Harish Gopalan

A unified RANS-LES model

Large eddy simulation (LES) is computationally extremely expensive for the investigation of wall-bounded turbulent flows at high Reynolds numbers. A way to reduce the computational cost of LES by orders of magnitude is to combine LES equations with Reynolds-averaged Navier-Stokes (RANS) equations used in the near-wall region. A large variety of such hybrid RANS-LES methods are currently in use such that there is the question of which hybrid RANS-LES method represents the optimal approach. The properties of an optimal hybrid RANS-LES model are formulated here by taking reference to fundamental properties of fluid flow equations. It is shown that unified RANS-LES models derived from an underlying stochastic turbulence model have the properties of optimal hybrid RANS-LES models. The rest of the paper is organized in two parts. First, a priori and a posteriori analyses of channel flow data are used to find the optimal computational formulation of the theoretically derived unified RANS-LES model and to show that this computational model, which is referred to as linear unified model (LUM), does also have all the properties of an optimal hybrid RANS-LES model. Second, a posteriori analyses of channel flow data are used to study the accuracy and cost features of the LUM. The following conclusions are obtained. (i) Compared to RANS, which require evidence for their predictions, the LUM has the significant advantage that the quality of predictions is relatively independent of the RANS model applied. (ii) Compared to LES, the significant advantage of the LUM is a cost reduction of high-Reynolds number simulations by a factor of 0.07 Re 0.46 . For coarse grids, the LUM has a significant accuracy advantage over corresponding LES. (iii) Compared to other usually applied hybrid RANS-LES models, it is shown that the LUM provides significantly improved predictions.

Realizable versus non-realizable dynamic subgrid-scale stress models

The existence of many different dynamic large eddy simulation (LES) methods leads to questions about the theoretical foundation of dynamic LES methods. It was shown recently that the use of stochastic analysis enables a theoretically well based systematic derivation of a realizable linear dynamic model (LDM) and a realizable nonlinear dynamic model (NDM). A priori and a posteriori analyses of turbulent channel flow are used here to study the characteristic properties of these dynamic models. The LDM and NDM are compared with other dynamic models: the non-stabilized and stabilized dynamic Smagorinsky model (DSM), which is used in many applications of LES, and Wang-Bergstroms dynamic model (WBDM), which represents an extension of the DSM. The DSM and WBDM do not represent realizable models because they are not derived as consequences of a realizable stochastic process. The comparisons reported here show that the LDM and NDM are based on a dynamic model formulation that avoids shortcomings of existing conce...

Simulation of Turbulent Channel Flow Using a Linear and Non Linear realizable Unified RANS-LES Model

Large Eddy Simulation (LES) is computationally expensive at high Reynolds numbers (Re), especially for the accurate resolution of the near-wall region in wall-bounded flows. Hence, hybrid turbulence models have been developed to reduce the computational cost of LES. In the hybridmodeling approach, the near-wall region is modeled using Reynolds-averaged Navier-Stokes (RANS) methods while regions away from the wall are modeled using LES. The most commonly used hybrid models do not accurately predict the mean streamwise velocity and the Reynolds normal stresses for attached flows without empirical modifications. Hence, there is a need for the development of unified turbulence models which can be used continuously as LES or RANS methods for both attached and separated flows. The current study investigates the performance of a unified turbulence model derived using stochastic analysis. Turbulent channel flow is used as the test case to evaluate the performance of the unified turbulence model. The current study provides encouraging results for the use of the unified turbulence model considered to investigate complex turbulent flows.

Lift Enhancement of Flapping Airfoils by Generalized Pitching Motion

The pitching and plungingmotions of airfoils have received a lot of attention recently, due to the increased interest in the design ofmicro air vehicles. The use of combined pitch–plungemotionwith phase difference between themhas often been used for the generation of thrust and lift. These aerodynamic forces could be significantly enhanced under similar operating conditions by using generalized pitch motion with variable center of wing rotation. The current study investigated the flowfield and aerodynamic forces for this generalized pitchingmotion. Two-dimensional rigid airfoils were taken as prototypes of micro air vehicle wings. First, the computational results were compared with the available measurements for an SD 7003 airfoil in pure-pitch and pure-plunge motions at Re 10; 000. Good agreement was observed between the numerical computations and the experimental results in terms of streamwise velocity, location of the vorticity contours, and wake profiles. Next, the pure-pitch case was considered with the stationary centers of rotation located at different positions along the chord of the airfoil. It was found that the maximum value of the computed average coefficient of lift was obtained when the pitching axis was positioned at either the leading edge or the trailing edge. The generalized pitching motion computations were performed next. It was observed that a phase difference of 90 deg between the pitching motion and the motion of the axis caused a twofold increase of the mean coefficient of lift compared to the pitching about leading edge and combined pitch– plungemotionwith a 90 deg phase difference. The stability of the leading-edge vortexwas found to be responsible for the enhancement of lift by reduction in pressure at the upper surface of the airfoil. However, thrust force was not generated by applying the generalized pitching alone, whereas it was generated by the combined pitch–plunge motion. Finally, a generalized pitching motion combined with a superimposed plunging motion was studied. It was found that for this motion, thrust was generated and the generated lift was higher than that for the generalized pitchingmotion. This resultmay help in the use of superposition of kinematicmotions of wings to produce the desired amount of lift and thrust.

