0

IN THIS ISSUE

### TECHNICAL PAPERS

J. Fluids Eng. 2004;128(2):209-215. doi:10.1115/1.2169814.

Recent developments in the methodology of large-eddy simulation applied to turbulent, reacting flows are reviewed, with specific emphasis on mixture-fraction-based approaches to nonpremixed reactions. Some typical results are presented, and the potential use of the methodology in applications and the future outlook are discussed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):216-222. doi:10.1115/1.2170125.

An important source of vibration and noise in piping systems is the fluctuating wall pressure produced by the turbulent boundary layer. One approach to calculating the wall pressure fluctuations is to use a stochastic model based on the Poisson pressure equation. If the model is developed in the wave-number domain, the solution to the wave-number-frequency spectrum can be expressed as an integral of the turbulent sources over the boundary layer thickness. Models based on this formulation have been reported in the literature which show good agreement with measured pressure spectra, but they have relied on adjustable “tuning” constants to account for the unknown properties of the turbulent velocity fluctuations. A variation on this approach is presented in this paper, in which only well-known “universal” constants are used to model the turbulent velocity spectrum. The resulting pressure spectrum predictions are shown to be in good agreement with canonical data sets over a wide range of Reynolds numbers.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):223-231. doi:10.1115/1.2170126.

Flow in annular space occurs in drilling operation of oil and gas wells. The correct prediction of the flow of the drilling mud in the annular space between the well wall and the drill pipe is essential to determine the variation in the mud pressure within the wellbore, the frictional pressure drop, and the efficiency of the transport of the rock drill cuttings. A complete analysis of this situation is extremely complex: the inner cylinder is usually rotating, the wellbore wall will depart significantly from cylindrical, the drill pipe is eccentric, and the eccentricity varies along the well. A complete analysis of this situation would require the solution of the three-dimensional momentum equation and would be computationally expensive and complex. Models available in the literature to study this situation do consider the rotation of the inner cylinder and the non-Newtonian behavior of the drilling fluids, but assume the relative position of the inner with respect to the outer cylinders fixed, i.e., they neglect the variation of the eccentricity along the length of the well, and the flow is considered to be fully developed. This approximation leads to a two-dimensional model to determine the three components of the velocity field in a cross-section of the annulus. The model presented in this work takes into account the variation of the eccentricity along the well; a more appropriate description of the geometric configuration of directional wells. As a consequence, the velocity field varies along the well length and the resulting flow model is three-dimensional. Lubrication theory is used to simplify the governing equations into a two-dimensional differential equation that describes the pressure field. The results show the effect of the variation of the eccentricity on the friction factor, maximum and minimum axial velocity in each cross section, and the presence of azimuthal flow even when the inner cylinder is not rotating.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):232-238. doi:10.1115/1.2170123.

In this paper, large-eddy simulation of the transition process in a separation bubble is compared to experimental results. The measurements and simulations are conducted under low freestream turbulence conditions over a flat plate with a streamwise pressure distribution typical of those encountered on the suction side of turbine airfoils. The computational grid is refined to the extent that the simulation qualifies as a “coarse” direct numerical simulation. The simulations are shown to accurately capture the transition process in the separated shear layer. The results of these simulations are used to gain further insight into the breakdown mechanisms in transitioning separation bubbles.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):239-246. doi:10.1115/1.2170128.

In order to assess the capability of the Sutton model to evaluate aero-optical effects in a turbulent boundary layer, large-eddy simulation (LES) evolving spatially and Reynolds averaged Navier-Stokes (RANS) computations are carried out at Mach number equal to 0.9. First aerodynamic fields are proved to compare favorably with theoretical and experimental results. Once validated, the characteristics of the boundary layer allow us to obtain information concerning optical beam degradation. On the one hand, the density field is used to compute phase distortion directly and, on the other hand, by means of the Sutton model. Therefore, LES and RANS simulations allow us to study optical models and the validity of their assumptions. Finally, LES is proved to be considered as a reference tool to evaluate aero-optical effects.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):247-257. doi:10.1115/1.2169813.

