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The objective of this study is to assess the predictive capabilities of low and high-fidelity turbulence models for low-Prflows. For this purpose, predictions by two-equation (k-ε) Reynolds-Averaged Navier Stokes (RANS), partially-averaged Navier Stokes (PANS) hybrid RANS/Large Eddy Simulation (LES), and DSM, WALE, filtered LES models are compared for four different test cases, namely vertical channel flow, vertical backward facing step, flow over a rod bundle and heat transfer in ascending and descending flow through a pipe with a constant wall heat flux. The test cases involve a range of complex flow conditions including separation/reattachment and aiding and opposing buoyant forcing (Re ranging from 640 to 40K; Ri ranging from −0.65 to 0.65) for water (Pr = 0.71) and liquid metals (Pr = 0.00585 to 0.025) flows. The validation study demonstrates that turbulence models are 4% more accurate for higher Prflows that for low-Pr flows; 6% more accurate for forced convective conditions than for flows involving mixed convective conditions; and predict aiding buoyant flow conditions better than the opposing buoyant flow conditions. Overall, LES performed the best and provided averaged error of 6% followed by 10% by PANS and RANS showed the largest error of 14%.

The predictive capability of RANS, hybrid RANS/LES and LES turbulence models are assessed for low-Pr flows involving complex flow features expected in engineering applications, such as forced convection, buoyancy, shear, reattachment, and recirculation. The models are tested for three test case vertical channel, vertical backward facing step with heated wall and rod bundle lattice cases for Pr ranging from 0.002 to 2.0. k-ε based model is used for RANS, partially averaged Navier Stokes (PANS) for hybrid RANS/LES and dynamic Smagorinsky model for LES. The channel and backward facing step cases show that LES performs the best followed by PANS and RANS. The RANS model performs reasonably well for flows with strong buoyant forces, which augments the mean flow more than the turbulent features. But for moderate buoyant flow condition, which augments turbulence characteristics and associated diffusion, RANS models do not perform well. Considering both the computational cost and accuracy, the PANS provides a good compromise between RANS and LES.

Numerical simulations of pulsatile flow in a feline aorta for hypertrophic cardiomyopathy (HCM) heart conditions are performed to predict flow details and to evaluate possible thrombus trajectory patterns. The study demonstrates that average flow rate boundary conditions (QBC) at artery outlets act as a resistance-type boundary condition for pulsatile flow. For simulations when the exact artery outflows are not known, specification of estimated values from physiological conditions is a plausible approach. This boundary condition is further improved using an iterative method (I-QBC) to accurately satisfy outflow conditions when expected outflow is known. The approach is validated against experimental data for the prediction of iliac artery flow and wall stresses in a human abdominal aorta. The feline aorta simulations including Lagrangian particle transport are performed on grids with up to 11M cells for a generalized feline aorta. It is found that LES on larger grids performs significantly better than URANS for the prediction of vortical structures. Simulations for both healthy and HCM conditions show similar flow patterns in the upper abdominal aorta. However, the HCM condition shows the presence of large recirculation regions in the thoracic aorta resulting in 50% lower flow through the iliac arteries and increased entrapment of fluid-borne particles near the trifurcation region compared to the healthy condition.

This study performs a comprehensive analysis of the effect of flow separation and reattachment, convective conditions and Pr to understand their effect on heat transfer characteristics and the predictive capability of low- and hig-fidelity turbulence models are assessed. To achieve the objective DNS is performed for plane channel flow at Reτ = 640, Pr = 0.71 and 0.025 involving mixed forced and natural convection condition, and RANS, hybrid RANS/LES, and LES calculations are performed for backward backing step with expansion ratio 1.5, Pr = 0.71 and 0.0088 and Ri = 0 and 0.338. Channel flow simulations reveal that the convective conditions affect the near-wall turbulent structures and thermal diffusion more significantly in high-Re flows that in low-Re flows. Thus, the generated DNS database provides a challenging test case for turbulence model validation. For backward facing step case, all the turbulence models predict the overall flow characteristics, and Ri = 0 case is a more challenging validation test case than Ri = 0.338, as the former involves complex turbulent diffusion, whereas the latter is dominated by large scale buoyancy driven convection. Results show that well resolved PANS and LES predictions can help in improve understanding of turbulent diffusion under complex convection and flow separation/ reattachment regimes. RANS results are also quite encouraging and indicates that they may represent a reasonable compromise between computational expense and accuracy for cases in which high resolution simulations are not feasible.

