which were seen to correspond to the capillary–gravity and gravity–capillary solutions of the dispersion relationship respectively. In light of this, the flowmap of the wavy region was recast into Weber number–Froude number coordinates, which provided a physical interpretation of the interaction between the developing turbulence and the changing wave patterns.

Influence of out-of-plane ply waviness on elastic properties of composite laminates under uniaxial loading

In this paper the influence of out-of-plane ply waviness on elastic properties of composite laminates subjected to uniaxial loading is investigated. A three-dimensional analytical method based on the classical lamination theory is developed to quantitatively determine the effective elastic constants of thick composite laminates with ply waviness. Various influential factors associated with ply waviness, such as the waviness geometry, the off-axis angle, the extent of wavy region, and the waviness pattern, are incorporated in the multi-parameter model. Analytical results for the Young’s moduli, the shear moduli and the Poisson’s ratios of wavy fiber composite laminates are obtained and discussed for a specific range of fiber volume fractions and two different lay-ups. Negative Poisson’s ratios are found for unidirectional laminates nested wavy ply/plies. The present method provides a useful and effective tool to evaluate the effects of out-of-plane ply waviness on three-dimensional elastic properties of composite laminates.

Solid state impact welding of BMG and copper by vaporizing foil actuator welding

The objective of this study was to create impact welds between a Zr-based Bulk Metallic Glass (BMG) and copper at a laboratory scale and subsequently investigate the relationship between interfacial structure and mechanical properties. Vaporizing Foil Actuator (VFA) has recently been demonstrated as a versatile tool for metalworking applications: impact welding of dissimilar materials being one of them. Its implementation for welding is termed as VFA Welding or VFAW. With 8kJ input energy into an aluminum foil actuator, a 0.5mm thick Cu110 alloy sheet was launched toward a BMG target resulting in an impact at a velocity of nearly 600m/s. For this experiment, the welded interface was straight with a few BMG fragments embedded in the copper sheet in some regions. Hardness tests across the interface showed increase in strength on the copper side. Instrumented peel test resulted in failure in the parent copper sheet. A slower impact velocity during a separate experiment resulted in a weld, which had wavy regions along the interface and in peel failure again happened in the parent copper sheet. Some through-thickness cracks were observed in the BMG plate and there was some spall damage in the copper flyers. TEM electron diffraction on a sample, cut out from the wavy weld interface region using a focused ion beam, showed that devitrification of the BMG was completely avoided in this welding process.