bluuebunny/crossref_metadata_2025 huggingface dataset:
Airfoil Shape Optimization Using Sensitivity Analysis on Viscous Flow Equations
Authors: Eleshaky Mohamed E. , Baysal Oktay
Published: May 2008

An aerodynamic shape optimization method has previously been developed by the authors using the Euler equations and has been applied to supersonic-hypersonic nozzle designs. This method has also included a flowfield extrapolation (or flow prediction) method based on the Taylor series expansion of an existing CFD solution. The present paper reports on the extension of this method to the thin-layer Navier-Stokes equations in order to account for the viscous effects. Also, to test the method under highly nonlinear conditions, it has been applied to the transonic flows. Initially, the success of the flow prediction method is tested. Then, the overall method is demonstrated by optimizing the shapes of two supercritical transonic airfoils at zero angle of attack. The first one is shape optimized to achieve a minimum drag while obtaining a lift above a specified value. Whereas, the second one is shape optimized for a maximum lift while attaining a drag below a specified value. The results of these two cases indicate that the present method can produce successfully optimized aerodynamic shapes.
The Flow Around Surface-Mounted, Prismatic Obstacles Placed in a Fully Developed Channel Flow (Data Bank Contribution)
Authors: Martinuzzi R. , Tropea C.
Published: May 2008

The flow field around surface-mounted, prismatic obstacles with different spanwise dimensions was investigated using the crystal violet, oil-film and laser-sheet visualization techniques as well as by static pressure measurements. The aim of this study is to highlight the fundamental differences between nominally two-dimensional and fully three-dimensional obstacle flows. All experiments were performed in a fully developed channel flow. The Reynolds number, based on the height of the channel, lay between 8 × 104 and 1.2 × 105. Results show that the middle region of the wake is nominally two-dimensional for width-to-height ratios (W/H) greater than 6. The separated region in front of wider obstacles is characterized by the appearance of a quasi-regular distribution of saddle and nodal points on the forward face of the obstacles. These three-dimensional effects are considered to be inherent to such separating flows with stagnation.
A Comparison of the Linear and Nonlinear k–ε Turbulence Models in Combustors
Authors: Hwang C. C. , Zhu Genxing , Massoudi M. , Ekmann J. M.
Published: May 2008

In swirling turbulent flows, the structure of turbulence is nonhomogeneous and anisotropic and it has been observed that the assumptions leading to the formulation of the k-ε model, which is used very often in many engineering applications, are inadequate for highly swirling flows. Furthermore, even with the various modifications made to the k-ε model, it is still not capable of describing secondary flows in noncircular ducts and it cannot predict non-zero normal-Reynolds-stress differences. Recently Speziale (1987) has developed a nonlinar k-ε model, which extends the range of validity of the standard k-ε model while maintaining most of the interesting features of the k-ε model; for example, the ease of application in existing Computational Fluid Dynamics (CFD) codes. In this work, we will use the nonlinear k-ε closure to model the turbulence in combustors. The particular combustor geometries selected for this study are (i) the flow in a round pipe entering an expansion into another coaxial round pipe, and (ii) the flow in two confined co-axial swirling jets. The results show that there are no significant differences in the performance of the two models. It is speculated that the inlet conditions for k and ε may play as crucial a role in achieving predicted accuracy as turbulence modeling details. Also it is possible that weaknesses in the performance of the modeled equations for k and ε may have masked differences in the two models.
One-Equation Near-Wall Turbulence Modeling With the Aid of Direct Simulation Data
Authors: Rodi W. , Mansour N. N. , Michelassi V.
Published: May 2008

The length scales appearing in the relations for the eddy viscosity and dissipation rate in one-equation models were evaluated from direct numerical (DNS) simulation data for developed channel and boundary-layer flow at two Reynolds numbers each. To prepare the ground for the evaluation, the distribution of the most relevant mean-flow and turbulence quantities is presented and discussed, also with respect to Reynolds-number influence and to differences between channel and boundary-layer flow. An alternative model is examined in which (v′2)1/2 is used as velocity scale instead of k1/2. With this velocity scale, the length scales now appearing in the model follow closely a linear relationship near the wall. The resulting length-scale relations together with a DNS based relation between v′2/k and y* = k1/2y/v form a new one-equation model for use in near-wall regions. The new model was tested as near wall component of a two-layer model by application to developed-channel, boundary-layer and backward-facing-step flows.
Frictional Drag Reduction by Injecting High-Viscosity Fluid Into Turbulent Boundary Layer
Authors: Kato H. , Fujii Y. , Yamaguchi H. , Miyanaga M.
Published: May 2008

