The influence of wing–wake interactions on the production of aerodynamic forces in flapping flight

We used two-dimensional digital particle image velocimetry (DPIV) to visualize flow patterns around the flapping wing of a dynamically scaled robot for a series of reciprocating strokes starting from rest. The base of the wing was equipped with strain gauges so that the pattern of fluid motion could be directly compared with the time history of force production. The results show that the development and shedding of vortices throughout each stroke are highly stereotyped and influence force generation in subsequent strokes. When a wing starts from rest, it generates a transient force as the leading edge vortex (LEV) grows. This early peak, previously attributed to added-mass acceleration, is not amenable to quasi-steady models but corresponds well to calculations based on the time derivative of the first moment of vorticity within a sectional slice of fluid. Forces decay to a stable level as the LEV reaches a constant size and remains attached throughout most of the stroke. The LEV grows as the wing supinates prior to stroke reversal, accompanied by an increase in total force. At stroke reversal, both the LEV and a rotational starting vortex (RSV) are shed into the wake, forming a counter-rotating pair that directs a jet of fluid towards the underside of the wing at the start of the next stroke. We isolated the aerodynamic influence of the wake by subtracting forces and flow fields generated in the first stroke, when the wake is just developing, from those produced during the fourth stroke, when the pattern of both the forces and wake dynamics has reached a limit cycle. This technique identified two effects of the wake on force production by the wing: an early augmentation followed by a small attenuation. The later decrease in force is consistent with the influence of a decreased aerodynamic angle of attack on translational forces caused by downwash within the wake and is well explained by a quasi-steady model. The early effect of the wake is not well approximated by a quasi-steady model, even when the magnitude and orientation of the instantaneous velocity field are taken into account. Thus, the wake capture force represents a truly unsteady phenomenon dependent on temporal changes in the distribution and magnitude of vorticity during stroke reversal

Modulation of swimming in the gastropod Melibe leonina by nitric oxide

Nitric oxide (NO) is a gaseous intercellular messenger produced by the enzyme nitric oxide synthase. It has been implicated as a neuromodulator in several groups of animals, including gastropods, crustaceans and mammals. In this study, we investigated the effects of NO on the swim motor program produced by isolated brains and by semi-intact preparations of the nudibranch Melibe leonina. The NO donors sodium nitroprusside (SNP, 1 mmol l–1) and S-nitroso-N-acetylpenicillamine (SNAP, 1 mmol l–1) both had a marked effect on the swim motor program expressed in isolated brains, causing an increase in the period of the swim cycle and a more erratic swim rhythm. In semi-intact preparations, the effect of NO donors was manifested as a significant decrease in the rate of actual swimming. An NO scavenger, reduced oxyhemoglobin, eliminated the effects of NO donors on isolated brains, supporting the assumption that the changes in swimming induced by donors were actually due to NO. The cGMP analogue 8-bromoguanosine 3′,5′-cyclic monophosphate (1 mmol l–1) produced effects that mimicked those of NO donors, suggesting that NO is working via a cGMP-dependent mechanism. These results, in combination with previous histological studies indicating the endogenous presence of nitric oxide synthase, suggest that NO is used in the central nervous system of Melibe leonina to modulate swimming

Defined mutations in the 5' nontranslated sequence of Sindbis virus RNA

We have constructed 24 deletion mutants which contain deletions of from 1 to 15 nucleotides in the 5' nontranslated region of Sindbis virus RNA and tested the effect of these mutations on virus replication. The results showed that the first 44 nucleotides, which are capable of forming a hairpin structure, are important for virus replication, as all deletions tested in this region were either lethal or resulted in virus that grew poorly in comparison to the parental virus. Many of these deletions had different effects in mosquito cells than in chicken cells, suggesting that cellular factors, presumably proteins, bind to this region. This domain may function in at least two processes in viral replication. It seems likely that in the minus strand, this sequence element is bound by the viral replicase and promotes RNA replication. In the plus strand, this element may modulate initiation of translation of the nonstructural proteins. The results suggest that the hairpin structure itself is important. All deletions within it had deleterious effects on virus replication, and in particular, deletion of one of the G residues at nucleotide 7 or 8 or of one of the C residues at nucleotide 36 or 37 which are theoretically base-paired with these G's resulted in temperature-sensitive viruses that behaved very similarly. In contrast, large deletions between the 44-nucleotide hairpin and the translation start site at nucleotides 60 to 62 resulted in virus that grew as well as or better than the parental virus in both chicken and mosquito cells. The A residue at position 5 of the HRSP strain used was examined in more detail. Deletion of this A was lethal, whereas substitution by G resulted in a virus that grew poorly, despite the fact that G is present at position 5 in the AR339 parent of HRSP. U at position 5 resulted in a virus that grew less well than the A5 strain but better than the G5 mutant

