Analysis of URANS capabilities on modeling airfoil pitching flows

Predicting aerodynamic forces on bluff bodies remains to be a challenging task due to the unpredictable flow behavior, specifically at higher Reynolds numbers. Experimental approaches to determine aerodynamic coefficients could be costly and time consuming. In the meantime, use of numerical techniques could also require a considerable computational cost and time depending on complexity of the flow behavior. The research focusses on developing an effective deep learning technique to predict aerodynamic force coefficients acting on elliptical bluff bodies for a given aspect ratio and given flow condition. Collecting data for drag and lift coefficients of several aspect ratios for flow conditions starting from onset of vortex shredding to verge of subcritical region is conducted by an accurate full order model. The specified region will provide a transient flow behavior and thus lift coefficient will be represented in terms of root mean square value and drag coefficient in terms of a mean value. With variations in flow behavior and vortex shredding frequencies, it requires to select an appropriate turbulence model, optimum discretization of fluid domain and time step to obtain an accurate result. Flow simulations are conducted primarily using Unsteady Reynolds Averaged Navier-Stokes Equations (URANS) model and Detached Eddy Simulations (DES) model. Effectiveness in using different turbulence models for specified flow regimes are also explored in comparison to available experimental results. At lower Reynolds numbers, aerodynamic force coefficients for a specified body will only depend on Reynolds number. But after a certain specific Reynolds number, aerodynamic forces are dependent on the Mach number in addition to Reynolds number. Therefore, for higher Reynolds numbers, aerodynamic force coefficients are recorded for multiple Mach numbers with same Reynolds number and will be fed to the neural network. With the development of the machine learning and neural network modelling, many of the fields have nourished and created effective and efficient technologies to ease complex functions and activities. Our goal is to ease the complexity in the computational fluid dynamic field with a deep neural network tool created to predict drag and lift coefficient of elliptical bluff bodies for a given aspect ratio with an acceptable accuracy level. Researchers have developed deep neural network tools to predict various flow conditions and have succeeded with sufficient accuracy and a satisfying reduction of computational cost. In our proposed deep learning neural network, we have chosen to model the network with inputs as the geometry setup and the flow conditions with validated drag and lift coefficients. The model will extract the necessary flow features into filters with the convolution operation performed on the inputs. Our main directive is to create a deep learned neural network tool to predict the target values within an acceptable range of accuracy while minimizing the computation cost.

The prediction of the flow in a gas turbine exhaust diffuser of a combined cycle power plant is particularly difficult as maximum performance is obtained with highly loaded diffusers, which operate close to boundary layer separation. CFD (computational fluid dynamics) simulations then need to cope with complex phenomena such as smooth wall separation, recirculation, reattachment, blockage and free shear layer mixing. Recent studies based on the RANS (Reynolds-Averaged-Navier-Stokes) approach demonstrate the challenge for two-equation turbulence models to predict separation and mixing of the flow correctly in such highly loaded diffusers and identify that more accurate methods are needed. Hence, the application of a hybrid Scale-Adaptive Simulation (SAS) is investigated and the CFD results are compared with experimental results from an in-house test rig. In the present study the flow in a model exhaust diffuser (for heavy-duty gas turbine diffuser applications typical Reynolds number 1.5×106 and inlet Mach number 0.6) is examined with unsteady RANS (URANS) simulations with the SST (Shear Stress Transport) model as well as a hybrid Scale-Adaptive Simulation (SAS) model. The SAS model switches from URANS to a mode similar to a Large Eddy Simulation (LES) in unsteady flow regions to resolve various scales of detached eddies. The current study shows that with the SST model similar results are obtained with RANS and URANS simulations, whereas the more complex SAS model leads to a much better resolution of the unsteady fluctuations. However, the time-averaged results of the SAS calculations overpredict the blockage of the separation and hub wake. This results in an underprediction of the pressure recovery and the mixing of the flow compared to the simpler two-equation models and also compared to experimental results.

