Search

One of the challenging points in the simulation of a nanofluid flowing through a porous medium is modeling the surface heat flux in the presence of nanoparticles and internal solid matrix. The question is how much energy is absorbed by the solid phase, fluid phase, and particles at the surface of imposing heat flux? To reach a suitable answer, a local thermal nonequilibrium approach (including three energy equations) is presented in this paper and three heat flux models are proposed for the first time. The proposed models are compared and analyzed. The effects of interstitial heat transfer coefficients on the heat transfer in a porous channel are completely studied. The fluid temperature distributions and heat transfer rate obtained by homogenous and nonhomogenous approaches (for the proposed models) are completely studied and compared. The results show that the nonhomogeneous approach experiences larger Nusselt number than the homogeneous one for all the recommended heat flux models. PACS Nos.: 47.61.-k, 47.56.+r. Une des difficultes dans la simulation de l'ecoulement d'un nanofluide a travers un milieu poreux est la modelisation de l'echange de chaleur a la surface en presence de nanoparticules et d'une matrice solide. La question est, combien de chaleur est absorbee par la phase solide, la phase fluide et les particules a la surface ou on retrouve le flux de chaleur. Afin de repondre a cette question, nous presentons ici pour la premiere fois une approche de non equilibre thermique local (incluant les equations d'energie libre) et proposons trois modeles de flux de chaleur. Les modeles proposes sont compares et analyses. Nous faisons une etude complete des effets des coefficients de transfert interstitiel de chaleur sur le transfert de chaleur dans un canal poreux. Nous completons une etude comparative des distributions de temperature du fluide et de transfert de chaleur obtenues par approches homogene et non homogene (pour nos modeles). Les resultats montrent que l'approche non homogene produit des valeurs plus elevees du nombre de Nusselt, et ce pour tous les modeles presentes de flux de chaleur. [Traduit par la Redaction], Introduction Convective flow and heat transfer in porous media has received considerable attention because of its applications, such as heat pipe, solid matrix heat exchangers, electronic cooling, and drying processes [...]

The aim of this article is twofold. First, an existing trailing edge noise model is validated by comparing with airfoil surface pressure fluctuations and far field sound pressure levels measured in three different experiments. The agreement is satisfactory in one case but poor in two other cases. Nevertheless, the model reproduces the main tendencies observed in the measurements with respect to varying flow conditions. Second, the model is implemented into an airfoil design code that is originally used for aerodynamic optimization. An existing wind turbine airfoil is optimized in order to reduce its noise emission, trying at the same time to preserve some of its aerodynamic and geometric characteristics. The new designs are characterized by less cambered airfoils and flatter suction sides. The resulting noise reductions seem to be mainly achieved by a reduction in the turbulent kinetic energy across the boundary layer near the trailing edge and to a lesser extent by a smaller boundary layer displacement thickness. [DOI: 10.1115/1.4001462]

A hybrid vortex filament scheme with the capability of simulating bounded turbulent flows is described. Viscous generation of new vortex elements at solid surfaces is accomplished through the intermediary step of solving the viscous vorticity transport equation on a thin boundary mesh via a finite difference and finite volume method. The transitional and turbulent boundary-layer flow past a wide, finite thickness, flat plate with rounded edges is computed with a view toward validating the methodology and gaining new insight into the structural aspects of transition. The predicted mean velocity and related statistics are well matched to experimental and numerical data. The representation of the flow through vortex filaments reveals that a distinction needs to be made between vortical structures, on one hand, and the rotational motion that they produce in the flowfield, on the other hand. In particular, hairpin-shaped regions that are commonly found by vortex identification schemes are found to not be structures in their own right, but rather the rotational signature of raised furrows in the surface vortex sheet. The latter overlie low-speed streaks and evolve to include ejected mushroom-shaped structures as well as spanwise vortices associated with roll-up. DOI: 10.2514/1.J050224

An experimental study was conducted to determine the effect of single dielectric barrier discharge plasma actuators on turbulent boundary-layer separation control. Two-component particle image velocimetry and laser Doppler velocimetry measurements showed the effect of plasma actuators on ambient air, a canonical zero-pressure gradient turbulent boundary layer, and a two-dimensional turbulent boundary-layer separation from a convex ramp section. Different actuator configurations and control strategies were implemented. Spanwise actuators were operated in both steady and unsteady modes. Plasma streamwise vortex generators were also used to enhance boundary-layer mixing. Flow visualization using a high-speed camera captured the temporal aspects of the separation control process and helped discern the physical mechanisms associated with each actuation strategy. The steady spanwise actuation produced a wall jet effect that augmented near-wall momentum and reattached the separated boundary layer. The streamwise oriented actuators also showed effective control authority by creating counter-rotating vortices within the boundary layer that promote mixing of high and low momentum fluid. DOI: 10.2514/1.J050009

A developing boundary layer starts at the spillway crest until it reaches the free surface at the so-called inception point, where the natural air entrainment is initiated. A detailed reanalysis of the turbulent velocity profiles on steep chutes is made herein, including mean values for the parameters of the complete turbulent velocity profile in the turbulent rough flow regime, given by the log-wake law. Accounting both for the laws of the wall and the wake, a new rational approach is proposed for a power-law velocity profile within the boundary layer of turbulent rough chute flow. This novel approach directly includes the power-law parameters and does not require for a profile matching, as is currently required. The results obtained for the turbulent velocity profiles were applied to analytically determine the resistance characteristics for chute flows. The results apply to the developing flow zone upstream of air inception in chute spillways. DOI: 10.1061/(ASCE)HY.1943-7900.0000190 CE Database subject headings: Air inception; Boundary layers; Flow resistance; Open channel flow; Velocity. Author keywords: Air inception; Boundary layer; Chute flow; Flow resistance; Open channel flow; Turbulent velocity profiles.