A verification and validation study was performed using the open source computational fluid dynamics solver OpenFOAM version 2.0.0 for incompressible bluff body fluid flows. This includes flow over a backward facing step, a sphere in the subcritical regime, and delta wing with sharp leading edge. The study investigates solver scalability, and accuracy of numerical methods and turbulence models available in the solver. Grid verification study shows mostly monotonic convergence with averaged grid uncertainty

The predictive capability of RANS, hybrid RANS/LES and LES turbulence models for momentum and thermal energy transfer in wall bounded low-Pr turbulent flows is investigated. Plane channel flow simulations are performed for Reτ = 150 and 640 for Pr = 0.025 and 0.71 with and without buoyancy effects, including both forced and mixed force/natural convection conditions, using the open source spectral element flow solver Nek5000. The prediction of one-point velocity and temperature statistics from the simulations are compared against available DNS results. Results are analyzed to understand the effect of flow conditions on turbulent thermal transport, and assess the relative strengths and weaknesses of the different modeling methods.

Robust automated vortex detection algorithms are needed to facilitate the exploration of large-scale turbulent fluid flow simulations. Unfortunately, robust non-local vortex detection algorithms are computationally intractable for large data sets and local algorithms, while computationally tractable, lack robustness. We argue that the deficiencies inherent to the local definitions occur because of two fundamental issues: the lack of a rigorous definition of a vortex and the fact that a vortex is an intrinsically non-local phenomenon. As a first step towards addressing this problem, we demonstrate the use of machine learning techniques to enhance the robustness of local vortex detection algorithms. We motivate the presence of an expert-in-the-loop using empirical results based on machine learning techniques. We employ adaptive boosting to combine a suite of widely used, local vortex detection algorithms, which we term weak classifiers, into a robust compound classifier. Fundamentally, the training phase of the algorithm, in which an expert manually labels small, spatially contiguous regions of the data, incorporates non-local information into the resulting compound classifier. We demonstrate the efficacy of our approach by applying the compound classifier to two data sets obtained from computational fluid dynamical simulations. Our results demonstrate that the compound classifier has a reduced misclassification rate relative to the component classifiers.

The performance of a pseudo-spectral solver, which uses FFT in the streamwise and spanwise directions and Chebyshev polynomials in the wall-normal direction, is improved by implementing MPI domain decomposition for the Chebyshev collocation method using the influence matrix method. The parallel code is validated for LES of plane channel flow at Reτ=590 and 2003, and applied for temporally developing flat plate transition simulations. The domain decomposition using the influence matrix method is implemented to achieve C1 continuity of the velocity across the domain interface. The method provides super linear scalability of solver computational performance. The predictions show kinks in the turbulent statistics on coarser grids, but agree very well with DNS data for finer grids. It is found that accurate turbulent predictions require a minimum of 48 collocation points per subdomain for Re ~ 105, with higher numbers required for Re ~ 106. The flat plate transitional flow simulation is performed for a...

Predictive capabilities of hybrid RANS/LES models are compared for single-phase flow over a surface combatant at Re = 5.3 × 106 and an appended DARPA Suboff model at Re = 1.2 × 107. The turbulence models used in the study are: k–ω shear stress transport (SST)-URANS; Spalart Allmaras based detached eddy simulation (SA-DDES); k–ω based improved delayed detached eddy simulation (KW-IDDES); and a dynamic hybrid RANS/LES (DHRL) model coupling SST and implicit LES. For the surface combatant case, both SA-DDES and KW-IDDES predicted 100% and 28%, respectively. Overall, the DHRL model performed best among the turbulence models tested, and KW-IDDES performed worst. The study indicates that the DHRL approach has the potential to provide accurate mean flow predictions while resolving small-scale turbulent structures. Results also highlight the importance of the wall function formulation for accurately resolving mean skin friction coefficient, especially over smooth regions of the hull.

