Leading Edge Velocity Research Articles

Computer Simulation Helps Increase Life of Impeller in Alumina Hydrate Precipitation from 2 to 8 Years

The rate at which an impeller wears is a strong function of the velocity at the leading edge. Blades often fail due to extreme erosion. Simulating the impeller and tank of a draft tube mixer using computational fluid dynamics (CFD) helped extend the life of an impeller by two to eight years. The computer simulation, using FLUENT CFD software from Fluent Inc., Lebanon, NH, made it possible to quickly evaluate the leading edge velocity of several proposed new designs. The analysis helped identify the erosion problem by predicting the locations of wear patterns.

Shape optimization of turbine blades with the integration ofaerodynamics and heat transfer

A multidisciplinary optimization procedure, with the integration of aerodynamic and heat transfer criteria, has been developed for the design of gas turbine blades. Two different optimization formulations have been used. In the first formulation, the maximum temperature in the blade section is chosen as the objective function to be minimized. An upper bound constraint is imposed on the blade average temperature and a lower bound constraint is imposed on the blade tangential force coefficient. In the second formulation, the blade average and maximum temperatures are chosen as objective functions. In both formulations, bounds are imposed on the velocity gradients at several points along the surface of the airfoil to eliminate leading edge velocity spikes which deteriorate aerodynamic performance. Shape optimization is performed using the blade external and coolant path geometric parameters as design variables. Aerodynamic analysis is performed using a panel code. Heat transfer analysis is performed using the finite element method. A gradient based procedure in conjunction with an approximate analysis technique is used for optimization. The results obtained using both optimization techniques are compared with a reference geometry. Both techniques yield significant improvements with the multiobjective formulation resulting in slightly superior design.

An unusual mechanism of airbag injury

An airbag mounted within the steering column of a car is an increasingly common safety feature in new motor vehicles sold in the UK. Together with the three-point lap-shoulder seatbelt it improves the level of protection for the driver in the event of a frontal collision. An airbag is intended to be a supplemental. It is triggered by a frontal impact within 30 degrees of each side of the head-on position, when an electronic crash sensor detects a sudden deceleration from a speed of over approximately 20 miles/h (32 km/h). If the sensor's crash conditions are satisfied then the inflator is fired. This is a pyrotechnic device containing sodium azide which on ignition produces a large volume of nitrogen gas, which then inflates the airbag up to a volume of 40 l (Eurobag design) or 70 l (full size) within 50 ms. The 'leading edge velocity' of the bag during deployment is between 46 and 184 mph but should be zero at the time of contact with a normally positioned driver. Evidence is accumulating to demonstrate that, with the airbag, overall mortality and serious injuries have been reduced, but there has been a new spectrum of injuries as a result of its deployment. Thermal injuries from the hot gases generated during firing, a chemical keratitis produced by the talcum or corn starch used to lubricate the neoprene surface or by a small amount of sodium hydroxide during sodium azide combustion, and the shearing forces produced by the 'punch out' of the bag are now well described, although uncommon. A case is reported in which the collision dynamics resulted in the driver being in close proximity to the steering column at the time of the bag's deployment.

Measuring the Burning Rate of Premixed Turbulent Flames in Stagnation Flows

Abstract A method is proposed to measure the burning rate of planar premixed turbulent flames in stagnation flows. The method is based on the balance of conditional mass fluxes in a control volume that extends from pure reactants to the stagnation plane. The principle is that the difference between the mass of reactants flowing into and out of the control volume is the mass that has reacted within the control volume. Similarly, if only reactants flow into the control volume then the mass flow rate of products out of the control volume constitutes the burning rate in that region of the flame. The measurement techniques necessary to use this approach are all available. Experimental measurements were conducted on the stagnation plate geometry. The burning rate characterized by a leading edge velocity overestimated the value determined from the conditioned mass fluxes by 41% The extension of this approach to other burner geometries and more general flame shapes is discussed and suggestions for its use in rod stabilized flames are made. In this case the shape and orientation of the control volume are set by the maximum gradient in space of the progress variable. For curved flame zones and more general flows the choice of control volume remains ambiguous.

