David Degani - Academia.edu (original) (raw)

Papers by David Degani

AIAA Journal, Jul 1, 1981

A model based on a solution of two-dimensional, time-dependent Navier-Stokes equations for compre... more A model based on a solution of two-dimensional, time-dependent Navier-Stokes equations for compressible turbulent flow, coupled with the solution of the heat conduction in the solid underlying a boundary layer, is presented. Computational results indicate that the sensitivity of the buried gage simulation increases with source strength and the conductivity decrease of the solid substrate, and that heat source influence on the flow is minimal even in cases where measurements of significant temperature increase exist.

[ Research paper thumbnail of Numerical simulation of transition to turbulence. [in boundary layer](https://a.academia-assets.com/images/blank-paper.jpg)

A numerical study of the conjugated problem of a separated supersonic flow field and a conductive... more A numerical study of the conjugated problem of a separated supersonic flow field and a conductive solid wall with an embedded heat source is presented. Implicit finite-difference schemes were used to solve the two-dimensional time-dependent compressible Navier-Stokes equations and the time-dependent heat-conduction equation for the solid in both general coordinate systems. A detailed comparison between the thin-layer and Navier-Stokes models was made for steady and unsteady supersonic flow and showed insignificant differences. Steady-state and transient cases were computed and the results show that a temperature pulse at the solid-fluid interface can be used to detect the flow direction near the wall in the vicinity of separation without significant distortion of the flow field.

A recently reported parabolized Navier-Stokes method has been extended to compute turbulent super... more A recently reported parabolized Navier-Stokes method has been extended to compute turbulent supersonic flows around cones and an ogive-cylinder body at large incidence. The algebraic eddy-viscosity turbulence model contained in the code was modified to properly account for the large regions of cross-flow separation that occur in these flows. Extensive comparisons between computed results and experimentally measured flow fields are presented. The results show good agreement for viscous-layer profiles and details of the external leeward-side vortex structure at angles of attack up to three times the cone half angles. Details of the modified turbulence model are presented and discussed.

Acta horticulturae, Jun 1, 1984

The occurrence of the flow about a slender body of revolution placed at incidence to an incoming ... more The occurrence of the flow about a slender body of revolution placed at incidence to an incoming stream is numerically examined for angles of attack ranging from 20 to 80 degrees and a Reynolds number of 200,000 based on maximum body diameter. Over a certain range of Reynolds numbers, the trend of flowfields around slender bodies at incidence can be roughly divided into three main categories: (1) at alpha = 0-30 deg, the flow is steady and symmetric; (2) at alpha = 30-60 deg, the flow under normal conditions is usually asymmetric, but the level of the asymmetry depends on the amount of disturbances present on the tip of the body; and (3) at alpha 60-90 deg, the flow in the wake of the body acts in a fashion similar to that of the Karman vortex shedding behind a two-dimensional circular cylinder. For each of these categories the range of incidence may change by + or - 10 degrees, depending on the quality of flow, or body finish.

An hypothesis advanced originally to explain computational observations is supported by theoretic... more An hypothesis advanced originally to explain computational observations is supported by theoretical considerations: The asymmetric mean flow observed on bodies of revolution at moderate to high angles of attack is the result of a convective instability of an originally symmetric flow to a time-invariant space-fixed disturbance. Additionally, the time-dependent fluctuations characteristic of the flow at higher angles of attack (up to 90°) are the result of an absolute instability of an originally steady flow to a small temporal disturbance of finite duration. Within a common domain, the instability mechanisms may coexist.-The experimentally confirmed existence of bistable states, wherein the side-force variation with nose roll angle approaches a square-wave distribution, is attributed to the dominant influence of a pair of trailing vortices from the ogival forebody. Their existence is made possible by the appearance of foci of separation in the skin-friction line pattern beyond a critical value of angle of attack. The extreme sensitivity of the asymmetric flow orientation to nose geometry, demonstrated experimentally, is attributed to the presence of an indeterminate phase in the family of possible solutions for the three-dimensional wave system.

APS Division of Fluid Dynamics Meeting Abstracts, Nov 1, 1996

In the present study, problems of laminar and turbulent two-dimensional flow of a viscous compres... more In the present study, problems of laminar and turbulent two-dimensional flow of a viscous compressible fluid near the trailing edge of a thin flat plate are considered. The complete set of Navier-Stokes equations is solved by the finite-difference method of MacCormack (MacCormack and Baldwin, 1975). It is an explicit, predictor-corrector, time-splitting method of second order acuracy. The computational mesh employed has sufficient resolution for all the characteristic lengths suggested by theory. In the laminar case, the present results are compared with the triple deck solution of Daniels (1974). This comparison indicates that the asymptotic triple deck theory for supersonic trailing edge flow is accurate within five percent for Reynolds numbers greater than 1000. In the turbulent case, the Prandtl-Van Driest-Clauser algebraic eddy viscosity model is used. The numerical results show that the region of upstream influence is approximately of the order of the boundary layer thickness. The solutions for skin-friction, pressure and wake center-line velocity are presented.

