Formation Of Boundary Layer Research Articles

Nano- and Micro-Tribological Investigations of Boundary Layers on Axial Bearing Washers Tested under WEC Critical Conditions

The formation of boundary layers on bearing surfaces due to the operational conditions has a significant influence on bearing lifetimes and frictional properties. Zinc dialkyldithiophosphate is an anti-wear additive widely used in oil and grease formulations that forms beneficial surface boundary layers. Under certain circumstances, this additive can cause early bearing failure due to white etching cracks (WEC) formation. By influencing chemical reactions and diffusion processes, the boundary films are suspected to be a reason for the emergence of WECs. The properties of these layers under WEC critical and uncritical conditions are of interest. To gain knowledge of these layers, nano- and micro-tribological tests were performed. One possibility is to measure the hardness by nanoindentation and scratching on and into the layers by nano scratch tests. Another way is to perform local resolved micro-pin-on-disk tests. Additionally, ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) was performed on the bearing surfaces to conclude the chemical compositions of the boundary layers. In the focus was, if the measured properties of the layers can be correlated to the bearing’s early failures due to WEC, frictional properties of the whole system, and the wear on the surfaces.

The impact of land-sea breezes on the formation of Brown haze in an urban isthmus environment

In some international cities, winter air pollution can manifest into a local-scale brown air pollution haze which has been associated with negative health outcomes. Land-sea breezes are known to impact urban air quality through the recirculation of air pollution and the formation of internal boundary layers (IBLs). However, research into land-sea breezes has primarily focused on summer air pollution and little is known about the influence of land-sea breezes on local-scale winter brown haze. Using continuous data (including surface meteorology, surface air quality, satellite-derived sea surface temperatures, and ceilometer-derived boundary-layer depths) observed over seven winters from 2013 to 2019, we present a novel investigation of the influence of land-sea breezes on brown haze in Auckland, New Zealand. Severe brown haze days are significantly more likely to coincide with a land-sea breeze circulation simultaneously occurring at both the east and west coasts when compared with days on which brown haze is expected but not observed (based on favourable meteorology and high surface air pollution levels). Both severe brown haze and high surface level PM2.5 concentrations (previously associated with the presence of brown haze) are found to be associated with a high degree of horizontal recirculation at Auckland's east coast. No relationship is found between the occurrence of sea-breeze-induced IBLs and the formation of brown haze. The results presented in this study offer insights into the physical mechanisms that influence the formation and persistence of local-scale winter brown haze in a complex coastal setting with correspondingly complex land-sea breezes.

Double-Diffusive MHD Viscous Fluid Flow in a Porous Medium in the Presence of Cattaneo-Christov Theories

The Cattaneo-Christov model will be used to examine the significance of heat generation, viscous dissipation, and thermal radiation on a double-diffusive MHD flow in this study. In this study, it was discovered that heat and mass transfer can be affected by nonlinear buoyancy significance. The flow direction was subjected to a uniform magnetic field. A set of partial differential equations governs the current design (PDEs). In order to simplify these equations, they are converted into ordinary differential equations (ODEs). In order to numerically solve the nonlinear ODEs, the spectral relaxation method (SRM) is utilized. In order to decouple and linearize the equation sets, the SRM employs the Gauss-Seidel relaxation method. Geothermal power generation and underground storage systems are just a few examples where this research could be put to use. When compared to previous findings, the current outcomes were discovered to be closely related. Owing to an increase in Lorentz force, the imposed magnetic field slows down fluid motion. Viscosity dissipation and heat generation all contribute to the formation of an ever-thicker thermal boundary layer. When the Cattaneo-Christov models are used, the thermal and concentration boundary layers get a lot thicker.

Novel numerical results for heat transfer along a stretching cylinder immersed in a shear-thinning fluid

Boundary layer formation over a moving or stationary surface has a wide range of applications in aerodynamics, heat transfer enhancement, and prediction of drag or resistance on a surface. Furthermore, viscosity changes with deformation cannot be ignored in almost all industrially vital fluids. Present work describes boundary layer development around a cylinder stretching axially in a shear-thinning fluid obeying Carreau model. Accompanying heat transfer problem is analyzed in the existence of frictional heating and Joule heating terms in the thermal energy equation. Previous studies concerning Carreau fluid flow driven by a stretching cylinder are discussed with a view to point out some errors concerning mathematical modeling. For computational results, a widely employed MATLAB package bvp4c is utilized, which is based on a collocation scheme. New numerical results of the resulting six parameter problem are worked out, which are utilized to assess the contribution of surface curvature on the flow fields in the presence of a shear-thinning effect. The results show that the fluid velocity is considerably enhanced for larger values of flow index behavior . In the presence of a magnetic field, momentum diffusion is significantly reduced while temperature profile is expanded.

