Laminar Case Research Articles

Wall-bounded flow over a realistically rough superhydrophobic surface

Direct numerical simulation (DNS) is performed for two wall-bounded flow configurations: laminar Couette flow at $Re=740$ and turbulent channel flow at $Re_{\unicode[STIX]{x1D70F}}=180$, where $\unicode[STIX]{x1D70F}$ is the shear stress at the wall. The top wall is smooth and the bottom wall is a realistically rough superhydrophobic surface (SHS), generated from a three-dimensional surface profile measurement. The air–water interface, which is assumed to be flat, is simulated using the volume-of-fluid (VOF) approach. The two flow cases are studied with varying interface heights $h$ to understand its effect on slip and drag reduction ($DR$). For the laminar Couette flow case, the presence of the surface roughness is felt up to $40\,\%$ of the channel height in the wall-normal direction. Nonlinear dependence of $DR$ on $h$ is observed with three distinct regions. A nonlinear curve fit is obtained for gas fraction $\unicode[STIX]{x1D719}_{g}$ as a function of $h$, where $\unicode[STIX]{x1D719}_{g}$ determines the amount of slip area exposed to the flow. A power law fit is obtained from the data for the effective slip length as a function of $\unicode[STIX]{x1D719}_{g}$ and is compared to those derived for structured geometry. For the turbulent channel flow, statistics of the flow field are compared to that of a smooth wall to understand the effects of roughness and $h$. Four cases are simulated ranging from fully wetted to fully covered and two intermediate regions in between. Scaling laws for slip length, slip velocity, roughness function and $DR$ are obtained for different penetration depths and are compared to past work for structured geometry. $DR$ is shown to depend on a competing effect between slip velocity and turbulent losses due to the Reynolds shear stress contribution. Presence of trapped air in the cavities significantly alters near-wall flow physics where we examine near-wall structures and propose a physical mechanism for their behaviour. The fully wetted roughness increases the peak value of turbulent intensities, whereas the presence of the interface suppresses them. The pressure fluctuations have competing contributions between turbulent pressure fluctuations and stagnation due to asperities, the near-wall structure is altered and breaks down with increasing slip. Overall, there exists a competing effect between the interface and the asperities, the interface suppresses turbulence whereas the asperities enhance them. The present work demonstrates DNS over a realistic multiphase SHS for the first time, to the best of our knowledge.

The role of eddies on pathways, transports, and entrainment in dense water flows along a slope

The flow of dense water along slopes has been investigated in several numerical investigations based on the Dynamics of Overflow Mixing and Entrainment (DOME) setup. In the present study, we try to obtain further insight into the pathways, transports, dynamics, and entrainment of such flows by performing numerical model studies with horizontal grid sizes of 10 km and 2.5 km. It is found that the rates of descent of the plumes along the slope are robust to the horizontal resolution. With a high vertical resolution and a bottom boundary condition that facilitates the representation of Ekman drainage, the plumes will follow a deeper path than when using quadratic bottom drag with a constant drag coefficient. In the results from the studies with 2.5 km horizontal grid, ambient lighter water inside anticyclonic eddies is sucked downward. Due to Ekman drainage, this water flows outwards near the bottom and underneath denser plume water. The water column around the core of the anticyclonic eddies becomes unstable, and ambient water is entrained into the plume. Due to the increased mixing and entrainment in the eddy-permitting regime, there is a substantial increase in the along slope plume transports when we reduce the grid size from 10 km (the laminar case) to 2.5 km (the eddy-permitting case).

Calculation of Friction Factors and Nusselt Numbers for Twisted Elliptical Tube Heat Exchangers Using Nek5000

Twisted elliptical tube geometries for heat exchanger design are expected to offer significant enhancement in heat transfer with a marginal increase in frictional losses. The presented work focuses on computational fluid dynamics (CFD) simulations of heated molten salt flows through twisted elliptical tube geometries at low modified Froude numbers. The objectives of this work are to evaluate the available correlations at low Froude numbers and to determine the impact of using a small gap to resolve contact points. The spectral element CFD code Nek5000 was used for all simulations, which were performed in periodic domains of triangular and square unit cells surrounding a single tube through a complete twist using an explicit filtering large eddy simulation (LES) method. The simulation methodology was validated by comparing to the experimental data of Tan et al. Excellent agreement was observed for this comparison. The Dzyubenko correlation for transitional flow was evaluated for a range of Froude numbers. This correlation demonstrated good agreement with LES results at high modified Froude number. As Froude number was decreased toward the bounds of the correlation, the agreement worsened. Cases were then simulated at low Froude number, testing the effects of tube spacing. It was determined that the laminar case for the square unit cell is the most affected by increasing gap size, which should be minimized to mitigate the effect. In the triangular unit cell, the laminar flow regime is also more significantly impacted by increasing gap size compared to the turbulent flow regime which was only marginally impacted.