This paper presents a new method to reduce turbulent frictional drag by injecting high-viscosity fluid into the boundary layer. When the turbulent region of the boundary layer is filled with high-viscosity fluid, and the viscosity of the viscous sublayer is kept low, the Reynolds stress in the turbulent region is reduced and therefore requires a greater velocity gradient to transfer the momentum. The greater velocity gradient in the turbulent region results in a reduction of the velocity gradient at the viscous sublayer, which causes a drop in shear stress at the wall. Such a boundary-layer structure could be created by injecting two different fluids from double slits on a wall. Sugar syrup and water were used as the high-viscosity fluid and the low-viscosity fluid, respectively. The shear stress was directly measured by shear stress pick-ups mounted flush on the wall. The shearing stress was reduced by more than 50 percent at the optimum injection condition. A water/water injection experiment was also performed to show the effect of injection itself.
Drag Characteristics of Extra-Thin-Fin-Riblets in an Air Flow Conduit
Authors: Wang Z. Y. , Jovanovic J.
Published: May 2008

An experimental study of riblets with extra thin fins (5 μm thick) is presented. A drag reduction of 2–3 percent per quarter conduit wall is indicated when h+ is around 3–15 in a square section of air flow conduit lined with the extra-thin-fin-riblets (ETFR) on one side wall. The pressure distributions along the conduit indicate the influence of the riblet front step on the drag reduction performance in the conduit flow. The measurement methods and the detailed structure of the ETFR are also discussed.
Use of Subdomains for Inverse Problems in Branching Flow Passages
Authors: Agrawal Ajay K. , Krishnan S. , Yang Tah-teh
Published: May 2008

For inverse problems in complex flow passages, a calculation procedure based on a multizone Navier-Stokes method was developed. A heuristic approach was employed to derive wall shape corrections from the wall pressure error. Only two subdomains sharing a row of control volumes were used. The grid work in the common region was identical for both subdomains. The flow solver, inverse calculation procedure, multizone Navier-Stokes method and subdomain inverse calculation procedure were validated independently against experimental data or numerical predictions. Then, the subdomain inverse calculation method was used to determine the wall shape of the main duct of a branching flow passage. A slightly adverse pressure gradient was prescribed downstream of the sidebranch. Inverse calculations resulted in a curved wall diffuser for which the wall pressure distribution matched the design (prescribed) wall pressure distribution. The present method was illustrated for laminar, incompressible flows in branching passages. However, the method presented is flexible and can be extended for turbulent flows in multiply connected domains.
Streamwise Computation of Three-Dimensional Flows Using Two Stream Functions
Authors: Greywall M. S.
Published: May 2008

An approach to compute three-dimensional flows using two stream functions is presented. The independent variables used are χ, a spatial coordinate, and ξ and η, values of stream functions along two sets of suitably chosen intersecting stream surfaces. The dependent variables used are the streamwise velocity, and two functions that describe the stream surfaces. Since the value of a stream function is constant along the solid boundaries, this choice of variables makes it easy to satisfy the boundary conditions. To illustrate the approach, computations of incompressible potential flow through a circular-to-rectangular transition duct are also presented.
Partial Cavities: Global Behavior and Mean Pressure Distribution
Authors: Le Q. , Franc J. P. , Michel J. M.
Published: May 2008

The results of an experimental work concerning the behavior of flows with partial cavities are presented. The tests were carried out using a plano-convex foil placed in the free surface channel of the I.M.G. Hydrodynamic Tunnel. The experimental conditions concerning ambient pressure, water velocity, and body size were such that various and realistic kinds of flows could be realized. The main flow regimes are described and correlated to the values of foil incidence and cavitation parameter. Attention is paid to the shedding of large vapor pockets into the cavity wake and its possible periodic character. Aside from classical consideration to the cavity length and shedding frequency in the periodic regime, results concerning the wall pressure distribution in the rear part of the cavity are given. They lead to distinguish thin, stable, and closed cavities from the thick ones in which the reentrant jet plays a dominant role for the shedding of vortical structures and the flow unsteadiness.
Partial Cavities: Pressure Pulse Distribution Around Cavity Closure
Authors: Le Q. , Franc J. P. , Michel J. M.
Published: May 2008

Pressure pulse height spectra (PPHS) are measured in the case of partial cavitation attached to the leading edge of a hydrofoil. It is shown that the distributions of pressure pulses around cavity closure may significantly differ according to the type of cavity. In the case of a thin, well-closed and stable cavity, the pressure pulse distributions exhibit a strong maximum centered on the visible cavity termination. As the cavity becomes thicker and increasingly open and unsteady, the pressure pulse distribution widens. In the limit case of a cavity periodically shedding bubble clusters, no definite maximum in the pressure pulse distribution is observed. In addition, scaling of pressure pulse height spectra is approached from measurements at two different velocities. It is shown that the pressure pulse height spectra can be correctly transposed from a velocity to another one from two basic scaling rules concerning pulse heights and production rates of bubbles.
The Flow Structure and Statistics of a Passive Mixing Tab
Authors: Gretta W. J. , Smith C. R.
Published: May 2008

