Presentations

This study compares k–ω SST, BSL EARSM, and SSG RSM predictions of a three-dimensional turbulent boundary layer subject to localized spanwise wall motion. High-resolution LES provides the reference for assessing shape factor, spanwise wall shear stress, pressure, and turbulent kinetic energy. The comparison examines the accuracy and limitations of the models in transverse-shear flow.

This study examines the response of Venturi cavitation to upstream pressure perturbations using URANS multiphase simulations with the Schnerr–Sauer model. A baseline case is compared with time-varying inlet-pressure cases using pressure signals, space–time vapor-fraction diagrams, and FFT analysis to characterize cavity behavior, collapse location, and pressure-fluctuation frequencies.

An agentic AI workflow connects geometry modeling, three-dimensional meshing, mesh-quality checks, OpenFOAM setup, solver execution, and postprocessing for selectable damper orifices. Steady laminar simulations vary hole diameter, flow rate, and taper orientation. The abstract reports distinct effects on overall pressure drop and local minimum pressure, motivating their separate assessment in designs intended to suppress cavitation.

This study compares single-phase and cavitating oil flow through a 1 mm damper orifice. OpenFOAM interPhaseChangeFoam with the Schnerr–Sauer model represents oil–vapor flow using Mobil DTE 25 properties at 40°C. With inlet flow rate fixed, two downstream-pressure conditions are compared through vapor fraction, pressure distribution, and time-dependent pressure drop to assess cavitation effects.

This study evaluates the NASA CC3 centrifugal compressor when ammonia replaces air without a geometry change. Steady compressible RANS simulations in Ansys CFX use the SST k–ω model. Operating conditions are matched using tip Mach number and flow coefficient, with the air performance curve checked against experiments. Pressure ratio, efficiency, work coefficient, shocks, separation, and tip leakage are compared to examine similarity conditions and their limitations.

This study investigates the sensitivity of steady RANS predictions of water–vapor Venturi cavitation to Schnerr–Sauer model parameters. Nuclei number density, nuclei diameter, evaporation coefficient, and condensation coefficient are varied. Cavity length, vapor fraction, pressure drop, and local static pressure are assessed using one-factor sensitivity analysis and Latin hypercube sampling to examine parameter influence, interactions, and assumption-conditioned uncertainty.

For a 6-inch liquid-hydrogen globe valve, this study separates errors caused by heat-transfer-coefficient magnitude from errors caused by neglecting its spatial distribution. Three-dimensional RANS and thermal–structural coupling compare local CFD coefficients, an equal-area-average uniform coefficient, and a correlation-based coefficient on the same solid mesh. The reported dominance of coefficient magnitude depends on the thermal-resistance conditions; under external natural convection, external resistance limits both effects.

URANS simulations with SST k–ω and Schnerr–Sauer cavitation modeling examine periodic cloud-cavitation dynamics in a Venturi tube. Surface tension is included, and centerline velocity and static pressure are compared with published experiments. After the initial transient, time-resolved flow fields and space–time diagrams are used to describe cavity formation and collapse over a recurring flow cycle.

This study examines how near-wall turbulence time-scale treatment affects RANS predictions of a three-dimensional boundary layer induced by localized spanwise wall motion. DNS with digital-filter synthetic inflow provides the reference. OpenFOAM comparisons include standard eddy-viscosity and Reynolds-stress models, ω-based time scales, and mixed ω/ε treatments. Wall-motion increments in mean velocity, Reynolds stresses, and wall shear stress are compared with DNS.

LBM simulations resolve a positive-displacement hydrogen compressor with its cylinder, discharge valves, and outlet passages to examine turbulence and noise.

The study compares flow rate, separation, and turbulence kinetic energy for different globe-valve openings at the same total pressure drop.

RANS and LBM predictions for hydrogen flow in an elbow are compared with DNS, and the influence of bend radius is examined.

The study couples gaseous-hydrogen flow, heat conduction in the steel stack, and the external hydrogen concentration field.

Public compressor geometry is adapted into a cylinder, discharge-valve, and elbow model, and voxels are generated for LBM flow and noise analysis.

A nonuniform inlet dynamic-pressure profile derived from a boundary layer is applied to NASA Rotor 67, and blade loading and flow behavior are compared with a uniform-inlet case.

