Local Reynolds Number Research Articles

Self‐similar turbulent boundary layer in pressure gradient

AbstractSelf‐similar flows in a turbulent boundary layer when the free‐stream velocity is specified as a power function of longitudinal coordinate are investigated. The self‐similar formulation not only simplifies solving of the problem by reducing the equations of motion to ordinary differential equations but also provides a mean for formulating closure conditions. It is shown that for the class of flows under consideration that depend on three governing parameters the dimensionless mixing length is a function of the normalised distance from the wall and the exponent in the law specifying the free‐stream velocity distribution in the outer region and a universal function of local Reynolds number in the wall region, the latter corollary being true even when the skin friction vanishes. In calculations this function is set to be independent of pressure gradient, which gives the results very close to experimental data.There exist four different self‐similar flow regimes. Each regime is related to its similarity parameter, one of which is the well‐known Clauser equilibrium parameter and the other three are established for the first time. In case of adverse pressure gradient when the exponent lies within certain limits, which depend on Reynolds number, the problem has two solutions with different values of the boundary layer thickness and skin friction, which points out the possibility of hysteresis in near‐separating flow. Separation occurs not at the minimum value of the exponent that corresponds to the strongest adverse pressure gradient but at a higher one whose dependence on Reynolds number is calculated in the paper. The results of the theory are in good agreement with experimental data. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Model for laminar and turbulent flows of viscous and nonlinear viscous non-Newtonian fluids

We develop a model that describes laminar and turbulent flows of viscous and nonlinear viscous fluids. A basis for the model is a characteristic, which we call a local Reynolds number, and which is calculated at each point of the domain of flow. In the subdomain wherein the local Reynolds number does not exceed some value, the flow is laminar; otherwise it is turbulent, i.e., the model identifies the areas of laminar and turbulent flows. The model predicts the existence of a laminar boundary layer at turbulent flows. It enables us to describe special features of turbulent flows such as a drastic increase in the resistance to flow and the variation of the velocity profile with the increase of the Reynolds number, and so on. The model is mathematically grounded. We prove the existence of global solutions to stationary and nonstationary flow problems with the nonhomogeneous Dirichlet and mixed boundary conditions, where velocities and surface forces are given on different parts of the boundary. Numerical methods for solving stationary and nonstationary flow problems are investigated.

The Lighthill–Weis-Fogh clap–fling–sweep mechanism revisited

The Lighthill–Weis-Fogh ‘clap–fling–sweep’ mechanism for lift generation in insect flight is re-examined. The novelty of this mechanism lies in the change of topology (the ‘break’) that occurs at a critical instanttcwhen two wings separate at their ‘hinge’ point as ‘fling’ gives way to ‘sweep’, and the appearance of equal and opposite circulations around the wings at this critical instant. Our primary aim is to elucidate the behaviour near the hinge point as timetpasses throughtc. First, Lighthill's inviscid potential flow theory is reconsidered. It is argued that provided the linear and angular accelerations of the wings are continuous, the velocity field varies continuously through the break, although the pressure field jumps instantaneously att=tc. Then, effects of viscosity are considered. Near the hinge, the local Reynolds number is very small and local similarity solutions imply a logarithmic (integrable) singularity of the pressure jump across the hinge just before separation, in contrast to the ‘negligible pressure jump’ of inviscid theory invoked by Lighthill. We also present numerical simulations of the flow using a volume penalization technique to represent the motion of the wings. For Reynolds number equal to unity (based on wing chord), the results are in good agreement with the analytical solution. At a realistic Reynolds number of about 20, the flow near the hinge is influenced by leading-edge vortices, but local effects still persist. The lift coefficient is found to be much greater than that in the corresponding inviscid flow.

The Lighthill–Weis-Fogh clap–fling–sweep mechanism revisited

The Lighthill–Weis-Fogh ‘clap–fling–sweep’ mechanism for lift generation in insect flight is re-examined. The novelty of this mechanism lies in the change of topology (the ‘break’) that occurs at a critical instant tc when two wings separate at their ‘hinge’ point as ‘fling’ gives way to ‘sweep’, and the appearance of equal and opposite circulations around the wings at this critical instant. Our primary aim is to elucidate the behaviour near the hinge point as time t passes through tc. First, Lighthill's inviscid potential flow theory is reconsidered. It is argued that provided the linear and angular accelerations of the wings are continuous, the velocity field varies continuously through the break, although the pressure field jumps instantaneously at t = tc. Then, effects of viscosity are considered. Near the hinge, the local Reynolds number is very small and local similarity solutions imply a logarithmic (integrable) singularity of the pressure jump across the hinge just before separation, in contrast to the ‘negligible pressure jump’ of inviscid theory invoked by Lighthill. We also present numerical simulations of the flow using a volume penalization technique to represent the motion of the wings. For Reynolds number equal to unity (based on wing chord), the results are in good agreement with the analytical solution. At a realistic Reynolds number of about 20, the flow near the hinge is influenced by leading-edge vortices, but local effects still persist. The lift coefficient is found to be much greater than that in the corresponding inviscid flow.