Computer Methods in Applied Mechanics and Engineering, 1981

ABSTRACT

The present paper deals with problems of two-dimensional laminar and turbulent flow of a viscous ... more The present paper deals with problems of two-dimensional laminar and turbulent flow of a viscous incompressible fluid near the trailing edge of a thin flat plate. The complete system of Navier-Stokes equations is solved by means of MacCormack's finite-difference scheme. In the laminar case, the results are compared with the triple deck solution of Daniels (1973). The comparisons indicate that the asymptotic triple deck theory for supersonic trailing-edge flow is accurate within five percent for Reynolds numbers greater than 1000. In the turbulent case, the Prandtl-Van Driest-Clausner algebraic eddy viscosity model is used, showing that the region of upstream influence is roughly of the order of the boundary layer thickness.

Physical review fluids, Jan 29, 2021

The fluid-structure interaction of an elastically restrained inclined tangent ogive-cylindrical b... more The fluid-structure interaction of an elastically restrained inclined tangent ogive-cylindrical body with a blunt elliptical base is investigated numerically. The flow is three-dimensional, compressible, and laminar, and the slender body is allowed to yaw at high incidence. The resulting response exhibits an intricate bifurcation structure that includes bistable periodic (finite amplitude) and nonstationary (small amplitude) limit cycles for moderate angles of attack, and nonstationary (finite amplitude) oscillations for high angles of attack.

Journal of applied mathematics and mechanics, 2017

An overview of progress in theoretical studies related to the assessment of averaged velocity dis... more An overview of progress in theoretical studies related to the assessment of averaged velocity distortion in turbulent and laminar shear layers (mixing layer, far wake, and boundary layer) caused by the coherent perturbations applied to the flow control is presented.

Physical Review Fluids

The evolution of nonstationary side forces acting upon an axisymmetric ogive-cylinder body with a... more The evolution of nonstationary side forces acting upon an axisymmetric ogive-cylinder body with a blunt base and subjected to flows at a wide range of angles of attack is studied numerically. To analyze the results, the body surface is divided into narrow cross-section disks and for each disk the time histories of rms and the mean of the side force are calculated and used to create bifurcation diagrams for angles of attack from 20 • to 80 •. These diagrams are compared with the diagrams obtained for the resultant side force acting on the entire body and those obtained for the ogive forebody and the cylindrical afterbody and its base. Results show that at low or medium angles of attack the flow is steady along the entire body except for some small oscillations near the body base. As the angle of attack increases beyond 40 • , the flow about the aft of the cylindrical afterbody becomes unsteady and the side force oscillates with nonzero mean; this unsteady region extends upstream toward the nose as the angle of attack increases. When the angle of attack exceeds 50 • the side force along the far aft of the body oscillates symmetrically with zero mean, and this symmetric region extends upstream with the increasing angle of attack until at α = 80 • it stretches virtually over the entire body.

Advances in Engineering Materials, Structures and Systems: Innovations, Mechanics and Applications, 2019

Journal of Biomechanical Engineering, 2020

A novel method for positioning and operating needle-like cryo-surgical probes in 2D convex target... more A novel method for positioning and operating needle-like cryo-surgical probes in 2D convex target areas is presented. The method is based on the recorded dynamic performance of a single probe, termed “unit circle,” (UC) embedded in a semi-infinite, tissue-like medium. Up to 15 cryo-probes, inserted into the same depth, are operated uniformly for 2–5 min. A predetermined number of probes are rearranged inside the target area until a “tight configuration” is obtained. The probes are initially arranged inside the target area such that the “lethal temperature” circles produced by them are tangent to its contour and to both adjacent lethal temperature circles. Subsequently, all probes are repositioned inwardly, each at a specific distance that depends on the local radius of curvature of the target area. Resulting total “defect areas”—internal and external—for a number of demonstrated cases, amounted to between 2.5% and 7.6% of the target area. The lower values of the defect areas were ob...

