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Showing papers in "AIAA Journal in 1993"


Journal ArticleDOI
Avi Seifert1, Tomer Bachar1, D. Koss1, M. Shepshelovich1, Israel Wygnanski1 
TL;DR: In this article, the effects of oscillatory blowing as a means of delaying separation are discussed, and experiments were carried out on a follow, flapped NACA 01115 airfoil equipped with a two-dimensional slot over the hinge of the flap.
Abstract: The effects of oscillatory blowing as a means of delaying separation are discussed. Experiments were carried out on a follow, flapped NACA 01115 airfoil equipped with a two-dimensional slot over the hinge of the flap. The flap extended over 25% of the chord and was detected at angles as high as 40 deg. The steady blowing momentum coefficients could be varied independently of the amplitudes and frequencies of the superimposed oscillations. The modulated blowing was a major factor in improving the performance of the airfoil at much lower energy inputs than was hitherto known. Optimum benefits in performance were obtained at reduced frequencies, based on the flap chord, of an order of unity. Significant increase in lift as well as cancellation of form drag were observed

571 citations


Journal ArticleDOI
TL;DR: In this paper, a solution to the plane elasticity problem for a symmetrically laminated composite panel with spatially varying fiber orientations has been obtained, and the effects of the variable fiber orientation on the displacement fields, stress resultants and global stiffness are analyzed.
Abstract: A solution to the plane elasticity problem for a symmetrically laminated composite panel with spatially varying fiber orientations has been obtained. The fiber angles vary along the length of the composite laminate, resulting in stiffness properties that change as a function of location. This work presents an analysis of the stiffness variation and its effects on the elastic response of the panel. The in-plane response of a variable stiff ness panel is governed by a system of coupled elliptic partial differential equations/Solving these equations yields the displacement fields, from which the strains, stresses, and stress resultants can be subsequently calculated. A numerical solution has been obtained using an iterative collocation technique. Corresponding closed-form solutions are presented for three sets of boundary conditions, two of which have exact solutions, and therefore serve to validate the numerical model. The effects of the variable fiber orientation on the displacement fields, stress resultants, and global stiffness are analyzed.

474 citations


Journal ArticleDOI
TL;DR: In this article, a finite element formulation for vibration control of a laminated plate with piezoelectric sensors/actuators is presented, and the static responses of a bimorph beam are calculated.
Abstract: A finite element formulation for vibration control of a laminated plate with piezoelectric sensors/actuators is presented. Classical laminate theory with the induced strain actuation and Hamilton's principle are used to formulate the equations of motion. The total charge developed on the sensor layer is calculated from the direct piezoelectric equation. The equations of motion and the total charge are discretized with four-node, 12-degreeof-freedom quadrilateral plate bending elements with one electrical degree of freedom. The piezoelectric sensor is distributed, but is also integrated since the output voltage is dependent on the integrated strain rates over the sensor area. Also, the piezoelectric actuator induces the control moments at the ends of the actuator. Therefore, the number, size, and locations of the sensors/actuators are very important in the control system design. By selective assembling of the element matrices for each electrode, responses with various sensor/actuator geometries can be investigated. The static responses of a piezoelectric bimorph beam are calculated. For a laminated plate under the negative velocity feedback control, the direct time responses are calculated by the Newmark-/? method, and the damped frequencies and modal damping ratios are derived by modal state space analysis.

464 citations


Journal ArticleDOI
Abstract: An efficient higher order plate theory for laminated composites is developed. A composite plate theory for general lamination configurations is obtained by superimposing a cubic varying displacement field on a zig-zag linearly varying displacement. The theory has the same number of dependent unknowns as first-order shear deformation theory, and the number of unknowns is independent of the number of layers. The displacement satisfies transverse shear stress continuity conditions at the interface between layers as well as shear free surface conditions. Thus, an artificial shear correction factor is not needed. To demonstrate and compare with other theories, the analytical solution for cylindrical bending is obtained. The present theory gives deflections and stresses that compare well with other known theories.