Effects of Turbulence Intensity and Biogas Composition on the Localized Forced Ignition of Turbulent Mixing Layers

ABSTRACTThree-dimensional compressible Direct Numerical Simulations (DNS) have been used to investigate the localized forced ignition of statistically planar mixing layers of biogas/air mixtures for different levels of turbulence intensity and biogas composition. The biogas is represented by a CH/CO mixture and a two-step mechanism involving incomplete oxidation of CH to CO and HO, and an equilibrium between the CO oxidation and the CO dissociation has been used. This two-step mechanism captures the variation of the unstrained laminar flame speed with equivalence ratio and CO dilution with sufficient accuracy when compared with detailed chemistry results. A successful ignition of CH/CO/air mixing layer initially gives rise to a tribrachial flame structure involving fuel-rich and lean premixed branches on either side of the diffusion flame stabilized on the stoichiometric mixture fraction iso-surface. Long after the energy deposition has ended, the lean branch may merge with the diffusion flame, but edge flame propagation along the stoichiometric mixture fraction iso-surface is observed at all stages. The highest heat release rate (HRR) is obtained on the rich premixed branch irrespective of the turbulence intensity and biogas composition, but its magnitude decreases with increasing CO dilution. The most probable edge flame displacement speed decreases in time and converges to its theoretical value for laminar cases. As the turbulence level increases, the edge flame displacement speed assumes a larger range of values, although its mean is consistently lower than the corresponding laminar one. Furthermore, in the turbulent cases, the probability of finding negative values of the edge flame displacement speed has been found to be non-zero. This probability is larger than the one of finding positive values for the cases failing to reach a self-sustained flame propagation without the energy deposition assistance. This becomes more probable as the turbulence intensity and CO dilution increase. This is reflected in the diminished burning extent, quantified in terms of burned gas volume, with increasing turbulence intensity and CO dilution.

Local Concentrating, Not Shear Stress, That May Lead to Possible Instability of Protein Molecules During Syringe Injection: A Fluid Dynamic Study with Two-Phase Flow Model.

A two-phase flow computational fluid dynamic (CFD) model was developed to study the hydrodynamic forces and the protein concentration changes of a protein solution in a syringe injector. Proteins were assumed to be nanosized solid spheres commensurate with their molecular weight and suspended in an aqueous environment, passing through the rapidly constricted sections of the syringe. Interaction between the solid and the liquid phase was taken into account, and four laminar flow cases were studied under the extensional flow. Profiles of pressure, velocity, and shear stress of the different cases were examined and compared. Hydrodynamic forces on a single protein particle were further analyzed. Our results indicate that the hydrodynamic forces are too small to affect significant conformational changes in proteins. The plunger rate showed limited impact on the distribution of protein particles inside the syringe. Nonetheless, the larger velocity gradient at the connection section of the hub toward the needle resulted in considerable accumulation of proteins. Such a concentrating effect may lead to protein aggregation and subsequent structural changes and will be examined in future studies.LAY ABSTRACT: Concentrated protein in the liquid form has become a preferred formulation strategy for delivering protein products, but it suffers from a high possibility of aggregation and precipitation, which may trigger a structural change and denaturation of the protein molecules and eventually cause the loss of the therapeutic functions of the protein products. To understand the effect of hydrodynamic forces on the change in local protein concentration in a syringe injector, we developed a two-phase flow computational fluid dynamic (CFD) model in this work. It was found that the local concentration of protein strongly depends on the velocity gradient of the fluid. A higher amount of proteins accumulates at the connection section of the hub toward the needle where the maximum velocity gradient is exhibited. The model results also imply a limited effect by hydrodynamic forces on possible unfolding of protein molecules. The local concentrating effect may enhance the possibility of molecular collision, resulting in aggregation and structural change.

