Download or read book Studies of Real Roughness Effects for Improved Modeling and Control of Practical Wall Bounded Turbulent Flows written by and published by . This book was released on 2008 with total page 183 pages. Available in PDF, EPUB and Kindle. Book excerpt: The present effort investigates the effects of practical roughness replicated from a turbine blade damaged by deposition of foreign materials or statistical and structural characteristics of wall turbulence. Two-dimensional particle image velocimetry (PIV) measurements are performed in the streamwise-wall-normal plane of turbulent boundary layers at momentum Reynolds numbers of 8000 and 13000. The surface conditions include a smooth wall and two highly-irregular rough walls. These rough surfaces have the same roughness topography but differ in spatial scaling. The validity of Townsend's wall similarity hypothesis in the presence of practical roughness is assessed and the impact of this roughness on the spatial structure of the flow is investigated. In addition, stereoscopic PIV measurements are made in streamwise-spanwise planes of smooth- and rough-wall turbulent boundary layers both within and at the outer edge of the roughness sublayer. This data is used to explore the impact of dominant topographical features on the near-wall flow as well as the influence of practical roughness on the spatial organization of the flow. Understanding such effects provides a steppingstone toward efficient modeling and control of practical flows in the presence of roughness.

Download or read book Simulation and Modeling of Compressible and Incompressible Turbulent Channel Flows Over Rough Walls written by Mostafa Aghaei Jouybari and published by . This book was released on 2020 with total page 144 pages. Available in PDF, EPUB and Kindle. Book excerpt: The effects of surface roughness on wall-bounded turbulent flows are important for fundamental turbulence research, and turbulence modeling and control, in both compressible and incompressible regimes. This dissertation studies these effects through statistical and structural analysis of turbulence, and provides practical insights for modeling of turbulence in the presence of roughness for incompressible flows. It also proposes an immersed boundary method to simulate compressible flows over rough walls with complex geometries, and studies the roughness effects on supersonic flows over wavy walls. Turbulence statistics in open channel flows over a smooth wall and three types of wall roughness: sand-grain, cube roughness and a realistic, multi-scale turbine-blade roughness, are examined using direct numerical simulations. Transport of the mean momentum, normal components of the Reynolds stress tensor, and normal components of the dispersive stress tensor are analyzed. The results show higher turbulence isotropy for the rough walls compared to the smooth wall. Wake production, the mechanism through which energy is transported from the wake field to the turbulence field (and vice versa), is strongly influenced by the kind of rough wall. For synthetic rough walls, the wake production has relatively large positive values, while it is negative with a smaller magnitude, for the turbine-blade surface. These results indicate a strong dependence of turbulence processes in the near wall regions on the roughness topography. Turbulent coherent motions in flows over rough walls are also analyzed. Two-point velocity correlations, length scales, inclination angles, and velocity spectra are studied. Results from linear stochastic estimation suggest that, near the wall, the quasi-streamwise vortices observed in smoothwall flow are present in the large-scale recessed regions of multi-scale roughness, whereas they are replaced by a pair of 'head-up, head-down' horseshoe structures in the sandgrain and cube roughnesses, similar to those observed in the previous studies. The configuration of conditional eddies near the wall suggests that the kinematic behavior of vortices differs for each kind of rough surfaces. Vortices over multiscale roughness are conjectured to obey a growth mechanism similar to those over smooth walls, while around the cube roughness the head-down horse-shoe vortices undergo a solid-body rotation on top of the element on account of the strong shear layer. This shortens the longitudinal extent of the near-wall structures and promotes turbulence production. Deep Neural Networks (DNN) and Gaussian Process Regression (GPR) are used to propose a high-fidelity prediction of the Nikuradse equivalent sandgrain height, (ks), which is frequently used in turbulence modeling of flows over rough walls. To provide a good database, 45 widely different surface geometries are generated and simulated at frictional Reynolds number of 1000, which are also accompanied by 15 fully rough experimental data. The designed DNN and GPR models predict ks with errrms

