Gary T. Chapman

Topology of Flow Separation on Three-Dimensional Bodies

In recent years there has been extensive research on three-dimensional flow separation. There are two different approaches: the phenomenological approach and a mathematical approach using topology. These two approaches are reviewed briefly and the shortcomings of some of the past works are discussed. A comprehensive approach applicable to incompressible and compressible steady-state flows as well as incompressible unsteady flow is then presented. The approach is similar to earlier topological approaches to separation but is more complete and in some cases adds more emphasis to certain points than in the past. To assist in the classification of various types of flow, nomenclature is introduced to describe the skin-friction portraits on the surface. This method of classification is then demonstrated on several categories of flow to illustrate particular points as well as the diversity of flow separation. The categories include attached, two-dimensional separation and three different types of simple, three-dimensional primary separation, secondary separation, and compound separation. Hypothetical experiments are utilized to illustrate the topological terminology and its role in characterizing these flows. These hypothetical experiments use colored oil injected onto the surface at singular points in the skin-friction portrait. Actual flow-visualization information, if available, is used to corroborate the hypothetical examples.

Streamlines, vorticity lines, and vortices around three-dimensional bodies

The properties of three-dimensional, steady, vortical flows are studied using both theoretical analysis and computed flowfields. The centerline of the vortex is described using minimum streamline curvature, maximum normalized helicity, and the edge of a separation sheet. Analysis indicates that several criteria must be met for these three descriptions of the centerline to agree. In certain regions of the flow, computed flowfields indicate that they do agree, that the velocity and vorticity fields are aligned at the centerline, and that extrema in the velocity magnitude, vorticity magnitude, pressure, and density occur at the centerline

Dynamics of planetary probes - Design and testing issues

Interest in planetary probes has increased significantly in the last few years. Many of the proposed missions will use aeroassist for the capture and descent phases of the missions. This use of aeroassist will put a greater demand on having accurate aerodynamics for these vehicles. Reviews are presented of some of the aerodynamic requirements with particular emphasis on dynamic stability as well as the aerodynamic testing requirements and with particular attention on the use of the ballistic range.

Streamlines, vorticity lines, and vortices

The properties of vortical flows are studied using both theoretical analysis and computational flow fields. The consequences of two definitions of the vortex core, a minimum in the streamline curvature and a maximum in the normalized helicity, are examined. Analysis indicates that several criteria must be met if the cores defined by these two methods are to coincide. In certain regions of the flow, computational flow fields indicate that these two definitions are coincident, that the velocity and vorticity fields are aligned at the core, and that extrema in the velocity magnitude, vorticity magnitude, pressure, and density occur at the core.

The shock tube as a device for testing transonic airfoils at high Reynolds numbers

A performance analysis of gas-driven shock tubes shows that transonic airfoil flows with chord Reynolds numbers in the range of 100 million can be generated behind the primary shock in a large shock tube. A study of flow over simple airfoils has been carried out at low and intermediate Reynolds numbers to assess the testing technique. Results obtained from schlieren photos and airfoil pressure measurements show that steady transonic flows similar to those observed for the airfoils in wind tunnels can be generated within the available testing time in a shock tube with either properly-contoured test section walls or a properly-designed slotted-wall test section. The study indicates that the shock tube is a useful facility for studying two-dimensional high Reynolds number transonic airfoil flows.

Free-Flight Testing in Support of the Mars Science Laboratory Aerodynamics Database

An extensive ballistic range test program to study the aerodynamic characteristics of two possible configurations for the Mars Science Laboratory entry vehicle is presented. Testing focused primarily on static and dynamic aerodynamic coefficients, in the pitch plane, for a tabbed geometry flying supersonically in carbon dioxide. Limited data were also obtained for tabbed models in air and for axisymmetric models, similar to the Viking/Pathfinder forebody shape, at supersonic and hypersonic Mach numbers in CO 2 . Innovations in testing and data-reduction procedures that enhanced experimental capabilities and reduced data uncertainties are described. In CO 2 , the tabbed configuration was dynamically stable, in pitch, for all trim angles and Mach numbers tested. Preliminary data showed the same vehicle to be unstable in air, suggesting the possibility of a significant gas-composition effect on pitch damping. In general, the experimental data agreed well with pretest computations.

Aerodynamic ballistic range analysis using generalized math models

Three approaches to math modeling are used for aeroballistic range data reduction: modeling before estimation, modeling during estimation, and modeling after estimation. The modeling before estimation approach uses polynomial expansions to approximate the aerodynamic coefficients. In modeling during estimation, the coefficients are described by continuous, piecewise linear functions. In the modeling after estimation approach, no aerodynamic coefficient model is specified, and values for the coefficients are identified at each data point. Each method is used to extract aerodynamic information from simulated trajectory data, with and without errors, and from real data; estimated errors are also calculated. The advantages and disadvantages of the three methods are discussed. (Author)

Experimental study of flow separation on an oscillating flap at Mach 2.4

Measurements of unsteady wall pressures have been made in the turbulent boundary layer just upstream of the hinge line of an oscillating flap. The flap, which creates a highly three-dimensional compression corner flowfield, was oscillated in fully attached, crossing incipient separation, and fully separated flow regimes over a range of frequencies. It was found that a substantial lag of the pressure on the flap was produced when oscillating across the point of incipient separation. This occurred at much lower reduced frequencies than for the case of dynamic stall on an airfoil in transonic flow. The dynamic hysteresis was much less in the fully separated case and negligible in the fully attached case.

Applications of dark central ground interferometry

The application of dark central ground interferometry in a supersonic wind tunnel is presented. Examples of this method as a quantitative flow visualization technique for several types of supersonic flowfields are presented. Theoretical work is presented regarding the effects of vibrations and filter size. The method is shown to be an inexpensive, easy to use tool for situations requiring interferograms for flow visualization.

Study of multibody aerodynamic interference at transonic Mach numbers

A wind-tunnel experiment involving single, double, and triple combinations of mutually interfering generic, unfinned aircraft stores has been conducted. Each combination of stores was tested at Mach numbers from 0.60 to 1.20, and at angles of attack from 0 to 25 deg for the single store and from 0 to 6 deg for the double- and triple-store configurations. Extensive axial and circumferential pressure and flow visualization data at each store location were obtained. Euler solutions for each configuration at 0 deg incidence have been generated and compared with experimental data. This comparison indicates that an Euler flow solver can yield accurate predictions of the location and magnitude of multibody interference provided an appropriate grid is used and the viscous effects associated with these configurations remain small. The data indicate that multibody interference in the transonic region increases as the freestream Mach number approaches 1 from either direction, and subsides as the Mach number moves away from sonic conditions. This interference is characterized by a large, localized reduction in pressure on the inboard surfaces of the bodies, which results in forces that draw the configuration closer together.