Investigation of the turbulent flame structure and topology at different Karlovitz numbers using the tangential stretching rate index

Turbulent premixed flames at high Karlovitz numbers exhibit highly complex structures in different reactive scalar fields to the extent that the definition of the flame front in an unambiguous manner is not straightforward. This poses a significant challenge in characterizing the observable turbulent flame behaviour such as the flame surface density, turbulent burning velocity, and so on. Turbulent premixed flames are reactive flows involving physical and chemical processes interacting over a wide range of time scales. By recognizing the multi-scale nature of reactive flows, we analyze the topology and structure of two direct numerical simulation cases of turbulent H2/air premixed flames, in the thin reaction zone and distributed combustion regimes, using tools derived from the computational singular perturbation (CSP) method and augmented by the tangential stretching rate (TSR) analysis. CSP allows to identify the local time scale decomposition of the multi-scale problem in its slow and fast components, while TSR allows to identify the most energetic time scale during both the explosive and dissipative regime of the reactive flow dynamics together with the identification of the flame front in an unambiguous manner. Before facing the complexity of the turbulent flow regime, we carry out a preliminary analysis of a one-dimensional laminar premixed flame in view of highlighting similarities and differences between laminar and turbulent cases. Subsequently, it is shown that the TSR metric provides a reliable way to identify the turbulent flame topologies.

Predicting thermophoresis induced particle deposition by using a modified Markov chain model

Thermophoresis is considered as one of the major causes leading to deposition of suspended submicron aerosols with the presence of temperature gradient. Prompt obtaining of information about particle deposition and spatial distribution is crucial to environment control. This study proposed a modified Markov chain model to include the effect of thermophoresis. A steady-state flow field and a constant temperature field are firstly established by using Computational Fluid Dynamics (CFD) tools. Then the data of flow and temperature fields are exported out as data files, which will be used by the proposed model to realize the particle phase simulation. A horizontal steady-state laminar duct flow case is used to validate the model. Results show that the proposed model is able to efficiently predict thermophoresis induced particle deposition and dispersion with reasonable accuracy. Detailed information also indicate that the particle deposition efficiency is dependent on both temperature gradients and particle diameters. However, the beginning of the particle escape (if any) is irrelevant to those two parameters. The highest deposition efficiency on the deposition surface is found to gradually increase over time and the location of highest deposition efficiency (peak) moves along with the main flow. The average speed of the moving peak is inversely proportional to temperature gradient, but the ultimate deposition efficiency is still proportional to temperature gradient. The proposed model is deemed a practical method in predicting aerosol particle deposition/transport as well as contaminant control in non-isothermal environment.

Viscoelasticity-induced pulsatile motion of 2D roll cell in laminar wall-bounded shear flow

For the clarification of the routes to elasto-inertial turbulence (EIT), it is essential to understand how viscoelasticity modulates coherent flow structures including the longitudinal vortices. We focused on a rotating plane Couette flow that provides two-dimensional (2D) roll cells for the steady laminar Newtonian-fluid case, and we investigated how the steady longitudinal vortices are modulated by viscoelasticity at different Weissenberg numbers. The viscoelasticity was found to induce an unsteady flow state where the 2D roll-cell structure was periodically enhanced and damped with a constant period, keeping the homogeneity in the streamwise direction. This pulsatile motion of the roll cell was caused by a time lag in the response of the viscoelastic force to the vortex development. Both the pulsation period and time lag were found to be scaled by the turnover time of cell rotation rather than by the relaxation time, despite the viscoelasticity-induced instability. We also discuss the counter torque on the roll cell and the net energy balance, considering their relevance to polymer drag reduction and EIT.

