A Rapid-Distortion-Theory Turbulence Model for Developed Unsteady Wall-Bounded Flow

A Rapid-Distortion-Theory Turbulence Model for Developed Unsteady Wall-Bounded Flow
Author: National Aeronautics and Space Administration (NASA)
Publisher: Createspace Independent Publishing Platform
Total Pages: 38
Release: 2018-08-10
Genre:
ISBN: 9781725023673

A new approach to turbulence modeling in unsteady developed flows has recently been introduced, based on results of rapid distortion theory. The approach involves closing the k-epsilon equations for the organized unsteady component of the flow by modeling local unsteadiness as a rapid distortion of the local structure of the parent turbulent flow, in terms of an effective strain parameter alpha(sub eff). In this paper, the phase-conditioned equations of motion are developed to accommodate a new unsteady dissipation model and local effects of the slow-relaxation time scale of the parent flow. The model equations are tested against measurements of the response of a fully-developed turbulent pipe flow to the superposition of sinusoidal streamwise oscillation. Good agreement is found between measurements and predictions over a wide range of frequencies of unsteadiness, indicating that this approach may be particularly well suited to modeling of unsteady turbulent flows which are perturbations about a well characterized mean. Brereton, G. J. and Mankbadi, R. R. Glenn Research Center NASA-TM-106249, ICOMP-93-22, E-7971, NAS 1.15:106249 NCC3-233; RTOP 505-90-5K...

Turbulent Flows

Turbulent Flows
Author: Jean Piquet
Publisher: Springer Science & Business Media
Total Pages: 767
Release: 2013-04-17
Genre: Technology & Engineering
ISBN: 3662035596

obtained are still severely limited to low Reynolds numbers (about only one decade better than direct numerical simulations), and the interpretation of such calculations for complex, curved geometries is still unclear. It is evident that a lot of work (and a very significant increase in available computing power) is required before such methods can be adopted in daily's engineering practice. I hope to l"Cport on all these topics in a near future. The book is divided into six chapters, each· chapter in subchapters, sections and subsections. The first part is introduced by Chapter 1 which summarizes the equations of fluid mechanies, it is developed in C~apters 2 to 4 devoted to the construction of turbulence models. What has been called "engineering methods" is considered in Chapter 2 where the Reynolds averaged equations al"C established and the closure problem studied (§1-3). A first detailed study of homogeneous turbulent flows follows (§4). It includes a review of available experimental data and their modeling. The eddy viscosity concept is analyzed in §5 with the l"Csulting ~alar-transport equation models such as the famous K-e model. Reynolds stl"Css models (Chapter 4) require a preliminary consideration of two-point turbulence concepts which are developed in Chapter 3 devoted to homogeneous turbulence. We review the two-point moments of velocity fields and their spectral transforms (§ 1), their general dynamics (§2) with the particular case of homogeneous, isotropie turbulence (§3) whel"C the so-called Kolmogorov's assumptions are discussed at length.

Gabor Mode Enrichment in Large Eddy Simulation of Turbulent Flows

Gabor Mode Enrichment in Large Eddy Simulation of Turbulent Flows
Author: Aditya Suresh Ghate
Publisher:
Total Pages:
Release: 2018
Genre:
ISBN:

