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In real application, once the pattern of fin is determined, fin spacing of tube bank fin heat exchanger can be adjusted in a small region, and air flow velocity in the front of the heat exchanger is not all the same. Therefore, the effects of fin spacing on heat transfer performance of such heat exchanger are needed. This paper numerically studied the optimal fin spacing regarding the different front flow velocities of a circular tube bank fin heat exchanger with vortex generators. To screen the optimal fin spacing, an appropriate evaluation criterion JF was used. The results show that when front velocity is 1.75 m/s, the optimal fin spacing is 2.25 mm, when front velocity is 2.5 m/s, the optimal fin spacing is 2 mm, and when front velocity is higher than 2.5 m/s, the optimal fin spacing is 1.75 mm. List of symbolsA Cross section area of flow passage (m 2 ) A front Cross section area of flow passage at front inlet (m 2 ) c p Specific heat capacity (J/kg K) D Diameter of the tube (m) d e Characteristic length of flow channel (m) F Total surface involved in heat transfer in the computational domain (m 2 ) f Friction factor, f = Dpd e /(L x qu max 2 /2) h Convective heat transfer coefficient (W/m 2 K) H Height of winglet type vortex generator (m) J n ABS Absolute vorticity flux along the normal direction of cross section (1/s) J n ABS;S Cross section average absolute vorticity flux (1/s) J n ABS;V Volume average absolute vorticity flux (1/s) L Base length of vortex generator (m) L x Stream wise length of fin (m) n Direction normal to the cross section or wall surface N 2 Number of the tubes Nu Nusselt number, Nu = hd e /k p Pressure (Pa) Re Reynolds number, Re = qu max d e /l S 1 Transversal pitch between the tubes (m) S 2 Longitudinal pitch between the tubes (m) T p Net fin spacing (m) T Temperature (K) u front Velocity of air at front inlet (m/s) u in Velocity of air at fin passage inlet (m/s) u max Maximum average velocity of air (m/s) u i , u, v, w Components of velocity vector (m/s) x, y, z CoordinatesGreek variables k Thermal conductivity (W/m K) lViscosity (kg/m s) qDensity (kg/m 3 )

Fish have appeared since Precambrian more than 500 million years ago. Yet, there are still much untamed areas for fish propulsion research. The swordfish has evolved a light thin/high crescent tail fin for pushing a large amount of water backward with a small velocity difference. Together with a streamlined forward-enlarged thin/high body and forwardbiased dorsal fin enclosing sizable muscles as the power source, the swordfish can thus achieve unimaginably high propulsion efficiency and an awesome maximum speed of 130 km/h as the speed champion at sea. This paper presents the innovative concepts of ''kidnapped airfoils'' and ''circulating horsepower'' using a vivid neat-digit model to illustrate the swordfish's superior swimming strategy. The body and tail work like two nimble deformable airfoils tightly linked to use their lift forces in a mutually beneficial manner. Moreover, they use sensitive rostrum/lateral-line sensors to detect upcoming/ambient water pressure and attain the best attack angle to capture the body lift power aided by the forwardbiased dorsal fin to compensate for most of the water resistance power. This strategy can thus enhance the propulsion efficiency greatly to easily exceed an astonishing 500%. Meanwhile, this amazing synergy of force/beauty also solves the perplexity of dolphin's Gray paradox lasting for more than 70 years and gives revelations for panoramic fascinating future studies.

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This paper focuses on flow visualization, normal force and pitch moment testing of the NASA TP-1803 strake-wing model at high attack angles. The dynamic aerocharacteristics for pitching with various reduced frequencies and two sideslip angles ¼ 0 and 10 in the water tunnel are compared with those for static case. For ¼ 20 -50 , the strake/wing vortices breakdown positions occur later for ¼ 10 than for ¼ 0 . The value of the normal force coefficient under sideslip angle ¼ 10 is greater than ¼ 0 at high attack angles. In the pitch-down process, the aerodynamic center creates a nose-up pitching moment and the model becomes unstable compared to the static condition. As the pitch reduced frequency increases, the wing vortices sustain longer flow lines and provide more normal force during pitch-up motion. In addition, the hysteresis loop of the normal force curve is larger for higher reduced frequencies.

Based on the dynamic simulation model, the dynamic response index of vehicle system under the action of track irregularity is divided into three areas: repair, deterioration and maintenance. The correlation between the track irregularity index and the dynamic response index domain of vehicle system components is calculated and statistically studied. The estimation model of dynamic response index domain of vehicle system and the domain boundaries of different dynamic response indexes are established and obtained Line. According to the principle of single variable method, the excitation source of vehicle track system is divided into track irregularity and other comprehensive factors (such as temperature load, material damage, etc.), and a simple inversion method of track foundation state is proposed based on the estimation model of dynamic response index domain. Its basic principle is: if the statistical characteristics of track irregularity remain unchanged and other influencing factors change, the estimation domain and measurement domain of dynamic response index will produce grade jump, so as to determine whether the basic state of the line is normal. The simulation results show that the accuracy of domain estimation of dynamic response index of vehicle system is more than 80%, and the accuracy of recognition is more than 70% for the damage condition of the line infrastructure, where the fastener is empty.