The influence of seepage (lateral flow) on the turbulence characteristics in free-surface flows over an immobile rough boundary is investigated. Steady flows having zero-pressure gradient over an immobile rough boundary created by uniform gravels of 4.1 mm in size were simulated experimentally with injection (upward seepage) and suction (downward seepage) applied through the boundary. A Vectrino (acoustic Doppler velocimeter) was used to measure the instantaneous velocities, which are analyzed to explore second-and third-order correlations, turbulent kinetic energy, turbulent energy budget, and conditional Reynolds shear stresses. It is observed that the second-order correlations decrease in presence of injection and increase in suction. The turbulent diffusivity and mixing length increase in presence of injection and decrease in suction. The third-order correlations suggest that the ejections are prevalent over the entire flow depth. The near-boundary flow is significantly influenced by the existence of upward seepage, which is manifested by a reduction in streamwise flux and the vertical advection of streamwise Reynolds normal stress. In addition, the upward flux and the streamwise advection of vertical Reynolds normal stress are also affected. The streamwise flux of turbulent kinetic is found to migrate upstream, while the vertical flux of turbulent kinetic energy is transported upward. The fluxes increase in presence of injection and decrease in suction. Energy budget evidences a lag between the turbulent dissipation and production and an opposing trend in the turbulent and pressure energy diffusions. A quadrant analysis for the conditional Reynolds shear stresses reveals that the ejection and sweep events are the primary contributions toward the total Reynolds shear stress production, with ejections dominating over the entire flow depth. The effect of seepage is shown to affect the magnitude of such events. However, in case of sweeps, this phenomenon is the opposite. The mean-time of occurrence of ejections and that of sweeps in suction are more persistent than those in no-seepage and injection. DOI: 10.1061/(ASCE)EM. 1943-7889.0000124 CE Database subject headings: Hydraulics; Open channel flow; Turbulent flow; Steady flow; River beds; Seepage; Turbulence. Author keywords: Hydraulics; Open channel flow; Turbulent flow; Flow characteristics; Steady flow; Streambeds; Seepage.

High-resolution three-dimensional cloud resolving model simulations of deep cumulus convection under a wide range of large-scale forcings are used to evaluate a mass flux closure based on boundary layer convective inhibition (CIN) that has previously been applied in parameterizations of shallow cumulus convection. With minor modifications, it is also found to perform well for deep oceanic and continental cumulus convection, and it matches simulated cloud-base mass flux much better than a closure based only on the boundary layer convective velocity scale. CIN closure maintains an important feedback among cumulus base mass flux, compensating subsidence, and CIN that keeps the boundary layer top close to cloud base. For deep convection, the proposed CIN closure requires prediction of a boundary layer mean turbulent kinetic energy (TKE) and a horizontal moisture variance, both of which are strongly correlated with precipitation. For our cases, CIN closure predicts cloud-base mass flux in deep convective environments as well as the best possible bulk entraining CAPE closure, but unlike the latter, CIN closure also works well for shallow cumulus convection without retuning of parameters. DOI: 10.1175/2010JAS3328.1

Doubts have been expressed about the validity of the critical depth defined in terms of the minimum specific energy head of the free-surface streamline when dealing with developing open-channel flows. This note examines the two approaches for defining critical flow, that based on the minimum specific energy of the free-surface streamline and that based on the mean energy head of the whole flow section. Large differences for the dimensionless critical depths are obtained with the two methods due to each critical depth proving to be a different control point on the free-surface profile. It is argued that both methods are different alternatives, although the critical depth concept was different in each case. Theoretical support to critical flow computations based on the free streamline is provided. An alternative approach for computing the discharge characteristics of broad-crested weirs based on the energy loss inside the boundary layer is also proposed. DOI: 10.1061/(ASCE)HY.1943-7900.0000142 CE Database subject headings: Open channel flow; Boundary layers; Critical flow; Weirs. Author keywords: Open channels; Boundary layers; Critical flow; Weirs.

This paper presents the classical approximation scheme to investigate the velocity profile associated with the Falkner--Skan boundary-layer problem. Solution of the boundary-layer equation is obtained for a model problem in which the flow field contains a substantial region of strongly reversed flow. The problem investigates the flow of a viscous liquid past a semi-infinite flat plate against an adverse pressure gradient. Optimized results for the dimensionless velocity profiles of reverse wedge flow are presented graphically for different values of wedge angle parameter [beta] taken from 0 [less than or equal to] [beta] [less than or equal to] 2.5. Weighted residual method (WRM) is used for determining the solution of nonlinear boundary-layer problem. Finally, for [beta] = 0 the results of WRM are compared with the results of homotopy perturbation method. [DOI: 10.1115/1.3173763]

In this paper, the nonsimilarity boundary-layer flows of second-order fluid over a flat sheet with arbitrary stretching velocity are studied. The boundary-layer equations describing the steady laminar flow of an incompressible viscoelastic fluid past a semi-infinite stretching flat sheet are transformed into a partial differential equation with variable coefficients. An analytic technique for highly nonlinear problems, namely, the homotopy analysis method, is applied to give convergent analytical approximations, which agree well with the numerical results given by the Keller box method. Furthermore, the effects of physical parameters on some important physical quantities, such as the local skin-friction coefficient and the boundary-layer thickness, are investigated in detail. Mathematically; this analytic approach is rather general in principle and can be applied to solve different types of nonlinear partial differential equations with variable coefficients in physics. [DO]: 10.1115/1.3173764] Keywords: nonsimilarity boundary-layer flow, second-order fluid, series solution, homotopy analysis method