Influence of Turbulence Parameters, Reynolds Number, and Body Shape on Stagnation-Region Heat Transfer

This experiment investigated the effects of free-stream turbulence intensity, length scale, Reynolds number, and leading-edge velocity gradient on stagnation-region heat transfer. Heat transfer was measured in the stagnation region of four models with elliptical leading edges downstream of five turbulence-generating grids. Stagnation-region heat transfer augmentation increased with decreasing length scale but ann optimum scale was not found. A correlation was developed that fit heat transfer data for isotropic turbulence to within ±4 percent but did not predict data for anisotropic turbulence. Stagnation heat transfer augmentation caused by turbulence was unaffected by the velocity gradient. The data of other researchers compared well with the correlation. A method of predicting heat transfer downstream of the stagnation point was developed.

Visualization of transitional pipe flow using the photochromic tracer method

Two noninvasive measurement techniques are described for investigating transitional flow in a rigid straight tube. Photochromic trace recordings, representing fluid displacement profiles, were made within the core and at the leading and trailing edges of turbulent slugs generated by a disturbance at the tube inlet. Consistent photochromic trace recordings were made possible through the discovery that Doppler ultrasound can be used to detect the passage of turbulent slugs, thereby allowing precise timing of the photochromic traces. Upstream and downstream Doppler probes enabled the leading and trailing edge velocities of slugs to be measured and also enabled the laminar/turbulent transition regions to be investigated in detail. Trailing edge interface velocities were in excellent agreement with existing data, while leading edge velocities agreed less well. Turbulence generation at the trailing edge appeared to be associated with a localized vortical structure which originated in the near wall region and then grew toward the tube centerline. The generation of this vortex is postulated to be induced by a shear layer instability. Mean velocity profiles obtained from within the core of turbulent slugs were similar to those obtained for fully developed turbulent flow.

A Design Optimization Procedure for Efficient Turbine Airfoil Design

An optimization procedure has been developed for the efficient design of turbine blade airfoil sections. A shape sensitivity study of the airfoils has been performed considering two leading edge shapes, circular and elliptic. Pressure and suction surfaces are approximated by polynomials. A two-level, nonlinear constrained optimization problem is formulated and is solved using the method of feasible directions. The aerodynamic analysis is performed using a two-dimensional panel code. Since several evaluations of the objective functions and the constraints are required within the optimizer, and exact aerodynamic analysis at each step is computationally prohibitive, a two-point exponential approximation technique has been used. The procedure developed successfully eliminates the sharp leading edge velocity spikes, characteristic of typical blade sections, without compromising blade performance. Circular leading edge airfoils appear to be more effective in eliminating the spikes than elliptic leading edge airfoils. However, the elliptic leading edge sections are more slender than the circular leading edge sections. Optimum results are compared with a reference design.

An Extending Dynamic Elastica: Impact With a Surface

The study of the dynamic transport of paper, film, and tape is of increasing importance as process speeds increase. In earlier machines, transport velocities were slow enough that inertia loads did not appreciably affect the deformation of the material. In many applications, material is pushed from a channel or clamp and can be modeled as a cantilevered beam or plate until it reaches another guide or barrier. In this paper, we are concerned with modeling the behavior of such a cantilevered sheet after it strikes a guide. The guide may be any two-dimensional curve. We are able to determine leading edge velocities and reaction forces necessary to keep the sheet from penetrating the guide. Friction is included in the model. The sheet is allowed to exit the channel at an arbitrary angle and may have a nonconstant transport velocity. Inertial loads become noticeable at speeds greater than one copy per second and are necessary for the resolution of impact loads.

Empiric limits of rod photocurrent component underlying a-wave response in the electroretinogram.

