Decrease In Mach Number Research Articles

Effect of the Hub Endwall Cavity Flow on the Flow-Field of a Transonic High-Pressure Turbine

In high-pressure turbines, a small amount of cold flow is ejected at the hub from the cavity that exists between the stator and the rotor disk. This prevents the ingestion of hot gases into the wheel-space cavity, thus avoiding possible damage. This paper analyzes the interaction between the hub-endwall cavity flow and the mainstream in a high-pressure transonic turbine stage. Several cooling flow ratios are investigated under engine representative conditions. Both time-averaged and time-resolved data are presented. The experimental data is successfully compared with the results of a three-dimensional steady Navier-Stokes computation. Despite the small amount of gas ejected, the hub-endwall cavity flow has a significant influence on the mainstream flow. The Navier-Stokes predictions show how the ejected cold flow is entrained by the rotor hub vortex. The time-resolved static pressure field around the rotor is greatly affected when traversing the non-uniform vane exit flow field. When the cavity flow rate is increased, the unsteady forces on the rotor airfoil are reduced. This is linked to the decrease of vane exit Mach number caused by the blockage of the ejected flow.

Perturbations of the solar wind flow by radial and latitudinal pick-up ion pressure gradients

Abstract. It has been found that pick-up ions at their dynamical incorporation into the solar wind modify the original conditions of the asymptotic solar wind plasma flow. In this respect, it has meanwhile been revealed in many papers that these type of solar wind modifications, i.e. deceleration and decrease of effective Mach number, are not only due to the pick-up ion loading effects, but also to the action of pick-up ion pressure gradients. Up to now only the effects of radial pick-up ion pressure gradients were considered, however, analogously but latitudinal pressure gradients also appear to be important. Here we study the effects of radial and latitudinal pick-up ion pressure gradients, occurring especially during solar minimum conditions at mid-latitude regions where slow solar wind streams change to fast solar wind streams. First, we give estimates of the latitudinal wind components connected with these gradients, and then after revealing its importance, present a more quantitative calculation of solar wind velocity and density perturbations resulting from these pressure forces. It is shown that the relative density perturbations near and in the ecliptic increase with radial distance and thus may well explain the measured non-spherically symmetric density decrease with distance. We also show that the solar wind decelerations actually seen with Voyager-1/2 are in conciliation with interstellar hydrogen densities of nH∞≥0.1cm-3, in contrast to earlier claims for nH∞=0.05cm-3.

Partially Implicit Scheme for Chemically Reacting Flows at All Mach Numbers

Chemical reactions make computations of chemically reacting e ows stiff. The degree of stiffness increases as the Mach number decreases because the e ow timescale increases while the reaction timescales remain constant. Thus, the computation of reacting e ows at low Mach numbers is more dife cult than that in high Mach number. In the present study a new implicit scheme that employs partially implicit treatment of chemical source terms at mixed time levels has been developed. The chemical Jacobian is reduced to a partial form of the full chemical Jacobian(a lower triangular matrix )and thus saves timein matrix inversion. In addition, robust calculation of the reacting e ows is made possible because a negative real eigenvalue of the partial chemical Jacobian allows larger time-step sizes than the full chemical Jacobian. For applications at all Mach numbers, a preconditioned lower upper symmetric Gauss ‐Seidel scheme employing an approximate e ux Jacobian splitting is incorporated. For high-Mach-number e ows the partially implicit scheme maintains similar convergence rates to the fully implicit one; therefore, it enhances computational efe ciency by reducing computing time per iteration. For low-Machnumber reacting e ows it also enables robust calculation of reactions as well as the fully implicit one; however, its convergence rate is rather slow. Furthermore, more stable and efe cient computations are possible when the fully implicit scheme is coupled with the partially implicit one.

Numerical simulation of plasma and fluid flow in a shock-tube-driven disk MHD generator

Three-dimensional numerical simulations have been carried out in order to explain the experimental results with a shock-tube-driven disk magnetohydrodynamics (MHD) generator with inlet swirl for the first time. The numerical results agree well with the experimental results especially in the enthalpy extraction ratio. The isentropic efficiency obtained numerically is higher than that in the experiment, for seed fractions lower than the optimum value. There may exist some additional mechanism causing a total pressure loss which cannot be, included in the present numerical model. The radial component of Mach number becomes less than unity for high seed fractions. This decrease in Mach number is not so steep but moderate, and seems to be different from that attributed to a shock wave observed in a fluid flow without MHD interaction. The plasma structure is almost uniform in the /spl theta/ direction near the optimum conditions. Under the condition far from the optimum value, however, a nonuniform and unsteady plasma structure owing to the partial ionization of seed material is observed.

Unsteady propagation of collisionless trans‐Alfvénic shocks at large timescales

In a collisional plasma, fast and slow shocks are time stationary. They evolve in such a way that the flow variations remain small under the action of a small perturbation. By contrast, in the case of a trans‐Alfvénic shock wave (TASW), at which the flow velocity passes over the Alfvén velocity, the small perturbation generates finite variation of the initial flow. As a result, the shock disintegrates or transforms into some other unsteady state. In the present paper, the problem of shock propagation is considered for a collisionless plasma with anisotropic pressure. Using a kinetic theory, it is shown that the transition of the flow velocity over the Alfvén velocity at a shock is crucial for its evolution. Namely, the small perturbation makes collisionless TASWs unsteady at timescales large enough compared to the ion gyroperiod. This conclusion is valid for both ordinary TASWs at which the Alfvén Mach number decreases and anomalous TASWs at which it increases. Possible manifestations of unsteady propagation of planetary bow shocks are discussed.

