Hybrid Reynolds-averaged Navier Research Articles

Four-dimensional particle tracking velocimetry measurements of unsteady three-dimensional vortex onset and progression for 5415 straight ahead, static drift, and pure sway

Large-scale towed system four-dimensional particle tracking velocimetry (4DPTV) implementation and measurements are presented for towing tank tests for a naval combatant 5415 ship model for straight ahead, static drift β = 10°, and pure sway βmax = 10° conditions. The results, for the first time, provide instantaneous volumetric flow field data around the ship model including near the hull surface, and a complete description of the sonar dome 3D unsteady vortex separation onset and progression due to the 4DPTV larger measurement volume and higher data rates in comparison with previous results using tomographic particle image velocimetry (TPIV); thereby, providing the essential data required for the assessment of current hybrid Reynolds-averaged Navier–Stokes/large eddy simulation turbulence modeling capabilities and guidance for the necessary modeling improvements. The 4DPTV system is summarized and compared with the previous TPIV, including the camera calibration procedures, trigger systems, and synchronization with the sway motion. Analysis procedures, including data reduction procedures and the 3D vortex core detection and voxel labeling techniques, are described. The identification of the complex vortex separations and breakdown is aided using complementary detached eddy simulations. The vortex–vortex interaction process is visualized from instantaneous volumetric datasets. The pros and cons of the 4DPTV vs TPIV, the comparison between the second- and third-generation vortex visualization technique, and the statistical convergence error analysis of sonar dome port vortex for β = 10° are discussed. Plans for increased spatial resolution of the 4DPTV system and additional data reduction techniques for detailed turbulence analysis are also discussed.

Wake tail plane interactions for a tandem wing configuration in high-speed stall conditions

In this work, wake-tail plane interactions are investigated for a tandem wing configuration in buffet conditions, consisting of two untapered and unswept wing segments, using hybrid Reynolds-Averaged Navier–Stokes / Large Eddy Simulations (RANS/LES) with the Automated Zonal Detached Eddy Simulation (AZDES) method. The buffet on the front wing and the development of its turbulent wake are characterized, including a spectral analysis of the fluctuations in the wake and a modal analysis of the flow. The impact of the wake on the aerodynamics and loads of the rear wing is then studied, with a spectral analysis of its lift and surface pressure oscillations. Finally, the influence of the position and the incidence angle of the rear wing is investigated. For the considered flow conditions, 2D buffet is present on the front wing. During the downstream movement of the shock, the amount of separation reaches its minimum and small vortices are present in the wake. During the upstream movement of the shock, the amount of separation is at its maximum and large turbulent structures are present accompanied by high fluctuation levels. A distinct peak in the corresponding spectra can be associated with vortex shedding behind the wing. The impingement of the wake leads to a strong variation of the loading of the rear wing. A low-frequent oscillation of the lift, attributed to the change of the intensity of the downwash generated by the front segment, can be distinguished from high-frequent fluctuations that are caused by the impingement of the wake’s turbulent structures.

Enhancing respiratory comfort with fan respirators: Computational analysis of carbon dioxide reduction, temperature regulation, and humidity control

Respirators provide protection from inhalation exposure to dangerous substances, such as chemicals and infectious particles, including SARS-COVID-laden droplets and aerosols. However, they are prone to exposure to stale air as masks create a microclimate influenced by the exhaled air. As a result, exhaled air from lungs accumulating in the mask produces a warm and humid environment that has a high concentration of carbon dioxide (CO2), unsuitable for re-inhalation. Fans are a favorable option for respirators to ventilate the mask and remove the stale air. This study utilized computational fluid dynamics simulation consisting of a hybrid Reynolds-averaged Navier–Stokes-large eddy simulation turbulence method to compare the inhalation flow properties for different fan locations (bottom, top, and side) with regular respirator breathing. Three mask positions, top, side, and bottom, were evaluated under two breathing cycles (approximately 9.65 s of breathing time). The results demonstrated that adding a fan respirator significantly decreased internal mask temperature, humidity, and CO2 concentration. The average CO2 concentration decreased by 87%, 67%, and 73% for locations bottom, top, and side, respectively. While the top and side fan locations enhanced the removal of the exhaled gas mixture, the bottom-fan respirator was more efficient in removing the nostril jet gas mixture and therefore provided the least barrier to respiratory function. The results provide valuable insight into the benefits of fan respirators for long-term use for reducing CO2 concentration, mask temperature, and humidity, improving wearer safety and comfort in hazardous environments, especially during the COVID-19 pandemic.