A spatially accelerating turbulent flow with longitudinally contracting walls

This paper describes a study of spatially accelerating turbulent flow based on the direct numerical simulation of a flow with longitudinally accelerating moving walls to create a relative acceleration between the fluid and the wall without inducing streamline curvature. The results show a broad similarity to those of previous investigations of spatial acceleration, albeit with some differences. A new interpretation has been proposed considering this spatially accelerating flow to be characterised by the formation of a new boundary layer superimposed on the pre-existing turbulent flow. This is followed by the transition of the flow in response to the development of this new boundary layer. This can be seen as an extension of the transition theory for temporally accelerating turbulent flows (He & Seddighi, J. Fluid Mech., vol. 715, 2013, pp. 60–102). The existing turbulent structures act as disturbances for the new boundary layer similar to the role of free-stream turbulence in bypass transition. This boundary layer modulates the pre-existing near-wall structures, amplifying and elongating the streaks. Some streaks eventually become unstable in a sinuous mechanism reminiscent of streak breakdown in near-wall turbulence, resulting in the formation localised turbulent spots which spread until the entire wall is covered in new turbulence. This interpretation naturally splits the flow into a new boundary layer region and a core (or free-stream) flow with interactions between the two dominating a significant length of the flow development and potentially offers a new explanation for the slow evolution of the turbulent stresses observed previously.

Electrical magneto hydrodynamic Jeffrey fluid flow with thermal radiation through stretched cylinder

The current study investigates the effects of electrical and magnetic fields on the stagnation point in Jeffrey fluid flow along a stretched cylinder with thermal radiation and chemical reaction. The Cattaneo-Christov model with double stratification and thermal relaxation was used. Heat and mass transfer are investigated. Governing partial differential equations are converted to ordinary differential equations using similarity transformation. FEM technique is used to calculate the numerical solution of a set of nonlinear coupled equations with coding implementation in MATLAB. The numerical outcomes of current work are more significant when they are correlated with previous work. The interest in computational attempts at the formation of boundary layers for concentration distribution, fluid velocity, and temperature increases as significant parameters are varied. The main outcomes of this study are that increasing electrical effects is to decreases fluid velocity while increasing magnetic parameters is to decreases the flow velocity. Another effect is that an increase in thermal radiation and heat transfer raises the temperature of the fluid. Furthermore, the fluid concentration is reduced due to the increased level of a chemical reaction.

Near-Wall Fluid Mechanics Of A Pulsatile Flow Through A Compliant Tube

The demand for heart transplantation far exceeds the supply. This shortage is mainly due to the high percentage of discarded hearts and the narrow time window of only six hours that the current standard preservation method, static cold storage (SCS), allows for completing the entire procedure from donor to recipient patient. An alternative method called ex-vivo heart perfusion (EVHP) maintains a human donor heart beating outside the body for the time preceding transplantation surgery. By keeping the donor heart beating in a quasi-physiological mode of operation and assessing its functions, the organ's health can be monitored so that, ultimately, the transplant time window can be extended to increase the geographic availability of donors. In order to improve and optimize the EVHP system, fundamental understanding of the system physics is needed. To achieve this, the present work aims to investigate the relationship between pulsatile flow and compliance, evaluating the near wall region of a compliant tube with a particular focus on the formation of the viscous boundary layer. Thus, an experimental setup was designed to replicate the flow loop of the EVHP system and an optical transparent compliant tube that mimics the functioning of the body's largest and most compliant artery, the aorta, was custom-made to allow optical access to the flow field matching the refractive index of the working fluid. A pulsatile waveform is generated using a computer-controlled mechanical diaphragm pump, also equipped with a tri-leaf exit valve to simulate the physiological working condition of the heart. In the present work, the flow field is assessed by first performing stereo particle image velocimetry (PIV) on the bulk flow to investigate the large-scale phenomena within a compliant tube and the results of these experiments will be further used as a guide to interrogate smaller near-wall regions of interest in the flow. The preliminary results show, in addition to the general characteristics of a pulsatile flow through a compliant tube, a potential complex fluid-structure relationship between the pulsatile flow and the moving wall of the tube has been observed. The presentation of this work will discuss the experimental setup and review the preliminary results presented here in addition to others findings that are still being analyzed, data processing is underway to do so.