In 2001, the authors presented a higher-order mass-flux model called 'assumed distributions with higher-order closure' (ADHOC 1), which represents the large eddies of the planetary boundary layer (PBL) in terms of an assumed joint distribution of the vertical velocity and scalars. In a subsequent version (ADHOC 2) the authors incorporated vertical momentum fluxes and second moments involving pressure perturbations into the framework. These versions of ADHOC, as well as all other higher-order closure models, are not suitable for use in large-scale models because of the high vertical and temporal resolution that is required. This high resolution is needed mainly because higher-order closure (HOC) models must resolve discontinuities at the PBL top, which can occur anywhere on a model's Eulerian vertical grid. This paper reports the development of ADHOC 3, in which the computational cost of the model is reduced by introducing the PBL depth as an explicit prognostic variable. ADHOC 3 uses a stretched vertical coordinate that is attached to the PBL top. The discontinuous jumps at the PBL top are 'hidden' in the layer edge that represents the PBL top. This new HOC model can use much coarser vertical resolution and a longer time step and is thus suitable for use in large-scale models. To predict the PBL depth, an entrainment parameterization is needed. In the development of the model, the authors have been led to a new view of the old problem of entrainment parameterization. The relatively detailed information available in the HOC model is used to parameterize the entrainment rate. The present approach thus borrows ideas from mixed-layer modeling to create a new, more economical type of HOC model that is better suited for use as a parameterization in large-scale models. DOI: 10.1175/2009JAS3205.1

Seiche-induced turbulence and the vertical distribution of dissolved oxygen above and within the sediment were analyzed to evaluate the sediment oxygen uptake rate ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]), diffusive boundary layer thickness ([[delta].sub.DBL]), and sediment oxic zone depth ([z.sub.max]) in situ. High temporal-resolution microprofiles across the sediment water interface and current velocity data within the bottom boundary layer in a medium-sized mesotrophic lake were obtained during a 12-h field study. We resolved the dynamic forcing of a full 8-h seiche cycle and evaluated [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] from both sides of the sediment-water interface. Turbulence (characterized by the energy dissipation rate, [epsilon]), the vertical distribution of dissolved oxygen across the sediment-water interface (characterized by [[delta].sub.DBL] and [z.sub.max]), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and the sediment oxygen consumption rate ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) are all strongly correlated in our freshwater system. Seiche-induced turbulence shifted from relatively active ([epsilon] = 1.2 x [10.sup.-8] W [kg.sup.-l]) to inactive ([epsilon] = 7.8 x [l0.sup.-12] W [kg.sup.-1]). In response to this dynamic forcing, [[delta].sub.DBL] increased from 1.0 mm to the point of becoming undefined, [z.sub.max] decreased from 2.2 to 0.3 mm as oxygen was depleted from the sediment, and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] decreased from 7.0 to 1.1 mmol [m.sup.2] [d.sup.-1] over a time span of hours. [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and oxygen consumption were found to be almost equivalent (within ~ 5% and thus close to steady state), with [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] adjusting rapidly to changes in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Our results reveal the transient nature of sediment oxygen uptake and the importance of accurately characterizing turbulence when estimating [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

Two-dimensional evolution equations are derived as applied to flows in the near-wall jet and the Blasius boundary layer on a flat plate on which a mechanism of inviscid--inviscid infraction controls the development of large-sized short-scaled disturbances. The first one is an extension of the Korteweg--de Vries equation. As distinct from the shallow-water wave motion underlying the Kadomtsev--Petviashvili equation, the fluid parameters are not assumed to depend only weakly on the direction transversal to the oncoming stream. The second dynamical system provides a two-dimensional analog of the Benjamin--Davis--Acrivos equation. Simple line-soliton solutions are presented in both cases. A generalized Hirota function allows a pair of crossed solitons to be obtained at some distance from a solid surface in the near-wall jet. An oblique periodic nonlinear wave train pointed out for the Blasius boundary layer comes in place of the Tollmien--Schlichting waves when their amplitude attains sufficiently large values. DOI: 10.2514/1.40761

To examine the potential of micro vortex generators for shock/boundary-layer interaction control, a detailed experimental and computational study in a supersonic boundary layer at M = 3.0 was undertaken. The experiments employed a flat-plate boundary layer with an impinging oblique shock with downstream total-pressure measurements. The moderate Reynolds number of 3800 allowed the computations to use monotone-integrated large eddy simulations. The monotone-integrated large eddy simulations predictions indicated that the shock changes the structure of the turbulent eddies and the primary vortices generated from the microramp. Furthermore, they generally reproduced the experimentally obtained mean velocity profiles, unlike similarly resolved Reynolds-averaged Navier-Stokes computations. The experiments and monotone-integrated large eddy simulations results indicate that the microramps, for which the height is h [approximately equal to] 0.5[delta], can significantly reduce boundary-layer thickness and improve downstream boundary-layer health as measured by the incompressible shape function H. Regions directly behind the ramp centerline tended to have increased boundary-layer thickness, indicating the significant three- dimensionality of the flowfield. Compared with baseline sizes, smaller microramps yielded improved total-pressure recovery. Moving the smaller ramps closer to the shock interaction also reduced the displacement thickness and the separated area. This effect is attributed to decreased wave drag and the closer proximity of the vortex pairs to the wall. DOI: 10.2514/1.41776