399 citations


Journal ArticleDOI
TL;DR: The effect of vortex generators, in the form of small tabs projecting normally into the flow at the nozzle exit, on the characteristics of an axisymmetric jet is investigated experimentally over the jet Mach number range of 0.3-1.81 as discussed by the authors.
Abstract: The effect of vortex generators, in the form of small tabs projecting normally into the flow at the nozzle exit, on the characteristics of an axisymmetric jet is investigated experimentally over the jet Mach number range of 0.3-1.81. The tabs eliminate screech noise from supersonic jets and alter the shock structure drastically. They distort the jet cross section and increase the jet spread rate significantly. The distortion produced is essentially the same at subsonic and underexpanded supersonic conditions. Thus, the underlying mechanism must be independent of compressibility effects. A tab with a height as small as 2 percent of the jet diameter, but larger than the efflux boundary-layer thickness, is found to produce a significant effect. Flow visualization reveals that each tab introduces an 'indentation' into the high speed side of the shear layer via the action of streamwise vortices. These vortices are inferred to be of the 'trailing vortex' type rather than of the 'necklace vortex' type. It is apparent that a substantial pressure differential must exist between the upstream and the downstream sides of the tab to effectively produce these trailing vortices. This explains why the tabs are ineffective in the overexpanded flow, as in that case an adverse pressure gradient exists near the nozzle exit which reduces the pressure differential produced by the tab.

346 citations


Journal ArticleDOI
TL;DR: In this article, a numerical scheme suitable for the computation of both the near field acoustic sources and the far field sound produced by turbulent free shear flows utilizing the Navier-Stokes equations is presented.
Abstract: A numerical scheme suitable for the computation of both the near field acoustic sources and the far field sound produced by turbulent free shear flows utilizing the Navier-Stokes equations is presented. To produce stable numerical schemes in the presence of shear, damping terms must be added to the boundary conditions. The numerical technique and boundary conditions are found to give stable results for computations of spatially evolving mixing layers.

345 citations


Journal ArticleDOI
TL;DR: In this paper, the performance of eight low Reynolds number k-epsilon and k-omega models for high Reynolds number, incompressible turbulent boundary layers with favorable, zero, and adverse pressure gradients was compared.
Abstract: This paper compares the performance of eight low Reynolds number k-epsilon and k-omega models for high Reynolds number, incompressible turbulent boundary layers with favorable, zero, and adverse pressure gradients. Results obtained underscore the k-epsilon model's unsuitability for such flows. Even more seriously, the k-epsilon model is demonstrated to be inconsistent with the well-established physical structure of the turbulent boundary layer, and low Reynolds number corrections cannot remove the inconsistency. By contrast, the k-omega model, with and without low Reynolds number modifications, proves to be very accurate for all of the tests conducted. 16 refs.

305 citations


Journal ArticleDOI
TL;DR: In this paper, a k-epsilon model for wall bonded turbulent flows is proposed and the damping function used in the eddy viscosity is chosen to be a function of R(sub y) = (k(sup 1/2)y)/v instead of y(+).
Abstract: A k-epsilon model is proposed for wall bonded turbulent flows. In this model, the eddy viscosity is characterized by a turbulent velocity scale and a turbulent time scale. The time scale is bounded from below by the Kolmogorov time scale. The dissipation equation is reformulated using this time scale and no singularity exists at the wall. The damping function used in the eddy viscosity is chosen to be a function of R(sub y) = (k(sup 1/2)y)/v instead of y(+). Hence, the model could be used for flows with separation. The model constants used are the same as in the high Reynolds number standard k-epsilon model. Thus, the proposed model will be also suitable for flows far from the wall. Turbulent channel flows at different Reynolds numbers and turbulent boundary layer flows with and without pressure gradient are calculated. Results show that the model predictions are in good agreement with direct numerical simulation and experimental data.

