Large Eddy Simulation Of Flow Research Articles

Large eddy simulation of flow through a periodic array of urban-like obstacles using a canopy stress method

Large-eddy simulation (LES) of a turbulent flow through an array of building-like obstacles is an idealized model to study transport of contaminants in the urban atmospheric boundary layer (urban ABL). A reasonably accurate LES prediction of turbulence in such an urban ABL must resolve a significant proportion of the small but energetic eddies in the roughness sublayer, which remains prohibitive even though computational power has been increased significantly. Recently, some researchers also reported a high level of inaccuracy in turbulence prediction if LES were coupled with an adaptive mesh refinement technique in order to optimize the cost of resolving the roughness sublayer. In this article, we present a turbulence closure methodology for LES of urban ABLs in which the roughness elements are represented through the canopy stress method, and the subgrid scale stress is modeled through a dynamically adaptive eddy viscosity method. Unlike the classical Smagorinsky model that considers only the ‘strain portion’ of the velocity gradient tensor, we consider both the ‘strain tensor’ and the ‘rotation tensor’ to compute the eddy viscosity. This allows us to dynamically adapt the rate of energy dissipation to the scale of the energetic eddies in the roughness sublayer. Without employing a mesh conforming to the urban roughness elements, the effect of such solid bodies are represented in the LES model through a canopy stress method in which the loss of pressure and the sink of momentum due to the interaction between eddies and roughness elements are parameterized using the instantaneous velocity field. Simulation results of the proposed canopy stress method is compared with that of a conventional Computational Fluid Dynamics (CFD) method employing a block-structured mesh conforming around the roughness elements. For urban flow simulations, the results demonstrate that the proposed canopy stress model is accurate in predicting vertical profiles of mean and variance, as well as the temporal intermittency of coherent structures.

Large-Eddy Simulation of Kerosene Spray Ignition in a Simplified Aeronautic Combustor

The current work presents the Large Eddy Simulation (LES) of a kerosene spray ignition phase in a simplified aeronautical combustor for which detailed experimental data are available. The carrier phase is simulated using an unstructured multi-species compressible Navier-Stokes solver while the dispersed liquid phase is modeled with a Lagrangian approach. An energy deposition model neglecting the presence of a plasma phase in the very first instants of the energy deposition process, a reduced kinetic scheme and a simplified spray injection model are combined to achieve both a reasonable computational expense and a satisfactory overall accuracy. Following a brief description of the validation of these models, non reactive gaseous and two-phase flow LES’s of the target combustor are performed. Excellent agreement with experiments is observed for the non reactive gaseous simulations. The dispersed phase velocity fields are also well reproduced while discrepancies appear for the spatial size distribution of the particles. Finally, numerical snapshots of a successful ignition phase are shown and discussed.

Large Eddy simulations of flow around tandem circular cylinders in the vicinity of a plane wall

To investigate the three-dimensional flow around free-spanning, tandem marine pipelines, Large Eddy Simulations (LES) with Smagorinsky subgrid scale model are performed using the open-source code OpenFOAM. Two circular cylinders in tandem arrangement are placed in the vicinity of a rigid, horizontal, plane wall. The cylinders are immersed in a steady current with a logarithmic boundary layer profile at an intermediate, subcritical Reynolds number (Re = 1.31 × 104). The non-dimensional distances between the cylinder centres are 2 and 5 (L/D = 2 and 5) and the gap to diameter ratios are G/D = 0.6 and 1, where gap G is the distance between the bottom of the cylinders and the wall. The present results are analysed through the values of drag and lift coefficients, as well as by the details of the flow fields in the wake of the cylinders. The results are compared with experimental results of the flow around tandem cylinders in an unlimited fluid for both L/D = 2 and 5, showing that, at chosen gaps, at L/D = 2, the flow belongs to the reattachment regime, and at L/D = 5 to the co-shedding regime. Compared with the case of a single cylinder near a plane wall, the flow around the tandem cylinders at G/D = 1 belongs to the wide gap regime, while at G/D = 0.6, tandem yields a stronger interaction with the wall than in the case of one cylinder. The presence of the wall modifies the flow in the spacing between the two cylinders, giving it some characteristics of the extended body flow regime.

