Showing papers on "Flow separation published in 1971"

The production of turbulence near a smooth wall in a turbulent boundary layer

Abstract: The structure of the flat plate incompressible smooth-surface boundary layer in a low-speed water flow is examined using hydrogen-bubble measurements and also hot-wire measurements with dye visualization. Particular emphasis is placed on the details of the process of turbulence production near the wall. In the zone 0 < y+ < 100, the data show that essentially all turbulence production occurs during intermittent ‘bursting’ periods. ‘Bursts’ are described in some detail.The uncertainties in the bubble data are large, but they have the distinct advantage of providing velocity profiles as a function of time and the time sequences of events. These data show that the velocity profiles during bursting periods assume a shape which is qualitatively distinct from the well-known mean profiles. The observations are also used as the basis for a discussion of possible appropriate mathematical models for turbulence production.

Chemically Reacting Viscous Flow Program for Multi-Component Gas Mixtures.

Abstract: A general computer program was developed for solving the laminar boundary layer equations with a finite-difference method. The governing equations are solved in an uncoupled manner in order that a gas mixture with a large number of chemical species can be readily handled. The program has been written with various options to provide a flexibility that allows a variety of problems to be solved with only a change in the input data.

The response of a turbulent boundary layer to a step change in surface roughness Part 1. Smooth to rough

Abstract: An experimental study of the structure of the internal layer which grows down-stream from a rough-to-smooth surface change shows it to be essentially different from that studied by Antonia & Luxton (1971 b) for the case of a smooth-to-rough perturbation. The rate of growth of the internal layer is less than that for the smooth-to-rough step and it appears that the more intense initial rough-wall flow dictates the rate of diffusion of the disturbance for a considerable distance. Inside the internal layer the mixing length I is increased relative to the equilibrium distribution I = KY. A turbulent energy budget shows that the advection is comparable with the production or dissipation, whilst there seems to be some diffusion of energy into the internal-layer region close to the wall. The boundary layer, as a whole, recovers much more slowly following a rough-to-smooth change than following a smooth-to-rough change, and at the last measuring station (16 boundary-layer thicknesses from the start of the smooth surface) the distributions of mean velocity and Reynolds shear stress are far from self-preserving.

Some experimental results on sphere and disk drag

Abstract: The drag on spheres and disks moving rectilinearly through an incompressible fluid has been measured for Reynolds numbers (Re) from 5 to 100,000. Test models were mounted on a carriage which rode along a linear air bearing track system. Tests were performed by towing the models through a channel filled with glycerine-water mixtures. Forces and moments on the models were sensed by strain gage transducers; hydrogen bubble flow visualization was utilized in relating these forces to the unsteady wake flows. Steady drag results agreed with existing data except for the disk at 100 < Re < 1000, in which the drag coefficient values were up to 50% below the level of existing data; drag force unsteadiness during steady motion was always <5% for the sphere and <3% for the disk. Sphere drag measurements under constant acceleration from rest showed the apparent mass concept to be valid (at high Re) until the sphere had traveled approximately one diameter, after which the quasi-steady drag (based on instantaneous velocity) showed good agreement with the actual drag. Interference effects of the sting supports used in these tests are discussed.

Survey of viscous interactions associated with high Mach number flight

Abstract: T most severe problems of atmospheric flight at high Mach numbers are associated with viscous-inviscid interactions. Cruise vehicles for Mach numbers above four and lifting re-entry vehicles have highly complex three-dimensional configurations in which exist many regions of high compression that can cause boundary layers to separate. Although separation can result in loss of control effectiveness or flow degradation in an engine inlet, flow reattachment gives rise to heat rates that can far exceed those for an attached boundary layer. A further, and possibly far more severe viscous interaction problem is the impingement of shock waves generated by the forebody and other external components of a vehicle on aft sections resulting in local heat rates that may be many times larger than stagnation point values. Peak heating conditions may be laminar for lifting re-entry configurations, though our knowledge of boundary layertransition is far from adequate so that transitional and turbulent flows cannot be ruled out. However, Reynolds num, bers of potential high Mach number cruise vehicles are high— 10 to 10—so that viscous interactions will be predominately associated with turbulent boundary layers and their attendant higher heat rates. The high local heat rates resulting from viscous interactions cause "hot spots" that could lead to catastrophic failure. Vivid examples of damage resulting from viscous interactions are given in Figs. 1 and 2. A ventral pylon on the X-15 airplane, shown in Fig. 1, caused high^local heating of the fuselage around its root, and developed large holes near its tip due to the impingement of the shock wave from a dummy ramjet it supported, during a flight at Mach 6.7 in 1967. A study of the flowfield of the pylon-mounted dummy ramjet configuration is reported in Ref. 1. Figure 2 shows considerable damage due to interaction heating to the underside of a sled and its supporting slipper as a result of a run at 7000 fps on the 7-mile test track at Holloman Air Force Base. Unlike stagnation-point heating where the location is obvious, the problem with complex configurations is to determine "where" high heat rates are likely to occur, as well as their magnitude. It is the purpose of this paper to identify some potentially critical areas of viscous interactions associated with high Mach number vehicles and briefly review our state of knowledge in these areas with emphasis on the basic flow phenomena.

