When a bluff body is placed in a crossflow, the total temperature in its wake can become substantially less than the incoming one, as manifested by the fact that the recovery factor R on its rearmost surface takes negative values at high subsonic flow: this is the phenomenon referred to here as the Eckert-Weise effect. Although a vortex street has been a suspected cause, the issue of whether this is so, and what the mechanism is, has remained unsettled. In this experimental and theoretical investigation, we first examine the cause of the Eckert-Weise effect by enhancing the vortex shedding through acoustic synchronization: resonance between the vortex shedding and transversely standing acoustic waves in a wind tunnel. At the lowest synchronization, where a ringing sound emanates from the wind tunnel, R at the rearmost section of the cylinder is found to become negative even at a Mach number of 0.2; the base pressure (Cpb) takes dips correspondingly, indicative of the intensification of the vortex street. At this lowest acoustic resonance, the decrease of R and Cpb, uniform along the span, agrees with the expectation based on the spanwise uniformity of the lowest standing wave. At the next acoustic resonance where the standing wave now varies along the span, the corresponding dips in R and Cpb, non-uniform along the span, reveals an interesting ‘strip-theory’-like behaviour of the vortex intensities in the vortex street. These results correlating the change in R with Cpb confirm that the Eckert-Weise effect is indeed caused by the vortex shedding, the mechanism of which is examined theoretically in the latter half of the paper.A simple theoretical argument, bolstered by a full numerical simulation, shows that the time-varying static pressure field due to the vortex movement separates the instantaneous total temperature into hot and cold spots located around vortices; once time-averaged, however, the total temperature distribution conceals the presence of hot spots and takes the guise of a colder wake, the Eckert-Weise effect. Therefore the correct explanation of the Eckert-Weise effect, a time-averaged phenomenon, emerges only out of, and only as a superposition of, instantaneous total temperature separation around vortices. Such a separation is not confined to the outside of vortex cores; every vortex in its entirety becomes thermally separated. Nor is it limited to the far downstream equilibrium configuration of the Kármán vortex street but applies to the important near-wake vortices, and to any three-dimensional vortical structure as well. For low subsonic flows in particular, this dynamical explanation also leads to a similar separation of total pressure; these features may thus be potentially exploited as a general marker to identify and quantify vortices.

This paper discusses the mechanism of rolling friction under conditions where the deformations involved are predominantly elastic. Experiments on the rolling of a metal cylinder over a rubber surface show that interfacial slip of the type described by Reynolds is minute and totally insufficient to account for the observed resistance to rolling. It is shown quantitatively that the rolling resistance under these conditions is due to elastic hysteresis losses in the rubber. This accounts for the ineffectiveness of lubricants in reducing the rolling friction. Similar results are obtained for hard spheres rolling on rubber surfaces. If, however, a sphere is rolled in a preformed rubber groove, interfacial slip between the ball and groove may occur since the central band on the ball measures out a larger circle than the bands at the edge of the groove. A simple quantitative theory of this effect is given. This type of slip, first described by Heathcote, is very marked when the groove is deep and of curvature very close to that of the ball; under these conditions a suitable lubricant can effect a considerable reduction in the rolling resistance. For shallower grooves the differential slip is reduced and the Heathcote contribution to the observed rolling resistance becomes trivial. These conclusions are applied to the rolling friction of a hard steel sphere in the equilibrium groove formed in the surface of a softer metal (part I). If the rolling friction is attributed to the Heathcote mechanism a coefficient of friction within the ellipse of contact of the order of u = 1 to 2 must be invoked even in the presence of the best boundary lubricants. This is impossibly high. If, on the other hand, the rolling friction is attributed to hysteresis losses, large loss factors, greater than 20 %, must be invoked for example for copper surfaces. In order to resolve this difficulty a new experimental approach has been adopted in which a copper ball is rolled over an identical copper ball. Because of symmetry conditions at the region of contact both the Reynolds and the Heathcote type of slip are eliminated. The equilibrium rolling friction is found to be as high as that observed when a hard steel sphere rolls in a copper groove. It is concluded that interfacial slip contributes little to the rolling friction although it may play an important part in surface wear. Consequently, lubricants may reduce the amount of wear but have little effect on the rolling resistance. The greater part of this arises from elastic hysteresis losses within the metals themselves.

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spanning two sidewalls; the flow is from left to right, the freestream velocity being uniform and equal to 4.57 cm/s. (The Reynolds number based on the diameter is 467.) The centerline of the tunnel is marked by a dark solid line drawn on the bottom, quarter-span lines by broken lines, eighth-span lines by chain lines. In Fig. 2a, where a dye is injected at the leeside and near a sidewall, the presence of a strong span wise crosscurrent is clearly visible. Its direction always remains away from the wall. Observe in particular the hooked shape of a darker filament near the wall, which indicates a change from the initial streamwise convection to the lateral transport, carried presumably through a vortex core.T HE three experimental results to be described here are all independently obtained, but they display a common feature: vortical structures induce vigorous crossflow current to such a degree that in a flowing apparatus, which may be presumed to produce reasonably two-dimensional flow, strong three-dimensonality appears. The reason is as follows. The cores of vortices, with their lower pressure, lift and draw the fluid out of the side-wall boundary layer of the apparatus, in a manner quite similar to that of the updraft induced by a tornado near the ground.The phenomenon first came to our attention through a study of large-scale structures in a shear flow, followed by the similar observations obtained for the von Kdrm£n vortex street. Figure 1 shows a shear layer (a plan view) formed between two streams of.air at 160 and 300 cm/s separated by a movable splitter plate. The layer is perturbed under conditions of a two-dimensional sinusoidal oscillation at 115 Hz, corresponding to the Kelvin-Helmholtz in viscid instability. The flow is from left to right, and the end of the splitter plate is slightly to the left, off the picture. The width of the flow is 10 cm. Smoke is introduced on both sides of the splitter plate and is used to view the flow at five spanwise locations. The largescale coherent structures show up through accumulation of smoke and show their transverse nature in the photograph. Of particular notice are the tornado-like structures at the end of each of the main coherent structures. This appears to be the result of a suction inflow from the wall boundary layer. For spanwise locations closer to the centerline, the apparent transverse flow is smaller and disappears altogether at the centerline. Thus, two-dimensional flow only seems to exist in a triangular region at the beginning of the flowfield, even though the unit Reynolds number of the flow is relatively small. This transverse flow was also seen and studied by Koochesfahani, 1 although in wake flows of airfoils rather than in a shear layer (see also Ref. 2).We now turn to the von K£rm£n vortex street. In Figs. 2a and 2b, a cylinder of 1.27 cm diam is placed in a water tunnel,

The comparison and ranking of risks is very important for safety and cost-benefit analysis.Most formats present risks in the form of probability distribution. Different ranking criteria for probability distributions are considered. It is demonstrated that significantly overlapping distributions lead to ambiguous results. For this reason, criteria of insignificant overlapping distributions are proposed. The first criterion uses the information theory approach; and the second criterion uses the statistical tests approach. Both approaches can be applied to decision theory to avoid questionable decisions based on statistically insignificant differences between two risks.