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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Turbulent flow past a backward-facing step - A critical evaluation of two-equation models

The ability of two-equation models to accurately predict separated flows is analyzed from a combined theoretical and computational standpoint. Turbulent flow past a backward facing step is chosen as a test case in an effort to resolve the variety of conflicting results that were published during the past decade concerning the performance of two-equation models. It is found that the errors in the reported predictions of the k-epsilon model have two major origins: (1) numerical problems arising from inadequate resolution, and (2) inaccurate predictions for normal Reynolds stress differences arising from the use of an isotropic eddy viscosity. Inadequacies in near wall modelling play a substantially smaller role. Detailed calculations are presented which strongly indicate the standard k-epsilon model - when modified with an independently calibrated anisotropic eddy viscosity - can yield surprisingly good predictions for the backstep problem.

Thangam, S.↗
NASA NTRS1995

Direct numerical simulation of transitional and turbulent flow over a heated flat plate using finite-difference schemes

This report deals with the direct numerical simulation of transitional and turbulent flow at low Mach numbers using high-order-accurate finite-difference techniques. A computation of transition to turbulence of the spatially-evolving boundary layer on a heated flat plate in the presence of relatively high freestream turbulence was performed. The geometry and flow conditions were chosen to match earlier experiments. The development of the momentum and thermal boundary layers was documented. Velocity and temperature profiles, as well as distributions of skin friction, surface heat transfer rate, Reynolds shear stress, and turbulent heat flux, were shown to compare well with experiment. The results indicate that the essential features of the transition process have been captured. The numerical method used here can be applied to complex geometries in a straightforward manner.

Madavan, Nateri K.↗
DOE OSTI2024

Coherent structures in stably stratified wall-bounded turbulent flows

To date, a growing body of literature has documented the existence and impacts of coherent structures known as large- and very-large-scale motions within wall-bounded turbulent flows under neutral and unstable thermal stratification. These coherent structures can account for a considerable fraction of the overall turbulent transport and have been found to modulate small-scale turbulent fluctuations near the wall. In the context of stably stratified flows, however, the examination of such coherent structures has garnered relatively little attention. Stable stratification limits vertical transport and turbulent mixing within flows, which makes it unclear the extent to which previous findings on coherent structures under unstable and neutral stratification are applicable to stably stratified flows. In this study, we investigate the existence and characteristics of coherent structures under stable stratification with a wide range of statistical and spectral analyses. Outer peaks in premultiplied spectrograms under weak stability indicate the presence of large-scale motions, but these peaks become weaker and eventually vanish with increasing stability. Quadrant analysis of turbulent transport efficiencies (the ratio of net fluxes to their respective downgradient components) demonstrates dependencies on both stability and height above ground, which is evidence of morphological differences in the coherent structures under increasing stability. Amplitude modulation by large-scale streamwise velocity was found to decrease with increasing gradient Richardson number, whereas modulation by large-scale vertical velocity was approximately zero across all stability ranges. For sufficiently stable stratification, large eddies are suppressed enough to limit any inner–outer scale interactions.

Greene, Brian R. (ORCID:0000000343766818)↗
DOE OSTI2022

Calibrating hypersonic turbulence flow models with the HIFiRE-1 experiment using data-driven machine-learned models.

In this paper we study the efficacy of combining machine-learning methods with projection-based model reduction techniques for creating data-driven surrogate models of computationally expensive, high-fidelity physics models. Such surrogate models are essential for many-query applications e.g., engineering design optimization and parameter estimation, where it is necessary to invoke the high-fidelity model sequentially, many times. Surrogate models are usually constructed for individual scalar quantities. However there are scenarios where a spatially varying field needs to be modeled as a function of the model’s input parameters. Here we develop a method to do so, using projections to represent spatial variability while a machine-learned model captures the dependence of the model’s response on the inputs. The method is demonstrated on modeling the heat flux and pressure on the surface of the HIFiRE-1 geometry in a Mach 7.16 turbulent flow. The surrogate model is then used to perform Bayesian estimation of freestream conditions and parameters of the SST (Shear Stress Transport) turbulence model embedded in the high-fidelity (Reynolds-Averaged Navier–Stokes) flow simulator, using shock-tunnel data. The paper provides the first-ever Bayesian calibration of a turbulence model for complex hypersonic turbulent flows. We find that the primary issues in estimating the SST model parameters are the limited information content of the heat flux and pressure measurements and the large model-form error encountered in a certain part of the flow.

42 ENGINEERING↗
NASA NTRS1993

Compressible turbulent flow simulation with a multigrid multiblock method

We describe a multigrid multiblock method for compressible turbulent flow simulations and present results obtained from calculations on a two-element airfoil. A vertex-based spatial discretization method and explicit multistage Runge-Kutta time-stepping are used. The slow convergence of a single grid method makes the multigrid method, which yields a speed factor of about 20, indispensable. The numerical predictions are in good agreement with experimental results. It is shown that the convergence of the multigrid process depends considerably on the ordering of the various loops. If the block loop is put inside the stage loop, the process converges more rapidly than if the block loop is situated outside the stage loop when a three-stage Runge-Kutta method is used. If a five-stage scheme is used, the process does not converge in the latter block ordering. Finally, the process based on the five-stage method is about 60 percent more efficient than with the three-stage method, if the block loop is inside the stage loop.

