Laminar-turbulent Transition Model Research Articles

Determination of Wind Force Acting on an Object in the Shape of a Bent Pipe

The aim of the paper is to determine the aerodynamic forces acting on a torus-shaped structure fragment at high wind velocity which are impossible to obtain from the existing standard EN 1991-1-4 (the so-called wind standard). The most important problem is the correct modeling of turbulence and laminar-turbulent transition in the conditions of flow interference resulting from the presence of other obstacles. For this reason, forces are obtained by two methods: Fluid-Structure Interaction (FSI, force transfer) and User-Defined Functions (UDF). Variations of the total aerodynamic lift force of the half of the torus with angle β and velocity of wind w, and the formula for estimating the horizontal force Pz perpendicular to drag force are presented. Additionally, useful engineering parameters (such as pressure distribution and air velocity field) are determined. The forces of wind influence on two cylinders and a torus-shaped object are obtained and compared

Aerodynamic Characterization of a Hypersonic Projectile Using Acceleration-Based Measurements and Numerical Methods

Armor-piercing fin-stabilized projectiles stand out by their exceptionally high slenderness ratios and ground-level flight at super- and hypersonic speeds. As space constraints limit the integration of measurement equipment into such slender test models, nonintrusive measurement techniques become favorable. The present analysis demonstrates a renewed approach to the free-flight technique, which sets models into unconstrained flight through the test section. It enabled the evaluation of the quasi-steady drag, lift, and pitching moment characteristics, and, to some extent, also the dynamic pitch damping characteristics. An alternative to the free-flight technique is the free-oscillation technique, which limits the motion to a rotation around the center of gravity. The free-oscillation technique allowed a higher-fidelity analysis of the static and dynamic pitching moments. Both methods were based on the analysis of the accelerations derived from trajectory tracking. This acceleration-based approach enabled the evaluation of the highly nonlinear characteristics. The high slenderness of the investigated projectile leads to a significant contribution of the viscous forces to the overall drag. These viscous forces are sensitive to the laminar-turbulent boundary-layer state, as well as the thermal boundary conditions. The application of a high-resolution schlieren system enabled the assessment of the local laminar-turbulent boundary-layer state, while the use of low- and high-enthalpy testing facilities enabled the assessment of the aerothermal influence. Steady-state Reynolds-averaged Navier–Stokes simulations, which included laminar-turbulent transition modeling, were employed to replicate the results of the experimental efforts.

Numerical study of the effects of discontinuous ice on three-dimensional wings

Ice accretion has adverse effects on aircraft, so it is very important to study the flow field of iced wings. Unlike previous studies that typically focused on continuous ice, this paper conducts a numerical study of the flow field around discontinuous ice. A modified laminar-turbulent transition model that incorporates large separation correction is employed for the study of discontinuous ice. An iced swept wing was chosen to validate the modified model's capability in predicting aerodynamic performance. The impact of discontinuous ice on both straight and swept wings was studied. For the straight wing, the effect of discontinuous ice with different gap widths on aerodynamic characteristics was investigated. The results show that the reduction in lift caused by discontinuous ice is typically lower than that caused by continuous ice. However, at angles of attack less than 7°, small-gap discontinuous ice causes more substantial reductions in lift compared to continuous ice. This is because small-gap ice causes greater separation at the leading edge, significantly reducing the suction peak of pressure. For the swept wing, the effect of sweep on discontinuous ice was studied. It was found that the swept effect causes asymmetric separation behind the ice shape. At higher angles of attack (greater than 4°), the impact of different ice configurations on lift shows an opposite trend on swept wings compared to straight wings.

Computable turbulence modeling of laminar-turbulent transition characterized boundary layer flows with the aid of artificial neural network

The continuous development of machine learning algorithms has stimulated the technological revolution on turbulence modeling for Reynolds-averaged Navier–Stokes (RANS) simulations. In this paper, a computable transition-enabled turbulence model is developed with the aid of an artificial neural network (ANN), which maps the relation between the mean flow variables from the shear stress transport (SST) model on coarse grid and the turbulent viscosity from SST-γ model on fine grid. It turns out that the ANN model can predict the aerodynamic integral quantities, transition onset location, laminar separation bubble structure, streamwise velocity distribution, etc. with superior accuracy and robustness for subsonic and moderate transonic airfoil flows. Meanwhile, the ANN model can significantly improve the convergence property with a much higher convergence speed of the model equation in comparison with traditional transition-enabled RANS model. Moreover, due to the lack of solving RANS model equations and use of grid interpolation technology, the computation speed of this ANN model is improved by a factor of about six over the conventional benchmark model. Therefore, the present computable ANN model provides a new perspective for high-efficiency simulation of transition-characterized flows in engineering applications.