Numerical Study on the Effect of Gender on the Airflow Characteristics inside the Nasal Cavity

Understanding the properties of airflow in the nasal cavity is very important in determining the nasal physiology and in diagnosis of various anomalies associated with the nose. This numerical study presents the characteristic flow features inside a female nasal cavity in comparison with the male models developed by other researchers. The study is based on the numerical model obtained from computed tomographic data of a healthy Malaysian subject. A steady state Reynold Averaged Navier-Stokes and continuity equations was solved for both inspiratory and expiratory phase with flow rates of 7.5, 10 and 15 L/min for laminar case and 20, 30 and 40 L/min studies were simulated for turbulent boundary conditions. The differences observed in the flow parameters can be attributed to the anatomical variations that exist between the male and female nasal cavity. It was found that the female models were slightly smaller in length when compared to the male models. The nasal valve region was located about 2.0 cm, 1.65 cm and 2.5 cm from the anterior tip of nose. The value of maximum velocity obtained for the three female models are 3.17, 2.68, and 2.23 m/s respectively as against 4.2 m/s for similar flow rate obtained for the male model. The values of pressure drop for female cases were lower than the male models in the literature. In general, the female model displayed lower values of pressure drop when compared with the male models.

Particle accretion onto planets in discs with hydrodynamic turbulence

The growth process of protoplanets can be sped up by accreting a large number of solid, pebble-sized objects that are still present in the protoplanetary disc. It is still an open question on how efficient this process works in realistic turbulent discs. We investigate the accretion of pebbles in turbulent discs that are driven by the purely hydrodynamical vertical shear instability (VSI). For this purpose, we performed global 3D simulations of locally isothermal, VSI turbulent discs that have embedded protoplanetary cores from 5 to 100 M⊕, which are placed at 5.2 au distance from the star. In addition, we followed the evolution of a swarm of embedded pebbles of different sizes under the action of drag forces between gas and particles in this turbulent flow. Simultaneously, we performed a set of comparison simulations for laminar viscous discs where the particles experience stochastic kicks. For both cases, we measured the accretion rate onto the cores as a function of core mass and Stokes number (τs) of the particles and compared these values to recent magneto-rotational instability (MRI) turbulence simulations. Overall the dynamic is very similar for the particles in the VSI turbulent disc and the laminar case with stochastic kicks. For small mass planets (i.e. 5–10 M⊕), well-coupled particles with τs = 1, which have a size of about 1 m at this location, we find an accretion efficiency (rate of particles accreted over drifting inwards) of about 1.6−3%. For smaller and larger particles, this efficiency is higher. However, the fast inwards drift for τs = 1 particles makes them the most effective for rapid growth, leading to mass doubling times of about 20 000 yr. For masses between 10 and 30 M⊕ the core reaches the pebble isolation mass and the particles are trapped at the pressure maximum just outside of the planet, shutting off further particle accretion.

A direct numerical simulation analysis of spherically expanding turbulent flames in fuel droplet-mists for an overall equivalence ratio of unity

Three-dimensional direct numerical simulations with a modified single-step Arrhenius chemistry have been used to analyze spherically expanding n-heptane flames propagating into mono-sized fuel droplet mists for different droplet diameters and an overall equivalence ratio of unity. The evolutions of flame surface area and burned gas volume for both laminar and turbulent spherically expanding droplet flames have been compared to the corresponding gaseous stoichiometric premixed spherically expanding flames with the same initial burned gas radius. It has been found that the initial droplet diameter significantly affects the burned gas volume and flame area generation, which increase with decreasing droplet diameter for both laminar and turbulent cases. The droplet-flame interaction plays a key role in determining flame wrinkling under laminar conditions, which is reflected in a range of local curvatures for a given reaction progress variable isosurface, whereas each progress variable isosurface in spherically expanding laminar premixed flames exhibits a single value of curvature. The effect of droplet-induced curvature becomes less distinguishable from the flow-induced wrinkling for the turbulent cases considered here, but the reaction progress variable isosurfaces in droplet cases exhibit wider curvature probability density functions than in the corresponding turbulent premixed flame cases. It has been found that the heat release rate arises principally from premixed mode in small droplet cases, whereas the contribution of the non-premixed mode to the overall heat release rate increases with increasing droplet diameter and turbulence intensity.