Numerical Investigation of Prediction on the NASA High-Lift Common Research Model

An assessment of a hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation (HRLES) approach for prediction is presented for the NASA High-Lift Common Research Model (CRM-HL). Both free-air and wind tunnel configurations of the CRM-HL are investigated, and the results are compared to the QinetiQ wind tunnel experiments and two other numerical approaches: RANS and wall-modeled LES (WMLES). For the free-air configuration, HRLES is shown to address some of the known shortcomings with RANS and prevent inboard and outboard flow separation particularly in the region of and poststall. To achieve these improvements over RANS, LES-appropriate grids and numerical discretizations are required. It was also found that, when applying HRLES to a RANS best practice grid and numerics, the HRLES method significantly underperformed RANS. For the in-tunnel configuration, HRLES showed good agreement with the loads, surface pressure, and oil-flow photographs obtained in the experiment. HRLES was able to improve upon the RANS simulations, which showed a sharp loss of lift at the two highest angles of attack due to large-scale inboard and outboard separation on the wing, by correctly predicting the corner flow separation and showing remarkably close agreement in the flow topologies with the experiment.

Investigation of low-dissipation low-dispersion schemes for incompressible and compressible flows in scale-resolving simulations

A comprehensive study is conducted on a second-order low-dissipation low-dispersion (LD2) scheme in scale-resolving simulations of both incompressible and compressible flows, using a node-based unstructured CFD solver. The scheme deploys a higher order central reconstruction of the face values (up to fourth-order on structured meshes) and a matrix dissipation formulation to reduce the dispersive and dissipative numerical errors. The LD2 scheme is examined for compressible flow cases involving shock discontinuities, LD2-Compressible (LD2C), and is verified in a classical shock-tube problem. The scheme is then further verified in Large-Eddy Simulations (LES) of decaying isotropic turbulence (DIT) in comparison with available experimental data. It is shown that in scale-resolving simulations, the LD2C scheme is able to significantly improve the prediction as compared to a conventional second-order central scheme. The scheme is then further assessed and verified in hybrid Reynolds-Averaged Navier–Stokes (RANS)-LES computations for the subsonic and supersonic turbulent channel flow, where excellent agreement with reference DNS and correlations are observed. Moreover, a supersonic base flow is simulated using hybrid RANS-LES, where improved predictions are observed. The LD2C scheme exploits a shock sensor incorporating vorticity and is shown to improve the prediction of the resolved shear stress in the shear layer of compression.

Unsteady flow and pressure pulsation characteristics in centrifugal pump based on dynamic mode decomposition method

Considering the computing accuracy and resources of numerical calculation, a hybrid Reynolds-averaged Navier–Stokes/large-eddy simulations method based on the von Kármán scale and the corrected eddy viscosity has been used to study the unsteady flow structure and pressure pulsation characteristics in a centrifugal pump. The unsteady flow characteristics of the vertical two-stage marine centrifugal pumps with complex structure have been studied. The dynamic mode decomposition method is used to study the internal unsteady flow structure and analyze the mechanism of pressure pulsation in the centrifugal pump. The results show that the unstable flow in impeller is mainly affected by the inflow state, system rotation, and the structure of the impeller. Different inflow states lead to obvious differences of the internal flow states and unsteady flow structures between the first-stage and second-stage impellers. There are complex pressure pulsation characteristics dominated by different frequencies in different parts of a two-stage centrifugal pump. The impeller blade main pass frequency has different causes at different locations in the downstream flow passages. The mutual matching of different numbers of impeller blades and guide vane blades will result in a kind of impeller guide vane blade interaction frequency in guide vane and volute, which will excite higher harmonics of the impeller blade frequency. Other important characteristic frequencies in centrifugal pumps had been analyzed. The pressure pulsation mechanism analysis of the centrifugal pump will help researchers to optimize the design of the centrifugal pump and improve the operation stability of the centrifugal pump. Some possible improvement measures for typical frequency pressure pulsation are recommended.

