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Showing papers by "Parviz Moin published in 2005"


Journal ArticleDOI
TL;DR: In this paper, the authors interpreted the data of polymer drag reduced flows in terms of modification of near-wall coherent structures, and showed that polymers are shown to reduce drag by damping nearwall vortices and sustain turbulence by injecting energy onto the streamwise velocity component in the very nearwall region.
Abstract: Numerical data of polymer drag reduced flows is interpreted in terms of modification of near-wall coherent structures. The originality of the method is based on numerical experiments in which boundary conditions or the governing equations are modified in a controlled manner to isolate certain features of the interaction between polymers and turbulence. As a result, polymers are shown to reduce drag by damping near-wall vortices and sustain turbulence by injecting energy onto the streamwise velocity component in the very near-wall region.

132 citations


Journal ArticleDOI
TL;DR: In this paper, a method for direct numerical simulation of polymer-induced friction drag reduction in turbulent boundary layers is described, where the effect of the polymer additives that induce spatial variations of skin-friction drag is included in the momentum equation through a continuum constitutive model for the viscoelastic stress, based on the evolution of a parameter describing the fluid microstructure.
Abstract: We describe a method for direct numerical simulation of polymer-induced friction drag reduction in turbulent boundary layers. The effect of the polymer additives that induce spatial variations of skin-friction drag is included in the momentum equation through a continuum constitutive model for the viscoelastic stress, which is based on the evolution of a parameter describing the fluid microstructure. We demonstrate that the turbulence structure and polymer microstructure evolve asynchronously as one moves in the streamwise direction. We observe an initial development length, which is followed by a quasisteady region where variations in drag reduction are weak. High drag reduction behavior can be present at short downstream distances from the inflow plane.

109 citations


Journal ArticleDOI
TL;DR: In this article, the drag reduction in a zero pressure gradient (ZPG) turbulent boundary layer (TBL) using a rigid rod-like polymer was experimentally and numerically investigated.
Abstract: The drag reduction in a zero pressure gradient (ZPG) turbulent boundary layer (TBL) using a rigid rodlike polymer was experimentally and numerically investigated. Using injection of the rigid polysaccharide scleroglucan, drag reductions of approximately 10–15 % were observed, with three distinct drag reduction regimes: a non-Newtonian flow region near the injector, followed by a region of nearly constant drag reduction, and finally a region of negligible drag reduction. Decreasing the effective rotary Peclet number reduced the drag reduction effectiveness. Increasing the concentration did not improve the drag reduction, but instead shifted the spatial development of the drag reduction further downstream. A complementary direct numerical simulation of the ZPG TBL using the rigid rod constitutive equation was performed at a matching inlet Reynolds number. The simulation assumed a homogeneous concentration distribution and used estimated effective parameters for the rodlike additive. Spatial evolution of the fiber stresses is rapid and develops asynchronously with the flow structure. The simulated turbulence statistics and experimental measurements at a position 23 boundary layer thicknesses downstream compare favorably, with the primary differences due to the concentration distribution assumed in the simulation.

43 citations


Journal Article
TL;DR: In this article, an internal layer was found in the turbulent flow through an asymmetric planar diffuser using large-eddy simulation; they discuss five issues relevant to the internal layer: definition and identification, conditions for occurrence, connection with its outer flow, similarity with other equilibrium flows, and growth.
Abstract: We report an internal layer found in the turbulent flow through an asymmetric planar diffuser using large-eddy simulation; we discuss five issues relevant to the internal layer: definition and identification, conditions for occurrence, connection with its outer flow, similarity with other equilibrium flows, and growth. The present internal layer exists in a region with stabilized positive skin friction downstream of a sharp reduction. The streamwise pressure gradient changes suddenly from slightly favourable to strongly adverse at the diffuser throat, and relaxes in a prolonged mildly adverse region corresponding to the skin friction plateau. Development of the internal layer into the outer region is slow, in contrast to the internal layers previously identified from certain external boundary-layer flows where the sudden change in streamwise pressure gradient is from strongly adverse to mildly favourable. Signatures of the internal layer include an inflectional point in the wall-normal profiles of streamwise turbulence intensity, and a well-defined logarithmic slope in the mean streamwise velocity underneath a linear distribution extending to the core region of the diffuser. Some of these characteristics bear a certain resemblance to those existing in the C-type of Couette–Poiseuille turbulent flows. Frequency spectrum results indicate that application of strong adverse pressure gradient at the diffuser throat enhances the low-frequency content of streamwise turbulent fluctuations. Inside the internal layer, the frequency energy spectra at different streamwise locations, but with the same wall-normal coordinate, nearly collapse. Two-point correlations with streamwise, wall-normal and temporal separations were used to examine connections between fluctuations inside the internal layer and those in the core region of the diffuser where the mean streamwise velocity varies linearly with distance from the wall. Galilean decomposition of instantaneous velocity vectors reveals a string of well-defined spanwise vortices outside the internal layer. The internal layer discovered from this study provides qualified support for a conjecture advanced by Azad &