285 citations


Journal ArticleDOI
TL;DR: In this article, families of two-dimensional, unsteady shock-induced vortical flows are simulated numerically and the effects of shock strength, light/heavy gas density ratio, and geometry on the mixing are investigated.
Abstract: Families of two-dimensional, unsteady shock-induced vortical flows are simulated numerically The flows consist of one or more regions of light gas, surrounded by heavy gas, being overtaken by a normal shock wave The interaction of the density gradient at each light/heavy interface with the pressure gradient from the shock wave generates vorticity This causes the light gas regions to roll up into one or more counter-rotating vortex pairs, which stir and mix the light and heavy gases The mixing is characterized by an asymptotic stretching rate The effects of shock strength, light/heavy gas density ratio, and geometry on the mixing are investigated These two-dimensional, unsteady flows are analogous to three-dimensional, steady flows that may be used in SCRAMJET combustors demanding rapid and efficient mixing of fuel and oxidizer For such applications, 1) the fuel injectors should be elongated in the direction of the shock; 2) multiple smaller injectors are preferable to a single larger injector; 3) injectors should be arranged in groups of closely spaced pairs, rather than uniformly; and 4) multiple shock waves should be utilized, if possible

283 citations


Journal ArticleDOI
TL;DR: In this paper, a sensitivity-based methodology for improving the finite element model of a given structure using test modal data and a few sensors is presented, which searches for both the location and sources of the mass and stiffness errors.
Abstract: A sensitivity-based methodology for improving the finite element model of a given structure using test modal data and a few sensors is presented. The proposed method searches for both the location and sources of the mass and stiffness errors and does not interfere with the theory behind the finite element model while correcting these errors. The updating algorithm is derived from the unconstrained minimization of the squared L sub 2 norms of the modal dynamic residuals via an iterative two-step staggered procedure. At each iteration, the measured mode shapes are first expanded assuming that the model is error free, then the model parameters are corrected assuming that the expanded mode shapes are exact. The numerical algorithm is implemented in an element-by-element fashion and is capable of 'zooming' on the detected error locations. Several simulation examples which demonstate the potential of the proposed methodology are discussed.

283 citations


Journal ArticleDOI
TL;DR: In this paper, the near wake of a circular cylinder aligned with a uniform Mach 2.5 flow has been experimentally investigated in a wind tunnel designed solely for this purpose, where mean static pressure measurements were used to assess the radial dependence of the base pressure and the mean pressure field approaching separation.
Abstract: The near wake of a circular cylinder aligned with a uniform Mach 2.5 flow has been experimentally investigated in a wind tunnel designed solely for this purpose. Mean static pressure measurements were used to assess the radial dependence of the base pressure and the mean pressure field approaching separation. In addition, two-component laser Doppler velocimeter (LDV) measurements were obtained throughout the near wake including the large separated region downstream of the base. The primary objective of the research was to gain a better understanding of the complex fluid dynamic processes found in supersonic base flowfields including separation, shear layer development, reattachment along the axis of symmetry, and subsequent development of the wake

Journal ArticleDOI
TL;DR: In this paper, the Strouhal number of a dimpled circular cylinder over the Reynolds number range from 2 x 104 to 3 x 10s is reported. And the ratio of the depth of the dimples to the diameter of the cylinder is 9xlO~ 3.
Abstract: Measurements are reported of the drag coefficient and Strouhal number of a dimpled circular cylinder over the Reynolds number range from 2 x 104 to 3 x 10s. The ratio of the depth of the dimples to the diameter of the cylinder is 9xlO~ 3. In common with sand-roughened cylinders, the dimpled cylinder has a lower critical Reynolds number than a smooth cylinder. After the drag coefficient minimum, the CD does not rise to the high values that are typical of cylinders with sand roughness but is found to be closer to that for a smooth cylinder. Over a Reynolds number range from about 4xl04 to 3xl05, a dimpled circular cylinder has a lower drag coefficient than a smooth cylinder.