Water channel flow visualization and anemometry studies were conducted to examine the flow structure and velocity statistics in the wake of a passive mixing tab designed for enhancement of cross-stream mixing by generation of flow structures characteristic of turbulent boundary layers. Flow visualization reveals that the mixing tab generates a wake comprising a combination of counter rotating, streamwise vortices enveloped by distinct hairpin vortex structures. The counter rotating streamwise vortices are observed to stimulate a strong ejection of fluid along the symmetry plane, which results in very rapid cross-stream mixing. The hairpin vortices are found to undergo successive amalgamation and coalescence downstream of the device, which aids in the streamwise mixing and outward penetration of ejected fluid. After an initially intense mixing process, the mixing tab wake rapidly develops mean velocity, turbulence intensity, and boundary layer integral properties characteristic of a significantly thickened turbulent boundary layer.
Film-Thickness, Pressure-Gradient, and Turbulent Velocity Profiles in Annular Dispersed Flows
Authors: Skouloudis A. N. , Wu¨rtz J.
Published: May 2008

A regional model has been described for dispersed turbulent two-phase flow which accounts for the transverse variation of velocity. The two-phase turbulence parameters are introduced in direct analogy to well-known single-phase flow parameters which are then correlated to experimental data. The advantages of this approach are its simplicity and the absence of arbitrary parameters which need calibration at different experimental ranges. Its generality has been tested by comparisons at high and low operating pressures with air-water and steam-water mixtures. Comparisons between calculated and measured values have been carried out for the film thickness and the pressure gradient at different experimental setups.
Comparison of Numerical and Experimental Results for a Turbulent Flow Field With a Longitudinal Vortex Pair
Authors: Zhu J. X. , Fiebig M. , Mitra N. K.
Published: May 2008

A numerical simulation of a three-dimensional turbulent flow with longitudinal vortices embedded in the boundary layer on a channel wall is presented. The flow is described by the unsteady incompressible Reynolds averaged Navier-Stokes equations and the standard k–ε turbulence model. A finite difference scheme based on the SOLA-algorithm is developed for the numerical solution of the governing equations. Comparison with the experimental data of Pauley and Eaton (1988 a, b) shows that the numerical computations predict the general characteristics of the flow correctly. Agreement to within 13 percent is obtained for the worst location in mean velocity fields. The average deviation of predicted mean streamwise velocity from the experimental data is 3.6 percent.
Hydraulic Forces Acting on a Circular Cylinder With Surface Source of Minute Air Bubbles and Its Cavitation Characteristics
Authors: Ihara A. , Watanabe Hideo , Hashimoto Hiroyuki
Published: May 2008

Experiments are performed by using two circular cylinders made of porous filter material, that is, bronze sintered compact, with a different filter size, respectively, in a water tunnel. The effects of minute air bubbles injected from the surface of the circular cylinder into its boundary layer on its hydraulic force characteristics are investigated over the Reynolds number (1.2~4.2)×105, changing the airflow rate of bubble injection. The pressure distributions on the porous cylinders are also measured. Their cavitation characteristics are studied, too. Compared with a smooth-surface cylinder, the critical Reynolds numbers of the porous cylinders decrease owing to the increase in the surface roughness caused by the bronze particles which compose the sintered compact. The effects of the bubble injection on the hydraulic force characteristics are different according to the size of the bronze particles. Though a little difference is recognized, the hydraulic force characteristics show a similar tendency in both cases of bubble injection and cavitation occurrence.
The Torsion Effect on Fully Developed Laminar Flow in Helical Square Ducts
Authors: Chen Wen-Hwa , Jan Ray
Published: May 2008

The continuity equation and Navier-Stokes equations derived from a non-orthogonal helical coordinate system are solved by the Galerkin finite-element method in an attempt to study the torsion effect on the fully developed laminar flow in the helical square duct. Since high-order terms of curvature and torsion are considered, the approach is also applicable to the problems with finite curvature and torsion. The interaction effects of curvature, torsion, and the inclined angle of the cross section on the secondary flow, axial velocity, and friction factor in the helical square duct are presented. The results show that the torsion has more pronounced effect on the secondary flow rather than the axial flow. In addition, unlike the flow in the toroidal square duct, Dean’s instability of the secondary flow, which occurs near the outer wall in the helical square duct, can be avoided due to the effects of torsion and/or inclined angle. In such cases, a decrease of the friction factor is observed. However, as the pressure gradient decreases to a small value, the friction factor for the toroidal square duct is also applicable to the helical square duct.
Probabilistic Simulation of Fragment Dynamics and Their Surface Impacts in the SSME Turbopump
Authors: Hamed A. , Moy H.
Published: May 2008

Numerical simulations are used to model the dynamics of solid fragments in the high pressure fuel turbopump of the space shuttle main engine (SSME HPFTP). The simulations are conducted in a probabilistic structure in order to simulate the variance in the geometrical configuration, material properties and initial conditions of the solid fragments. The results of the particle dynamics simulations are presented in the form of the probability of surface impacts, and probability distributions of the impacting velocity and impingement angle.
Flow Measurements in a Model Burner—Part 2
Authors: Dura˜o D. F. G. , Heitor M. V. , Moreira A. L. N.
Published: May 2008