Download or read book Numerical and Theoretical Study of Wall bounded Turbulence written by Haosen Xu and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation presents numerical and theoretical studies of wall-bounded turbulence. The dissertation is divided into three major parts. The first part focuses on flow physics and modeling of statistical quantities in turbulent boundary layers. We model pressure statistics. The modeling strategy is based on a combination of Townsend's attached eddy hypothesis with Komolgorov's 1941 theory on small-scale turbulence. Specifically, we account for small-scale motions related to pressure inside larger scale wall-attached eddies. With this strategy, we are able to model the even order moments and the scaling of pressure spectrum. In the second part of the dissertation, we study rough wall turbulent boundary layer with densely packed roughness elements. The roughness elements are cubes. Direct numerical simulations are carried out. We report mean flow statistics, Reynolds and dispersive stresses, as well as terms in the turbulent/dispersive kinetic energy budget equations. We show that roughness with high packing densities do not necessarily have similar behaviors and therefore they cannot simply be categorized as d-type roughness. The third part of this dissertation aims to apply computational fluid dynamics to realistic engineering problems. A few problems are considered. We start by studying turbulent boundary layers with heat transfer. We focus on low-speed flows(Mach number within 0.2) with heat transfer, and the performance of wall-modeled large eddy simulation with equilibrium wall model is assessed. The study shows that the Mach number limit for incompressible assumption for thermal fields is lower than the often-quoted value 0.2 due to the associated viscous heating. In addition, we show that the first grid point implementation of the equilibrium wall model outperforms the third grid point implementation for heat transfer problems. Next, we apply wall-modeled large eddy simulation for flows in turbo-machinery. The study focuses specifically on the return channel of a multistage centrifugal compressor. We compare the results from wall-modeled large eddy simulation with those from Reynolds-averaged Navier-Stokes equations and experimental measurements. We show the potential advantages of wall-modeled large eddy simulations for practical engineering applications.

Download or read book Turbulent Flows written by Jean Piquet and published by Springer Science & Business Media. This book was released on 2001-03-26 with total page 778 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Turbulence Modelling Approaches written by Konstantin Volkov and published by BoD – Books on Demand. This book was released on 2017-07-26 with total page 252 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Flow Control written by Mohamed Gad-el-Hak and published by Springer Science & Business Media. This book was released on 2003-07-01 with total page 533 pages. Available in PDF, EPUB and Kindle. Book excerpt: No be certain it can is not based mathematics. knowledge if upon da Vinci, (Leonardo 1452 1519) the humankind. Thinking is one greatest of Joys of Galilei, (Galileo 1564 1642) Now I think is to be the root all hydrodynamics and is at of physical science, second the to none in its mathematics. present beauty of Thomson (William (Lord Kelvin), 1824 1907) The book contains the lecture notes of of the nine instructors at present eight the short Flow Control: Fundamentals and which held course was Practices, in the week 24 28 June and Carg6se, Corsica, France, during 1996, repeated at the of Notre 9 13 1996. University Dame, Indiana, September Following the week in the course a on same was held. Corsica, 5 day workshop topic Selected from the scheduled to 1998 workshop are papers appear early special volume of the International Journal Heat Thermo of Experimental Transfer, and Fluid All Mechanics. three events were Jean Paul dynamics, organized by Bonnet of Universit6 de Andrew Pollard of Univer Poitiers, France, Queen's at and Mohamed Gad el Hak of the of sity Kingston, Canada, University Notre U.S.A.