Journal ArticleDOI
TL;DR: In this article, the results obtained from this theory are compared with those obtained from a full-fledged three-dimensional elasticity analysis and various equivalent single-layer theories that are available, such as the classical laminated plate theory (CLPT), the first-order shear deformation laminated plates theory (FSDPT), and the third-order Shear Deformation Plate theory (THSDPT).
Abstract: Reddy's layerwise theory is used. The results obtained from this theory are compared with those obtained from a full-fledged three-dimensional elasticity analysis and various equivalent single-layer theories that are available. These include the classical laminated plate theory (CLPT), the first-order shear deformation laminated plate theory (FSDPT), and the third-order shear deformation plate theory (THSDPT). The elasticity equations are solved by utilizing the state space variables and the transfer matrix


Journal ArticleDOI
TL;DR: In this article, the selection and reproduction schemes of the genetic algorithm are modified, and a new operator called forced mutation is introduced to improve the convergence of the algorithm and to lead to near-optimal sensor locations.
Abstract: The selection and reproduction schemes of the genetic algorithm are modified, and a new operator called forced mutation is introduced. These changes are shown to improve the convergence of the algorithm and to lead to near-optimal sensor locations. Two practical examples are investigated: sensor placement for an early version of the space station and an individual space station photovoltaic array

Journal ArticleDOI
TL;DR: In this paper, a numerical procedure called TURNS (transonic unsteady rotor Navier-Stokes) is proposed to calculate the aerodynamics and acoustics of a single rotor out to several rotor diameters.
Abstract: Computational capabilities of a numerical procedure, called TURNS (transonic unsteady rotor Navier-Stokes), to calculate the aerodynamics and acoustics (high-speed impulsive noise) out to several rotor diameters are summarized. The procedure makes it possible to obtain the aerodynamics and acoustics information in one single calculation. The vortical wave and its influence, as well as the acoustics, are captured as part of the overall flowfield solution. The accuracy and suitability of the TURNS method is demonstrated through comparisons with experimental data.

Journal ArticleDOI
TL;DR: In this article, a comparison of two passive approaches for controlling the shock interaction with a turbulent boundary layer: low-profile vortex generators and a passive cavity (porous wall with a shallow cavity underneath) is presented.
Abstract: This paper describes an experimental comparison of two passive approaches for controlling the shock interaction with a turbulent boundary layer: low-profile vortex generators and a passive cavity (porous wall with a shallow cavity underneath). This investigation is the first known direct comparison of the two methods wherein the advantages and disadvantages of both are revealed. The experiments were conducted with a normal shock wave in an axisymmetric wind tunnel. The shock strength (M = 1.56-1.65) was of sufficient magnitude to induce a large separation bubble, thus causing substantial boundary-layer losses. The low-profile vortex generators were found to significantly suppress the shock-induced separation and improve the boundary-layer characteristics downstream of the shock. However, the suppression of the separation bubble decreased the extent of the low total pressure loss region associated with the lambda foot shock system which results in a lower mass-averaged total pressure downstream of the shock. The passive cavity substantially reduced the total pressure loss through the shock system (and thus wave drag) by causing a more isentropic compression over a larger lateral extent. However, the boundary-layer losses downstream of the shock were significantly increased.

Journal ArticleDOI
TL;DR: In this paper, a high-order numerical scheme is used to perform large-eddy simulations of a supersonic jet flow with emphasis on capturing the time-dependent flow structure representating the sound source.
Abstract: The present paper explores the use of large-eddy simulations as a tool for predicting noise from first principles. A high-order numerical scheme is used to perform large-eddy simulations of a supersonic jet flow with emphasis on capturing the time-dependent flow structure representating the sound source. The wavelike nature of this structure under random inflow disturbances is demonstrated. This wavelike structure is then enhanced by taking the inflow disturbances to be purely harmonic. Application of Lighthill's theory to calculate the far-field noise, with the sound source obtained from the calculated time-dependent near field, is demonstrated. Alternative approaches to coupling the near-field sound source to the far-field sound are discussed.