Analysis of tip leakage flow unsteadiness in a transonic turbine cascade using data-driven modal decomposition methods

The evolution of tip leakage flow and subsequent vortical structures is accompanied by inherent unsteadiness. This paper presents a novel characterization for the unsteady dynamics of turbine tip flow using data-driven, i.e., equation-free modal decomposition methods, which are applied to the hybrid Reynolds-averaged Navier–Stokes/large eddy simulation data at a transonic condition. By combining these techniques, the identified eigen-tuples (eigenvalues, eigenvectors, and time evolution) are well determined, and the differences between the obtained patterns (modes) are also pronounced. The snapshot proper orthogonal decomposition (POD) analysis can isolate the large-scale fluctuating structures that populate the rear part of the suction-side, which is mainly attributed to the shock-induced vortex instability. Similar to the turbulence cascade, macro-scale coherent structures that correspond to the tip leakage vortex shedding phenomena and the subsequently transitional and noisy parts closely related to the dissipation can be well derived by a quadruple reconstruction. Three dynamic mode decomposition (DMD) variants including the amplitude selecting-DMD method, the DMD with criterion method, and the sparsity promoting (SP)-DMD method are also compared in extracting dominant modes from the periodic tip flow, and the SP-DMD method which can distill modes of broadband frequencies and low dissipation is proved to be more conducive to representing and reconstructing the complex tip flow. Additional spectral-kernel-based POD (SPOD) analysis that can identify the similar primary unsteadiness frequencies as the DMD method is also encompassed in this study. Specifically, although it manifests that a physical resemblance of the pattern of pressure fluctuations to tip eddy unsteadiness can be captured by all these approaches, the behavior of small-scale vortical interaction downstream of the trailing edge can be clearly isolated with the intrinsic Karman-type vortex layer shedding process via DMD and SPOD approaches, which also demonstrates that these two techniques are more favorable to decomposing the complex tip flows into uncoupled single-frequency coherent structures compared to the conventional POD method. On this basis, resulting modes of velocity components have been accounted for verifying their contributions to the turbulent kinetic energy fields. The ensuing observations can offer a glimpse of the complex dynamics in the tip region, which also sheds light on features previously masked by conventional analysis approaches.

Experimental and Numerical Investigation of Stall on the NACA Airfoil

In this paper, measurements of the stall of a NACA airfoil section are presented and compared to a simulation with a hybrid Reynolds-averaged Navier–Stokes (RANS)/large-eddy simulation (LES) model with the goal of gaining deeper insight into the flow physics of a stall cell and the turbulent wake in such conditions. Detailed measurements of the evolution of the velocity deficit and turbulence within the wake along with surface pressure measurements and flow visualization provided a comprehensive database for code validation and flow physics studies. The measurements in the Laminar Wind Tunnel of the Institute of Aerodynamics and Gas Dynamics were performed at a Reynolds number of and comprised surface pressures, oil flow surface visualizations, and hot-wire measurements of the wake. The results at were used as validation data for a hybrid RANS/LES run at the same flow conditions. It was shown that the Bernoulli-based detached eddy simulation (BDES) model, which uses a shielding function based on a localized Bernoulli formulation, combined with effective gray area mitigation can capture the stall cell pattern and the point of flow separation at an angle of attack of . Furthermore, the velocity deficit and the Reynolds stresses in the wake show good agreement between simulation and experiment under these conditions.