Download or read book A New Rough Wall Layer Modeling for Turbulent Flows Using the Brinkman Equation written by Meng-Huang Lu and published by . This book was released on 2008 with total page 414 pages. Available in PDF, EPUB and Kindle. Book excerpt: A new flow physics-based modeling of surface roughness effect is developed for the Reynolds averaged Navier-Stokes equation numerical calculations of high-Reynolds-number turbulent flows over a wide range of rough surfaces. In light of the geometric and the dynamic similarities between the porous medium flow and the surface roughness flow, it is proposed herein to use the Brinkman equation to model the averaged flow in the surface roughness layer of the turbulent boundary layer flow. The averaged model equations for the mean flows are derived and turbulence transport equations are developed based on existing smooth wall turbulence closures. The roughness-related model parameters are also introduced. In the proposed approach, the fluid dynamics of the averaged flow in the near-wall rough layer is modeled by using the Brinkman equation. The porosity can be calculated based on the volumetric characteristics of the roughness and the permeability is modeled. The Reynolds averaged Navier-Stokes equations are solved numerically in the outer free flow region, which is above the near-wall rough layer, while a low-Reynolds-number [kappa-epsilon] model and a second-order Reynolds stress model are employed in all regions. The interface conditions are applied to enforce the continuity of velocity, pressure, and turbulence properties, and the stress jump at the interface between the near-wall rough layer and the free flow region. The computational results, including the skin friction coefficient, the log-law mean velocity, the roughness function, the turbulent kinetic energy, and the Reynolds stresses, are presented. The results show that the new rough wall layer modeling approach predicts well the skin friction coefficient, the log-law mean velocity, the roughness function, and the Reynolds shear stress. The results indicate that the developed rough wall layer modeling approach improves the current predictive capability of the roughness effects, and is applicable to a wider range of surface roughness.

Download or read book An Improved Near wall K epsilon Model for the Calculation of Wall Bounded Turbulent Flows written by Arup Sarkar and published by . This book was released on 1995 with total page 272 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Recent Developments in Turbulence Management written by Kwing-So Choi and published by Springer Science & Business Media. This book was released on 1991 with total page 368 pages. Available in PDF, EPUB and Kindle. Book excerpt: Including papers on riblets, this book covers their practical applications to aircraft and to a model ship, near-wall coherent structure of boundary layer and effects of flow three-dimensionality. This volume also includes LEBUs (Large Eddy Break-Up devices), surface roughness, compliant surfaces and polymer additives, and more.

Download or read book Wall Modeling for Turbulent Flow Over Complex Roughness written by Xiaowei Zhu (Mechanical engineer) and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Turbulent flow over complex rough surfaces is crucial in both engineering and boundary layer meteorology science. The surface morphology has significant effects on the flow, but it is computationally expensive to solve all the turbulent scales especially when the air flows over complex roughness. Large-eddy simulation (LES) with wall model is therefore employed in this situation. This dissertation focuses on the wall modeling of turbulent flow over complex roughness such as urban topography. As the wall effects can be accurately represented by the equilibrium logarithmic law via roughness length, z0, this dissertation aims to parameterize z0 over complex roughness. In this dissertation, two types of complex roughness are discussed: spatially heterogenous urban-like topography (Chapter 3) and multiscale fractal urban-like topography (Chapter 4). For the spatially heterogeneous urban-like topography, a priori prediction method for z0 based on the statistical moments of surface height is proposed especially for the boundarylayer turbulent flow. Using a posteriori LES results, we demonstrate that the skewness of surface height (as measures of the presence of the extreme value, or the “heavy tail” events) has non-negligible effects, which received less attention as topographic parameters in the past. This finding is reconciled with a model recently proposed by Flack and Schultz (2010) who demonstrate that z0 can be modeled with standard deviation and skewness, and two empirical coefficients (one for each moment). We find that the empirical coefficient related to skewness is not constant but exhibits a dependence on standard deviation over certain ranges. For idealized, quasi-uniform cubic topographies and complex, fully random urbanlike topographies, we demonstrate robust performance of the generalized Flack and Schultz model against contemporary roughness correlations. The multiscale fractal-like topographies pose a particular challenge to numerical simulation schemes since the large-scale elements are resolved, but the small-scale descendant elements cannot be resolved on the computational mesh grid. A local wall model representing the effects of unresolved sub-generation roughness is needed in such scenario. By virtue of selfsimilarity among scales, we develop a methodology and a roughness model for the unresolved scales by learning from the large-scale momentum fluxes. And then the roughness model for the unresolved scales via the equilibrium logarithmic law is established. The research shows that aerodynamic stress associated with descendant, sub-generation scale elements can be parameterized, thus that the turbulent flow over fractal-like geometry can be simulated with only the large generations resolved on the computational mesh grid. The key questions we ask in this dissertation are: How does the spatial heterogeneity affect the transport of the turbulent flow? How to model the sub-generation scales which are smaller than the mesh grid for a fractal topography? The results, and the modeling framework developed herein, have practical implications for the operation of numerical weather prediction models and the initialization of high-resolution solutions.