36 citations


Journal ArticleDOI
TL;DR: In this paper, a method based on a virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of an LES flow domain in order to take downstream flow effects computed by a RANS flow solver into account.
Abstract: The numerical flow prediction of highly complex flow systems, such as the aerothermal flow through an entire aircraftgasturbineengine,requirestheapplicationofmultiplespecializedflowsolvers,whichhavetorunsimultaneously in order to capture unsteady multicomponent effects. The different mathematical approaches of different flow solvers, especially large eddy simulation (LES) and Reynolds-averaged Navier‐Stokes (RANS) flow solvers, pose challenges in the definition of boundary conditions at the interfaces. Here, a method based on a virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of an LES flow domain in order to take downstream flow effects computed by a RANS flow solver into account. This method shows good results in the test case of a swirl flow, where the influence of a flow contraction downstream of the LES domain is represented entirely by the Reynolds-averaged velocity field at the outlet of the LES domain. I. Motivation N UMERICALsimulationsofcomplexlarge-scaleflowsystems must capture a variety of physical phenomena in order to predicttheflowaccurately.Currently,manyflowsolversarespecialized to simulate one part of a flow system effectively, but are either inadequate or too expensive to be applied to a generic problem. Asanexample,theaerothermalflowthroughagasturbineengine can be considered. In the compressor and the turbine section, the flow solver has to be able to handle the moving blades, model the wall turbulence, and predict the pressure and density distributions properly. This can be done efficiently by a flow solver based on the Reynolds-averaged Navier‐Stokes (RANS) approach. On the other hand, the flow in the combustion chamber is governed by largescale turbulence, complex mixing processes, chemical reactions, and the presence of fuel spray. Experience shows that these phenomena require an unsteady approach. 1 Hence, the use of a large

35 citations


Proceedings ArticleDOI
06 Jun 2005
Abstract: Optical aberrations induced by turbulent flows are a serious concern in airborne communication and imaging systems. In these applications an optical beam is required to be transmitted through a relatively long distance, over which the quality of the beam can degrade due to variations of the index of refraction along its path. For air and many fluids, the refractive index is linearly related to the density of the fluid through the Gladstone-Dale relation (see Wolf & Zizzis 1978), and therefore density fluctuations due to flow turbulence are the root cause of optical aberrations. An airborne optical beam generally encounters two distinct turbulent flow regimes: the turbulence in the vicinity of the aperture produced by the presence of solid boundaries, and atmospheric turbulence. Aero-optics is the study of optical distortions by the near-field turbulent flows, typically involving turbulent boundary layers, mixing layers, and wakes (see Gilbert 1982). The depth of the aberrating flowfield is usually smaller than or comparable to the projecting (or imaging) aperture. When an initially planar optical wavefront passes a compressible flow, different parts of the wavefront experience different density in the medium and hence have different propagation speeds. Consequently the wavefront becomes deformed. A small initial deformation of the wavefront can lead to large errors on a distant target. The consequences of such deformations include optical beam deflection (bore-sight error) and jitter, beam spread, and loss of intensity. Wavefront distortions can also cause reductions of resolution, contrast, effective range, and sensitivity for airborne electro-optical sensors and imaging systems (Jones & Bender 2001). Research in the area of turbulent distortions of optical waves can be traced back to the 1950s and 1960s (see, for example, Chernov 1960; Tatarski 1961) when the scattering of acoustic and electromagnetic waves due to random fluctuations of refractive index were studied, mostly in the context of atmosphere propagation. Most of the early studies are based on statistical analysis with simplifying assumptions such as homogeneous and isotropic turbulence, and therefore are not directly applicable in realistic aero-optical flowfields. Sutton (1969) characterized different regimes based on optical and flow parameters for the case of homogeneous and isotropic turbulence and developed statistical models to predict far-field optical aberrations. It was in the late 1980s when aero-optics in the modern sense, i.e., the study of optical distortions due to near-aperture turbulence, came into consideration. Many experimental studies have been performed to develop high-speed wavefront measurement tools (e.g., Jumper & Fitzgerald 2001; Cheung & Jumper 2004), study the refractive index structures (e.g., Catrakis & Aguirre 2004; Dimotakis et al. 2001; Fitzgerald & Jumper 2004), develop distortion scaling laws (e.g., Gordeyev et al. 2003), and devise control techniques to suppress or modify optically important turbulence structures (e.g., Gordeyev et al. 2004; Sinha et al. 2004). Despite advances in wavefront sensor technology, significant limitations