The isothermal swirling flow in the vicinity of a model oxy-fuel industrial burner is analyzed with laser-Doppler velocimetry together with laser-sheet visualization. The burner consists of a central axisymmetric swirling jet surrounded by sixteen circular jets, simulating the injection of oxygen in practical burners. The results extend those obtained for non-swirling flows, and presented in Part 1 of this paper, to the analysis of the dependence of the mixing efficiency of the burner assembly upon the swirl motion of the central jet and have the necessary detail to allow to assess the accuracy of calculation procedures of the flow in industrial burners. It is shown that swirl attenuates the three-dimensional structure typical of multijet flows in such a way that turbulence production and transport in the near burner zone are dominated by swirl-induced processes.
Correlation of Adiabatic Two-Phase Pressure Drop Data Using the Frictional Law of Corresponding States
Authors: Obot N. T. , Wambsganss M. W. , France D. M. , Jendrzejczyk J. A.
Published: May 2008

A method, based on that developed for single-phase flow, is proposed for the correlation of two-phase frictional pressure drop data. It is validated using air-water data obtained on small horizontal passages of rectangular and circular cross-section for values of total mass flux G in the 50-2000 kg/m2s range. The pressure drop for air-water mixtures can be predicted from the proposed correlations provided the critical quality (or superficial gas Reynolds number) and the critical pressure gradient for transition from bubble/plug-to-slug flow are known. A comparison of the proposed method with that of Lockhart and Martinelli is presented and discussed.
Transient Interface Shape of a Two-Layer Liquid in an Abruptly Rotating Cylinder
Authors: Lim Tae Gyu , Choi Sangmin , Hyun Jae Min
Published: May 2008

A description is made of the transient shape of interface of a two-layer liquid in an abruptly rotating circular cylinder. The density of the lower layer is higher than that of the upper layer, but the viscosities may assume arbitrary values. The overall Ekman number is much smaller than unity, and the cylinder aspect ratio is 0(1). The classical Wedemeyer model, which deals with the spin-up from rest of a homogeneous fluid, is extended to tackle the two-layer liquid system. If the upper-layer fluid is of higher viscosity, the interface, at small and intermediate times, rises (sinks) in the center (periphery). After reaching a maximum height at the center, the interface tends to the parabolic shape characteristic of the final-state rigid-body rotation. If the lower-layer fluid is of higher viscosity, the interface, at small and intermediate times, sinks (rises) in the center (periphery). The deformation at the center reaches a minimum height, after which the interface approaches the final-state parabola. The gross adjustment process is accomplished over the spin-up time scale, En−1/2Ω−1, where En and Ω denote the lower value of the Ekman numbers of the two layers and the angular velocity of the cylindrical container, respectively. These depictions are consistent with the physical explanations offered earlier. A turntable experiment is performed to portray the transient interface shape. The model predictions of the interface form are in satisfactory agreement with the laboratory measurements.
Unsteady Flow of a Power-Law Dusty Fluid With Suction
Authors: Chamkha Ali J.
Published: May 2008

Equations governing flow of a particulate suspension exhibiting finite volume fraction in non-Newtonian power-law fluids are developed and applied to the problem of unsteady flow past an infinite porous flat plate with suction. Numerical results for small volume fraction for the displacement thicknesses for both phases and the skin-friction coefficient for the fluid phase are obtained using an implicit finite difference scheme and presented graphically to elucidate interesting features of the solutions.
Fluid Dynamics of Sprays—1992 Freeman Scholar Lecture
Authors: Sirignano William A.
Published: May 2008

Various theoretical and computational aspects of the fluid dynamics of sprays are reviewed. Emphasis is given to rapidy vaporizing sprays on account of the richness of the scientific phenomena and the several, often disparate, time scales. Attention is given to the behavior of individual droplets including the effects of forced convection due to relative droplet-gas motion, Stefan convection due to the vaporization or condensation of the liquid, internal circulation of the liquid, interactions with neighboring droplets, and interactions with vortical eddies. Flow field details in the gas boundary layer and wake and in the liquid droplet interior are examined. Also, the determinations of droplet lift and drag coefficients and Nusselt and Sherwood numbers and their relationships with Reynolds number, transfer number, Prandtl and Schmidt numbers, and spacing between neighboring droplets are extensively discussed. The spray equations are examined from several aspects; in particular, two-continua, multi-continua, discrete-particle, and probabilistic formulations are given. The choice of Eulerian or Lagrangian representation of the liquid-phase equations within these formulations is discussed including important computational issues and the relationship between the Lagrangian method and the method of characteristcis. Topics for future research are suggested.
Relaxation of the Turbulent Boundary Layer After an Abrupt Change From Rough to Smooth Wall (Data Bank Contribution)
Authors: Taylor R. P. , Taylor J. K. , Hosni M. H. , Coleman H. W.
Published: May 2008

Measurements of velocity and turbulence intensity profiles and skin friction coefficient are presented for turbulent flat-plate boundary layer flow over a test surface with a rough-to-smooth step change in surface roughness. The first 0.9 m length of the test surface is roughened with 1.27 mm diameter hemispheres spaced 2 base diameters apart in a uniform staggered array, and the remaining 1.5 m length is smooth. The profiles are compared with previous data for all-rough cases under closely matched conditions in the same facility. The skin friction data are compared with previous data for both all-rough and all-smooth cases.
Ventilated Flow in the Unobstructed Space Between Corotating Disks in a Cylindrical Enclosure
Authors: Gor D. , Humphrey J. A. C. , Greif R.
Published: May 2008

