J. F. Keffer

The round turbulent jet in a cross-wind

The flow of a jet directed normal to a uniform, steady cross-wind is considered. Experimental results show that for various jet strengths, the position of the jet in space, when stretched by the ratio of jet to cross-wind momenta, is described by a single function. Exceptions exist at very low velocity ratios where a shift of the potential core is evident. A natural system of axes is used to define important directions of the flow. The integrated equation of motion along the primary jet flow direction is made dimensionless after the general method of Morton (1961) and a virtual source is defined for the flow. It is shown that a single functional behaviour of the axial jet velocity exists for various velocity ratios if the jet is considered to originate from this source. Lateral velocity profiles show a similarity when scaled by appropriate lengths and velocities but true self-preservation is not attained.

Spread of a heated plane turbulent jet

A heated two‐dimensional turbulent jet flow has been studied experimentally using conditional sampling techniques. The aim of the investigation was to establish the details of the spread of the temperature and velocity fields. Results confirmed previous observations showing that the conventionally measured mean temperature profile was wider than the velocity. However, when considered in terms of the turbulent fluid only, the spread of thermal and of momentum fields was found to be coincident.

On the structures in the near-wake region of an elevated turbulent jet in a crossflow

A pattern-recognition technique, applied to multi-point simultaneous velocity measurements obtained with 45° X -wire anemometer probes, is used to extract and characterize the underlying organized motions, i.e. coherent structures, within the near-wake region of a turbulent round jet discharged perpendicularly from a pipe into a crossflow. This flow has been found to be quite complex owing to its three-dimensional nature and the interactions between several flow regions. Analyses of the underlying coherent structures, which play an important role in the physics of the flow, are still rare and are mostly based on flow-visualization techniques. Using a pattern-recognition technique in conjunction with hot-wire measurements, we recently examined the wake regions of the pipe and jet at levels near the tip of the pipe, and found that Karman-like vortex structures in the wake of the pipe are locked to similar structures in the jet-wake. In this paper we expand upon our previous work and characterize these structures throughout the wake of the jet up into the region of the bent-over jet – a region where they have not been identified previously. The complex geometry of these structures in the wake of the jet as well as their interaction with the bent-over jet are discussed. The results show that these structures split before they link to similar structures on the opposite side of the symmetry plane in the jet region. The results further suggest that the vorticity due to the structures in the wake of the jet contributes to the motion of the well-known counter-rotating vortex pair.

Turbulent Wake in a Passively Stratified Field

A two‐dimensional turbulent wake superimposed upon a linearly stratified main stream has been examined. The buoyancy forces were negligibly small and the temperature could be treated as a passive scalar contaminant. Mean and fluctuating temperature and velocity distributions were measured across the wake and a self‐preserving analysis was developed for the growth of the thermal wake in terms of a length and intensity scale. The results showed that the maximum temperature defect increases in the streamwise direction as the wake spreads laterally.

The uniform distortion of thermal and velocity mixing layers

Two similar problems are considered: what is the effect of applying a uniform and constant rate of strain (i) to the two-dimensional thermal mixing region in a homogeneous grid-generated turbulent field, and (ii) to the two-dimensional velocity mixing region formed between two uniform streams moving with different mean velocities? The imposed strain field is orientated so as to compress or separate the isothermal and isokinetic surfaces in the plane of interest. Two theoretical models are presented; in the first, the profiles of temperature and velocity are assumed to be self-preserving and an assumption is made about the velocity scales; in the second, the statistical, rapid-distortion approach to dispersion due to Hunt & Mulhearn (1973) is applied. The circumstances in which these models differ and those where the simpler self-preserving model can be applied are determined. The measurements presented here indicate that the widths of both mixing layers decrease within the strain field, the width of the thermal mixing layer decreasing at a greater rate than that of the velocity mixing layer. However, the measured length scales were found to be 5 yo larger than the scales predicted by either of the analyses, which differed from each other by 5 yo. It is suggested that selective amplification of the energy-containing eddies by the strain field is responsible.

The turbulent mixing layer with an asymmetrical distribution of temperature

An experimental programme has been carried out to examine the spread of heat as a passive scalar contaminant in a turbulent shear flow. The situation involves a slightly heated two-dimensional jet expanding into a quiescent medium on one side and a uniform stream with velocity equal to that of the warm jet on the other. Thus the developed flow is a typical mixing layer with an asymmetric mean temperature profile superimposed on it. Measurements of the mean and fluctuating velocity and temperature fields show the existence of a region where the production of temperature fluctuations is negative. Spectral analysis in this zone indicates a separation of large and small wavenumber components of the cospectrum into two regimes. The sign of the high-frequency portion is consistent with gradient-transport concepts while the low-frequency component is of opposite sign. From this it is inferred that the large eddies are mainly responsible for the negative production. A mathematical model has been developed to describe the transport within this region.

Spanwise structure of the plane turbulent wake

An experimental investigation of a developed plane turbulent wake has been carried out to gain some insight into the spanwise structure of the large scale intermittent turbulent bulges. The results indicate that these bulges are three dimensional, and that their extent in the spanwise direction is less than in the streamwise direction and greater than in the lateral direction. No evidence of periodicity was found.

An experimental investigation of an asymmetrical turbulent wake

Experiments on the two-dimensional turbulent wake generated by pairs of cylinders of unequal diameter have revealed some interesting flow characteristics. The wake width grew asymmetrically in the downstream direction, spread rate and entrainment coefficients proving larger on the small diameter cylinder side. Mean velocity profiles were also skewed to this side while maximum values of Reynolds stresses were larger on the other. Close to the cylinder, a region or turbulent ‘energy reversal’ was measured. The level of turbulence and the diffusion mechanism were high at this point and some comments are made concerning the structure of the flow under these conditions.

Structural features of the plane turbulent jet

An experimental investigation of a two‐dimensional turbulent jet in a quiescent environment has been carried out to determine the structural characteristics of the outer, intermittent turbulent motion. An overall picture of this motion was deduced from the results which indicate that, on the average, the turbulent bulges are of the same order of magnitude in the three coordinate directions and are tilted backward at an angle of approximately 26° with respect to the lateral axis but have no spanwise yaw. Furthermore, it was found that there is no periodicity associated with the motion and that no correlation exists between bulges on opposite sides of the jet center line.

Coherent structures within the plane turbulent jet

A fully developed plane turbulent jet was investigated experimentally by means of hot‐wire anemometry and digital data analysis. Pairs of normal hot‐wire signals were digitized simultaneously and computer processed to produce autospectra, auto‐ and cross‐correlations, coherence functions, and phase spectra. The results establish that this flow does not contain regularly occurring structures. But, distinct frequency‐centered activity near 20 Hz was detected via the coherence functions, and this implies that randomly occurring large‐scale coherent structures exist within the flow. Moreover, convection velocities estimated via the corresponding phase spectra suggest that these structures move more slowly than the smaller turbulent eddies.