Stability Of Flow Field Research Articles

Experimental Study on Thermal Protection System by Opposing Jet in Supersonic Flow

Introduction C URRENTLY, developments of reusable launch vehicle (RLV) for a low-cost space transportation system are in progress. In the development of RLV, one of the most important problems is the severe aerodynamic heating at the nose and leading edges of the vehicle. In such supersonic and hypersonic flights, prediction of aerodynamic heating and construction of proper thermal protection system are especially important. Heat-resistant tiles and ablators are currently used for thermal protection systems. However, those thermal protection systems are not reusable. In the present study, the method using an opposing jet is proposed for fully reusable thermal protection system of RLV. The method can be considered to have almost the same effect of heat reduction at nose region as the method with mechanical spike.1 The opposing jet works as an aerodynamic spike to move the detached shock wave away from the nose and form a recirculation region, which is quite effective to reduce aerodynamic heating at the nose region. The schematic diagram of supersonic flowfields with opposing jet injected at the nose of a blunt body is shown in Fig. 1. In the flowfield, the opposing jet forms a Mach disk and contact surface with freestream. The jet layer reattaches to the body surface and forms a recirculation region between the nozzle exit and reattachment point of the jet layer. The recompression shock wave is formed near the reattachment point of the jet layer. Many studies on opposing jet flow have been conducted in order to reveal the flow mechanism.2−7 However, most of those studies are related to the stability of flowfield and oscillations of shock waves. Except for Warren,6 not much study has been conducted to reveal the effects of opposing jet on reduction of aerodynamic heating. In the present study, geometric ratio of diameters and Mach number are fixed. The flow stability is determined by the total pressure ratio of freestream to opposing jet. We define the total pressure ratio

Aerodynamic-Rotordynamic Interaction in Axial Compression Systems—Part II: Impact of Interaction on Overall System Stability

This paper presents an integrated treatment of the dynamic coupling between the flow field (aerodynamics) and rotor structural vibration (rotordynamics) in axial compression systems. This work is motivated by documented observations of tip clearance effects on axial compressor flow field stability, the destabilizing effect of fluid-induced aerodynamic forces on rotordynamics, and their potential interaction. This investigation is aimed at identifying the main nondimensional design parameters governing this interaction, and assessing its impact on overall stability of the coupled system. The model developed in this work employs a reduced-order Moore-Greitzer model for the flow field, and a Jeffcott-type model for the rotordynamics. The coupling between the fluid and structural dynamics is captured by incorporating a compressor pressure rise sensitivity to tip clearance, together with a momentum based model for the aerodynamic forces on the rotor (presented in Part I of this paper). The resulting dynamic model suggests that the interaction is largely governed by two nondimensional parameters: the sensitivity of the compressor to tip clearance and the ratio of fluid mass to rotor mass. The aerodynamic-rotordynamic coupling is shown to generally have an adverse effect on system stability. For a supercritical rotor and a typical value of the coupling parameter, the stability margin to the left of the design point is shown to decrease by about 5% in flow coefficient (from 20% for the uncoupled case). Doubling the value of the coupling parameter not only produces a reduction of about 8% in the stability margin at low flow coefficients, but also gives rise to a rotordynamic instability at flow coefficients 7% higher than the design point.

Lattice Boltzmann Numerical Simulation of a Circular Cylinder

The lattice Boltzmann equation (LBE) model based on the Boltzmann equation issuitable for the numerical simulation of various flow fields. The fluiddynamics equation can be recovered from the LBE model. However, compared tothe Navier-Stokes transport equation, the fluid dynamics equation derivedfrom the LBE model is somewhat different in the viscosity transport term,which contains not only the Navier-Stokes transport equation but also nonsteadypressure and momentum flux terms. The two nonsteady terms can produce thesame function as the random stirring force term introduced in the directnumerical or large-eddy vortex simulation of turbulence. Through computationof a circular cylinder, it is verified that the influence of the twononsteady terms on flow field stability cannot be ignored, which is helpfulfor the study of turbulence.

Variable viscosity effects on free and mixed convection boundary-layer flow from a horizontal surface in a saturated porous medium - variable heat flux

In recent years, the study of convective heat transfer from surfaces embedded in porous media has received considerable attention in the literature. The interest for such studies is motivated by several thermal engineering applications, such as geothermal systems, the storage of nuclear wastes, oil extraction, ground water pollution and thermal insulation. The boundary-layer mixed convection flow over horizontal surface with constant or variable wall temperature or heat flux has been studied by many investigators [1-8]. Recently, the flow over an horizontal surface in saturated porous media for the entire regime of forced to free convection has been investigated by Aldoss et al. [9], Chen [10] for Newtonian fluids and by Kumari et al. [11] for non-Newtonian fluids. In all these studies, it was assumed that the thermophysical properties of the fluid are constant. However, it is known that these properties may change with temperature, especially for fluid viscosity. Variable viscosity effects on the stability of flow and temperature fields were considered by Kassoy and Zebib [12], Strans and Schubert [13], Horne and Sullivan [14] and Gary et al. [15]. The effect of variable viscosity for mixed convection flow over a vertical plate with constant wall temperature in saturated porous media has been investigated by Lai and Kulacki [16]. The free or mixed convection flow taking in to account the effect of variable viscosity over a horizontal surface with variable heat flux has not been studied so far.