A grid-adaptive simulation model for turbulent flow predictions

Hybrid Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) methods, abbreviated as hybrid RANS-LES, have been rapidly developed and increasingly used for predicting complex turbulent flows. In this study, a new high-fidelity hybrid RANS-LES strategy that modifies the turbulent viscosity equation using the ratio of grid length scale to turbulent integral length scale based on the Kolmogorov energy spectrum, termed the grid-adaptive simulation (GAS) model, is proposed to achieve high accuracy for turbulent flows using different grid resolutions. Using the shear-stress transport (SST) k–ω model as the baseline turbulence model, the GAS-SST model is validated by predicting three typical turbulent flows with coarse and fine meshes, including periodic hill flow, circular cylinder flow, and simplified tip leakage flow. As a reference, the scale-adaptive simulation (SAS) and delayed detached-eddy simulation (DDES) models are also employed to predict the above three turbulent flows. Solutions of GAS-SST, SAS-SST, and DDES-SST are compared against the high-fidelity data from the experiments or LES solutions. Detailed comparisons show that the GAS-SST model could achieve high accuracy with different grid resolutions for all three validation cases, which means that the GAS model has strong grid-adaptive ability. The results predicted by the GAS-SST model using coarse meshes are usually much more in agreement with the high-fidelity data than those predicted by SAS-SST and DDES-SST models. The GAS model demonstrates the potential to address the accuracy and computational efficiency requirements for predicting turbulent flows.

Impact of Porous Media on Boundary Layer Turbulence

The subsonic flows around NACA 0012 aerofoils with a solid, a porous, and a poro-serrated trailing edge (TE) at a Reynolds number of 1 × 106 are investigated by a hybrid Reynolds-averaged Navier–Stokes (RANS)/large-eddy simulation (LES) approach. The porosity is treated by the method-of-volume averaging. In the RANS, a two-equation low Reynolds number k-ε turbulence model is modified to include the porous treatment. Similarly the equations in the LES are extended by the Darcy–Forchheimer model. The simulation is set up with the broadband turbulent boundary layer trailing edge (TBL-TE) noise prediction as a future objective in mind, i.e., the noise sources in the trailing edge region are captured by the LES. To enforce a physically realistic transition from an averaged RANS solution towards a resolved turbulent flow field, at the inflow of the LES coherent structures are generated by means of the reformulated synthetic turbulence generation (RSTG) method. For the poro-serrated TE, turbulence statistics vary in the spanwise direction between the two extremes of a pure solid and a rectangular porous TE, where porosity locally increases the level of turbulence intensity and alters the near wall turbulence anisotropy.

Vorticity Transport in Different Regions of a Strut-Based Scramjet

A hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation investigation of a strut-based scramjet is performed in the present paper. Both nonreacting and reacting cases in which the same sonic fuel jet mixes/reacts with two different supersonic airstreams ( vs ) are comparatively studied. The vorticity and its influencing factors are analyzed to understand the transport behaviors in the scramjet. Multiple mechanisms contribute to vorticity simultaneously but unequally in different regions. A large part of the flowfield is full of the supersonic airstream with small vorticity. In the mixing region, the averaged vorticity magnitude decreases gradually along the chamber length, and vortex stretch is the leading mechanism of vorticity transport within this region. The baroclinic torque effect is strengthened dramatically by combustion, but is weakened by the incoming flow with higher stagnation enthalpy. Although the near-wall region is small, the averaged vorticity magnitude in this region is extremely large due to the confinement and viscosity effects. Diffusion is the leading mechanism in the near-wall region, but dilatation can dominate vorticity transport due to shock-wave/boundary-layer interaction and pressure rise resulting from combustion.

Scale-Resolving Simulations of a Statistically Targeted Forcing Method for Synthetic Turbulence Generation in a Freestream Turbulence