Large eddy simulation of flow over inclined wavy cylinders

The wavy cylinders have been proven effective in suppressing the Kármán vortex shedding and mitigating the aerodynamic forces. As yet their performance in the inclined incoming flow has not been well understood. In the current work, large eddy simulations are carried out to unveil the aerodynamics of the flow around inclined wavy cylinders at the Reynolds number of 5000. Three inclination angles, 0°, 30° and 45°, together with 2 × 2 combinations of shape parameters, namely λ∕Dm=2 and 6, a∕Dm=0.1 and 0.15, have been taken into consideration. Firstly, the aerodynamic force coefficients in terms of both the span-wise averaged and sectional values are discussed at length. It is revealed that the growing inclination angle invites not only a surge in their span-wise averaged values, but also an enlargement in the sectional difference of the force coefficients. The span-wise correlation of the lift force is found to be enhanced for the wavy cylinders with the increase of inclination angle, while the opposite is true for the normal cylinders. By examining the mean wake properties, it is disclosed that the increase in the span-wise averaged drag force coefficients is closely related to the shrinkage in the vortex formation length and the regression of the base pressure, whereas the sectional distribution of the drag coefficients is largely affected by the stagnation pressure. The mean wall shear stress fields are exploited to shed light on the flow topology of the cylinder surfaces. It is discovered that the wavy cylinders, especially the short-wavelength one, exhibit significant span-wise variation in the separation structure in the inclined flow. Besides, the chaotic surface flow in the rear side of the non-inclined cylinders could be regulated to maintain symmetry by the secondary axial flow in the near wake of the inclined cylinders. The instantaneous three-dimensional vortical structures are visualized by the Q isosurfaces at last to collaborate the previous discussions. A preliminary mechanism for the cessation of flow control efficacy for the wavy cylinders in inclined flow is also presented.

3D vortex structure investigation using Large Eddy Simulation of flow around a rotary oscillating circular cylinder

Three-dimensional turbulent flow over a circular cylinder performing sinusoidal rotational motions around its axis is investigated using Large Eddy Simulation (LES). The simulation is carried out for Reynolds number equal to 3900 with rotation rate (Ω=θosc*r∕U0, where θosc is oscillation amplitude, r cylinder radius, and U0 flow velocity) varying from 0.5 to 2; and with non-dimensional forcing frequencies (ratio of the frequency of the cylinder oscillation and that of the vortex-shedding from a stationary cylinder) from 0 to 8. Time-averaged flow parameters as well as mean drag and lift coefficients, mean base pressure coefficient, and separation angle and mean length of the vortex formation region are obtained and thoroughly analyzed. It was found that under forcing control, 50% of drag reduction was achieved and the flow three-dimensionality was reduced in the lock-on range.

Overdamped large-eddy simulations of turbulent pipe flow up to Reτ = 1500

We present results from large-eddy simulations (LES) of turbulent pipe flow in a computational domain of 42 radii in length. Wide ranges of shear the Reynolds number and Smagorinsky model parameter are covered, 180 ≤ Reτ ≤ 1500 and 0.05 ≤ Cs ≤ 1.2, respectively. The aim is to asses the effect of Cs on the resolved flow field and turbulence statistics as well as to test whether very large scale motions (VLSM) in pipe flow can be isolated from the near-wall cycle by enhancing the dissipative character of the static Smagorinsky model with elevated Cs values. We found that the optimal Cs to achieve best agreement with reference data varies with Reτ and further depends on the wall normal location and the quantity of interest. Furthermore, for increasing Reτ, the optimal Cs for pipe flow LES seems to approach the theoretically optimal value for LES of isotropic turbulence. In agreement with previous studies, we found that for increasing Cs small-scale streaks in simple flow field visualisations are gradually quenched and replaced by much larger smooth streaks. Our analysis of low-order turbulence statistics suggests, that these structures originate from an effective reduction of the Reynolds number and thus represent modified low-Reynolds number near-wall streaks rather than VLSM. We argue that overdamped LES with the static Smagorinsky model cannot be used to unambiguously determine the origin and the dynamics of VLSM in pipe flow. The approach might be salvaged by e.g. using more sophisticated LES models accounting for energy flux towards large scales or explicit anisotropic filter kernels.