Rayleigh Light Scattering Measurements of Transient Gas Temperature in a Rapid Chemical Vapor Deposition Reactor

Abstract A laser-induced Rayleigh light scattering (RLS) system was used to measure transient gas temperatures in a simulated rapid chemical vapor deposition (RCVD) reactor. The test section geometry was an axisymmetric jet of carrier gas directed down, impinging on a heated wafer surface. RLS was used to measure instantaneous gas temperature at several locations above the wafer as it was heated from room temperature to 475 K. Gas flow rate and wafer temperature correspond to jet Reynolds number Rei=60, wafer maximum Grashof number GrH=4.4×106, and maximum mixed convection parameter GrH/Rei2=1200; all conditions typical of impinging jet reactors common in the numerical literature. Uncertainty of RLS transient temperature from a propagated error analysis was ±2–4 K. Peak gas temperature fluctuations were large (in the order of 25 to 75 °C). Both flow visualization and RLS measurements showed that the flow field was momentum dominated prior to heating initiation, but became unstable by GrH/Rei2=5. It then consisted of buoyancy-induced plumes and recirculations. Up to the peak wafer temperature, the flow field continued to be highly three-dimensional, unsteady, and dominated by buoyancy. RLS measurements are shown to provide information on carrier gas instantaneous temperature and flow field stability, both critical issues in RCVD processing. [S0022-1481(00)02401-4]

Development of a Lean Premixed Low-Swirl Burner for Low NOx Practical Applications

Laboratory experiments have been performed to evaluate the performance of a premixed low-swirl burner (LSB) in configurations that simulate commercial heating appliances. Laser diagnostics were used to investigate changes in flame stabilization mechanism, flowfield, and flame stability when the LSB flame was confined within quartz cylinders of various diameters and end constrictions. The LSB adapted well to enclosures without generating (lame oscillations and the stabilization mechanism remained unchanged. The feasibility of using the LSB as a low NO x commercial burner has also been verified in a laboratory test station that simulates the operation of a water heater. It was determined that the LSB can generate NO x emissions < 10 ppm (at 3% O2) without significant effect on the thermal efficiency of the conventional system. Our study has demonstrated that the lean premixed LSB has commercial potential for use as a simple economical and versatile burner for many low emission gas appliances.

A Time-Dependent Nodal-Integral Method for the Investigation of Bifurcation and Nonlinear Phenomena in Fluid Flow and Natural Convection

Two new “low-” and “high-order” time-dependent nodal-integral methods were developed and applied to both incompressible fluid flow and natural convection. These new methods have a high level of accuracy on a coarse mesh, high efficiency, and an ability to reproduce results using various time-step sizes independent of a Courant condition. These new methods are applied to various benchmark problems, such as double-glazing, to verify their accuracy in space and time. Other applications to bifurcation searches and stability of flow fields verify their accuracy and their ability to duplicate natural phenomena without exhibiting problems with spurious solutions, turning points, and bifurcation points. The new methods are also used to verify the existence of critical values of the aspect ratio. The means by which alternative stable solutions can be obtained from a no-flow initial condition for a critical aspect ratio are also examined.

Thermohaline convection in a nonhomogeneous aquifer

Geothermal activity creates destabilizing temperature gradients in groundwater. Very often stabilizing salinity gradients also exist in the aquifer. The combination of temperature and salinity distributions in the aquifer may induce various kinds of hydrodynamic instability. Such instabilities are possible mainly in thick aquifers, which usually consist of various nonhomogeneous formations. In this study we investigate the effect of the aquifer nonhomogeneity on the stability of the flow field as well as on heat and salinity transport through the aquifer. By Galerkin expansion the criteria of the flow field stability were identified and calculated quantitatively. It was found that the point of instability is demonstrated by marginal stability or overstability. The nonhomogeneity of the aquifer leads to asymmetry of the convection cells.

Seminumerical simulation of singly dispersive convection in groundwater

Geothermal activity may lead to convection currents in groundwater. Such currents considerably affect transport phenomena in the aquifer. In this study, the applicability of a seminumerical approach for the simulation of flow conditions in such an aquifer is presented. Flow field variables are expanded through truncated sets of eigenfunctions, which leads to a system of general first-order differential equations that can be solved by applying available subroutines. Criteria of flow field stability, parameters affecting flow conditions, and quantitative analysis of transport processes in the aquifer are studied.

Horizontal Buoyant Jets in Quiescent Shallow Water

The flow behavior of a single round buoyant jet discharging into a shallow quiescent body of water is experimentally investigated. The temperature distributions are measured in a large test basin for jet densimetric Froude numbers ranging from 5 - 20 and submergence ratios ranging from 2.5 - 7. Empirical relations for the flow-field stability of the near field, the temperature rise in the zone of surface inpingement and the water surface area covered by the temperaturee rise isotherms are proposed.

Effect of solute dispersion on thermal convection in a porous medium layer, 2

In some situations associated with geothermal activity, groundwater motions are affected by convection currents due to large temperature gradients. In such cases, usually saline hot water is located in the deep layers of the aquifer from which salt and heat are transferred to the upper layers. In part 1 of this study the parameters of the two‐dimensional flow field stability were determined. In this paper, further analysis of the phenomenon in three dimensions is presented. It was found that the convection cells have the shape of rolls whose axes are perpendicular to the steady state flow velocity. However, there is also a possibility of overstability of the flow field caused by rolls whose axes are parallel to the steady state velocity. The parameters of these two kinds of instability were determined in this study.