Experiments are described in which well-defined weak Free Stream Nonuniformity (FSN) is introduced by placing fine wires upstream of the leading edge of a flat plate. Large amplitude spanwise thickness variations form in the boundary layer as a result of the interaction between the steady laminar wakes from the wires and the leading edge. The centerline of a region of elevated layer thickness is aligned with the centerline of the wake in the freestream and the response is shown to be remarkably sensitive to the spanwise length-scale of the wakes. The region of elevated thickness is equivalent to a long narrow low speed streak in the layer. Elevated Free Stream Turbulence (FST) levels are known to produce randomly forming arrays of long narrow low speed streaks in laminar boundary layers. Therefore the characteristics of the streaks resulting from the FSN are studied in detail in an effort to gain some insight into bypass transition that occurs at elevated FST levels. The shape factors of the profiles in the vicinity of the streak appear to be unaltered from the Blasius value, even though the magnitude of the local thickness variations are as large as 60% of that of the undisturbed layer. Regions of elevated background unsteadiness appear on either side of the streak and it is shown that they are most likely the result of small amplitude spanwise modulation of the layer thickness. The background unsteadiness shares many of the characteristics of Klebanoff modes observed at elevated FST levels. However, the layer remains laminar to the end of the test section $(Rx≈1.4×106)$ and there is no evidence of bursting or other phenomena associated with breakdown to turbulence. A vibrating ribbon apparatus is used to examine interactions between the streak and Tollmien-Schlichting (TS) waves. The deformation of the mean flow introduced by the streak is responsible for substantial phase and amplitude distortion of the waves and the breakdown of the distorted waves is more complex and it occurs at a lower Reynolds number than the breakdown of the $K$-type secondary instability that is observed when the FSN is not present.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):258-265. doi:10.1115/1.2169818.

In the present study, the flow in a rotating helical pipe with an elliptical cross section is considered. The axes of the elliptical cross section are in arbitrary directions. Using the perturbation method, the Navier-Stokes equations in a rotating helical coordinate system are solved. The combined effects of rotation, torsion, and geometry on the characteristics of secondary flow and fluid particle trajectory are discussed. Some new and interesting conclusions are obtained, such as how the number of secondary flow cells and the secondary flow intensity depends on the ratio of the Coroilis force to the centrifugal force. The results show that the increase of torsion has the tendency to transfer the structure of secondary flow into a saddle flow, and that the incline angle $α$ increases or decreases the secondary flow intensity depending on the resultant force between the Corilois force and centrifugal force.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2006;128(2):266-275. doi:10.1115/1.2171713.

The dynamic response, stability, and noise characteristics of fluid components and systems can be strongly influenced by the inertance of the fluid in passageways, which are often of complex geometry. The inertance is a parameter that has often proved to be very difficult to accurately quantify, either theoretically or experimentally. This paper presents a method of numerical calculation of the inertance in a passageway, assuming inviscid, incompressible flow and zero mean flow. The method is simple to apply and can be applied to geometries of arbitrary complexity. Two simple but unorthodox ways of calculating inertance using a computational fluid dynamics and a finite element solid-modeling package are also demonstrated. Results are presented for a simple cylindrical orifice, a simple spool valve, and a conical poppet valve. The effect of the inertance on the response of a poppet valve is demonstrated.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):276-283. doi:10.1115/1.2169811.

Thin films of oil flowing down a nearly-vertical plate were subjected to a strong normal electrostatic field. Steady-state height profiles were measured by fluorescence imaging. For electrode potentials less than that required to produce an instability, the two-dimensional response of the interface was $<1%$. Calculations of the fluid height coupled with the electric field solution were identical to uncoupled calculations for electric fields below the stability threshold. Pressure profiles under the film and three-dimensional effects are also discussed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):284-296. doi:10.1115/1.2169816.