22 citations


Book
01 Jan 2005
TL;DR: In this article, a wall model based on optimal control theory has been developed that differs from previous approaches in two significant ways: first, the optimization problem is defined only near the boundary and carefully constructing the equations governing the optimization.
Abstract: : Large-eddy simulation (LES) requires very high resolution in high Reynolds number, attached turbulent boundary layers due to the need to capture the small, dynamically important near-wall eddies. Wall modeling enables LES to be performed on grids that do not resolve these eddies by providing approximate boundary conditions to the simulation. Unfortunately, wall models based on purely physical reasoning often lead to an inaccurate LES, particularly on coarse grids and at high Reynolds numbers, because they do not account for numerical and subgrid scale modeling errors. To compensate for these errors, a wall model based on optimal control theory has been developed that differs from previous approaches in two significant ways. First, the computational expense of the optimization procedure has been reduced by an order of magnitude (with respect to previous control-based wall models) by defining the optimization problem only near the boundaries and carefully constructing the equations governing the optimization problem. Second, no a priori information is required since a near-wall RANS solver is coupled with the LES to provide the controller with information about the mean velocity profile. This approach has been successfully tested in high Reynolds number plane channel flow.

10 citations


DOI
27 Jun 2005
TL;DR: The tip-leakage flow dynamics in axial turbomachines have been studied using large-eddy simulation (LES) with particular emphasis on understanding the underlying mechanisms for viscous losses, low-pressure fluctuations, and tip leakage vortex oscillations as discussed by the authors.
Abstract: The tip-leakage flow dynamics in axial turbomachines has been studied using large-eddy simulation (LES) with particular emphasis on understanding the underlying mechanisms for viscous losses, low-pressure fluctuations, and tip-leakage vortex oscillations. Such an understanding is essential to predicting and eventually controlling cavitation, noise, and vibration in liquid handling systems such as pumps and ducted propellers, and improving their performance. An overview of the computational challenges and major accomplishments of this Challenge Project is presented. These include: (i) improvements of parallel performance, optimization, and portability of the LES code, (ii) LES of the tip-leakage flow in a linear cascade, (iii) detailed analysis of flow statistics, vortex dynamics, and pressure fluctuations, (iv) exploration of tip-leakage flow control strategies by modifying the tip-gap size and end-wall shape, and (v) extension of the flow solver for multiphase simulations of cavitating flow in a rotating twisted blade.

3 citations


Proceedings ArticleDOI
01 Jan 2005
TL;DR: In this article, the tip-clearance flow in axial turbomachines is studied using large-eddy simulation with particular emphasis on understanding the underlying mechanisms for viscous losses in the end-wall region and the unsteady characteristics of the tip leakage vortical structures.
Abstract: The tip-clearance flow in axial turbomachines is studied using large-eddy simulation with particular emphasis on understanding the underlying mechanisms for viscous losses in the end-wall region and the unsteady characteristics of the tip-leakage vortical structures. Systematic and detailed analysis of the mean flow field and turbulence statistics has been made in a linear cascade with a moving end-wall. The tip-leakage jet and tip-leakage vortex are found to produce significant mean velocity gradients, leading to the production of vorticity and turbulent kinetic energy. These are the major causes for viscous losses in the cascade end-wall region. An analysis of the energy spectra and space-time correlations of the velocity fluctuations suggests that the tip-leakage vortex is subject to a pitchwise low frequency wandering motion.Copyright © 2005 by ASME

3 citations