Ice accretion simulation incorporating roughness effects and separation correction through a transition model

In the simulations of ice accretion, accurately modeling the impact of surface roughness and calculating the separated flow are crucial aspects. A modified laminar–turbulent transition model that incorporates both separation and roughness-induced transition correction is employed for icing simulations. An iced airfoil was selected to validate the modified model's predictive capability for large separation, and a rough airfoil was utilized to assess the modified model's capability in simulating roughness effects. Icing simulations under different conditions are conducted for an airfoil. The results indicate that roughness induces an earlier laminar–turbulent transition, thereby influencing the heat transfer coefficients. The effects of separation and roughness correction on rime icing simulation are minimal, but they are significant for glaze icing simulation, resulting in more accurate predictions of maximum ice thickness and ice horn orientation. The simulated glaze ice causes more pronounced separation and more substantial reductions in lift compared to simulated rime ice. The modified model can accurately predict the separation bubble and reduce the prediction error of lift over 60%.

Numerical Approach Based on Solving 3D Navier–Stokes Equations for Simulation of the Marine Propeller Flow Problems

The report presents the approach implemented in the Russian LOGOS software package for the numerical simulation of the marine propeller flow problems using unstructured computational meshes automatically generated by the mesh generator. This approach includes a computational model based on the Navier–Stokes equation system and written with respect to the physical process: the turbulent nature of flow with transient points is accounted using the Reynolds Averaged Navier–Stokes method and the k–ω SST model of turbulence by Menter along with the γ–Reθ (Gamma Re Theta) laminar-turbulent transition model; the Volume of Fluid method supplemented with the Schnerr–Sauer cavitation model is used to simulate the cavitation processes; a rotating propeller is simulated by a moving computational mesh and the GGI method to provide conformity of the solutions on adjacent boundaries of arbitrarily-shaped unstructured meshes of the two domains. The specific features of the numerical algorithms in use are described. The method validation results are given; they were obtained because of the problems of finding the performance curves of model-scale propellers in open water, namely the problems of finding the performance of propellers KP505 and IB without consideration of cavitation and the performance of propellers VP1304 and C5 under cavitation conditions. The paper demonstrates that the numerical simulation method presented allows for obtaining sufficiently accurate results to predict the main hydrodynamic characteristics for most modes of operation of the propellers.

Simulation of the flow past a circular cylinder from sub-critical to super-critical Reynolds numbers using an intermittency-based hybrid model

In this work, we investigate the prediction of the flow around a circular cylinder from sub-critical to super-critical Reynolds numbers using a hybrid approach which combines a dynamic variational multiscale (DVMS) large-eddy simulation (LES) model and a transitional RANS model. In the proposed hybrid approach, the variational multiscale model, aiming to limit the effects of the subgrid-scale (SGS) model to the smallest resolved scales, is combined with the dynamic procedure which provides a tuning of the SGS dissipation in space and time. For representing laminar to turbulent boundary layer transition, the equations of the RANS part of the hybrid approach are completed with a transition model based on an intermittency transport equation. Results are compared to those of other numerical simulations in the literature and with experimental data. They highlight the overall good prediction capabilities of the proposed hybrid strategy for the simulation of such massively separated flows, even with the use of coarse meshes. In particular, the intermittency-based hybrid model was found to be able to predict the drag crisis of a circular cylinder, as well as the sharp increase in vortex shedding frequency, unlike the equivalent hybrid approach when no laminar-turbulent transition model is introduced.

Numerical Study of the Influence of the Critical Reynolds Number on the Aerodynamic Characteristics of the Wing Airfoil

The paper reports the results of a study concerned with the influence of the size of the leading edge laminar bubble on the aerodynamic characteristics of the HGR01 airfoil. The completely turbulent and transient flows are considered. The mechanism of the appearance and interaction of laminar and turbulent flow separation near the leading and trailing edges of the airfoil is studied in detail. In the paper, the dependence of aerodynamic forces on the critical Reynolds number for the HGR01 airfoil is discussed. It has been established that the separation bubble at the leading edge can only be obtained using the laminar–turbulent transition model. Fully turbulent models are not able to show this feature of the airfoil flow. Graphs of the lift coefficient as a function of the critical Reynolds number, as well as the pressure distribution as a function of the size of the laminar bubble, are shown.

