Problem Of Separated Flow Research Articles

Study of the SST turbulence model for the 2D NASA wall-mounted hump separated FLOW problem with plenum case

The article presents the results of the standard SST turbulence model in the Comsol Multiphysics 6.1 software package for 2D NASA wall-mounted hump separated flow with plenum case problems, which are presented in the NASA database. In this work, the finite element method is used for the numerical implementation of the turbulence equations. To stabilize the discretized equations, Galerkin least squares (GLS) stabilization and crosswind propagation stabilization were used. The results were taken for different computational grids. The results obtained are compared with the results of experimental data.

Comparison modern of turbulence models for the 2D NASA wall-mounted hump separated flow problem

Comparison modern of turbulence models for the 2D NASA wall-mounted hump separated flow problem

Evaluation of generalized k-ω turbulence model in strong separated flow estimation of thrust optimized parabolic nozzle

One of the frequently reported defects of RANS-based turbulence models is overestimation of turbulent kinetic energy production in high speed separated flow problems, which causes significant prediction errors. The correct estimation of such flow in thrust optimized parabolic nozzles extremely depends upon the accurate prediction of the onset of flow separation. In this paper, firstly, the significant error of conventional RANS-based turbulence models is shown to predict the onset of flow separation in this type of nozzles. Then, the prediction accuracy is improved through the modification of the essential parameters of the generalized k-ω (GEKO) turbulence model. It was found that modifying the separation and mixing parameters of the GEKO model to realize the turbulent kinetic energy production resulted in the accurate prediction of onset of flow separation at the extensive range of nozzle pressure ratios. Using this modified model with new coefficients reduced the error of about 30% of the k-ω-sst model in estimating the onset of flow separation. Also, the nozzle pressure value at which the transition from free shock separation (FSS) to restricted shock separation (RSS) occurs is well predicted by this approach. After strengthening the turbulence model, the flow physics has been investigated with increasing and decreasing nozzle chamber pressure. The length of the separation shock and reflected shock waves which impose the presence of FSS or RSS patterns and transitional phenomena are discussed. Our new findings show that unlike the transition from FSS to RSS, the inverse transition from RSS to FSS did not depend on the length of the separation and reflective shocks.

Unsteady aerodynamic characteristics of slender body at extra-wide angle-of-attack range

For complicated unsteady separated flow problem of slender body at extra-wide of angle of attack range, delayed detached eddy simulation based on shear stress transport turbulence model is adopted to simulate the unsteady flows of slender body numerically in Mach number 0.6 and at the angle of attack of 0° to 180°. The calculated results of unsteady aerodynamic forces and the fluctuating pressure are analyzed which have a good agreement with experimental results. The unsteady amplitude of the lateral force is particularly strong at the angle of attack of 45 to 165°. The Strouhal number of lateral force range from 0.177 to 0.31. The investigation of unsteady vortical flow characteristics indicate the dominant frequency characteristic of aerodynamic force is mainly generated by Karman-vortex-street-like flow induced by the cylindrical body and no obvious dominant frequency is found in the massively separation past the canard surface.

Leveraging reduced-order models for state estimation using deep learning

State estimation is key to both analysing physical mechanisms and enabling real-time control of fluid flows. A common estimation approach is to relate sensor measurements to a reduced state governed by a reduced-order model (ROM). (When desired, the full state can be recovered via the ROM.) Current methods in this category nearly always use a linear model to relate the sensor data to the reduced state, which often leads to restrictions on sensor locations and has inherent limitations in representing the generally nonlinear relationship between the measurements and reduced state. We propose an alternative methodology whereby a neural network architecture is used to learn this nonlinear relationship. A neural network is a natural choice for this estimation problem, as a physical interpretation of the reduced state–sensor measurement relationship is rarely obvious. The proposed estimation framework is agnostic to the ROM employed, and can be incorporated into any choice of ROMs derived on a linear subspace (e.g. proper orthogonal decomposition) or a nonlinear manifold. The proposed approach is demonstrated on a two-dimensional model problem of separated flow around a flat plate, and is found to outperform common linear estimation alternatives.

Application of mosaic-skeleton approximations in the simulation of three-dimensional vortex flows by vortex segments

The computations in a vortex method for three-dimensional fluid dynamics simulation are accelerated by applying mosaic-skeleton approximations of matrices in the velocity computations. A modified vortex segment method is proposed in which mosaic-skeleton matrix approximations are effectively used to solve the problem of a vorticity field developing in an unbounded three-dimensional domain and in separated flow problems. Examples of the numerical solution of model fluid dynamic problems, such as the motion of a pair of vortex rings, the flow past a hemisphere, and the flow past an octahedral cylinder, are given that illustrate the capability of accelerating the computations while preserving the qualitative and quantitative description of the flow.