An experimental investigation has been performed for the ventilated flow of air in the unobstructed space between the center pair of four disks corotating in a fixed cylindrical enclosure. This configuration is of fundamental interest and considerable practical utility in the computer industry. Time-resolved and time-averaged measurements of the circumferential velocity component were obtained using a laser-Doppler velocimeter in back-scatter mode. The data were collected along the radial coordinate direction on the midplane between the disks (Z = 0) and along the axial coordinate at two radial locations (R = 0.71 and 0.86). Three values of the Reynolds number (Re = 2.73 × 103, 2.22 × 104, and 2.66 × 105) were investigated for a limited but significant range of the Rossby number (|Ro| < 0.85), including air sucked radially inward (Ro < 0) and air blown radially outward (Ro > 0). The experimental data present a challenging target for numerical procedures purporting to predict this class of flows. In agreement with earlier preliminary calculations performed by Humphrey et al. (1992), the imposition of a radial ventilation condition in the experiments is observed to have a pronounced effect on the inter-disk flow characteristics. However, the predicted results were found to depend strongly on the boundary conditions imposed and present measurements show the importance of knowing these accurately. These and related findings are discussed with special consideration given to their potential impact on the improved design of ventilated disk storage systems.
On the Flow Structure Due to Corotating Disks With a Ventilated Hub
Authors: Girard John , Abrahamson Scott
Published: May 2008

The effects of hub ventilation on the flow structure between corotating disks enclosed in a stationary cylindrical shroud have been studied using quantitative flow visualization. The ventilation flow resulted from the pumping of the rotating disks. The geometry of the ventilation holes and the total open area were varied and the flow structure was compared to the solid hub case. Many of the features present in the solid hub case remained in modified form. The radial extent of the region purged by the through-flow was independent of the hub open area, which ranged from 3.4 to 37.5 percent of the total hub area. Additionally, the radial extent of the purged region was found to be independent of the Reynolds number.
Influence of Vibration Amplitude on Laminar Flow Over a Plate Vibrating at Low Strouhal Number
Authors: Venkat N. K. , Spaulding M.
Published: May 2008

The spectral and hydrodynamic response of laminar flow over a flat plate with a vibrating section forced in sinusoidal motion with a dimensionless amplitude ratio, H0 (vibration amplitude divided by plate length) varying in the range 0.0 < H0 < 0.1 is analyzed using numerical simulations. The Reynolds number, Re, based on the length of the vibrating plate, is fixed at 1000. The flow is simulated for Strouhal number, St, = 0.25 (low frequency). The spectral characteristics are obtained by performing Fast Fourier Transform (FFT) on the pressure coefficient time series data. The hydrodynamic analysis is performed by plotting stream function contour plot in the vicinity of the vibrating section for one vibration cycle. The model predicted results show that the friction and pressure coefficients over the vibrating body vary with vibration amplitude. For low amplitude ratios, the interaction of the external flow with the vibrating section is linear and there is little up or downstream influence. For high H0, there is considerable downstream influence of the disturbance. Nonlinear energy transfer, as evidenced by the existence of a significant first harmonic in the pressure wave, takes place between the vibrating plate and the flow field. Energy transfer to the higher harmonics is less significant.
Improved Approximate Method for Simulating Frequency-Dependent Friction in Transient Laminar Flow
Authors: Schohl G. A.
Published: May 2008

A new approximation to the weighting function in Zielke’s (1967) equation is used in an improved implementation of Trikha’s (1975) method for including frequency-dependent friction in transient laminar flow calculations. The new, five-term approximation was fitted to the weighting function using a nonlinear least squares approach. Transient results obtained using the new approximating function are nearly indistinguishable from results obtained using the exact expression for the weighting function.
A Study on the Noise Generation Mechanism in a Circular Air Jet
Authors: Tang S. K. , Ko N. W. M.
Published: May 2008

An investigation of the noise generation mechanism of turbulent structures in a subsonic air jet has been done theoretically as well as experimentally. Based on the results obtained, it is proposed that the rate of change of velocities of the jet structures plays an important role in the generation of sound in an air jet.
Experimental Observations of Flow Instability in a Helical Coil (Data Bank Contribution)
Authors: Webster D. R. , Humphrey J. A. C.
Published: May 2008

Experimental observations were made for the nominally fully developed flow through a helically coiled pipe of circular cross-section with a curvature radius to pipe radius ratio Rc/a = 18.2. Laser-Doppler measurements of the instantaneous streamwise velocity, uθ, and the cross-stream circumferential velocity, uφ, components were obtained along the midplane of the pipe cross-section. The Reynolds number range explored was 3800 < Re < 10500 (890 < De < 2460) and spans the laminar and turbulent flow regimes. Time integration of the velocity records has yielded previously unavailable mean and rms velocity profiles. In the range 5060 < Re < 6330, the time records of the velocity components reveal periodic flow oscillations with St ≈ 0.25 in the inner half of the pipe cross-section while the flow near the outer wall remains steady. A frequency doubling (St ≈ 0.5) is also observed at some midplane locations. This low frequency unsteadiness is distinct from the shear-induced turbulent fluctuations produced with increasing Re first at the outer wall and later at the inner wall of the coiled pipe. Simple considerations suggest that the midplane jet in the recirculating cross-stream flow is the source of instability.
Turbulent Swirling Flow in Short Cylindrical Chambers
Authors: Riahi A. , Hill P. G.
Published: May 2008