Uncertainty Quantification of Laminar–Turbulent Transition on Airfoil and Fuselage

The effect of uncertainty in Reynolds-averaged Navier–Stokes (RANS) simulations is determined through the application of uncertainty quantification (UQ). In this study, the sensitivity of aerodynamic coefficients to uncertainty in the freestream turbulence intensity (FSTI) and surface roughness is computed for an airfoil (SC1095) and a rotorcraft fuselage model (Rotor-Body-Interaction-Mod7). A laminar–turbulent transition model has been extended to consider roughness-induced transition by incorporating an existing roughness-induced transition model. The current UQ analysis is based on the Monte Carlo method with the Gaussian distributions of uncertain input parameters. A surrogate model allows for incorporating the results of intensive RANS simulations into the Monte Carlo method. The surrogate model is generated using a cubic interpolation for a single-uncertain parameter or a radial basis function for multiple-uncertain parameters. The stochastic standard deviation is measured using combined FSTI and roughness uncertainties. The standard deviation of each aerodynamic coefficient depends on flow conditions (angle of attack, mean FSTI, and mean roughness) and the transition type (natural or separation-induced). The main source of output uncertainty is identified between the unknown FSTI and roughness under various flow conditions.

Numerical investigation of the unsteady effect owing to oscillation on airfoil icing

Modeling the unsteadiness owing to body motion is essential for accurately simulating ice accretion on a moving object. For an oscillating airfoil, high-frequency oscillations have been assumed to occur in slow motion to apply a quasi-steady approach to numerical analysis. The method raised arbitrariness in the number of data exchanges and the initial angle of attack. Simultaneously, the oscillation owing to a change in the angle of attack was considered for each step. Subsequently, the quasi-unsteady approach was proposed, the results from which exhibited good agreement for a few ice shapes, unlike that of the extended quasi-steady approach. However, whether a quasi-unsteady approach is sufficiently accurate for simulating ice accretion on a two-dimensional oscillating airfoil under various conditions still remains unknown. This study analyzed the unsteady effect on collection efficiency and convective heat transfer using the quasi-unsteady approach. Furthermore, as the static case, this approach demonstrated that the roughness and laminar-turbulent transition model for the oscillating airfoil can improve the prediction of the ice shape. The results obtained from this approach using the improved model exhibited good agreement with previously reported experimental results.Fig. 11 compared predicted ice shape with different roughness height. Fig. 11(a) is Run 61 with a reduced frequency of 0.044 at an LWC = 1 g/m3, and Fig. 11(b) is Run 50 with a reduced frequency of 0.027 at an LWC =.55 g/m3 The dotted line indicates the experimental ice shape and the dashed line indicates the numerical results applied to the empirical roughness model. The solid line indicates ice shape with the roughness size when the direction and size of the ice horn closely match the experimental values. When an empirical roughness model is applied, the ice horns are usually concentrated near the leading edge. Roughness increases convective heat transfer and premature laminar turbulence in ice accretion. The convective heat transfer peaks near the leading edge with a high roughness value; therefore, the ice horn gathers near the leading edge. Low roughness delayed the formation of the ice horn due to the decreased heat transfer rate, which balances the latent heat of icing, thereby allowing the droplets to freeze before moving to the trailing edge. In Run 61, the ice shape was predicted accurately at 20 % of the empirical roughness and 40 % roughness height of Run 55. When the roughness height was reduced, such as in the case of Run 61 with 10 % empirical roughness, the primary horn reduced, whereas the secondary horns increased.