An unsteady VOF simulation process represents the complex pad/head geometry and rotation in a CMP system, with comparisons of slurry distribution and pressure across mesh resolutions.

RANS and SPH simulations examine internal flow and resistance for different relative positions of an elbow and a swirl generator.

GIS terrain data and VOF/DPM methods are used to model runoff on slopes and compare the effects of precipitation and ground absorption.

SST-based URANS and DDES are compared for separation, flow rate, pressure drop, and computational cost in a liquid-hydrogen globe valve.

The work develops a conjugate heat-transfer simulation process coupling liquid-hydrogen flow with conduction in the valve body and compares mesh resolutions.

Multiphase RANS simulations examine free-surface and turbulent flow around a human-body model as water level and velocity vary.

The talk covers CFD applications from DNS-based turbulence-structure analysis to hybrid RANS/LES and aeroacoustic analysis for turbomachinery.

The study examines a gas-turbine blade failure mechanism using material analysis, thermal-stress and vibration-mode analysis, and risk assessment.

Accurate prediction of thermal field in high pressure turbines is a critical aspect of aerodynamic and durability design. This is particularly true when the flow at turbine inlet exhibits large gradients in temperature, both radially and circumferentially. In other words, in the presence of hot streaks from the combustor. In the numerical study presented in this paper, coupled highfidelity eddy-resolving simulations of a combustor and a turbine are used to study the differences in the temperature profile at the exit of the first vane and the heat flux on the first blade, resulting from different positioning, or clocking, between the combustor fuel nozzles and turbine vanes. The resolved unsteadiness and turbulence from the combustor impacts mixing and secondary flow in the high pressure turbine. Temperature profiles from both actual combustor CFD simulations, as well as and modulated profiles with more pronounced variation, or pattern factor, are used at the turbine inlet. A threshold of the pattern factor that brings the benefit of clocking is identified. Clocking positioning between the combustor and vanes was studied for the most benefit.

In this paper, wall-resolved LES computations for a compressor cascade from Ecole Centrale de Lyon [1] are presented. A computational grid containing about 600 million computational cells was used in these simulations. This grid resolves the details of tripping strips used in the experiments, located near the leading edge of the blade on both suction and pressure sides. Endwall turbulent boundary layer at cascade inlet was measured to be at a momentum thickness based Reynolds number of about 7000 to 8000, with quite a bit of variation in the pitchwise direction. In order to avoid the cost of simulating the entire duct upstream of the cascade, and any auxiliary flat plate boundary layer simulations, the inlet fluctuations for LES computations were generated using digital filtering method for synthetic turbulence generation [27]. Turbulence statistics from a database of high fidelity eddy simulations of flat plate boundary layers (at similar Reynolds numbers) from KTH Royal Institute of Technology in Stockholm [28] were used to fully define the properties of the cascade inlet boundary layer. In this paper, time-averaged results from three LES computations for this configuration are presented – one with no inlet fluctuations at the cascade endwall at the domain inlet, and then two computations with inlet fluctuations and boundary layers at Re  of 7000 and 8183. These provide a sensitivity of LES predictions of corner separation in the cascade to the boundary layer thickness at cascade inlet. A comparison of these simulations with prior DDES (and RANS) simulations from UTRC [26], as well as existing LES results from Ecole Centrale de Lyon [12], allows to further the understanding of critical elements of the endwall flow physics. More specifically, it provides more insight into 1 1 Corresponding author medicg@utrc.utc.com which phenomena need to be sufficiently resolved (e.g. horseshoe vortex) in order to capture both the average behavior of the corner separation, as well as its unsteady dynamics. In addition, it provides new information which will help define best practice guidelines for the use of eddy simulations to resolve endwall features in compressors at off-design conditions.

Development of an axisymmetric turbulent boundary layer (ATBL) over a stationaryrotating-stationary cylinder was investigated using large-eddy simulation (LES) for a turbomachinery-relevant configuration. In this case, the rotating speed was set to twice the axial free-stream velocity, aΩ = 2U∞ , where a is the cylinder radius, and the inlet Reθ to 4060. The rotating cylinder creates significant turbulence production through a Taylor-Couette-type flow regime and its coupling with the boundary layer. The nearwall transverse flow propagates to the outer region and contributes to a transverse mean flow. As a result, the upstream two-dimensional (2D) boundary layer turns into a threedimensional (3D) ATBL. The LES results show that the strong smaller-scale turbulence near the rotating wall propagates toward the outer region of the downstream stationary region. Finally, flow simulation of the same configuration using the standard k-ω model was also conducted for comparison.