The corneally recorded rod photocurrent component (photoresponse) underlying the a-wave feature of the electroretinogram was analyzed. The results set empiric limits on critical photoresponse variables. Measurements were obtained from four normal adult subjects on a-wave amplitude, a-wave velocity, b-wave amplitude, b-wave implicit time and b-wave height above baseline. At high intensity, interference from the b-wave component was minimized and the amplitude of the saturated photoresponse component was approximated by the a-wave feature. At lower intensities, the a-wave feature represented progressively less of the underlying photoresponse amplitude. Photoresponse amplitude saturation was signaled by the abrupt slowing of the rate of decline of b-wave peak latency and occurred at an intensity about 2.5 log units above the first appearance of the b-wave. At the intensity of photoresponse saturation, the peak amplitude of the a-wave feature was only about 25% of the maximum amplitude of the underlying photoresponse component. A-wave leading edge velocity was found to increase up to 3 log units above the intensity of photoresponse amplitude saturation and to provide a good estimate of photoresponse velocity at higher intensities. A cascaded low-pass filter model with modifications to accommodate amplitude and timing nonlinearities was used to generate a set of probable underlying photoresponses from the analysis of a-wave amplitude and velocity. Movement of the a-wave leading edge to the left at higher intensities in algebraic combination with a static b-wave leading edge above the intensity of photoresponse amplitude saturation was found to explain the second rise of the b-wave amplitude function and the decline of b-wave amplitude above baseline at high intensities. This analysis provides a basis for modeling the underlying photoresponse on a biochemical level and for interpreting photoreceptor damage in disease states.

BUMPER DEBRIS CLOUD STRUCTURE ESTIMATED BY SHOCK CALCULATIONS

BUMPER DEBRIS CLOUD STRUCTURE ESTIMATED BY SHOCK CALCULATIONS

Properties of metre-wavelength solar bursts associated with coronal mass ejections

An investigation is made to determine the relationship between a coronal mass ejection (CME) and the characteristics of associated metre-wave activity. It is found that (1) the CME width and leading edge velocity can be highly influential in determining the intensity, spectral complexity and frequency coverage of both type II and continuum bursts; (2) the presence of a CME is possibly a necessary condition for the production of a metric continuum event and (3) metric continuum bursts as well as intense, complex type II events are preferentially associated with strong, long lasting soft X-ray events.

Spreading of a Pseudowave Front

Using analytic, nonlinear techniques the spreading of the leading edge of a pseudowave (here assumed to be a slab of ions injected at hypersonic velocity into a dense background plasma) is computed. It is found that the front spreads due to collective effects with a characteristic velocity of 3(n20/n10)1/2va, where n20 is the initial injected ion density, n10 is the background ion density, and va is the ion-acoustic velocity. The computation also predicts a finite leading edge velocity of v0+2(n20/n10)1/2va, where v0 is the initial velocity of injection. Neither shock steepening nor an accelerating leading edge as in a free expansion of plasma is found.

Drop formation in liquid—liquid systems—I prediction of drop volumes at moderate speed of formation

Correlations are presented for the prediction of drop volumes for drops formed at nonwetted capillaries in liquid—liquid systems. The analysis is based on a two-stage drop formation. The first stage is one of pure growth of the drop ending with equilibrium of forces. Equations for the drop volume in this stage are given. For the second stage, the period of release, models have been developed for the essential variables, which are the forces acting upon a drop, the way the dispersed phase enters the drop, the necking of the drop as a function of time and finally the velocity of rise of the neck. An experimental relation was obtained for the leading-edge velocity from a high speed ciné-film piecures. An experimental correction factor was introduced. Also for this period equations are presented from which the extra increase in drop volume can be predicted.

Some Interference Effects Between Two Flat Surfaces Planing Parallel to Each Other at High Speed

temperature. In line with current work on the laminar boundary layer, the theory also has been extended to allow arbitrary variation of specific heat, thermal conductivity, and Prandtl Number, and the use of imperfect gas relations. These changes cause no basic complications in the work, requiring only the use of stagnation enthalpy in place of stagnation temperature and certain addi­ tional terms of the type employed for the arbitrary viscosity law. A report is in preparation which uses stagnation conditions for the basic thermal variable and includes the variable properties listed above. In reference 1, values of gi(0) and ^(0) had been obtained as eigenvalues of the equations representing the initial conditions for finite leading-edge velocity and a stagnation point, respec­ tively. It is now clear that the equation for the stagnation point does not possess a singularity as Tw* —* 1 and u0 —• 0 (as stated in reference 1) since u0 at x = 0 is always identically zero. There­ fore no special conditions need be imposed upon g2(0), and it assumes the constant value of —1/60 for a sixth-order poly­ nomial.