Low Dorsal Fin Research Articles

Effect of low dorsal fin on the breakdown of vortices over a slender delta wing

Previous theoretical and experimental works show that the originally steady symmetric vortices over a flat-plate delta wing at zero side slip but high angles of attack are destabilized by the addition of a low dorsal fin, which renders the separation vortices asymmetric, unsteady, or non-conical. This paper examines the effect of the dorsal fin on the onset of vortex breakdown. A sharp-edged flat-plate delta wing with a 7.5 deg semi-apex angle is tested in a low-speed wind tunnel. The unsteady velocity and vorticity fields are mapped out at the 60% wing root chord location to quantify the vortex burst process caused by the addition of the low dorsal fins by using the laser Particle-Image-Visualization (PIV) technique. Two fin heights with the ratio of the local fin height to the local wing semi-span 0.3 and 0.6 are tested. The results demonstrate that the loss of global stability of the vortex configuration due to the addition of the low dorsal fin accelerates the onset of vortex breakdown and causes one of the vortices in the pair to burst periodically. The frequency and size of the burst are related to the height ratio of the fin.

Effect of Dorsal Fin on Symmetry of Vortices over Slender Delta Wing

Three different sets of experiments are designed to reveal the behavior of the vortex flow over a delta wing with different fin heights. A sharp-edged flat-plate delta wing with an 82.6-deg-sweep angle is tested in low-speed wind tunnels at angles of attack up to 35 deg. The same tests are performed on an identical delta-wing model but with a flat-plate dorsal fin. The ratio of the local fin height to the local wing semispan varies from 0.3 to 1.5. The results of smoke laser-sheet visualization, particle image velocimetry, and force measurement are obtained and presented. The test results show that the vortices over the delta-wing-alone model at high angles of attack and zero sideslip are stable, symmetric, and steady before vortex breakdown, but a low dorsal fin renders the originally symmetric vortices asymmetric at the same angle of attack; the flow recovers symmetry and conicity when a higher fin is added. These results are in agreement with predictions by the stability theoryCaiet al., (“Stability of Symmetric Vortices in Two Dimensions and over Three-Dimensional Slender Conical Bodies,” Journal of Fluid Mechanics, Vol. null, 2003, pp. 65–94). This agreement demonstrates that the absolute type of instability is responsible for loss of symmetry of the vortices over a delta wing with a fin within the range of tested conditions.

Description of Apistogramma paulmuelleri sp. n., a new geophagine cichlid species (Teleostei: Perciformes) from the Amazon river basin in Loreto, Peru

A new species of Apistogramma is described from Peru, based on a total of 28 specimens collected in a small forest stream in the catchment of a nameless tributary of the Rio Amazonas about 80 kilometres south of Iquitos, Departamento Loreto (approximately 73° 34′ W / 04°24′ S). At first sight Apistogramma paulmuelleri sp. n. resembles A. regani, but is differentiated from the latter and all other Apistogramma species by the combination of a large band-like spot on the caudal-fin base, four distinct abdominal stripes, a roundish, banded caudal fin, a low dorsal fin without any striking features, in adult males yellow on the chin between the gill covers and on parts of the branchiostegal membranes, ivory sides to the head below the cheeks, and reversal of the band pattern during aggression and courtship display. Apistogramma paulmuelleri sp. n. is currently thought to be a representative of the Apistogramma eunotus complex within the Apistogramma regani lineage.

Description of a new species of Laetacara KULLANDER,1986 from central Brazil and re-description of Laetacara dorsigera (HECKEL,1840) (Labroidei: Cichlidae: Cichlasomatinae)

A new species of Laetacara is described from the rio Verde, rio Araguaia basin, São Miguel do Araguaia, Goiás, Brazil; L. dorsigera is re-described. The new species from the rio Verde, rio Araguaia differs from all the species of the genus by its low dorsal-fin ray number. It is distinguished from L. thayeri by the presence of a caudal-fin peduncle spot and by the presence of cycloid scales on side of head. The new species also differs from L. fulvipinnis and L. fl avilabris by some meristic characters. It differs from L. dorsigera by a narrow ectopterygoid and by some meristic characters, and from L. curviceps and L. dorsigera by the lack of a spot on dorsal fin. Laetacara dorsigera differs from L. curviceps and the new species from the rio Verde, rio Araguaia basin by a wide ectopterygoid and by some meristic and morphometric characters; it is distinguished from the other species of the genus by some morphometric and meristic characters.

Two new species of Apistogramma Regan (Teleostei: Cichlidae) from the rio Trombetas, Pará State, Brazil

Apistogramma angayuara is described from the rio Trombetas close to the cachoeira Vira Mundo where it is found in association with rapids. It is assigned to the A. pertensis species group, distinguished by the following characters in combination: three prominent stripes composed of dark spots along the sides of the abdomen, 2 vs. 3 postlachrymal infraorbital pores, 5 vs. 4 dentary pores, low dorsal fin in adult males, and presence of a caudal spot. It is the smallest species of Apistogramma reported so far, with the largest male 24.7 mm SL and the largest female 22.7 mm SL, and the first cichlid species found with a significant proportion of rhizopods in the stomach content. Apistogramma salpinction is described from lentic habitats at the margin of road BR-163, circa 70 km from Cachoeira Porteira village, in a swamp most probably connected to the igarapé Caxipacoré. It is compared to members of the Apistogramma cacatuoides group with which it shares prolonged anterior dorsal fin lappets and marginal caudal fin streamers in adult males. It is distinguished from all other species of Apistogramma by the color pattern which includes a lateral band and abdominal stripes that become darker and have much lighter interspaces on the caudal peduncle, and a caudal spot that is divided into elongated blotches continuing the lateral band and upper two abdominal stripes, respectively.

Mouth and Body Form Relative to Feeding Ecology in the Fish Fauna of a Small Lake, Lake Opinicon, Ontario

In 14 cohabiting fish species in a small freshwater lake, mouth and body structures combine with food specializations and habitat preferences to greatly restrict interspecific competition within the fauna.The species differ quantitatively in a large number of structures and, individually and in combination, these are clearly adapted for distinctive roles. The mouth is particularly plastic, varying in position, in aperture width, and in overall form, with structures as diverse as a scoop, a beak, and a tube being found. Six basic body types occur and these, combined with varying fin morphologies, result in a range of distinctive forms, including the following. Micropterus salmoides, with a compressed fusiform body and a wide mouth, is a strong-swimming, widely ranging piscivore. Notemigonus crysopterus, with a long, slender caudal peduncle, subfalcate pectoral fins, and a deeply forked caudal fin, has great maneuverability that permits it to catch individual zooplankters. Lepomis macrochirus is a "sedentary," gibbose-bodied water-hanger. Umbra limi has a stubby, cylindrical body that favours life in dense vegetation. Labidesthes sicculus, with an almost straight dorsal line to the body, a low dorsal fin, and a beak-like snout with tweezer-like teeth, is modified for surface feeding and leaping out of the water. Ictalurus nebulosus has chemotactile barbels that favour bottom feeding and paired fins that function partly as hydrofoils, keeping the body inclined downwards as the mouth sweeps the bottom.The structural specializations give their owners a decided ecological advantage in certain situations. Only a few species, however, are limited by them to restricted ways of life. In most cases, a considerable measure of feeding flexibility is retained, presumably important for survival in cold temperate lakes.