In a companion paper (2008, 'Vorticity Dynamics in Axial Compressor Flow Diagnosis and Design,' ASME J. Fluids Eng., 130, p. 041102), a study has been made on the critical role of circumferential vorticity (CV) in the performance of axial compressor in through-flow design (TFD). It has been shown there that to enhance the pressure ratio, the positive and negative CV peaks should be pushed to the casing and hub, respectively. This criterion has led to an optimal TFD that indeed improves the pressure ratio and efficiency. The CV also has great impact on the stall margin as it reflects the end wall blockage, especially at the tip region of the compressor. While that work was based on inviscid and axisymmetric theory, in this paper, we move on to the diagnosis and optimal design of fully three-dimensional (3D) viscous flow in axial compressors, focusing on the boundary vorticity flux (BVF), which captures the highly localized peaks of pressure gradient on the surface of the compressor blade, and thereby signifies the boundary layer separation and dominates the work rate done to the fluid by the compressor For the 2D cascade flow we show that the BVF is directly related to the blade geometry. BVF-based 2D and 3D optimal blade design methodologies are developed to control the velocity diffusion, of which the results are confirmed by Reynolds-averaged Navier--Stokes simulations to more significantly improve the compressor performance than that of CV-based TFD. The methodology enriches the current aerodynamic design system of compressors. [DOI: 10.1115/1.4000650]

Numerical simulation of shock waves interacting with multimaterial interface is immensely challenging, particularly when the embedded interface is retained as a sharp entity. The challenge lies in accurately capturing and representing the interface dynamics and the wave patterns at the interface. In this regard, the ghost fluid method has been successfully used to capture the interface conditions for both fluid-fluid and solid-fluid interfaces. However, the ghost fluid method results in over/underheating errors when shocks impact interfaces, and hence must be supplemented with numerical corrective measures to mitigate these errors. Such corrections typically fail for strong shock applications. Variants and extensions of the ghost fluid method have been proposed to remedy its shortcomings with mixed success. In this paper, the performance of approaches based on the ghost fluid method, in the case of strong shocks impinging on immersed solid boundaries in compressible flows, is evaluated. It is found that (from the viewpoint of simplicity, robustness, and accuracy) a reflective boundary condition used in conjunction with a local Riemann solver at the interface proves to be a good choice. The method is found to be stable, accurate, and robust for wide range problems involving strong shocks interacting with embedded solid objects. DOI: 10.2514/1.43153

Numerical studies were carried out to investigate the effects of turbulent boundary layers on panel flutter at supersonic speeds. In this study, Reynolds-averaged Navier-Stokes equations were solved to take into account the turbulent boundary layer and its viscous effects. First, the fluid-structure coupling code was validated. The computed flutter boundaries agreed well with experimental data. Moreover, the results showed that the viscous effects were important and should be taken into account for flutter computation. Second, the boundary-layer effects were investigated in the Mach number range of 1.0-2.4. We compared the Reynolds-averaged Navier-Stokes computation with the inviscid computation and discussed the differences between them. We found that the boundary layer has not only a stabilizing effect but also a destabilizing effect, depending on the Mach number. The most important finding is that the flutter dynamic pressure slowly increases due to the boundary layer as the Mach number increases. In addition, the design boundary methodology was reviewed in terms of the turbulent boundary-layer effect, which will be helpful for the development of a new boundary-layer correction for the design boundary. DOI: 10.2514/1.35786

Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFe[O.sub.3]) films. Electric field--dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications. 29 May 2009; accepted 1 September 2009 10.1126/science.1177046

An Euler solver with boundary-layer option, overset/sheared Cartesian mesh, and automated mesh-generation capability has been developed for aeroelastic applications to complex aircraftlike configurations. The automated mesh-generation scheme call automatically generate a block mesh by extending the grid lines from surface meshes to the entire flowfield, in this way, the surface mesh can adopt the paneling definition required by commonly practiced aerodynamic panel methods as input. The overset/sheared-Cartesian-mesh capability allows convenient modeling of very complex configurations, such as an aircraft with external stores, in which the aircraft and stores are modeled by different blocks of meshes. This paper does not describe a novel computational method nor provide new insight into the characteristics of physical problem. Rather, it offers a user-friendly and cost-effective computational methodology that can be readily adopted by industry for rapid aeroelastic applications. DOI: 10.2514/1.43434

A new method for predicting the noise generated by a ducted rotor interacting with inhomogeneous and nonisotropic turbulence has been developed. The analytical formulation used a model of the two-point correlation function of the turbulent velocity in the space--time domain. The study focused on a specific condition where the dominant noise source was the interaction between a rotor and a casing turbulent boundary layer. The axial length scale of this turbulence was found to be large enough to generate unsteady lift that was correlated between multiple rotor blades. This led to tonal sound at the blade passing frequency in the absence of mean velocity variations. The analytical formulation was validated with a set of measurements obtained in a ducted rotor facility. The prediction accurately modeled both the tonal and broadband features of the sound spectra. DOI: 10.2514/1.43271

The interface condition provides flexibility to handle complex geometries on a set of structured grids. However, when used with a compact scheme to apply in computational aeroacoustics, its accuracy and stability may be degraded seriously by the boundary treatment that arises at a grid interface. To minimize this effect, a modified characteristic condition is incorporated in the finite difference compact scheme by using an explicit difference form at the boundary. The extension of this approach is naturally attained through the coordinate transformation of convective terms across an interface in multidimensions. The validity of current development is demonstrated through a simulation of trailing-edge noise generation from an airfoil at a low Mach number. DOI: 10.2514/1.42110