Buoyed by the rise in computational capabilities, eddy resolving simulation methodologies such as Large Eddy Simulation (LES), first developed in 1970s, have now come to dominate academic research in numerical simulation of turbulent flows. Wall-modeled LES (WMLES) which alleviates the no-slip boundary condition at walls with a stress condition computed using either a physics based or a mathematically derived boundary condition based on the filtering kernel, has further enabled the use of LES in applications involving very high Reynolds number wall bounded flows. Motivated by this growing success and push for use of WMLES in both engineering and geophysics communities, we address a new emerging challenge in this field: Given a robust characterization of the large scales, can we come up with a reasonably consistent synthesis of the unresolved/subfilter scales at low computational cost? This is a particularly pertinent question in wall bounded flows since several applications that involve unsteady loads, surface vibrations, acoustic radiation, etc. require near wall representation of turbulent scales with a spatio-temporal bandwidth substantially larger than that implied by WMLES. This problem naturally arises in wind energy applications where the Planetary Boundary Layer (PBL) turbulence at extreme Reynolds numbers can only be resolved up to scales much larger than the chord lengths of wind turbine blades. Other multiphysics applications such as particle laden turbulence also require spectral resolution of the explicitly resolved fluid turbulence to be larger than what is typically available in LES. The method developed in this thesis is based on combining a scale-resolving turbulence simulation (such as WMLES) with limited bandwidth, with an efficient, physics-based scale-enrichment model. The enrichment scheme generates a physically consistent realization of the finer-scale eddies locally which dynamically respond to the resolved larger scales and enable statistical predictions of physical quantities with a substantially larger bandwidth. This physics based model is carefully constructed to obtain small scales with accurate second-order space-time correlations. By representing the small scales using \emph{Gabor modes/wavepackets}, there is a substantial compression in degrees of freedom. Furthermore, the use of WKB-variant of Rapid Distortion Theory (RDT) along with a spectral viscosity closure allows for temporal evolution of the Gabor modes via a set of ODEs described in an x-k frame. Use of spatially and spectrally localized Gabor modes ensures correct inter-scale energy transfer and allows for treatment of spatial inhomogeneities. The transform of physical field variables from their Gabor modes representation to the physical space is accomplished using modern non-uniform FFT algorithms thereby achieving an overall nlog(n) computational cost. Robustness of this new method is assessed on various wall-bounded flow configurations (including a problem with Coriolis acceleration and stratification) using a representation of large scales obtained using both ideal WMLES (filtered DNS) and realistic/true WMLES simulations at high Reynolds numbers. While the enrichment of large scales obtained via idealized LES is excellent, the results for realistic LES are also very promising. Finally, we discuss a highly idealized problem to study the interaction of free-stream (isotropic) turbulence with a turbulent wake generated by an actuator disk. Beyond characterizing the flow physics in detail, we present evidence suggesting that the recovery and entrainment in the wake is much more sensitive to the integral length scale of the ambient turbulence, than its intensity. Furthermore, the incident flow seen by a downstream object present in the wake, has no low-rank behavior in terms of space-time coherence. We subsequently demonstrate that a truncated reconstruction of the flow field using space-time POD modes can be enriched with fine scales using Gabor modes to recover the full bandwidth of the high resolution simulation.

Turbulent Flows

Turbulent Flows
Author: G. Biswas
Publisher: CRC Press
Total Pages: 478
Release: 2002
Genre: Technology & Engineering
ISBN: 9780849310140

This book allows readers to tackle the challenges of turbulent flow problems with confidence. It covers the fundamentals of turbulence, various modeling approaches, and experimental studies. The fundamentals section includes isotropic turbulence and anistropic turbulence, turbulent flow dynamics, free shear layers, turbulent boundary layers and plumes. The modeling section focuses on topics such as eddy viscosity models, standard K-E Models, Direct Numerical Stimulation, Large Eddy Simulation, and their applications. The measurement of turbulent fluctuations experiments in isothermal and stratified turbulent flows are explored in the experimental methods section. Special topics include modeling of near wall turbulent flows, compressible turbulent flows, and more.

Turbulence Modelling Approaches

Turbulence Modelling Approaches
Author: Konstantin Volkov
Publisher: BoD – Books on Demand
Total Pages: 252
Release: 2017-07-26
Genre: Science
ISBN: 9535133497

Accurate prediction of turbulent flows remains a challenging task despite considerable work in this area and the acceptance of CFD as a design tool. The quality of the CFD calculations of the flows in engineering applications strongly depends on the proper prediction of turbulence phenomena. Investigations of flow instability, heat transfer, skin friction, secondary flows, flow separation, and reattachment effects demand a reliable modelling and simulation of the turbulence, reliable methods, accurate programming, and robust working practices. The current scientific status of simulation of turbulent flows as well as some advances in computational techniques and practical applications of turbulence research is reviewed and considered in the book.