Turbulent swirling flow in a short closed cylindrical chamber has been measured with laser Doppler anemometry. The swirl was generated by a rotating roughened disk and measured during steady and transient conditions with a smooth disk. The velocity and turbulence fields were found to be strongly dependent on swirl Reynolds numbers (in the range 0.3 × 106 < ΩR2/v < 0.6 × 106) and on chamber length-to-diameter ratio (in the range 0.1 ≤ L/D ≤ 0.5). With a roughened disk the flow was nearly independent of Reynolds number though still strongly dependent on chamber length-to-diameter ratio.
Added Mass Coefficients for Submerged Bodies by a Low-Order Panel Method
Authors: Sahin I. , Crane J. , Watson K.
Published: May 2008

The added mass coefficients for two and three-dimensional submerged bodies were calculated using a low-order panel code. The source and dipole strengths, and the panel surface area for each panel, were used to compute the integrals needed for added mass in all six degrees of motions. Several applications of this method were used in comparing the results with the theoretical, when available, experimental or other numerical results. The method was found to be successful in predicting the added mass coefficients using relatively low numbers of panels.
Prediction of Three-Dimensional Steady Incompressible Flows Using Body-Fitted Coordinates
Authors: Tamamidis P. , Assanis D. N.
Published: May 2008

A finite-volume method for three-dimensional, steady, incompressible flows in complex geometries is presented. The method uses generalized Body-Fitted Coordinates to accurately take into account the shape of the boundary. A collocated scheme is employed, which uses the three covariant velocities and the pressure as main variables. Continuity is coupled with the momentum equations using the SIMPLEC algorithm. It is found that the SIMPLEC algorithm can provide savings in computational time of up to 40 percent compared to calculations with SIMPLE. Sensitivity studies are also performed to find optimum values of the underrelaxation parameters. The method is validated against experimental results for the case of the flow in a 90 deg curved duct of square cross-section and comparatively strong curvature. The application of the method to the prediction of flows in complex geometries is then illustrated.
Interpretation of Impeller Flow Calculations
Authors: Tuzson J.
Published: May 2008

Most available computer programs are analysis and not design programs. Therefore the intervention of the designer is indispensable. Guidelines are needed to evaluate the degree of fluid mechanic perfection of a design which is compromised for practical reasons. A new way of plotting the computer output is proposed here which illustrates the energy distribution throughout the flow. The consequences of deviating from optimal flow pattern is discussed and specific cases are reviewed. A criterion is derived for the existence of a jet/wake flow pattern and for the minimum wake mixing loss.
Turbulent Flows in a Model SDR Combustor
Authors: Liou T.-M. , Wu Y.-Y.
Published: May 2008

An experimental study is reported on the isothermal flow fields in a model solid-propellant ducted rocket combustor with two opposing side inlets. The measurements were made by using a four-beam two-color laser-Doppler velocimeter (LDV). Three values of momentum ratio (Ma/Ms) of the axial- to side-inlet jet—0.025, 0.11, and 1.28—were selected to investigate their effects on the flow characteristics. The Reynolds number, based on the air density, combustor height, and bulk velocity, was 4.56 × 104. The flow field was characterized in terms of the mean-velocity vectors and contours, joint probability functions, mean reattachment lengths, spreading rate of the axial-jet width, and Reynolds stress and turbulence kinetic energy contours. The LDV measured mean reattachment lengths were found to well agree with the corresponding flow-visualization photograph. In addition, the three Ma/Ms values provided three characteristic flows which are useful in testing the computational models. Further, correlations between the present cold-flow and previous reading-flow studies were documented in detail. It was found that from the fluid dynamic point of view Ma/Ms = 0.11 was preferable to the other two values of Ma/Ms.
Simulation of the Nonreacting Flow in a Bluff-Body Burner; Effect of the Diameter Ratio
Authors: Martins Luis-Filipe , Ghoniem Ahmed F.
Published: May 2008

Axisymmetric vortex simulation is used to study the unsteady dynamics of the flowfield generated by the interaction between two concentric jets initially separated by a thick bluff-body. The computational scheme treats convective transport in a Lagrangian sense by discretizing the vorticity into a number of finite-area vortex ring elements which move along particle trajectories during each convective substep, thus reducing the numerical diffusion and allowing simulations at high Reynolds number. In this paper, investigation is focused on the time-dependent dynamics and the effect of the diameter ratio across the bluff-body on the wake flow. In both cases simulated, the dynamics is governed by the shedding of large vortex eddies from the inner and outer sides of the bluff-body. Mixing between the two streams is enhanced by the merging of these eddies downstream the bluff-body and the formation of composite structures. We find that the frequency of shedding, the level of fluctuations and the degree of organization are strongly dependent on the diameter ratio. The fluctuation associated with this shedding increases as the diameter ratio becomes larger. The origin and mechanism of shedding in each case are determined from the results.
Analysis of Flow Induced Vibration Using the Vorticity Transport Equation
Authors: Marn Jure , Catton Ivan
Published: May 2008

