摘要:

The stability of stratified flows consisting of a middle layer of homogeneous fluid in linear shear flow and contiguous upper and lower stratified layers of constant velocities is considered. The density and velocity are continuous throughout. It is shown that there are infinitely many pairs of neutral modes with the phase velocitycrin the range of the velocity which, in general, are not represented by the stability boundary, but which merge as the parameterN(the inverse of the Froude number squared) increases. AsNincreases further the coalesced neutral modes become modes with complexc, one of which is unstable. The instability may be considered to be caused by the resonance of the original pair of separate modes. In addition, instability of stratified flows is considered for general density and velocity distributions without a layer of constant density and linear velocity, and it is concluded that in that case the modes with complexcon the unstable side of a stability boundary do not continue into any neutral normal modes on the stable side.

ISSN:0031-9171    

DOI:10.1063/1.1694922

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A theory for internal progressive waves of permanent form in any continuously stratified fluid is presented, and a calculation for the flow and the wave velocity is carried out for an exponential stratified fluid. The most important conclusion from this calculation, also valid for other weak stratifications, is that the wave velocity alwaysdecreaseswith the amplitude, provided the density gradient is weak and the wavelength is not too short. This conclusion is significant because it entails the existence or nonexistence of solitary waves in weakly stratified fluids. The validity of the Boussinesq approximation and the significance of the well‐known exactly linear cases are also discussed.

ISSN:0031-9171    

DOI:10.1063/1.1694923

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The effects of upstream influence on the thermal field in the vicinity of the leading edge of the new thermal boundary layer that develops within an already well‐established thermal and velocity boundary layer at a point of discontinuous thermal boundary conditions are studied. In contrast to ordinary boundary layer theory, the streamwise conduction terms are retained in the Navier‐Stokes energy equation for forced convection and locally valid series solutions obtained for the leading edge region.

ISSN:0031-9171    

DOI:10.1063/1.1694924

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The velocity fluctuations at one point of a strongly turbulent air flow, produced by arrays of jets, are analyzed up to eighth order correlations. The odd order correlations show the proportionality factors already found in grid and boundary layer turbulence. Two numerical models are described, which rely on a simple nonlinear process to attempt to explain the observed results. The first model generates a signal where the relations between the phases of the Fourier components are limited to triplets. Except for short range correlations, the agreement with turbulent results is good. However, in the range of small, but still inertial, eddies, significant differences appear. The second model simulates abrupt jumps in velocity. It gives qualitative agreement through the entire range, thus showing that strong coupling of the phases, by occasional discontinuities can produce the same appearance of weak phase relations in the range of medium to large eddies, as well as the short range behavior.

ISSN:0031-9171    

DOI:10.1063/1.1694925

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The positive spikes observed in the time derivative of turbulent velocity fluctuations constitute the principal non‐Gaussian contribution. They are directly related to the irreversible turbulent process. Coincidence between strong spikes detected at four points was measured. The spikes appear in clusters, about as large as the integral scale. The results suggest that they represent crumpled vortex sheets, rather than vortex ligaments or plane vortex sheets.

ISSN:0031-9171    

DOI:10.1063/1.1694926

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The detection of the interface between turbulent and nonturbulent zones in slightly heated turbulent shear flows on the basis of a temperature signal is discussed. Two detection parameters, a threshold level or gate &dgr;&thgr;gand a hold time &tgr;h, are used. A complete description of the detection process and of the procedure used to select nominal values for the detection param eters is presented. It is shown that the detection process is credible in the sense that the statistics of the intermittency function are nearly independent of the values of the detection parameters. Comparison of the intermittency function statistics with previous results is made.

ISSN:0031-9171    

DOI:10.1063/1.1694927

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The interaction of a plane shock wave of finite strength with a vortex line, point vortex, doublet or quadrupole of weak strength is studied. Based upon the physical condition that a free vortex line cannot support a pressure difference, rules are established which define the change of the linear intensity of the segment of the vortex line after its passage through the shock. The rules for point vortex, doublet, and quadrupole are then established as limiting cases. These rules can be useful for the construction of the solution of the entire flow field and for its physical interpretation. However, the solution can be obtained directly by the technique developed for shock diffraction problems. Explicit solutions and the associated sound generation are obtained for the passage of a point vortex through the shock wave.

ISSN:0031-9171    

DOI:10.1063/1.1694928

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The structure of a steady plane shock in a partially ionized gas has been investigated using the Boltzmann equation with a kinetic model as the governing equation and the discrete ordinate method as a tool. The effects of the electric field induced by the charge separation on the shock structure have also been studied. No overshoot of upstream velocity has been observed. Although the three species of an ionized gas travel with approximately the same macroscopic velocity, the individual distribution functions are found to be very different. In a strong shock the atom distribution function may have double peaks, while the ion distribution function has only one peak. Electrons are heated up much earlier than ions and atoms in a partially ionized gas. Because the intereactions of electrons with atoms and with ions are different, the ion temperature can be different from the atom temperature. No undershoot of the upstream temperature has been found. The present results for neutral particles and electrons are generally in fairly good agreement with the continuum solutions at the low Mach number and with the Mott‐Smith solutions at the high Mach number. It has been found that the effects of the induced electric field on the shock‐wave structure are insignificant for all cases investigated and can be neglected.

ISSN:0031-9171    

DOI:10.1063/1.1694929

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The results of an experimental and theoretical investigation of the flow upstream of a thin foil model aligned with a low density, hypersonic stream are presented. Measurements of number density distributions ahead of the foil for two different foil thicknesses are compared in order to assess the relative importance of leading edge bluntness and aerodynamic surface contributions to the upstream disturbance. Numerical computation of a first collision model for the nearly free molecular flow field confirms the proposed scaling of the flow field. The upstream disturbance can be scaled on &lgr;bf, a mean free path for body emitted (wall temperature) molecules traveling into a cloud of free stream molecules.

ISSN:0031-9171    

DOI:10.1063/1.1694930

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The degree of coupling between free electron and nitrogen vibrational temperatures in weakly ionized nozzle expansions of shock‐heated nitrogen has been determined. Free electron temperatures were measured with Langmuir probes while nitrogen vibrational temperatures were determined with a new instrument based on the electron beam technique. The resultant data were analytically correlated by coupling chemical nonequilibrium with nitrogen vibrational and free electron temperature nonequilibrium. It was found that a vibrational loss factor of 7 × 10−4and an effective translational‐rotational loss factor of 4.26 × 10−4best predicted the measured electron temperature distributions.

ISSN:0031-9171    

DOI:10.1063/1.1694931

出版商:AIP    

年代:1974

数据来源: AIP