Mutagenesis of the conserved 51-nucleotide region of Sindbis virus

We have constructed 25 site-specific mutations in a domain of 51 nucleotides in Sindbis virus that is highly conserved among all alphaviruses sequenced to date. These 51 nucleotides are capable of forming two hairpin structures and are found from nucleotides 155 to 205 in Sindbis virus within the region encoding nsP1. Of the mutations, 21 were silent and did not lead to a change in the amino acid sequence encoded. These silent mutations changed not only the linear sequence but also the stability of the hairpins in most cases. Two double mutants that were constructed led to the replacement of one base pair by another so that the linear sequence was altered but the nature of the hairpins was not. All of the mutants with silent mutations were viable, but 19 of the 21 mutants were severely impaired for growth in both chicken and mosquito cells. Compared with the parental virus, they grew slowly and produced virus at rates of 10(-1) to 10(-4) times the parental rate. Surprisingly, however, the plaques produced by these mutants were indistinguishable from those produced by the parental virus. Two of the silent mutations, found within the first hairpin structure, produced virus at a faster rate than the parental virus. It is clear that the exact sequence of this region is important for some aspect of virus replication. We suggest that one or more proteins, either virus encoded or cellular, bind to the hairpin structures in a sequence-specific fashion in a step that promotes replication of the viral RNA. Of the mutations that resulted in a change of coding, only one of four was viable, suggesting that the amino acid sequence encoded in this domain is essential for virus replication

Recent Evidence on Budget Deficits and National Savings

This paper examines the recent United States experience with sustained budget deficits and concludes that the events of the last five years cast significant doubt on the proposition that the timing of taxes does not affect national savings. Rather than raising private saving, the recent deficits have if anything coincided with reduced saving and increased consumption. These findings suggest that realistic analysis of fiscal policies must recognize that consumers are liquidity constrained and/or myopic.

Finite Lifetimes and the Crowding Out Effects of Budget Deficits

This note explores the sensitivity of the short-run savings effects of government deficits to assumptions about household planning horizons. Using a lifecycle simulation model, we show that even though deficit policies shift sizable tax burdens to future generations, individuals live long enough to make the assumption of an infinite horizon a good approximation for analyzing the short-run savings effects. In practice, periods of debt accumulation such as that in the United States during World War II are reversed sufficiently rapidly to make their short-run effects on consumption and national savings relatively small.

Exchange rates and fundamentals: a generalization

Exchange rates have raised the ire of economists for more than twenty years. The problem is that few, if any, exchange rate models are known to systematically beat a naive random walk in out-of-sample forecasts. Engel and West (2005) show that these failures can be explained by the standard present value model (PVM) because it predicts random walk exchange rate dynamics if the discount factor approaches one and fundamentals have a unit root. This paper generalizes the Engel and West hypothesis to the larger class of open economy dynamic stochastic general equilibrium (DSGE) models. The Engel and West hypothesis is shown to hold for a canonical open economy DSGE model. We show that all the predictions of the standard PVM carry over to the DSGE PVM. The DSGE PVM also yields unobserved components (UC) models that we estimate using Bayesian methods and a quarterly Canadian-U.S. sample. Bayesian model evaluation reveals that the data support a UC model that calibrates the discount factor to one, implying the Canadian dollar–U.S. dollar exchange rate is a random walk dominated by permanent cross-country monetary and productivity shocks.Foreign exchange rates

Tectonic setting of Martian volcanoes and deep-seated intrusives

More than 50 volcanoes have been mapped on Mars, and recent geologic studies indicate structural evidence of deep seated intrusive bodies. Most volcanoes in the Tharsis region are volcanotectonic features; they have been associated with large scale tectonic and volcanic processes. They occur along complex systems of faults and grabens having a dominant northwest to southwest trend closely coincident with a great circle, which extends along 90 deg of arc from Tempe Patera to probable volcanic mountains near lat. 40 deg S, long. 150 deg. Deep seated intrusive bodies are also concentrated in the Tharsis region and are recognized mostly where faults have been deflected around their cores. The Elysium Mons-Amphitrites Patera volcanic alignment is subparallel to that of Tharsis but is longer, extending through about 120 deg of arc; it transects the dichotomy boundary and is radial to the Hellas basin. Volcanoes in the Tharsis region have the widest age range of all volcanoes on Mars, as determined by the size-frequency distribution of their craters having diameters of 2, 5, and 16 km

Dividend Taxes, Corporate Investment, and "Q"

Taxes on corporate distributions have traditionally been regarded as a "double tax" on corporate income. This view implies that while the total effective tax rate on corporate source income affects real economic decisions, the distribution of this tax burden between the shareholders and the corporation is irrelevant. Recent research has suggested an alter- native to this traditional view. One explanation of why firms in the U.S. pay dividends in spite of the heavy tax liabilities associated with this form of distribution is that the stock market capitalizes the tax payments associated with corporate distributions. This capitalization leaves investors indifferent at the margin between corporations paying our dividends and retaining earnings. This alternative view holds that while changes in the dividend tax rate will affect shareholder wealth, they will have no impact on corporate investment decisions. This paper develops econometric tests which distinguish between these two views of dividend taxation. By extending Tobin's "q" theory of investment to incorporate taxes at both the corporate and personal levels, the implications of each view for corporate investment decisions can be derived. The competing views may be tested by comparing the performance of investment equations estimates under each theory's predict ions. British time series data are particularly appropriate for testing hypotheses about dividend taxes because of the substantial postwar variation in effective tax rates on corporate distributions. The econometric results suggest that dividend taxes have important effects on investment decisions.