Laminar-turbulent transition remains a critical issue in a number of cases, including drag reduction, performance prediction of high-lift systems, improved accuracy in general computational fluid dynamics, and reduction of computation cycles for development of optimization tools. Transition delay remains one of the most promising technologies for reducing air transport energy consumption, through natural or hybrid laminar flow control. The use of linear stability theory, either local or nonlocal, remains rather demanding in terms of knowledge and user interaction. Hence, a demand exists for simplified, robust, and accurate transition prediction tools to be inserted into general flow solvers, of boundary-layer or Reynolds-averaged Navier--Stokes types. The problem can be solved by developing transition criteria or database methods. In this last case, characteristics of an actual flow are derived from known solutions of model flows. ONERA, the French Aerospace Laboratory, has long been involved in the development of such methods, and the present paper aims at providing a comprehensive view of the tools developed in the second category, applicable from low-speed two-dimensional to transonic three-dimensional flows, and even to three-dimensional supersonic flows. DOI: 10.2514/1.42990

As a first step toward predicting rough-wall boundary-layer noise, the sound radiated from a single hemispherical roughness element and a pair of roughness elements in a turbulent boundary layer at [Re.sub.[theta]] = 7500 is investigated. The roughness height is 3.6% of the boundary-layer thickness, or 95 wall units. The flowfield is obtained from large-eddy simulation, and the results are validated against experimental measurements. Acoustic calculations are performed based on the Curle--Powell integral solution to the Lighthill equation for an acoustically compact hemisphere. The sound radiation is dominated by unsteady drag dipoles and their images in the wall. It is found that the spanwise dipole, which has been overlooked in previous studies of roughness noise, is of larger or similar strength compared with the streamwise dipole, and the viscous contribution to the drag dipoles is negligible in comparison with the pressure contribution. Important flow features contributing to sound radiation are identified by examining the unsteady surface-pressure field and the surrounding flow structures. Pressure fluctuations are strongest on the upstream part of the hemispheric surface near the base due to impingement of incoming turbulent eddies and their interaction with horseshoe vortices. On the back surface of the hemisphere, pressure fluctuations are relatively weak, indicating that shear-layer separation and vortex shedding do not produce significant self-noise from the hemisphere. In the case with two hemispheres, the wake of the upstream hemisphere is found to significantly enhance sound radiation from the downstream hemisphere, particularly in the streamwise direction and at high frequencies. DOI: 10.2514/1.42343

An analytical model for the unswept normal shock wave/turbulent-boundary-layer interaction control using an upstream and downstream unimorph piezoelectric flap actuator has been proposed. The amount of flap deflection controls the bleed/suction rate through a plenum chamber. The cavity allows rapid thickening of the boundary layer approaching a normal shock wave, which splits into a series of weaker shocks forming a lambda shock foot, leading to a reduction in the wave drag. The analysis provides an understanding of the control influences produced in an experimental investigation of an unswept normal shock wave/turbulent-boundary-layer interaction at a Mach number of 1.5. It has also been validated by application to the normal shock wave/boundary-layer interaction control system using mesoflaps for aeroelastic transpiration described in previous transonic/supersonic shock wave/ boundary-layer interaction studies.

A two-dimensional Shack--Hartmann wave-front sensor is used to study aero-optic distortion in turbulent boundary layers at transonic and hypersonic speeds, with and without gas injection. The large-scale motions in the outer layer, of the order of the boundary-layer thickness in size, are shown to dominate the aero-optic distortion. Gas injection always reduced the Strehl ratio, with helium injection generally giving lower Strehl ratios than nitrogen injection. The large aperture approximation is shown to be accurate for a wide variety of aberrations regardless of Mach number and gas injection. A new scaling argument for the root-mean-square phase distortion is proposed that appears to collapse the data better than previous models. DOI: 10.2514/1.41453

Simulations of compressible boundary-layer flow at three different Reynolds numbers ([Re.sub.[delta]] = 5.59 x [10.sup.4], 1.78 x [10.sup.5], and 1.58 x [10.sup.6]) are performed using a hybrid large-eddy simulation/Reynolds-averaged Navier--Stokes method. Variations in the recycling/rescaling method, the higher order extension, the choice of primitive variables, the Reynolds-averaged Navier--Stokes to large eddy simulation transition parameters, and the mesh resolution are considered in order to assess the model. The results indicate that the present model can provide good predictions of the mean-flow properties, second-moment statistics, and structural features of the boundary layers considered. Normalized turbulent statistics in the outer layer are found to be independent of Reynolds number, similar to incompressible turbulent boundary layers. DOI: 10.2514/1.41598

This paper presents an experimental and computational study of a passive version of the air-jet vortex-generator flow control system for the delay or suppression of trailing-edge boundary-layer separation and subsequent stall. The results show that a passive vortex-generating jet flow control system, which uses the high pressure front the leading-edge attachment line to feed the upper-surface air jets, can effectively delay trailing-edge separation and subsequent stall to higher angles of attack, without the need for any active energy input and without significant drag penalty. DOI: 10.2514/1.41986

