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IN THIS ISSUE

### TECHNICAL PAPERS

J. Fluids Eng. 2005;128(3):413-420. doi:10.1115/1.2173289.

The time-dependent fluid flow in a square cavity was studied using model fluids of glycerol-water solution at different frequencies and amplitudes of motion of the top plate. The range of Reynolds numbers in our investigation varied from 5 to 3700. The experiments were carried out in a square cavity with a periodically driven lid, and planar velocity measurements were obtained using particle image velocimetry. The flow was driven by moving the top surface of the cavity in a simple harmonic motion. The aspect ratio, defined as the ratio of cavity length to the cavity height, is unity. The ratio of cavity spanwise width to the length of the cavity is 0.2. The temporal variation of velocity at fixed locations in the cavity exhibits a periodic variation. The basic frequency of the fluid motion at a point in the flow domain was observed to be the same as that of plate motion for low Reynolds number Re. However, existence of dominant secondary frequencies was observed along the central vertical plane. The velocity variation as a function of time at a fixed position and the velocity profiles along horizontal and vertical planes are also quantitatively described. These were compared to computational fluid dynamics (CFD) simulations based on the finite volume technique. Comprehensive details of the flow as a function of Reynolds number are analyzed. The evolution of secondary vortices at different plate positions as a function of Reynolds number is also presented. The planar velocity measurements acquired are indicative of the flow behavior in a periodically driven cavity with a narrow span width even at high Re. At very low Re, the flow throughout the periodically driven cavity qualitatively resembles the classical steady lid-driven cavity flow. At high Re, the entire cavity is occupied with multiple vortices. The qualitative features of the bulk flow observed are valid even for cavities with infinite span width.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):421-429. doi:10.1115/1.2173290.

Tip leakage vortex cavitations originating from the tip clearance of an oscillating hydrofoil were observed experimentally. It was found that the delay between the unsteady and the steady-state results of the tip leakage vortex cavitation increase, and that the maximum cavity size decreases when the reduced oscillating frequency increases. To simulate the unsteady characteristics of tip leakage vortex cavitation, a simple calculation based on slender body approximation was conducted taking into account the effect of cavity growth. The calculation and experimental results of the cavity volume fluctuation were found to be in qualitative agreement.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):430-438. doi:10.1115/1.2173291.

It has been shown by experimental and numerical studies that various cavitation instabilities occur in inducers for rocket engines when the cavity length exceeds about 65% of the blade spacing. On the other hand, it has been pointed out by an experimental study that the cavitation instabilities occur when the pressure gradient near the throat becomes small to some degree. The present study is motivated to examine the latter criterion based on pressure gradient for cavitation instabilities from the viewpoint of theoretical analysis. For this purpose, analyses of steady flow and its stability were carried out for cavitating flow in cascades with circular arc and plano-convex blades by a singularity method based on closed cavity model. It was found that the criterion based on the cavity length for the occurrence of cavitation instabilities is more adequate than the criterion based on the pressure gradient. It was also found that the steady cavity length and the stability of the flow in both cascades can be practically correlated with a parameter $σ∕[2(α−α0)]$, where $σ$ is a cavitation number, $α$ is an angle of attack, and $α0$ is a shockless angle of attack.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):439-445. doi:10.1115/1.2173292.

Characteristics of small vortices were studied in axisymmetric jets wherein the Kolmogorov scale was approached by progressively decreasing the Reynolds number while still maintaining turbulent flow. A periodic forcing introduced far upstream of the jet nozzle ensured that the jet was turbulent. A vortex eduction tool was developed and applied to the high-pass filtered $2D$ velocity field in the axial plane of a turbulent jet while varying Re between 140 and 2600. Vortex population, energy, vorticity, and rms (root-mean-square velocity fluctuations) of the high-pass filtered field were measured to elucidate vortex characteristics. The observed population of vortices decreases dramatically at the Kolmogorov scale. The observed increase in vortex population with decreasing vortex size appears to be in accord with the space-filling argument, in that the vortex population in a two-dimensional domain should grow as $R−2$. The energy density curve obtained from vortex statistics reproduces the $−5∕3$ slope for the inertial subrange, and the high-pass filtered field accounts for approximately two-thirds of the total rms.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):446-453. doi:10.1115/1.2173293.