Large-eddy simulation of laminar transonic buffet

A large-eddy simulation of laminar transonic buffet on an airfoil at a Mach number $M=0.735$, an angle of attack $\unicode[STIX]{x1D6FC}=4^{\circ }$, a Reynolds number $Re_{c}=3\times 10^{6}$ has been carried out. The boundary layer is laminar up to the shock foot and laminar/turbulent transition occurs in the separation bubble at the shock foot. Contrary to the turbulent case for which wall pressure spectra are characterised by well-marked peaks at low frequencies ($St=f\cdot c/U_{\infty }\simeq 0.06{-}0.07$, where $St$ is the Strouhal number, $f$ the shock oscillation frequency, $c$ the chord length and $U_{\infty }$ the free-stream velocity), in the laminar case, there are also well-marked peaks but at a much higher frequency ($St=1.2$). The shock oscillation amplitude is also lower: 6 % of chord and limited to the shock foot area in the laminar case instead of 20 % with a whole shock oscillation and intermittent boundary layer separation and reattachment in the turbulent case. The analysis of the phase-averaged fields allowed linking of the frequency of the laminar transonic buffet to a separation bubble breathing phenomenon associated with a vortex shedding mechanism. These vortices are convected at $U_{c}/U_{\infty }\simeq 0.4$ (where $U_{c}$ is the convection velocity). The main finding of the present paper is that the higher frequency of the shock oscillation in the laminar regime is due to a different mechanism than in the turbulent one: laminar transonic buffet is due to a separation bubble breathing phenomenon occurring at the shock foot.

Characterization of pseudo-boiling in a transcritical nitrogen jet

This study is devoted to the investigation, by means of direct numerical simulation, of the interaction between turbulent motions and the pseudo-boiling process. To this end, fully resolved data of a transcritical nitrogen jet are used, obtained via high order methods and using detailed thermodynamic and transport properties. A laminar pseudo-boiling process is simulated in a quiescent setting and used as a consistent reference to shed light on the mutual effects of the jet evolution and thermodynamic non-linearities. In the turbulent scenario, pseudo-boiling is shown to be faster, in an average sense, to the laminar reference case. A consistent definition of the pseudo-boiling rate, based on the concept of the displacement speed, commonly used in premixed flame propagation, is introduced and, for a better physical interpretation, split into a normal diffusion component and a curvature component. The pseudo-boiling rate is statistically analyzed to evaluate the rate of mass transfer from the liquid-like state to the gas-like state during the jet evolution. Normal diffusion is found to be the dominant component of the pseudo-boiling rate, while the curvature component is shown to have a role only when warm fluid pockets are deeply entrained in the jet cold core.

Structure of tetrabrachial flames in non-premixed autoigniting dimethyl ether/air mixtures

Structures of tetrabrachial flames in two-stage autoigniting dimethyl ether (DME)/air mixture under diesel engine relevant conditions are investigated by direct numerical simulation. Three cases with different initial turbulent integral length scales are studied. Results show that the first stage of autoignition initiates in lean mixtures, and subsequently develops into a diffusion-supported cool flame propagating into rich mixtures; the second-stage autoignition features spatially distributed kernels in fuel-rich mixtures, followed by hybrid autoignition/tetrabrachial flames. The detailed chemical structures of the tetrabrachial flames are analyzed in terms of reactant concentrations and the reaction rate profiles. The cool flame branch is dominated by low temperature reactions, while the other branches are mainly involved in high temperature oxidation of the remaining fuel and intermediate species. The excess DME is consumed in the premixed flame branches and decomposed into more stable fuels including H2, CH4 and CO in the trailing diffusion flame, where H2 and CO are mainly oxidized by intermediate species OH and O. The structures and reaction rates in the tetrabrachial flame exhibit significant asymmetry, which is more distinct in the mixture fraction-temperature phase space. Effects of turbulence on the timing and location of two-stage ignition are then studied. In this study, turbulence tends to advance ignition compared with laminar cases, while the first high-temperature ignition time is similar for the three cases with different initial turbulence integral length scales.

Periodically driven Taylor–Couette turbulence

We study periodically driven Taylor–Couette turbulence, i.e. the flow confined between two concentric, independently rotating cylinders. Here, the inner cylinder is driven sinusoidally while the outer cylinder is kept at rest (time-averaged Reynolds number is $Re_{i}=5\times 10^{5}$). Using particle image velocimetry, we measure the velocity over a wide range of modulation periods, corresponding to a change in Womersley number in the range $15\leqslant Wo\leqslant 114$. To understand how the flow responds to a given modulation, we calculate the phase delay and amplitude response of the azimuthal velocity. In agreement with earlier theoretical and numerical work, we find that for large modulation periods the system follows the given modulation of the driving, i.e. the behaviour of the system is quasi-stationary. For smaller modulation periods, the flow cannot follow the modulation, and the flow velocity responds with a phase delay and a smaller amplitude response to the given modulation. If we compare our results with numerical and theoretical results for the laminar case, we find that the scalings of the phase delay and the amplitude response are similar. However, the local response in the bulk of the flow is independent of the distance to the modulated boundary. Apparently, the turbulent mixing is strong enough to prevent the flow from having radius-dependent responses to the given modulation.