Journal ArticleDOI
TL;DR: In this article, the in-plane and flexural stiffness of a laminated composite plate are modeled as functions of the lamination parameters that are the functions of their stacking sequences.
Abstract: When laminated composite plates are symmetric and orthotropic, their in-plane and flexural stiffnesses become the functions of the lamination parameters that are the functions of their stacking sequences. We use the lamination parameters as fundamental design variables in designing laminates. The feasible region of the lamination parameters is obtained on a two-dimensional plane. Optimum design points can be obtained from the geometric relations between the feasible region and an objective function

Journal ArticleDOI
TL;DR: In this article, the quasistatic equations of piezoelectricity and thermopiezoeetics are used to develop a finite element formulation of distributed PDE and TPE media.
Abstract: The quasistatic equations of piezoelectricity and thermopiezoelectricity are used to develop a finite element formulation of distributed piezoelectric and thermopiezoelectric media. The formulation is then integrated with the distributed sensing and control of advanced intelligent structure design. The procedure is illustrated with the help of two example problems. The purpose of the first example, which consists of two piezoelectric layers used as a bimorph robotic finger, is to check the accuracy of the finite element solution with the analytical one. As a second example, an aluminum beam is utilized along with two polyvinylidene fluoride layers acting as distributed actuator and sensor to study the distributed control of the beam when thermal effects are present. It is concluded that the thermal effects are important in the precision distributed control of intelligent structures.

Journal ArticleDOI
TL;DR: A new method of background grid construction is introduced for generation of unstructured grids using the advancing-front technique that exploits the simplicity of uniform Cartesian meshes and the superiority of the elliptic grid point distribution for unstructuring grid generation.
Abstract: A new method of background grid construction is introduced for generation of unstructured grids using the advancing-front technique. Unlike the conventional triangular/tetrahedral background grids that are difficult to construct and usually inadequate in performance, the new method exploits the simplicity of uniform Cartesian meshes and the superiority of the elliptic grid point distribution for unstructured grid generation. The approach is analogous to solving a steady-state heat conduction problem with discrete heat sources. The spacing parameters of grid points are distributed over the nodes of a Cartesian background grid by solving a Poisson equation. To increase the control over the grid point distribution, a directional clustering approach is developed. The new method is convenient to use and provides better grid quality and flexibility. Some sample two-dimensional results are presented to demonstrate the power of the method.

Journal ArticleDOI
TL;DR: In this article, an intelligent structure composed of a laminated substrate of graphite/epox y composite coupled with distributed sensor and actuator layers of a biaxially polarized piezoelectric polymer, polyvinylidene fluoride (PVDF), is presented.
Abstract: Exact solutions for static analysis of simply supported rectangular plate-type intelligent structures are presented. The intelligent structure proposed here is composed of a laminated substrate of graphite/epox y composite coupled with distributed sensor and actuator layers of a biaxially polarized piezoelectric polymer, polyvinylidene fluoride (PVDF). The study aims at investigating the capability of the actuator and sensor layers to cause and sense the deformations, respectively, of the substrate of the intelligent structure. The results show that the effectiveness of the piezoelectric actuator layer to cause induced strain actuation in the structure significantly increases with the decrease in the length to thickness ratio of the substrate.

Journal ArticleDOI
TL;DR: In this article, an experimental investigation of the vortical and turbulent structure in a free shear lager downstream of a lobed mixer has been conducted, and a new vortex structure was confirmed to exist for the lobed mixers in addition to the well-known streamwise vortex array, consistent with the work of Manning.
Abstract: An experimental investigation of the vortical and turbulent structure in a free shear lager downstream of a lobed mixer has been conducted. Pulsed-laser sheet flow visualization with smoke and three-dimensional velocity measurements with hot-film anemometry were obtained for a lobed-mixer configuration and a baseline, planar configuration. Laminar and turbulent initial boundary-lager conditions were documented for both cases. The main result of this investigation is that a new vortex structure was confirmed to exist for the lobed mixer in addition to the well-known streamwise vortex array, consistent with the work of Manning