Direct numerical simulations are carried out on boundary-layer flow at Mach 6 over a porous surface, in which a Mack mode of instability is excited. The pores are resolved rather than modeled, allowing an evaluation to be made of the accuracy of simplified analytical models used in previous investigations based on linear stability theory. It is shown that the stabilizing effect of porosity is stronger in the simulations than in the corresponding theory for both two- and three-dimensional pores. From comparisons of spanwise grooves, streamwise slots, and square pores, it appears that the detailed surface structure is not as important as the overall porosity, and the hydraulic diameter is able to collapse the results for different pore shapes to good accuracy. When the porous surface consists of fewer larger pores, the flow is noisier, with sound waves generated at the pore edges. DOI: 10.2514/1.43388

This paper describes a modeling method for closed-loop unsteady fluid transport systems based on 1D unsteady Euler equations with nonlinear forced periodic boundary conditions. A significant feature of this model is the incorporation of dynamic constraints on the variables that control the transport process at the system boundaries as they often exist in many transport systems. These constraints result in a coupling of the Euler equations with a system of ordinary differential equations that model the dynamics of auxiliary processes connected to the transport system. Another important feature of the transport model is the use of a quasilinear form instead of the flux-conserved form. This form lends itself to modeling with measurable conserved fluid transport variables and represents an intermediate model between the primitive variable approach and the conserved variable approach. A wave-splitting finite-difference upwind method is presented as a numerical solution of the model. An iterative procedure is implemented to solve the nonlinear forced periodic boundary conditions prior to the time-marching procedure for the upwind method. A shock fitting method to handle transonic flow for the quasilinear form of the Euler equations is presented. A closed-loop wind tunnel is used for demonstration of the accuracy of this modeling method. [DOI: 10.1115/1.3130244]

Means of controlling the flow over a large spherical protuberance were examined. The role of suction around the base of the protuberance in reducing or even eliminating the necklace vortex created by the protuberance was considered. In the absence of suction, this vortex was lifted by the low base pressure existing behind the protuberance into the wake, thus affecting the turbulence level along its path. Large vortex generators placed upstream of the protuberance were able to delay local separation of the flow over the protuberance, thus affecting the symmetry of the wake and the level of turbulence on one side or the other. Observations made using flow visualization were supplemented by hot-wire measurements. The experiments were carried out at low speed at Reynolds numbers that did not exceed 3 x [10.sup.5].

Experiments were performed to investigate the flow in a Mach 2.9 shock wave turbulent boundary-layer interaction at a Reynolds number based on momentum thickness of 2400. The flow configuration was a nominally two-dimensional 24 deg compression ramp, which exhibited a separation bubble in the corner region. Mean flow quantities, including the velocity profile upstream and downstream of the corner, and the wall pressure through the interaction were measured. Filtered Rayleigh scattering was used to visualize the flow structure in the interaction and provide quantitative measurements of the turbulent structure angle and the intermittency of the boundary-layer edge turbulence. The shock motion was characterized by measuring the fluctuating wall pressure. The results indicate that, compared to previous measurements obtained at higher Reynolds numbers of 60-80,000, the separation bubble is approximately twice as long, the root mean square of the wall-pressure fluctuations has a relatively smaller peak, and the intermittency of the wall-pressure signal in the shock-foot region is attenuated. As in the high-Reynolds-number case, the shock motion has a broadband frequency distribution with a peak slightly below 1 kHz. The mean flow quantities, root mean square wall-pressure-fluctuation profile, wall-pressure signal, and shock-motion frequency agree well with the direct numerical simulation of a previous study at matching conditions.

The wall shear-stress sensor [MPS.sup.3] based on flexible micropillars has been used to experimentally assess the two- dimensional wall shear-stress distribution in turbulent duct flow at moderate Reynolds number. The sensor covers an area of 90 x 125 viscous length scales along the streamwise and spanwise directions, respectively. Applying Taylor's hypothesis allows a further increase in the streamwise extension. Preliminary results evidence the coexistence of low- and high-shear regions representing footprints of near-wall coherent structures. The low-shear regions resemble long meandering bands interrupted by local high-shear-stress regions. A qualitative comparison evidences the structures detected in the present study to be similar to wall shear-stress distributions reported in the literature and to structures found in higher regions of the boundary layer. These preliminary findings indicate the potential of the micropillar sensor concept to be capable of detecting the multidirectional planar wall shear-stress distribution in turbulent flows. A detailed discussion of the wall shear-stress characteristics and of the geometric properties of the streaklike structures is beyond the scope of this paper, and a more intrusive discussion needs to be done in the future.

This paper first discusses the constitutive relations and surface catalytic model for direct numerical simulation of wall-bounded turbulence including finite-rate chemistry and gas--surface interaction and then provides a systematic procedure to test the validity of the simulations by dividing the whole problem into different components and testing each component separately. Namely, comparisons against similarity solutions and other established hypersonic boundary-layer solutions are used to test the validity of laminar mean flow with and without gas-phase chemical reactions; comparisons against the analytic solution for the one-dimensional diffusion equation are used to test the validity of the surface catalysis boundary condition; and comparisons against empirical predictions, detailed experimental data and linear stability theory are used to test the validity of turbulent boundary-layer solutions. DOI: 10.2514/1.38318