An algorithm based on the combination of time-derivative preconditioning strategies with low-diffusion upwinding methods is developed and applied to multiphase, compressible flows characteristic of underwater projectile motion. Multiphase compressible flows are assumed to be in kinematic and thermodynamic equilibrium and are modeled using a homogeneous mixture formulation. Compressibility effects in liquid-phase water are modeled using a temperature-adjusted Tait equation, and gaseous phases (water vapor and air) are treated as an ideal gas. The algorithm is applied to subsonic and supersonic projectiles in water, general multiphase shock tubes, and a high-speed water entry problem. Low-speed solutions are presented and compared to experimental results for validation. Solutions for high-subsonic and transonic projectile flows are compared to experimental imaging results and theoretical results. Results are also presented for several multiphase shock tube calculations. Finally, calculations are presented for a high-speed axisymmetric supercavitating projectile during the important water entry phase of flight.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):297-304. doi:10.1115/1.2170124.

In this paper, general motion of a two-dimensional body is modeled using a new moving mesh concept. Solution domain is divided into the three zones. The first zone, with a circular boundary, includes the moving body and facilitates the rotational motion of it. The second zone, with a square boundary, includes the first zone and facilitates the translational motion of the body. The third zone is a background grid in which the second zone moves. With this configuration of grids any two-dimensional motion of a body can be modeled with almost no grid insertion or deletion. However, in some stages of motion we merge or split a few number of elements. The discretization method is control-volume based finite-element, and the unsteady form of the Euler equations are solved using AUSM algorithm. To demonstrate the excellent performance of the present method two moving cases including rotational and translational motions are solved. The results show excellent agreement with experimental data or other numerical results.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):305-315. doi:10.1115/1.2170122.

Microscopic particle image velocimetry (microPIV) was used to measure velocities in rectangular microchannels with aspect ratios ranging from 0.97 to 5.69 for $200. Mean velocity profiles, velocity fluctuations, and Reynolds stresses were determined from the microPIV data. Transition to turbulence was observed at $Re=1765–2315$ for the five aspect ratios studied, agreeing very well with both recent microscale experiments and macroscale duct flow and indicating no evidence of early transition for any of the aspect ratios studied. The onset of fully turbulent flow was observed at $Re=2600–3200$. For the fully turbulent flow, the $⟨u′⟩∕umax$ and $⟨v′⟩∕umax$ fluctuations at the channel centerline were 6% and 3%–3.5% and generally agreed well with macroscale results. As aspect ratio increased, the $⟨u⟩∕umax$ and $⟨u′⟩∕umax$ profiles became flatter, with nearly uniform values extending for some distance from the centerline of the channel. This region of uniform $⟨u⟩∕umax$ and $⟨u′⟩∕umax$ became larger with increasing aspect ratio. The Reynolds shear stress for fully turbulent flow also displayed a strong dependence on aspect ratio. For the $W∕H=0.97$ microchannel, $⟨u′v′⟩∕umax2$ steadily increased in value moving from the centerline to the wall, but for the higher aspect ratio microchannels, $⟨u′v′⟩∕umax2$ remained close to zero in the center region of the microchannel before increasing in value at locations close to the wall, and this region of near zero $⟨u′v′⟩∕umax2$ became larger with increasing aspect ratio. This behavior in the Reynolds shear stress is due to the region of uniform velocity and, hence, small mean shear, near the channel centerline of the high aspect ratio microchannels.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2006;128(2):316-325. doi:10.1115/1.2170132.

A new method to predict traveling bubble cavitation inception is devised. The crux of the method consists in combining the enhanced predictive capabilities of large-eddy-simulation (LES) for flow computation with a simple but carefully designed stability criterion for the cavitation nuclei. For LES a second-order accurate finite element model based on the Galerkin/least-squares method with Runge-Kutta time integration is applied. The incoming nucleus’ spectrum is approximated by a Weibull distribution. Moreover, it is shown that under typical conditions the stability of the nuclei can be evaluated with an algebraic criterion emerging from the Rayleigh-Plesset equation. This criterion can be expressed as modified critical Thoma number and fits well into the LES approach. The method was applied to study cavitation inception in a flow past a square cylinder. A good agreement with experimental results was achieved. Furthermore, the principal advantage over statistical (time-averaged) methods could be clearly demonstrated, even though the spatial resolution and application of the LES were restricted by limited computational resources. As the latter keep on growing, a wider range of applications will become accessible methods for cavitation prediction based on algebraic stability criteria combined with LES.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):326-331. doi:10.1115/1.2169808.