The separated turbulent flow past an inclined flat plate with sharp leading and trailing edges was computed based on three different simulation approaches for a reynolds number Rec= 20000 and a high angle of attack α=18°. The simulation techniques applied were the Reynolds‐averaged Navier–Stokes (RANS) equations combined with a one‐equation Spalart–Allmaras turbulence model, the large‐eddy simulation (LES) based on an algebraic eddy‐viscosity model, and a hybrid approach known as detached‐eddy simulation (DES) applying a slightly modified Spalart–Allmaras model in the entire integration domain. DES is a non‐zonal coupling technique of RANS and LES developed in the hope of reducing the large computational resources required for LES computations of turbulent flows with practical relevance. However, the objective of the present study was not to compare the resources (CPU time, memory) required for all three techniques but to investigate and evaluate the quality of the predicted results for RANS, DES and LES. For this purpose, a test case which is favourable to the basic concept of DES and which places special emphasis on the LES part of the DES concept was chosen. This last issue was important since the modified Spalart–Allmaras model applied as a subgrid scale model in the LES part is not as well validated as other models usually applied in LES. For this purpose, an LES prediction on a very fine grid served as a reference case for the evaluation. For all three techniques, predictions with different grid resolutions were carried out and compared with each other based on important integral parameters (e.g. Strouhal number, mean drag and lift coefficients and their standard deviations), the instantaneous and time‐averaged flow structures, and higher‐order statistics. As expected, the pure RANS calculation, although applied as unsteady RANS, failed to predict the unsteady characteristics of the separated flow. In contrast, the DES approach yielded reasonably the shedding phenomenon and some integral parameters. However, analysing the results in more detail led to remarkable deviations between the DES and LES predictions also when the same grid resolution was applied. Especially the free shear layer originating from the leading edge of the plate was not well reproduced by DES, showing strong deficiencies of the model applied as a subgrid scale model. The reasons for this behaviour of the model were analysed in detail. Two basic causes were identified; the first is given by some near‐wall corrections in the finite Reynolds number version of the model which are not working properly in the LES mode of DES. The second is a modified definition of the filter width typically applied for DES, leading to strongly increased values of the eddy viscosity. A revised version of the S–A model taking both issues seriously into account was used as a subgrid scale model in the LES mode. As a direct consequence, much better agreement with the reference LES solution was found for the DES prediction on the coarse grid, eliminating the deficiencies of the original formulation. Copyright© 2003 John Wiley & Sons, Ltd.

Three-dimensional numerical simulations of the flow around the NACA 0015 airfoil were conducted by using the unsteady Reynolds-averaged Navier-Stokes (URANS) and improved delayed detached eddy simulation (IDDES) based on k-ω shear stress transport (SST) turbulence model. The angle of attack range (AoA) is from 0° to 75°. The solver in this paper is developed on an in-house platform HRAPIF based on the finite volume method (FVM) with the elemental velocity vector transformation (EVVT) approach. It uses a density-based method with a low Mach preconditioning technique to accelerate convergence. The inviscid spatial discretization is the 5th-order modified weighted essentially non-oscillatory (WENO-Z) scheme. The results show that the IDDES model can effectively predict lift and drag coefficients at all the studied AoAs and has significant advantages over URANS, both in predicting aerodynamic forces and in simulating flow vortex structures at post-stall AoAs.

The entrapped water in subsea structures with openings significantly influences hydrodynamic forces and structural stability. This study utilises Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the k − ω Shear Stress Transport (SST) turbulence model to investigate oscillatory flows around a subsea cover under varying conditions, including different levels of entrapped water (obtained by different opening dimensions) and KC numbers. Numerical simulations are validated through experimental data from oscillatory flows over a subsea cover on a plate. The study examines lift and in-line force coefficients to assess the impact of opening dimensions and KC numbers on hydrodynamic forces. Findings show that opening size primarily affects the lift coefficient due to changes in the behaviour of entrapped water. The influence of entrapped water decreases as both the KC number and opening size increase. A 30% opening yields the smallest lift-to-in-line ratio ( C Lrms / C Drms ) and reduced C Lrms and C Lstd values. Additionally, the flow fields around the cover are also discussed. Smaller openings reduce the amplitude of velocity, vorticity, and pressure inside the cover. Openings mitigate vortex formation and shedding caused by jets between the structure and the wall. The results also reveal a phase shift in lift coefficients for different opening sizes. These findings provide critical insights for the hydrodynamic behaviour and design of similar subsea structures.