Centrifugal fans with an electric motor included in the hub are commonly used in HVAC (heating, ventilation, and air conditioning) systems. A design of experiments (DOE) has been performed to study the effect of the entrance conditions of a backward-inclined centrifugal fan on its efficiency. The parameters involved are the base radius of the motor hub, the radius of the fan entry section, the deceleration factor throughout the entry zone (from the entry of the fan to the entry of the blade), and the solidity factor. Numerical simulation coupled with the DOE has been used for the sensitivity analysis of the entrance parameters. Initially, a complete factorial plan $(24)$ was performed to screen the most influent parameters and interactions. This has shown that the motor’s cap radius, as well as its interactions with other parameters, is not significant. A second DOE, using composite central design (CCD which has a second order of accuracy) has then been performed on the remaining parameters (radius of the fan entry section, deceleration factor, and the solidity factor). The effects of these parameters and their interactions on the fan efficiency are now presented. A linear regression with three parameters has been performed to establish the efficiency distribution map. The methodology employed is validated by comparing the predicted results from the DOE and those from the numerical simulation of the corresponding fan.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):454-462. doi:10.1115/1.2173294.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):463-466. doi:10.1115/1.2175157.

This paper presents an image processing technique in order to predict the shape of a levitated aspherical droplet. The technique is of great importance to containerless materials processing. A majority of the electromagnetic levitation techniques utilizes two cameras at right angles to observe both transversal and frontal views. This allows obtaining two images of the droplet at instant time. In many cases, the portion of the frontal image is missing due to the heating coil. The newly developed technique allows restoration of the missing portion of the image information. The through image can be reconstructed by combining the recovered shapes. A special computer program is generated to simulate a normalized volume of the droplet.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):467-480. doi:10.1115/1.2174058.

Static mixers are increasingly being used to perform a variety of mixing tasks in industries, ranging from simple blending to complex multiphase reaction systems. Use of static mixers to process non-Newtonian fluids is quite common. Data on the pressure drop of non-Newtonian fluids in static mixers and the degree of mixing of materials through the mixer are very useful in the design and engineering application of these tools. This paper extends a previous study by the authors on an industrial helical static mixer and illustrates how static mixing processes of single-phase viscous liquids can be simulated numerically. A further aim is to provide an improved understanding of the flow pattern of pseudoplastic liquids through the mixer. A three-dimensional finite volume simulation is used to study the performance of the mixer. The flow velocities, pressure drops, etc., are calculated for various flow rates, using the Carreau and the power law models for non-Newtonian fluids. The numerical predictions by these two models are compared to existing experimental data. Also, a comparison of the mixer performance for both Newtonian and pseudoplastic fluids is presented. The effects of the Reynolds number of the flow and also properties of pseudoplastic fluids on the static mixer performance have been studied. It is shown that for the materials studied here, the fluid type is not effective on the degree of mixing. It is also shown that the fluid type has a major impact on the pressure drop across the mixer.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):481-487. doi:10.1115/1.2174059.

The velocity and pressure field of a ship’s Weis-Fogh-type propulsion mechanism are studied in this paper using an advanced vortex method. The wing (NACA0010 airfoil) and channel are approximated by source and vortex panels, and free vortices are introduced away from the body surfaces. The viscous diffusion of fluid is represented using the core-spreading model to the discrete vortices. The velocity is calculated on the basis of the generalized Biot-Savart law and the pressure field is calculated from an integral, based on the instantaneous velocity and vorticity distributions in the flow field. Two-dimensional unsteady viscous flow calculations of this propulsion mechanism are shown, and the calculated results agree qualitatively with the measured thrust and drag due to unmodeled large fluctuations in the measured data.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):488-493. doi:10.1115/1.2174060.