Improved vortex method for large-eddy simulation inflow generation

The generation of turbulent inflow conditions in large-eddy simulation (LES) is a key ingredient for general applications of LES in both academic turbulent flows and industrial designs with complicated engineering flows. This is because accurate predictions of the fluid behaviour are strongly dependent on the inflow conditions, particularly in turbulent flows at high turbulent Reynolds numbers. This paper aims at improving the vortex method (VM) of Sergent that demands long adaptive distances (12 times the half channel height, for a channel flow at Reτ=395) to achieve high quality turbulence, and evaluating the equilibrium of the flow field obtained in terms of both the equilibrium of the mean flow and that of the turbulence (inter-scale turbulent energy transfer). The mean flow equilibrium is checked with classic criteria such as the friction velocity. In order to assess the equilibrium of turbulence, we propose using the velocity-derivative skewness, because it associates with the balance of energy transfer between large- and small-scale fluid motions. Numerical tests with the optimised set of model parameters reveal that the IVM is very efficient, in terms of adaptive distance, in generating high-quality synthetic turbulent fluctuations over a moderate distance: 6h for channel flow and 21δ for flat-plate boundary layer, with h and δ being respectively the half channel height and the nominal boundary layer thickness.

LES and Wind Tunnel Test of Flow around Two Tall Buildings in Staggered Arrangement

Wind flow structures and their consequent wind loads on two high-rise buildings in staggered arrangement are investigated by Large Eddy Simulation (LES). Synchronized pressure and flow field measurements by particle image velocimetry (PIV) are conducted in a boundary layer wind tunnel to validate the numerical simulations. The instantaneous and time-averaged flow fields are analyzed and discussed in detail. The coherent flow structures in the building gap are clearly observed and the upstream building wake is found to oscillate sideways and meander down to the downstream building in a coherent manner. The disruptive effect on the downstream building wake induced by the upstream building is also observed. Furthermore, the connection between the upstream building wake and the wind loads on the downstream building is explored by the simultaneous data of wind pressures and wind flow fields.

Modeling space‐time correlations of velocity fluctuations in wind farms

AbstractAn analytical model for the streamwise velocity space‐time correlations in turbulent flows is derived and applied to the special case of velocity fluctuations in large wind farms. The model is based on the Kraichnan‐Tennekes random sweeping hypothesis, capturing the decorrelation in time while including a mean wind velocity in the streamwise direction. In the resulting model, the streamwise velocity space‐time correlation is expressed as a convolution of the pure space correlation with an analytical temporal decorrelation kernel. Hence, the spatiotemporal structure of velocity fluctuations in wind farms can be derived from the spatial correlations only. We then explore the applicability of the model to predict spatiotemporal correlations in turbulent flows in wind farms. Comparisons of the model with data from a large eddy simulation of flow in a large, spatially periodic wind farm are performed, where needed model parameters such as spatial and temporal integral scales and spatial correlations are determined from the large eddy simulation. Good agreement is obtained between the model and large eddy simulation data showing that spatial data may be used to model the full spatiotemporal structure of fluctuations in wind farms.

Detailed SGS atomization model and its implementation to two-phase flow LES

A novel turbulent atomization model, which is physically closed itself and free of case-by-case parameter tuning using experimental data, has been formulated and demonstrated in the framework of turbulent spray combustion large-eddy simulation (LES). Based on our accumulated research findings that elementary droplet/ligament generation is a deterministic phenomenon, not something random as considered in the conventional understanding, the model describes two dominant modes of turbulent atomization, i.e. the turbulent resonant mode and the Rayleigh–Taylor (RT) mode, in a physically straightforward manner. Extending the baseline theory proposed in Umemura (2016), to a hybrid turbulent spray LES formulation which includes both an Eulerian liquid jet core and Lagrangian droplets, the subgrid-scale (SGS) atomization characteristics are completely detailed in this study. Using the LES-resolved turbulent Weber and Bond numbers on the liquid core surface, the atomization mode and the SGS atomization characteristics such as droplet size, number, ejection velocity and core regression velocity are all identified locally, and the information is transferred back to the LES code as input information. Test cases of Diesel fuel jets demonstrate that the present formulation well reproduces the turbulent spray behavior. Thanks to the obtained detailed data, the spray formation process can be tracked both temporally and spatially, from the initial head formation with edge atomization to the later core atomization and spray spreading. It is essentially featured that the present turbulent atomization model works well without ambiguous user input, contrary to the conventional way of spray simulation. This is a significant breakthrough to urge paradigm shift in spray simulation, from unclosed/unpredictable to closed/predictable, which enables drastic improvement in the accuracy of spray simulation and may exert a large impact on both research studies and industrial applications.