An investigation of the forces on flat plates normal to a turbulent flow

Abstract: Measurements on square and circular plates in turbulent flow show the mean base pressure to be considerably lower than that measured in smooth flow. Power spectral density measurements of the fluctuating component of the drag on square plates in both smooth and turbulent flow are presented. The measurements in turbulent flow show the importance of the ratio of turbulence scale to plate size. There is shown to be a strong correlation between the fluctuating drag force and the velocity fluctuations in the approaching flow. The distortion of the turbulence structure approaching a plate is also discussed.

Accurate numerical methods for boundary layer flows I. Two dimensional laminar flows

Abstract: A very simple and accurate numerical scheme which is applicable t o quite general boundary layer flow problems has been devised. It has been tested extensively on laminar flows, turbulent flows (using the eddy viscosity and eddy conductivity formulations), wake flows and many other such problems. In the brief space alloted to us here we shall illustrate the method by showing its application in some detail to nonsimilar plane laminar incompressible boundary layers and in particular to the well known case of Howarth's flow [3] .

Calculation of three-dimensional turbulent boundary layers

Abstract: The two-dimensional prediction method of Bradshaw, Ferriss & Atwell (1967), which was based on the empirical conversion of the turbulent energy equation into a ‘transport’ equation for shear stress, is extended to three-dimensional flows satisfying the boundary-layer approximation (which excludes flows near bluff obstacles or streamwise corners). Predictions, using exactly the same empirical data as in two-dimensional flow, agree to within the likely experimental error with a variety of experiments on ‘infinite’ swept wings.

On the reverse transition of a turbulent flow under the action of buoyancy forces

Abstract: Experiments were conducted in an ascending laminar flow through a vertical pipe under combined free and forced convection at constant heat flux through the wall.Mean velocity and temperature profiles were measured with a hot-wire probe. This velocity profile which is deformed by the buoyancy forces, enabled us to compute the reduced acceleration parameter. The profiles obtained showed that the value of the parameter at which reverse transition takes place is approximately the same as that found in isothermal boundary-layer flow. By measuring the autocorrelation function of the velocity after the reverse transition it was shown that the flow in the boundary layer becomes laminar as well as fluctuating and that it oscillates with a predominating period.

Separation in oscillating laminar boundary-layer flows

Abstract: The results of an experimental investigation of separation in oscillating laminar boundary layers is reported. Instantaneous velocity profiles obtained with multiple hot-wire anemometer arrays reveal that the onset of wake formation is preceded by the initial vanishing of shear at the wall, or reverse flow, throughout the entire cycle of oscillation. Correlation of the experimental data indicates that the frequency, Reynolds number and dynamic history of the boundary layer are the dominant parameters and oscillation amplitude has a negligible effect on separation-point displacement.

Measurements of the wave-number/phase velocity spectrum of wall pressure beneath a turbulent boundary layer

Abstract: Measurements are presented of the wave-number/frequency and wave-number/phase velocity spectrum of wall pressure for a two-dimensional turbulent boundary layer in zero pressure gradient, obtained from a Fourier transform of experimental filtered spatial correlations. This method allows the results to be corrected for acoustic disturbances in the wind tunnel, and for finite transducer size. An empirical form for the pressure field is proposed, based on the measurements, and is used to predict a frequency spectrum correction for transducer size which agrees well with measured values.

Laminar or turbulent boundary-layer flows of perfect gases or reacting gas mixtures in chemical equilibrium

Abstract: Turbulent boundary layer flows of non-reacting gases are predicted for both interal (nozzle) and external flows Effects of favorable pressure gradients on two eddy viscosity models were studied in rocket and hypervelocity wind tunnel flows Nozzle flows of equilibrium air with stagnation temperatures up to 10,000 K were computed Predictions of equilibrium nitrogen flows through hypervelocity nozzles were compared with experimental data A slender spherically blunted cone was studied at 70,000 ft altitude and 19,000 ft/sec in the earth's atmosphere Comparisons with available experimental data showed good agreement A computer program was developed and fully documented during this investigation for use by interested individuals

Measurements of boundary layer transition, separation and streamline direction on rotating blades

Abstract: Laminar boundary layer transition, separation and streamline direction on rotating helicopter blades

Experiments on laminar flow in curved channels of square section

Abstract: Recently Cheng & Akiyama (1970) published a numerical analysis of laminar flow in curved channels of square and rectangular section. Experimental results are presented here for flow in curved channels of square section. The channels were toroidal in shape, and the flow was driven electromagnetically. Various ratios of the channel dimension d to the channel radius of curvature, R, were used to investigate the dependence of friction factor, f, on the Dean number K, and the Reynolds number, Re. For 5 × 102 < K < 7 × 104 the formula (fRe) = 1·51 K½ was found to fit all the results, although R/d was varied from 17·5 down to the low value of 1·75. At lower values of K the analysis of Cheng & Akiyama was approximately validated.