Journal ArticleDOI
TL;DR: In this paper, a finite element frequency domain method for predicting nonlinear flutter response of panels with temperature effects is presented, using the principle of virtual work, the element nonlinear stiffness formulation for a panel under a combined thermal and aerodynamic loads is derived on the bases of von Karman's large deflection plate theory, the first-order piston theory aerodynamics and the quasi-steady thermal stress theory.
Abstract: A finite element frequency domain method for predicting nonlinear flutter response of panels with temperature effects is presented. By using the principle of virtual work, the element nonlinear stiffness formulation for a panel under a combined thermal and aerodynamic loads is derived on the bases of von Karman's large deflection plate theory, the first-order piston theory aerodynamics and the quasi-steady thermal stress theory. The system equations of motion can be mathematically separated into two sets of equations and then solved in sequence. The first set of equations yields the panel thermal-aerodynamic equilibrium and the second set of equations of motion leads to the flutter limit-cycle oscillations. Stability and flutter boundaries can also be obtained from the two sets of system equations. Finite element large amplitude limit-cycle flutter results at different uniform temperatures are obtained for a simply supported square panel and are compared with existing Galerkin/time integration and other finite element solutions. Effects of nonuniform temperature distributions, panel length-to-width ratios, and boundary conditions on flutter responses of rectangular and triangular panels are presented.

Journal ArticleDOI
TL;DR: In this article, a photoluminescent coating is used for aerodynamic applications to measure the static pressure of paint and its luminescence is related to the static static pressure.
Abstract: A pressure measurement technique based on a photoluminescent coating is being developed and used for aerodynamic applications. Visible light excites probe molecules in the paint and their luminescence is related to the static pressure. Details of the illumination, luminescence detection, and data reduction for this technique are presented. These include key issues such as temperature effects, camera calibration, and model movement. Results from this technique in a variety of flowfields are given. Comparisons with pressures measured using standard wall taps show good agreement

Journal ArticleDOI
TL;DR: The structure of turbulence near a free surface is examined by using results obtained from a direct simulation of flow between a no-slip wall and a shear free boundary, which serves as a model of a waveless free surface as mentioned in this paper.
Abstract: The structure of turbulence near a free surface is examined by using results obtained from a direct simulation of flow between a no-slip wall and a shear free boundary, which serves as a model of a waveless free surface. An energy balance analysis shows that the pressure-strain term is the dominant producing term for the spanwise component of the turbulent kinetic energy. In addition, the dissipation rates for the horizontal components of the turbulence are reduced near the free surface, whereas the dissipation rate for the vertical component remains approximately constant. Two-point correlations, energy spectra, and length scales reveal important free surface induced effects. The length scales near the free surface are compared with the scales near the centerline of normal turbulent channel flow. This comparison reveals an increase by a factor of three in the streamwise length scales associated with the spanwise velocity fluctuations and an increase by a factor of two in the spanwise length scales for the streamwise velocity fluctuations. The length scales normal to the free surface are decreased for all velocity components. This indicates a more pancake-like eddy structure near the free surface compared with the structure near the centerline of a normal channel. The energy spectra show qualitative agreement with the Hunt-Graham model, though higher resolution calculations will be required to make more quantitative comparisons. The streaky structure in free surface bounded turbulent channel flow is noticeably more persistent than in normal turbulent channel flow. This is principally due to the attachment of streamwise oriented wall eddies to the free surface.

Journal ArticleDOI
TL;DR: In this article, a new set of equations termed the augmented Burnett equations was developed and shown to be stable both by a linearized stability analysis and by direct numerical computations for one-dimensional and plane-two-dimensional flows.
Abstract: Numerical solutions of the Burnett equations for hypersonic flow at high altitudes in the continuum transitional regime were not possible except for some one-dimensional flows. It is shown from both analytical investigation and numerical computations that the Burnett equations are unstable to disturbances of small wavelengths. This fundamental instability arises in numerical computations when the grid spacing is less than the order of a mean free path and precludes Burnett flowfield computations above a certain maximum altitude for any given aerospace vehicle. A new set of equations termed the "augmented Burnett equations" has been developed and shown to be stable both by a linearized stability analysis and by direct numerical computations for one-dimensional and plane-two-dimensional flows. The latter represents the first known Burnett solutions for two-dimensional hypersonic flow over blunt leading edges. The comparison of these solutions with the conventional Navier-Stokes solutions reveals that the difference is small in low-altitude low-speed flows but significant in high-altitude hypersonic flows.