Enhancements to AERMOD's building downwash algorithms based on wind-tunnel and Embedded-LES modeling

Knowing the fate of effluent from an industrial stack is important for assessing its impact on human health. AERMOD is one of several Gaussian plume models containing algorithms to evaluate the effect of buildings on the movement of the effluent from a stack. The goal of this study is to improve AERMOD's ability to accurately model important and complex building downwash scenarios by incorporating knowledge gained from a recently completed series of wind tunnel studies and complementary large eddy simulations of flow and dispersion around simple structures for a variety of building dimensions, stack locations, stack heights, and wind angles. This study presents three modifications to the building downwash algorithm in AERMOD that improve the physical basis and internal consistency of the model, and one modification to AERMOD's building pre-processor to better represent elongated buildings in oblique winds. These modifications are demonstrated to improve the ability of AERMOD to model observed ground-level concentrations in the vicinity of a building for the variety of conditions examined in the wind tunnel and numerical studies.

A-priori assessment of subgrid scale models for large-eddy simulation of multiphase primary breakup

While Large-Eddy Simulation (LES) of single phase flows is well established in computational fluid dynamics, LES of multiphase flow is at an early development stage. The presence of the phase interface causes additional subgrid scale (SGS) terms to appear in the LES formalism. Not only turbulent structures but also interfacial deformations need to be captured by SGS models. The fidelity of multiphase LES depends on the complex interaction between turbulence and the phase interface at the unresolved scale. A variety of traditional and more recent SGS models of eddy viscosity or scale similarity type is suggested for the SGS terms in multiphase LES. Two new models for the SGS stress and for the SGS scalar flux are proposed. The first is based on a Taylor series expansion of the convolution filter, the latter is inspired by flamelet theory of turbulent premixed combustion. The closure models are a-priori assessed with respect to explicitly filtered direct numerical simulation (DNS) data of primary breakup of a liquid jet. The model performance is evaluated based on a correlation analysis and an order of magnitude study. Model accuracy is discussed depending on the filter size and the flow region. Promising candidates for a-posteriori analysis are identified.

Large-eddy simulation of flow around two off-shore wind turbines in tandem arrangement

Large-eddy simulation of flow around two off-shore wind turbines in tandem arrangement

Toward Development of a Stochastic Wake Model: Validation Using LES and Turbine Loads

Wind turbines within an array do not experience free-stream undisturbed flow fields. Rather, the flow fields on internal turbines are influenced by wakes generated by upwind unit and exhibit different dynamic characteristics relative to the free stream. The International Electrotechnical Commission (IEC) standard 61400-1 for the design of wind turbines only considers a deterministic wake model for the design of a wind plant. This study is focused on the development of a stochastic model for waked wind fields. First, high-fidelity physics-based waked wind velocity fields are generated using Large-Eddy Simulation (LES). Stochastic characteristics of these LES waked wind velocity field, including mean and turbulence components, are analyzed. Wake-related mean and turbulence field-related parameters are then estimated for use with a stochastic model, using Multivariate Multiple Linear Regression (MMLR) with the LES data. To validate the simulated wind fields based on the stochastic model, wind turbine tower and blade loads are generated using aeroelastic simulation for utility-scale wind turbine models and compared with those based directly on the LES inflow. The study’s overall objective is to offer efficient and validated stochastic approaches that are computationally tractable for assessing the performance and loads of turbines operating in wakes.

Large-eddy simulation of flow over a grooved cylinder up to transcritical Reynolds numbers

