Free-stream Speed Research Articles

Measurement and Analysis of the Flowfields Induced by Suction Perforations

The flow physics of laminar flow control suction surfaces is revealed by performing a detailed fundamental experimental investigation of an isolated suction perforation. A unique series of nonintrusive, high-resolution measurements are obtained using a three-component laser Doppler velocimetry system, and experiments are conducted in a low-speed, low-turbulence wind tunnel. The suction perforation flowfields are mapped for a range of sub- and supercritical suction rates and are found to be highly three dimensional. A rich variety of flowfield features is observed, including a pair of counter-rotating longitudinal vortices, multiple corotating longitudinal vortices, spanwise variations of the mean flow, and inherently unstable boundary-layer profiles. Critical suction limits, over a range of freestream speeds, are determined, and a new design criterion for critical suction is established. It is also shown that for sufficiently small perforations, irrespective of suction flow, boundary-layer transition does not occur. Further analyses of the measurements explore the possibility of interaction between the crossflow vortices and the suction-induced longitudinal vortices

Positive flow dependence in the initial colonization of a fouling community: results from in situ water current manipulations

During late winter/early spring (1992–1993), a series of 12 tapered channels were placed subtidally in a Long Island, New York (U.S.A) estuary to examine total fouling community development. Rather than relying upon flow-dependent correlations among spatially segregated sites, these channels directly accelerated or decelerated the existing tidal currents ± 50% (ambient flow speeds ranged from < 2 to > 50 cm/s). In response to experimentally manipulated water flows over five months, the arborescent hydroid Obelia longissima Pallas (1766) accumulated onto Tygon tubing in direct proportion to mean free-stream flow speed, whereas barnacle ( Balanus eburneus Gould) initial settlement and cumulative recruitment along plywood ceilings increased three-fold and five-fold, respectively, with a long-term doubling of water speed. These positive flow-dependent results directly contrast those from previous experiments in the same channel apparatus which failed to document any flow enhancement on the growth of suspension-feeding bivalves.

Chemical Induction of Larval Settlement Behavior in Flow.

The ability of dissolved chemical cues to induce larval settlement from the water column has long been debated. Through computer-assisted video motion analysis, we quantified the movements of individual oyster (Crassostrea virginica) larvae in a small racetrack flume at free-stream flow speeds of 2.8, 6.2, and 10.4 cm/s. In response to waterborne chemical cues, but not to seawater (control), oyster larvae moved downward in the water column and swam in slow curved paths before attaching to the flume bottom. Effective stimuli were adult-oyster-conditioned seawater (OCW) and a synthetic peptide analog (glycyl-glycyl-L-arginine) for the natural cue. The chemically mediated behavioral responses of oyster larvae in flow were essentially identical to those responses previously reported in still water. Our experimental results therefore demonstrate the capacity of waterborne cues to evoke settlement behavior in oyster pediveligers under varying hydrodynamic conditions.

Upwind Movement of Achenes of Centaurea eriophoraL. on the Ground

The lightly compressed achenes of Centaurea eriophoraL. bear a pappus composed of stiff bristles at their apex and have an elaiosome appendage at their base. The pappus is ineffective in keeping the achene wind-borne but does serve to regulate the movement of the achene on the ground in response to wind. In wind the achene swivels like a weather vane with the base of the achene pointing into the wind. In weak wind the pappus bristles prevent the achene from blowing away. In stronger wind the bristles move due to their flattened, flexible, hinge-like bases and act like ratchets against the substratum, thus enabling the achene to move upwind. In either case achenes remain in groups. Ants are attracted to the elaiosome and disperse the achenes. Wind-induced movement was explored by testing achenes on various substrata in a wind tunnel at free-stream speeds between 2 and 7 m s −1.

Effect of flow velocity and body size on swimming trajectories of sea scallops, Placopecten magellanicus (Gmelin): a comparison of laboratory and field measurements

The effect of flow velocity and body size on swimming trajectories of Placopecten magellanicus (Gmelin) was studied in a 5 m, flow-through laboratory flume and in a shallow (7 m depth) tidal channel in Lunenburg Bay, Nova Scotia, Canada. In the flume, scallops of 4 size classes, 11–20, 21–30, 31–50 and 51–80 mm shell height, (SH) were induced to swim at mean free-stream flow speeds of 0, 5, 9, 14 and 21 cm s −1. Swimming (active) and post-swimming (passive sinking) phases of individual trajectories were quantified in terms of swimming height, net displacement and direction, swimming time and swimming speed. Smaller scallops (≤ 30 mm) were adverted by the flow during both phases while larger scallops (> 30 mm) were advected only during the passive phase. Net horizontal displacement (active and passive stages combined) was positively related to flow speed for all size classes but the proportion of the variance explained by flow speed decreased with increasing scallop size. Most scallops swimming under flow had a downstream net swimming direction (finished their swimming phase downstream of the start point). For small scallops (≤ 30 mm SH), the proportion swimming downstream was independent of flow speed at flows of 0–5 cm s ( p > 0.30), but not independent of the flow speed at higher flows ( p < 0.01). For large scallops (> 30 mm SH), the proportion swimming downstream was independent of flow speed. In the field study, scallops ranging from 17–72 mm were induced to swim at current speeds ranging from 7–55 cm s −1. Model II regression and circular-linear correlation analysis showed a positive relationship between net horizontal displacement and current speed for scallops landing downstream and a negative relationship for scallops landing upstream, indicating the effect of advection. For small scallops that swam at medium (~ 18 cm s −1) current speeds, the distribution of displacement directions was significantly non-random and the mean direction did not differ significantly from the mean current direction. For large scallops that swam at both medium and high (~ 38 cm s −1) current speeds, the distribution of displacement directions was not significantly different from random. However, at high current speeds the mean size of scallops that landed upstream was significantly greater and the mean swimming height was significantly lower than for those that landed downstream. Flume and field measurements of swimming trajectories demonstrated that predictability of dispersion patterns of P. magellanicus released in bottom culture based on near-bed current data will be poor, especially when scallops larger than 30 mm SH are used.

Some effects of ground clearance and ground plane boundary layer thickness on the mean base pressure of a bluff vehicle type body

Abstract The use of a moving ground plane is an established wind tunnel technique for ground simulation in surface vehicle tests. However, the data available relating to the influence of variations in ground plane boundary layer thickness on bluff body flowfields is limited. Results from a short series of wind tunnel tests, using a moving ground facility, are presented, in which mean base pressure measurements are made on three simple bluff body models while; (i) the moving ground speed, relative to the freestream speed, and (ii) the model ground clearance, are varied. The results show that reducing the moving ground speed has a similar effect on the base pressure to a reduction in the model ground clearance. This effect on base pressure is limited to moving ground speeds less than those which result in a significant increase in the ground boundary layer displacement thickness, except in the case of a more complex tandem module body. The data for this model, chosen to simulate a typical cab-container commercial vehicle, suggest that the presence of a gap between the front and rear modules increases significantly the sensitivity of the base pressure to the moving ground speed. Data from this limited study may be useful when comparing measurements made on vehicles with particularly small ground clearances above fixed and/or moving ground planes. They may also serve as a guide to the assessment of the tolerance of a moving ground system to belt speed variations in terms of the ground boundary layer displacement thickness.

High-enthalpy, hypersonic compression corner flow

The results of an experimental investigation of high-enthalpy, hypersonic flow over sharp leading-edge compression corners are presented and discussed. In particular, the possible effects of real gas behavior are examined. Measurements have been made of the heat transfer and pressure distributions for flat plate and compression corner flow. Some flow visualization data have also been obtained. Test flows were generated using a free-piston shock tunnel operating in the reflected mode. The reservoir enthalpy ranged from 3 to 19 MJ kg -1 , giving freestream speeds of 2.3-5.5 km s -1 . For these conditions, the flow remains laminar throughout. The flat plate data for both high- and low-enthalpy flows are in agreement with the reference enthalpy method for heat transfer and the weak interaction theory for pressure. Also, the measured flat plate boundary-layer thickness compares well with an expression strictly valid for perfect gas flows only. The high- and low-enthalpy compression corner flows have upstream influence and plateau pressure behavior similar to perfect gas flow. That is, real gas effects for the present flows appear to be negligible. This is consistent with the essentially chemically frozen viscous and inviscid flow upstream of the interaction.

Flutter stability of a detuned cascade in subsonic compressible flow

A mathematical model is developed to predict the unsteady aerodynamics of a detuned two-dimensional flat plate cascade in subsonic compressible flow. Aerodynamic detuning is introduced by nonuniform circumferential spacing and chordwise offset. Combined aerodynamic-structural detuning is accomplished by replacing alternate airfoils with splitter blades. A torsion mode stability analysis that considers aerodynamic and combined aerodynamic-structural detuning is developed by combining the unsteady aerodynamic model with a single degreeof-freedom structural model. The effect of these detuning techniques on flutter stability is then demonstrated by applying this model to a baseline unstable 12-bladed rotor and detuned variations of this rotor. This study demonstrates that detuning is a viable passive flutter control technique. Nomenclature A = cascade A airfoil index a — freestream speed of sound B - cascade B airfoil index CL = lift coefficient CM - moment coefficient C, = ratio of chord length of cascade B to cascade A CA = chord length of cascade A CB = chord length of cascade B ea = elastic axis location k - reduced frequency a>c/W n = airfoil index os - chordwise offset of cascade B relative to cascade A

Spectral mapping of quasiperiodic structures in a vortex flow

Steady vortex flows over highly swept wings develop quasiperiodic velocity fluctuations. The nature of such fluctuations was explored extensively using a 59.3-deg cropped delta wing at 25-deg incidence in a low-speed wind tunnel. Additional single point tests were conducted over a range of incidences (16-32 deg) and Reynolds numbers (1.2 x 105 to 5.6 x 10 s) based on the root chord. Cross-spectral analysis of velocity fluctuations, sensed by two hot-wire sensors, was used to track these phenomena to the region of their origin as well as study the evolution and growth of the fluctuations. Results show the existence of narrow, dominant frequency bands containing the majority of the fluctuation energy. At 25 deg the quasiperiodicity originates near the 30% span region and the intensity amplifies downstream as the corresponding peak frequency decreases. Further downstream the frequency levels off while the intensity peaks, then decreases. Coherence trajectory mapping displays a helical path around the core of the vortex system. At a fixed location relative to the model, the product of the Strouhal number and the nominal wake scale was relatively constant with respect to freestream speed.

Numerical investigation of high incidence flow over a double-delta wing

The vortical flowfield over a double-delta wing configuration, consisting of a sharp leading-edge 76-deg sweep strake and a 40-deg sweep wing section is investigated numerically. The governing equations are solved with a partially upwind, finite difference, two-factor algorithm. The leeward-side vortex system resulting from the strake and wing vortices is investigated for a subsonic freestream speed of M^ = 0.22, high Reynolds number turbulent flow at various angles of incidence. At low angles of attack the strake and wing vortices remain separate over the wing section, whereas for flows at higher angles of attack the two vortices merge and vortex breakdown develops. Vortex breakdown appears initially in the trailing-edge region of the wing section. As the angle of attack increases, bursting occurs further upstream closer to the strake section. The effect of numerical grid density is investigated, and the solutions are compared with available experimental data. The computed surface pressures are in good agreement with the experimental measurements for the lower angles of attack, but the agreement deteriorates as the angle of attack increases.

Building downwash of plumes and plume interactions

This paper presents a number of wind tunnel measurements of building induced downwash effects on the plume in a neutrally stable boundary layer. Vertical and horizontal profiles of mass concentrations of the exhausts from stacks on a rectangular model building were measured downwind of the buoyant source in the near wake. These experiments were conducted in a simulated natural wind over open country terrain for various freestream wind speeds with different stack and building configurations. Significant downwash effects from the building below the stack were observed in all tested configurations. In this preliminary study, only mean concentrations were evaluated and are presented in non-dimensional form.

Aspiration efficiency of a thin-walled cylindrical probe rear-facing the wind

The sampling mechanism of a thin-walled, cylindrical aerosol sampling probe which faces directly away from the wind (180° orientation) is numerically investigated. The turbulent fluid flow is predicted by employing the control volume, finite-difference method and κ-ε turbulence model. The particle trajectories are calculated by integrating the particle equations of motion and thus the aspiration efficiency of the sampler is determined. The numerical results have been obtained for two very long thin-walled cylindrical samplers of diameters D = 2 and 5 cm which have zero thickness and the operating conditions as used in the experimental investigations of Vincent et al. (1986, J. Aerosol Sci. 17, 211–224) have been considered. The dependence of the characteristics of the fluid flow and the aspiration efficiency on the freestream air speed, the diameter of the sampler and the ratio of the freestream speed to the sampling speed, have been thoroughy investigated. It is concluded that the numerical results for the aspiration efficiency are in reasonable agrement with all the existing experimental data for thin-walled samplers which are placed at 180° to the direction of the wind.

Structure and stability of a laminar diffusion flame in a compressible, three-dimensional mixing layer

In this paper we consider the structure and stability of a three-dimensional, finite rate, reacting compressible mixing layer lying between two streams of reactants with different freestream speeds and temperatures. Using both numerical calculations and asymptotic analyses, the structure of the ignition and diffusion flame regimes was studied; in particular the effect of crossflow on the structure. The results of both approaches are in good agreement. In the stability analysis the general case of three-dimensional disturbances was studied. It was shown that certain general results, in particular the circle theorem, could be easily extended to three-dimensional disturbances in a compressible fluid with crossflow. The introduction of a chemical reaction, in the form of a flame sheet, was found to have complex effects on flow stability. These included the appearance of additional neutral modes compared with the nonreacting case and substantial changes in growth rates with heat release, the skew angle of the mean flow, and the direction of propagation of the disturbance wave. These results are discussed in detail.

Disklike body sampling in a turbulent wind

In this paper a study is made of the sampling characteristics of a diskshaped sampler facing a turbulent wind. The Reynolds-averaged Navier-Stokes equation and the k- ε turbulence model are used to predict the turbulent mean fluid motion past an axially symmetric disklike sampler, where the sampler is taken to have a single orifice on the side facing the oncoming flow. Based on the resulting velocity distribution of the fluid flow, the equations of motion of the particle are traced in order to obtain the particle trajectories which are then used to determine the aspiration efficiency of the sampler. The dependence of the aspiration efficiency on the five parameters, namely, the Stokes number St, the velocity ratio of the free stream speed to the suction speed ( U O U s ), the ratio of the sampler diameter to the orifice diameter ( D O D ), the Reynolds number (Re) and the turbulence intensity ( I) has systematically been investigated. It is of interest to note that a comparison of the present results with the proposed impaction model reveals some interesting phenomena. Of all the theoretical predictions, the present computational results give the best agreement with all the available experimental data and this agreement is good. Thus the model may be extended to study the flow and particle characteristics for three-dimensional samplers.

An inverse method with regularity condition for transonic airfoil design

It is known from Lighthill's exact solution of the incompressible inverse problem that in the inverse design problem the surface pressure distribution and the free stream speed cannot both be prescribed independently. This implies the existence of a constraint (regularity condition) on the prescribed pressure distribution. The same constraint exists at compressible speeds. In this paper, an inverse design method for transonic airfoil is presented. In the method, the target pressure distribution contains a free parameter that is adjusted during the computation to satisfy the regularity condition derived in this paper. A few design results are presented here in order to demonstrate the capability of the method.

Microbubble skin friction reduction on an axisymmetric body under the influence of applied axial pressure gradients

The influence of both a favorable and an adverse applied axial pressure gradient on microbubble-induced skin friction reduction was examined. An 87 mm diameter, 632 mm long model equipped with a 273 mm long cylindrical force balance was employed. Experiments were carried out in a 305 mm diameter water tunnel, at free-stream speeds of 4.6, 7.6, 10.7, 13.7, and 16.8 m/sec. Air was injected at rates as high as 12×10−3 m3/sec. Measurement of the static pressure along the body with gas injection demonstrated that gas injection did not alter the pressure gradient and that the flow remained axisymmetric. Reductions in skin friction for the zero pressure gradient case agreed well with the earlier results of Deutsch and Castano [Phys. Fluids 29, 3590 (1986)]. The adverse-gradient-induced separation of the boundary layer for speeds at and above 7.6 m/sec, for air injection rates in excess of 5.0×10−3 m3/sec. The favorable gradient strongly inhibited the drag reduction mechanism [47].

Three-dimensional simulations of large eddies in the compressible mixing layer

The effect of Mach number on the evolution of instabilities in the compressible mixing layer is investigated. The full time-dependent compressible Navier–Stokes equations are solved numerically for a temporally evolving mixing layer using a mixed spectral and high-order finite difference method. The convective Mach number Mc (the ratio of the velocity difference to the sum of the free-stream sound speeds) is used as the compressibility parameter. Simulations with random initial conditions confirm the prediction of linear stability theory that at high Mach numbers (Mc &gt; 0.6) oblique waves grow more rapidly than two-dimensional waves. Simulations are then presented of the nonlinear temporal evolution of the most rapidly amplified linear instability waves. A change in the developed large-scale structure is observed as the Mach number is increased, with vortical regions oriented in a more oblique manner at the higher Mach numbers. At convective Mach numbers above unity the two-dimensional instability is found to have little effect on the flow development, which is dominated by the oblique instability waves. The nonlinear structure which develops from a pair of equal and opposite oblique instability waves is found to resemble a pair of inclined A-vortices which are staggered in the streamwise direction. A fully nonlinear computation with a random initial condition shows the development of large-scale structure similar to the simulations with forcing. It is concluded that there are strong compressibility effects on the structure of the mixing layer and that highly three-dimensional structures develop from the primary inflexional instability of the flow at high Mach numbers.

Moving surface boundary-layer control as applied to two-dimensional airfoils

The concept of moving surface boundary-layer control, as applied to a Joukowsky airfoil, is investigated through an experimental program complemented by a flow visualisation study. The moving surface was provided by rotating cylinders located at the leading edge and upper surface of the airfoil. The results suggest that the leading-edge rotating cylinder effectively extends the lift curve without substantially affecting its slope, thus increasing the maximum lift and delaying stall. When used in conjunction with a second cylinder on the upper surface, further improvements in the maximum lift and stall angle are possible. The maximum coefficient of lift realised was around 2.73, approximately three times that of the base airfoil. The maximum delay in stall was around 48 deg. In general, the performance improves with an increase in the ratio of cylinder surface speed Uc to the freestream speed U.

Wind tunnel simulation of airflow and pollutant diffusion over complex terrain

A wind tunnel simulation of airflow and pollutant diffusion has been conducted for Mt. Tsukuba, a rather isolated mountain. the wind tunnel simulation was carried out in the Thermally Stratified Wind Tunnel of the National Research Institute for Pollution and Resources. The scale of the topographic model was 1/7500 for the horizontal and 1/5000 for the vertical. A nocturnal inversion was simulated in the wind tunnel by heating the airflow to 40°C and chilling the floor of the test section at 10°C. The wind tunnel free stream speed was 0.5 m s −1. The airflow was found to be satisfactorily simulated in the wind tunnel. The upstream flow stagnation, the dividing streamline and the wake in the lee of the mountain were reproduced.

Local shear stress measurements on an axisymmetric body in a microbubble modified flow field

An array of flush-mounted hot film probes has been used to measure the local shear stress reduction as a result of microbubble injection over an axisymmetric body at the four discrete, free-stream speeds of 4.6, 10.7, 13.2, and 16.8 m/sec. Visualization of the bubble flow pattern supplement these results at intermediate free-stream speeds. At speeds of 10.7 m/sec and above, a circumferential gradient in skin friction, with skin friction reduction larger at the top than at the bottom of the model occurs at some distance downstream of injection. For these speeds, the gradient is stronger at the lower speeds and higher gas injection conditions. Higher speeds tend to drive the axial location of the gradient farther from the injection location. At speeds below 10.7 m/sec, the flow is dominated by a double vortex structure that entrains the bubbles at the bottom and sides of the model and transports them to the top. At sufficiently high gas flow rates a cavity, large enough to be observed visually, is formed just upstream of the vortices, centered near the body midline. The axial position of the cavity is roughly independent of flow speed and gas flow conditions. The transport of bubbles by the vortices, to the top of the body, is the cause of the poor skin friction reduction performance of microbubble injection at low speeds on an axisymmetric shape. Integration of the current local skin friction results gives good agreement with earlier drag balance measurements. The persistence of the drag reduction phenomenon with axial distance as well as the statistics of the shear stress fluctuations are quite similar to what was observed earlier on flat plates.