G. R. Hall

Bio: G. R. Hall is an academic researcher from Northrop Corporation. The author has contributed to research in topics: Boundary layer & Nozzle. The author has an hindex of 4, co-authored 8 publications receiving 85 citations.

Interaction of the wake from bluff bodies with an initially laminar boundary layer.

Abstract: An experimental investigation was conducted to determine the mechanism and conditions governing transition induced by small bluff bodies suspended in a laminar boundary layer. The experiment was performed in the inlet section of a tube. The mechanism by which transition occurred appeared to be a local effect dependent upon the stability characteristics of the element wake rather than the stability characteristics of the boundary layer over the range of boundary-layer Reynolds numbers examined. That is, transition of the boundary layer appeared to involve the direct seeding of turbulence in the boundary layer from the wake of the element, rather than amplification of wavelike disturbances as considered in classical boundary-layer stability theory. In the central region of the boundary layer, the critical value of the element Reynolds number which resulted in transition of the boundary layer coincided, approximately, with the Reynolds number for incipient transition in the wake of the element. Laminar vortices shed by disturbance elements at subcritical values of element Reynolds numbers were found to decay in propagating downstream. For elements located outside the boundary layer, transverse turbulence contamination of the boundary layer was found to depend on diffusion of the turbulent wake below a critical (trigger) value of y/d. Turbulent patches were generated by elements traversing the boundary layer when the instantaneous slip velocity Reynolds number was slightly higher than the value of critical Reynolds number for elements fixed within the boundary layer.

Strong shock boundary layer interaction control system

Abstract: A control system for suppression of shock-induced flow separation caused by the interaction of turbulent boundary layer air and a strong normal shock in the air inlet of a supersonic aircraft by application of continuous bleeding upstream and across the shock boundary layer interaction region of the inlet through a porous cover which leads to a row of plenums from whence the bleed air is exhausted to atmosphere, through a controlled incrementally operated door. The control system can control interactions with normal shock strength approaching Mach=2 in strength.

The ''fountain effect'' and VTOL exhaust ingestion.

Abstract: This paper presents the results of an experimental study of the ingestion and flowfield characteristics of the interaction of two parallel jets of heated air, a quiescent environment, a perpendicular "ground" plane, and a pair of inlets. The flowfield was observed visually, and the transient response of the inlet thermocouples was recorded on an oscillograph over a range of configuration and flow parameters, e.g., spacing ratios, angles, and velocities. The major contribution of this study is the obtaining of a detailed qualitative picture of the upwash flowfield and its relation to ingestion levels. Data were obtained also with the "image plane" technique (and with the addition of simulated fuselage and wings to better approximate a VTOL aircraft). Some apparent discrepancies between previous full-scale and small-scale VTOL exhaust ingestion tests are explained. This study also points out that inlet temperature fluctuations are a random process and that a statistical approach to data analysis is desirable. No menclature D = nozzle diameter / = frequency of temperature fluctuation H = nozzle height above ground plane NPR = nozzle pressure ratio (= nozzle pressure/atmospheric pressure) S = distance between nozzle exit centers AT = inlet temperature rise above ambient ATm = time mean of AT 7 ATy = exhaust jet temperature minus ambient temperature V = jet exit velocity a = nozzle cant angle |8 = ground plane inclination

A study of hairpin vortices in a laminar boundary layer. Part 1. Hairpin vortices generated by a hemisphere protuberance

Abstract: It has been suggested that hairpin vortices may play a key role in developing and sustaining the turbulence process in the near-wall region of turbulent boundary layers. To examine this suggestion, a study was done of the hairpin vortices generated by the interaction of a hemisphere protuberancee within a developing laminar boundary layer. Under the proper conditions, hairpin vortices are shed extremely periodically, which allows detailed examination of their behaviour. Shedding characteristics of the hemispheres were determined using hot-film-anemometry techniques. The flow patterns created by the presence of the hairpin vortices have been documented using flow visualization and hot-film-anemometry techniques, and cross-compared with the patterns observed in the near-wall of a fully turbulent boundary layer. In general, it has been observed that many of the visual patterns observed in the near-wall region of a turbulent boundary layer can also be observed in the wake of the hairpin-shedding hemisphere, which appears supportive of the importance of hairpin vortices in the near-wall turbulence production process. Furthermore, velocity measurements indicate the presence of strong inflexional profiles just downstream of the hairpin-vortex generation region which evolve into fuller profiles with downstream distance, eventually developing a remarkable similarity to a turbulent-boundary-layer velocity profile.