Experimental and Numerical Cross-Over Jet Impingement in an Airfoil Trailing-Edge Cooling Channel

Trailing edge cooling cavities in modern gas turbine airfoils play an important role in maintaining the trailing-edge temperature at levels consistent with airfoil design life. In this study, local and average heat transfer coefficients were measured in a test section, simulating the trailing-edge cooling cavity of a turbine airfoil using the steady-state liquid crystal technique. The test rig was made up of two adjacent channels, each with a trapezoidal cross-sectional area. The first channel, simulating the cooling cavity adjacent to the trailing-edge cavity, supplied the cooling air to the trailing-edge channel through a row of racetrack-shaped slots on the partition wall between the two channels. Eleven crossover jets issued from these slots entered the trailing-edge channel and exited from a second row of race-track shaped slots on the opposite wall in staggered or inline arrangement. Two jet angles were examined. The baseline tests were for zero angle between the jet axis and the trailing-edge channel centerline. The jets were then tilted toward one wall (pressure or suction side) of the trailing-edge channel by 5 deg. Results of the two set of tests for a range of local jet Reynolds number from 10,000 to 35,000 were compared. The numerical models contained the entire trailing-edge and supply channels with all slots to simulate exactly the tested geometries. They were meshed with all-hexa structured mesh of high near-wall concentration. A pressure-correction based, multiblock, multigrid, unstructured/adaptive commercial software was used in this investigation. Standard high Reynolds number k−ε turbulence model in conjunction with the generalized wall function for most parts was used for turbulence closure. Boundary conditions identical to those of the experiments were applied and several turbulence model results were compared. The numerical analyses also provided the share of each cross-over and each exit hole from the total flow for different geometries. The major conclusions of this study were (a) except for the first and last cross-flow jets which had different flow structures, other jets produced the same heat transfer results on their target surfaces, (b) jets tilted at an angle of 5 deg produced higher heat transfer coefficients on the target surface. The tilted jets also produced the same level of heat transfer coefficients on the wall opposite the target wall, and (c) the numerical predictions of impingement heat transfer coefficients were in good agreement with the measured values for most cases; thus, computational fluid dynamics could be considered a viable tool in airfoil cooling circuit designs.

Simulation of low pressure impactor collection efficiency curves

The collection efficiency of the low pressure impactors has been studied using numerical simulations. Flow field was modeled by solving the equations describing the time-average flow field (RANS) with a commercial CFD solver. Particle tracks were calculated separately using Lagrangian methods. Simulation results were verified against published experimental results. Effect of turbulent velocity fluctuations on the impactor resolution was investigated by comparing the ratio of the simulated to experimental impactor resolutions as a function of the turbulence level of the jet. It was found that the turbulence is the dominant mechanism reducing the resolution when the local Reynolds number is over 1800. Effect of jet-to-plate distance on the resolution of the low pressure impactor was studied in the case of low turbulence level. Highest resolution was achieved when the ratio of jet diameter to jet to plate distance ( S/ W) is 2. When the ratio is lower or higher, resolution is reduced because there is an increase in nonuniformity of the impaction conditions across the jet.

Parametric study on the effect of end walls on heat transfer and fluid flow across a micro pin-fin

