In natural convection in a porous medium heated from below, the convective flow in two dimensions becomes unsteady above a certain critical Rayleigh number and exhibits a fluctuating or oscillatory behaviour (depending on the confinement in the horizontal dimension). This fluctuating behaviour is due to a combination of the instability of the thermal boundary layers at horizontal boundaries together with a ‘triggering’ effect of earlier disturbances. The point of the origin of the instability of the thermal boundary layer appears to play a dominant role in determining the regularity of the fluctuating flow. This numerical study investigates the importance of this point of evolution and concludes that there may exist more than one oscillatory mode of convection, depending on its position. The investigation focuses on the symmetry of the flow and demonstrates that with stable and accurate numerical schemes, an artificial symmetry may be imposed in the absence of realistic physical noise. If an initially symmetric perturbation is imposed the flow retains an essentially symmetric flow pattern with a high degree of regularity in the oscillatory behaviour. The imposition of an asymmetric perturbation results in a degradation of regularity. The appearance of the symmetric, regularly oscillatory flow is characterized by a symmetric (and stationary) arrangement of the points of origin of the instability of the upper and lower thermal boundary layers; in the case of the irregular oscillations the points of origin are not symmetric and their locations are not fixed.

The response of a subsonic round jet to internal excitation by an acoustic wave has been examined using flow visualization and noise measurements. The jet was subjected to pulse excitation and to harmonic excitation. In both cases large-scale vortex ring structures were observed in the jet mixing layer. With pulse excitation, interactions between the vortex rings were observed, but no associated noise emission could be detected.

Equations for the instantaneous velocity and temperature fluctuations in a turbulent flow are used to assess the effect of a fluctuating convection velocity on Taylor's hypothesis when certain simplifying assumptions are made. The probability density function of the velocity or temperature derivative is calculated, with an assumed Gaussian probability density function of the spatial derivative, for two cases of the fluctuating convection velocity. In the first case, the convection velocity is the instantaneous longitudinal velocity, assumed to be Gaussian. In the second, the magnitude of the convection velocity is equal to that of the total velocity vector whose components are Gaussian. The calculated probability density function shows a significant departure, in both cases, from the Gaussian distribution for relatively large amplitudes of the derivative, at only moderate values of the turbulence intensity level. The fluctuating convection velocity affects normalized moments of measured velocity and temperature derivatives in the atmospheric surface layer. The effect increases with increasing order of the moment and is more significant for odd-order moments than even-order moments.

The response of the near field of a free, plane air jet (aspect ratio 44:1) to a controlled, sinusoidal perturbation was investigated by hot-wire measurements. The experiments were carried out at an exit excitation amplitude of 1·4 % for the Strouhal number range 0·15 [les ] StH [les ] 0·6 and the Reynolds-number range 8 × 103 [les ] ReH [les ] 3·1 × 104. The influence of the excitation, introduced with a loudspeaker attached to the jet settling chamber, on the mean and fluctuating velocity fields is much weaker than that in the circular jet. The amplitude and phase profiles of the fundamental, educed through phase-locked measurements, show that the induced symmetric mode remains symmetric as it travels downstream. The wave growth rate is much higher and the wavelength much smaller in the shear layer than on the centre-line of the jet. The wave fundamental attains its maximum amplitude at StH ≃ 0·18 on the jet centre-line and at StH ≃ 0·45 in the shear layer. The amplitude profiles of the fundamental in the shear layer agree quite well with the spatial stability theory of Michalke (1965b); however, the phase data do not agree well with the theoretical predictions. The growth rate and the disturbance wavenumber increase monotonically with the StH both in the shear layer and on the centre-line but tend to approach constant values at higher StH. The phase velocity data show that, in the lower Strouhal-number range, the plane jet acts as a non-dispersive waveguide.

Fink & Soh (1978) reported a technique to calculate numerically the motion of vortex sheets. They claim it gives reliable results. This paper re-examines the error in the calculation of the velocity of mesh points representing the sheet and shows that the test case used by Fink & Soh is not an adequate one. Instead the roll-up of a vortex sheet shed by a ring wing is studied. The results obtained proved unreliable. (Although several possibilities are discussed) the reason for the breakdown in results remains unknown.

The spectral turbulent diffusivity (STD) theory, originally deduced from a spectral generalization of the gradient-transfer theory (Berkowicz & Prahm 1979), is here derived from a basic concept of turbulent mixing for the case of homogeneous turbulence. The turbulent mixing is treated in a way similar to Prandtl's mixing-length concept. The contribution to the turbulent flux from eddies of different length is represented by a linear superposition. The spatial variation of the concentration distribution is described in terms of Fourier series. This procedure results in the spectral diffusivity formulation, which is Eulerian and scale dependent. If the concentration distribution is approximated by a truncated Taylor expansion instead of an exact representation by the Fourier series, the gradient-transfer approximation is retrieved.

The turbulent energy density, as function of the eddy length, is related to the eddy transport velocity and a probability of the occurrence of the eddies. The eddy transport velocity, derived from the relation between the energy spectrum and the Lagrangian correlation function, is used for computation of the spectral turbulent diffusivity. The turbulent energy spectrum is approximated by the inertial sub-range form $(-\frac{5}{3}$ law). The STD coefficient obtained here has, for large wavenumbers, a slope of $k^{-\frac{4}{3}}$ as predicted previously.