Download or read book Biofluid Dynamics written by Clement Kleinstreuer and published by CRC Press. This book was released on 2016-04-19 with total page 525 pages. Available in PDF, EPUB and Kindle. Book excerpt: Biofluid Dynamics builds a solid understanding of medical implants and devices from a bioengineering standpoint. The text features extensive worked examples and mathematical appendices; exercises and project assignments to stimulate critical thinking and build problem solving skills; numerous illustrations, including a 16-page full-color insert; computer simulations of biofluid dynamics processes and medical device operations; tools for solving basic biofluid problems; and a glossary of terms. The text can be used as a primary selection for a comprehensive course or for a two-course sequence or as a reference for professionals in biomedical engineering and medicine.

Download or read book A Rapid Distortion Theory Turbulence Model for Developed Unsteady Wall Bounded Flow written by National Aeronautics and Space Administration (NASA) and published by Createspace Independent Publishing Platform. This book was released on 2018-08-10 with total page 38 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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...

Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1995 with total page 542 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Modeling Turbulence Using Optimal Large Eddy Simulation written by Henry Chang and published by . This book was released on 2012 with total page 272 pages. Available in PDF, EPUB and Kindle. Book excerpt: Most flows in nature and engineering are turbulent, and many are wall-bounded. Further, in turbulent flows, the turbulence generally has a large impact on the behavior of the flow. It is therefore important to be able to predict the effects of turbulence in such flows. The Navier-Stokes equations are known to be an excellent model of the turbulence phenomenon. In simple geometries and low Reynolds numbers, very accurate numerical solutions of the Navier-Stokes equations (direct numerical simulation, or DNS) have been used to study the details of turbulent flows. However, DNS of high Reynolds number turbulent flows in complex geometries is impractical because of the escalation of computational cost with Reynolds number, due to the increasing range of spatial and temporal scales. In Large Eddy Simulation (LES), only the large-scale turbulence is simulated, while the effects of the small scales are modeled (subgrid models). LES therefore reduces computational expense, allowing flows of higher Reynolds number and more complexity to be simulated. However, this is at the cost of the subgrid modeling problem. The goal of the current research is then to develop new subgrid models consistent with the statistical properties of turbulence. The modeling approach pursued here is that of "Optimal LES". Optimal LES is a framework for constructing models with minimum error relative to an ideal LES model. The multi-point statistics used as input to the optimal LES procedure can be gathered from DNS of the same flow. However, for an optimal LES to be truly predictive, we must free ourselves from dependence on existing DNS data. We have done this by obtaining the required statistics from theoretical models which we have developed. We derived a theoretical model for the three-point third-order velocity correlation for homogeneous, isotropic turbulence in the inertial range. This model is shown be a good representation of DNS data, and it is used to construct optimal quadratic subgrid models for LES of forced isotropic turbulence with results which agree well with theory and DNS. The model can also be filtered to determine the filtered two-point third-order correlation, which describes energy transfer among filtered (large) scales in LES. LES of wall-bounded flows with unresolved wall layers commonly exhibit good prediction of mean velocities and significant over-prediction of streamwise component energies in the near-wall region. We developed improved models for the nonlinear term in the filtered Navier-Stokes equation which result in better predicted streamwise component energies. These models involve (1) Reynolds decomposition of the nonlinear term and (2) evaluation of the pressure term, which removes the divergent part of the nonlinear models. These considerations significantly improved the performance of our optimal models, and we expect them to apply to other subgrid models as well.