Turbulence and the Isolated Wind Turbine

The turbulence in the atmospheric inflow to a wind turbine as well as the turbulence produced by the wind turbine are considered. A good understanding and ability to model this turbulence is critical for designing turbines with higher efficiencies and greater reliability. The atmospheric boundary layer is first discussed broadly followed by a presentation of past and present measurement and modeling efforts. The effects of the atmospheric boundary layer turbulence on wind turbine aerodynamics and aeroelastics are then discussed, and the importance of wake turbulence is considered. Attempts to use control to mitigate the effects of turbulence on the wind turbine are presented, and the extreme challenges of modeling all the relevant scales of turbulence required to accurately model wind turbines is discussed. As each of these items is presented, future needs for bettering our understanding of turbulence relevant to a wind turbine are identified. The presentation of these subjects makes it clear that there is still much to be learned about the turbulence associated with wind turbines that could impact future designs.

Stream function-potential function coordinates for aeroacoustics and unsteady aerodynamics

‘Stream function as a coordinate approach’ (SFC) combined with compact high-order finite difference schemes has been developed and applied to aeroacoustics and unsteady aerodynamics problems. Straightforward implementation of SFC creates coarse grids at the vicinity of stagnation points that smears high-order numerical computations. Grid clustering is employed to resolve coarse grid near stagnations points. The agreement between numerical results and particle image velocimetry (PIV) measurements for flapping airfoil shows the robustness of the current approach for performing high-order computations.

Numerical Investigation of Mini Wind Turbines Near Highways

High-speed vehicle motion on the highways produces localized winds whose energy can be harnessed. These local winds have less variability especially if the highway traffic is constant. The idea of extracting energy from highway winds has been conceptualized in many studies before. However, the feasibility of this idea has never been tested using analytical, computation, or experimental methods. In this study, we numerically compute the amount of power that can be extracted from local highway winds due to vehicular motion. A unsteady Reynolds-averaged Navier–Stokes (URANS) method is used for modeling the atmospheric boundary layer (ABL). Realistic computer-aided design (CAD) models of cars and trucks separated by spacing information obtained from the existing standards are used to model the vehicle motion. A vertical axis wind turbine (VAWT) is used for extracting energy from the wind. The entire framework of ABL, vehicles, and turbine is simulated using overset grids and multiple translating and rotating frames of reference. Many vehicle motion scenarios were compared to the case of an isolated wind turbine. The initial results show a significant increase in the power that can be extracted by these turbines. The average extracted power increases about 317% when compared to the case without any vehicular motion. Field measurements or wind tunnel studies are required to provide validation for the computations and to determine if more advanced turbulence modeling methodologies have to be employed for these studies.

Unified RANS-LES Simulations of Turbulent Swirling Jets and Channel Flows

The accurate and efficient simulation of both attached and separated flows represents a huge challenge. RANS methods suffer from the lack of ability to simulate instantaneous turbulence structures, and LES methods are computationally very expensive regarding the simulation of wall-bounded flows, which have to be considered very often. A promising alternative is the use of hybrid RANS-LES methods, but existing hybrid methods like DES face many questions. The paper focuses on the use of unified RANS-LES methods implied by stochastic analysis as an alternative to using existing hybrid RANS-LES methods. The theoretical basis of the approach applied and applications to turbulent channel flows and turbulent swirling jet flows will be presented. The accuracy and cost features of the unified RANS-LES model will be discussed in comparison with other (in particular DES) hybrid methods.

A Study of the Sensitivity of Wind Turbine Response to Inflow Temporal and Spatial Resolution

The effect of spatial and temporal resolution of the wind inflow on the dynamic response of a wind turbine was studied. Large eddy simulation of a neutral stability atmospheric boundary layer was used to generate the wind inflows. These inflows were then used with an aero-elastic code FAST to assess the turbine’s response to different spatial and temporal resolutions. Frequency analysis and the variance of turbine parameters representative of turbine loading showed that the smaller scales of turbulence captured by the wind inflow play an important role in the turbine response. Even though these small scales in the atmospheric boundary layer have small energy content compared to the larger scales, they occur in a frequency range where the turbine responds dynamically to disturbances. It was also noticed that the turbine exhibits different response depending on the performance parameter analyzed. An investigation of varying the temporal resolution of the inflow did not produce noticeable variations in the turbine response, but the range of temporal resolution studied was insufficient to draw general conclusions.

Investigation of Non-Swirling and Swirling Turbulent Jet Flows using Unified LES-RANS Models

Large-eddy simulation (LES) has proven to be an accurate and computationally feasible approach for swirling turbulent jet flow simulations. However, the inflow conditions of swirling turbulent jet flows are often determined by a nozzle flow, which has a significant influence on the jet flow development. LES simulations of the nozzle flow is often infeasible: the simulation of near-wall fluid motions requires computational costs that are comparable with those of direct numerical simulation. Hence, nozzle flow simulations have to be performed using Reynolds-averaged Navier-Stokes (RANS) methods. The numerical predictions of RANS models are unreliable when no experimental or benchmark data is available for comparison. Hence, there is a need for the development of models which can simulate both the nozzle flow and the jet flow region accurately. This study investigates the combined numerical simulation of a nozzle flow and swirling turbulent jet flow by a unified turbulence model. The unified turbulence model combines RANS methods in the near-wall region with LES methods away from the wall. The numerical simulations performed using the unified model have a much lesser computational cost than the combined nozzle and jet flow simulations using LES. The accuracy of the numerical predictions are validated against experimental data for non-swirling and swirling turbulent jet flows. The application of the validated model to studies of the mechanism of swirl effects shows the following. Swirl breaks apart the typical ring structures of non-swirling turbulent jets into two modes: a helical mode and streamwise braid structures. The interaction of these two modes generates unorganized turbulence that enhances the turbulent mixing.