Laminar–Turbulent Transition Prediction on Industrial Computational Fluid Dynamics Applications

This paper presents Ansys Fluent laminar–turbulent transition results using the shear stress transport model applied to the workshop cases of the First American Institute of Aeronautics and Astronautics Computational Fluid Dynamics (CFD) Transition Modeling Prediction Workshop. The key objectives of this workshop were to assess the current state-of-the-art laminar–turbulent transition models in an industrial Computational Fluid Dynamics environment and to determine and document the best practices to simulate laminar–turbulent transition flows. Sensitivity of the shear stress transport model to mesh refinement was established on a zero-pressure-gradient flat plate. Two other cases [a two-dimensional natural laminar flow (NLF) (1)-0416F airfoil, and a scaled Common Research Model (CRM)-NLF aircraft model] were selected as validation cases using a hierarchy of structured and unstructured meshes. Due to the complexity of the geometry and the airflow around the Common Research Model (CRM)- Natural laminar Flow (NLF) aircraft model, mesh adaptation cycles were also conducted to capture the shock, the wake, and the wing-tip vortices produced by the CRM-NLF. The accuracy of the model is evaluated using transition location measurements obtained with temperature-sensitive paint, pressure coefficient distributions at multiple wingspan stations, and aerodynamic coefficients at numerous angles of attack. The outcome of these comparisons will provide guidelines to conduct laminar–turbulent transition simulations with the model on simple and complex aerospace designs.

Effect of Turbulence Modeling Selection Within Helios for Small Quadrotor Aerodynamics

The high-fidelity rotorcraft simulation framework HPCMP CREATE-AV Helios was used to study the impact of turbulence modeling methods on the prediction of a small quadrotor unmanned aerial vehicle. First, the vehicle’s rotor was simulated as an isolated rotor in both hover and forward flight for turbulence model assessment. Predictions using a fully turbulent turbulence model were compared with predictions from several laminar–turbulent transition models. Both comparison with experiments and computational performance were considered. The laminar–turbulent transition models predicted flow separation near the tip for all flight conditions simulated, leading to lower torque and thrust than the fully turbulent model, which predicted attached turbulent flow. The fully turbulent model was shown to provide reasonable comparison with experiments with the lowest computational cost and was used for full vehicle simulations. The full quadrotor configuration calculations were performed in hover and forward flight. The forward flight simulations were performed with and without a detached eddy simulation method, and similar interactional aerodynamics were predicted. The Helios simulation suite is shown to be well-suited for prediction of aerodynamic performance of small-scale rotorcraft.

Numerical Investigation of Unsteady Boundary Layer Transition on a Dynamically Pitching Rotor

Predictions of unsteady boundary layer transition are performed on a four-bladed rotor in axial inflow using a computational fluid dynamics approach. The configuration is based on experiments performed at the German Aerospace Center (DLR) in the 1.6-m × 3.4-m wind tunnel in the rotor test facility (RTG). Simulations are performed using the NASA OVERFLOW 2.3 solver with hybrid RANS/LES and laminar turbulent transition modeling. Solutions are based on a hover tip Mach number of 0.143 with prescribed cyclic pitching conditions. Computational methods and grid generation are described. The rotor flow field is analyzed, and the effect of transition modeling on unsteady boundary layer transition prediction is assessed. Laminar-turbulent transition predictions and rotor performance are compared to experimental measurements obtained at the DLR RTG. A study of sensitivity was performed on freestream turbulence intensity to investigate its effect on predicted rotor transition.

High-Lift Computational Strategy for Slotted, Natural-Laminar-Flow Airfoils

This work uses a computational fluid dynamics approach to evaluate the ability of the aft element of a slotted, natural-laminar-flow airfoil, designed for transonic applications, to function as a high-lift device. The analysis is based on the Reynolds-averaged Navier–Stokes equations with a laminar–turbulent transition model for subsonic flow at representative flight conditions and a fully turbulent model for the aft-element optimization. Results obtained at angles of attack near maximum lift contribute to the understanding of stall characteristics and show that maximum aerodynamic efficiency is obtained with a constant slot width between the flap and main element. Results indicate that the microflap can augment the effectiveness of the aft element. Drag calculations when compared with angle of attack and lift show insight on the aerodynamic efficiency of the microflap system in landing as a lift effector, as well as a drag device. Pitching-moment data are also presented for completeness. Results obtained with Fowler flaps are consistent with other studies of extended-flap configurations; more specifically, the aforementioned velocity ratio decreases toward the aft-element trailing edge, indicating that the multi-element high-lift system is operating as intended.