Crossflow induced vibrations are the subject of this work. The analysis is two dimensional. The governing equations for fluid motion are solved using linearized perturbation theory and coupled with the equations of motion for cylinders to yield the threshold of dynamic instability for an array of cylinders. Parametric analysis is performed to determine the lowest instability threshold for a rotated square array and correlations are developed relating the dominant parameters. The results are compared with theoretical and experimental data for similar arrays and the discrepancies are discussed.
A Note on Laminar Flow in Uniformly Porous Tubes
Authors: Schnitzlein K.
Published: May 2008

A numerical analysis of the steady, laminar, incompressible flow in a constant diameter tube with uniform mass suction at the wall is made to study the development of the flow for the entire range of wall Reynolds numbers. It is shown that for small suction rates the state of flow can reach the similarity condition at least at distinct positions at the tube axis. For higher suction rates similarity profiles are found to occur only if the flow exhibits an asymmetric behavior with respect to the point of complete mass extraction.
Propeller Tip Vortex Cavitation Suppression Using Selective Polymer Injection
Authors: Chahine G. L. , Frederick G. F. , Bateman R. D.
Published: May 2008

This paper presents results of experiments where selective injection of a drag-reducing polymer solution into the tip vortex region of the blades of an 11.5 in. diameter propeller was effective in significantly delaying tip vortex cavitation. The most critical phase of the investigation was the selection of the position of the injection ports. For well-positioned injection ports, at a fixed water channel speed the propeller cavitation number had to be decreased by as much as 35 percent in order to reestablish cavitation inception. Injections of water and a viscous mixture of water and glycerin for the same conditions did not affect the inception characteristics of the modified blades. Preliminary analysis of the results indicates that the viscoelastic properties of the Polyox solution injected in the vortex core played a significant role in thickening the viscous core of the tip vortex and thus reducing the pressure drop at the vortex center without affecting circulation or lift.
A Comparison of Four Velocity Bias Correction Techniques in Laser Doppler Velocimetry
Authors: Gould R. D. , Loseke K. W.
Published: May 2008

Data from a two-component dual-beam laser-Doppler Velocimeter were used to evaluate four different velocity bias correction methods. The data were collected over a range of valid data rates from 100 Hz to 20 kHz at four flow locations in a shear layer. Turbulence intensities ranged from 15 to 100 percent. Flow statistics were calculated using these proposed velocity bias correction techniques. The results were compared to the unbiased values, which were obtained using an approximated equal-time sampling technique. Some of the correction schemes performed well at low turbulence levels, but became ineffective at high turbulence levels. One ad hoc empirical correction scheme showed relatively good results throughout.
Holographic Visualization of Convective Flow Around a Heated Rotating Cone of Finite Length
Authors: Tieng S. M. , Wang Y. C.
Published: May 2008

The convective flow around a heated rotating cone of finite length is investigated experimentally by means of 3-D and real-time holographic interferometry at rotational speeds of up to 4600 rpm and rotational angles from 0 to 90 deg. This advanced optical technique allows visualization of the thermal boundary layer, it’s unstable transition region and the turbulent zones, as well as providing a detailed visual analysis of the holographic fringe patterns, information about the thickness of the thermal boundary layer, and a new method for detecting and determining the critical points and critical Reynolds numbers of the transition region. The results are compared with previously obtained experimental data.
“Heavy” Particle Dispersion Measurements With Mono- and Polydisperse Particle Size Distributions
Authors: Ferguson J. R. , Stock D. E.
Published: May 2008

A method is presented to estimate the effects of a polydisperse particle size distribution on the measured turbulent dispersion of particles. In addition, the analysis provides a means to estimate the standard deviation of the size distribution for which a class of particles may be considered monodisperse. If monodisperse particles are unavailable because of practical considerations (e.g., the required standard deviation of particle size is too small to obtain a sufficient quantity) then the method provides a means to correct the data of near monodisperse size distributions to reflect the dispersion of monodisperse particles.
Capillary Instability of a Jet of Liquid Metal
Authors: Wallace David B.
Published: May 2008

A system based on ink-jet technology has been used to demonstrate the controlled generation of monodispersed 100 μm diameter molten solder droplets. Oxide formation on the surface of the molten solder jet was shown to have a drastic effect on the droplet formation process. If the oxygen is not removed from the environment, no jet break up occurs. The growth rate of a radial disturbance on a capillary jet of molten solder is similar in magnitude to that predicted by Rayleigh and Weber theory, but the agreement is not good enough to say that these theories are valid for the liquid metal jet.
Perspective—Aerodynamic Control of Combustion
Authors: Oppenheim A. K.
Published: May 2008