Coral Reef Coupling to the Atmospheric Boundary Layer Through Exchanges of Heat, Moisture, and Momentum: Case Studies From Tropical and Desert Fringing Coral Reefs

Coral reefs represent abrupt changes in surface roughness, temperature, and humidity in oceanic and coastal locations. This leads to formation of internal atmospheric boundary layers that grow vertically in response to turbulent mixing which conveys changes in surface properties into the prevailing wind. As a result, coral reefs under favourable conditions couple to the overlying atmosphere. Here we present rare observations of coral reef – atmospheric interactions during summer monsoon conditions on the Great Barrier Reef, Australia, and a desert fringing coral reef in the Gulf of Eilat, Israel. We show that in the hyper-arid location of the Gulf of Eilat where air temperatures are greater than water temperatures, a stable atmospheric boundary inhibits coupling of the reef to the atmosphere. In contrast, under monsoon conditions on the Great Barrier Reef, the coral reef is shown to couple to the overlying atmosphere leading to the formation of a convective internal boundary layer in which convective exchange of heat and moisture influences cloud and possibly precipitation. We conclude that understanding these processes is essential for determining the role of coral reefs in coastal meteorology, and whether through coupling with the overlying atmosphere coral reefs may regulate their meteorology by triggering formation of cloud and/or vertical exchange on aerosols and their precursor gases such as Dimethyl sulphide.

Influence of the ocean on tropical cyclone intensity using a high resolution coupled atmosphere–ocean model: A case study of very severe cyclonic storm Ockhi over the North Indian Ocean

AbstractA very severe cyclonic storm (VSCS), Ockhi was conceived in the Bay of Bengal and moved over the Arabian Sea undergoing rapid intensification (RI) in the early stages of its life cycle. Ockhi is simulated using the high‐resolution coupled Hybrid Coordinate Ocean Model and the Hurricane Weather Research Forecast model. The study suggests that sea surface temperature warmer than the seasonal climatological mean with the availability of a large tropical cyclone heat potential (TCHP) has acted as a potential cause for RI when the TC moved over that region. We have conducted experiments with (CP) and without (UCP) ocean coupling. The predicted tracks of UCP and CP experiments agree well with the India Meteorological Department's best track data. However, CP showed a significant improvement in intensity prediction with 61 and 19% for mean sea level pressure and maximum sustained surface wind at 10 m respectively in comparison to UCP. The presence of thermal and haline stratification in the subsurface affects surface cooling. The subsurface turbulent heat flux calculated for the high‐TCHP region under the TC suggests that the upward transport of heat, which increases the enthalpy flux, becomes necessary to bring about RI. The primary circulation, secondary circulation, inflow, thermal structure, moisture distribution and mass flux within the TC environment during the 24‐hr RI period are analyzed to investigate the evolution of TC characteristics for both UCP and CP experiments. The analyses suggest that the UCP experiments show higher values of intensity as compared to CP experiments. Further, a more organized evolution of updrafts is present in the CP simulation, maybe due to the formation of a stable boundary layer. The study shows the significant influence of coupling on surface enthalpy controlling boundary layer characteristics and thus ultimately producing a more organized TC.

An iterative transfer matrix approach for estimating the sound speed and attenuation constant of air in a standing wave tube.

In this work, an iterative method based on the four-microphone transfer matrix approach was developed for evaluating the sound speed and attenuation constant of air within a standing wave tube. When the air inside the standing wave tube is treated as the material under test, i.e., as if it were a sample of porous material, the transfer matrix approach can be used to identify the air's acoustic properties. The wavenumber within the tube is complex owing to the formation of a visco-thermal boundary layer on the inner circumference of the tube. Starting from an assumed knowledge of the air properties, an iterative method can be applied in the post-processing stage to estimate the complex wavenumber. Experimental results presented here show that although the results are sensitive to ambient temperature, a semi-empirical formula previously proposed by Temkin [(1981). Elements of Acoustics (John Wiley & Sons)] matches closely with the measured sound speed and attenuation constant, as does a theoretical formulation proposed by Lahiri et al. [(2014). J. Sound Vib. 333(15), 3440-3458]. Further, it is shown that the Temkin [(1981). Elements of Acoustics (John Wiley & Sons)] and Lahiri et al. [(2014). J. Sound Vib. 333(15), 3440-3458] predictions accurately represent the variation of sound speed with frequency, in contrast to the formula recommended in the ASTM E1050 standard [(2019). American Society for Testing and Materials], in which the sound speed is assumed to be independent of frequency.