This paper presents a review of data on controlling the boundary layer attachment by a plasma actuator with high-voltage pulsed-periodic nanosecond excitation. Actuator-induced gas velocities show near-zero values for nanosecond pulses. The measurements performed show overheating in the discharge region at fast ([tau] [equivalent] 1 [micro]s) thermalization of the plasma inputed energy. The mean values of such heating of the plasma layer can reach 70, 200, and even 400 K for 7-, 12-, and 50-ns pulse durations, respectively. The emerging shock wave together with the secondary vortex flows disturbs the main flow. The resulting pulsed-periodic disturbance causes an efficient transversal momentum transfer into the boundary layer and further flow attachment to the airfoil surface. Thus, for periodic pulsed nanosecond dielectric barrier discharge, the main mechanism of impact is the energy transfer to and heating of the near-surface gas layer. The following pulse-periodic vortex movement stimulates redistribution of the main flow momentum. The experiments performed here have shown high efficiency of the given mechanism to control boundary layer separation, lift, and drag force coefficients, and acoustic noise reduction in the Mach number range of 0.05 to 0.85. DOI: 10.2514/1.38113

Oil-film visualization patterns of the flow on a classical wing and on a wing with a wavy surface at various angles of attack are analyzed. For low Reynolds numbers (Re < 2 x [10.sup.5]), the boundary-layer structure on a wavy wing is demonstrated to differ significantly from that on a classical wing. As a result, leading-edge stall on a wavy wing occurs later than on a smooth wing. The present activities form the basis for a systematic study of the boundary-layer structure on a wavy wing. For this purpose, a series of investigations is performed. Boundary-layer parameters on classical and wavy-surface wings at the angle of attack [alpha] = 0 deg and the chord Reynolds number Re = 1.7 x [10.sup.5] are studied experimentally. The combined experiment includes oil-film visualization, measurements of pressure distributions, and flow velocity. On both wings, substantial differences are found in the transition position and in the transition scenarios realized on the wavy-surface wing along its streamwise grooves and humps. However, some similarity in the disturbance growth in both separation-bubble regions is found.

An experimental study has been conducted to investigate the effects of oscillation at a range of Strouhal numbers (0.0006 to 0.0113) on the boundary-layer development of a flat plate under the influence of favorable pressure gradients. The Reynolds number was [10.sup.6] based on the flat-plate chord length. An inverted airfoil placed in the turbulent region over the flat plate was used to create the effects of favorable pressure gradients. Hot-wire anemometry was used to measure the velocities in the streamwise direction. The flow separates near the rounded leading edge, causing the formation of the separation bubble. It is found that the time-averaged velocity and turbulence intensity profiles remain mostly unaffected by oscillation in the turbulent flow regions (II and III). The boundary-layer properties such as the boundary-layer displacement thickness, momentum thickness, and the shape factor remain almost constant. However, in region I, in which the flow is in its transitional state, oscillation slows the reattachment process under the influence of favorable pressure gradients.

An experimental investigation of turbulent flow subjected to variable adverse and favorable pressure gradients in two-dimensional asymmetric channels is reported. The floors of the diverging and converging channels were flat while the roofs of the channels were curved. Adverse pressure gradient flows at [Re.sub.h]=27,050 and 12,450 and favorable pressure gradient flow at [Re.sub.h] = 19,280 were studied. A particle image velocimetry was used to conduct detailed measurements at several planes upstream, within the variable section and within the downstream sections. The boundary layer parameters were obtained in the upper and lower boundary layers to study the effects of pressure gradients on the development of the mean flow on the floor and roof of the channels. The profiles of the mean velocities, turbulence intensities, Reynolds shear stress, mixing length, eddy viscosity, and turbulence production were also obtained to document the salient features of pressure gradient turbulent flows in asymmetric converging and diverging channels. [DOI: 10.1115/1.2829590]

Boundary-layer separation can be prevented or delayed by sucking part of the boundary layer into the surface, but in a straightforward application the required hydraulics entail significant penalties in terms of weight and cost. By means of computational techniques, this paper explores the possibility of autogenous suction, in which the local pressure differences that lead to separation drive the suction used to prevent it. The chosen examples include steady and unsteady laminar flows around leading edges of thin airfoils. No fundamental theoretical limit to autogenous suction was found in the range of angles of attack that could be studied, but rapidly increasing suction volumes suggest that practical application will become increasingly difficult for more severe adverse pressure gradients. [DOI: 10.1115/1.2813135]

The interpolation technique is presented for the Dirichlet and Neumann or mixed boundary conditions using immersed-boundary methods. The validation of the interpolation algorithm is assessed and shows second-order accuracy in space and time suitable for heat transfer phenomena.

An experimental study has been conducted to investigate the effects of oscillation, of varying amplitude (0 to 0.03 m) and frequency (0 to 6 Hz), on the boundary-layer development of a flat plate under the influence of adverse pressure gradients. The Reynolds number was [10.sup.6] based on the flat plate chord length. The study employed a subsonic wind tunnel facility at a freestream velocity of 16 m/s. A cylinder placed at various locations over the flat plate within the boundary layer was used to create the effects of adverse pressure gradients. Surface pressures were measured along the centerline of the flat plate to study the flow quantitatively. Oil flow visualizations were also obtained to examine the flow qualitatively. Boundary-layer surveys were conducted to examine the effect of oscillation on the mean velocity, turbulence intensity, and boundary-layer thickness profiles.