A water channel has been used as a statistically steady experiment to investigate the development of a buoyant plane wake. Parallel streams of hot and cold water are initially separated by a splitter plate and are oriented to create an unstable stratification. At the end of the splitter plate, the two streams are allowed to mix and a buoyancy-driven mixing layer develops. The continuous, unstable stratification inside the developing mixing layer provides the necessary environment to study the buoyant wake. Downstream a cylinder was placed at the center of the mixing layer. As a result the dynamic flows of the plane wake and buoyancy-driven mixing layer interact. Particle image velocimetry and a high-resolution thermocouple system have been used to measure the response of the plane wake to buoyancy driven turbulence. Velocity and density measurements are used as a basis from which we describe the transition, and return to equilibrium, of the buoyancy-driven mixing layer. Visual observation of the wake does not show the usual vortex street associated with a cylinder wake, but the effect of the wake is apparent in the measured vertical velocity fluctuations. An expected peak in velocity fluctuations in the wake is found, however the decay of vertical velocity fluctuations occurs at a reduced rate due to vertical momentum transport into the wake region from buoyancy-driven turbulence. Therefore for wakes where buoyancy is driving the motion, a remarkably fast recovery of a buoyancy-driven Rayleigh-Taylor mixing in the wake region is found.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):494-506. doi:10.1115/1.2174061.

The effects of oscillatory backpressure on the air induction system for pulse detonation engines were examined for a two-dimensional, mixed-compression configuration at a freestream Mach number of 3.5. The pressure perturbations at the diffuser exit were produced by injecting air through four ports located at the corners of the exit cross section. The frequency, coupling of the ports and airflow rates through the ports were varied, simulating the operation of detonation tubes. A terminal normal shock in the diffuser oscillated in the excited inlet, causing large pressure fluctuation amplitudes at some locations. Large injection mass flows resulted in inlet flow oscillations throughout the inlet, increased the spillage, yet did not cause inlet unstart.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):507-519. doi:10.1115/1.2174062.

Particle tracking velocimetry has been used to measure the velocity fields of both continuous phase and dispersed microbubble phase, in a turbulent boundary layer, of a channel flow. Hydrogen and oxygen microbubbles were generated by electrolysis. The average size of the microbubbles was $15μm$ in radius. Drag reductions up to 40% were obtained, when the accumulation of microbubbles took place in a critical zone within the buffer layer. It is confirmed that a combination of concentration and distribution of microbubbles in the boundary layer can achieve high drag reduction values. Microbubble distribution across the boundary layer and their influence on the profile of the components of the liquid mean velocity vector are presented. The spanwise component of the mean vorticity field was inferred from the measured velocity fields. A decrease in the magnitude of the vorticity is found, leading to an increase of the viscous sublayer thickness. This behavior is similar to the observation of drag reduction by polymer and surfactant injection into liquid flows. The results obtained indicate that drag reduction by microbubble injection is not a simple consequence of density effects, but is an active and dynamic interaction between the turbulence structure in the buffer zone and the distribution of the microbubbles.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):520-530. doi:10.1115/1.2175158.

The flow around an isolated wheel in contact with the ground is computed by the Unsteady Reynolds-Averaged Navier-Stokes (URANS) method. Two cases are considered, a stationary wheel on a stationary ground and a rotating wheel on a moving ground. The computed wheel geometry is a detailed and accurate representation of the geometry used in the experiments of Fackrell and Harvey. The time-averaged computed flow is examined to reveal both new flow structures and new details of flow structures known from previous experiments. The mechanisms of formation of the flow structures are explained. A general schematic picture of the flow is presented. Surface pressures and pressure lift and drag forces are computed and compared to experimental results and show good agreement. The grid sensitivity of the computations is examined and shown to be small. The results have application to the design of road vehicles.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):531-540. doi:10.1115/1.2175159.

