Turbulent transport in the transition flow region

The present author's model of turbulent viscosity published in Int. J. Heat Mass Transfer26, pp. 479–508, 1983 was applied to analyse the conditions in the transition region between laminar and developed turbulent pipe flow, making use of the Rothfus' experimental data from Chem. Eng. Prog.4, pp. 533–538, 1953. Theoretical value of the critical Reynolds number Re_crit = 2295.18 for transition from laminar to turbulent pipe flow was determined from the limit values of turbulent transport characteristics.     

Turbulent transport of passive scalar behind line sources in an unstably stratified open channel flow

This study employs a direct numerical simulation (DNS) technique to study the flow, turbulence structure, and passive scalar plume transport behind line sources in an unstably stratified open channel flow. The scalar transport behaviors for five emission heights (z_s = 0, 0.25H, 0.5H, 0.75H, and H, where H is the channel height) at a Reynolds number of 3000, a Prandtl number and a Schmidt number of 0.72, and a Richardson number of −0.2 are investigated. The vertically meandering mean plume heights and dispersion coefficients calculated by the current DNS model agree well with laboratory results and field measurements in literature. It is found that the plume meandering is due to the movement of the positive and negative vertical turbulent scalar fluxes above and below the mean plume heights, respectively. These findings help explaining the plume meandering mechanism in the unstably stratified atmospheric boundary layer.     

Heat transfer in a supersonic flow

At present it is commonly assumed that some peculiar surface exists which bounds a viscous region around a body in a supersonic flow. This region is identified with a laminar or turbulent Prandtl layer, the process of transition of visible motion into heat being described by the procedures developed by Prandtl and von Kármán for subsonic flows.

In the present work another approach is used based on the ideas of Osborne Reynolds and the so-called resolution equation where transition of thermal motion into pulsating one is fixed which, in its turn establishes relationship between the Nusselt and Reynolds numbers. This is a power-law relationship in which the coefficients and power exponent may be pre-calculated based on simple considerations. This has been done in the present work.     

Turbulent flow in a composite channel

Numerical solutions for turbulent flow in a composite channel are presented. Here, a channel with a centered porous material is considered. The interface between the porous medium and the clear flow was assumed to have different transversal positions and the porous matrix was simulated with distinct permeabilities. Governing equations were discretized and solved for both domains making use of one unique numerical methodology. Increasing the size of the porous material pushes the flow outwards, increasing the levels of turbulent kinetic energy at the macroscopic interface. For high permeability media, a large amount of mechanical energy is converted into turbulence inside the porous structure.     

Turbulent pipe flow of Herschel-Bulkley fluids

A modified two-equation turbulence model is used to compute the turbulent flow of Herschel-Bulkley fluids in smooth pipes. Numerical results are presented for the fully-developed friction factor, and compared with existing empirical correlations. The model is used to produce flow resistance curves over a wide range of values for the power-law index and the generalised Reynolds and Hedstrom numbers.     

Turbulent flow with combustion in a moving bed

This paper presents a mathematical model for treating turbulent combusting flows in a moving porous bed, which might be useful to design and analysis of modern and advanced biomass gasification systems. Here, one explicitly considers the intra-pore levels of turbulent kinetic energy and the movement of the rigid solid matrix is considered to occur at a steady speed. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. The rate of fuel consumption is described by an Arrhenius expression involving the product of the fuel and oxidant mass fractions. Results indicate that fixing the gas speed and increasing the speed of the solid matrix pushes the flame front towards the end of the reactor. Also, since the rate of production of turbulence is dependent on the relative velocity between phases, as the solid velocity approaches that of the gas stream, the level of turbulence in the flow is reduced.     

Nonequilibrium hydrogen combustion in one- and two-phase supersonic flow

A time-splitting method for the numerical simulation of stiff nonequilibrium combustion problem was developed. The algorithm has been applied to simulate the shock-induced combustion and to investigate a supersonic one- and two-phase flowfield. The results are physically reasonable and demonstrate that the presence of particles has a dramatic effect on the nozzle flowfield and the the thrust.     