Kuerten, Hans↗
NASA NTRS2012

Discrete Adjoint-Based Design for Unsteady Turbulent Flows On Dynamic Overset Unstructured Grids

A discrete adjoint-based design methodology for unsteady turbulent flows on three-dimensional dynamic overset unstructured grids is formulated, implemented, and verified. The methodology supports both compressible and incompressible flows and is amenable to massively parallel computing environments. The approach provides a general framework for performing highly efficient and discretely consistent sensitivity analysis for problems involving arbitrary combinations of overset unstructured grids which may be static, undergoing rigid or deforming motions, or any combination thereof. General parent-child motions are also accommodated, and the accuracy of the implementation is established using an independent verification based on a complex-variable approach. The methodology is used to demonstrate aerodynamic optimizations of a wind turbine geometry, a biologically-inspired flapping wing, and a complex helicopter configuration subject to trimming constraints. The objective function for each problem is successfully reduced and all specified constraints are satisfied.

Nielsen, Eric J.↗
DOE OSTI2023

Coupling turbulent flow with blade aeroelastics and control modules in large-eddy simulation of utility-scale wind turbines

We present a large-eddy simulation framework capable of control co-design of large wind turbines, coupling the turbulent flow environment with blade aeroelastics and turbine controllers. The geometry and aerodynamics of the rotor blades and the turbine nacelle are parameterized using an actuator surface model. The baseline collective pitch control and individual pitch control (IPC) algorithms, consisting of a single-input, single-output proportional–integral controller and two integral controllers, respectively, are incorporated into the simulation framework. Furthermore, a second-order model based on the Euler–Bernoulli beam theory is implemented to describe the blade deformation. Simulations are carried out to investigate the impact of collective and individual pitch control strategies on the deflection of turbine blades. Our results show that the IPC reduces the blade tip deflection fluctuations in the out-of-plane direction, while the fluctuations of the blade tip deflection along the in-plane direction are barely affected by the IPC. Furthermore, the blade out-of-plane deformation fluctuation is underestimated by the one-way coupling approach compared to the two-way coupling approach. Furthermore, the findings of this study reveal the importance of advanced control systems in reducing the dynamic loads on wind turbine blades and underscore the potential of control co-design to reduce the levelized cost of wind energy.

42 ENGINEERING↗
NASA NTRSDate not supplied

Assessment of UMUSCL Scheme for DNS of Turbulent Flows

Direct numerical simulation (DNS) is performed using an unstructured node-based finite-volume methodology for canonical turbulent flow configurations to assess the effect of the upwinding parameter (κ) in the unstructured monotonic upstream scheme for conservation laws (UMUSCL) reconstruction scheme. The NASA FUN3D solver is used, and four turbulent validation cases are considered: periodic Taylor-Green vortex, (2) flow through a plane channel, (3) flow through a channel with a constriction, and (4) flow over a flat plate. The results are presented for κ = 0.5, 0.9 and 1 on hexahedral grids, with the last value corresponding to a zero-numerical dissipation scheme. Detailed comparisons with available reference data indicate that for the smallest length-scales simulated, κ = 0.5 is too dissipative while κ = 1 could lead to an energy build-up. Overall, κ =0.9 yields satisfactory results in terms of accuracy and robustness for all the cases considered in this study.

Direct Numerical Simulation↗
NASA NTRS1993

On streak spacing in wall-bounded turbulent flows

The present study is a continuation of the examination by Hamilton et al. of the regeneration mechanisms of near-wall turbulence and an attempt to investigate the conjecture of Waleffe et al. The basis of this study is an extension of the 'minimal channel' approach of Jimenez and Moin that emphasizes the near-wall region and reduces the complexity of the turbulent flow by considering a plane Couette flow of near minimum Reynolds number and stream-wise and span-wise extent. Reduction of the flow Reynolds number to the minimum value which will allow turbulence to be sustained has the effect of reducing the ratio of the largest scales to the smallest scales or, equivalently, of causing the near-wall region to fill more of the area between the channel walls. A plane Couette flow was chosen for study since this type of flow has a mean shear of a single sign, and at low Reynolds numbers, the two wall regions are found to share a single set of structures.