SUMMARY Static model coefficients for an algebraic subgrid stress (SGS) model are determined using a dynamic approach, based on results from simulations of isotropic decaying turbulence. The study was motivated by the discrepancies in energy transfer predictions using the previously documented coefficients (Bhushan and Warsi, Int. J. Numer. Meth. Fluids 2005; 49: 489–519). The discrepancies are identified to be due to inconsistent filter functions used in the analytic estimates and the simulations. The study emphasizes that SGS model development should use filter functions compatible with those inherent in CFD application solvers. The dynamic approach predicts consistent model and transfer coefficients for different grid resolutions and is judged to be a reliable basis for model coefficient adjustments. The predicted Leonard's stress coefficient and associated energy transfer coefficients agree very well with the analytic estimates using a Gaussian/cutoff combination filter. This suggests that the modeling of Leonard's stress term using a truncated Taylor series expansion is robust and may not benefit significantly from dynamic modeling. Validation simulations were performed for turbulent channel flow at Reτ = 180 and 590. The dynamic approach was found to be reliable only for the lower log-layer of the Reτ = 590 case, where the scale invariance condition was satisfied. Nonetheless, in this narrow range, the model and transfer coefficients compare well with the isotropic case. The static coefficient algebraic model with new adjusted coefficients shows improved predictions compared with the previous coefficients, for both isotropic decaying turbulence and channel flow cases. Copyright © 2013 John Wiley & Sons, Ltd.

This paper investigates a recently proposed dynamic hybrid RANS-LES framework using a general-purpose finite-volume flow solver. The new method is highly generalized, allowing coupling of any selected RANS model with any selected LES model and containing no explicit grid dependence in its formulation. Selected results are presented for three test cases: two-dimensional channel flow, backward facing step, and a nozzle flow relevant to biomedical applications. Comparison with experimental and DNS data, and with other hybrid RANS-LES approaches, highlights the advantages of the new method and suggests that further investigation is warranted.

In large-eddy simulation, the subgrid-stress model is included to represent the energy-transfer mechanisms between the resolved and unresolved scales of motion. Although most models represent forward scatter (dissipation) of energy, they fail to account for backscatter properly. The algebraic model, obtained directly from the closure of the subgrid stresses, has an additional term over the mixed model that accounts for the backscatter of energy explicitly. This model has been previously documented in the open literature and shows promising results in accounting for the subgrid-scale energy-transfer mechanisms. Detailed documentation is provided of the backscatter capability of the algebraic model, for a series of test cases including isotropic turbulence, plane channel flow, and two free shear flows. The model performace in predicting turbulent flow quantities is also compared with commonly used subgrid-stress models and with the available direct-numerical-simulation data.

A dynamic hybrid RANS/LES (DHRL) model has been implemented in the spectral-element solver Nek5000 to reduce computational expense for high Reynolds number applications. The model couples a k-e URANS model and the dynamic Smagorinsky model for LES. The model is validated for plane channel flow at Reτ = 590 using DNS data, and compared with LES predictions. The model is then applied for the ANL-MAX case, which is a test case relevant to nuclear reactor cooling flow simulations. For the channel flow case, DHRL predictions were similar to LES on finer grids, but on coarser grids, the former predicted velocity profiles closer to DNS than the latter in the log-layer region. The improved prediction by the DHRL model was identified to be due to a 30% additional contribution of RANS stresses. For the ANL-MAX case, the URANS simulation predicts quasi-steady flow, with dominant large-scale turbulent structures, whereas LES predicts small-scale turbulent structures comparable with results in rapid mixing of cool and warm flow jets. DHRL simulations predict LES mode in the inlet jet region, and URANS mode elsewhere, as expected.Copyright © 2014 by ASME