Abstract Computational fluid dynamics results for synthetic turbulence generation of freestream turbulence by a proposed statistically targeted forcing (STF) method are presented. The STF method has been previously documented for homogeneous isotropic and anisotropic turbulence and formulated to introduce a fluctuating velocity field with a distribution of first and second moments that match a user-specified target mean velocity and Reynolds stress tensor, by incorporating deterministic time-dependent forcing terms into the momentum equation for the resolved flow. This study extends its applicability to generation of freestream turbulence in scale-resolving simulations in which flow is spatially developing. The method provides flexibility in regions where synthetic turbulence needs to be generated or damped, for use in engineering level scale-resolving simulations such as large eddy simulation (LES), and hybrid Reynolds-averaged Navier–Stokes (RANS)-LES. The objective of this study is to evaluate the performance of the proposed STF method in simulations that incorporate monotonically integrated LES (MILES), Smagorinsky (SMAG) LES subgrid stress model, and dynamic hybrid RANS-LES (DHRL) model in reproducing inflow freestream turbulent flow, and the capability of each model to reproduce proper energy decay characteristics downstream of forcing. Results are inter-rogated and compared to target statistical velocity and turbulent stress distributions for inflow turbulence and evaluated in terms of energy spectra. Analysis of the influence of STF model parameters, mesh resolution, and LES subgrid stress model on the results is investigated. Results show that the STF method can successfully reproduce desired statistical distributions in a nearly isotropic freestream turbulent flow.

Open-channel flows through emergent rigid vegetation: Effects of bed roughness and shallowness on the flow structure and surface waves

Free-surface flows through a staggered cylinder array were investigated in an open-channel flume. The cylinders simulated rigid emergent vegetation. Specifically, we studied four flow cases with a two-factor design comprising flow rate (7 and 13 l s−1) and bed-surface state (hydraulically rough and smooth). We have primarily assessed the effects of bed roughness and shallowness on the time-averaged flow structure and the transverse fluctuating flow motion in the cylinder wake. Secondarily, the effects of the former on the vortex-shedding-induced surface waves were quantified. To gain further insight into the bed roughness effect on flow structure, we conducted transient flow simulations using a hybrid Reynolds-Averaged Navier–Stokes/Large Eddy Simulation turbulence model. For all cases, downstream of a cylinder, an upward flow occurs and two counterrotating secondary current cells develop. The two cells bring high-momentum fluid from the high-speed region into the cylinder wake, resulting in a near-bed streamwise velocity-bulge. The measured upward flow and velocity-bulge are greater for the rough-bed cases than for the smooth-bed cases. The simulated upward flow and velocity-bulge increase with an increasing roughness height, while secondary currents decay faster in the longitudinal direction. For the rough-bed cases, in the cylinder wake, the transverse fluctuating flow motion is hindered by the rough-bed induced turbulence over the whole water column, irrespective of the shallowness level. Coupled with the fluctuating flow motion, we have observed for three flow cases noticeable surface oscillations (termed “seiche waves”), whose amplitude decreases with decreasing flow depth. Under the combined effects of strong shallowness and a rough bed, seiche waves vanished.

Significance of generating synthetic turbulence for zonal detached eddy simulation of shallow water flows

A zonal detached eddy simulation (ZDES) is suitable for large-scale open-channel flows with a free surface, e.g. river flow. In ZDES, only a local region is required to be investigated using large eddy simulation (LES) to obtain a high resolution of vortical structures in hydraulic engineering. In this study, a fully non-hydrostatic free-surface flow model is developed for a ZDES-type model. The focus is to analyse the critical strategy of the turbulent forcing method, which is very important for the ZDES-type model. A well-validated synthetic eddy method was established using a numerical framework and applied to generate velocity fluctuations. Based on the synthetic turbulent velocity, a new formulation of the body force term was fed into the momentum equations. A test case of a fully developed open-channel flow with a free surface was used to verify the numerical strategy. Then, another common flow problem of a free-surface flow over a fixed dune was adopted to further validate the numerical approach. The results show that the proposed numerical strategy can promote the application of the hybrid Reynolds-averaged Navier–Stokes/LES method in natural large-scale open-channel flow simulation.