On the Many Faces of Transition

Abstract: For a given shear-layer geometry the high-Reynolds-numbers turbulent flows possess strong, stable in-the-large, nearly universal features associated with the large number of degrees of freedom in the flows. These features include a range of scales of dominant energetic motion, a range of operating intensity levels, and average three-dimensional phase relations which, together, somehow insure the maintenance of a self-regenerative process. In seeking a rational approach to the bewildering variety of transitional behavior (Ref. 1), the author conjectures that many instability paths to turbulence are admissible and that their effectiveness hinges on whether the given mechanism supplies the proper scales, intensity level, and 3D phase relations needed for self-regeneration.

Critical evaluation of transition from laminar to turbulent shear layers with emphasis on hypersonically traveling bodies

Abstract: : The review report represents an attempt to evaluate critically the available data on high-speed boundary layer transition to turbulence and to interpret the apparent agreements and contradictions within some rational framework. Special attention was paid to the more documentable discrepancies between reported results as touchstones of conceptual models and instability theories. Experiments with 'microscopic' information are used as backbone of conceptual models, both linear and nonlinear. Linear instability results are used as a point of departure for the examination of current controversial questions of transition reversal with cooling, unit Reynolds number effect, effect of aerodynamic noise in supersonic wind tunnels, et.

On the evolution of a turbulent boundary layer induced by a three-dimensional roughness element

Abstract: An experimental investigation is described which has as its objectives the extension of the technical data base pertaining to roughness-induced transition and the advancement of the understanding of the physical processes by which three-dimensional roughness elements induce transition from laminar to turbulent flow in boundary layers The investigation was carried out primarily with single hemispherical roughness elements surface mounted in a well-characterized zero-pressure-gradient laminar boundary layer on a flat plate The critical roughness Reynolds number at which turbulence is regarded as originating at the roughness was determined for the roughness elements herein considered and evaluated in the context of data existing in the literature The effect of a steady and oscillatory free-stream velocity on eddy shedding was also investigated The Strouhal behaviour of the ‘hairpin’ eddies shed by the roughness and role they play in the evolution of a fully developed turbulent boundary layer, as well as whether their generation is governed by an inflexional instability, are examined Distributions of mean velocity and intensity of the u-fluctuation demonstrating the evolution toward such distributions for a fully developed turbulent boundary layer were measured on the centreline at Reynolds numbers below and above the critical Reynolds number of transition A two-region model is postulated for the evolutionary change toward a fully developed turbulent boundary layer: an inner region where the turbulence is generated by the complex interaction of the hairpin eddies with the pre-existing stationary vortices that lie near the surface and are inherent to a flow about a three-dimensional obstacle in a laminar boundary layer; and an outer region where the hairpin eddies deform and generate turbulent vortex rings The structure of the resulting fully developed turbulent boundary layer is discussed in the light of the proposed model for the evolutionary process

The formation mechanism and shedding frequency of vortices from a sphere in uniform shear flow

Abstract: Experiments to investigate the formation mechanism and frequency of vortex shedding from a sphere in uniform shear flow were conducted in a water channel using flow visualization and velocity measurement. The Reynolds number, defined in terms of the sphere diameter and approach velocity at its centre, ranged from 200 to 3000. The shear parameter K, defined as the transverse velocity gradient of the shear flow non-dimensionalized by the above two parameters, was varied from 0 to 0.25. The critical Reynolds number beyond which vortex shedding from the sphere occurred was found to be lower than that for uniform flow and decreased approximately linearly with increasing shear parameter. Also, the Strouhal number of the hairpin-shaped vortex loops became larger than that for uniform flow and increased as the shear parameter increased.The formation mechanism and the structure of vortex shedding were examined on the basis of series of photographs and subsequent image processing using computer graphics. The range of Reynolds number in the present investigation, extending up to 3000, could be classified into three regions on the basis of this study, and it was observed that the wake configuration did not differ substantially from that for uniform flow. Also, unlike the detachment point of vortex loops in uniform flow, which was irregularly located along the circumference of the sphere, the detachment point in shear flow was always on the high-velocity side.