The present paper illustrates the main results of an experimental campaign conducted in the Thermal Cavitation Tunnel of the Cavitating Pump Rotordynamic Test Facility (CPRTF) at Centrospazio/Alta S.p.A. Experiments were carried out on a NACA 0015 hydrofoil at various incidence angles, cavitation numbers, and freestream temperatures, in order to investigate the characteristics of cavitation instabilities and the impact of thermal cavitation effects. Measured cavity length, surface pressure coefficients, and unsteady pressure spectra are in good agreement with the data available in the open literature and suggest the existence of a strong correlation between the onset of the various forms of cavitation and instabilities, the thermal cavitation effects, and the effects induced by the presence of the walls of the tunnel. Further analytical investigations are planned in order to provide a better interpretation of the above results.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):332-340. doi:10.1115/1.2169809.

Flow fields near the exit and the global performance parameters of the various types of axial flow fans are studied with Particle Image Velocimetry and a standard AMCA 210 flow bench. The fans used in this study included the shrouded, shroudless, and winglet-blade types. The velocity vectors, streamlines, vorticity contours, velocity distributions, and performances are presented and discussed. The flow patterns on the radial and axial planes show that a vortex always exists near the exit of the fans at various impeller angles. The experimental results demonstrate that the shrouded fan with winglets has the most stable flow field and the best fan performance.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):341-349. doi:10.1115/1.2169815.

In very low specific speed range $(ns<0.25)$, the efficiency of the centrifugal pump designed by the conventional method becomes remarkably low. Therefore, positive-displacement pumps have been widely used for long. However, the positive-displacement pumps remain associated with problems such as noise and vibration and they require high manufacturing precision. Since the recently used centrifugal pumps are becoming higher in rotational speed and smaller in size, there appear to be many expectations to develop a new centrifugal pump with high performance in the very low specific speed range. The purpose of this study is to investigate the internal flow characteristics and its influence on the performance of a very low specific speed centrifugal pump. The results show that large reverse flow at the semi-open impeller outlet decreases absolute tangential velocity considerably which in turn decreases the pumping head.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):350-358. doi:10.1115/1.2170120.

The aim of the paper is to present the results of investigations conducted on the free surface flow in a Pelton turbine model bucket. Unsteady numerical simulations, based on the two-phase homogeneous model, are performed together with wall pressure measurements and flow visualizations. The results obtained allow defining five distinct zones in the bucket from the flow patterns and the pressure signal shapes. The results provided by the numerical simulation are compared for each zone. The flow patterns in the buckets are analyzed from the results. An investigation of the momentum transfer between the water particles and the bucket is performed, showing the regions of the bucket surface that contribute the most to the torque. The study is also conducted for the backside of the bucket, evidencing a probable Coanda interaction between the bucket cutout area and the water jet.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):359-369. doi:10.1115/1.2170121.

In this work, a numerical model has been applied in order to obtain the wall pressure fluctuations at the volute of an industrial centrifugal fan. The numerical results have been compared to experimental results obtained in the same machine. A three-dimensional numerical simulation of the complete unsteady flow on the whole impeller-volute configuration has been carried out using the computational fluid dynamics code FLUENT® . This code has been employed to calculate the time-dependent pressure both in the impeller and in the volute. In this way, the pressure fluctuations in some locations over the volute wall have been obtained. The power spectra of these fluctuations have been obtained, showing an important peak at the blade passing frequency. The amplitude of this peak presents the highest values near the volute tongue, but the spatial pattern over the volute extension is different depending on the operating conditions. A good agreement has been found between the numerical and the experimental results.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):370-377. doi:10.1115/1.2169810.