In this paper we propose a method to reconstruct the flow at a given time over a region of space using partial instantaneous measurements and full-space proper orthogonal decomposition (POD) statistical information. The procedure is tested for the flow past an open cavity. 3D and 2D POD analysis are used to characterize the physics of the flow. We show that the full 3D flow can be estimated from a 2D section at an instant in time provided that some 3D statistical information—i.e., the largest POD modes of the flow— is made available.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):541-547. doi:10.1115/1.2175160.

This paper presents the results of experimental and numerical studies of the hot distortion phenomenon in the phenolic urethane cold box systems used in metal casting. Dual Pushrod Dilatometry has been used to measure a thermal expansion/contraction of phenolic urethane cold box sand core specimens at temperatures ranging from $20°C$ to $600°C$. High temperature tensile tests showed that the tensile strength of the phenolic urethane cold box sand cores is significantly affected by the bench life, temperature and binders level. High temperature hot distortion furnace tests on cylindrical cores showed that some coatings increase the temperature limit when distortion starts, but application of coating cannot prevent distortion. The hot distortion test during metal casting showed that regardless of the application of coating, the type of coating, and anti-veining additives, all cores with density greater than the density of the molten metal (magnesium alloy) were significantly distorted. Numerical simulations of the liquid metal flow around the cylindrical sand core and analysis of dynamic forces acting on the core during the fill process showed that a buoyancy force is the major contributor to the hot distortion. It is concluded that the one of the solutions in preventing the hot distortion of sand cores is optimizing their weight, which will balance the buoyancy force and will bring the resultant force to the minimum. The hot distortion test castings using optimized sand cores with density almost equal to the density of the molten magnesium proved our predictions, and hot distortion has been prevented.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):548-558. doi:10.1115/1.2175161.

Alternate power law velocity profile $u+=Aζα$ in transitional rough pipe fully turbulent flow, has been proposed, in terms of new appropriate inner rough wall variables ($ζ=Z+∕ϕ$, $uϕ=u∕ϕ$), and new parameters $Rϕ=Rτ∕ϕ$ termed as the roughness friction Reynolds number, $Reϕ=Re∕ϕ$ termed as the roughness Reynolds number and $ϕ$ termed as roughness scale (along with normal wall coordinate $Z=y+ϵr$ where $ϵr$ is the shift of the origin of boundary layer due to the rough wall, $Z+=Zuτ∕ν$ and $u+=u∕uτ$). The envelope of the power law shows that the power law constants $α$ and $A$ depend on the parameter $Rϕ$ (i.e., $α=α(Rϕ)$ and $A=A(Rϕ)$) but explicitly independent of the wall roughness parameter $h∕δ$ (roughness height $h$ in pipe of radius $δ$). The roughness scale $ϕ$ has been related to the roughness function $ΔU+$ of Clauser representing the velocity shift caused by wall roughness. The present results of the velocity profile, just slightly above the wall roughness level $h$, remain valid for all types of wall roughness. The data of Nikuradse for sand-grain roughness, in transitional and fully rough pipes, has been considered, which provides good support to the predictions of an alternate power law velocity profile, based on single parameter $Rϕ$, the roughness friction Reynolds number.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):559-567. doi:10.1115/1.2175162.

Turbulence investigations of the flow past an unswept wing at a high angle of attack are reported. Detailed predictions were carried out using large-eddy simulations (LES) with very fine grids in the vicinity of the wall in order to resolve the near-wall structures. Since only a well-resolved LES ensures reliable results and hence allows a detailed analysis of turbulence, the Reynolds number investigated was restricted to $Rec=105$ based on the chord length $c$. Admittedly, under real flight conditions $Rec$ is considerably higher (about $(35–40)∙106$). However, in combination with the inclination angle of attack $α=18$ deg this $Rec$ value guarantees a practically relevant flow behavior, i.e., the flow exhibits a trailing-edge separation including some interesting flow phenomena such as a thin separation bubble, transition, separation of the turbulent boundary layer, and large-scale vortical structures in the wake. Due to the fine grid resolution applied, the aforementioned flow features are predicted in detail. Thus, reliable results are obtained which form the basis for advanced turbulence analysis. In order to provide a deeper insight into the nature of turbulence, the flow was analyzed using the invariant theory of turbulence by Lumley and Newman (J. Fluid Mech., 82, 161–178, 1977). Therefore, the anisotropy of various portions of the flow was extracted and displayed in the invariant map. This allowed us to examine the state of turbulence in distinct regions and provided an improved illustration of what happens in the turbulent flow. Thus, turbulence itself and the way in which it develops were extensively investigated, leading to an improved understanding of the physical mechanisms involved, not restricted to a standard test case such as channel flow but for a realistic, practically relevant flow problem at a moderate Reynolds number.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):568-578. doi:10.1115/1.2175163.