Micro heat sinks have a broad applicability in many fields such as aerospace applications, micro turbine cooling, micro reactors, electronics cooling, and micro biological applications. Among different types of micro heat sinks, those with micro pin-fins are becoming popular due to their enhanced heat removal performance. However, relevant experimental data in current literature is still scarce to adequately explain their differences from their macro size counter parts. In previous studies in literature, it was shown that thermal and hydrodynamic characteristics of micro pin-fin heat sinks are strongly affected by height over diameter ( H/ D) ratio of pin-fins. To address the lack of information about this subject, the objective of this work is to show how velocity boundary layer around pin-fins and consequently, the thermal and hydrodynamic characteristics are affected when H/ D ratio and local Reynolds number ( Re) vary. To investigate end wall effects, a small portion of a typical micro pin-fin heat sink is modeled. This portion is represented by a simplified model, which consists of a single pin-fin positioned in a rectangular micro channel. This approach simplified the micro heat sink, which is simulated for only half of it by using a symmetry plane. Moreover, the transverse channel walls are kept as close as the minimum distance (1.5 D) between pin-fins available in the literature. In this paper, the pin-fin height over diameter ratio, H/ D, varies from 0.5 to 5, while Reynolds number and heat flux provided from the fluid interacting surfaces of the micro pin-fin are in the range of 20 ≤ Re ≤ 150 and 100 ≤ q in (W/cm 2) ≤ 500, respectively. In this research, micro pin-fin heat sinks are three dimensionally modeled on a one-to-one scale with the use of commercially available software COMSOL Multiphysics 3.5a. Full and temperature dependent Navier–Stokes equations subjected to compressibility and energy equations are solved under steady state conditions. In order to validate the use of numerical models, simulation results are compared against theoretical predictions. The numerical results and theoretical predictions show a good agreement. After this validation, parametric analysis is performed using the three dimensional model developed with COMSOL Multiphysics 3.5a. The end wall effects are quantified, and this amount decreases with Re and H/ D. It is revealed that end walls play an important role on the total fluidic force acting on the micro pin-fin and on the heat transfer coefficients. Moreover, the trends in the amount of end walls effects, the ratio of viscous over total forces on the pin-fin, friction factors, and Nusselt numbers change at various critical Reynolds numbers. It is also demonstrated that increasing H/ D ratio leads to a less stable flow, higher fluidic forces on the micro pin-fin with an increased partial role of viscous forces relative to pressure forces, smaller friction factors, and higher heat transfer coefficients. There are maxima and minima in Nusselt number profiles for different H/ D ratios. It is found that increasing Re has a positive role in Nusselt numbers, as well as a parallel effect with H/ D on fluidic forces on micro pin-fin, friction factors, and heat transfer coefficients. Different than the effect of H/ D, Re decreases the partial role of viscous forces relative to pressure forces.

Revisiting the resolution requirements for turbulence simulations in nuclear heat transfer

Direct numerical simulations (DNSs) require the resolution of all relevant turbulence scales in space and time, whereas large eddy simulations (LESs) need only to resolve the dominant energy carrying large scales. Important influences from physics and numerics on the small-scale resolution are discussed. Quantitative criteria for turbulent flows are re-evaluated. Experience shows, resolving the microscales is usually by far not achieved in DNS; this is less relevant than the adequate resolution of the anisotropic coherent fine flow structures. These structures depend on the flow type, so that general criteria cannot be given. Resolving the large scales is a serious problem. When the computational domain covers only part of the flow domain, the large-scale resolution is coupled to the artificial boundary conditions for open boundaries. Each measure and criteria have to be carefully considered to ensure that the simulations meet the expectations. Special emphasis is given to liquid metal flows because related nuclear applications are often in the transition range between LES and DNS of the temperature field. A new model is given to predict local turbulent Prandtl numbers for subgrid scale heat flux modeling. It covers the required most important influences: local resolution, a local turbulence parameter, and Reynolds and Prandtl numbers.

Universal threshold of the transition to localized turbulence in shear flows

Experimental and numerical studies have shown similarities between localized turbulence in channel and pipe flows. By scaling analysis of a disturbed-flow model, this paper proposes a local Reynolds number ReM to characterize the threshold of transition triggered by finite-amplitude disturbances. The ReM represents the maximum contribution of the basic flow to the momentum ratio between the nonlinear convection and the viscous diffusion. The lower critical ReM observed in experiments of plane Poiseuille flow, pipe Poiseuille flow and plane Couette flow are all close to 323, indicating the uniformity of mechanism governing the transition to localized turbulence.

On the role of copepod antennae in the production of hydrodynamic force during hopping

We integrate high-resolution experimental observations of a freely hopping copepod with three-dimensional numerical simulations to investigate the role of the copepod antennae in production of hydrodynamic force during hopping. The experimental observations revealed a distinctive asymmetrical deformation of the antennae during the power and return strokes, which lead us to the hypothesis that the antennae are active contributors to the production of propulsive force with kinematics selected in nature in order to maximize net thrust. To examine the validity of this hypothesis we carried out numerical experiments using an anatomically realistic, tethered, virtual copepod, by prescribing two sets of antenna kinematics. In the first set, each antenna moves as a rigid, oar-like structure in a reversible manner, whereas in the second set, the antenna is made to move asymmetrically as a deformable structure as revealed by the experiments. The computed results show that for both cases the antennae are major contributors to the net thrust force during hopping, and the results also clearly demonstrate the significant hydrodynamic benefit in terms of thrust enhancement and drag reduction derived from the biologically realistic, asymmetric antenna motion. This finding is not surprising given the low local Reynolds number environment within which the antenna operates, and points to striking similarities between the copepod antenna motion and ciliary propulsion. Finally, the simulations provide the first glimpse into the complex, highly 3-D structure of copepod wakes.