Direct Numerical Simulation of Head-On Quenching of Statistically Planar Turbulent Premixed Methane-Air Flames Using a Detailed Chemical Mechanism

A three-dimensional compressible Direct Numerical Simulation (DNS) analysis has been carried out for head-on quenching of a statistically planar stoichiometric methane-air flame by an isothermal inert wall. A multi-step chemical mechanism for methane-air combustion is used for the purpose of detailed chemistry DNS. For head-on quenching of stoichiometric methane-air flames, the mass fractions of major reactant species such as methane and oxygen tend to vanish at the wall during flame quenching. The absence of text {OH} at the wall gives rise to accumulation of carbon monoxide during flame quenching because text {CO} cannot be oxidised anymore. Furthermore, it has been found that low-temperature reactions give rise to accumulation of text {HO}_{2} and mathrm {H}_{2}mathrm {O}_{2} at the wall during flame quenching. Moreover, these low temperature reactions are responsible for non-zero heat release rate at the wall during flame-wall interaction. In order to perform an in-depth comparison between simple and detailed chemistry DNS results, a corresponding simulation has been carried out for the same turbulence parameters for a representative single-step Arrhenius type irreversible chemical mechanism. In the corresponding simple chemistry simulation, heat release rate vanishes once the flame reaches a threshold distance from the wall. The distributions of reaction progress variable c and non-dimensional temperature T are found to be identical to each other away from the wall for the simple chemistry simulation but this equality does not hold during head-on quenching. The inequality between c (defined based on text {CH}_{4} mass fraction) and T holds both away from and close to the wall for the detailed chemistry simulation but it becomes particularly prominent in the near-wall region. The temporal evolutions of wall heat flux and wall Peclet number (i.e. normalised wall-normal distance of T = 0.9 isosurface) for both simple and detailed chemistry laminar and turbulent cases have been found to be qualitatively similar. However, small differences have been observed in the numerical values of the maximum normalised wall heat flux magnitude left ({Phi }_{max } right )_{mathrm {L}} and the minimum Peclet number (Pe_{min })_{mathrm {L}} obtained from simple and detailed chemistry based laminar head-on quenching calculations. Detailed explanations have been provided for the observed differences in behaviours of left ({Phi }_{max }right )_{mathrm {L}} and (Pe_{min })_{mathrm {L}}. The usual Flame Surface Density (FSD) and scalar dissipation rate (SDR) based reaction rate closures do not adequately predict the mean reaction rate of reaction progress variable in the near-wall region for both simple and detailed chemistry simulations. It has been found that recently proposed FSD and SDR based reaction rate closures based on a-priori DNS analysis of simple chemistry data perform satisfactorily also for the detailed chemistry case both away from and close to the wall without any adjustment to the model parameters.

Direct numerical simulation study of hydrogen/air auto-ignition in turbulent mixing layer at elevated pressures

Auto-ignition of turbulent stratified mixing layer between hydrogen and hot air under elevated pressures p = 1–30 atm is studied using direct numerical simulations (DNS) in this work. Homogeneous isotropic turbulence is superimposed on the field. Detailed chemical mechanism and multicomponent diffusion model are employed. Other than turbulent mixing ignition (TMI), homogeneous mixing ignition (HMI) and laminar mixing ignition (LMI) are also investigated for comparison. For both laminar and turbulent cases, the onset of auto-ignition always happens at the same most reactive mixture fraction isosurfaces. Most reactive mixture fractions in diffusion auto-ignition are inconsistent with HMI calculations and shift to the rich side owing to diffusion for all pressures. At elevated pressures, auto-ignition chemistry is different from low pressures. The importance of H2O2 and HO2 is highlighted as radical sinks during the ignition process, and can also be used as an indicator for locating the ignition spots. Moreover, OH radicals can be used as a marker variable for the transition of auto-ignition to flame propagation under high pressures. Two stages are involved in the diffusion ignition process: radical explosion and thermal runaway. According to our study, under elevated pressures, turbulence has little influence on the radical explosion stage. The role of turbulence is to accelerate the thermal runaway stage in the kernels to make the ignition delay time (IDT) shorter than laminar cases.