Turbulent flow and heat transfer past a cube (L × L × L) mounted on one wall of a channel of height 2L have served as a benchmark problem to validate computational methods and turbulence models because of the richness in the flow features induced by an unassumingly simple geometry. These include an unstable bi-modal horse-shoe vertical structure upstream of the cube, vortex shedding from the two sides of the cube that extend to the aft of the cube, and periodic separation from the top of the cube. In this study, simulations based on steady and unsteady Reynolds-Averaged Navier-Stokes (RANS and URANS) and large-eddy simulations (LES) were performed to understand the nature of the flow and how that flow affects pressure, drag, and surface heat transfer distributions. For RANS and URANS, the following turbulence models were assessed: the realizable k−ε model, the shear-stress transport (SST) model with and without corrections for curvature, the Reynolds stress model with linear pressure strain (RSM-LPS), and the stress-omega Reynolds stress model. In this study, the RANS and URANS results were generated by using the ANYSYS Fluent code, and the LES results were generated by using the PowerFlow code. Results obtained show RANS to be unacceptable in predicting the reattachment of the separation bubble downstream of the cube. URANS by being able to resolve the oscillatory shedding from the two sides of the cube was able to predict the reattachment with reasonable accuracy. URANS with the SST model that has the curvature correction gave the best results for the reattachment length downstream of the cube (relative error within 5% of the experimental data). Also, this model was able to capture all details of the flow features revealed in the experimental oil-flow study with considerable accuracy. However even with this model, the predicted pressure just downstream of the cube is incorrect by up to 50%. This is because URANS was unable to predict the unstable bi-modal, horse-shoe vortex system upstream of the cube induced by interactions between the spanwise vortices in the turbulent boundary layer and the horsehoe vortex system. This interaction was found to cause the separation bubble on top of the cube to shed periodically. By being able to capture this interaction, LES was able to better predict the pressure just downstream of the cube. This shows why LES is needed and why RANS and URANS are inadequate unless those models can be modified to account for these unsteady interactions. Steady and unsteady RANS based on the SST model were also employed to study the heat transfer about the surface of the cube. Reasonably good results were obtained for the leading and trailing faces of the cube but not for the top and the two side faces. A parametric study on the effects of the turbulent Prandtl number show the connection between the turbulent eddy viscosity and the turbulent thermal conductivity cannot be represented by a simple algebraic equation, the Prandtl number. Thus, better RANS models are needed to capture the physics embedded in the correlations between the fluctuating velocities and temperatures. A paper on this work is being prepared. The order of authorship will be X. Chi, K, Adeel Ahmad, Surya Muthukannan Chinnamani, and T. I-P. Shih. The author of this abstract thanks his students who worked on the different parts of this problem.

This paper studies Co-Flow Jet (CFJ) active flow control (AFC) on a NACA 0015 wingsail airfoil to improve its performance at various angles of attack (AoA). The objective is to enhance lift at minimal energy expenditure for maximizing thrust and net power saving of the wingsail. The parametric study is conducted on the jet momentum coefficient (Cµ), and various locations and sizes of the injection and suction slots. The 2D Unsteady Reynolds Averaged Navier-Stokes (URANS) simulation is conducted with a two-equation k-ω shear stress transport (SST) turbulence model and a Gamma-ReTheta (γ−Reθ) transition model. Numerical validation is conducted on the NACA 0015 airfoil by comparing the lift coefficient (CL) and drag coefficient (CD) with those of experiments, which achieves a good agreement up to the post-stall condition of 17° AoA. From the parametric study of CFJ location and size, the optimized design has the injection slot size of 0.5%C located at 3%C and a suction slot size of 1.2%C located at 85%C. Compared to the baseline airfoil with the maximum CL acquired at 15° AoA, the optimum CFJ airfoil is able to extend the stall AoA to 24° and achieve a CL increase of 122% and a CD reduction of 24%. Using the optimized CFJ wingsail airfoil for ship propulsion, the net power saving is increased by 111% as compared to the baseline wingsail at an apparent wind angle (AWA) of 90°. Furthermore, the practical energy saving of the wingsail is evaluated along three representative shipping routes using the real-world wind and operation data. Depending on the selected route, the CFJ wingsail can increase the energy savings by 63.3% to 109% as compared to the baseline wingsail, which corresponds to a 0.98% to 3.0% reduction of the total energy consumption over the routes.