Turbulent mixing and molecular transport in highly under-expanded hydrogen jets

Highly under-expanded hydrogen jets releasing in quiescent air atmosphere are studied using highly resolved numerical simulations accounting for complex multicomponent molecular transport phenomena. In a first step of the analysis, the main overall features of the hydrogen jet structure are described and compared to those of the classical under-expanded air jet at the same nozzle pressure ratio (NPR). Even if the global flow topology remains quite similar in both cases (i.e., hydrogen and air discharges), the modification of both mean density and mean velocity gradients leads to different relative energy levels for each velocity component. The corresponding change of fluid properties mainly leads to an enhanced mixing at the jet periphery. In comparison to the air case, the turbulence development within the internal part of the under-expanded hydrogen jet surrounding the subsonic core also yields a different structure. While a significantly higher peak of streamwise turbulent stress is observed downstream of the reflected shock, the vorticity dynamics is dampened by viscous diffusion and velocity divergence (i.e., volumetric expansion) contributions. Then, the performance of the simplified Hirschfelder and Curtiss approximation of the multicomponent molecular diffusion phenomena is evaluated with respect to the detailed multicomponent transport representation, as deduced from the EGLIB library. The detailed representation of molecular phenomena is shown to have a significant influence on the estimated local levels of hydrogen mass flux, leading to a non-negligible alteration of the global jet structure.     

Turbulent large-scale structures in natural convection vertical channel flow

We analyzed a database of a direct numerical simulation of natural convection in a vertical channel. The flow is driven by a constant temperature difference imposed at the walls (Ra = 5.4 × 10⁵, Pr = 0.7). The averaged flow and turbulent statistics are in good agreement with previous direct numerical simulations reported in the literature. Contrary to forced convection flows, the fluctuations of the heat transfer rate are uncorrelated with the fluctuations of the wall shear stress, which exhibit a symmetric probability density function. At the low Rayleigh number considered, the large-scale structures, which consist mainly in two counter-rotating vortices, with sizes comparable to the separation of the walls, are responsible for the extreme fluctuations of the wall heat transfer rate. The occurrence and the averaged topology of these structures have been determined using a conditional sampling technique.     

Flow Patterns and Thermal Drag in Supersonic Duct Flow with Heating

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Numerical simulation on supersonic flow in High-Velocity Oxy-Fuel thermal spray gun

This paper analyzes the behaviour of coating particles as well as the gas flow both inside and outside of the High-Velocity Oxy-Fuel (HVOF) thermal spray gun by using a quasi-one-dimensional analysis and a numerical simulation. The HVOF gun in the present analysis is an axially symmetric convergent-divergent nozzle with the design Mach number of 2.0. From the present analysis, the distributions of velocity and temperature of the coating particles flying inside and outside of the HVOF gun are predicted. The velocity and temperature of the coating particles at the exit of the gun calculated by the present method agree well with the previous experimental results. Therefore, the present method of calculation is considered to be useful for predicting the HVOF gas and particle flows.     

The second-order boundary layer along a circular cylinder in supersonic flow

Tha sacond-ordar laminar boundary layar along a circular cylindar in suparsonic (low with and without surfaca mass transfar is studiad using tha mathod of matchad asymptotic axpansions. Spacifically, tha analysis obtains tha sacond-ordar tarms for tha prassura, tha shaar strass, and tha haat transfar at tha wall dua to transvarsa surfaca curvatura and displacamant. For tha flow naar tha laading adga of tha cylindar numarical rasults ara prasantad.Thara ara thraa sacond-ordar affacts, namaly dua to transvarsa curvatura, dua to displacamant, and dua to intaraction of curvatura and displacamant.For tha solid wall all thraa sacond-ordar affacts incraasa tha shaar strass and this rasult incraasas with incraasing wall tamparatura. In tha casa of haat transfar, tha displacamant affact vanishas, tha intaraction affact is always positiva (incraasing tha haat transfar), and tha transvarsa curvatura affact is positiva for small wall tamparaturas and nagativa for larga wall tamparaturas.Tha affact of mass flow at tha wall is ganarally m tha axpactad diraction; i.a. tha injaction of mass thickans tha boundary layar and incraasas sacond-ordar affacts wharaas wall suction, by axtracting mass from tha boundary layar, raducas tha importanca of displacamant and transvarsa curvatura. An axcaption to tha ganaral rula statad abova occurs at high wall tamparaturas.     