The Tesla valve is a passive-type check valve used for flow control in micro- or minichannel systems for a variety of applications. Although the design and effectiveness of a singular Tesla valve is somewhat well understood, the effects of using multiple, identically shaped Tesla valves in series—forming a multistaged Tesla valve (MSTV)—have not been well documented in the open literature. Therefore, using high-performance computing (HPC) and three-dimensional (3D) computational fluid dynamics (CFD), the effectiveness of an MSTV using Tesla valves with preoptimized designs was quantified in terms of diodicity for laminar flow conditions. The number of Tesla valves/stages (up to 20), valve-to-valve distance (up to 3.375 hydraulic diameters), and Reynolds number (up to 200) was varied to determine their effect on MSTV diodicity. Results clearly indicate that the MSTV provides for a significantly higher diodicity than a single Tesla valve and that this difference increases with Reynolds number. Minimizing the distance between adjacent Tesla valves can significantly increase the MSTV diodicity, however, for very low Reynolds number (Re

The paper discusses the setup of a light duty axle efficiency test in evaluating gear lubricants for their fuel economy performance. Data collected with an internal reference oil highlight the repeatability of the test in different axles. Comparisons between single grade SAE 90 to multigrade gear lubricants were made under a variety of pinion torques and speeds to simulate highway and city driving conditions. A discussion of lubricant rheology and its importance to maintaining film strength for adequate bearing and gear lubrication for optimum torque efficiency and axle temperature is provided.

Numerical results are presented for a three-dimensional discrete-jet in crossflow problem typical of a realistic film-cooling application in gas turbines. Key aspects of the study include: (1) application of a systematic computational methodology that stresses accurate computational model of the physical problem, including simultaneous, fully elliptic solution of the crossflow, film-hole, and plenum regions; high-quality three-dimensional unstructured grid generation techniques, which have yet to be documented for this class of problems; the use of a high-order discretization scheme to reduce numerical errors significantly; and effective turbulence modeling; (2) a three-way comparison of results to both code validation quality experimental data and a previously documented structured grid simulation; and (3) identification of sources of discrepancy between predicted and measured results, as well as recommendations to alleviate these discrepancies. Solutions were obtained with a multiblock, unstructured/adaptive grid, fully explicit, time-marching, Reynolds-averaged Navier–Stokes code with multigrid, local time stepping, and residual smoothing type acceleration techniques. The computational methodology was applied to the validation test case of a row of discrete jets on a flat plate with a streamwise injection angle of 35 deg, and two film-hole length-to-diameter ratios of 3.5 and 1.75. The density ratio for all cases was 2.0, blowing ratio was varied from 0.5 to 2.0, and free-stream turbulence intensity was 2 percent. The results demonstrate that the prescribed computational methodology yields consistently more accurate solutions for this class of problems than previous attempts published in the open literature. Sources of disagreement between measured and computed results have been identified, and recommendations made for future prediction of film-cooling problems.

This paper presents a new model concept for prediction of boundary layer transition using a linear eddy-viscosity RANS approach. It is a single-point, physics-based method that adopts an alternative to the Laminar Kinetic Energy (LKE) framework. The model is based on a description of the transition process previously discussed by Walters (2009). The version of the model presented here uses the k-ω SST model as the baseline, and includes the effects of transition through one additional transport equation for v2. Here v2 is interpreted as the energy of fully turbulent, 3D velocity fluctuations, while k represents the energy of both fully turbulent and pre-transitional velocity fluctuations. This modelling approach leads to slow growth of fluctuating energy in the pre-transitional region and relaxation towards a fully turbulent model result downstream of transition. Simplicity of the formulation and ease of extension to other baseline models are two potential advantages of the new method. An initial version of the model has been implemented as a UDF subroutine in the commercial CFD code FLUENT and tested for canonical flat plate boundary layer test cases with different freestream turbulence conditions.Copyright © 2012 by ASME

One of the key challenges for computational fluid dynamics (CFD) simulations of human lung airflow is the sheer size and complexity of the complete, multiscale geometry of the bronchopulmonary tree. Since 3-D CFD simulations of the full airway tree are currently intractable, researchers have proposed reduced geometry models in which multiple airway paths are truncated downstream of the first few generations. This paper investigates a recently proposed method for closing the CFD model by application of physiologically correct boundary conditions at truncated outlets. A realistic, reduced geometry model of the lung airway based on CT data has been constructed up to generation 18, including extrathoracic, bronchi, and bronchiole regions. Results indicate that the new method yields reasonable results for pressure drop through the airway, at a small fraction of the cost of fully resolved simulations.