The effects of surface roughness on the aerodynamic performance of a turbine vane are investigated for three Mach number distributions, one of which results in transonic flow. Four turbine vanes, each with the same shape and exterior dimensions, are employed with different rough surfaces. The nonuniform, irregular, three-dimensional roughness on the tested vanes is employed to match the roughness which exists on operating turbine vanes subject to extended operating times with significant particulate deposition on the surfaces. Wake profiles are measured for two different positions downstream the vane trailing edge. The contributions of varying surface roughness to aerodynamic losses, Mach number profiles, normalized kinetic energy profiles, Integrated Aerodynamics Losses (IAL), area-averaged loss coefficients, and mass-averaged loss coefficients are quantified. Total pressure losses, Mach number deficits, and deficits of kinetic energy all increase at each profile location within the wake as the size of equivalent sandgrain roughness increases, provided the roughness on the surfaces is uniform. Corresponding Integrated Aerodynamic Loss IAL magnitudes increase either as Mach numbers along the airfoil are higher, or as the size of surface roughness increases. Data are also provided which illustrate the larger loss magnitudes which are present with flow turning and cambered airfoils, than with symmetric airfoils. Also described are wake broadening, profile asymmetry, and effects of increased turbulent diffusion, variable surface roughness, and streamwise development.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):579-586. doi:10.1115/1.2175164.

The discrete-element surface roughness model is used to provide insight into the importance of the mean elevation of surface roughness in predicting skin friction over rough surfaces. Comparison of experimental data and extensive computational results using the discrete-element model confirm that the appropriate surface for the imposition of the no-slip condition is the mean elevation of the surface roughness. Additionally, the use of the mean elevation in the Sigal-Danberg approach relating their parameter to the equivalent sand-grain roughness height results in replacing three different piecewise expressions with a single relation. The appropriate mean elevation for closely-packed spherical roughness is also examined.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):587-595. doi:10.1115/1.2175165.

The boundary-layer separation and wake structure of a NACA 0025 airfoil and the effect of external excitations in presence of structural vibrations on airfoil performance were studied experimentally. Wind tunnel experiments were carried out for three Reynolds numbers and three angles of attack, involving hot-wire measurements and complementary surface flow visualization. The results establish that external acoustic excitation at a particular frequency and appropriate amplitude suppresses or reduces the separation region and decreases the airfoil wake, i.e., produces an increase of the lift and∕or decrease of the drag. The acoustic excitation also alters characteristics of the vortical structures in the wake, decreasing the vortex length scale and coherency. Optimum excitation frequencies were found to correlate with the fundamental frequencies of the naturally amplified disturbances in the separated shear layer. The results suggest that acoustic waves play a dominant role in exciting the separated shear layer of the airfoil. Moreover, low-frequency structural vibrations are found to have a significant effect on airfoil performance, as they enhance the sound pressure levels within the test section.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):596-601. doi:10.1115/1.2175166.