Experiments on Plane Couette Flow

Abstract: A simple method for generating plane Couette flow is described. In this technique the water surface of a covered open-channel flume represents the moving boundary, and measurements are performed in the overlying (sandwiched) air layer. Experimental results are presented which suggest transition from laminar to turbulent plane Couette flow at a Reynolds number (based on center-line velocity and channel half-depth) of about 280. Turbulent mean velocities from all available sources are correlated in terms of inner and outer law coordinates, and universal skin friction laws are derived therefrom for the case of hydrodynamically smooth boundaries. However, it is concluded that none of the presently available experimental data on turbulent plane Couette flow render support for von Karman's theory of homologous turbulence. Results of a cursory exploration of the effects of unilateral boundary roughness suggest that mean-flow characteristics on the smooth side of a Couette channel assume the features prevailing on the rough side.

Numerical solution of the equations for compressible laminar, transitional, and turbulent boundary layers and comparisons with experimental data

Abstract: Numerical solution of flow equations for laminar, transitional, and turbulent compressible boundary layers for planar or axisymmetric flows

Turbulent Boundary-Layer Shock Interaction with and without Injection

Abstract: : Experimental results of turbulent boundary layer-shock interaction at Mach 6 using a 10 deg axisymmetric wedge around an axisymmetric centerbody are presented and an effort is made to obtain a simple numerical solution of this problem by the time dependent unsteady method. Static pressure, pitot pressure and total temperature profiles, wall static pressure, heat transfer measurements and optical data were obtained. Air and hydrogen are injected to study their effects on the interaction region. Wall heat transfer and pressure distributions for different injection rates of both air and hydrogen are presented. Detailed mappings of Mach number, pressure distributions and shock wave locations are derived for the air injection as well as the no injection case. Experimental results show a natural boundary layer thickness of about .85 inches, a separated flow region about two inches long and a pressure plateau characteristic of turbulent flow. (Author)

Hypersonic boundary-layer transition in the presence of wind-tunnel noise.

Abstract: Description of additional results indicating that a unique relationship exists between rms sound pressure level and transition Reynolds number, provided all other parameters are held nearly invariant. The results illustrate the different levels of the transition Reynolds numbers obtained in six facilities with nearly invariant laminar boundary layers on the different models (at the same local unit Reynolds number) and indicate that some facility characteristic has a strong influence on the transition Reynolds numbers.

Temperature Distributions in Supersonic Turbulent Boundary Layers

Abstract: : Experimental investigations of total temperature and Mach number distributions in turbulent boundary layers have been performed in the Mach number range from 1.75 to 4.5, using a newly developed combined temperature and pressure probe. The measurements carried out so far have been used to reanalyze the process of heat transport by calculating the turbulent Prandtl number distribution throughout the boundary layer. The results show an increase of the turbulent Prandtl number close to the surface, thus indicating that the turbulent transport of heat decreases more rapidly towards the wall than the turbulent transport of momentum. A simple relation, based on the Prndtl mixing length theory, is proposed for the variation of the turbulent Prandtl number within the viscous layer. This relation gives temperature distributions that are in good agreement with the experimental data in the region of constant shear stress. (Author)

Laminar boundary layer on a cone at incidence in supersonic flow

Abstract: Results of three-dimensional boundary-layer calculations for a pointed cone and comparisons with experimental data are presented. Using a similarity transformation, the explicit dependence of the equations on axial position is removed. A further transformation is made to remove a singularity at the leeward symmetry plane. The resulting equations, in two independent variables, are then solved with an implicit finite difference technique by marching around the cone from the windward to the leeward symmetry plane. A technique for computing the edge properties from an experimental pressure distribution is developed. The comparisons with experimental data demonstrate that the theory provides an adequate representation of the viscous flow over most of the cone surface, although the physical model breaks down near the leeward symmetry plane.

On laminar boundary-layer blow-off. Part 2

Abstract: Several examples of incipient blow-off phenomena described by the compressible similar laminar boundary-layer equations are considered. An asymptotic technique based on the limit of small wall shear, and the use of a novel form of Prandtl's transposition theorem, leads to a complete analytical description of the blow-off behaviour. Of particular interest are the results for overall boundarylayer thickness, which imply that, for a given large Reynolds number, classical theory fails for a sufficiently small wall shear. A derivation of a new distinguished limit of the Navier–Stokes equations, the use of which will lead to uniformly valid solutions to blow-off type problems for Re → ∞, is included. A solution for uniform flow past a flat plate with classical similarity type injection, based on the new limit, is presented. It is shown that interaction of the injectant layers and the external flow results in a favourable pressure gradient, which precludes the classical blow-off catastrophy.