This paper presents results from an ongoing effort to develop and validate a two-equation eddy-viscosity turbulence model for computational fluid dynamics (CFD) prediction of transitional and turbulent flow. The new model is based on a k-ω model framework, making it more easily implemented into existing general-purpose CFD solvers than other recently proposed model forms. The model incorporates inviscid and viscous damping functions for the eddy viscosity, as well as a production damping term, in order to reproduce the appropriate effects of laminar, transitional, and turbulent boundary layer flow. The new model has been implemented into a Mississippi State University (MSU) Computational Simulation and Design Center (SimCenter) developed flow solver (U2NCLE), as well as a commercially available CFD code (FLUENT). For model validation, comparisons were made to experimental data for an incompressible, zero-pressure gradient, flat plate geometry over a range of freestream turbulence quantities, using both of the flow solvers. Additional test cases were performed with the in-house flow solver and compared to experimental data for two sharp-cone geometries. The Mach number for the cone cases ranged from 0.4 to 2. The results presented in this document show that the new model performed well for the 2-D test cases and showed agreement with the experimental data of the 3-D geometries. The results illustrate the ability of the model to yield reasonable predictions of transitional flow behavior using a very simple modeling framework, including an appropriate response to freestream turbulence quantities.Copyright © 2007 by ASME

This paper presents the initial development and validation of a modified two-equation eddy-viscosity turbulence model for computational fluid dynamics (CFD) prediction of transitional and turbulent flow. The new model is based on a k-ω model framework, making it more easily implemented into existing general-purpose CFD solvers than other recently proposed model forms. The model incorporates inviscid and viscous damping functions for the eddy viscosity, as well as a production damping term, in order to reproduce the appropriate effects of laminar, transitional, and turbulent boundary layer flow. It has been implemented into a commercially available flow solver (FLUENT) and evaluated for simple attached and separated flow conditions, including 2-D flow over a flat plate and a circular cylinder. The results presented show that the new model is able to yield reasonable predictions of transitional flow behavior using a very simple modeling framework, including an appropriate response to freestream turbulence and boundary layer separation.Copyright © 2006 by ASME

A dynamic framework for hybrid Reynolds-averaged Navier-Stokes (RANS)—large eddy simulation (LES) modeling is proposed, wherein the RANS-to-LES transition parameter is adjusted to maintain continuity in turbulence production. The model is applied for temporally developing plane channel flow at varying Reynolds numbers with different initial turbulence intensity and grid resolution. On sufficiently fine grids, dynamic hybrid RANS-LES model (DHRL) yields similar results to LES simulations. On coarse grids, DHRL activates RANS mode in the log layer, thus improving the mean flow predictions. The model framework addresses grid sensitivity issues observed in other hybrid RANS-LES approaches and may be used with any desired combination of LES and RANS basis models.