Numerical investigation of flows around an axisymmetric body of revolution by using Reynolds-stress model based hybrid Reynolds-averaged Navier–Stokes/large eddy simulation

Flows around an axisymmetric body of revolution at a zero yaw angle were studied using a hybrid Reynolds-averaged Navier–Stokes (RANS)/large eddy simulation (LES) approach, which employed a full Reynolds stress model (RSM) in the RANS branch with the aim of accounting for the Reynolds stress anisotropy, streamline curvature, and flow separations in the boundary layer. The SUBOFF model without appendages was applied to conduct the simulations, and the Reynolds number based on the free-stream velocity and the length of the body is ReL=1.2×106. The results, including time-averaged Cp, Cf, and velocity statistics, were compared with the experimental data and wall-resolved LES results available in the literature, and the overall agreement of the comparisons was satisfactory. To assess the performance of the RSM-based hybrid RANS/LES approach, we carried out shear-stress transport-based hybrid RANS/LES approach simulations under identical free-stream conditions for comparison. The sensitivity of the hybrid RANS/LES approach to the RANS models was observed for separated flow with surface curvature and adverse pressure gradient-induced separation. The RSM-based hybrid RANS/LES approach was found to provide a better prediction for the unsteady flows near the stern. That is because the effects of the streamline curvature and the strong interactions between individual stresses can be captured by the exact production terms in the RSM-based hybrid RANS/LES approach. These effects are important for predicting the development of turbulent boundary layers along the stern.

Predicting large-scale pool fire dynamics using an unsteady flamelet- and large-eddy simulation-based model suite

A low-Mach, unstructured, large-eddy-simulation-based, unsteady flamelet approach with a generalized heat loss combustion methodology (including soot generation and consumption mechanisms) is deployed to support a large-scale, quiescent, 5-m JP-8 pool fire validation study. The quiescent pool fire validation study deploys solution sensitivity procedures, i.e., the effect of mesh and time step refinement on capturing key fire dynamics such as fingering and puffing, as mesh resolutions approach O(1) cm. A novel design-order, discrete-ordinate-method discretization methodology is established by use of an analytical thermal/participating media radiation solution on both low-order hexahedral and tetrahedral mesh topologies in addition to quadratic hexahedral elements. The coupling between heat losses and the flamelet thermochemical state is achieved by augmenting the unsteady flamelet equation set with a heat loss source term. Soot and radiation source terms are determined using flamelet approaches for the full range of heat losses experienced in fire applications including radiative extinction. The proposed modeling and simulation paradigm are validated using pool surface radiative heat flux, maximum centerline temperature location, and puffing frequency data, all of which are predicted within 10% accuracy. Simulations demonstrate that under-resolved meshes predict an overly conservative radiative heat flux magnitude with improved comparisons as compared to a previously deployed hybrid Reynolds-averaged Navier–Stokes/eddy dissipation concept-based methodology.

Unsteady flow characteristics in centrifugal pump based on proper orthogonal decomposition method

Considering the complexity flow characteristics and the rotation curvature effect on the internal unsteady flow in a centrifugal pump, a hybrid Reynolds-averaged Navier–Stokes/large eddy simulations method based on the von Kármán scale and the correction of the rotation curvature has been established, and its effectiveness has also been verified. Proper orthogonal decomposition method is used to study the internal unsteady flow structure in a centrifugal pump for its advantages on turbulence analysis. Further, the complex flow characteristics of the vertical single-stage marine centrifugal pumps have been studied, and the relationship between the energy distribution characteristics of the internal flow structure and hydraulic performance has been explored under different operating conditions. The results show that when the fluid flows into the impeller, which is affected by the angle of attack of the inflow, system rotation, and bending blades, the flow separation occurs and leads to an unstable flow in the impeller. The obvious pressure fluctuations emerge on the blades surface, which affects the energy conversion ability of blades. The poor inflow in the impeller causes the low local pressure could also affect the anti-cavitation ability of the centrifugal pump. In addition, there are multi-scales of the flow structures in the centrifugal pumps, and the energy distribution of flow in the centrifugal pump with good hydraulic performance is more concentrated in low-order and large-scale flows structures, and the high-order and small-scale flow structure contains less energy, which indicates that the energy distribution characteristics of the flow structures could be used as a criterion to evaluate the performance of the centrifugal pump.