Effect of Crossflow Transition on the Pressure-Sensitive-Paint Rotor in Hover

The amplification factor transport transition model has been extended to include crossflow transition and applied to a rotor in hover. Validation of the crossflow model implementation has been performed using standard test cases. The pressure-sensitive-paint rotor is simulated in hover for various collective pitch angles as a test case for the transition model. The OVERFLOW 2.2n solver is used with laminar–turbulent transition modeling and hybrid Reynolds–averaged Navier–Stokes/large-eddy simulation turbulence modeling enabled. Grid generation, numerical methods, and transition modeling are described. The effects of crossflow transition are qualitatively and quantitatively assessed through comparisons to experiments, to a transition model without crossflow, and to a fully turbulent model. Additionally, several techniques for evaluating unsteady boundary-layer transition are described and are applied to solutions for the pressure-sensitive-paint rotor in hover.

Recurrent neural network for end-to-end modeling of laminar-turbulent transition

Abstract Accurate prediction of laminar-turbulent transition is a critical element of computational fluid dynamics simulations for aerodynamic design across multiple flow regimes. Traditional methods of transition prediction cannot be easily extended to flow configurations where the transition process depends on a large set of parameters. In comparison, neural network methods allow higher dimensional input features to be considered without compromising the efficiency and accuracy of the traditional data-driven models. Neural network methods proposed earlier follow a cumbersome methodology of predicting instability growth rates over a broad range of frequencies, which are then processed to obtain the N-factor envelope, and then, the transition location based on the correlating N-factor. This paper presents an end-to-end transition model based on a recurrent neural network, which sequentially processes the mean boundary-layer profiles along the surface of the aerodynamic body to directly predict the N-factor envelope and the transition locations over a two-dimensional airfoil. The proposed transition model has been developed and assessed using a large database of 53 airfoils over a wide range of chord Reynolds numbers and angles of attack. The large universe of airfoils encountered in various applications causes additional difficulties. As such, we provide further insights on selecting training datasets from large amounts of available data. Although the proposed model has been analyzed for two-dimensional boundary layers in this paper, it can be easily generalized to other flows due to embedded feature extraction capability of convolutional neural network in the model.

Testing of a new algebraic laminar-turbulent transition model on a range of airfoils at moderate Reynolds numbers

High quality prediction of laminar-turbulent transition for flow around airfoils is an important task for industrial applications. Prediction of such flows is usually carried out using the RANS framework in combination with transition models. The present study shows the capability of a recently developed algebraic transition model for prediction flows around different airfoils. The results of the model demonstrate satisfactory agreement with the experimental data and with results of existing differential models.

A two-equation local-correlation-based laminar-turbulent transition modeling scheme for external aerodynamics

A local-correlation-based laminar-turbulent transition model has been developed for external aerodynamic problems and coupled with the Spalart-Allmaras (S−A) turbulence model. This laminar-turbulent transition modeling scheme is then termed as the γ−SA model. It has been validated against a series of two- and three-dimensional test cases with a relatively wide range of Reynolds numbers and free-stream turbulence intensity levels. Results show that the γ−SA model can give accurate transition-onset-location predictions, as well as more accurate predictions on the force coefficients than the underlying S−A turbulence model at various angles of attack. Comparison with the classic SST−γ−Reθ transition model shows that the γ−SA model can achieve comparable results, with the expect of less time or memory consumption, as the γ−SA model has two equations fewer than the former one. With a newly proposed local-farfield mixed turbulence intensity strategy, the turbulence intensity on the inlet or farfield boundary can be conveniently set directly with the value tested for the wind tunnel or the value as expected, while achieving local turbulence intensity variation. Moreover, the γ−SA model employs only local variables, which makes it totally modern-CFD-compatible and very appropriate for large-scale parallel running using unstructured grids. In the future, it can be modified into a robust and efficient DES-type model with the consideration of transition process.

Stability-Based Transition Model Using Transport Equations

A laminar–turbulent transition model is presented. The model accounts for longitudinal transition mechanisms (i.e., Tollmien–Schlichting-induced transition) and was calibrated using stability computations on similar boundary-layer profiles at Mach number ranging from zero to four on both adiabatic and isothermal walls. The model embeds the so-called C1 criterion for transverse transition mechanisms (i.e., crossflow-wave-induced transition). Finally, the model accounts for separation-induced transition by means of the Gleyzes criterion. The transition model is written under a transport equation formalism. Validations are performed on three-dimensional configurations, and comparisons against results from a transition database method for natural transition, exact linear stability computations, and experimental data are shown.

Investigation of Various Approaches to the Modeling of Laminar-Turbulent Transition in Compressible Separated Flows

Investigation of Various Approaches to the Modeling of Laminar-Turbulent Transition in Compressible Separated Flows