The boundary-layer feature and the forces on the particle are analyzed in detail, and the motion parameters of the particle in the gas-solid rotary flow are divided into two parts according to the $r-z$ meridian and $r-θ$ cross section. The Lagrange method is then applied, the 3-D mathematical model of particle motion in the gas-solid rotary flow is presented, and the Gear integral method is applied to simulate the motion characteristics of the particles. The results show that the centrifugal force and Saffman lift force play important roles in the process of the particle being separated from the gas-solid rotary flow in the rotary boundary layer. The velocity gradient of radial direction is the biggest, and that of tangent direction is the smallest. For a higher density ratio of gas to solid, the deposition performance of the particle depends not only on the inlet flow velocity but also on the range of the particle diameter. Reasonable velocity gradient matching of the three directions $(r,z,θ)$ in the gas-solid rotary flow is useful to improve the separation efficiency of the rotary separators.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):602-610. doi:10.1115/1.2175167.

The development and testing of a rotating single-disk viscous pump are described. This pump consists of a $10.16mm$ diameter spinning disk, and a pump chamber, which are separated by a small gap that forms the fluid passage. The walls of the pump chamber form a C-shaped channel with an inner radius of $1.19mm$, an outer radius of $2.38mm$, and a depth of 40, 73, 117, or $246μm$. Fluid inlet and outlet ports are located at the ends of the C-shaped channel. Experimental flow rate and pressure rise data are obtained for rotational speeds from $100to5000rpm$, fluid chamber heights from $40to246μm$, flow rates from $0to4.75ml∕min$, pressure rises from $0to31.1kPa$, and fluid viscosities from $1to62mPas$. An analytical expression for the net flow rate and pressure rise, as dependent on the fluid chamber geometry, disk rotational speed, and fluid viscosity, is derived and found to agree with the experimental data. The flow rate and pressure rise of the pump vary nearly linearly with rotational speed. The volumetric flow rate does not change significantly with changes in fluid viscosity for the same rotational speed and pumping circuit. Advantages of the disk pumps include simplicity, ease of manufacture, ability to produce continuous flow with a flow rate that does not vary significantly in time, and ability to pump biological samples without significant alteration or destruction of cells, protein suspension, or other delicate matter.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):611-617. doi:10.1115/1.2175168.

The effects of localized blowing through a porous strip on a turbulent channel were studied experimentally. The measurements were conducted downstream of the porous strip for three blowing rates: 3%, 5%, and 8% (of the velocity at the centerline of the channel). It was found that the injection affects several turbulence parameters. Indeed, blowing decreases the skin friction while it increases the turbulence intensities and the Reynolds stresses. A study of cross-correlations in the streamwise and spanwise direction’s showed that both inclination angles in the $(x,y)$ and $(x,z)$ planes were increased with blowing.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):618-627. doi:10.1115/1.2175169.

The paper investigates the effect of channel aspect ratio on the flow performance of a newly introduced spiral-channel viscous micropump. An approximate 2D analytical solution for the flow field, which ignores channel curvature but accounts for a finite wall height, is first developed at the lubrication limit. A number of 3D models for spiral pumps with different aspect ratios are then built and analyzed using the finite volume method. Numerical and analytical results are in good agreement and tend to support one another. The results are compared with an approximate 2D analytical solution developed for infinite aspect ratio, which neglects the effect of side walls, and assumes uniform velocity distribution across the channel width. The error in this approximation was found to exceed 5% for aspect ratios less than 10. Pressure and drag shape factors were introduced in the present work to express the effect of the pressure difference and boundary velocity on the flow rate at various aspect ratios for both moving and stationary walls. Also, it has been found numerically that the flow rate varies linearly with both the pressure difference and boundary velocity, which supports the validity of the linear lubrication model employed.