We report wall-resolved large-eddy simulation (LES) of flow over a grooved cylinder up to the transcritical regime. The stretched-vortex subgrid-scale model is embedded in a general fourth-order finite-difference code discretization on a curvilinear mesh. In the present study $32$ grooves are equally distributed around the circumference of the cylinder, each of sinusoidal shape with height $\unicode[STIX]{x1D716}$, invariant in the spanwise direction. Based on the two parameters, $\unicode[STIX]{x1D716}/D$ and the Reynolds number $Re_{D}=U_{\infty }D/\unicode[STIX]{x1D708}$ where $U_{\infty }$ is the free-stream velocity, $D$ the diameter of the cylinder and $\unicode[STIX]{x1D708}$ the kinematic viscosity, two main sets of simulations are described. The first set varies $\unicode[STIX]{x1D716}/D$ from $0$ to $1/32$ while fixing $Re_{D}=3.9\times 10^{3}$. We study the flow deviation from the smooth-cylinder case, with emphasis on several important statistics such as the length of the mean-flow recirculation bubble $L_{B}$, the pressure coefficient $C_{p}$, the skin-friction coefficient $C_{f\unicode[STIX]{x1D703}}$ and the non-dimensional pressure gradient parameter $\unicode[STIX]{x1D6FD}$. It is found that, with increasing $\unicode[STIX]{x1D716}/D$ at fixed $Re_{D}$, some properties of the mean flow behave somewhat similarly to changes in the smooth-cylinder flow when $Re_{D}$ is increased. This includes shrinking $L_{B}$ and nearly constant minimum pressure coefficient. In contrast, while the non-dimensional pressure gradient parameter $\unicode[STIX]{x1D6FD}$ remains nearly constant for the front part of the smooth cylinder flow, $\unicode[STIX]{x1D6FD}$ shows an oscillatory variation for the grooved-cylinder case. The second main set of LES varies $Re_{D}$ from $3.9\times 10^{3}$ to $6\times 10^{4}$ with fixed $\unicode[STIX]{x1D716}/D=1/32$. It is found that this $Re_{D}$ range spans the subcritical and supercritical regimes and reaches the beginning of the transcritical flow regime. Mean-flow properties are diagnosed and compared with available experimental data including $C_{p}$ and the drag coefficient $C_{D}$. The timewise variation of the lift and drag coefficients are also studied to elucidate the transition among three regimes. Instantaneous images of the surface, skin-friction vector field and also of the three-dimensional Q-criterion field are utilized to further understand the dynamics of the near-surface flow structures and vortex shedding. Comparison of the grooved-cylinder flow with the equivalent flow over a smooth-wall cylinder shows structural similarities but significant differences. Both flows exhibit a clear common signature, which is the formation of mean-flow secondary separation bubbles that transform to other local flow features upstream of the main separation region (prior separation bubbles) as $Re_{D}$ is increased through the respective drag crises. Based on these similarities it is hypothesized that the drag crises known to occur for flow past a cylinder with different surface topographies is the result of a change in the global flow state generated by an interaction of primary flow separation with secondary flow recirculating motions that manifest as a mean-flow secondary bubble. For the smooth-wall flow this is accompanied by local boundary-layer flow transition to turbulence and a strong drag crisis, while for the grooved-cylinder case the flow remains laminar but unsteady through its drag crisis and into the early transcritical flow range.

Large Eddy Simulations of flow around two circular cylinders in tandem in the vicinity of a plane wall at small gap ratios

Large Eddy Simulations (LES) with Smagorinsky subgrid scale model have been performed for the flow past two circular cylinders in tandem placed in the vicinity of a horizontal plane wall at very small gap ratios, namely G∕D=0.1,0.3 and 0.5, in three-dimension (3D). The ratio of cylinder center-to-center distance to cylinder diameter, or the pitch ratio, L∕D, considered in the simulations is L∕D=2 and 5. This work serves as an extension of Abrahamsen Prsic et al. (2015). In essence, six sets of simulations have been performed in the subcritical Reynolds number regime at Re=1.31×104. Our major findings can be summarized as follows. (1) At both pitch ratios, the wall proximity has a decreasing effect on the mean drag coefficient of the upstream cylinder. At L∕D=2, the mean drag coefficient of the downstream cylinder is negative since it is located within the drag inversion separation distance. (2) At L∕D=2, a squarish cavity-like flow exists between the cylinders and the flow circulates within the cavity. A long lee-wake recirculation zone is found behind the downstream cylinder at G∕D=0.1. However, a much smaller lee-wake recirculation zone is noticed at L∕D=5 with G∕D=0.1. (3) At L∕D=2, the reattachment is biased to the bottom shear layer due to the deflection from the plane wall, which leads to the formation of the slanted squarish cavity-like flow. At both pitch ratios, as G∕D becomes smaller, stronger vortices are found between the two cylinders. Less intense vortices are observed in the near wake of the downstream cylinder due to the vortex shedding suppression of the neighboring wall.