Journal ArticleDOI
TL;DR: In this paper, the authors apply the dynamic subgrid-scale model to a large-eddy simulation of turbulent channel flow with a square rib mounted on one wall and show that the results show better agreement with direct numerical simulation than largeeddy simulation with a fixed model constant.
Abstract: We apply the dynamic subgrid-scale model to a large-eddy simulation of turbulent channel flow with a square rib mounted on one wall. The Reynolds number Re is 3.21 x 10 3 based on the mean velocity above the obstacle and the obstacle's height. Near-wall structures are resolved with the no-slip boundary condition. The results show better agreement with direct numerical simulation than large-eddy simulation with a fixed model constant, verifying the value of the dynamic subgrid-scale model for simulating complex turbulent flows. ARGE-EDDY simulation (LES) is an accurate method of simulating complex turbulent flows in which the large flow structures are computed while small scales are modeled. The rationale behind this method is based on two observations: most of the turbulent energy is in the large structures, and the small scales are more isotropic and universal. Therefore, LES may be more general and less geometry dependent than Reynolds-averaged modeling, although it comes at higher cost. Even though LES has been used by many investigators, most research has been limited to flows with simple geometry. In engineering applications, however, one encounters more complicated geometries. Here we shall consider a rectangular parallelepiped mounted on a flat surface. Related flows are those over surfaces protruding from submarines (conning towers or control fins), wind flows around buildings, and airflows over computer chips, among others. The most distinctive features associated with these flows are three dimensionality, flow separation due to protruding surfaces, and large-scale unsteadiness. As a model flow, we consider a plane channel flow in which a two-dimensional obstacle is mounted on one surface (see Fig. 1). This relatively simple geometry contains flow separation and reattachment. Flow in this geometry has been studied by Tropea and Gackstatter1 for low Re and Werner and Wengle2 and Dimaczek et al.3 for high Re, among others. Recently, Germano et al. 4 suggested a dynamic subgridscale model (DSGSM) in which the model coefficient is dynamically computed as computation progresses rather than specified a priori. This approach is based on an algebraic identity between the subgrid-scale stresses at two different filter levels and the resolved turbulent stresses. They applied the model to transitional and fully turbulent channel flows and showed that the model contributes nothing in laminar flow and exhibits the correct asymptotic behavior in the nearwall region of turbulent flows without an ad hoc damping function. This is a significant improvement over conventional subgrid-scale modeling.

Journal ArticleDOI
TL;DR: The accuracy and the performance of three two-dimensional compressible flow codes at freestream Mach numbers as low as 0.001 are examined.
Abstract: The accuracy and the performance of three two-dimensional compressible flow codes at freestream Mach numbers as low as 0.001 are examined. Two of the codes employ a finite volume discretization scheme along with a multistage time-stepping algorithm to solve the Euler equations. The two codes differ in their respective use of cell-centered and node-centered differencing schemes. The third code uses an implicit finite difference procedure to solve the unsteady Navier-Stokes equations. Computational test cases are the inviscid steady flow over a circular cylinder and the impulsively started viscous flow over a cylinder

Journal ArticleDOI
TL;DR: In this paper, the authors present various methods to influence the disturbed flow downstream of the region of interest, such that the disturbance level at the outflow boundary is significantly reduced, and hence the possibility of reflections is minimized.
Abstract: For numerical simulations of the spatially evolving laminar-turbulent transition process in boundary layers using the complete Navier-Stokes equations, the treatment of the outflow boundary requires special attention. The disturbances must pass through this boundary without causing reflections that would significantly alter the flow upstream. In this paper, we present various methods to influence the disturbed flow downstream of the region of interest, such that the disturbance level at the outflow boundary is significantly reduced, and hence the possibility of reflections is minimized. To demonstrate the effectiveness of the various techniques to alter the disturbance flow near the outflow boundary, the fundamental breakdown of a strongly decelerated boundary layer is simulated. Our results show that the most effective method is to spatially suppress the disturbance vorticity within a so-called "relaminarization zone." The suppression of the disturbance vorticity is gradually imposed within this zone by means of a weighting function. The enforced decay of the disturbance vorticity leads to a practically complete dissipation of any fluctuating component. Most importantly, this technique causes only a negligible upstream effect. The "relaminarized" boundary-layer flow then passes through the outflow boundary without significant reflections.