Downstream Wake Research Articles

Measurements of a laminar wake in a confined stratified fluid

Two-dimensional upstream and downstream wakes are produced by extremely slowly moving cylinders in a tank containing a linearly stratified salt solution. The upstream and downstream wakes appear to be of comparable strength, consistent with the symmetry predicted by theories that take account of density diffusion as well as viscosity. This contrasts with the quasi-non-diffusive régime recently studied experimentally by Browand & Winant (1972). The present velocity profiles are essentially similar to those predicted by diffusion theories, althought the magnitudes of the velocities appear to be greatly attenuated by the proximity of the upstream and downstream walls.

Dissipative magnetogasdynamic flow

An analysis of the structure of the wakes and waves in steady compressible magnetohydrodynamics is presented. No restriction is made on the equation of state of the gas or on the ratios of the various dissipative parameters. An asymptotic solution is obtained which furnishes directly the flow far from a body and which may be used in the construction of the entire flow field. The non-dissipative solutions are obtained as a non-uniform limit for vanishing dissipation; no matter how small the dissipation, one can go far enough from the origin that the flow is essentially dissipative. For non-aligned fields the wave pattern consists of a downstream wake and either two or four standing waves, depending on the flow regime. For aligned fields, two of these waves become wakes, so that the wake is a superposition of three structured layers, with either all downstream or two downstream and one up-stream. It is found that the nondissipative limit of the wake is non-unique for the aligned fields case. Different limits are obtained depending on how the various dissipative parameters vanish. In this paper we consider steady magnetohydrodynamic flow in the Oseen approximation. No restriction is placed on the equation of state of the gas or on the various dissipative parameters (viscosity, thermal and electrical conductivity). The method of approach is to obtain the fundamental solution. It was shown in an earlier paper (Salathe & Sirovich 1967) how this could be used to obtain the solution for arbitrary boundary-value problems. In 5 2 of the present paper we demonstrate that the fundamental solutions themselves provide the far field flow past a finite body. This is obtained simply in terms of such quantities as the total force on the body, heat added, etc. In 53 we obtain the fundamental solutions for the non-aligned fields case. An asymptotic solution is obtained applicable for distance from the origin large compared to the mean free path. It is well known that there exist two distinct flow regimes in magnetohydrodynamics, the doubly hyperbolic and the hyperliptic. In each of these we obtain a downstream wake which is a pure entropy wake. That is, it carries only density and temperature disturbances and is structured by thermal conductivity. In the doubly hyperbolic regime, the flow exhibits, in addition, four structured waves, while in the hyperliptic case, t Present address: Center for the Application of Mathematics, Lehigh University, Bethlehem, Pennsylvania.

An experimental study of the dynamic and steady-state flow disturbances encountered by aircraft during a carrier landing approach.

Abstract : A series of investigations were undertaken in a water tunnel to obtain greater insight into the disturbances encountered by aircraft engaged in the process of landing on a carrier. Water tunnel tests were selected because the use of water as the test medium permitted the disturbed flow areas to be identified by the cavitation phenomenon. In addition, these studies were undertaken at Reynolds numbers higher than those normally achieved in wind tunnel investigations. Key conclusions are that dynamic conditions produce flow regimes significantly different from those produced with steady state conditions; that the downstream wake is periodic, with the periodicity having a direct relationship with the pitching and heaving motions of the ship; that the overhang of the deck is the principal cause of the flow disturbances (far outweighing the disturbances caused by the island); and that steady state measurements are of considerable value and do permit some quantitative estimation of the velocity fluctuations occurring in a wake resulting from carrier motions.

Experiments on the Plasma flow past Bodies in a Magnetic Field

Plasma flows past bodies in a magnetic field are investigated experimentally by the use of a plasma from a plasma gun. The downstream wake of an ordinary hydrodynamic shock wave is observed for a collision-dominant plasma flow. The structure of the shock is investigated in detail for a wedge-shaped conductor. The plasma flow is divided into four regions by three characteristic lines: (1) the shock front, (2) the expansion wave from the tail of the wedge, and (3) the magnetic line of force passing through the tail of the wedge. When a wake of the shock wave interacts with the other wake, they join into one along the magnetic lines of force. Plasma flows past a wave-shaped wall are also investigated. As the plasma flow becomes collision-free, this downstream wake gradually disappears and no collision-free shock wave is observable. When the plasma flow is sub-Alfvénic, a disturbance propagating upstream along the magnetic lines of force is observed.