Download or read book Open Channel Turbulent Boundary Layers and Wall Jets on Rough Surfaces written by and published by . This book was released on 2001 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: For many industrial and environmental flows, the momentum and convective heat transfer rates at the surface are determined by the turbulence structure in the near-wall region. Although many flows of practical interest occur on rough surfaces, our understanding and ability to predict rough wall turbulent flows lags far behind the corresponding technology for smooth surfaces. This provides reasonable grounds for additional refined rough wall measurements with the expectation of improving our physical understanding of practically relevant turbulent flows. This thesis reports an experimental investigation of wall roughness effects on the characteristics of turbulent boundary layers and wall jets. The measurements are obtained for smooth wall and three different roughness elements using a laser-Doppler anemometer. An insightful presentation of the results requires that the correct scaling laws must be used. In the case of the turbulent boundary layer, the appropriateness of the log law proposed by Millikan (1938) to model the overlap region of the mean flow and the power laws proposed by Barenblatt (1993) and George and Castillo (1997) is compared. The boundary layer results show that the theory proposed by George and Castillo (1997) has important advantages over the log law in modeling the mean velocity profiles as well as predicting the wall shear stress. The results also show that wall roughness increases the turbulence fluctuations and transport terms, which suggests that rough wall turbulence models must explicitly account for roughness effects in order to predict the mixing characteristics accurately. This promises to provide significant challenges to rough wall turbulence models. The wall jet results show that wall roughness increases the inner layer thickness but the jet half-width does not show any important sensitivity to surface roughness. The spread rates for the jet half-width are higher than the values obtained in earlier measurements. This may be attribute.

Download or read book Turbulence Models and Their Application written by Tuncer Cebeci and published by Springer Science & Business Media. This book was released on 2003-12-04 with total page 140 pages. Available in PDF, EPUB and Kindle. Book excerpt: After a brief review of the more popular turbulence models, the author presents and discusses accurate and efficient numerical methods for solving the boundary-layer equations with turbulence models based on algebraic formulas (mixing length, eddy viscosity) or partial-differential transport equations. A computer program employing the Cebeci-Smith model and the k-e model for obtaining the solution of two-dimensional incompressible turbulent flows without separation is discussed in detail and is presented in the accompanying CD.

Download or read book On the Theory of Wall Bounded Turbulence written by Eylül Bilgin and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The study of turbulence has been ongoing for over seven decades now, and it is without a doubt one of the most challenging problems in classical physics. The immense difficulty of understanding and analyzing turbulent flows is why researchers have thrown all their resources at the problem, including supercomputers, in hopes of gaining some insight. While this thesis was written during the era of supercomputers and advanced CFD tools (in other words, in the 21st century), it takes on a different approach- we introduce analytical concepts to deal with turbulent flows, and we do so in a manner that encompasses all Reynolds number flows, from laminar to turbulent. It is our objective to show that the conventional view that turbulence cannot be analyzed analytically and that sophisticated numerical methods must be applied to solve fundamental turbulent flows of Newtonian fluids is not necessarily accurate. In this work, we develop physically meaningful and universal tools to analyze turbulent flows over smooth and rough walls, demonstrate their sufficiency in matching data very well and demonstrate their suitability for practical problems. As of now, direct numerical simulation is only suitable for very low Reynolds number and simple geometry investigations; we do not have enough computational power for the needs of most investigations. However, in far future developments might make it possible to supercompute the movement of each molecule in a fluid, providing a deeper understanding of the fluid dynamics of highly excited fluids. We can, in the meantime, gain a deeper understanding of turbulence through semi-analytical and practical methods. Incorporating semi-analytical models, such as those presented here, into numerical programs will also be extremely helpful for improving the computation of turbulence, since this can reduce computation times and improve the stability of numerical computations.