Effect of Urban Expansion Indices Change by (R.S.) on Height of Convective Radix Layer around Baghdad Airport (Iraq)

Change in structure of land surface can affect atmospheric boundary layer and formation of internal boundary layer. This will change shear stresses and turbulent boundary layer. Change in low level wind shear and turbulent can affect air craft performance and has potentially adverse effects on flight safety during landing and taking-off stages. In this study, change in land area around Baghdad Airport and its effects on structure of boundary layer and radix layer through summer season (July from years 1985 and 2014) is examined. The examination is done through atmospheric data of radiosonde (at altitude more than 1500 m) and remote sensing by Landsat 5 and 8 images (circular region around airport has radius 3.250 km and the airport is considered the center of this circle). From analysis of hourly wind speed and temperature profile with height observed by radiosonde, change in convective radix layer can be determined at this period resulted from change structure of boundary layer depending on characteristic of landcover used in supervised classification. Building area increases from 18% in 1985 to 41% in 2014. These results enforced by positive values of built up index BI, that showed a decreases in 2014. All these elements changed convective radix layer from less than 500 m in summer 1985 to more than 700 m. There is a large fluctuation in radix layer height depended on convective and friction velocity at 2014 data. Thus indirect change in structure of convective layer will affect navigation movement of Baghdad International Airport.

Synergy of phospholipid and hyaluronan based super-lubricated hydrogels

Synthetic hydrogels have been widely studied for the application of artificial joints and other biomedicals. In this work, the synergy of phospholipid and hyaluronan (HA) based super-lubricated hydrogel was synthesized with zwitterionic polymers through free-radical polymerization. The coefficient of friction could reduce down to 0.004, reaching a super-lubricated state, which exhibited a remarkable reduction of friction (approximately 85%) in comparison to that of the original hydrogel in the absence of lipids and HA. The friction reduction was observed under a wide range of sliding velocities, normal loads, and different upper friction materials (including steel, glass, polytetrafluoroethylene, and silicon nitride). The enhanced lubrication performance was mainly attributed to the formation of a robust lipid-based hydration boundary layer at the hydrogel interface with the synergistic lubrication effect between lipids and HA arising from electrostatic interaction. Our approach may provide the potential application for cartilage replacement and artificial joints and help us develop hydrogels with good lubrication characteristics.

Analysis of spray/wall impingement using an ECN single-hole injector and a controlled-temperature wall under realistic engine conditions

Spray–wall interactions (SWI) directly affect fuel–air mixture and emissions formation. Therefore, they are considered among the most critical physical processes in engine research nowadays. However, the physics of the wall film formation, propagation, and breakup is not fully understood yet. This work aims to use a thermoregulated steel wall to study the spray–wall interaction phenomenon and its influence on the macroscopic spray behavior. A single-hole injector known as “Spray D” in the Engine Combustion Network was used. n-Dodecane was employed as fuel and the wall has been positioned in four different configurations, varying angle and distance respect to the injector tip. For this work, not only diesel combustion is reproduced in the test rig due to ambient gas engine-like thermodynamic conditions, but wall temperature has been controlled to emulate characteristic values that could be found in the piston of an internal combustion engine. This implies that the spray–wall heat transfer is simulated and its effects on ignition and spray development can be analyzed. Heat flux was measured by employing high-speed thermocouples fitted in the wall and by the use of an one-dimensional transient wall heat model. Three high speed cameras were simultaneously used to observe the SWI, one for the Schlieren optical technique which allows to study the vapor phase of the spray and to determine the ignition delay, another one to observe the natural luminosity of the flame, and finally, an intensified camera was used to determine the lift-off length by observing the chemiluminescence of the OH*. An interesting finding obtained in this work was a boundary layer formation due to the thermal diffusion that contributes to cool down the spray and to delay the high-temperature chemical reactions. Results show a substantial increment of the heat flux and the wall temperature variation with both ambient temperature and density by increasing the flame temperature and gas entrainment. The exposure to the cold wall affects the ignition delay variation with the injection pressure and the wall distance. It was found that the wall temperature (in the range of tested conditions) did not affect the lift-off length location. • The exposure to the cold wall delayed the ignition. • Flame thickness around the wall undergoes a narrowing at lower wall temperatures. • The injection pressure increments the convective coefficient. • A boundary layer due to thermal diffusion was formed on the wall.