Decay of Turbulence in Axisymmetric Wakes

Turbulent wakes formed downstream of flat, circular disks were surveyed with hot-wire anemometers in a low-speed wind tunnel. Three regions were discernible in the wake. Between the disk and x = 50 disk diameters D downstream, the turbulence was highly anisotropic and new turbulence was generated locally. An approximate similarity region (100 ≲ x/D ≲ 400) existed where isotropic turbulence relations were adequate for estimating decay. The far downstream wake (x/D ≳ 400) was highly intermittent throughout; the decay rate lessened in this final period. Over a 4-fold range of disk size and a 7 1/2-fold range of mean flow velocity, Taylor’s microscale was correlated by the mean residence time x/U∞; this relation was independent of disk size.

Laboratory experiment on the disturbance of environmental conditions by presence of sounding rockets

A laboratory experiment on the disturbance in the surrounding medium caused by the re-entry of traveling rockets into the environmental atmosphere has been performed with the use of a plasma from a plasma gun. The experiment is so arranged as to simulate the phenomena in the ionospheric region. The magnetic field is applied both perpendicular and parallel to the plasma stream. The formation of a downstream wake has been observed in the collision-dominant region. This downstream wake gradually disappears as the plasma stream becomes collision-free. For a parallel magnetic field, a disturbance propagating upstream has also been observed when the plasma flow was sub-Alfvénic.

Aligned-fields magnetogasdynamic wakes

The effect of compressibility is included here in a study of wakes created in two-dimensional, steady, aligned-fields, magnetogasdynamic flow past obstacles. The gas is assumed to be viscous, resistive, and thermally conducting. With the Oseen type of approximation as well as the magnetogasdynamic boundary-layer approximation, a great simplification in the formulation of wakes results. The boundary-layer equations, although linearized, still retain the coupling between the velocity, the magnetic, and the temperature fields. The solution of the magnetogasdynamic wake, in general, is a superposition of three individual-wake components, each satisfying a diffusion type of equation. Only one of them is capable of extending upstream. Hence the wake picture is generally characterized by a conventional downstream wake with also the possibility of the existence of an upstream one.To be sure, the general features of the magnetogasdynamic wakes in aligned-fields flows are similar to those of an incompressible fluid; however, the flow now, instead of just being subalfvénic, must be subcritical for the upstream wake to occur. That is, the flow condition corresponding to the occurrence of the upstream wake is such that the sum of the square of the Mach number M∞ and the square of the Alfvén number A∞ is less than unity. Evidently it is the mechanism of magneto-sonic-wave propagation that modifies the transition of wakes from the Alfvénic point A∞ = 1 (for an incompressible fluid) to the subcritical are $A^2_\infty + M^2_\infty = 1$ in the Taniuti-Resler diagram.

Magneto-fluid dynamics of bodies in alined fields

The motion of a perfectly conducting fluid past a fixed body in the presence of an alined magnetic field can be regarded as the limit of a number of problems, namely (i) when the fluid is a finite conductor, (ii) an unsteady problem, (iii) when themagnetic and velocity fields are not parallel at infinity. The limits of these m ore general problems agree in predicting a force on the body and on the existence of upstream and downstream wakes. However, if the magnetic field is fairly strong (i) does not predict a downstream wake while (ii) and (iii) do. An attempt to reconcile these limit results is made here. In the first part we show that the unsteady problem associated with a finitely conducting fluid and an alined field is nonunique but that by making an appeal to the theory of real fluids the conclusion of (i) and (ii) can be obtained as special cases of the general solution. In the second part the steady problem, assuming that the conductivity is finite and the magnetic field is nearly alined, is considered and it is shown that the flow fields predicted in (i) and (ii) are limiting cases of this more general problem. The manner of the changeover from the solution for (i) to that for (iii) is elucidated.