Formation of vortices in a combined pressure-driven electro-osmotic flow through the insulated sharp tips under finite Debye length effects

Formation of vortices in an electro-osmotic flow possesses engineering applications in enhancing and controlling the microfluidic mixing. In this paper, we investigate the combined pressure-driven electro-osmotic flow through the insulated sharp tips in a straight microchannel when a direct current electric field is imposed. Maximum vorticity generated near the tip back is influenced by the local Reynolds number and tip sharpness. Under a finite Debye length, the way to control the recirculation region for the single tip is discussed. Such control is applied to a pair of sharp tips which are designed as the symmetrical and asymmetrical ones in shape, or the ones staggered in position. Poisson–Boltzmann model is solved to simulate the flow by the finite element method. Results are shown to support the assumption of finite Debye length and expected to be helpful in controlling the vorticity and recirculation in the relevant microfluidic devices.

Intermittency of near‐bottom turbulence in tidal flow on a shallow shelf

The higher‐order structure functions of vertical velocity fluctuations (transverse structure functions (TSF)) were employed to study the characteristics of turbulence intermittency in a reversing tidal flow on a 19 m deep shallow shelf of the East China Sea. Measurements from a downward‐looking, bottom‐mounted Acoustic Doppler Velocimeter, positioned 0.45 m above the seafloor, which spanned two semidiurnal tidal cycles, were analyzed. A classical lognormal single‐parameter (μ) model for intermittency and the universal multifractal approach (specifically, the two‐parameter (C1 and α) log‐Levy model) were employed to analyze the TSF exponent ξ(q) in tidally driven turbulent boundary layer and to estimate μ, C1, and α. During the energetic flooding tidal phases, the parameters of intermittency models approached the mean values of ≈ 0.24, 1 ≈ 0.15, and ≈ 1.5, which are accepted as the universal values for fully developed turbulence at high Reynolds numbers. With the decrease of advection velocity, μ and C1 increased up to μ ≈ 0.5–0.6 and C1 ≈ 0.25–0.35, but α decreased to about 1.4. The results explain the reported disparities between the smaller “universal” values of intermittency parameters μ and C1 (mostly measured in laboratory and atmospheric high Reynolds number flows) and those (μ = 0.4–0.5) reported for oceanic stratified turbulence in the pycnocline, which is associated with relatively low local Reynolds numbers Rλw. The scaling exponents ξ(2) of the second‐order TSF, relative to the third‐order structure function, was also found to be a decreasing function of Rλw, approaching the classical value of 2/3 only at very high Rλw. A larger departure from the universal turbulent regime at lower Reynolds numbers could be attributed to the higher anisotropy and associated intermittency of underdeveloped turbulence.

Estimation of instantaneous local heat transfer coefficient in spark-ignition engines

Optimizing heat transfer for internal combustion engines requires application of advanced development tools. In addition to experimental method, numerical 3D-CFD calculations are needed in order to obtain an insight into the complex phenomenas in-cylinder processes. In this context, fluid flow and heat transfer inside a four-valve engine cylinder is modeled and effects of changing engine speed on dimensionless parameters, instantaneous local Nusselt number and Reynolds number near the surface of combustion chamber are studied. Based on the numerical simulation new correlations for instantaneous local heat transfer on the combustion chamber of SI engines are derived. Results for several engine speeds are compared for total heat transfer coefficient of the cylinder engine with available correlation proposed by experimental measurements and a close agreement is observed. It is found that the local value of heat transfer coefficient varies considerably in different parts of the cylinder, but it has equivalent trend with crank angle position.