Numerical investigation of unsteady cavitating turbulent flows around a three-dimensional hydrofoil using stress-blended eddy simulation

The mechanism of flow instability, which involves complex gas–liquid interactions and multiscale vortical structures, is one of the hot research areas in cavitating flow. The role of turbulence modeling is crucial in the numerical investigation of unsteady flow characteristics. Although large-eddy simulation (LES) has been used as a reliable numerical method, it is computationally costly. In this work, we used a hybrid Reynolds-averaged Navier–Stokes (RANS) and LES model, that is, stress-blended eddy simulation (SBES), to improve the prediction capability for the cloud cavitating flow. Our hybrid approach introduces a shielding function to integrate the RANS model with the LES applied only regionally, such as to large-scale separated flow regions. The results showed that the periodic shedding of cavity growth, break off, and collapse around a three-dimensional Clark-Y hydrofoil was reproduced in accordance with experimental observations. The lift/drag coefficients, streamwise velocity profiles, and cavity patterns obtained by the SBES model were in better agreement with the experimental data than those obtained by the modified RANS model. The re-entrant jet dynamics responsible for the break off of the attached cavity were discussed. Further analysis of vorticity transportation indicated that the stretching and dilatation terms dominated the development of vorticity around the hydrofoil. In conclusion, the SBES model can be used to predict cavitating turbulent flows in practical engineering applications.

Hybrid RANS–LES Simulation of Turbulent Heat Transfer in a Channel Flow With Imposed Streamwise or Spanwise Mean Temperature Gradient

Abstract Computational fluid dynamics (CFD) results are presented for turbulent flow and heat transfer in a plane channel. This study investigates an idealized fully developed planar channel flow case for which the mean velocity gradient is nonzero only in the wall-normal direction, and the mean temperature gradient is imposed to be uniform and nonzero in the streamwise or spanwise direction. The objective is to evaluate the accuracy of turbulent heat flux predictions using hybrid Reynolds-averaged Navier–Stokes (RANS)–large eddy simulation (LES) models in wall-bounded flows. Results are obtained at Prandtl number of 0.71 and Reynolds number of 180 based on wall friction velocity and channel half-height and are compared to available direct numerical simulation (DNS) data and to a well-validated RANS model (k–ω shear-stress transport (SST)). The specific hybrid RANS–LES (HRL) models investigated include delayed detached eddy simulation (DDES), improved delayed detached eddy simulation (IDDES), and dynamic hybrid RANS–LES (DHRL). The DHRL model includes both the standard formulation that has been previously documented in the literature as well as a modified version specifically developed to improve predictive capability for flows in which the mean velocity and mean temperature gradients are not closely aligned. The modification consists of using separate RANS-to-LES blending parameters in the momentum and energy equations. Results are interrogated to evaluate the performance of the three different model types and specifically to evaluate the performance of the new modified DHRL variant compared with the baseline version.

Numerical Simulation of the Streamwise Transport of a Delta Wing Leading-Edge Vortex

Longitudinal vortices are a common flow phenomenon in the flow around aircraft. They emerge wherever sharp edges are encountered and affect the stability of the boundary layers with which they interact. Hence, for an optimal aircraft design, it is necessary to accurately predict not only the formation of these vortices, but also their downstream evolution. This paper presents a numerical simulation approach for the formation and downstream transport of longitudinal vortices. The approach consists in a hybrid Reynolds-averaged Navier–Stokes/large eddy simulation (RANS/LES) method with synthetic turbulence forcing for which different RANS/LES interface locations are investigated. To provide a quantitative analysis, an isolated sharp-edged delta wing configuration is adopted that allows to get rid of unwanted uncertainties. The presented RANS/LES approach is validated against particle image velocimetry data and compared with a wall-modeled LES and to a differential Reynolds stress model simulation. The results show that the RANS simulation is able to accurately predict the formation of the bulk vortical structure. However, its downstream transport can be accurately predicted only by a scale-resolving simulation. The hybrid RANS/LES approach allows to achieve this while saving computational resources by modeling the roll-up of the vortex. Nevertheless, its predictive accuracy highly depends on the appropriate placement of the RANS/LES interface.