Effect of blade skew, endplate and casing groove on the aerodynamic performance of Wells turbine for OWC: a review

Abstract The geometry modification of a bidirectional flow Wells turbine used in a wave energy converter can assist in overcoming the mammoth stall triggered by tip leakage, secondary flow on the blade surface, corner vortex, passage vortex, tip scrubbing, tip trailing edge vortex, boundary layer formation on hub and casing, and combination of all the listed making it a complex 3D flow. As the flow varies almost sinusoidally in each wave cycle in the ocean, it is inevitable to get a turbine having a higher operating range and average power output. Skewing a blade reduces the radial flow on the blade suction surface. A casing groove and endplate reduce tip leakage flow. These modifications give a significant improvement in the turbine performance. There are several design alternatives reported in the articles, and they have shown different results. In this article, the modifications and their effects are discussed in detail and concluded for future recommendations.

Phononic crystal as a homogeneous viscous metamaterial

At low frequencies, a phononic crystal behaves like a homogeneous medium. Different variants of homogenization technique have been recently proposed to calculate the effective elastic parameters of periodic medium. Here, we develop a homogenization theory for a phononic crystal of solid rods in a viscous fluid. Using the plane wave expansion method, we derive analytical formula for the decay coefficient of low frequency sound propagating in a 2D Bravais lattice with arbitrary unit cell. It is shown that due to the formation of a viscous boundary layer around each cylinder, the losses are enhanced by two to four orders of magnitude as compared to the losses in the free fluid. This enhancement depends on the filling fraction of solid rod and it becomes very strong for almost touching scatterers when sound propagates through narrow slits between the neighbouring rods. Also, the decay coefficient in a phononic crystal scales with frequency as ω2, unlike scaling √ω known for free viscous fluid. The enhanced viscous losses are associated with high effective viscosity of phononic crystal. Like other effective parameters the effective viscosity exhibits strong anisotropy, if the unit cell is asymmetric. The proposed theory can be used for evaluation of efficiency of acoustic devices based on phononic crystals. [This study is supported by NSF Grant No. 1741677.]

Extraordinary simultaneous enhancement of the coercivity and remanence of dual alloy HRE‐free Nd‐Fe‐B sintered magnets by post‐sinter annealing

• Both the remanence and coercivity are improved after the 2nd annealing process. • The coercivity is hugely increased from 10.09 kOe for the as-sintered sample to 17.19 kOe for the 2nd annealed magnet, with a significant increment of 70.37%. • The matrix grains are enveloped by a (Pr,Nd,Cu,Ga)-enriched layer after the 2nd annealing process. • HRE-free Nd-Fe-B sintered magnets with excellent comprehensive magnetic properties were manufactured by Pr-Fe-Ga boundary addition. Post-sinter annealing process plays an important role in the microstructures and magnetic properties of the Nd-Fe-B sintered magnets. In this paper, systematically investigated are the magnetic properties and microstructures of the as-sintered and post-sinter annealed Nd-Fe-B magnets with Pr-Fe-Ga boundary addition. Two choice consecutive annealing methods are adopted at high and low temperatures, namely the 1 st annealing at 880 ºC for 2 h and then the 2 nd annealing at 440 ºC for 3 h. It is exceptional to find out that both the remanence and coercivity are improved after 2 nd annealing process for this type of magnet. The coercivity is hugely increased from 10.09 kOe for the as-sintered sample to 17.19 kOe for the 2 nd annealed magnet, with a significant increment of 70.37% in coercivity. The extraordinary magnetic properties of B r =14.44 kGs, H cj =17.19 kOe and ( BH ) max =51.08 MGOe are obtained for the designated Nd-Fe-B sintered magnets without heavy rare earth (HRE) elements manufactured by dual alloy method. The Curie temperature is monotonically decreased from 634 K to 602 K while the c -axis alignment degree is optimized after annealing. Microstructural observation and analysis indicate that the elemental distribution patterns are altered after the 2 nd annealing. The diffusion of the aggregate (Pr,Nd,Cu,Ga)-rich phase from triple junctions into the grain boundary regions is ascribed to the formation of thin and continuous grain boundary layer, which is critical to improve the microstructures and magnetic properties.