Use of digitally filtered inflow conditions for LES of flows over backward facing steps

Large eddy simulations (LES) of subsonic and supersonic boundary layers separating at backward facing steps are performed for validating a hybrid flow solver and testing the digital filtering approach for specifying the inflow turbulence. The broadband spectra of eddies in the approaching boundary layers resulting from filtering properly trigger the shear layer instabilities leading to significant improvements in predictions of first and second order turbulence statistics when compared to those resulting from use of uncorrelated noise for generating inflow turbulence. This seems to be true even though the distance between the inflow boundary and the step is about the same or less than in most of the LES of this kind of flows reported in literature and not sufficient to establish an equilibrium boundary layer with correct phase information before the flow reaches the step. The density/entropy disturbances resulting from non-solenoidal inflow do not seem to adversely affect the predictions in the subsonic case. The digital filtering approach does, however, generate acoustic disturbances that contaminate the expansion fan generated at the corner and lead to slight overpredictions in turbulence levels downstream in the supersonic case.

Large Eddy Simulation of liquid sheet breakup using a two-phase lattice Boltzmann method

A two-phase flow Large Eddy Simulation (LES) model is developed within the framework of phase-field lattice Boltzmann methods (LBM). The proposed model is based on a standard Smagorinsky model for subgrid closure and uses the Multiple-Relaxation-Time (MRT) collision operator. Relevant test cases are considered to demonstrate the capability and accuracy of the new model in dealing with two-phase flows at wide range of density contrasts. These include the Rayleigh–Taylor instability at two distinct density ratios, the breakup of a turbulent liquid column, and assisted breakup of a liquid layer. The validated model is then utilized to examine a liquid sheet emerging from a nozzle into a stagnant gas at different Reynolds numbers ranging from 1000 to 6000. The influence of jet injection velocity on the flow structure, variation of the jet centerline velocity, and jet area is also investigated. This work reports for the first time the extension of the Smagorinsky model for LES to the binary fluid model of Fakhari & Lee [1] and its application in simulation of turbulent liquid sheet breakup at the density ratios as high as 250.

Large-Eddy Simulation of Flow over a Wall-Mounted Hump with Separation and Reattachment

Wall-resolved large-eddy simulation of a model turbulent flow involving favorable and adverse pressure gradients, imposed by surface curvature of a wall-mounted hump, is performed for a spanwise-periodic computational domain. The flow acceleration over the front portion of the hump is strong enough to exceed the relaminarization criterion of Narasimha and Sreenivasan (“Relaminarization in Highly Accelerated Turbulent Boundary Layers,” Journal of Fluid Mechanics, Vol. 61, No. 3, 1973, pp. 417–447) but only over a relatively short streamwise extent. Although the flow does not relaminarize, turbulent skin-friction variation exhibits a plateau, also observed in the experiment of Greenblatt et al. (“A Separation Control CFD Validation Test Case, Part 1: Baseline and Steady Suction,” AIAA Journal, Vol. 44, No. 12, 2006, pp. 2820–2830), before it again continues to rise. The subsequent adverse pressure gradient is strong enough to cause flow separation. The location and extent of flow separation, including skin-friction distributions, compare reasonably well with experimental data. Computed velocity and Reynolds stress profiles are also compared with the experimental results. A systematic study of the effect of computational domain span, subgrid-scale model, tunnel backpressure, incoming boundary layer, grid refinement, Mach number, and top tunnel wall contour (which models the blockage effect of the experimental setup) is carried out, and sensitivity of the results to these parameters is discussed.

Navier slip model of drag reduction by Leidenfrost vapor layers

Recent experiments found that a hot solid sphere that is able to sustain a stable Leidenfrost vapor layer in a liquid exhibits significant drag reduction during free fall. The variation of the drag coefficient with Reynolds number deviates substantially from the characteristic drag crisis behavior at high Reynolds numbers. Measurements based on liquids of different viscosities show that the onset of the drag crisis depends on the viscosity ratio of the vapor to the liquid. Here we attempt to characterize the complexity of the Leidenfrost vapor layer with respect to its variable thickness and possible vapor circulation within, in terms of the Navier slip model that is defined by a slip length. Such a model can facilitate tangential flow and thereby alter the behavior of the boundary layer. Direct numerical and large eddy simulations of flow past a sphere at moderate to high Reynolds numbers (102≤Re≤4×104) are employed to quantify comparisons with experimental results, including the drag coefficient and the form of the downstream wake on the sphere. This provides a simple one parameter characterization of the drag reduction phenomenon due to a stable vapor layer that envelops a solid body.