This paper discusses the requirements for the propulsion system of supersonic cruise aircraft that are quiet enough to fly over land and operate from civil airports, have trans-pacific range in the order of $11,112km$$(6000nmi)$, and payload in the order of $4545kg$$(10,000lb.)$. It is concluded that the resulting requirements for both the fuel consumption and engine thrust/weight ratio for such aircraft will require high compressor exit and turbine inlet temperatures, together with bypass ratios that are significantly higher than typical supersonic-capable engines. Several technologies for improving both the fuel consumption and weight of the propulsion system are suggested. Some of these directly reduce engine weight while others, by improving individual component performance, will enable higher bypass ratios. The latter should therefore also indirectly reduce the bare engine weight. It is emphasized, however, that these specific technologies require considerable further development. While the use of higher bypass ratio is a significant departure from more usual engines designed for supersonic cruise, it is nonetheless considered to be a practical option for an aircraft of this kind.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):378-387. doi:10.1115/1.2169807.

Two-dimensional and quasi-3D in-flight ice accretion simulation codes have been widely used by the aerospace industry for the last two decades as an aid to the certification process. The present paper proposes an efficient numerical method for calculating ice shapes on simple or complex 3D geometries. The resulting ice simulation system, FENSAP-ICE, is built in a modular fashion to successively solve each flow, impingement and accretion via field models based on partial differential equations (PDEs). The FENSAP-ICE system results are compared to other numerical and experimental results on 2D and slightly complex 3D geometries. It is concluded that FENSAP-ICE gives results in agreement with other code calculation results, for the geometries available in the open literature.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):388-393. doi:10.1115/1.2170127.

We investigate theoretically the periodic shear environment of a cone-and-plate bioreactor. The imposed frequency is designated to reflect the periodic nature of mammalian cardiac cycles. The working formula obtained for the distribution of shear stresses shall be of substantial interest for applying periodic shear stresses to cell cultures in vitro.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2006;128(2):394-401. doi:10.1115/1.2170131.

When nanostructured powder particles are used for thermal spray coatings, the retention of the original nanostructure that is engineered into the raw stock is a principal objective, along with production of some molten material in order to adhere the sprayed material to the surface being coated. Therefore, in contrast with spraying conventional powders, complete melting of the nanostructured raw stock is to be avoided. In this study, the melting and resolidification of sprayed material is correlated to a spray processing parameter that has been introduced in the literature by some of the spray processing practitioners. Using computer modeling, processing of zirconia agglomerates with plasma spraying has been simulated. Transition regions for the phase change response of the sprayed material to the thermal processing conditions are identified. The retained nanostructure content and liquid fraction of the sprayed material are correlated to particle diameters, injection velocities, as well as this thermal spray processing parameter. Finally, a novel method to produce desired coatings composed of partially molten material using a bimodal particle size distribution of the sprayed powder is suggested.

Commentary by Dr. Valentin Fuster

### TECHNICAL BRIEFS

J. Fluids Eng. 2005;128(2):402-405. doi:10.1115/1.2170129.

This study proposes a new concept for quantifying the energy of flowing compressed air, called air power. Air power is defined as the work-producing potential of compressed air, and its definition and general equation are presented. The properties of air power are also discussed. Air power is comprised of two components, transmission power and expansion power, while air temperature and kinetic energy can generally be neglected.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(2):406-409. doi:10.1115/1.2170130.

$Ck$

amplitude of error at $kth$ iteration

$i$

$−1$

$k$

iteration index

$M$

number of nodes

$PeL$

Peclet number based on the total length $L$

$Ṡϕ$

production rate of scalar $ϕ$

$U$

fluid velocity vector (m/s)

$x$

space variable (m)

$α$

inertial damping factor

$δ$

nondimensional grid spacing

$εk$

error between exact numerical solution and solution at $kth$ iteration

$λ$

eigenvalue of iteration matrix

$λmax$

$ϕ$

general scalar

$ρ$

mixture density $(kg∕m3)$

$θ$

$Γϕ$
diffusion coefficient $(m2∕s)$