Physics of Fluids, 2017

The current work investigates numerically rolling instabilities of a free-to-roll slender rigid-b... more The current work investigates numerically rolling instabilities of a free-to-roll slender rigid-body of revolution placed in a wind tunnel at a high angle of attack. The resistance to the roll moment is represented by a linear torsion spring and equivalent linear damping representing friction in the bearings of a simulated wind tunnel model. The body is subjected to a three-dimensional, compressible, laminar flow. The full Navier-Stokes equations are solved using the second-order implicit finite difference Beam-Warming scheme, adapted to a curvilinear coordinate system, whereas the coupled structural second order equation of motion for roll is solved by a fourth-order Runge-Kutta method. The body consists of a 3.5-diameter tangent ogive forebody with a 7.0-diameter long cylindrical afterbody extending aft of the nose-body junction to x/D = 10.5. We describe in detail the investigation of three angles of attack 20°, 40°, and 65°, at a Reynolds number of 30 000 (based on body diameter...

34th Aerospace Sciences Meeting and Exhibit, 1996

At high angles of attack the use of the thin-layer approximation of Navier-Stokes equations (TLNS... more At high angles of attack the use of the thin-layer approximation of Navier-Stokes equations (TLNS) may not be justified. Investigations of the effect of adding all viscous terms to Beam-Warming TLNS algorithm have been performed. Results show that solutions for symmetric flow fields (even at high angles of attack) are identical. Differences were found in asymmetric high angle-of-attack computations especially when the wake is no longer steady. A method for parallelizing the above computations was developed and tested. This method will produce identical results to those obtained using standard non-parallelized methods for implicit approximately factored schemes. INTRODUCTION The forebody of a flight vehicle can be typified by a slender body of revolution. Since such an axisymmetric body is numerically and experimentally tractable, it has been the object of much research over the years. A number of experimenters have noted (e.g., Refs. 1-5) that for such bodies, there is an angle-of-attack range (roughly, from 30° to 65°) for which minute imperfections at the tip (e.g. dust accumulation, surface roughness) can cause large asymmetries in the flow pattern. They found that as the angle of attack increases from 30° to 65°, the form of the side-force response as a function of the roll-angle changes from a continuous, almost periodic variation to a virtually discontinuous, square-wave commonly called a 'bistable" variation. For angles of attack beyond 65°, the crossflow past the cylindrical part of the body becomes virtually identical to the flow past a two-dimensional cylinder: Vortex shedding occurs, and as the angle of attack tends toward 90°, the mean side force tends toward zero. The fact that minute perturbations of body shape can result in finite asymmetries suggests the existence of inherent instabilities in the expected symmetric flow. This is certainly the case as the angle of attack approaches 90° since the observed Karman vortex street beyond the body is well known to be caused by an absolute instability of the symmetric flow—one which would remain even after removal of any perturbation that initiated it. However, in numerical predictions for angles of attack between 30° to 65° both for laminar" and for turbulent flows, the existence ofconvective instability of the symmetric flow was found. More recent evidence to support the existence of convective instability has been obtained from several experiments". Numerical methods which can simulate the asymmetry phenomenon must account for the fluid viscosity. The degree to which the viscous terms are considered usually affects the complexity and execution time requirements of the computer models. In previous work, we have employed the thin layer Navier Stokes (TLNS) model. This model considers only the effect of the viscosity on the flow normal to the body. Objections have been raised regarding the justification for utilizing the TLNS model to study flow asymmetries, since the viscosity effect of the circumferential and streamwise flow components might become important at the high attack angles of interest. The first objective of this work is to test this question by comparing the 3D Beam-Warming thin-layer Navier-Stokes (BW TLNS) algorithm and Beam-Warming full Navier-Stokes (BW FNS) algorithm where all viscous terms are kept. The 3D flux-splitting thin-layer Navier-Stokes (FS TLNS) algorithm will also be used as an additional reference. The numerical methods mentioned above involve large systems the solution of which may be parallelized by decomposing the flow domain into overlapping or adjacent sub-domains, with computations on each sub-domain performed by a different parallel processor. After each step or iteration in the solution process, information can be exchanged among the processors although this should be kept to a minimum. A 'Chimera" scheme for defining overlapping domains for this purpose has been described [e.g. Steger et al]. The solution in each sub-domain is coupled to neighboring sub-domains through conditions along their common 'pseudo-boundaries" using a Schwarz type principle. As its name implies, this Chimera scheme is quite complex, and represents a major pre-processing effort for the solution of realistic problems. Moreover, preliminary results of a study on propagation of disturbances in a three dimensional flow field solution using a Chimera scheme showed a severe distortion of pressure waves as they passed through the interface of the overlapped sub-domains. On the other hand, a recent study on domain decomposition * Professor, Associate Fellow ALAA + Senior Research Associate Copyright 1996 by authors. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission.