Sidewall Geometry Research Articles

Experimental Study on Band Dispersion in Channels Structured with Micropillars

The band dispersion in channels with an orderly pillar structure with a pressure-driven liquid flow was determined. Several channels with different geometries were etched in a silicon wafer and enclosed by a glass wafer. The microchannels obtained had the same depth, pillar disposition, and overall porosity, but different pillar diameters and channel widths. The broadening of narrow bands of a fluorescent sample solution flowing through the channels was measured using a fluorescence microscope. It was shown that the peak dispersion occurring in the channels can be much lower than in conventional packed columns, as a consequence of the higher degree of order of the solid structure. Reduced plate heights of approximately 0.2 could be obtained for (nonretained) bands. No correlation was found between the aspect ratio (the ratio of the channel width and the pillar diameter) and band dispersion. The geometrical construction of the sidewall region was shown to play a critical role for channel performance. A good agreement was found with predictions for the optimal sidewall geometry obtained previously with simulation studies.

The mechanical strength of polysilicon films: Part 2. Size effects associated with elliptical and circular perforations

A systematic study of failure initiation in small-scale specimens has been performed to assess the effect of size scale on “failure properties” by drawing on the classical analysis of elliptically perforated specimens. Limitations imposed by photolithography restricted the minimum radii of curvature of the specimen perforations to one micron. By varying the radius of curvature and the size of the ellipses, the effects of domain size and stress concentration amplitude could be assessed separately to the point where the size of individual grains (∼0.3 μm) becomes important. The measurements demonstrate a strong influence of the domain size under elevated stress on the “failure strength” of MEMS scale specimens, while the amplitude, or the variation, of the stress concentration factor is less significant. In agreement with probabilistic considerations of failure, the “local failure strength” at the root of a notch clearly increases as the radius of curvature becomes smaller. Accordingly, the statistical scatter also increases with decreasing size of the (super)stressed domain. When the notch radius becomes as small as 1 μm the failure stress increases on average by a factor of two relative to the tension values derived from unnotched specimens. This effect becomes moderate for larger radii of curvature, up to a radius of 8 μm (25 times the grain size), for which the failure stress at the notch tip closely approaches the value of the tensile strength for un-notched tensile configurations. We deduce that standard tests, performed on micron-sized, non-perforated, tension specimens, provide conservative strength values for design purposes. In addition, a Weibull analysis shows for surface-micromachined specimens a dependence of the strength on the specimen length, rather than the surface area or volume, which implies that the sidewall geometry, dimensions and surface conditions can dominate the failure process.

Dynamics and structure of transitional buoyant jet diffusion flames with side-wall effects

Transitional buoyant jet diffusion flames are investigated by 3D spatial direct numerical simulations (DNS). To examine the fuel source and sidewall geometry effects, a comparative study has been performed with four computational cases: a free-square and a free-round case in an open boundary domain, and a corner-square and a corner-round case in a half-confined domain. A grid system of 192 × 192 × 288 nodes is employed to obtain the near field flame structure. Due to the absolute buoyancy instability and the geometry effects, the jet diffusion flames display unsteady dynamic behavior. The free-square case is more vortical and entrains more efficiently than the free-round case because of the self-induced vortex deformation associated with the noncircular source geometry. The sidewall effects are found to be significant on the flame structure because of the wall-induced vortex deformation. Compared with the free cases, the corner cases have a stronger tendency of transition to turbulence. The corner-round case has a much stronger wall-induced vortex deformation than the corner-square case. Although the corner cases are half-confined in the cross-streamwise directions, entrainment of the corner cases is not lower than that of the free cases due to the wall-induced vortex deformation. In contrast with the comparison between the free-round and free-square cases, entrainment of the corner-round case is higher than that of the corner-square case.

Numerical study of laminar-turbulent transition on a plate in a low-speed tunnel with contoured wall

Laminar-turbulent transition flow phenomena on a flat plate in a low-speed wind tunnel at different Reynolds numbers were studied numerically. The flow calculation is based on an inviscid/boundary layer interaction method with modified Abu-Ghannam/Shaw(AGS) transition criterion. The test section has non-symmetrical contoured walls, and the plate is located biased the bottom side with a height ratio of 26:14. In test case of steady flow, a laminar-turbulent transition takes place and a small separation bubble occurs on the upper side of the plate, when the inlet Reynolds number is as small as 0.631×10−6. The predicted transition location agrees well with that of the test results, but the separation bubble is hardly to see from the calculated velocity profiles though the printed data of velocity in this region do show the negative values. The further numerical predictions with different Reynolds numbers corresponding to the incoming flow velocities show that when the Reynolds number is greater than 1.379×10−6, the separation bubble does not occur, which is coincident with the experimental results. The influence of the side wall geometry on the transition on the plate is also studied.

Numerical study of the performance of swept, curved compression surface scramjet inlets

A computational performance enhancement study was performed employing systematic modifications to a planarsidewall compression scramjet inlet operating at an entrance Mach number of 4 and at a dynamic pressure of 2040 psf. The variations included modifying the planar-side wall compression angle as a function of height, utilizing sidewall curvature, and employing, simultaneously, both forward-swept and reverse-swept compression surfaces. Turbulent flowfield solutions were generated by solving the Reynolds-averaged Navier-Stokes equations to obtain inlet performance parameters such as total-pressure recovery, mass capture, and flowfield pressure distortion (the ratio of maximum static pressure to minimum static pressure generated at the inlet exit plane). Additionally, an inviscid parametric study was performed by employing solutions to the Euler equations to optimize a cubic polynomial that defined the longitudinal sidewall geometry. A final viscous flowfield solution of the optimized inviscid inlet geometry yielded inlet performance improvements; however, inlet top-wall surface boundary-layer shock wave separation interactions persisted. Hence, this numerical study demonstrated that enhanced performance is obtainable via curved-wall geometric modifications to the standard planar-sidewall inlet design, although future work should employ constraints to mitigate detrimental flow separation effects.

Laboratory simulation of exchange through Fram Strait

Laboratory experiments and theory were conducted to observe the flow patterns and transport in both buoyancy‐driven and wind‐driven rotating fluids. In “lock‐exchange” experiments, water with one density flows into a second basin after a sliding gate is removed. Water of a second density flows back into the first basin. The size and location of the currents for various values of density difference, rotation rate, and assorted sidewall geometries was recorded. Volume flux of the fluid was also measured and compared with a theory for lock‐exchange flow of a rotating fluid. In a separate group of experiments with a passive upper layer, easterly winds (like those in the Arctic Ocean) drive the upper level water into the Arctic Ocean and therefore oppose the buoyant exchange. Westerly winds would drive the water out of the Arctic Ocean. This indicates that the exchange between the Arctic Ocean and the Greenland‐Norwegian Sea is likely to be driven by buoyancy rather than by driven by wind. Crude estimates of the volumetric and fresh water exchange rate from the lock‐exchange formulas are compared with observed ocean fluxes, and approximate agreement is found.

Heteroep1Taxial Nucleation and Structural Properties of MBE GaAs on Recessed Si: Etching Implications.

AbstractGaAs is grown embedded in pre-etched wells in the Si substrate. HNO3:HF, KOH:H2O and a dry etch technique are used as alternatives to form the wells. Cross-sectional SEM views of AlGaAs/GaAs heterostructures reveal the growth front and facet formation for the different sidewall geometries. Transmission electron microscopy is used to study epilayer degradation in relation to the substrate damage and the presence of edge induced defects in the GaAs. Cathodoluminescence reveals the uniformity of strain present in the GaAs layer embedded in the wet etched well. The impact of the different etching techniques on device integration is briefly discussed.

Discharge characteristics of local, discontinuous contractions

This first investigation on discharge characteristics in local discontinuous contractions with straight channel side-wall geometry considers theoretically the effects of the inlet angle, the outlet angle, the relative contraction width and length using an elementary hydraulic approach. This study accounts for critical flow conditions at the hydrodynamically contracted cross-section. Results are compared with observations and a good agreement is found. Design diagrams then enable a rapid determination of global flow features in the considered channel transitions.

Critical flow condition in open channel hydraulics

Transition between sub- and supercritical flows in open channel occurs at the so-called critical point, for which critical flow conditions appear. This particular flow state has been originally introduced for flows with parallel streamlines. However, as streamlines are considerably sloped and inclined in the vicinity of the critical point, these effects have to be accounted for. The present investigation explores in detail these effects. In particular, present investigations include i) first order analysis expressing discharge in terms of upstream head and channel geometry (calibration of discharge measurement structures) and ii) determination of surface profiles for transitional flow states by accounting for the effective pressure and velocity distributions. Analysis is based on a first order model with restriction to typical channel bottom and sidewall geometry. Results are compared with observations, and a fair agreement between the two is noted.

Combutational studies of end-wall flame quenching at low pressure: The effects of heterogeneous radical recombination and crevices

A computational study of the effect of heterogeneous radical destruction and crevices on end-wall flame quenching in a lean CH 4O 2Ar mixture at low pressure has been performed. The unsteady conservation equations for a chemically reacting multicomponent gas have been solved with the commercially available partial differential equation solver package PDEPACK. Heterogeneous radical destruction at a cooled surface was found to have a minor effect on flame quenching compared to homogeneous destruction in the cooled flame gases near the surface. These results are in qualitative agreement with experiments performed in a side-wall geometry. Cooled crevices, however, had a much greater effect on the postquench oxidation of the fuel. Whereas the fuel layer left after quenching near a flat wall burns up more rapidly as the pressure is increased, unburned fuel left in a crevice after quencing burns up more slowly as the pressure is increased. This reaffirms the potential importance of crevices as sources of unburned hydrocarbons in an engine.

Some further studies on the transition to turbulent convection

In a horizontal convecting layer of fluid, several distinct transitions occur at certain distinct Rayleigh numbersR, for a given Prandtl numberPr.The regime diagram has been extended to include the Prandtl-number range2·5 × 10−2[les ]Pr[les ] 0·85 × 104.In particular it is found that distinct changes in the slope of the heat-flux curve occur even forPr= 2·5 × 10−2. The flow is steady up toR=Rt= 2·4 × 103. ForR>Rt, the period of oscillation is compared with the theoretical values of Busse. ForPr= 0·71 decreases as well as increases in the slope of the heat-flux curve are observed.ForRjust greater thanRc, the preferred orientation of rolls in various side-wall geometries is investigated. For high Prandtl number, the effect of curvature of the roll axis, forced by curved side walls, upon the second transition atRIIis investigated. It is found that curvature, as well as previously discussed effects, leads to a lowering ofRII. These results, along with the observed hysteresis, support the view that there are metastable states attainable by finite amplitude instability. Finally the nature of the time-dependent flow at largeRand high Prandtl number is investigated in a Hele-Shaw cell. It is shown unequivocally that the observed periodicity at a fixed point is due to hot or cold plumes moving past the point.