Hamilton, James M.↗
NASA NTRS1987

Laser velocimetry in turbulent flow fields - Particle response

Measurements of the particle response in a decelerating flow and a highly turbulent two-phase flow were obtained. Simultaneous measurements of the particle size and velocity served to quantify the particle response to the prevailing flow field. In the case of a flow incident upon a cylinder, the particle lag for a range of size classes was recorded. Results were also obtained in the flow generated by an atomizer operating on the leeward side of a flat disk bluff body in a coflowing air stream. Measurements of the mean axial, mean radial, and rms velocities and angles of trajectories were obtained for representative particle size classes. The air velocity and turbulence intensity were inferred from the seed particles on the order of one micrometer in diameter. Particles 9 micrometers and larger showed significant differences with respect to the gas phase mean velocity and turbulence intensity even at low velocities. In two-phase flows, reliable measurements of the continuous phase velocity and turbulence parameters requires the simultaneous measurement of particle size as a means for rejecting readings from large particles from the velocity pdf's.

Bachalo, W. D.↗
NASA NTRS1986

Applications of a new wall function to turbulent flow computations

A new wall function approach is developed based on a wall law suitable for incompressible turbulent boundary layers under strong adverse pressure gradients. This wall law was derived from a one-dimensional analysis of the turbulent kinetic energy equation with gradient diffusion concept employed in modeling the near-wall shear stress gradient. Numerical testing cases for the present wall functions include turbulent separating flows around an airfoil and turbulent recirculating flows in several confined regions. Improvements on the predictions using the present wall functions are illustrated. For cases of internal recirculating flows, one modification factor for improving the performance of the k-epsilon turbulence model in the flow recirculation regions is also included.

Chen, Y. S.↗
NASA NTRS1987

Investigation of turbulent flow in highly curved ducts with application to turbomachinery components

Numerical predictions were performed using a semi-elliptic calculation procedure for the case of turbulent flow in passage through a 90 deg bend of square cross section. Two versions of the isotropic turbulent viscosity two equation k-epsilon model were used. The first, the wall function model (WFM), employs the logarithmic law-of-the-wall relation and the notion of equilibrium flow to set all the necessary boundary conditions at the first grid point adjacent to a solid wall. The second model, the Van Driest model (VDM), employs Prandtl's original mixing length formulation, in conjunction with Van Driest's semi-empirical relation for the mixing length, to calculate the turbulent viscosity in the near wall regions of the flow. In this case, boundary conditions for k and epsilon, required to calculate these quantities in the core of the flow, are obtained by matching the mixing length and Reynolds number model formulations in an overlapping region of the flow near the walls. In both cases the results obtained show an improvement over earlier calculations using an elliptic numerical procedure. This is attributed to the finer grids possible in the present work. Of the two models, the VDM formulation shows better overall conformity with the mean flow measurements. Neither model reproduces well the details of the stress distribution as a result of the implied isotropic turbulent viscosity.

Humphrey, J. A. C.↗
NASA NTRS1984

Optical measurements of fluctuating temperatures in a supersonic turbulent flow using one- and two-photon, laser-induced fluorescence

A laser-induced fluorescence technique was developed that provides a practical means of nonintrusively measuring the instantaneous temperatures in low-temperature turbulent flows. The capabilities of the method are reviewed, and its application to a simple, two-dimensional, turbulent boundary-layer flow at Mach 2 is reported. Measurements of the average temperature distribution through the boundary layer and the magnitudes of temperature fluctuations about their average values are presented.

Gross, K. P.↗
NASA NTRS1975

Viscous shock layer solutions for turbulent flow of radiating gas mixtures in chemical equilibrium

The viscous shock layer equations for hypersonic laminar and turbulent flows of radiating or nonradiating gas mixtures in chemical equilibrium are presented for two-dimensional and axially symmetric flow fields. Solutions are obtained using an implicit finite difference scheme and results are presented for hypersonic flow over spherically blunted cone configurations at free stream conditions representative of entry into the atmosphere of Venus. These data are compared with solutions obtained using other methods of analysis.

Anderson, E. C.↗
NASA NTRS1989

Turbulent flow separation control through passive techniques

Several passive separation control techniques for controlling moderate two-dimensional turbulent flow separation over a backward-facing ramp are studied. Small transverse and swept grooves, passive porous surfaces, large longitudinal grooves, and vortex generators were among the techniques used. It was found that, unlike the transverse and longitudinal grooves of an equivalent size, the 45-deg swept-groove configurations tested tended to enhance separation.

Lin, J. C.↗
NASA NTRS1985

On the Computation of Structural Vibrations Induced by a Low-speed Turbulent Flow

A method for numerical evaluation of the vibrations of a cylindrical shell structure induced by a low speed external turbulent flow is discussed. The direction of flow is along the axis of revolution of the shell, and the source of excitation is the pressure fluctuations in the turbulent boundary layer. For the investigation of vibration and noise problems it is usually more desirable to utilize the modal expansion approach. The axisymmetric shell structure can be modeled by the assemblage of conical-shell finite-elements. This modeling allows the eigenfunction psi sub mn (x,theta) to be represented in a rectangular product of a longitudinal modal function f sub mn (x) and a circular harmonic function cos m theta (or sin m theta).

Hwang, Y. F.↗