Transitional boundary layers feature long coherent motions of streamwise velocity fluctuation, u0 , in both the laminar and turbulent regions. In the former, Klebanoff streaks amplify and become seats for breakdown to turbulence. In the fully turbulent region, large-scale motions contribute appreciably to the turbulence energy and shear stresses. Direct numerical simulation (DNS) of boundary-layer bypass transition over a flat plate with a leading edge is performed. Instantaneous realizations of spatially and temporally resolved fields are stored in a database. Structure identification techniques are used to identify these coherent flow structures [Lee, Sung & Zaki, J. Fluid Mech. 819, 165-187 (2017)]. The inception rate, lifetime and amplification rate of Klebanoff streaks are evaluated in the laminar region, and conditional averaging is used to examine the early stages of streak formation. Structure identification and tracking is also used to study the inception of large-scale coherent motion in the nascent turbulent spots and fully turbulent boundary layer downstream.

It has been widely investigated that there is a substantial intermittency between high and low drag states in wallbounded shear flows. Recent experimental and computational studies in a turbulent channel flow have identified low-drag time intervals based on wall shear stress measurements. These intervals are a weak turbulence state characterized by low-speed streaks and weak streamwise vortices. In this study, the spatiotemporaldynamics of low-drag intervals in a turbulent boundary layer is investigated using experiments and simulations. The low-drag intervals are monitored based on the wall shear stress measurement. We show that near the wall conditionally-sampled mean velocity profiles during low-drag intervals closely approach that of a low-drag nonlinear traveling wave solution as well as that of the so-called maximum drag reduction asymptote. This observation is consistent with the channel flow studies. Interestingly, the large spatial stretching of the streak is very evident in the wall-normal direction during low-drag intervals. Lastly, a possible connection between the mean velocity profile during the low-drag intervals and the Blasius profile will be discussed.

The turbulent/non-turbulent (T/NT) interface in a transitional boundary layer is examined using direct numerical simulation (DNS) flow field. The interface is identified using the local vorticity, and its statistical properties are reported. Similarities between the turbulence spots and fully turbulent region are evaluated using conditional sampling. In addition, the modulation of the interface by largescale motion (LSM) is investigated with the aid of a streak detection algorithm.

The vorticity response to small-amplitude surface undulations in a viscoelastic Couette flow is examined. An integral measure is defined to quantify the depth of vorticity penetration into the bulk flow. Three regimes of penetration are identified, and each occupies a sector of a phase diagram parameterized by the ratios of the channel depth and of the critical-layer (CL) height to the lengthscale of the surface roughness. The CL emerges due to the propagation of vorticity waves along the tensioned mean-flow streamlines. The most intriguing behavior, the trans-critical regime, occurs when the CL lies within one wavelength from the wall. In trans-critical flows a kinematic mechanism associated with the relative realignment of the polymer by the mean shear amplifies the vorticity in ‘sheets’ around the CL. The effect is dominant at this location since the vorticity waves appear steady to an observer advecting with the mean flow.

Characteristics of the boundary separating the laminar and turbulent regions in a transitional boundary layer are examined using a time series of three-dimensional flow fields extracted from direct numerical simulations (DNS). In order to accurately mimic boundary-layer experiments perturbed by grid turbulence, the current simulation includes the leading edge of the flat plate and the incoming homogeneous isotropic turbulence. The Reynolds number based on the momentum thickness reaches up to 1400, and high-resolution three-dimensional flow fields of the DNS data will be publicly accessible via the Johns Hopkins Turbulence Database (JHTDB). The laminar-turbulence discrimination algorithm isolates the turbulence spots within the transition zone and the bounding surface of the fully-turbulent flow. Attention is placed on the cross-stream surface between the transition zone and fully-turbulent boundary layer. The shape of this interface is dictated by a balance between downstream advection, destabilization of upstream flow and merging of turbulence spots. Conditionally sampled statistics are examined across the interface, and are also compared to the downstream equilibrium turbulent boundary layer.