Key Factors in Determining the Magnitude of Vorticity in Turbulent Plane Wakes

We examine the effects of local turbulence Reynolds number Rλ and inflow conditions on the magnitude of vorticity in plane turbulent wakes. Measurements of the spanwise component (ω3) of the fluctuating vorticity vector ω = ω1i + ω2j + ω3k (here the subscripts 1, 2 and 3 denote the streamwise, lateral and spanwise directions, respectively) are made in turbulent wakes of a screen and a circular cylinder. Lateral distributions of ω3* (normalized) in general depend on both Rλ and inflow conditions. In the developing region, as the downstream distance x1 increases, ω3* increases significantly in the screen wake but decreases slightly in the cylinder wake. Far downstream in the self-preserving region, ω3* increases linearly with Rλ while it no longer varies with x1 and depends weakly on the influence of inflow conditions. Our analysis suggests that these findings from the measurement of ω3* should apply for the magnitude of ω.

An Experimental and Numerical Investigation of Impingement Heat Transfer in Airfoils Leading-Edge Cooling Channel

To enhance the heat transfer coefficient along the leading edge of an airfoil, the cooling flow enters the leading edge cavity from the adjacent cavity through a series of crossover holes, cast on the partition wall between the two cavities. The crossover jets impinge on the leading-edge wall, then form a crossflow that moves towards the airfoil tip. In this experimental setup, there were nine crossover holes with racetrack-shaped cross sections on the partition wall. To investigate the effects of crossflow created by the upstream jets on the flow through each hole and on the impingement heat transfer coefficients, five crossover flow arrangements were studied. These flow arrangements were for 0, 1, 2, 3, and 4 jets upstream of the crossover hole # 5 for which the impingement heat transfer coefficients were measured. Jet to target wall distance ratio, Z/Dh, of 2.81 and local jet Reynolds numbers ranging from 7000 to 32,000 were tested. All tested geometries were meshed with all-hexa structured mesh of high near-wall concentration. Boundary conditions identical to those of experiments were applied and several turbulence model results were compared. The major conclusions of this study were: a) the crossflow produced by the upstream jets caused a slight reduction in impingemen heat transfer coefficients, b) there could be a significant variation in mass flow rate through the crossover holes, and c) the numerical predictions using the standard high Reynolds number k−ε turbulence model along with the generalized wall function, were in good agreement with the measured values for most cases, thus CFD could be considered a viable tool in airfoil cooling circuit designs.

Investigating the effects of the important parameters on magnetohydrodynamics flow and heat transfer over a stretching sheet

Sheet extrusion is a technique for making flat plastic sheets. Thermoplastic sheet production is a significant sector of plastics processing. In order to improve the physical properties of the sheet, it is consequential to study heat and fluid flow over the sheet as it solidifies. To understand the process, in the present work, the two-dimensional steady-state incompressible viscoelastic boundary layer magnetohydrodynamics flow and heat transfer over a stretching sheet have been studied in the presence of electric and magnetic fields. The highly non-linear momentum and heat transfer equations are solved analytically using the homotopy analysis method. The effect of various physical parameters including the viscoelastic parameter, the Prandtl number, the local Reynolds number, the Eckert number, the Hartmann number, and the local electric parameter on momentum and heat transfer characteristics have been studied. Ultimately some results could be found from this analytical treatment, such as the following: by increasing the values of the local Reynolds number and the electric parameter, the value of the skin friction coefficient is decreased. If the fluid is viscoelastic, the electric field is present, and the Prandtl number is lower, then there would be an obvious decrease of temperature near the boundary sheet. The other result is in the presence of the magnetic field, the effect of the electric field is to decrease the temperature near the stretching boundary.

Upstream condition effects on the evolution of symmetric and asymmetric near-wakes of a flat plate

An experimental study was conducted in order to investigate the influence of upstream conditions with respect to near-wakes evolution by using a two-dimensional flat plate. A position of x B . L / C = 0.98 for the flat plate with a blunt trailing edge was settled upon as an upstream condition point. Three upstream conditions (i.e., natural laminar, bypass transitional, and turbulent boundary layers) were applied by tripping wires that were placed on the upper surface of the plate. The free-stream velocity was 6.0 m/s and the local Reynolds numbers based on momentum thickness at the upstream conditions point were 259, 303, and 331. The boundary layers and the near-wakes were measured by hot-wire anemometry. It was observed that for the symmetric near-wakes, the local similarity profiles of the mean defect velocity matched well with the traditional similarity profile in the center part, whereas some difference in the outer part occurred due to the different trailing edge shape. For the asymmetric upstream conditions, the center of the local similarity profiles moved to one side, and the amplitude of the local similarity profiles over the wakes edge varied according to the momentum thickness of the upstream conditions. Also, the collapse of the normalized Reynolds shear stress depended on turbulent quantities included in the upstream conditions, and these results showed that the evolution of the near-wakes is clearly affected by upstream conditions.