Separation in the flow through peristaltic motion of a carreau fluid in uniform tube

The problem of separated flow (trapping) through peristaltic motion of an incompressible carreau fluid in uniform tube has been investigated under long wavelength approximation neglecting the wave number and using a perturbation series in the Weissenberg number ( Wi) was used to obtain explicit forms for the velocity field, a relation between the flow rate and the pressure gradient and a relation between the friction force and the flow rate, in terms of parameter Weissenberg number ( Wi), power-law index ( n) and amplitude ratio φ. A condition frequently used to predict separated flow in boundary layer theory is to set the vorticity equal to zero on the boundary. We solve the problem numerically using Newton Raphson method to get separated flow points on the wall surface. From our results, the trapping region at the wall decreases with increasing volume flow rate and we found that there exist two values of longitudinal component of separated flow points one of them approaches to inlet of contraction and the other approaches to outlet of contraction. Also, the magnitude of radial velocity of the fluid at the separated flow points for Newtonian fluid is greater than non-Newtonian fluid and the magnitude of pressure rise at separated flow points approximately has the same values at the two values of the longitudinal component of separated flow points. Moreover, the friction force at separated flow points declare reflux phenomena in the contraction region. We discuss behavior of the vertical velocity, the pressure rise and the friction force at separated flow points for the physical parameters of interest.

Separated Flow in Bends of Arbitrary Turning Angles, Using the Hodograph Method and Kirchhoff’s Free Streamline Theory

The problem of separated flow in bends of arbitrary turning angles has been examined. The method of analysis is based on the inviscid flow theory coupled with Kirchhoff’s separation model. The physical flow problem is first transformed to the hodograph domain, and then into a rectangular computational region using properly selected flow parameters. The solution is first established in the hodograph plane. The final flow pattern including the inner and outer walls of the bend, the separation streamline, and other flow properties in the physical plane are subsequently obtained through direct integration. The results of the present analysis are compared with those of Lichtarowicz and Markland as well as Mankbadi and Zaki.

Study on computing separating flows within a diffusion inlet S-duct

A three-dimensional implicit full Navier-Stokes (FNS) analysis and a three-dimensional reduced Navier-Stokes (RNS) initial value space marching solution technique have been applied to a class of separated flow problems within a diffusing S-duct configuration characterized as vortex-liftof f. Both the FNS and the RNS solution technique were able to capture the overall flow physics of vortex liftoff, and gave remarkably similar results which agreed reasonably well with the experimental performance data. However, the FNS and RNS also consistently predicted separation further downstream in the M2129 inlet S-duct than was indicated by experimental data. The separated region associated with vortex liftoff in M2129 inlet S-duct increased in size as the inlet mass flow increased, although the radial extent of the reverse flow region remained very small. Since the reverse flow velocity was also very small, the separated flow in the M2129 S-duct might be regarded as incipient. This, along with the fact that the RNS analysis was able to capture the topology of vortex liftoff, indicates that the parabolized form of the Navier-Stokes equations are appropriate for separated flows represented by the M2129 inlet S-duct.

Prediction of vortex shedding from forebodies with chines

An engineering prediction method and associated computer code VTXCHN to predict nose vortex shedding from circular and noncircular forebodies with sharp chines edges in subsonic flow at angles of attack and roll are presented. Axisymmetric bodies are represented by point sources and doublets, and noncircular cross sections are transformed to a circle by either analytical or numerical conformal transformations. The lee side vortex wake is modeled by discrete vortices in crossflow planes along the body; thus the three-dimensional steady flow problem is reduced to a two-dimensional, unsteady, separated flow problem for solution. Comparison of measured and predicted pressure distributions, flow field surveys, and aerodynamic characteristics are presented for noncircular bodies alone and forebodies with sharp chines.

Convergence of coupled elliptic equations for separated flow problem

An overrelaxation procedure, that includes accelerating parameters is applied to the coupled elliptic equations in order to reduce the computational time. The specific system is the finite-difference form of the Navier-Stokes equations for separated flow in a symmetric sudden-expansion channel. The convergence domain for the successive overrelaxation method and the values of accelerating parameters which allow one to choose optimum values in this domain are given. Numerically solutions are obtained for Reynolds numbers up to 2000. Reduction in computing time by a factor of 4 is obtained by using the optimum values of accelerating parameters.

Application of a vortex lattice numerical model in the calculation of inviscid incompressible flow around delta wings

The flow over a flat plate delta wing at incidence and in sideslip is studied using vortex lattice models based on streamwise penelling. For the attached flow problem the effect of sideslip is simulated by modifying the standard vortex lattice model for zero sideslip by aligning the trailing vortices aft of the wing along the resultant flow direction. For the separated flow problem a non-linear vortex lattice model is developed for both zero and non-zero sideslip angles in which the shape and position of the leading edge separation vortices are calculated by an iterative procedure starting from an assumed initial shape. The theoretical values are compared with available theoretical and experimental results.