In the late eighties and up to the beginning of nineties computation of turbulent flows is mostly dominated by RANS (Reynolds Averaged Navier-Stokes Simulation) type modelling. During the last few years URANS (Unsteady RANS) and LES (Large Eddy Simulation) type of approaches have been attempted with some success. Yet, there have been many difficulties when LES is applied to practical engineering problems and to high Reynolds number flows as energy dissipating eddies become really small and mesh resolution required for a reasonably resolved LES approaches that of DNS (Direct Numerical Simulation). An alternative solution suggested was to combine RANS and LES, which in general referred to as Hybrid LES. There have been many proposals for combining RANS and LES in different ways. In this article, some of the issues involved in performing hybrid LES reported in the recent literature is briefly reviewed.

Turbulence model adaptability at critical Reynolds numbers and applications in wake control via fairings

An unsteady three-dimensional numerical study is performed to explore flow and heat transfer in a periodic array of cubic pin‐fins housed inside a channel. The pin‐fins are arranged in an in-line pattern with both streamwise and transverse periodicity set to 2.5 times the pin-fin dimension. Calculations are done in the turbulent flow regime for Reynolds numbers in the range of 7,090–13,280. The unsteady Reynolds-averaged Navier–Stokes (URANS) and energy equations are solved using higher-order temporal and spatial discretization schemes. An unsteady k – ϵ turbulence model is employed to model the unresolved turbulence fluctuations. Large eddy simulations (LES) are also performed for a Reynolds number of 7,090 for validation purposes. The URANS results are able to resolve the discrete large-scale spatial and temporal fluctuations in the flow, and the time-averaged predictions with URANS match the LES results very well. The large-scale fluctuations in the flow appear primarily in the region between the cubic fins, but are linked to low-amplitude oscillations in the outer flow.Three thermal boundary conditions are studied: (1) only channel wall heated (2) only pin‐fins heated, and (3) both channel wall and pin‐fins heated. The overall heat transfer enhancement is about 1.8–2.0 times the heat transfer from a smooth duct flow. The heat transfer from pin‐fins is found to be 5–9% higher than that from the top wall at low Reynolds numbers (7,090 and 8,900), while it is of comparable magnitude at higher Reynolds number (13,280).

This paper presents the workflow and the results of fluid dynamics and aeroacoustic simulations for an air-cooling system as used in electronic devices. The setup represents a generic electronic device with several electronic assemblies with forced convection cooling by two axial fans. The aeroacoustic performance is computed using a hybrid method. In a first step, two unsteady CFD simulations using the Unsteady Reynolds-Averaged Navier-Stokes simulation with Shear Stress Transport (URANS-SST) turbulence model and the Scale Adaptive Simulation with Shear Stress Transport (SAS-SST) models were performed. Based on the unsteady flow results, the acoustic source terms were calculated using Lighthill’s acoustic analogy. Propagation of the flow-induced sound was computed using the Finite Element Method. Finally, the results of the acoustic simulation are compared with measurements and show good agreement.

In this research, the flow physics and aerodynamic performance of dragonfly cross sections, used in Micro Aerial Vehicles (MAVs), in low Reynolds are investigated. The main objective of the research is to study the performance of dragonfly wing cross-sections flapping motion in Reynolds 5000 and 10,000. Pitching motion is one of the most important mechanisms in force lifting generation, and the effects of Reynolds number and mean angle of attack on aerodynamic coefficients have been extensively investigated for the pitching motion. In the present study, the geometry of two cross sections of dragonfly was extracted. Incompressible, two-dimensional and unsteady Navier–Stokes equations have been used to simulate the flow. k − ɛ RNG model was used for turbulence modeling. To simulate the wing pitching motion, the dynamic mesh method was used. The results showed that in flapping motion, pitching-up rotation has caused a rapid increase in lift coefficient. Furthermore, it was found that the absence of stall does not increase the lift and drag coefficients, while formation of new strong vorticity layers have caused an increase in lift coefficient. On the other hand, corrugations on the cross sections of the dragonfly in the pitching motion cause the delay of separation and increasing the lift coefficient. In flapping motion and the pitching motion, the lift coefficients of three cross sections were increased due to stronger vorticity layers by reducing the Reynolds number. Due to the existence of corrugations, the first and the second cross sections have good aerodynamic performance, compared to the flat plate. The comparison carried out in the current research showed that the second cross section is a proper replacement for the flat plate in MAVs.