A parametric study of laminar and transitional oblique shock wave reflections

High-resolution particle image velocimetry measurements were performed on laminar and transitional oblique shock wave reflections for a range of Mach numbers ($M=1.6{-}2.3$), Reynolds numbers ($Re_{x_{sh}}=1.4\times 10^{6}{-}3.5\times 10^{6}$) and flow deflection angles ($\unicode[STIX]{x1D703}=1^{\circ }{-}5^{\circ }$ or $p_{3}/p_{1}=1.11{-}1.64$). The laminar interactions revealed a long, flat and triangular shaped separation bubble. For relatively strong interactions ($p_{3}/p_{1}>1.2$), the bubble grows linearly in the upstream direction with increasing shock strength. Under these conditions, the boundary layer keeps an on average laminar velocity profile up to the shock impingement location, followed by a quick transition and subsequent reattachment of the boundary layer. For weaker interactions ($p_{3}/p_{1}<1.2$), the boundary layer is able to remain laminar further downstream of the bubble, which consequently results in a later reattachment of the boundary layer. The pressure distribution at the interaction onset for all laminar cases shows excellent agreement with the free-interaction theory, therefore supporting its validity even for incipiently separated laminar oblique shock wave reflections.

A dissipation-based method for improving the accuracy of computational fluid dynamics simulations of high level non-Newtonian wastes

This paper investigates a new viscosity model for laminar, transitional, and turbulent pipe flow simulations of a non-Newtonian slurry using the Star-CCM+ software. Herschel-Bulkley (HB) rheology equation was used in its original and modified forms to model the fluid viscosity. The realizable k-ε model was used for modeling of turbulence. It was observed that simulations using the HB model predicted inaccurate profiles of U (mean velocity) and U+ (mean velocity in wall units). Therefore, we developed alternative values of viscosity obtained directly from non-linearity of stress–strain variation (α) in each iteration. The developed method, “Alpha method”, applied changes in places where dissipation rate of turbulent kinetic energy occurred and was designed based on local or global dissipation rate values. Inverse values of α were used to test effects of more viscosity reduction for all flow regimes. In prediction of the mean velocities, significant improvements were obtained through global adjustment of viscosity in inverse form during simulation of the laminar and turbulent flow cases. For the transitional flow, obtaining improvements in prediction of U and U+ from the same model version faced challenges. While the global adjustments in direct form were found significantly effective in improving the prediction of U, they failed to predict the U+ accurately. For this purpose, Local viscosity adjustments were found significantly effective.

Modeling Inspiratory Flow in a Porcine Lung Airway.

Inspiratory flow in a multigeneration pig lung airways was numerically studied at a steady inlet flow rate of 3.2 × 10-4 m3/s corresponding to a Reynolds number of 1150 in the trachea. The model was validated by comparing velocity distributions with previous measurements and simulations in simplified airway geometries. Simulation results provided detailed maps of the axial and secondary flow patterns at different cross sections of the airway tree. The vortex core regions in the airways were visualized using absolute helicity values and suggested the presence of secondary flow vortices where two counter-rotating vortices were observed at the main bifurcation and in many other bifurcations. Both laminar and turbulent flows were considered. Results showed that axial and secondary flows were comparable in the laminar and turbulent cases. Turbulent kinetic energy (TKE) vanished in the more distal airways, which indicates that the flow in these airways approaches laminar flow conditions. The simulation results suggested viscous pressure drop values comparable to earlier studies. The monopodial asymmetric nature of airway branching in pigs resulted in airflow patterns that are different from the less asymmetric human airways. The major daughters of the pig airways tended to have high airflow ratios, which may lead to different particle distribution and sound generation patterns. These differences need to be taken into consideration when interpreting the results of animal studies involving pigs before generalizing these results to humans.