An iterative approach for estimating the sound speed and attenuation constant of air in a standing wave tube

In the present work, an iterative method based on the four-microphone transfer matrix approach was developed for evaluating the sound speed and attenuation constant of air within a standing wave tube. In particular, when the air inside the standing wave tube is treated as the material under test, i.e., as if it were a sample of porous material, the transfer matrix approach can be used to identify the air’s acoustic properties. Note that the wavenumber within the tube is complex owing to the formation of a visco-thermal boundary layer on the inner circumference of the tube. Starting from an assumed prior knowledge of the air properties, an iterative method can be applied in the post-processing stage to estimate the complex wavenumber accurately. Experimental results presented here show that although the results are sensitive to ambient temperature, a formula previously proposed by Temkin matches closely with the measured sound speed and attenuation constant. Furthermore, it is shown that the Temkin prediction accurately represents the variation of sound speed with frequency, in contrast to the formula recommended in the ASTM E1050 standard, in which the sound speed is assumed to be independent of frequency.

Experimental and numerical investigation of micro-scale effusion and transpiration air cooling on cascaded turbine blades

Micro cooling utilizing the micro-sized cooling hole or a porous structure on the surface is a promising technology in cooling applications of gas turbine blades. Although there have been numerous parametric studies on basic geometries to enhance the performance of micro cooling, little has been done for experimental study on blade geometry. This study comprehensively investigated micro cooling performance by applying effusion and transpiration cooling to C3X blades arranged in a cascade. The overall cooling effectiveness distribution on the blade surface was estimated by using infrared thermometry. In addition, the velocity and thermal boundary layer formation by cooling air were qualitatively investigated by a flow visualization using the smoke-laser sheet technique and numerical simulation using the shear stress transport k-ω turbulence model. Micro cooling performs effectively because of the convective heat transfer through the microstructure of the blade wall but also the reduction of heat transfer from the hot mainstream due to the formation of a uniform coolant layer on the blade surface. Especially at the mass flow ratio of 5.3% used for typical gas turbine cooling, effusion cooling and transpiration cooling achieve the overall cooling effectiveness of 0.4 and 0.6, respectively.

Vacuum UV (VUV) Photo-Oxidation of Polyethersulfone (PES)

International need for water quality is placing a high demand on separation technology to develop advanced oxidative processes for polyethersulfone (PES) membranes to help improve water purification. Therefore, VUV photo-oxidation with a low pressure Ar plasma was studied to improve the hydrophilicity of PES by flowing oxygen over the surface during treatment. X-ray photoelectron spectroscopy (XPS) detected a decrease in the C at% (4.4 ± 1.7 at%), increase in O at% (3.7 ± 1.0 at%), and a constant S at% (5.4 ± 0.2 at%). Curve fitting of the XPS spectra showed a decrease in sp2 C-C aromatic group bonding, and an increase in C-O, C-S, O=C-OH, sulphonate (-SO3) and sulphate (-SO4) functional groups with treatment time. The water contact angle decreased from 71.9° for untreated PES down to a saturation level of 41.9° with treatment. Since scanning electron microscopy (SEM) showed no major changes in surface roughness, the increase in hydrophilicity was mainly due to oxidation of the surface. Washing the VUV photo-oxidized PES samples with water or ethanol increased the water contact angle saturation level up to 66° indicating the formation of a weak boundary layer.

Stability of gravity-driven particle-laden flows – roles of shear-induced migration and normal stresses

In this paper, we study the role of shear-induced migration and particle-induced normal stresses in the formation and stability of a particle-laden, gravity-driven shallow flow. We first examine the modification of the base-state Nusselt flow due to the underlying microstructure, how shear-induced migration leads to viscosity stratification. We inspect the development of the base state via the boundary layer formation in the ‘shallow’ limit and find a reduction in entrance length with increasing bulk particle concentration and an increase in entrance length with increasing Péclet number ($Pe_p = \dot {\gamma } a^2 / D_0$, where $\dot{\gamma}$ is the average shear rate, a is the particle size and $D_0$ is the single particle diffusivity). A linear stability analysis is then performed on the fully developed state to identify two modes of instability typically found in gravity-driven falling films – the long-wave surface and the short-wave shear modes. We find that when the associated Péclet number is $Pe_p \ll 1$, increasing bulk particle volume fraction delays the onset of instability for both the surface mode and shear mode. However, with $Pe_p = {O}(1)$, we find an enhancement in both modes of instability. We also find that, beyond a critical Péclet number, for a fixed particle volume fraction, the surface mode is unstable even in the absence of fluid inertia. The enhanced destabilisation is attributed to the combined effects of base-state viscosity stratification and momentum forcing via particle concentration perturbations. We also show that the physics behind the enhancement of instability is independent of the choice of the constitutive model used to describe the dynamics of the particle phase, provided the chosen model has elements of shear-induced migration.