The velocity spectra and turbulence length scale distributions in the near to intermediate regions of a round free turbulent jet

Stationary and flying single hot-wire measurements were made to investigate the near to intermediate field in a round free turbulent jet. Measurements were carried out at Reynolds numbers ReD based on the jet exit mean velocity and the nozzle diameter, between 6000 and 100 000. The objective of this study was to investigate the concept of a mixing transition proposed by Dimotakis [“The mixing transition in turbulent flows,” J. Fluid Mech. 409, 69 (2000)]. This was done through the measurement of the velocity spectra and the calculation of Kolmogorov λK, Taylor microscale λT, the Liepmann–Taylor microscale λL, and the viscous length scale λν at different positions in the near to intermediate regions of a round free jet. The present results demonstrate the local nature of mixing transition. It is found that the Kolmogorov, Taylor, and viscous length scales all decrease in magnitude with the local Reynolds number Reδ based on the local center line mean velocity and the local time-averaged diameter of the jet and appear to be only weakly dependent on the radial position, although the ratio ReT∕Reδ1∕2 varies nonlinearly across the jet radius. The ratio of the laminar to viscous length scale exceeds unity beyond the potential core of the jet. Moreover, the skewness of the axial velocity is approximately −0.4 over 30<ReT<400.

Modelling of the fin type heat exchanger for the HTS current leads of W7-X and JT-60SA

The Forschungszentrum Karlsruhe has taken over the responsibility for the design, construction and testing of the high temperature superconductor (HTS) current leads for two fusion experiments, i.e. the stellarator Wendelstein 7-X (W7-X) and the satellite tokamak JT-60SA. One important task for the design of the HTS current lead is the heat exchanger (HEX). In the current leads for W7-X and JT-60SA the HEX consists of the central conductor with meander flow fins to achieve a cross flow HEX. A design optimisation requires the knowledge of the heat transfer characteristics of the HEX. Therefore, 3D CFD simulations were performed to compute local Nusselt and Reynolds numbers. From the CFD results a 1D system code description and heat transfer correlations have been deduced. This paper describes the 3D-CFD and 1D system code as well as the results of the numerical calculations. The results were validated using experimental results of HEX mock-ups. The model was extended to the HEX covering the temperature range between 60 K and room temperature leading to a single Nusselt–Reynolds number correlation. Finally the parameters for optimised HTS current leads for W7-X and JT-60SA are presented.

Time-resolved velocity and concentration measurements in variable-viscosity turbulent jet flow

This paper addresses the ability to reliably measure the fluctuating velocity field in variable-viscosity flows (herein, a propane–air mixture), using hot-wire anemometry. Because the latter is sensitive to both velocity and concentration fluctuations, the instantaneous concentration field also needs to be inferred experimentally. To overcome this difficulty, we show that the hot-wire response becomes insensitive to the concentration of the field, when a small amount of neon is added to the air. In this way, velocity measurements can be made independently of the concentration field. Although not necessary to velocity measurements, Rayleigh light-scattering technique is also used to infer the local (fluctuating) concentration, and, therefore, the viscosity of the fluid. Velocity and concentration measurements are performed in a turbulent propane jet discharging into an air–neon co-flow, for which the density and viscosity ratios are 1.52 and 1/5.5, respectively. The Reynolds number (based on injection diameter and velocity) is 15400. These measurements are first validated: the axial decay of the mean velocity and concentration, as well as the lateral mean and RMS profiles of velocity and concentration, is in full agreement with the existing literature. The variable-viscosity flow along the axis of the round jet is then characterized and compared with a turbulent air jet discharging into still air, for which the Reynolds number (based on injection diameter and velocity) is 5400. Both flows have the same initial jet momentum. As mixing with the viscous co-flow is enhanced with increasing downstream position, the viscosity of the fluid increases rapidly for the case of the propane jet. In comparison with the air jet, the propane jet exhibits: (1) a lower local Reynolds number based on the Taylor microscale (by a factor of four); (2) a reduced range of scales present in the flow; (3) the isotropic form of the mean energy dissipation rate is first more enhanced and then drastically diminishes and (4) a progressively increasing local Schmidt number (from 1.36 to 7.5) for increasing downstream positions. Therefore, the scalar spectra exhibit an increasingly prominent Batchelor regime with a ~ k −1 scaling law. The experimental technique developed herein provides a reliable method for the study of variable-viscosity flows.