摘要:

Measurements have been made of nearly isotropic, low Reynolds number, grid‐generated turbulence. The decay results agree with the predictions for ’’final period’’ isotropic turbulence decay (in time) by the linear approximation of von Ka´rma´n and Howarth, and Batchelor and Townsend. This agreement occurs in spite of the fact that at the smallest turbulence Reynolds number attained,R&lgr;≈4, the inertial (triple velocity correlation) term in the double velocity correlation equation is not negligible as assumed in the theoretical estimates. As in experiments of Batchelor and Stewart, the turbulence shows appreciable departure from isotropy, indicated by component energy inequality growth downstream. It is also shown by nonzero velocity skewness, not reported heretofore at smallR&lgr;. Measurement of the effect of a secondary contraction on small Reynolds number turbulence indicates that the contraction has less effect than at larger Reynolds numbers.

ISSN:0031-9171    

DOI:10.1063/1.862168

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Numerical computations have been made for the laminar boundary‐layer development and stability of a series of similar Falkner–Skan wedge flows in freon‐114 with surface heating. The variation in physical properties (primarily viscosity) of the fluid alters the thickness and velocity profile of the boundary layers and consequently, their stability characteristics. Heating of the surface, with an accompanying reduction of viscosity close to the wall, has the qualitative effect as a more negative (i.e., more favorable) pressure gradient. The effects of various combinations of pressure gradient and heat transfer on the characteristics of these variable‐property laminar boundary layers, on their neutral stability, and on the growth of disturbances by the linear stability theory are described. The minimum critical Reynolds number is presented as a function of pressure gradient, temperature difference, and free‐stream temperature. The latter has an appreciable effect because of the rapid change in the temperature derivative of the viscosity with temperature level.

ISSN:0031-9171    

DOI:10.1063/1.862169

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The flow due to super‐imposed torsional oscillations on a rotating disk in a semi‐infinite expanse of viscous fluid is studied for the large and small frequencies separately. Analytical‐numerical solutions are obtained for the oscillatory and time mean flows. Expressions for the skin friction on the disk and the axial flow in the far field are given.

ISSN:0031-9171    

DOI:10.1063/1.862170

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The beginning of a continuous buoyancy (heat) injection into a stationary fluid results in a cap of buoyant fluid followed by a steady plume. This cap undergoes a transition in which a breakup is observed at a height from the source which increases with decreasing buoyancy injection rates. Sequences of cine film frames are presented illustrating this transition. A numerical criterion for the transition was found, namely, that the product of the power injected times the time at which transition occurs is a constant (19 cal). This can be related to an effective Rayleigh or Reynolds number, but the transition mechanism remains unknown. A critical effective Rayleigh number for the thermal may be estimated from these results to be 105which in this case represents an injected heat of 10 cal.

ISSN:0031-9171    

DOI:10.1063/1.862171

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

For a medium whose motion is described by v˙+B×v=0, where B is perpendicular to the two‐dimensional velocity v, the trajectory of each element of the medium is a circle that is determined by the initial value of v. Through a transformation of coordinates the differential equation for the (transformed) velocity derivative matrix is U˙+U2=−I, a special Ricatti equation for which the solution is known. This allows the determination of ∇⋅v, ∇×v, and other functions of the velocity derivatives until the time of formation of a shock.

ISSN:0031-9171    

DOI:10.1063/1.862172

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

It is shown that the apparent divergences in the Meeron expansion for the properties of the classical equilibrium electron gas, suggested by Cohen and Murphy, can be resolved by a more careful treatment of the region of small particle separation. Explicit expressions are given for the &egr;4(ln&egr;)2and &egr;4ln&egr; corrections to the equation of state.

ISSN:0031-9171    

DOI:10.1063/1.862173

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Incompressible magnetohydrodynamic turbulence is considered for an axisymmetric plasma in cylindrical geometry. The equilibrium statistical states to which the plasma evolves are examined by imposing an external constraint of constant toroidal current on the equilibrium Gibbs ensemble. Only the state with maximal magnetic helicity to energy ratio has zero‐mean kinetic energy for intially quiescent systems. There is an upper bound on the toroidal current to toroidal flux ratio below which this unique stable force free state can exist and whose magnetic flux surfaces are basically concentric circles. Above this critical value, the plasma is unstable with islands present in the flux surfaces. This stability result is in sharp contrast with both the results of a linearized &dgr;Wanalysis as well as considering the stability of the linearized spectral coefficient evolution equations for nonforce‐free states.

ISSN:0031-9171    

DOI:10.1063/1.862174

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The transient behavior of the cw laser produced melt layer when subjected to an over‐pressure/impulse has been analyzed. Results show that the over‐pressure and impulse required for significant melt removal by individual microsecond laser pulses are very high (≳103atm and ≳104dyn sec/cm2, respectively) and melt removal by such pulses is unlikely to be achieved in practice. The quasi‐steady‐state melt removal resulting from long‐duration over‐pressure produced either by single long‐duration pulses (≳100 &mgr;sec), by high repetition rate pulses or by the cw laser itself has also been analyzed, with results indicating the possible practical operating regimes for such a mechanism.

ISSN:0031-9171    

DOI:10.1063/1.862175

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

General conditions under which rarefaction shocks can exist in the expanding corona of a plasma heated by a laser are derived. In particular, for the case of a two‐electron temperature isothermal plasma with temperaturesThandTc, such a shock is shown to occur ifTh/Tc≳5+&sqrt;24. The case of rarefaction shocks induced by the ponderomotive force is also briefly discussed.

ISSN:0031-9171    

DOI:10.1063/1.862176

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

It is shown from first principles, using the formalism of Wigner distributions, that the divergences in the plasma kinetic equation due to the long range nature of the Coulomb interaction can be eliminated even if the plasma is rapidly varying, inhomogeneous, and immersed in external fields. This is due to the fact that for plasmas which have a screening length much larger than either the electron thermal wavelength or the classical distance of closest approach there is a large region in impact parameter space in which collision terms based on binary collisions and collision terms based on small momentum transfer give the same result.

ISSN:0031-9171    

DOI:10.1063/1.862177

出版商:AIP    

年代:1978

数据来源: AIP