We investigate the performance of two recently developed subgrid-scale models, the approximate deconvolution model and the high-pass filtered Smagorinsky model, in large-eddy simulations of laminar-turbulent transition in a supersonic boundary layer. Subharmonic transition in a boundary layer at a freestream Mach number of 4.5 and a Reynolds number (based on initial displacement thickness) of 10,000 is considered, which has been studied previously in detail by direct numerical simulations. For computational efficiency, the temporal simulation approach has been adopted. The discretization is based on Fourier collocation and various high-order finite difference schemes in the wall-parallel and wall-normal directions, respectively. Large-eddy simulations results are assessed by comparing statistical and instantaneous quantities during transition with data obtained from a sufficiently resolved direct numerical simulation. The results show that the large-eddy simulations accurately reproduce the direct numerical simulations data from the slightly disturbed laminar flow through transition into the turbulent stage, with a computational effort of two orders of magnitude less than the direct numerical simulations. Both subgrid-scale models are formulated locally in space and in a fully three-dimensional manner and do not need an ad hoc adaptation to nonturbulent or near-wall regions. DOI: 10.2514/1.15048

A numerical study has been carried out for the laminar flow of Newtonian and non-Newtonian power-law fluids through a suddenly expanded axisymmetric geometry. Mathematical correlations are proposed for the prediction of the length of the recirculating eddy in terms of Reynolds number, expansion ratio and rheological parameters. A wide range of expansion ratios (1.25 [less than or equal to] ER [less than or greater than] 8.0) has been covered for the Newtonian fluid and both the shear-thinning and shear-thickening flow characteristic fluids have been considered for the non-Newtonian fluids. [DOI: 10.1115/1.2409361]

The present study investigates turbulent boundary layer separation control by means of streamwise vortices with focus on the instantaneous vortex behavior. A turbulent boundary layer is exposed to a pressure gradient that generates a separation bubble with substantial backflow. The separation bubble is controlled by conventional passive vortex generators creating pairs of counterrotating vortices. Quantitative information is achieved by applying Particle Image Velocimetry (PIV) to the cross-stream plane of the vortices. The characteristics of a pair of counter-rotating vortices shed from a vortex generator is investigated in the near-field downstream of the vortex generator. The vortices were found to grow with the boundary layer in the downstream direction, and the maximum vorticity decreases as the circulation is conserved. The vortices are nonstationary, and the movements in the spanwise direction are larger than those in the wallnormal direction, due to the presence of the wall. The vortices fluctuate substantially and move over a spanwise distance, which is approximately equal to their size. The most probable instantaneous separation between the two vortices shed from one vortex generator equals the difference between their mean positions. The unsteadiness of the vortices contributes to the observed maxima in the Reynolds stresses around the mean vortex centers. The instantaneous vortex size and the instantaneous maximum vorticity are also fluctuating properties, and the instantaneous vortex is generally smaller and stronger than the mean vortex. A correlation was found between a large instantaneous vortex size and a low instantaneous maximum vorticity (and vice versa), suggesting that the vortices are subjected to vortex stretching. [DOI: 10.1115/1.2409327]

The mean velocity, Reynolds stress, and mean vorticity regions of a separated shear layer over a surface mounted block are investigated by 2D Digital Particle Image Velocimetry (DPIV) for three Reynolds numbers ([Re.sub.a] = 500, 1000, and 2500) and two channel-to-block height ratios (H/a = 1.825 and 4.6). The recirculation region's height and length are determined for the separated shear layer by means of [bar.U] = 0 contours. It is observed that the high Reynolds stress regions lay just outside of the [bar.U]=0 contours. The flow visualization and DPIV measurement of vorticity indicate that the differing normalized Reynolds stresses between [Re.sub.a] = 500 and 1000 are most probably due to the initiation of the vortex shedding between these two Reynolds numbers while, differences are minimal between [Re.sub.a] = 1000 and 2500. A sign change in the Reynolds shear stress distribution of the separated shear layer near the leading edge of the block was recognized for every Reynolds number and channel width. [DOI: 10.1115/1.2409359]

A new law of the wall accounting for curvature effects in swirling axial flows is derived. The influence of the curvature on the turbulence mixing lengths in both axial and tangential directions is examined theoretically using the Reynolds stress transport equations. For equilibrium flows with weak curvature, identical mixing lengths are derived for the axial and tangential directions. Additionally, the effect of finite local curvature and shear stress ratio on the near-wall velocities is systematically explored. It is found that the curvature effect in swirling axial flows is suppressed by a factor of 1/(1 + [[sigma].sup.2.sub.w]) compared to that in curved channel flows, where [[sigma].sub.w] is the ratio of the axial to swirl shear stress. For a given curvature radius, the maximum velocity deviation occurs when the axial-to-swirl shear stress ratio is zero. Finally, the performance of the new curvature law is evaluated by implementing it as a wall function in a well-established CFD code. The new wall function provides improved agreement for swirl velocity distributions inside labyrinth cavities in comparison with existing experimental laser Doppler anemometry measurements. [DOI: 10.1115/1.2409360] Keywords: dynamic modeling, curvature, law of the wail swirling axial flow

Linear hydrostatic 3D mountain wave theory is extended to include a thin frictional boundary layer (BL), parameterized using two characteristic relaxation times for wind adjustment. The character of the BL is described using a 'compliance coefficient,' defined as the ratio of BL thickness change to imposed pressure. In this formulation the simplest model that captures the two-way interaction between mountain waves and the boundary layer is sought. The slower BL wind speed amplifies the wind response and shifts it upstream so that the wind maxima occur in regions of favorable pressure gradient, not at points of minimum pressure. Variations in BE thickness reduce the mountain wave amplitude. The BL effect is sensitive to the wind profile convexity. The boundary layer improves the linear theory description of windy peaks. Low-level flow splitting is enhanced and wave breaking aloft is reduced. The BL also decreases the amount of upslope orographic precipitation. The wave momentum flux reduction by the BL is greater than the pressure drag reduction, indicating that part of the pressure drag is taken from BL momentum.