Commentary by Dr. Valentin Fuster

### TECHNICAL BRIEFS

J. Fluids Eng. 2005;128(3):628-631. doi:10.1115/1.2175170.
Lachmann, G. V., 1961, Boundary Layer and Flow Control. Its Principles and Application, Pergamon Press, New York.Simpson, R. J., 1966, “ Aspects of Turbulent Boundary Layer Separation,” Prog. Aeronaut. Sci.PRAEAQ 0079-6026, 32, pp. 457–521.Chang, P. K., 1976, Control of Boundary Layer Separation, McGraw-Hill, New York.Bragg, M. B., and Gregorek, G. M., 1987, “ Experimental Study of Airfoil Performance with Vortex Generators,” J. Aircr.JAIRAM 0021-8669, 4, pp. 305–309.Simpson, R. Y., 1989, “ Turbulent Boundary Layer Separation,” Annu. Rev. Fluid Mech.ARVFA3 0066-4189[[XSLOpenURL/10.1146/annurev.fl.21.010189.001225]], 21, pp. 205–234.Gad-el-Haq, M., and Bushnell, D. M., 1991, “ Separation Control: Review,” J. Fluids Eng.JFEGA4 0098-2202, 113, pp. 1–28.Gad-el-Haq, M., 2000, Flow Control, Cambridge University Press, Cambridge, UK.Rao, D. M., and Koriya, 1988, “ Boundary-Layer Submerged Vortex Generators for Separation Control—An Exploratory Study,” AIAA Paper 88-3546CP, pp. 839–846.McCormick, D. C., 1993, “ Shock/Boundary-Layer Interaction Control with Vortex Generation and Passive Cavity,” AIAA J.AIAJAH 0001-1452, 31(1), pp. 91–96.Wallis, R. A., and Stuard, C. M., 1962, “ On the Control of Shock-Induced Boundary Layer Separation with Discrete Air Jets,” Aerodynamic Research Council C.P. No. 595.Johnson, J. P., and Nishi, M., 1990, “ Vortex Generator Jets—Means for Flow Separation Control,” AIAA J.AIAJAH 0001-1452, 28(6), pp. 989–994.Szumowski, A., and Wojciechowski, J., 2005, “ Use of Vortex Generators to Control Internal Supersonic Flow Separation,” AIAA J.AIAJAH 0001-1452, 43(1), pp. 216–218.Urzynicok, F., 2003, “ Separation Control by Flow-Induced Oscillations of a Resonator,” Ph.D. thesis, T.U. Berlin, D83; edocs.tu-berlin.de/diss/2003/urzynicko_frank.htmGreenblatt, D., and Wygnanski, I. J., 2000, “ The Control of Flow Separation by Periodic Oscillation,” Prog. Aerosp. Sci.PAESD6 0376-0421, 36, pp. 487–545.
Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):632-637. doi:10.1115/1.2175171.

The authors gratefully acknowledge the financial support of the Centre for Microelectronics Assembly and Packaging, CMAP and the Natural Sciences and Engineering Research Council of Canada, NSERC. Our thanks go to Mr. K. Narimani for his helpful comments on Sec. 4.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2005;128(3):638-642. doi:10.1115/1.2175172.

Permeability $(K)$ and form coefficient $(C)$ are the characteristic hydraulic properties of any porous medium. They are determined simultaneously, for known fluid thermo-physical properties by using the Hazen-Dupuit-Darcy model (HDD) to curve-fit the longitudinal global pressure-drop versus average fluid speed data from an isothermal, steady flow, hydraulic experiment across a test section of the porous medium. The $K$ and $C$ thus measured are global parameters, i.e., valid for the entire porous medium and universal provided the flow throughout the porous medium is of plug flow nature. We report here experimental evidence on the influence of non-plug flow velocity profiles at the inlet, on the simultaneous determination of $K$ and $C$ of fissure- and rod bundle-type porous inserts. Although variation in $K$ is minimal, as much as 12.1% variation in $C$ is observed, when going from a fully developed velocity profile to a plug flow profile at the inlet.

Commentary by Dr. Valentin Fuster

### DISCUSSIONS

J. Fluids Eng. 2005;128(3):643-645. doi:10.1115/1.2175173.
FREE TO VIEW

The paper by Uchiyama and Fukase (1) proposes a three-dimensional vortex method for particulate flows, which is applied to a circular jet of air laden with glass particles. The numerical method presented has the following characteristics: (1) it includes the two-way coupling of the particles and the fluid, (2) it discretizes the vorticity in the gas phase in the usual manner when vortex blob methods are used, and (3) it accounts for the effects of viscosity in the fluid by means of the less-than-usual core spreading method.

Commentary by Dr. Valentin Fuster