Turbulent flow in the near field of a round impinging jet

This paper reports on detailed measurements of the turbulent flow in the stagnation region of a single impinging jet issuing from a round pipe with diameter D and a length of 76D. The distance between the pipe exit and the flat impingement plate is 2D, and the Reynolds number (based on the bulk velocity and pipe diameter) is 2.3 × 10⁴. Mean velocity components and Reynolds stresses were determined by using a two-component LDA. A modified one-component LDA was used to perform near-wall measurements with minimum wall distances of approximately 40 μm. PIV measurements were taken in a small field of view (approximately 4 × 5 mm²) to study instantaneous reversals in the near wall region.     

Turbulent flow and heat transfer in discrete double inclined ribs tube

The turbulent heat transfer and flow resistance in an enhanced heat transfer tube, the DDIR tube, were studied experimentally and numerically. Water was used as the working fluid with Reynolds numbers between 15,000 and 60,000. The numerical simulations solved the three dimensional Reynolds-averaged Navier–Stokes equations with the standard k-ε model in the commercial CFD code, Fluent. The numerical results agree well with the experimental data, with the largest discrepancy of 10% for the Nusselt numbers and 15% for the friction factors. The heat transfer in the DDIR tube is enhanced 100 ～ 120% compared with a plain tube and the pressure drop is increased 170 ～ 250%. The heat transfer rate for the same pumping power is enhanced 30 ～ 50%. Visualization of the flow field shows that in addition to the front and rear vortices around the ribs, main vortices and induced vortices are also generated by the ribs in the DDIR tube. The rear vortex and the main vortex contribute much to the heat transfer enhancement in the DDIR tubes. Optimum DDIR tube parameters are proposed for heat transfer enhancement at the same pumping power.     

Hydrogen tube vehicle for supersonic transport: 5. Aerodynamic tunneling

Aerodynamic tunneling is the process of transporting a vehicle from terrestrial point A to B via a closed tube containing an atmosphere more aerodynamically favorable than air. Equations are derived for “gas efficacy,” Γ, a measure of how well a gas increases Mach-1 speed or decreases drag of a vehicle. Theoretical results, Γ_p and its Mach-normalized form Γ₁, based on reducing the vehicle to a flat plate, allows efficacy to be calculated ab initio as a function of only four gas parameters: ratio of specific heats, pressure, density, and viscosity. Hydrogen has the highest normalized gas efficacy (Γ₁ = 48.5 s/kg). Ammonia, hydrocarbon gases, and helium have above-average efficacies. Xenon has the lowest (10.1 s/kg). Binary mixtures of hydrogen and methane (or natural gas) are proposed for lowering the upper flammability limit at a relatively small penalty in efficacy.     

Hydrogen tube vehicle for supersonic transport: 4. Hydrogen propeller

A hydrogen propeller is the method of propulsion of a conceptual supersonic vehicle that operates within a hydrogen-filled tube at cruise speed of 1 km/s. Because Mach number governs formation of shock waves at the blade tips, the high sonic speed of hydrogen allows a rotational frequency 3.85 times faster than the same propeller operating in air immediately outside the tube. Rankine–Froude propulsive efficiency and – for a given vehicle Mach number–propeller pitch and helix angle are invariant with respect to the atmosphere. To achieve constant efficiency at a given thrust and for adequate acceleration, the low density of hydrogen requires some combination of higher frequency, more blades, or larger diameter. The hydrogen propeller conceptual design employs 14 contra-rotating blades, 4.11 m diameter, and rotational frequency of 40.4 s⁻¹ at translational velocity of 970 m/s.     

Hydrogen tube vehicle for supersonic transport: 3. Atmospheric merit

To compare 24 common gases as potential operating atmospheres for tube vehicles, equations are derived for aerodynamic (tunneling) performance of the atmosphere and molar energy density of the tube vehicle. Aerodynamic performance is a function of the speed of sound and Reynolds number, and energy density is a function of the free energy of reduction or oxidation of the tube gas and the stoichiometric coefficients of the stored reactants. The product of these two parameters determines the rank of atmospheric merit. Hydrogen exhibits the highest aerodynamic performance, yields the fourth highest energy density, and has the highest overall merit. Acetylene, ammonia, and methane, in decreasing order, follow hydrogen in merit. Although superficially a promising tube gas, helium is below average in merit because of low energy density of the vehicle.