Euler solution of axisymmetric jets in supersonic external flow

Conclusions A numerical experiment of axisymmetric turbulent viscous flow over a bulbous heat shield is performed by employing a three-stage Runge-Kutta time-stepping scheme. Turbulence closure is achieved using the Baldwin-Lomax turbulence model. The flowfield visualization of the terminal shock and the separated region helps in a systematic understanding of flow structure under various freestream Mach numbers. The following observations are made based on the described numerical simulations. 1) The terminal shock moves downstream with increasing freestream Mach number. The strength of the terminal shock initially increases with Mach number and later decreases. The location of the terminal shock is found as a nonlinear function of the freestream Mach number. 2) The separation zone in the boat-tail region is found as a function of the freestream Mach number. The flowfield features of the separated region are found to be different in the transonic and supersonic Mach number ranges.

The effect of heat and mass addition on the base pressure of bodies of revolution at supersonic speeds

The results of an experimental investigation of the influence of mass addition and afterburning of combustion products of pyrotechnic mixtures in the base region of axisymmetric bodies in supersonic air flow at Mach numbers of 1.2–3.0 are reported. It is shown that the base-pressure increment increases monotonically to a maximum value with an optimum injected-mass flow rate. The value of this increment reduces as the Mach number decreases. A dependence generalizing the experimental data is presented.

57 MHD相互作用を伴う超音速プラズマ流れの可視化

The purpose of this paper is to report the experimental studies of the gas dynamic characteristics, especially those such as supersonic plasma flow with MHD interaction in the constant area channel. The plasma for exeperiment is produced by a shock tube. In order to obtain a clear and sharp image of the supersonic plasma flow with MHD interaction, the color schlieren photographic method has been employed. The schlieren photographs show clealy the image of the shock-boundary layer interaction in the MHD channel. The experimental results show that the flow Mach number decreases and velocity of propagating shock wave increases in the channel, with an increase in the Lorentz force for the supersonic plasma flow with MHD interaction.

The configuration of slow‐mode shocks

Interest in slow‐mode shocks in space physics applications has increased in recent years. These shocks, which can arise from the steepening of linear slow‐mode magnetohydrodynamic waves, might form ahead of objects moving through a magnetized, highly conducting gas at speeds greater than the slow‐mode speed. In particular, slow shocks are the only shocks possible for speeds between those of the slow and fast modes. This appears to be the speed range, for example, of many and perhaps most coronal mass ejections. Despite the interest in and detection of slow‐mode shocks in the heliosphere, there appear to date to be no calculations of flows involving slow shocks. Basic properties like shock configuration and standoff distance are therefore unknown, although several authors have pointed out that slow shocks should extend upstream of the obstacle giving rise to the shock. This paper presents a simple method for computing slow shock flows in an infinitely conducting plasma, based on the solution of a free boundary problem for the shock configuration that matches a post shock potential flow to a uniform preshock flow. Such potential‐flow methods have been used in ordinary gasdynamics to obtain crude approximations to blunt‐body flows containing shocks; arguments given here suggest that the potential‐flow approximation should be more accurate in the case of slow MHD shocks. The method is applied to shocks arising from flow about spherical and paraboloidal obstacles. A parametric study of shock formation as a function of sonic and Alfvén Mach numbers shows standoff distance increasing as either Mach number decreases. Streamlines and shock configurations for several slow shock flows are also presented.

Transonic dip mechanism of flutter of a sweptback wing. II

Conclusions An experimental investigation of the RR—-MR and MR-*RR transitions over concave and convex, smooth and rough wedges revealed that 1) as the surface roughness increases, the wedge angle at which transition takes place (for a given Mach number) decreases and 2) for a mesh 40 sand paper type surface roughness, it seems that the transition angle becomes independent of the incident shock wave Mach number, i.e., a) MR—>RR transition occurs at 6W =^54 deg and b) RR-MR transition takes place at Ow-21 deg. These results should be of great importance to those dealing with reflections of blast waves, since this type of problem involves nonstationary flows and high degrees of surface (ground) roughness.

Propagation of MHD shock waves into regions of various uniform densities

In the presence of uniform magnetic field in an infinitely conducting fluid, the pressure behind the MHD shock wave is investigated after the shock wave has interacted with the density variation parallel to the shock front. The case is generalized to another density variation perpendicular to the shock front. The density variation is due to a small discontinuity jump across the interfaces. The problem is formulated under the assumption of small change in density and the shock profile is obtained. It is observed that the curvature of the shock increases as the Mach number decreases or the strength of the magnetic field increases.

Supersonic combustion tests with a double- oblique-shock SCRAMjet ina shock tunnel

Abstract : Some results of a continuing research program to develop a capability for testing integrated scramjets are reported. During this research program, an integrated double-oblique-shock scramjet model was developed to provide a test bed for supersonic combustion tests and for instrumentation development essential for analysis of combustion test results. Results are presented for tests in which hydrogen fuel was injected into the combustor. Injection of the fuel, from sonic orifices in the wall, normal to the flow did not lead to satisfactory combustion data, supposedly because of the cold boundary layer. Injection through sonic orifices in a series of diamond airfoil injectors led to combustion confirmed by all the following measurements: (1) an increase in static pressure within the combustor downstream of the injection station, (2) an increase in surface heat-transfer rate, (3) an increase in static temperature as measured by the sodium line reversal technique, (4) an increase in output of radiation sensor gages, and (5) a decrease in flow Mach number inferred from static to pitot pressure measurements. The measured increases were proportionate to increases in computed average equivalence ratio. The combustion results are compared with numerical solutions. In general, the measured temperatures and pressures in the combustor with heat addition were higher than the calculated values.