This paper presents an evaluation of five different turbulence models by comparing the numerical data derived from these models using ANSYS Fluent with experimental data at a Reynolds number and a Mach number of 0.05 × 106 and 0.015 respectively based on the centerline chord of the airfoil for the flow over NACA 0012 and NACA 2412 airfoils. Moreover, the aim of the present study is to demonstrate the difference in aerodynamic characteristics of the airfoils in order to find aerodynamically more advantageous airfoil. It is concluded that Spalart-Allmaras model and k-ω SST model are capable of providing the most accurate prediction for lift coefficient at a low angle of attack for both airfoils. Standard k - ε model gives a slightly low value of lift coefficient at low angle of attack and slightly high value of lift coefficient at high angle of attack for both airfoils. k-ω SST model, Spalart-Allmaras model, Transition k-kL - ω model, and γ-Rⅇθ Transition SST model can predict drag coefficient reasonably at low angle of attack. At a high angle of attack, however, no turbulence model is able to give a satisfactory prediction for lift coefficient as well as drag coefficient, which implies that these models are unable to predict post-stall characteristics. NACA 2412 airfoil produces more lift coefficient than that of the NACA 0012 airfoil at all angles of attack. Moreover, the drag coefficient of NACA 2412 airfoil is less than that of the NACA 0012 airfoil, which implies that NACA 2412 airfoil exhibits better aerodynamic performance. The lift to drag coefficient ratio of NACA 2412 airfoil is also higher than that of the NACA 0012 airfoil indicating NACA 2412 airfoil to be more fuel economic.

An accurate noise prediction is important in order to reduce noise emission significantly and to prevent expensive after-design treatments. This study aims to examine the aerodynamics and aeroacoustics performance of an open system consisting of an axial fan and a heat exchanger where hybrid method incorporating CFD (Computational Fluid Dynamics) and CAA (Computational Aeroacoustics) is used to predict the noise behavior. The hybrid model method used consists of three steps. Firstly, the flow is computed by means of flow-computed fluids and the pressure fluctuations are obtained. This is followed by the acquisition of acoustic signals from these fluctuations and the attainment of a sound pressure level approach with the FW-H (Ffowcs Williams & Hawkings) model.  Unsteady flow field of the air channel case was obtained by using different turbulence models. The SAS model is capable of resolving largescale turbulent structures without the time and grid-scale resolution restrictions of LES (Large Eddy Simulations), often allowing the use of existing grids created for URANS simulations. For this reason, two different turbulence models, namely URANS (Unsteady Reynolds Averaged Navier Stokes) model, SAS (Scale Adaptive Simulations) model have been applied. Acoustic sources were computed based on the pressure fluctuations and sound pressure level and frequency dependent graphics were plotted with Fast Fourier Transform. On the other hand, acoustic measurements were performed in a semi-anechoic chamber for both of them. When the experimental and numerical results were compared with the previously determined receiver points, the accuracy rate was obtained as SAS, URANS respectively.

In this paper, 2-D computational analyses were conducted for unsteady high Reynolds number flows around a smooth circular cylinder in the supercritical and upper-transition flow regimes, i.e. 8.21×104<Re<1.54×106. The calculations were performed by means of solving the 2-D Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with a k-ɛ turbulence model. The calculated results, produced flow structure drag and lift coefficients, as well as Strouhal numbers. The findings were in good agreement with previous published data, which also supplied us with a good understanding of the flow across cylinders of different high Reynolds numbers. Meanwhile, an effective measure was presented to control the lift force on a cylinder, which points the way to decrease the vortex induced vibration of marine structure in future.