Analysis of three-dimensional separated flow with the boundary-layerequations

An analysis based on the boundary-layer equations is presented for the p rediction of threedimensional separated flow. In this analysis, t he boundary-layer equations are solved with finite difference techniques in four i nverse (pressure is unknown) modes. One inverse mode, in which the component of vorticity normal to the surface is specified at the boundary-layer e dge, is shown to result in an elliptic system of boundary-layer equations which has departure solutions when solved with a f orward marching technique. The accuracy o f the p resent analysis in the other t hree inverse modes using forward marching techniques is demonstrated for: (1) separated flow over an infinite swept wing which has been rendered three dimensional with a rotated coordinate system; and (2) a three-dimensiona l separated flow problem analyzed by Smith. The principal conclusion o f this paper is that the methodology is now in place to solve the boundary-layer equations for three-dimensiona l separated flow and hence work should now be focused on the development of three-dimensiona l viscousinviscid interaction procedures.

Predicted Vortex Shedding from Noncircular Bodies in Supersonic Flow

A method to predict nose vortex shedding from circular and noncircular slender bodies in supersonic flow at angles of attack and roll is presented. The body is represented by a supersonic panel method, and the lee side vortex wake is modeled by discrete vortices in cross-flow planes. The three-dimensional steady flow problem is reduced to a two-dimensional, unsteady, separated flow problem for solution. Comparison of measured and predicted surface pressure distributions, flowfield surveys, and aerodynamic characteristics are presented for circular and elliptic bodies. Good agreement between experiment and theory for a range of flow conditions indicate that the principal features of the flow phenomena are modeled correctly.

Axisymmetric Transonic Turbulent Base Pressures

R studies of incompressible and transonic separated flow problems* led to the development of a model accounting for the viscid-inviscid interaction within these flow regimes. It is stipulated that the inviscid flowfield is produced from an equivalent body whose geometry can be characterized by certain parameters. The inviscid flow so established guides the viscous flow processes of mixing and recompression along the wake, including the subsequent reattachment and flow redevelopment in the sense of boundary-layer concept. The viscid-inviscid interaction is manifested by the fact that the characteristic parameters required to establish the corresponding inviscid flow are determined through the viscous flow considerations. Extension of this approach to study the base pressure of a transonic flow past a backward facing step in axisymmetric configuration is reported and discussed.

Viscid-Inviscid Interaction of Incompressible Separated Flows

A flow model has been devised to deal with the viscid-inviscid interaction of a class of two-dimensional incompressible separated flow problems. It is suggested that the corresponding inviscid flow of these problems is described by the free streamline theory with few unspecified parameters and their values are, in turn, determined by the viscous flow considerations. The problem of a flow past a backward facing step is selected for study in detail. The viscous flow components of turbulent jet mixing, recompression, and reattachment are delineated and studied individually. When they are later combined, it is found that the point of reattachment behaves as a saddle-point-type singularity in the system of differential equations describing the viscous flow process. This feature is employed to the determination of the aforementioned free parameters and thus the establishment of the overall corresponding inviscid flow field. The resulting base pressure coefficient for the specific case agrees reasonably well with the available experimental data. Additional calculations are performed to demonstrate the influence of higher Reynolds numbers and the values of the similarity (or spread rate) parameter σ of the “constant pressure” turbulent jet mixing process. Further studies of redevelopment of the viscous flow after reattachment, the turbulent exchange within the recompression and redevelopment regions, and the effect of wind tunnel-wall interference on the overall flow patterns have been suggested and discussed.

Separated flow over a body of revolution

A method is developed to determine the flowfield of a body of revolution in separated flow. The technique employed is the use of the computer to integrate various solutions and solution properties of the sub-flowfields which make up the entire flowfield without resorting to a finite difference solution to the complete Navier-Stokes equations. The technique entails the use of the unsteady cross flow analogy and a new solution to the required two-dimensional unsteady separated flow problem based upon an unsteady discrete-vorticity wake. Data for the forces and moments on bodies of revolution at high angle of attack (outside the range of linear inviscid theories) such that the flow is substantially separated are produced which compare well with experimental data at low speeds. In addition, three-dimensional steady separation regions and wake vortex patterns are determined.

Unsteady control surface loads of lifting re-entry vehicles at very high speeds.

Recent research on control surface instabilities of lifting body configurations at very high speeds and with separated flows has led to a new insight into several possible unsteady control surface load problems. These separated flow problems are examined by presenting the most instructive of the numerous dynamic measurements obtained from wind-tunnel tests on simple aeroelastic models at Mach numbers from 3 to 10. The discussion is intended to provide a conceptual framework within which the test results can be interpreted and assessed as to their more general validity and to draw some guidelines as to their possible design significance. The observations include an instability of a type that involved a fluid-dynamical self-excitation of the separated pocket, feeding upon or modulated by tunnel and model disturbances without the necessity for participation of the control surface motion. The key to this type of instability appears to be contained in the recognition and identification of the various feedback mechanisms in the complex fluid and elastic systems and the conditions under which these might become regenerative or self-sustaining.

An approximate solution of the problem of separated flow past an axially symmetric body

An approximate solution of the problem of separated flow past an axially symmetric body