Direct numerical simulations (DNSs) of a fully developed turbulent channel flow with the slip and no-slip boundary conditions were performed to investigate the spatial characteristics of large-scale structures in the dragreducing flow and the effect of the slip wall on those structures. The slip boundary condition was used in the streamwise direction and the overall mean skin-friction coefficient was reduced by 35% relative to the channel flow with the no-slip condition at the same bulk Reynolds number of 10,333. The wall-normal variations of average streamwise and spanwise length scales of the channel flow with the slip condition are relatively greater than those with the no-slip condition and the difference is significant in the outer region (y/δ > 0.2). The populations of low- and highmomentum structures are mainly due to the large populations of low-momentum structures in the channel flow with the slip condition. Based on the conditional twopoint correlations, the average streamwise length scales of negative-u structures in the flow with the slip condition are larger than those with the no-slip condition in the outer layer. Conditionally averaged velocity fields present that the lowmomentum structures in the flow with the slip condition have the weak congregative motions and ejections, by which the near-wall streamwise velocity fluctuations are less affected.

A direct numerical simulation of turbulent pipe flow is performed at Re τ = 3008 with a long streamwise domain length (30R) to investigate wall scaling laws in the physical and wavenumber spaces. The power law follows the streamwise mean velocity in the overlap region, y + = 90 – 300, evaluated by the power law indicator function. The scale-separated Reynolds shear stress shows that the largescale motions (λ x + > 3000) are more responsible to construct the constant-stress layer than the small-scale motions (λ x + < 3000), and thus it is proposed that the large-scale motions more contribute to the growth of the mean velocity in the overlap region than the small-scale motions. In the premultiplied energy spectra of the streamwise velocity fluctuations, the k x -1 region ssociates with the attached eddies appeared in λ x /R = 2 – 5 at y + = 90 – 300 with the bimodal distribution. Linear growth of small-scale energies to large-scale energies helps to appear the k x –1 region at high Reynolds number.

The spatial and temporal formation of very-large-scale motions (VLSMs) is investigated using data from direct numerical simulation (DNS) of turbulent channel flow at a friction Reynolds number Re τ = 930. The physical structures of negative and positive streamwise velocity disturbances (u) are examined. The population and area fraction of the structures reveal that negative-u regions are organized into long streamwise extent as compared with positive-u regions whose streamwise length is dominantly less than 3δ in the log layer. The conditional correlations show that the footprint of negative-u structure is relatively narrower but longer than that of positive counterpart. This difference is due to the opposite spanwise motions in the vicinity of their footprints which are the congregative (negative-u) and dispersive (positive-u) motions in the near-wall region induced by the ejection and sweep events associated with the outer large-scale structures. Instantaneous perturbations fields show that near-wall streaks beneath the merging event undergo a spanwise congregative migration. The time series of conditionally-averaged velocity fields demonstrate that this congregative behavior is generated by a roll motion in the outer region, and enhances the magnitude of the nearwall low-speed streaks. This reduces downstream structure convection speed thus promoting the merging process and the formation of VLSMs. The present study confirms that the top-down and bottom-up interactions are the origin of the local difference in convection velocities of outer large-scale structures, and hence their merging into VLSMs.

The slip over a super-hydrophobic surface has an effect on the drag reduction in wall-bounded turbulent flows. The slip condition mimics a super-hydrophobic surface, which leads to drag reduction via weakened mean-shear rate on the surface. Direct numerical simulations (DNSs) of turbulent channel flows with the slip condition were performed to see the changes of large- and very-large-scale motions (LSMs and VLSMs) over the surface. For comparison, a DNS of turbulent channel flow with the no-slip condition was also carried out. The slip condition was chosen such that the drag reduction rate decreased by 35%. The Reynolds number Re H (≡ U CL H/ν) was 15,500, where U CL , H and ν are the laminar centerline velocity, the channel half-height and the kinematic viscosity, respectively. The drag reduction over a slip surface was examined numerically by Min & Kim (2004) [1]. Recently, Park et al. (2014) experimentally showed that the drag reduction was obtained up to 75% [2]. Although a little information on the slip flow structure was revealed by the pioneers, much research has focused on the turbulence statistics. Because the drag reduction is directly affected by the change of turbulence structures [3], the investigation of drag-reduced turbulent structures is of great importance in the field of the flow control. The objective of the present study, therefore, is to understand turbulent structures including LSMs and VLSMs and the mechanism of the drag reduction over the slip surface, by comparing with the turbulent channel flows with the no-slip condition. A very-long computational domain, which is larger than 30 times of H, should be adopted to describe the LSMs and VLSMs. The computation domain was 10πH × 2H × 3πH in the streamwise, wall-normal, and spanwise directions, respectively.

The relationship between large-scale motions (LSMs) and the shape of the turbulent/non-turbulent interface (TNTI) is examined using data from direct numerical simulation (DNS) of turbulent boundary layer (TBL) flow. The Reynolds number based on the momentum thickness and the free-stream velocity ranges from 1180 to 3500. Feature extraction techniques are used to identify cores of the large-scale motions in the perturbation fields. Since turbulence kinetic energy and enstrophy are different inside low- and high-speed LSMs, the wall-normal elevation of TNTI is correlated with the streamwise momentum of these structures. The large-scale crests and troughs of TNTI are matched to the locations of the wall-detached LSMs of low- and high-speed streaks, respectively. In addition, abrupt changes in turbulence statistics across the TNTI reported in previous studies are associated with population trends of the wall-detached LSMs near the TNTI.

Direct numerical simulation (DNS) of turbulent pipe flow at Re τ = 3008 with a very long streamwise domain length (Lx = 30R, R is a pipe radius) was performed to explore the wall scaling laws in the overlap region. The high-Re turbulent pipe flow was found not to follow a log law, but rather to follow a power law. By contrast, the high-Re turbulent channel flow (Re τ ≥ 2006) followed a log law. A mesolayer was observed in both the pipe and channel flows, in agreement with the power law. The retarded log law in the turbulent pipe flow was attributed to the presence of large-scale structures in the outer region of the pipe flow. These large-scale structures were more dominant in the turbulent channel flows than in the turbulent pipe flows. As the Reynolds number increased, the fluids transitioned from a power law to a log law. The development of large-scale structures in the pipe flow was slower than the corresponding development in the channel flow The proportion of largescale and very-large-scale motions (LSMs and VLSMs) was obtained in comparison with the low-Reynolds-number pipe flow at Re τ = 934.

Direct numerical simulation (DNS) data of a turbulent channel flow at Re τ =930 was scrutinized to investigate the formation of very large-scale motions (VLSMs) by merging of two large-scale motions (LSMs), aligned in the streamwise direction. We mainly focused on the supportive motions by the near-wall streaks during the merging of the outer LSMs. From visualization of the instantaneous flow fields, several low-speed streaks in the near-wall region were collected in the spanwise direction, when LSMs were concatenated in the outer region. The magnitude of the streamwise velocity fluctuations in the streaks was intensified during the spanwise merging of the near-wall streaks. Conditionally-averaged velocity fields around the merging of the outer LSMs showed that the intensified near-wall motions were induced by the outer LSMs and extended over the near-wall regions. The intense near-wall motions influence the formation of the outer low-speed regions as well as the reduction of the convection velocity of the downstream LSMs. The interaction between the near-wall and the outer motions is the essential origin of the different convection velocities of the upstream and downstream LSMs for the formation process of VLSMs by merging.

The hydrodynamic patterns in the wake of swimming jellyfish are based on the bell morphology. Jellyfish with a prolate bell morphology form a clear jet structure in the wake. A three-dimensional computational model was used to analyze the hydrodynamic patterns. The Froude propulsion efficiency, defined by the ratio of the value of the energy required to deform the elastic bell to the value of the average center velocity multiplied by the thrust, was compared with different forms of the elastic bell deformation. The immersed boundary method was adopted to consider the interaction between the swimming jellyfish and surrounding fluid. Due to the effect of the momentum transferred to the surrounding fluid by the bell deformation, the rotational fluid mass was formed, called vortices. The vortex structures in the wake of prolate swimming jellyfish were elucidated in detail in both quantitative and qualitative ways. A dimensionless temporal parameter was employed to investigate the vortex formation process quantitatively. The starting/stopping vortex structures were generated during the contraction/relaxation phase. During the early stage of the contraction, the vortex structures were mainly generated by the stroke, then the ejected fluid was entrained into the vortex structures.

Turbulent pipe and channel flows have been generally accepted as having a similar turbulent flow structure. However, turbulence statistics in the core region of pipe flow are different from that of channel flow owing to the difference in the averaged spanwise dimension of the low-speed structures. In particular, wall-normal and spanwise stresses of channel flow are smaller than those of pipe flow. In the present study, DNS dataset of turbulent channel and pipe flows with the friction Reynolds number Re=934 have been compared to elucidate the difference of statistics in terms of the populations of large and very-large scales in the low-speed region. To this end, large and very-large scales were extracted by a low-pass filtered streak detection algorithm. We found that the population density of large scales of pipe flow is more increased in the core region than that of channel flow. Although the density of very-large scales of pipe flow decreases, the area of low-speed region increases due to the large number of large scales. Further comparison of pipe and channel flows showed that the higher turbulence intensity of pipe flow is caused by the interference of large scales with the azimuthal distance.

Direct numerical simulation was carried out to investigate the spatial features of very large-scale motions (VLSMs) in a turbulent channel flow with Re τ ≈ 930. Using the streak detection algorithm and the population trend, statistics of the spatial extents for both large-scale motions (LSMs) and VLSMs were obtained. This result showed that, at least one VLSM, on average, can be found in the wall-parallel 10δ × δ plane (here δ is the channel half height), which provides a statistical evidence of the presence of VLSMs. Moreover, to clarify one of previous hypotheses regarding the formation of VLSMs, the upstream tracking of individual streaks was utilized for temporal analysis. The present study statistically supported that the connection of the upstream LSMs is mainly contributed to the formation of VLSMs; that is, the concatenation of the small structures to form VLSMs. The downstream tracking of VLSMs indicated that over than 30% of VLSMs survive for 9 wall-unit time. Finally, the spatial organization of LSMs and VLSMs was examined by the conditional averaging.

Direct numerical simulations (DNS) of turbulent boundary layers over isothermally-heated walls were performed to investigate the effect of viscosity stratification on the turbulent and thermal boundary layer flows. An empirical relation of temperature-dependent viscosity for water was adopted. Based on the free-stream temperature (30 ◦ C), two wall temperatures (70 ◦ C and 99 ◦ C) were selected. In the heated flows, the turbulence energy diminishes in the buffer layer, but increases near the wall. The reduction in turbulence kinetic energy in the buffer layer is accompanied by smaller levels of Reynolds shear stresses and, hence, weaker turbulence production. The enhanced turbulence energy near the wall is attributed to enhanced transfer of energy via additional diffusion-like terms due to the viscosity stratification. Wall heating also results in increased scalar flux in the sublayer. Large wall-normal gradients of U and Θ lead to increased production in the scalar flux budget.

Direct numerical simulation (DNS) database of a zero pressure-gradient turbulent boundary layer was scrutinized to investigate the spatial distribution of vortices around a streamwise-elongated low-speed structure. The turbulent flow field has been numerically produced by preceding the bypass transition simulation using the isotropic free-stream turbulence, in which the Reynolds number reaches up to Reθ = 3280. In the present study, the low-speed structures and vortices have been identified and tracked in order to obtain spatial properties of the large-scale structures. The information about the inclination angle, distance and population of individual vortices on the long streamwise structure has been investigated. Finally, the present study provides statistical evidence on the formation of largescale packet-like structure and its variation along the downstream.

This presentation introduces spectral simulation and parallel computing for turbulent channel flow.

Direct numerical simulations of turbulent flows over isothermally-heated walls were performed to investigate the influence of viscosity stratification on boundary-layer turbulence and drag. The adopted model for temperature-dependent viscosity was typical of water. The free-stream temperature was set to 30◦ C, and two wall temperatures, 70◦ C and 99◦ C, were simulated. In the heated flows, the mean shear-rate is enhanced near the wall and reduced in the buffer region, which induces a reduction in turbulence production. On the other hand, the turbulence dissipation is enhanced near the wall, despite the the reduction in fluid viscosity. The higher dissipation is attributed to a decrease in the smallest length scales and near-wall fine-scale motions. The combined effect of the reduced production and enhanced dissipation leads to lower Reynolds shear stresses and, as a result, reduction of the skin-friction coefficient.

Direct numerical simulations of turbulent boundary layers with temperature-dependent viscosity were performed to investigate the influence of wall-heating on skin friction. The Arrhenius type viscosity model was chosen to represent water at atmospheric pressure. Based on the free-stream temperature of T∞=30C, two wall temperatures (Tw=70C and 99C) were selected. These wall temperatures correspond to viscosity ratios (w∞) of approximately 50% and 35% at the wall. When the wall is heated, a low-viscosity layer near the wall is formed. The reduced viscosity leads to a decrease in the Kolmogorov length-scale () of flow in the near-wall region. The smallest scale of motion near the wall enhances the dissipation range of the turbulent kinetic energy (TKE) spectrum (figure 2), which results in augmented dissipation in the TKE budget. Furthermore, the viscosity drop due to wall-heating also gives rise to increased mean shear rate in the near-wall region and decreased mean shear rate in the buffer region. Reduction of the shear rate in the buffer region induces a decreased production of TKE which can weaken vortical structures as shown in figure 3. Subsequently, the combined increase of the dissipation and decrease of the production lead to a reduction of the Reynolds shear stress, which is the largest contribution to the skin friction coefficient. The skin-friction coefficient and its reduction rate are reported in figure 4. The drag reduction mechanism will be discussed with focus on turbulent structures and their role.

Numerical simulation is used to examine turbulent structures and adverse-pressure-gradient effects in a conical diffuser.

The study examines how wall-heating-induced viscosity changes affect turbulent structures and momentum transport in water boundary layers.

Direct numerical simulation (DNS) of open-channel flow in the presence of an air-water interface was performed to examine the effects of interface deformation on the turbulence structures. In the water-driven turbulence, flows characterized by either of two Froude numbers (Fr=0.2 and 0.8) were examined and compared. A coupled level-set and volume-offluid (CLSVOF) method was employed to track the interface. The air that contacted the water was entrained into the turbulent flow. At high Fr, all turbulent normal stresses on the air side of the interface were high near the sidewall. Moreover, all terms of the Reynolds shear stress were intensified at the mixed-boundary corner on the air side of the interface. Twopoint correlations between the streamwise vortex and the velocity fluctuations provided structural information about the near-wall streaky structures and the inner secondary flows in the cross-stream plane. Linear stochastic estimates of the conditionally averaged flow field showed that the inner secondary flow consisted of not only in-plane velocity components but also streamwise velocity components.

Direct numerical simulation (DNS) of open-channel flow in the presence of an air-water interface was performed to examine the effects of interface deformation on the turbulence structures. In the waterdriven turbulence, flows characterized by either of two Froude numbers (Fr=0.2 and 0.8) were examined and compared. A coupled level-set and volume-of-fluid (CLSVOF) method was employed to track the interface. At high Fr, all turbulent normal stresses on the air side of the interface were high near the sidewall. Two-point correlations between the streamwise vortex and the velocity fluctuations provided structural information about the near-wall streaky structures and the inner secondary flows in the cross-stream plane. Linear stochastic estimates of the conditionally averaged flow field showed that the inner secondary flow consisted of not only in-plane velocity components but also streamwise velocity components.

Coherent wall-normal structures in turbulent boundary layer (TBL) have been investigated by scrutinizing the direct numerical simulation (DNS) dataset with Re θ =2500. The spatial signatures of hairpin vortex legs were frequently observed in the vector fields of the wall-normal/spanwise (yz ) plane and it was found that groups of such hairpin legs induce wall-normal-aligned elongated structures with a large Reynolds shear stress; this result strongly supports the typical hairpin packet model. The two-point correlation of the velocity fluctuations showed that the wall-normal length scales vary linearly with the distance from the wall and the two-point correlations between the signed swirling motions showed that interactions between counter-rotating vortices are predominant throughout the boundary layer and especially frequent in the near-wall region.

DNS data are used to investigate spanwise-vortex populations and coherent structures in adverse-pressure-gradient turbulent boundary layers.

Turbulent structures in spanwise/wall-normal plane of the turbulent boundary layers (TBLs) subjected to adverse pressure gradients (APGs) were investigated by analyzing the DNS database of Lee & Sung (2009). Probability density functions of the strength of the vortex cores normalized by their r.m.s. values displayed that strong swirling motions are frequently observed on the APG TBLs than zero pressure gradient TBLs. Influence of APGs on the population trends of spanwise vortex cores showed that those have a local maximum at the outer region for APG TBLs which might be due to the maximum Reynolds shear stress. Moreover, two-point correlations and linear stochastic estimations were scrutinized to provide statistical evidence for hairpin packet motions in the vertical plane of the TBLs with APGs. We found that wall-normal extent of the contours is elongated vertically owing to the strong swirling motion of the individual vortex located in the wake region or wake-type detached structures.