To do useful work, the exothermic process of combustion should be carried out in an enclosure, as is typically the case with i.c. engines—the subject of this paper’s particular concern. To meet the requirements of high efficiency and low pollutant production, this process should be executed at a relatively low temperature—a condition attainable by the use of lean air-fuel mixtures. For this purpose it has to be distributed in space upon multipoint initiation and kept away from the walls to minimize their detrimental effects. In principle, all this can be accomplished by a system referred to as fireball combustion that takes advantage of entrainment and spiral mixing associated with large scale vortex structures of jet plumes. As demonstrated in this paper, the success in such an endeavor depends crucially upon the utilization of the essential elements of classical aerodynamics: the properly distributed sources, expressed in terms of velocity divergences prescribed by the thermodynamic process of combustion and of the vorticity field generated by shear between the jets and the fluid into which they are injected.
Tilt Rotor Broadband Hover Aeroacoustics
Authors: Coffen C. D. , George A. R.
Published: May 2008

To improve the acoustic characteristics of tilt rotor aircraft, the dominant noise mechanisms must be understood. Toward this goal, a method was developed to identify and predict the dominant broadband noise mechanism of a hovering tilt rotor. Predictions are presented for a range of azimuthal observer locations and polar observer angles and are compared to NASA full scale tilt rotor hover noise data. Comparisons between experiment and prediction indicate that the polar and azimuthal directionality trends are captured. The predicted sound spectrum levels are generally within 5 dB of the experiment. The results of this study indicate that the highly turbulent recirculating fountain flow is the dominant broadband noise mechanism for a tilt rotor aircraft in hover.
Direct Calculation of Sound Radiated From Bodies in Nonuniform Flows
Authors: Atassi H. M. , Fang J. , Patrick S.
Published: May 2008

Sound radiated from a single airfoil and a cascade of airfoils in three-dimensional gusts is directly calculated. Euler’s equations are linearized about the mean flow of the airfoil or cascade. The velocity field is split into a vortical part and a potential part. The latter is governed by a single nonconstant-coefficient convective wave equation. For a single airfoil, the radiated sound is calculated using Kirchhoff’s method from the mid field of the unsteady pressure obtained through the unsteady aerodynamic solver. The results indicate the importance of the contribution of the quadrupole effects to the sound field. For a cascade of airfoils, the acoustic pressure is directly obtained by solving the partial differential equation. The results show that, as the maximum Mach number on the blade surface nears unity, there is a significant rise in the local unsteady pressure, and also a significant increase in the upstream acoustic pressure.
Response of a Thin Airfoil Encountering a Strong Density
Discontinuity
Authors: Marble F. E.
Published: May 2008

Airfoil theory for unsteady motion has been developed extensively assuming the undisturbed medium to be of uniform density, a restriction accurate for motion in the atmosphere, Glauert (1929), Burgers (1935), Theodorsen (1935), Kussner (1936), Karman and Sears (1938), Kinney and Sears (1975). In some instances, notably for airfoils comprising fan, compressor and turbine blade rows, the undisturbed medium may carry density variations or “spots,” resulting from non-uniformities in temperature or composition, of a size comparable to the blade chord. This condition exists for turbine blades, Marble (1975), Giles and Krouthen (1988), immediately downstream of the main burner of a gas turbine engine where the density fluctuations of the order of 50 percent may occur. Disturbances of a somewhat smaller magnitude arise from the ingestion of hot boundary layers into fans, Wortman (1975), and exhaust into hovercraft. Because these regions of non-uniform density convect with the moving medium, the airfoil experiences a time varying load and moment which we propose to calculate.
The Interaction of Coherent Vortices With Short Flat Plates (Data Bank Contribution)
Authors: Swirydczuk J. , Wilder M. C. , Telionis D. P.
Published: May 2008

The interaction between a vortex and a small plate, are studied experimentally in a water tunnel using a computer-controlled laser-Doppler velocimeter. The interaction is proved to be a strong source of secondary and tertiary vorticity, the formation of which can be controlled by a selection of the plate dimensions, and cross-sectional shape. It is demonstrated that shorter plates with sharp edges are more efficient in breaking up oncoming vortical structures, while they create lower levels of secondary and tertiary vorticity. Such devices could be employed to control the amplitude of pressure fluctuations and the generation of noise in many practical applications.
The Rayleigh Problem for the Interior of a Torus
Authors: Rae W. J. , Ollila C. J.
Published: May 2008

This paper contains a description of the low Reynolds number flow inside a torus which has been set impulsively in motion about its central axis. The moving wall drags with it a primary flow confined to a sheath that grows steadily toward the center of the torus cross section. The primary flow in turn produces centrifugal and Coriolis accelerations which lead to secondary flows in the cross-sectional plane. At very long time the secondary flows subside, and the primary flow approaches a condition of solid-body rotation. The present analysis treats this problem in the thin-torus limit, where the cross-sectional radius is small compared to the toroidal radius, and is restricted to wall velocities small enough to support a low Reynolds-number assumption. At this level of approximation, the flow is characterized by a single dimensionless parameter, analogous to the Dean number.