Subjects with active stretch reflexes responded to an imposed sinusoidal movement of the ankle joint with a reflex force whose amplitude and timing varied widely with changes in the frequency of movement. At some frequency between 6 and 8 Hz, the reflex force tended to offset the non-reflex component of resistance, and thus to reduce the total resistance to movement. At this frequency the reflex response was particularly vigorous, with a deep modulation of electromyogram (e.m.g.) activity and a displacement of the joint stiffness vectors far from their high frequency values. The total resistance to movement might then be small, or it might be zero, or the reflex might actually assist the movement. As the frequency of movement was decreased through this critical range, the timing of the reflex response to movement changed rapidly with an abrupt advancement of the triceps surae e.m.g. signal, and a wide separation of the joint stiffness vectors as they passed close to the origin. This result was attributed to a changing distribution of the movement between the muscle fibres and an elastic Achilles tendon. It was assumed that at most frequencies the muscle fibres resisted extension, so that a major part of the imposed movement went into stretching the tendon; when, however, at 6-8 Hz, the reflex response was so timed as to reduce or abolish the resistance of the muscle fibres, more of the movement would take place in them. The muscle spindles would 'see' this larger movement of the muscle fibres, and generate correspondingly more reflex activity. A simplified model of the muscle-tendon combination behaves in a way that supports this view, and the available information about the human Achilles tendon indicates that it is sufficiently compliant for such an explanation. Therefore, movements imposed on the ankle joint would not necessarily be 'seen' by the muscle spindles, since they would be modified by transmission through a compliant tendon. By assuming a value for the tendon stiffness, it was possible to calculate the course of movements that actually occurred in the muscle fibres and spindles. Records of these spindle movements indicated how some non-linearities might arise.(ABSTRACT TRUNCATED AT 400 WORDS)

Ankle joints were subjected to sinusoidal movements at a range of amplitudes and frequencies. Records were made of electromyograms (e.m.g.s) in calf muscles, and of the forces at the joints. When the leg is relaxed, the ankle joint resists an imposed sinusoidal movement with a small approximately sinusoidal force. It is stiffer in its resistance to small movements than to large ones, and this resistance is greater when the joint is dorsiflexed than when it is plantarflexed. If the subject exerts a steady mean flexing force, the imposed sinusoidal movement generates reflex activity which may be recorded as a modulation of the gastrocnemius and soleus e.m.g.s. The e.m.g. response to the sinusoidal movement occurs later in cycles of movement at high than at low frequencies, as one could expect of a reflex pathway that involves a delay. The results suggest that this delay is between 50 and 60 ms, and we conclude that under these circumstances spinal stretch reflexes are playing the important part. The relation of the resisting force to the movement has been displayed as a vector. As the frequency changes, this vector describes the circular path that is characteristic of a system which includes delays or lags; this path enables one to draw conclusions about the amplitude and timing of the reflex resistance to the movement. When a subject exerts a moderate flexing force against the sinusoidal movement for some minutes, the reflex response becomes progressively potentiated. A subject whose reflex responses are normally slight may then exhibit a vigorous reflex response to the movement of that ankle. This enhancement of spinal reflex activity was accompanied by an increase in the myotatic reflex response at the ankle. Reflex responses to sinusoidal movement were most clearly seen when the subject exerted a mean flexing force that amounted to about one-fifth of his maximum. Very small movements (+/- 0.5 degrees) generated little or no reflex response. With large amplitudes of movement there was more reflex activity, but at some amplitude (which varied from subject to subject and from time to time) the reflex mechanism appeared to 'saturate', and further increases in amplitude were not accompanied by comparable increases in the reflex response. With movements at 10-15 Hz the e.m.g. response often became large in alternate cycles, with less activity in the intervening cycles.(ABSTRACT TRUNCATED AT 400 WORDS)

A laboratory engine test procedure was employed to develop lubricants suitable for highly turbocharged diesel engines in response to three significant areas of legislation for truck manufacturers in Europe which have affected engine design and placed more stringent requirements on lubricants. These areas include maximum gross vehicle train weight (GVTW), minimum brake horsepower per ton of GVTW, and maximum smoke emission. European legislative pressures and a market demand for low fuel consumption have led to the development of turbocharged direct-injection diesel engines with brake mean effective pressure ratings in excess of 160 psi for truck operation. This trend has resulted in increased field problems involving high oil consumption and ring seizure due to cylinder wear or bore polishing. The Ford Tornado truck engine was used in laboratory testing of two reference oils and two candidate oils. A series of new candidate lubricants was devised using the Ford Tornado engine test procedure, and typical results obtained with commercial, reference, and candidate oils are tabulated. Additive packages developed to control bore polishing have been extensively engine tested to assure they will meet all other aspects of diesel crankcase oil performance. Field testing in highly turbocharged diesel engines is in progress as a final demonstration of performance.